Out-of-Plane Resistance Switching of 2D Bi2O2Se at the Nanoscale

Two-dimensional (2D) bismuth oxyselenide (Bi₂O₂Se) with high electron mobility shows great potential for nanoelectronics. Although the in-plane properties of Bi₂O₂Se have been widely studied, its out-of-plane electrical transport behavior remains elusive, despite its importance in fabricating devices with new functionality and high integration density. Here, the out-of-plane electrical properties of 2D Bi₂O₂Se at nanoscale are revealed by conductive atomic force microscope. This work finds that hillocks with tunable heights and sizes are formed on Bi₂O₂Se after applying a vertical electric field. Intriguingly, such hillocks are conductive in the vertical direction, resulting in a previously unknown out-of-plane resistance switching in thick Bi₂O₂Se flakes while ohmic conductive characteristic in thin ones. Furthermore, the transformation is observed from bipolar to stable unipolar conduction in thick Bi₂O₂Se flake possessing such hillocks, suggesting its potential to function as a selector in vertical devices. This work reveals the unique out-of-plane transport behavior of 2D Bi₂O₂Se, providing the basis for fabricating vertical devices based on this emerging 2D material.     

Solid-Phase Reactions and Phase Transformations in a Nanoscale Bismuth/Selenium Film Structure

Experimental results of a study concerned with solid-phase reactions and phase transformations in a Bi/Se nanoscale film structure under heat treatment in vacuum are presented. Nanocrystalline Bi2Se3, BiSe, and Bi₄Se₃ films are obtained for the first time by solid-phase synthesis at various ratios between the Bi and Se layer thicknesses. The phase-transformation temperatures at which Se, BiSe, and Bi4Se3 crystalline phases are formed are determined. The average crystallite sizes in the Bi₂Se₃, BiSe, and Bi₄Se₃ films are found to be 21, 23, and 33 nm, respectively.     

Ultrahigh Responsivity of Ternary Sb–Bi–Se Nanowire Photodetectors

High‐quality single‐crystalline ternary (Sb_1‐xBi_x)₂Se₃ nanowires (NWs) (x = 0–0.88) are synthesized by chemical vapor deposition. Nanowires with x from 0 to 0.75 are indexed as an orthorhombic structure. With increasing Bi incorporation ratio, (Sb_1‐xBi_x)₂Se₃ NWs exhibit remarkable photoresponsivities, which originate from growing surface Se vacancies and augmented oxygen chemisorptions. Notably, spectra responsivity and external quantum efficiency of an (Sb_0.44Bi_0.56)₂Se₃ NW photodetector reach as high as ≈8261.4 A/W and ≈1.6 × 10⁶ %, respectively. Those excellent performances unambiguously demonstrate that Sb–Bi–Se NWs are promising for the utilizations of high‐sensitivity and high‐speed photodetectors and photoelectronic switches.     

Metabolizable Bi2Se3 Nanoplates: Biodistribution,Toxicity, and Uses for Cancer Radiation Therapy and Imaging

Bi, a high atomic number element, has a high photoelectric absorption coefficient, and Se has anticancer activity. Hence, their compound chalcogenide (Bi₂Se₃) deserves a thorough investigation for biomedical applications. This study reveals that Bi₂Se₃ nanoplates (54 nm wide) protected with poly(vinylpyrollidone) (PVP) could be presumed to have low toxicity even at a high dose of 20 mg/kg in mice. This conclusion is made through studies on the biodistribution and 90‐day long term in vivo clearance of the nanoplates. The liver and spleen are dominant organs for accumulation of the nanoplates, which is mainly due to RES absorption. 93% of the nanoplates are cleared after 90 days of treatment. Concentrations of Bi and Se in tumor tissue continuously increased until 72 h after intraperitoneal injection into mice. Such selective accumulation of Bi is utilized to enhance the contrast of X‐ray computerized tomography (CT) images. Bi element concentrated in a tumor leads to damage on the tumor cells when exposed to gamma radiation. Growth of the tumor is significantly delayed and stopped in 16 days after the tumor is treated by radiation with Bi₂Se₃ nanoplates. This work clearly shows that Bi₂Se₃ nanoplates may be used for cancer radiation therapy and CT imaging. The nanoplates deserve further study for biological and medical applications.     

