Nanocrystal Quantum Dots - Federation of American Scientists

Number 28 2003 Los Alamos Science 214

Semiconductor lasers are ubiqui-tous in modern society and play akey role in technologies ranging

from CD players to optical telecommu-nications. Current-generation lasershave high power output and low lasingthresholds, are stable over a wide rangeof temperatures, and are cheap and easyto produce. Still, there is room forimprovement. We are developing a newtype of laser based on ultrasmall bits ofsemiconductor material called quantumdots (QDs). Consisting of only a fewhundred to a few hundred thousandatoms, QDs bridge the gap between thesolid state and single atoms, and hencethese specks of matter exhibit a mix ofsolid-state and atomic properties. In ourwork, we concentrate on nanoparticlesthat are synthesized by colloidal chem-istry, and therefore, they are oftencalled colloidal or nanocrystal QDs(NQDs). Interestingly, the emissionwavelength (that is, the emission color)of QDs depends on the dot size, and inthe case of semiconductor nanocrystals,color can be controlled preciselythrough simple chemistry. We are there-fore developing an altogether new typeof color-selectable lasing medium.

Although this paper focuses on ourNQD laser work, quantum dots are“bigger” than lasers. Because of theirsmall dimensions and size-controlledelectronic spectra, NQDs can beviewed as tunable artificial atoms with

(b) CdSe Quantum Dot (QD)

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Figure 1. Quantum Dots (QDs)(a) A bulk semiconductor such as CdSe has continuous conduction and valenceenergy bands separated by a “fixed” energy gap, Eg(bulk). Electrons normally occupyall states up to the edge of the valence band, whereas states in the conduction bandare empty. (b) A QD is characterized by discrete atomic-like states with energies thatare determined by the QD radius R. These well-separated QD states can be labeledwith atomic-like notations, such as 1S, 1P, and 1D. (c) The expression for the size-dependence separation between the lowest electron and hole QD states—Eg(QD), theQD energy gap—was obtained with the spherical “quantum box” model. (d) Thisschematic represents the continuous absorption spectrum of a bulk semiconductor(black line) compared with a discrete absorption spectrum of a QD (colored bars).

Nanocrystal Quantum Dots From fundamental photophysics to multicolor lasing

Victor I. Klimov

properties that can be engineered tosuit either the needs of a certainexperiment or a specific technologicalapplication. When coated with a suit-able, chemically active surface layer,NQDs can be coupled to each other orto different inorganic or organic enti-ties and thus serve as useful opticaltags. We can now chemically manipu-late NQDs almost as well as standardmolecules, and can assemble theminto close-packed ordered or disor-dered arrays that mimic naturallyoccurring solids. Furthermore,because their dimensions, shapes, andsurface properties can be manipulatedwith ease, NQDs are ideally suited toserve as nanoscale laboratories forstudies of fundamental quantummechanical effects.

The Quantum Size Effect and QDs

One of the defining features of asemiconductor is the energy gap sepa-rating the conduction and valenceenergy bands. The color of light emit-ted by the semiconductor material isdetermined by the width of the gap. Insemiconductors of macroscopicsizes—bulk semiconductors—the gap

width is a fixed parameter determinedby the material’s identity.

The situation changes, however, inthe case of nanoscale semiconductorparticles with sizes smaller than about10 nanometers. This size range corre-sponds to the regime of quantum con-finement, for which the spatial extentof the electronic wave function iscomparable with the dot size. As aresult of these “geometrical” con-straints, electrons “feel” the presenceof the particle boundaries and respondto changes in particle size by adjust-ing their energy. This phenomenon isknown as the quantum-size effect, andit plays a very important role in QDs.

In the first approximation, thequantum-size effect can be describedby a simple “quantum box” model(Efros and Efros 1982), in which theelectron motion is restricted in allthree dimensions by impenetrablewalls. For a spherical QD with radiusR, this model predicts that a size-dependent contribution to the energygap is simply proportional to 1/R2,implying that the gap increases as theQD size decreases. In addition, quan-tum confinement leads to a collapse ofthe continuous energy bands of a bulkmaterial into discrete, atomic-likeenergy levels. The discrete structure

of energy states leads to a discreteabsorption spectrum of QDs, which isin contrast to the continuous absorp-tion spectrum of a bulk semiconductor(see Figure 1).

