Jiuxiang Dai

Hardware Security

In article number 2400661, Akshay Wali, Harikrishnan Ravichandran, and Saptarshi Das present a novel hardware security authentication system that employs programmable integrated circuits (ICs) crafted from atomically thin monolayers of molybdenum disulfide (MoS₂), a two-dimensional material. It capitalizes on the inherent randomness of charge trapping and de-trapping in 2D memtransistors to generate cryptographic keys. Additionally, it utilizes the properties of a NOR gate to establish a secure one-way hash function.

This work presents a novel 2D ferromagnetic semiconducting material with an unprecedented Curie temperature (T _c) of 230 K, over four times higher than previous records for 2D magnetic semiconductors. Such a high T _c arises from the synergy between in-plane and out-of-plane superexchange interactions, and puts forward a novel concept of regulating properties in the vertical dimension for 2D materials.

Abstract

2D magnetic semiconductors exhibit great potential for next-generation spintronics, but realizing their full capabilities has been hindered by the low Curie temperatures (T _c) below 50 K observed in current materials. Here, a new mechanism to substantially enhance the T _c of 2D semiconducting materials through incorporating both in-plane and out-of-plane superexchange interactions enabled by structural design is demonstrated. Specifically, monolayer Cr₂Se₃ is synthesized with a five-layer Se–Cr–Se–Cr–Se atomic structure using molecular beam epitaxy (MBE). This unique structure not only possesses optimized in-plane superexchange interaction but also incorporates out-of-plane Cr–Se–Cr couplings. Scanning tunneling spectroscopy (STS) and angular-resolved photoemission spectroscopy (ARPES) confirm its semiconducting nature. Remarkably, the ferromagnetic phase transition observed by ARPES and Magnetic Force Microscopy (MFM) indicated that its T _c is up to 230 K. This not only establishes a new record for T _c in 2D ferromagnetic semiconductor materials but also introduces a novel approach to modulating materials' properties by manipulating the vertical dimension in 2D materials.

Tomographic PFM imaging delineates a 3D domain arrangement in the van der Waals ferroelectric semiconductor α-In₂Se₃ by revealing antiparallel domain layers stacked along the polar direction separated by the stable H-H and T-T domain boundaries. A considerable electroresistance effect with the OFF/ON ratio of at least two orders of magnitude is observed via local transport measurements.

Abstract

One of the exceptional features of the van der Waals (vdW) ferroelectrics is the existence of stable polarization at a level of atomically thin monolayers. This ability to withstand a detrimental effect of the depolarization fields gives rise to complex domain configurations characterized, among others, by the presence of layered “antipolar” head-to-head (H-H) or tail-to-tail (T-T) dipole arrangements. In this study, tomographic piezoresponse force microscopy (TPFM) is employed to study the 3D polarization arrangement in vdW ferroelectric α-In₂Se₃. Sequential removal of thin layers from the polar surface using the PFM tip reveals a complex 3D profile of the domain walls in the α-In₂Se₃ crystals. Antiparallel domain layers stacked along the polar direction are also observed by PFM imaging of the non-polar surfaces showing that H-H and T-T domain boundaries are commonly present in α-In₂Se₃. Application of TPFM to the electrically written domains allows evaluation of their geometrical lateral-to-vertical size aspect ratio, which shows a strong prevalence for the sidewise expansion in comparison to the forward growth. Local I–V measurements reveal a strong polarization direction dependence of conductivity due to the modulation of the energy barrier height as corroborated by theoretical modeling.

A miniature piezoelectric robot inspired by earthworms is proposed, and the principle of longitudinal-vibration-compound actuation with multilegged collaboration is designed to improve surface adaptability. The experimental results show that the robot achieves miniature size, lightweight, and high adaptability. The robot can adapt to flat, fold, concave, and convex surfaces, and even incline and dynamically rotating tubes.

