FACULTY OF ENGINEERING
LAB SHEET
ENT4066 NANOELECTRONIC
MATERIALS AND DEVICES
TRIMESTER 1 2012-2013
NM1: STUDY OF PHOTON ABSORPTION
BEHAVIOUR OF SEMICONDUCTOR
NANOCRYSTAL QUANTUM DOTS
*Note: On-the-spot evaluation may be carried out during or at the end of the experiment.
Students are advised to read through this lab sheet before doing the experiment. Your
performance, teamwork effort, and learning attitude will count towards the marks.
STUDY OF PHOTON ABSORPTION BEHAVIOUR OF SEMICONDUCTOR
NANOCRYSTAL QUANTUM DOTS
Contents:
1. Introduction to Photon Absorption Due to Interband Transitions
2. The Beer-Lambert Law
3. Density of States
4. Quantum Confinement in Low Dimensional Structures
5. Non-Radiative Auger Recombination Process
6. Quantum Dot Fabrication by Chemical Synthesis
7. Procedure for the Study
8. References
1 Introduction to Photon Absorption Due to Interband Transitions
The room-temperature energy bandgap for CdSe semiconductor is 1.70 eV. To
raise an electron from the valence band (VB) to the conduction band (CB) requires a
photon energy greater than or equal to 1.70 eV. Since the optical absorption process
involves elevating an electron to a new state, such transitions are called interband
transitions, or electronic transitions. In principle, photons with longer wavelengths, such
as visible light, cannot excite electrons across the bandgap, so they will not be absorbed
by the material. In practice, the sharp absorption edge predicted by the simple band
model is not observed. The transition from absorbing to transmitting usually follows a
soft (exponential) curve as the wavelength changes. The bandgap of most semiconductor
materials is not well defined in energy, and there is not a sharp edge energy that defines
the conduction or valence bands. In addition, the thermal vibrations, the variety of
molecular bonds or configurations in the material lead to alterations in individual electron
energy distribution, and thus the band structure of the material. As a result, the onset of
optical absorption in semiconductor is not a sharp function of wavelength, but rather is a
smooth function of wavelength.
2 The Beer-Lambert Law
Figure 1 Absorption of photons within a small elemental volume of thickness x .
Suppose that Io is the intensity of a beam of photons incident on a semiconductor
material. Thus, Io is the energy incident per unit area per unit time. If  ph is the photon
flux, then
I 0  hph .
(1)
Suppose that I(x) is the light intensity at x and I is the change in the light intensity in the
small elemental volume of thickness x at x due to photon absorption. Then, I will
depend on the number of photons arriving at this volume I(x) and the thickness x . Thus
(2)
I  Ix
where  is called the absorption coefficient of the semiconductor and it is therefore
defined by
I
 
(3)
I x
When we integrate Eqn. (3) for illumination with constant wavelength light, we obtain
the Beer-Lambert Law, the transmitted intensity decreases exponentially with the
thickness of the semiconductor,
(4)
I ( x)  I 0 exp( x)
The absorption coefficient depends on the photon absorption processes occurring
in the semiconductor. In the case of intraband absorption, the absorption coefficient
increases rapidly with the photon energy above the bandgap. The general trend of the
absorption coefficient vs the photon energy behaviour can be intuitively understood from
the density of states diagram (see below and Figure 5).
3 Density of States
Density of states, g(E) represents the number of states per unit energy per unit
volume. The photon absorption process increases when there are more VB states
available as more electrons can be excited. We also need available CB states into which
the electrons can be excited, otherwise the electrons cannot find empty states to fill. The
probability of photon absorption depends on both the density of VB states and the density
of CB states.
Figure 2 The absorption coefficient depends on the photon energy and hence
the wavelength. Density of states increases from band edges and usually
exhibits peaks and troughs.
4 Quantum Confinement in Low Dimensional Structures
The use of semiconductor quantum well (QW) structures as lasing and opticalgain media resulted in important advances in semiconductor laser and LED technology
[1]. It is well known that the quantum confinement in one dimension restricts chargecarrier motion in QWs to the remaining two dimensions. Consequently, QWs have a two
dimensional step-like density of electronic states that is non-zero at the band edge,
enabling a higher concentration of charge carriers to contribute to the band-edge emission
and leading to a reduced lasing threshold, improved temperature stability, and a narrower
emission line.
