Electrochemical Modulation of Fluorescence of
Nitrogen Vacancy Centers in Nanodiamonds for
Voltage Sensing Applications
by
Reyu Sakakibara
B.S., Chemical Biology
University of California, Berkeley (2012)
Submitted to the Department of Electrical Engineering and Computer
Science
in partial fulfillment of the requirements for the degree of
Master of Science in Electrical Engineering and Computer Science
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Feburary 2015
© Massachusetts Institute of Technology 2015. All rights reserved.
Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Department of Electrical Engineering and Computer Science
January 30, 2015
Certified by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dirk Englund
Assistant Professor of Electrical Engineering
Thesis Supervisor
Accepted by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Leslie Kolodziejski
Chair of the Committee on Graduate Students
2
Electrochemical Modulation of Fluorescence of Nitrogen
Vacancy Centers in Nanodiamonds for Voltage Sensing
Applications
by
Reyu Sakakibara
Submitted to the Department of Electrical Engineering and Computer Science
on January 30, 2015, in partial fulfillment of the
requirements for the degree of
Master of Science in Electrical Engineering and Computer Science
Abstract
The nitrogen vacancy (NV) color center in diamond has been used to sense environmental variables such as temperature and electric and magnetic fields. Most sensing
protocols depend on the optically detectable magnetic resonance of the negatively
charged NV− spin state. As such, fluctuations in the NV charge state present a challenge for NV− spin-based sensing. This thesis discusses the electrochemical modulation
of NV charge state and fluorescence as the basis for an alternative sensing scheme.
An externally applied electrochemical potential shifts the occupation probabilities of
the NV in each charge state, which manifest as changes in NV fluorescence intensity
and emission spectra.
In this thesis, the voltage dependence of fluorescence in high pressure high temperature nanodiamonds is demonstrated in an electrochemical cell. Following this, the
mechanisms for NV response to externally applied electrical bias are investigated in
other electrochemical cell morphologies, capacitors, and interdigitated electrode arrays.
Finally, a design of an optical microscope setup for future studies of NV sensing in
nanodiamond is outlined.
Thesis Supervisor: Dirk Englund
Title: Assistant Professor of Electrical Engineering
3
4
Acknowledgments
I would like to thank my research supervisor Professor Dirk Englund for his patience,
guidance, and support.
My thanks extend to mentors and collaborators in the Quantum Photonics Group.
I am grateful to Sinan Karaveli, Edward Chen, Tim Schröder, Matt Trusheim, Hannah
Clevenson, and Sara Mouradian for technical advice and moral support. In particular, I
would like to thank Sinan for his extensive mentorship. I would also like to mention my
officemates, including Sara Mouradian, a great friend; Mihir Pant, Darius Bunandar,
and Tsung-Ju Jeff Lu whose banter makes me laugh.
I would also like to thank my undergraduate mentor Professor Naomi Ginsberg, for
her boundless optimism and encouragement, and my academic advisor and graduate
officer Professor Leslie Kolodziejski, also for encouragement and for valuable advice.
In addition, I would like to thank my family for their love, especially my parents
and my grandparents; my Friday night dinner friends; and my close friends Samantha
Dale Strasser and Jessica Weaver. Last but not least, I would like to thank Clément
Pit--Claudel for chocolate cakes and LATEX tips.
5
6
Contents
1 Introduction
1.1
17
The nitrogen vacancy center in diamond . . . . . . . . . . . . . . . .
18
1.1.1
Overview of the NV− defect center . . . . . . . . . . . . . . .
18
1.1.2
Basics of NV− spin based sensing . . . . . . . . . . . . . . . .
20
1.1.3
NV charge state control . . . . . . . . . . . . . . . . . . . . .
22
1.2
Thesis objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
1.3
Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
2 Spectroelectrochemical setup and measurement
2.1
2.2
27
Spectroelectrochemical setup . . . . . . . . . . . . . . . . . . . . . . .
27
2.1.1
Microscope setup . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.1.2
Sample preparation . . . . . . . . . . . . . . . . . . . . . . . .
28
2.1.3
Measurement procedure and analysis . . . . . . . . . . . . . .
32
Initial results on voltage dependent nanodiamond fluorescence . . . .
34
2.2.1
Wide field measurements . . . . . . . . . . . . . . . . . . . . .
35
2.2.2
Confocal measurements . . . . . . . . . . . . . . . . . . . . . .
38
2.2.3
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
3 Theoretical analysis of voltage dependent nanodiamond fluorescence 45
3.1
Possible mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
3.1.1
Schematic of the electrochemical cell interfaces . . . . . . . . .
46
3.1.2
Band bending change with applied electric field . . . . . . . .
48
3.1.3
Charge injection
49
. . . . . . . . . . . . . . . . . . . . . . . . .
7
3.2
Fluorescence time trace fits . . . . . . . . . . . . . . . . . . . . . . .
3.2.1
3.2.2
49
Calculations for energy levels in diamond and diamond-ITO
junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
Fits to the experimental fluorescence time traces . . . . . . . .
53
4 Investigation of charge injection with variations on the electrochemical cell
59
4.1
Isolation of electrochemical cell interfaces . . . . . . . . . . . . . . . .
60
4.1.1
Spacer types and their fabrication . . . . . . . . . . . . . . . .
60
4.1.2
Isolating the diamond-working electrode interface . . . . . . .
63
4.1.3
Isolating the diamond-electrolyte interface . . . . . . . . . . .
64
4.1.4
Isolating both interfaces . . . . . . . . . . . . . . . . . . . . .
65
4.1.5
Summary of results . . . . . . . . . . . . . . . . . . . . . . . .
65
Indium zinc oxide working electrode . . . . . . . . . . . . . . . . . . .
65
4.2.1
Diamond-IZO interface . . . . . . . . . . . . . . . . . . . . . .
67
4.2.2
Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
4.2.3
Measurement results . . . . . . . . . . . . . . . . . . . . . . .
67
4.2
5 Investigation of electric field effect with dry cells
5.1
5.2
73
Parallel plate capacitors . . . . . . . . . . . . . . . . . . . . . . . . .
73
5.1.1
Polystyrene bead spacer . . . . . . . . . . . . . . . . . . . . .
76
5.1.2
Poly(methyl methacrylate) spacer . . . . . . . . . . . . . . . .
77
5.1.3
Hydrogen silsesquioxane spacer . . . . . . . . . . . . . . . . .
79
5.1.4
Sputtered silicon dioxide spacer . . . . . . . . . . . . . . . . .
80
5.1.5
SiO2 spacer made with plasma enhanced chemical vapor deposition 82
5.1.6
Summary of parallel plate capacitor results . . . . . . . . . . .
84
Interdigitated Electrodes . . . . . . . . . . . . . . . . . . . . . . . . .
84
5.2.1
Fabrication process flow . . . . . . . . . . . . . . . . . . . . .
85
5.2.2
Measurement results . . . . . . . . . . . . . . . . . . . . . . .
88
5.2.3
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
8
6 Microscope setup modification design
95
6.1
Microscope reflector turret . . . . . . . . . . . . . . . . . . . . . . . .
96
6.2
Excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
6.2.1
Wide field excitation design . . . . . . . . . . . . . . . . . . .
97
6.3
Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
99
6.4
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
7 Conclusions
101
9
10
List of Figures
1-1 Schematic of the NV center in a diamond lattice. . . . . . . . . . . .
18
1-2 Schematic of the NV− electronic structure. Based on [21] and [23]. The
relative energies of the triplet and singlet states are currently unknown. 18
1-3 NV− center emission spectrum in 25 nm high pressure high temperature
(HPHT) nanodiamond. Image printed with permission from Sinan
Karaveli. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
1-4 ODMR spectrum of bulk IIa diamond at zero field. Red dots indicate
experimental data points, with error bars. Blue line is the fitted curve.
Image printed with permission from Christopher Foy. . . . . . . . . .
20
1-5 Simulation of ODMR spectrum showing two resonances. Image printed
with permission from Christopher Foy. . . . . . . . . . . . . . . . . .
21
1-6 Schematic of the NV0 electronic structure. Based on [21]. The relative
energies of the doublet and quartet are currently unknown. . . . . . .
23
1-7 NV− and NV0 emission spectra in 25 nm high pressure high temperature
(HPHT) nanodiamond. Image printed with permission from Sinan
Karaveli. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
2-1 Schematic of the spectroelectrochemical setup depicting the electrochemical cell and wide field and confocal imaging modes. The difference
between the two setups is that the initial setup did not have galvanometer mirrors for scanning on the sample. Modified version of original
image by Sinan Karaveli, modified and printed with permission. . . .
11
29
2-2 Modulation contrast of two spots, whose time traces are shown, as a
function of power, for voltage scans of -0.5 V to 0.5 V, 0.5 Hz. Note
that the voltage signal time traces are not shown with the fluorescence
time traces because the voltage readouts from the DAQ were not stored. 35
2-3 Histograms of modulation contrast for fluorescence modulation measurements taken at different excitation powers. . . . . . . . . . . . . .
36
2-4 Histograms of modulation contrast for varying voltage scans. The
modulation contrast for the ±0.5 V time trace is 14 % and for ±0.3 V
is 8 %. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
2-5 APD time traces of two spots showing the fluorescence response to
“turn on” and “turn off” voltage scan sequences. The “on” and “off”
sequence parameters are shown in Table 2.3. . . . . . . . . . . . . . .
39
2-6 Spectral measurements, with 200 s acquisition each, for a spot . . . .
40
2-7 Wide field fluorescence time trace, APD time traces, and spectra for
one spot. The modulation contrast observed in wide field is 12.7 %;
the excitation power was around 60 mW. The “turn on” and “turn off”
voltage sequences are described in Table 2.3. Spectral acquisition time
was 200 s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
2-8 Fluorescence responses of two spots to 100 mV voltage scan with a DC
bias. Image printed with permission from Sinan Karaveli. . . . . . . .
42
2-9 Spectra of a nanodiamond cluster at different applied potentials. NV−
fluorescence increases for decreased potential. Image printed with
permission from Sinan Karaveli. . . . . . . . . . . . . . . . . . . . . .
43
3-1 Simple schematic for band bending in one dimension . . . . . . . . .
47
3-2 Results for fluorescence time trace fits on two spots . . . . . . . . . .
56
3-3 Histograms for values of m and u with different initial values . . . . .
57
4-1 Modulation results for IZO electrochemical cell with horn sonicated
nanodiamonds, for 0.25 Hz scans with amplitudes within ±1 V . . . .
12
68
4-2 Modulation results for IZO electrochemical cell with horn sonicated
nanodiamonds, for 0.25 Hz scans with amplitudes 0 to -1.2V . . . . .
69
4-3 Modulation results for IZO electrochemical cell with non-horn sonicated
and non-treated nanodiamonds, for 0.25 Hz scans with amplitudes 0 to
−1 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
4-4 Modulation results for IZO electrochemical cell with piranha treated
nanodiamonds, for 0.25 Hz scans with amplitudes 0 to −1 V . . . . .
71
5-1 Schematic for the parallel plate capacitor, where the spacer is a transparent dielectric material. Offset in the ITO cover slips is left to make
electrical contacts. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
5-2 Schematic for the capacitor with an ITO cover slip with “matchstick”
gold overhang as the top electrode. Size of gold overhang is exaggerated. 81
5-3 SEM image of interdigitated electrodes with 8 µm spacing and 8 µm
width electrodes, with nanodiamonds dropcast. The lines around
the electrodes came from dropcasting the nanodiamond solution; the
nanodiamonds appear as white flecks. Not included in the picture are
large contact pads on each side. . . . . . . . . . . . . . . . . . . . . .
86
5-4 Schematic of PCB. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
5-5 8 µm spacing interdigitated electrodes: Effect of voltage amplitude on
how many spots modulate, with respect to their location relative to the
electrode, their modulation behavior, and their modulation contrast .
89
5-6 Wide field modulation results for 90 V, 0.125 Hz voltage scans on nanodiamonds on 8 µm spacing interdigitated electrodes. Spot whose
fluorescence time trace and spectra are shown was located on the signal
electrode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
90
5-7 Wide field modulation results for 100 V, 0.25 Hz scans on nanodiamonds
on 8 µm spacing interdigitated electrodes. Spot whose fluorescence time
trace is shown was located near the signal electrode. . . . . . . . . . .
13
91
5-8 Spectral measurements for 100 V voltage amplitude on nanodiamonds
near the signal electrode on 8 µm spacing interdigitated electrodes.
Fluorescence modulation was different from that observed in wide field
measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
5-9 Wide field modulation results for 100 V voltage, 0.125 Hz scans on
nanodiamonds on 6 µm spacing interdigitated electrodes. . . . . . . .
93
6-1 The Zeiss reflector turret and module. Figure created with images from
the Zeiss manual [89]. . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
6-2 Schematic of the optical components to add wide field excitation to the
right of the microscope. Top view. Not to scale. . . . . . . . . . . . .
98
6-3 Replacement head piece includes a top port for the camera. Figure
created with images from the Zeiss manual [89]. . . . . . . . . . . . . 100
14
List of Tables
2.1
Properties of ITO coated glass coverslips . . . . . . . . . . . . . . . .
30
2.2
Modulation results for measurements at different powers . . . . . . .
36
2.3
Parameters for voltage “turn on” and “turn off” sequences used during
fluorescence measurements in confocal mode on the APDs . . . . . .
40
3.1
Workfunction values for ITO ashed with oxygen plasma [79] . . . . .
52
3.2
Fitting parameter results for spot 1 . . . . . . . . . . . . . . . . . . .
55
3.3
Fitting parameter results for spot 2 . . . . . . . . . . . . . . . . . . .
55
4.1
Thicknesses for HSQ films on silicon
. . . . . . . . . . . . . . . . . .
61
4.2
PECVD SiO2 film spacers in electrochemical cell interfaces . . . . . .
63
4.3
Summary of electrochemical cell samples with isolated interfaces . . .
66
5.1
Third set of parallel plate capacitors with PECVD SiO2 film spacers .
83
5.2
Result for second set of parallel plate capacitors with PECVD SiO2
film spacers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3
83
Summary of parallel plate capacitors with different spacers. Asterisk
indicates cases where capacitor spacing and spacer film thickness were
assumed to be the same. . . . . . . . . . . . . . . . . . . . . . . . . .
15
84
16
Chapter 1
Introduction
The negatively charged nitrogen vacancy (NV− ) color center in diamond has been
subject to intense research efforts in recent decades because its spin state can be
optically initialized and read out. Not only is the NV− center very promising as
a solid state spin quit for quantum information processing [1, 2] , it has also been
shown to sense environmental variables ranging from temperature [3, 4] to magnetic
[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] and electric fields [16].
Because of its photostability [17], brightness (emission in excess of 106 photons
per second [17]), as well as the biocompatibility [18] and relative chemical stability
[8, 19, 20] of its host diamond, the NV center is seen as especially suitable for biological
applications. In particular, its sensitivity to electric field may make it promising as
a fluorescent voltage indicator for real time imaging and measurement of action
potentials in neurons.
This thesis demonstrates the sensitivity of NV fluorescence to applied electrochemical potential, documents the investigations of the mechanism for the NV center’s
voltage-dependent charge state and fluorescence modulation, and describes the design
of a microscope setup to be used for future experiments in NV sensing.
17
1.1
1.1.1
The nitrogen vacancy center in diamond
Overview of the NV− defect center
The nitrogen vacancy center is a point defect consisting of a substitutional nitrogen
atom and neighboring vacancy in the diamond lattice (Figure 1-1). It is a deep level
defect in diamond bandgap, as shown in Figure 1-2 [21, 22].
N
V
Figure 1-1: Schematic of the NV center in a diamond lattice.
Conduction band
3
E
1
A1 Meta-
637nm
1
3
A2
ms=±1
E
stable
singlet
state
2.87GHz
ms=0
Valence Band
Figure 1-2: Schematic of the NV− electronic structure. Based on [21] and [23]. The
relative energies of the triplet and singlet states are currently unknown.
The basic electronic structure of the NV− center is characterized by a spin triplet
ground state 3 A2 , a spin triplet excited state 3 E, and metastable singlet state with
two levels 1 A1 and 1 E. A chemical bonding model attributes six electrons to the
3
A2 triplet state; five of the electrons occupy the dangling bonds of the carbons
and nitrogen that surround the vacancy, while a (typically nitrogen) donor in the
lattice provides the sixth [24]. The NV− emission spectrum shows a zero phonon line
18
(ZPL) at 637 nm distinct from its phonon side bands that range from about 630 nm to
800 nm (Figure 1-3). The zero phonon line at 637 nm of the NV− emission spectrum
corresponds to the resonant wavelength of the ground-state-to-excited-state transition.
The spin triplet ground and excited states are each split into sublevels ms = 0 and
ms = ±1, which are characterized by a zero field splitting. The ms = ±1 states are
degenerate and energetically higher than the ms = 0 state, with zero field splitting
values of 2.87 GHz for the ground state and 1.42 GHz for the excited state. The
transitions between excited state and ground state are spin-preserving [25, 26, 21, 22].
NV - spectrum in 25nm HPHT nanodiamond
6000
intensity (a.u.)
5000
4000
3000
2000
1000
0
-1000
550
600
650
700
750
800
wavelength (nm)
Figure 1-3: NV− center emission spectrum in 25 nm high pressure high temperature
(HPHT) nanodiamond. Image printed with permission from Sinan Karaveli.
The NV− center is often identified by this magnetic resonance at 2.87 GHz, measured through electron paramagnetic resonance (EPR) or optically detected magnetic
resonance technique (ODMR) [21]. In the ODMR technique a microwave field sweeps
in frequency, and at 2.87 GHz resonance excitation from ms = 0 to ms = ±1 occurs.
