Epitaxial growth of topological insulator Bi2Se3 film on Si(111) with atomically sharp interface

Atomically sharp epitaxial growth of Bi₂Se₃ films is achieved on Si(111) substrate with molecular beam epitaxy. Two-step growth process is found to be a key to achieve interfacial-layer-free epitaxial Bi₂Se₃ films on Si substrates. With a single-step high temperature growth, second phase clusters are formed at an early stage. On the other hand, with low temperature growth, the film tends to be disordered even in the absence of a second phase. With a low temperature initial growth followed by a high temperature growth, second-phase-free atomically sharp interface is obtained between Bi₂Se₃ and Si substrate, as verified by reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM) and X-ray diffraction. The lattice constant of Bi₂Se₃ is observed to relax to its bulk value during the first quintuple layer according to RHEED analysis, implying the absence of strain from the substrate. TEM shows a fully epitaxial structure of Bi₂Se₃ film down to the first quintuple layer without any second phase or an amorphous layer.     

Structure and Microscopic Magnetism of Epitaxial Ni–Mn–Ga Films

We report on the structural and magnetic properties of epitaxial thin films of the ferromagnetic shape memory material Ni–Mn–Ga prepared by DC magnetron sputter deposition. Different substrate materials, i.e., MgO(100) and Al₂O₃(11?20) allow for a tailored epitaxial growth. Using a sacrificial chromium buffer layer freestanding epitaxial films are obtained. In combination with photolithography partially freestanding structures such as microbridges are fabricated. The complex martensite crystal structure in substrate‐constrained and freestanding films is studied by means of X‐ray diffraction. The identified asymmetric twin variant configuration is associated with a macroscopic surface pattern observed by optical microscopy. The absence of magnetic‐field induced strain in the (100) oriented samples is explained on basis of the detected twin variant configuration using a simplified model. Taking advantage of the thin film geometry spectroscopic methods are applied to the samples. The measurements provide the first experimental test for changes in the electronic structure of the involved 3d metals during a martensitic transition. Exploiting the X‐ray magnetic circular dichroism quantitative information on the element‐specific spin and orbital magnetic moments are accessed. In addition, angular‐dependent experiments allow us to trace the microscopic origin of the magnetic anisotropy in Ni₂MnGa improving the fundamental understanding of this material.     

Physical properties of Bi2 (Te,Se)3 and Bi2Se1.2Te1.8 prepared using solid-state microwave synthesis

Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 bulk products were synthesised using standard solid-state microwave synthesis. The Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 were then deposited thermally onto glass substrates at a pressure of 10^? 6 Torr. The structure of the samples was analysed using X-ray diffraction (XRD), and the powders and thin films were observed to be polycrystalline and rhombohedral in structure. The surface morphology of the samples was determined using scanning electron microscopy (SEM). From the measurements of optical properties, the energy gap values for the Bi₂Te₃, Bi₂Se₃, and Bi₂Se_1.2Te_1.8 thin films were 0.43, 0.73, and 0.65 eV, respectively.     

Structural, electrical and optical properties of Bi2Se3 and Bi2Se(3−x)Tex thin films

Thin films of Bi₂Se₃, Bi₂Se_2.9Te_0.1, Bi₂Se_2.7Te_0.3 and Bi₂Se_2.6Te_0.4 are prepared by compound evaporation. Micro structural, optical and electrical measurements are carried out on these films. X-ray diffraction pattern indicates that the as-prepared films are polycrystalline in nature with exact matching of standard pattern. The composition and morphology are determined using energy dispersive X-ray analysis and scanning electron microscopy (SEM). The optical band gap, which is direct allowed, is 0.67 eV for Bi₂Se₃ thin films and the activation energy is 53 meV. Tellurium doped thin films also show strong optical absorption corresponding to a band gap of 0.70-0.78 eV. Absolute value of electrical conductivity in the case of tellurium doped thin film shows a decreasing trend with respect to parent structure.     

Crystallization and melting of Se0.99Bi0.01 thin films by means of D.T.A. technique

The Se_0.99Bi_0.01 thin films deposited on aluminium and kapton substrates either by flash or by thermal evaporation have been characterized by means of the differential thermal analysis technique (D.T.A.). This paper deals with crystallization and melting of such films. Furthermore, in order to determine the activation energy associated with the crystallization and the frequency factor, three methods are considered. The values obtained do not vary more than 10 %. The nature of the substrate and the method of evaporation seem to modify the thermal behaviour. It is worth noting that a sharper peak of crystallization occurs in the case of Se_1−x Bi_x deposited on kapton substrates due to the small size of crystals.     

