Nanocrystal Quantum Dots
From fundamental photophysics to multicolor lasing
Victor I. Klimov
emiconductor lasers are ubiqui- (a) CdSe Bulk Semiconductor (b) CdSe Quantum Dot (QD)
tous in modern society and play a 1D(e)
key role in technologies ranging
from CD players to optical telecommu- 1P(e)
nications. Current-generation lasers
have high power output and low lasing Conduction 1S(e)
thresholds, are stable over a wide range band
of temperatures, and are cheap and easy
to produce. Still, there is room for
improvement. We are developing a new Eg(bulk) Eg(QD)
type of laser based on ultrasmall bits of
semiconductor material called quantum
dots (QDs). Consisting of only a few Valence
hundred to a few hundred thousand band
atoms, QDs bridge the gap between the
solid state and single atoms, and hence
these specks of matter exhibit a mix of (c) (d)
solid-state and atomic properties. In our h π
E g (QD) ≈ E g0 +
work, we concentrate on nanoparticles 2m ehR 2
that are synthesized by colloidal chem- 1D
m em h
istry, and therefore, they are often m eh =
me + mh 1P
called colloidal or nanocrystal QDs
(NQDs). Interestingly, the emission me = effective electron mass
wavelength (that is, the emission color) Photon energy
of QDs depends on the dot size, and in mh = effective hole mass Eg(bulk) Eg(QD)
the case of semiconductor nanocrystals,
color can be controlled precisely Figure 1. Quantum Dots (QDs)
through simple chemistry. We are there- (a) A bulk semiconductor such as CdSe has continuous conduction and valence
fore developing an altogether new type energy bands separated by a “fixed” energy gap, Eg(bulk). Electrons normally occupy
of color-selectable lasing medium. all states up to the edge of the valence band, whereas states in the conduction band
are empty. (b) A QD is characterized by discrete atomic-like states with energies that
Although this paper focuses on our
are determined by the QD radius R. These well-separated QD states can be labeled
NQD laser work, quantum dots are with atomic-like notations, such as 1S, 1P, and 1D. (c) The expression for the size-
“bigger” than lasers. Because of their dependence separation between the lowest electron and hole QD states—Eg(QD), the
small dimensions and size-controlled QD energy gap—was obtained with the spherical “quantum box” model. (d) This
electronic spectra, NQDs can be schematic represents the continuous absorption spectrum of a bulk semiconductor
viewed as tunable artificial atoms with (black line) compared with a discrete absorption spectrum of a QD (colored bars).
214 Number 28 2003 Los Alamos Science
Nanocrystal Quantum Dots
(a) Me2Cd + Se(TOP) (b) TOPO ligand (c) Radius (nm)
0.9 1.4 1.9 2.4
P O= P
O= CdSe O=P
TOPO at 360°C CdSe-TOPO/TOP
Figure 2. Nanocrystal Quantum Dots (NQDs)
(a) An organometallic method is used for the fabrication of highly monodisperse CdSe NQDs. Nucleation and subsequent growth
of 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 heat
source stops the reaction. As a result, NQDs of a particular size form. (b) The colloidal NQDs obtained by the method illustrated
in (a) consist of an inorganic CdSe core capped with a layer of TOPO/TOP molecules. (c) Solutions of CdSe NQDs of different
radii, under ultraviolet illumination, emit different colors because of the quantum size effect. A 2.4-nm-radius dot has an energy
gap 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.
properties that can be engineered to width is a fixed parameter determined of energy states leads to a discrete
suit either the needs of a certain by the material’s identity. absorption spectrum of QDs, which is
experiment or a specific technological The situation changes, however, in in contrast to the continuous absorp-
application. When coated with a suit- the case of nanoscale semiconductor tion spectrum of a bulk semiconductor
able, chemically active surface layer, particles with sizes smaller than about (see Figure 1).
