Current research into semiconductor clusters is focused on the properties of quantum dots-fragments of semiconductor consisting of hundreds to many thousands of atoms-with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Semiconductor Clusters, Nanocrystals, and Quantum Dots

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

We report a synthesis of highly luminescent (CdSe)ZnS composite quantum dots with CdSe cores ranging in diameter from 23 to 55 A. The narrow photoluminescence (fwhm ≤ 40 nm) from these composite dots spans most of the visible spectrum from blue through red with quantum yields of 30−50% at room temperature. We characterize these materials using a range of optical and structural techniques. Optical absorption and photoluminescence spectroscopies probe the effect of ZnS passivation on the electronic structure of the dots. We use a combination of wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, small and wide angle X-ray scattering, and transmission electron microscopy to analyze the composite dots and determine their chemical composition, average size, size distribution, shape, and internal structure. Using a simple effective mass theory, we model the energy shift for the first excited state for (CdSe)ZnS and (CdSe)CdS dots with varying shell thickness. Finally, we characterize the...

(CdSe)ZnS Core-Shell Quantum Dots - Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites

Semiconductor nanocrystals exhibit a wide range of size-dependent properties. Variations in fundamental characteristics ranging from phase transitions to electrical conductivity can be induced by controlling the size of the crystals. The present status and new opportunities for research in this area of materials physical chemistry are reviewed.

Perspectives on the Physical Chemistry of Semiconductor Nanocrystals

The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

http://science.sciencemag.org/content/sci/290/5490/314.full.pdf

Optical gain and stimulated emission in nanocrystal quantum dots.

Quantum dots of InP, GaP and GaInP{sub 2} with diameters ranging from 20-65 {Angstrom} were synthesized as well-crystallized nanoparticles with bulk zinc blende structure. The high sample quality of the InP and GaP QDs results in excitonic features in the absorption spectra. The GaP and GaInP{sub 2} QD colloids exhibited very intense (quantum yields of 15-25%) visible photoluminescence at room temperature. The photoluminescence for InP preparations showed two emission bands: one band in the visible at the band edge of the QD (50 nm Stokes shift), and a second band above 800 nm. The near-IR PL is attributed to deep traps, presumably phosphorus vacancies on the QD surface. This band disappears after activation of particles by controlled addition of etchant. In that case very intense band-edge emission (quantum yield 30% at room temperature and 60% at 10 K) was obtained. The QDs were characterized by TEM, SAXS, AFM, powder x-ray diffraction, steady state optical absorption and photoluminescence spectroscopy, ps to ns transient photoluminescence spectroscopy, and fs to ps pump-probe absorption (i.e., hole-burning) spectroscopy. Results will also be reported on efforts to produce ordered arrays of InP QDs.

Synthesis and Characterization of InP, GaP, and GaInP2 Quantum Dots

The coherent interaction of femtosecond laser pulses and thin CdSe and GaAs samples is investigated experimentally and theoretically Oscillatory structures in the differential probe transmission around the exciton resonance and around the pump frequency are observed when the probe pulse precedes the pump Comparison with theory attributes the oscillations to the coherent coupling between the light field and the electron-hole transitions in the semiconductor For nonresonant excitation, the oscillatory structures around the exciton are identified as the early stages of the optical Stark effect

Femtosecond Studies of Coherent Transients in Semiconductors

Femtosecond differential absorption measurements of the quantum-confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The hole width increases with pump intensity and the magnitude of the induced transparency saturates at the highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition. >

Femtosecond optical nonlinearities of CdSe quantum dots

By use of femtosecond differential absorption spectroscopy the electron-hole pair relaxation times have been determined in CdSe quantum dots with excited state energies larger than the LO-phonon energy. The fast energy relaxation within 500 fs is independent of the relation between level spacing and LO-phonon energy and shows no bottleneck effect. The population dynamics is mainly governed by the hole relaxation process. With increasing intensity, the formation of two-pair states (biexcitons) of various energies dominates the population dynamics due to the strong influence of Coulomb interaction in quantum dots even in the strong confinement. At high pair densities, the created two-pair states mediate a new relaxation path due to their stimulated decay into one photon and one electron-hole pair with lower energy.

Ultrafast energy relaxation in quantum dots

Microcrystallites of CdTe were grown in a special-formula glass, yielding CdTe dots; the linear-absorption spectrum exhibits as many as six quantum-confinement peaks. Excitation of the sample with pulses ranging from femtoseconds to microseconds has allowed us to distinguish between various mechanisms for optical nonlinearity. At very early times, phase-space filling and Coulomb interaction between the excited charged carriers is responsible for the absorption changes, \ensuremath{\delta}\ensuremath{\alpha}. At later times, Coulomb effects due to trapped carriers remain and last for nanoseconds or microseconds.

Brian Fluegel

Papers

Synthesis and Characterization of InP, GaP, and GaInP2 Quantum Dots

Femtosecond Studies of Coherent Transients in Semiconductors

Femtosecond optical nonlinearities of CdSe quantum dots

Ultrafast energy relaxation in quantum dots

State filling, Coulomb, and trapping effects in the optical nonlinearity of CdTe quantum dots in glass.