During the past decade, enormous progress has been made in growing ultrathin organic films and multilayer structures with a wide range of exciting optoelectronic properties. This progress has been made possible by several important advances in our understanding of organic films and their modes of growth. Perhaps the single most important advance has been the use of ultrahigh vacuum (UHV) as a means to achieve, for the first time, monolayer control over the growth of organic thin films with extremely high chemical purity and structural precision.1-3 Such monolayer control has been possible for many years using well-known techniques such as Langmuir-Blodgett film deposition,4 and more recently, self-assembled monolayers from solution have also been achieved.5 However, ultrahighvacuum growth, sometimes referred to as organic molecular beam deposition (OMBD) or organic molecular beam epitaxy (OMBE), has the advantage of providing both layer thickness control and an atomically clean environment and substrate. When combined with the ability to perform in situ highresolution structural diagnostics of the films as they are being deposited, techniques such as OMBD have provided an entirely new prospect for understanding many of the fundamental structural and optoelectronic properties of ultrathin organic film systems. Since such systems are both of intrinsic as well as practical interest, substantial effort worldwide has been invested in attempting to grow and investigate the properties of such thin-film systems. This paper is a review of recent progress made in organic thin films grown in ultrahigh vacuum or using other vapor-phase deposition methods. We will describe the most important work which has been published in this field since the emergence of OMBD in the mid-1980s. Both the nature of thin-film growth and structural ordering will be discussed, as well as some of the more interesting consequences to the physical properties of such organic thin-film systems will be considered both from a theoretical as well as an experimental viewpoint. Indeed, it will 1793 Chem. Rev. 1997, 97, 1793−1896

Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques.

Amorphous semiconductors, being intrinsically metastable in nature, exhibit a wide variety of changes in their physical properties, particularly when photoinduced using bandgap illumination. This article reviews the photoinduced phenomena exhibited by amorphous semiconductors such as amorphous hydrogenated silicon (and other tetrahedrally coordinated materials) and chalcogenide glasses. Features exhibited in common by all types of amorphous semiconductors, whether in the experimentally observed photoinduced metastability or the theoretical models used to account for such behaviour, are stressed.

Photoinduced effects and metastability in amorphous semiconductors and insulators

Organometal halide perovskites have recently emerged as a highly promising class of functional materials for a variety of applications. The exceptional structural tunability enables these materials to possess three- (3D), two- (2D), one- (1D), and zero-dimensional (0D) structures at the molecular level. Remarkable progress has been realized in the research of perovskites in recent years, focusing mainly on 3D and 2D structures but leaving low-dimensional 1D and 0D structures significantly underexplored. Here we offer our perspective on the most exciting developments in the low-dimensional organometal halide perovskites. Due to the strong quantum confinement and site isolation, 1D and 0D perovskites exhibit remarkable and useful properties that are significantly different from those of 3D and 2D perovskites. The excitement about the recent developments lies not only in the specific achievements but also in what these materials represent in terms of a new paradigm in materials design.

http://pubs.acs.org/doi/pdf/10.1021/acsenergylett.7b00926

Low-Dimensional Organometal Halide Perovskites

Small particles of metals in solution often behave like electrodes although they are not connected to a battery which determines their potential. However, when a chemical reaction occurs in the solution of such particles intermediate free radicals may transfer electrons to them. The particles are thus charged chemically and are able to act as a metal electrode on cathodic potential. Electron transfer reactions become possible at these micro-electrodes which cannot be brought about by the radicals in the absence of the colloidal catalyst.

Mechanism of reactions on colloidal microelectrodes and size quantization effects

A thorough discussion of the various features of the photoluminescence spectra of undoped, p‐doped and n‐doped AlxGa1−xAs (0≤x≤1) alloys is given. This review covers spectral features in the energy region ranging from the energy band gap down to ≂0.8 eV, doping densities from isolated impurities to strongly interacting impurities (heavy‐doping effects) and lattice temperatures from 2 to 300 K. The relevance of photoluminescence as a simple but very powerful characterization technique is stressed also in comparison with other experimental methods. The most recent determinations of the Al concentration dependence of some physical properties of the alloy (energy gaps, carrier effective masses, dielectric constants, phonon energies, donor and acceptor binding energies, etc.) are given. The main physical mechanisms of the radiative recombination process in semiconductors are summarized with particular emphasis on the experimental data available for AlxGa1−xAs. The effects of the nature of the band gap (direct ...

