Mechanism and efficiency of photocatalytic triclosan degradation by TiO2/BiFeO3 nanomaterials.

Hierarchical porous TiO2 photocatalytic nanomaterials were fabricated by impregnation and calcination using a peanut shell biotemplate, and TiO2/BiFeO3 composite nanomaterials with different doping amounts were fabricated using hydrothermal synthesis. The micromorphology, structure, element composition and valence state of the photocatalyst were analyzed using a series of characterization methods, including X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), BET surface area (BET), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance (UV-vis), fluorescence spectroscopy (PL) and other technological means. Finally, the degradation mechanism and efficiency of BiFeO3 composite photocatalyst on the target pollutant triclosan were analyzed using a xenon lamp to simulate sunlight. The results showed that TiO2/BiFeO3 catalyst fabricated using a peanut shell biotemplate has a specific surface area of 153.64 m2/g, a band gap of 1.92 eV, and forms heterostructures. The optimum doping amount of TiO2/BiFeO3 catalyst was 1 mol/mol, and the degradation rate was 81.2%. The main active substances degraded were ·O2-and ·OH. The degradation process measured is consistent with the pseudo-first-order kinetic model.

Tunable and nonvolatile multibit data storage memory based on MoTe2/boron nitride/graphene heterostructures through contact engineering.

Heterostructures formed by stacking atomically thin two-dimensional materials are promising candidates for flash memory devices to achieve premium performances, due to the capability of effective carrier modulation and unique charge trapping behavior at the interfaces with atomic flatness. Here, we report a nonvolatile floating-gate flash memory based on MoTe2/h-BN/graphene van der Waals heterostructure, which possesses increased data storage capacity per cell and versatile tunability. The decent memory behavior of the device is enabled by the carriers stored in the floating gate of graphene layer, which tunnel through the dielectric layer of h-BN from the channel layer of MoTe2 under static-electrical field. Consequently, the developed memory device is capable to store 2 bits per cell by applying varied gate bias to implement multi-distinctive current levels. The device also exhibits remarkable erase/program current ratio of ∼105 with 1 µs switch speed and stable retention with estimated ∼30% charge loss after 10 yr. Furthermore, the memory device can operate in both p- and n-type modes through contact engineering, offering wide adaptability for emerging applications in electronic technologies, such as neuromorphic computing, data-adaptive energy efficient memory, and complex digital circuits.

The effect of air stable n-doping through mild plasma on the mechanical property of WSe2 layers.

Two-dimensional transition metal dichalcogenides have been widely applied to electronic and optoelectronic device owing to their remarkable material properties. Many studies present the platform for regulating the contact resistance via various doping schemes. Here, we report the alteration of mechanical properties of few top layers of the WSe2 flake which are processed by air stable n-doping of N2O with a constant gas flow through mild plasma and present better manufacturability and friability. The single-line nanoscratching experiments on the WSe2 flakes with different doping time reveal that the manufacturable depths are positively correlated with the exposure time at a certain range and tend to be stable afterwards. Meanwhile, material characterization by x-ray photoelectron spectroscopy confirms that the alteration of mechanical properties is owing to the creation of Se vacancies and substitution of O atoms, which breaks the primary molecular structure of the WSe2 flakes. The synchronous Kelvin probe force microscopy and topography results of ROI nanoscratching of a stepped WSe2 sample confirmed that the depth of the degenerate doping is five layers, which was consistent with the single-line scratching experiments. Our results reveal the interrelationship of the mechanical property, chemical bonds and work function changes of the doped WSe2 flakes.

Highly-sensitive gas sensor based on two-dimensional material field effect transistor.

Atomically thin two-dimensional (2D) materials are ideal gas sensing materials for achieving an ultra-low detection limit, due to the high surface-to-volume ratio, low electronic noise and sensitively tunable Fermi level. However, the sensitivity of 2D materials to their surrounding environment may also severely degrade the long-term stability of sensing devices, since most of them use the same 2D material flake as both the sensing and conduction material. In this work, we report a gas sensor based on a 2D material field effect transistor (FET) which uses few-layer black phosphorus (BP), boron nitride (BN) and molybdenum disulfide (MoS2) as the top-gate, dielectric layer and conduction channel, respectively. In this device configuration, the top-gate of BP with a superior gas adsorption capability serves as the sensing material, while the conduction channel of MoS2 is isolated from ambient environment by the coverage of the BN dielectric layer. The separation of the sensing and conduction materials not only improves the long-term stability of the device, but also enables us to use different materials for gas adsorption and conduction purposes to achieve optimum sensing performances. In addition, the adsorption kinetics of the gas molecules on the sensing channel can be sensitively detected by the current/resistance variation of the conduction channel, since the adsorbed gas molecules can effectively tune the Fermi level of sensing and conduction materials (BP and MoS2, respectively) through band alignment. We experimentally demonstrated that the proposed 2D material FET not only achieved a detection limit of 3.3 ppb to NO2, but was also capable to differentiate oxidizing and reducing gases.

