Xingxing Zhang

A single‐crystalline gallium nitride (GaN) film with atomic‐step terraces is realized on a complementary metal‐oxide‐semiconductor‐compatible Si(100) substrate by using a one‐atom‐thick single‐crystalline graphene buffer layer. The monolayer single‐crystalline graphene provides an in‐plane driving force for the uniform alignment of nitrides domains. This approach can also enable the growth of wafer‐scale hexagonal single‐crystalline films on amorphous or flexible substrates.

Abstract

Fabricating single‐crystalline gallium nitride (GaN)‐based devices on a Si(100) substrate, which is compatible with the mainstream complementary metal‐oxide‐semiconductor circuits, is a prerequisite for next‐generation high‐performance electronics and optoelectronics. However, the direct epitaxy of single‐crystalline GaN on a Si(100) substrate remains challenging due to the asymmetric surface domains of Si(100), which can lead to polycrystalline GaN with a two‐domain structure. Here, by utilizing single‐crystalline graphene as a buffer layer, the epitaxy of a single‐crystalline GaN film on a Si(100) substrate is demonstrated. The in situ treatment of graphene with NH₃ can generate sp³ CN bonds, which then triggers the nucleation of nitrides. The one‐atom‐thick single‐crystalline graphene provides an in‐plane driving force to align all GaN domains to form a single crystal. The nucleation mechanisms and domain evolutions are further clarified by surface science exploration and first‐principle calculations. This work lays the foundation for the integration of GaN‐based devices into Si‐based integrated circuits and also broadens the choice for the epitaxy of nitrides on unconventional amorphous or flexible substrates.

A nickel‐based 2D metal–organic nanosheet is reported using polyvinylpyrrolidone as the structure‐directing agent. The heterobilayers of Ni₇S₆/graphene are derived from the 2D metal–organic framework using thiourea under hydrothermal conditions. The composite is used for electrochemical lithium storage application.

Abstract

Herein, a novel polymer‐templated strategy is described to obtain 2D nickel‐based MOF nanosheets using Ni(OH)₂, squaric acid, and polyvinylpyrrolidone (PVP), where PVP has a dual role as a structure‐directing agent, as well as preventing agglomeration of the MOF nanosheets. Furthermore, a scalable method is developed to transform the 2D MOF sheets to Ni₇S₆/graphene nanosheet (GNS) heterobilayers by in situ sulfidation using thiourea as a sulfur source. The Ni₇S₆/GNS composite shows an excellent reversible capacity of 1010 mAh g⁻¹ at 0.12 A g⁻¹ with a Coulombic efficiency of 98% capacity retention. The electrochemical performance of the Ni₇S₆/GNS composite is superior not only to nickel sulfide/graphene‐based composites but also to other metal disulfide–based composite electrodes. Moreover, the Ni₇S₆/GNS anode exhibits excellent cycle stability (≈95% capacity retention after 2000 cycles). This outstanding electrochemical performance can be attributed to the synergistic effects of Ni₇S₆ and GNS, where GNS serves as a conducting matrix to support Ni₇S₆ nanosheets while Ni₇S₆ prevents restacking of GNS. This work opens up new opportunities in the design of novel functional heterostructures by hybridizing 2D MOF nanosheets with other 2D nanomaterials for electrochemical energy storage/conversion applications.

Nature Nanotechnology, Published online: 10 June 2019; doi:10.1038/s41565-019-0466-2

Nature Nanotechnology, Published online: 17 June 2019; doi:10.1038/s41565-019-0467-1

Nature Nanotechnology, Published online: 18 June 2019; doi:10.1038/s41565-019-0491-1

Nature Materials, Published online: 08 July 2019; doi:10.1038/s41563-019-0415-3

Nature Materials, Published online: 15 July 2019; doi:10.1038/s41563-019-0423-3

The coassembly of chiral gelators and chiral iron disulfide quantum dots is carried out. It is found that the helical pitch and diameter of the cogels are remarkably regulated by illumination with circularly polarized light. This discovery opens up avenues to precisely synthesize chiral composite nanosystems and control their nanohelical chirality.

