Jiuxiang Dai

Wafer-scale low-power hetero-CMOS inverters are realized by integrating monolayer MoS₂ and SWCNT networks. An ultralow standby power consumption of ≈5 pW at a reduced supply voltage of 0.25 V, high NMs (>70%), and dynamic analysis in a push-pull configuration are achieved. It paves the way toward the wafer-scale integration of low-dimensional materials for low-power nanoelectronics.

Abstract

Scalable nanoelectronics with energy-efficient logic technology is crucial for next-generation edge devices. Low-dimensional semiconductors, such as transition metal dichalcogenides and single-walled carbon nanotubes (SWCNTs), have tunable properties with reduced short-channel effects. The unique properties of each material can be utilized owing to the heterogeneous integration of multiple semiconducting channels to form complementary metal-oxide-semiconductor (CMOS) logic. However, the integration remains challenging. This study reveals the realization of low static power hetero-CMOS inverters by the integration of n-type monolayer MoS₂ and p-type SWCNT networks. The balanced inverter exhibits a large peak gain of ≈67 at a supply voltage of 2 V with the customized design of the wafer-scale synthetic process and channel integration. An ultralow standby power consumption of ≈5 pW and a practical peak gain of ≈7 at a reduced supply voltage of 0.25 V are achieved. A high noise margin (>70%) validates the circuit's tolerance to external noises and the dynamic analysis of the inverting amplifier in push–pull configuration exhibits a large AC gain. This work paves the way toward the wafer-scale integration of low-dimensional materials for low-power nanoelectronics.

Nature Materials, Published online: 17 April 2023; doi:10.1038/s41563-023-01539-8

An optical spectroscopy approach unravels different layer-dependent correlated electron phases in a two-dimensional semiconductor heterobilayer.

Nature Materials, Published online: 17 April 2023; doi:10.1038/s41563-023-01521-4

The authors demonstrate electrical on/off switching of interlayer interactions in tungsten diselenide/molybdenum disulfide heterobilayers, the phase diagram of which contains layer-dependent correlated regions that reveal the role of strong correlations in interlayer exciton dynamics.

Applied Physics Letters, Volume 122, Issue 15, April 2023.
We investigate the low-frequency noise characteristics of indium–gallium–zinc oxide ferroelectric thin-film transistors (FeTFTs) with a metal–ferroelectric–metal–insulator–semiconductor (MFMIS) structure. MFMIS FeTFTs are fabricated with different metal-to-FE area ratios (AM/AF's). It is revealed that the noise generation mechanism differs depending on the operation region [low and high drain current (ID) regions] and AM/AF. Excess noise in the low ID region is observed in the MFMIS FeTFTs with AM/AF's of 4 and 6 due to carrier mobility fluctuations. In the high ID region, the carrier number fluctuation generates the 1/f noise of the devices regardless of the AM/AF.

Applied Physics Letters, Volume 122, Issue 15, April 2023.
Magnetic tunnel junction (MTJ) based on van der Waals (vdW) magnetic layers has been found to present excellent tunneling magnetoresistance (TMR) property, which has great potential applications in field sensing, nonvolatile magnetic random access memories, and spin logics. Although MTJs composed of multilayer vdW magnetic homojunction have been extensively investigated, the ones composed of vdW magnetic heterojunction are still to be explored. Here, we use first-principles approaches to reveal that the magnetic heterojunction MTJs have much more distinguishable TMR values than the homojunction ones. In the MTJ composed of bilayer CrI3/bilayer Cr2Ge2Te6 heterojunction, we find there are eight stable magnetic states, leading to six distinguishable electronic resistances. As a result, five sizable TMRs larger than 300% can be obtained (the maximum TMR is up to 620 000%). Six distinguishable memories are obtained, which is two times larger than that of a four-layered homojunction MTJ. The underlying relationships among magnetic state, spin-polarized band structures, and transmission spectra are further revealed to explain the multiple TMR values. We also find that the magnetic states, and thus TMRs, can be efficiently modulated by an external electric field. This study opens an avenue to the design of high-performance MTJ devices based on vdW heterojunctions.

Nature Communications, Published online: 14 April 2023; doi:10.1038/s41467-023-37769-2

Here, the authors report evidence of unconventional correlated insulating states in bilayer graphene/CrOCl heterostructures over wide doping ranges and demonstrate their application for the realization of low-temperature logic inverters.