Flexible gas sensor based on graphene/ethyl cellulose nanocomposite with ultra-low strain response for volatile organic compounds rapid detection.

Minimizing the strain-induced undesirable effects is one of the major efforts to be made for flexible electronics. This work demonstrates a highly sensitive flexible gas sensor with ultra-low strain response, which is potentially suitable for wearable electronics applications. The gas sensing material is a free-standing and flexible thin film made of graphene/ethyl cellulose (EC) nanocomposite, which is then integrated with flexible substrate of polyethylene terephthalate. The sensor exhibits relative resistance change within 0.3% at a minimum bending radius of 3.18 mm and 0.2% at the bending radius of 5 mm after 400 bending cycles. The limited strain response attributes to several applied strategies, including using EC with high Young's modulus as the matrix material, maintaining high graphene concentration and adopting suspended device structure. In contrast to the almost negligible strain sensitivity, the sensor presents large and rapid responses toward volatile organic compounds (VOCs) at room temperature. Specifically, the sensor resistance rapidly increases upon the exposure to VOCs with detection limits ranging from 37 to 167 ppm. A preliminary demo of wearable gas sensing capability is also implemented by wearing the sensor on human hand, which successfully detects several VOCs, instead of normal hand gestures.

Volatile organic compounds discrimination based on dual mode detection.

We report on a volatile organic compound (VOC) sensor that can provide concentration-independent signals toward target gases. The device is based on a dual-mode detection mechanism that can simultaneously record the mechanical (resonant frequency, f r) and electrical (current, I) responses of the same gas adsorption event. The two independent signals form a unique I-f r trace for each target VOC as the concentration varies. The mechanical response (frequency shift, Δf r) resulting from mass load on the device is directly related to the amount of surface adsorptions, while the electrical response (current variation, ΔI) is associated with charge transfer across the sensing interface and changes in carrier mobility. The two responses resulting from independent physical processes reflect intrinsic physical properties of each target gas. The ΔI-Δf r trace combined with the concentration dependent frequency (or current) signals can therefore be used to achieve target both recognition and quantification. The dual-mode device is designed and fabricated using standard complementary metal oxide semiconductor (CMOS) compatible processes. It exhibits consistent and stable performance in our tests with six different VOCs including ethanol, methanol, acetone, formaldehyde, benzene and hexane.

Hypersonic Poration: A New Versatile Cell Poration Method to Enhance Cellular Uptake Using a Piezoelectric Nano-Electromechanical Device.

Efficient delivery of genes and therapeutic agents to the interior of the cell is critical for modern biotechnology. Herein, a new type of chemical-free cell poration method-hypersonic poration-is developed to improve the cellular uptake, especially the nucleus uptake. The hypersound (≈GHz) is generated by a designed piezoelectric nano-electromechanical resonator, which directly induces normal/shear stress and "molecular bombardment" effects on the bilayer membranes, and creates reversible temporal nanopores improving the membrane permeability. Both theory analysis and cellular uptake experiments of exogenous compounds prove the high delivery efficiency of hypersonic poration. Since target molecules in cells are accumulated with the treatment, the delivered amount can be controlled by tuning the treatment time. Furthermore, owing to the intrinsic miniature of the resonator, localized drug delivery at a confined spatial location and tunable arrays of the resonators that are compatible with multiwell plate can be achieved. The hypersonic poration method shows great delivery efficacy combined with advantage of scalability, tunable throughput, and simplification in operation and provides a potentially powerful strategy in the field of molecule delivery, cell transfection, and gene therapy.

Enhanced non-volatile resistive switching in suspended single-crystalline ZnO nanowire with controllable multiple states.

Resistive switching nanostructures are a promising candidate for next-generation non-volatile memories. In this report, we investigate the switching behaviors of single-crystalline ZnO nanowires suspended in air. They exhibit significantly higher current density, lower switching voltage, and more pronounced multiple conductance states compared to nanowires in direct contact with substrate. We attribute the effect to enhanced Joule heating efficiency, reduced surface scattering, and more significantly, the positive feedback established between the current density and local temperature in the suspended nanowires. The proposed mechanism has been quantitatively examined by finite element simulations. We have also demonstrated an innovative approach to initiating the current-temperature mutual enhancement through illumination by ultraviolet light, which further confirmed our hypothesis and enabled even greater enhancement. Our work provides further insight into the resistive switching mechanism of single-crystalline one-dimensional nanostructures, and suggests an effective means of performance enhancement and device optimization.

