STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
METALLIC NANOSTRUCTURES FOR CONTROLLING
LIGHT – MOLECULES INTERACTIONS
Simion Astilean
Molecular Spectroscopy Department,
Faculty of Physics, Babes-Bolyai University,
3400 Cluj-Napoca, Romania
Abstract. We investigate the optical properties of ordered silver
nanostructures with the goal of optimizing and controlling the specific
spectroscopic signatures of molecules, i.e. the fluorescence decay and
surface-enhanced Raman scattering.
Introduction
The density of states of electromagnetic excitations D() at optical
frequencies, both photons and surface plasmons, is a basic concept underlying the
theory of light-matter interaction. For instance the probability of spontaneous
emission of an emitter is given by Fermi’s [1] golden rule as:
ij
2
M ij D(ij )
(1)
where ij is the rate for the transition between the excited state i and lower energy
state j, Mij is a matrix element that connects the excited and lower energy levels
and is determined by the wavefunctions associated with those levels and D(ij) is
the density of the optical field at the transition frequency. A similar relation works
for the probability of spontaneous Raman scattering of light [2]. According to
relation (1), the fluorescence emission rate or the Raman scattering probability of
molecules are not an intrinsic property of the molecule but a possible exchange
between molecule and all available light states. This idea was first pointed out at
radio frequencies by Purcell [3] in 1946.
To control the density of optical modes D() in order to manipulate the
optical properties of fundamental light emitters (atoms, molecules, quantum dots)
has been the great motivating force of intensive investigations during the last
decade in the field of photonic crystals (PCs). Photonic crystals are dielectric or
metallic materials which possess a periodic modulation of their refractive index on
the scale of the wavelength of light. Contrary to homogeneous materials, PCs
exhibit a discrete density of states D() , that is, a range of frequencies () and
wavevectors (k) wherein the propagation of light can be allowed or blocked [4].
In this paper we employ an inexpensive nanofabrication method, i.e. the
nanosphere lithography [5], to produce two-dimensional PCs consisting of arrays
of silver nanoparticles or ordered nanoscopic holes in metallic films. We
SIMION ASTILEAN
investigate their photonic structure and hereafter their potential to control light –
molecules interaction. Specifically, we demonstrate that the spectroscopic
signature, i.e. the fluorescence decay or surface-enhanced Raman scattering of
molecules are strongly dependent on the “photonic environment”.
Metallic PCs with specific “plasmonic” band structures D() are
consistent with the manipulate of both the spontaneous emission rate and the
Raman scattering efficiency. The control of spectroscopic signal on the nanoscale
is useful in ultrasensitive analysis and chemo- or biosensing applications.
Experimental
a) Nanostrctures preparation. The samples were prepared according to the
nanosphere lithography procedure [5]. A suspension of polystyrene nanospheres
was dropcoated onto the substrate where they self-assembled into hexagonally
close-packed 2D colloidal crystal that served as a deposition mask. Polystyrene
nanospheres were supplied by Interfacial Dynamics Corporation and by Duke
Scientific Ltd as monodispersed suspensions in deionised water. Once the 2D
colloidal crystal deposition mask was formed, the substrate were mounted into the
vacuum chamber of a vapour deposition system. Silver or gold films of controlled
thickness (usually between 20-120 nm) were thermally evaporated onto the
substrate under a pressure of 5×10-6 Torr. The film thickness was monitored using
a calibrated quartz crystal oscillator. The silver was evaporated from a
molybdenum boat, having been thoroughly out-gassed under high vacuum. A
voltage of ~90V was required to melt and then vaporise the metal, and once the
metal had been deposited, the sample was left under vacuum for half an hour in
order to allow the film to cool and stabilize, so that there was the minimum of
oxidation and sulphidisation on the metal surface. The evaporated silver or gold
coats the regions of the substrate not covered with polymer spheres. A solvent
(dichloromethane or chloroform) wash is then used to remove the polystyrene
nanospheres from the substrate. The solvent wash removes the nanospheres but
does not affect the silver or gold that has been deposited onto the substrate,
resulting in a glass substrate metallized with repeatable spaced metallic nanotriangles. The period of nanostructures is determined by the initial period of
hexagonally close-packed nanosphere arrays and the size of metallic features can
be easily tuned by the diameter of spheres.The versatility of nanosphere
lithography was extended by using reactive ion etching (RIE) of polystyrene
nanospheres. The sample consisting of crystalline assemblies of polystyrene
nanospheres was placed in a RIE chamber and exposed to oxygen (O2) plasma. The
nanospheres are reduced in size by RIE after which silver film was thermally
evaporated. The thinned spheres were removed by sonication in dichloromethane
leaving the silver film patterned with periodic array of nanoholes.
