MoS2 Van der Waals p–n Junctions Enabling Highly Selective Room‐Temperature NO2 Sensor

Van der Waals p–n junctions of 2D materials present great potential for electronic devices due to the fascinating properties at the junction interface. In this work, an efficient gas sensor based on planar 2D van der Waals junctions is reported by stacking n‐type and p‐type atomically thin MoS₂ films, which are synthesized by chemical vapor deposition (CVD) and soft‐chemistry route, respectively. The electrical conductivity of the van der Waals p–n junctions is found to be strongly affected by the exposure to NO₂ at room temperature (RT). The MoS₂ p–n junction sensor exhibits an outstanding sensitivity and selectivity to NO₂ at RT, which are unavailable in sensors based on individual n‐type or p‐type MoS₂. The sensitivity of 20 ppm NO₂ is improved by 60 times compared to a p‐type MoS₂ sensor, and an extremely low limit of detection of 8 ppb is obtained under ultraviolet irradiation. Complete and very fast sensor recovery is achieved within 30 s. These results are superior to most of the previous reports related to NO₂ detection. This work establishes an entirely new sensing platform and proves the feasibility of using such materials for the high‐performance detection of gaseous molecules at RT.     

Controllable gas selectivity at room temperature based on Ph5T2-modified CuPc nanowire field-effect transistors

A dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2)-modified copper phthalocyanine (CuPc) single crystal nanowire field-effect transistor (FET) with gas dielectric was fabricated as an organic gas sensor. This device exhibits the high response and the excellent controllable selectivity at room temperature. Its detection limit for NO₂, NO, and H₂S is down to sub-ppm level. Prior to surface modification, the CuPc nanowire FET shows the response as high as 1088% to 10 ppm H₂S, but only 97.5% to 10 ppm NO₂. After Ph5T2 modification, the response to 10 ppm H₂S is decreased by one order of magnitude, but is dramatically improved up to 460% to 10 ppm NO₂. The responses towards H₂S and NO₂ respectively for pristine and the modified sensor are higher than those of most reported organic sensors. The gas-sensing results reveal that Ph5T2 modification can transform the selectivity of the sensor from H₂S to NO₂. The controllable modulation of gas selectivity is related to the formed organic heterojunctions between CuPc and Ph5T2, where the hole carriers of CuPc nanowire are modulated by these heterojunctions, resulting in the changed adsorption behavior towards different gases.     

Impact of Bonding on the Stacking Defects in Layered Chalcogenides

Phase‐change materials for high‐density data storage traditionally exploit the amorphous‐to‐crystalline phase transition. A number of these compounds are organized in blocks, separated by van der Waals‐like gaps. Such layered chalcogenides are attracting interest due to their unique material properties and the possibility to change their properties upon local rearrangements at the gap, giving rise to novel applications. To better understand the behavior of layered chalcogenides, the connection between structural defects, physical properties, and the bonding situation is highlighted here using electron microscopy, X‐ray diffraction, and density functional theory. In particular, stacking defects in hexagonal Ge₄Se₃Te, GaSe, and Sb₂Te₃ are characterized experimentally, followed by an investigation of the influence of observed and hypothetical stacking defects on optical and electronic properties by theoretical means. Then, a connection between observed defects and the bonding situation in these materials is drawn and related to the presence of van der Waals and metavalent bonding in chalcogenides. Finally, additional experiments are performed to validate the conclusions for other metavalently bonded layered chalcogenides. Transmission electron microscopy provides a powerful tool for direct detection of defects and, when combined with diffraction experiments and ab initio theory, it facilitates the precise investigation of the bonding mechanisms in layered chalcogenides.     