Mobile solitons are discovered in a van der Waals material as kinks and antikinks along domain walls in the 2D layer with spontaneously broken symmetry. The mobility, electronic states, and topology of kink solitons are elucidated, which makes the application of solitons in van der Waals electronic materials promising.

Abstract

Kinks, point-like geometrical defects along dislocations, domain walls, and DNA, are stable and mobile, as solutions of a sine-Gordon wave equation. While they are widely investigated for crystal deformations and domain wall motions, electronic properties of individual kinks have received little attention. In this work, electronically and topologically distinct kinks are discovered along electronic domain walls in a correlated van der Waals insulator of 1T-TaS₂. Mobile kinks and antikinks are identified as trapped by pinning defects and imaged in scanning tunneling microscopy. Their atomic structures and in-gap electronic states are unveiled, which are mapped approximately into Su–Schrieffer–Heeger solitons. The twelvefold degeneracy of the domain walls in the present system guarantees an extraordinarily large number of distinct kinks and antikinks to emerge. Such large degeneracy together with the robust geometrical nature may be useful for handling multilevel information in van der Waals materials architectures.

This work proposes a strategy to create 1D-EFBs in untwisted heterostructures and experimentally realize it by van der Waals (vdW) epitaxy. The idea is that, the atomic relaxations of the anisotropic rectangular vdW layers introduce 1D buckling reversal regions, where electronic states are isolated in one direction and are localised in 1D extended states along the orthogonal direction. Therefore, 1D-EFBs are naturally emergent.

Abstract

After the preparation of 2D electronic flat band (EFB) in van der Waals (vdW) superlattices, recent measurements suggest the existence of 1D electronic flat bands (1D-EFBs) in twisted vdW bilayers. However, the realization of 1D-EFBs is experimentally elusive in untwisted 2D layers, which is desired considering their fabrication and scalability. Herein, the discovery of 1D-EFBs is reported in an untwisted in situ-grown two atomic-layer Bi(110) superlattice self-aligned on an SnSe(001) substrate using scanning probe microscopy measurements and density functional theory calculations. While the Bi–Bi dimers of Bi zigzag (ZZ) chains are buckled, the epitaxial lattice mismatch between the Bi and SnSe layers induces two 1D buckling reversal regions (BRRs) extending along the ZZ direction in each Bi(110)-11 × 11 supercell. A series of 1D-EFBs arises spatially following BRRs that isolate electronic states along the armchair (AC) direction and localize electrons in 1D extended states along ZZ due to quantum interference at a topological node. This work provides a generalized strategy for engineering 1D-EFBs in utilizing lattice mismatch between untwisted rectangular vdW layers.

After the V atom doping into WS₂, it will spontaneously form an interface similar to heterojunction, and the doping concentration of the V atom at both sides of the interface is significantly different. This indicates that the doping distribution of doping atoms in monolayer transition metal dichalcogenides is worthy of further exploration.

Abstract

The density and spatial distribution of substituted dopants affect the transition metal dichalcogenides (TMDCs) materials properties. Previous studies have demonstrated that the density of dopants in TMDCs increases with the amount of doping, and the phenomenon of doping concentration difference between the nucleation center and the edge is observed, but the spatial distribution law of doping atoms has not been carefully studied. Here, it is demonstrated that the spatial distribution of dopants changes at high doping concentrations. The spontaneous formation of an interface with a steep doping concentration change is named concentration phase separation (CPS). The difference in the spatial distribution of dopants on both sides of the interface can be identified by an optical microscope. This is consistent with the results of spectral analysis and microstructure characterization of scanning transmission electron microscope. According to the calculation results of density functional theory, the chemical potential has two relatively stable energies as the doping concentration increases, which leads to the spontaneous formation of CPS. Understanding the abnormal phenomena is important for the design of TMDCs devices. This work has great significance in the establishment and improvement of the doping theory and the design of the doping process for 2D materials.

The formation of clean and defect-free van der Waals stackings at the Sb–PdSe₂ heterointerfaces boosts the transport characteristics of few-layer PdSe₂ field-effect transistors, including low contact resistance down to 0.55 kΩ µm, high on-current density reaching 96 µA µm⁻¹, and high electron mobility of 383 cm² V⁻¹ s⁻¹ at room temperature and 2,184 cm² V⁻¹ s⁻¹ at 10 K.