Controlled Vapor–Solid Deposition of Millimeter‐Size Single Crystal 2D Bi2O2Se for High‐Performance Phototransistors

Atomically thin 2D materials have received intense interest both scientifically and technologically. Bismuth oxyselenide (Bi₂O₂Se) is a semiconducting 2D material with high electron mobility and good stability, making it promising for high‐performance electronics and optoelectronics. Here, an ambient‐pressure vapor–solid (VS) deposition approach for the growth of millimeter‐size 2D Bi₂O₂Se single crystal domains with thicknesses down to one monolayer is reported. The VS‐grown 2D Bi₂O₂Se has good crystalline quality, chemical uniformity, and stoichiometry. Field‐effect transistors (FETs) are fabricated using this material and they show a small contact resistivity of 55.2 Ω cm measured by a transfer line method. Upon light irradiation, a phototransistor based on the Bi₂O₂Se FETs exhibits a maximum responsivity of 22 100 AW^?1, which is a record among currently reported 2D semiconductors and approximately two orders of magnitude higher than monolayer MoS₂. The Bi₂O₂Se phototransistor shows a gate tunable photodetectivity up to 3.4 × 10¹⁵ Jones and an on/off ratio up to ≈10⁹, both of which are much higher than phototransistors based on other 2D materials reported so far. The results of this study indicate a method to grow large 2D Bi₂O₂Se single crystals that have great potential for use in optoelectronic applications.     

Seed‐Induced Vertical Growth of 2D Bi2O2Se Nanoplates by Chemical Vapor Transport

As two‐dimensional (2D) layered materials attract more attention owing to their unique optical, electrical, and thermal properties, there are persistent efforts to grow high‐quality 2D layered materials for fundamental research and device applications. While large‐area 2D layered materials with high crystal quality can be obtained through chemical vapor transport, the strong binding between 2D layered materials and substrates poses a significant challenge for attempts to reveal their intrinsic properties and to use these 2D building blocks for constructing advanced heterostructured devices. Therefore, it would be ideal to grow high‐quality 2D materials with minimized contact and binding with substrate. Through both calculation and experiment, it is demonstrated that by introducing a seed layer at the nucleation stage, the crystallographic disregistry and the corresponding adhesion energy between 2D materials and substrate can be altered, resulting in a change of crystal surface in contact with the substrate, and therefore vertical growth of 2D materials on substrates. As an example, it is demonstrated that with Bi₂O₃ serving as a seed layer, vertical growth of 2D plates of Bi₂O₂Se on mica substrates can be realized. These vertically grown 2D nanoplates of Bi₂O₂Se can be conveniently transferred with their thermal properties investigated for the first time.     

Monolayer Epitaxial Heterostructures for Selective Visible‐Light‐Driven Photocatalytic NO Oxidation

Construction of vertical heterostructures by stacking two‐dimensional (2D) layered materials via chemical bonds can be an effective strategy to explore advanced solar‐energy‐conversion systems. However, it remains a great challenge to fabricate such heterostructures based on conversional oxide‐based compounds, as they either do not possess a 2D layered structure or are not suitable for epitaxial growth due to large lattice mismatch. Here, a vertical heterostructure of bismuth oxyhalide semiconductors fabricated through a heteroepitaxial anion exchange method is reported. Monolayer Bi₂WO₆ is epitaxially grown on the exposed surface of BiOI to inhibit photocorrosion and introduce active sites. Theoretical and experimental results reveal that electrons generated under visible‐light irradiation can directly transfer to surface coordinatively unsaturated (CUS) Bi atoms, which contribute to the adsorption and activation of reactant molecules. As a result, the Bi₂WO₆/BiOI vertical heterostructures exhibit significantly enhanced visible‐light‐driven NO oxidation activity compared with BiOI and Bi₂WO₆.     

