8706
`
`J. Am. Chem. Soc. 1993, 115, 8706-8715
`
`Synthesis and Characterization of Nearly Monodisperse CdE
`(E = S, Se, Te) Semiconductor Nanocrystallites
`
`C. B. Murray, D. J. Norris, and M. G. Bawendi*
`
`Contribution from the Department of Chemistry, Massachusetts Institute of Technology,
`Cambridge, Massachusetts 02139
`
`Received March 22, 1993
`
`Abstract: A simple route to the production of high-quality CdE (E = S, Se, Te) semiconductor nanocrystallites is
`presented. Crystallites from ~ 12 A to ~ 115 A in diameter with consistent crystal structure, surface derivatization,
`and a high degree of monodispersity are prepared in a single reaction. The synthesis is based on the pyrolysis of
`organometallic reagents by injection into a hot coordinating solvent. This provides temporally discrete nucleation and
`permits controlled growth of macroscopic quantities of nanocrystallites. Size selective precipitation of crystallites from
`portions of the growth solution isolates samples with narrow size distributions ( <5% rms in diameter). High sample
`quality results in sharp absorption features and strong "band-edge" emission which is tunable with particle size and
`choice of material. Transmission electron microscopy and X-ray powder diffraction in combination with computer
`simulations indicate the presence of bulk structural properties in crystallites as small as 20 A in diameter.
`
`I. Introduction
`
`The study of nanometer sized crystallites provides an oppor(cid:173)
`tunity to observe the evolution of material properties with size.
`This intermediate size regime is where the collective behavior of
`bulk materials emerges from the discrete nature of molecular
`properties. The differing rates with which each of the bulk
`properties develops provides the possibility of observing and
`perhaps controlling novel behavior. Nonlinear optical effects
`from highly polarizable excited states and novel photochemical
`behavior are two such examples. 1
`The physical properties of semiconductor nanocrystallites are
`dominated by the spatial confinement of excitations (electronic
`and vibrational). Quantum confinement, the widening HOMO
`LUMO gap with decreasing crystallite size, and its implications
`for the electronic structure and photophysics of the crystallites
`have generated considerable interest. 1•2 A number of optical
`studies have begun probing the photoexcited states in such
`crystallites. 1•2
`Although considerable progress has been made in the controlled
`synthesis of II-VI semiconductor crystallites, 1•3 interpretation of
`sophisticated optical experiments often remains difficult due to
`
`(I) Recent reviews include: (a) Brus, L. E. Appl. Phys. A 1991, 53, 465.
`(b) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (c) Wang, Y.; Herron,
`N. J. Phys. Chem. 1991, 95, 525. (d) Bawendi, M. G.; Steigerwald, M. L.;
`Brus, L. E. Annu. Rev. Phys. Chem. 1990, 41, 477.
`(2) Recent optical studies include the following: (a) Roussignol, P.; Ricard,
`D.; Flytzanis, C.; Neuroth, N. Phys. Rev. Lett. 1989, 62, 312. (b) Alivisatos,
`A. P.; Harris, A.; Levinos, N.; Steigerwald, M.; Brus, L. E. J. Chem. Phys.
`1989, 89, 4001. (c) Peyghambarian, N.; Fluegel, B.; Hulin, D.; Migus, A.;
`Joffre, M.; Antonetti, A.; Koch, S. W.; Lindberg, M. IEEE J. Quantum
`Electron. 1989, 25, 2516. (d) Bawendi, M. G.; Wilson, W. L.; Rothberg, L.;
`Carroll, P. J.; Jedju, T. M.; Steigerwald, M. L.; Brus, L. E. Phys. Rev. Lett.
`1990, 65, 1623. (e) Bawendi, M. G.; Carroll, P. J.; Wilson, W. L.; Brus, L.
`E. J. Chem. Phys. 1992, 96, 946. (f) Alivisatos, A. P.; Harris, T. D.; Carroll,
`P. J.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1989, 90, 3463. (g)
`O'Neil, M.; Marohn, J.; McLendon, G. J. Phys. Chem. 1990, 94, 4356. (h)
`Eychmuller, A.; Hasserlbarth, A.; Katsikas, L.; Weller, H. Ber. Bunseges.
