Jiuxiang Dai

This article reports measurements of quantum transport in strained graphene transistors which agree quantitatively with models based on mechanically-induced gauge potentials. Mechanically generated vector potentials suppress the ballistic conductance of graphene by up to 30% and control its quantum interferences. This work opens opportunities to harness straintronics effects in 2DM quantum technologies.

Abstract

2D materials (2DMs) are fundamentally electro-mechanical systems. Their environment unavoidably strains them and modifies their quantum transport properties. For instance, a simple uniaxial strain can completely turn off the conductance of ballistic graphene or switch on/off the superconducting phase of magic-angle bilayer graphene. This article reports measurements of quantum transport in strained graphene transistors which agree quantitatively with models based on mechanically-induced gauge potentials. A scalar potential is mechanically induced in situ to modify graphene's work function by up to 25 meV. Mechanically generated vector potentials suppress the ballistic conductance of graphene by up to 30% and control its quantum interferences. The data are measured with a custom experimental platform able to precisely tune both the mechanics and electrostatics of suspended graphene transistors at low-temperature over a broad range of strain (up to 2.6%). This work opens many opportunities to harness quantitative strain effects in 2DM quantum transport and technologies.

Wafer-scale atomic assembly method to produce 2D multinary (binary, ternary, and quaternary) semiconductors for broadband photodetection is accomplished using a succinct coating of the single-precursor and subsequent thermal decomposition combined with thermal evaporation of the chalcogen powder. The MoS₂-, Ni_0.06Mo_0.26S_0.68-, and Ni_0.1Mo_0.9S_1.79Se_0.21-based photodetector exhibit excellent photoelectrical properties, which is highly beneficial for visible and near-infrared photodetection.

Abstract

The tunable properties of 2D transition-metal dichalcogenide (TMDs) materials are extensively investigated for high-performance and wavelength-tunable optoelectronic applications. However, the precise modification of large-scale systems for practical optoelectronic applications remains a challenge. In this study, a wafer-scale atomic assembly process to produce 2D multinary (binary, ternary, and quaternary) TMDs for broadband photodetection is demonstrated. The large-area growth of homogeneous MoS₂, Ni_0.06Mo_0.26S_0.68, and Ni_0.1Mo_0.9S_1.79Se_0.21 is carried out using a succinct coating of the single-source precursor and subsequent thermal decomposition combined with thermal evaporation of the chalcogen powder. The optoelectrical properties of the multinary TMDs are dependent on the combination of heteroatoms. The maximum photoresponsivity of the MoS₂-, Ni_0.06Mo_0.26S_0.68-, and Ni_0.1Mo_0.9S_1.79Se_0.21-based photodetectors is 3.51 × 10⁻⁴, 1.48, and 0.9 A W⁻¹ for 532 nm and 0.063, 0.42, and 1.4 A W⁻¹ for 1064 nm, respectively. The devices exhibited excellent photoelectrical properties, which is highly beneficial for visible and near-infrared (NIR) photodetection.

Light: Science & Applications, Published online: 29 March 2024; doi:10.1038/s41377-024-01422-4

Nonvolatile and reconfigurable two-terminal electro-optic duplex memristor based on III-nitride semiconductors

The work demonstrates the application of photostriction to wirelessly drive freestanding ferroelectric membranes as resonators/actuators. Photoexcited electrons screen the ferroelectric polarization generating an in-plane strain: the underlying mechanism behind photostriction. Consequently, ferroelectric membranes exhibit orders of magnitude larger responses than non-ferroelectric ones. The findings provide significant insight for future applications in optical micro-electromechanical systems and sensors using ferroelectrics.

Abstract

Complex oxides offer a wide range of functional properties, and recent advances in the fabrication of freestanding membranes of these oxides are adding new mechanical degrees of freedom to this already rich functional ecosystem. Here, photoactuation is demonstrated in freestanding thin film resonators of ferroelectric Barium Titanate (BaTiO₃) and paraelectric Strontium Titanate (SrTiO₃). The free-standing films, transferred onto perforated supports, act as nano-drums, oscillating at their natural resonance frequency when illuminated by a frequency-modulated laser. The light-induced deflections in the ferroelectric BaTiO₃ membranes are two orders of magnitude larger than in the paraelectric SrTiO₃ ones. Time-resolved X-ray micro-diffraction under illumination and temperature-dependent holographic interferometry provide combined evidence for the photostrictive strain in BaTiO₃ originating from a partial screening of ferroelectric polarization by photo-excited carriers, which decreases the tetragonality of the unit cell. These findings showcase the potential of photostrictive freestanding ferroelectric films as wireless actuators operated by light.