Submillimeter 2D Bi2Se3 Flakes toward High‐Performance Infrared Photodetection at Optical Communication Wavelength

Infrared detection at optical communication wavelength is of great significance because of their diverse commercial and military communication applications. The layered Bi₂Se₃ with a narrow band gap of 0.3 eV is regarded as a promising candidate toward high‐performance terahertz to infrared applications. However, the controllable synthesis of large‐size ultrathin Bi₂Se₃ flakes remains a challenge owing to complex nucleation process and infrared telecommunication photodetectors based on Bi₂Se₃ flakes are rarely reported. Here, large size (submillimeter: 0.2–0.4 mm in lateral dimensions) and ultrathin (thickness: 3 nm to few nanometers) 2D Bi₂Se₃ flakes with high crystal quality are obtained by suppressing the nucleation density. More importantly, back‐gate field‐effect transistor based on Bi₂Se₃ flake exhibits an ultrahigh on/off current ratio of 10⁶ and competitive mobility of 39.4 cm² V^?1 s^?1. Moreover, excellent on/off ratio of 972.5, responsivity of 23.8 A W^?1, and external quantum efficiency of 2035% are obtained from Bi₂Se₃‐based photodetector at 1456 nm in the E‐band of the telecommunication range. With controlled morphology and excellent photoresponse performance, the Bi₂Se₃ photodetector shows great potential in the optoelectronic field including communications, military, and remote sensing.     

Broadband Bi2O2Se Photodetectors from Infrared to Terahertz

2D Bi₂O₂Se has shown great potential in photodetector from visible to infrared (IR) owing to its high mobility, ambient stability, and layer-tunable bandgaps. However, for the terahertz (THz) band with longer wavelength and richer spectral information, there are few reports on the research of THz detection based on 2D materials. Herein, an antenna-assisted Bi₂O₂Se photodetector is constructed to achieve broadband photodetection from IR to THz ranges driven by multi-mechanism of electromagnetic waves to electrical conversion. The good tradeoff between the bandgap and high mobility results in a broad spectral detection. In the IR region, the nonequilibrium carriers result from photo-induced electron-hole pairs in the Bi₂O₂Se body. While in the THz region, the carriers are caused by the injected electrons from the metal electrodes by the electromagnetic-induced well. The Bi₂O₂Se photodetector achieves a broadband responsivity of 58 A W^-1 at 1550 nm, 2.7 × 10⁴ V W^-1 at 0.17 THz, and 1.9 × 10⁸ V W^-1 at 0.029 THz, respectively. Surprisingly, an ultrafast response time of 476 ns and a quite low noise equivalent power of 0.2 pW Hz^−1/2 are acquired at room temperature. Our researches exhibit promising prospects of Bi₂O₂Se in broadband detection, THz imaging, and ultrafast sensing.     

High‐Performance Near‐Infrared Photodetector Based on Ultrathin Bi2O2Se Nanosheets

As an emerging 2D layered material, Bi₂O₂Se has shown great potential for applications in thermoelectric and electronics, due to its high carrier mobility, near‐ideal subthreshold swing, and high air‐stability. Although Bi₂O₂Se has a suitable band gap for infrared (IR) applications, its photoresponse properties have not been investigated. Here, high‐quality ultrathin Bi₂O₂Se sheets are synthesized via a low‐pressure chemical vapor deposition method. The thickness of 90% Bi₂O₂Se sheets is below 10 nm and lateral sizes mainly distribute in the range of 7–11 µm. In addition, it is found that triangular sheets largely lack “O” content, even only 0.2 for Bi₂O_0.2Se. The near‐IR photodetection performance of Bi₂O₂Se nanosheets is systematically studied by variable temperature measurements. The response time, responsivity, and detectivity can approach up to 2.8 ms, 6.5 A W^?1, and 8.3 × 10¹¹ Jones, respectively. Additionally, the critical performance parameters, including responsivity, rising time, and decay time, remain at almost the same level when the temperature is changed from 80 to 300 K. These phenomena are likely due to the fact that as‐grown ultrathin Bi₂O₂Se sheets have no surface trap states and shallow defect energy levels. The findings indicate ultrathin Bi₂O₂Se sheets have great potentials for future applications in ultrafast, flexible near‐IR optoelectronic devices.     

