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We report the growth and characterization of Sb2Se3/Bi2Se3 bilayer films fabricated by molecular beam epitaxy. High quality heterostructures are obtained as evidenced from the X-ray diffraction (XRD), atomic force microscopy and high-resolution transmission electron microscopic (HRTEM) analysis. Interestingly, Sb2Se3 grows as a (120) hexacrystal film in orthorhombic phase on rhombohedral Bi2Se3 (0001) plane, as verified by the out-of-plane and in-plane XRD scans. The cross-sectional TEM studies indicate a sharp interface between Sb2Se3 and Bi2Se3, which is important for the protection of surface states Bi2Se3. The ultraviolet photoelectron spectroscopy indicates that the Fermi level located 0.95 eV above the valence band maximum in Sb2Se3. The insulating nature of Sb2Se3 is confirmed by the nonlinear current-voltage curve via the vertical junction electrical measurement. By four point probe measurements, we confirm the charge transfer effect from Sb2Se3 into Bi2Se3, and such effect can be reduced in the Sb2Se3/(Bi0.7Sb0.3)2Se3 bilayer. This work opens a new avenue for the synthesis of multilayers consisting of topological insulators and ordinary insulator, which is important for harvesting of the multiple surface states for advanced electronic and spintronic applications.

The enhancement of spin-to-charge conversion in Py/Cu/Bi2Se3 is achieved when increasing the Cu layer thickness up to 7nm. The conversion rate is studied using spin pumping technique. The inverse IEE length λIEE is found to increase up to ∼2.7nm when 7nm Cu layer is introduced. Interestingly, maximized λIEE is obtained when the effective spin mixing conductance (and thus Js) is decreased due to the Cu insertion. Monotonic increase of λIEE with decreasing Js suggests that IEE relaxation time τ is enhanced due to additional tunnelling barrier (Cu) that limits the interface transmission rate. The results have shown the importance of interface engineering in Py/TI magnetic heterostructure, which is the key factor for optimizing spin-to-charge conversion efficiency.

Tuning the Fermi level (EF) in Bi2Te3 topological-insulator (TI) films is demonstrated on controlling the temperature of growth with molecular-beam epitaxy (MBE). Angle-resolved photoemission spectra (ARPES) reveal that EF of Bi2Te3 thin films shifts systematically with the growth temperature (Tg). The key role that a Bi-on-Te(1) (BiTe1) antisite defect plays in the electronic structure is identified through extended X-ray-absorption fine-structure (EXAFS) spectra at the Bi L3-edge. Calculations of electronic structure support the results of fitting the EXAFS, indicating that the variation of EF is due to the formation and suppression of BiTe1 that is tunable with the growth temperature. Our findings provide not only insight into the correlation between the defect structure and electronic properties but also a simple route to control the intrinsic topological surface states, which could be useful for applications in TI-based advanced electronic and spintronic devices.

In this work, Cobalt-Ferrite (CFO) films were grown on silicon substrates with 300 nm amorphous silicon dioxide by Pulsed Laser Deposition (PLD) with different annealing conditions. The results of structural analysis prove that the CFO films have high crystalline quality with (1 1 1) preferred orientation. The Raman spectra and X-ray absorption spectra (XAS) indicate that the Co ions can transfer from tetrahedral sites to octahedral sites with increasing the annealing pressure. The site exchange of Co and Fe ions leads to the change of saturation magnetization in the CFO films. Our experiments provide not only a way to control the magnetism of CFO films, but also a suitable magnetic layer to develop silicon and semiconductor based spintronic devices.

In this study, we investigate the proximity effect in topological insulator (TI) and magnetic insulator bilayer system. (Bi1−xSbx)2Se3/CoFe2O4 (CFO) heterostructure was fabricated using molecular beam epitaxy and pulsed laser deposition system respectively. As revealed from the magnetoresistance measurement, the weak anti-localization (WAL) is strongly suppressed by proximity effect in (Bi1−xSbx)2Se3/CFO interface. Modified Hikama-Larkin-Nagaoka equation was used to fit the WAL results so that the size of surface state gap can be extracted successfully. The temperature-dependent resistance of the heterostructures at small and large perpendicular magnetic fields were also measured and analyzed. The results indicate that the surface band gap can be induced in TI and continuously enlarged up to 9 T, indicating the gradual alignment of the magnetic moment in CFO under perpendicular magnetic field. The approaches and results accommodated in this work show that CFO can effectively magnetize (Bi1−xSbx)2Se3 and the heterostructures are promising for TI-based spintronic device applications.