Abstract

Chiral inorganic nanomaterials have recently attracted significant attention because of their many important applications, such as in asymmetric catalysis and chiral sensing. Here, chiral iron disulfide quantum dots (FeS₂ QDs) are synthesized via chirality transfer using l/d‐cysteine (Cys) as chiral ligands. The chiral FeS₂ QDs are coassembled with two gelators to produce a cogel (l‐ or d‐[Gel+FeS₂]) with a g‐factor value of ±0.06. Interestingly, the cogels display intense circularly polarized luminescence. More significantly, the degree of twisting (twist pitch) and the diameter of the cogels can be markedly regulated by illumination with circularly polarized light (CPL) in the ranges of 120–213 and 37–65 nm, respectively, which is caused by the CPL‐induced electron transfer. This research opens the way for the design of chiroptical devices with a wide range of functions and applications.

Nature Communications, Published online: 16 July 2019; doi:10.1038/s41467-019-10939-x

Solution‐processed heterojunction and superlattice channel transistors composed of sequentially deposited In₂O₃ and ZnO layers show remarkably different operating characteristics depending on the stack configuration. The difference relates to the quality of the heterointerface and its dependence on the material deposition sequence. Optimized superlattice transistors fabricated on plastic substrates operate at ±1.5 V with a maximum electron mobility of 25 cm² V^‐1 s^‐1.

Abstract

The astonishing recent progress in the field of metal oxide thin‐film transistors (TFTs) and their debut in commercial displays is accomplished using vacuum‐processed multicomponent oxide semiconductors. However, emulating this success with their solution‐processable counterparts poses numerous scientific challenges. Here, the development of high mobility n‐channel TFTs based on ultrathin (<10 nm) alternating layers of In₂O₃ and ZnO that are sequentially deposited to form heterojunction and superlattice channels is reported. The resulting TFTs exhibit high electron saturation mobility (13 cm² V⁻¹ s⁻¹), excellent current on/off ratios (>10⁸) with nearly zero onset voltages and hysteresis‐free operation despite the low temperature processing (≤200 °C). The enhanced performance is attributed to the formation of a quasi‐2D electron gas‐like system at the In₂O₃/ZnO heterointerface due to the conduction band offset. It is shown that altering the oxide deposition sequence has an adverse effect on electron transport due to formation of trap states. Optimized multilayer TFTs are shown to exhibit improved bias‐stress stability compared to single‐layer TFTs. Modulating the electron concentration within the superlattice channel via selective n‐doping of the ZnO interlayers leads to almost 100% saturation mobility increase (≈25 cm² V⁻¹ s⁻¹) even when the TFTs are fabricated on flexible plastic substrates.

The potential of self‐propagating defects in black phosphorus (BP) is exploited to create functional optoelectronic capabilities, particularly unique wavelength‐selective photoresponse characteristics. To unveil the potential offered by defect engineering of 2D materials, three distinct optoelectronic applications for UV‐A/B discrimination, light‐stimulated logic operations, and neuromorphic computation are demonstrated in BP devices.

Abstract

Layered black phosphorus (BP), a promising 2D material, tends to oxidize under ambient conditions. While such defective BP is typically considered undesirable, defect engineering has in fact been exploited in contemporary materials to create new behaviors and functionalities. In this spirit, new opportunities arising from intrinsic defect states in BP, particularly through harnessing unique photoresponse characteristics, and demonstrating three distinct optoelectronic applications are demonstrated. First, the ability to distinguish between UV‐A and UV‐B radiations using a single material that has tremendous implications for skin health management is shown. Second, the same device is utilized to show an optically stimulated mimicry of synaptic behavior opening new possibilities in neuromorphic computing. Third, it is shown that serially connected devices can be used to perform digital logic operations using light. The underpinning photoresponse is further translated on flexible substrates, highlighting the viability of the technology for mechanically conformable and wearable systems. This demonstration paves the way toward utilizing the unexplored potential offered by defect engineering of 2D materials for applications spanning across a broad range of disciplines.

Self‐assembly of chiral copper (Cu) heterostructures in black phosphorus (BP) is triggered using simple and reproducible methods. Multi‐modal electron microscopy techniques reveal the formation of atomically thin Cu heterostructures templated by the structural anisotropy of the BP nanosheets. Using density functional theory calculations, the growth process, the atomic structure, and metallic nature of this novel hybrid material are also investigated.

