Xingxing Zhang

0D PbS nanocrystals are epitaxially grown from 2D PbI₂ nanosheet templates by a simple wet‐chemical sulfuration process. The resulting 0D‐PbS/2D‐PbI₂ based Structure II photodetector device demonstrates a wide detection range up to 2000 nm, fast photoresponse of ≈400 µs and specific detectivity of more than 10¹² Jones due to Au–PbI₂ Schottky contact and epitaxial growth.

Abstract

Heterostructures of 0D/2D materials synergistically combine the advantages of two different materials, and photodetectors (PDs) based on such heterostructures demonstrate superior detection properties as compared to those based on each individual material. Here, a facile solution method is proposed for the fabrication of 0D/2D heterostructures to overcome the limitations of the traditional chemical vapor deposition method. The 0D PbS nanocrystals (NCs) are epitaxially grown from 2D‐PbI₂ nanosheet templates by a simple wet‐chemical sulfuration process. The resulting heterojunction PDs demonstrate wide detection range up to 2000 nm, a fast photoresponse of ≈400 µs, and a specific detectivity of more than 10¹² Jones. The superior device performances are attributed to the underneath PbI₂ layer, which can not only be used as the precursor layer but also help to passivate the defect states of PbS NCs. More importantly, the developed Structured II device with Au–PbI₂ Schottky contact effect can suppress the dark current of PD to extend its depletion region and improve the response speed. Overall, this study demonstrates a novel strategy for the growth of 0D/2D heterostructures in conjunction with fast near‐infrared detection, which surmount the limitations of photoconductive detectors in terms of large‐scale synthesis and response speed.

This study uses standard nanofabrication techniques to create plasmonic structures of a widely used thin film semiconductor material, Sn‐doped indium oxide (ITO). Through electron microscopy and optical characterization, the tunability of these structures is demonstrated by altering the crystallinity through high‐temperature annealing. The ease of modifying its properties makes ITO an exciting alternative plasmonic material to noble metals.

Abstract

Transparent conductive oxides (TCOs) have strong potential for plasmonic applications. Given their easily tunable properties and low energy response, significant challenges in the controlled fabrication and precise characterization of TCOs must be better understood before this potential can be realized. Here, the mid‐ to near‐infrared plasmonic response of Sn‐doped In₂O₃ (ITO) nanostructures is presented, fabricated top‐down using electron beam lithography and radio‐frequency sputtering. These equilateral ITO triangles of different side lengths are imaged at high spatial and energy resolution with monochromated electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope. Applying the Richardson–Lucy (RL) deconvolution algorithm to experimental EELS spectra reveals localized surface plasmon (LSP) excitations between 150 and 550 meV and a 730 meV bulk plasmon. This very‐low‐energy response to an electron beam is compared with boundary element method simulations of nanostructures. These simulations use the dielectric functions of continuous thin films of the same materials, characterized by ellipsometry, 4‐point probe, and Hall effect tests. Additionally, upon rapid thermal annealing of ITO, blue‐shifts in LSP energy, and longer LSP lifetimes are examined as a consequence of an amorphous‐to‐polycrystalline transformation and an increase in the free carrier density.

In situ environmental transmission electron microscopy (TEM) measurements show that pit formation by thermal oxidation is preferentially initiated at the interface between adventitious carbon (C) nanoparticles and monolayer molybdenum disulfide (MoS₂), rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS₂ interface favors the sequential adsorption of oxygen atoms with facile kinetics.

Abstract

Forming pits on molybdenum disulfide (MoS₂) monolayers is desirable for (opto)electrical, catalytic, and biological applications. Thermal oxidation is a potentially scalable method to generate pits on monolayer MoS₂, and pits are assumed to preferentially form around undercoordinated sites, such as sulfur vacancies. However, studies on thermal oxidation of MoS₂ monolayers have not considered the effect of adventitious carbon (C) that is ubiquitous and interacts with oxygen at elevated temperatures. Herein, the effect of adventitious C on the pit formation on MoS₂ monolayers during thermal oxidation is studied. The in situ environmental transmission electron microscopy measurements herein show that pit formation is preferentially initiated at the interface between adventitious C nanoparticles and MoS₂, rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS₂ interface favors the sequential adsorption of oxygen atoms with facile kinetics. These results illustrate the important role of adventitious C on pit formation on monolayer MoS₂.