`Phys. Chem. 1991, 95, 79. (i) Wang, Y.; Suna, A.; McHugh, J.; Hillinski,
`E.; Lucas, P.; Johnson, R. D. J. Chem. Phys. 1990, 92, 6927. U) Esch, V.;
`Fluegel, B.; Khitrova, G.; Gibbs, M.; Jiajin, X.; Chang, S. W.; Koch, S. W.;
`Liu, L. C.; Risbud, S. W.; Peyghambarian, N. Phys. Rev. B 1990, 42, 7450.
`(k) Ekimov, A. I.; Hache, F.; Schanne-Klein, M. C.; Ricard, D.; Flytzanis,
`C.; Kudryatsev, I. A.; Yazeva, T. V.; Rodina, A. F.; Efros, Al. L. J. Opt. Soc.
`Am. B 1993, JO, 100.
`(3) (a) Steigerwald, M. L.; Alivisatos, A. P.; Gibson, J.M.; Harris, T. D.;
`Kortan, R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglas, D. C.;
`Brus, L. E. J. Am. Chem. Soc. 1987, 110, 3046. (b) Brennan, J. G.; Siegrist,
`T.; Carroll, P. J.; Stuczynski, S. M.; Brus, L. E.; Steigerwald, M. L. J. Am.
`Chem.Soc.1989, 111,4141. (c)Spanhel,L.;Haase,M.; Weller,H.;Henglein,
`A. J. Am. Chem. Soc. 1987, 109, 5649.
`
`polydispersities in size and shape, surface electronic defects due
`to uneven surface derivatization, and poor crystallinity. The study
`of an appropriate high quality model system is essential in
`distinguishing properties truly inherent to the nanometer size
`regime from those associated with variations in sample quality.
`Each sample must display a high degree of monodispersity (size,
`shape, etc.), regularity in crystallite core structure, and a consistent
`surface derivatization (cap).
`This paper presents a relatively simple synthetic route to the
`production of high-quality nearly monodisperse ( <5% rms in
`diameter) samples of CdE (E = S, Se, Te) nanometer size
`crystallites, with the emphasis on CdSe. The synthesis begins
`with the rapid injection of organometallic reagents into a hot
`coordinating solvent to produce a temporally discrete homoge(cid:173)
`neous nucleation. Slow growth and annealing in the coordinating
`solvent results in uniform surface derivatization and regularity
`in core structure. Size selective precipitation provides powders
`of nearly monodisperse nanocrystallites which can be dispersed
`in a variety of solvents. The crystallites are slightly prolate with
`an aspect ratio of 1.1 to 1.3. The average crystallite size, defined
`by its major axis, is tunable from ~ 12 to ~ 115 A. Room
`temperature optical absorption and luminescence experiments
`show that the samples are of high optical quality. Transmission
`electron microscopy and X-ray powder diffraction are used in
`combination with computer simulations to characterize nano(cid:173)
`crystallite structural features.
`
`II. Experimental Section
`
`General. All manipulations involving alkylcadmium, silylchalconides,
`phosphines, and phosphine chalconides were carried out using standard
`airless procedures. Tri-n-octylphosphine [TOP] and bis(trimethylsilyl)
`sulfide [ (TMS)2S] were used as purchased from Fluka. Electronic grade
`(99.99+%) selenium and tellurium shot were purchased from Alfa.
`Anhydrous methanol, 1-butanol, pyridine, and hexane were purchased
`from a variety of sources. Tri-n-octylphosphine oxide [TOPO] was
`purchased from Alfa and purified by distillation, retaining the fraction
`transferred between 260 and 300 °C at -1 Torr. Dimethylcadmium
`[Me2Cd] was purchased from Organometallics Inc. and purified by
`filtration (0.250 µm) and vacuum transfer. Bis(trimethylsilyl)selenium
`[(TMS)iSe] and Bis(tert-butyldimethylsilyl)tellurium [(BDMS)iTe]
`were prepared via literature methods 3•·4 and stored at-35 °C in a drybox.
