Revealing Strong Flexoelectricity and Optoelectronic Coupling in 2D Ferroelectric CuInP2S6 Via Large Strain Gradient.

The interplay between flexoelectric and optoelectronic characteristics provides a paradigm for studying emerging phenomena in various 2D materials. However, an effective way to induce a large and tunable strain gradient in 2D devices remains to be exploited. Herein, we propose a strategy to induce large flexoelectric effect in 2D ferroelectric CuInP2S6 by constructing a 1D-2D mixed-dimensional heterostructure. The strong flexoelectric effect is induced by enormous strain gradient up to 4.2 × 106 m-1 resulting from the underlying ZnO nanowires, which is further confirmed by the asymmetric coercive field and the red-shift in the absorption edge. The induced flexoelectric polarization efficiently boosts the self-powered photodetection performance. In addition, the improved photoresponse has a good correlation with the induced strain gradient, showing a consistent size-dependent flexoelectric effect. The mechanism of flexoelectric and optoelectronic coupling is proposed based on the Landau-Ginzburg-Devonshire double-well model, supported by density functional theory (DFT) calculations. This work provides a brand-new method to induce a strong flexoelectric effect in 2D materials, which is not restricted to crystal symmetry and thus offers unprecedented opportunities for state-of-the-art 2D devices.

Polarization enhanced photoresponse of InSe via 2D ferroelectric CuCrP2S6.

Two-dimensional CuCrP2S6 possesses significant potential for low-power non-volatile devices owing to its multiferroic properties. Nonetheless, comprehensive investigations regarding the modulation of CuCrP2S6 polarization for enhancing semiconductor photodetection capabilities and its potential applications in ferroelectric non-volatile devices are still relatively scarce. In this study, we present a novel, non-volatile, tunable photodetector engineered through the integration of a ferroelectric heterostructure comprising CuCrP2S6 and InSe. Our findings reveal that distinct ferroelectric polarization states of CuCrP2S6 exert varying modulation effects on the InSe photodetection performance. Notably, optimized results give a responsivity of 1839 A W-1 and a detectivity of 1.9 × 1012 Jones at a 300 nm wavelength, featuring a substantial 20.7-fold difference in responsivity between the two polarization states. This investigation underscores the immense potential of CuCrP2S6 in the development of non-volatile, multi-state optoelectronic devices.

Recent Advances in Ferroelectric-Enhanced Low-Dimensional Optoelectronic Devices.

Ferroelectric (FE) materials, including BiFeO3 , P(VDF-TrFE), and CuInP2 S6 , are a type of dielectric material with a unique, spontaneous electric polarization that can be reversed by applying an external electric field. The combination of FE and low-dimensional materials produces synergies, sparking significant research interest in solar cells, photodetectors (PDs), nonvolatile memory, and so on. The fundamental aspects of FE materials, including the origin of FE polarization, extrinsic FE materials, and FE polarization quantification are first discussed. Next, the state-of-the-art of FE-based optoelectronic devices is focused. How FE materials affect the energy band of channel materials and how device structures influence PD performance are also summarized. Finally, the future directions of this rapidly growing field are discussed.

Ferroelectric Tuning of ZnO Ultraviolet Photodetectors.

The ferroelectric field effect transistor (Fe-FET) is considered to be one of the most important low-power and high-performance devices. It is promising to combine a ferroelectric field effect with a photodetector to improve the photodetection performance. This study proposes a strategy for ZnO ultraviolet (UV) photodetectors regulated by a ferroelectric gate. The ZnO nanowire (NW) UV photodetector was tuned by a 2D CuInP2S6 (CIPS) ferroelectric gate, which decreased the dark current and enhanced the responsivity and detectivity to 2.40 × 104 A/W and 7.17 × 1011 Jones, respectively. This strategy was also applied to a ZnO film UV photodetector that was tuned by a P(VDF-TrFE) ferroelectric gate. Lower power consumption and higher performance can be enabled by ferroelectric tuning of ZnO ultraviolet photodetectors, providing new inspiration for the fabrication of high-performance photodetectors.

Progress on two-dimensional binary oxide materials.

Two-dimensional van der Waals (2D vdW) materials have attracted much attention because of their unique electronic and optical properties. Since the successful isolation of graphene in 2004, many interesting 2D materials have emerged, including elemental olefins (silicene, germanene, etc.), transition metal chalcogenides, transition metal carbides (nitrides), hexagonal boron, etc. On the other hand, 2D binary oxide materials are an important group in the 2D family owing to their high structural diversity, low cost, high stability, and strong adjustability. This review systematically summarizes the research progress on 2D binary oxide materials. We discuss their composition and structure in terms of vdW and non-vdW categories in detail, followed by a discussion of their synthesis methods. In particular, we focus on strategies to tailor the properties of 2D oxides and their emerging applications in different fields. Finally, the challenges and future developments of 2D binary oxides are provided.