The NQDs discussed earlier aresmall quantum dots that are made byorganometallic chemical methods andare composed of a semiconductor corecapped with a layer of organic mole-cules (Murray et al. 1993). (SeeFigure 2.) The organic capping pre-vents uncontrolled growth andagglomeration of the nanoparticles. Italso allows NQDs to be chemicallymanipulated as if they were large mol-ecules, with solubility and chemicalreactivity determined by the identityof the organic molecules. The cappingalso provides “electronic” passivationof NQDs; that is, it terminates dan-gling bonds that remain on the semi-conductor’s surface. As discussedbelow, the unterminated danglingbonds can affect the NQD’s emissionefficiency because they lead to a lossmechanism wherein electrons are rap-idly trapped at the surface before theyhave a chance to emit a photon. Usingcolloidal chemical syntheses, one canprepare NQDs with nearly atomic pre-cision; their diameters range fromnanometers to tens of nanometers and


Nanocrystal Quantum Dots

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Figure 2. Nanocrystal Quantum Dots (NQDs)(a) An organometallic method is used for the fabrication of highly monodisperse CdSe NQDs. Nucleation and subsequent growthof NQDs occurs after a quick injection of metal and chalcogenide precursors into the hot, strongly coordinating solvent—a mix-ture of trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO) in the case shown. After a fixed period, removing the heatsource stops the reaction. As a result, NQDs of a particular size form. (b) The colloidal NQDs obtained by the method illustratedin (a) consist of an inorganic CdSe core capped with a layer of TOPO/TOP molecules. (c) Solutions of CdSe NQDs of differentradii, under ultraviolet illumination, emit different colors because of the quantum size effect. A 2.4-nm-radius dot has an energygap of about 2 eV and emits in the orange, whereas a dot of radius 0.9 nm has a gap of about 2.7 eV and emits a blue color.

size dispersions as narrow as 5 per-cent. Because of the quantum-sizeeffect, this ability to tune the NQDsize translates into a means of control-ling various NQD properties, such asemission and absorption wavelengths.The emission of cadmium-selenium(CdSe) NQDs, for example, can betuned from deep red to blue by areduction in the dot radius from 5nanometers to 0.7 nanometer.

Nanocrystal Lasers:Advantages and Problems

Lasers made from bulk semicon-ductor materials have been used forseveral decades. (Laser fundamentalsare described in Figure 3.) Althoughnumerous advances were madethroughout those years, laser perform-ance improved dramatically with theintroduction of so-called quantumwell lasers, in which charge carriers—electrons and holes—were confined tomove in a plane—that is, they werefree to move in a two-dimensional (2-D) quantum well. Compared withbulk semiconductors, the quantumwell has a higher density of electronicstates near the edges of the conduc-tion and valence bands, and thereforea higher concentration of carriers cancontribute to the band-edge emission.Consequently, it takes less intense“pumping” of energy into a quantum-well laser to get it to lase (the lasingthreshold is lower). Additionally,quantum-well lasers show improvedtemperature stability and a narroweremission line.

In QDs, the charge carriers areconfined in all three dimensions, withthe result that the electrons exhibit adiscrete atomic-like energy spectrum.In very small QDs, the spacingbetween these atomic-like states isgreater than the available thermalenergy, so thermal depopulation ofthe lowest electronic states is inhibit-ed. It was therefore anticipated that a

QD laser would have a temperature-insensitive lasing threshold at an exci-tation level of only one electron-hole(e-h) pair per dot.

Lasing in QDs was first reported in1991 (Vandyshev et al. 1991) and wasachieved in an optically pumpeddevice with relatively large (approxi-mately 10-nanometer) CdSe nanopar-ticles. The QDs were fabricated byhigh-temperature precipitation inmolten glass. Later, lasing was alsoobserved for QDs grown by epitaxialtechniques (Ledentsov et al. 1994). Asexpected, the QD lasers showed animproved performance and featured alower lasing threshold and enhancedtemperature stability by comparisonwith quantum-well lasers.

These successes provided us withstrong motivation for the developmentof lasers based on NQDs less than

10 nanometers in diameter. In this sizerange, spacing between electronic lev-els can exceed hundreds of milli-elec-tron-volts (meV), a much larger valuethan the room temperature energyscale of about 24 meV. Size-con-trolled spectral tunability over anenergy range of 1 electron volt wasexpected. However, after a decade ofresearch that provided some tantaliz-ing hints of optical gain, NQDs failedto demonstrate lasing action.

