Jiuxiang Dai

Proceedings of the National Academy of Sciences, Volume 120, Issue 3, January 2023.

Nature, Published online: 11 January 2023; doi:10.1038/s41586-022-05431-4

The electrical contact of two-dimensional transistors is pushed close to the quantum limit by hybridization of the energy bands with antimony; the contacts have low contact resistance and excellent stability.

Applied Physics Letters, Volume 122, Issue 2, January 2023.
Two-dimensional van der Waals (2D vdW) materials have opened up an opportunity to explore an innovative spin-based magnetic nanodevice. However, controllable fabrication of 2D vdW ferromagnets with high Curie temperature remains challenging. In this paper, we reported the growth of 2D CrTe2 single-crystal films epitaxially on Al2O3 substrates using pulsed laser deposition. We find that it shows a typical paramagnetic–ferromagnetic (PM–FM) phase transition around 200 K. The precise Curie temperature and Weiss temperature are 189 and 206.7 K, respectively. The saturation magnetization reaches 73.64 emu/g for the film thickness of 30 nm. The critical exponent β = 0.329 indicates that the magnetic interactions obey the 3D-Ising model. Electronic transport measurement confirms that a CrTe2 film always remains a metallic behavior at 5 K [math] T [math] 320 K and the resistivity of room temperature is 1.5 mΩ/cm. The first-principles calculation uncovers that the FM ordering state mainly stems from an exchange coupling of the adjacent Cr-spin t2g polarized electrons and the metallic conductivity is due to p–d orbital hybridization between Cr and Te atoms. This work would shed new light on studying large-scale growth of 2D magnets and developing 2D magnet-based nanodevices of room temperature.

Nature Materials, Published online: 10 January 2023; doi:10.1038/s41563-022-01466-0

Transforming atomically thin materials by their magnetic neighbours reveals a surprising asymmetry that allows a versatile control of the valley degrees of freedom and band topology in van der Waals heterostructures.

The review focuses on the synthesis, reaction mechanism, and applications of 2D ZnO nanosheets categorically. The different methodology for the synthesis of 2D ZnO nanosheets, its current status for applications in electronics/optoelectronics, electrocatalysis, energy storage, solar cells, photocatalysis, sensing and piezotronics are also discussed.

Abstract

Zinc oxide (ZnO) is a thermally stable n-type semiconducting material. ZnO 2D nanosheets have mainly gained substantial attention due to their unique properties, such as direct bandgap and strong excitonic binding energy at room temperature. These are widely utilized in piezotronics, energy storage, photodetectors, light-emitting diodes, solar cells, gas sensors, and photocatalysis. Notably, the chemical properties and performances of ZnO nanosheets largely depend on the nano-structuring that can be regulated and controlled through modulating synthetic strategies. Two synthetic approaches, top–down and bottom–up, are mainly employed for preparing ZnO 2D nanomaterials. However, owing to better results in producing defect-free nanostructures, homogenous chemical composition, etc., the bottom–up approach is extensively used compared to the top–down method for preparing ZnO 2D nanosheets. This review presents a comprehensive study on designing and developing 2D ZnO nanomaterials, followed by accenting its potential applications. To begin with, various synthetic strategies and attributes of ZnO 2D nanosheets are discussed, followed by focusing on methodologies and reaction mechanisms. Then, their deliberation toward batteries, supercapacitors, electronics/optoelectronics, photocatalysis, sensing, and piezoelectronic platforms are further discussed. Finally, the challenges and future opportunities are featured based on its current development.

Molybdenum disulfide nanoribbons and nanotubes grown from vapor phase are a low-defect-density nanomaterial highly promising for quantum electronic applications. Devices integrating them with bismuth-based contacts show strongly reduced contact resistances and a marked absence of trap states at cryogenic temperatures. This allows for quantum-dot Coulomb blockade measurements. Single-level quantum transport is observed at temperatures below 100mK.

Abstract

Molybdenum disulfide nanoribbons and nanotubes are quasi-1D semiconductors with strong spin–orbit interaction, a nanomaterial highly promising for quantum electronic applications. Here, it is demonstrated that a bismuth semimetal layer between the contact metal and this nanomaterial strongly improves the properties of the contacts. Two-point resistances on the order of 100 kΩ are observed at room temperature. At cryogenic temperature, Coulomb blockade is visible. The resulting stability diagrams indicate a marked absence of trap states at the contacts and the corresponding disorder, compared to previous devices that use low-work-function metals as contacts. Single-level quantum transport is observed at temperatures below 100 mK.