Ultrathin three-dimensional topological insulator films are promising for use in field effect devices. (Bi1–xSbx)2Se3 ultrathin films were fabricated on SrTiO3 substrate, where large resistance changes of ∼25 000% could be achieved using the back gate voltage. We suggest that the large ON/OFF ratio was caused by the combined effect of Sb-doping and the reduction of film thickness down to the ultrathin regime. The crossover of different quantum transport under an electric field may form the basis for topological insulators (TI)-based spin transistors with large ON/OFF ratios in the future.

High-quality crystalline (Cr,Sb)-doped Bi2Se3 (Cr-BSS) films were synthesized using molecular beam epitaxy (MBE). The effect of Cr- and Sb-doping on the transport and magnetic properties of Cr-BSS films was systematically investigated. The sheet carrier density N2D was found to be reduced to ∼6 × 1012 cm−2 in this quaternary compound at room temperature. This has not previously been observed in a Cr–Bi2Se3-based magnetic topological insulator (TI). Moreover, owing to the Sb dopants, the weak localization (WL)-like positive magnetoconductance in magnetic Cr–Bi2Se3 (Cr-BS) was enhanced. The enhancement is attributed to the emergence of ferromagnetism, as evidenced from the field-dependent Hall resistance and magnetic moment. The obvious tunable electrical and magnetic properties by the Sb dopant in this system are well suited for applications based on magnetic TI devices.

In this study, a perovskite is integrated with an ultra-thin Cu/Cu

Phase-change memory materials such as the pseudobinary GeTe-Sb2Te3 compounds have recently gained attention for their good thermoelectric properties, which can be used for power-generation/cooling applications. In this work, GeTe-rich Ge–Sb–Te thin films deposited using a radio-frequency magnetron sputtering method readily exhibit the metastable face-centered cubic (FCC) phase at room temperature. This is in stark contrast to its bulk form, which only transforms to its FCC phase after a transition temperature of around 350 °C. Based on previous works, the FCC phase contributes to the superior thermoelectric properties of this material system. In this study, by decreasing the working deposition pressure, the preferred orientation of (200) plane is observed that translates to improved carrier mobility. Moreover, increasing the annealing temperature has been shown to decrease the carrier concentration due to Te deficiency, leading to a significant improvement in the Seebeck coefficient of the film. By combining these effects, an optimized thermoelectric power factor (21 μW/cm K2) was obtained at an operating temperature of 350 °C.

Long-range electronic interaction between Bismuth (Bi) adatoms on graphene formed on a 4H-SiC (0 0 0 1) substrate are clearly observed at room temperature (T = 300 K). Using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we have demonstrated that such oscillatory interaction results mainly from the mediation of graphene Dirac-like electrons and the effect of the corrugated surface of SiC substrate. These two factors cause the observed oscillatory interaction with characteristic distribution distances and linear arrangements of Bi adatoms. The present study sheds light on understanding and controlling the nucleation of adatoms and subsequent growth of nanostructures on graphene surface.

