Flexoelectric manipulation of ferroelectric polarization in self-strained tellurium

Yan Yan, Xiongyi Liang, Liqiang Wang, Yuxuan Zhang, Jiaming Zhou, Weijun Wang, Zhibo Zhang, Yu Zhou, Irum Firdous, Zhengxun Lai, Wei Wang, Pengshan Xie,  Yuecheng Xiong, Walid A. Daoud, Zhiyong Fan, Dong- Myeong Shin, Yong Yang, Yang Lu, Xiao Cheng Zeng, You Meng, Johnny C. Ho 

Beyond conventional ferroelectric compounds, the realization of single-element ferroelectricity expands the scope of ferroelectric materials and diversifies polarization mechanisms. However, strategies for manipulating ferroelectric dipoles in elemental ferroelectrics remain underexplored, limiting their broader applications. Here, we introduce a universal flexoelectric manipulation strategy to tune the ferroelectric and piezoelectric polarization of one-dimensional self-strained tellurium (Te) ferroelectrics. A substantial flexoelectric field of 9.55 microcoulombs per square centimeter was observed in self-strained Te, inducing a polarization rotation of 18°, comparable to the typical 15° rotation in ferroelectric PbTiO₃ compounds. This substantial polarization rotation enhances ferroelectric coercivity by 165% and piezoelectric responses by 75% compared to unstrained Te. Moreover, the flexoelectric manipulation of ferroelectric polarization demonstrated improved energy harvesting performance at the device level, surpassing most existing counterparts. Our findings highlight the crucial role of flexoelectricity-ferroelectricity coupling in developing high-performance single-element electromechanical devices and ferroelectronics.

An inorganic-blended p-type semiconductor with robust electrical and mechanical properties

You Meng, Weijun Wang, Rong Fan, Zhengxun Lai, Wei Wang, Dengji Li, Xiaocui Li, Quan Quan, Pengshan Xie, Dong Chen, He Shao, Bowen Li, Zenghui Wu, Zhe Yang, SenPo Yip, Chun-Yuen Wong, Yang Lu & Johnny C. Ho

Inorganic semiconductors typically have limited p-type behavior due to the scarcity of holes and the localized valence band maximum, hindering the progress of complementary devices and circuits. In this work, we propose an inorganic blending strategy to activate the hole-transporting character in an inorganic semiconductor compound, namely tellurium-selenium-oxygen (TeSeO). By rationally combining intrinsic p-type semimetal, semiconductor, and wide-bandgap semiconductor into a single compound, the TeSeO system displays tunable bandgaps ranging from 0.7 to 2.2 eV. Wafer-scale ultrathin TeSeO films, which can be deposited at room temperature, display high hole field-effect mobility of 48.5 cm²/(Vs) and robust hole transport properties, facilitated by Te-Te (Se) portions and O-Te-O portions, respectively. The nanosphere lithography process is employed to create nanopatterned honeycomb TeSeO broadband photodetectors, demonstrating a high responsibility of 603 A/W, an ultrafast response of 5 μs, and superior mechanical flexibility. The p-type TeSeO system is highly adaptable, scalable, and reliable, which can address emerging technological needs that current semiconductor solutions may not fulfill.

Fused computing and storage in a 2D transistor

Du Xiang, Tao Liu & Wei Chen

The development of devices that exploit the intrinsic properties of two-dimensional materials can provide opportunities for in-memory computing and area-efficient integrated circuits based on Moore’s law.

Improving carrier mobility in two-dimensional semiconductors with rippled materials

Hong Kuan Ng, Du Xiang, Ady Suwardi, Guangwei Hu, Ke Yang, Yunshan Zhao, Tao Liu, Zhonghan Cao, Huajun Liu, Shisheng Li, Jing Cao, Qiang Zhu, Zhaogang Dong, Chee Kiang Ivan Tan, Dongzhi Chi, Cheng-Wei Qiu, Kedar Hippalgaonkar, Goki Eda, Ming Yang & Jing Wu 

Two-dimensional (2D) semiconductors could potentially replace silicon in future electronic devices. However, the low carrier mobility in 2D semiconductors at room temperature, caused by strong phonon scattering, remains a critical challenge. Here we show that lattice distortions can reduce electron–phonon scattering in 2D materials and thus improve the charge carrier mobility. We introduce lattice distortions into 2D molybdenum disulfide (MoS₂) using bulged substrates, which create ripples in the 2D material leading to a change in the dielectric constant and a suppressed phonon scattering. A two orders of magnitude enhancement in room-temperature mobility is observed in rippled MoS₂, reaching ∼900 cm² V⁻¹ s⁻¹, which exceeds the predicted phonon-limited mobility of flat MoS₂ of 200–410 cm² V⁻¹ s⁻¹. We show that our approach can be used to create high-performance room-temperature field-effect transistors and thermoelectric devices.