The failures were often attributedto material defects or dangling bondson the surface of the NQDs, whichwere a natural consequence of thelarge surface-to-volume ratio of thesub-10-nanometer particles. Thedefects lead to electronic states thatlie within the material’s energy gap.Electrons can relax into those states,whereupon they typically undergo

216 Los Alamos Science Number 28 2003


(a) Band Edge Transition (c) Transparency

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Figure 3. Laser Basics(a) “Pumping” energy into a semiconductor can excite an electron, e, into the con-duction band. That electron leaves behind a hole, h, in the normally filled valenceband, and thus an e-h pair is created. The electron and hole each relax to therespective band-edge states by nonradiative processes. During the band-edge tran-sition, a photon is emitted as the excited electron spontaneously recombines withthe hole. (b) Stimulated emission occurs when a photon induces the excited elec-tron to decay. The emitted photon has the exact frequency, phase, and polarizationof the initial photon. (c) For a ground state that contains two electrons, excitingonly one electron (population equality) can lead to two equally probable outcomes:The incoming photon stimulates the excited electron to decay, producing an extraphoton (left), or the photon excites the ground-state electron and is absorbed (right).There is no net gain or loss of photons. In this case, the medium is in the trans-parency regime. (d) Optical gain can occur if there are more electrons in the excitedstate than in the ground state (population inversion) because photon absorptionsinhibited. If a population inversion is established in a bulk system and if the gain fromstimulated emission is larger than losses that absorb or scatter photons, the systemwill exhibit amplified spontaneous emission (ASE). In a laser, an ASE-capablemedium is placed in a reflecting cavity, and thus the photon field builds on itself.

either nonradiative or radiative (in-gap“deep-trap” emission) decay to theground state. Thus the surface defectsintroduce carrier losses that inhibit theoptical gain. Another concern raisedin several theoretical papers was thereduced efficiency of electron-phononinteractions that results from the dis-crete, atomic-like energy structures,an effect that reduces the ability ofcarriers to enter into the band-edgestates and hence reduces lumines-cence efficiencies. However, ourresearch team eventually realized thatthe main difficulty in getting ourultrasmall NQDs to lase stemmedfrom a largely unforeseen problemknown as multiparticle Auger recom-bination (Klimov et al. 2000).

Multiparticle AugerRecombination vs

Optical Gain

As in the case of other lasingmedia, QDs require a populationinversion in order to produce opticalgain (refer to Figure 3). The popula-tion inversion corresponds to the situ-ation in which the number of elec-trons in a high-energy excited state isgreater than that in the low-energy

ground state. In small dots, the lowest“emitting” transition can be treated asa two-level system that contains twoelectrons in its ground state. To invertsuch a system, one has to promoteboth electrons from the ground to theexcited state, meaning that opticalgain in QDs originate from nanoparti-cles that contain two e-h pairs (doublyexcited nanoparticles).

Paradoxically, whereas the intrinsicdecay of singly excited QDs is due tothe e-h recombination and the emis-sion of a photon, the deactivation oftwo e-h pair states is dominated bynonradiative Auger recombination(Klimov et al. 2000a). In the lattercase, the e-h recombination energy isnot released as a photon but is trans-ferred to a third particle (an electronor a hole) that is re-excited to a higherenergy state (see Figure 4). Augerrecombination has a relatively lowefficiency in bulk semiconductorsbecause of restrictions imposed byenergy and momentum conservation.But linear, or translational, momen-tum conservation is a consequence ofthe translation symmetry of bulk crys-tals, and this symmetry is broken inQDs (the electrons feel the dot’sboundaries). Therefore, translational-momentum conservation does not

apply to QDs, so the probability ofAuger effects is greatly enhanced.

Since Auger recombination and opti-cal gain develop from the same initialstate (that is, two e-h pairs in a dot), theAuger decay is unavoidable in theregime of optical amplification and willalways impose an intrinsic limit onoptical gain lifetimes. In CdSe NQDs,for example, Auger recombinationleads to the deactivation of doublyexcited nanoparticles on time scalesfrom approximately 400 picoseconds toapproximately 10 picoseconds, depend-ing on the dot size (the smaller the dot,the faster the recombination). Thesetime scales are significantly shorterthan the time of the radiative decay(approximately 20 to 30 nanoseconds),which obviously should hinder thedevelopment of lasing.

