Cory A. Nelson

Charge Transfer Excitons at van der Waals Interfaces.

The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation.

Rare-earth cation effects on three-dimensional metal–organic rotaxane framework (MORF) self assembly

A set of metal-organic rotaxane frameworks (MORFs) are constructed with the use of a tetraimidazolium macrocycle, the terephthalate dianion, and the trivalent lanthanide metal cations Nd(III), Sm(III), Eu(III) or Tb(III) and are reported herein. The specific choice of the metal cation allows for control of the structure and luminescent properties of the resulting molecular frameworks.

Reversible surface electronic traps in PbS quantum dot solids induced by an order-disorder phase transition in capping molecules.

The electronic properties of semiconductor quantum dots (QDs) are critically dependent on the nature of the ligand molecules on their surfaces. Here we show the reversible formation of surface electronic trap states in the model system of solid thin films of PbS QDs capped with thiol molecules. As the temperature was increased from cryogenic to room temperature, we discovered a phase transition in the fluorescence spectra from excitonic emission to trap emission. The critical temperature (T(c)) of the phase transition scales with molecular length and in each case is close to the bulk melting temperature of the capping molecules. We conclude that an order-disorder transition in the molecular monolayer above T(c) introduces surface mobility and the formation of a disordered atomic lead layer at the QD/capping molecule interface, leading to electronic trap formation.

Exceeding the Shockley-Queisser Limit in Solar Energy Conversion

We summarize our recent explorations of photophysical mechanisms that may be utilized in solar cells with power conversion efficiency theoretically exceeding the Shockley–Queisser limit. The dominant losses responsible for the Shockley–Queisser limit are below band-gap and thermalization (hot carrier) losses; together, they account for >55% of the total absorbed solar energy. This perspective focuses on two photophysical mechanisms, hot carrier equilibration and carrier multiplication, which may be used to reduce these losses through their utilization in novel solar cell designs. For the implementation of a hot carrier solar cell, we discuss the necessity of hot carrier scattering as well as the photon flux challenge. Although recent experiments have demonstrated the feasibility of hot-electron extraction from photo-excited semiconductor nano-crystals, the photon flux challenge cannot be met in these materials. We propose graphene and related materials as potentially ideal chromophores for hot carrier solar cells. For the multi-exciton solar cell, we focus on the molecular analog called singlet fission. Recent experiments in our lab revealed a quantum coherent mechanism in which photo-excitation of the organic semiconductor pentacene or tetracene creates a quantum superposition of singlet exciton and multi-exciton states. This quantum superposition and the corresponding decoherence time (i.e., singlet fission time) are critical to the competing dynamics of charge or energy harvesting. We discuss design principles for solar cells based on singlet fission materials.

Ultrafast lattice dynamics in lead selenide quantum dot induced by laser excitation

We directly monitored the lattice dynamics in PbSe quantum dots (QD) induced by laser excitation using ultrafast electron diffraction. The energy relaxation between the carriers and the lattice took place within 10 ps, showing no evidence of any significant phonon bottleneck effect. Meanwhile, the lattice dilation exhibited some unusual features that could not be explained by the available mechanisms of photon-induced acoustic vibrations in semiconductors alone. The heat transport between the QDs and the substrate deviates significantly from Fouriers Law, which opens questions about the heat transfer under nonequilibrium conditions in nanoscale materials.

Correction to “Charge Transfer Excitons at van der Waals Interfaces”

J. Am. Chem. Soc. 2015, 137, 8313−8320. DOI:10.1021/jacs.5b03141 Page 8316. In the text following eq (2), the value “3.1 nm−1 ” for the parallel momentum vector at the zone boundary is wrong, which also affects the subsequent comments about the momentum uncertainty. The correct number should be 22.7 nm−1. The corrected text should read as follows: “...the parallel momentum vector at the zone boundary is 22.7 nm−1. Thus, the momentum uncertainty resulting from excitonic localization can cover 4−7% of the Brillouin zone and satisfy part of the requirement for momentum conservation in interfacial charge transfer.” Addition/Correction