Switchable ferroelectric polarization in Bi₂O₂Se semiconductors is experimentally and theoretically investigated. The interplay between ferroelectricity and semiconducting characteristics of Bi₂O₂Se is explored on device-level operation. Leveraging its ferroelectric polarization, the fabricated device exhibits “smart” photoresponse tunability. This research provides valuable insights into the synergistic effect of ferroelectricity with semiconducting characteristics in 2D ferroelectric semiconductors, toward device simplification and miniaturization.

Abstract

Atomically 2D layered ferroelectric semiconductors, in which the polarization switching process occurs within the channel material itself, offer a new material platform that can drive electronic components toward structural simplification and high-density integration. Here, a room-temperature 2D layered ferroelectric semiconductor, bismuth oxychalcogenides (Bi₂O₂Se), is investigated with a thickness down to 7.3 nm (≈12 layers) and piezoelectric coefficient (d₃₃) of 4.4 ± 0.1 pm V⁻¹. The random orientations and electrically dependent polarization of the dipoles in Bi₂O₂Se are separately uncovered owing to the structural symmetry-breaking at room temperature. Specifically, the interplay between ferroelectricity and semiconducting characteristics of Bi₂O₂Se is explored on device-level operation, revealing the hysteresis behavior and memory window (MW) formation. Leveraging the ferroelectric polarization originating from Bi₂O₂Se, the fabricated device exhibits “smart” photoresponse tunability and excellent electronic characteristics, e.g., a high on/off current ratio > 10⁴ and a large MW to the sweeping range of 47% at V_GS = ±5 V. These results demonstrate the synergistic combination of ferroelectricity with semiconducting characteristics in Bi₂O₂Se, laying the foundation for integrating sensing, logic, and memory functions into a single material system that can overcome the bottlenecks in von Neumann architecture.

Highlights

• Double-cation etching induces abundant vacancies serving as active sites and accelerates the surface reconstruction.

• NMO-30M with cation deficiencies and oxygen vacancies exhibits outstanding OER performance and remarkable stability.

• In situ Raman spectroscopy directly captures the surface reconstruction process.

Nature Reviews Chemistry, Published online: 09 January 2023; doi:10.1038/s41570-022-00455-w

Liquid-like surfaces (LLSs) are emerging omniphobic systems with promising abilities to minimize interfacial adhesion. This Review summarizes the concept, mechanism, fabrication and applications of LLSs, and discusses the challenges and future opportunities in this field.

The development of spin logic devices and spin-FETs in graphene marks an important step for the future development of spin-based computing technology. Their low on/off switching energy, low Joule heating losses, and scalability are attractive features from a device perspective. The non-volatile nature of graphene spin logic devices can be exploited for logic-in-memory applications for neuromorphic computing architectures.

Abstract

An alternative to charge-based electronics identifies the spin degree of freedom for information communication and processing. The long spin-diffusion length in graphene at room temperature demonstrates its ability for highly scalable spintronics. The development of the graphene spin valve (SV) has inspired spin devices in graphene including spin field-effect transistors and spin majority logic gates. A comprehensive picture of spin transport in graphene SVs is required for further development of spin logic. This review examines the advances in graphene SVs and their role in the development of spin logic devices. Different transport and scattering mechanisms in charge and spin are discussed. Furthermore, the on/off switching energy between graphene SVs and charge-based FETs is compared to highlight their prospects for low-power devices. The challenges and perspectives that need to be addressed for the future development of spin logic devices are then outlined.