QD Solids: A New Type ofLasing Medium

We realized the hindering role ofAuger recombination only toward theend of 1999, after we had conducteddetailed studies of multiparticledynamics in CdSe NQDs (Klimov etal. 2000a). Soon after, we also realizedhow to overcome this problem. Optical



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Figure 4. Nonradiative Multiparticle AugerRecombination in NQDs(a) In NQDs, the lowest optical transition can be approximated by atwo-level system that has two electrons in the ground state. Whenboth electrons are excited, a population inversion occurs, and theNQD can exhibit optical gain. An incoming photon stimulates one electron to decay, producing an extra photon. (b) The two-electronexcited state also allows for a loss mechanism called nonradiative Auger recombination, whereby the energy from e-h recombina-tion is not released as a photon but is transferred to either an electron or a hole. (c) Experiments show that the smaller the dot, theshorter the Auger recombination time (τ2). Even the largest dot has a significantly shorter τ2 than the radiative decay time.



gain relies on the effect of stimulatedemission, the rate of which can beenhanced by simply increasing theconcentration of NQDs in the sample.

We estimated that the stimulatedemission rate would exceed the Augerdecay rate in a medium with NQD fillingfactors of 0.2 to 1 percent (Klimov et al.2000b). Such densities are readilyachieved in close-packed NQD films(also known as NQD solids). For exam-ple, NQDs capped with trioctylphos-phine oxide (TOPO) will self-assembleinto a thin film that can have filling fac-tors as high as 20 percent, well above theestimated critical loading required for thedevelopment of stimulated emission.1

We demonstrated optical gain forthe first time by using close-packed,

matrix-free films of CdSe NQDs(Klimov et al. 2000b). In these experi-ments, the NQD samples were opti-cally excited by the output of anamplified titanium:sapphire pumplaser (see Figure 5). At pump intensi-ties of approximately 8 milliwatts, weobserved the development of a sharp,amplified spontaneous emission(ASE) peak situated on the low-ener-gy side of the spontaneous emissionband. The dependence of this peak onthe pump-laser intensity showed athreshold behavior that was a clearsignature of the transition to the opti-cal-amplification regime. We alsoconfirmed that the frequency of theASE peak changed with the size ofthe dot. Because of strong quantum

confinement, the peak in the smallestdots was blue-shifted with respect tothat in bulk CdSe by more than0.5 electron volt.

In order to slow down the NQDdegradation that results when the sam-ple heats up, we performed initialexperiments at cryogenic tempera-tures. More recently, after improvingthe optical quality of our NQD solids,we were also able to demonstrateoptical gain in NQDs at room temper-ature (Mikhailovsky et al. 2002).

Interestingly, the same pump flu-

Figure 5. Observation of Amplified Spontaneous Emission(a) This is a typical transmission electron microscopy (TEM) image of a matrix-free NQD solid film.The black dots are the semiconduc-tor cores, whereas the space between the dots is taken up by the capping molecules. (b) The figure shows images of three CdSe NQDsolid films taken under ultraviolet illumination.The films are fabricated from dots whose radii are 1.2, 1.5, and 2.1 nm. If the TOPO hasa 1.1-nm length, these films have filling factors ranging from ~17% to ~26%. (c) This illustration shows our experimental setup.Thecylindrical lens focuses the pump beam into a stripe on the NQD film.The ASE was detected at the edge of the film, which acted as anoptical waveguide. (d) As the intensity of the pump beam increased, a sharp ASE band developed in the emission spectra of the NQDfilm. (Inset) The intensity of the ASE peak (circles) rose sharply once a pump laser intensity of 8 mW was reached, indicating the startof stimulated emission and optical gain (the NQD radius was 2.1 nm, and the sample temperature, T = 80 K).The open squares showthe sublinear dependence of the emission intensity outside the sharp ASE peak.

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1Another approach to achieving high-den-sity NQD materials is to incorporate theNQDs into transparent sol-gel matrices.See Sundar et al. 2002 and Petruska et al.2003 for details.

ences (number of photons per pulse percentimeter squared) that were used toexcite room temperature ASE in CdSeNQDs were not sufficient to producelight amplification in bulk CdSe sam-ples. The reason is that light amplifica-tion in bulk CdSe can be due to bothlow-threshold excitonic and high-threshold e-h plasma mechanisms.Excitons are bound states of e-h pairsthat are “naturally” confined in spacebecause of the Coulomb attractionbetween opposite charges. The e-hinteraction energy in the exciton(approximately 16 meV in bulk CdSe)provides a barrier for the re-excitationof electrons and holes into the “dense”continuum of unbound e-h pair states.The existence of this “natural” barrierreduces the threshold for the “exciton-ic” optical gain compared to that forunbound charge carriers. However, atroom temperature, excitons dissociatebecause of large electron thermal ener-gies (approximately 24 meV). This

process quenches the exciton-relatedgain and results in a significantlyincreased ASE threshold. Because ofthe large interlevel spacing in NQDs,“quantum-confined” excitons are morerobust than bulk excitons, allowing oneto excite room temperature ASE atpump levels comparable to those atcryogenic temperatures. This is anillustrative example of enhanced tem-perature stability in lasing applicationsexpected for strongly confined dots.