Abstract

Even though atomically thin 2D semiconductors have shown great potential for next-generation electronics, the low carrier mobility caused by poor metal–semiconductor contacts and the inherently high density of impurity scatterings remains a critical issue. Herein, high-mobility field-effect transistors (FETs) by introducing few-layer PdSe₂ flakes as channels is achieved, via directly depositing semimetal antimony (Sb) as drain–source electrodes. The formation of clean and defect-free van der Waals (vdW) stackings at the Sb–PdSe₂ heterointerfaces boosts the room temperature transport characteristics, including low contact resistance down to 0.55 kΩ µm, high on-current density reaching 96 µA µm⁻¹, and high electron mobility of 383 cm² V⁻¹ s⁻¹. Furthermore, metal–insulator transition (MIT) is observed in the PdSe₂ FETs with and without hexagonal boron nitride (h–BN) as buffer layers. However, the layered h–BN/PdSe₂ vdW stacking eliminates the interference of interfacial disorders, and thus the corresponding device exhibits a lower MIT crossing point, larger mobility exponent of γ ∼ 1.73, significantly decreased hopping parameter of T ₀, and ultrahigh electron mobility of 2,184 cm² V⁻¹ s⁻¹ at 10 K. These findings are expected to be significant for developing high mobility 2D-based quantum devices.

Numerous nanoscale heterointerfaces created via laser embedding ferroelectric nanodomains in the bulk of CuBi₂O₄ films are constructed to efficiently reduce the photogenerated charge carriers transport barrier and boost the separation of the charge carriers, which results in a CuBi₂O₄ photocathode with a photocurrent density of 3.21 mA cm^-2 at 0.6 V_RHE.

Abstract

It is widely accepted that metal oxide-based photoelectrodes (MOPs) hold great promise for future solar hydrogen generation but are facing awkward challenge arising from their low intrinsic carrier mobility. The highly polarized nature of the predominantly ionic metal-oxygen bond always leads to the formation of small polarons that are responsible for the localized trapping of photo-generated carriers. Present study explores the reduction of carriers transport barrier via bulk embedding of ferroelectric nanodomains (FNDs) in MOPs that results in a new performance benchmark for the CuBi₂O₄ photocathode. By embedding laser-generated sub-10 nm BaTiO₃ nanocrystals in the bulk of CuBi₂O₄ photocathode, numerous FNDs are created that can lead to two times enhancement of the carrier mobility, which is proposed to originate from the overlaying of the internal electric fields and effective electrons transport channel at the heterointerfaces of BaTiO₃/CuBi₂O₄. Such strategy leads to the CuBi₂O₄ photocathode with the photocurrent density of up to 3.21 mA cm⁻² at 0.6 V_RHE, as well as a pronounced absorbed photon-to-current efficiency up to 80% at 400 nm. The universal feature of present technology is further verified by laser embedding of SrTiO₃ FNDs, providing an effective route for addressing the charge transport limitations in MOPs.

Charge transfer across heterostructure interfaces crucially determines device applications, including light sensing. In this work, an optical switching of the hole-transfer direction and efficiency across strongly coupled double-perovskite Cs₂AgBiBr₆/graphene heterostructures by tuning the excitation wavelength is reported. This provides a novel way to control the interfacial charge flow across a semiconductor/graphene heterojunction.

Abstract

Synergically combining their respective ultrahigh charge mobility and strong light absorption, graphene (Gr)/semiconductor heterostructures are promising building blocks for efficient optoelectronics, particularly photodetectors. Charge transfer (CT) across the heterostructure interface crucially determines device efficiency and functionality. Here, it is reported that hole-transfer processes dominate the ultrafast CT across strongly coupled double-perovskite Cs₂AgBiBr₆/graphene (DP/Gr) heterostructures following optical excitation. While holes are the primary charges flowing across interfaces, their transfer direction, as well as efficiency, show a remarkable dependence on the excitation wavelength. For excitation with photon energies below the bandgap of DPs, the photoexcited hot holes in Gr can compete with the thermalization process and inject into in-gap defect states in DPs. In contrast, above-bandgap excitation of DP reverses the hole-transfer direction, leading to hole transfer from the valence band of DPs to Gr. Experimental evidence that increasing the excitation photon energy enhances CT efficiency for both below- and above-bandgap photoexcitation regimes is further provided, unveiling the positive role of excess energy in enhancing interfacial CT. The possibility of switching the hole-transfer direction and thus the interfacial photogating field by tuning the excitation wavelength, provides a novel way to control the interfacial charge flow across a DP/Gr heterojunction.