On-chip surface modified nanostructured ZnO as functional pH sensors.

Zinc oxide (ZnO) nanostructures are promising candidates as electronic components for biological and chemical applications. In this study, ZnO ultra-fine nanowire (NW) and nanoflake (NF) hybrid structures have been prepared by Au-assisted chemical vapor deposition (CVD) under ambient pressure. Their surface morphology, lattice structures, and crystal orientation were investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). Two types of ZnO nanostructures were successfully integrated as gate electrodes in extended-gate field-effect transistors (EGFETs). Due to the amphoteric properties of ZnO, such devices function as pH sensors. We found that the ultra-fine NWs, which were more than 50 µm in length and less than 100 nm in diameter, performed better in the pH sensing process than NW-NF hybrid structures because of their higher surface-to-volume ratio, considering the Nernst equation and the Gouy-Chapman-Stern model. Furthermore, the surface coating of (3-Aminopropyl)triethoxysilane (APTES) protects ZnO nanostructures in both acidic and alkaline environments, thus enhancing the device stability and extending its pH sensing dynamic range.

A Sensitivity-Enhanced Electrolyte-Gated Graphene Field-Effect Transistor Biosensor by Acoustic Tweezers.

Low-abundance biomolecule detection is very crucial in many biological and medical applications. In this paper, we present a novel electrolyte-gated graphene field-effect transistor (EGFET) biosensor consisting of acoustic tweezers to increase the sensitivity. The acoustic tweezers are based on a high-frequency bulk acoustic resonator with thousands of MHz, which has excellent ability to concentrate nanoparticles. The operating principle of the acoustic tweezers to concentrate biomolecules is analyzed and verified by experiments. After the actuation of acoustic tweezers for 10 min, the IgG molecules are accumulated onto the graphene. The sensitivities of the EGFET biosensor with accumulation and without accumulation are compared. As a result, the sensitivity of the graphene-based biosensor is remarkably increased using SMR as the biomolecule concentrator. Since the device has advantages such as miniaturized size, low reagent consumption, high sensitivity, and rapid detection, we expect it to be readily applied to many biological and medical applications.

Modulation of MoTe2/MoS2 van der Waals heterojunctions for multifunctional devices using N2O plasma with an opposite doping effect.

van der Waals layered heterojunctions have a variety of band offsets that open up possibilities for a wide range of novel and multifunctional devices. However, due to their poor pristine carrier concentrations and limited band modulation methods, multifunctional p-n heterojunctions are very difficult to achieve. In this report, we developed a highly effective N2O plasma process to treat MoTe2/MoS2 heterojunctions. This allowed us to adjust the hole and electron concentrations in the two materials independently and simultaneously. More importantly, for the first time, we were able to create opposite doping on the two sides of the junction through a single-step treatment. With a very wide doping range from pristine to degenerate levels, a MoTe2/MoS2 heterojunction can be modulated to behave as a forward rectifying diode with enhanced rectifying ratio and as a tunneling transistor with negative differential resistance at room temperature. The new approach provides an effective and generic doping scheme for heterojunctions to construct versatile and multifunctional electronic devices.

Multi-level flash memory device based on stacked anisotropic ReS2-boron nitride-graphene heterostructures.

Charge-trapping memory devices based on two-dimensional (2D) material heterostructures possess an atomically thin structure and excellent charge transport capability, making them promising candidates for next-generation flash memories to achieve miniaturized size, high storage capacity, fast switch speed, and low power consumption. Here, we report a nonvolatile floating-gate memory device based on an ReS2/boron nitride/graphene heterostructure. The implemented ReS2 memory device displays a large memory window exceeding 100 V, leading to an ultrahigh current ratio over 108 between programming and erasing states. The ReS2 memory device also exhibits an ultrafast switch speed of 1 µs. In addition, the device can endure hundreds of switching cycles and shows stable retention characteristics with ∼40% charge remaining after 10 years. More importantly, taking advantage of its anisotropic electrical properties, a single ReS2 flake can achieve direction-sensitive multi-level data storage to enhance the data storage density. On the basis of these characteristics, the proposed ReS2 memory device is potentially able to serve the entire memory device hierarchy, meeting the need for scalability, capacity, speed, retention, and endurance at each level.