Understanding Microscopic Operating Mechanisms of a van der Waals Planar Ferroelectric Memristor

Ferroelectric memristors represent a promising new generation of devices that have a wide range of applications in memory, digital information processing, and neuromorphic computing. Recently, van der Waals ferroelectric In₂Se₃ with unique interlinked out-of-plane and in-plane polarizations has enabled multidirectional resistance switching, providing unprecedented flexibility in planar and vertical device integrations. However, the operating mechanisms of these devices have remained unclear. Here, through the demonstration of van der Waals In₂Se₃-based planar ferroelectric memristors with the device resistance continuously tunable over three orders of magnitude, and by correlating device resistance states, ferroelectric domain configurations, and surface electric potential, the studies reveal that the resistive switching is controlled by the multidomain formations and the associated energy barriers between domains, as opposed to the commonly assumed Schottky barrier modulations at the metal-ferroelectric interface. The findings reveal new device physics through elucidating the microscopic operating mechanisms of this new generation of devices, and provide a critical guide for future device development and integration efforts.     

Enhanced Sb2Se3 solar cell performance through theory‐guided defect control

Defects present in the absorber layer largely dictate photovoltaic device performance. Recently, a binary photovoltaic material, Sb₂Se₃, has drawn much attention due to its low‐cost and nontoxic constituents and rapid performance promotion. So far, however, the intrinsic defects of Sb₂Se₃ remain elusive. Here, through a combined theoretical and experimental investigation, we revealed that shallow acceptors, Se_Sb antisites, are the dominant defects in Sb₂Se₃ produced in an Se‐rich environment, where deep donors, Sb_Se and V_Se, dominate in Sb₂Se₃ produced in an Se‐poor environment. We further constructed a superstrate CdS/Sb₂Se₃ thin‐film solar cell achieving 5.76% efficiency through in situ Se compensation during Sb₂Se₃ evaporation and through careful optimization of absorber layer thickness. The understanding of intrinsic defects in Sb₂Se₃ film and the demonstrated success of in situ Se compensation strategy pave the way for further efficiency improvement of this very promising photovoltaic technology. Copyright © 2017 John Wiley & Sons, Ltd.     

Room temperature NO2 gas sensing properties of DBSA doped PPy–WO3 hybrid nanocomposite sensor

We demonstrate the chemiresistive NO₂ gas sensor based on DBSA doped PPy–WO₃ hybrid nanocomposites operating at room temperature. The sensor was fabricated on glass substrate using simple and cost effective drop casting method. The gas sensing performance of sensor was studied for various toxic/flammable analytes like NO₂, C₂H₅OH, CH₃OH, H₂S and NH₃. The sensor shows higher selectivity towards NO₂ gas with 72% response at 100 ppm. Also the sensor can successfully detect low concentration of NO₂ gas upto 5 ppm with reasonable response of 12%. Structural, morphological and compositional analyses evidenced the successful formation of DBSA doped PPy–WO₃ hybrid nanocomposite with uniform dispersion of DBSA into PPy–WO₃ hybrid nanocomposite and enhance the gas sensing behavior. We demonstrated that DBSA doped PPy–WO₃ hybrid nanocomposite sensor films shows excellent reproducibility, high stability, moderate response and recovery time for NO₂ gas in the concentration range of 5–100 ppm. A gas sensing mechanism based on the formation of random nano p–n junctions distributed over the surface of the sensor film has been proposed. In addition modulation of depletion width takes place in sensor on interaction with the target NO₂ gas has been depicted on the basis of schematic energy band diagram. Impedance spectroscopy was employed to study bulk, grain boundary resistance and capacitance before and after exposure of NO₂ gas. The structural and intermolecular interaction within the hybrid nanocomposites were explored by Raman and X-ray photoelectron spectroscopy (XPS), while field emission scanning electron microscopy (FESEM) was used to characterize surface morphology. The present method can be extended to fabricate other organic dopent-conducting polymer–metal oxide hybrid nanocomposite materials and could find better application in the gas sensing.     