NQDs can be coupled to each other or 10 nanometers. This size range corre- The NQDs discussed earlier are
to different inorganic or organic enti- sponds to the regime of quantum con- small quantum dots that are made by
ties and thus serve as useful optical finement, for which the spatial extent organometallic chemical methods and
tags. We can now chemically manipu- of the electronic wave function is are composed of a semiconductor core
late NQDs almost as well as standard comparable with the dot size. As a capped with a layer of organic mole-
molecules, and can assemble them result of these “geometrical” con- cules (Murray et al. 1993). (See
into close-packed ordered or disor- straints, electrons “feel” the presence Figure 2.) The organic capping pre-
dered arrays that mimic naturally of the particle boundaries and respond vents uncontrolled growth and
occurring solids. Furthermore, to changes in particle size by adjust- agglomeration of the nanoparticles. It
because their dimensions, shapes, and ing their energy. This phenomenon is also allows NQDs to be chemically
surface properties can be manipulated known as the quantum-size effect, and manipulated as if they were large mol-
with ease, NQDs are ideally suited to it plays a very important role in QDs. ecules, with solubility and chemical
serve as nanoscale laboratories for In the first approximation, the reactivity determined by the identity
studies of fundamental quantum quantum-size effect can be described of the organic molecules. The capping
mechanical effects. by a simple “quantum box” model also provides “electronic” passivation
(Efros and Efros 1982), in which the of NQDs; that is, it terminates dan-
electron motion is restricted in all gling bonds that remain on the semi-
The Quantum Size Effect three dimensions by impenetrable conductor’s surface. As discussed
and QDs walls. For a spherical QD with radius below, the unterminated dangling
R, this model predicts that a size- bonds can affect the NQD’s emission
One of the defining features of a dependent contribution to the energy efficiency because they lead to a loss
semiconductor is the energy gap sepa- gap is simply proportional to 1/R2, mechanism wherein electrons are rap-
rating the conduction and valence implying that the gap increases as the idly trapped at the surface before they
energy bands. The color of light emit- QD size decreases. In addition, quan- have a chance to emit a photon. Using
ted by the semiconductor material is tum confinement leads to a collapse of colloidal chemical syntheses, one can
determined by the width of the gap. In the continuous energy bands of a bulk prepare NQDs with nearly atomic pre-
semiconductors of macroscopic material into discrete, atomic-like cision; their diameters range from
sizes—bulk semiconductors—the gap energy levels. The discrete structure nanometers to tens of nanometers and
Number 28 2003 Los Alamos Science 215
Nanocrystal Quantum Dots
(a) Band Edge Transition (b) Stimulated (c) Transparency (d) Optical Gain
size dispersions as narrow as 5 per- Emission
cent. Because of the quantum-size Electron Relaxation
effect, this ability to tune the NQD Conduction
size translates into a means of control- band or
ling various NQD properties, such as Band
emission and absorption wavelengths. edge
The emission of cadmium-selenium
(CdSe) NQDs, for example, can be
tuned from deep red to blue by a band
reduction in the dot radius from 5 Hole
nanometers to 0.7 nanometer.
Figure 3. Laser Basics
(a) “Pumping” energy into a semiconductor can excite an electron, e, into the con-
Nanocrystal Lasers: duction band. That electron leaves behind a hole, h, in the normally filled valence
Advantages and Problems band, and thus an e-h pair is created. The electron and hole each relax to the
respective band-edge states by nonradiative processes. During the band-edge tran-
Lasers made from bulk semicon- sition, a photon is emitted as the excited electron spontaneously recombines with
ductor materials have been used for the hole. (b) Stimulated emission occurs when a photon induces the excited elec-
several decades. (Laser fundamentals tron to decay. The emitted photon has the exact frequency, phase, and polarization
are described in Figure 3.) Although of the initial photon. (c) For a ground state that contains two electrons, exciting
only one electron (population equality) can lead to two equally probable outcomes:
numerous advances were made
The incoming photon stimulates the excited electron to decay, producing an extra
throughout those years, laser perform-
photon (left), or the photon excites the ground-state electron and is absorbed (right).