In order to demonstrate true lasingaction, the NQD gain medium must becombined with an optical cavity thatprovides efficient positive feedback.Figure 6 shows one example of a “laserfabricated in a beaker” that we madeby incorporating NQD solids (Klimovet al. 2001, Malko et al. 2002) into amicrocapillary tube. The cylindricalmicrocavity can support two types ofoptical modes: planar waveguide-likemodes that develop along the tubelength, and whispering gallery (WG)

modes that develop (because of totalinternal reflection) around the innercircumference of the tube. The modespropagating along the tube can onlyachieve the ASE regime because nooptical feedback is present. The WGmodes can support a true lasing action(microring lasing). After severalattempts, we were able to uniformlyfill the interior of the tube with theNQDs and achieved the first occur-rence of NQD lasing (Klimov et al.2001, Malko et al. 2002). Several typesof cavities have since been utilized todemonstrate NQD lasing, includingpolystyrene microspheres (Klimov andBawendi 2001), and distributed-feed-back resonators (Eisler et al. 2002).

Outlook

The first demonstrations of NQDlasing devices indicate a high poten-tial for NQD materials to be new



Figure 6. NQD Lasing(a) This microphotograph of an NQD microcavity fabricated by incorporation of an NQD solid layer into a microcapillary tube wastaken under ultraviolet illumination (the NQD layer on the inner side of the tube appears pink). (b) A cross-sectional view of the NQDmicrocavity illustrates an optical path of a WG mode. (c) This plot shows the development of lasing into sharp WG modes.The spec-tra are taken at higher and higher pump fluences. Lasing into a single, sharp WG mode develops at ~1 mJ cm–2.The position of thismode (612.0 nm) corresponds to the optical gain maximum (Malko et al. 2002). As the pump fluence is further increased, additionalWG modes develop on the low-energy side of the 612.0-nm mode.The insets show a schematic and photo of the laser setup.

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types of lasing media, characterizedby wide-ranging color tunability, hightemperature stability, and chemicalflexibility. Thus far, we have onlyachieved lasing action by using apump laser to create the populationinversion in NQDs. An important con-ceptual challenge, however, awaits usin the area of electrical injectionpumping. Currently, our lasing mediaconsist of NQDs suspended in a non-conducting matrix, and it is not possi-ble to excite the dots electrically.

One possible strategy to achieveelectrical injection is by combining“soft” colloidal fabrication methodswith traditional, epitaxial crystal-growing techniques and incorporatedots into high-quality injection layersof wide gap semiconductors. A possi-ble technique that is “gentle” enoughto be compatible with colloidal dots isenergetic neutral-atom-beam epitaxy.This method utilizes a beam of neutralatoms carrying significant kineticenergy of several electron volts. Thebeam energy is sufficient for the acti-vation of nonthermal surface chemicalreactions, eliminating the need to heatthe substrate in order to grow high-quality films for NQD encapsulation.

Because of Auger recombination,however, electrical pumping of NQDlasing devices would still be signifi-cantly more difficult than pumping ofsimple, “nonlasing” light emitters.Interestingly, there is a possibleapproach to completely eliminateAuger recombination from NQDs. Itstems from the realization that theoptical-gain requirement of two e-hpairs (the same initial state that allowsAuger recombination to occur) is aconsequence of the electron-spindegeneracy of the lowest emittingtransition. Two electrons occupy thesame ground state; therefore, bothmust be excited to achieve a popula-tion inversion. If the ground-statedegeneracy could be broken (perhapsthrough interactions with magneticimpurities) the gain can, in principle,

be realized with a single e-h pair, andAuger decay would no longer be aproblem for either optically or electri-cally pumped NQDs. �

Acknowledgments

I would like to acknowledge contribu-tions of Alexandre A. Mikhailovsky,Jennifer A. Hollingsworth, Melissa A.Petruska, Anton V. Malko, and HanHtoon to the work reviewed here. Ialso thank Jay Schecker and NeciaCooper for a thorough editorial workon the manuscript. This work was sup-ported by Los Alamos DirectedResearch and Development Funds andthe U. S. Department of Energy,Office of Sciences, Division ofChemical Sciences.

