Synthesis and Analysis of Quantum Dots

1
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

•  
• 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
•  
• 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. 
• Assume you have 100 mg in which the percentage
  equals the amount in mg.
• 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)
• 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).
• These values were then substituted for the AA
  data in the AA method calculations above.

  Table 1 CdSe Spectral Data
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
120C. The TOPO solution was cooled to 70C 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 150C. 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 170C 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 190C, 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 Beers 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. 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. Quantum Confinement4,6 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 p2 (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) ab exciton
Bohr radius (CdSe 4.9 nm) adot radius of the
Q.D Ry Rydberg constant (CdSe 0.016
eV)
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. 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 nanocrystals fluorescence.

• 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).
• 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.
• 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)
•  
• Equations 1) VTOTAL (4/3)? (dTOTAL/2)3

Introduction

Discussion

• 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.
• References
• Evident Technologies. 2003. Evident
  Technologies. March 15, 2004
• ltwww.evidenttech.com gt.
• 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
  (ES, MZn, 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.
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• Supported in part by
• NSF grant DMR-0303992
• Through the Nanotechnology
• Undergradate Education
• (NUE) program

CdSe(amine capped)-3,2 Hexadecylamine (6 grams,
24.84 mmoles) was degassed under vacuum for 30
minutes at 60C. While under Nitrogen, the
solution was cooled to below 40C, at which point
the septum was removed, and 0.1g (0.028 mmol) of
(Li)4Cd10Se4(SPh)16 was added to the top of
the solidified Hexadecylamine. The septum was
reattached, and while under vacuum, the
temperature was raised to 120C. At 120C the
system was switched to nitrogen atmosphere, and
heating continued until 130C. Aliquots (5-6
drops) were taken starting at 130C using a
syringe, and quenched in 1 mL toluene. Subsequent
aliquots were taken at 10-15 degree intervals.
When the solution reached 250C, 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.
Procedures
Results