Effect of Se content on the structural,morphological and optical properties of Bi2Te3−ySey thin films electrodeposited by under potential deposition technique

Bi₂Te_3−ySe_y thin films with different Se contents ranging from 0.3 to 2.5 were successfully electrodeposited by under potential deposition (UPD) technique onto gold foil substrates from an electrolyte consist of Bi(NO₃)₃, TeO₂, and SeO₂ at ambient conditions. The effects of Se content on structural, morphological and optical properties of the products were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and FT-IR spectroscopy, respectively. The XRD analysis revealed that the diffraction peaks positions of Bi₂Te_3−ySe_y thin films shifts gradually towards the higher angle side due to replacement of Te by Se atoms in the crystal structure with increasing Se content. The SEM results showed that the particle size of Bi₂Te_3−ySe_y thin films decreased as the Se content increased. The optical constants of ternary Bi₂Te_3−ySe_y thin films such as refractive index, extinction coefficient, and dielectric constant were obtained from the transmission spectra in the range of 2500–10.000 nm. The direct allowed band gap energies were estimated using Tauc equation and found to increase from 0.210 to 0.282 eV with increasing Se content from 0.3 to 2.5. The dispersion behavior of refractive index was studied by the single oscillator Wemple-DiDomenico model.     

Stoichiometry Controlled,Single‐Crystalline Bi2Te3 Nanowires for Transport in the Basal Plane

Thermoelectric Bi₂Te₃ based bulk materials are widely used for solid‐state refrigeration and power‐generation at room temperature. For low‐dimensional and nanostructured thermoelectric materials an increase of the thermoelectric figure of merit ZT is predicted due to quantum confinement and phonon scattering at interfaces. Therefore, the fabrication of Bi₂Te₃ nanowires, thin films, and nanostructured bulk materials has become an important and active field of research. Stoichiometric Bi₂Te₃ nanowires with diameters of 50–80 nm and a length of 56 μm are grown by a potential‐pulsed electrochemical deposition in a nanostructured Al₂O₃ matrix. By transmission electron microscopy (TEM), dark‐field images together with electron diffraction reveal single‐crystalline wires, no grain boundaries can be detected. The stoichiometry control of the wires by high‐accuracy, quantitative enegy‐dispersive X‐ray spectroscopy (EDX) in the TEM instrument is of paramount importance for successfully implementing the growth technology. Combined electron diffraction and EDX spectroscopy in the TEM unambiguously prove the correct crystal structure and stoichiometry of the Bi₂Te₃ nanowires. X‐ray and electron diffraction reveal growth along the [110] and [210] directions and the c axis of the Bi₂Te₃ structure lies perpendicular to the wire axis. For the first time single crystalline, stoichiometric Bi₂Te₃ nanowires are grown that allow transport in the basal plane without being affected by grain boundaries.     

Catalyst-Free Growth of Atomically Thin Bi2O2Se Nanoribbons for High-Performance Electronics and Optoelectronics

1D materials have attracted significant research interest due to their unique quantum confinement effects and edge-related properties. Atomically thin 1D nanoribbons are particularly interesting because it is a valuable platform with the physical limits of both thickness and width. Here, a catalyst-free growth method is developed and the growth of Bi₂O₂Se nanostructures with tunable dimensionality is achieved. Significantly, Bi₂O₂Se nanoribbons with a thickness down to 0.65 nm, corresponding to a monolayer, are successfully grown for the first time. Electrical and optoelectronic measurements show that Bi₂O₂Se nanoribbons possess decent performance in terms of mobility, on/off ratio, and photoresponsivity, suggesting their promise for devices. This work not only reports a new method for the growth of atomically thin nanoribbons but also provides a platform to study properties and applications of such nanoribbon materials at a thickness limit.     

Sensitive and Ultrabroadband Phototransistor Based on Two‐Dimensional Bi2O2Se Nanosheets