This can be detected optically as a decrease in the fluorescence intensity at 2.87 GHz,
which is enabled through the optical contrast of the ms = 0 and ms = ±1 states due
to spin-selective nonradiative decay pathways. Electrons in excited state ms = ±1
sublevels preferentially decay via the long-lived metastable singlet state, while those in
the excited state ms = 0 sublevel decay through fast radiative transition. The resulting
optical contrast is about 30% [21, 22]. The spin state can be optically initialized using
a long laser excitation due to the preferential decay of the metastable singlet state to
19
the ms = 0 ground state [23].
ODMR spectrum of IIa bulk diamond at zero field
1
Normalized contrast
0.99
0.98
0.97
0.96
0.95
0.94
0.93
0.92
2.84
2.85
2.86
2.87
2.88
2.89
Frequency (Hz)
2.9
×109
Figure 1-4: ODMR spectrum of bulk IIa diamond at zero field. Red dots indicate
experimental data points, with error bars. Blue line is the fitted curve. Image printed
with permission from Christopher Foy.
1.1.2
Basics of NV− spin based sensing
Most sensing protocols based on NV− depend on optical detection of interactions of
the NV− spin. Magnetometry with the NV has led to the development of sensing of
physical quantities such as electric fields [16, 27], electron spin [28, 29], temperature
[30, 3], orientation [31], and charge [32].
Under an externally applied magnetic field the degeneracy of the ms = ±1 is
lifted, which is reflected as two resonances in the ODMR spectrum. The frequency
separation between the two resonances is 2γBz , where Bz is the applied magnetic field
parallel to the NV axis (conventionally the NV axis is along the z axis) and γ is the
electron gyromagnetic ratio 2π × 28GHz/T. This Zeeman effect is reflected in the spin
Hamiltonian, which neglecting hyperfine interactions can be written [22]:
2
2
H
2
2
~
~
= Dgs Sz −
+ γ B · S + z Ez Sz −
h̄
3
3
+ xy Ex (Sx Sy + Sy Sx ) + Ey Sx2 + Sy2
~ is the vector magnetic
where Dgs is the ground state zero field splitting 2.87 GHz, B
20
~ is the spin, E
~ is a vector electric field, and z and xy are coupling constants.
field, S
The sensitivity of the NV to magnetic field is greater than that to electric fields;
the spin’s interaction with electric field is indirectly caused by the Stark effect and
spin-orbit coupling [16, 22]. Other physical quantities such as temperature affect NV
electronic energy levels by perturbing the ground state zero field splitting D [22].
Normalized contrast
ODMR simulation spectrum of IIa bulk diamond
2.8
2.82
2.84
2.86
2.88
2.9
2.92
2.94
Frequency (GHz)
Figure 1-5: Simulation of ODMR spectrum showing two resonances. Image printed
with permission from Christopher Foy.
Biological sensing
In addition, the NV center is suitable for biological applications because it is photostable and bright, emitting above 106 photons per second [17]. Its photostability is one
key advantage over other fluorescent biomarkers, such as organic fluorescent proteins,
which often suffer from photobleaching [33, 34]. A number of super resolution imaging
techniques with NVs have also been developed [35, 36, 37].
Diamond itself is chemically stable and biocompatible, lending itself to specific
targeting without causing unwanted side reactions [8, 19, 20]. Studies of the uptake of
nanodiamonds in HeLa human cancer cells [38] as well as in C. elegans ringworms [39]
have found the cytotoxicity of nanodiamonds to be low. Other studies have performed
NV− spin based sensing in biological systems, including tracking the orientation of
NVs in nanodiamonds in living HeLa cells [31] and thermometry in living cells [40].
The desirable properties of the NV in nanodiamond make it promising as a fluorescent voltage indicator for neuroscience. Optical methods for measuring membrane
21
potentials, unlike electrical single-cell recording with patch clamping, can enable
simultaneous optical recording from hundreds of individually addressable neurons[41].
During an action potential, the membrane voltage spikes from a resting potential of
∼−70 mV to about 10 mV–30 mV over a membrane thickness of 5 nm–10 nm, with a
pulse duration on the order of milliseconds. This yields a high electric field change
of 107 V/m–108 V/m. Some challenges to measuring the action potential include the
requirement that the nanodiamonds be embedded in or localized very close to the
plasma membrane, and that the number of nanodiamonds embedded should not affect
cellular function [41]. One proposed method to measure action potentials is based on
detecting the ODMR resonance shift due to the electric field [22]. Others are based
on detecting changes to the ODMR spectrum induced by the weak magnetic field
generated by the action potential. Some suggest using complex pulse sequences to
detect the magnetic field [42].
1.1.3
NV charge state control
Although many of the applications for the NV center leverage the spin properties of
the singly negatively charged NV− , the NV center’s charge state is not necessarily
stable. Notably, the NV center is known to stochastically switch between NV− and
NV0 , another fluorescent charge state, under constant laser illumination [43, 44, 45].
The neutrally charged NV0 center’s photoluminescence is analogous to that of
NV− : its emission spectra is characterized by a sharp ZPL at 575 nm with phonon
side bands ranging from about 580 nm to 750 nm (Figure 1-7) [46]. The basic NV0
electronic structure consists of a spin doublet ground (2 E) and excited (2 A) states
as well as a quartet state (4 A2 )[47]. See Figure 1-6. Like NV− , it is known to be
photostable under off-resonant excitation [48, 49]. The two NV charge states can be
distinguished by their emission spectra, as shown in Figure 1-7 [25, 46]. However,
unlike NV− , NV0 does not have an optically detectable magnetic resonance between
the ground and excited states.
Because of these charge state fluctuations, there have been efforts to stabilize
the NV charge state through both static and dynamic control. Static control of
22
Conduction band
2
A
4
575nm
2
A2
Quartet
state
E
Valence Band
Figure 1-6: Schematic of the NV0 electronic structure. Based on [21]. The relative
energies of the doublet and quartet are currently unknown.
0
NV - and NV
spectra in 25nm HPHT nanodiamond
6000
NV 0
NV -
intensity (a.u.)
5000
4000
3000
2000
1000
0
-1000
550
600
650
700
750
800
wavelength (nm)
Figure 1-7: NV− and NV0 emission spectra in 25 nm high pressure high temperature
(HPHT) nanodiamond. Image printed with permission from Sinan Karaveli.
the NV charge state has involved chemical treatment of the diamond at different
stages of diamond fabrication and NV center creation. Also, it has been shown
that diamond surface termination and other local charges often dictate the charge
state of shallow NVs in bulk diamond. For an NV center close to the surface of a
bulk diamond, hydrogen surface termination is found to yield a higher percentage
of NV0 , while oxygen surface termination increases the NV− percentage [50]. In a
study on nanodiamonds, the NV− percentage is found to decrease with the size of the
nanodiamond, which is thought to be because surface electron traps ionize nitrogen
atoms that would normally provide an electron to convert NV0 to NV− . This study
suggests that the surface electron traps are graphitic defects that can be partially
23
removed by a thermal oxidation; indeed, it is found that thermal oxidation recovers
some NV− centers [51]. A theoretical study has posited that for NV− charge state
stability, the best diamond surface termination is a combination of hydrogen, hydroxyl,
and ether groups [52].
On the other hand, many other studies have sought to dynamically control the
NV charge state in bulk diamond through an externally applied electrical bias. One
line of research has studied NV fluorescence and charge state in diamond-based p-i-n
diodes. These studies explain that when external bias is applied across the device, the
flow of holes converts NV− to NV0 [53, 54]. Other studies have focused on the effect
of diamond surface band bending on the NV charge state using bulk diamond devices
patterned with oxygen and hydrogen terminated sections. One device was used as
an in-plane gate field effect transistor [55], while in another study the potential was
applied electrochemically through the use of an electrolytic gate [56]. However, the
majority of NV charge state control studies have been done in bulk diamond.
1.2
Thesis objectives
This thesis presents the voltage dependence of the NV charge state and fluorescence
in nanodiamonds as the basis for an alternative sensing scheme based on the NV
charge state rather than on NV− spin. The dependence of the NV charge state
on its electrostatic environment indicates that it can be used to sense local charges
and voltage. In particular, the suitability of NVs and nanodiamonds for biological
applications can make NVs in nanodiamonds promising as a voltage indicator to detect
neuron action potentials.
The dynamics of NV fluorescence and charge state as a function of applied electrochemical potential are studied. Hydroxyl terminated, 25 nm HPHT nanodiamonds are
studied in an electrochemical cell with an indium tin oxide (ITO) working electrode.
NV fluorescence is measured in both wide field and confocal modes and charge state
fluctuations are assessed via spectral measurements. Fits are performed to voltage
dependent fluorescence time traces using a model based on Fermi Dirac statistics.
24
Other devices are fabricated to investigate the mechanism for voltage dependent
NV charge state and fluorescence. These devices include variations on the ITO
electrochemical cell morphology to investigate voltage induced charge injection, as
well as parallel plate capacitor devices and interdigitated electrode arrays for electric
field induced surface band bending.
The design of modifications to an inverted microscope setup, for the purpose
of expanding the setup’s capabilities for further sensing experiments with NVs in
nanodiamonds, is outlined.
The study of voltage dependent NV charge state and fluorescence was performed
jointly with a colleague, postdoctoral researcher Dr. Sinan Karaveli. Some of Sinan’s
data on NVs in the ITO electrochemical cell are mentioned, with due credit, but the
thesis focuses mainly on independent work.
1.3
Thesis outline
In Chapter 2, the experimental setup and wide field and confocal measurements for
the ITO electrochemical cell are described. Chapter 3 provides a discussion of the
two possible mechanisms for the observed voltage dependent NV fluorescence and a
quantitative model for fitting the experimental time traces. Chapter 4 and 5 each
discuss the fabrication of and measurements performed on devices to study the two
mechanisms described in Chapter 3. The focus of Chapter 4 is the charge injection
mechanism while that of Chapter 5 is the electric field induced band bending. Chapter
6 details the design of new capabilities added to an inverted microscope setup. Lastly,
Chapter 7 provides a summary of the thesis, with conclusions and recommendations
for future work.
25
26
Chapter 2
Spectroelectrochemical setup and
measurement
This chapter focuses on the measurements performed to observe the voltage dependent
fluorescence modulation of NV centers in nanodiamonds in an electrochemical cell with
an indium tin oxide (ITO) working electrode. Descriptions of the microscope setup,
electrochemical cell sample, and software analysis are provided. In addition, excitation
power, nanodiamond size, and excitation wavelength were varied in order to optimize
fluorescence modulation contrast. Details on the possible underlying mechanisms of
the NV charge state and fluorescence modulation are discussed in Chapter 3.
2.1
2.1.1
Spectroelectrochemical setup
Microscope setup
The chief aims of the experiment were to demonstrate that applied voltage could
change both the fluorescence intensity and charge state of NVs in nanodiamond,
and to investigate the mechanism for such a fluorescence modulation. Because of
the variation in any given batch of nanodiamonds, there was a need to screen for
nanodiamonds with the desired voltage-dependent fluorescence, prior to addressing a
nanodiamond of interest to look at the NV charge state as a function of voltage. Thus
27
the measurements included two imaging modalities, with wide field imaging used for
nanodiamond screening and confocal imaging for spectral measurements.
Measurements on the ITO electrochemical cell were performed on an inverted
microscope (Zeiss Observer.Z1m) setup. A schematic diagram of the setup is shown
in Figure 2-1. A 300 mW 532 nm CW laser excitation beam entering the right side
port of the microscope was focused to the back aperture of an 100x immersion oil, 1.4
NA objective. Emission from the sample was collected through the same objective.
For wide field, the fluorescence signal was filtered through a 650 nm long pass filter
and imaged onto a CCD camera (ProEM-512K CCD) attached to the top port of the
microscope. Confocal measurements were performed on a home built setup (built by
colleague Edward Chen) outside the left side port of the microscope. The same 532 nm
laser, but without the wide field lens, was focused to a diffraction limited spot on the
sample, and the sample was moved using the stage micrometers. The fluorescence
was collected through the confocal setup, filtered with either a 550 nm, 600 nm, or
650 nm long pass filter and imaged on a 4.6 µm core optical fiber connected to a
single avalanche photodiode (APD). For spectral measurements emission was filtered
with a 550 nm long pass filter and the fiber was connected to a grating spectrometer
(SP2500i).
The measurements to investigate the mechanism of voltage-dependent fluorescence
modulation were performed on another similar microscope setup. The inverted
microscope was a Zeiss Axiovert 200M with the immersion oil objective 100x with NA
1.4. For wide field fluorescence collection the Photometrics Cascade 1k emCCD camera
was used. The confocal setup included galvanometer mirrors on a plane conjugate to
the microscope objective back aperture. These galvanometers were used to scan the
532 nm excitation laser on the sample.
2.1.2
Sample preparation
Initial measurements on voltage dependent NV fluorescence were performed in a
three electrode electrochemical cell that held nanodiamonds. The application of
electrochemical potential is analogous to that in a study of the NV charge state in bulk
28
DM: Dichroic Mirror
FM: Flip Mirror
GM: Galvo. Mirror
LP: Long Pass Filter
SP: Short Pass Filter
DM: Dichroic Mirror
FM: Flip Mirror A
GM: Galvo. Mirror
Auxiliary Electrode
LP: Long Pass Filter
Reference Electrode
SP: Short
Pass Filter
Working Ref. Counter
NDs
Working Ref. Counter
Electrolyte solution
Micro.
Obj.
V
Diamond
Nanocrystals
SP
FM1
DM1
Micro.
Obj.
532 nm Laser
NDs
LP1
Working Electrode
Electrochemical Cell
SP
FM1
Fiber
APD
532 nm Laser
FM2
LP2
LP1
DM2
Spectro.
Fiber
Spectro.
Confocal
Mode
FM2
GM1
LP2
DM2
GM1
DM1
Pol
GM2
Tube
Lens
emCCD
APD
Tube
Lens
Pol
GM2
emCCD
Wide Field Mode
Figure 2-1: Schematic of the spectroelectrochemical setup depicting the electrochemical
2
cell and wide field and confocal imaging modes. The difference between the two setups
is that the initial setup did not have galvanometer mirrors for scanning on the sample.
Modified version of original image by Sinan Karaveli, modified and printed with
permission.
MIT Quantum Photonics Group
diamond [56] as well as a study of photoluminescence blinking in quantum dots [57].
In addition, the electrochemical cell contained an aqueous solution that was intended
to approximate the aqueous nature of cerebrospinal fluid (or interstitial fluid), which
is an aqueous fluid that surrounds neurons and contains salts [58].
Electrochemical cell
The electrodes of the three-electrode electrochemical cell correspond to the working,
reference, and auxiliary electrode. The working electrode applies a desired potential
while the reference electrode, which has a known potential, is the standard against
which the working electrode’s potential is measured. The auxiliary electrode passes
a current to counteract the current formed as charge transfer events happen at the
working electrode. The electrochemical cell also includes an electrolyte solution. When
potential is applied at the working electrode and current passes, the charge is carried
in the electrode by electrons or holes, while in the electrolyte phase the charge is
carried by ions [59].
29
Manufacturer
SPI Supplies
Nanocs
ITO film thickness
140 nm
150 nm
Resistance
30 – 60 Ω/square
50 Ω/square
Optical transmission
> 88 %
> 85 %
Table 2.1: Properties of ITO coated glass coverslips
The sample holder consisted of an indium tin oxide (ITO) coated glass cover
slip with a cut up microcentrifuge tube epoxied onto it to create a reservoir for the
electrolyte solution. This sample holder was essentially the same as that used for
a study of the photoluminescence behavior of quantum dots [57]. The ITO cover
slip was the working electrode of the electrochemical cell was chosen as a working
electrode because it is an optically transparent conductor. ITO cover slips with
resistances of 30 Ω/square–60 Ω/square were purchased from SPI Supplies or Nanocs.
Their properties are listed in Table 2.1.
The auxiliary electrode was a platinum wire and the reference electrode a silver
/ silver chloride (Ag/AgCl) electrode filled with saturated potassium chloride (KCl)
solution. The reference electrode had a half cell potential of 0.197 V with respect to
SHE (standard hydrogen electrode) [59]. The reference electrode was later replaced
with a commercial leak free silver / silver chloride reference electrode from Harvard
Apparatus.
The electrolyte solution was composed of electroinactive species (do not undergo
redox reactions at the potential window used) in aqueous solution. It was based
on that used in a bulk diamond study which had 10 mM K-PBS buffer and 50 mM
with a pH of 7 [56]. The electrolyte solution was prepared by adding 0.259 g of
powdered potassium chloride salt (VWR) to 56.7 ml commercial Corning CellGro
10x PBS buffer, and then adding ultrapure water until the solution volume reached
100 ml. This gave concentrations of 10 mM KH2 PO4 , 50 mM KCl, 776.18 mM NaCl,
and 57.51 mM Na2 PO4 .
The ionic strength I [60] and Debye length K −1 [61] of the electrolyte solution
30
were calculated based on the following equations:
n
1X 2
I=
ci z
2 i=1 i
r
r 0 kB T
−1
K =
2NA e2 I
where ci is the molar concentration of the ion in mol/m3 , zi is the charge number
of the ion, r is the dielectric constant of water, 0 is vacuum permittivity, kB is the
Boltzmann constant, T is temperature, NA is Avogadro’s number, e is the charge
of an electron. The ionic strength for the electrolyte solution is 1008.71 mM, or
1.008 71 mol/m3 . The Debye length for this solution is about 11 nm.
Nanodiamonds
The measurements involved HPHT nanodiamonds that had primarily hydroxyl group
(OH) termination, about one NV center per diamond, and average size 18 nm–25 nm.
While NVs in oxygen terminated nanodiamonds tend to show the highest charge
state stability for shallow NVs and NVs small nanodiamonds [50], oxygen terminated
nanodiamonds were chosen for their biofunctionalization applications. Oxygen termination (hydroxyl, carboxyl, carbonyl, and ether groups) is known to make diamonds
hydrophilic and yield suspensions in aqueous media [22]. In addition, several protocols for functionalizing oxygen terminated diamonds for biological applications have
been developed [62]. Small nanodiamonds with one NV center were chosen in order
to the behavior of one or a few NV centers, and also because the NVs in smaller
nanodiamonds tend to have lower charge state stability [63].
