Highly Anisotropic Sb2Se3 Nanosheets: Gentle Exfoliation from the Bulk Precursors Possessing 1D Crystal Structure

2D materials, particularly those bearing in‐plane anisotropic optical and electrical properties such as black phosphorus and ReS₂, have spurred great research interest very recently as promising building blocks for future electronics. However, current progress is limited to layered compounds that feature atomic arrangement asymmetry within the covalently bonded planes. Herein, a series of highly anisotropic nanosheets (Sb₂Se₃, Sb₂S₃, Bi₂S₃, and Sb₂(S, Se)₃), which are composed of 1D covalently linked ribbons stacked together via van der Waals force, is introduced as a new member to the anisotropic 2D material family. These unique anisotropic nanosheets are successfully fabricated from their polymer‐like bulk counterparts through a gentle water freezing‐thawing approach. Angle‐resolved polarized Raman spectroscopy characterization confirms the strong in‐plane asymmetry of Sb₂Se₃ nanosheets, and photodetection study reveals their high responsivity and anisotropic in‐plane transport. This work can enlighten the synthesis and application of new anisotropic 2D nanosheets that can be potentially applied for future electronic and optoelectronic devices.     

Thermally Phase‐Transformed In2Se3 Nanowires for Highly Sensitive Photodetectors

The photoresponse characteristics of In₂Se₃ nanowire photodetectors with the κ‐phase and α‐phase structures are investigated. The as‐grown κ‐phase In₂Se₃ nanowires by the vapor‐liquid‐solid technique are phase‐transformed to the α‐phase nanowires by thermal annealing. The photoresponse performances of the κ‐phase and α‐phase In₂Se₃ nanowire photodetectors are characterized over a wide range of wavelengths (300–900 nm). The phase of the nanowires is analyzed using a high‐resolution transmission microscopy equipped with energy dispersive X‐ray spectroscopy and X‐ray diffraction. The electrical conductivity and photoresponse characteristics are significantly enhanced in the α‐phase due to smaller bandgap structure compared to the κ‐phase nanowires. The spectral responsivities of the α‐phase devices are 200 times larger than those of the κ‐phase devices. The superior performance of the thermally phase‐transformed In₂Se₃ nanowire devices offers an avenue to develop highly sensitive photodetector applications.     

Orientation Engineering in Low‐Dimensional Crystal‐Structural Materials via Seed Screening

The orientation of low‐dimensional crystal‐structural (LDCS) films significantly affects the performance of photoelectric devices, particularly in vertical conducting devices such as solar cells and light‐emitting diodes. According to film growth theory, the initial seeds determine the final orientation of the film. Ruled by the minimum energy principle, lying (chains or layers parallel to the substrate) seeds bonding with the substrate through van der Waals forces are easier to form than standing (chains or layers perpendicular to the substrate) seeds bonding with the substrate by a covalent bond. Utilizing high substrate temperature to re‐evaporate the lying seeds and preserve the standing seeds, the orientation of 1D crystal‐structural Sb₂Se₃ is successfully controlled. Guided by this seed screening model, highly [211]‐ and [221]‐oriented Sb₂Se₃ films on an inert TiO₂ substrate are obtained; consequently, a record efficiency of 7.62% in TiO₂/Sb₂Se₃ solar cells is achieved. This universal model of seed screening provides an effective method for orientation control of other LDCS films.     

Facile Synthesis of γ‐In2Se3 Nanoflowers toward High Performance Self‐Powered Broadband γ‐In2Se3/Si Heterojunction Photodiode

An effective colloidal process involving the hot‐injection method is developed to synthesize uniform nanoflowers consisting of 2D γ‐In₂Se₃ nanosheets. By exploiting the narrow direct bandgap and high absorption coefficient in the visible light range of In₂Se₃, a high‐quality γ‐In₂Se₃/Si heterojunction photodiode is fabricated. This photodiode shows a high photoresponse under light illumination, short response/recovery times, and long‐term durability. In addition, the γ‐In₂Se₃/Si heterojunction photodiode is self‐powered and displays a broadband spectral response ranging from UV to IR with a high responsivity and detectivity. These excellent performances make the γ‐In₂Se₃/Si heterojunction very interesting as highly efficient photodetectors.     

