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.Comment: 35 pages, 12 figure

Frequency-dependent spontaneous emission rate from CdSe and CdTe nanocrystals: influence of dark states

We studied the rate of spontaneous emission from colloidal CdSe and CdTe nanocrystals at room temperature. The decay rate, obtained from luminescence decay curves, increases with the emission frequency in a supra-linear way. This dependence is explained by the thermal occupation of dark exciton states at room temperature, giving rise to a strong attenuation of the rate of emission. The supra-linear dependence is in agreement with the results of tight-binding calculations.Comment: 11 page

Efficient electron spin manipulation in a quantum well by an in-plane electric field

Electron spins in a semiconductor quantum well couple to an electric field {\it via} spin-orbit interaction. We show that the standard spin-orbit coupling mechanisms can provide extraordinary efficient electron spin manipulation by an in-plane ac electric field

Comment on "Self-Purification in Semiconductor Nanocrystals"

In a recent Letter [PRL 96, 226802 (2006)], Dalpian and Chelikowsky claimed that formation energies of Mn impurities in CdSe nanocrystals increase as the size of the nanocrystal decreases, and argued that this size dependence leads to "self-purification" of small nanocrystals. They presented density-functional-theory (DFT) calculations showing a strong size dependence for Mn impurity formation energies, and proposed a general explanation. In this Comment we show that several different DFT codes, pseudopotentials, and exchange-correlation functionals give a markedly different result: We find no such size dependence. More generally, we argue that formation energies are not relevant to substitutional doping in most colloidally grown nanocrystals.Comment: 1 page, 1 figur

Electron spin synchronization induced by optical nuclear magnetic resonance feedback

We predict a new physical mechanism explaining the electron spin precession frequency focusing effect observed recently in singly charged quantum dots exposed to a periodic train of resonant circularly polarized short optical pulses [A. Greilich et al, Science 317, 1896 (2007), Ref. 1]. We show that electron spin precession in an external magnetic field and a field of nuclei creates a Knight field oscillating at the frequency of nuclear spin resonance. This field drives the projection of the nuclear spin onto magnetic field to the value that makes the electron spin precession frequency a multiple of the train cyclic repetition frequency, which is the condition at which the Knight field vanishes.Comment: 4+ pages, 3 figure

Single-particle states in spherical Si/SiO2_2 quantum dots

We calculate ground and excited electron and hole levels in spherical Si quantum dots inside SiO$_2$ in a multiband effective mass approximation. Luttinger Hamiltonian is used for holes and the strong anisotropy of the conduction electron effective mass in Si is taken into account. As boundary conditions for electron and hole wave functions we use continuity of the wave functions and the velocity density at the boundary of the quantum dots.Comment: 8 pages, 5 figure

Anomalous circular polarization of magneto-photoluminescence from individual CdSe nanocrystals

We study the low-temperature magneto-photoluminescence (PL) from individual CdSe nanocrystals. Nanocrystals having a small "bright" exciton fine structure splitting ($<$0.5 meV) exhibit a conventional left- and right-circularly polarized Zeeman PL doublet in applied magnetic fields. In contrast, nanocrystals with large fine structure splitting ($>$1 meV) show an anomalous magneto-PL polarization, wherein the lower-energy peak becomes circularly polarized with increasing field, while the higher-energy peak remains linearly polarized. This unusual behavior arises from strong mixing between the absorbing and emitting bright exciton levels due to strong anisotropic exchange interactions.Comment: 15 pages, 3 figures; submitte