Gate-Tunable Photodetection/Voltaic Device Based on BP/MoTe2 Heterostructure.

van der Waals heterostructures based on two-dimensional (2D) materials have attracted tremendous attention for their potential applications in optoelectronic devices, such as solar cells and photodetectors. In addition, the widely tunable Fermi levels of these atomically thin 2D materials enable tuning the device performances/functions dynamically. Herein, we demonstrated a MoTe2/BP heterostructure, which can be dynamically tuned to be either p-n or p-p junction by gate modulation due to compatible band structures and electrically tunable Fermi levels of MoTe2 and BP. Consequently, the electrostatic gating can further accurately control the photoresponse of this heterostructure in terms of the polarity and the value of photoresponsivity. Besides, the heterostructure showed outstanding photodetection/voltaic performances. The optimum photoresponsivity, external quantum efficiency, and response time as a photodetector were 0.2 A/W, 48.1%, and 2 ms, respectively. Our study enhances the understanding of 2D heterostructures for designing gate-tunable devices and reveals promising potentials of these devices in future optoelectronic applications.

Photoinduced Doping To Enable Tunable and High-Performance Anti-Ambipolar MoTe2/MoS2 Heterotransistors.

van der Waals (vdW) p-n heterojunctions formed by two-dimensional nanomaterials exhibit many physical properties and deliver functionalities to enable future electronic and optoelectronic devices. In this report, we demonstrate a tunable and high-performance anti-ambipolar transistor based on MoTe2/MoS2 heterojunction through in situ photoinduced doping. The device demonstrates a high on/off ratio of 105 with a large on-state current of several micro-amps. The peak position of the drain-source current in the transfer curve can be adjusted through the doping level across a large dynamic range. In addition, we have fabricated a tunable multivalue inverter based on the heterojunction that demonstrates precise control over its output logic states and window of midlogic through source-drain bias adjustment. The heterojunction also exhibits excellent photodetection and photovoltaic performances. Dynamic and precise modulation of the anti-ambipolar transport properties may inspire functional devices and applications of two-dimensional nanomaterials and their heterostructures of various kinds.

Ambipolar and n/p-type conduction enhancement of two-dimensional materials by surface charge transfer doping.

The controllable and wide-range modulation of the carrier type and mobility in atomically thin two-dimensional (2D) materials is one of the most critical issues to be addressed before 2D materials can be practically used for future electronic and optoelectronic devices. In this work, we propose using a novel surface charge transfer mechanism to accomplish the controllable and wide-range modulation of the carrier type and mobility in 2D materials. Our methodology uses a solution of triphenylboron (TPB) to physically coat 2D materials; the TPB molecule contains positive and negative charge centers that are spatially separable when induced by an electrical field. Consequently, the TPB can transfer either positive or negative charges to 2D materials depending on the direction of the applied electrical field and thus enhance the ambipolar behavior of the 2D-material FET. This method is so versatile that seven types of 2D materials including graphene, black phosphorus and five transition metal dichalcogenides (TMDCs) can be modulated to strong ambipolar behavior with significantly increased conduction. In addition, selectively suppressing or enhancing the negative charge center enables solely p-type and n-type doping. We also accomplish the precise tuning of carrier mobility in TMDCs from ambipolar to p-type by coating a mixture of TPB/BCF in certain concentration ratios.

Dynamically controllable polarity modulation of MoTe2 field-effect transistors through ultraviolet light and electrostatic activation.

Energy band engineering is of fundamental importance in nanoelectronics. Compared to chemical approaches such as doping and surface functionalization, electrical and optical methods provide greater flexibility that enables continuous, reversible, and in situ band tuning on electronic devices of various kinds. In this report, we demonstrate highly effective band modulation of MoTe2 field-effect transistors through the combination of electrostatic gating and ultraviolet light illumination. The scheme can achieve reversible doping modulation from deep n-type to deep p-type with ultrafast switching speed. The treatment also enables noticeable improvement in field-effect mobility by roughly 30 and 2 times for holes and electrons, respectively. The doping scheme also provides good spatial selectivity and allows the building of a photo diode on a single MoTe2 flake with excellent photo detection and photovoltaic performances. The findings provide an effective and generic doping approach for a wide variety of 2D materials.

Wet Chemical Method for Black Phosphorus Thinning and Passivation.