Further Reading

Arakawa, Y., and H. Sakaki. 1982.Multidimensional Quantum Well Laser andTemperature Dependence of its ThresholdCurrent. Appl. Phys. Lett. 40 (11): 939.

Efros, Al. L., and A. L. Efros. 1982. PioneeringEffort I. Sov. Phys. Semicond. 16: 772.

Eisler, H.-J., V. C. Sundar, M. G. Bawendi, M.Walsh, H. I. Smith, and V. I. Klimov. 2002.Color-Selective Semiconductor NanocrystalLaser. Appl. Phys. Lett. 80 (24): 4614.

Klimov, V. I., and M. G. Bawendi. 2001.Ultrafast Carrier Dynamics, OpticalAmplification, and Lasing in NanocrystalQuantum Dots. MRS Bulletin 26 (12): 998.

Klimov, V. I., A. A. Mikhailovsky, J. A.Hollingsworth, A. Malko, C. A. Leatherdale,H.-J. Eisler, and M. G. Bawendi. 2001.“Stimulated Emission and Lasing inNanocrystal Quantum Dots.” In QuantumConfinement: Nanostructured Materials andDevices. Proceedings of the ElectrochemicalSociety 19, 321. Edited by M. Cahay, J. P.Leburton, D. J. Lockwood, S.Bandyopadhyay, and J. S. Harris.

Klimov, V. I., A. A. Mikhailovsky, D. W.McBranch, C. A. Leatherdale, and M. G.Bawendi. 2000. Quantization ofMultiparticle Auger Rates in SemiconductorQuantum Dots. Science 287: 1011.

Klimov, V. I., A. A. Mikhailovsky, Su Xu, A.Malko, J. A. Hollingsworth, C. A.Leatherdale et al. 2000. Optical Gain and

Stimulated Emission in Nanocrystal QuantumDots. Science 290: 314.

Ledentsov, N. N., V. M. Ustinov, A. Yu. Egorov,A. E. Zhukov, M. V. Maksimov, I. G.Tabatadze, and P. S. Koplev. 1994. OpticalProperties of Heterostructures withInGaAs–GaAs Quantum Clusters. Semicond.28 (8): 832.

Malko, A. V., A. A. Mikhailovsky, M. A.Petruska, J. A. Hollingsworth, H. Htoon, M.G. Bawendi, and V. I. Klimov. 2002. FromAmplified Spontaneous Emission toMicroring Lasing Using NanocrystalQuantum Dot Solids. Appl. Phys. Lett. 81(7): 1303.

Mikhailovsky, A. A., A. V. Malko, J. A.Hollingsworth, M. G. Bawendi, and V. I.Klimov. 2002. Multiparticle Interactions andStimulated Emission in ChemicallySynthesized Quantum Dots. Appl. Phys. Lett.80 (13): 2380.

Murray, C. B., D. J. Norris, and M. G. Bawendi.1993. Synthesis and Characterization ofNearly Monodisperse CdE (E = S, Se, Te)Semiconductor Nanocrystallites. J. Am.Chem. Soc. 115: 8706.

Petruska, M. A., A. V. Malko, P. M. Voyles, andV. I. Klimov. 2003. High-Performance,Quantum Dot Nanocomposites for NonlinearOptical and Optical Gain Applications. Adv.Mater. 15 (7-8): 610.

Sundar, V. C., H.-J. Eisler, and M. Bawendi.2002. Room-Temperature, Tunable GainMedia from Novel II-VI Nanocrystal-TitaniaComposite Matrices. Adv. Mater. 14 (10):739.

Vandyshev, Yu. V., V. S. Dneprovskii, V. I.Klimov, and D. K. Okorokov. 1991. Lasingon a Transition Between Quantum-WellLevels in a Quantum Dot. JETP Lett. 54 (8):442.

Victor Klimov has been at Los Alamos since1995. As a team leader in the ChemistryDivision, Victor directsresearch projects on thedevelopment, advancedcharacterization, andapplications of soft-matternanostructures based onsemiconductor colloidalnanoparticles (quantumdots). Victor received hisM.S., Ph.D., and D.Sci. degrees in 1978, 1981,and 1993, respectively, from Moscow StateUniversity. Over the years, he held jointappointments at Moscow State University andMoscow Institute of Geodesy and was a visit-ing professor at Humboldt University inBerlin, the Technical University of Aachen,Germany, and the Madrid AutonomousUniversity, Spain.

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Nanocrystal Quantum Dots - Federation of American Scientists