`Appropriate masses of selenium and tellurium shot were dissolved directly
`
`(4) Detty, M. R.; Seidler, M. D. J. Org. Chem. 1982, 47, 1354.
`
`Nanoco Technologies, Ltd
`0002-7863/93/1515-8706$04.00/0
`@ 1993 Ame,;con Che1 .. _________________ _
`EXHIBIT 1007
`1 of 10
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`Published on May 1, 2002 on http://pubs.acs.org | doi: 10.1021/ja00072a025
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`Synthesis of CdE Semiconductor Nanocrysta/lites
`
`J. Am. Chem. Soc., Vol. 115, No. 19, 1993 8707
`
`in sufficient TOP to produce 1.0 M stock solutions of trioctylphosphine
`selenide [TOPSe] and trioctylphosphine telluride [TOPTe].5
`Method 1. The typical preparation of TOP/TOPO capped CdSe
`nanocrystallites follows: Fifty grams of TOPO is dried and degassed in
`the reaction vessel by heating to -200 °C at -I Torr for -20 min,
`flushing periodically with argon. The temperature of the reaction flask
`is then stabilized at - 300 °C under -1 atm of argon.
`Solution A is prepared by adding 1.00 mL (13.3S mmol) of Me2Cd
`to 2S.O mL of TOP in the dry box. Solution Bis prepared by adding I 0.0
`mLofthe 1.0 MTOPSestocksolution (10.00mmol) to IS.OmLofTOP.
`Solutions A and B are combined and loaded into a SO-mL syringe in the
`dry box.
`The heat is removed from the reaction vessel. The syringe containing
`the reagent mixture is quickly removed from the dry box and its contents
`delivered to the vigorously stirring reaction flask in a single injection
`through a rubber septum. The rapid introduction of the reagent mixture
`produces a deep yellow/ orange solution with an absorption feature at
`440-460 nm. This is also accompanied by a sudden decrease in
`temperature to -180 °C. Heating is restored to the reaction flask and
`the temperature is gradually raised to 230--260 °C.
`Aliquots of the reaction solution are removed at regular intervals (S-
`10 min) and absorption spectra taken to monitor the growth of the
`crystallites. The best quality samples are prepared over a period of a few
`hours of steady growth by modulating the growth temperature in response
`to changes in the size distribution as estimated from the absorption spectra.
`The temperature is lowered in response to a spreading of the size
`distribution and increased when growth appears to stop. When the desired
`absorption characteristics are observed, a portion of the growth solution
`is transferred by cannula and stored in a vial. In this way, a series of sizes
`ranging from -1 S to 11 S A in diameter can be isolated from a single
`preparation.
`CdTe nanocrystallites are prepared by Method I with TOPTe as the
`chalcogen source, an injection temperature of -240 °C, and growth
`temperatures between -190 and -220 °C.
`Method 2. A second route to the production of CdE (E = S, Se, Te)
`nanocrystallites replaces the phosphine chalconide precursors in Method
`I with (TMS)iS, (TMS)iSe, and (BDMS)iTe, respectively. Growth
`temperatures between -290 and -320 °C were found to provide the
`best CdS samples. The smallest ( -12 A) CdS, CdSe, and Cd Te species
`are produced under milder conditions with injection and growth carried
`out at -100 °C.
`Isolation and Purification of Crystallites. A I 0-mL aliquot of the
`reaction solution is removed by cannula and cooled to -60 °C, slightly
`above the melting point of TOPO. Addition of 20 mL of anhydrous
`methanol to the aliquot results in the reversible flocculation of the
`nanocrystallites. The flocculate is separated from the supernatant by
`centrifugation. Dispersion of the flocculate in 2S mL of anhydrous
`1-butanol followed by further centrifugation results in an optically clear
`solution of nanocrystallites and a gray precipitate containing byproducts
`of the reaction. Powder X-ray diffraction and energy dispersive X-ray
`analysis indicate these byproducts consist mostly of elemental Cd and Se.