ance improved dramatically with the There is no net gain or loss of photons. In this case, the medium is in the trans-
introduction of so-called quantum parency regime. (d) Optical gain can occur if there are more electrons in the excited
well lasers, in which charge carriers— state than in the ground state (population inversion) because photon absorptionis
electrons and holes—were confined to inhibited. If a population inversion is established in a bulk system and if the gain from
move in a plane—that is, they were stimulated emission is larger than losses that absorb or scatter photons, the system
free to move in a two-dimensional (2- will exhibit amplified spontaneous emission (ASE). In a laser, an ASE-capable
D) quantum well. Compared with medium is placed in a reflecting cavity, and thus the photon field builds on itself.
bulk semiconductors, the quantum
well has a higher density of electronic QD laser would have a temperature- 10 nanometers in diameter. In this size
states near the edges of the conduc- insensitive lasing threshold at an exci- range, spacing between electronic lev-
tion and valence bands, and therefore tation level of only one electron-hole els can exceed hundreds of milli-elec-
a higher concentration of carriers can (e-h) pair per dot. tron-volts (meV), a much larger value
contribute to the band-edge emission. Lasing in QDs was first reported in than the room temperature energy
Consequently, it takes less intense 1991 (Vandyshev et al. 1991) and was scale of about 24 meV. Size-con-
“pumping” of energy into a quantum- achieved in an optically pumped trolled spectral tunability over an
well laser to get it to lase (the lasing device with relatively large (approxi- energy range of 1 electron volt was
threshold is lower). Additionally, mately 10-nanometer) CdSe nanopar- expected. However, after a decade of
quantum-well lasers show improved ticles. The QDs were fabricated by research that provided some tantaliz-
temperature stability and a narrower high-temperature precipitation in ing hints of optical gain, NQDs failed
emission line. molten glass. Later, lasing was also to demonstrate lasing action.
In QDs, the charge carriers are observed for QDs grown by epitaxial The failures were often attributed
confined in all three dimensions, with techniques (Ledentsov et al. 1994). As to material defects or dangling bonds
the result that the electrons exhibit a expected, the QD lasers showed an on the surface of the NQDs, which
discrete atomic-like energy spectrum. improved performance and featured a were a natural consequence of the
In very small QDs, the spacing lower lasing threshold and enhanced large surface-to-volume ratio of the
between these atomic-like states is temperature stability by comparison sub-10-nanometer particles. The
greater than the available thermal with quantum-well lasers. defects lead to electronic states that
energy, so thermal depopulation of These successes provided us with lie within the material’s energy gap.
the lowest electronic states is inhibit- strong motivation for the development Electrons can relax into those states,
ed. It was therefore anticipated that a of lasers based on NQDs less than whereupon they typically undergo
216 Los Alamos Science Number 28 2003
Nanocrystal Quantum Dots
(a) Stimulated Emission (b) Auger Recombination (c)
Number of doubly excited NQDs
0.01 R (nm) τ2 (ps)
Figure 4. Nonradiative Multiparticle Auger 2.77 147
Recominbation in NQDs 0.001
(a) In NQDs, the lowest optical transition can be approximated by a 0 50 100 150 200 250 300 350
two-level system that has two electrons in the ground state. When Time (ps)
both electrons are excited, a population inversion occurs, and the
NQD can exhibit optical gain. An incoming photon stimulates one electron to decay, producing an extra photon. (b) The two-electron
excited 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, the
shorter the Auger recombination time (τ2). Even the largest dot has a significantly shorter τ2 than the radiative decay time.
either nonradiative or radiative (in-gap ground state. In small dots, the lowest apply to QDs, so the probability of
“deep-trap” emission) decay to the “emitting” transition can be treated as Auger effects is greatly enhanced.
