Eran Rabani

DRYING-MEDIATED SELF-ASSEMBLY OF NANOPARTICLES

Systems far from equilibrium can exhibit complex transitory structures, even when equilibrium fluctuations are mundane. A dramatic example of this phenomenon has recently been demonstrated for thin-film solutions of passivated nanocrystals during the irreversible evaporation of the solvent. The relatively weak attractions between nanocrystals, which are efficiently screened in solution, become manifest as the solvent evaporates, initiating assembly of intricate, slowly evolving structures. Although certain aspects of this aggregation process can be explained using thermodynamic arguments alone, it is in principle a non-equilibrium process. A representation of this process as arising from the phase separation between a dense nanocrystal ‘liquid’ and dilute nanocrystal ‘vapour’ captures some of the behaviour observed in experiments, but neglects entirely the role of solvent fluctuations, which can be considerable on the nanometre length scale. Here we present a coarse-grained model of nanoparticle self-assembly that explicitly includes the dynamics of the evaporating solvent. Simulations using this model not only account for all observed spatial and temporal patterns, but also predict network structures that have yet to be explored. Two distinct mechanisms of ordering emerge, corresponding to the homogeneous and heterogeneous limits of evaporation dynamics. Our calculations show how different choices of solvent, nanoparticle size (and identity) and thermodynamic state give rise to the various morphologies of the final structures. The resulting guide for designing statistically patterned arrays of nanoparticles suggests the possibility of fabricating spontaneously organized nanoscale devices.

Heavily Doped Semiconductor Nanocrystal Quantum Dots

Impurities can be added into semiconductor nanoparticles to control their electronic and optical properties. Doping of semiconductors by impurity atoms enabled their widespread technological application in microelectronics and optoelectronics. However, doping has proven elusive for strongly confined colloidal semiconductor nanocrystals because of the synthetic challenge of how to introduce single impurities, as well as a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We developed a method to dope semiconductor nanocrystals with metal impurities, enabling control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing. Our method yields n- and p-doped semiconductor nanocrystals, which have potential applications in solar cells, thin-film transistors, and optoelectronic devices.

Electronic properties of CdSe nanocrystals in the absence and presence of a dielectric medium

We present a detailed study of the electronic properties of CdSe nanocrystals in the absence and presence of a dielectric medium. The electronic structure of the nanocrystal is modeled within the framework of the empirical pseudopotential method. We use a real-space grid representation of the wave function, and obtain the eigenvalues and eigenstates of the one-electron Hamiltonian using a slightly modified version of the filter-diagonalization method. The band gap, density of states, charge density, multipole moments, and electronic polarizabilities are studied in detail for an isolated nanocrystal. We discuss the implications of the results for the long range electrostatic and dispersion interactions between two CdSe nanocrystals. To study the effects of the surroundings we develop a self-consistent reaction field method consistent with the empirical pseudopotential method. We use the eigenstates of the isolated nanocrystal and iterate the self-consistent equations until converged results are obtained. The results show that the electronic properties of polar CdSe nanocrystals are quite sensitive to the environment.

Real-Time Path Integral Approach to Nonequilibrium Many-Body Quantum Systems

A real-time path-integral Monte Carlo approach is developed to study the dynamics in a many-body quantum system coupled to a phonon background until reaching a nonequilibrium stationary state. The approach is based on augmenting an exact reduced equation for the evolution of the system in the interaction picture which is amenable to an efficient path integral (worldline) Monte Carlo approach. Results obtained for a model of inelastic tunneling spectroscopy reveal the applicability of the approach to a wide range of physically important regimes, including high (classical) and low (quantum) temperatures, and weak (perturbative) and strong electron-phonon couplings.