`This precipitate is discarded. Addition of 2S mL of anhydrous methanol
`to the supernatant produces flocculation of the crystallites and removes
`excess TOP and TOPO. A final rinse of the flocculate with SO mL of
`methanol and subsequent vacuum drying produces - 300 mg of free
`flowing TOP /TOPO capped CdSenanocrystallites. The resulting powder
`is readily dispersed in a variety of alkanes, aroma tics, long-chain alcohols,
`chlorinated solvents, and organic bases (amines, pyridines, furans,
`phosphines).
`Size-Selectile Precipitation. Purified nanocrystallites are dispersed
`in anhydrous 1-butanol forming an optically clear solution. Anhydrous
`methanol is then added dropwise to the dispersion until opalescence persists
`upon stirring or sonication. Separation of supernatant and flocculate by
`centrifugation produces a precipitate enriched with the largest crystallites
`in the sample. Dispersion of the precipitate in 1-butanol and size-selective
`precipitation with methanol is repeated until no further sharpening of the
`optical absorption spectrum is noted. Size-selective precipitation can be
`carried out in a variety of solvent/nonsolvent pairs, including pyridine/
`hexane and chloroform/methanol.
`Surface Exchange. Crystallite surface derivatization can be modified
`by repeated exposure to an excess of a competing capping group. Heating
`to -60 °C a mixture of -so mg ofTOPO/TOP capped crystallites and
`S-10 mL of pyridine gradually disperses the crystallites in the solvent.
`
`(5) Zingaro, R. A.; Steeves, B. H.; Irgorlic, K. J. Organomet. Chem. 1965,
`4, 320.
`
`Treatment of the dispersion with excess hexane results in the flocculation
`of the crystallites which are then isolated by centrifugation. The process
`of dispersion in pyridine and flocculation with hexane is repeated a number
`of times to produce crystallites which disperse readily in pyridine, methanol,
`and aromatics but no longer disperse in aliphatics.
`Optical Characterization. Optical absorption spectra were collected
`at room temperature on a Hewlett-Packard 84S2 diode array spectrometer
`using 1-cm quartz cuvettes. Samples were prepared by dispersing washed
`CdSe nanocrystallites in hexane. Luminescence experiments were carried
`out on a SPEX Fluorolog-2 spectrometer with use of front face collection
`with SOO-µrn slits. Estimates of quantum yield were obtained by comparing
`the integrated emission from Rhodamine 640 in methanol and that of
`3 SA diameter CdSe nanocrystallites dispersed in hexane. Concentrations
`of both were adjusted to provide optical densities of 0.30 at 460 nm in
`matched I-mm quartz cuvettes. Fluorescence spectra were collected
`between 480 and 800 nm at room temperature with 460-nm excitation.
`Transmission Electron Microscopy. A Topcon EM002B electron
`microscope operating at 200 kV was used for transmission electron
`microscopy (TEM). Imaging was carried out in bright field with an
`objective aperture selected to permit lattice imaging of the (100), (002),
`and (101) Wurtzite planes. Copper grids (300 mesh) coated with a -so
`A amorphous carbon film were purchased from Ernest Fullam. Samples
`were prepared by placing a drop of a dilute pyridine dispersion of
`nanocrystallites on the surface of a grid, waiting for -1 min, and then
`wicking away the solution. The coverage level of crystallites was adjusted
`by varying the initial dispersion concentration and the contact time.
`X-ray Powder Diffraction. Powder X-ray diffraction spectra were
`collected on a Rigaku 300 Rotaflex diffractometer operating in the Bragg
`configuration using Cu Ka radiation. The accelerating voltage was set
`at 2SO kV with a 200 milliamp flux. Scatter and diffraction slits of0.50°
`and a 0.15-mm collection slit were used. Samples for X-ray diffraction
`were prepared from - 500 mg of thoroughly washed and dried nano(cid:173)
`crystallite powder. The free-flowing powders were pressed at 5000 psi
`to form 0.5 in. diameter pellets with mirror flat surfaces.
`
`III. Results and Discussion
`
`The production and phenomenology of monodisperse lyophobic
`colloids has been investigated since Faraday's production of gold
`sols in 1857.6 Classic work by La Mer and Dinegar7 has shown
`that the production of a series of monodisperse lyophobic colloids
`depends on a temporally discrete nucleation event followed by
`controlled growth on the existing nuclei. Temporally discrete
`nucleation in our synthesis is attained by a rapid increase in the
`reagent concentrations upon injection, resulting in an abrupt
`supersaturation which is relieved by the formation of nuclei and
`followed by growth on the initially formed nuclei.