However, for biological sensing it may be desirable to use larger nanodiamonds.
Preliminary results from collaborators in the research group of Edward Boyden have
shown that larger nanodiamonds may be attached to the cell membrane, but this has yet
to be shown for smaller nanodiamonds. In addition, larger nanodiamonds with multiple
NV centers can more easily yield a higher fluorescence signal, which is important in
cases of high cellular autofluorescence [64]. Therefore, 100 nm nanodiamonds from
31
Adamas Diamond Corp were studied for some measurements.
Sample assembly
Because the nanodiamonds tended to self-aggregate in the aqueous solution, they were
bath sonicated for approximately half an hour to two hours prior to dropcasting onto
the ITO cover slip. The ITO cover slips were cleaned with deionized water, acetone,
isopropyl alcohol, and oxygen plasma. After the nanodiamonds were dropcast in the
reservoir on the ITO cover slip, the amount of nanodiamonds precipitating out of
the solution onto the ITO cover slip was monitored on the CCD camera. Following
this, the reservoir was filled with an aqueous electrolyte solution. The reference and
auxiliary electrodes were positioned such that they did not touch each other, the sides
of the reservoir, or the ITO working electrodes.
2.1.3
Measurement procedure and analysis
Cyclic voltammetry
Typical wide field measurements involved cyclic voltammetry measurements, where
the applied waveform is a triangular wave and the potentiostat records the current at
the working electrode. The cyclic voltammetry measurement is a reversible technique:
the potential sweeps linearly in one direction before reversing. If the scan begins at a
potential that is positive relative to the reduction (loss of electrons) potential of a redox
species, initially there is only a current caused by the movement of charged species in
solution. When the electrode potential sweeps close to the reduction potential of the
redox species, the reduction starts. The surface concentration of the redox species
drops due to the reduction and flux of the remaining redox species from the bulk
solution to the surface increases, which increases the current as well. The current drops
as the redox species is depleted. The reduction manifests as a peak in the current
vs. voltage (IV curve, also known as cyclic voltammogram). Similarly, the oxidation
(release of electrons) during the reverse scan manifests as a dip in the current. The IV
curve typically shows a hysteresis.
32
For these experiments, the electrochemical cell was used as a means to to apply
potential rather than to study redox species. The CHI 630D model potentiostat
applied voltages of different waveforms with magnitudes typically no larger than
±1 V. Limiting the voltage magnitudes was intended to prevent the electrolysis of
the water in the aqueous electrolyte solution (1.229 V with respect to SHE [59]) and
redox reactions at the diamond surface. The current was monitored to check for redox
reactions. Neither peaks nor dips were observed for the potential windows used.
Acquisition and analysis
As the potentiostat applied voltage, the CCD camera acquired a series of fluorescence
images over an area of approximately 20 µm by 20 µm. The exposure time of the CCD
camera varied according to the size of the acquisition image, so that the sum of the
readout and exposure time would add up to 100 ms. This camera acquisition rate was
sufficient to sample the voltage sweep frequency.
Analysis was performed after each acquisition with custom analysis code. The
intensity of each pixel of the resulting images was plotted over time, and a discrete
Fourier transform was used to identify the pixels whose intensity modulated at the
frequency of the applied voltage.
If the spot showed fluorescence modulation that corresponded with applied voltage
modulation, then it was addressed in confocal mode. Compared to wide field, confocal
mode offered advantages such as faster timing resolution of single spots (1 ms on the
DAQ vs. the camera’s 30 ms for a 200 by 200 pixel array), better signal to noise ratio,
and also the ability to take spectral measurements. First to confirm the fluorescence
modulation, the measurement was repeated but with the fluorescence monitored on
the APDs.
Following this, spectra were taken at different biases to investigate the voltage
dependence of the NV charge state. However, a full conversion between the charge
states was not expected. Since the NV center fluctuates between both charge states,
it was expected instead that the bias would affect the occupation probabilities of each
NV charge state, and thus the relative NV− to NV0 character ratio. The change in
33
relative NV− to NV0 character would be reflected in the spectra, but the spectra
would not show a full conversion.
2.2
Initial results on voltage dependent nanodiamond fluorescence
The main goal of the measurements on the ITO electrochemical cell was to find
nanodiamonds whose fluorescence modulation would show voltage sensitivity down
to 100 mV. To this end, initial wide field measurements focused on screening for
nanodiamonds and optimizing for their modulation contrast. The main parameters
varied to optimize for modulation contrast included 532 nm excitation power, nanodiamond size dependence, and excitation wavelength dependence. The aim of the
initial measurements was to find the “needle in the haystack,” i.e. a nanodiamond
with desired fluorescence modulation characteristics, rather than to build statistics on
the nanodiamonds with the wide field measurements. Confocal measurements were
performed on some of the modulating spots.
To calculate contrast, the fluorescence intensity was averaged over each voltage
cycle or period and the maximum intensity Imax and the minimum intensity Imin
values were determined. Contrast was then calculated as
Imax − Imin
Imax + Imin
For initial experiments with the ITO electrochemical cell the background was not subtracted, but in subsequent experiments in samples other than the ITO electrochemical
cell the background was accounted for.
34
2.2.1
Wide field measurements
Excitation power
Studying the excitation power dependence involved two approaches. In both cases,
the voltage scans were ±0.5 V, 0.5 Hz scans. One approach was to monitor the change
in modulation contrast in two spots as power was varied. The results for two spots
in the same area are depicted in Figure 2-2, which also shows the time traces of two
spots at the powers where the modulation contrast was highest. There was no clear
pattern for the dependence of the two spots’ modulation contrast on excitation power,
Norm.
Norm.
Fluorescence IntensityModulation contrast
Fluorescence Intensity
although the modulation contrast was generally higher at higher powers.
Modulation contrast of two spots with respect to power
0.2
Bright spot
Dim spot
0.15
0.1
0.05
0
20
30
40
50
60
70
80
90
Excitation power (mW)
Fluorescence time trace of a dim spot at 90mW
3
2.8
2.6
2.4
2.2
2
0
10
20
30
40
50
60
70
80
90
100
90
100
Time (s)
Fluorescence time trace of a bright spot at 77mW
15
14
13
12
0
10
20
30
40
50
60
70
80
Time (s)
Figure 2-2: Modulation contrast of two spots, whose time traces are shown, as a
function of power, for voltage scans of -0.5 V to 0.5 V, 0.5 Hz. Note that the voltage
signal time traces are not shown with the fluorescence time traces because the voltage
readouts from the DAQ were not stored.
The second approach, instead of studying individual spots, was to compare overall
changes in the modulation contrast results from measurements at varying powers
over different areas on the sample. The comparison across measurements was not
complete, because the number of spots studied at each excitation power were not
35
similar. This was likely in part because the NV fluorescence depends on the excitation
power. However, this approach was intended to serve as an overall assessment for
how higher modulation contrast would potentially be achieved by changing the power.
This approach excluded the two spots measured for the first approach.
Figure 2-3 shows the results for the second approach. The results appear to be
consistent with the general trend observed in the first approach that the modulation
contrast increased for higher power. More modulation details are in Table 2.2.
Histogram of modulation
contrast at 25mW
5
4
3
2
4
3
2
0
0
0.05
0.5
Modulation contrast
5
4
3
2
4
3
2
0
0.2
5
4
3
2
1
0
0
0.2
0.15
6
1
0.1
0.1
0
0.3
0.05
0.1
0.15
Modulation contrast
Histogram of modulation
contrast at 90mW
7
Number of spots
6
Modulation contrast
5
Modulation contrast
Histogram of modulation
contrast at 75mW
0
6
1
Histogram of modulation
contrast at 100mW
7
Number of spots
0
-0.5
Number of spots
5
1
1
7
6
Histogram of modulation
contrast at 53mW
7
Number of spots
Number of spots
Number of spots
6
Histogram of modulation
contrast at 35mW
7
7
6
5
4
3
2
1
0
0.05
0.1
0.15
0
0
Modulation contrast
0.1
0.2
0.3
Modulation contrast
Figure 2-3: Histograms of modulation contrast for fluorescence modulation measurements taken at different excitation powers.
Table 2.2: Modulation results for measurements at different powers
Excitation power
(mW)
25
35
53
75
90
100
Number of modu- Total number of
lating spots
spots measured
1
121
4
65
33
398
5
169
22
576
13
180
36
Percentage of spots
modulating
0.83 %
6.2 %
8.3 %
3.0 %
3.8 %
7.2 %
Nanodiamond size
For this study 100 nm commercial Adamas Diamond nanodiamonds containing about
50 NV centers were selected. It was considered that these larger diamonds could
yield higher modulation contrast if the NVs all showed the same modulation behavior.
However, though 585 Adamas nanodiamonds were observed under 60 mW excitation
power, with voltage scans of -0.5 to 0.5 V and 0.5 Hz, none of these modulated. The
Adamas diamonds were very bright, and therefore had high signal, but none showed
fluorescence modulation and they were not studied further.
Excitation wavelength
Blue light was selected because it was considered that, due to its high energy, it
could ionize the electron in the NV− center into the conduction band. A 473 nm
laser was aligned to the same beam path as the green laser. In all cases, the sample
was illuminated with green light before adding blue light. Unfortunately, both
in wide field and in confocal imaging, the addition of blue light caused a high
background fluorescence to appear across the field of view, rendering individual spots
indistinguishable. The background fluorescence could not be bleached. ITO has low
absorption at 473 nm [65], and the cause of the high background fluorescence was
unknown. Repeating the procedure on another area of the sample caused the same
high background fluorescence to appear.
Worse, the blue light appeared to cause damage to the sample: it seemed to
burn the ITO, turning it brown. Although the ITO does not absorb at 473 nm, the
combined effect of laser illumination and applied bias may have damaged the ITO
film.
Voltage scan variation
The voltage scan was varied by decreasing the amplitude and increasing the voltage
sweep frequency. The voltage scan used for most of the ITO electrochemical cell
measurements was ±0.5 V at 0.5 Hz. One measurement with these scan parameters
37
and another measurement with a voltage scan of ±0.3 V at 0.83 Hz were performed
on the same area. Figure 2-4 shows that the modulation contrast decreased with
voltage amplitude. Another measurement was repeated for a ±0.1 V, 2.5 Hz scan, but
no spots were found to modulate.
Histogram of modulation contrast
for±0.5V, 0.5Hz scan
3
2.5
Number of spots
Number of spots
3
2
1.5
1
0.5
0
0
Histogram of modulation contrast
for±0.3V, 0.8333Hz scan
2.5
2
1.5
1
0.5
0.05
0.1
0.15
0
0.02
0.2
0.04
2.4
0.06
0.08
0.1
0.12
Modulation contrast
Fluorescence time trace of a spot
for±0.5V, 0.5Hz scan
Norm.
Fluorescence Intensity
Norm.
Fluorescence Intensity
Modulation contrast
2.2
2
1.8
1.6
1.4
2.2
Fluorescence time trace of a spot
for±0.3V, 0.8333Hz scan
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.2
1.3
1
0
20
40
60
80
1.2
0
100
Time (s)
20
40
60
80
100
Time (s)
Figure 2-4: Histograms of modulation contrast for varying voltage scans. The modulation contrast for the ±0.5 V time trace is 14 % and for ±0.3 V is 8 %.
2.2.2
Confocal measurements
Confocal measurements were performed only on 25 nm diamonds with 532 nm laser
excitation. After identifying spots of interest with wide field, individual spots were
addressed in confocal mode. On the APDs the collection was filtered with a 600 nm
long pass filter. To confirm that the individual spot addressed showed fluorescence
modulation behavior, the fluorescence was monitored on the APDs as triangular wave
voltage scans were applied.
The fluorescence of the spot could be increased or decreased by changing the
38
parameters of the voltage scan sequence the fluorescence. The APDs monitored
the fluorescence of the spots before and after the voltage scans as well as during.
Depending on the voltage scan sequences, the fluorescence could be turned “on,”
increased, or turned “off,” decreased. The fluorescence time traces responding to such
voltage scan sequences “on” and “off” are shown in Figure 2-5, on two different spots.
The parameters for different “on” and “off” sequences are shown in Table 2.3. The
sequences varied in amplitude and sweep frequency, as well as in number of sweeps.
Spot 1: APD counts
2.6
On 2
On 1
2.2
0
100
3
On 1
2.6
2.4
20
40
80
On 1
On 2
4
On 1
3.5
3
2.5
0
20
40
60
80
100
50
150
On 1
Off 1
On 1
Off 1Off 1
8000
6000
100
200
300
400
500
Time (s)
Spot 2: APD counts
On 1
On 3
10000
On 1
On 3
8000
6000
0
120
100
Time (s)
Spot 2: APD counts
12000
Counts/s
4.5×10
10000 On 1
4000
0
100
Time (s)
Spot 1: APD counts
4
Counts/s
60
0
12000
Off 1
2.8
2.2
0
On 2
Off 2
1.5
1
150
Time (s)
Spot 1: APD counts
3.2×10
Off 1
2
Off 2
50
4
Counts/s
Counts/s
3
2.8
2.4
Spot 2: APD counts
4
2.5×10
Counts/s
Counts/s
4
3.2×10
50
Time (s)
100
150
200
Time (s)
Figure 2-5: APD time traces of two spots showing the fluorescence response to “turn
on” and “turn off” voltage scan sequences. The “on” and “off” sequence parameters
are shown in Table 2.3.
The key distinction between the “on” and “off” sequences was the polarity of
the last voltage sweep: that is, whether during the last sweep the voltage increased
or decreased. For all “on” sequences the voltage of the last sweep decreased from
positive to negative, while for “off” sequences the voltage of the last sweep increased.
Therefore, by controlling the polarity of the sweep the fluorescence of the spot could
be controlled. In particular, decreasing voltage would increase the fluorescence of the
spot, while increasing the voltage would dim the fluorescence.
Spectra were taken both at high fluorescence after an “on” sequence and at low
39
Table 2.3: Parameters for voltage “turn on” and “turn off” sequences used during
fluorescence measurements in confocal mode on the APDs
Name
On 1
Off 1a
Off 1
On 2
Off 2
On 3
Number of sweeps (in- First sweep Rate
clude first and last)
(V/s)
11
0 to -0.5 V 0.5
21
0 to 0.5 V 0.5
10
0 to -0.5 V 0.5
25
0 to -0.3 V 0.5
26
0 to -0.3 V 0.5
31
0 to -0.5 V 0.5
Voltage
range
±0.5 V
0 - 0.5 V
±0.5 V
±0.3 V
±0.3 V
±0.5 V
Last sweep
+0.5 to -0.5 V
0 to +0.5 V
-0.5 to +0.5 V
+0.3 to -0.3 V
-0.3 to +0.3 V
+0.2 to -0.5 V
fluorescence after an “off” sequence. The collection was filtered above 550 nm and the
spectral acquisition times were 200 s. The spectra for one such spot are shown together
in Figure 2-6. The spectra taken after the “off” sequence shows two narrow blue
peaks, which are attributed to cosmic rays causing charge readout in the spectrometer
CCD. The spectra suggest that the spot is an NV center, though the zero phonon
lines are not clear in the spectra. The spectral change suggests an increase in the
NV− portion of the spectrum. As expected, the overall fluorescence is higher after the
“on” sequence. However, from these spectra it is not conclusive whether this spot is an
NV center and whether the fluorescence change is due to the shift in the charge state
occupation probabilities.
Intensity (a.u.)
Spectra after "On 1" and
"Off 1a" Sequences
After On 1
After Off 1a
800
750
700
650
600
500
550
600
650
700
750
800
Wavelength (nm)
Figure 2-6: Spectral measurements, with 200 s acquisition each, for a spot
The wide field fluorescence time trace, confocal time traces for “on” and “off”
voltage scans, and spectral measurements all for one spot are shown in Figure 2-7.
40
The modulation contrast observed in wide field was 12.7 %. As in the case of the
other two spots shown in Figure 2-5, voltage scans ending with decreasing voltage
increased the fluorescence while those ending with increasing voltage decreased the
fluorescence. The sharp spurious peak spectrum after the “off” sequence is attributed
to a cosmic ray. The spectra suggest that this spot is also an NV center, and the
fluorescence increases after the “on” voltage scan sequence. As before, however, these
spectra are not conclusive about the voltage dependent change in the relative NV− to
Fluorescence time trace of a spot
for±0.5V, 0.5Hz scan
20
19
3.2
18
3.1
17
16
3
2.8
14
2.7
20
40
60
80
2.6
0
100
50
100
150
Time (s)
Off 1a
Intensity (a.u.)
Counts/s
3.1
200
250
300
350
Time (s)
APD counts showing
"turn off" and "on" sequences
4
3.2×10
On 1
On 1
2.9
15
13
0
APD counts showing
2 "turn on" sequences
4
3.3×10
Counts/s
Norm.
Fluorescence Intensity
NV0 character.
On 1
Off 1a
3
2.9
2.8
2.7
2.6
Spectra after "On 1" and
"Off 1a" Sequences
After On 1
After Off 1a
850
800
750
700
650
2.5
2.4
0
50
100
150
200
250
600
500
300
Time (s)
550
600
650
700
750
800
Wavelength (nm)
Figure 2-7: Wide field fluorescence time trace, APD time traces, and spectra for one
spot. The modulation contrast observed in wide field is 12.7 %; the excitation power
was around 60 mW. The “turn on” and “turn off” voltage sequences are described in
Table 2.3. Spectral acquisition time was 200 s.
2.2.3
Summary
In wide field, higher 532 nm excitation power and higher voltage amplitudes generally
increased the modulation contrast of 25 nm nanodiamonds. The best modulation
contrast of 22.0 % observed was at 100 mW power with a ±0.5 V and 0.5 Hz voltage
scan. Modulation was observed for voltage scans with amplitudes down to ±0.3 V.