ground state. Thus the surface defects a two-level system that contains two Since Auger recombination and opti-
introduce carrier losses that inhibit the electrons in its ground state. To invert cal gain develop from the same initial
optical gain. Another concern raised such a system, one has to promote state (that is, two e-h pairs in a dot), the
in several theoretical papers was the both electrons from the ground to the Auger decay is unavoidable in the
reduced efficiency of electron-phonon excited state, meaning that optical regime of optical amplification and will
interactions that results from the dis- gain in QDs originate from nanoparti- always impose an intrinsic limit on
crete, atomic-like energy structures, cles that contain two e-h pairs (doubly optical gain lifetimes. In CdSe NQDs,
an effect that reduces the ability of excited nanoparticles). for example, Auger recombination
carriers to enter into the band-edge Paradoxically, whereas the intrinsic leads to the deactivation of doubly
states and hence reduces lumines- decay of singly excited QDs is due to excited nanoparticles on time scales
cence efficiencies. However, our the e-h recombination and the emis- from approximately 400 picoseconds to
research team eventually realized that sion of a photon, the deactivation of approximately 10 picoseconds, depend-
the main difficulty in getting our two e-h pair states is dominated by ing on the dot size (the smaller the dot,
ultrasmall NQDs to lase stemmed nonradiative Auger recombination the faster the recombination). These
from a largely unforeseen problem (Klimov et al. 2000a). In the latter time scales are significantly shorter
known as multiparticle Auger recom- case, the e-h recombination energy is than the time of the radiative decay
bination (Klimov et al. 2000). not released as a photon but is trans- (approximately 20 to 30 nanoseconds),
ferred to a third particle (an electron which obviously should hinder the
or a hole) that is re-excited to a higher development of lasing.
Multiparticle Auger energy state (see Figure 4). Auger
Recombination vs recombination has a relatively low
Optical Gain efficiency in bulk semiconductors QD Solids: A New Type of
because of restrictions imposed by Lasing Medium
As in the case of other lasing energy and momentum conservation.
media, QDs require a population But linear, or translational, momen- We realized the hindering role of
inversion in order to produce optical tum conservation is a consequence of Auger recombination only toward the
gain (refer to Figure 3). The popula- the translation symmetry of bulk crys- end of 1999, after we had conducted
tion inversion corresponds to the situ- tals, and this symmetry is broken in detailed studies of multiparticle
ation in which the number of elec- QDs (the electrons feel the dot’s dynamics in CdSe NQDs (Klimov et
trons in a high-energy excited state is boundaries). Therefore, translational- al. 2000a). Soon after, we also realized
greater than that in the low-energy momentum conservation does not how to overcome this problem. Optical
Number 28 2003 Los Alamos Science 217
Nanocrystal Quantum Dots
10 2.5 mW
Emission intensity (a.u.)
Emission intensity (a.u.)
R = 1.2 nm 1.5 nm 2.1 nm
(c) Laser 4 8 12
Slit Pump intensity (mW)
beam Cylindrical 4
1.9 2.0 2.1 2.2 2.3 2.4
detector Sample Photon energy (eV)
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 NQD
solid 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 has
a 1.1-nm length, these films have filling factors ranging from ~17% to ~26%. (c) This illustration shows our experimental setup. The
cylindrical 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 an
optical waveguide. (d) As the intensity of the pump beam increased, a sharp ASE band developed in the emission spectra of the NQD
film. (Inset) The intensity of the ASE peak (circles) rose sharply once a pump laser intensity of 8 mW was reached, indicating the start
of stimulated emission and optical gain (the NQD radius was 2.1 nm, and the sample temperature, T = 80 K). The open squares show
the sublinear dependence of the emission intensity outside the sharp ASE peak.
gain relies on the effect of stimulated matrix-free films of CdSe NQDs confinement, the peak in the smallest
emission, the rate of which can be (Klimov et al. 2000b). In these experi- dots was blue-shifted with respect to
enhanced by simply increasing the ments, the NQD samples were opti- that in bulk CdSe by more than
concentration of NQDs in the sample. cally excited by the output of an 0.5 electron volt.
