Jiuxiang Dai

Nature Materials, Published online: 02 February 2024; doi:10.1038/s41563-023-01789-6

Metal monochalcogenides — a class of van der Waals layered semiconductors — can exhibit ultrahigh plasticity. Investigation of the deformation mechanism reveals that on mechanical loading, these materials undergo local phase transitions that, coupled with the concurrent generation of a microcrack network, give rise to the ultrahigh plasticity.

The crystal structure and optical properties of the van der Waals layered CuInP₂Se₆ are systematically studied. A dual-function detector based on 2D CuInP₂Se₆, exhibiting an ultralow and stable dark current, is fabricated. This detector demonstrates remarkable sensitivity in detecting UV-vis light and X-rays, and it is successfully employed in high-resolution image sensing applications.

Abstract

Metal thio(seleno)phosphates are renowned for their multifaceted physical characteristics and versatile applications, particularly in optoelectronics. In detection applications, a low and stable dark current is crucial, enhancing the sensitivity and signal-to-noise ratio of detectors. Herein, a van der Waals layered material has synthesized, CuInP₂Se₆. Despite its nanometric scale, 2D CuInP₂Se₆ detector transcends the conventional absorption inefficiencies tied to ultrathin materials. It delivers exceptional ultraviolet–visible detection, characterized by an ultralow, stable dark current of 150 fA, and a noise power density of 27.7 fA Hz^−1/2 at room temperature. The in-depth investigation reveals a responsivity of 4.47 A W⁻¹, an external quantum efficiency of 1369%, a special detectivity of 1.44 × 10¹³ Jones, and a rapid response speed of 280 µs, positioning it at the pinnacle of 2D photodetector performance. The CuInP₂Se₆’s ultralow, stable dark current paves the way for X-ray detection, achieving an unprecedented sensitivity of 1.32 × 10⁵ µC Gy_air ⁻¹ cm⁻² and a low detection limit of 0.15 µGy_air s⁻¹. Furthermore, 2D CuInP₂Se₆ detector exhibits a remarkable image-sensing capability, adeptly capturing intricate patterns with high resolution. This discovery indicates its promise in revolutionizing integrated micro/nano optoelectronic devices, opening avenues for advancements in light and X-ray detection and imaging technologies.

This article presents the design and characterization of a new 2D organic–inorganic hybrid perovskite ferroelectric, (6-BHA)₂CdBr₄ (6-BHA = 6-bromohexylamine), which crystallizes in point group C_c , possesses multiaxial ferroelectric properties, and exhibits a large spontaneous polarization of 3.26 µC cm⁻² in the thin film. A proof-of-concept device based on this material shows potential for next-generation in-memory computing, nanoelectronics, and ultra-high-density memories.

Abstract

2D ferroelectric materials with out-of-plane polarization are crucial for future nanoscale logic devices due to the increasing demand for energy-efficient architectures in artificial intelligence. However, only a few 2D out-of-plane ferroelectrics are confirmed experimentally. As an important branch of ferroelectrics, organic–inorganic hybrid perovskite ferroelectrics show flexible structures, making them eligible for constructing multifunctional materials. Here, a 2D organic–inorganic hybrid perovskite ferroelectric (6-BHA)₂CdBr₄ (6-BHA is 6-bromohexylamine) is designed, which crystallizes in polar point group C_c . It experiences the reversal phase transition at 317.8 K and possesses multiaxial ferroelectric properties. More interestingly, it exhibits a large spontaneous polarization value of 3.26 µC cm⁻² in out-of-plane direction of the film compared with typical 2D ferroelectrics. Moreover, an inverter based on (6-BHA)₂CdBr₄ is fabricated, which serves as a proof of concept for the feasibility for logic-in-memory devices. This work not only enriches the family of molecular ferroelectrics but also shows the potential to create the next generation of in-memory computing devices, nanoelectronics devices, and ultra-high-density memories.

