Jiuxiang Dai

A photoelectrochemical-type photosensor using monocrystalline p-GaN nanowires on the Si platform is built with two primary interfaces, at which the charge separation and transport determine the photoresponses. With rational Platinum decoration, the balance of the charge carriers is tuned, making the device exhibit either positive or negative photocurrent upon different light illumination.

Abstract

The carrier transport dynamics at the surface/interface of semiconductors determine the electronic and optical properties of devices. Thus, precise control of their dynamic processes while understanding the nature of these characteristics is crucial for modulating device functionalities. Here, a photoelectrochemical-type photosensor is built using monocrystalline p-GaN nanowires on the Si platform, which unambiguously exhibits either positive or negative photocurrent upon different light illumination. Such dual-polarity photocurrents are attributed to photogenerated-carrier-transport competition at two interfaces: the GaN/electrolyte and GaN/Si interface. Particularly, a rational Pt decoration successfully accelerates the carrier migration at the GaN/electrolyte interface that breaks the original balance of carrier transport. This mechanism is further elaborated by Kelvin-probe-force-microscopy characterization, which intuitively reveals the impact of Pt decoration on modulating nanowires’ surface band bending and the consequent carrier dynamics at the interfaces. These insights into the control of carrier dynamics shed light on achieving multi-functional PEC devices built upon simple semiconductor architectures.

Ultrathin CrTe₃ films are successfully synthesized. Monolayer (ML) CrTe₃ represents a unique van der Waals (vdW) magnet with intralayer antiferromagnetism, which is identified by scanning probe microscopy. A facile method is developed to fabricate CrTe₃/CrTe₂ magnetic heterostructures from ML CrTe₃ with an atomically sharp and seamless interface, which is significant for the development of miniaturized spintronic devices based on vdW magnets.

Abstract

Ultrathin van der Waals (vdW) magnets are heavily pursued for potential applications in developing high-density miniaturized electronic/spintronic devices as well as for topological physics in low-dimensional structures. Despite the rapid advances in ultrathin ferromagnetic vdW magnets, the antiferromagnetic counterparts, as well as the antiferromagnetic junctions, are much less studied owing to the difficulties in both material fabrication and magnetism characterization. Ultrathin CrTe₃ layers have been theoretically proposed to be a vdW antiferromagnetic semiconductor with intrinsic intralayer antiferromagnetism. Herein, the epitaxial growth of monolayer (ML) and bilayer CrTe₃ on graphite surface is demonstrated. The structure, electronic and magnetic properties of the ML CrTe₃ are characterized by combining scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy and confirmed by density functional theory calculations. The CrTe₃ MLs can be further utilized for the fabrication of a lateral heterojunction consisting of ML CrTe₂ and ML CrTe₃ with an atomically sharp and seamless interface. Since ML CrTe₂ is a metallic vdW magnet, such a heterostructure presents the first in-plane magnetic metal–semiconductor heterojunction made of two vdW materials. The successful fabrication of ultrathin antiferromagnetic CrTe₃, as well as the magnetic heterojunction, will stimulate the development of miniaturized antiferromagnetic spintronic devices based on vdW materials.

A new family of 2D transition metal carbo-chalcogenides (TMCCs) can be considered an atomic-level combination of TM carbides (MXenes) and TM dichalcogenides (TMDCs). Realized by electrochemical-assisted delamination, Nb₂S₂C and Ta₂S₂C are the first examples of TMCCs to be delaminated into single layers. These newly developed 2D TMCCs are 50% stronger than TMDC counterparts and have a wide range of applications.

Abstract

Here, a new family of 2D transition metal carbo-chalcogenides (TMCCs) is reported, which can be considered a combination of two well-known families, TM carbides (MXenes) and TM dichalcogenides (TMDCs), at the atomic level. Single sheets are successfully obtained from multilayered Nb₂S₂C and Ta₂S₂C using electrochemical lithiation followed by sonication in water. The parent multilayered TMCCs are synthesized using a simple, scalable solid-state synthesis followed by a topochemical reaction. Superconductivity transition is observed at 7.55 K for Nb₂S₂C. The delaminated Nb₂S₂C outperforms both multilayered Nb₂S₂C and delaminated NbS₂ as an electrode material for Li-ion batteries. Ab initio calculations predict the elastic constant of TMCC to be over 50% higher than that of TMDC.

