Jiuxiang Dai

Ultrathin boron flakes are grown in the Volmer–Weber mode by atmospheric-pressure chemical vapor deposition. The metal-catalyzed, ultrafast gasification of boron flakes at room temperature, exemplified by the complete, spontaneous vanishment of 200 nm thick islands in 3 h is revealed. The two-step mechanism related to boron surface chemistry is then confirmed. Furthermore, oxygen-free chemical vapor depositiongrowth for air-sensitive materials is proposed.

Abstract

The trivalent outer shell of boron renders this element electron-poor but chemically rich, exhibiting more than one dozen allotropes. Its 2D polymorph has been recently synthesized on metal substrates under ultrahigh vacuum and has attracted intense interest. However, probing its properties ex situ has been challenging due to the quality degradation—surface oxidation—that occurs upon exposure to ambient environments. Herein, this surface chemistry is investigated in regard to the air stability of ultrathin boron flakes on metals prepared by atmospheric-pressure chemical vapor deposition. The characteristic Volmer–Weber growth is recognized by the stacking of polygon-shaped, thin flakes as isolated islands. Significantly, the metal-catalyzed, ultrafast gasification of boron flakes at room temperature, exemplified by the complete, spontaneous vanishment of 200 nm-thick boron islands in 3 h is observed. A two-step mechanism, first oxygen-involved surface oxidation and then subsequent reactions with water forming a highly volatile boric acid layer, is unambiguously revealed by combined surface characterizations. The catalysis by metal substrates, corroborated by theoretical calculations, is attributed as the crucial cause of the unprecedented gasification. The concept of oxygen-free growth is thereby proposed for air-sensitive material growth by introducing in situ oxygen scavengers. These findings significantly expand the fundamental understanding of the surface chemistry of boron and pave the way for the chemical vapor deposition growth of hydrophobic materials.

Nature Materials, Published online: 22 November 2022; doi:10.1038/s41563-022-01416-w

Ferro-Floating Memory (FFM) which operates in dual-mode mechanisms (ferroelectric switching of FeFET and charge injection of Floating gate memory) was implemented by employing a-In2Se3 as a floating gate. The dual-mode operation allowed the multiple stages of conductance and improved the linearity and dynamic range of conductance. On the cover, the authors illustrate the dual-mode operation of the FFM by combining FeFET and Flash memory. (DOI : 10.1002/inf2.12367)

Abstract

Ferro-Floating Memory (FFM) which operates in dual-mode mechanisms (ferroelectric switching of FeFET and charge injection of Floating gate memory) was implemented by employing a-In2Se3 as a floating gate. The dual-mode operation allowed the multiple stages of conductance and improved the linearity and dynamic range of conductance. On the cover, the authors illustrate the dual-mode operation of the FFM by combining FeFET and Flash memory. (DOI : 10.1002/inf2.12367)

Nature Nanotechnology, Published online: 21 November 2022; doi:10.1038/s41565-022-01252-8

Nature Nanotechnology, Published online: 21 November 2022; doi:10.1038/s41565-022-01248-4

Nature Electronics, Published online: 21 November 2022; doi:10.1038/s41928-022-00883-y

Chemical synthesis and functionalization of 2D germananes (hydrogen atom and covalent group(s) termination) via topotactical deintercalation offer customizable properties or new functionalities. The fundamentals from synthesis and exfoliation to properties; for optoelectronics, catalysis, energy conversion and storage, sensors, and biomedical applications are reviewed. The review presents the recent progress and challenges, and provides insight for future exploration of these materials.

Abstract

In the realm of 2D layered materials, the monoelemental group 14 Xene, germanene, as the germanium analog of graphene, has emerged as the next prospective candidate. Preceded by silicon, germanium is widely used in the semiconductor industry; thus, germanene is deemed compatible with existing semiconductor technologies. Germanene consists of mixed sp²–sp³-hybridized networks in a buckled hexagonal honeycomb structure. Chemical exfoliation of Zintl phases, such as CaGe₂, specifically the topotactical deintercalation in acidic media, removes the alkaline earth metal ions Ca²⁺, giving rise to layered germanane (germanene with the Ge centers covalently saturated with terminal hydrogen atoms). Diverse variants of functionalized germananes (with covalent group(s) termination) can be obtained by varying the topotactical deintercalation precursors, elevating the game with limitless functionalization possibilities for customizable properties or new functionalities. The preparation of Zintl phases to the details of functionalized and modified germananes and their properties, and the additional exfoliation step to achieve mono- or few-layer germananes, are comprehensively covered. The progress and challenges of 2D functionalized germananes in optoelectronics, catalysis, energy conversion and storage, sensors, and biomedical areas are reviewed. This review provides insight into designing and exploring this class of atomically thin semiconductors in realizing future nanoarchitectonics.