Abstract

Miniature resonant piezoelectric robots have the advantages of compact structure, fast response, high speed, and easy control, which have attracted the interest of many scholars in recent years. However, piezoelectric robots usually suffer from the problem of poor adaptability due to the micron-level amplitude at the feet. Inspired by the fact that earthworms have actuation trajectories all around their bodies to move flexibly under the ground, a miniature piezoelectric robot with circumferentially arranged driving feet to improve adaptability is proposed. Notably, a longitudinal-vibration-compound actuation principle with multilegged collaboration is designed to achieve the actuation trajectories around the robot, similar to the earthworms. The structure and operating principle are simulated by the finite element method, and the prototype is fabricated. The robot weighs 22.7 g and has dimensions of 35.5 × 36.5 × 47 mm³. The robot is tethered to an ultrasonic power supply, and the experimental results show that the speed reaches 179.35 mm s⁻¹  under an exciting signal with a frequency of 58.5 kHz and a voltage of 200 V_p-p. High adaptability is achieved by the proposed robot, it can move on flat, fold, concave, and convex surfaces, and even in an inclined or rotating tube.

Nature Nanotechnology, Published online: 06 June 2024; doi:10.1038/s41565-024-01685-3

Multiwalled boron nitride nanotubes, featuring coherently stacked structures with monochirality, homo-handedness and unipolarity among the component tubes, show a large nonlinear chiroptical response.

Spontaneous chirality emerges in a highly polar liquid of achiral molecules

Nature Physics, Published online: 06 June 2024; doi:10.1038/s41567-024-02515-y

Some features resembling superconductivity at high temperature have been seen under pressure in La3Ni2O7, but a transition to a zero-resistance state has not been observed. Now transport studies demonstrate this transition, along with strange metallicity.

Recently, van der Waals (vdW) sliding ferroelectricity is confirmed in twisted 2D materials by artificially stack design. Here, controllable vapor deposition growth of bilayer 3R MoS₂ is achieved. The ferroelectricity of as-grown bilayer 3R MoS₂ is investigated via in situ atomic force probe technique. This work can be a big step toward industrial applications of 2D vdW ferroelectric memory.

Abstract

Two-dimensional ultrathin ferroelectrics have attracted much interest due to their potential application in high-density integration of non-volatile memory devices. Recently, 2D van der Waals ferroelectric based on interlayer translation has been reported in twisted bilayer h-BN and transition metal dichalcogenides (TMDs). However, sliding ferroelectricity is not well studied in non-twisted homo-bilayer TMD grown directly by chemical vapor deposition (CVD). In this paper, for the first time, experimental observation of a room-temperature out-of-plane ferroelectric switch in semiconducting bilayer 3R MoS₂ synthesized by reverse-flow CVD is reported. Piezoelectric force microscopy (PFM) hysteretic loops and first principle calculations demonstrate that the ferroelectric nature and polarization switching processes are based on interlayer sliding. The vertical Au/3R MoS₂/Pt device exhibits a switchable diode effect. Polarization modulated Schottky barrier height and polarization coupling of interfacial deep states trapping/detrapping may serve in coordination to determine switchable diode effect. The room-temperature ferroelectricity of CVD-grown MoS₂ will proceed with the potential wafer-scale integration of 2D TMDs in the logic circuit.

By harnessing the unique property changes induced by photothermal effects in 2D graphene oxide (GO) films, three novel functionalities beyond the capability of photonic integrated circuits are demonstrated, including all-optical control and tuning, optical power limiting, and nonreciprocal light transmission. The experimental results are theoretically analyzed, reflecting intriguing insights into the physics of 2D GO films.

Abstract

On-chip integration of 2D materials with unique structures and properties endow integrated devices with new functionalities and improved performance. With high flexibility in ways to modify its properties and compatibility with integrated platforms, graphene oxide (GO) is an exceptionally attractive 2D material for hybrid integrated photonic chips. Here, by harnessing unique property changes induced by photothermal effects in 2D GO films, novel functionalities beyond the capability of photonic integrated circuits are demonstrated. These include all-optical control and tuning, optical power limiting, and nonreciprocal light transmission. The 2D layered GO films are integrated onto photonic chips with precise control of their thickness and size. Benefitting from the broadband optical response of 2D GO films, all three functionalities feature a very wide operational optical bandwidth. By fitting the experimental results with theory, the changes in GO film properties induced by the photothermal effects are analyzed, revealing interesting insights about the physics of 2D GO films. These results highlight the versatility of 2D GO films in implementing new functions for integrated photonic devices for a wide range of applications.

Nature Synthesis, Published online: 04 June 2024; doi:10.1038/s44160-024-00562-0

A van der Waals epitaxial strategy is reported for growing intrinsic quantum dots (QDs) by modulating interfacial couplings on van der Waals surfaces. This method overcomes lattice mismatch constraints and produces versatile III–V and IV–VI QDs with controllable morphologies, broadening near-infrared photoresponse in InSb QDs/MoS2 by efficient interlayer charge transfer.