Spectroscopy signatures of electron correlations in a trilayer graphene/hBN moiré superlattice

Jixiang Yang, Guorui Chen, Tianyi Han, Qihang Zhang, Ya-Hui Zhang, Lili Jiang, Bosai Lyu, Hongyuan Li, Kenji Watanabe, Takashi Taniguchi, Zhiwen Shi, Todadri Senthil, Yuanbo Zhang, Feng Wang, Long Ju 

ABC-stacked trilayer graphene/hexagonal boron nitride moiré superlattice (TLG/hBN) has emerged as a playground for correlated electron physics. We report spectroscopy measurements of dual-gated TLG/hBN using Fourier transform infrared photocurrent spectroscopy. We observed a strong optical transition between moiré minibands that narrows continuously as a bandgap is opened by gating, indicating a reduction of the single-particle bandwidth. At half-filling of the valence flat band, a broad absorption peak emerges at ~18 milli–electron volts, indicating direct optical excitation across an emerging Mott gap. Similar photocurrent spectra are observed in two other correlated insulating states at quarter- and half-filling of the first conduction band. Our findings provide key parameters of the Hubbard model for the understanding of electron correlation in TLG/hBN.

Van der Waals heterostructures

A. K. Geim & I. V. Grigorieva 

Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as ‘van der Waals’) have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene’s springboard, van der Waals heterostructures should develop into a large field of their own.

Room temperature near unity spin polarization in 2D Van der Waals heterostructures

Danliang Zhang, Ying Liu, Mai He, Ao Zhang, Shula Chen, Qingjun Tong, Lanyu Huang, Zhiyuan Zhou, Weihao Zheng, Mingxing Chen, Kai Braun, Alfred J. Meixner, Xiao Wang & Anlian Pan

The generation and manipulation of spin polarization at room temperature are essential for 2D van der Waals (vdW) materials-based spin-photonic and spintronic applications. However, most of the high degree polarization is achieved at cryogenic temperatures, where the spin-valley polarization lifetime is increased. Here, we report on room temperature high-spin polarization in 2D layers by reducing its carrier lifetime via the construction of vdW heterostructures. A near unity degree of polarization is observed in PbI₂ layers with the formation of type-I and type-II band aligned vdW heterostructures with monolayer WS₂ and WSe₂. We demonstrate that the spin polarization is related to the carrier lifetime and can be manipulated by the layer thickness, temperature, and excitation wavelength. We further elucidate the carrier dynamics and measure the polarization lifetime in these heterostructures. Our work provides a promising approach to achieve room temperature high-spin polarizations, which contribute to spin-photonics applications.

Near-Unity Polarization of Valley-Dependent Second-Harmonic Generation in Stacked TMDC Layers and Heterostructures at Room Temperature

Danliang Zhang, Zhouxiaosong Zeng, Qingjun Tong, Ying Jiang, Shula Chen, Biyuan Zheng, Junyu Qu, Fang Li, Weihao Zheng, Feng Jiang, Hepeng Zhao, Lanyu Huang, Kai Braun, Alfred J Meixner, Xiao Wang, Anlian Pan

With unique valley-dependent optical and optoelectronic properties, 2D transition metal dichalcogenides (2D TMDCs) are promising materials for valleytronics. Second-harmonic generation (SHG) in 2D TMDCs monolayers has shown valley-dependent optical selection rules. However, SHG in monolayer TMDCs is generally weak; it is important to obtain materials with both strong SHG signals and a large degree of polarization. In the work, a variety of inversion-symmetry-breaking (3R-like phase) TMDCs (WSe₂, WS₂, MoS₂) atomic layers, spiral structures, and heterostructures are prepared, and their SHG polarization is studied. Through circular-polarization-resolved SHG experiments, it is demonstrated that the SHG intensity is enhanced in thicker samples by breaking inversion symmetry while maintaining the degree of polarization close to unity at room temperature. By studying TMDCs with different twist angles and the spiral structures, it is found that there is no significant effect of multilayer interlayer interaction on valley-dependent SHG. The realization of strong SHG with high degree of polarization may pave the way toward a new platform for nonlinear optical valleytronics devices based on 2D semiconductors.

