Xingxing Zhang

Planar and van der Waals heterostructures for vertical tunnelling single electron transistors

Planar and van der Waals heterostructures for vertical tunnelling single electron transistors, Published online: 16 January 2019; doi:10.1038/s41467-018-08227-1

The possibility to combine planar and van der Waals heterostructures holds great promise for nanoscale electronic devices. Here, the authors report an innovative method to synthesise embedded graphene quantum dots within hexagonal boron nitride matrix for vertical tunnelling single electron transistor applications.

Lithium intercalation into bilayer graphene

Lithium intercalation into bilayer graphene, Published online: 17 January 2019; doi:10.1038/s41467-018-07942-z

The mechanism of lithium storage in graphenic carbon remains a fundamental question to be addressed. Here the authors employ suitable bilayer graphene foam to investigate various physiochemical phenomena of lithium intercalation and propose a storage model.

Atomically thin transition metal dichalcogenides (TMDCs) have drawn much interest for their promising applications in electronic, optoelectronic, valleytronic and sensing fields. Controlled growth of large-scale and high-quality TMDC nanostructures is highly desirable but remains challenging. In the present work, large-scale monolayer, bilayer and few-layer MoSe 2 films have been controllably synthesized by ambient pressure chemical vapor deposition (APCVD). Hydrogen flow rate, growth temperature as well as selenium–metal flux ratio have been systematically investigated, which were demonstrated to play a key role in the synthesis of MoSe 2 nanostructures. We have also reported the successful growth of MoSe 2 nanoribbons with controlled width and length on diverse substrates by APCVD with the assistance of sodium chloride and corresponding growth mechanism was proposed. Our findings highlight the prospects for the controlled growth of novel 1D and 2D...

Discovery of colossal Seebeck effect in metallic Cu₂Se

Discovery of colossal Seebeck effect in metallic Cu₂Se, Published online: 08 January 2019; doi:10.1038/s41467-018-07877-5

Recent research efforts have aimed at discovering thermoelectric materials with high efficiency in the middle-low temperature range, where a majority of waste heat is lost to the ambient. Here, the authors discover colossal Seebeck coefficient values in metallic copper selenide from 340 K to 400 K.

2D materials hold immense potential for future, lightweight, flexible solar cells. In article number 1802722, Jayan Thomas, Tania Roy, and co‐workers describe recent progress made with graphene and other 2D materials for silicon, organic, and perovskite solar photovoltaics. They also present an outlook for meeting the challenges and requirements for 2D‐materials‐based next‐generation solar photovoltaics at low cost and high production rate.

Solution‐processed polymeric contacts used in 2D semiconductor devices are reported here. Predoping of the benzyl viologen alters the contact surface to obtain electron‐doped materials with high work functions. Ohmic contacts are induced by the polymer and the thus formed devices produce 3‐, 700‐, 3000‐, and 17‐fold increases in the mobilities for MoS₂, WSe₂, MoTe₂, and black phosphorus devices, respectively.

Abstract

The fabrication of a polymeric Ohmic contact interlayer between a metal and a 2D material using solution‐processed benzyl viologen (BV) is reported here. Predoping of the polymer alters the contact surface to obtain electron‐doped materials with ultrahigh work functions that significantly enhance the current density across the contact and reduce the contact resistance and Schottky barrier height. The fabrication of solution‐processed polymeric contacts for the preparation of high mobility MoS₂, WSe₂, MoTe₂, and BP (black phosphorous) FETs with significantly lowered contact resistance is demonstrated. Ohmic contacts are achieved and produce 3‐, 700‐, 3000‐, and 17‐fold increases in electron mobilities, respectively, when the bottom gate voltage is 10 V compared to those respective materials alone. Ambipolar and p‐type 2D material based FETs could, therefore, be transformed into n‐type FETs. Most importantly, the devices exhibit excellent stability in both ambient and vacuum.

In‐plane anisotropic thermal conductivities are experimentally demonstrated in suspended Td‐WTe₂ of different thicknesses. Theoretical calculation implies that the in‐plane anisotropy is attributed to the different mean free paths along the two orientations. As the thickness decreases, the phonon‐boundary scattering increases faster along the armchair direction, resulting in the stronger anisotropy.

Abstract

2D Td‐WTe₂ has attracted increasing attention due to its promising applications in spintronic, field‐effect chiral, and high‐efficiency thermoelectric devices. It is known that thermal conductivity plays a crucial role in condensed matter devices, especially in 2D systems where phonons, electrons, and magnons are highly confined and coupled. This work reports the first experimental evidence of in‐plane anisotropic thermal conductivities in suspended Td‐WTe₂ samples of different thicknesses, and is also the first demonstration of such anisotropy in 2D transition metal dichalcogenides. The results reveal an obvious anisotropy in the thermal conductivities between the zigzag and armchair axes. The theoretical calculation implies that the in‐plane anisotropy is attributed to the different mean free paths along the two orientations. As thickness decreases, the phonon‐boundary scattering increases faster along the armchair direction, resulting in stronger anisotropy. The findings here are crucial for developing efficient thermal management schemes when engineering thermal‐related applications of a 2D system.

A triboiontronic field‐effect transistor of molybdenum disulfide is proposed to bridge triboelectric potential modulation and ion‐controlled semiconductor devices. The operation mechanism of the triboiontronic transistor is investigated and high current on/off ratio (>10⁷), low threshold value (75 µm), and steep switching properties (20 µm dec⁻¹) are achieved.

Abstract

Electric double layers (EDLs) formed in electrolyte‐gated field‐effect transistors (FETs) induce an extremely large local electric field that gives a highly efficient charge carrier control in the semiconductor channel. To achieve highly efficient triboelectric potential gating on the FET and explore diversified applications of electric double layer FETs (EDL‐FETs), a triboiontronic transistor is proposed to bridge triboelectric potential modulation and ion‐controlled semiconductor devices. Utilizing the triboelectric potential instead of applying an external gate voltage, the triboiontronic MoS₂ transistor is efficiently operated owing to the formation of EDLs in the ion‐gel dielectric layer. The operation mechanism of the triboiontronic transistor is proposed, and high current on/off ratio over 10⁷, low threshold value (75 μm), and steep switching properties (20 µm dec⁻¹) are achieved. A triboiontronic logic inverter with desirable gain (8.3 V mm⁻¹), low power consumption, and high stability is also demonstrated. This work presents a low‐power‐consuming, active, and a general approach to efficiently modulate semiconductor devices through mechanical instructions, which has great potential in human–machine interaction, electronic skin, and intelligent wearable devices. The proposed triboiontronics utilize ion migration and arrangement triggered by mechanical stimuli to control electronic properties, which are ready to deliver new interdisciplinary research directions.

Heterostructures composed of a 2D layered material (2DLM) and complex transition metal oxides (TMO) provide a new platform for design, realization, and examination of artificial materials that are physically intriguing and technologically useful. An overview is presented for some of the examples, where various functional properties emerge at the novel 2DLM/TMO heterostructures.

Abstract

The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO₃, graphene/LaAlO₃/SrTiO₃, graphene/ferroelectric oxide, MoS₂/SrTiO₃, and FeSe/SrTiO₃ heterostructures include voltage scaling in field‐effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this context, various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems are reviewed. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically as each constituent material is a promising candidate for next‐generation optoelectronic devices.