Nonvolatile Ferroelectric Memory Effect in Ultrathin α‐In2Se3

High‐density memory is integral in solid‐state electronics. 2D ferroelectrics offer a new platform for developing ultrathin electronic devices with nonvolatile functionality. Recent experiments on layered α‐In₂Se₃ confirm its room‐temperature out‐of‐plane ferroelectricity under ambient conditions. Here, a nonvolatile memory effect in a hybrid 2D ferroelectric field‐effect transistor (FeFET) made of ultrathin α‐In₂Se₃ and graphene is demonstrated. The resistance of the graphene channel in the FeFET is effectively controllable and retentive due to the electrostatic doping, which stems from the electric polarization of the ferroelectric α‐In₂Se₃. The electronic logic bit can be represented and stored with different orientations of electric dipoles in the top‐gate ferroelectric. The 2D FeFET can be randomly rewritten over more than 10⁵ cycles without losing the nonvolatility. The approach demonstrates a prototype of rewritable nonvolatile memory with ferroelectricity in van der Waals 2D materials.     

纳米氧化钨薄膜改性的大孔硅气敏传感器

通过双槽电化学腐蚀法在P型单晶硅表面制备了大孔硅。然后通过直流对靶反应磁控溅射法在大孔硅表面淀积了纳米氧化钨薄膜。使用场发射扫描电子显微镜（FESEM）观察大孔硅和氧化钨/大孔硅样品的形貌。分别使用X射线衍射（XRD）图谱和X射线光电子能谱（XPS）分析氧化钨晶体结构和钨的化合价。在室温下测试大孔硅和氧化钨/大孔硅气敏传感器的气敏特性。结果表明：氧化钨/大孔硅气敏传感器表现出了P型半导体气敏传感器的气敏特性。它对1ppm的二氧化氮显示了良好的恢复特性和重复性。氧化钨/大孔硅气敏传感器的长期稳定性要好于大孔硅气敏传感器。氧化钨的添加提高了大孔硅气敏传感器对二氧化氮的灵敏度。氧化钨/大孔硅气敏传感器对于二氧化氮的灵敏度要高于其对氨气和乙醇的灵敏度。通过淀积纳米氧化钨薄膜,改善了大孔硅对二氧化氮的选择性。     

Improving the performance of Sb2Se3 thin film solar cells over 4% by controlled addition of oxygen during film deposition

Sb₂Se₃ has attracted great research interest very recently as a promising absorber material for thin film photovoltaics due to its suitable bandgap, high absorption coefficient, and non‐toxic, low cost, and earth abundant nature. In this work, a significant efficiency improvement to 4.8% of superstrate cadmium sulfide (CdS)/Sb₂Se₃ solar cells is obtained by the controlled addition of oxygen during thermal evaporation of Sb₂Se₃ films. Systematic materials and device physics characterization reveal that oxygen addition during Sb₂Se₃ film evaporation significantly improves the CdS/Sb₂Se₃ heterojunction quality through effective passivation of interfacial defect states, resulting in a substantial enhancement in device circuit voltage and short circuit current density. The 4.8% device is the highest efficiency thus far reported for Sb₂Se₃ thin film solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Ultrathin Non‐van der Waals Magnetic Rhombohedral Cr2S3: Space‐Confined Chemical Vapor Deposition Synthesis and Raman Scattering Investigation

Two dimensional (2D) magnetic materials display enormous application potential in spintronic fields. However, most of currently reported magnetic materials are van der Waals layered structure that is easy to be isolated via exfoliation method. By contrast, the studies on non‐van der Waals ultrathin magnetic materials are rare, largely due to the difficulty in fabrication. Rhombohedral Cr₂S₃, an intensively studied antiferromagnetic transition metal chalcogenide with Neel temperature of ≈120 K, has a typical non‐van der Waals structure. Restricted by the strong covalent bonding in all the three dimensions of non‐van der Waals structure, the synthesis of ultrathin Cr₂S₃ single crystals is still a challenge that is not achieved yet. Besides, the study on the Raman modes of rhombohedral Cr₂S₃ is also absent. Herein, by employing space‐confined chemical vapor deposition strategy, ultrathin rhombohedral Cr₂S₃ single crystals with a thickness down to ≈2.5 nm for the first time are successfully grown. Moreover, a systematically investigation is also conducted on the Raman vibrations of ultrathin rhombohedral Cr₂S₃. With the aid of angle‐resolved polarized Raman technique, the Raman modes of rhombohedral Cr₂S₃ for the first time based on crystal symmetry and Raman selection rules are rationally assigned.     