From Strong Dichroic Nanomotor to Polarotactic Microswimmer

Light‐driven micro/nanomotors are promising candidates for long‐envisioned next‐generation nanorobotics for targeted drug delivery, noninvasive surgery, nanofabrication, and beyond. To achieve these fantastic applications, effective control of the micro/nanomotor is essential. Light has been proved as the most versatile method for microswimmer manipulation, while the light propagation direction, intensity, and wavelength have been explored as controlling signals for light‐responsive nanomotors. Here, the controlling method is expanded to the polarization state of the light, and a nanomotor with a significant dichroic ratio is demonstrated. Due to the anisotropic crystal structure, light polarized parallel to the Sb₂Se₃ nanowires is preferentially absorbed. The core–shell Sb₂Se₃/ZnO nanomotor exhibits strong dichroic swimming behavior: the swimming speed is ≈3 times faster when illuminated with parallel polarized light than perpendicular polarized light. Furthermore, by incorporating two cross‐aligned dichroic nanomotors, a polarotactic artificial microswimmer is achieved, which can be navigated by controlling the polarization direction of the incident light. Compared to the well‐studied light‐driven rotary motors based on optical tweezers, this dichroic microswimmer offers eight orders of magnitude light‐intensity reduction, which may enable large‐scale nanomanipulation as well as other heat‐sensitive applications.     

Suppression of Persistent Photoconductivity of Rubrene Crystals using Gate‐Tunable Rubrene/Bi2Se3 Diodes with Photoinduced Negative Differential Resistance

Organic single‐crystalline semiconductors show great potential in high‐performance photodetectors. However, they suffer from persistent photoconductivity (PPC) due to the charge trapping, which has severely hindered high‐speed imaging applications. Here, a universal strategy of solving the PPC by integrating with topological insulator Bi₂Se₃ is provided. The rubrene/Bi₂Se₃ heterojunctions are selected as an example for general demonstration due to the reproducibly high mobility and broad optoelectronic applications of rubrene crystals. By virtue of high carrier concentration on Bi₂Se₃ surface and the strong built‐in electrical field, the photoresponse of the heterotransistor is significantly reduced for more than two orders (from over 10 s to 54 ms), meanwhile the photoresponsivity can reach 124 A W^?1. To the best of knowledge, this operating speed is among the fastest responses in organic–inorganic heterojunctions. The heterotransistor also shows unique negative differential resistance under positive gate bias, which can be explained by photoinduced de‐trapping of electron trap states in the bulk rubrene crystals. Besides, the rubrene/Bi₂Se₃ heterojunction behaves as a gate‐tunable backward‐like diode due to the inhomogenous carrier distribution in the thick rubrene crystal and inversion of relative Fermi level positions. The findings demonstrate versatile functionalities of the rubrene/Bi₂Se₃ heterojunctions for various emerging optoelectronic applications.     

Preparation and characterization of belt-like Sb2Se3 crystals

Orthorhombic Sb₂Se₃ belts are synthesized via a hydrothermal treatment at 200 °C and pH = 9-10 for 16 h. Transmission electron microscopic (TEM) images show that as-obtained Sb₂Se₃ belts are with a typical width in the range of 200-400 nm and length up to 20 μm. High-resolution transmission electron microscopic (HRTEM) studies reveal that the Sb₂Se₃ are oriented in the [001] growth direction. The influences of poly vinyl alcohol (PVA) and the reaction conditions on the formation of belt-like Sb₂Se₃ microstructures are discussed.     

Symmetry‐Reduction Enhanced Polarization‐Sensitive Photodetection in Core–Shell SbI3/Sb2O3 van der Waals Heterostructure

Structural symmetry is a simple way to quantify the anisotropic properties of materials toward unique device applications including anisotropic transportation and polarization‐sensitive photodetection. The enhancement of anisotropy can be achieved by artificial symmetry‐reduction design. A core–shell SbI₃/Sb₂O₃ nanowire, a heterostructure bonded by van der Waals forces, is introduced as an example of enhancing the performance of polarization‐sensitive photodetectors via symmetry reduction. The structural, vibrational, and optical anisotropies of such core–shell nanostructures are systematically investigated. It is found that the anisotropic absorbance of a core–shell nanowire is obviously higher than that of two single compounds from both theoretical and experimental investigations. Anisotropic photocurrents of the polarization‐sensitive photodetectors based on these core–shell SbI₃/Sb₂O₃ van der Waals nanowires are measured ranging from ultraviolet (UV) to visible light (360–532 nm). Compared with other van der Waals 1D materials, low anisotropy ratio (I_max/I_min) is measured based on SbI₃ but a device based on this core–shell nanowire possesses a relatively high anisotropy ratio of ≈3.14 under 450 nm polarized light. This work shows that the low‐symmetrical core–shell van der Waals heterostructure has large potential to be applied in wide range polarization‐sensitive photodetectors.     