`The work of Steigerwald and co-workers on the use of
`organometallic precursors in the solution-phase synthesis of bulk
`and nanocrystalline materials provides guidance in our selection
`of reagents. 3a.b,s Me2Cd is chosen as the Cd source and (TMS)iE
`(E = S, Se, Te) or TOPSe and TOPTe are selected as chalcogen
`sources with TOPSe and TOPTe preferred due to their ease of
`preparation and their stability. Me2Cd and (TMS)iE reagents
`have been shown to undergo dealkylsilylation in a variety of
`solvents as a route to the production of bulk materials.8
`Trimethylphosphine telluride is known as a good source of Te0.9
`Mixed phosphine/ phosphine oxide solutions have previously been
`found to be good solvents for the high-temperature growth and
`annealing of CdSe crystallites.2<l,e,io The coordinating solvent
`plays a crucial role in controlling the growth process, stabilizing
`the resulting colloidal dispersion, and electronically passivating
`the semiconductor surface.
`Nucleation and Growth. Injection of reagents into the hot
`reaction pot results in a short burst of homogeneous nucleation.
`
`(6) Overbeek, J. Th. G. Adv. Colloid Interface Sci. 1982, 15, 2Sl.
`(7) LaMer, V. K.; Dinegar, R. H.J. Am. Chem. Soc. 1950, 72, 4847.
`(8) Stuczynski, S. M.; Brennan, J. G.; Steigerwald, M. L. Inorg. Chem.
`1989, 28, 4431.
`(9) (a) Steigerwald, M. L.; Sprinkle, C. R. J. Am. Chem. Soc. 1987, 109,
`7200. (b) Steigerwald, M. L. Chem. Mater. 1989, 1, 52.
`(10) Bawendi, M. G.; Kortan, A. R.; Steigerwald, M. L.; Brus, L. E. J.
`Chem. Phys. 1989, 91, 7282.
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`Murray et al.
`
`600
`
`350
`
`400
`
`550
`500
`450
`Wavelength (nm)
`Figure 1. Example of the effect of size-selective precipitation on the
`absorption spectrum of -37 A diameter CdSe nanocrystallites. (a) Room
`temperature optical absorption spectrum of the nanocrystallites in the
`growth solution before size-selective precipitation. (b) Spectrum after
`one size-selective precipitation from the growth solution witl methanol.
`(c) Spectrum after dispersion in 1-butanol and size-selective precipitation
`with methanol. (d) Spectrum after a final size-selective precipitation
`from 1-butanol/methanol.
`
`sterically stabilized by a lyophylic coat of alkyl groups anchored
`to the crystallite surface by phosphine oxide/ chalconide moieties.
`The efficiency of the steric stabilization is strongly dependent on
`the interaction of the alkyl groups with the solvent. Gradual
`addition of a nonsolvent can produce size-dependent flocculation
`of the nanocrystallite dispersion. This phenomenon is exploited
`in further narrowing the particle size distribution.
`The addition of methanol increases the average polarity of the
`solvent and reduces the energetic barrier to flocculation. The
`largest particles in a dispersion experience the greatest attractive
`forces. These large particles have a higher probability of
`overcoming the reduced barrier and are thus enriched in the
`flocculate produced. The removal of a specific subset of particles
`from the initial size distribution narrows the size distribution of
`both supernatant and precipitate. Depending on the cap molecule,
`a number of solvent/nonsolvent systems can be used for size(cid:173)
`selective precipitation (e.g. hexane/ethanol, chloroform/meth(cid:173)
`anol, pyridine/hexane, etc .... ).
`Figure 1 illustrates the result of size-selective precipitation.