The introduction of ferromagnetism at the surface of a topological insulator (TI) produces fascinating spin-charge phenomena. It has been assumed that these fascinating effects are associated with a homogeneous ferromagnetic (FM) layer possessing a single type of magnetic phase. However, we obtained phase separation within the FM layer of a Ni80Fe20/Bi2Se3 heterostructure. This phase separation was caused by the diffusion of Ni into Bi2Se3, forming a ternary magnetic phase of Ni:Bi2Se3. The inward diffusion of Ni led to the formation of an FeSe phase outward, transforming the original Ni80Fe20/Bi2Se3 into a sandwich structure comprising FeSe/Ni:Bi2Se3/Bi2Se3 with dual-phase magnetic characteristics similar to that driven by the proximity effect. Such a phenomenon might have been overlooked in previous studies with a strong focus on the proximity effect. X-ray magnetic spectroscopy revealed that FeSe and Ni:Bi2Se3 possess horizontal and perpendicular magnetic anisotropy, respectively. The overall magnetic order of the heterostructure can be easily tuned by adjusting the thickness of the Bi2Se3 as it compromises the magnetic orders of the two magnetic phases. This discovery is essential to the quantification of spin-charge phenomena in similar material combinations where the FM layer is composed of multiple elements.

In this study, we investigate the effects of fluorinated poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) buffer layer on the performance of polymer photovoltaic cells. We demonstrate for the first time, the deterioration of the device performance can be effectively mended by modifying the interface between the active layer and buffer layer with heptadecafluoro-1,1,2,2-tetra-hydro-decyl trimethoxysilane (PFDS) and perfluorononane. Device performance shows a substantial enhancement of short-circuit current from 7.90 to 9.39 mA/cm(2) and fill factor from 27% to 53%. The overall device efficiency was improved from 0.98% to 3.12% for PFDS modified device. The mechanism of S-shape curing is also discussed. In addition, the stability of modified devices shows significant improvement than those without modification. The efficiency of the modified devices retains about half (1.88%) of its initial efficiency (4.1%) after 30 d compared to the unmodified ones (0.61%), under air atmosphere.

Controlled dispersion of single-walled carbon nanotubes (SWCNTs) in common solvents is a challenging issue, especially for the rising need of low cost flexible transparent conducting films (TCFs). Utilizing conductive polymer as surfactant to facilitate SWCNTs solubility is the most successful pragmatic approach to such problem. Here, we show that dispersion of SWCNT with polymer significantly relies on the length of polymer side groups, which not only influences the diameter distribution of SWCNTs in solution, also eventually affects their effective TCF performance. Surfactants with longer side groups covering larger nanotube surface area could induce adequate steric effect to stabilize the wrapped SWCNTs against the nonspecific aggregation, as discerned by the optical and microscopic measurements, also evidenced from the resultant higher electrokinetic potential. This approach demonstrates a facile route to fabricate large-area SWCNTs-TCFs exhibiting high transmittance and high conductivity, with considerable uniformity over 10 cm × 10 cm.

In this study, we integrated bilayer structure of covered Pt on nickel zinc ferrite (NZFO) and CoFe/Pt/NZFO tri-layer structure by pulsed laser deposition system for a spin Hall magnetoresistance (SMR) study. In the bilayer structure, the angular-dependent magnetoresistance (MR) results indicate that Pt/NZFO has a well-defined SMR behavior. Moreover, the spin Hall angle and the spin diffusion length, which were 0.0648 and 1.31 nm, respectively, can be fitted by changing the Pt thickness in the longitudinal SMR function. Particularly, the MR ratio of the bilayer structure (Pt/NZFO) has the highest changing ratio (about 0.135%), compared to the prototype structure Pt/Y3Fe5O12 (YIG) because the NZFO has higher magnetization. Meanwhile, the tri-layer samples (CoFe/Pt/NZFO) indicate that the MR behavior is related with CoFe thickness as revealed in angular-dependent MR measurement. Additionally, comparison between the tri-layer structure with Pt/NZFO and CoFe/Pt bilayer systems suggests that the SMR ratio can be enhanced by more than 70%, indicating that additional spin current should be injected into Pt layer.

Ultrathin three-dimensional topological insulator films are promising for use in field effect devices. (Bi

In this study, a perovskite is integrated with an ultra-thin Cu/Cu2O (CCO) composite film, a transparent material with high mobility, to achieve a double-side and low-voltage operable photodetector. Compared to photodetectors that utilize metal electrode with perovskite, the use of CCO significantly enhances the photocurrent (from nA up to mA). It acts as a large-scale hole transport layer. The photodetector exhibits high responsivities of 6.79 AW-1 [illuminated via the fluorine-doped tin oxide (FTO) side] and 10.23 AW-1 (illuminated via CCO side). The detectivities obtained at both illuminated sides are as high as over 1011 Jones. Additionally, the Cu/Cu2O-covered perovskite effectively prevents the reaction of perovskite in the interface. This work reveals that the perovskite/CCO heterojunction photodetector can be considered a promising candidate for applications in bifacial-illuminated and flexible/wearable optoelectronic technologies.