We estimated that the stimulated amplified titanium:sapphire pump In order to slow down the NQD
emission rate would exceed the Auger laser (see Figure 5). At pump intensi- degradation that results when the sam-
decay rate in a medium with NQD filling ties of approximately 8 milliwatts, we ple heats up, we performed initial
factors of 0.2 to 1 percent (Klimov et al. observed the development of a sharp, experiments at cryogenic tempera-
2000b). Such densities are readily amplified spontaneous emission tures. More recently, after improving
achieved in close-packed NQD films (ASE) peak situated on the low-ener- the optical quality of our NQD solids,
(also known as NQD solids). For exam- gy side of the spontaneous emission we were also able to demonstrate
ple, NQDs capped with trioctylphos- band. The dependence of this peak on optical gain in NQDs at room temper-
phine oxide (TOPO) will self-assemble the pump-laser intensity showed a ature (Mikhailovsky et al. 2002).
into a thin film that can have filling fac- threshold behavior that was a clear Interestingly, the same pump flu-
tors as high as 20 percent, well above the signature of the transition to the opti-
estimated critical loading required for the cal-amplification regime. We also 1Another approach to achieving high-
development of stimulated emission.1 confirmed that the frequency of the density NQD materials is to incorporate
the NQDs into transparent sol-gel matri-
We demonstrated optical gain for ASE peak changed with the size of ces. See Sundar et al. 2002 and Petruska
the first time by using close-packed, the dot. Because of strong quantum et al. 2003 for details.
218 Los Alamos Science Number 28 2003
Nanocrystal Quantum Dots
(a) (c) 180
2.8 mJ/cm2 Capillary
Emission intensity (mW)
(b) laser light
WG mode Glass
80 µm 605 610 615 620 625 630
Figure 6. NQD Lasing
(a) This microphotograph of an NQD microcavity fabricated by incorporation of an NQD solid layer into a microcapillary tube was
taken under ultraviolet illumination (the NQD layer on the inner side of the tube appears pink). (b) A cross-sectional view of the
NQD microcavity illustrates an optical path of a WG mode. (c) This plot shows the development of lasing into sharp WG modes.
The spectra are taken at higher and higher pump fluences. Lasing into a single, sharp WG mode develops at ~1 mJ cm–2. The
position of this mode (612.0 nm) corresponds to the optical gain maximum (Malko et al. 2002). As the pump fluence is further
increased, additional WG modes develop on the low-energy side of the 612.0-nm mode. The insets show a schematic and photo
of the laser setup.
ences (number of photons per pulse per process quenches the exciton-related modes that develop (because of total
centimeter squared) that were used to gain and results in a significantly internal reflection) around the inner
excite room temperature ASE in CdSe increased ASE threshold. Because of circumference of the tube. The modes
NQDs were not sufficient to produce the large interlevel spacing in NQDs, propagating along the tube can only
light amplification in bulk CdSe sam- “quantum-confined” excitons are more achieve the ASE regime because no
ples. The reason is that light amplifica- robust than bulk excitons, allowing one optical feedback is present. The WG
tion in bulk CdSe can be due to both to excite room temperature ASE at modes can support a true lasing action
low-threshold excitonic and high- pump levels comparable to those at (microring lasing). After several
threshold e-h plasma mechanisms. cryogenic temperatures. This is an attempts, we were able to uniformly
Excitons are bound states of e-h pairs illustrative example of enhanced tem- fill the interior of the tube with the
that are “naturally” confined in space perature stability in lasing applications NQDs and achieved the first occur-
because of the Coulomb attraction expected for strongly confined dots. rence of NQD lasing (Klimov et al.
between opposite charges. The e-h In order to demonstrate true lasing 2001, Malko et al. 2002). Several types
interaction energy in the exciton action, the NQD gain medium must be of cavities have since been utilized to
(approximately 16 meV in bulk CdSe) combined with an optical cavity that demonstrate NQD lasing, including
provides a barrier for the re-excitation provides efficient positive feedback. polystyrene microspheres (Klimov and
of electrons and holes into the “dense” Figure 6 shows one example of a “laser Bawendi 2001), and distributed-feed-
continuum of unbound e-h pair states. fabricated in a beaker” that we made back resonators (Eisler et al. 2002).