Abstract

Recently, graphene has drawn considerable attention in the field of electronics, owing to its favorable conductivity and high carrier mobility. Crucial to the industrialization of graphene is its high-quality microfabrication via chemical vapor deposition. However, many problems remain in its preparation, such as the not fully understood cracking mechanism of the carbon source, the mechanism of its substrate oxidation, and insufficient defect repair theory. To help close this capability gap, this study leverages density functional theory to explore the role of O in graphene growth. The effects of Cu substrate oxidation on carbon source cracking, nucleation barriers, crystal nucleus growth, and defect repairs are discussed. O_Cu was found to reduce energy change during dehydrogenation, rendering the process easier. Moreover, the adsorbed O in graphene or its Cu substrate can promote defect repair and edge growth.

The bi-directional growth enables the simultaneous control of in-plane and out-of-plane orbital occupation. This paves the way for unprecedented thin films with an additional degree of freedom compared to thin films grown on conventional substrates, thereby enabling anisotropic transport properties such as ferromagnetism and superconductivity.

Abstract

The pursuit of breakthroughs in thin film technology drives the exploration of novel growth strategies for quantum materials that surpass conventional limitations. Departing from the prevailing unidirectional growth approach, the methodology allows for atomic precision in both the in-plane and out-of-plane directions. To demonstrate the capabilities of this transformative technique, a bi-directionally grown superlattice comprised of alternating LaMnO₃ and SrMnO₃ layers is engineered, enabling the emergence of interfacial ferromagnetism. By adopting this superlattice as a model system, the vast potential of the approach is highlighted through comprehensive analysis and characterization. Furthermore, the application of the method is extended to the growth of superconducting La_1.84Sr_0.16CuO₄ thin films on various offcut substrates. Remarkably, these substrates induce an anisotropic critical current originating from two distinct mechanisms.

npj 2D Materials and Applications, Published online: 08 February 2024; doi:10.1038/s41699-024-00443-2

Van der Waals enabled formation and integration of ultrathin high-κ dielectrics on 2D semiconductors

Twisted bilayer transitionmetal dichalcogenides provide a unique platform for investigating intriguing fundamental physical properties. A tilted SiO₂/Si substrate is introduced into chemical vapordeposition to synthesize twisted bilayer WS₂ with a wide twist angle range from 0° to 120°. The strong correlation between the SHG response and the twist angle suggests atunable SHG performance with twist angles.

Abstract

The introduction of rotational freedom by twist angles in twisted bilayer (TB) transition metal dichalcogenides (TMDCs) can tailor the inherent properties of the TMDCs, which provides a promising platform to investigate the exotic physical properties. However, direct synthesis of high-quality TB-TMDCs with full twist angles is significantly challenging due to the substantial energy barriers during crystal growth. Here, a modified chemical vapor deposition strategy is proposed to synthesize TB-WS₂ with a wide twist angle range from 0° to 120°. Utilizing a tilted SiO₂/Si substrate, a gas flow disturbance is generated in the furnace tube to create a heterogeneous concentration gradient of the metal precursor, which provides an extra driving force for the growth of TB-WS₂. The Raman and photoluminescence results confirm a weak interlayer coupling of the TB-WS₂. High-quality periodic Moiré patterns are observed in the scanning transmission electron microscopy images. Moreover, owing to the strong correlation between the nonlinear optical response and the twisted crystal structure, tunable second harmonic generation behaviors are realized in the TB-WS₂. This approach opens up a new avenue for the direct growth of high-crystalline-quality and pristine TB-TMDCs and their potential applications in nonlinear optical devices.

The unipolar barrier detector allows for the flow of minority carrier current while effectively inhibiting the flow of majority carriers. The existence of a wide-bandgap barrier plays a crucial role in preventing Shockley–Reed–Hall current within the depletion region of photodiodes. Therefore, the barrier structure allows the passage of photocurrent, while effectively blocking dark current.