Carrier Transport Enhancement Mechanism in Highly Efficient Antimony Selenide Thin-Film Solar Cell

Exhibiting outstanding optoelectronic properties, antimony selenide (Sb₂Se₃) has attracted considerable interest and has been developed as a light absorber layer for thin-film solar cells over the decade. However, current state-of-the-art Sb₂Se₃ devices suffer from unsatisfactory “cliff-like” band alignment and severe interface recombination loss, which deteriorates device performance. In this study, the heterojunction interface of an Sb₂Se₃ solar cell is improved by introducing effective aluminum (Al³⁺) cation into the CdS buffer layer. Then, the energy band alignment of Sb₂Se₃/CdS:Al heterojunction is modified from a “cliff-like” structure to a “spike-like” structure. Finally, heterojunction interface engineering suppresses recombination losses and strengthens carrier transport, resulting in a high efficiency of 8.41% for the substrate-structured Sb₂Se₃ solar cell. This study proposes a facile strategy for interfacial treatment and elucidates the related carrier transport enhancement mechanism, paving a bright avenue to overcome the efficiency bottleneck of Sb₂Se₃ thin-film solar cells.     

Giant and Nonvolatile Control of Exchange Bias in Fe3GeTe2/Irradiated Fe3GeTe2/MgO Heterostructure Through Ultralow Voltage

The discovery of van der Waals magnets has provided a new platform for the electrical control of magnetism. Recent experiments have demonstrated that the magnetic properties of van der Waals magnets can be tuned by various gate modulations, although most of them are volatile and require gate voltages no lower than several volts. Here, the realization of nonvolatile control of exchange bias and coercive fields in Fe₃GeTe₂/MgO heterostructures, and the gate voltage is as low as tens of mV which is two orders of magnitude smaller than those in previous experiments is presented. The discovery of an ionic-irradiated phase formed in Fe₃GeTe₂ by MgO sputtering revealed that an exchange bias effect can be obtained in this heterostructure and tuned from ≈700 to 0 Oe through voltages ranging from 5 to 20 mV. Owing to the high stability of oxidized Fe₃GeTe₂, the voltage-driven oxygen incorporated into Fe₃GeTe₂ from the irradiated phase induces a nonvolatile magnetism modulation that can be retained after turning off the gate voltage. These findings demonstrate a methodology to modulate the magnetism of van der Waals magnets, opening new opportunities to fabricate all-solid, long-retention, and low-dissipation nano-electronic devices using van der Waals materials.     

Spin-Orbit Readout Using Thin Films of Topological Insulator Sb2Te3 Deposited by Industrial Magnetron Sputtering

Driving a spin-logic circuit requires the production of a large output signal by spin-charge interconversion in spin-orbit readout devices. This should be possible by using topological insulators, which are known for their high spin-charge interconversion efficiency. However, high-quality topological insulators have so far only been obtained on a small scale, or with large scale deposition techniques that are not compatible with conventional industrial deposition processes. The nanopatterning and electrical spin injection into these materials have also proven difficult due to their fragile structure and low spin conductance. The fabrication of a spin-orbit readout device from the topological insulator Sb₂Te₃ deposited by large-scale industrial magnetron sputtering on SiO₂ is presented. Despite a modification of the Sb₂Te₃ layer structural properties during the device nanofabrication, a sizeable output voltage is measured that can be unambiguously ascribed to a spin-charge interconversion process. The results pave the way for the integration of layered van der Waals materials in spin-logic devices.     