A further enhancement in the density of states at the band edge and an associated
reduction in the lasing threshold is, in principle, possible with quantum wires (QWs) and
quantum dots (QDs), where the quantum confinement is in two- or three- dimensions,
respectively. Figure 4 shows the general realization of those structures, viz., two-, one-,
or even zero- dimensional depending on whether the potential barriers confine the charge
carrier in one- (2-DEG), two- (QWs) or three- (QDs) dimension. These structures are
known as low dimensional structures, whose dimensions are comparable with the
interatomic distances in solids. The movement of charge carriers in these structures is
constrained by potential barriers.
Figure 3 The lowest energy level in a QW lies above the CB edge.
The electronic-configuration spectrum of QDs consists of well-separated atomiclike states with an energy spacing that increases as the quantum dot size is reduced. In
very small QDs, the spacing of the electronic states is much greater than the available
thermal energy (strong quantum confinement), inhibiting thermal de-population of the
lowest electronic states. Additionally, QDs in the strong quantum confinement regime
have an emission wavelength that is a pronounced function of size, adding the advantage
of continuous spectral tunability over a wide energy range simply by changing the size of
the quantum dots. The design prospect of QD laser output-color can be controlled by
manipulation of QD size and semiconductor composition has been a driving force in
nanocrystal QD research for more than a decade.
E2
E1
Figure 4 In low dimensional structure realizations, the quantum confinement
changes the density of electron states, or specific energy levels, that will be filled
by incoming electrons.
5 Non-Radiative Auger Recombination Process
Excitation of a gas-phase or near-surface atom can eject a low-lying electron and
leave a “hole” state within the atomic levels. A higher lying electron can recombine with
this hole state, and the energy released in electron-hole (e-h) recombination can be
emitted as a photon (radiative decay) or as an electron in the non-radiative Auger
recombination process. The Auger process also occurs in bulk semiconductors, in which
the emitted (re-excited) particle can be either an electron or a hole.
There are substantial differences between the electronic and optical properties of
nanocrystal QDs and those of epitaxially-grown quantum dots, mostly due to the smaller
size of nanocrystal QDs. In particular, strong quantum confinement in nanocrystal QDs
results in a large splitting of band-edge states and in an enhancement of intrinsic
nonradiative Auger recombinations. In a “quantized” regime, Auger recombination is
characterized by a set of discrete (Auger recombination) constants, characteristic of the
decay of the 2-, 3-, …electron-hole pair QD states.
Figure 5 The absorption coefficient  depends on the photon energy and hence
the wavelength. Density of states g(E) increases from band edges and usually
exhibits peaks and troughs. Generally  increases with the photon energy greater
than Eg because more energetic photons can excite electrons from populated
regions of the VB to numerous available states deep in the CB.
Competition between radiative and non-radiative processes crucially affects
optical gain. In nanocrystal QDs, non-radiative charge-carrier losses are dominated by
surface trapping and multi-particle Auger relaxation. One can model the band-edge
emissions in QDs using a simple, two-level system with twofold spin-degenerate states.
Figure 6 Modeling of band-edge emissions in QDs. Schematics of transitions
with “absorption” and “emission” in CdSe QDs along with intraband relaxation
processes leading to a population buildup of the emitting transition.
In this model, we find that the optical gain, i.e., population inversion begins at a carrier
density of Neh = 1 (Neh is the number of e-h pairs per quantum dot on average.), with gain
saturation (i.e., complete population inversion) at Neh = 2. This implies that the QD bandedge gain is primarily due to two e-h pair states.
In CdSe nanocrystal QDs, non-radiative Auger relaxation of doubly excited
nanoparticles, t2 is strongly size-dependent, i.e., approximately proportional to R3, where
R is QD mean radius. The average QD population buildups are usually monitored with
the high-sensitivity femtosecond (fs) transient absorption (TA) dynamics as well as with
the continuous wave (cw) time-resolved photoluminescence (PL) spectra at room and
liquid nitrogen temperatures, respectively.
6 Quantum Dot Fabrication by Chemical Synthesis
An alternative approach to fabricating QDs that are small enough to show strong
quantum confinement is through chemical synthesis. Chemical methods can provide
routine preparations of semiconductor nanoparticles (nanocrystal QDs) with radii 1 nm to
6 nm and with size dispersions as small as 5%. In this size range, electronic interlevel
spacings can exceed hundreds of meV, and size-controlled spectral tunability over an
energy range as wide as 1 eV can be achieved. Additionally, nanocrystal QDs can be
chemically manipulated and incorporated into polymer, glass matrices, microcavitities
and photonic crystals. Nanocrystal QDs can also be assembled into close-packed ordered
and disordered arrays (QD solids).