A 3D trenched-structure metal-insulator-metal (MIM) nanocapacitor array with an ultrahigh equivalent planar capacitance (EPC) of ~300 μF cm(-2) is demonstrated. Zinc oxide (ZnO) and aluminum oxide (Al2O3) bilayer dielectric is deposited on 1 μm high biomimetic silicon nanotip (SiNT) substrate using the atomic layer deposition method. The large EPC is achieved by utilizing the large surface area of the densely packed SiNT (!5 × 10(10) cm(-2)) coated conformally with an ultrahigh dielectric constant of ZnO. The EPC value is 30 times higher than those previously reported in metal-insulator-metal or metal-insulator-semiconductor nanocapacitors using similar porosity dimensions of the support materials.

In this study, we investigate the proximity effect in topological insulator (TI) and magnetic insulator bilayer system. (Bi1−xSbx)2Se3/CoFe2O4 (CFO) heterostructure was fabricated using molecular beam epitaxy and pulsed laser deposition system respectively. As revealed from the magnetoresistance measurement, the weak anti-localization (WAL) is strongly suppressed by proximity effect in (Bi1−xSbx)2Se3/CFO interface. Modified Hikama-Larkin-Nagaoka equation was used to fit the WAL results so that the size of surface state gap can be extracted successfully. The temperature-dependent resistance of the heterostructures at small and large perpendicular magnetic fields were also measured and analyzed. The results indicate that the surface band gap can be induced in TI and continuously enlarged up to 9 T, indicating the gradual alignment of the magnetic moment in CFO under perpendicular magnetic field. The approaches and results accommodated in this work show that CFO can effectively magnetize (Bi1−xSbx)2Se3 and the heterostructures are promising for TI-based spintronic device applications.

The current–voltage relations and magnetotransport properties of La0.7Sr0.3MnO3/Si-nanotips and La0.7Sr0.3MnO3/Si-film junctions are studied, revealing that their transport properties are dominated by various mechanisms depending on temperature and bias voltage. A giant positive magnetoresistance (PMR) of ∼200% is observed at 40 K in La0.7Sr0.3MnO3/Si-nanotips junction. The temperature dependence of resistance in the presence of magnetic field suggests the origin of giant PMR to be the strong electron–electron interaction and electron–magnon scattering. Interestingly, such behavior is not observed in regular La0.7Sr0.3MnO3/Si-film junction, implying that the coupling between spin and charge could be greatly enhanced at the interface of La0.7Sr0.3MnO3 and Si nanotips.

(Bi1− x Sb x )2Se3 thin films were prepared by molecular beam epitaxy (MBE). The existence of strong and robust topological surface states was demonstrated in the (Bi1− x Sb x )2Se3 ternary system by angle-resolved photoemission spectroscopy (ARPES). The sheet carrier density n 2D was found to be decreased by 75% by doping Sb into Bi2Se3, compared with that in the case of undoped Bi2Se3. The enhancement of the surface state transport due to Sb doping was also revealed via the high-field Hall effect and weak antilocalization measurement. Our results reveal the potential of this system for the study of tunable topological-insulator based device physics.

Polycrystalline Bi-Ti-O thin films were prepared by multilayer deposition method using electron beam evaporation. The thin films were obtained by sequentially evaporating Bi 2 O 3 / TiO 2 layers on Si / SiO 2 substrate followed by a heat treatment for 2 hours in air at 900 °C. The piezoelectric response of the sample was measured by pneumatic loading method. A pressure sensor was fabricated by annealed the deposited multilayer thin films on Si / SiO 2 /Au substrate and Al was then deposited as top electrode. When an air pressure was applied and imparted on the sensor, electrical voltage was generated and measured using an electrometer. The sensor’s response was measured at three response cycles. It was found that the sensor has good voltage sensitivity and repeatability. The study shows the possibility to obtain Bi-Ti-O thin film pressure sensor by electron beam multilayer deposition.

