Jiuxiang Dai

Nature Electronics, Published online: 02 December 2022; doi:10.1038/s41928-022-00905-9

Nature Electronics, Published online: 02 December 2022; doi:10.1038/s41928-022-00902-y

Quantum chemical bonding descriptors enable a quantitative description of bonding in chalcogenides. These descriptors are employed as property predictors for optical and transport properties. Hence, they can be utilized to tailor thermoelectrics, photovoltaics, and phase-change memories. Their unusual property portfolio is attributed to a novel bonding mechanism, different from ionic, metallic, and covalent bonding, which is called “metavalent.”

Abstract

Quantum chemical bonding descriptors have recently been utilized to design materials with tailored properties. Their usage to facilitate a quantitative description of bonding in chalcogenides as well as the transition between different bonding mechanisms is reviewed. More importantly, these descriptors can also be employed as property predictors for several important material characteristics, including optical and transport properties. Hence, these quantum chemical bonding descriptors can be utilized to tailor material properties of chalcogenides relevant for thermoelectrics, photovoltaics, and phase-change memories. Relating material properties to bonding mechanisms also shows that there is a class of materials, which are characterized by unconventional properties such as a pronounced anharmonicity, a large chemical bond polarizability, and strong optical absorption. This unusual property portfolio is attributed to a novel bonding mechanism, fundamentally different from ionic, metallic, and covalent bonding, which is called “metavalent.” In the concluding section, a number of promising research directions are sketched, which explore the nature of the property changes upon changing bonding mechanism and extend the concept of quantum chemical property predictors to more complex compounds.

It is found that epitaxial growth and facile detachment of crystallographically aligned domains with a substrate are possible by thru-hole epitaxy. Unlike remote epitaxy, no stringent requirements on the number of a layer or the polarity of a 2D space layer are necessary. The proposed thru-hole epitaxy can be implemented even with SiO₂ as a 3D space layer.

Abstract

Controllable growth and facile transferability of a crystalline film with desired characteristics, acquired by tuning composition and crystallographic orientation, become highly demanded for advanced flexible devices. Here the desired crystallographic orientations and facile transferability of a crystalline film can be achieved by “thru-hole epitaxy” in a straightforward and undemanding manner with no limitation on the layer number and polarity of a 2D space layer and the surface characteristics. The crystallographic alignment can be established by the connectedness of the grown material to the substrate through a small net cross-sectional area of thru-holes, which also allows the straightforward detachment of the grown material. Thru-hole epitaxy can be adopted for the realization of advanced flexible devices on large scale with desired crystallographic orientation and facile transferability.

The opto-electronic properties of 2D WS₂ can be manipulated via its inclusion in van der Waals heterostacks. This spectroscopic ellipsometry investigation highlights the tunability of the excitonic features and provides a set of reference values for the dielectric function of WS₂ in monolayer configuration, 2H and 3R bilayer stacking, and WS₂/MoS₂ vertical heterostructure.

Abstract

The opto-electronic properties of semiconducting 2D materials can be flexibly manipulated by engineering the atomic-scale environment. This can be done by including 2D materials in tailored van der Waals (vdW) stacks, whose optical response is a function of the number and the type of adjacent 2D layers. This work reports a systematic investigation of the dielectric function of 2D semiconducting WS₂ in various stacking configurations: monolayer, 3R/2H homobilayer, and WS₂/MoS₂ heterobilayer. Reliable, Kramers–Kronig-consistent dielectric functions are obtained for WS₂ in each configuration by means of spectroscopic ellipsometry (SE) and related parametric optical modeling in a wide spectral range (1.55–3.10 eV). The results of SE are combined with photoluminescence and absorbance spectra to identify the spectral position of the main excitonic features in WS₂, which manifest sizable redshifts depending on the stacking configuration. These results represent a consistent reference set for the dielectric function of WS₂ in vdW stacking configurations of particular interest for the scientific and technological field, and can be fruitfully exploited for reliable predictions of the optical response of WS₂-containing systems.

Large van der Waals antiferromagnet CrCl₃ flakes down to monolayer thickness have been synthesized by the physical vapor transport technique. The samples have the monoclinic structure with high crystallinity and homogeneous stoichiometry. Tunneling magnetoresistance of graphite/CrCl₃/graphite tunnel junctions reveals in-plane magnetic anisotropy and T _N of 17 K for few-layer CrCl₃. This work paves the path for developing CrCl₃-based scalable 2D spintronics.