Chemical vapor deposition and temperature-dependent Raman characterization of two-dimensional vanadium ditelluride.

Recently, ultrathin two-dimensional (2D) metallic vanadium dichalcogenides have attracted widespread attention because of the charge density wave (CDW) phase transition and possible ferromagnetism. Herein, we report the synthesis and temperature-dependent Raman characterization of the 2D vanadium ditelluride (VTe2). The synthesis is done by atmospheric pressure chemical vapor deposition (APCVD) using vanadium chloride (VCl3) precursor on fluorphlogopite mica, sapphire, and h-BN substrates. A large area of the thin film with thickness ∼10 nm is grown on the hexagonal boron nitride (h-BN) substrate. Temperature-dependent Raman characterization of VTe2 is conducted from room temperature to 513 K. Remarkable changes of Raman modes at around 413 K are observed, indicating the structural phase transition.

Ultralow-Transition-Energy Organic Complex on Graphene for High-Performance Shortwave Infrared Photodetection.

Room-temperature, high-sensitivity, and broadband photodetection up to the shortwave infrared (SWIR) region is extremely significant for a wide variety of optoelectronic applications, including contamination identification, thermal imaging, night vision, agricultural inspection, and atmospheric remote sensing. Small-bandgap semiconductor-based SWIR photodetectors generally require deep cooling to suppress thermally generated charge carriers to achieve increased sensitivity. Meanwhile, the photogating effect can provide an alternative way to achieve superior photosensitivity without the need for cooling. The optical photogating effect originates from charge trapping of photoinduced carriers at defects or interfaces, resulting in an extremely high photogain (106 or higher). Here, a highly sensitive SWIR hybrid photodetector, fabricated by integrating an organic charge transfer complex on a graphene transistor, is reported. The organic charge transfer complex (tetrathiafulvalene-chloranil) has an exceptional low-energy intermolecular electronic transition down to 0.5 eV, with the aim of achieving efficient SWIR absorption for wavelengths greater than 2 µm. The photogating effect at the organic complex and graphene interface enables an extremely high photogain and a high detectivity of ≈1013 Jones, along with a response time of 8 ms, at room temperature for a wavelength of 2 µm.

In situ growth of a CaAl-NO3--layered double hydroxide film directly on an aluminum alloy for corrosion resistance.

In this study, a calcium-aluminum-layered double hydroxide (CaAl-LDH) thin film was grown on an AA6082 aluminum alloy, for the very first time, by using a facile in situ growth method in an effort to investigate the CaAl-LDH structural geometry and corresponding corrosion resistance properties. The structure and surface morphologies of the CaAl-LDH thin film were studied using a scanning electron microscope (SEM) equipped with an EDS detector, a transmission electron microscope (TEM), an X-ray diffractometer (XRD), and a Fourier transform infrared spectrometer (FT-IR), while the electron impendence spectra (EIS) and potentiodynamic curves were recorded to understand the LDH anticorrosion behavior. The findings demonstrated that thin, well-developed CaAl-LDH coatings with different surface morphologies can be prepared with eminent corrosion resistance properties. Specifically, the CaAl-LDH thin film synthesized at 140 °C-24 h synthetic conditions showed a large impedance modulus of 7.3 Ω cm2 at 0.01 Hz (|Z|f = 0.01 Hz), along with a low corrosion current density (Icorr) of 0.0007 µA cm-2, while a vertically orientated rod like structure with a uniform surface morphology was observed.

Organic charge transfer complexes on graphene with ultrahigh near infrared photogain.

Photodetectors have widespread applications in fields including telecommunications, thermal imaging and bio-medical imaging. The photogating effect, arising from charge trapping at defects and/or interfaces, can have extremely high photoelectric gain which can be a benefit to high-sensitivity room temperature photodetection. Here, we introduce thin layered organic charge transfer complexes (CPXs) integrated on graphene transistors for the development of hybrid phototransistors with ultra-high photoresponsivity of ∼106 A W-1 in the near infrared (NIR) region at room temperature. Our study has demonstrated a graphene-organic CPX with a broadband photoresponse ranging from the visible to the NIR region. The high photoelectric gain was from the photogating effect at the graphene/CPX interface. In addition, the photoresponse properties of the graphene-organic CPX can be regulated by electrical gating of graphene.

Potential 2D Materials with Phase Transitions: Structure, Synthesis, and Device Applications.

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-transition materials with CDWs and magnetic and dipole ordering, including transition metal dichalcogenides, transition metal halides, metal thio/selenophosphates, chromium silicon/germanium tellurides, and more, are introduced. The structures and experimental phase-transition properties are summarized for the bulk materials and some of the obtained monolayers. In addition, recent experimental progress on the synthesis and measurement of monolayers, such as 1T-TaS2 , CrI3 , and Cr2 Ge2 Te6 , is reviewed.