Light: Science & Applications, Published online: 14 April 2022; doi:10.1038/s41377-022-00754-3

We demonstrate that plexcitons in monolayer semiconductors sustain giant nonlinearity at room temperature and thus very hopeful to realize the practical implementation of polaritonic devices.

Author(s): Yiqing Zhou, D. N. Sheng, and Eun-Ah Kim

Moiré systems provide a rich platform for studies of strong correlation physics. Recent experiments on heterobilayer transition metal dichalcogenide Moiré systems are exciting in that they manifest a relatively simple model system of an extended Hubbard model on a triangular lattice. Inspired by the…

[Phys. Rev. Lett. 128, 157602] Published Thu Apr 14, 2022

Nature Materials, Published online: 14 April 2022; doi:10.1038/s41563-022-01229-x

Slit-like nanochannels of pristine graphite and activated carbon, fabricated by van der Waals assembly of pristine or sculpted graphite crystals, enable comprehensive ionic response measurements and the systematic realization of their ion transport properties. These are attributed to optimal combinations of (mobile) surface charge and slippage effects at the channel wall surface in both pristine and activated nanochannels.

Nature Materials, Published online: 14 April 2022; doi:10.1038/s41563-022-01223-3

Nanometre-sized clusters can self-organize into centimetre-scale hierarchical structures, mimicking the complex constructions seen in nature and providing a platform to design synthetically directed advanced materials with sophisticated functions.

Ferroelectric Field-Effect Transistors

In article number 2200032, Hang Luo, Jian Sun, and co-workers report how a ferroelectric field-effect transistor (FeFET) can work as both a hysteresis-free low-power-consumption negative-capacitance field-effect transistor and a memory device for neural computing. The functions are reconfigured and controlled by modulating the behaviors of the interfacial oxygen vacancies. The frontispiece shows the chips consisting of the reconfigurable FeFET devices.

Exciton Linewidth

Due to strong multiparticle interactions in 2D transition metal dichalcogenides at room temperature, their homogeneous exciton linewidths are significantly broadened, degrading the quality of their excitonic mode and emission. In article number 2108721, Yuebing Zheng and co-workers achieve near-intrinsic exciton linewidth in monolayer WS₂ at room temperature, approaching the theoretical limit at 0 K. A dielectric nanosphere is designed to boost the dynamic competition between exciton and trion decay channels, rebuilding the excitonic relaxation processes with suppressed exciton nonradiative recombination.

Conformal Amorphous Carbon

Ultrathin Cu diffusion barriers with high conformality have gained great attention for next-generation ultrahigh-density semiconductor device miniaturization. In article number 2110454, Sun Hwa Lee, Rodney S. Ruoff, Ki-Bum Kim, Sang Ouk Kim, and co-workers introduce a handy and reliable method for the preparation of conformal amorphous carbon (a-C) barrier layers with nanometer-level thickness. A polystyrene brush layer is grafted onto a 3D copper structure with self-limiting chemistry, and subsequent carbonization yields large-area uniform 1 nm-level a-C layers with excellent Cu blocking performance.

In-plane polarization in MoS₂ is realized through contact with low-symmetric CrOCl. The emergence of asymmetric second harmonic generation pattern in MoS₂/CrOCl heterojunction indicates the variation of lattice symmetry in MoS₂, which stems from lattice-mismatch-induced uniaxial strain because of the strong interlayer interactions. More importantly, the strong linear polarization-sensitive photodetection is realized.

Abstract

2D materials with low-symmetry exhibit anisotropic physical properties, making them promising candidates for various applications. However, the lack of matured synthesis methods in anisotropic 2D materials is still the main obstacle to their future applications. Given the mature synthesis method of transition metal dichalcogenides (TMDCs), manipulating anisotropy in 2D TMDCs becomes a promising way to tune or trigger functional properties. Herein, for the first time, a van der Waals symmetry engineering is reported to introduce in-plane polarization in MoS₂ through contact with low-symmetric CrOCl. The emergence of asymmetric second harmonic generation pattern in MoS₂/CrOCl heterojunction indicates the variation of lattice symmetry in MoS₂. Furthermore, the theoretical simulation shows that such change stems from lattice-mismatch-induced uniaxial strain because of the strong interlayer interactions. The angle-dependent Raman and photoluminescence spectra further identify that the uniaxial strain gives rise to the in-plane polarization in MoS₂. In addition, the polarized MoS₂ exhibits excellent orientation-sensitive electrical characteristics with a conductance anisotropy ratio of ≈1.5. More importantly, the strong linear polarization-sensitive photodetection is realized, and the anisotropic ratio reached 1.25 with 532 nm. The results suggest that symmetric engineering potentially opens up a new field to endow high-symmetry 2D materials with anisotropic functionalities.