Bi₂O₂Se, a high‐mobility and air‐stable 2D material, has attracted substantial attention for application in integrated logic electronics and optoelectronics. However, achieving an overall high performance over a wide spectral range for Bi₂O₂Se‐based devices remains a challenge. A broadband phototransistor with high photoresponsivity (R) is reported that comprises high‐quality large‐area ( ≈ 180 µm) Bi₂O₂Se nanosheets synthesized via a modified chemical vapor deposition method with a face‐down configuration. The device covers the ultraviolet (UV), visible (Vis), and near‐infrared (NIR) wavelength ranges (360–1800 nm) at room temperature, exhibiting a maximum R of 108 696 A W^?1 at 360 nm. Upon illumination at 405 nm, the external quantum efficiency, R, and detectivity (D*) of the device reach up to 1.5 × 10⁷%, 50055 A W^?1, and 8.2 × 10¹² Jones, respectively, which is attributable to a combination of the photogating, photovoltaic, and photothermal effects. The devices reach a ?3 dB bandwidth of 5.4 kHz, accounting for a fast rise time (τ_rise) of 32 µs. The high sensitivity, fast response time, and environmental stability achieved simultaneously in these 2D Bi₂O₂Se phototransistors are promising for high‐quality UV and IR imaging applications.     

Self-Driven WSe2/Bi2O2Se Van der Waals Heterostructure Photodetectors with High Light On/Off Ratio and Fast Response

Benefiting from the superior electron mobility and good air-stability, the emerging layered bismuth oxyselenide (Bi₂O₂Se) nanosheet has received considerable attention with the promising prospects for electronics and optoelectronics applications. However, the high charge carrier concentration and bolometric effect of Bi₂O₂Se give rise to the high dark current and relatively slow photoresponse, which severely impede further improvement of the performance of Bi₂O₂Se based photodetectors. Here, a WSe₂/Bi₂O₂Se Van der Waals p-n heterostructure is reported with a pronounced rectification ratio of 10⁵ and a low reverse dark current of 10⁻¹¹ A, as well as an enhanced light on/off ratio up to 618 under 532 nm light illumination. The device also exhibits a fast response speed of 2.6 µs and a broadband detection capability from 365 to 2000 nm due to the efficient charge separation and strong interlayer coupling at the interface of the two flakes. Importantly, the built-in potential in the WSe₂/Bi₂O₂Se heterostructure offers a competitive self-powered photodetector with the light on/off ratio above 10⁵ and a photovoltaic responsivity of 284 mA W⁻¹. The WSe₂/Bi₂O₂Se heterostructure shows promising potentials for high-performance self-driven photodetector applications.     

Optoelectronic Coincidence Detection with Two-Dimensional Bi2O2Se Ferroelectric Field-Effect Transistors

Information processing with optoelectronic devices provides an alternative way to efficiently process hybrid optical and electronic signals. Ferroelectric field-effect transistors (FeFETs) can effectively respond to external optical and electrical stimuli by modulating their polarization states. Here, a 2D FeFET is demonstrated by the epitaxial growth of high-quality 2D bismuth layered oxyselenide (Bi₂O₂Se) films on PMN-PT(001) ferroelectric single-crystal substrates. Upon switching the polarization direction of PMN-PT, the authors realize in situ, reversible, and nonvolatile manipulation of the resistance of Bi₂O₂Se thin film ( ≈ 877%). The device simultaneously exhibits a polarization-dependent photoresponse through visible light (λ  =  405 nm) and infrared light (IR, λ  =  980 nm) illumination. Combining optical stimuli with ferroelectric gating, it is demonstrated that the devices not only show nonvolatile memory and optoelectronic responses, but also show coincidence detection of visible and IR light. This work holds great potential in constructing new multiresponse and multifunction 2D-FeFETs.     

Modulation Doping Leads to Optimized Thermoelectric Properties in n-Type Bi6Cu2Se4O6 due to Interface Effects

Heterogeneous composites consisting of Bi₆Cu₂Se_3.6Cl_0.4O₆ and Bi₂O₂Se are prepared according to the concept of modulation doping. With prominently increased carrier mobility and almost unchanged effective mass, the electrical transport properties are considerably optimized resulting in a peak power factor ≈1.8 µW cm⁻¹ K⁻² at 873 K, although the carrier concentration is slightly deteriorated. Meanwhile, the lattice thermal conductivity is lowered to ≈0.62 W m⁻¹ K⁻¹ due to the introduction of the second phase. The modified Self-consistent Effective Medium Theory is utilized to explain the deeper mechanism of modulation doping. The enhancement of apparent carrier mobility is derived from the highly active phase interfaces as fast carrier transport channels, while the reduced apparent thermal conductivity is ascribed to the existence of thermal resistance at the phase interfaces. Ultimately, an optimized ZT ≈0.23 is obtained at 873 K in Bi₆Cu₂Se_3.6Cl_0.4O₆ + 13% Bi₂O₂Se. This research demonstrates the effectiveness of modulation doping for optimizing thermoelectric properties once again, and provides the direct microstructure observation and consistent theoretical model calculation to emphasize the role of interface effects in modulation doping, which should be probably applicable to other thermoelectrics.     