Direct chemical synthesis method can produce colloidal QDs with narrow size
distributions (dispersions) and improved surface passivation. In case of Evident
Technologies EviDots™ a core QD of CdSe is overcoated with a shell (or capping) of
ZnS (see Figure 7). In our present studies, we use the above, CdSe/CdS core shell QDs
dispersed in toluene. One advantage of using the core shell QDs is the substantial
suppression of surface trapping and resultant high quantum-efficiency PL spectra.
Figure 7 CdSe/ZnS Core-Shell EviDots™ (Evident Technologies).
References
1. V. I Klimov. et al., Optical Gain and Stimulated Emission in Nanocrystal Quantum
Dots, Science, 290 (2000) 314-317.
2. M. Henini, The physics and technology of low dimensional structures,
Microelectronics Journal, 25, 5 (1994) L29.
3. Images from www.evidenttech.com .
4. S. O. Kasap, Principles of Electronic Materials and Devices, McGraw-Hill, 2004.
5. C. R. Pollock, Fundamentals of Optoelectronics, Irwin, 1995.
6. C. Harenza, Quantum Dots ; Advanced Materials Catalog, Evident Technologies Inc.
Figure 8 Sample: PbSe Core EviDots™ (Evident Technologies).
Figure 9 Sample: PbSe Core EviDots™ (Evident Technologies) Absorption
and Emission spectra.
STUDY OF PHOTON ABSORPTION BEHAVIOUR OF SEMICONDUCTOR
NANOCRYSTAL QUANTUM DOTS
INTRODUCTION
The Ocean Optics USB4000 Spectrometer is a compact, asymmetrically-crossed
Czerny-Turner mounted spectrometer system, scanning the wavelengths from the visible
to the near infrared (370nm - 985nm). The SpectraSuite is the operating software
controlling the Ocean Optics USB spectrometer instrumentation and devices. The
SpectraSuite framework is a completely modular, Java-based spectroscopy software
platform that operates on Windows, Linus or Macintosh operating systems and every
function in it can be altered or replaced. For instance, the data acquisition functions, the
scheduling functions, the data processing function and rendering functions are all
separate modules. You can add or delete modules to create a propriety user interface or
functionality; create modules to perform calculations; automate experiment routines and
more. You or an Ocean Optics application developer can easily customize SpectraSuite
through Java code. The SpectraSuite is a platform-independent application software that
provides graphical and numeric representation of spectra in one window.
The SpectraSuite software provides the user with advanced control of episodic
data capture attributes. For instance, a user can acquire data for a fixed number of scans
or for a specific interval. Initiation of each scan can be externally triggered or eventdriven. Captured data is quickly stored into a system memory at speeds as fast as 1 scan
per msec with speeds limited by hardware performance.
The SpectraSuite software allows you to perform the three basic spectroscopic
experiments: (1) absorbance, (2) reflectance, and (3) emission, as well as signalprocessing functions such as
(a) electrical dark-signal correction,
(b) stray light correction,
(c) boxcar pixel smoothing and signal averaging.
Scope mode, the spectrometer operating mode in which raw data (signal) is acquired. The
basic concept for the software is that real-time display of data allows users to evaluate the
effectiveness of their experimental setups and data processing selections, make changes
to these parameters, instantly see the effects and save the data. Most spectrometer-system
operating software does not allow such signal-conditioning flexibility.
OBJECTIVES
At the end of this experiment, students will learn the operations required for conducting
experiment on measurement of photon absorption by semiconductor nanocrystal quantum
dots, by using Ocean Optics USB4000 spectrometer and SpectraSuite spectrometersystem operating software. The objectives are:
 To perform data acquisition for photon absorption experiment
 To establish the flexible and signal-conditioning parameters
 To compute reference and dark spectra monitoring
 To control the setting system-function parameters
 To analyze the absorption spectra obtained from quantum dots
NM- 1: STUDY OF PHOTON ABSORPTION BEHAVIOUR OF
SEMICONDUCTOR NANOCRYSTAL QUANTUM DOTS
PROCEDURE:
1. Installing the spectrometer
a. Switch on the PC and the monitor.
b. Connect the spectrometer and PC by using an USB cable provided.
c. Click on the SpectraSuite icon to open it.
