Zhibin Lin

Size Dependence of the Multiple Exciton Generation Rate in CdSe Quantum Dots

The multiplication rates of hot carriers in CdSe quantum dots are quantified using an atomistic pseudopotential approach and first-order perturbation theory. We consider both the case of an individual carrier (electron or hole) decaying into a trion and the case of an electron-hole pair decaying into a biexciton. The dependence on quantum dot volume of multiplication rate, density of final states, and effective Coulomb interaction are determined. We show that the multiplication rate of a photogenerated electron-hole pair decreases with dot size for a given absolute photon energy. However, if the photon energy is rescaled by the volume-dependent optical gap, then smaller dots exhibit an enhancement in carrier multiplication rate for a given relative photon energy. We find that holes have much higher multiplication rates than electrons of the same excess energy due to the larger density of final states (positive trions). When electron-hole pairs are generated by photon absorption, however, the net carrier multiplication rate is dominated by electrons because they have much higher excess energy on average. We also find, contrary to earlier studies, that the effective Coulomb coupling governing carrier multiplication is energy-dependent.

Efficient Exciton Transport Between Strongly Quantum-Confined Silicon Quantum Dots

Many-body Green function analysis and first-order perturbation theory are used to quantify the influence of size, surface reconstruction, and surface treatment on exciton transport between small silicon quantum dots. Competing radiative processes are also considered in order to determine how exciton transport efficiency is influenced. The analysis shows that quantum confinement causes small (~1 nm) Si quantum dots to exhibit exciton transport efficiencies far exceeding that of their larger counterparts for the same center-to-center separation. This surprising result offers the prospect of designing assemblies of quantum dots through which excitons can travel for long distances, a game-changing paradigm shift for next-generation solar energy harvesting. We also find that surface reconstruction significantly influences the absorption cross section and leads to a large reduction in both transport rate and efficiency. Further, exciton transport efficiency is higher for hydrogen-passivated dots as compared with those terminated with more electronegative ligands, a result not predicted by Förster theory.

Two-dimensional carbon semiconductor: Density functional theory calculations

We show that patterned defects can be used to disrupt the sub-lattice symmetry of graphene so as to open up a band gap. This way of modifying graphenes electronic structure does not rely on external agencies, the addition of new elements or special boundaries. The method is used to predict a planar, low energy, graphene allotrope with a band gap of 1.2 eV. This defect engineering also allows semiconducting ribbons of carbon to be fabricated within graphene. Linear arrangements of defects lead to naturally embedded ribbons of the semiconducting material in graphene, offering the prospect of two-dimensional circuit logic composed entirely of carbon.

First principles analysis of the initial oxidation of Si(001) and Si(111) surfaces terminated with H and CH3

Transition state analyses have been carried out within a density functional theory setting to explain and quantify the distinctly different ways in which hydrogen and methyl terminations serve to protect silicon surfaces from the earliest onset of oxidation. We find that oxidation occurs via direct dissociative adsorption, without any energy barrier, on Si(111) and reconstructed Si(001) that have been hydrogen terminated; oxidation initiates with a barrier of only 0.05 eV on unreconstructed Si(001). The commonly measured protection afforded by hydrogen is shown to derive from a coverage-dependent dissociation rate combined with barriers to the hopping of adsorbed oxygen atoms. Methyl termination, in contrast, offers an additional level of protection because oxygen must first undergo interactions with these ligands in a three-step process with significant energy barriers: adsorption of O(2) into a C-H bond to form a C-O-O-H intermediate; decomposition of C-O-O-H into C-O-H and C=O intermediates; and, finally, hopping of oxygen atoms from ligands to the substrate.

A Two-Dimensional Carbon Semiconductor

We show that patterned defects can be used to disrupt the sub-lattice symmetry of graphene so as to open up a band gap. This way of modifying graphenes electronic structure does not rely on external agencies, the addition of new elements or special boundaries. The method is used to predict a planar, low energy, graphene allotrope with a band gap of 1.2 eV. This defect engineering also allows semiconducting ribbons of carbon to be fabricated within graphene. Linear arrangements of defects lead to naturally embedded ribbons of the semiconducting material in graphene, offering the prospect of two-dimensional circuit logic composed entirely of carbon.

Charge separation at nanoscale interfaces: Energy-level alignment including two-quasiparticle interactions

The universal and fundamental criteria for charge separation at interfaces involving nanoscale materials are investigated. In addition to the single-quasiparticle excitation, all the two-quasiparticle effects including exciton binding, Coulomb stabilization, and exciton transfer are considered, which play critical roles on nanoscale interfaces for optoelectronic applications. We propose a scheme allowing adding these two-quasiparticle interactions on top of the single-quasiparticle energy level alignment for determining and illuminating charge separation at nanoscale interfaces. Employing the many-body perturbation theory based on Greens functions, we quantitatively demonstrate that neglecting or simplifying these crucial two-quasiparticle interactions using less accurate methods is likely to predict qualitatively incorrect charge separation behaviors at nanoscale interfaces where quantum confinement dominates.

A Semiconducting Graphene Allotrope

We show that patterned defects can be used to disrupt the sub-lattice symmetry of graphene so as to open up a band gap. This way of modifying graphenes electronic structure does not rely on external agencies, the addition of new elements or special boundaries. The method is used to predict a planar, low energy, graphene allotrope with a band gap of 1.2 eV. This defect engineering also allows semiconducting ribbons of carbon to be fabricated within graphene. Linear arrangements of defects lead to naturally embedded ribbons of the semiconducting material in graphene, offering the prospect of two-dimensional circuit logic composed entirely of carbon.