Electrochemical Biosensor Composed of Silver Ion‐Mediated dsDNA on Au‐Encapsulated Bi2Se3 Nanoparticles for the Detection of H2O2 Released from Breast Cancer Cells

A newly developed electrochemical biosensor composed of a topological insulator (TI) and metallic DNA (mDNA) is fabricated. The bismuth selenide nanoparticle (Bi₂Se₃ NP) is synthesized and sandwiched between the gold electrode and another Au‐deposited thin layer (Bi₂Se₃@Au). Then, eight‐silver‐ion mediated double‐stranded DNA (mDNA) is immobilized onto the substrate (Bi₂Se₃@Au‐mDNA) for the further detection of hydrogen peroxide. The Bi₂Se₃ NP acts as the electrochemical‐signal booster, while unprecedentedly its encapsulation by the Au thin layer keeps the TI surface states protected, improves its electrochemical‐signal stability and provides an excellent platform for the subsequent covalent immobilization of the mDNA through Au–thiol interaction. Electrochemical results show that the fabricated biosensor represents much higher Ag⁺ redox current (≈10 times) than those electrodes prepared without Bi₂Se₃@Au. The characterization of the Bi₂Se₃@Au‐mDNA film is confirmed by atomic force microscopy, scanning tunneling microscopy, and cyclic voltammetry. The proposed biosensor shows a dynamic range of 00.10 × 10^?6m to 27.30 × 10^?6m , very low detection limit (10 × 10^?9m ), unique current response (1.6 s), sound H₂O₂ recovery in serum, and substantial capability to classify two breast cancer subtypes (MCF‐7 and MDA‐MB‐231) based on their difference in the H₂O₂ generation, offering potential applications in the biomedicine and pharmacology fields.     

Structure evolution and ferroelectric properties of Bi3.4Yb0.6Ti3O12 thin films crystallized under a moderate temperature

Ytterbium-doped Bi₄Ti₃O₁₂ (Bi_3.4Yb_0.6Ti₃O₁₂, BYT) ferroelectric thin films were successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by chemical solution deposition (CSD). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to identify the crystal structure, the surface and cross-section morphology of the deposited ferroelectric films. Structure evolution and ferroelectric properties of the as-prepared thin films annealed under different temperatures (600 °C-750 °C) were studied in detail. Additionally, the mechanism concerning the dependence of electrical properties of the BYT ferroelectric thin films on the annealing temperature was discussed.     

Fabrication of thin films of bismuth selenide using novel single-source precursors by metal organic chemical vapor deposition

A metal-organic compound, Bi[(SePⁱPr₂)₂N]₃ has been synthesized and used as a single-source precursor for the deposition of bismuth selenide thin films via low-pressure metal-organic chemical vapor deposition. Crystalline thin films of rhombohedral Bi₂Se₃ have been deposited on glass substrates. The films have been characterized by X-ray powder diffraction, scanning electron microscopy and energy dispersive analysis of X-rays.     

Photoelectrochemical characterization of Bi2Se3 thin films deposited by SILAR technique

Bi₂Se₃ thin films have been deposited by simple and less investigated successive ionic layer adsorption and reaction (SILAR) technique onto fluorine-doped tin oxide (FTO) coated glass substrates at room temperature (27 °C). The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies indicate that Bi₂Se₃ thin films are nanocrystalline. These films are used for photoelectrochemical (PEC) characterization in cell with configuration as n-Bi₂Se₃/0.1 M polysulfide/C. The studies such as current–voltage characteristics in dark and light, photovoltaic power output, transient photoresponse, and capacitance–voltage have been carried out. The study reveals that fill factor and power conversion efficiency of the cell are low (0.43 and 0.032%, respectively).The flat bond potential is found to be −0.04 V (SCE).     

Effect of the substrate on the electrodeposition of Bi2Te3−ySey thin films

The potentiostatic electrodeposition of n-type Bi₂Te_3−ySe_y thermoelectric films onto stainless steel and gold substrates from nitric acid aqueous solutions has been carried out at room temperature. The cathodic process during the electrodeposition of Bi₂Te_3−ySe_y films was investigated by cyclic voltammetric experiments. The structure and surface morphology of Bi₂Te_3−ySe_y films deposited on both substrates were characterized by X-ray diffraction (XRD) and environment scanning electron microscopy (ESEM) coupled with energy dispersive spectroscopy (EDS). Electrical and thermoelectric properties of as-deposited films were also measured at room temperature. The results show that the reduction process under the same depositing conditions on gold and stainless steel substrates is very different. On gold substrates, H₂SeO₃ in the electrolyte is firstly reduced to elemental Se, and then the deposited Se reacts with HTeO₂⁺ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. On stainless steel substrates, HTeO₂⁺ in the electrolyte is firstly replaced by elemental Fe to produce elemental Te, and subsequently the generated Te reacts with H₂SeO₃ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. Analysis of ESEM show that the surface morphology of the films electrodeposited on gold substrates is more compact than that on stainless steel substrates. The XRD patterns indicate that the films electrodeposited on both substrates exhibit preferential orientation along (1 1 0) plane, but the relative peak intensity of (0 1 5) and (2 0 5) planes on stainless steel substrates is stronger than that on gold substrates. The Seebeck coefficient and electrical resistivity of the films deposited on stainless steel substrates are higher than that on gold substrates.     