2. Running Ocean Optics SpectraSuite
a. The spectrum graph window appears under the standard toolbars.
b. If you have followed the correct installation of the spectrometer, the
spectrometer is already acquiring data in Scope (S) mode. Even with no
light in the spectrometer, SpectraSuite should display a dynamic trace in
the bottom of the graph window.
3. Data Sources and Data Views panes
a. The spectrometer model that you have installed is listed in upper left pane.
b. The acquisition parameters that you set via the (integration time, scans-toaverage, boxcar smoothing).
c. The properties (serial no., firmware level, total pixels, wavelengths)
d. The optical bench (grating, filter wavelength, slit size)
e. The detector (array coating mfg., array wavelength, etc.)
f. Whether reference and/or dark spectra have been stored, the graph (A, B,
C, etc.) associated with the spectrometer appears in lower left pane.
4. Measurement (A, T, R, I)
a. Choose the type of measurement A (for Absorbance), T (for
Transmission), R (for Reflection), I (for Irradiance), etc.
The type of measurement you will take determines the configuration of the
sampling optics for your system. Furthermore, your choice of reference
and data analysis determines how SpectraSuite presents the results.
-----------------------------------------------------------------------------------------------------------Note: For each measurement, you must first take a reference and dark
spectrum before the experiment mode icon (A, T, R, I) on the toolbar becomes
active. After you take a reference and a dark spectrum, you can take as many
measurement scans as needed. However, if you change any sampling variable
(integration time, scans-to-average, boxcar smoothing, fiber size, etc.), you
must store a new reference and dark spectrum.
------------------------------------------------------------------------------------------------------------
Figure 10 Absorbance measurement set up.
b. If the signal you collect is saturating the spectrometer (intensity greater
than 4000 counts), you can decrease the light level on scale in Scope (S)
mode by decreasing the integration time (e.g., 100ms-->60ms), using a
smaller diameter fiber, or using a correct, neutral density filter (NDF).
c. If the signal you collect has too little light, you can increase the light level
on scale in Scope (S) mode by increasing the integration time, using a
larger diameter fiber, or removing any optical filters.
5. Storing Reference and Dark Spectra
Reference and dark spectra must be stored before collecting experimental
data. If you pass the cursor over No-preprocessor line in either Data
Sources or Data Views pane, a dialog box informs you whether reference
and/or dark spectra have been stored.
1. Reference spectra
a. Go to File  NewAbsorbance measurement.
b. Under Select spectral source wizard, select one spectral source
from the table.
c. Next.
d. Check on Strobe/Lamp Enable.
e. Change the integration time appropriately until the Last peak value
has come closer to the Recommended peak value.
You may select “Set Automatically” if Step (1.e) is not achieved.
f. Next.
g. Click on the Yellow lamp to store the Reference spectrum. You
may load the Reference spectrum from a file.
h. Next.
2. Dark spectra
i. Uncheck on Strobe/Lamp Enable.
j. Change the integration time to the value of Step (1.e). Press Next.
k. Click on the Dark lamp to store the Dark spectrum. You may load
the Dark spectrum from a file.
l. Choose Finish to get the Reference and Dark spectrum on Graph
window.
6. Absorbance Measurement
Absorbance spectra are a measure of how much light a sample absorbs. For
most samples absorbance relates linearly to the concentration of the substance.
SpectraSuite calculates Absorbance (Aλ) using the following equation:
 S  D 

(5)
A   log 10  
 R  D 
where Sλ is the sample intensity at wavelength λ,
Dλ is the dark intensity at wavelength λ, and
Rλ is the reference intensity at wavelength λ.
6.1 Absorbance measurement setup
a. The light source sends light via an input fiber into a cuvette in a cuvette
holder. The light interacts with the sample. The output fiber carries light
from the sample to the spectrometer connected to the computer.
6.2 Procedure
b. Place SpectraSuite in Scope mode by clicking the “S” icon in the
Experiment mode toolbar or selecting ProcessingProcessing
ModeScope from the menu. Ensure that the entire signal is on scale.
The intensity of the reference signal’s peak differs depending on the
device being used. If necessary, adjust the integration time until the
intensity is appropriate for your device.
c. Select FileNewAbsorbance Measurement from the menu or click ‘A’
to start the Absorbance measurement Wizard.
d. Select the source of your absorbance measurement and click ‘Next’. The
second page of the wizard appears.
e. Turn on your light source.
f. Set your acquisition parameters so that the peak value reached the
recommended level.
g. Click ‘Next’. The third page of the wizard appears.
h. If you have not already done so, place a sample of solvent into a cuvette to
take a reference spectrum. You must take a reference spectrum before
measuring absorbance.
