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In article number https://doi.org/10.1002/adfm.2019010701901070, Husam N. Alshareef and co‐workers successfully develop an epitaxial phase conversion process to make continuous and homogeneous monolayer MoS₂ films at the wafer scale. The process was enabled by careful epitaxial growth of MoO₂ and a subsequent sulfurization process. The resulting epitaxial films exhibit high‐performance excitonic and electronic properties that can be engineered during the growth process.

The controllable synthesis of high‐quality two‐dimensional (2D) ytterbium oxychloride (YbOCl) nanosheets is demonstrated. High‐resolution transmission electron microscopy and selected‐area electron diffraction reveal its high crystal quality and single‐crystalline structure. The systematic Raman scattering spectra of YbOCl single crystals from both experimental and theoretical perspectives are studied.

Abstract

MOX (M = Fe, Co, Mn, Cr, Lanthanide, or Actinide metals; O = oxygen, X = F, Cl, Br, I), an emerging type of 2D layered materials, have been theoretically predicted to possess unique electronic and magnetic properties. However, 2D MOX have rarely been investigated. Herein, for the first time, ultrathin high‐quality ytterbium oxychloride (YbOCl) single crystals are successfully synthesized via an atmospheric pressure chemical vapor deposition method. Both theoretical simulations and experimental measurements are utilized to systematically investigate the Raman properties of 2D YbOCl nanosheets. The experimentally observed E_g mode at 85.53 cm⁻¹ and A_1g mode at 138.17 cm⁻¹ demonstrate a good match to the results from density functional theory calculations. Furthermore, the temperature‐dependent and thickness‐dependent Raman scattering spectra reveal the adjacent layers in YbOCl nanosheets show a relatively weak van der Waals interaction. Additionally, the polarized‐dependent Raman scattering spectra show the intensity of A_1g mode exhibits twofold patterns while the intensity of the E_g mode remains constant as the rotation angle changes. These findings could provide the first‐hand experimental information about the 2D YbOCl crystals.

The origins and critical factors, which lead to organic field‐effect transistors (OFETs) with deviations from the ideal device models, are comprehensively uncovered from the view point of device physics. Also, the recent progress in optimizing strategies and new perspectives on nonideal OFETs are presented.

Abstract

Many advanced materials have been developed for organic field‐effect transistors (OFETs) or thin‐film transistors (TFTs) based on organic and organic hybrid materials. However, although many new OFETs exhibit superior characteristic parameters (such as high mobility), most of them show nonideal performances that have strongly limited progress in the design of molecules, the understanding of transport mechanisms, and the circuit applications of OFETs. In this review, the device physics of ideal and nonideal OFETs is discussed first to understand the factors that limit effective mobility in semiconducting channels, distort the potential distribution, or reduce the drift electric field. Then, recent advances in optimizing the material combinations, device structures, and fabrications of OFETs toward ideal transistors are discussed. Based on the good control of materials and interfaces, some new and novel concepts to utilize the nonideal properties of OFETs to build low‐power circuits and integrated sensors are also discussed.

A multibeam interference model is developed to analyze irregular scattering‐type scanning near‐field optical microscopy images of polaritons induced by small sample size or complex edges. This model extracts the polariton wave vectors and ratio of scattering rate to reflectivity at edge, which is important for studying van der Waals nanomaterials smaller than 10 µm and designing integrated nanophotonic devices.

Abstract

Van der Waals (vdW) materials are among the most promising candidates for photonic integrated circuits because they support a full set of polaritons that can manipulate light at deep subdiffraction nanoscale. It is possible to directly probe the propagating polaritons in vdW materials in real space via scattering‐type scanning near‐field optical microscopy, such that the wave vector and lifetime of the polaritons can be extracted from as‐measured interference fringes by Fourier analysis. However, this method is unsuitable for clutter interference patterns in samples exhibiting inadequate fringes due to small size (less than 10 µm) or complex edges that are often encountered in nanophotonic devices and new material characterization. Here, a multibeam interference model is developed to analyze complex images by disentangling them into periodic patterns and residue. By employing phase stationary approximation, polariton wave vector can be derived from offset ratio of the center point, and the ratio of polariton reflection and scattering rates at the edge is obtained from the ratio of the periodic and aperiodic patterns. This method can be widely used in the optical characterization of new vdW materials that are difficult to synthesize into large crystals, as well as nanophotonic integrated devices with unique boundaries.

Various theoretical and experimental researches regarding the mechanism of degradation and passivation strategies are proposed and reported to overcome the problem of the ambient instability of phosphorene. Here, not only an extensive summary of these passivation strategies but also an overview of the fabrication methods, challenges, and suitable applications of phosphorene are provided.