Applied Physics Letters, Volume 122, Issue 1, January 2023.
We report on transport characteristics of field effect two-dimensional electron gases (2DEGs) in surface indium antimonide quantum wells. The topmost 5 nm of the 30 nm wide quantum well is doped and shown to promote the formation of reliable, low resistance Ohmic contacts to surface InSb 2DEGs. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus is observed to filling factor ν = 1 in magnetic fields of up to B = 18 T. We show that the electron density is gate-tunable, reproducible, and stable from pinch-off to 4 [math] cm−2, and peak mobilities exceed 24 000 cm2/V s. Large Rashba spin–orbit coefficients up to 110 meV [math]Å are obtained through weak anti-localization measurements. An effective mass of 0.019me is determined from temperature-dependent magnetoresistance measurements, and a g-factor of 41 at a density of 3.6 [math] cm−2 is obtained from coincidence measurements in tilted magnetic fields. By comparing two heterostructures with and without a delta-doped layer beneath the quantum well, we find that the carrier density is stable with time when doping in the ternary Al0.1In0.9Sb barrier is not present. Finally, the effect of modulation doping on structural asymmetry between the two heterostructures is characterized.

Journal of Applied Physics, Volume 133, Issue 1, January 2023.
We demonstrate a method for fabricating a high-quality AlOx-based planar tunnel junction using atomic layer deposition, integrated with the exfoliation and transfer techniques for van der Waals (vdW) materials. The tunneling spectroscopy results on exfoliated Bi2Sr2CaCu2O8+δ and 2H-NbSe2 vdW superconductors are highly consistent with that obtained by ultrahigh vacuum scanning tunneling spectroscopy on atomically clean surfaces. The planar tunneling devices enable high-precision spectroscopy over a wide range of temperatures and magnetic fields and reveal novel features and stark contrast between high-TC cuprates and conventional superconductors. This method represents a universally applicable technique for probing the electronic structure of various two-dimensional vdW materials.

Journal of Applied Physics, Volume 133, Issue 1, January 2023.
An 8-band [math] theory is implemented for studying the electronic properties of near-surface lateral InxGa(1−x)As/InP heterostructure nanowires in the (100) direction. The change in bandgap and effective mass due to inhomogeneous strain are compared to unstrained scenario nanowires, and the lattice mismatch is varied from −3.2% to 1%. The nanowires' height is H = 5, 13 nm, and the width varies from the smallest possible width for a given height to 100 nm. The change in the bandgap exhibited a nonlinear trait with strain for all sizes of the nanowire. The tensile strain reduces the bandgap irrespective of the width of the nanowire for a given height, while the effect of compressive strain on change in the bandgap becomes width dependent. Minima and maxima in the change in effective mass with respect to the nanowire width are observed in compressively and tensile strained nanowires, respectively, due to the interplay of quantum confinement and strain. The electrical performance of a single nanowire In0.85Ga0.15As/InP MOSFET in quantum capacitance limit is discussed for various nanowire sizes. The implemented 8-band [math] method is verified with the available experimental work and demonstrated that the developed model can be extended to study electronic parameters of arbitrarily shaped core–shell structures over a wide range of strain.

Nature Communications, Published online: 07 January 2023; doi:10.1038/s41467-022-35760-x

2D semiconductors are attracting increasing attention as potentially scalable channels for future transistors, but the scaling of their contact length remains challenging. Here, the authors report the realization of 1D semimetal-2D semiconductor contacts based on individual carbon nanotubes with contact length down to 2 nm.

2D bimetallic phosphorous trisulfides M^IM^IIIP₂S₆ (M^I = Cu, Ag; M^III = Sc, V, Cr, In) are tested for the first time as electrocatalysts in alkaline media. AgScP₂S₆ exhibits a higher mass activity toward the oxygen reduction reaction. For the hydrogen evolution reaction, CuScP₂S₆ shows an overpotential of 407 mV and a Tafel slope of 90 mV dec⁻¹.

Abstract

Considerable improvements in the electrocatalytic activity of 2D metal phosphorous trichalcogenides (M₂P₂X₆) have been achieved for water electrolysis, mostly with M^II ₂[P₂X₆]⁴⁻ as catalysts for hydrogen evolution reaction (HER). Herein, M^IM^IIIP₂S₆ (M^I = Cu, Ag; M^III = Sc, V, Cr, In) are synthesized and tested for the first time as electrocatalysts in alkaline media, towards oxygen reduction reaction (ORR) and HER. AgScP₂S₆ follows a 4 e⁻ pathway for the ORR at 0.74 V versus reversible hydrogen electrode; CuScP₂S₆ is active for HER, exhibiting an overpotential of 407 mV and a Tafel slope of 90 mV dec⁻¹. Density functional theory models reveal that bulk AgScP₂S₆ and CuScP₂S₆ are both semiconductors with computed bandgaps of 2.42 and 2.23 eV, respectively and overall similar electronic properties. Besides composition, the largest difference in both materials is in their molecular structure, as Ag atoms sit at the midpoint of each layer alongside Sc atoms, while Cu atoms are raised to a similar height to S atoms, in the external segment of the 2D layers. This structural difference probably plays a fundamental role in the different catalytic performances of these materials. These findings show that M^I(Cu, Ag) together with Sc(M^III) leads to promising achievements in M^IM^IIIP₂S₆ materials as electrocatalysts.