Poly(Vinylpyrollidone)‐ and Selenocysteine‐Modified Bi2Se3 Nanoparticles Enhance Radiotherapy Efficacy in Tumors and Promote Radioprotection in Normal Tissues

The development of a new generation of nanoscaled radiosensitizers that can not only enhance radiosensitization of tumor tissues, but also increase radioresistance of healthy tissue is highly desirable, but remains a great challenge. Here, this paper reports a new versatile theranostics based on poly(vinylpyrollidone)‐ and selenocysteine‐modified Bi₂Se₃ nanoparicles (PVP‐Bi₂Se₃@Sec NPs) for simultaneously enhancing radiotherapeutic effects and reducing the side‐effects of radiation. The as‐prepared nanoparticles exhibit significantly enhanced free‐radical generation upon X‐ray radiation, and remarkable photothermal effects under 808 nm NIR laser irradiation because of their strong X‐ray attenuation ability and high NIR absorption capability. Moreover, these PVP‐Bi₂Se₃@Sec NPs are biodegradable. In vivo, part of selenium can be released from NPs and enter the blood circulation system, which can enhance the immune function and reduce the side‐effects of radiation in the whole body. As a consequence, improved superoxide dismutase and glutathione peroxidase activities, promoted secretion of cytokines, increased number of white blood cell, and reduced marrow DNA suppression are found after radiation treatment in vivo. Moreover, there is no significant in vitro and in vivo toxicity of PVP‐Bi₂Se₃@Sec NPs during the treatment, which demonstrates that PVP‐Bi₂Se₃@Sec NPs have good biocompatibility.     

Preparation and characterization of indium doped CdS0.2Se0.8 thin films by spray pyrolysis

The CdS_0.2Se_0.8 and indium doped CdS_0.2Se_0.8 thin films have been deposited onto the amorphous glass and fluorine doped tin oxide coated glass substrates by spray pyrolysis. The doping concentration of indium has been optimized by photoelectrochemical characterization technique. The structural, surface morphological, optical and electrical properties of CdS_0.2Se_0.8 and indium doped CdS_0.2Se_0.8 thin films have been studied. X-ray diffraction studies reveal that the films are polycrystalline in nature with hexagonal crystal structure. Scanning electron microscopy studies reveal that the grains are uniform with uneven spherically shaped, distributed over the entire substrate surface. The complete surface morphology has been changed after doping. In optical studies, the transition of the deposited films is found to be direct allowed with optical energy gaps decreasing from 1.91 to 1.67 eV with indium doping. Semiconducting behavior has been observed from resistivity measurements. The thermoelectric power measurements reveal that the films exhibit n-type conductivity.     

Passivating gallium arsenide surface by gallium chalcogenide

We have studied the protective properties of thin films of gallium selenide formed by the method of heterovalent substitution on the surface of GaAs substrates. The data of transmission (Hitachi H-800) and scanning (JEOL JSM-638 OLV) electron microscopy showed that GaAs substrates treated with selenium vapor produced a more pronounced orienting action on the subsequent deposition of GaAs as compared to the substrates covered with a natural oxide. The processing of a GaAs substrate in selenium vapor followed by the removal of the resulting Ga₂Se₃ layer increases the degree of smoothness of the substrate surface on the atomic level.     

How Does Graphene Grow? Easy Access to Well‐Ordered Graphene Films

The selective formation of large‐scale graphene layers on a Rh‐YSZ‐Si(111) multilayer substrate by a surface‐induced chemical growth mechanism is investigated using low‐energy electron diffraction, X‐ray photoelectron spectroscopy, X‐ray photoelectron diffraction, and scanning tunneling microscopy. It is shown that well‐ordered graphene layers can be grown using simple and controllable procedures. In addition, temperature‐dependent experiments provide insight into the details of the growth mechanisms. A comparison of different precursors shows that a mobile dicarbon species (e.g., C₂H₂ or C₂) acts as a common intermediate for graphene formation. These new approaches offer scalable methods for the large‐scale production of high‐quality graphene layers on silicon‐based multilayer substrates.     

