High-mobility semiconducting ultrathin films form the basis of modern electronics, and may lead to the scalable fabrication of highly performing devices. Because the ultrathin limit cannot be reached for traditional semiconductors, identifying new two-dimensional materials with both high carrier mobility and a large electronic bandgap is a pivotal goal of fundamental research. However, air-stable ultrathin semiconducting materials with superior performances remain elusive at present. Here, we report ultrathin films of non-encapsulated layered BiOSe, grown by chemical vapour deposition, which demonstrate excellent air stability and high-mobility semiconducting behaviour. We observe bandgap values of ∼0.8 eV, which are strongly dependent on the film thickness due to quantum-confinement effects. An ultrahigh Hall mobility value of >20,000 cm V s is measured in as-grown BiOSe nanoflakes at low temperatures. This value is comparable to what is observed in graphene grown by chemical vapour deposition and at the LaAlO-SrTiO interface, making the detection of Shubnikov-de Haas quantum oscillations possible. Top-gated field-effect transistors based on BiOSe crystals down to the bilayer limit exhibit high Hall mobility values (up to 450 cm V s), large current on/off ratios (>10) and near-ideal subthreshold swing values (∼65 mV dec) at room temperature. Our results make BiOSe a promising candidate for future high-speed and low-power electronic applications.

Two-dimensional (2D) layered hybrid perovskites of (RNH)PbX (R is an alkyl and X is a halide) have been recently synthesized and exhibited rich optical properties including fluorescence and exciton effects. However, few studies on transport and optoelectronic measurements of individual 2D perovskite crystals have been reported, presumably owing to the instability issue during electronic device fabrications. Here we report the first photodetector based on individual 2D (CHNH)PbBr perovskite crystals, built with the protection and top contact of graphene film. Both a high responsivity (∼2100 A/W) and extremely low dark current (∼10 A) are achieved with a design of interdigital graphene electrodes. Our study paves the way to build high-performance optoelectronic devices based on the emerging 2D single-crystal perovskite materials.

Non-neutral layered crystals, another group of two-dimensional (2D) materials that lack a well-defined van der Waals (vdWs) gap, are those that form strong chemical bonds in-plane but display weak out-of-plane electrostatic interactions, exhibiting intriguing properties for the bulk counterpart. However, investigation of the properties of their atomically thin counterpart are very rare presumably due to the absence of efficient ways to achieve large-area high-quality 2D crystals. Here, high-mobility atomically thin BiOSe, a typical non-neutral layered crystal without a standard vdWs gap, was synthesized via a facial chemical vapor deposition (CVD) method, showing excellent controllability for thickness, domain size, nucleation site, and crystal-phase evolution. Atomically thin, large single crystals of BiOSe with lateral size up to ∼200 μm and thickness down to a bilayer were obtained. Moreover, optical and electrical properties of the CVD-grown 2D BiOSe crystals were investigated, displaying a size-tunable band gap upon thinning and an ultrahigh Hall mobility of >20000 cm V s at 2 K. Our results on the high-mobility 2D BiOSe semiconductor may activate the synthesis and related fundamental research of other non-neutral 2D materials.

We present the controlled synthesis of high-quality two-dimensional (2D) GaSe crystals on flexible transparent mica substrates via a facile van der Waals epitaxy method. Single- and few-layer GaSe nanoplates with the lateral size of up to tens of micrometers were produced. The orientation and nucleation sites of GaSe nanoplates were well-controlled. The 2D GaSe crystal-based photodetectors were demonstrated on both mechanically rigid SiO2/Si and flexible mica substrates. Efficient photoresponse was observed in 2D GaSe crystal devices on transparent flexible mica substrates, regardless of repeated bending with different radii. The controlled growth of 2D GaSe crystals with efficient photoresponsivity opens up opportunities for both fundamental aspects and new applications in photodetectors.

Infrared light detection and sensing is deeply embedded in modern technology and human society and its development has always been benefitting from the discovery of various photoelectric materials. The rise of two-dimensional materials, thanks to their distinct electronic structures, extreme dimensional confinement and strong light–matter interactions, provides a material platform for next-generation infrared photodetection. Ideal infrared detectors should have fast respond, high sensitivity and air-stability, which are rare to meet at the same time in one two-dimensional material. Herein we demonstrate an infrared photodetector based on two-dimensional Bi2O2Se crystal, whose main characteristics are outstanding in the whole two-dimensional family: high sensitivity of 65 AW−1 at 1200 nm and ultrafast photoresponse of ~1 ps at room temperature, implying an intrinsic material-limited bandwidth up to 500 GHz. Such great performance is attributed to the suitable electronic bandgap and high carrier mobility of two-dimensional oxyselenide.