Layered black phosphorus (BP) has been expected to be a promising material for future electronic and optoelectronic applications since its discovery. However, the difficulty in mass fabricating layered air-stable BP severely obstructs its potential industry applications. Here, we report a new BP chemical modification method to implement all-solution-based mass production of layered air-stable BP. This method uses the combination of two electron-deficient reagents 2,2,6,6-tetramethylpiperidinyl- N-oxyl (TEMPO) and triphenylcarbenium tetrafluorobor ([Ph3C]BF4) to accomplish thinning and/or passivation of BP in organic solvent. The field-effect transistor and photodetection devices constructed from the chemically modified BP flakes exhibit enhanced performances with environmental stability up to 4 months. A proof-of-concept BP thin-film transistor fabricated through the all-solution-based exfoliation and modification displays an air-stable and a typical p-type transistor behavior. This all-solution-based method improves the prospects of BP for industry applications.

The Opposite Anisotropic Piezoresistive Effect of ReS2.

Mechanical strain induced changes in the electronic properties of two-dimensional (2D) materials is of great interest for both fundamental studies and practical applications. The anisotropic 2D materials may further exhibit different electronic changes when the strain is applied along different crystalline axes. The resulting anisotropic piezoresistive phenomenon not only reveals distinct lattice-electron interaction along different principle axes in low-dimensional materials but also can accurately sense/recognize multidimensional strain signals for the development of strain sensors, electronic skin, human-machine interfaces, etc. In this work, we systematically studied the piezoresistive effect of an anisotropic 2D material of rhenium disulfide (ReS2), which has large anisotropic ratio. The measurement of ReS2 piezoresistance was experimentally performed on the devices fabricated on a flexible substrate with electrical channels made along the two principle axes, which were identified noninvasively by the reflectance difference microscopy developed in our lab. The result indicated that ReS2 had completely opposite (positive and negative) piezoresistance along two principle axes, which differed from any previously reported anisotropic piezoresistive effect in other 2D materials. We attributed the opposite anisotropic piezoresistive effect of ReS2 to the strain-induced broadening and narrowing of the bandgap along two principle axes, respectively, which was demonstrated by both reflectance difference spectroscopy and theoretical calculations.

Ultrasensitive and Fully Reversible NO2 Gas Sensing Based on p-Type MoTe2 under Ultraviolet Illumination.

The unique properties of two-dimensional (2D) materials make them promising candidates for chemical and biological sensing applications. However, most 2D material sensors suffer from extremely long recovery time due to the slow molecular desorption at room temperature. Here, we report an ultrasensitive p-type molybdenum ditelluride (MoTe2) gas sensor for NO2 detection with greatly enhanced sensitivity and recovery rate under ultraviolet (UV) illumination. Specifically, the sensitivity of the sensor to NO2 is dramatically enhanced by 1 order of magnitude under 254 nm UV illumination as compared to that in the dark condition, leading to a remarkable low detection limit of 252 ppt. More importantly, the p-type MoTe2 sensor can achieve full recovery after each sensing cycle well within 160 s at room temperature. Finally, the p-type MoTe2 sensor also exhibits excellent sensing performance to NO2 in ambient air and negligible response to H2O, indicating its great potential in practical applications, such as breath analysis and ambient NO2 detection. Such impressive features originate from the activated interface interaction between the gas molecules and p-type MoTe2 surface under UV illumination. This work provides a promising and easily applicable strategy to improve the performance of the gas sensors based on 2D materials.

Ultraviolet Light-Induced Persistent and Degenerated Doping in MoS2 for Potential Photocontrollable Electronics Applications.

Efficient modulation of carrier concentration is fundamentally important for tailoring the electronic and photoelectronic properties of semiconducting materials. Photoinduced doping is potentially a promising way to realize such a goal for atomically thin nanomaterials in a rapid and defect-free manner. However, the wide applications of photoinduced doping in nanomaterials are severely constrained by the low doping concentration and poor stability that can be reached. Here, we propose a novel photoinduced doping mechanism based on the external photoelectric effect of metal coating on nanomaterials to significantly enhance the achievable doping concentration and stability. This approach is preliminarily demonstrated by an MX2 (M is Mo or Re; X is S or Se) nanoflake modified through a simple process of sequentially depositing and annealing an Au layer on the surface of the flake. Under ultraviolet (UV) light illumination, the modified MX2 achieves degenerated n-type doping density of 1014 cm-2 rapidly according to the experimentally observed >104 times increment in the channel current. The doping level persists after the removal of UV illumination with a nonobservable decrease over 1 day in vacuum (less than 23% over 7 days under an ambient environment). This photoinduced doping approach may contribute a major leap to the development of photocontrollable nanoelectronics.