41
The confocal measurements showed that the fluorescence the nanodiamonds could
be controlled by the polarity of the applied voltage scans, where decreasing voltage
increases the fluorescence. Spectral measurement suggested that the fluorescence came
from NV centers and that the variation in applied bias changed the NV charge state
character. However, the spectral measurements were not conclusive.
After these initial measurements, the experiments with the ITO electrochemical
cell were continued jointly with Sinan Karaveli. Most of the joint measurements were
performed at 300 mW excitation power over an approximately 40 µm by 40 µm area,
with voltage scans of ±0.75 V and 0.125 Hz. Some of the results in wide field were that
approximately 20 % of all nanodiamonds studied showed modulation, with highest
contrast being on the order of 20 % as well. Modulation down to 100 mV was observed
in wide field measurements but for acquisition times of less than 10 ms (Figure 2-8).
Also, in most of the nanodiamonds decreasing voltage increased the fluorescence,
consistent with the initial confocal fluorescence results. However, a small fraction of
spots showed increased fluorescence with increased voltage. The predominant behavior
of fluorescence increase with decreased voltage was reflected in the spectra, where the
fluorescence increase corresponded with an increase in NV− fluorescence (Figure 2-9).
Norm Intensity (a.u.) Voltage (V)
a)
6
4
2
1.7
1.6
1.5
1.4
1.3
0
20
40
60
80
100
120
Time (s)
Fluorescence (a.u.) Voltage (V)
b)
Figure 2-8: Fluorescence
responses of two spots to 100 mV voltage scan with a DC
0.3
bias. Image printed0.25with permission from Sinan Karaveli.
0.2
Subsequent chapters of this thesis distinguish these two modulation patterns as “in
0.55
phase” (simultaneous increase) and “out of phase” (fluorescence increasing as voltage
0.5
0.45
0.4
42
b)
V
1000
V=0V
0
550
600
650
700
750
800
c)
Figure 2-9: Spectra of a nanodiamond cluster at different applied potentials. NV−
fluorescence increases for decreased potential. Image printed with permission from
Sinan Karaveli.
decreases). The majority of spots showed “out of phase” modulation during wide field
measurements.
43
44
Chapter 3
Theoretical analysis of voltage
dependent nanodiamond
fluorescence
This chapter discusses the mechanisms that might explain the modulation of the
NV charge state and fluorescence due to an externally applied potential. The first
section will describe two mechanisms that give rise to NV charge state and fluorescence
modulation in nanodiamonds: one where the applied voltage changes the band bending
at the diamond surface, and the other where the applied voltage induces charge transfer
from the ITO to diamond surface trap states.
In addition, the band bending mechanism is further developed quantitatively to
perform fits to the experimental fluorescence time traces.
3.1
Possible mechanisms
In the two mechanisms, which both assume thermal equilibrium, the nanodiamond in
the electrochemical cell, where it is in contact both with the ITO working electrode
and electrolyte solution, is considered in terms of the energy band diagram commonly
used in semiconductor physics. While the diamond is nanoscale, in these mechanisms
the Fermi level in the nanodiamond is considered to be the same as the Fermi level in
45
bulk diamond.
3.1.1
Schematic of the electrochemical cell interfaces
The electrochemical cell is made up of different components: nanodiamond, ITO, and
electrolyte; band bending arises when these come into contact. The diamond-ITO
and diamond-electrolyte interfaces can each be thought of as a metal-semiconductor
junction. Separately, each of the three materials has a Fermi level with respect to the
vacuum level, but bringing these into contact initiates the transfer of charge carriers.
Assuming good contact, the transfer of charge carriers continues until the Fermi levels
equilibrate.
The high pressure high temperature nanodiamonds studied in this experiment are
doped with 200 ppm of nitrogen atoms [66], and therefore are an n-type semiconductor.
When the nanodiamond comes into contact with the ITO, electrons are transferred
from the semiconductor to the metal [67, 68, 69]. A simple, one dimensional schematic
for band bending is presented in Figure 3-1. This transfer creates a depletion region
where there are only cations. The potential at interface is known as the built in
potential [67, 68, 69].
The diamond-electrolyte interface can also be considered as a metal-semiconductor
junction, but with a distinction: in the metal, the charges are at the surface, so
that the electric field within the metal is zero; thus, the potential drop at the
metal-semiconductor interface occurs almost entirely in the semiconductor. In the
semiconductor-electrolyte interface case, on the other hand, the potential drop occurs
across both the semiconductor and what is known as the electrical double layer in the
electrolyte solution [68].
The electrical double layer in the electrolyte interface consists of solvent molecules
and ions that balance the charge observed at the working electrode surface. For
example, at negative bias at the ITO, there is a negative charge on the ITO surface
that attracts electrolytic cations to the interface. The double layer is usually modeled
as an inner layer (called the inner Helmholtz plane) that includes solvent molecules at
the interface, and an outer layer (the outer Helmholtz plane) with solvated ions. The
46
ITO
EF
Nanodiamo
Nanodiamond
nd Electrolyte
EF
EF
EF
Figure 3-1: Simple schematic for band bending in one dimension
ions are distributed from the outer Helmholtz plane into the bulk of the solution [59].
Because charge cannot cross the electrode-electrolyte interface, the interface can
to first order be described as a parallel plate capacitor. In the Helmholtz model, the
charge on the working electrode and the charge in the electrolyte phase correspond to
the charges on the two sheets of the parallel plate capacitor. The electrode-electrolyte
interface is characterized by a double layer capacitance. The Gouy-Chapman-Stern
model posits that the electrode-electrolyte interface – and thus the capacitor – has
finite thickness; due to the finite size of molecules and solvated ions, it can take a
significant thickness of electrolyte solution to counteract the charge at the surface of
the electrolyte. The double layer of the electrolyte solution and the semiconductor
depletion layer can be thought of as capacitors connected in series. However, unlike
a true capacitor, the capacitance of the electrolyte solution usually varies with the
potential and electrolyte solution concentration [59].
47
3.1.2
Band bending change with applied electric field
The first mechanism describes nanodiamond fluorescence in terms of the effect of
the applied bias on band bending at the diamond surface. This is similar to the
explanation given for a bulk diamond study [56], where a gate voltage is applied
between a bulk diamond and an electrolyte solution in contact with the bulk diamond.
The gate voltage changes the band bending in the diamond and shifts the Fermi level
at the diamond surface. The change in the band bending affects the energy difference
between the Fermi level and the NV charge state transition level (the NV0/− ionization
level). This variation of the energy difference affects the occupation probability of
the NV charge state. Therefore modulating the gate voltage also modulates the NV
charge state occupation probability and thus the NV fluorescence.
In the case of the electrochemical cell studied in this thesis, the voltage is applied
between the ITO working electrode and the reference electrode in the electrolyte. There
is a large double layer capacitance in the electrolyte solution. The capacitance at the
interface changes with the external applied voltage. As in the study of electrochemical
potential on bulk diamond, the resulting electric field can induce band bending in
the nanodiamond [56]. As the band bending varies with applied voltage, the energy
difference between the Fermi level and the NV charge state transition level changes as
well. The modulation of the electrochemical potential modulates the relative time the
NV spends in each charge state, leading to a fluorescence modulation.
In this band bending picture, the amount of band bending would depend on the
characteristic length across which the applied potential drops across. As described
before, the potential drops across both the Helmholtz layer and the depletion layer.
The size of the Helmholtz layer can varied by changing concentration of the electrolyte
solution. It is expected then that a shorter characteristic length and thus larger
potential drop would result in an increased band bending and stronger fluorescence
modulation.
48
3.1.3
Charge injection
In the second mechanism, nanodiamond fluorescence is affected by voltage-induced
charge injection from the ITO to diamond, through mediation by trap states on the
nanodiamond surface. Surface trap states with energy levels in the band gap are
known to be present in nanodiamonds with oxygen termination [63, 70, 71, 52]. These
surface trap states can affect the charge state of nearby NV centers, based on the
distance of the NV to the surface. A study has shown that NV− centers convert to
NV0 as nanodiamond size and therefore the NV to surface trap state distance are
both reduced [63, 52].
Applying a bias at the ITO working electrode of the electrochemical cell can change
the occupation of surface trap states, as in the case for CdSe/CdS quantum dots in a
similar electrochemical cell system [57]. The applied bias changes the Fermi level’s
energetic position with respect to that of trap states. In particular, negative bias raises
the Fermi level of the ITO and can lead to the filling of the surface trap state. The
occupation of the trap state can affect the charge state of NVs close to the surface.
Because the nanodiamond fluorescence was found to increase with negative applied
voltage, it is possible that the negative voltage leads to charge injection from the ITO
to trap states, whose occupation prevents the conversion of NV− centers nearby to
NV0 . In this mechanism, the negative applied voltage stabilizes the NV charge state
by passivating the diamond surface trap states.
Notably, the difference between the first mechanism and second mechanism is that
in the first mechanism the band bending depends on both the applied potential and
ionic concentration, while in the second mechanism the charge transfer depends only
on the applied potential.
3.2
Fluorescence time trace fits
The focus of this section is to develop a quantitative model for the effect of the
applied voltage on the energy separation between the Fermi level and the NV charge
state transition level. This model is then compared to a energy band diagram of
49
diamond-ITO interface.
Some complications to drawing an accurate energy band diagram included the
variations in literature values as well as the small size of the nanodiamond. Because
the diamond’s size is nanoscale it is unclear whether there is a “bulk” region within
the diamond where there is a constant, flat-band potential.
3.2.1
Calculations for energy levels in diamond and diamondITO junction
In order to make the two mechanisms above more quantitative, some of the energy
levels in diamond and diamond-ITO are calculated [69].
Energy levels in diamond
The distance of the Fermi level EF from the conduction band, EC − EF is calculated
for nanodiamond. The doping density of the diamond is 200 ppm nitrogen. Given
that in the diamond lattice there are 8 atoms per unit cell and the cubic volume
3 , the dopant concentration is calculated to be
of the unit cell is given by (3.567 A)
1019 cm−3 –1020 cm−3 .
The calculation for EC − EF uses the approximation n ' ND , where the n the
number of electrons per cm3 is approximately equal to ND the concentration of donors
per cm3 . This is valid when ND − NA ≈ ND ni , where ND is greater than the
concentration f acceptors NA and much greater than the intrinsic concentration ni .
This approximation is valid because ND is 1019 cm−3 –1020 cm−3 , the diamond is not
p-doped, and the diamond intrinsic carrier concentration is ni = 109 cm−3 at 300 K
for single crystalline diamond [72].
EC − EF can be calculated based on the relation between n and NC , which is the
effective density of conduction band states:
n ' NC e
(EF −EC )/kB T
, NC = 2
50
m* n kB T
2πh̄2
(3/2)
so that
EC − EF = −kB T ln
n
NC
For diamond m* n = 1.9me (me mass of an electron) [73], which gives that EC − EF
is 1.7 eV. This value is consistent with the level given in literature for the flat band
potential for nitrogen dopants in N-doped diamond [74]. Some studies [74, 75, 76]
give the energy levels for the NV0 and NV− ground states based on density functional
theory calculations as 1.2 eV and 2.0 eV above the valence band edge. Based on the
diamond energy band gap value 5.47 eV, the diamond Fermi level is calculated as
1.7 eV and 2.8 eV above the NV− and NV0 charge state ground states respectively.
There are inconsistencies in the literature about where the energy levels of trap
states lie. Some of the highest values for trap state energy levels are (2.6 ± 0.2) eV
or 2 eV–3 eV above the valence band edge [77, 70], while other studies put them at
about 1.2 eV–1.6 eV above the valence band edge [71, 78].
Diamond-ITO interface
A more quantitative picture of the diamond-ITO interface is built based on the metalsemiconductor junction from semiconductor physics. The notation in this section is
somewhat different from that used to describe the diamond-electrolyte interface. The
energy barrier for electrons in the metal is ΦB = ΦM −χ, ΦM is the metal workfunction
and χ is the semiconductor electron affinity χ = (Evacuum − EC )|surface . The built in
potential Vbi is given as [69]
Vbi =
1
(ΦB − (EC − EF )F B )
q
where (EC − EF )F B is the distance of the Fermi level from the conduction band edge
for the flat band or zero field conditions. This is 1.7 eV from calculations above.
Table 3.1 gives the workfunction values for ITO. The average of these three values is
4.99 eV, which was taken as the workfunction value for ITO in the following calculations.
51
Table 3.1: Workfunction values for ITO ashed with oxygen plasma [79]
Measurement type
XPS prior to UPS
UPS
XPS after UPS
Workfunction (eV)
5.24
4.85
4.89
The electron affinity χ for a n-doped diamond with concentration 1020 cm−3 is 0.5 eV
[80]. This gives ΦB = 4.94 eV. Vbi is
1
q
(4.94eV − 1.7eV) = 3.2 V.
The Poisson equation relates the potential distribution, electric field, and charge
density in this region [67, 68, 69]. Based on Poisson’s equation, the electric field E(x)
and potential V (x) distribution are given as follows [69]:
E(x) = −
V (x) = −
qND
(W − x) , 0 ≤ x ≤ W
Ks 0
qND
(W − x)2 , 0 ≤ x ≤ W
Ks 0
where Ks is the semiconductor dielectric constant, which for diamond is 5.7 [72] and
0 is the vacuum permittivity. W is the depletion width is a function of the applied
bias VA :
W =
2
qND
(Vbi − VA )
Ks 0
At zero bias VA = 0, the depletion width W is 4.520 nm. The electric field E(x) at
the diamond-ITO interface is 1.43 × 107 V/cm for zero bias.
The bias applied by the potentiostat is with respect to the silver / silver chloride
reference electrode, which in turn has a potential given with respect to the standard
hydrogen electrode (SHE). The standard hydrogen electrode is said to lie −4.5 eV
with respect to the vacuum level [68]. The silver / silver chloride reference electrode
filled with saturated potassium chloride electrolyte solution has a potential of 0.197 V
with respect to SHE [59]. So for a voltage of ±1.0 V applied by the potentiostat,
VA = −4.5 + 0.197± 1.0 V which gives a depletion width range of 6.42 nm to 7.34 nm
52
at the diamond-ITO interface.
3.2.2
Fits to the experimental fluorescence time traces
Model
A model is considered for the analysis of the fluorescence as a function of voltage.
Because thermal equilibrium is assumed, one can write that the relative probabilities
of NV− and NV0 , P− and P0 respectively, can be written as a Boltzmann distribution:
P−
= e−∆E/kB T
P0
(3.1)
where ∆E is the energy difference that can be expressed in terms of the applied voltage
∆V : ∆E = u + m∆V , which considers a built in energy difference u (a constant) and
a slope m that expresses how much of a voltage drop the NV experiences compared
to the voltage drop over the entire characteristic length.
The fluorescence intensity I depends on the occupation and intensity of each NV
charge state: I = P− γ− + P0 γ0 , where γ− and γ0 are the fluorescence intensities of
NV− and NV0 alone. Because the NV is either in the NV− or the NV0 charge state
then the probabilities must add up to 1: P− + P0 = 1. From this the fluorescence
intensity can be expressed in terms of ∆E:
I=
γ− e−∆E/kB T + γ0
e−∆E/kB T + 1
(3.2)
An alternative form of the model is written by expressing the probabilities in terms
of Fermi-Dirac statistics, where f (E) is the probability that the NV− will be occupied
by an electron at a given energy E and 1 − f (E) is the probability that NV0 is filled
at a given energy E.
f (E) =
1
e−(E−EF )/kB T
53
+1
This gives an expression for fluorescence intensity as follows:
I = (1 − f (E)) γ0 + f (E)γ−
that is equivalent to the expression for fluorescence intensity
I=
γ− e−∆E/kB T + γ0
e−∆E/kB T + 1
(3.3)
described previously.
Implementation of fits and results
The implementation of the fit is performed in Matlab using the lsqcurvefit
function. The function lsqcurvefit takes as its arguments the fit function in terms
of the fitting parameters, the x data, and y data, as well as the initial values for the
fitting parameters. The fitting parameters are γ− , γ0 , u, and m. The initial values γ−
and γ0 are respectively the maximum and minimum fluorescence intensities.
The initial values for u and m are varied; the fits are performed with four combinations of u and m, or in other words using four sets of initial values. The initial
value combinations are chosen to give the lowest norm of the residual values. The u
and m combinations were ±0.1 and ±0.1, ±0.1 and ∓0.1. The units are eV.
If in a fit result the value for γ0 is greater than γ− , this fit result is discarded. For
collection of wavelengths above 600 nm or 650 nm, NV− is brighter than NV0 and
therefore γ− should be greater than γ0 . In addition, if the fit result for m is less than 0
or greater than 1 it is discarded as well. The requirement that 0 < m < 1 is consistent
with the representation of m as a ratio: it represents the distance over which the NV
experiences the voltage drop to the entire characteristic length.
The fit parameter results for for two spots are shown in Tables 3.2 and 3.3, while
the plot of the model with the experimental fluorescence time traces is shown in
Figure 3-2. Visually the fits to the fluorescence time trace appear to capture the
fluorescence modulation well. For spot 2, despite variation in the initial values the fit
54
Table 3.2: Fitting parameter results for spot 1
Parameter
γ−
γ0
u
m
norm of residual
Fit u0 = 0.1, m0 = 0.1
198.4100
1.6873
0.1525
0.0510
115.4193
Table 3.3: Fitting parameter results for spot 2
Parameter
γ−
γ0
u
m
norm of residual
Fit u0 = 0.1, m0 = 0.1
4.8451
3.1992
0.0482
0.1481
77.1666
Fit u0 = −0.1, m0 = 0.1
4.8455
3.1992
0.0482
0.1481
77.1666
parameters converge.