The spectroscopic experiments were performed on two representative dyes:
rhodamine 700 (R700) and rhodamine 6G. All materials involved in sample and
substrate preparation were purchased from commercial sources as analytical pure
METALLIC NANOSTRUCTURES FOR CONTROLLING LIGHT – MOLECULES INTERACTIONS
reagents. For recording the fluorescence and SERS spectra rhodamine 10-6 M
methanol solution was used.
b) Experimental measurements. Scanning electron microscopy investigations
were performed with a Hitachi electronic microscope.
Fluorescence emission decays were measured with a time correlated single
photon counting (TCSPC) system. A pulsed laser diode emitting at 635 nm served
as the excitation source. It provides a pulse of < 200 ps (FWHM) duration at a
repetition rate of 1 MHz. The output optics of the laser diode allows collimating or
focusing the beam to a spot size of approx. 200 m in diameter. Measurements
were performed with an average power of less than 1mW at the sample. In order to
acquire time-resolved data, the signal was recorded with a time-correlated single
photon counting module and fluorescence decay signals were collected in typical
fluorescence lifetime histograms with a timing resolution 6.1 ps/channel. The
experimental determination of fluorescence lifetime was performed by using a nonlinear fitting procedure to fit sampling photon arrival times to a single exponential
decay function.
The SERS measurements were performed on a Dilor Labram system for
excitation with 514 nm laser line and Renishaw's inVia Raman microscopes for
excitation with 785 diode laser line.
Results and discussion
a) Structural and optical characterization of fabricated metallic
nanostructures. Figure 1a and 1b show the scanning electron microscope (SEM)
pictures of fabricated periodic metallic nanostructures. The first picture represents
an array of triangular silver particles left on the substrate after polystyrene spheres
removing. One can see the regular arrangement of metal particles as well as their
shape reflecting the voids between polystyrene spheres (about of 65 nm height and
120 nm length). The second picture shows highly regular hole-arrays in the
deposited film. The diameter of holes (250 nm) can be easily tuned by the etching
time, as Haginoya et al. had reported partially [6].
We measured the dispersion of the surface plasmon modes by recording
optical transmission through the samples as a function of light frequency and inplane wave vector. The experimental set-up used to obtain dispersion data consists
of a white light source focused through two apertures to reduce beam divergence
and grating spectrometer to select light with frequency ranging from 1.15 m-1 to
2.3 m-1 (where ω(m-1) = ω(rad/s)/2πc). The in-plane wave vector, k// was varied
by adjusting the angle of incidence to take a value dependent on the frequency ω of
the incident radiation according to the equation k// = k0 sin θ, where k0 = ω/c for
which n is the refractive index of the dielectric
Experimental plasmonic bands for the two nanostructures are presented in
Fig 2b and Fig. 2b, respectively. As mentioned, the signature optical property of
noble metal nanoparticles is the localized surface plasmon resonances (LSPR). The
first experimental plasmonic band illustrates SPR modes localized on nanoparticles
SIMION ASTILEAN
whereas the second one that of SPR modes localized inside of hole-arrays [7]. The
primary consequence of SPR excitation is the enhancement of the local
electromagnetic field, that is why such metallic nanostructures play the role of
active substrates for surface-enhanced Raman spectroscopy (SERS) and controlling
molecular fluorescence.
Figure 1. SEM images: a) periodic array of silver nanoparticle b) periodic array of
nanoholes in silver film.