Using a self-assembled monolayer (SAM) of MeO-2PACz as an interface layer between the Sb₂S₃ and Spiro-OMeTAD, an interfacial engineering is created that improves band alignment, increases built-in potential, produces a more uniform surface potential, and increases the champion power conversion efficiency (PCE) of Sb₂S₃ solar cells by >13%, reaching 8.06%.

Abstract

Antimony chalcogenides films and devices have drawn much attention in recent years because of their notable advantages. Unfortunately, the performance of Sb-based solar cells is still underdeveloped compared to the theoretical value, which is closely related to charge carrier separation and transfer, highlighting the importance of enhancing the interface quality. In this work, an interfacial engineering by utilizing a self-assembled monolayer (SAM) of MeO-2PACz as an interface layer between the Sb₂S₃ and Spiro-OMeTAD is developed to assist hole transport. The strong interface interaction between Sb₂S₃ and MeO-2PACz is systematically investigated by Raman, X-ray photoelectron spectroscopy (XPS) measurements, and Density functional theory (DFT) calculations. Through such interfacial engineering, a more uniform surface potential, bigger built-in potential, better energy-level match as well as outstanding photoelectric properties are achieved. Finally, the champion power conversion efficiency (PCE = 8.06%) of Sb₂S₃ solar cells with SAMs is inspiringly enhanced by >13%. It is expected that this effort will bring fresh insights and strategies for improving the performance of Sb-based solar devices.

The progress was summarized in the flexible electronics empowered by the two-dimensional materials, including electronic skins (for sweat and temperature sensors), gas sensors, touch pads, nanogenerators for mechanical energy collection, flexible supercapacitors and batteries, transistors and logic circuits, as well as memristors for neuromorphic computing. The readers may collect the stat-of-the-art research on graphene and MXene based flexible electronics.

Abstract

Flexible electronics has emerged as a continuously growing field of study. Two-dimensional (2D) materials often act as conductors and electrodes in electronic devices, holding significant promise in the design of high-performance, flexible electronics. Numerous studies have focused on harnessing the potential of these materials for the development of such devices. However, to date, the incorporation of 2D materials in flexible electronics has rarely been summarized or reviewed. Consequently, there is an urgent need to develop comprehensive reviews for rapid updates on this evolving landscape. This review covers progress in complex material architectures based on 2D materials, including interfaces, heterostructures, and 2D/polymer composites. Additionally, it explores flexible and wearable energy storage and conversion, display and touch technologies, and biomedical applications, together with integrated design solutions. Although the pursuit of high-performance and high-sensitivity instruments remains a primary objective, the integrated design of flexible electronics with 2D materials also warrants consideration. By combining multiple functionalities into a singular device, augmented by machine learning and algorithms, we can potentially surpass the performance of existing wearable technologies. Finally, we briefly discuss the future trajectory of this burgeoning field. This review discusses the recent advancements in flexible sensors made from 2D materials and their applications in integrated architecture and device design.

A morphotropic phase boundary comprising a coexistence of four distinct crystallographic phases is uncovered through A-site doping and strain engineering. The various ferroelectric, polar, and nonpolar phases form a nanoscale mixture without the presence of crystallographically hard boundaries. Polarization rotation and extension, which release the polarization from its crystallographic constraint, promote robust ferroelectric properties and giant electromechanical responses.

Abstract

In ferroic materials, giant susceptibilities can be realized at artificially constructed phase boundaries through deterministic manipulation of the order parameter. Here, emergent ferroelectric structural phase evolution behavior is demonstrated through a synergistic combination of A-site doping and strain engineering. Using chemical solution deposition derived (001)-oriented Sm-substituted bismuth ferrite (Bi_{1- x} Sm _x FeO₃) films as a prototypical system, a morphotropic phase boundary comprising a coexistence of four distinct crystallographic phases is uncovered. These ferroelectric, polar, and nonpolar phases form a nanoscale mixture without the presence of crystallographically hard boundaries. The system thus possesses the ability to show both polarization rotation and extension, effectively releasing the polarization from its crystallographic constraint. Consequently, both robust ferroelectric properties and giant electromechanical responses are obtained. For instance, the optimized composition with x = 0.14 has a remnant polarization of 2P_r = 103 µC cm⁻² and electromechanical response 175% that of undoped BFO. These findings showcase the tremendous potential of synthetic phase boundaries, particularly in the context of lead-free functional multiferroics.