The field effect transistors (FETs) based on thin layer MoS 2 often have large hysteresis and unstable threshold voltage in their transfer curves, mainly due to the charge trapping at the oxidesemiconductor interface. In this paper, the charge trapping and de-trapping processes at the SiO 2 -MoS 2 interface are studied. The trapping charge density and time constant at different temperatures are extracted. Making use of the trapped charges, the threshold voltage of the MoS 2 based metaloxide-semiconductor FETs is adjusted from 4 V to À45 V. Furthermore, the impact of the trapped charges on the carrier transport is evaluated. The trapped charges are suggested to give rise to the unscreened Coulomb scattering and/or the variable range hopping in the carrier transport of the MoS 2 sheet. 6-12 Besides in low power consumption electronics, devices based on MoS 2 , WS 2 , etc., also show potential for application in chemical sensing, light emitting, photo detecting, photovoltaics, and integrated flexible circuits. [13][14][15][16][17][18][19][20][21][22][23][24] Among these devices, the structure of metal-oxide-semiconductor (MOS) is mostly used. Charge trapping at the interface of the oxide and the semiconductor is common, 25,26 and MOS devices based on MoS 2 are not exceptional. 27 On one hand, charge trapping causes hysteresis and relaxation, hindering the stability of operational circuits, sensors, and photo detectors;28-30 on the other hand, charge trapping plays a key role in MoS 2 based multifunctional photoresponsive devices and flexible transparent multibit memory devices; [31][32][33][34] what is more, charge trapping offers a potential techniques for threshold voltage adjustment. 30 To better develop devices based on MoS 2 , understanding the charge trapping at the interface between the oxide and MoS 2 is necessary. SiO 2 is one of the most commonly used dielectric in MoS 2 based devices. Although trapped charges in the SiO 2 /Si substrate has been suggested to be the dominant source of potential fluctuations in MoS 2 field effect transistors (FETs), 35 there is no study on the charge trapping process for the SiO 2 in the MoS 2 based devices, and the effects of such trapped charges on the carrier transport is not clear so far. Here, the charge trapping and de-trapping processes at the MoS 2 -SiO 2 interface are studied, and their impact on the carrier transport is evaluated.MoS 2 devices were fabricated using cleaved MoS 2 sheets on SiO 2 /Si substrates (Figure 1(a)). To ensure the cleanness at the SiO 2 /MoS 2 interface, the process was done with extreme care. 36 The source and drain electrodes were patterned using electron beam lithography, followed by the deposition of Ti/ Au 5 nm/80 nm using electron beam evaporation. The heavily doped Si substrate was used as the back gate electrode. The thickness of the MoS 2 sheet is 3 nm, characterized by atomic force microscope (AFM), corresponding to three layers, as confirmed by the Raman spectrum. [36][37][38] The homogeneous contrast in the optical microscope and...

Graphene with ultra-high carrier mobility and ultra-short photoresponse time has shown remarkable potential in ultrafast photodetection. However, the broad and weak optical absorption (∼2.3%) of monolayer graphene hinders its practical application in photodetectors with high responsivity and selectivity. Here we demonstrate that twisted bilayer graphene, a stack of two graphene monolayers with an interlayer twist angle, exhibits a strong light–matter interaction and selectively enhanced photocurrent generation. Such enhancement is attributed to the emergence of unique twist-angle-dependent van Hove singularities, which are directly revealed by spatially resolved angle-resolved photoemission spectroscopy. When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons, the photocurrent generated can be significantly enhanced (up to ∼80 times with the integration of plasmonic structures in our devices). These results provide valuable insight for designing graphene photodetectors with enhanced sensitivity for variable wavelength.

The atomically thin 2D nature of suspended graphene membranes holds promising in numerous technological applications. In particular, the outstanding transparency to electron beam endows graphene membranes great potential as a candidate for specimen support of transmission electron microscopy (TEM). However, major hurdles remain to be addressed to acquire an ultraclean, high-intactness, and defect-free suspended graphene membrane. Here, a polymer-free clean transfer of sub-centimeter-sized graphene single crystals onto TEM grids to fabricate large-area and high-quality suspended graphene membranes has been achieved. Through the control of interfacial force during the transfer, the intactness of large-area graphene membranes can be as high as 95%, prominently larger than reported values in previous works. Graphene liquid cells are readily prepared by π-π stacking two clean single-crystal graphene TEM grids, in which atomic-scale resolution imaging and temporal evolution of colloid Au nanoparticles are recorded. This facile and scalable production of clean and high-quality suspended graphene membrane is promising toward their wide applications for electron and optical microscopy.