Nitrogen dioxide (NO2) sensing performance of p-polypyrrole/n-tungsten oxide hybrid nanocomposites at room temperature

Polypyrrole (PPy)–tungsten oxide (WO₃) hybrid nanocomposite have been successfully synthesized using different weight percentages of tungsten oxide (10–50%) dispersed in polypyrrole matrix by solid state synthesis method. The sensor based on PPy–WO₃ was fabricated on glass substrate using cost effective spin coating method for detection of NO₂ gas in the low concentration range of 5–100 ppm. The gas sensing performance of hybrid material was studied and compared with those of pure PPy and WO₃. It was found that PPy–WO₃ hybrid nanocomposite sensor can complement the drawbacks of pure PPy and WO₃. The structure, morphology and surface composition properties of PPy–WO₃ hybrid nanocomposites were employed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The presence of WO₃ in PPy matrix and their interaction was confirmed using XRD, FTIR techniques. The porous surface morphology was observed with addition of WO₃ in PPy matrix which is useful morphology for gas sensing applications. TEM image of PPy–WO₃ hybrid nanocomposites shows the average diameter of 80–90 nm. X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical composition of nanocomposites. It was observed that 50% WO₃ loaded PPy sensor operating at room temperature exhibit maximum response of 61% towards 100 ppm of NO₂ gas and able to detect low concentration of 5 ppm NO₂ gas with reasonable response of 8%. The hybrid sensor shows better sensitivity, selectivity, reproducibility and stability compared to pure PPy and WO₃. The proposed sensing mechanism of hybrid nanocomposite in presence of air and NO₂ atmosphere was discussed with the help of energy band diagram. Furthermore, the interaction of NO₂ gas with PPy–WO₃ hybrid nanocomposites sensor was studied by cole–cole plot using impedance spectroscopy.     

Layered Sb2Te3 Nanoflakes as Chalcogenide Dielectrics

Dielectric nanoflakes of Sb₂Te₃ represent an important advance in science and technology due to their extraordinary properties. Polycrystalline layered Sb₂Te₃ nanoflakes have been successfully synthesized via a high-throughput chemical route at 60°C. The frequency and temperature dependence of the dielectric constant and dielectric loss of the layered Sb₂Te₃ nanoflakes have been measured in the frequency range from 30 Hz to 758,000 Hz and temperature range from 313 K to 373 K. As-synthesized Sb₂Te₃ nanoflakes are shown to be promising alternative dielectrics because of their high dielectric constant (ε′ ≈ 7.3 to 6022) and low dielectric loss (tan δ ≈ 0.2 to 9.2). These higher values of ε′ and lower values of tan δ of Sb₂Te₃ nanoflakes confirm that capacitors with capacity (C) of ～5.2 pF to 4336 pF may be fabricated for storing renewable energy. Raman spectroscopy confirms that the peak located at ～142 cm^?1 corresponds to one in-plane vibrational mode (E _g ² ) of layered Te–Sb–Te–Sb–Te lattice vibration.     

In situ sulfurization to generate Sb2(Se1 − xSx)3 alloyed films and their application for photovoltaics

Because of its tunable band gap and band position, Sb₂(Se_{1 − x} S_x )₃ (0 ≤ x ≤ 1) is a promising light‐absorbing material for photovoltaic device applications. However, no systematic study on the synthesis and characterization of single‐phase polycrystalline Sb₂(Se_{1 − x} S_x )₃ thin films has been reported. Through introducing in situ sulfurization into the rapid thermal evaporation process, a series of single‐phase, highly crystalline Sb₂(Se_{1 − x} S_x )₃ films with x = 0.09, 0.20, 0.31, and 0.43 were successfully obtained, with the corresponding band gap, band position and film morphology fully revealed. Futhermore, solar cells with superstrate ITO/CdS/Sb₂(Se_{1 − x} S_x )₃/Au structure were fabricated and carefully optimized. Finally, a champion device having 5.79% solar conversion efficiency was obtained employing uniform Sb₂(Se_0.80S_0.20)₃ absorber layer. Our experimental investigation confirmed that Sb₂(Se_{1 − x} S_x )₃ is indeed a very promising absorber material worth further optimization. Copyright © 2016 John Wiley & Sons, Ltd.     