Abstract

The fabrication of 2D systems for electronic devices is not straightforward, with top‐down low‐yield methods often employed leading to irregular nanostructures and lower quality devices. Here, a simple and reproducible method to trigger self‐assembly of arrays of high aspect‐ratio chiral copper heterostructures templated by the structural anisotropy in black phosphorus (BP) nanosheets is presented. Using quantitative atomic resolution aberration‐corrected scanning transmission electron microscopy imaging, in situ heating transmission electron microscopy and electron energy‐loss spectroscopy arrays of heterostructures forming at speeds exceeding 100 nm s⁻¹ and displaying long‐range order over micrometers are observed. The controlled instigation of the self‐assembly of the Cu heterostructures embedded in BP is achieved using conventional electron beam lithography combined with site specific placement of Cu nanoparticles. Density functional theory calculations are used to investigate the atomic structure and suggest a metallic nature of the Cu heterostructures grown in BP. The findings of this new hybrid material with unique dimensionality, chirality, and metallic nature and its triggered self‐assembly open new and exciting opportunities for next generation, self‐assembling devices.

Boron‐doped graphene quantum dots (B‐GQDs) with a strong second harmonic generation (SHG) signal are first synthesized. Mouse mesenchymal stem cells with internalized B‐GQDs are seeded on an acellular dermal matrix, which are subsequently transplanted to the mouse skin wound site. The B‐GQDs with SHG are applied for stem cell imaging and tracking in wound healing in vivo.

Abstract

Second harmonic generation (SHG) has recently emerged, having the advantages of no bleaching, no blinking, and no signal saturation, as well as a high signal to‐noise ratio compared to fluorescence. Existing SHG probes are based on heavy metal or organic dye molecules, which have the shortcomings of toxicity, a large size, or photoinstability. To address the urgent need for long‐term tracking and imaging in organisms, boron‐doped graphene quantum dots (B‐GQDs), a highly biocompatible graphene based with a strong and photostable SHG signal is first synthesized and is further applied for stem cell imaging and tracking in wounds. The results demonstrate the possibility of stem cell internalization of B‐GQDs as a SHG probe and show no hindering of the stem cell's central physiological activities such as differentiation. Most importantly, B‐GQDs are successfully tracked in skin tissue in vivo after the labeled mouse mesenchymal stem cells being implanted over 35 days. This work will inspire the development of doped graphene quantum dot materials and promote the broad use of B‐GQDs in future molecular imaging, drug delivery, and stem cell therapy.

Ultrahigh thermoelectric performance in black phosphorus is predicted through group VA doping by resonant band manipulation. Strikingly, ZT values can reach up to 1.21 and 0.87 at 300 K in p‐type BiP₇ and n‐type NP₃, respectively. Further significant enhancement is also found in N–P systems at 800 K, indicating promising candidates for nontoxic, metal‐free, and ultralight thermoelectrics.

Abstract

Black phosphorus (BP) has emerged as a promising thermoelectric candidate because of its strong electronic and thermal anisotropy, suggesting a large σ/κ ratio can be realized by controlling carrier transport orientation for a potentially high ZT. Nevertheless, to date, low conversion efficiency (ZT ≈0.08, 300 K) and poor stability of BP remain the major issues that have hampered its practical applications. This work reports a material family in simple composition XP₇, XP₃, and XP (X = N, As, Sb, Bi) with high‐performance thermoelectric properties by first‐principles calculations. Strikingly, an ultrahigh ZT up to 1.21 at 300 K is achieved in p‐type BiP₇ with an optimal carrier concentration of 5.48 × 10¹⁹ cm⁻³ and ZT in n‐type NP₃ can reach up to ≈0.87 at the electron concentration of 3.67 × 10¹⁹ cm⁻³ along the zigzag direction, owing to their enhanced density of states and multivalley band structures around the Fermi level through the resonant effects of VA guest and host atoms. Additionally, the calculations demonstrate further improvement in thermoelectric performance of pristine BP by ≈4.8 and 4.5 times at 800 K in p‐type NP and n‐type NP₃, respectively. Considering the high stability, current results indicate that N–P based systems are highly promising for novel metal‐free, nontoxic, and ultralight thermoelectrics.