Abstract

The van der Waals magnets CrX₃ (X = I, Br, and Cl) exhibit highly tunable magnetic properties and are promising candidates for developing novel two-dimensional (2D) spintronic devices such as magnetic tunnel junctions and spin tunneling transistors. Previous studies of the antiferromagnetic CrCl₃ have mainly focused on mechanically exfoliated samples. Controlled synthesis of high quality atomically thin flakes is critical for their technological implementation but has not been achieved to date. This work reports the growth of large CrCl₃ flakes down to monolayer thickness via the physical vapor transport technique. Both isolated flakes with well-defined facets and long stripe samples with the trilayer portion exceeding 60 µm have been obtained. High-resolution transmission electron microscopy studies show that the CrCl₃ flakes are single crystalline in the monoclinic structure, consistent with the Raman results. The room temperature stability of the CrCl₃ flakes decreases with decreasing thickness. The tunneling magnetoresistance of graphite/CrCl₃/graphite tunnel junctions confirms that few-layer CrCl₃ possesses in-plane magnetic anisotropy and Néel temperature of 17 K. This study paves the path for developing CrCl₃-based scalable 2D spintronic applications.

Nature Physics, Published online: 01 December 2022; doi:10.1038/s41567-022-01829-z

A modified spatially confined strategy create slow rising of S:W to maintain a mild growth kinetics of WS₂, leading to the facile regulation of selective lateral/vertical growth. Referring to SW-T diagram, the growth routes for 2D WS₂ are facilely designed, leading to the versatile growth control of atomically thin WS₂ on domain size, layer number, and lateral/vertical heterostructures.

Abstract

Chemical vapor deposition (CVD) has been widely used to produce high quality 2D transitional metal dichalcogenides (2D TMDCs). However, violent evaporation and large diffusivity discrepancy of metal and chalcogen precursors at elevated temperatures often result in poor regulation on X:M molar ratio (M = Mo, W etc.; X = S, Se, and Te), and thus it is rather challenging to achieve the desired products of 2D TMDCs. Here, a modified spatially confined strategy (MSCS) is utilized to suppress the rising S vapor concentration between two aspectant substrates, upon which the lateral/vertical growth of 2D WS₂ can be selectively regulated via proper S:W zones correspond to greatly broadened time/growth windows. An S:W-time (SW-T) growth diagram was thus proposed as a mapping guide for the general understanding of CVD growth of 2D WS₂ and the design of growth routes for the desired 2D WS₂. Consequently, a comprehensive growth management of atomically thin WS₂ is achieved, including the versatile controls of domain size, layer number, and lateral/vertical heterostructures (MoS₂-WS₂). The lateral heterostructures show an enhanced hydrogen evolution reaction performance. This study advances the substantial understanding to the growth kinetics and provides an effective MSCS protocol for growth design and management of 2D TMDCs.

The new architecture of self-powered photodetectors allows for simultaneously enhancing the generation of photocurrent and photo-response speed in van der Waals heterojunction photodetectors of InSe/WSe₂ in InSe/WSe₂/SnS₂ through a light-induced electric field.

Abstract

Self-powered photodetectors have attracted widespread attention due to their low power consumption which can be driven by the built-in electric field instead of external power, but it is very difficult to achieve high responsivity and fast response speed concurrently. Here, a self-powered photodetector with light-induced electric field enhancement based on a 2D InSe/WSe₂/SnS₂ van der Waals heterojunction is designed. The light-induced electric field derived from the photo-generated electrons of SnS₂ accumulated at the SnS₂/WSe₂ interface produces an additional negative gate voltage applied to the WSe₂ layer, which enhances the built-in electric field in the InSe/WSe₂/SnS₂ heterojunction. Accordingly, the photocurrent and photoresponse speed of the heterostructure device are largely improved. The self-powered photodetector based on the InSe/WSe₂/SnS₂ heterostructure exhibits a high responsivity of 550 mA W⁻¹, which is a 50 times increase compared to the InSe/WSe₂ photodetector, and the response speed (110/120 µs) is one order of magnitude faster than that of the InSe/WSe₂ photodetector. The high responsivity and fast speed are caused by the stronger built-in electric field modulated by a light-induced electric field, which can separate carriers effectively and reduce drift times. This device architecture can provide a new avenue to fabricate high-responsivity, fast self-power photodetectors by utilizing the van der Waals heterojunction.

One of the ultimate goals in polymer science is how to approach physical limits such as density, modulus, conductivity, etc. Our work demonstrates the creation of crystalline polymer monolayers for approaching the physical limits of polymer electronic materials and also provides an opportunity to challenge the synthetically iterative limit of an isolated ultra-long polymer.

Abstract

The synthesis of crystalline polymer with a well-defined orientated state and a two-dimensional crystalline size beyond a micrometer will be essential to achieve the highest physical feature of polymer material but remain challenging. Herein, we show the synthesis of the crystalline unipolymer monolayer with an unusual ultrahigh modulus that is higher than the ITO substrate and high conductance by simultaneous electrosynthesis and manipulation. We find that the polymer monolayer has fully extended in the vertical and unidirectional orientation, which is proposed to approach their theoretically highest density, modulus, and conductivity among all aggregation formations of the current polymer. The modulus and current density can reach 40 and 1000 times higher than their amorphous counterpart. It is also found that these monolayers exhibit the bias- and length-dependent multiple charge states and asymmetrically negative differential resistance (NDR) effect, indicating that this unique molecular tailoring and ordering design is promising for multilevel resistive memory devices. Our work demonstrates the creation of a crystalline polymer monolayer for approaching the physical limit of polymer electronic materials and also provides an opportunity to challenge the synthetically iterative limit of an isolated ultra-long polymer.