Realization of fractional-layer transition metal dichalcogenides

Ya-Xin Zhao, Heng Jin, Zi-Yi Han, Xinlei Zhao, Ya-Ning Ren, Ruo-Han Zhang, Xiao-Feng Zhou, Wenhui Duan, Bing Huang, Yu Zhang & Lin He

Layered van der Waals transition metal dichalcogenides (TMDCs), generally composed of three atomic X-M-X planes in each layer (M = transition metal, X = chalcogen), provide versatile platforms for exploring diverse quantum phenomena. In each MX₂ layer, the M-X bonds are predominantly covalent in nature and, as a result, the cleavage of TMDC crystals normally occurs between the layers. Here we report the controllable realization of fractional-layer WTe₂ via an in-situ scanning tunneling microscopy (STM) tip manipulation technique. By applying STM tip pulses, hundreds of the topmost Te atoms are removed to form a nanoscale monolayer Te pit in the 1 T′-WTe₂, thus realizing a 2/3-layer WTe₂ film. Such a configuration undergoes a spontaneous atomic reconstruction, yielding a unidirectional charge density redistribution with the wavevector and geometry quite distinct from that of pristine 1 T′-WTe₂. Our results expand the conventional understanding of the TMDCs and are expected to stimulate further research on the structure and properties of fractional-layer TMDCs.

Unraveling the Role of Interfacial Interactions in Electrical Contacts of Atomically Thin Transition-Metal Dichalcogenides

Meiying Gong, Dabao Xie, Yiqian Tian, Zeqi Hua, Congmin Zhang, Meng Li, Dan Cao, Jing Zhou, Xiaoshuang Chen, Haibo Shu

Van der Waals (vdW) contact has been widely regarded as one of the most potential strategies for exploiting low-resistance metal–semiconductor junctions (MSJs) based on atomically thin transition-metal dichalcogenides (TMDs), but this method is still not efficient due to weak metal–TMD interfacial interactions. Therefore, an understanding of interfacial interactions between metals and TMDs is essential for achieving low-resistance contacts with weak Fermi level pinning (FLP). Herein, we report how the interfacial interactions between metals and TMDs affect the electrical contacts by considering more than 90 MSJs consisting of a semiconducting TMD channel and different types of metal electrodes, including bulk metals, MXenes, and metallic TMDs. We reveal that the vdW contact scheme cannot ensure the formation of low-resistance metal–TMD contacts. The interfacial coupling between metals and TMDs leads to a delicate competition between the FLP and carrier tunneling efficiency, which explains the broad experimental observations in which the weakly coupled van der Waals contacts usually show high contact resistance, while the strongly coupled metal–TMD contacts suffer from strong FLP. Benefiting from the low Schottky barrier and weak FLP, bulk Ag is a promising electrode for n-type MoS2 devices with a contact resistance of 83 Ω μm at a carrier concentration of 5.95 × 10¹³ cm^-2, and 1T′-phase MoS₂ and Sc₂NO₂ are identified as superior contact electrodes for p-type WSe₂ devices. This work offers a general rule to exploit high-performance MSJs and clarifies the key role of interfacial coupling in the electrical contacts of TMD-based devices.

Bright and stable anti-counterfeiting devices with independent stochastic processes covering multiple length scales

Junfang Zhang, Adam Creamer, Kai Xie, Jiaqing Tang, Luke Salter, Jonathan P. Wojciechowski & Molly M. Stevens

Physical unclonable functions (PUFs) are considered the most promising approach to address the global issue of counterfeiting. Current PUF devices are often based on a single stochastic process, which can be broken, especially since their practical encoding capacities can be significantly lower than the theoretical value. Here we present stochastic PUF devices with features across multiple length scales, which incorporate semiconducting polymer nanoparticles (SPNs) as fluorescent taggants. The SPNs exhibit high brightness, photostability and size tunability when compared to the current state-of-the-art taggants. As a result, they are easily detectable and highly resilient to UV radiation. By embedding SPNs in photoresists, we generate PUFs consisting of nanoscale (distribution of SPNs within microspots), microscale (fractal edges on microspots), and macroscale (random microspot array) designs. With the assistance of a deep-learning model, the resulting PUFs show both near-ideal performance and accessibility for general end users, offering a strategy for next-generation security devices.