Post-synthesis doping of InAs nanocrystals with Zn enables the fabrication of heavy metal free, RoHS compliant nanocrystal-based field effect transistors, which exhibit either n- or p-type characteristics. Advanced structural characterization of the doped nanocrystals highlights the importance of Zn dopant chemistry on the doping efficiency. This study sets the stage for future development of nanocrystals-based opto-electronics applications.

Abstract

Doped heavy metal-free III–V semiconductor nanocrystal quantum dots (QDs) are of great interest both from the fundamental aspects of doping in highly confined structures, and from the applicative side of utilizing such building blocks in the fabrication of p–n homojunction devices. InAs nanocrystals (NCs), that are of particular relevance for short-wave IR detection and emission applications, manifest heavy n-type character poising a challenge for their transition to p-type behavior. The p-type doping of InAs NCs is presented with Zn – enabling control over the charge carrier type in InAs QDs field effect transistors. The post-synthesis doping reaction mechanism is studied for Zn precursors with varying reactivity. Successful p-type doping is achieved by the more reactive precursor, diethylzinc. Substitutional doping by Zn²⁺ replacing In³⁺ is established by X-ray absorption spectroscopy analysis. Furthermore, enhanced near infrared photoluminescence is observed due to surface passivation by Zn as indicated from elemental mapping utilizing high-resolution electron microscopy corroborated by X-ray photoelectron spectroscopy study. The demonstrated ability to control the carrier type, along with the improved emission characteristics, paves the way towards fabrication of optoelectronic devices active in the short-wave infrared region utilizing heavy-metal free nanocrystal building blocks.

A strategic synthesis technique based on a bicontinuous microemulsion system is developed to design nanostructures of conducting polymers including 2D, 3D, and nanocomposites with tailored electrical properties and morphologies.

Abstract

In recent decades, there has been a great deal of interest in conducting polymers due to their broad applications. At the same time, various synthetic techniques have been developed to produce various nanostructures of the conducting polymers with their fascinating properties. However, the techniques for the manufacture of 2D nanosheets are either complex or expensive. No comprehensive approach for constructing 2D and 3D materials or their composites has been documented. Herein, a simple and scalable synthetic protocol is reported for the design of 2D, 3D, and related conducting polymer nanocomposites by interface manipulation in a bicontinuous microemulsion system. In this method, diverse bicontinuous thin layers of oil and water are employed to produce 2D nanosheets of conducting polymers. For the fabrication of 3D polypyrrole (PPY) and their composites, specially designed linkers of the monomers are applied to lock the 3D networks of the conducting polymers and their composites. The technique can be extended to the fabrication of most conducting polymer composites, being cost-effective and easily scalable. The optimum electrical conductivity obtained for 2D PPY nanosheets is 219 S cm⁻¹, the highest literature value reported to date to the best of knowledge.

Prototypical gate-programmable memory that seamlessly integrates the functionality of both ferroelectric memristor and metal-oxide-semiconductor field effect transistor (MOS-FET), in a vertical fashion is demonstrated. Its memristive characteristics can be quenched (enabled), by setting the Fermi level of MoS₂ inside (outside) of its band gap via a top gate, yielding a gate programmable non-volatile memory for multi-bit data storage and more.

Abstract

Ferroelectricity, one of the keys to realize non-volatile memories owing to the remanent electric polarization, is an emerging phenomenon in the 2D limit. Yet the demonstrations of van der Waals (vdW) memories using 2D ferroelectric materials as an ingredient are very limited. Especially, gate-tunable ferroelectric vdW memristive device, which holds promises in future multi-bit data storage applications, remains challenging. Here, a gate-programmable multi-state memory is shown by vertically assembling graphite, CuInP₂S₆, and MoS₂ layers into a metal(M)-ferroelectric(FE)-semiconductor(S) architecture. The resulted devices seamlessly integrate the functionality of both FE-memristor (with ON–OFF ratios exceeding 10⁵ and long-term retention) and metal-oxide-semiconductor field effect transistor (MOS-FET). Thus, it yields a prototype of gate tunable giant electroresistance with multi-levelled ON-states in the FE-memristor in the vertical vdW assembly. First-principles calculations further reveal that such behaviors originate from the specific band alignment between the FE-S interface. Our findings pave the way for the engineering of ferroelectricity-mediated memories in future implementations of 2D nanoelectronics.

