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,Upper division students synthesized quantum dot samples using modified literature procedures a
2、nd 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 ra
3、tio 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 tha
4、t 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-hol
5、e 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
6、, 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
7、 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 surfa
8、ce. This prevents the passive relaxation of the exciton and forces the exciton to relax via emission, increasing the intensity of the nanocrystals fluorescence.,Introduction,Abstract,Preparation of Amine-Capped Zinc Selenium (ZnSe) Nanoparticles-2 A synthetic procedure analogous to the one described
9、 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 nan
10、oparticles 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
11、). 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
12、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
13、 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 centr
14、ifuge 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 li
15、quid 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 concentratio
16、n determined by way of Beers Law, by UV-VIS spectrum and molar absorptivity of CdSe available on website: . 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 t
17、o 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 t
18、ube and a small hole was punched in the filter paper to allow airflow. The centrifuge tube was then placed under vacuum at 70C 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 70C for an additional hour. This p
19、rocess 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 cen
20、trifuge 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 orde
21、r to determine the constant mass of the dried crystals.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 + 2 (ab/adot)2 Ry* - 1.786 (ab/adot) Ry* - 0.248 Ry*
22、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),Table 1: CdSe Spectral Data:,Procedures,CdSe(amine capped)-3,2 Hexadecylamine (6 grams, 24.84 mmoles) was degassed
23、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 temperatur
24、e 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 weretaken at 10-15 degree intervals. When the solution reached 250C, the
25、 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 wa
26、s 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.,CdSe UV-Vis and Fluorescence Wavelength Shifts:Sizes of the CdSe nanoparticles were estimated using absorption spectroscopy.1 A relationship between sample
27、 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 nanopart
28、icles (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
29、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 concentrat
30、ion 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
31、 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: Diam
32、eter 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 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
33、. 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 Avogadros number to get grams of CdSe/dot and converted to milligrams.,2. Using equation 4, the mg of ZnS/dot was determined utilizing both the m
34、g 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
35、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 method1. Determine the concentration
36、 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 pe
37、rcentage 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
38、then substituted for the AA data in the AA method calculations above.,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
39、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.
40、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 wa
41、s 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 . Cumberland, S; Hanif, K; Javier;
42、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
43、 of (Me4N)4 E4M10(SPh)16 (E=S, M=Zn, Cd) MolecularSupertetrahedral 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: CambridgeUniversity Press, 1998. Hines, Margaret A., and Philippe G
44、uyot-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,Results,