2.2
2.0
-1
 /2 p c (  m )
-1
 /2 p c (  m )
2.2
1.8
1.6
1.4
1.2
0.0 0.2 0.4 0.6 0.8 1.0
-1
k x /2 p ( m )
2.0
1.8
1.6
1.4
1.2
0.0 0.2 0.4 0.6 0.8 1.0
-1
k x /2 p ( m )
Figure 2. Experimental plots of surface plasmons bands: a) nanoparticle array and b) hole-array
(white color means high extinction coefficient)
METALLIC NANOSTRUCTURES FOR CONTROLLING LIGHT – MOLECULES INTERACTIONS
Counts
Fluorescence Intensity [au]
b) Controlling the fluorescence emission. Figure 3 shows representative examples
of fluorescence data, steady-state fluorescence spectra (Fig.3a) and time-resolved
profiles (Fig. 3b) of rhodamine 700 (R700). We measured the fluorescence of dye
positioned directly on the silica substrate in two distinct cases. In the first case
(data marked with (1)) the molecules were located on silica substrate in the
presence of nearby
regular arrays of silver
nanoparticles whereas
in the second case (data
(2)) without any metal
particles. Both tyes of
measurements
show
1
important
effects
2
induced by the presence
of
metallic
nanostructures . As we
have
studied
the
emission in a virtually
600
650
700
750
800
identical
chemical
Wavelength [nm]
environment on silica
10
substrate, the relative
1xe
differences in data
9
1xe
result exclusively from
8
different D(). The
1xe
alteration
in
the
7
1xe
fluorescence
lifetime
is
2
6
important
and
prove
1xe
clearly the ability of
5
1xe
such
metallic
4
nanostructure
to
1xe
manipulate
the
1
3
1xe
fluorescence signal of
2
dye molecules. The
1xe
theoretical model to
1
e
study the emission rate
0
of dye induced by a
e
nearby flat silver mirror
-1
e
is described in literature
0
10
20
30
[8]. The lifetime and
Time [ns]
quantum efficiency of
R700 was previously
Fig 3. Steady-state fluorescence spectra and time-resolved
determined and found
fluorescence profiles of rhodamine 700 molecules located on
to be 2.8 ns and 0.65.
flat silica substrate (1) with regular arrays of silver
nanoparticles and (2) without metal.
respectively.
(1) without
SIMION ASTILEAN
Raman Intensity [au]
c) Controlling the surface-enhance Raman scattering (SERS). We investigate the
SERS efficiency of the structure presented in Fig 1b using rhodamine 6G
molecules as model compound. The SERS spectrum of Rh6G recorded with the
3
2
1
200
400
600
800
1000
1200
1400
1600
1800
-1
Raman shift [cm ]
Fig. 4. (1) Ordinary FT-Raman spectrum of Rh6G polycrystalline sample (excitation
line: 1064 nm); (2) SERRS spectrum (excitation line: 514 nm) of adsorbed Rh6G
molecules; (3) SERS spectrum (excitation line: 785 nm) of adsorbed Rh6G.
785 nm line, presented together with the SERRS recorded with the 514 nm laser
line and FT-Raman spectra in Fig. 4, demonstrates the great capability of this
substrate to provide SERS enhancements, when various laser lines are employed
for excitation. The surface plasmons dispersion band (Fig 2b) provides only
electromagnetic enhancement when using for excitation the laser line of 785 nm.
Rh6G exhibits an electronic absorption maximum at 526 nm and therefore both
resonance and surface enhancements are expected to contribute to the observed
SERS spectra recorded with the 514 nm laser line. The differences between the
SERRS and SERS spectra can be explained by considering the resonant
contribution to the overall SERS enhancement. The electromagnetic field
responsible for SERS is likely localized inside the holes, where giant local fields
corresponding to Raman enhancement factors of the order of 1011 are conceivable.
This implies that the majority of the SERS signal measured from our sample is due
to the excitation of very small percentage of adsorbate situated inside the holes, the
individual enhancements being greater than the surface-averaged values [9].
METALLIC NANOSTRUCTURES FOR CONTROLLING LIGHT – MOLECULES INTERACTIONS
Conclusions
We have investigated the optical properties of ordered silver
nanostructures with the goal of controlling the specific spectroscopic signatures of
molecules, i.e. the fluorescence decay and surface-enhanced Raman scattering. The
fluorescence emission depends strongly of the coupling between molecules and
regular arrays of metallic nanoparticles via their plasmonic environment and the
enhancement of SERS signal can be correlated with the plasmonic band structure
D().
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old materials with new applications, Adv. Materials, 12 (10), 693, (2000).
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