Epitaxial thin films of topological insulator Bi2Te3 with two-dimensional weak anti-localization effect grown by pulsed laser deposition

We have grown high quality epitaxial topological insulator Bi₂Te₃ thin films on silicon (111) substrates by pulsed laser deposition. Systematic structural characterization of the films using X-ray diffraction and transmission electron microscopy has demonstrated that a low laser pulse rate is the key to achieving epitaxial films. The films show n-type metallic behavior, consistent with Te deficiency as determined by Rutherford backscattering spectrometry measurements. The A_1g longitudinal optical phonon mode of Bi₂Te₃ was detected by time-resolved reflectivity measurements. A 2-dimensional (2-D) weak-antilocalization effect was also observed at low temperatures, which indicates the existence of topologically protected 2-D surface states in our thin films. This growth and characterization effort paves the way to fabricate multi-layer heterostructures of topological insulators along with ferromagnetic oxides and high temperature superconductors by the same growth technique in the search for physics arising from their interfacial couplings.     

Growth and Properties of Fully Strained SrCoOx (x ≈ 2.8) Thin Films on DyScO3

High quality epitaxial SrCoO_x (oxygen deficient SrCoO₃) thin films were grown on (110) DyScO3 substrates by pulsed laser deposition.The disappearance of half order peaks in X‐ray diffraction as well as the XAS at the O K‐edge indicates an oxygen content of x ≈ 2.8 in the thin films. Magnetization measurements reveal that the specific substrate strain suppresses the ferromagnetism found in the corresponding bulk material and the emergence of an antiferromagnetic‐type spin correlation as predicted by theoretical calculations. Our work demonstrates that the magnetism can be tuned by in‐plane strain in SrCoO_x thin films.     

Anisotropic Properties of Bi2Sr2−xLa<Emphasis Type="Italic">x</Emphasis>Cu1O6+δ Thin-Films Grown on Vicinal SrTiO3 Substrates

Epitaxial Bi₂Sr_2?xLa _x Cu₁O_6+δ thin films were grown on vicinal substrates by magnetron sputtering. The growth mode and the superconductivity of the thin films were studied. The films were deposited on 5–7° misoriented (100) SrTiO₃ (STO). The surface of misoriented substrate is tilted to the 〈111〉 azimuth. The topographies of the Bi-2201 films show strip-like shapes. The transmission electron microscopy (TEM) study revealed that the films grew with very good in-plane alignments which were comparable to those of single crystals. Both of the temperature dependence of the a- and c-axis resistivities were studied under magnetic fields. The corresponding anisotropic activation energies and upper critical magnetic fields were investigated.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Preparation of P-type indium phosphide films by close space vapor transport

InP films have been grown by close space transport employing 0.8 mol% PCl₃ in H₂. For deposition on InP single crystals, 700C source and 650C substrate temperatures produced epitaxial films on (100), polycrystalline films on (111)A, and powdery layers on (111)B. Growth rates are 6 to 10 |Gmm/hr on (100) InP and ~50 μm/hr on (111)A InP. Regardless of InP source doping, deposits exhibit net donor concentrations of 5×10¹⁷ to 1×10¹⁸cm^?3. Zn doping with 0.02 to 0.5 mol% Zn(C₂H₅)₂ in the gas phase resulted in partially compensated p-InP with net acceptor concentrations up to 7×10¹⁸cm^?3. Polycrystalline films have been grown on Mg-coated carbon or molybdenum substrates at 700C source and 590C substrate temperatures. Growth rates lie between 40 and 50 μm/hr. Substantial recrystallization and grain growth are observed after 2 day anneals at 950C under 5 atm of phosphorus.     

Enhancement of photosensitivity by annealing in Bi2S3 thin films grown using SILAR method

Bi₂S₃ thin films were grown by successive ionic layer adsorption and reaction method (SILAR) onto the glass substrates at room temperature. The as prepared thin film were annealed at 250 °C in air for 30 min. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrical measurement systems. The X-ray diffraction patterns reveal that Bi₂S₃ thin film have orthorhombic crystal structure. SEM images showed uniform deposition of the material over the entire glass substrate. The optical energy band gap observed to be decreased from 1.69 to 1.62 eV for as deposited and annealed films respectively. The I–V measurement under dark and illumination condition (100 W) show annealed Bi₂S₃ thin film gives good photoresponse as compared to as deposited thin film and Bi₂S₃ thin film exhibits photoconductivity phenomena suggesting its useful in sensors device. The thermo-emf measurements of Bi₂S₃ thin films revealed n-type electrical conductivity.