We present the first in a series of microscopic studies of electrical transport through individual molecules with metallic contacts. We view the molecules as ``heterostructures'' composed of chemically well-defined atomic groups, and analyze the device characteristics in terms of the charge and potential response of these atomic groups to the perturbation induced by the metal-molecule coupling and the applied bias voltage, which are modeled using a first-principles based self-consistent matrix Green's function (SCMGF) method. As the first example, we examine the devices formed by attaching two benzene-based molecular radicals---phenyl dithiol (PDT) and biphenyl dithiol (BPD)---symmetrically onto two semi-infinite gold electrodes through the end sulfur atoms. We find that both molecules acquire a fractional number of electrons with similar magnitude and spatial distribution upon contact with the electrodes. The charge transfer creates a potential barrier at the metal-molecule interface that modifies significantly the frontier molecular states depending on the corresponding electron density distribution. For both molecules, the metal Fermi level is found to lie closer to the highest-occupied-molecular-orbital (HOMO) than to the lowest-unoccupied-molecular-orbital (LUMO). Transmission in the HOMO-LUMO gap for both molecules is due to the metal-induced gap states arising from the hybridization of the metal surface states with the occupied molecular states. Applying a finite bias voltage leads to only minor net charge injection due to the symmetric device structure assumed in this work. But as current flows, the electrons within the molecular junction redistribute substantially, with resistivity dipoles developing in the vicinity of potential barriers. Only the delocalized $\ensuremath{\pi}$ electrons in the benzene ring can effectively screen the applied electric field. For the PDT molecule, the majority of the bias voltage drops at the metal-molecule interface. But for the BPD molecule, a significant amount of the voltage also drops in the molecule core. The field-induced modification of the molecular states (the static Stark effect) becomes significant as the bias voltage increases beyond the linear-transport region. A bias-induced reduction of the HOMO-LUMO gap is observed for both molecules at large bias. The Stark effect is found to be stronger for the BPD molecule than the PDT molecule despite the longer length of the former. For both molecules, the peaks in the conductance are due to electron transmission through the occupied rather than the unoccupied molecular states. The calculation is done at room temperature, and we find that the thermionic-emission contribution to the current-voltage characteristics of both molecules is negligible.

Microscopic study of electrical transport through individual molecules with metallic contacts. I. Band lineup, voltage drop, and high-field transport

Electronic transport measurements were performed on Y-junction carbon nanotubes. These novel junctions contain a large diameter tube branched into smaller ones. Independent measurements using good quality contacts on both individual Y junctions and many in parallel show intrinsic nonlinear transport and reproducible rectifying behavior at room temperature. The results were modeled using classic interface physics for a junction with an abrupt change in band gap due to the change in tube diameter. These Y-junction tubes represent new heterojunctions for nanoelectronics.

Electronic transport in Y-junction carbon nanotubes

In this paper, we present a first principles high throughput screening system to search for new water-splitting photocatalysts. We use the approach to screen through nitrides and oxynitrides. Most of the known photocatalytic materials in the screened chemical space are reproduced. In addition, sixteen new materials are suggested by the screening approach as promising photocatalysts, including three binary nitrides, two ternary oxynitrides and eleven quaternary oxynitrides.

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First principles high throughput screening of oxynitrides for water-splitting photocatalysts

Dielectric relaxations in ferroelectric ceramics occur at various frequencies, depending on the type of chemical or physical defects These defects depend on either intrinsic or extrinsic heterogeneities due to special heat treatments (quenching, annealing,…), ionic substitutions, grain size additives, and grain boundary nature The value of the relaxation frequency fr increases from about 102 to 1012 Hz as the scale of the defect phenomenon decreases from microstructure, to nanostructure, to unit-cell, to atomic vibrations This type of study requires a pluridisciplinary approach involving solid state chemistry, materials science, solid state physics and various industrial aspects The applications in the area of electronic ceramics are related to the value of fr In absorbants, the usable frequency is close to fr, whereas it is far from fr in good
insulators In view of numerous experimental examples, a classification is proposed to predict which materials may be suitable for a given application

Ferroelectric ceramics: defects and dielectric relaxations

Publisher Summary This chapter discusses field of ferroelectricity. It deals with selected salient properties and associated physics. The chapter reviews the phenomenological, soft mode, and first-principles theories of ferroelectrics. There are similarities between spin glasses and dielectric dipolar glasses. The dipolar interaction in both a normal, or ordinary, polarizable medium leads to the formation of a dipolar glass at low temperatures, and in a highly polarizable medium that leads to either long-range ferroelectric order or to a glass-like relaxor state. The properties of relaxor ferroelectrics and the contrast between properties and normal ferroelectrics have also been discussed in the chapter. Discoveries coupled with the scientific challenges of the many facets of ferroelectric phenomena and the technological importance of the field ensure health and vitality of research in ferroelectricity.

Ferroelectricity revisited—Advances in materials and physics

The dynamics of the freezing process in relaxor ferroelectrics are investigated by Monte Carlo computer simulation. The concepts of ``frozen dipole'' and ``slow dipole'' are proposed according to the flipping frequency during the observation time of simulation. The deviation from Curie-Weiss behavior and the frequency dispersion in relaxor ferroelectrics have been proved to be caused by the increasing proportion of slow dipoles. The simulation shows that the external field reduces the dielectric constant and increases the temperature of dielectric maximum, which agrees with the experiment results. The polarization behavior with various electrical and thermal histories and starting from different initial states is also simulated and a systematic explanation based on the bifurcation tree of components in phase space is given. It is proposed that the relaxor characteristics are caused by a gradual freezing process of polar microregions, which arises from the inhomogeneous crystal structure of this kind of material.

Dynamics of the freezing process in relaxor ferroelectrics.

Transient ordered phases with different structure types along a phase-decomposition path

The dielectric constant e*(ω,T)=e′(ω,T)+ie″(ω,T) of relaxor ferroelectrics is simulated by the Monte Carlo method. It is proved that the polar microregions switch among different polarization directions as a whole under the electrical field, giving rise to the high dielectric constant. The relaxor characteristics are caused by the randomly distributed interactions between the polar microregions. Factors which influence the dielectric response process are discussed.

Dielectric response process in relaxor ferroelectrics

The distribution of relaxation times in perovskite‐type relaxor ferroelectrics is studied by computer simulation. It has been proved that the spectra move to longer relaxation times with the decreasing of temperature and the increasing of the external field. The frequency dispersion of relaxors is explained by a gradual freezing process of polar microregions. The relationship between the time dependence of the polarization and the relaxation time distribution at various temperatures and external fields has been investigated. The macroscopic polarization is suggested to be sustained mainly by frozen dipoles during the observation time.

Distribution of relaxation times in perovskite‐type relaxor ferroelectrics

A theoretical model is proposed to calculate the spontaneous polarization in PMN relaxor. It is supposed that the Nb ions in the ordered microregions displace cooperatively due to the structure homogeneity, giving rise to the polarization below Td (600 K). The results agree with the polarization deduced from the electro-optic and electrostrictive effects. The ordered microregions have been proved to be the locations of the polar clusters at Tf ≪ T < Td , where Tf is the freezing temperature (near 210 K). The spontaneous polarization process in the ordered microregions is suggested to be similar to that of normal ferroelectrics. The dipole glass model of relaxors is discussed.

Hong Gui

Papers

Dynamics of the freezing process in relaxor ferroelectrics.

Transient ordered phases with different structure types along a phase-decomposition path

Dielectric response process in relaxor ferroelectrics

Distribution of relaxation times in perovskite‐type relaxor ferroelectrics

Spontaneous polarization in lead magnesium niobate relaxor