FeSe2‐Decorated Bi2Se3 Nanosheets Fabricated via Cation Exchange for Chelator‐Free 64Cu‐Labeling and Multimodal Image‐Guided Photothermal‐Radiation Therapy

Multifunctional theranostic agents have become rather attractive to realize image‐guided combination cancer therapy. Herein, a novel method is developed to synthesize Bi₂Se₃ nanosheets decorated with mono‐dispersed FeSe₂ nanoparticles (FeSe₂/Bi₂Se₃) for tetra‐modal image‐guided combined photothermal and radiation tumor therapy. Interestingly, upon addition of Bi(NO₃)₃, pre‐made FeSe₂ nanoparticles via cation exchange would be gradually converted into Bi₂Se₃ nanosheets, on which remaining FeSe₂ nanoparticles are decorated. The yielded FeSe₂/Bi₂Se₃ composite‐nanostructures are then modified with polyethylene glycol (PEG). Taking advantages of the high r ₂ relaxivity of FeSe₂, the X‐ray attenuation ability of Bi₂Se₃, the strong near‐infrared optical absorbance of the whole nanostructure, as well as the chelate‐free radiolabeling of ⁶⁴Cu on FeSe₂/Bi₂Se₃‐PEG, in vivo magnetic resonance/computer tomography/photoacoustic/position emission tomography multimodal imaging is carried out, revealing efficient tumor homing of FeSe₂/Bi₂Se₃‐PEG after intravenous injection. Utilizing the intrinsic physical properties of FeSe₂/Bi₂Se₃‐PEG, in vivo photothermal and radiation therapy to achieve synergistic tumor destruction is then realized, without causing obvious toxicity to the treated animals. This work presents a unique method to synthesize composite‐nanostructures with highly integrated functionalities, promising not only for nano‐biomedicine but also potentially for other different nanotechnology fields.     

Molecular Beam Epitaxial Growth and Properties of Bi2Se3 Topological Insulator Layers on Different Substrate Surfaces

Growth of high-quality Bi₂Se₃ films is crucial not only for study of topological insulators but also for manufacture of technologically important materials. We report a study of the heteroepitaxy of single-crystal Bi₂Se₃ thin films grown on GaAs and InP substrates by use of molecular beam epitaxy. Surface topography, crystal structure, and electrical transport properties of these Bi₂Se₃ epitaxial films are indicative of highly c-axis oriented films with atomically sharp interfaces.     

Nanostructured β‐Bi2O3 Fractals on Carbon Fibers for Highly Selective CO2 Electroreduction to Formate

3D Bi₂O₃ fractal nanostructures (f‐Bi₂O₃) are directly self‐assembled on carbon fiber papers (CFP) using a scalable hot‐aerosol synthesis strategy. This approach provides high versatility in modulating the physiochemical properties of the Bi₂O₃ catalyst by a tailorable control of its crystalline size, loading, electron density as well as providing exposed stacking of the nanomaterials on the porous CFP substrate. As a result, when tested for electrochemical CO₂ reduction reactions (CO₂RR), these f‐Bi₂O₃ electrodes demonstrate superior conversion of CO₂ to formate (HCOO^?) with low onset overpotential and a high mass‐specific formate partial current density of ?52.2 mA mg^?1, which is ≈3 times higher than that of the drop‐casted control Bi₂O₃ catalyst (?15.5 mA mg^?1), and a high Faradaic efficiency (FE_HCOO^?) of 87% at an applied potential of ?1.2 V versus reversible hydrogen electrode. The findings reveal that the high exposure of roughened β‐phase Bi₂O₃/Bi edges and the improved electron density of these fractal structures are key contributors in attainment of high CO₂RR activity.