-------------------------------------------------------------------------------------------------------Note: Do not put the sample itself in the path when taking a reference
spectrum, only the solvent.
-------------------------------------------------------------------------------------------------------i. Click the Store Reference Spectrum icon on the screen. This command
merely stores a reference spectrum in memory.
Click the Save Spectra icon on the toolbar or select FileSaveSave
Spectra Collection from the menu bar to permanently save the reference
spectrum to disk.
j. Click ‘Next’. The fourth page of the wizard appears.
k. Block the light path to the spectrometer, uncheck the Strobe/Lamp Enable
box, or turn the light source off. Take a dark spectrum before measuring
absorbance.
l. This command merely stores a dark spectrum in memory.
Click the Save Spectra icon on the toolbar or select FileSaveSave
Spectra Collection from the menu bar to permanently save the dark
spectrum to disk.
--------------------------------------------------------------------------------------------------------Note: If possible, do not turn off the light source when taking a dark
spectrum. If you must turn off your light source to store a dark spectrum,
allow enough time for the lamp to warm up again before continuing your
experiment. After the lamp warms up again, store a new reference.
---------------------------------------------------------------------------------------------------------m. Put the sample in place and ensure that the light path is clear. Then, click
‘Finish’.
If you have already taken one or more absorbance measurements, a dialog
box appears asking you to specify whether to display the new data in a
new graph, or on the existing graph.
Note the following changes on the screen:
1. The experiment mode listed in the Data Sources and Data Views panes
changes to Absorbance Mode.
2. The units listed on the Graph pane changes to Absorbance (OD).
n. To permanently save the spectrum to disk, click the Save Spectra icon on
the toolbar.
--------------------------------------------------------------------------------------------------------Note: If you change any sampling variable (integration time, averaging,
smoothing, fiber size, etc., you must store a new reference and dark spectrum.
---------------------------------------------------------------------------------------------------------
REPORT WRITING
The lab report should include the following information:
(i)
(ii)
(iii)
(iv)
The processed spectra of all samples provided.
Analysis of the Results
Discussions
Conclusion
Marking Scheme
Lab
(10%)
Assessment Components
Hands-On & Efforts (2%)
On the Spot Evaluation
(2%)
Lab Report (6%)
Details
The hands-on capability of the students and their efforts during the
lab sessions will be assessed.
The students will be evaluated on the spot based on the lab
experiments and the observations on the quantum dots absorption
characteristics.
Each student will have to submit his/her lab final report within 7
days of performing the lab experiment. The report should cover
the followings:
1. Introduction, which includes background information on
absorption measurement and their relationship with
semiconductor nanomaterials.
2. Experimental section, which includes the general summary
of the lab experiment work.
3. Results and Discussions, which include the absorption
measurement results, analysis, and evaluations, with neat
graphs/images of the results and recorded data.
4. Conclusion, which includes a conclusion on the
experimental.
5. List of References, which includes all the technical
references cited throughout the entire lab report.
The report must have references taken from online scientific
journals (e.g. www.sciencedirect.com,
http://ieeexplore.ieee.org/xpl/periodicals.jsp,
http://www.aip.org/pubs/) and/or conference proceedings (e.g.
http://ieeexplore.ieee.org/xpl/conferences.jsp).
Format of references: The references to scientific journals and text
books should follow following standard format:
Examples:
[1] William K, Bunte E, Stiebig H, Knipp D, Influence of
low temperature thermal annealing on the performance of
microcrystalline silicon thin-film transistors, Journal of
Applied Physics, 2007, 101, p. 074503.
[2] Hodges DA, Jackson HG, Analysis and design of digital
integrated circuits, New York, McGraw-Hill Book
Company, 1983, p. 76.
Reports must be typed and single-spaced, and adopt a 12-point
Times New Roman font for normal texts in the report.
Any student found plagiarizing their reports will have the
assessment marks for this component (6%) forfeited.
The lab report has to be submitted to the NanoLab1 staff. Please
make sure you sign the student list for your submission. No
plagiarism is allowed. Though the absorption spectrum of the
measured sample from the same group can be similar, the report
write-up cannot be duplicated for group members. The individual
report has to be submitted within 7 days from the date of your lab
session. Late submission is strictly not allowed.