Diamond-like carbon coating (DLC) exhibits notably distinct frictional behavior across macro-, micro-, and nanoscale friction tests conducted in inert gas environments. This peculiar dependence of DLC friction on the sliding contact scale is attributed to the instability of transfer films within the small contact area at the nanoscale, which hinders the occurrence of shear-induced graphitization and hydrogenation at the sliding interface.

Abstract

Hydrogenated diamond-like carbon (HDLC) is a promising solid lubricant for its superlubricity which can benefit various industrial applications. While HDLC exhibits notable friction reduction in macroscale tests in inert or reducing environmental conditions, ultralow friction is rarely observed at the nanoscale. This study investigates this rather peculiar dependence of HDLC superlubricity on the contact scale. To attain superlubricity, HDLC requires i) removal of ≈2 nm-thick air-oxidized surface layer and ii) shear-induced transformation of amorphous carbon to highly graphitic and hydrogenated structure. The nanoscale wear depth exceeds the typical thickness of the air-oxidized layer, ruling out the possibility of incomplete removal of the air-oxidized layer. Raman analysis of transfer films indicates that shear-induced graphitization readily occurs at shear stresses lower than or comparable to those in the nanoscale test. Thus, the same is expected to occur at the nanoscale test. However, the graphitic transfer films are not detected in ex-situ analyses after nanoscale friction tests, indicating that the graphitic transfer films are pushed out of the nanoscale contact area due to the instability of transfer films within a small contact area. Combining all these observations, this study concludes the retention of highly graphitic transfer films is crucial to achieving HDLC superlubricity.

This work reports the dopant engineering method of hafnia-based ferroelectrics to achieve high thermal stability. By utilizing Al as a dopant element for hafnia-based ferroelectrics, high thermal stability is achieved in ferroelectric capacitors and transistors. Further analysis demonstrates that defect concentration and phase transition can be controlled by optimizing the concentration of dopant elements.

Abstract

Hafnia-based ferroelectrics have gained much attention because they can be used in highly scaled, advanced complementary metal-oxide semiconductor (CMOS) memory devices. However, thermal stability should be considered when integrating hafnia-based ferroelectric transistors in advanced CMOS devices, as they can be exposed to high-temperature processes. This work proposed that doping of Al in hafnia-based ferroelectric material can lead to high thermal stability. A ferroelectric capacitor based on Al-doped hafnia, which can be used for one-transistor-one-capacitor applications, exhibits stable operation even after annealing at 900 °C. Moreover, it demonstrates that the ferroelectric transistors based on Al-doped hafnia for one-transistor applications, such as ferroelectric NAND, retain their memory states for 10 years at 100 °C. This study presents a practical method to achieve thermally stable ferroelectric memories capable of enduring high-temperature processes and operation conditions.

Highly customizable 2D and 3D microstructures with sub-micron to micron resolution are achieved by combining mussel-inspired materials (MIMs) with multiphoton lithography microfabrication technique. The presented MIM nano- and microstructures can be easily post-functionalized and are used for DNA detection and electroless metallization with silver nanoparticles. The proposed approach enables precise surface functionalization of choice in various miniaturized systems.

Abstract

This work addresses the critical need for multifunctional materials and substrate-independent high-precision surface modification techniques that are essential for advancing microdevices and sensing elements. To overcome existing limitations, the versatility of mussel-inspired materials (MIMs) is combined with state-of-the-art multiphoton direct laser writing (DLW) microfabrication. In this way, 2D and 3D MIM microstructures of complex designs are demonstrated with sub-micron to micron resolution and extensive post-functionalization capabilities. This study includes polydopamine (PDA), mussel-inspired linear, and dendritic polyglycerols (MI-lPG and MI-dPG), allowing their direct microstructure on the substrate of choice with the option to tailor the patterned topography and morphology in a controllable manner. The functionality potential of MIMs is demonstrated by successfully immobilizing and detecting single-stranded DNA on MIM micropattern and nanoarray surfaces. In addition, easy modification of MIM microstructure with silver nanoparticles without the need of any reducing agent is shown. The methodology developed here enables the integration of MIMs in advanced applications where precise surface functionalization is essential.