Thermoelectric materials based on Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions with optimal performance in the range 100–400 K

We have optimized the compositions of thermoelectric materials based on Sb₂Te₃-Bi₂Te₃ solid solutions using Czochralski-grown single crystals. The thermoelectric performance of Sb₂Te₃-Bi₂Te₃ solid solutions containing 0–100 mol % Bi₂Te₃ and Bi₂Te₃-Sb₂Te₃-Bi₂-Bi₂Se₃ solid solutions containing 2, 4, or 7 mol % Bi₂Se₃ has been investigated. The Bi₂Se₃-doped crystals are found to have higher thermoelectric figures of merit compared to the undoped crystals. The optimal crystal compositions are selected for different temperatures in the range 100–400 K.     

Understanding the Structure and Properties of Sesqui‐Chalcogenides (i.e., V2VI3 or Pn2Ch3 (Pn = Pnictogen,Ch = Chalcogen) Compounds) from a Bonding Perspective

A number of sesqui‐chalcogenides show remarkable properties, which make them attractive for applications as thermoelectrics, topological insulators, and phase‐change materials. To see if these properties can be related to a special bonding mechanism, seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Bi₂S₃, Sb₂Te₃, Sb₂Se₃, Sb₂S₃, and β‐As₂Te₃) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and β‐As₂Te₃) show an unconventional bond‐breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal‐like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui‐chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum‐chemistry‐based bonding analysis employing charge partitioning, which reveals that the four sesqui‐chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.     

A Facile One‐Step Approach for Constructing Multidimensional Ordered Nanowire Micropatterns via Fibrous Elastocapillary Coalescence

Nanowire (NW) based micropatterns have attracted research interests for their applications in electric microdevices. Particularly, aligning NWs represents an important process due to the as‐generated integrated physicochemical advantages. Here, a facile and general strategy is developed to align NWs using fibrous elastocapillary coalescence of carbon nanotube arrays (ACNTs), which enables constructing multidimensional ordered NW micropatterns in one step without any external energy input. It is proposed that the liquid film of NW solution is capable of shrinking unidirectionally on the top of ACNTs, driven by the dewetting‐induced elastocapillary coalescence of the ACNTs. Consequently, the randomly distributed NWs individually rotate and move into dense alignment. Meanwhile, the aggregating and bundling of ACNTs is helpful to produce carbon nanotube (CNT) yarns connecting neighboring bundles. Thus, a micropatterned NW network composed of a top‐layer of horizontally aligned NWs and an under‐layer of vertical ACNT bundles connected by CNT yarns is prepared, showing excellent performance in sensing external pressure with a sensitivity of 0.32 kPa^?1. Moreover, the aligned NWs can be transferred onto various substrates for constructing electronic circuits. The strategy is applicable for aligning various NWs of Ag, ZnO, Al₂O₃, and even living microbes. The result may offer new inspiration for fabricating NW‐based functional micropatterns.     

Ultrathin Single‐Crystalline CdTe Nanosheets Realized via Van der Waals Epitaxy

Due to the novel physical properties, high flexibility, and strong compatibility with Si‐based electronic techniques, 2D nonlayered structures have become one of the hottest topics. However, the realization of 2D structures from nonlayered crystals is still a critical challenge, which requires breaking the bulk crystal symmetry and guaranteeing the highly anisotropic crystal growth. CdTe owns a typical wurtzite crystal structure, which hinders the 2D anisotropic growth of hexagonal‐symmetry CdTe. Here, for the first time, the 2D anisotropic growth of ultrathin nonlayered CdTe as thin as 4.8 nm via an effective van der Waals epitaxy method is demonstrated. The anisotropic ratio exceeds 10³. Highly crystalline nanosheets with uniform thickness and large lateral dimensions are obtained. The in situ fabricated ultrathin 2D CdTe photodetector shows ultralow dark current (≈100 fA), as well as high detectivity, stable photoswitching, and fast photoresponse speed (τ_rising = 18.4 ms, τ_decay = 14.7 ms). Besides, benefitting from its 2D planar geometry, CdTe nanosheet exhibits high compatibility with flexible substrates and traditional microfabrication techniques, indicating its significant potential in the applications of flexible electronic and optoelectronic devices.     