Author(s): Shilei Ding, Min-Gu Kang, William Legrand, and Pietro Gambardella

Orbital currents have recently emerged as a promising tool to achieve electrical control of the magnetization in thin-film ferromagnets. Efficient orbital-to-spin conversion is required in order to torque the magnetization. Here, we show that the injection of an orbital current in a ferrimagnetic Gd…

[Phys. Rev. Lett. 132, 236702] Published Wed Jun 05, 2024

Field-effect thermoelectrics in monolayer graphene transistor is reported. Spatially localized hotspot is observed at Dirac point and arises from both Joule and Peltier heating, characterized by scanning thermal microscopy and analyzed through simulation. The position of the observed Dirac hotspot can shift in the gated region, highly controllable by the gate bias.

Abstract

Graphene is a promising candidate for the thermal management of downscaled microelectronic devices owing to its exceptional electrical and thermal properties. Nevertheless, a comprehensive understanding of the intricate electrical and thermal interconversions at a nanoscale, particularly in field-effect transistors with prevalent gate operations, remains elusive. In this study, nanothermometric imaging is used to examine a current-carrying monolayer graphene channel sandwiched between hexagonal boron nitride dielectrics. It is revealed for the first time that beyond the expected Joule heating, the thermoelectric Peltier effect actively plays a significant role in generating hotspots beneath the gated region. With gate-controlled charge redistribution and a shift in the Dirac point position, an unprecedented systematic evolution of thermoelectric hotspots, underscoring their remarkable tenability is demonstrated. This study reveals the field-effect Peltier contribution in a single graphene-material channel of transistors, offering valuable insights into field-effect thermoelectrics and future on-chip energy management.

This review provides perspectives on the chemical intercalation of layered materials. The characteristics of the different intercalation methods and their chemical mechanisms are discussed. The properties and applications of intercalation compounds are discussed. Finally, brief insights into the challenges and future opportunities for the chemical intercalation of layered materials are provided.

Abstract

The intercalation of layered materials offers a flexible approach for tailoring their structures and generating unexpected properties. This review provides perspectives on the chemical intercalation of layered materials, including graphite/graphene, transition metal dichalcogenides, MXenes, and some particular materials. The characteristics of the different intercalation methods and their chemical mechanisms are discussed. The influence of intercalation on the structural changes of the host materials and the structural change how to affect the intrinsic properties of the intercalation compounds are discussed. Furthermore, a perspective on the applications of intercalation compounds in fields such as energy conversion and storage, catalysis, smart devices, biomedical applications, and environmental remediation is provided. Finally, brief insights into the challenges and future opportunities for the chemical intercalation of layered materials are provided.

This paper encapsulates the review's focus on recent advances in strain engineering of 2D layered materials like graphene, h-BN, TMDs, and BP. It outlines the methods, effects on properties, and potential applications in devices, providing a roadmap for further research.

Abstract

This review explores the growing interest in 2D layered materials, such as graphene, h-BN, transition metal dichalcogenides (TMDs), and black phosphorus (BP), with a specific focus on recent advances in strain engineering. Both experimental and theoretical results are delved into, highlighting the potential of strain to modulate physical properties, thereby enhancing device performance. Various strain engineering methods are summarized, and the impact of strain on the electrical, optical, magnetic, thermal, and valleytronic properties of 2D materials is thoroughly examined. Finally, the review concludes by addressing potential applications and challenges in utilizing strain engineering for functional devices, offering valuable insights for further research and applications in optoelectronics, thermionics, and spintronics.

This work proposes that hydrogen-bonding significantly enhances the tunneling effect to achieve contact resistance approaching the quantum limit that enables overcoming the van der Waals gap. Experiments further demonstrate that hydrogen-bonding integrated flexible transistors have low contact resistance and high mobility with values an order of magnitude higher than their van der Waals counterparts.