Enhanced charge transport in 2D inorganic molecular crystals, by self-assembling ring-like Se₈ molecule to achieve charge delocalization and strong intermolecular orbitals overlap, is reported for the first time, which further supports a remarkable optoelectronic performance.

Abstract

Outstanding charge transport in molecular crystals is of great importance in modern electronics and optoelectronics. The widely adopted strategies to enhance charge transport, such as restraining intermolecular vibration, are mostly limited to organic molecules, which are nearly inoperative in 2D inorganic molecular crystals currently. In this contribution, charge transport in 2D inorganic molecular crystals is improved by integrating charge-delocalized Se₈ rings as building blocks, where the delocalized electrons on Se₈ rings lift the intermolecular orbitals overlap, offering efficient charge transfer channels. Besides, α-Se flakes composed of charge-delocalized Se₈ rings possess small exciton binding energy. Benefitting from these, α-Se flake exhibits excellent photodetection performance with an ultrafast response rate (~5 μs) and a high detectivity of 1.08 × 10¹¹ Jones. These findings contribute to a deeper understanding of the charge transport of 2D inorganic molecular crystals composed of electron-delocalized inorganic molecules and pave the way for their potential application in optoelectronics.

Dynamic regulation and customized manufacturing of WC single crystal and WC/graphene (WC-G) heterostructure can be achieved through a substrate engineering of Cu-Zn alloys. The composition of alloys can be adjusted by varying the ratio of Cu and Zn, wherein low Zn concentration contribute to the growth of WC-G heterostructure and high Zn concentration result in the formation of WC.

Abstract

Transition metal carbides (TMCs) grown by chemical vapor deposition (CVD) offer promise for numerous novel phenomena and applications in the 2D limit. Despite considerable efforts thus far, the flexible customization of TMCs and their heterostructures still remains challenging. Herein, a substrate engineering is developed to achieve customized manufacturing of ultrathin WC single crystals and WC/graphene (WC-G) heterostructures by varying the concentration of Zn in Cu-Zn alloy substrate. It is worth noting that Zn atoms can remarkably reduce the nucleation density of graphene and promote the nucleation of WC. Thus, an increasing Zn content is applied to synergistically modulate the growth of graphene and WC, enabling the controllable fabrication of WC and WC-G heterostructures. The synthesized WC crystals exhibit an ultrathin nature down to 3 nm, as well as high crystalline, ultra-clean surface, and superb chemical stability. Based on that, the typical metallic properties with a temperature-dependent resistance (nearly 1.30 Ω at 300 K and nearly 0.08 Ω at 1.7 K) and low resistance as well as excellent nonlinear optical performance of WC are demonstrated. This work provides fresh insights into regulating the growth behavior of multiblock-structured carbides and promotes the study of their optic and electronic properties.

A universal and effective vacuum evaporation crystallization (VEC) method is developed for growing high-quality perovskite single crystals. The method overcomes the adverse effects of temperature fluctuations and auxiliary substances, and is based on the natural volatilization of solvents at chamber pressure lower than the saturated vapor pressure of the solution.

Abstract

Perovskite single crystals have attracted tremendous attention owing to their excellent optoelectronic properties and stability compared to typical multicrystal structures. However, the growth of high-quality perovskite single crystals (PSCs) generally relies on temperature gradients or the introduction of additives to promote crystal growth. In this study, a vacuum evaporation crystallization technique is developed that allows PSCs to be grown under extremely stable conditions at constant temperature and without requiring additives to promote crystal growth. The new method enables the growth of PSCs of unprecedented quality, that is, MAPbBr₃ single crystals that exhibit an ultranarrow full width at half maximum of 0.00701°, which surpasses that of all previously reported values. In addition, the MAPbBr₃ single crystals deliver exceptional optoelectronic performance, including a long carrier lifetime of 1006 ns, an ultralow trap-state density of 3.67 × 10⁹ cm⁻³, and an ultrahigh carrier mobility of 185.86 cm² V⁻¹ s⁻¹. This method is applicable to various types of PSCs, including organic–inorganic hybrids, fully inorganic structures, and low-dimensional structures.

Epitaxial regrowth processes for achieving Al-rich aluminum gallium nitride (AlGaN) high electron mobility transistor (HEMTs) with p-type gates with large, positive threshold voltage for enhancement mode operation and low resistance Ohmic contacts are reported. The combination of low-leakage, large positive threshold p-gates, and low resistance Ohmic contacts by the described regrowth processes provide a pathway to realizing high-current, enhancement-mode, Al-rich AlGaN-based ultra-wide bandgap transistors.