Fullerphene Nanosheets

In article number 2102241, Jingwen Song, Jonathan P. Hill, Lok Kumar Shrestha, Katsuhiko Ariga, and co-workers describe the synthesis of large area ultrathin fullerphene nanosheets using a Pickering emulsion precursor to obtain homogeneous fullerene film. Pyrolytic transformation of the film yields fullerphene film used to establish selective sensing of acid analytes.

Mediating point defects simultaneously modulates the carrier concentration and microstructure in n-type Bi₂Te₃ materials further generating superior thermoelectric and mechanical performance. With high strength and conversion efficiency of over 5% in a wide temperature range, the power generation module fabricated with this n-type Bi₂Te₃ has comprehensive advances compared with commercial modules and may be greatly capable of serving in a hostile environment.

Abstract

Bi₂Te₃-related alloys dominate the commercial thermoelectric market, but the layered crystal structure leads to the dissociation and intrinsic brittle fracture, especially for single crystals that may worsen the practical efficiency. In this work, point defect configuration by S/Te/I defects engineering is engaged to boost thermoelectric and mechanical properties of n-type Bi₂Te₃ alloy, which, coupled with p-type BiSbTe, shows a competitive conversion efficiency for the fabricated module. First, as S alloying suppresses the intrinsic BiTe, antisite defects and forms a donor-like effect, electronic transport properties are optimized, associated with the decreased thermal conductivity due to the point defect scattering. The periodide compound TeI₄ is afterward adopted to further tune carrier concentration for the realization of an optimal ZT. Finally, an advanced average ZT of 1.05 with ultra-high compressive strength of 230 MPa is achieved for Bi₂Te_2.9S_0.1(TeI₄)_0.0012. Based on this optimum composition, a fabricated 17-pair module demonstrates a maximum conversion efficiency of 5.37% under the temperature difference of 250 K, rivaling the current state-of-the-art Bi₂Te₃ modules. This work reveals the novel mechanism of point defect reconfiguration in synergistic enhancement of thermoelectric and mechanical properties for durably commercial application, which may be applicable to other thermoelectric systems.

Cyano functionalization of thienylene–vinylene–thienylene donor unit in n-type polymer f-BTI2g-TVTCN leads to simultaneously enhanced ion-uptake ability, film structural order, and charge-transport property compared to its non-cyanated analogue, subsequently enabling a record-high μ_e,OECT of 0.24 cm² V⁻¹ s⁻¹ and µC* of 41.3 F cm⁻¹ V⁻¹ s⁻¹ in n-type OECTs.

Abstract

n-Type organic mixed ionic–electronic conductors (OMIECs) with high electron mobility are scarce and highly challenging to develop. As a result, the figure-of-merit (µC*) of n-type organic electrochemical transistors (OECTs) lags far behind the p-type analogs, restraining the development of OECT-based low-power complementary circuits and biosensors. Here, two n-type donor–acceptor (D–A) polymers based on fused bithiophene imide dimer f-BTI2 as the acceptor unit and thienylene–vinylene–thienylene (TVT) as the donor co-unit are reported. The cyanation of TVT enables polymer f-BTI2g-TVTCN with simultaneously enhanced ion-uptake ability, film structural order, and charge-transport property. As a result, it is able to obtain a high volumetric capacitance (C*) of 170 ± 22 F cm⁻³ and a record OECT electron mobility (μ_e,OECT) of 0.24 cm² V⁻¹ s⁻¹ for f-BTI2g-TVTCN, subsequently achieving a state-of-the-art µC* of 41.3 F cm⁻¹ V⁻¹ s⁻¹ and geometry-normalized transconductance (g _m,norm) of 12.8 S cm⁻¹ in n-type accumulation-mode OECTs. In contrast, only a moderate µC* of 1.50 F cm⁻¹ V⁻¹ s⁻¹ is measured for the non-cyanated polymer f-BTI2g-TVT. These remarkable results demonstrate the great power of cyano functionalization of polymer semiconductors in developing n-type OMIECs with substantial electron mobility in aqueous environment for high-performance n-type OECTs.