Abstract

Van der Waals (vdW) integration enables clean contacts for low-dimensional electronic devices. The limitation remains; however, that an additional tunneling contact resistance occurs owing to the inherent vdW gap between the metal and the semiconductor. Here, it is demonstrated from theoretical calculations that stronger non-covalent hydrogen-bonding interactions facilitate electron tunneling and significantly reduce the contact resistance; thus, promising to break the limitations of the vdW contact. π-plane hydrogen-bonding contacts in surface-engineered MXene/carbon nanotube metal/semiconductor heterojunctions are realized, and an anomalous temperature-dependent tunneling resistance is observed. Low-dimensional flexible thin-film transistors integrated by hydrogen-bonding contacts exhibit both excellent flexibility and carrier mobility orders of magnitude higher than their counterparts with vdW contacts. This strategy demonstrates a scalable solution for realizing high-performance and low-power flexible electronics beyond vdW contacts.

This review outlines emerging engineering methods aimed at producing ultralow disordered heterostructures, crucial for exploring quantum phenomena. Despite significant progress, barriers to widespread implementation remain, highlighting the need for continued research in further advancing the fabrication toolkits.

Abstract

The exploration of emerging quantum phenomena by stacking dissimilar atomic layered materials into van der Waals (vdW) heterostructures has driven the development of layer assembly techniques. Achieving ultralow disorder within these heterostructures is crucial for unlocking their novel physical properties. However, current fabrication methods for designer heterostructures have limitations in throughput, yield, and scalability. Over the past decade, engineering toolkits have evolved to address some of these challenges, but their adoption for fabricating designer heterostructures remains limited. In this review, an overview of these emerging engineering toolkits is provided, and examine their utility and limitations in achieving ultralow disordered heterostructures. It is hoped that the insights from this review article can help guide future research directions on advancing the fabrication process of designer heterostructures.

A unipolar barrier van der Waals heterostructure (UB-vdWH) photodetector is reported to realize filter-free visible-blind UV detection with good stability, robustness, selectivity, and high detection performance. The UB-vdWH shows a responsivity of 2452 A W⁻¹, a photo on-off ratio of 2.94 × 10⁵, and a detectivity of 1.26 × 10¹⁵ Jones, owing to the intentionally designed barrier height.

Abstract

Visible-blind ultraviolet (UV) light detection has a wide application range in scenes like space environment monitoring and medical imaging. To realize miniaturized UV detectors with high performance and high integration ability, new device structures without bulky light filters need to be developed based on advanced mechanisms. Here the unipolar barrier van der Waals heterostructure (UB-vdWH) photodetector is reported that realizes filter-free visible-blind UV detection with good stability, robustness, selectivity, and high detection performance. The UB-vdWH shows a responsivity of 2452 A W⁻¹, a photo on-off ratio of 2.94 × 10⁵ and a detectivity of 1.26 × 10¹⁵ Jones as a UV detector, owing to the intentionally designed barrier height that suppresses dark current and photoresponse to visible light during the transport process. The good performance remains intact during 10⁴ test cycles or even under high temperatures, which proves the stability, and robustness of the UB-vdWH, thus shows the huge potential for a wider application range.

2D Cr₂S₃ prepared by liquid phase exfoliation forms a heterojunction with titanium dioxide for photoelectrochemical water splitting. 2D Cr₂S₃ is obtained from bulk Cr₂S₃ by liquid phase stripping in NMP. The results show that 2D Cr₂S₃ can form an S-scheme heterojunction with TiO₂ giving a PEC water decomposition efficiency of the heterojunction of H₂: 124.3 and O₂: 62.2 µmol cm⁻² h⁻¹.

Abstract

Exfoliation of 2D non-Van der Waals (non-vdW) semiconductor nanoplates (NPs) from inorganic analogs presents many challenges ahead for further exploring of their advanced applications on account of the strong bonding energies. In this study, the exfoliation of ultrathin 2D non-vdW chromium sulfide (2D Cr₂S₃) by means of a combined facile liquid-phase exfoliation (LPE) method is successfully demonstrated. The morphology and structure of the 2D Cr₂S₃ material are systematically examined. Magnetic studies show an obvious temperature-dependent uncompensated antiferromagnetic behavior of 2D Cr₂S₃. The material is further loaded on TiO₂ nanorod arrays to form an S-scheme heterojunction. Experimental measurements and density functional theory (DFT) calculations confirm that the formed TiO₂@Cr₂S₃ S-scheme heterojunction facilitates the separation and transmission of photo-induced electron/hole pairs, resulting in a significantly enhanced photocatalytic activity in the visible region.