The values u and m are expected to be different for every nanodiamond, but
generally the model predicts values of u and m in the range in the range of 10−2 to
10−1 , as shown in the histograms for u and m values in Figure 3-3. The results for
u suggest that the NV charge state transition level and Fermi level are closer than
described in the energy band diagram above. It is possible the assumption that the
nanodiamond Fermi level is the same as the bulk Fermi level does not hold and instead
the nanoscale diamond surface Fermi level is closer energetically to the NV charge
state transition level.
The values for m suggest that the voltage drop over the NV is on the order of
to 2 nm (considering the depletion width and electrical double layer length
a few A
to 2 nm suggest that the NVs that
are on the order of 20 nm). The range of a few A
experience charge state modulation are close to the surface and have unstable charge
states. This appears reasonable given that the minimum depth for stable NV− centers
in bulk diamond is 2 nm from the surface [81, 22], and that NVs are more likely to
convert to NV0 the closer they are to the surface [63]. It is possible that NVs at this
shallow depth experience charge state fluctuations affected by applied voltage.
For many of the modulating spots, however, the fits fail to capture the dynamics
55
3
0
2
-0.5
0
6
0.5
5
4
0
3
-0.5
20
40
60
80
100
120
1
0
20
40
Time (s)
Fit for spot 1 with u=0.15249, m=0.050967
4
3.5
3
2.5
2
80
100
120
2
5.5
Fit for spot 2 with u=0.048206, m=0.14807
5
4.5
4
3.5
3
1.5
1
60
Time (s)
Norm.Fluorescence
Intensity
Norm.Fluorescence
Intensity
4.5
Applied Voltage (V)
4
Applied Voltage (V)
5
0.5
Fluorescence time trace of spot 2
Norm.
Fluorescence Intensity
Norm.
Fluorescence Intensity
Fluorescence time trace of spot 1
20
40
60
80
100
2.5
120
Time (s)
20
40
60
80
100
120
Time (s)
Figure 3-2: Results for fluorescence time trace fits on two spots
of the fluorescence modulation: this is evidenced by large values for norms of the
residual. Essentially, the fits do not provide a means to model all observed fluorescence
modulation behavior, and are thus imperfect. However, while they were imperfect,
the fits suggest that the modulating NVs are shallow in the nanodiamond and that
the diamond Fermi levels are energetically close to the charge state transition level.
In other words, the fits suggest that the modulating NVs are unstable NVs.
The following chapters describe experimental methods to investigate the two
mechanisms outlined in this chapter, the electric field induced band bending and
charge injection mechanisms.
56
u value for fit
u =-0.1,m=0.1
0
0
50
50
40
30
20
10
70
45
Number of spots
Number of spots
Number of spots
60
u value for fit
u =0.1,m=-0.1
0
40
35
30
25
20
15
10
0
-0.5
0.5
u value
45
40
Number of spots
Number of spots
50
m value for fit
u0=0.1,m0=0.1
40
35
30
25
20
15
10
50
40
30
20
0
0
0.5
0
m value for fit
u0=-0.1,m
=0.1
0
35
70
30
25
20
15
10
0
0.5
1
m value
m value for fit
u0=0.1,m0=-0.1
20
0
1
40
30
20
0
-1
0
70
m value for fit
u0=-0.1,m
=-0.1
0
60
50
40
30
20
10
0.2
m value
0.4
0
0
0.5
m value
Figure 3-3: Histograms for values of m and u with different initial values
57
1
u value
30
10
0.5
50
0
-2
0.4
40
0
m value
0.2
50
5
0
10
60
0
0
60
u value
5
0
70
60
u value
Number of spots
0
u value for fit
u =-0.1,m=-0.1
0
10
5
0
-0.5
0
Number of spots
0
Number of spots
u value for fit
u =0.1,m=0.1
1
58
Chapter 4
Investigation of charge injection
with variations on the
electrochemical cell
This chapter describes the preparation and measurement of devices that are variations
on the ITO electrochemical cell for the purpose of investigating whether voltage
dependent fluorescence modulation in nanodiamonds can be explained primarily by
the injection of electrons from the ITO working electrode into the nanodiamond
surface trap states. The filling of these surface trap states can affect the occupation
probability of the NV charge sate.
One approach to the device design involved separating one or both of the following
interfaces in the electrochemical cell: nanodiamond-ITO or nanodiamond-electrolyte.
Transparent spacers inserted in any of these interfaces were made of different dielectric materials. The spacers were characterized with spectroscopic ellipsometry
measurements of the spacer films on a silicon substrate.
Another approach involved changing the working electrode to a different transparent
conductor, indium zinc oxide (IZO), which has a different workfuntion from ITO.
59
4.1
4.1.1
Isolation of electrochemical cell interfaces
Spacer types and their fabrication
ITO cover slips were consistently used as the working electrode for the electrochemical cell samples with isolated interfaces. Following the addition of the spacer and
nanodiamonds, a cut up microcentrifuge tube was glued onto the cover slip to create
a reservoir, which was reservoir was then filled with electrolyte solution. Typically
the spacer film was characterized through spectroscopic ellipsometry measurements
(M-2000 Ellipsometer, J. A. Woollam Co., Inc.) of the spacer film on silicon wafers.
Nanodiamonds of average size 25 nm were added to bare ITO or the spacer films
either by dropcasting or spincoating. In some cases, the bare ITO films were treated
with Hellmanex III to promote the distribution of the aqueous nanodiamond solution
on a more hydrophilic ITO surface. Dropcasting involved sonicating the nanodiamonds
first. For spincoating 0.01 % polyvinyl alcohol (PVA) was added before spincoating at
2000 rpm for 30 s.
Atomic layer deposition alumina
Atomic layer deposition (ALD) was chosen as the fabrication method for alumina
(Al2 O3 ) because ALD would give thin and uniform films of the material and the ALD
conditions (200 ◦C for 1.5 hour) were best suited for the ITO cover slips. The edges of
the ITO cover slip were masked with Kapton® tape to leave room to make electrical
contacts. ALD of 1 and 5 nm alumina fabrication and characterization were performed
at Brookhaven National Laboratory by colleague Dr. Igal Bayn.
The ALD alumina spacers separated the diamond-working electrolyte interface.
Hydrogen silsesquioxane
Hydrogen silsesquioxane (HSQ) is a well characterized resist for electron-beam lithography that was chosen because it is easy to spincoat onto a substrate. It gives thin
films on the order of 10 nm–100 nm.
60
Spacers were made with 1 % and 6 % HSQ. After spincoating, HSQ was removed
at the edges of the cover slip using a 1 % sodium hydroxide and 4 % sodium chloride
“salty developer” to leave room for electrical contacts. The HSQ films were cured by
placing the samples in an oxygen plasma asher for 20 s at 100 W.
After curing, the HSQ film thickness were characterized via ellipsometry measurements on HSQ films on a dummy silicon wafer. The thicknesses for 1 % HSQ with
spin speeds of 2000, 3000, and 4000 rpm are shown in Table 4.1.
Table 4.1: Thicknesses for HSQ films on silicon
HSQ spin speed (rpm)
2000
3000
4000
Before O2 plasma (A)
623
520
476
After O2 plasma (A)
408
345
311
Some of the 1 % HSQ films cracked after curing. This was thought to be either
because the film was too thin or the film lacked sufficient adhesion to the substrate.
These could be solved respectively by spinning 6 % HSQ to yield thicker films, or
pretreating the substrate with the adhesion promoter TMAH (tetramethylammonium
hydroxide).
To test the thickness of 6 % HSQ films, 6 % HSQ was spun on silicon. However,
the HSQ film was nonuniform on the substrate and was full of “pockmarks.” It
was considered that the pretreatment of the silicon was insufficient, which could be
remedied by treating the silicon with MIBK (methylisobutylketone) and then isopropyl
alcohol, or that the particular 6 % HSQ resist used had degraded.
It was also found that curing thicker HSQ films would be difficult. Oxygen plasma
would only cure just the top of a thick HSQ film. A thermal anneal would be required
to properly cure HSQ, but the anneal temperature of 400 ◦C or above would degrade
the ITO film (based on discussions with the ITO manufacturer). Unless it is annealed
at 400 ◦C or above, the HSQ film remains porous [82].
In addition, HSQ films are possibly permeable to potassium ions [83]. Because
one of the goals of the spacer was to isolate the diamond from the electrolyte, which
contains potassium chloride, the ion permeability of HSQ would render it unsuitable
61
for the purpose. Therefore, the fabrication of HSQ films was discontinued.
Two types were electrochemical cells were prepared with HSQ: one isolated the
diamond-electrolyte interface, and other isolated both diamond-working electrode and
diamond-electrolyte interfaces.
Sputtered silicon dioxide
RF-based sputter deposition was another method for the fabrication of thin films.
SiO2 was chosen out of the two transparent electrically insulating materials, SiO2 and
TiO2 .
Before, sputtering, the the SiO2 sputter target was loaded in to the sputter tool
and the tool was pumped down for about 24 hours to reach 10−6 Torr base pressure.
After the sample was loaded, argon gas was flowed at 10 sccm into the chamber, whose
pressure was kept to 40 mTorr. To make plasma RF was struck at 30 W–50 W before
the power was increased to 130 W. The growth pressure was then adjusted to 4 mTorr.
The growth time was 1850 s, at a rate of 0.5429 A/s.
During the deposition, the sample
was rotated to sputter uniformly across the sample. The edges of the ITO cover slip
were masked to make copper tape contacts; the SiO2 was sputtered in the center of
the ITO cover slip. The sputtered SiO2 films were characterized with ellipsometry
measurements.
A collaborator from the research group of Vladimir Bulović, Wendi Chang, sputtered 100 nm SiO2 on two samples: one bare ITO cover slip and an ITO cover slip
with nanodiamonds dropcast. Both ITO cover slips were bath sonicated in water,
acetone, isopropyl alcohol, and Hellmanex III before ashing with oxygen plasma.
However, as sputtered SiO2 films tend to be porous [84], the devices with sputtered
SiO2 were deemed unsuitable for isolating the diamond-electrolyte interface and not
used.
Silicon dioxide made with plasma enhanced chemical vapor deposition
SiO2 fabricated through plasma enhanced chemical vapor deposition (PECVD) was
chosen as an alternative to sputtered SiO2 . According to unpublished results from
62
members of the Bulović group, the PECVD SiO2 has a higher breakdown voltage than
sputtered SiO2 : the breakdown voltage of PECVD SiO2 is 7 MV/cm, while that of
sputtered SiO2 was 1 V/10nm.
Prior to PECVD, ITO cover slips were cleaned or pretreated with sonicating water,
acetone, isopropyl alcohol, Hellmanex III, and oxygen plasma. On some cover slips,
nanodiamonds were dropcast or spuncoat. The ITO cover slips were wrapped in foil
to mask the edges.
The PECVD SiO2 samples were fabricated to isolate all interfaces in the electrochemical cell, as detailed in Table 4.2.
Table 4.2: PECVD SiO2 film spacers in electrochemical cell interfaces
Sample
Spacer
1
160 nm
PECVD
SiO2
1/200 NDs dropcast
230 nm
PECVD
SiO2
Working elec- ITO cover slip
trode
4.1.2
2
3
310 nm
PECVD
SiO2
1/200 NDs drop- 390 nm
cast
PECVD
SiO2
1/200 NDs drop230 nm
PECVD
cast
SiO2
ITO cover slip ITO cover slip
4
1/200
NDs drop-
cast
160 nm
PECVD
SiO2
ITO cover slip
Isolating the diamond-working electrode interface
For this series of electrochemical cells, the spacer was added to the ITO cover slip
before adding nanodiamonds and the electrolyte solution. Two types of samples were
studied: one type with ALD alumina, and another with PECVD SiO2 .
For electrochemical cells with ALD alumina coated ITO, voltage dependent fluorescence modulation was observed for the same range of experimental parameters, but
with reduced contrast.
In order to account the possibility that electrons could propagate through these
relatively thin layers of alumina, samples with thicker layers of PECVD SiO2 were
63
prepared. Specifically, the sample had 160 nm of PECVD SiO2 . However, the PECVD
SiO2 electrochemical cell exhibited high background fluorescence. Because the background fluorescence was not easily bleachable, the fluorescence of nanodiamonds
containing NVs could not be distinguished. As will be explained in the following
sections, the high fluorescence was a characteristic problem with PECVD SiO2 films
for every thickness prepared.
4.1.3
Isolating the diamond-electrolyte interface
Fabrication of this series of electrochemical cells involved adding nanodiamonds to
the ITO cover slip via dropcasting or spincoating prior to spincoating or depositing
the spacer film. However, it was required that the spacer films be thick. Unlike
the case of covering just the ITO, thicker films were required to cover the large
nanodiamond aggregates to prevent the aggregates from being in contact with the
electrolyte solution.
One sample included an HSQ film of about 40 nm on top of dropcast nanodiamonds.
However, though fluorescence modulation was observed during the measurements, it
was found that the HSQ film had cracks. The cracks were found where there was
less background fluorescence and no diamonds. The film was thought to have cracked
perhaps because it was too thin (nanodiamond aggregates perhaps penetrated the
film) or the adhesion to the ITO substrate was poor. However, further efforts in
the fabrication of HSQ films were not pursued because of the likelihood that HSQ
films were porous to potassium ions [83]. Since potassium chloride was present in the
electrolyte solution, HSQ would not make a good barrier to isolate the diamond from
the electrolyte solution.
Another electrochemical cell with 390 nm of PECVD SiO2 was tested, but very
high background fluorescence, which did not easily photobleach despite long durations
of laser illumination, made it difficult to see the nanodiamond fluorescence.
The results for diamond-electrolyte isolation were not conclusive, mostly due to
difficulties with the isolation of the interface or high background fluorescence.
64
4.1.4
Isolating both interfaces
Isolating both the diamond-working electrode and diamond-electrolyte interfaces
involved two spacers for each sample. Preparation of each sample involved the
construction of a “sandwich.” Steps for making each sample included cleaning the
ITO, adding a spacer on the ITO, dropcasting or spincoating nanodiamonds on the
spacer, adding another spacer, and then gluing the reservoir. These “sandwich” cells
were constructed with HSQ and PECVD SiO2 films.
The electrochemical cell sample with two layers of 40 nm HSQ did not show
any modulation. Also, shortly after increasing the voltage amplitudes, the HSQ
films degraded. From the camera acquisition the HSQ film’s fluorescence seemed to
photobleach. The degradation in the HSQ film was also visible to the naked eye as a
change in color.
The other electrochemical cell sample had 230 nm and 160 nm of PECVD SiO2
film (where 160 nm of SiO2 was on the ITO) but also suffered from high fluorescence
as the previous samples of PECVD SiO2 .
Therefore, while it appeared that nanodiamond fluorescence modulation could not
be observed in electrochemical cells whose interfaces were isolated, the results were
not reproducible with the samples used and inconclusive.
4.1.5
Summary of results
Table 4.3 summarizes the results for the electrochemical cell samples prepared and
tested.
4.2
Indium zinc oxide working electrode
As described previously in Chapter 3, the diamond-ITO interface can be described as
a metal-semiconductor junction, where the ITO is the metal and the nanodiamond
the semiconductor. Since the Fermi levels depend on the workfunctions of the metal
and semiconductors, varying the workfunction of the metal can in turn offer insight
65
Table 4.3: Summary of electrochemical cell samples with isolated interfaces
Interface isolated
Diamond-ITO
DiamondElectrolyte
Both
Spacer
ALD Al2 O3
Thickness
1 nm
ALD Al2 O3
5 nm
PECVD SiO2
160 nm
HSQ
40 nm
PECVD SiO2
390 nm
HSQ
40 nm, 40 nm
PECVD SiO2
230 nm, 160 nm
Outcome
Modulation
observed
with
reduced contrast
Modulation
observed
with
reduced contrast
High background
fluorescence
Modulation
observed but HSQ
cracked
High background
fluorescence
No modulation observed and HSQ
film degraded
High background
fluorescence
on the how voltage dependent nanodiamond fluorescence may be described through
changes in the Fermi level.
To change the workfunction of the metal the ITO working electrode was replaced
with a different transparent conducting film, indium zinc oxide, or IZO. The fluorescence behavior of nanodiamonds with different surface treatments were studied in the
IZO electrochemical cells. This was to ensure that any changes in the percentage of
modulating spots or modulation contrast seen for all nanodiamonds were caused by
the IZO rather than the diamond surface termination. All of the nanodiamonds were
of average size 25 nm, with hydroxyl group termination: one sample had been horn
sonicated (by colleague Dr. Abraham Wolcott), one sample had been treated with
piranha solution (by Diamond Nanotechnologies), and the third sample was neither
treated nor horn sonicated.
66
4.2.1
Diamond-IZO interface
A quantitative picture of the diamond-ITO interface is given in Chapter 3. The
diamond-IZO interface is similar, except the workfunction is different. For IZO
films made with RF-based sputter deposition (the fabrication is detailed below), the
workfunction is low at 4.46 eV [85]. Therefore the energy barrier for electrons in the
metal ΦB = ΦM − χ becomes ΦB = 4.46 − 0.5 = 3.96 eV. The electrons in the metal
see a lower energy barrier. The built in voltage Vbi is also lower at
1
q
(3.96eV − 1.7eV)
= 2.26 V, as well as the depletion width W at zero bias VA = 0, 3.775 nm.
4.2.2
Fabrication
IZO films were fabricated on glass cover slips via sputter deposition, in a procedure
similar to that used for fabricating IZO-based solar cells [85]. The sputter target
was composed of 90 % In2 O3 and 10 % ZnO. The glass cover slips were cleaned with
sonicating water, acetone, and isopropyl alcohol, and oxygen plasma. The growth
conditions were 130 W RF power, Ar gas at 10 sccm, and 3 mTorr for a growth rate
as characterized
of 0.4 A/s.
The thicknesses of the resulting IZO film were 2000 A/s,
through spectroscopic ellipsometry.
Compared to ITO, sputtered IZO had lower optical transparency, a pink color
(as opposed to ITO’s slightly green hue), and higher resistance (about 60 Ω/square–
100 Ω/square). Electrochemical cell samples were constructed in the usual fashion by
adding a reservoir for electrolyte solution.
