Synthesis and Analysis of Quantum Dots
Karen S. Quaal1, Justin LaRocque1, Shazmeen Mamdani1, Luke Nally1, Jennifer Z. Gillies2 and Daniel Landry2
(1) Siena College, Loudonville, NY, (2) Evident Technologies
Abstract
Upper division students synthesized quantum dot samples using modified literature
procedures and analyzed some of the optical properties between CdSe and ZnSe, and CdSe
and shelled CdSe/ZnS Quantum Dots. The spectra (UV/VIS, Fluor.) obtained also constituted a
model quantum system that was accessible to experimental and theoretical study by
undergraduates. A method for analyzing the Cd/Zn ratio by Atomic Absorption (AA) Spectroscopy
was developed and used to calculate the number of ZnS shells on the CdSe Nanoparticles. An
alternative method for calculating the number of shells was also developed for use by students
without access to AA instrumentation.
Introduction
A semiconductor is a material that is neither a conductor nor an insulator of electric current.
Nanocrystals, also referred to as Quantum Dots (QD), are the newest wave of semiconductor
technology. The size of quantum dots ranges between 2-10 nm. The diameter of a QD is so small;
it is actually smaller than the excited electron-hole Bohr radius. This results in a phenomenon
known as quantum confinement. Quantum confinement leads to increased stress on the excited
electron-hole relationship (exciton), which results in increased energy of the emitted photon. The
smaller the dot, the less room there is for the exciton separation, and the more energy required to
form the exciton. The energy and wavelength of the emitted photon are directly related to the size
of the particle and the respective degree of confinement. When a QD is placed under ultraviolet light
fluorescence will occur which is a form of energy release. As the diameter of a CdSe QD increases,
the color changes from blue to red. Inorganic materials of larger band gap coat nanocrystals
(shelling) which improves confinement and increases the intensity of emitted fluorescence. A Zinc
Sulfide (ZnS) shell added to the Cadmium Selenide core passivates the surface. This prevents the
passive relaxation of the exciton and forces the exciton to relax via emission, increasing the intensity
of the nanocrystal’s fluorescence.
Preparation of Amine-Capped Zinc Selenium (ZnSe) Nanoparticles-2 A synthetic procedure
analogous to the one described above was used to synthesize ZnSe nanoparticles.
Cadmium Selenide/Zinc Sulfide Core/Shell-2 Using standard airless techniques, Trioctylphosphine
oxide (10g, TOPO) was degassed under vacuum for 30 minutes at 120°C. The TOPO solution was
cooled to 70°C and 100 nmoles** of previously synthesized CdSe nanoparticles in toluene was
added to the TOPO. While under Nitrogen, the TOPO and CdSe solution was heated to 150°C. In a
glovebox, a 1000 fold excess of dimethylzinc (1M in heptane) and bis(trimethylsilyl) sulfide (TMS) in
equal molar amounts were dissolved in a 4 fold excess of trioctylphosphine (TOP). At 150°, the
solution prepared in the glovebox was added slowly into the reaction vessel using a dropping funnel.
The temperature was raised to 170°C and allowed to stir for one hour. An aliquot was removed using
a syringe, quenched in toluene, and the UV spectrum obtained was compared to the CdSe core
spectrum. The solution was heated to 190°C, and allowed to stir for 30 minutes, at which point
another aliquot was removed and the UV spectrum obtained.
After the growth of the
nanocrystals/shell stabilized, as indicated by no additional change in the UV spectrum, the
nanoparticles were isolated by precipitation using methanol (see selective precipitation procedure for
amine capped CdSe nanoparticles).
Determination of CdSe/ZnS Nanocrystal Shell Thickness (AA method)
Crash and Suspend Process: Approximately 1 micromole** of a CdSe/ZnS in toluene nanocrystal
sample was added to a glass centrifuge tube. The centrifuge tube was filled 3/4 full with methanol.
The tube was shaken, and put in ice for twenty minutes. After icing, the tube was placed in a
centrifuge for twenty minutes. When removed from the centrifuge the liquid was clear and crystals
were in the bottom of the tube. If the liquid is not clear, repeat the icing and centrifuge procedure.
The liquid was decanted and a minimum of toluene was added in order to re-suspend crystals. A
sonicator was used to aid in re-suspension. Methanol was added, and procedure was repeated two
additional times.
** - Moles based on concentration determined by way of Beer’s Law, by UV-VIS spectrum and molar
absorptivity of CdSe available on website: www.evidenttech.com. The molecular weight of the dot
can be determined by dividing the diameter of the dot by the bond length of Cd-Se (.36nm). This
result (x) is then inserted in the following equation in order to find the number of CdSe units in the
dot: (4/3)(x/2)3. The result is multiplied by the molecular mass of CdSe (191.371 g/mole) to obtain
the molecular mass of the dot.