Abstract

The barrier structure is designed to enhance the operating temperature of the infrared detector, thereby improving the efficiency of collecting photogenerated carriers and reducing dark current generation, without suppressing the photocurrent. However, the development of barrier detectors using conventional materials is limited due to the strict requirements for lattice and band matching. In this study, a high-performance unipolar barrier detector is designed utilizing a black arsenic phosphorus/molybdenum disulfide/black phosphorus van der Waals heterojunction. The device exhibits a broad response bandwidth ranging from visible light to mid-wave infrared (520 nm to 4.6 µm), with a blackbody detectivity of 2.7 × 10¹⁰ cmHz^−1/2 W⁻¹ in the mid-wave infrared range at room temperature. Moreover, the optical absorption anisotropy of black arsenic phosphorus enables polarization resolution detection, achieving a polarization extinction ratio of 35.5 at 4.6 µm. Mid-wave infrared imaging of the device is successfully demonstrated at room temperature, highlighting the significant potential of barrier devices based on van der Waals heterojunctions in mid-wave infrared detection.

Epitaxial ferroelectric Hf_0.5Zr_0.5O₂ (HZO) film has been deposited using Atomic Layer Deposition onto single crystalline YSZ in low temperature (280 °C). Optical and structural characteristics of epitaxial HZO film has been performed by Second Harmonic Generation and Scanning Transmission Microscope. Additionally, epitaxial HZO film has been obtained on CMOS compatible YSZ buffered Si substrate, with distinct ferroelectric switching currents.

Abstract

The groundbreaking discovery of unconventional ferroelectricity in HfO₂ opens exciting prospects for next-generation memory devices. However, the practical implementation, particularly its epitaxial stabilization and a clearer understanding of its intrinsic ferroelectricity has been a significant challenge. The study arouses the potential importance of atomic layer deposition (ALD) for mass production in modern industries, demonstrating its proficiency in achieving epitaxial growth of ferroelectric Hf_0.5Zr_0.5O₂ (HZO) thin films on Yttria-stabilized zirconia (YSZ) substrates. Moreover, with distinct ferroelectric switching currents, the work reveals the ferroelectric characteristics of epitaxial HZO thin films deposited through ALD on YSZ-buffered Si substrates, which aligns well with CMOS technology. Overall, the results pave the way for a scalable synthesis system for ferroelectric HfO₂-based materials, hinting at a bright future for low-temperature epitaxial nanoelectronics.

This study reports a novel approach to eliminate the magnetic dead layer in atomically thin oxides, by using the epitaxial interface that reconciles both strong exchange interaction and large uniaxial magnetic anisotropy. A largely enhanced saturation magnetization (2 μ_B Mn⁻¹) and Curie temperature (80 K) are observed for the single manganite monolayer when interfacing with 5d oxides.

Abstract

Discoveries of ferromagnetic materials with ultrathin thickness are of great importance for both fundamental science and technological applications. Transition metal oxides (TMOs) provide promising candidates in the context of next-generation spintronics, despite the severe decay of ferromagnetism as the thickness reduces to the nanometer regime. Here, an efficient strategy to eliminate the magnetic dead layer in atomically thin oxides is presented, by using the epitaxial interface of 3d and 5d oxide monolayers that reconciles both strong exchange interaction and large uniaxial magnetic anisotropy. Combining multiple experimental methods, a ferromagnetic transition in an ultrathin oxide heterostructure comprised of only one La_0.2Sr_0.8MnO₃ monolayer sandwiched by SrIrO₃ monolayer (total thickness of three unit-cells) is unambiguously demonstrated. Remarkably, a largely enhanced saturation magnetization (2 µ_B Mn⁻¹) and Curie temperature (80 K) are observed for the single manganite monolayer, as compared to previously reported ferromagnetic monolayer oxides. The results demonstrate a general strategy for creating robust ferromagnetism in ultrathin TMOs, potentially enabling novel oxide spin-orbitronic devices.