Terraced graphene, formed by laminating single‐layer graphene on a terraced substrate, shows a colossal magnetoresistance of up to 5000% at 9 T and 300 K. The magnetoresistance enhancement is attributed to the topographic corrugations and inhomogeneous charge puddles induced by the terraced structure. The concept of inducing colossal magnetoresistance by stepped surfaces is certainly boosting room‐temperature graphene magnetic sensors.

Abstract

Disorder‐induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in 2D materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single‐layer regime. Here, a room‐temperature colossal MR of up to 5000% at 9 T is reported in terraced single‐layer graphene. By laminating single‐layer graphene on a terraced substrate, such as TiO₂‐terminated SrTiO₃, a universal one order of magnitude enhancement in the MR compared to conventional single‐layer graphene devices is demonstrated. Strikingly, a colossal MR of >1000% is also achieved in the terraced graphene even at a high carrier density of ≈10¹² cm⁻². Systematic studies of the MR of single‐layer graphene on various oxide‐ and non‐oxide‐based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. The results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.

An outstanding feature of the topological Weyl semimetal WTe₂ is its novel spin topologies in the electronic band structure. An unconventional charge–spin conversion in WTe₂ due to its lower crystal symmetry combined with large Berry curvature and spin‐texture of the Fermi states is demonstrated. These findings have great potential for utilizing WTe₂ for spintronic circuits and quantum technologies.

Abstract

An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge‐to‐spin conversion efficiency. Here, a charge‐current‐induced spin polarization in the type‐II Weyl semimetal candidate WTe₂ and efficient spin injection and detection in a graphene channel up to room temperature are reported. Contrary to the conventional spin Hall and Rashba–Edelstein effects, the measurements indicate an unconventional charge‐to‐spin conversion in WTe₂, which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe₂ due to a reduced crystal symmetry combined with its large spin Berry curvature, spin–orbit interaction with a novel spin‐texture of the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization using both charge‐to‐spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as nonmagnetic spin sources in all‐electrical van der Waals spintronic circuits and for low‐power and high‐performance nonvolatile spintronic technologies.

A blue quantum dot‐based light‐emitting diode is fabricated on a scalable emission area of 2 × 2 in. with a patterned cathode accompanied by an evaporated TiO₂ thin film as an electron transport layer, which allows to perform conventional photolithography. A highly reproducible vaporized inorganic TiO₂ thin film secures a high performance and enhances the stability on the process.

Abstract

Quantum dot‐based light‐emitting diodes (QD‐LEDs) have excellent optical properties; however, their limitations of stability, reproducibility, and scalability due to the solution process are the major drawback. Herein, blue QD‐LEDs fabricated with the conventional vacuum process using an e‐beam‐evaporated TiO₂ thin film as an electron transport layer (ETL) are demonstrated. CdZnS/ZnS‐based blue LEDs with a TiO₂ thin film are fabricated under ambient conditions. They exhibit maximum external quantum efficiencies of 3.53% and a peak luminance of 2847 cd m⁻². These values are retained, which minimizes performance degradation under high potential bias. In addition, the optimized evaporated TiO₂ thin film has a negligible red shift (0.5 nm) of the peak wavelength between the photoluminescence spectrum and electroluminescence spectrum with stable full‐width at half‐maximum changing by less than 2 nm at high voltage. Finally, a blue QD‐LED is fabricated on a scalable emission area of 2 × 2 in. with a patterned cathode accompanied by an evaporated TiO₂ thin film, which allows to perform conventional photolithography. A highly stable and reproducible vaporized inorganic thin film as the ETL supports the multilayer architecture to minimize the process damage.

In article number https://doi.org/10.1002/adma.2020027022002702, Ting Zhang, Lei Wei, and co‐workers demonstrate a two‐step method to achieve ultralong single‐crystal SnSe wires with a stable rock‐salt structure and high thermoelectric performance. Large‐area, lightweight, breathable, and high‐performance thermoelectric fabrics are achieved, which can continuously generate electricity, offering potential in flexible and wearable electronics.