Constant Weight Process: After the final centrifugation, the supernatant was removed, filter paper
was secured over a centrifuge tube and a small hole was punched in the filter paper to allow airflow.
The centrifuge tube was then placed under vacuum at 70C for 1 hour. The tube was allowed to cool
in a desiccator then weighed. The filter paper was replaced and the tube was dried in a vacuum at
70C for an additional hour. This process was repeated until a constant weight was obtained.
Procedures
CdSe(amine capped)-3,2 Hexadecylamine (6 grams, 24.84 mmoles) was degassed under
vacuum for 30 minutes at 60°C. While under Nitrogen, the solution was cooled to below 40°C,
at which point the septum was removed, and 0.1g (0.028 mmol) of (Li)4[Cd10Se4(SPh)16] was
added to the top of the solidified Hexadecylamine. The septum was reattached, and while
under vacuum, the temperature was raised to 120°C. At 120°C the system was switched to
nitrogen atmosphere, and heating continued until 130°C. Aliquots (5-6 drops) were taken
starting at 130°C using a syringe, and quenched in 1 mL toluene. Subsequent aliquots were
taken at 10-15 degree intervals. When the solution reached 250°C, the remaining reaction
mixture was cooled to 60°, and transferred to glass centrifuge tubes. The CdSe was isolated
using selective precipitation by the addition of methanol to each of the centrifuge tubes. The
tubes were placed in an ice bath (15-20 minutes), centrifuged (10 minutes) and the methanol
was removed. A minimum amount of toluene was added to each tube to dissolve the CdSe.
The contents of each tube were transferred to a single vial.
Digestion Process: High purity concentrated nitric acid (2 mL) was added to the centrifuge tube
containing the dry crystals. This solution was allowed to sit overnight. High purity concentrated
hydrochloric acid (5-6 drops) was added to a centrifuge tube which was then placed in a hot water
bath and left until the solution was clear and contained no solids. The solution was then diluted for
AA determination of cadmium and zinc concentrations. After the solution was removed from the
centrifuge tube, the tube was dried and weighed in order to determine the constant mass of the dried
crystals.
Results
Quantum Confinement:4,6
Several quantum mechanical models were used to predict the size of the Q.D. The best agreement
With TEM values were found with the strong confinement model.
E1s1s = Eg + π2 (ab/adot)2 Ry* - 1.786 (ab/adot) Ry* - 0.248 Ry*
Where E1S1S = Energy calculated from UV/VIS spectrum
Eg= bang gap (CdSe= 1.84 eV)
adot= radius of the Q.D
ab= exciton Bohr radius (CdSe= 4.9 nm)
Ry* = Rydberg constant (CdSe= 0.016 eV)
Table 1: CdSe Spectral Data:
Temp (C)
128
138
148
158
168
178
188
198
208
218
Lambda Max (nm)
453
473
501
510
521
522
530
538
546
555
Energy (J)
4.38515E-19
4.19974E-19
3.96502E-19
3.89505E-19
3.81281E-19
3.80551E-19
3.74807E-19
3.69233E-19
3.63823E-19
3.57923E-19
radius (nm)
1.97
2.11
2.33
2.41
2.51
2.52
2.61
2.70
2.79
2.91
2. Using equation 4, the mg of ZnS/dot was determined utilizing both the mg of Cd/Se dot
calculated in step 1 and AA data for the left side of the equation.
3. VSHELL was determined by dividing the mg Zn/dot by the density of ZnS to obtain the
volume of the shell in nm3.
4. VCORE was determined using equation 1 and substituting the diameter of the core in
place of dTOTAL..
5. Equation 2 was then used to determine VTOTAL .
6. Using equation 1, dTotal was determined.
7. Equation 3 was then used to determine the d1 + d2 which was then divided by two in
order to find the thickness of the shell.
Determination of Number of ZnS Shells - UV method
CdSe UV-Vis and Fluorescence Wavelength Shifts:
Sizes of the CdSe nanoparticles were estimated using absorption spectroscopy.1 A
relationship between sample temperature and the wavelength was observed.
Absorbance spectra of CdSe nanoparticle and CdSe/ZnS nanoparticle were
compared. After a shell was added, a small red shift to lower energies was
observed. The ratio of a known sample (Rhodamine 6G) was used to determine the
quantum yield of two nanoparticles (CdSe and CdSe/ZnS). The quantum yield
increased by a factor of 2.3 from CdSe to CdSe/ZnS.
Determination of Thickness of Zn/S Shell on CdSe/ZnS Nanocrystals:
A. Constant Mass: The constant mass of the dry crystals was divided by the original
sample size used for the crash/suspend procedure to give the constant mass of
crystals per mL (g/mL).