4.2.3
Measurement results
For the first sample, spectral measurements of the nanodiamonds were taken prior to
measurements in order to assess the effect of IZO mounting and lack of electrolyte
solution on the NV charge state character. These nanodiamonds had been horn
sonicated by Dr. Abraham Wolcott three months prior to the experiments in order to
increase their dispersion in water. Most of the NVs measured had a combination of
both NV− and NV0 , though a few NVs were strongly NV− or NV0 .
67
Initially, the IZO electrochemical cell sample was studied under the conditions for
which voltage dependent fluorescence modulation was observed in the ITO electrochemical cell. In particular, at voltage ranges from ±1 V, with scan rate of 0.25 Hz, a
total of 9 spots out of 304 spots (about 3.0 % of the total spots) measured showed
fluorescence modulation. The types of modulation included “in phase,” in which the
fluorescence increases as the voltage increases; “out of phase,” in which the fluorescence
increases as the voltage decreases; and “double dip,” where the fluorescence shows dips
at both high and low voltages. Typical time traces with these different modulation
behaviors, as well as a histogram for the modulation contrasts of these spots, are
0.5
1.6227
0
1.5788
1.5349
-0.5
1.4910
1.4471
0
20
40
60
80
100
120
-1
4
2.0346 ×10
Sample "out of phase" time trace
2 out of 302 spots
0.2
1.9709
0
1.9072
-0.2
1.8436
-0.4
-0.6
1.7799
1.7162
-0.8
0
20
40
60
80
100
120
-1
Time (s)
"Double dip" time trace
1 out of 302 spots
Modulation contrast of modulating spots
1.5
4
1
1.6215
0.5
1.4793
0
1.3371
-0.5
Number of spots
3.5
1.7637
Applied Voltage (V)
Norm. Fluorescence Intensity
Time (s)
4
1.9060 ×10
0.4
Applied Voltage (V)
Sample "in phase" time trace
7 out of 302 spots
4
1.6666 ×10
Applied Voltage (V)
Norm. Fluorescence Intensity
Norm. Fluorescence Intensity
shown in Figure 4-1.
3
2.5
2
1.5
1
0.5
1.1948
0
20
40
60
80
100
120
-1
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Modulation contrast
Time (s)
Figure 4-1: Modulation results for IZO electrochemical cell with horn sonicated
nanodiamonds, for 0.25 Hz scans with amplitudes within ±1 V
Then the voltage scan amplitude was changed to the range 0 to −1.2 V, although
the scan rate was kept at 0.25 Hz, to see if the voltage amplitude would change
the nanodiamond fluorescence. There was an small increase in the percentage of
modulating spots: 42 out of 743, or 5.65 %, total fluorescent spots modulated. All
68
except for one of the spots showed “in phase” type modulation, as opposed to the
“out of phase” modulation that was the predominant behavior observed in the ITO
electrochemical cell sample. The results are shown in Figure 4-2. Although a higher
percentage of spots did modulate with the increased voltage amplitude, the modulation
contrast did not increase significantly. Spectral measurements were not performed
0
Voltage (V)
1
1
-1
0
Norm.
Fluorescence
Intensity
Sample "in phase" time trace
41 out of 743 spots
×104
1.1
20
40
60
80
100
120
Time (s)
Sample "out of phase" time trace
1 out of 743 spots
4
×10
-2
1
2
0
1.8
0
Voltage (V)
Norm.
Fluorescence
Intensity
due to problems with sample drift.
-1
20
40
60
80
100
120
-2
Number of spots
Time (s)
Modulation contrast of modulating spots
30
20
10
0
0.01
0.02
0.03
0.04
Modulation contrast
0.05
0.06
0.07
Figure 4-2: Modulation results for IZO electrochemical cell with horn sonicated
nanodiamonds, for 0.25 Hz scans with amplitudes 0 to -1.2V
The second sample with IZO working electrode had nanodiamonds with no special
chemical treatment. The spectra showed many of the fluorescent spots were actually
not NVs; of those that were NVs, many of these were a combination of NV− and NV0 .
16 spots out of 1002 measured spots showed fluorescence modulation with low contrast
as shown in Figure 4-3.
A third electrochemical cell sample contained nanodiamonds treated with piranha
solution (piranha treatment was performed by Diamond Nanotechnologies, Inc. and
the ratio of sulfuric acid to hydrogen peroxide was unknown). Initially 6 out of 117
spots showed fluorescence modulation, as shown in Figure 4-4. However, as the wide
field measurements were repeated to confirm the reproducibility of the fluorescence
69
-0.5
6500
0
20
40
60
80
100
120
Sample "out of phase" time trace
3 out of 1002 spots
0
Voltage (V)
Voltage (V)
0
Norm. Fluorescence Intensity
Norm.
Fluorescence
Intensity
Sample "in phase" time trace
12 out of 1002 spots
-0.5
7500
-1
0
20
Time (s)
40
60
80
100
120
-1
Time (s)
"Double dip" time trace
1 out of 1002 spots
0
6
Modulation contrast of modulating spots
9000
-0.4
8500
-0.6
8000
-0.8
0
20
40
60
80
100
120
-1
Number of spots
-0.2
Voltage (V)
Norm.
Fluorescence
Intensity
9500
5
4
3
2
1
0
0.005
Time (s)
0.01
0.015 0.02 0.025 0.03
Modulation contrast
0.035
Figure 4-3: Modulation results for IZO electrochemical cell with non-horn sonicated
and non-treated nanodiamonds, for 0.25 Hz scans with amplitudes 0 to −1 V
modulation, the modulation contrast decreased and most spots stopped modulating
or showed noisy signal. After the wide field measurements, spectral measurements
were taken of both the “out of phase” spot and “in phase” spot whose fluorescence
time traces are depicted in Figure 4-4. The spectra showed the two spots had strongly
NV0 character. Voltage dependent spectra were not taken because at the time of
measurement both the NV centers had stopped showing voltage dependent fluorescence
modulation.
Compared to the electrochemical cell with the ITO working electrode, both the
percentage of spots modulating and the modulation contrasts were lower. Furthermore, while a few spots showed the “out of phase” modulation behavior typified by
increasing fluorescence with decreasing voltage, the majority of spots showed “in phase”
modulation behavior, where the fluorescence increased with voltage. Because the “in
phase” modulation behavior in the IZO electrochemcial cell was predominant for even
for nanodiamonds with different surface terminations, the IZO was thought to have
caused the “in phase” modulation behavior. The surface terminations in the diamond,
70
Voltage (V)
2.4375
-0.5
2.3481
2.2588
4
2.0270×10
1.9713
1.9157
1.8600
1.8044
1.7488
0
20
40
60
80
100
120
Time (s)
Sample "out of phase" time trace
3 out of 117 spots
-1
0
Voltage (V)
Norm.
Fluorescence
Intensity
0
2.5268
2.1695
0
Norm.
Fluorescence
Intensity
Sample "in phase" time trace
3 out of 117 spots
4
2.6162×10
-0.5
20
40
60
80
100
120
-1
Number of spots
Time (s)
Modulation contrast of modulating spots
2
1
0
0.005
0.01
0.015
0.02
Modulation contrast
0.025
0.03
0.035
Figure 4-4: Modulation results for IZO electrochemical cell with piranha treated
nanodiamonds, for 0.25 Hz scans with amplitudes 0 to −1 V
however, could play a role in the modulation contrast, which in turn would affect the
utility of the nanodiamonds as sensors. The results suggested that the nanodiamond
surface termination would need to be optimized to use the NV charge state as a sensor.
71
72
Chapter 5
Investigation of electric field effect
with dry cells
This chapter describes the fabrication of and measurements on parallel plate capacitors
and interdigitated electrode arrays to investigate the role of electric field induced
band bending on the fluorescence modulation of NVs in nanodiamonds. Parallel
plate capacitors were fabricated because to first order the electrochemical cell could
be described as a capacitor. Interdigitated electrode arrays were chosen as another
device for applying electric field across nanodiamonds. The parallel plate capacitors
consisted of nanodiamonds and a dielectric spacer between two ITO electrodes, and
the interdigitated electrodes were made of gold and the nanodiamonds were spuncoat
or dropcast on top. For measurement, the voltage was applied between the two ITO
electrodes or interdigitated electrodes.
5.1
Parallel plate capacitors
The components of most of the parallel plate capacitor included an ITO cover slip as
each of the conductive plates, and a dielectric material as the spacer between the two
plates. For construction, a dielectric material was added to one ITO cover slip and
nanodiamonds were dropcast or spuncoat on the other ITO cover slip. In all cases the
concentration stated for the nanodiamonds refers to the fractional concentration of
73
the original nanodiamond solution prepared by Diamond Nanotechnologies, Inc.
The electric field in the original electrochemical cell was estimated from the
voltage applied, the Debye length based on the electrolyte solution concentration,
and the dielectric constant of water. The electric field in the electrochemical cell was
calculated as
V
,
kd
where V is the applied voltage, k is the dielectric constant, and
d, the characteristic length over which the voltage drops, is the Debye length. For
the original electrochemical cell, the applied voltage ranged from 100 mV to 1 V, the
Debye length was estimated to be 10 nm–20 nm, and the dielectric constant of water
is 80. The electric field was on the order of 105 V/cm–106 V/cm.
Because the width of the ITO cover slips (with area about 2.5 cm by 2.5 cm) was
significantly greater than the spacing between the plates (which ranged from nanometers to microns), the simple parallel plate capacitor model was deemed appropriate for
these devices. The applied voltage required for the parallel plate capacitor was estimated with a similar calculation to the one for the electric field in the electrochemical
cell, and with the dielectric constants of spacers ranging from 1 to 5. This calculation
showed that voltages of magnitude 10 V–100 V would achieve the same electric field
magnitude as that in the electrochemical cell.
For assembly of the parallel plate capacitor, the two ITO cover slips were put
together with either tape or UV cured glue. The two ITO cover slips were offset so
that the overlapping areas would include both the spacer and nanodiamonds, while
copper tape would be added to the exposed ITO areas to make electrical contacts.
The UV cured glue was added only to the corners of the cover slip, and while the
corners were exposed with UV light for 20 s, the rest of the device was covered to
prevent or minimize exposure to the UV light. Figure 5-1 shows a schematic of the
parallel plate capacitor. After assembly a voltmeter measured the resistance of a
device, to check that it had not shorted.
Further characterization of the device involved spectral measurements under white
light illumination to calculate the distance between the two ITO cover slips. The two
plates of the parallel plate capacitor could correspond to the two surfaces of a FabryPerot cavity. A transmission spectrum of a Fabry-Perot cavity is a periodic function
74
ary results: Parallel plate capacitor
ITO
spacers
tyrene beads 1um size
~40nm-3um
Spacer
A (~30-50nm)
Cu tape
ITO
(~40-60nm)
ds
Figure 5-1: Schematic for the parallel plate capacitor, where the spacer is a transparent
dielectric material. Offset in the ITO cover slips is left to make electrical contacts.
odulation observed
even for 100V swing
that depends on the spacing of the cavity, where the peaks and dips corresponding
e shorted, ~6V
respectively constructive and destructive interference. The condition for the distance
between the peaks is given by the free spectral range of the cavity, νF SR =
c
,
2nd
where
odulation observed
for ~13V
swingand(higher
d is the length
of the cavity
n is the would
index of exceed
refraction of the medium [86].
akdown voltage)
For the parallel plate capacitor, first the background was accounted for by dividing
oup
the spectrum taken with the device by the background spectrum. Calculations of
the spacings of the device were based on the distances between the peaks in the
background-accounted spectrum.
10
The same measurement procedure as that for the ITO electrochemical cell was
used to investigate fluorescence modulation in the parallel plate capacitors. The
potentiostat applied voltages of magnitudes up to and at ±6.5 V, and a piezo driver
applied any voltages of higher magnitudes. An arbitrary waveform generator was
used to generate the input signal for the piezo driver. The parameters varied for
the arbitrary waveform generator included wave amplitude, offset, and frequency. A
National Instruments DAQ recorded the arbitrary waveform generator output, but
the piezo driver multiplied the amplitude of the arbitrary waveform generator by 20.
The ITO electrode chosen as the working or signal electrode was in direct contact
with the spacer and not the nanodiamonds, to avoid charge injection directly from the
ITO to the nanodiamonds. In addition, spectral measurements were taken to confirm
that NV centers were present in the devices.
In some of the devices, the top electrode was made with ITO sputtered directly
on top of the spacer. For these devices neither glue nor tape were required, and the
75
spacer film thickness was assumed to be the spacing between the two electrodes.
5.1.1
Polystyrene bead spacer
Fabrication
Polystyrene beads of sizes 0.05 µm and 1 µm (in aqueous solution from Polysciences,
Inc.) were chosen because 50 nm was about twice the size of the average nanodiamond,
but 1 µm would accommodate even larger aggregates in the nanodiamond solution.
Several methods were attempted to yield a sparse, uniform distribution of polystyrene
beads on the ITO cover slips. First, polystyrene beads in an aqueous with concentrations of 1/100 and 1/1000 of the original solution were dropcast on the cover slips
and dried on a hot plate. However, these left “coffee stain” like markings, which
indicated areas of high concentration. Another attempt was to mix UV cured glue
with polystyrene beads (at 1/1000 and undiluted concentrations), dot 1 µl of the solution
on each of the corners and the center of the coverslip, and then press another cover
slip on top to spread the dots of nanodiamond solution. However, the UV glue and
polystyrene beads did not mix well: pockets of the polystyrene beads, unmixed with
the glue, were observed in the microscope.
A different approach involved a dip coating method: the ITO cover slip was dipped
into a solution of polystyrene beads (1/100 concentration) and rinsed successively with
deionized water for a few times. The dip coating method yielded a sparse distribution
of diamonds. This was combined with an ITO cover slip with dropcast 25 nm NDs.
The two cover slips with glued together with small dots of UV cured glue at each
corner of the overlapping areas.
Another approach was to spincoat the polystyrene beads. Spinning parameters
of 2500 rpm spin speed and 30 s spin time yielded the best results for 1 µm size, 1/100
concentration polystyrene beads. These were combined with ITO cover slips with
nanodiamonds either dropcast or spuncoat. Spincoating the nanodiamonds involved
preparing a nanodiamond solution with 0.01 % polyvinyl alcohol (PVA) by weight,
then spinning at 2000 rpm for 30 s. Unlike the cover slips with dropcast nanodiamond,
76
the cover slips with spuncoat nanodiamonds did not show the “coffee stains.”
One simple approach to yield sparse polystyrene beads was to add drops of less
than 0.5 µl at each corner of cover slip and then spread the beads across the cover slip
with nitrogen or air flow.
Measurement results
The first device measured had dropcast 25 nm nanodiamonds and the 1 µm polystyrene
beads added by dipcoating. Prior fluorescence measurements, the resistance was
measured to confirm the device was not shorted and spectra were taken to confirm
the presence of NVs and to measure the capacitor spacing. Based on the Fabry-Perot
cavity model, the spacing was calculated to be 3 µm.
For the fluorescence measurements, the voltage scan rates, amplitudes, and waveform type were varied. Initial voltage amplitudes were at 100 mV to prevent any
breakdown of the capacitor, but then were increased to 100 V. Scan rates varied
from 0.0125 Hz to 0.5 Hz, for both triangular and square waves. No modulation was
observed in this device.
The other polystyrene bead capacitor shorted but the reason for the shorting was
inconclusive. Some possible reasons were that due to the lack of overage with the
beads, ITO surface roughness, or presence of contaminating particles on the ITO
surface, the electrodes had come into contact.
Because it was not trivial to get a sparse, uniform distribution of polystyrene
beads, parallel plate capacitors with other spacers were fabricated, as described in the
following sections.
5.1.2
Poly(methyl methacrylate) spacer
Poly(methyl methacrylate), or PMMA, was chosen as a spacer because it is transparent
and well characterized as a resist for electron beam lithography.
77
Fabrication
PMMA is commonly added to substrates by spincoating and the film thicknesses as
a function of spin speeds are well documented in spin curves. Because PMMA A11
(950,000 molecular weight PMMA in anisole with weight 11 % from MicroChem) yields
films with micron thicknesses, it was diluted in anisole (1 part PMMA A11 to 7 parts
anisole) to yield films with nanometer thicknesses. The thicknesses of films from this
diluted PMMA were characterized with ellipsometry measurements. PMMA was spun
for 60 s at spin speeds 1000 and 4000 rpm and baked at 180 ◦C for 90 s, which yielded
film thicknesses of 430 and 224 A.
To make contacts, the PMMA on the edge of the cover slip was removed by dipping
the edge in acetone and copper tape was added. The devices were assembled with
ITO cover slips with nanodiamonds spuncoat on them, using the method described
previously: 0.01 % PVA and 2000 rpm spin for 60 s. The cover slips were glued together
with dots of UV cured glue on the corners.
For some devices, small pieces of the ITO cover slip were used as the top electrode
to minimize the surface that could lead to shorting the device.
Measurement results and insight
The PMMA devices held together with UV cured glue all shorted, including those
with the smaller top electrodes. This was confirmed using both the voltmeter and the
potentiostat. The IV curve generated by the potentiostat depicted a linear relation
between current and voltage, which indicated the device behaved as a resistor. It was
likely that the UV light exposure had degraded PMMA [87] and therefore had shorted
the devices.
Other alternatives to UV glue were suggested but not used: superglue would not
work because its solvent would also probably degrade the polymer spacer; heating
the device would melt the PMMA and glue the two plates together but affect the
integrity of the film. In addition, the breakdown voltage of PMMA is 1 MV/cm [88],
which shows that for a spacing of 2 µm PMMA breaks down at about 600 V, but for a
78
spacing of 60 nm the breakdown is much lower at around 6 V.