`Spectrum a shows the optical absorption of the initial growth
`solution. The broad absorption features correspond to a sample
`with an average size of 35 A :I: 10% (sized by TEM). Slow
`addition of methanol results in the flocculation of the larger
`particles in the distribution which give spectrum b after dispersing
`in 1-butanol. Titration of methanol in sample b again produces
`flocculation of the larger particles, giving spectrum c upon
`dispersion in 1-butanol. A final size-selective precipitation from
`1-butanol yields a sample with optical absorption d and with an
`average size of - 37 A :I: 5%. Spectrum d is dramatically
`sharpened relative to that of the initial growth solution and reveals
`transitions at 530 and -400 nm which were previously cloaked
`by polydispersity. For the fractionation process to work well it
`is crucially important that the shape and surface derivatization
`of the initial crystallites be uniform and that the initial poly(cid:173)
`dispersity in size be relatively small.
`
`8708
`
`J. Am. Chem. Soc., Vol. 115, No. 19, 1993
`
`The depletion of reagents through nucleation and the sudden
`temperature drop associated with the introduction of room
`temperature reagents prevents further nucleation. Gently re(cid:173)
`heating allows slow growth and annealing of the crystallites.
`Crystallite growth appears consistent with "Ostwald ripening",
`where the higher surface free energy of small crystallites makes
`them less stable with respect to dissolution in the solvent than
`larger crystallites. The net result of this stability gradient within
`a dispersion is slow diffusion of material from small particles to
`the surface of larger particles.11 Reiss has shown how growth by
`this kind of transport can result in the production of highly
`monodisperse colloidal dispersions from systems that may initially
`be polydisperse.12
`Both the average size and the size distribution of crystallites
`in a sample are dependent on the growth temperature, consistent
`with surface free energy considerations. The growth temperature
`necessary to maintain steady growth increases with increasing
`average crystallite size. As the size distribution sharpens, the
`reaction temperature must be raised to maintain steady growth.
`Conversely, if the size distribution begins to spread, the tem(cid:173)
`perature necessary for slow steady growth drops. Size distri(cid:173)
`butions during growth are crudely estimated from absorption
`line widths (typically 50 nm fwhm). Modulation of the reaction
`temperature in response to changes in the absorption spectrum
`allows the maintenance of a sharp size distribution as the sample
`grows.
`The Ostwald ripening process accentuates any kinetic or
`thermodynamic "bottleneck" in the growth of crystallites. As a
`bottleneck is approached (e.g. a closed structural shell}, sharpening
`of the sample size distribution reduces the thermodynamic driving
`force for further growth. Sharpening in the absorption features
`as the average particle size approaches 12, 20, 35, 45, and 51 A.
`in diameter may point to the presence of such bottlenecks.
`Capping groups present a significant steric barrier to the
`addition of material to the surface of a growing crystallite, slowing
`the growth kinetics. The TOP /TOPO solvent coordinates the
`surface of the crystallites and permits slow steady growth at
`temperatures above 280 °C. Replacing the octyl chains with
`shorter groups reduces the temperature for controlled growth.
`Mixed alkyl phosphine/ alkylphosphine oxide solvents with butyl,
`ethyl, and methyl groups show uncontrolled growth at 230, 100,
`and 50 ° C, respectively. Steady controlled growth results in highly
`monodisperse particles of consistent crystal structure and allows
`size selection by extracting samples periodically from the reaction
`vessel.
`A wealth of potential organometallic precursors and high boiling
`point coordinating solvents exist. Although phosphine/phosphine
`oxide have been found to provide the most controlled growth
`conditions, injections of reagents into hot pyridines, tertiary
`amines, and furans all allow production of nanocrystallites.
`We are beginning the extension of this synthetic method to the
`production of ZnE and HgE materials using diethylzinc and
`dibenzylmercury as group II sources. Growth conditions have
`not yet been optimized to provide sample quality comparable to
`that of CdE materials.
`Colloid Stabilization and Size-Selective Precipitation. Lyo(cid:173)
`phobic colloidal particles attract each other by van der Waals
`forces. The attraction is strong due to the near additivity of
`forces between pairs of unit cells in different particles. 13 Colloids
`remain stable with respect to aggregation only if there exists a
`repulsive force of sufficient strength and range to counteract the
`van der Waals attraction. Chemisorption of ambiphylic species
`on the surface of the particles gives rise to a steric barrier to
`aggregation. The dispersions of CdSe nanocrystallites are
`
`( 11) Smith, A. L. Particle Growth in Suspensions; Academic Press: London,
`1983; pp 3-15.