A high-yield crystal growth method utilizing a polymer flux in a compressed growth space is invented to cast molecules or precursors into dimension-defined single crystals. This compressed flux growth is universally applicable to various materials and scalable to mass production, and the grown single crystals can be directly integrated into electronic devices.

Abstract

Single crystals possess the most perfect and stable morphology and represent the intrinsic and upper limits of performance when integrated into various application scenarios. However, for a large portion of the newly emerging low-dimensional and molecular materials, the mass production of crystals with a desirable shape is still challenging. Here, a universal and high-yield method to grow functional single crystals with controlled dimensions is provided that can be directly integrated into a device. By utilizing a polymeric flux in combination with a compressed growth space, numerous materials can be grown into size-controllable single crystalline flakes, with millions produced in one batch. This scalable growth method shows promise for the large-scale integration of micro-single-crystals as functional components, as exemplified by the construction of a 5 in. field-effect transistor array.

By proper structure engineering, exotic n-type GeTe-based materials with hierarchical architectural structures are reported. This unique architecture can simultaneously increase electrical performance and decrease thermal performance, and demonstrating high thermoelectric conversion efficiency. The high-performance n-type GeTe-based material matching well with p-type GeTe-based materials can promote the corresponding practical applications.

Abstract

Compatible p- and n-type materials are necessary for high-performance GeTe thermoelectric modules, where the n-type counterparts are in urgent need. Here, it is reported that the p-type GeTe can be tuned into n-type by decreasing the formation energy of Te vacancies via AgBiTe₂ alloying. AgBiTe₂ alloying induces Ag₂Te precipitates and tunes the carrier concentration close to the optimal level, leading to a high-power factor of 6.2 µW cm⁻¹ K⁻² at 423 K. Particularly, the observed hierarchical architectural structures, including phase boundaries, nano-precipitates, and point defects, contribute an ultralow lattice thermal conductivity of 0.39 W m⁻¹ K⁻¹ at 423 K. Correspondingly, an increased ZT of 0.5 at 423 K is observed in n-type (GeTe)_0.45(AgBiTe₂)_0.55. Furthermore, a single-leg module demonstrates a maximum η of 6.6% at the temperature range from 300 to 500 K. This study indicates that AgBiTe₂ alloying can successfully turn GeTe into n-type with simultaneously optimized thermoelectric performance.

A tunneling dominant imaging photodetector based on WS₂/Te heterostructure with type-I band alignment is proposed. This device integrates carrier transmission barriers and photoinduced tunneling mechanism, which demonstrates competitive photodetection performance. Moreover, high-resolution polarized light imaging and ultra-weak light imaging are realized based on this device.

Abstract

Next-generation imaging systems require photodetectors with high sensitivity, polarization sensitivity, miniaturization, and integration. By virtue of their intriguing attributes, emerging 2D materials offer innovative avenues to meet these requirements. However, the current performance of 2D photodetectors is still below the requirements for practical application owing to the severe interfacial recombination, the lack of photoconductive gain, and insufficient photocarrier collection. Here, a tunneling dominant imaging photodetector based on WS₂/Te heterostructure is reported. This device demonstrates competitive performance, including a remarkable responsivity of 402 A W⁻¹, an outstanding detectivity of 9.28 × 10¹³ Jones, a fast rise/decay time of 1.7/3.2 ms, and a high photocurrent anisotropic ratio of 2.5. These outstanding performances can be attributed to the type-I band alignment with carrier transmission barriers and photoinduced tunneling mechanism, allowing reduced interfacial trapping effect, effective photoconductive gains, and anisotropic collection of photocarriers. Significantly, the constructed photodetector is successfully integrated into a polarized light imaging system and an ultra-weak light imaging system to illustrate the imaging capability. These results suggest the promising application prospect of the device in future imaging systems.