bandgap can overcome the short channel effect, and then can scale down transistors to sub-10 nm [13,14] and even sub-5 nm [10,[15][16][17][18] to extend the Moore's law. Over last decades, typical 2D semiconductors, such as MoS 2 and WSe 2 , have been intensively studied. [19][20][21][22] On the other hand, 2D materials with versatile phase-transition properties offer new opportunities in both fundamental research and future device applications. In the last few years, a lot of exciting experiments have been conducted on this subfamily of 2D materials in the monolayer limit, such as charge density wave (CDW) 1T-TaS 2 , [23][24][25][26] superconducting (SC) 2H-NbSe 2 , [27][28][29] ferromagnetic (FM) CrI 3 , [12,30,31] and Cr 2 Ge 2 Te 6 . [11,32] CDW is a phenomenon of forming a standing-wave-like electron density usually accompanied with a lattice distortion and a bandgap opening due to electronphonon interaction. [33] SC is a phenomenon of zero electrical resistance and expulsion of magnetic flux fields also originated from electron-phonon interaction. [34] Magnetism and electric polarization stem from spontaneous spin/dipole ordering, respectively. [35,36] Parallel and antiparallel spins constitute long-range ferromagnetic (FM) and antiferromagnetic (AFM) ordering, respectively. Similarly, parallel and antiparallel dipoles form ferroelectric (FE) and antiferroelectric (AFE) ordering, respectively. Van der Waals layered materials can show a lot of novel properties, especially in the atomic thickness limit. This includes the follow ing terms: (1) The odd/even layer effect may emerge due to weak interlayer coupling. For example, FM or FE ordering may appear in the odd layers, while AFM or AFE ordering may appear in the even layers; (2) Due to confinement effect in the out-of-plane direction, new ordering structures may appear;(3) The properties can be easily tuned by strain, external fields, interface interaction, and so on. Toward fundamental property measurement and device applications, the challenge remains, such as predicting/discovering new 2D phase-transition materials, controlling the chemical synthesis, overcoming the poor air stability, and so on. Here, a literature survey has been carried out to search possible 2D phase-transition materials. Recent progress on the chemical synthesis and property investigation of some obtained monolayers has been reviewed, including CDW 1T-TaS 2 monolayers, FM CrI 3 mono layers, and so on.Layered materials with phase transitions, such as charge density wave (CDW) and magnetic and dipole ordering, have potential to be exfoliated into monolayers and few-layers and then become a large and important subfamily of two-dimensional (2D) materials. Benefitting from enriched physical properties from the collective interactions, long-range ordering, and related phase transitions, as well as the atomic thickness yet having nondangling bonds on the surface, 2D phase-transition materials have vast potential for use in new-concept and functional devices. Here, potential 2D phase-transit...
A sol-gel-based ultrathin TiO2 lamination coating was adapted to a hydrothermally grown ZnO nanowire array to realize an all-oxide ultra-sensitive p-n photodiode. The core-shell heterojunction—the key component of the device—is composed of a 5–10 nm thick p-type Cr-doped TiO2 nanoshell and n-type single-crystalline ZnO nanowires (50 nm radius). Owing to the enhanced light scattering and carrier separation in the core-shell architecture, this device exhibits the highest performance among the ZnO nanowire-based photodetectors. At a moderate reverse bias of −5 V and under ultraviolet light illumination at 104 μW, it shows a switch current ratio of 140 and a responsivity as large as 250 A/W, while it shows nearly no response to the infrared and visible light.
Two-dimensional layered 1T-TaS 2 exhibits rich charge-density-wave (CDW) states with distinct electronic structures and physical properties, leading to broad potential applications, such as phase-transition memories, electrical oscillators and photodetectors. Besides the various CDW ground states at different temperatures, multiple intermediate phases in 1T-TaS 2 have been observed by applying optical and electrical stimulations. Here, we investigated the electric-fielddriven multistate CDW phase transition by Raman spectroscopy and voltage oscillations in 1T-TaS 2 . Strong correlation was observed between electrical conductivity and intensity of fold-back acoustic and optical phonon modes in 1T-TaS 2 . This indicates that the multistate transitions arise from serial transitions, from the nearly commensurate (NC) CDW phase to out-of-equilibrium intermediate states, and finally to the incommensurate (IC) CDW phase. The dynamics of phase transition under an electric field was investigated. As the electrical field increased, the dwell time of different CDW states changed. At lower temperatures, the multistate oscillations disappeared because of higher-energy barriers between the intermediate phases and/or lower thermal excitation energies at lower temperatures.
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