Nature Electronics, Published online: 09 February 2024; doi:10.1038/s41928-024-01117-z

Tapes whose adhesive force is controlled by ultraviolet illumination can be used to cleanly transfer large-area graphene, molybdenum disulfide and other two-dimensional materials with a low thermal budget and using no organic solvents.

Nature Electronics, Published online: 09 February 2024; doi:10.1038/s41928-024-01121-3

Large-area two-dimensional materials can be transferred at low temperatures and without solvents using conformable tapes whose adhesive force varies with ultraviolet illumination, allowing transfer to various planar and non-planar substrates.

2D materials hold great promise in strain engineering. By introducing out-of-plane deformations, localized and non-uniform strain can be induced, leading to novel phenomena and unique properties. This review systematically outlines the recent progress in the preparation, properties, and applications of locally strained 2D materials, aiming to broaden the scope of the 2D family for diverse applications.

Abstract

2D materials are promising for strain engineering due to their atomic thickness and exceptional mechanical properties. In particular, non-uniform and localized strain can be induced in 2D materials by generating out-of-plane deformations, resulting in novel phenomena and properties, as witnessed in recent years. Therefore, the locally strained 2D materials are of great value for both fundamental studies and practical applications. This review discusses techniques for introducing local strains to 2D materials, and their feasibility, advantages, and challenges. Then, the unique effects and properties that arise from local strain are explored. The representative applications based on locally strained 2D materials are illustrated, including memristor, single photon emitter, and photodetector. Finally, concluding remarks on the challenges and opportunities in the emerging field of locally strained 2D materials are provided.

The strong coupling of topological surface acoustic wave resonators with a large Rabi splitting and a high-quality factor operating at gigahertz frequencies based on a single-crystal lithium niobate acoustic-electric integrated system is realized. The coupling resonators are prepared by etching nanoscale grooves on the surface of lithium niobate. Then dense wavelength division multiplexers based on multiresonator coupling are also demonstrated.

Abstract

Coherent phonon transfer via high-quality factor (Q) mechanical resonator strong coupling has garnered significant interest. Yet, the practical applications of these strongly coupled resonator devices are largely constrained by their vulnerability to fabrication defects. In this study, topological strong coupling of gigahertz frequency surface acoustic wave (SAW) resonators with lithium niobate is achieved. The nanoscale grooves are etched onto the lithium niobate surface to establish robust SAW topological interface states (TISs). By constructing phononic crystal (PnC) heterostructures, a strong coupling of two SAW TISs, achieving a maximum Rabi splitting of 22 MHz and frequency quality factor product fQ _m of ≈1.2 × 10¹³ Hz, is realized. This coupling can be tuned by adjusting geometric parameters and a distinct spectral anticrossing is experimentally observed. Furthermore, a dense wavelength division multiplexing device based on the coupling of multiple TISs is demonstrated. These findings open new avenues for the development of practical topological acoustic devices for on-chip sensing, filtering, phonon entanglement, and beyond.

Reversible optical control of the ferroelectric polarization is accomplished in epitaxial PbZr _x Ti_1−x O₃ (PZT) thin films using UV light. For single-domain films, a transient modulation of the polarization magnitude occurs, mediated by the separation of photoexcited charge carriers in the Schottky interface. In PZT films at the morphotropic phase boundary, room-temperature remanent optical poling is realized that can be reversed via thermal annealing.