Graphene In their Research Article (e202200321), Kumar Varoon Agrawal et al. demonstrate CO₂ as a promising etchant for the controlled manipulation of graphene edges and vacancy defects.

Nature Communications, Published online: 13 April 2022; doi:10.1038/s41467-022-29527-7

Graphene and related two-dimensional (2D) materials have been at the core of intense research and development for over fifteen years, however the market penetration of products based on this technology has lagged behind expectations. At Nature Communications we wish to support research providing insights into the path towards the industrialisation of 2D materials. We introduce a Collection that encapsulates the recent progress and outstanding challenges faced by research on atomically thin materials, and we focus, in particular, on the potential of 2D technologies for future impact at the commercial level.

A complete process flow to form photodiode stacks sensitive for short-wave infrared (SWIR) light based on non-restricted In(As,P) quantum dots (QDs) is proposed. Films made of semiconducting n-In(As,P) QDs inks, formulated through apolar/polar QD phase transfer, form a rectifying junction with p-NiO that is photosensitive beyond 1400 nm. This result highlights the prospect of printable SWIR opto-electronics based on InAs QDs.

Abstract

Short-wave infrared (SWIR) image sensors based on colloidal quantum dots (QDs) are characterized by low cost, small pixel pitch, and spectral tunability. Adoption of QD-SWIR imagers is, however, hampered by a reliance on restricted elements such as Pb and Hg. Here, QD photodiodes, the central element of a QD image sensor, made from non-restricted In(As,P) QDs that operate at wavelengths up to 1400 nm are demonstrated. Three different In(As,P) QD batches that are made using a scalable, one-size-one-batch reaction and feature a band-edge absorption at 1140, 1270, and 1400 nm are implemented. These QDs are post-processed to obtain In(As,P) nanocolloids stabilized by short-chain ligands, from which semiconducting films of n-In(As,P) are formed through spincoating. For all three sizes, sandwiching such films between p-NiO as the hole transport layer and Nb:TiO₂ as the electron transport layer yields In(As,P) QD photodiodes that exhibit best internal quantum efficiencies at the QD band gap of 46±5% and are sensitive for SWIR light up to 1400 nm.

The atomic structure changes from 2H MoTe₂ to 1T′ MoTe₂ are monitored directly by in situ transmission electron microscopy observation and the seamless interface can be formed between these two phases. By patterning hexagonal boron nitride on MoTe₂, a lateral 1T′-enriched MoTe₂/2H MoTe₂ homojunction photodetector can be built, which exhibits fast photo response due to the gradient bandgap alignment.

Abstract

Direct atomic-scale observation of the local phase transition in transition metal dichalcogenides (TMDCs) is critically required to carry out in-depth studies of their atomic structures and electronic features. However, the structural aspects including crystal symmetries tend to be unclear and unintuitive in real-time monitoring of the phase transition process. Herein, by using in situ transmission electron microscopy, information about the phase transition mechanism of MoTe₂ from hexagonal structure (2H phase) to monoclinic structure (1T′ phase) driven by sublimation of Te atoms after a spike annealing is obtained directly. Furthermore, with the control of Te atom sublimation by modulating the hexagonal boron nitride (h-BN) coverage in the desired area, the lateral 1T′-enriched MoTe₂/2H MoTe₂ homojunction can be one-step constructed via an annealing treatment. Owing to the gradient bandgap provided by 1T′-enriched MoTe₂ and 2H MoTe₂, the photodetector composed of the 1T′-enriched MoTe₂/2H MoTe₂ homojunction shows fast photoresponse and ten times larger photocurrents than that consisting of a pure 2H MoTe₂ channel. The study reveals a route to improve the performance of optoelectronic and electronic devices based on TMDCs with both semiconducting and semimetallic phases.