Abstract

Phosphorene as a rising star is a monolayer or few‐layer form of black phosphorus (BP), which is used as a 2D material, in addition to graphene. This monoelemental 2D material has gained considerable attention in the fields of electronics, optoelectronics, and biomedicine due to its extraordinary physical properties. However, as both theoretical and experimental works show, the intrinsic instability of phosphorene under ambient conditions is a major challenge in practical applications. Various theoretical and experimental researches regarding the mechanism of the degradation and passivation strategies are proposed and reported to overcome the problem of the ambient instability of phosphorene. These strategies have enabled researchers to conduct fundamental studies on phosphorene's extraordinary properties. Here, not only an extensive summary of these passivation strategies but also an overview of the fabrication methods, challenges, and suitable applications of phosphorene are provided.

An improved understanding of charge transport physics has enabled high‐performance organic field‐effect transistors through microstructure and electronic structure control by altering various donor and acceptor units. This report discusses in detail the relationship between donor–acceptor‐conjugated polymer structure and charge transport and summarizes the key features of the molecular design strategies.

Abstract

Polymeric semiconductors have demonstrated great potential in the mass production of low‐cost, lightweight, flexible, and stretchable electronic devices, making them very attractive for commercial applications. Over the past three decades, remarkable progress has been made in donor–acceptor (D–A) polymer‐based field‐effect transistors, with their charge‐carrier mobility exceeding 10 cm² V⁻¹ s⁻¹. Numerous molecular designs of D–A polymers have emerged and evolved along with progress in understanding the charge transport physics behind their high mobility. In this review, the current understanding of charge transport in polymeric semiconductors is covered along with significant features observed in high‐mobility D–A polymers, with a particular focus on polymeric microstructures. Subsequently, emerging molecular designs with further prospective improvements in charge‐carrier mobility are described. Moreover, the current issues and outlook for future generations of polymeric semiconductors are discussed.

Porous carbon nitrides enable “facile functionalization” when its domain is wetted by solution of metal halides with mobile cations/anions. During the wetting process, each cavity can be functionalized by a unit of metal halide, endowing the systems with various desirable properties for potential applications in multiferroicity, piezoelectronics, valleytronics, and photovoltaics.

Abstract

Ab initio calculation evidence has shown that two‐dimensional (2D) carbon nitrides may enable “facile functionalization” when a domain of carbon nitride is wetted by a solution of metal halides with mobile cations/anions. During the wetting process, each cavity can be functionalized by a unit of metal halide. Compared with prevailing functionalization or doping strategies through either high‐temperature diffusion of source ions or ion implantation by using accelerators, such a room‐temperature “wet‐lab” functionalization approach is more facile and efficient. The wet‐lab functionalization not only can facilitate isolation of the 2D monolayer, but also, with applying different metal halides, enable various new and desirable properties for broad applications, e.g., 2D magnetism and 2D ferroelectricity with high piezoelectric coefficient. The latter can be implemented in spin‐independent valleytronics for non‐volatile electrical manipulations. Notably, tunable bandgaps, ranging from 1.0 to 2.5 eV, can be realized by controlling the metal‐halide functionalization density, while the separation of electrons/holes can be facilitated by the ferroelectric polarizations and heterostructure band alignments. Moreover, multifunctional domains like P/N doped or magnetic/ferroelectric domains can be selectively constructed through such solution‐processed functionalization with different halides, followed by seamless integration into a single sheet of carbon nitride, akin to the P/N channels in silicon wafers.

Nature Nanotechnology, Published online: 19 August 2019; doi:10.1038/s41565-019-0525-8

The Weyl semimetal WTe2 possesses strong spin–orbit coupling and time-reversal-protected spin polarization in surface and bulk states. In a WTe2/permalloy heterostructure, WTe2 can act as a spin current source that enables magnetization switching at low current densities.

Nature Nanotechnology, Published online: 19 August 2019; doi:10.1038/s41565-019-0541-8

Energy-efficient magnetization manipulation is a prerequisite for competitive spintronic devices. The Weyl semimetal WTe2 can act as a spin current source that enables magnetization switching of an adjacent ferromagnet at low power consumption and additionally induces chiral magnetism.

Nature Nanotechnology, Published online: 19 August 2019; doi:10.1038/s41565-019-0520-0

Local changes of the Coulomb interaction due to external dielectric environment fluctuations present a new type of disorder in monolayer transition-metal dichalcogenides.

Nature Physics, Published online: 26 August 2019; doi:10.1038/s41567-019-0621-6

The authors demonstrate magnetoresistance of 80% from a two-dimensional electron gas proximity coupled to a ferromagnetic layer. This extends spintronics functionality to semiconductor devices.

Current approaches for electric power generation from nanoscale conducting or semiconducting layers in contact with moving aqueous droplets are promising as they show efficiencies of around 30%, yet even the most successful ones pose challenges regarding fabrication and scaling. Here, we report stable, all-inorganic single-element structures synthesized in a single...