Abstract

Epitaxial regrowth processes are presented for achieving Al-rich aluminum gallium nitride (AlGaN) high electron mobility transistor (HEMTs) with p-type gates with large, positive threshold voltage for enhancement mode operation and low resistance Ohmic contacts. Utilizing a deep gate recess etch into the channel and an epitaxial regrown p-AlGaN gate structure, an Al_0.85Ga_0.15N barrier/Al_0.50Ga_0.50N channel HEMT with a large positive threshold voltage (V_TH = +3.5 V) and negligible gate leakage is demonstrated. Epitaxial regrowth of AlGaN avoids the use of gate insulators which can suffer from charge trapping effects observed in typical dielectric layers deposited on AlGaN. Low resistance Ohmic contacts (minimum specific contact resistance = 4 × 10⁻⁶ Ω cm² _, average = 1.8 × 10⁻⁴ Ω cm²) are demonstrated in an Al_0.85Ga_0.15N barrier/Al_0.68Ga_0.32N channel HEMT by employing epitaxial regrowth of a heavily doped, n-type, reverse compositionally graded epitaxial structure. The combination of low-leakage, large positive threshold p-gates and low resistance Ohmic contacts by the described regrowth processes provide a pathway to realizing high-current, enhancement-mode, Al-rich AlGaN-based ultra-wide bandgap transistors.

Symmetries and magnetoelectric responses of van der Waals multiferroic CuCrP₂S₆ are investigated by second harmonic generation (SHG). Structural and magnetic phase transitions are successfully probed. The polarization dependence of the SHG signals is significantly modified, and the signal magnitude is enhanced by one order of magnitude under a finite magnetic field, which can be explained by the magnetoelectric effect.

Abstract

Multiferroic materials have attracted considerable attention owing to their unique magnetoelectric or magnetooptical properties. The recent discovery of few-layer van der Waals multiferroic crystals provides a new research direction for controlling the multiferroic properties in the atomic layer limit. However, research on few-layer multiferroic crystals is limited and the effect of thickness-dependent symmetries on those properties is less explored. In this study, the symmetries and magnetoelectric responses of van der Waals multiferroic CuCrP₂S₆ are investigated by optical second harmonic generation (SHG). Structural and magnetic phase transitions are successfully probed by the temperature-dependent SHG signals, revealing significant changes by applying the magnetic field reflecting the magnetoelectric effect. Moreover, it is found that symmetries and resultant magnetoelectric responses can be modulated by the number of layers. These results offer a new principle of controlling the multiferroicity and indicate that 2D van der Waals multiferroic material is a promising building block for functional nanodevices.

This study introduces a versatile platform with the capability to achieve ultrasensitive, label-free biomolecular detection in a potentially high-throughput imaging method, employing a plasmonic sensor based on 2D ferroelectric Bi₂O₂Te nanoflakes. Notably, the research stands as the pioneering demonstration of harnessing the unique ferroelectric properties inherent in 2D materials to enable ultrasensitive biomolecule detection through precise modulation of electric polarization.

Abstract

Ultrasensitive detection of biomarkers, particularly proteins, and microRNA, is critical for disease early diagnosis. Although surface plasmon resonance biosensors offer label-free, real-time detection, it is challenging to detect biomolecules at low concentrations that only induce a minor mass or refractive index change on the analyte molecules. Here an ultrasensitive plasmonic biosensor strategy is reported by utilizing the ferroelectric properties of Bi₂O₂Te as a sensitive-layer material. The polarization alteration of ferroelectric Bi₂O₂Te produces a significant plasmonic biosensing response, enabling the detection of charged biomolecules even at ultralow concentrations. An extraordinary ultralow detection limit of 1 fm is achieved for protein molecules and an unprecedented 0.1 fm for miRNA molecules, demonstrating exceptional specificity. The finding opens a promising avenue for the integration of 2D ferroelectric materials into plasmonic biosensors, with potential applications spanning a wide range.

Indium and gallium selenide films are innovatively grown in a sequential physical vapor deposition process starting from metal precursor layers of various thicknesses, which are then subject to chalcogenization in different selenium contents. By Raman and XRD spectroscopy, all the compounds In₂Se₃, InSe, In₄Se₃, Ga₂Se₃, and GaSe as well as different polytypes are identified. Optical bandgaps range up to 2 eV.