The existence of this “natural” barrier by incorporating NQD solids (Klimov
reduces the threshold for the “exciton- et al. 2001, Malko et al. 2002) into a
ic” optical gain compared to that for microcapillary tube. The cylindrical Outlook
unbound charge carriers. However, at microcavity can support two types of
room temperature, excitons dissociate optical modes: planar waveguide-like The first demonstrations of NQD
because of large electron thermal ener- modes that develop along the tube lasing devices indicate a high poten-
gies (approximately 24 meV). This length, and whispering gallery (WG) tial for NQD materials to be new
Number 28 2003 Los Alamos Science 219
Nanocrystal Quantum Dots
types of lasing media, characterized be realized with a single e-h pair, and Quantum Dots. Science 290: 314.
Ledentsov, N. N., V. M. Ustinov, A. Yu. Egorov,
by wide-ranging color tunability, high Auger decay would no longer be a
A. E. Zhukov, M. V. Maksimov, I. G.
temperature stability, and chemical problem for either optically or electri- Tabatadze, and P. S. Koplev. 1994. Optical
flexibility. Thus far, we have only cally pumped NQDs. Properties of Heterostructures with
achieved lasing action by using a InGaAs–GaAs Quantum Clusters. Semicond.
pump laser to create the population 28 (8): 832.
Malko, A. V., A. A. Mikhailovsky, M. A.
inversion in NQDs. An important con- Acknowledgments Petruska, J. A. Hollingsworth, H. Htoon, M.
ceptual challenge, however, awaits us G. Bawendi, and V. I. Klimov. 2002. From
in the area of electrical injection I would like to acknowledge contribu- Amplified Spontaneous Emission to
pumping. Currently, our lasing media tions of Alexandre A. Mikhailovsky, Microring Lasing Using Nanocrystal
Quantum Dot Solids. Appl. Phys. Lett. 81
consist of NQDs suspended in a non- Jennifer A. Hollingsworth, Melissa A.
conducting matrix, and it is not possi- Petruska, Anton V. Malko, and Han Mikhailovsky, A. A., A. V. Malko, J. A.
ble to excite the dots electrically. Htoon to the work reviewed here. I Hollingsworth, M. G. Bawendi, and V. I.
One possible strategy to achieve also thank Jay Schecker and Necia Klimov. 2002. Multiparticle Interactions and
electrical injection is by combining Cooper for a thorough editorial work Stimulated Emission in Chemically
Synthesized Quantum Dots. Appl. Phys. Lett.
“soft” colloidal fabrication methods on the manuscript. This work was sup- 80 (13): 2380.
with traditional, epitaxial crystal- ported by Los Alamos Directed Murray, C. B., D. J. Norris, and M. G. Bawendi.
growing techniques and incorporate Research and Development Funds and 1993. Synthesis and Characterization of
dots into high-quality injection layers the U. S. Department of Energy, Nearly Monodisperse CdE (E = S, Se, Te)
Semiconductor Nanocrystallites. J. Am.
of wide gap semiconductors. A possi- Office of Sciences, Division of Chem. Soc. 115: 8706.
ble technique that is “gentle” enough Chemical Sciences. Petruska, M. A., A. V. Malko, P. M. Voyles, and
to be compatible with colloidal dots is V. I. Klimov. 2003. High-Performance,
energetic neutral-atom-beam epitaxy. Quantum Dot Nanocomposites for Nonlinear
Optical and Optical Gain Applications. Adv.