Abstract

Light is an effective tool to probe the polarization and domain distribution in ferroelectric materials passively, that is, non-invasively, for example, via optical second harmonic generation (SHG). With the emergence of oxide electronics, there is now a strong demand to expand the role of light toward active control of the polarization. In this work, optical control of the ferroelectric polarization is demonstrated in prototypical epitaxial PbZr _x Ti_1−x O₃ (PZT)-based heterostructures. This is accomplished in three steps, using above-bandgap UV light, while tracking the response of the polarization with optical SHG. First, it is found that UV-light exposure induces a transient enhancement or suppression of the ferroelectric polarization in films with an upward- or downward-oriented polarization, respectively. This behavior is attributed to a modified charge screening driven by the separation of photoexcited charge carriers at the Schottky interface of the ferroelectric thin film. Second, by taking advantage of this optical handle on electrostatics, remanent optical poling from a pristine multi-domain into a single-domain configuration is accomplished. Third, via thermal annealing or engineered electrostatic boundary conditions, a complete reversibility of the optical poling is further achieved. Hence, this work paves the way for the all-optical control of the spontaneous polarization in ferroelectric thin films.

InP-InAsP quantum well nanomembranes are demonstrated for enhancing and manipulating the second-harmonic generation. Up to 100 times enhancement of SHG is achieved in the short-wave infrared range. The enhanced SHG peak wavelengths can also be tuned by changing the QW composition.

Abstract

Second-harmonic generation (SHG) offers a convenient approach for infrared-to-visible light conversion in tunable nanoscale light sources and optical communication. Semiconductor nanostructures offer rich possibilities to tailor their nonlinear optical properties. In this study, strong second-harmonic generation in InP nanomembranes with InAsP quantum well (QW) is demonstrated. Compared with bulk InP, up to 100 times enhancement of SHG is achieved in the short-wave infrared range. This enhancement is shown to be predominantly induced by the resonance-enhanced absorption and quantum confinement of fundamental wavelengths in the InAsP QW. The thin nanomembrane structure will also provide nanocavity enhancement for second-harmonic wavelengths. The enhanced SHG peak wavelengths can also be tuned by changing the QW composition. These findings provide an effective strategy for enhancing and manipulating the second-harmonic generation in semiconductor quantum-confined nanostructures for on-chip all-optical applications.

Author(s): Mengmeng Wu, Xiao Liu, Renfei Wang, Yoon Jang Chung, Adbhut Gupta, Kirk W. Baldwin, Loren Pfeiffer, Xi Lin, and Yang Liu

Deforming the quantum phase with a nondestructive current flowing through the sample shows that the system becomes more incompressible when hosting a current.

[Phys. Rev. Lett. 132, 076501] Published Mon Feb 12, 2024

Nature Materials, Published online: 12 February 2024; doi:10.1038/s41563-024-01803-5

Optically detected magnetic resonance (ODMR) is an efficient mechanism for quantum sensors and has been discovered in a few systems, but all have technological limitations. Here the authors report room temperature ODMR in single defects in GaN, promising for integrated quantum sensing applications.

A regioselective and stepwise cyclodehydrogenation procedure has led to the synthesis of a negatively curved aza-nanographene containing two octagons, as confirmed by X-ray crystallography. The electron-rich nature and curved π-surface of the saddle-shaped structure lead to intriguing photophysical and electrochemical properties as well as tight association with fullerenes (K _a=9.5×10³ M⁻¹ with C₆₀ and K _a=3.7×10⁴ M⁻¹ with C₇₀).

Abstract

A negatively curved aza-nanographene (NG) containing two octagons was synthesized by a regioselective and stepwise cyclodehydrogenation procedure, in which a double aza[7]helicene was simultaneously formed as an intermediate. Their saddle-shaped structures with negative curvature were unambiguously confirmed by X-ray crystallography, thereby enabling the exploration of the structure–property relationship by photophysical, electrochemical and conformational studies. Moreover, the assembly of the octagon-embedded aza-NG with fullerenes was probed by fluorescence spectral titration, with record-high binding constants (K _a=9.5×10³ M⁻¹ with C₆₀, K _a=3.7×10⁴ M⁻¹ with C₇₀) found among reported negatively curved polycyclic aromatic compounds. The tight association of aza-NG with C₆₀ was further elucidated by X-ray diffraction analysis of their co-crystal, which showed the formation of a 1 : 1 complex with substantial concave-convex interactions.