The symmetry-adapted artificial neural networks are designed to predict the interlayer magnetic exchange interactions in the twisted bilayer CrI3 (TBCI). These make the magnetic state simulations for TBCIs with arbitrary twist angles possible. The simulation results show that the K and T type TBCIs contain both the interlayer ferromagnetic and antiferromagnetic domains, and the T-TBCIs have rich robust skyrmion lattices.

Abstract

Magnetic skyrmions are topologically protected spin swirling vertices, which are promising in device applications due to their particle-like nature and excellent controlability. Magnetic skyrmions are extensively studied in a variety of materials and proposed to exist in the extreme 2D limit, i.e., in twisted bilayer CrI₃ (TBCI). Unfortunately, the magnetic states of TBCIs with small twist angles are disorderly distributed ferromagnetic and antiferromagnetic (AFM) domains in recent experiments, and thus the method to get rid of disorders in TBCIs is highly desirable. Here, intralayer exchange interactions up to the third nearest neighbors without empirical parameters and very accurate interlayer exchange interactions are used to study the magnetic states of TBCIs. The functions of interlayer exchange interactions obtained using first-principles calculations and stored in symmetry-adapted artificial neural networks are proposed. Based on these, the subsequent Landau–Lifshitz–Gillbert equation calculations explain the disorderly distributed FM-AFM domains in TBCIs with small twist angles and predict the orderly distributed skyrmions in TBCIs with large twist angles. This novel twisted 2D bilayer magnet can be used to design memory devices, monochromatic spin wave generators and many kinds of skyrmion lattices.

Nature Materials, Published online: 28 November 2022; doi:10.1038/s41563-022-01407-x

Nature Electronics, Published online: 28 November 2022; doi:10.1038/s41928-022-00874-z

2D semiconductor nanocircuits are engineered thanks to a novel additive nanofabrication approach based on the thermal-scanning probe lithography (t-SPL). Large-area growth of few-layer MoS₂ combined with t-SPL promotes the additive nanolithography of 2D semiconductor nanostructure devices, locally resolved in conductive nanoscopy. This approach opens new avenues to the integration of 2D interconnects in real world electronic and photonic devices.

Abstract

2D transition metal dichalcogenide semiconductor (TMDs) nanocircuits are deterministically engineered over large-scale substrates. This original additive nanolithography approach combines large-area physical growth of 2D TMDs layer with high resolution thermal-scanning probe lithography, to reshape the ultra-thin semiconducting layers at the nanoscale level. The additive nanofabrication of few-layer MoS₂ nanostructures of controlled thickness, grown in the 2H-semiconducting phase, is demonstrated as shown by their Raman vibrational fingerprints and by their optoelectronic response. The electronic signatures of the MoS₂ nanostructures are locally identified by Kelvin probe force microscopy providing chemical and compositional contrast at the nanometer scale. Finally, the potential role of the 2D TMDs nanocircuits as building blocks of deterministic 2D semiconducting interconnections is demonstrated by high-resolution local conductivity maps showing the competitive transport properties of these large-area nanolayers. This work thus provides a powerful approach to scalable nanofabrication of 2D nano-interconnects and van der Waals heterostructures, and to their integration in real-world ultra-compact electronic and photonic nanodevices.

Development of a strategy for stabilizing 1T materials is highly desirable, since the poor phase stability has hindered its widespread applications. The nanotubular system stabilizes the phase by changing the original bond identities. With a high 1T-to-2H-phase ratio (92%), Ru dichalcogenide nanotubes effectively catalyze the oxygen reduction reaction [half-wave-potential (E _1/2) = 0.864 eV, and maximum-power-density (W _max) = 0.526 W cm⁻²].

Abstract

Owing to their remarkable electrochemical activities, 1T phase transition metal dichalcogenide (TMD) materials have attracted considerable interest in recent decades. However, metastable 1T phases are difficult to prepare and readily change phases. Therefore, for the first time, a monolayer nanotubular 1T Ru dichalcogenide comprising 92% of the 1T phase is synthesized, which is the highest value ever obtained using solvothermal methods. In the tubular geometry, the 1T phase exhibits superior durability against various external stimuli and electrocatalytic activity toward the oxygen reduction reaction. According to density-functional-theory-based and molecular dynamics calculations, sufficiently curved architectures can change their bond identities to safely maintain 1T phases, hence providing a strategy for stabilizing metastable phases. The study results form a basis for extensively applying 1T phases and will stimulate interest for applying tubular structures for stabilizing metastable materials.