Another method to fabricate PMMA-based devices was to use the adhesion promoter hexamethyldisilazane (HMDS) to render the ITO surface hydrophobic before
spinning hydrophobic PMMA in chlorobenzene. However, since this would affect the
adhesion of the hydrophilic, hydroxyl terminated nanodiamonds to the PMMA, this
method was not pursued.
5.1.3
Hydrogen silsesquioxane spacer
Hydrogen silsesquioxane, or HSQ, is another common electron beam lithography resist.
The devices with HSQ spacer included two types: for one type, the capacitor spacer
included two HSQ films sandwiching a film of nanodiamonds, while for the other type
there was one film each of HSQ and nanodiamond.
Fabrication
1 % HSQ was spun at spin speeds 2000, 3000, 4000 rpm and the thicknesses were
characterized with spectroscopic ellipsometry. The resulting thicknesses were 634, 536,
respectively. HSQ was removed from the edge by rubbing the edge of the ITO
484 A
cover slip with a “salty developer” composed of 1 % NaOH and 4 % NaCl (prepared
by Sam Nicaise from the research group of Karl Berggren). An electrical contact pad
was made by adding copper tape to the edge where HSQ was removed. To cure the
HSQ, it was placed in an oxygen plasma etcher for 60 s at 100 W.
The cover slips for the nanodiamonds were pretreated with a commercial chemical
solution Hellmanex III to make the ITO surface more hydrophilic. Specifically, the
cover slips were sonicated in 1 % volume Hellmanex III at 35 ◦C for 30 minutes. Then
10 µl of 1/100 concentration of 25 nm nanodiamonds was dropcast on the pretreated
cover slip. The effect of the Hellmanex III treatment was obvious to the naked eye, as
the contact angle of the nanodiamond solution droplet was reduced and the spread of
the droplet on the cover slip was greater.
The devices were assembled by taping the cover slips together with cellophane
79
tape.
Measurement results
For the device measured the spacer consisted of nanodiamonds sandwiched between
of HSQ (1 % HSQ spun at 3000rpm) and 408 A
of HSQ
two layers of HSQ: 345 A
(1 % HSQ spun at 2000 rpm). No interference fringes were observed in the spectra
taken of the device with white light, which made it difficult to calculate the spacing
between the plates based on the Fabry-Perot model. The capacitor spacing was instead
characterized through ellipsometry measurements of the thin films. No nanodiamond
fluorescence modulation was observed for a voltage range of ±6.5 Vs. This meant that,
nanodiamonds did not
based on the assumption that the capacitor spacing was 753 A,
show modulation even for 1.7 × 106 V/cm.
5.1.4
Sputtered silicon dioxide spacer
The use of HSQ as a spacer was discontinued because in some other devices (where
HSQ was intended to isolate interfaces in electrochemical cells) it was found that the
HSQ film cracked. Sputtered SiO2 was an alternative spacer to HSQ. The breakdown
voltage of sputtered SiO2 is 1 V/10 nm (according to members of the Bulović group).
Fabrication
The RF-based sputter deposition method was detailed in Chapter 4. Some samples were
prepared by treating the ITO cover slip withHellmanex III and then adding 190 nm of
SiO2 and dropcast nanodiamonds. For 190 nm sputtered SiO2 , the breakdown voltage
is 19 V, at which the electric field is 106 V/cm.
A new approach was used for the the top electrode of the parallel plate capacitor.
It consisted of a 1 cm by 1 cm square of an ITO cover slip (about a quarter of the size
of the bottom cover slip) with 2 nm of titanium and 20 nm gold evaporated on one
edge (see Figure 5-2. Due to the orientation or the cover slip during evaporation, a
“matchstick” like pattern formed on the edge of the small cover slip. This design was
80
chosen for a number of reasons. A smaller cover slip would reduce the area that could
short, metal would provide good conductivity, and the overhang shape of the metal
would allow contact to the top surface of the device. The evaporation was performed
by NanoStructures Laboratory cleanroom facility manager James Daley. As a contact,
a metal wire was glued to the gold on the top surface with silver paste.
ITO
Gold
SiO2
ITO
Figure 5-2: Schematic for the capacitor with an ITO cover slip with “matchstick” gold
overhang as the top electrode. Size of gold overhang is exaggerated.
Another new approach to making the top electrode of the parallel plate capacitor
was to sputter ITO directly on top of the spacer film. Since the ITO would be
sputtered directly on the spacer film, without leaving any air gaps, the thickness of
the spacer film was assumed to be the same the spacing of the parallel plate capacitor.
The ITO sputtering was also performed with RF-based sputter deposition, with
the following conditions: argon gas at 10 sccm, growth pressure at 4 mTorr, RF struck
at 30 W and increased to 70 W. The growth rate of ITO was approximately 0.89 A/s,
so the growth time was 1170 s to get 100 nm of ITO.
Measurement results
The sputtered SiO2 device with gold matchstick top electrode did not short. The
spacing was inconclusive from the spectral measurements. No fluorescence modulation
was observed for voltage amplitudes up to 18 V, just short of the breakdown voltage.
The sputtered SiO2 device with sputtered ITO top electrode shorted. This was
considered to be because the sputtered SiO2 was porous, though further investigations
were not pursued.
81
5.1.5
SiO2 spacer made with plasma enhanced chemical vapor deposition
Because the breakdown voltage for sputtered SiO2 is less than that of SiO2 films made
with plasma enhanced chemical vapor deposition (PECVD), PECVD SiO2 spacers were
made to check the fluorescence modulation of the nanodiamonds at higher voltages.
The breakdown voltage of PECVD SiO2 is 7 MV/cm for a deposition temperature of
150 ◦C.
Fabrication
ITO cover slips were cleaned and then pretreated with Hellmanex III. In some cases
nanodiamonds were dropcast or spuncoat on the ITO cover slips prior to PECVD.
The edges were then masked by wrapping the cover slips in foil. The top electrode
consisted of sputtered ITO.
Three batches of PECVD SiO2 samples were fabricated. One set of samples
had naondiamonds simply dropcast on the ITO cover slips, with 150 nm and 260 nm
thicknesses of SiO2 .
For the second set of samples, the nanodiamonds underwent more processing and
treatment. This was because it was considered that the aggregates of nanodiamonds
could be shorting many of the devices; coffee stains would usually indicate high
concentration or large agglomerates of nanodiamonds. Therefore, the nanodiamonds
were horn sonicated and then dropcast or spuncoat. Four ITO cover slips had
nanodiamonds with 1/1000 concentration dropcast, and another four ITO cover slips
had 1/10 nanodiamond solution spuncoat with PVA. SiO2 was then fabricated and
100 nm of ITO sputtered. Because there were four thicknesses of SiO2 , there were
eight samples total (see Table 5.2).
The third set of parallel plate capacitor substrates were prepared with various
thicknesses of PECVD SiO2 . In some of these devices the nanodiamonds were
sandwiched between two layers of spacers, while in other devices nanodiamonds
directly contacted the ITO. The fabricated device types are detailed in Table 5.1.
82
Table 5.1: Third set of parallel plate capacitors with PECVD SiO2 film spacers
Sample
1
Top
elec- 100 nm
trode
sputtered
ITO
160 nm
Spacer
PECVD
SiO2
1/200
NDs
dropcast
2
100 nm
sputtered
ITO
310 nm
PECVD
SiO2
1/200
NDs
dropcast
230 nm
PECVD
SiO2
ITO cover
slip
230 nm
PECVD
SiO2
ITO cover
slip
Bottom
electrode
3
100 nm
sputtered
ITO
4
100 nm
sputtered
ITO
5
100 nm
sputtered
ITO
390 nm
PECVD
SiO2
1/500
NDs
dropcast
390 nm
PECVD
SiO2
1/200
NDs
dropcast
390 nm
PECVD
SiO2
1/10
NDs
spuncoat
ITO cover
slip
ITO cover
slip
ITO cover
slip
Measurement results
The first batch of devices with the sputtered ITO top electrode, with 150 nm and
260 nm PECVD SiO2 , shorted. This was thought to be because sputtering ITO could
penetrate the PECVD SiO2 layer, though this was not investigated.
All but one device from the second batch shorted, as shown in Table 5.2. The one
device that did not short had dropcast nanodiamonds with 600 nm of SiO2 .
Table 5.2: Result for second set of parallel plate capacitors with PECVD SiO2 film
spacers
Sample
Spuncoat nanodiamond
Dropcast nanodiamond
150 nm
shorted
shorted
260 nm
shorted
shorted
400 nm
shorted
shorted
600 nm
shorted
not shorted
High background fluorescence was observed for this device with 600 nm of PECVD
SiO2 . Although typically background fluorescence would photobleach, the background
fluorescence in this device did not easily bleach and was illuminated with the laser for
some duration. No modulation was observed for voltage measurements in the range of
±1 V, ±3 V, and ±5 V. Although no higher voltage was applied, the device was later
found to be shorted, with a resistance on the order of hundreds of kΩ. Based on the
breakdown voltage of 7 MV/cm, the breakdown for a 600 nm film would have been
83
420 V, which was lower than the voyage applied. It was possible that the SiO2 film
had degraded through other means, but this was not verified.
The third set of devices was found to similarly suffer from high background
fluorescence as well, although the PECVD SiO2 films were thinner than 600 nm.
Similarly the devices also shorted after some duration of laser illumination.
5.1.6
Summary of parallel plate capacitor results
A summary of the devices with different spacers and the outcomes are detailed in
Table 5.3.
Table 5.3: Summary of parallel plate capacitors with different spacers. Asterisk
indicates cases where capacitor spacing and spacer film thickness were assumed to be
the same.
Spacer
Polystyrene beads
Thickness
1 µm
PMMA
20 nm–50 nm
HSQ
80 nm
Sputtered SiO2
100 nm–200 nm
Sputtered SiO2
190 nm
PECVD SiO2
300 nm–600 nm
5.2
Top electrode
ITO cover slip
Outcome
No modulation up
to 3 × 105 V/cm
ITO cover slip
Shorted due to UV
light illumination
ITO cover slip
No
modulation
observed
for
1.7 × 106 V/cm*
Sputtered ITO
Device
shorted,
sputtered SiO2 is
porous
ITO cover slip with No modulation for
Au overhang
9 × 105 V/cm*
Sputtered ITO
High background
fluorescence
Interdigitated Electrodes
Because the parallel plate capacitor devices had often shorted, interdigitated electrodes
were introduced as an alternative device geometry for investigating the effect of electric
field on nanodiamond fluorescence. The design, from colleague Hannah Clevenson,
included three sets of interdigitated electrodes, with spacings of 8 µm, 6 µm and 4 µm.
84
An SEM image of part of the 8 µm interdigitated electrodes set is depicted in Figure 5-3.
In each of the sets the width of the electrodes matched the width of the spacings; each
electrode had contact pads of size 1 mm by 1 mm. The interdigitated electrodes were
fabricated on glass and then mounted on a customized printed circuit board (PCB).
5.2.1
Fabrication process flow
Interdigitated electrodes on glass
The fabrication process was adapted from an existing recipe developed by Hannah
Clevenson, with the help of James Daley. The process included resist spinning,
exposure, resist development, metal evaporation, and liftoff. Initially, the glass cover
slip substrates (square cover slips of size 20 mm by 20 mm or 22 mm by 22 mm) were
cleaned by bath sonication in ultrapure water, acetone, and isopropyl alcohol, and by
oxygen plasma ashing.
Prior to spinning resist, the adhesion promoter HMDS (hexamethyldisilazane, as
described previously) was spun on the glass cover slips at 4500 rpm for 60 s. There
was a ten minute pause after spinning HMDS to let the HMDS vapors, which could
potentially interfere with the spinning of the resist, dissipate. The resist S1813 was
spun at 4500 rpm for 60 s, then baked at 110 ◦C for 60 s.
Exposure was performed in a mask aligner, with a mercury arc lamp as the UV light
source. The mask was a chrome mask written by Hannah Clevenson in a Heidelberg
model laser writer. The exposures were timed to a desired interval with a resolution
of one second. The total exposure was 30 s, but for every second of exposure there
was a wait time of 30 s to allow the resist to degas.
After exposure, the resist was developed in MF321 for 25 s, then the device was
rinsed with deionized water. Then 25 nm of Ti and 100 nm of Au were evaporated by
James Daley. After liftoff in sonicating acetone, the device was rinsed in DI water and
dried.
The characterization of the devices included optical microscopy and scanning
electron microscopy (SEM), and resistance measurements. This was done to check the
85
proper fabrication of the electrodes and to see whether the devices were shorted.
Figure 5-3: SEM image of interdigitated electrodes with 8 µm spacing and 8 µm width
electrodes, with nanodiamonds dropcast. The lines around the electrodes came from
dropcasting the nanodiamond solution; the nanodiamonds appear as white flecks. Not
included in the picture are large contact pads on each side.
Printed circuit board
The interdigitated electrodes were mounted on a custom designed PCB in order to
apply a voltage across the interdigitated electrodes. The boards were manufactured by
ExpressPCBTM and the designs were made in the ExpressPCBTM commercial software.
Each PCB was approximately the size of a microscope cover glass. The contact pads
and wires were all made with tin/lead plating and the laminate of the board itself
was made of FR-4 epoxy glass with copper. See Figure 5-4 for the PCB schematic.
86
for SMA
connectors
signal
pad
ground
pad
Figure 5-4: Schematic of PCB.
In each PCB there was a hole in the center to mount the glass cover slip with
interdigitated electrodes. Outside of the hole were six contact pads: one signal
electrode and one ground electrode for each set of interdigitated electrodes. Near the
edges of the PCB were three sets of five small holes (four surrounding one central
hole) to accommodate the pins of SMA connectors. The central pin was for signal and
the four surrounding pins for ground. The contact pads and SMA connector holes for
ground were all connected to the ground plane. The signal contact pad was connected
to the central hole for SMA connector with a wire.
Assembly and experimental setup
The glass cover slip with interdigitated electrodes was mounted on the bottom of the
PCB so that the interdigitated electrodes would be visible through the center hole of
the PCB. The contact pads of the interdigitated electrodes were then wire bonded to
the PCB contact pads with a gold ball wire bonder. However, as the connection of the
gold wire to the PCB’s tin/lead plated contact pads was often tenuous, a dab of silver
paste was added to each connection to secure it. Once the interdigitated electrodes
were mounted on the PCB, the SMA connectors (from Digikey, with female sockets
with external threads) were soldered to the PCB.
On each set of interdigitated electrodes, a very small amount of sonicated nanodia87
monds, less than 1 µl, was dropcast. The amount of sonicated nanodiamond solution
was restricted to prevent nanodiamonds from spreading to other interdigitated electrodes on the same cover slip.
After the device was mounted on the microscope, it was connected via SMA cable
to the output of a piezo driver. As before, an arbitrary waveform generator was used:
its output was recorded by a DAQ and served as the input for the piezo driver. In the
camera images, the gold electrodes appeared as dark areas.
5.2.2
Measurement results
The first sample studied was a set of interdigitated electrodes with 8 µm spacing with
piranha treated nanodiamonds. The voltage amplitudes applied on this sample were
varied, to investigate the effect of this variation on the number of modulating spots
with respect to their location relative to the electrodes, their modulation behavior,
and their modulation contrast. The voltage frequency was 0.125 Hz and the 532 nm
excitation power at 125 mW. The results are shown in Figure 5-5 for a total of 231
nanodiamonds on ∼40 µm by 40 µm studied. A total of 28 nanodiamonds, or 12 %
showed modulation. The modulation contrast did not significantly improve as the
voltage amplitude increased; however the number of modulating spots overall and on
the signal and ground electrodes increased. The predominant modulation behavior
was “out of phase.”
Subsequently the measurements were performed at 90 V or 100 V amplitude and
125 mW excitation power. It was found that increases in the excitation power led
to the melting of the gold and disfigurement of the gold electrode edges; therefore
excitation power could not be usedto increase the modulation contrast or signal.
Results for 90 V modulation are shown in Figure 5-6. Out of 557 spots studied,
114 (20 %) showed modulation. Of these the majority modulation behavior was “out
of phase,” with 107 spots showing this behavior. The “in phase” modulating spots
had weak modulation and low contrast.
Modulation contrast was also depicted with respect to location relative to the
electrodes, where 1 denotes on the signal electrode, 2 on ground electrode, 3 near
88
Modulation contrast
Modulation contrast as function of applied voltage
0.2
0.1
0
0
10
20
30
40
50
60
70
80
90
Number of spots
Voltage (V)
Location of modulating spots as function of applied voltage
30
On Signal
On Ground
Near Signal
Near Ground
Isolated
20
10
0
5
10
20
30
40
50
60
70
80
90
Number of spots
Voltage (V)
Type of modulating behavior as function of applied voltage
Out of phase
In phase
Double dip
20
10
0
5
10
20
30
40
50
60
70
80
90
Voltage (V)
Figure 5-5: 8 µm spacing interdigitated electrodes: Effect of voltage amplitude on how
many spots modulate, with respect to their location relative to the electrode, their
modulation behavior, and their modulation contrast
signal, 4 near ground, and 5 is an isolated spot not near any electrodes. Quantitatively,
“on” an electrode meant the spot was within 7 pixels of the electrode edge, and “near”
an electrode meant the spot was within a quarter of the distance between electrodes,
from an electrode edge. The lack of spots near the ground electrode was likely due
to pattern of dropcast nanodiamonds: based on the camera image, there were few
nanodiamonds near the ground electrode. Modulation contrast appeared to be greater
for spots on or near the signal electrode.
For one particular spot (which was located on the signal electrode), whose time
trace is shown in Figure 5-6, spectra were acquired at 0 V and 100 V with an acquisition
time of 10 s. As in the wide field time trace, fluorescence was greater at 0 V than
at 100 V. The spectra show both NV− and NV0 character, and the NV− character
increased with decreasing voltage.