Photoluminescence of AlxGa1−xAs alloys

Self-trapping of a Frenkel exciton in binary mixed crystal A c B 1- c , with excitation energy difference Δ = e B - e A and exciton-phonon coupling constant g , is studied. Impurity-assisted self-trapping is possible through cooperation of Δ and g even when neither of them alone is enough for localization. In this situation, one expects amalgamated absorption spectrum representing unrelaxed extended exciton and Stokes-shifted broad emission band representing relaxed localized exciton. We study whether relaxed exciton is localized (S-state) at a particular site of, or extended (B-state) throughout, the A -cltuster of various sizes, and how such cluster state changes, as c increases, into free (F) state extended throughout the crystal. The results are compared with recent experiments on TlBr c Cl 1- c . Sudden appearance of edge emission (due to F) at c ∼0.3, at the cost of broad band (due to S and B), is ascribed to the combined effect of size-dependent discontinuity of S-B transition in Br-clusters and th...

Self-Trapping in Mixed Crystal –Clustering, Dimensionality, Percolation–

Variational calculations are presented for Wannier excitons in quantum wells for the ground ($1s$) and the excited ($2s$ and $2p$) states, with careful attention to a variety of spatial extensions of these excitons in directions parallel and perpendicular to the well interface. The results are shown to be valid throughout the entire well thickness, corresponding in the thin and the thick limits to two- and three-dimensional situations, respectively.

Wannier exciton in quantum wells

Local and global stabilities of an electron in deformable lattice with dimensionality δ and force range index λ are studied within the adiabatic approximation. Depending on the sign of the stability index: σ=δ-2λ-2, one finds the free state to be locally stable (σ>0) ith discontinuous transition to globally stable self-trapped state beyond a critical value of coupling constant g , to be marginal (σ=0) with local (as well as global) stability being governed by g , or to be locally instabilized (σ <0) towards the optimum lattice distortion which grows continuously from zero with increasing g . A phase diagram is presented for an electron interacting simultaneously with two types of phonon fields with $\sigma \lessgtr 0$. The potential barrier for self-trapping of a Wannier exciton is studied with particular attention to the effect of its internal motion, and is compared with the observation.

Stability of an Electron in Deformable Lattice –Force Range, Dimensionality and Potential Barrier–

Electron-lattice interactions in nonmetallic materials are reexamined in the many-electron scheme. The difference in the stable atomic configuration between two electronic states is the origin of the electron-lattice interaction. We show the relationship among the adiabatic potentials, one electron (hole) energy and the lattice elastic energy, paying attention to the electron-hole symmetry. Correct configuration coordinate diagrams for deep-level defects in semiconductors are presented which can be used even when the number of carriers changes due to creation and recombination. Radiative and nonradiative carrier capture and recombination processes at deep-level defects are described consistently with particular attention to the charge of a defect, the thermal and the optical depths of a bound carrier, the correlation between successive electron and hole captures, and the energy dissipation to the lattice through the interaction mode.

https://iopscience.iop.org/article/10.1143/JJAP.32.4560/pdf

Electron-Lattice Interaction in Nonmetallic Materials: Configuration Coordinate Diagram and Lattice Relaxation

Variational calculations are performed to investigate the subband dependence of the ground (1 s type) and a few excited (2 s and 2 p type) states of a Wannier exciton in a quantum well. It is shown that (i) binding energies and oscillator strengths of excitons depend strongly not only on the width of the well but also on (electron and hole) subbands of the well with which excitons are associated and (ii) as the well changes from a thin well to a thick one, binding energies of most excitons for higher subbands do not change monotonically between two- and three-dimensional results.

Yuzo Shinozuka

Papers

Self-Trapping in Mixed Crystal –Clustering, Dimensionality, Percolation–

Wannier exciton in quantum wells

Stability of an Electron in Deformable Lattice –Force Range, Dimensionality and Potential Barrier–

Electron-Lattice Interaction in Nonmetallic Materials: Configuration Coordinate Diagram and Lattice Relaxation

A Wannier Exciton in a Quantum Well:Subband Dependence