This paper describes the enhancement of the quantum yield of photoluminescence (PL) of CdSe quantum dots (QDs) upon the adsorption of an exciton-delocalizing ligand, phenyldithiocarbamate. Increasing the apparent excitonic radius by only 10% increases the value of the radiative rate constant by a factor of 1.8 and the PL quantum yield by a factor of 2.4. Ligand exchange therefore simultaneously perturbs the confinement energy of charge carriers and enhances the probability of band-edge transitions.

Displacement of cadmium oleate (Cd(oleate)2) ligands for the exciton-delocalizing ligand 4-hexylphenyldithiocarbamate (C6-PTC) on the surfaces of CdS quantum dots (QDs) causes a decrease in the band gap (Eg) of the QD of ∼100 meV for QDs with a radius of 1.9 nm and ∼50 meV for QDs with a radius of 2.5 nm. The primary mechanism of this decrease in band gap, deduced in previous work, is a decrease in the confinement barrier for the excitonic hole. The increase in apparent excitonic radius of the QD that corresponds to this decrease in Eg is denoted ΔR. The dependence of ΔR on the surface coverage of C6-PTC, measured by (1)H NMR spectroscopy, appears to be nonlinear. Calculations of the excitonic energy of a CdS QD upon displacement of native insulating ligands with exciton-delocalizing ligands using a 3D spherical potential well model show that this response includes the contributions to ΔR from both isolated, bound C6-PTC ligands and groups of adjacent C6-PTC ligands. Fits to the experimental plots of ΔR vs surface coverage of C6-PTC with a statistical model that includes the probability of formation of clusters of bound C6-PTC on the QD surface allow for the extraction of the height of the confinement barrier presented by a single, isolated C6-PTC molecule to the excitonic hole. This barrier height is less than 0.6 eV for QDs with a radius of 1.9 nm and between 0.6 and 1.2 eV for QDs with a radius of 2.5 nm.

Displacement of native octylphosphonate (OPA) ligands for methylthiophenolate (CH 3 -TP) on the surfaces of CdSe quantum dots (QDs) causes a moderate (up to 50 meV) decrease in the band gap (E g ) of the QD. Plots of the corresponding increase in apparent excitonic radius, ΔR, of the QDs versus the surface coverage of CH 3 -TP, measured by 1 H NMR, for several sizes of QDs reveal that this ligand adsorbs in two distinct binding modes, (1) a tightly bound mode (K a = 1.0 ± 0.3 × 10 4 M −1 ) capable of exciton delocalization, and (2) a more weakly bound mode (K a = 8.3 ± 9.9 × 10 2 M −1 ) that has no discernible effect on exciton confinement. For tightly bound CH 3 -TP, the degree of delocalization induced in the QD is approximately linearly related to the fractional surface area occupied by the ligand for all sizes of QDs. Comparison of the dependence of ΔR on surface coverage of CH 3 -TP over a range of physical radii of the QDs, R = 1.1−2.4 nm, to analogous plots simulated using a 3D spherical potential well model yield a value for the confinement barrier presented to the excitonic hole by tightly bound CH 3 -TP of ∼1 eV. ■ INTRODUCTIONThis paper describes the dependence of the excitonic radius of CdSe quantum dots, QDs, on the surface coverage of an exciton-delocalizing ligand, methylthiophenolate, CH 3 -TP, and the use of this dependence to estimate both the number of binding geometries (with corresponding adsorption constants) for the ligand and the magnitude of the confinement potential that each binding mode presents for the exciton. Exciton delocalizing ligands, such as thiolates 1−4 and dithiocarbamates, 5−8 allow for increased electronic coupling of a quantum-confined exciton with the immediate surroundings of the QD, and therefore facilitate charge carrier or exciton extraction into proximate redox or energy acceptors, without changing the physical size or chemical composition of the QD core or broadening their optical spectra. 5−9 Exciton delocalization is also associated with an increase in the oscillator strength of band-edge transitions 3,4 and a resultant increase in the photoluminescence quantum yield of ensembles of QDs. 10 In addition to the benefits of delocalizing ligands for use of QDs as photovoltaic active materials, photocatalysts, and luminescent tags, the response of the excitonic energy of the QD to its surface chemistry is a sensitive probe of the degree of quantum confinement of its carriers, 6,8 and, as we show here, the chemical and electronic structure of the QD−ligand interface, which is often difficult to probe using traditional analytical techniques, especially in the solution phase.Displacement of native octylphosphonate ligands (OPA) by CH 3 -TP on the surfaces of the QDs, Figure 1A, causes a moderate (up to 50 meV) decrease in the band gap (E g ) of the QDs, which we measure by monitoring the position of the first excitonic peak of the QDs with UV−vis absorption spectroscopy, Figure 1B. We report the decrease in E g as an increase in the apparent radius of its quantum-confine...