Abstract

Group IIIA metal chalcogenides are an auspicious material system due to their variability of properties and hence the multitude of application options, for example, in the fields of optoelectronic, thermoelectric, piezo-, and ferroelectric devices. Indium and gallium selenide films are innovatively grown in a sequential PVD (physical vapor deposition) process starting from metal precursor layers of various thicknesses, which are then subject to chalcogenization in different selenium contents. The resulting thin films are investigated for structural and optical properties by Raman, XRD (X-ray diffraction), and UV–Vis–NIR spectrometry, revealing that all the compounds In₂Se₃, InSe, In₄Se₃, Ga₂Se₃, and GaSe as well as different polytypes can be achieved depending on the metal/chalcogen ratio. Results from Raman and XRD spectroscopy are highly consistent, and also from the optical measurements changes in absorption characteristics can be correlated. The results indicate, that by fine-tuning the selenium content, deliberately growing ultra-thin layers of the different indium and gallium phases will be possible, thus opening up a promising route for 2D material fabrication. Given the scalability of the fabrication method, it is highly promising for large-scale deployment of the materials.

Nature, Published online: 27 March 2024; doi:10.1038/s41586-024-07243-0

A strategy for the transfer-free direct growth of ultralong, high-quality graphene nanoribbons, which have desirable electronic properties, between layers of a boron nitride insulator is reported.

A ferroelectric molecular crystal displays characteristics required for implantation

Nature Synthesis, Published online: 28 March 2024; doi:10.1038/s44160-024-00511-x

A series of molecular rare-earth telluride clusters incorporating a three-centre, four-electron, tri-tellurido ligand (Te34−) are reported. These atomically precise clusters, possessing ultralow band gaps comparable to those of monocrystalline silicon and gallium arsenide, are potentially applicable as quantum materials and for optoelectronic applications.

Highlights

• Atomic substitution applied in the synthesis of different dimensional transition metal chalcogenide (TMC) is dissertated.

• The controllable synthesis and property modification realization with atomic substitution or ion exchange are introduced.

• The substitution principle and mechanism in different TMCs are concluded.

Nature Communications, Published online: 27 March 2024; doi:10.1038/s41467-024-46878-5

Using Zr-doped HfO2 and ultra-thin indium tin oxide, Li et al. develop flexible field-effect transistors with a memory window of 2.78 V and bending reliability to enable high-performance back-end-of-line compatible wearable devices.

npj 2D Materials and Applications, Published online: 27 March 2024; doi:10.1038/s41699-024-00464-x

Enhancing dielectric passivation on monolayer WS₂ via a sacrificial graphene oxide seeding layer

npj 2D Materials and Applications, Published online: 27 March 2024; doi:10.1038/s41699-024-00465-w

Atomistic description of conductive bridge formation in two-dimensional material based memristor

An exchange bias effect at an all-antiferromagnetic polycrystalline heterointerface is observed. The chiral antiferromagnet Mn₃Sn works as the active layer, replacing conventional ferromagnets. The unidirectional magnetic anisotropy can be determined omnidirectionally due to the absence of the shape anisotropy. These findings are significant for developing various antiferromagnetic spintronic devices, including antiferromagnetic tunnel junctions, essential for ultrafast and ultra-power-efficient computing.

Abstract

Due to promising functionalities that may dramatically enhance spintronics performance, antiferromagnets are the subject of intensive research for developing the next-generation active elements to replace ferromagnets. In particular, the recent experimental demonstration of tunneling magnetoresistance and electrical switching using chiral antiferromagnets has sparked expectations for the practical integration of antiferromagnetic materials into device architectures. To further develop the technology to manipulate the magnetic anisotropies in all-antiferromagnetic devices, it is essential to realize exchange bias through the interface between antiferromagnetic multilayers. Here, the first observation on the omnidirectional exchange bias at an all-antiferromagnetic polycrystalline heterointerface is reported. This experiment demonstrates that the interfacial energy causing the exchange bias between the chiral-antiferromagnet Mn₃Sn/collinear-antiferromagnet MnN layers is comparable to those found at the conventional ferromagnet/antiferromagnet interface at room temperature. In sharp contrast with previous reports using ferromagnets, the magnetic field control of the unidirectional anisotropy is found to be omnidirectional due to the absence of the shape anisotropy in the antiferromagnetic multilayer. The realization of the omnidirectional exchange bias at the interface between polycrystalline antiferromagnets on amorphous templates, highly compatible with existing Si-based devices, paves the way for developing ultra-low power and ultra-high speed memory devices based on antiferromagnets.