Enhancement of Carrier Mobility in Multilayer InSe Transistors by van der Waals Integration.

Two-dimensional material indium selenide (InSe) holds great promise for applications in electronics and optoelectronics by virtue of its fascinating properties. However, most multilayer InSe-based transistors suffer from extrinsic scattering effects from interface disorders and the environment, which cause carrier mobility and density fluctuations and hinder their practical application. In this work, we employ the non-destructive method of van der Waals (vdW) integration to improve the electron mobility of back-gated multilayer InSe FETs. After introducing the hexagonal boron nitride (h-BN) as both an encapsulation layer and back-gate dielectric with the vdW interface, as well as graphene serving as a buffer contact layer, the electron mobilities of InSe FETs are substantially enhanced. The vdW-integrated devices exhibit a high electron mobility exceeding 103 cm2 V-1 s-1 and current on/off ratios of ~108 at room temperature. Meanwhile, the electron densities are found to exceed 1012 cm-2. In addition, the fabricated devices show an excellent stability with a negligible electrical degradation after storage in ambient conditions for one month. Electrical transport measurements on InSe FETs in different configurations suggest that a performance enhancement with vdW integration should arise from a sufficient screening effect on the interface impurities and an effective passivation of the air-sensitive surface.

Boosting the Sensitivity of WSe2 Phototransistor via Janus Interfaces with 2D Perovskite and Ferroelectric Layers.

Hybrid systems have recently attracted increasing attention, which combine the special attributes of each constitute and create interesting functionalities through multiple heterointerface interactions. Here, we design a two-dimensional (2D) hybrid phototransistor utilizing Janus-interface engineering, in which the WSe2 channel combines light-sensitive perovskite and spontaneously polarized ferroelectrics, achieving collective ultrasensitive detection performance. The top perovskite (BA2(MA)3Pb4I13) layer can absorb the light efficiently and provide generous photoexcited holes to WSe2. WSe2 exhibit p-type semiconducting states of different degrees due to the selective light-operated doping effect, which also enables the ultrahigh photocurrent of the device. The bottom ferroelectric (Hf0.5Zr0.5O2) layer dramatically decreases the dark current, which should be attributed to the ferroelectric polarization assisted charge trapping effect and improved gate control. As a whole, our phototransistors show excellent photoelectric performances across the ultraviolet to near-infrared range (360-1050 nm), including an ultrahigh ON/OFF current ratio > 109 and low noise-equivalent power of 1.3 fW/Hz1/2, all of which are highly competitive in 2D semiconductor-based optoelectronic devices. In particular, the devices show excellent weak light detection ability, where the distinguishable photoswitching signal is obtained even under a record-low light intensity down to 1.6 nW/cm2, while showing a high responsivity of 2.3 × 105 A/W and a specific detectivity of 4.1 × 1014 Jones. Our work demonstrates that Janus-interface design makes the upper and lower interfaces complement each other for the joint advancement into high-performance optoelectronic applications, providing a picture to realize the integrated engineering on carrier dynamics by light irradiation, electric field, interfacial trapping, and band alignment.

Enhanced Performance of Two-Photon Excited Amplified Spontaneous Emission by Cd-Alloyed CsPbBr3 Nanocrystals.

It has been demonstrated that the alloyed perovskite nanocrystals (NCs) with a small amount of Cd element may passivate the inherent halide vacancies in perovskite NCs and improve their stability. However, the study of the optical properties of such alloyed perovskite NCs still remains essentially untouched, which will seriously hinder relevant applications. Herein, using different amounts of CdBr2 as an alloyed metal precursor, a series of CsPbxCd1-xBr3 NCs (x = 1, 0.93, and 0.88) were synthesized. Compared with bare CsPbBr3 NCs, the Cd-alloyed NCs exhibited a near-unity photoluminescence quantum yield (99%), efficiently improved stability, and enhanced electron-phonon coupling strength. As the Cd-alloyed amount was increased, their hot-carrier cooling time became faster and the exciton-biexciton interaction decreased, causing a decreased threshold of two-photon excited amplified spontaneous emission (ASE) from 1.58 to 1.23 mJ cm-2. In addition, the Cd-alloying method can also improve the photostability of ASE, resulting in the initial ASE intensity remaining at 90% even after 7.5 × 105 pulse shots. This work implies that the Cd-alloyed CsPbBr3 NCs will be promising for application in a laser gain medium.