`(12) Reiss, H.J. Chem. Phys. 1951, 19, 482.
`(13) Sato, T.; Ruch, R. Stabilization of Colloidal Dispersions by Polymer
`Absorption; Marcel Dekker: New York, 1980; pp 46-51.
`
`Published on May 1, 2002 on http://pubs.acs.org | doi: 10.1021/ja00072a025
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`Synthesis of CdE Semiconductor Nanocrysta//ites
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`J. Am. Chem. Soc .. Vol. 115, No. 19, 1993 8709
`
`0
`
`400
`500
`Wavelength (nm)
`Figure 2. Room temperature optical absorption spectra of -20-30 A
`diameter CdS, CdSe, and CdTe crystallites.
`
`600
`
`700
`
`300
`
`Surface Exchange. Preliminary studies of surface exchange
`have been used to tailor the compatibility of crystallites with a
`variety of solvents. Exposure of purified crystallites to a large
`excess of pyridine begins the exchange of the surface cap. Addition
`of solvents compatible with TOPO and TOP but incompatible
`with the new cap results in the flocculation of the crystallites and
`the removal of displaced TOP and TOPO species. The repeated
`dispersion in pyridine and isolation with alkanes drives the surface
`exchange with mass action. Crystallites capped with pyridine
`are dispersible in polar solvents and aromatics but not aliphatics.
`Surface exchange results in a slight decrease in average crystallite
`size and a small broadening of the size distribution, probably due
`to the loss of species containing Cd and Se. Sharp optical features
`can be recovered by size-selective precipitation from pyridine
`with the titration of hexane. Crystallites can be stabilized in a
`variety of solvents with a range of functionalized caps. There is
`obvious potential for customizing photochemical activity of these
`robust chromophores through manipulation of their immediate
`chemical environment.
`Optical Properties. The absorption spectra of ""'20--30 A
`diameter CdS, CdSe, and Cd Te nanocrystallite samples are shown
`in Figure 2. All three clearly show the effect of quantum
`confinement. CdS, CdSe, and CdTe absorptions are shifted
`dramatically from their 512, 716, and 827 nm bulk band gaps,
`respectively. The CdSe spectrum shows three clearly resolved
`transitions while the CdS and CdTe samples show less structure.
`We do not believe the difference in quality of the optical spectra
`reflects any fundamental material limitations but rather the
`amount of effort spent in optimizing the growth conditions for
`each material.
`Figure 3 shows the evolution of the optical spectrum with size
`in a series of room temperature absorption spectra for CdSe
`crystallites ranging from""' 12 to 115 A in diameter. The series
`spans a range of sizes from nearly molecular species containing
`fewer than 100 atoms to fragments of the bulk lattice containing
`more than 30 000 atoms. Figure 4 compares the experimentally
`observed HOMO LUMO gap as a function of particle size with
`the prediction of the simple effective mass approximation with
`the Coulomb interaction treated in first-order perturbation. id All
`particle sizes were determined by TEM and confirmed by X-ray
`line-shape analysis. Although experiment and theory agree
`reasonably well at large sizes, the simple theory diverges from
`the experimental values for small sizes as expected from the non(cid:173)
`parabolicity of the bands at higher wave vectors and the finite
`potential barrier at the surface of the particles. Lippens and
`
`300 350 400 450 500 550 600 650 700 750
`Wavelength (nm)
`
`Figure 3. Room temperature optical absorption spectra of CdSe
`nanocrystallites dispersed in hexane and ranging in size from -12 to 115
`A.
`
`6.0 ,--,..----.----r---r----,-----.,
`
`5.5
`
`5.0
`
`4.5
`
`> 4.0
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`40
`60
`80
`100
`120
`20
`Diameter (A)
`Figure 4. HOMO LUMO transition energy of CdSe crystallites as a
`function of size (diamonds) compared with the prediction of the effective
`mass approximation (solid line).
`
`Lannoo14 have shown that tight binding calculations can yield
`better agreement for these smaller sizes.