Other areas on the set of 8 µm interdigitated electrodes were studied for 100 V
modulation; these results are depicted in Figure 5-7. Of total 683 nanodiamonds
89
Modulation contrast of modulating spots
Modulation contrast sorted by location of spots
Modulation contrast
Number of spots
60
50
40
30
20
10
0
0
0.05
0.1
0.15
0.2
0.25
0.2
0.15
0.1
0.05
0
0
0.25
1
50
1.1804
1.0822
20
40
4
5
6
60
0
2000
Intensity (a.u.)
1.2785
Applied Voltage (V)
Norm. Fluorescence Intensity
100
1.3766
0.9841
0
3
Spectra at 0V and 100V
Fluorescence time trace
4
x 10
1.4748
2
Location: 1/3=on/near signal,
2/4=on/near ground, 5=isolated
Modulation contrast
0V
100V
1500
1000
500
500
Time (s)
550
600
650
700
750
800
Wavelength (nm)
Figure 5-6: Wide field modulation results for 90 V, 0.125 Hz voltage scans on nanodiamonds on 8 µm spacing interdigitated electrodes. Spot whose fluorescence time trace
and spectra are shown was located on the signal electrode.
observed, a total of 118 spots (17.3 %) showed modulation, and of these 114 spots
showed “out of phase” modulation. Most of the modulating spots were on the electrode
edges, and the spots with the highest modulation contrast were on the signal electrode
or near the signal electrode.
Confocal and spectral measurements were taken of some spots near the signal
electrode. The spectral measurements are shown in Figure 5-8. Unlike the dominant
behavior in the wide field measurements, the fluorescence of these spots increased
with increasing voltage. The subtraction spectra suggested a simultaneous increase in
NV0 behavior with decrease in NV− behavior as voltage increased. These spectral
measurements suggested that the NV charge state was indeed influenced by the
variation in applied voltage. However, this fluorescence modulation was not observed
in wide field measurements, and it was different from the predominant behavior
observed in wide field measurements. Also, the spectra were not reproducible for other
spots.
90
Histogram of modulating spot contrast
40
35
30
25
20
15
10
5
0
0.05
0.1
0.15
0.2
60
50
40
30
20
0
0.25
Modulation contrast sorted by
location of spots
0.25
0.2
0.15
0.1
0.05
0
1
2
3
4
1
2
3
4
5
Location: 1/3=on/near signal, 2/4=on/near ground, 5=isolated
Norm. Fluorescence Intensity
Modulation contrast
70
10
Modulation contrast
0
80
5
6
4
1.2187×10
Fluorescence time trace of spot near signal
100
1.1438
80
1.0690
60
0.9941
40
0.9192
20
0.8444
0
Location: 1/3=on/near signal, 2/4=on/near ground,
5=isolated
10
20
30
40
50
60
70
Voltage (V)
0
Histogram of modulating spot location
90
Number of spots
Number of spots
45
0
Time (s)
Figure 5-7: Wide field modulation results for 100 V, 0.25 Hz scans on nanodiamonds
on 8 µm spacing interdigitated electrodes. Spot whose fluorescence time trace is shown
was located near the signal electrode.
The same wide field measurements were repeated for 6 µm spacing interdigitated
electrodes. On this sample there was a dense amount of nanodiamonds, often in
clusters. A total of 2166 spots were studied; of these the total number of modulating
spots was 287 spots (13.3 %). Of these 23 spots showed “in phase” and 264 showed “out
of phase” modulation. Results are shown in Figure 5-9. However, these modulation
results were not reproducible: the nanodiamonds on this 6 µm electrodes sample
stopped modulating for subsequent measurements.
Other samples were prepared but did not show modulation. A set of 8 µm spacing
interdigitated electrodes with piranha treated nanodiamonds did not show modulation. A set of 8 µm spacing interdigitated electrodes with spuncoat piranha treated
nanodiamonds did show modulation but spectral measurements showed that none of
modulation was from NV centers. A set of interdigitated electrodes with no diamonds
was studied to assess the presence of non-NV fluorescent material, but no modulation
was observed.
SEM images of interdigitated electrodes with and without nanodiamonds were
91
Subtraction spectra
Intensity (a.u.)
Intensity (a.u.)
Spectra at 0V and 100V
1400
0V
100V
1200
1000
800
600
550
200
100
0
−100
600
650
700
750
−200
550
800
600
Spectra at 0V and 100V
700
750
800
Subtraction spectra
2500
Intensity (a.u.)
Intensity (a.u.)
650
Wavelength (nm)
Wavelength (nm)
0V
100V
2000
1500
200
100
0
−100
1000
−200
500
550
600
650
700
750
−300
550
800
Wavelength (nm)
600
650
700
750
800
Wavelength (nm)
Figure 5-8: Spectral measurements for 100 V voltage amplitude on nanodiamonds
near the signal electrode on 8 µm spacing interdigitated electrodes. Fluorescence
modulation was different from that observed in wide field measurements.
taken but these were not conclusive. The interdigitated electrodes themselves did not
appear to show any degradation, other than locations where the high laser illumination
combined with high applied voltage melted the gold. On the interdigitated electrodes
that had been measured clusters of nanodiamonds as well as some dust were visible, but
other than this the electrodes seemed intact and undamaged. Voltmeter measurements
also showed that the electrodes had not shorted.
5.2.3
Summary
The results for nanodiamond modulation on 8 µm and 6 µm interdigitated electrodes
were inconclusive. The percentage of nanodiamonds showing fluorescence modulation
was lower slightly lower than that of the original ITO electrochemical cell. The
dominant modulation behavior was “out of phase,” as in the case for nanodiamonds in
the ITO electrochemical cell. For some NVs in the nanodiamonds the voltage did shift
the charge state, as seen in the spectral measurements, though the spectral results
92
Histogram of modulating spot contrast
Histogram of modulating spot location
25
Number of spots
Number of spots
20
15
10
5
20
15
10
5
0.05
0.1
0.15
0
1
0.2
Norm. Fluorescence Intensity
Modulation contrast
Modulation contrast sorted by location of spots
0.2
0.15
0.1
0.05
0
0
1
2
3
4
2
3
4
5
Location: 1/3=on/near signal, 2/4=on/near ground, 5=isolated
Modulation contrast
5
Fluorescence time trace of spot near signal
100
3.0219
2.7153
50
2.4086
2.102
1.7953
0
6
4
x 10
3.3286
Location: 1/3=on/near signal, 2/4=on/near ground, 5=isolated
20
40
60
Applied Voltage (V)
0
0
0
Time (s)
Figure 5-9: Wide field modulation results for 100 V voltage, 0.125 Hz scans on nanodiamonds on 6 µm spacing interdigitated electrodes.
were inconsistent.
The fact that most of the fluorescence modulation was observed in spots on or
near the electrodes seemed to suggest the charge transfer mechanism as the dominant
mechanism for fluorescence modulation. This also was consistent with the observation
that many nanodiamonds in clusters exhibited similar modulation behavior, which
suggested that the charge transfer between nanodiamonds in a cluster could have
been mediated by surface trap states. However, this does not explain the spectral
measurements shown for two NVs on the signal electrode, where the fluorescence
increased as voltage increased. If charge transfer were the dominant mechanism the
fluorescence would be expected to increase as voltage on the signal electrode was
decreased. This behavior may be more consistent with the electric field mechanism.
Because nanodiamonds on interdigitated electrodes stopped showing fluorescence
modulation it was considered that their modulation could have been a transient effect.
As the modulation results were not reproducible and the spectral measurements were
inconsistent, the results were inconclusive.
93
94
Chapter 6
Microscope setup modification
design
This chapter details the design for modifications to an inverted Zeiss Observer.Z1m
microscope setup to extend its capabilities. The intended use of the microscope setup
is for future studies of nanodiamond conjugates, where the nanodiamond may be
bound to gold nanoparticles, superparamagnetic particles, and quantum dots in order
to enhance the its fluorescence emission or its sensitivity to external fields. The aims
of the modification are to add wide field and confocal imaging capabilities to the
microscope. Wide field allows for screening of nanodiamonds and building statistics,
while confocal allows for addressing individual nanodiamonds and measuring their
fluorescence lifetimes.
The current microscope setup does not allow for wide field laser excitation with
camera detection. There is, to the microscope’s left, an optical setup used for
polarization measurements. But otherwise, the area to the right of the microscope is
underutilized and can accommodate more optical components (see Figure 6-2).
Therefore, the goal of the design to add the capability for excitation from right side
of the microscope and for collection on a CCD camera on the top of the microscope.
95
6.1
Microscope reflector turret
To enable excitation from the right side of the microscope, a specially designed and
machined reflector turret has been commissioned from Zeiss. The reflector turret,
or revolving filter wheel, plays a role in both excitation collection (see Figure 6-1.
It can switch among six reflector modules which can each hold an emission filter,
an excitation filter, and a dichroic beam splitter. The excitation filter separates the
beam from excitation source from the back of the microscope. The dichroic beam
splitter reflects the excitation beam onto the back aperture of the microscope objective.
Emission from the sample is collected through the same objective, is transmitted
through the dichroic beam splitter onto the collection path, and filtered through the
emission filter. Collection can be directed to the front port, the left side port, or the
right side port. At the front port, emission from the sample may be imaged on the
eyepieces or a camera.
Reflector
turret
Dichroic
beamsplitter
along
diagonal
Emission
and
excitation
filters
Reflector
module
Figure 6-1: The Zeiss reflector turret and module. Figure created with images from
the Zeiss manual [89].
The customized reflector turret has been commissioned because the current turret
96
is configured to receive the excitation beam from the back and not from the side. The
modified turret has one reflector module oriented so that excitation from the right
side of the microscope can be reflected on to the back aperture of the objective.
For the particular purpose of studying NVs in nanodiamonds, the reflector module
contains a dichroic beam splitter with a 532 nm cutoff wavelength. In this configuration
the 532 nm excitation beam is reflected and the NV emission from the sample above
532 nm is transmitted.
The emission is further filtered through filters in the “analyzer slider” below the
reflector turret. These hold additional filters, including 550 nm or 650 nm long pass
filters that can be removed or replaced easily.
6.2
Excitation
The excitation includes two imaging modalities: it can switch between a CW laser
(for wide field and confocal CW measurements) and a fiber port to couple excitation
from a pulsed laser source for fluorescence lifetime measurements (see Figure 6-2). To
switch between the CW laser and the fiber port there is a mirror on a flip mount. The
flip mirror is up for the CW and down for the fiber port. From either the fiber port
or CW laser via the flip mount, the beam enters a periscope assembly (a set of two
mirrors at different z heights with respect to the table) to adjust its height before it
enters the microscope right side port. From the right side port the beam enters the
dichroic beam splitter in the turret and then the back aperture of the objective.
6.2.1
Wide field excitation design
Wide field excitation is performed using a 532 nm CW laser. The optical components
include filters for filtering the beam from laser, and a beam expander and wide field
lens used for wide field excitation. For the laser, A 532 nm laser-line filter and a
neutral density filter are added to the beam path to filter out excess emission and
reduce the intensity of the laser.
In wide field excitation, in order to have a large area of illumination on the sample
97
Left
Back
Right
Mirror
CW laser
Microscope
Right
side port
532nm
Neutral
laser line density
filter
filter Beam expander
14-16.5cm
Fiber
port
7-8cm
Periscope
assembly
Wide
field
lens
Mirror
on flip
mount
Front
Figure 6-2: Schematic of the optical components to add wide field excitation to the
right of the microscope. Top view. Not to scale.
plane, the beam entering the back aperture of the microscope must be tightly focused.
A wide field lens in front of the microscope right side port focuses the incoming beam
into a tightly focused spot. The wide field lens is mounted on a flip mount so that
it can be removed for confocal excitation, which involves a small beam size on the
sample plane. Micrometers in the lens mount for alignment adjustment of the wide
field lens.
The wide field lens is positioned to outside the right port of the microscope to
focus the laser to the back aperture of the objective. In particular, the wide field lens
is positioned so that the distance of the wide field lens to the back aperture matches
the focal length of the lens. Because the distances from the edge of the microscope to
dichroic and dichroic to back aperture are respectively 5 cm and 7 cm–8 cm, the focal
length needs to be at least 12 cm–13 cm. The wide field lens focal length is chosen to
be 200 mm and the lens situated 7 cm–8 cm from the microscope right side port.
The beam expander, which consist of two lenses, increases the beam size out of the
laser so that the incoming beam of the wide field lens is large. A larger magnification
yields a larger beam size, but if the beam size is too large, too much light is lost
during confocal excitation, where the wide field lens is removed. The size of the back
98
aperture of the objective used is 5.5 mm. Therefore the target size for the expanded
beam is 9 mm–12 mm.
Calculations for wide field excitation optical components
Calculations for the lens sizes and distances assume the laser is a collimated Gaussian
beam. The laser beam size is 3.82 mm.
When a collimated Gaussian beam is focused by a thin lens, the size or diameter
of the focused spot is given as [86]:
0
2W 0 =
4 f
λ
π 2W0
(6.1)
where 2W0 is the diameter of the incoming beam, λ is the wavelength, and f is the
focal length of the lens. For this laser system λ is 532 nm and 2W0 is 3.82 mm.
The desired magnification can be accomplished with a set of 40 mm and 100 mm
lenses or a set of 40 mm and 125 mm lenses. The lenses of the beam expander should
be spaced the sum of their focal lengths f1 + f2 apart, so the lenses are either spaced
140 mm or 165 mm apart. The first lens the beam enters is the 40 mm lens, after which
the beam diameter is 7.09 µm. This is the input beam size for the 100 mm or 125 mm
lens, where the output beam size is, respectively, 9.55 mm or 11.94 mm.
A 200 mm wide field lens focuses these down to 14.19 µm or 11.35 µm spot on the
back aperture.
6.3
Collection
A camera is added to the microscope for wide field imaging. Because the microscope
setup does not have a top port for mounting cameras, a replacement head piece with
a eyepieces and a top port was purchased from Zeiss (see Figure 6-3). A CCD camera
was mounted on the top port by means of a camera mount adapter.
99
Top port
for camera
Figure 6-3: Replacement head piece includes a top port for the camera. Figure created
with images from the Zeiss manual [89].
6.4
Summary
The design to expand the capabilities of the microscope setup includes the following
optical components: a custom machined reflector turret with 532 nm dichroic beam
splitter; a flip mount for switching between a CW laser and a fiber coupled laser;
a fiber port; a periscope assembly; a 532 nm CW laser with a laser line filter and
ND filters, beam expander consisting of two lenses, and wide field lens for wide field
excitation; a camera and a camera mount adapter; and 550 nm and 650 nm long pass
filters for filtering emission from the sample.
These optical components are to add wide field and confocal excitation and
collection, as well as the possibilities of coupling in pulsed laser excitation via a fiber
port.
100
Chapter 7
Conclusions
The work in this thesis demonstrates the dependence of NV center fluorescence on
externally applied electrochemical potential. The fluorescence change is attributed
to the shift in occupation probabilities of the NV− and NV0 charge sates and can be
also observed as a spectral change. For most of the modulating spots, the fluorescence
increases as the applied electrochemical potential decreases. The mechanism for a
voltage-induced fluorescence is thought be due to either an electric field induced
diamond surface band bending or charge transfer from theITO that is mediated by
trap states on the diamond surface. Fits to the fluorescence time traces based on
a model for NV− and NV0 charge state occupation probabilities are imperfect but
suggest that the NVs that show modulation are located close to the diamond surface
and that their Fermi levels are energetically close to the charge state transition level.
Various devices are fabricated in order to investigate the mechanism for NV
charge state and fluorescence in other systems. The devices fabricated to investigate
charge injection include variations on the electrochemical cell where the diamondITO and/or diamond-electrolyte interfaces are isolated, and an electrochemical cell
with an indium zinc oxide (IZO) working electrode. The devices fabricated to study
the electric field effect include parallel plate capacitors and interdigitated electrode
arrays. Despite the variety of spacers fabricated for the parallel plate capacitor spacers
and electrochemical cell interface spacers, many of the spacers are not electrically
isolating, are ion permeable, or show high autofluorescence. Nanodiamonds in the IZO
101
electrochemical cell are observed to show fluorescence modulation but the percentage
of modulating spots and modulation contest are both lower compared to those of the
electrochemical cell. The predominant modulation behavior is that of fluorescence
increase corresponding with voltage increase, which is the opposite to that of NVs
in the ITO electrochemical cell. The change in modulation behavior is attributed to
IZO, which has a different workfunction from ITO. In the case of the interdigitated
electrode arrays, the predominant modulation behavior is consistent with that for
NVs in the ITO electrochemical cell. Most of the modulating nanodiamonds on the
interdigitated electrode arrays that show modulation are on the edges of or close to
the electrodes. Some NV spectra show a change in relative NV− to NV0 character.
The result that most of the modulating spots are close to the electrode edges suggest
that charge injection is the dominant mechanism for NV fluorescence modulation.
However, the results for the interdigitated electrode arrays have not been reproducible.
The studies of NV charge state and fluorescence modulation in the different devices
are inconclusive.
Although the mechanistic studies are inconclusive, the charge state and fluorescence
modulation of NV due to applied electrochemical potential indicate that the NV charge
state can indeed serve as a sensor of its environment. In particular, its sensitivity
down to 100 mV makes it promising for sensing membrane action potentials in neurons.
The next steps towards a better NV charge state based sensor include optimizing its
sensitivity through surface functionalization and conjugation. For example, conjugating
gold nanoparticles to nanodiamond may lead to plasmonic enhancement of NV emission,
as shown in a study where gold nanoparticles are positioned next to a bulk diamond
[90]. This may decrease the required excitation power, which is important for imaging
in cells, because high intensity laser illumination can be phototoxic to cells [91]. The
eventual goal is to image the membrane action potentials of neurons in real time with
NVs in nanodiamonds.
In conclusion, the dependence of NV charge state and fluorescence on applied
electrochemical potential make the NV charge state a promising sensor for local
charges and potentials. Better engineering of nanodiamonds as well as optimizing
102
surface functionalization and conjugation will lead to a new class of NV based sensors
in nanodiamonds that need not rely on NV− spin.
103
104
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