This method utilizes a beam of neutral Further Reading Mater. 15 (7-8): 610.
atoms carrying significant kinetic Sundar, V. C., H.-J. Eisler, and M. Bawendi.
Arakawa, Y., and H. Sakaki. 1982.
energy of several electron volts. The 2002. Room-Temperature, Tunable Gain
Multidimensional Quantum Well Laser and
beam energy is sufficient for the acti- Temperature Dependence of its Threshold
Media from Novel II-VI Nanocrystal-Titania
vation of nonthermal surface chemical Composite Matrices. Adv. Mater. 14 (10):
Current. Appl. Phys. Lett. 40 (11): 939.
reactions, eliminating the need to heat Efros, Al. L., and A. L. Efros. 1982. Pioneering
Vandyshev, Yu. V., V. S. Dneprovskii, V. I.
the substrate in order to grow high- Effort I. Sov. Phys. Semicond. 16: 772.
Klimov, and D. K. Okorokov. 1991. Lasing
Eisler, H.-J., V. C. Sundar, M. G. Bawendi, M.
quality films for NQD encapsulation. Walsh, H. I. Smith, and V. I. Klimov. 2002.
on a Transition Between Quantum-Well
Because of Auger recombination, Levels in a Quantum Dot. JETP Lett. 54 (8):
Color-Selective Semiconductor Nanocrystal
however, electrical pumping of NQD Laser. Appl. Phys. Lett. 80 (24): 4614.
lasing devices would still be signifi- Klimov, V. I., and M. G. Bawendi. 2001.
Ultrafast Carrier Dynamics, Optical
cantly more difficult than pumping of
Amplification, and Lasing in Nanocrystal
simple, “nonlasing” light emitters. Quantum Dots. MRS Bulletin 26 (12): 998. Victor Klimov has been at Los Alamos since
Interestingly, there is a possible Klimov, V. I., A. A. Mikhailovsky, J. A. 1995. As a team leader in the Chemistry
approach to completely eliminate Hollingsworth, A. Malko, C. A. Leatherdale, Division, Victor directs
H.-J. Eisler, and M. G. Bawendi. 2001. research projects on the
Auger recombination from NQDs. It development, advanced
“Stimulated Emission and Lasing in
stems from the realization that the Nanocrystal Quantum Dots.” In Quantum characterization, and
optical-gain requirement of two e-h Confinement: Nanostructured Materials and applications of soft-matter
pairs (the same initial state that allows Devices. Proceedings of the Electrochemical nanostructures based on
Society 19, 321. Edited by M. Cahay, J. P. semiconductor colloidal
Auger recombination to occur) is a
Leburton, D. J. Lockwood, S. nanoparticles (quantum
consequence of the electron-spin Bandyopadhyay, and J. S. Harris. dots). Victor received his
degeneracy of the lowest emitting Klimov, V. I., A. A. Mikhailovsky, D. W. M.S., Ph.D., and D.Sci. degrees in 1978, 1981,
transition. Two electrons occupy the McBranch, C. A. Leatherdale, and M. G. and 1993, respectively, from Moscow State
same ground state; therefore, both Bawendi. 2000. Quantization of University. Over the years, he held joint
Multiparticle Auger Rates in Semiconductor appointments at Moscow State University and
must be excited to achieve a popula- Moscow Institute of Geodesy and was a visit-
Quantum Dots. Science 287: 1011.
tion inversion. If the ground-state Klimov, V. I., A. A. Mikhailovsky, Su Xu, A. ing professor at Humboldt University in
degeneracy could be broken (perhaps Malko, J. A. Hollingsworth, C. A. Berlin, the Technical University of Aachen,
through interactions with magnetic Leatherdale et al. 2000. Optical Gain and Germany, and the Madrid Autonomous
impurities) the gain can, in principle, Stimulated Emission in Nanocrystal
220 Los Alamos Science Number 28 2003