Diego Alducin

Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs

Borophene: Boron in two dimensions Although bulk allotropes of carbon and boron differ greatly, small clusters of these elements show remarkable similarities. Boron analogs of two-dimensional carbon allotropes such as graphene have been predicted. Now Mannix et al. report the formation of two-dimensional boron by depositing the elemental boron onto a silver surface under ultrahigh-vacuum conditions (see the Perspective by Sachdev). The graphene-like structure was buckled, weakly bonded to the substrate, and metallic. Science, this issue p. 1513; see also p. 1468 A two-dimensional boron allotrope forms after depositing its elemental vapor on a silver surface in vacuum. [Also see Perspective by Sachdev] At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes. Theoretical studies predict that two-dimensional (2D) boron sheets will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal.

Thickness sorting of two-dimensional transition metal dichalcogenides via copolymer-assisted density gradient ultracentrifugation

Two-dimensional transition metal dichalcogenides have emerged as leading successors to graphene due to their diverse properties, which depend sensitively on sample thickness. Although solution-based exfoliation methods hold promise for scalable production of these materials, existing techniques introduce irreversible structural defects and/or lack sufficient control over the sample thickness. In contrast, previous work on carbon nanotubes and graphene has shown that isopycnic density gradient ultracentrifugation can produce structurally and electronically monodisperse nanomaterial populations. However, this approach cannot be directly applied to transition metal dichalcogenides due to their high intrinsic buoyant densities when encapsulated with ionic small molecule surfactants. Here, we overcome this limitation and thus demonstrate thickness sorting of pristine molybdenum disulfide (MoS2) by employing a block copolymer dispersant composed of a central hydrophobic unit flanked by hydrophilic chains that effectively reduces the overall buoyant density in aqueous solution. The resulting solution-processed monolayer MoS2 samples exhibit strong photoluminescence without further chemical treatment.

The structure and properties of amorphous indium oxide

A series of In2O3 thin films, ranging from X-ray diffraction amorphous to highly crystalline, were grown on amorphous silica substrates using pulsed laser deposition by varying the film growth temperature. The amorphous-to-crystalline transition and the structure of amorphous In2O3 were investigated by grazing angle X-ray diffraction (GIXRD), Hall transport measurement, high resolution transmission electron microscopy (HRTEM), electron diffraction, extended X-ray absorption fine structure (EXAFS), and ab initio molecular dynamics (MD) liquid-quench simulation. On the basis of excellent agreement between the EXAFS and MD results, a model of the amorphous oxide structure as a network of InOx polyhedra was constructed. Mechanisms for the transport properties observed in the crystalline, amorphous-to-crystalline, and amorphous deposition regions are presented, highlighting a unique structure–property relationship.

High-Concentration Aqueous Dispersions of Nanoscale 2D Materials Using Nonionic, Biocompatible Block Copolymers.

Conditions for the dispersion of molybdenum disulfide (MoS2) in aqueous solution at concentrations up to 0.12 mg mL(-1) using a range of nonionic, biocompatible block copolymers (i.e., Pluronics and Tetronics) are identified. Furthermore, the optimal Pluronic dispersant for MoS2 is found to be effective for a range of other 2D materials such as molybdenum diselenide, tungsten diselenide, tungsten disulfide, tin selenide, and boron nitride.

Elasticity of MoS2 sheets by mechanical deformation observed by in situ electron microscopy

MoS2 has been the focus of extensive research due to its potential applications. More recently, the mechanical properties of MoS2 layers have raised interest due to applications in flexible electronics. In this article, we show in situ transmission electron microcsopy (TEM) observation of the mechanical response of a few layers of MoS2 to an external load. We used a scanning tunneling microscope (STM) tip mounted on a TEM stage to induce deformation on nanosheets of MoS2 containing few layers. The results confirm the outstanding mechanical properties on the MoS2. The layers can be bent close to 180°. However, when the tip is retrieved the initial structure is recovered. Evidence indicates that there is a significant bond reconstruction during the bending with an outstanding capability to recover the initial bond structure. The results show that flexibility of three layers of MoS2 remains the same as a single layer while increasing the bending modulus by 3 orders of magnitude. Our findings are consistent with theoretical calculations and confirm the great potential of MoS2 for applications.