`Figure 5 shows the room temperature photoluminescence
`spectrum of a sample of 35 A diameter CdSe crystallites and
`compares it with the absorption spectrum. The luminescence
`quantum yield for this sample is ""'9.6% relative to Rhodamine
`640 at room temperature. The line width in emission is equal to
`that in absorption, with the peak of the emission shifted 4 nm to
`the red of the absorption maximum. This shift is the result of
`a combination of relaxation into shallow trap states and the size
`distribution.20 No deep trap emission was detected.
`
`(14) (a) Lippens, P. E.; Lannoo, M. Phys. Rev. B 1989, 39, 10935. (b)
`Lippens, P. E.; Lannoo, M. Mater. Sci. Eng. B 1991, 9, 485.
`
`4 of 10
`
`Published on May 1, 2002 on http://pubs.acs.org | doi: 10.1021/ja00072a025
`
`Downloaded by Scott Reese on July 2, 2009
`
`

`

`8710 J. Am. Chem. Soc .. Vol. 115, No. 19, 1993
`
`Murray et al.
`
`Fipre 6. TEM image taken in bright field with lattice contrast shows
`a collection of slightly prolate particles. The elongated (002) axis measures
`35.0 A % 5% while the perpendicular axis measures 30 A % 6%. The
`particles are well dispersed and not aggregated.
`
`10 DID
`Fipre 7. An 80 A diameter CdSe crystallite imaged in bright field with
`atom contrastshows the presence of stackbg faults in the (002tdire.:1ion.
`
`High-magnification imaging (590 X 103 to 1 X 106) allows the
`detection of planar disorder in individual crystallites. Figure 7
`shows an 80 A diameter CdSe nanocrystallite atom imaged
`perpendicular to the ( 100) and (002) axes. Planar disorder along
`the (002) axis is clearly observed. Figure 8 shows a -110 A
`diameter crystallite with a 1.33 aspect ratio and four clear stacking
`faults in the (002) direction (the left particle). A neighboring
`crystallite displays the hexagonal atom imaging pattern resulting
`from imaging along the (002) axis. No planar disorder is
`detectable in this projection. The analysis of a series of samples
`leads to some general results. The (002) planes (perpendicular
`to the long axis) show little disorder while loss of lattice image
`contrast is most commonly seen in the (100) planes (parallel to
`the long axis) and in the (101) planes. This loss of contrast is
`believed to result from the presence of stacking faults along the
`(002) direction. These stacking faults are the predominant form
`
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`
`500
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`600
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`
`Wavelength (nm)
`Fipre 5. Typical room temperature band edge luminescence and
`absorption spectra for 35 A diameter CdSe crystallites. No deep trap
`luminescence is detected.
`
`The sharp absorption features suggest highly monodisperse
`samples. High quantum yields and narrow emission line widths
`indicate growth of crystallites with few electronic defect sites.
`The sharp luminescence is a dramatic example of the efficiency
`of the capping group in electronically passivating the crystallites.
`The capping groups also protect the individual crystallites from
`chemical degradation yielding robust systems. Samples stored in
`the original growth solution still show strong, sharp emission
`after storage for more than a year.
`These room temperature optical experiments, carried out on
`common laboratory equipment, demonstrate the benefits ofhigh(cid:173)
`quality samples and point to the potential of more sophisticated
`optical studies on these samples.
`Transmission Electron Microscopy. Transmission electron
`microscopy allows imaging of individual crystallites and the
`development of a statistical description of the size and shape of
`the particles in a sample. High magnification imaging with lattice
`contrast allows the determination of individual crystallite mor(cid:173)
`phology.15
`Imaging at :>YU "' 1uJ umes magnmcauon w1tn moaerate
`crystallite coverage allows careful size measurements of 30 to 50
`individual nanocrystallites on a single image and shows that the
`particles are not aggregated. Figure 6 shows a collection of slightly
`prolate CdSe nanocrystallites averaging 35 A± 5% in the direction
`of the (002) wurtzite axis and 30 A ± 6% perpendicular to the
`(002) axis. Particles with the (002) axis perpendicular to the
`grid are identified by a characteristic hexagonal pattern in atom