The rational manipulation of work function difference and interfacial charge transfer is very effective in introducing the optimization effects of hole injection, energy filtering and modulation doping. This approach is validated in p-type (MnTe) _x (Sb₂Te₃) _y superlattice-like films and quantum-dots superlattices in order for synergistically optimized hole density p, carrier mobility μ, carrier effective mass m*, contributing to much improved thermoelectric power factor.

Abstract

Interfacial charge transfer has a vital role in tailoring the thermoelectric performance of superlattices (SLs), which, however, is rarely clarified by experiments. Herein, based on epitaxially grown p-type (MnTe) _x (Sb₂Te₃) _y superlattice-like films, synergistically optimized thermoelectric parameters of carrier density, carrier mobility, and Seebeck coefficient are achieved by introducing interfacial charge transfer, in which effects of hole injection, modulation doping, and energy filtering are involved. Carrier transport measurements and angle-resolved photoemission spectroscopy (ARPES) characterizations reveal a strong hole injection from the MnTe layer to the Sb₂Te₃ layer in the SLs, originating from the work function difference between MnTe and Sb₂Te₃. By reducing the thickness of MnTe less than one monolayer, all electronic transport parameters are synergistically optimized in the quantum-dots (MnTe) _x (Sb₂Te₃)₁₂ superlattice-like films, leading to much improved thermoelectric power factors (PFs). The (MnTe)_0.1(Sb₂Te₃)₁₂ obtains the highest room-temperature PF of 2.50 mWm⁻¹K⁻², while the (MnTe)_0.25(Sb₂Te₃)₁₂ possesses the highest PF of 2.79 mWm⁻¹K⁻² at 381 K, remarkably superior to the values acquired in binary MnTe and Sb₂Te₃ films. This research provides valuable guidance on understanding and rationally tailoring the interfacial charge transfer of thermoelectric SLs to further enhance thermoelectric performances.

Abstract

The ultrathin body of two-dimensional (2D) materials provides potential for next-generation electronics and optoelectronics. The unavoidable atomic defects substantially determine the physical properties of atomic-level thin 2D materials, thus enabling new functionalities that are impossible in three-dimensional semiconductors. Therefore, rational design of atomic defects provides an alternative approach to modulate the physical properties of 2D materials. In this review, we summarize the recent progress of defect engineering in 2D materials, particularly in device performance enhancement. Firstly, the common defects in 2D materials and approaches for generating and repairing defects, including synthesis and post-growth treatments, are systematically introduced. The correlations between defects and optical, electronic, and magnetic properties of 2D materials are then highlighted. Subsequently, defect engineering for high performance electronics and optoelectronics is emphasized. At last, we provide our perspective on challenges and opportunities in defect engineering of 2D materials.

Using van der Waals epitaxial method, ultrathin topological insulator Ag₂Te single crystals are synthesized. The Ag₂Te single crystals exhibit p-type conduction behavior with high carrier mobility of 3336 cm² V^–1 s^–1 at room temperature. A self-driven broad-spectrum high-performance photodetector with high responsivity, high on/off ratio, and fast response speed is obtained based Ag₂Te/WSe₂ heterojunction_.

Abstract

β-Ag₂Te has attracted considerable attention in the application of electronics and optoelectronics due to its narrow bandgap, high mobility, and topological insulator properties. However, it remains a significant challenge to synthesize 2D Ag₂Te because of the non-layered structure of Ag₂Te. Herein, the synthesis of large-size, ultrathin single crystal topological insulator 2D Ag₂Te via the van der Waals epitaxial method for the first time is reported. The 2D Ag₂Te crystal exhibits p-type conduction behavior with high carrier mobility of 3336 cm² V⁻¹ s⁻¹ at room temperature. Taking advantage of the high mobility and perfect electron structure of Ag₂Te, the Ag₂Te/WSe₂ heterojunctions are fabricated via mechanical stacking and show an ultrahigh rectification ratio of 2 × 10⁵. Ag₂Te/WSe₂ photodetector also exhibits self-driven properties with a fast response speed (40 µs/60 µs) in the near-infrared region. High responsivity (219 mA W⁻¹) and light ON/OFF ratio of 6 × 10⁵ are obtained under the photovoltaic mode. The overall performance of the Ag₂Te/WSe₂ photodetector is significantly competitive among all reported 2D photodetectors. These results indicate that 2D Ag₂Te is a promising candidate for future electronic and optoelectronic applications.