Structure and electrostatic properties of passivated CdSe nanocrystals

The electrostatic and structural properties of CdSe nanocrystals are discussed using an atomistic model that treats both the nanocrystal core and its passivation layer. The model predicts the presence of a permanent dipole moment for CdSe nanocrystals in the wurtzite crystal structure. The dipole moment is mildly screened by the nanocrystal’s passivation layer, and is significantly reduced due to surface reconstruction and relaxation. Possible explanation for preferential growth of CdSe particles along the c-axis is provided with a detailed analysis of the structural reconstruction of the different surfaces.

Electrostatic Force Microscopy Study of Single Au−CdSe Hybrid Nanodumbbells: Evidence for Light-Induced Charge Separation

Electrostatic force microscopy is used to study light-induced charging in single hybrid Au-CdSe nanodumbbells. Upon illumination, nanodumbbells show negative charging, which is in contrast with CdSe rods and Au particles that show positive charging. This different behavior is attributed to charge separation in the nanodumbbells, where after excitation the electron is transferred to the gold tips and the hole is subsequently filled through tunneling interactions with the substrate. The process of light-induced charge separation at the metal-semiconductor interface is key for the photocatalytic activity of such hybrid metal-semiconductor nanostructures.

Direct Observation of Nanoparticle Superlattice Formation by Using Liquid Cell Transmission Electron Microscopy

Direct imaging of nanoparticle solutions by liquid phase transmission electron microscopy has enabled unique in situ studies of nanoparticle motion and growth. In the present work, we report on real-time formation of two-dimensional nanoparticle arrays in the very low diffusive limit, where nanoparticles are mainly driven by capillary forces and solvent fluctuations. We find that superlattice formation appears to be segregated into multiple regimes. Initially, the solvent front drags the nanoparticles, condensing them into an amorphous agglomerate. Subsequently, the nanoparticle crystallization into an array is driven by local fluctuations. Following the crystallization event, superlattice growth can also occur via the addition of individual nanoparticles drawn from outlying regions by different solvent fronts. The dragging mechanism is consistent with simulations based on a coarse-grained lattice gas model at the same limit.

An interatomic pair potential for cadmium selenide

We have developed a set of interatomic pair potentials for cadmium selenide based on a form similar to the Born–Mayer model. We show that this simple form of the pair potential, which has been used to describe the properties of alkali halides in the sixfold-coordinate structure, provides a realistic description of the properties of cadmium selenide in all three crystal structures: wurtzite, zinc blende, and rocksalt. Using the new pair potential we have studied the pressure-induced phase transition from the fourfold-coordinate wurtzite structure to the sixfold-coordinate rocksalt structure. The pressure transformation and the equation of state are in good agreement with experimental observations. Using the dispersion term in our pair potential we have also calculated the Hamaker constant for cadmium selenide within the framework of the original microscopic approach due to Hamaker. The results indicate that for ionic materials many-body terms that are included in the Lifshitz theory are well captured by th...

The calculation of transport properties in quantum liquids using the maximum entropy numerical analytic continuation method: Application to liquid para-hydrogen

We present a method based on augmenting an exact relation between a frequency-dependent diffusion constant and the imaginary time velocity autocorrelation function, combined with the maximum entropy numerical analytic continuation approach to study transport properties in quantum liquids. The method is applied to the case of liquid para-hydrogen at two thermodynamic state points: a liquid near the triple point and a high-temperature liquid. Good agreement for the self-diffusion constant and for the real-time velocity autocorrelation function is obtained in comparison to experimental measurements and other theoretical predictions. Improvement of the methodology and future applications are discussed.

Distribution of Multiexciton Generation Rates in CdSe and InAs Nanocrystals

The distribution of rates of multiexciton generation following photon absorption is calculated for semiconductor nanocrystals (NCs). The rates of biexciton generation are calculated using Fermis golden rule with all relevant Coulomb matrix elements, taking into account proper selection rules within a screened semiempirical pseudopotential approach. In CdSe and InAs NCs, we find a broad distribution of biexciton generation rates depending strongly on the exciton energy and size of the NC. Multiexciton generation becomes inefficientfor NCs exceeding 3 nm in diameter in the photon energy range of 2-3 times the band gap.