Al. L. Efros

Quantum size effect in semiconductor microcrystals

A growth technique of the semiconductor microcrystals in a glassy dielectric matrix has been developed. This technique permits to vary the size of the grown microcrystals in a controlled manner from some tens to thousands of angstroms. The size dependence of absorption spectra of a number of I–VII and II–VI compounds grown by this technique have been studied. The size of the microcrystals being decreased, a considerable short-wave-length shift of the exciton lines and the fundamental absorption edge has been observed. This phenomenon is due to the size quantization of the free carrier and exciton energy spectra in the microcrystals.

Colloidal nanoplatelets with two-dimensional electronic structure

The syntheses of strongly anisotropic nanocrystals with one dimension much smaller than the two others, such as nanoplatelets, are still greatly underdeveloped. Here, we demonstrate the formation of atomically flat quasi-two-dimensional colloidal CdSe, CdS and CdTe nanoplatelets with well-defined thicknesses ranging from 4 to 11 monolayers. These nanoplatelets have the electronic properties of two-dimensional quantum wells formed by molecular beam epitaxy, and their thickness-dependent absorption and emission spectra are described very well within an eight-band Pidgeon-Brown model. They present an extremely narrow emission spectrum with full-width at half-maximum less than 40 meV at room temperature. The radiative fluorescent lifetime measured in CdSe nanoplatelets decreases with temperature, reaching 1 ns at 6 K, two orders of magnitude less than for spherical CdSe nanoparticles. This makes the nanoplatelets the fastest colloidal fluorescent emitters and strongly suggests that they show a giant oscillator strength transition.

Electron spin relaxation by nuclei in semiconductor quantum dots

We have studied theoretically electron spin relaxation in semiconductor quantum dots via interaction with nuclear spins. The relaxation is shown to be determined by three processes: (i) the precession of the electron spin in the hyperfine field of the frozen fluctuation of the nuclear spins; (ii) the precession of the nuclear spins in the hyperfine field of the electron; and (iii) the precession of the nuclear spin in the dipole field of its nuclear neighbors. In external magnetic fields the relaxation of electron spins directed along the magnetic field is suppressed. Electron spins directed transverse to the magnetic field relax completely in a time on the order of the precession period of its spin in the field of the frozen fluctuation of the nuclear spins. Comparison with experiment shows that the hyperfine interaction with nuclei may be the dominant mechanism of electron spin relaxation in quantum dots.

Breaking the phonon bottleneck in nanometer quantum dots: role of Auger-like processes

Abstract An Auger-like mechanism is described for the rapid transfer of the electron energy to the hole in the valence band via their Coulomb interaction in the nanometer size quantum dots. The relaxation time and its dependence on the nanocrystal energy band parameters and radius is obtained. A value of ∼ 2 ps has been calculated for the electron thermalization from the first excited to the ground state of spherical CdSe nanocrystals. Our results show that Auger-like processes remove the thermalization “phonon bottleneck” problem in nanometer quantum dots with level spacings greater than typical phonon energies.

Optical pumping of the electronic and nuclear spin of single charge-tunable quantum dots.

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.

Orbital mechanisms of electron-spin manipulation by an electric field.

A theory of spin manipulation of quasi-two-dimensional (2D) electrons by a time-dependent gate voltage applied to a quantum well is developed. The Dresselhaus and Rashba spin-orbit coupling mechanisms are shown to be rather efficient for this purpose. The spin response to a perpendicular-to-plane electric field is due to a deviation from the strict 2D limit and is controlled by the ratios of the spin, cyclotron, and confinement frequencies. The dependence of this response on the magnetic field direction is indicative of the strengths of the competing spin-orbit coupling mechanisms.

Electronic states and optical properties of PbSe nanorods and nanowires

A theory of the electronic-structure and excitonic absorption spectra of PbS and PbSe nanowires and nanorods in the framework of a four-band effective-mass model is presented. Calculations conducted for PbSe show that dielectric contrast dramatically strengthens the exciton binding in narrow nanowires and nanorods. However, the self-interaction energies of the electron and hole nearly cancel the Coulomb binding, and as a result the optical absorption spectra are practically unaffected by the strong dielectric contrast between PbSe and the surrounding medium. Measurements of the size-dependent absorption spectra of colloidal PbSe nanorods are also presented. Using room-temperature energy-band parameters extracted from the optical spectra of spherical PbSe nanocrystals, the theory provides good quantitative agreement with the measured spectra.

Size-dependent electronic level structure of InAs nanocrystal quantum dots: Test of multiband effective mass theory

The size dependence of the electronic spectrum of InAs nanocrystals ranging in radius from 10–35 A has been studied by size-selective spectroscopy. An eight-band effective mass theory of the quantum size levels has been developed which describes the observed absorption level structure and transition intensities very well down to smallest crystal size using bulk band parameters. This model generalizes the six-band model which works well in CdSe nanocrystals and should adequately describe most direct semiconductor nanocrystals with band edge at the Γ-point of the Brillouin zone.

Donor-like exciton in zero-dimension semiconductor structures

Abstract The absorption spectra of CuBr and CdS microcrystal have been investigated. The donor-like exciton with the hole location within a small region near the centre of microcrystal is found. The localization of the hole is due to the potential of electron localized by the boundaries of microcrystal. The quantum levels of hole produce the fine structure in the absorption spectra which has been observed.

Giant internal magnetic fields in Mn doped nanocrystal quantum dots

Abstract We observed a giant splitting of exciton spin sublevels in CdS nanocrystals, each doped on the average by one Mn ion. The splitting, which exists in zero external magnetic field, is caused by the gigantic internal magnetic field of the Mn ion and results from the enhancement of the short range spin–spin interactions in nanocrystal quantum dots. The splitting is seen in the strong magnetic circular dichroism of the CdS band edge transitions. The magnitude of the observed band edge splitting is in good agreement with a theoretical calculation of the effective magnetic field.