Mass Transport Determined Silica Nanowires Growth on Spherical Photonic Crystals with Nanostructure‐Enabled Functionalities

A robust and facile method has been developed to obtain directional growth of silica nanowires (SiO₂NWs) by regulating mass transport of silicon monoxide (SiO) vapor. SiO₂NWs are grown by vapor–liquid–solid (VLS) process on a surface of gold‐covered spherical photonic crystals (SPCs) annealed at high temperature in an inert gas atmosphere in the vicinity of a SiO source. The SPCs are prepared from droplet confined colloidal self‐assembly. SiO₂NW morphology is governed by diffusion‐reaction process of SiO vapor, whereby directional growth of SiO₂NWs toward the low SiO concentration is obtained at locations with a high SiO concentration gradient, while random growth is observed at locations with a low SiO concentration gradient. Growth of NWs parallel to the supporting substrate surface is of great importance for various applications, and this is the first demonstration of surface‐parallel growth by controlling mass transport. This controllable NW morphology enables production of SPCs covered with a large number of NWs, showing multilevel micro‐nano feature and high specific surface area for potential applications in superwetting surfaces, oil/water separation, microreactors, and scaffolds. In addition, the controllable photonic stop band properties of this hybrid structure of SPCs enable the potential applications in photocatalysis, sensing, and light harvesting.     

Glass formation in the GeSe2 (As2Se3)–Sb2Se3–CdTe systems

Glass-forming regions in GeSe₂–Sb₂Se₃–CdTe (I) and As₂Se₃–Sb₂Se₃–CdTe (II) systems have been determined by visual, X-ray diffraction and electron microscope analyses. Glasses have been obtained in the GeSe₂-rich region in system I and As₂Se₃-rich region in system II, respectively. Glassformation has been observed along the tie-lines GeSe₂–Sb₂Se₃-line (30–100 mol% GeSe₂) and As₂Se₃–CdTe-line (45–100 mol% As₂Se₃). In the binary GeSe₂–CdTe and As₂Se₃–CdTe systems, glasses has found homogeneous in the concentration ranges 0–5 mol% CdTe (I) and 0–7 mol% CdTe (II), respectively. Up to 15 mol% CdTe are dissolved in the system GeSe₂–Sb₂Se₃ and in the system As₂Se₃–Sb₂Se₃, up to 12 mol% CdTe are dissolved. The basic physical–chemical characteristics of the glasses are investigated—density, microhardness, transformation temperature, crystallization temperature, melting temperature. A compositional dependence of these properties is shown. IR spectra of the samples are investigated.     

Investigation of Sb<Subscript>65</Subscript>Se<Subscript>35</Subscript>/Sb multilayer thin films for high speed and high thermal stability application in phase change memory

Sb₆₅Se₃₅/Sb multilayer composite thin films were prepared by depositing the Sb₆₅Se₃₅ and Sb layers alternately. In situ resistance vs. temperature was measured and the crystallization temperature increased with thickening the Sb₆₅Se₃₅ layer in Sb₆₅Se₃₅/Sb thin films. The data retention temperature of 10 years increased greatly from 14 °C of pure Sb to 103 °C of [Sb₆₅Se₃₅(3 nm)/Sb(7 nm)]₃. Also, the band gap was broadened and the surface became smoother. X-ray diffraction patterns for the studied materials revealed that Sb and Sb₂Se₃ phases coexisted in Sb₆₅Se₃₅/Sb thin films. Absorbing the advantages of the fast phase change for Sb, the [Sb₆₅Se₃₅(1 nm)/Sb(9 nm)]₅ multilayer thin film had an ultrafast amorphization speed of 1.6 ns. The results indicated that Sb₆₅Se₃₅/Sb multilayer thin film was a potential phase change material for fast speed and good stability.     

Broadband photodetector of high quality Sb2S3 nanowire grown by chemical vapor deposition

Low dimensional semiconductors can be used for various electronic and optoelectronic devices because of their unique structure and property.In this work,one-dimensional Sb2S3 nanowires(NWs)with high crystallinity were grown via chemical vapor deposition(CVD)technique on SiO2/Si substrates.The Sb2S 3 NWs exhibited needle-like structures with inclined cross-sections.The lengths of Sb2S3 nanowires changed from 7 to 13 pm.The photodetection properties of Sb2S3 nanowires were comprehensively and systematically characterized.The Sb2S3 photodetectors show a broadband photoresponse ranging from ultraviolet(360 nm)to near-infrared(785 nm).An excellent specific detectivity of 2.1×1014 Jones,high external quantum efficiency of 1.5×104％,sensitivity of 2.2×104 cm2W-1 and short response time of less than 100 ms was achieved for the Sb2S3 NW photodetectors.Moreover,the Sb2S3 NWs showed out-standing switch cycling stability that was beneficial to the practical applications.The high-quality Sb2S3 nanowires fabricated by CVD have great application potential in semiconductor and optoelectronic fields.     