Defect-Assisted Anchoring of Pt Single Atoms on MoS2 Nanosheets Produces High-Performance Catalyst for Industrial Hydrogen Evolution Reaction.

Pt-based catalysts are currently the most efficient electrocatalysts for the hydrogen evolution reaction (HER), but the scarcity and high cost of Pt limit industrial applications. Downsizing Pt nanoparticles (NPs) to single atoms (SAs) can expose more active sites and increase atomic utilization, thus decreasing the cost. Here, a solar-irradiation strategy is used to prepare hybrid SA-Pt/MoS2 nanosheets (NSs) that demonstrate excellent HER activity (the overpotential at a current density of 10 mA cm-2 (η10 ) of 44 mV, and Tafel slope of 34.83 mV dec-1 in acidic media; η10 of 123 mV, and Tafel slope of 76.71 mV dec-1 in alkaline media). Defects and deformations introduced by thermal pretreatment of the hydrothermal MoS2 NSs promote anchoring and stability of Pt SAs. The fabrication of Pt SAs and NPs is easily controlled using different Pt-precursor concentrations. Moreover, SA-Pt/MoS2 produced under natural sunlight exhibits high HER performance (η10 of 55 mV, and Tafel slope of 43.54 mV dec-1 ), which indicates its viability for mass production. Theoretical simulations show that Pt improves the absorption of H atoms and the charge-transfer kinetics of MoS2 , which significantly enhance HER activity. A simple, inexpensive strategy for preparing SA-Pt/MoS2 hybrid catalysts for industrial HER is provided.

Ultrathin Single-Crystalline 2D Perovskite Photoconductor for High-Performance Narrowband and Wide Linear Dynamic Range Photodetection.

For next-generation Internet-of-Everything applications, for example, artificial-neural-network image sensors, artificial retina, visible light communication, on-chip light interconnection, and flexible devices, etc., high-performance microscale photodetectors are in urgent demands. 2D material (2DM) photodetectors have been researched and demonstrated impressive performances. However, they have not met the demands in filterless narrowband photoresponse, wide linear dynamic range (LDR), ultralow dark current, and large on/off ratio, which are key performances for these applications. 2D Ruddlesden-Popper perovskites (2D-RPPs) are recently highlighted photovoltaic and optoelectronic materials. Embedding ultrathin 2D-RPPs into 2DM photodetectors holds potentials to improve these performances. Herein, a single-crystalline ultrathin (PEA)2 PbI4 is integrated into a vertical-stacked graphene-(PEA)2 PbI4 -graphene micro photoconductor (V-PEPI-PC). V-PEPI-PC exhibits narrowband photoresponses at 517 nm with a full-width-at-half-maximum of 15 nm and a wide LDR of 122 dB. Due to the multiple quantum wells in (PEA)2 PbI4 , V-PEPI-PC demonstrates an ultralow dark current of 1.1 × 10-14 A (44 pA mm-2 ), a high specific detectivity of 1.2 × 1013 Jones, and a high on/off ratio of 1.6 × 106 . Owing to the short vertical channel, V-PEPI-PC shows a fast response rise time of 486 µs. Therefore, the vertical-stacked photodetectors based on hybrid 2D-RPPs and 2DMs may have great potentials in future optoelectronics.

Low Damage Reductive Patterning of Oxidized Alkyl Self-Assembled Monolayers through Vacuum Ultraviolet Light Irradiation in an Evacuated Environment.

Through 172 nm vacuum ultraviolet light irradiation in a high vacuum condition (HV-VUV), well-defined micropatterns with a varied periodic friction were fabricated at the surface of self-assembled monolayers (SAMs) terminated with oxygenated groups. No apparent height contrast between the HV-VUV-irradiated and -masked areas was observed, which indicated the stability of the C-C skeleton of the assembled molecules. The trimming of oxygenated groups occurred through dissociating the C-O bonds and promoting the occurrence of α- and ß-cleavages in the C═O-containing components. Hence, the HV-VUV treatment trimmed the oxygenated groups without degrading the C-C skeleton. The HV-VUV treatment influenced the order of the assembled molecules, and the step-terrace structure was distorted. The decrease in friction at the HV-VUV-irradiated domains was attributed to the dissociation of oxygenated groups. (3-Aminopropyl)trimethoxysilane (APTMS) aggregated at the masked areas of the HV-VUV-patterned SAM, where the oxygenated groups worked as anchors. APTMS aggregations did not exist at the irradiated areas, indicating the trimming of the oxygenated groups at these areas. The direct assembling of APTMS on the Si substrate at the irradiated areas was prevented by the remaining C-C skeleton.