1. Determine the concentration of CdSe (mg/mL) using UV spectroscopy**.
2. This concentration was divided by the constant mass of sample/mL and multiplied by
100. This yields the percent CdSe in a 1 mL sample. This number was subtracted
from 100 to determine the percent ZnS in a 1 mL sample.
3. Assume you have 100 mg in which the percentage equals the amount in mg.
4. Calculate the mg of Cd in CdSe by multiplying the mg CdSe in the sample by the
percentage of Cd in each unit of CdSe (58.7%)
5. Calculate the mg of Zn in ZnS by multiplying the mg ZnS in the sample by the
percentage of Zn in each unit of ZnS (67.1%).
6. These values were then substituted for the AA data in the AA method calculations
above.
Discussion
B. Mass of CdSe: Divide the mass of Cd from the AA sample by the molar mass of Cd
(112.411 g/mole), the result was then multiplied by the molar mass of Se (78.96
g/mol), this result was then added to the mass of Cd in the AA sample.
Increasing size and a corresponding red shift in the absorbance spectrum was
observed for a series of CdSe samples synthesized at progressively higher growth
temperatures (table 1). The observed shift and particle size was modeled using several
theoretically models including: 1-dimensional particle in a box particle, particle in a
spherical well using m, and particle in a spherical well using me, and strong
confinement model. The best agreement with experimental data was observed for the
strong confinement model. A red shift in the fluorescence spectra was also observed
when the CdSe core samples were compared to the CdSe/ZnS shelled samples. An
increase in quantum yield was also observed because the shell removes an avenue of
relaxation and enhances emission. A method using Atomic Absorption Spectroscopy
was developed to determine the number of ZnS shells on the CdSe/ZnS nanoparticles.
If AA is unavailable, the number of shells can also be determined using a UV/VIS and
constant mass method.
C. The concentration of CdSe in mg/mL by AA: Divide the mass of CdSe obtained by
AA by the original sample size.
D. Mass of ZnS: Divide the mass of Zn from the AA sample by the molar mass of Zn
(65.39 g/mol), the result was then multiplied by the molar mass of S (32.066 g/mol),
this result was then added to the mass of Zn in the AA sample.
E. The total mass of the sample was then determined by adding the mass of CdSe to
the mass of ZnS. The percent error between the AA total mass and the constant
weight was then determined.
Determination of Number of ZnS Shells – Atomic Absorption method:
Information needed:
Diameter of CdSe (nm)
# units of CdSe across diameter
# units of CdSe/dot
Density of ZnS (4.1x10-21 g/nm3)
Single ZnS shell thickness (0.31 nm)
/2)3
Equations:
1) VTOTAL = (4/3) (dTOTAL
2) VTOTAL = VCORE + VSHELL
3) dTOTAL = d1 +d2 + dCORE
4) (mg Cd divided by mg Zn) = (mg CdSe/dot divided by mg ZnS/dot)
1. The milligrams of CdSe/dot were determined by multiplying the number of units of
CdSe/dot by the molecular mass of CdSe (191 g/mol) and then dividing by
Avogadro’s number to get grams of CdSe/dot and converted to milligrams.
References:
1.
2.
3.
4.
5.
6.
Evident Technologies. 2003. Evident Technologies. March 15, 2004
<www.evidenttech.com >.
Cumberland, S; Hanif, K; Javier; Artjay; Khitrov, Gregory; Strouse, Geoffrey, Woessner; Yun, S.
Inorganic Clusters as Single-Source Precursors for Preparation of CdSe, ZnSe, and
CdSe/ZnS Nanomaterials. Chem. Mater. 2002, 14, 1576-1584.
Dance, I; Choy, Anna; Scudder, Marcia. Synthesis, Properties and Molecular and
Crystal Structures of (Me4N)4 [E4M10(SPh)16] (E=S, M=Zn, Cd) Molecular
Supertetrahedral Fragments of the Cubic Metal Chalcogenide Lattice. J. Am.
Chem. Soc. 1984, 106, 6285-6295.
Gaponenko, S.V. Optical Properties of Semiconductor Nanocrystals. New York: Cambridge
University Press, 1998.
Hines, Margaret A., and Philippe Guyot-Sionnest. Bright UV-Blue Luminescent Colloidal ZnSe
Nanocrystals. The Journal of Physical Chemistry B: 1998, 102, 19.
Yu, W. William, Lianhua Qu, Wenzhuo Guo, Xiaogang Peng. Experimental Determination of the
Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chemical Mater. 2003.
Supported in part by:
* NSF grant DMR-0303992
Through the Nanotechnology
Undergradate Education
(NUE) program
* Siena College