Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron–hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials’ properties.

We investigate the electronic structure of the GaN(101¯0) prototype surface for GaN nanowire sidewalls. We find a paradoxical situation that a surface state at all k points in the bandgap cannot be probed by conventional scanning tunneling microscopy, due to a dispersion characterized by a steep minimum with low density of states (DOS) and an extremely flat maximum with high DOS. Based on an analysis of the decay behavior into the vacuum, we identify experimentally the surface state minimum 0.6 ± 0.2 eV below the bulk conduction band in the gap. Hence, GaN nanowires with clean (101¯0) sidewall facets are intrinsically pinned.

A quantitative description of photoexcited scanning tunneling spectra is developed and applied to photoexcited spectra measured on p-doped nonpolar GaAs(110) surfaces. Under illumination, the experimental spectra exhibit an increase of the tunnel current at negative sample voltages only. In order to analyze the experimental data quantitatively, the potential and charge-carrier distributions of the photoexcited tip-vacuum-semiconductor system are calculated by solving the Poisson as well as the hole and electron continuity equations by a finite-difference algorithm. On this basis, the different contributions to the tunnel current are calculated using an extension of the model of Feenstra and Stroscio to include the light-excited carrier concentrations. The best fit of the calculated tunnel currents to the experimental data is obtained for a tip-induced band bending, which is limited by the partial occupation of the C 3 surface state by light-excited electrons. The tunnel current at negative voltages is then composed of a valence band contribution and a photoinduced tunnel current of excited electrons in the conduction band. The quantitative description of the tunnel current developed here is generally applicable and provides a solid foundation for the quantitative interpretation of photoexcited scanning tunneling spectroscopy.

Stabilizing ordering instead of randomness in alloy semiconductor materials is a powerful means to change their physical properties. We used scanning tunneling and transmission electron microscopies to reveal the existence of an unrecognized ordering in ternary III-V materials. The lazarevicite short-range order, found in the shell of InAs 1−x Sb x nanowires, is driven by the strong Sb-Sb repulsion along 110 atomic chains during their incorporation on unreconstructed {110} sidewalls. Its spontaneous formation under group-III-rich conditions of growth offers the prospect to broaden the limited classes of ordered structures occurring in III-V semiconductor alloys.Binary III-V compound semiconductors cover only discrete values of materials properties, such as band gaps and lattice parameters. Many technological applications require, however, materials with properties intermediate between those of the binary compounds. Commonly this is achieved by creating solid solutions of different binary compounds. The electronic properties of such ternary or quaternary III-V alloys depend, however, sensitively on the chemical ordering. Although alloys with long-range order may exhibit significantly deviating band gaps from random alloys with identical compositions [1,2], alloys with short-range order (SRO) have been shown to affect the carrier localization in the alloys and modify their optical properties [3]. Studying the nature of SRO in III-V alloys and the physical mechanisms leading to its spontaneous formation is thus of prime importance to further understand the interplay between atomic-scale structures and electronic properties in these alloys.Chemical ordering has been investigated rather well for thin films of III-V compound semiconductors [2,4,5]. Recently, ordering was found to occur in III-V semiconductor nanowires (NWs) too [6,7]. However, despite the considerable interest in a better control of the ordering and the large number of possible ordered spatial arrangements that are predicted by the space-group theory, only a few ordered structures have been reproducibly stabilized from the parent zinc-blende (ZB) lattice: CuPt, triple period, CuAu-I, chalcopyrite, and famatinite [8]. This restriction is intimately related to the limited numbers of achievable surface reconstructions and the limited ranges of the growth parameters, such as temperature, growth rate, III/V ratio, and substrate orientation. Recent developments in the epitaxial growth of III-V semiconductors, such as the ones achieved with liquid-droplet epitaxy or obtained during the growth of semiconductor nanowires, offer the potential to reach unattained growth conditions and growth regimes [9,10]. This situation raises the question whether new ordered structures could be tailored in III-V semiconductor materials.To address this question, we used scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM) to provide direct evidence of SRO in ZB InAs 1−x Sb x nanowires. In addition to a CuPt SRO typically found in InAs 1−x Sb ...

The geometric and electronic structure of overgrown v-shaped defects in GaN epitaxial layers are investigated by cross-sectional scanning tunneling microscopy and spectroscopy. The v-defects are found to be hexagonal pit structures delimited by six f11 22g planes. The electronic properties are inhomogeneous. In some areas the center of the v-defects exhibits a strongly inhibited tunneling current, indicating the presence of deep traps. V