Water–solid interfaces play important roles in a wide range of fields, including atmospheric science, geochemistry, electrochemistry, and food science. Herein, we report simulation evidence of 2-dimensional (2D) ice formation on various surfaces and the dependence of the 2D crystalline structure on the hydrophobicity and morphology of the underlying surface. Contrary...

Two-dimensional monolayer materials, with thicknesses of up to several atoms, can be obtained from almost every layer-structured material. It is believed that the catalogs of known 2D materials are almost complete, with fewer new graphene-like materials being discovered. Here, we report 2D graphene-like monolayers from monoxides such as BeO, MgO,...

We report an in situ synthesis of ruthenium-reduced graphene oxide (Ru-rGO) using 6 MeV electron beam assisted radiolytic reduction method and its supercapacitive behavior. X-ray diffraction (XRD), transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) explore Ru nanoparticles of size ~2 nm are decorated on rGO sheets. Raman spectroscopy shows I D / I G ratio increased and formation of bilayer rGO after electron beam irradiation. The defect density in Ru-rGO is increased due to the electron beam irradiation as compared to its counterpart GO. The Ru-rGO based supercapacitor exhibits specific capacitance (128.1  ±  5.59) F g −1 at 10 mV s −1 scan rate. The specific capacitance retention of Ru-rGO is up to 99.4% at 900 cycles while it increases to 130% at 5000 cycles. Discharge curve of the supercapacitor involves three current decay processes viz. activation polarization, ohmic pol...

The design of efficient graphene-silicon (GSi) Schottky junction photodetectors requires detailed understanding of the spatial origin of the photoresponse. Scanning-photocurrent-microscopy (SPM) studies have been carried out in the visible wavelengths regions only, in which the response due to silicon is dominant. Here we present comparative SPM studies in the visible ( ##IMG## [http://ej.iop.org/images/2053-1583/6/4/041004/tdmab32f5ieqn001.gif] nm) and infrared ( ##IMG## [http://ej.iop.org/images/2053-1583/6/4/041004/tdmab32f5ieqn002.gif] nm) wavelength regions for a number of GSi Schottky junction photodetector architectures, revealing the photoresponse mechanisms for silicon and graphene dominated responses, respectively, and demonstrating the influence of electrostatics on the device performance. Local electric field enhancement at the graphene edges leads to a more than ten-fold increased photoresponse compared to the bulk of ...

In large-scale electronic applications of graphene, imperfections play a key role in controlling the electrical properties. Here we directly probe the electrical-degradation effects induced by wrinkles, grain boundaries, multilayered islands, cracks, holes, and adsorbates on millimeter-scale graphene on a SiO 2 /Si substrate using a four-probe scanning tunneling microscope. By comparing the local measurements near and far away from these imperfections, we quantify their impact on the most important figures of merit including sheet resistance, carrier mobility, and residual carrier-density variations in the vicinity of the imperfections. Angle-dependent measurements via a van der Pauw geometry are then performed to determine the influence of imperfections on the whole graphene flake. A key result is that, as long as the imperfections do not extend continuously over the entire flake, the overall electrical properties of a graphene flake are not distinctly impacted by the ...

The chemical vapor deposition (CVD) of graphene on thin Cu film wafers is highly desirable for the development of technological applications as it offers superior flatness, rigidity, high purity, and compatibility with conventional thin film techniques. Here, we report the high-throughput synthesis of uniform single-layer highly crystalline graphene on a batch of 3-inch wafers. The production throughput is optimized by using closely-packed vertically-standing wafers instead of placing them flat on a horizontal support. Significantly reducing convective gas transport is found to be essential to minimize the variation of graphene single crystal seeding density and growth rate across individual wafers. Given the very small amount of carbon required to grow an atomically-thin layer, we show that graphene can be grown under static gas flow conditions and that the growth rate does not significantly vary from one wafer to another, even within a large batch (∼25 wafers). These findings ...

Lattice defects (mainly chalcogen vacancies) drastically affect the optoelectronic properties of monolayer transition metal dichalcogenides (TMDs) grown by chemical vapor deposition (CVD). They can be passivated through charge-transfer doping by laser irradiation in air. Here we perform a systematic in situ study to elucidate the passivation mechanism upon laser irradiation and show a way to controllably n-dope CVD-grown monolayer MoS 2 on SiO 2 substrates. By combining resonance Raman and photoluminescence (PL) spectroscopy we show that an increase in defect density correlates with a redshifted PL emission and hence an increase in electron density. Density functional theory (DFT) calculations identify chalcogen vacancies to be facilitators (not the source) of n-doping, and population of mid-gap levels upon doping lowers the activation barrier for O 2 adsorption from 0.3 to 0.03 eV. Laser irradiation aids in the oxygen-passivation of chalcoge...