Precession electron diffraction-assisted crystal phase mapping of metastable c-GaN films grown on (001) GaAs

The control growth of the cubic meta‐stable nitride phase is a challenge because of the crystalline nature of the nitrides to grow in the hexagonal phase, and accurately identifying the phases and crystal orientations in local areas of the nitride semiconductor films is important for device applications. In this study, we obtained phase and orientation maps of a metastable cubic GaN thin film using precession electron diffraction (PED) under scanning mode with a point‐to‐point 1 nm probe size beam. The phase maps revealed a cubic GaN thin film with hexagonal GaN inclusions of columnar shape. The orientation maps showed that the inclusions have nucleation sites at the cubic GaN {111} facets. Different growth orientations of the inclusions were observed due to the possibility of the hexagonal {0001} plane to grow on any different {111} cubic facet. However, the generation of the hexagonal GaN inclusions is not always due to a 60° rotation of a {111} plane. These findings show the advantage of using PED along with phase and orientation mapping, and the analysis can be extended to differently composed semiconductor thin films. Microsc. Res. Tech. 77:980–985, 2014.

Structural damage reduction in protected gold clusters by electron diffraction methods

The present work explores electron diffraction methods for studying the structure of metallic clusters stabilized with thiol groups, which are susceptible to structural damage caused by electron beam irradiation. There is a compromise between the electron dose used and the size of the clusters since they have small interaction volume with electrons and as a consequence weak reflections in the diffraction patterns. The common approach of recording individual clusters using nanobeam diffraction has the problem of an increased current density. Dosage can be reduced with the use of a smaller condenser aperture and a higher condenser lens excitation, but even with those set ups collection times tend to be high. For that reason, the methods reported herein collects in a faster way diffraction patterns through the scanning across the clusters under nanobeam diffraction mode. In this way, we are able to collect a map of diffraction patterns, in areas with dispersed clusters, with short exposure times (milliseconds) using a high sensitive CMOS camera. When these maps are compared with their theoretical counterparts, oscillations of the clusters can be observed. The stability of the patterns acquired demonstrates that our methods provide a systematic and precise way to unveil the structure of atomic clusters without extensive detrimental damage of their crystallinity.

Electrical Probing of Silver Nanowires in situ Transmission Electron Microscopy

One of the most important aspects of the area of material sciences is the characterization of nanostructured and advanced materials. Silver is one of the most promising materials due to the large thermal and electrical conductivity compared to other metals. However, not all properties of materials scale down in proportion to its one-dimensional nanostructure. In this work we present the electrical behavior of single silver nanowires. By employing an in situ transmission electrical holder, one can measure the resistivity and conductance of individual nanowires while observing the changes of the structure due to the high resolution transmission electron microscope. By applying different voltages we observed the how the nanowire is affected by this increase until failure.

Nano-manipulation of Ag/ZnO Nanoantennas for in-situ TEM Electrical Measurements

Nowadays metal-semiconductor systems have attracted more attention due to their role as an active element in opto-electronic applications, especially in nanoantenna devices where the coupling of a metal with a semiconductor material could lead to the appearing of a resonant system responding to a specific frequency useful to be employed in solar cells, detectors, and nonlinear optical devices [1–2]. Moreover, the in-situ monitoring of physical properties associated with different arrays and configurations will lead to controllable opto-electronic properties. In this context, zinc oxide has been recognized as an excellent semiconductor material that meets most of the features required for those applications due to its easiness to be modifiable, i.e. controlling not only its arrangement distribution, leading to a variety of internal structures, but also its morphological configurations. It is known that using a specific method for the synthesis, different features and morphologies could be achieved.

Fast Scanning Electron Diffraction and Electron Holography as Methods to Acquire Structural Information on Au102(p-MBA)44 Nanoclusters

Electron diffraction analysis in Transmission Electron Microscopy (TEM) is one resourceful but problematic technique for the characterization of sensitive materials due the high intensity on the convergent beam. In the case of ligand-protected clusters there are two key damage mechanisms that can appear during the data acquisition of nanobeam diffraction patterns (NBD) in the TEM: (i) the knock-on effect, arising from the momentum transfer between the electron beam and the nanoclusters and (ii) the ionization damage (radiolysis).