A light‐emitting transistor based on a van der Waals heterostructure is demonstrated. Utilizing tunable graphene contacts, holes and electrons are separately injected into the ambipolar WSe₂ monolayer. By balanced recombination of electrons and holes, the external quantum efficiency reaches ≈6% at room temperature. Multimode operation of this device powered by combination of electrical and optical states would be beneficial for optical circuitry.

Abstract

2D semiconductors have shown great potential for application to electrically tunable optoelectronics. Despite the strong excitonic photoluminescence (PL) of monolayer transition metal dichalcogenides (TMDs), their efficient electroluminescence (EL) has not been achieved due to the low efficiency of charge injection and electron–hole recombination. Here, multioperation‐mode light‐emitting field‐effect transistors (LEFETs) consisting of a monolayer WSe₂ channel and graphene contacts coupled with two top gates for selective and balanced injection of charge carriers are demonstrated. Visibly observable EL is achieved with the high external quantum efficiency of ≈6% at room temperature due to efficient recombination of injected electrons and holes in a confined 2D channel. Further, electrical tunability of both the channel and contacts enables multioperation modes, such as antiambipolar, depletion,and unipolar regions, which can be utilized for polarity‐tunable field‐effect transistors and photodetectors. The work exhibits great potential for use in 2D semiconductor LEFETs for novel optoelectronics capable of high efficiency, multifunctions, and heterointegration.

High‐performance thin‐film‐transistor (TFT) channel materials are essential components in future displays with novel functions and form factors. The basic materials properties and their TFT applications are discussed, focusing mainly on their performance. In addition, other key considerations such as bias‐ and light‐stability are also covered.

Abstract

As the need for super‐high‐resolution displays with various form factors has increased, it has become necessary to produce high‐performance thin‐film transistors (TFTs) that enable faster switching and higher current driving of each pixel in the display. Over the past few decades, hydrogenated amorphous silicon (a‐Si:H) has been widely utilized as a TFT channel material. More recently, to meet the requirement of new types of displays such as organic light‐emitting diode displays, and also to overcome the performance and reliability issues of a‐Si:H, low‐temperature polycrystalline silicon and amorphous oxide semiconductors have partly replaced a‐Si:H channel materials. Basic material properties and device structures of TFTs in commercial displays are explored, and then the potential of atomically thin layered transition metal dichalcogenides as next‐generation channel materials is discussed.

The space‐charge region between Cu₂Se host matrix and in‐situ‐formed BiCuSeO under a direct current causes drastic suppression of the Cu⁺ ion migration in such composites and obstructs the reduction reaction of Cu⁺ into Cu metal. This, together with the effective regulation of carrier concentration as well as enhanced interfacial phonon scattering, greatly stabilizes the improved thermoelectric performance.

Abstract

The applications of mixed ionic–electronic conductors are limited due to phase instability under a high direct current and large temperature difference. Here, it is shown that Cu₂Se is stabilized through regulating the behaviors of Cu⁺ ions and electrons in a Schottky heterojunction between the Cu₂Se host matrix and in‐situ‐formed BiCuSeO nanoparticles. The accumulation of Cu⁺ ions via an ionic capacitive effect at the Schottky junction under the direct current modifies the space‐charge distribution in the electric double layer, which blocks the long‐range migration of Cu⁺ and produces a drastic reduction of Cu⁺ ion migration by nearly two orders of magnitude. Moreover, this heterojunction impedes electrons transferring from BiCuSeO to Cu₂Se, obstructing the reduction reaction of Cu⁺ into Cu metal at the interface and hence stabilizes the β‐Cu₂Se phase. Furthermore, incorporation of BiCuSeO in Cu₂Se optimizes the carrier concentration and intensifies phonon scattering, contributing to the peak figure of merit ZT value of ≈2.7 at 973 K and high average ZT value of ≈1.5 between 400 and 973 K for the Cu₂Se/BiCuSeO composites. This discovery provides a new avenue for stabilizing mixed ionic–electronic conduction thermoelectrics, and gives fresh insights into controlling ion migration in these ionic‐transport‐dominated materials.