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.

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.

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.

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.

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.

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.

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.

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.

Solution-Based Property Tuning of Black Phosphorus.

The air instability of black phosphorus (BP) severely hinders the development of its electronic and optoelectronic applications. Although a lot of effort has been made to passivate it against degradation in ambient conditions, approaches to further manipulate the properties of passivated BP are still very limited. Herein, we report a simple and low-cost chemical method that can achieve BP passivation and property tailoring simultaneously. The method is conducted by immersing a BP sample in the solution containing both 2,2,6,6-tetramethylpiperidinyl- N-oxyl (TEMPO) and triphenylcarbenium tetrafluorobor in a mixture of water and acetone (v/v = 1:1). After the treatment, the BP sample is functionalized with TEMPO, which not only efficiently passivates BP but also p-dopes BP to a degenerated density level of 1013 cm-2. The performance of the BP field effect transistor is improved after functionalization with a high Ion/ Ioff ratio of 106 and carrier mobility of 881.5 cm2/(V·s). The functionalization-induced doping also significantly reduces the contact resistance between BP and the Cr/Au electrode to 0.97 kΩ·µm. Additionally, we observe a great reduction of BP electrical and optical anisotropies after functionalization. This chemical functionalization method provides a viable route to simultaneously passivate and tune the properties of BP.

Specific and Highly Sensitive Detection of Ketone Compounds Based on p-Type MoTe2 under Ultraviolet Illumination.

The selective and sensitive detection of chemical agents is demanded by a wide range of practical applications. In particular, sensing of volatile organic compounds (VOCs) at parts-per-billion level is critical for environmental monitoring, process control, and early diagnosis of human diseases. In this report, we demonstrate a specific and highly sensitive detection of ketone compounds using two-dimensional (2D) molybdenum ditelluride (MoTe2). We investigated the effects of UV activation on the sensing performance to a variety of VOCs. It is found that the MoTe2 field-effect transistor (FET) exhibits an opposite sensing response to ketone compounds before and after UV light activation, whereas the responses to other types of VOCs remain in the same direction regardless of the illumination. This unique behavior enables the discriminative detection of ketone molecules including acetone and pentanone from other VOCs in a gas mixture. The activation of UV light also results in a very high sensitivity and low detection limit toward acetone (∼0.2 ppm). Moreover, the MoTe2 FET shows a stable sensing performance in a high humidity environment. The results demonstrate the potential of MoTe2 as a promising candidate for high-performance acetone sensors in important applications such as human breath analysis. The scheme of light-tunable sensing can be applied to a broad range of sensing platforms 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.

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.

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.

Implementing Lateral MoSe2 P-N Homojunction by Efficient Carrier-Type Modulation.

High-performance p-n junctions based on atomically thin two-dimensional (2D) materials are the fundamental building blocks for many nanoscale functional devices that are ideal for future electronic and optoelectronic applications. The lateral p-n homojunctions with conveniently tunable band offset outperform vertically stacked ones, however, the realization of lateral p-n homojunctions usually require efficient carrier-type modulation in a single 2D material flake, which remains a tech challenge. In this work, we have realized effective carrier-type modulation in a single MoSe2 flake, and thus, a lateral MoSe2 p-n homojunction is achieved by sequential treatment of air rapid thermal annealing and triphenylphosphine (PPh3) solution coating. The rapid thermal annealing modulates MoSe2 flakes from naturally n-type doping to degenerated p-type doping and improves the hole mobility of the MoSe2 field effect transistors from 0.2 to 71.5 cm2·V-1·s-1. Meanwhile, the n-doping of MoSe2 is increased by drop-coating PPh3 solution on the MoSe2 surface with increased electron mobility from 78.6 to 412.8 cm2·V-1·s-1. The as-fabricated lateral MoSe2 p-n homojunction presents a high rectification ratio of 104, an ideality factor of 1.2, and enhanced photoresponse of 1.3 A·W-1 to visible light. This efficient carrier-type modulation within a single MoSe2 flake has potential for use in various functional devices.

Enhancing electronic and optoelectronic performances of tungsten diselenide by plasma treatment.

Transition metal dichalcogenides (TMDCs) have recently become spotlighted as nanomaterials for future electronic and optoelectronic devices. In this work, we develop an effective approach to enhance the electronic and optoelectronic performances of WSe2-based devices by N2O plasma treatment. The hole mobility and sheet density increase by 2 and 5 orders of magnitude, reaching 110 cm2 V-1 s-1 and 2.2 × 1012 cm-2, respectively, after the treatment. At the same time, the contact resistance (Rc) between WSe2 and its metal electrode drop by 5 orders of magnitude from 1.0 GΩ µm to 28.4 kΩ µm. The WSe2 photoconductor exhibits superior performance with high responsivity (1.5 × 105 A W-1), short response time (<2 ms), high detectivity (3.6 × 1013 Jones) and very large photoconductive gain (>106). We have also built a lateral p-n junction on a single piece of WSe2 flake by selective plasma exposure. The junction reaches an exceedingly high rectifying ratio of 106, an excellent photoresponsivity of 2.49 A W-1 and a fast response of 8 ms. The enhanced optoelectronic performance is attributed to band-engineering through the N2O plasma treatment, which can potentially serve as an effective and versatile approach for device engineering and optimization in a wide range of electronic and optoelectronic devices based on 2D materials.

Two-Dimensional High-Quality Monolayered Triangular WS2 Flakes for Field-Effect Transistors.

Large-area uniform of single-crystal tungsten disulfide (WS2) is important for advanced optoelectronics based on two-dimensional (2D) atomic crystals. However, difficulties in controlling the interrelated growth parameters restrict its development in devices. Herein, we present the synthesis of triangular monolayered WS2 flakes with good uniformity and single crystal by adjusting the introduction time of sulfur precursor and the distances between the sources and substrates to control the nucleation density. Investigation of the morphology and structure by transmission electron microscopy and Raman spectroscopy indicates that a series of triangular (side length of 233 µm) monolayered WS2 flakes shows high-quality structure and homogenous crystallinity. Field-effect transistors based on the fabricated triangular monolayered WS2 with single crystal demonstrate environmentally stable charge transport with a field-effect mobility of 50.5 cm2/V s and current modulation Ion/ Ioff of ∼107. The results of this study pave the way for the application of monolayered WS2 in a multitude of 2D-material-based devices.

Acoustically enhanced photodetection by a black phosphorus-MoS2 van der Waals heterojunction p-n diode.

We developed a new way to enhance the photoresponsivity of a van der Waals heterojunction p-n diode using surface acoustic waves (SAWs). The diode was constructed on top of a piezoelectric LiNbO3 substrate and composed of p-type black phosphorus (BP) and n-type molybdenum disulfide (MoS2) flakes that partly overlapped with each other. This layout facilitated the applied SAWs to rapidly drive carriers out of the depletion region. In this structural design, SAWs promoted the separation of photogenerated carriers, and thus greatly increased the photocurrent. The measured photocurrent for the device with SAWs was about 103 times higher than that of the device without SAWs. The device using SAWs showed a photoresponsivity as high as 2.17 A W-1 at the wavelength of 582 nm. This excellent performance was attributed to the SAWs suppressing electron-hole recombination in the device under light illumination. Our device exhibits promise as a high-performance photodetector and reveals new possibilities for acoustic devices in optoelectronics.