Highly crystalline single layer 2D polymers (SL-2DPs) are prepared and further used as an active layer in the memristor. The devices exhibit low variability, high reliability in terms of yield, stability and durability, nanometer scalability, as well as distinguished bending endurance, which verified the unambiguous role of SL-2DP film in diverse applications from high density information storage to ultra-thin flexible electronics.

Abstract

Large-scale growth of highly crystalline single layer 2D polymers (SL-2DPs) and their subsequent integration into memristors is key to advancing the development of high-density data storage devices. However, leakage problems resulting from the porous structure of 2DPs continue to make such advances extremely challenging. Herein, we overcome this issue by incorporating long alkoxy chains into key molecular building blocks to obtain a highly crystalline 2DP, as visualized by scanning tunneling microscopy, and prevent metal permeation in the subsequent device fabrication process. SL-2DP memristors constructed via direct evaporation of the top electrodes exhibit low variability (σ_Vset = 0.14) due to the single-monomer-thick feature together with the high regular structure and coordination ability which minimizes the stochastic spatial distribution of conductive filaments (CFs) in both vertical and lateral dimensions. The variability is further decreased to 0.04 by confining the formation and fracture of CFs to the interface through the utilization of bilayer junctions. Using peak force tunneling atomic force microscopy, the nanometer scalability (< 50 nm²) and low power consumption of these molecular memristor devices are demonstrated.

Scanning Tunneling Microscopy

In article number 2208048, Jérôme Lagoute and co-workers report on the realization and properties of in-plane junctions in nitrogen doped graphene. Using scanning tunneling microscopy they visualize the spatial variation of the doping level along the junctions. The width of the transition region is found to be around 7 nm, which corresponds to a sharp junction regime.

A highly efficient (completes within a few minutes) focused laser treatment is proposed to construct a continuous artificial cathode/electrolyte interface (CEI) for the high-mass-loading, high-voltage cathode paired with solid polymer electrolyte. The pulsed laser beam enables the tailorable, yet localized temperature gradient that can customize the CEI species from the target precursor salts in the solution.

Abstract

Poly(ethylene oxide) (PEO)-based solid polymer electrolyte promises interfacial compatibility with the high-capacity metallic anodes in all-solid-state batteries (ASSBs). However, the prototype construction is severely hindered by the parasitic ohmic resistance at the electrode-electrolyte interface, insufficient ionic pathway of the high loading cathode, as well as the PEO oxidation tendency at the high voltage. Herein, a laser-assisted strategy is presented toward ultra-efficient cathode modification (completes within 240 s) by constructing continuous, multi-scale artificial cathode/electrolyte interface (CEI). The tailorable, yet localized temperature gradient induced by the pulsed laser beam can customize the CEI species from the target precursor salts for the on-demand protection purpose. Derived from the tris(trimethylsilyl)phosphate, the proof-of-concept model achieves phosphorus-rich, ion-diffusion network across the high-mass-loading LiNi_0.8Co_0.1Mn_0.1O₂ cathode, which enables the high-rate operation of the ASSBs prototype as well as the extended shelf life at the oxidized idling state. Transmission-mode operando X-ray phase tracking unravels the electrochemical stability origin at the cathode/PEO interface due to the insulation of electron shuttling, where the layered to spinel phase transition and the lattice oxygen release are alleviated. This generic, readily tailorable, highly-efficient laser processing strategy thus provides unprecedented opportunities to secure the varieties of energy-dense, polymer-based ASSBs.

npj 2D Materials and Applications, Published online: 19 November 2022; doi:10.1038/s41699-022-00354-0

Nature Materials, Published online: 17 November 2022; doi:10.1038/s41563-022-01398-9

Nature Communications, Published online: 17 November 2022; doi:10.1038/s41467-022-34874-6