Heterojunction solar cells with integrated Si and ZnO nanowires and a chalcopyrite thin film

ZnO nanowires (NWs) have been successfully synthesized using a hydrothermal technique on both glass and silicon substrates initially coated with a sputtered ZnO thin film layer. Varying ZnO seed layer thicknesses were deposited to determine the effect of seed layer thickness on the quality of ZnO NW growth. The effect of growth time on the formation of ZnO NWs was also studied. Experimental results show that these two parameters have an important effect on formation, homogeneity and vertical orientation of ZnO NWs. Silicon nanowires were synthesized by a Ag-assisted electroless etching technique on an n-type Si (100) wafer. SEM observations have revealed the formation of vertically-aligned Si NWs with etching depth of ∼700 nm distributed over the surface of the Si. An electron-beam evaporated chalcopyrite thin film consisting of p-type AgGa_0.5In_0.5Se₂ with ∼800 nm thickness was deposited on the n-type ZnO and Si NWs for the construction of nanowire based heterojunction solar cells. For the Si NW based solar cell, from a partially illuminated area of the solar cell, the open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were 0.34 V, 25.38 mA cm⁻², 63% and 5.50%, respectively. On the other hand, these respective parameters were 0.26 V, 3.18 mA cm⁻², 35% and 0.37% for the ZnO NW solar cell.     

Single step electrochemical synthesis of Sb<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films: effect of molarities of precursor solution

In the present investigation, we have successfully synthesized polycrystalline Sb₂Se₃ thin films by single-step electrochemical method. Effect of concentration of precursor solution on structural, morphological, optical, and wettability properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption, and contact angle measurement have been investigated. It is evident from XRD pattern that Sb₂Se₃ thin films are polycrystalline having orthorhombic crystal structure. Also, as precursor concentration increases the diffraction peak intensity also increases. Scanning electron micrographs reveal that the increase in precursor concentration causes the formation of soap foam like microstructure which is spread in the form of ellipsoids over whole substrate surface. The optical band gap decreases from 1.49 to 1.35 eV and contact angle decreases from 40° to 13°, i.e., the surface of Sb₂Se₃ thin films converts from hydrophilic to superhydrophilic nature due to increase in precursor concentration. In addition, the holographic interferometric properties have been studied. The thickness, stress to substrate and deposited mass of the thin films is determined using double exposure holographic interferometry (DEHI) technique.     

The growth and properties of (Sbx Bi1–x)2Te3 crystals

Abstract

Vertical Bridgman systems with programmable temperature control are used to grow (Sb_xBi_1:x)₂Te₃ crystals. High purity Bi, Sb and Te are used as sources and the diameter of 1.1 cm, little soft bulk crystals of (Sb_xBi_1–x)₂ Te₃ can be obtained. Scanning electron microscope (SEM) and electron probe microanalysis (EPMA) are used to analyze the micro‐structure and the compositions of the crystal. From the X‐ray diffraction patterns it appears that the grown crystal is single crystal or directive polycrystal. If the uniformity of the source solution and grown temperature are under control, then the high quality of single crystals can be obtained. The dependence of crystal structure and the thermoelectric characteristics on the changed compositions of grown crystals are discussed. The optimum composition for the thermoelectric properties is Sb_1.00 Bi_1.04Te_2.96. When the DC current, 3A, is applied to the Sb_1.00 Bi_1.04 Te_2.96 crystal with suitable electrodes, the temperature difference (△T) between two sides of the crystal can be as high as 60°C. It is 2 times larger than that ever obtained by Sb₂Te₃ crystal. It appeared that the grown (Sb_xBi_1‐x)₂Te₃ crystals have the potential on the fabrication of thermoelectric devices and electronic cooling system.     

A physicochemical study of the Sb<Subscript>2</Subscript>Se<Subscript>3</Subscript>–Nd<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

We have studied phase relations in the Sb₂Se₃–Nd₂Se₃ system and mapped out its T–x phase diagram using differential thermal analysis, X-ray diffraction, microstructural analysis, microhardness tests, and density measurements. The system contains one compound, with the composition NdSbSe₃, which melts incongruently at 865 K and crystallizes in orthorhombic symmetry with the following lattice parameters: а = 12.77(1) Å, b = 14.08(1) Å, and c = 5.82(5) Å (Z = 8, ρ_meas = 6.20 g/cm³, ρ_x = 6.38 g/cm³). At room temperature, the Nd₂Se₃ solubility in Sb₂Se₃ is 5 mol % and the Sb₂Se₃ solubility in Nd₂Se₃ is 2.5 mol %. The Sb₂Se₃–Nd₂Se₃ system has a eutectic located at 15 mol % Nd₂Se₃, with a melting point at 755 K. The electrical conductivity and thermoelectric power of the (Sb₂Se₃)_1–x (Nd₂Se₃) _x solid solutions have been measured as functions of temperature.