Epitaxial Growth of 2D Bi2O2Se Nanoplates/1D CsPbBr3 Nanowires Mixed-Dimensional Heterostructures with Enhanced Optoelectronic Properties

The 2D/1D mixed-dimensional van der Waals heterostructures have great potential for electronics and optoelectronics with high performance and multifunctionality. The epitaxy of 1D micro/nanowires on 2D layered materials may efficiently realize the large-scale preparation of 2D/1D heterostructures, which is critically important for their practical applications. So far, however, only the wires of Bi₂S₃, Te, and Sb₂Se₃ have been epitaxially grown on MoS₂ or WS₂. Here, it is reported that the epitaxial growth of 1D CsPbBr₃ nanowires on 2D Bi₂O₂Se nanoplates through a facile vertical vapor deposition method. The CsPbBr₃ wires are well aligned on the Bi₂O₂Se plates in fourfold symmetry with the epitaxial relationships of [001]_CsPbBr3||[200]_Bi2O2Se and [1-10]_CsPbBr3||[020]_Bi2O2Se. The photoluminescence results reveal that the emission from CsPbBr₃ is significantly quenched in the heterostructure, which implies the charge carriers transfer from CsPbBr₃ to Bi₂O₂Se. The waveguide characterization shows that the epitaxial CsPbBr₃ wires may efficiently confine and guide their emission, which favors the light absorption of Bi₂O₂Se. Importantly, the photocurrent mapping and spectra of the devices based on these 2D/1D heterostructures prove that the epitaxial CsPbBr₃ wires remarkably enhances the photoresponse of Bi₂O₂Se, which indicates these heterostructures can be applied in high-performance optoelectronic devices or on-chip integrated photonic circuits.     

A comparative study on the electronic and optical properties of Sb<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin film

The thin film of Sb₂Se₃ was deposited by thermal evaporation method and the film was annealed in N₂ flow in a three zone furnace at a temperature of 290°С for 30 min. The structural properties were characterized by scanning electron microscopy (SEM), transmission electron microscopy (ТЕМ), X-ray diffraction (XRD) and Raman spectroscopy, respectively. It is seen that the as-deposited film is amorphous and the annealed film is polycrystalline in nature. The surface of Sb₂Se₃ film is oxidized with a thickness of 1.15 nm investigated by X-ray photolecetron spectroscopy (XPS) measurement. Spectroscopic ellipsometry (SE) and UV–vis spectroscopy measurements were carried out to study the optical properties of Sb₂Se₃ film. In addition, the first principles calculations were applied to study the electronic and optical properties of Sb₂Se₃. From the theoretical calculation it is seen that Sb₂Se₃ is intrinsically an indirect band gap semiconductor. Importantly, the experimental band gap is in good agreement with the theoretical band gap. Furthermore, the experimental values of n, k, ε₁, and ε₂ are much closer to the theoretical results. However, the obtained large dielectric constants and refractive index values suggest that exciton binding energy in Sb₂Se₃ should be relatively small and an antireflective coating is recommended to enhance the light absorption of Sb₂Se₃ for thin film solar cells application.     

Electrical Conduction at the Interface between Insulating van der Waals Materials

Emergent properties of 2D materials attract considerable interest in condensed matter physics and materials science due to their distinguished features that are missing in their bulk counterparts. A mainstream in this research field is to broaden the scope of material to expand the horizons of the research area, while developing functional interfaces between different 2D materials is another indispensable research direction. Here, the emergence of electrical conduction at the interface between insulating 2D materials is demonstrated. A new class of van der Waals heterostructures consisting of two sets of insulating transition‐metal dichalcogenides, group‐VI WSe₂ and group‐IV TMSe₂ (TM = Zr, Hf), is developed via molecular‐beam epitaxy, and it is found that those heterostructures are highly conducting although all the constituent materials are highly insulating. The WSe₂/ZrSe₂ interface exhibits more conducting behavior than the WSe₂/HfSe₂ interface, which can be understood by considering the band alignments between constituent materials. Moreover, by increasing Se flux during heterostructure fabrication, the WSe₂/ZrSe₂ interface becomes more conducting, reaching nearly metallic behavior. Further improvement of the crystalline quality as well as exploring different material combinations are expected to lead to metallic conduction, providing a novel functionality emerging at van der Waals heterostructures.