Statistical electrical measurements of 2D memristors show the extraordinary improvement of yield and endurance by optimizing the metal deposition and MoS₂ condition. The intriguing convergence of switching voltages and resistance ratio is revealed. An “effective switching layer” model compatible with both monolayer and few-layer MoS₂, is proposed to understand the reliability improvement and the convergence of switching metrics.

Abstract

2D memristors have demonstrated attractive resistive switching characteristics recently but also suffer from the reliability issue, which limits practical applications. Previous efforts on 2D memristors have primarily focused on exploring new material systems, while damage from the metallization step remains a practical concern for the reliability of 2D memristors. Here, the impact of metallization conditions and the thickness of MoS₂ films on the reliability and other device metrics of MoS₂-based memristors is carefully studied. The statistical electrical measurements show that the reliability can be improved to 92% for yield and improved by ≈16× for average DC cycling endurance in the devices by reducing the top electrode (TE) deposition rate and increasing the thickness of MoS₂ films. Intriguing convergence of switching voltages and resistance ratio is revealed by the statistical analysis of experimental switching cycles. An “effective switching layer” model compatible with both monolayer and few-layer MoS₂, is proposed to understand the reliability improvement related to the optimization of fabrication configuration and the convergence of switching metrics. The Monte Carlo simulations help illustrate the underlying physics of endurance failure associated with cluster formation and provide additional insight into endurance improvement with device fabrication optimization.

Applied Physics Letters, Volume 122, Issue 15, April 2023.
As a promising two-dimensional (2D) layered material, black arsenic phosphorus (b-AsP) alloys have received growing attention due to their unique properties and their ability for high-performance broadband photodetection. However, high dark current and slow response speed have already become bottlenecks for further development. Manual vertical van der Waals heterojunctions made of different 2D materials offer opportunities to alleviate such bottlenecks in a simple and low-energy way. The rational design of band alignment can facilitate device performance. In this work, we design and achieve a type-I vertically stacked WSe2/b-As0.084P0.916 device, which exhibits a rectification ratio of 102 along with an unusual backward current as low as 10−12 A. As such, this device can function as an ultrasensitive photodetector, which shows excellent photoresponse properties from the visible to near-infrared region (275–850 nm), with a responsivity of 244 A/W, a specific detectivity of 2.27 × 1012 Jones, and a fast response speed of τrise ≈ 5.1 ms and τdecay ≈ 4 ms. Compared to the dark state, the hole mobility under light stimulation is raised more than ten times (from 1.1 to 12.1 cm2 V−1 s−1), which contributes to numerous excited electron–hole pair transfers from WSe2 to b-As0.084P0.916. The responsivity and detectivity increase by 5 and 3 orders of magnitude, respectively, after applying gate voltage, indicating remarkable gate-controlled properties. These results suggest that the WSe2/b-As0.084P0.916 heterostructure is a promising candidate for future electronic and optoelectronic applications.

Applied Physics Letters, Volume 122, Issue 15, April 2023.
High p-conductivity (0.7 Ω−1 cm−1) was achieved in high-Al content AlGaN via Mg doping and compositional grading. A clear transition between the valence band and impurity band conduction mechanisms was observed. The transition temperature depended strongly on the compositional gradient and to some degree on the Mg doping level. A model is proposed to explain the role of the polarization field in enhancing the conductivity in Mg-doped graded AlGaN films and the transition between the two conduction types. This study offers a viable path to technologically useful p-conductivity in AlGaN.

Applied Physics Letters, Volume 122, Issue 15, April 2023.
Continuum mechanics break down in bending stiffness calculations of mono- and few-layered two-dimensional (2D) van der Waals crystal sheets, because their layered atomistic structures are uniquely characterized by strong in-plane bonding coupled with weak interlayer interactions. Here, we elucidate how the bending rigidities of pristine mono- and few-layered molybdenum disulfide (MoS2), graphene, and hexagonal boron nitride (hBN) are governed by their structural geometry and intra- and inter-layer bonding interactions. Atomic force microscopy experiments on the self-folded conformations of these 2D materials on flat substrates show that the bending rigidity of MoS2 significantly exceeds those of graphene or hBN of comparable layers, despite its much lower tensile modulus. Even on a per-thickness basis, MoS2 is found to possess similar bending stiffness to hBN and is much stiffer than graphene. Density functional theory calculations suggest that this high bending rigidity of MoS2 is due to its large interlayer thickness and strong interlayer shear, which prevail over its weak in-plane bonding.