Compared to common density functionals, ab initio wave function methods can provide greater reliability and accuracy, which could prove useful when modeling adsorbates or defects of otherwise periodic systems. However, the breaking of translational symmetry necessitates large supercells that are often prohibitive for correlated wave function methods. As an alternative, this paper introduces the regional embedding approach, which enables correlated wave function treatments of only a target fragment of interest through small, fragment-localized orbital spaces constructed using a simple overlap criterion. Applications to the adsorption of water on lithium hydride, hexagonal boron nitride, and graphene substrates show that regional embedding combined with focal-point corrections can provide converged CCSD(T) (coupled-cluster) adsorption energies with very small fragment sizes.

Ratchets are nonequilibrium devices that produce directional motion of particles from nondirectional forces without using a bias, and are responsible for many types of biological transport, which occur with high yield despite strongly damped and noisy environments. Ratchets operate by breaking time-reversal and spatial symmetries in the direction of transport through application of a time-dependent potential with repeating, asymmetric features. This work demonstrates the ratcheting of electrons within a highly scattering organic bulk-heterojunction layer, and within a device architecture that enables the application of arbitrarily shaped oscillating electric potentials. Light is used to modulate the carrier density, which modifies the current with a nonmonotonic response predicted by theory. This system is driven with a single unbiased sine wave source, enabling the future use of natural oscillation sources such as electromagnetic radiation.ratchet | nonequilibrium | charge transport | organic semiconductor B iological environments are noisy, chaotic, and highly damped (1). Transporting particles under those circumstances is a challenge, for which nature developed molecular motors, such as the myosin-actin system responsible for muscle contraction (2), the kinesin molecular walker (3), and ATP synthase (4). Such systems couple inherent structural asymmetries and relaxation with nondirectional sources of energy, like chemical energy, to obtain directional motion in the presence of strong damping and thermal noise through a mechanism called "ratcheting" (5, 6). For example, in the myosin-actin system, thermal fluctuations of the myosin head on an elastic tether lead to occasional binding of the head to an actin filament, at which point thermal energy is transduced to elastic energy, and the filaments translate relative to one another. A chemical reaction-coupled conformational change in the myosin head upon translation induces release of the head from the actin filament, and renders the translation irreversible. The system thereby uses a chemical trigger to rectify random thermal motion (7). The design principles of natural systems are today being used to develop a variety of molecular machines (8) and to achieve, experimentally, ratcheting of micrometer-sized particles (9), DNA (10), and cold atoms (11).The concept of an electron ratchet has been explored theoretically (12-14) and, in rare cases, experimentally (15-21). There are two major types of electron ratchets: "flashing," in which the electron moves along a periodic potential surface with locally asymmetric repeat units that oscillates between two states, while the source-drain bias along the direction of transport is constantly zero (5, 9); and "tilting," in which the shape of the potential surface remains constant, but the source-drain bias oscillates with a time average of zero (5). Fig. 1 shows a mechanism of transport in perhaps the simplest 1D flashing ratchet system, an "on/off" ratchet. Our interest lies in adapting the flashing ratchet mechanism t...