Modifiable resistance switching (RS) is demonstrated in devices made of La0.7Sr0.3MnO3 (LSMO) on heavy-doped n-type silicon nanotips (n+-SiNTs) template. The high RS ratio of 900% with low switching voltage (±2 V) and read voltage (+0.1 V) prove the applicability of such devices for resistance memory cells. The RS mechanism can be modulated from interfacial charge transfer to Mott metal-insulator transition, dependent on the number of defect state at the surface of LSMO in association with the morphology of SiNTs.

A Taiwan-based team led by Kuei-Hsien Chen has improved the room-temperature gas-sensing properties of silica nanowire-based systems by incorporating gold nanoparticles inside the nanowire in a ‘peas-in-a-pod’ manner. Upon shining light on the device, surface plasmon resonance occurs at gold nanoparticles that absorb the visible light (from a 532 nm laser) at room temperature. Under this irradiation, a gold-embedded silica nanowire undergoes photogeneration of electrons and holes at a higher rate than its plain silica counterpart. As a result, the nanowire's conductivity is enhanced and the sensing of oxygen at the wire's surface is facilitated by the molecule's interaction with photogenerated electrons, while its recovery (desorption) is improved by the presence of the holes. As neither gold nor silica nanoparticles are known to be toxic, the gold-silica nanowire system holds promise for measuring oxygen in biological and medical applications.

This study presents a simple approach to improve the performance of small molecule based organic solar cells (OSCs) by inserting a fluorinated buffer layer (e.g., PFAS) at the hetero-interface of bilayer devices. As demonstrated in this work, the PFAS modification reduces the surface energy of the conventional PEDOT : PSS photoanode and results in a significant improvement in the pentacene based OSC. The passivated PEDOT : PSS surface after PFAS modification has a lower interface energy with pentacene and facilitates 3D single crystalline (dendritic-like) phase pentacene growth. Concurrently, the accumulated negative charges of the fluorinated PFAS layer result in the development of interfacial dipole moments that in turn lead to an enhanced built-in potential across the devices, and consequently enhanced hole transport efficiency. Improved performance of the modified OSCs is evident from the ∼97% enhancement in efficiency from 0.88% to 1.73%, along with the open-circuit voltage improvement from 0.29 to 0.42 V. As well as improving the photovoltaic performance, the PFAS treatment also enhances the stability of the device under high temperature annealing, which is essential in the fabrication process.

We report the enhanced electrical conductivity properties of single gold-peapodded amorphous silica nanowires synthesized using microwave plasma enhanced chemical vapor deposition. Dark conductivity of the gold-peapodded silica nanowires can be adjusted by controlling the number of incorporated metal nanoparticles. The temperature-dependent conductivity measurement reveals that the band tail hopping mechanism dominates the electron transport in the gold-peapodded silica nanowires. The high conductivity in the nano-peapodded nanowires with more embedded gold-nanoparticles can be explained by the higher density of hopping states and shorter hopping distance. These Au-embedded amorphous silica nanowires have provided a new approach to enhance not only the electron conduction, but also the chemical-sensor response/sensitivity.

An on-chip approach for fabricating ferromagnetic/semiconductor-nanotip heterojunctions is demonstrated. The high-density array of Si nanotips (SiNTs) is employed as a template for depositing La(0.7)Sr(0.3)MnO(3) (LSMO) rods with a pulsed-laser deposition method. Compared with the planar LSMO/Si thin film, the heterojunction shows a large enhancement of room temperature magnetoresistance (MR) ratio up to 20% under 0.5 T and a bias current of 20 µA. The MR ratio is found to be tunable, which increases with increasing external bias and the aspect ratios of the nanotips. Electric-field-induced metallization, in conjunction with nanotip geometry, is proposed to be the origin for the giant MR ratio.