Immobilization of Reduced Graphene Oxide on Hydrogen-Terminated Silicon Substrate as a Transparent Conductive Protector.

Silicon is a promising electrode material for photoelectrochemical and photocatalytic reactions. However, the chemically active surface of silicon will be easily oxidized when exposed to the oxidation environment. We immobilized graphene oxide (GO) onto hydrogen-terminated silicon (H-Si) and reduced it through ultraviolet (UV) and vacuum-ultraviolet (VUV) irradiation. This acted as an ultrathin conductive layer to protect H-Si from oxidation. The elemental evolution of GO was studied by X-ray photoelectron spectroscopy, and it was found that GO was partially reduced soon after the deposition onto H-Si and further reduced after UV or VUV light irradiation. The VUV photoreduction demonstrated ca. 100 times higher efficiency compared to the UV reduction based on the irradiation dose. The saturated oxygen-to-carbon ratio (RO/C) of the reduced graphene oxide (rGO) was 0.21 ± 0.01, which is lower than the photoreduction of GO on SiO2 substrate. This indicated the H-Si played an important role in assisting the photoreduction of GO. No obvious exfoliation of rGO was observed after sonicating the rGO-covered H-Si sample in water, which indicated rGO was immobilized on H-Si. The electrical conductivity of H-Si surface was maintained in the rGO-covered region while the exposed H-Si region became insulating, which was observed by conductive atomic force microscopy. The rGO was verified capable to protect the active H-Si against the oxidation under an ambient environment.

Decoration of reduced graphene oxide by gold nanoparticles: an enhanced negative photoconductivity.

Photodetection in a visible light region is important in various applications, including computation, environmental monitoring, biological detection and industrial control. Due to this, research studies to develop photoconductive devices have great significance. We report a study on the photoconductivity of reduced graphene oxide (rGO)/gold nanoparticle (AuNP) nanocomposites, emphasizing the enhancement effect induced by AuNPs. rGO/AuNP photoelectric devices were prepared by spincoating rGO onto an AuNP-array-covered silicon substrate. Photoelectric responses under visible light illumination were measured and the results showed that the negative photoelectric responsivity of rGO was improved by 3 orders of magnitude due to AuNPs. The effects of AuNPs on negative photoconductivity (NPC) properties of rGO were investigated, and it was found that AuNPs affected NPC in three aspects: (1) AuNPs form discrete electrodes separated by nanoscale gaps which generated new conduction paths, and hence the conductivity of rGO was enhanced by 3 orders of magnitude; (2) localized surface plasmon resonance (LSPR) of AuNPs effectively enhances total light absorption of rGO; (3) photocurrent between AuNPs and rGO can weaken the NPC property of rGO. The low-cost and mass-producible rGO/AuNP nanocomposites demonstrate high photoelectric responsivity, which hold much promise for NPC devices.

Vacuum-Ultraviolet Promoted Oxidative Micro Photoetching of Graphene Oxide.

Microprocessing of graphene oxide (GO) films is of fundamental importance in fabricating graphene-based devices. We demonstrate the photoetching of GO sheets using vacuum-ultraviolet (VUV, λ = 172 nm) light under controlled atmospheric pressure. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and differential interference contrast microscopy (DIC) studies revealed that the photoetching of GO films successfully proceeded in the regions exposed to VUV irradiation in the oxygen-containing atmosphere. Precise photoetching of the GO sheets was achieved at a vacuum pressure of 5 × 10(3) Pa with VUV light irradiation for 20 min. This was followed by VUV irradiation in a high vacuum (<10(-3) Pa) and sonication in water. The photoetched GO sheets then transformed into reduced GO (rGO) patterns. The minimum feature fabricated by this method was 2 µm wide lines aligned at an interval of 4 µm. This method provides a cost-effective way to fabricate rGO patterns with fewer boundaries between rGO sheets and offers a better integrity of rGO, which can be promising for further applications in micro mechanics, micro electrochemistry, optoelectronics, etc.