Photomodulation of Charge Transport in All-Semiconducting 2D–1D van der Waals Heterostructures with Suppressed Persistent Photoconductivity Effect

Van der Waals heterostructures (VDWHs), obtained via the controlled assembly of 2D atomically thin crystals, exhibit unique physicochemical properties, rendering them prototypical building blocks to explore new physics and for applications in optoelectronics. As the emerging alternatives to graphene, monolayer transition metal dichalcogenides and bottom-up synthesized graphene nanoribbons (GNRs) are promising candidates for overcoming the shortcomings of graphene, such as the absence of a bandgap in its electronic structure, which is essential in optoelectronics. Herein, VDWHs comprising GNRs onto monolayer MoS₂ are fabricated. Field-effect transistors (FETs) based on such VDWHs show an efficient suppression of the persistent photoconductivity typical of MoS₂, resulting from the interfacial charge transfer process. The MoS₂-GNR FETs exhibit drastically reduced hysteresis and more stable behavior in the transfer characteristics, which is a prerequisite for the further photomodulation of charge transport behavior within the MoS₂-GNR VDWHs. The physisorption of photochromic molecules onto the MoS₂-GNR VDWHs enables reversible light-driven control over charge transport. In particular, the drain current of the MoS₂-GNR FET can be photomodulated by 52%, without displaying significant fatigue over at least 10 cycles. Moreover, four distinguishable output current levels can be achieved, demonstrating the great potential of MoS₂-GNR VDWHs for multilevel memory devices.     

Large-Area Synthesis of Ferromagnetic Fe5−xGeTe2/Graphene van der Waals Heterostructures with Curie Temperature above Room Temperature

Van der Waals (vdW) heterostructures combining layered ferromagnets and other 2D crystals are promising building blocks for the realization of ultracompact devices with integrated magnetic, electronic, and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing for realizing highly uniform heterostructures with well-defined interfaces between different 2D-layered materials. It is also required that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, it is demonstrated that the large-area growth of Fe_5−xGeTe₂/graphene heterostructures is achieved by vdW epitaxy of Fe_5−xGeTe₂ on epitaxial graphene. Structural characterization confirms the realization of a continuous vdW heterostructure film with a sharp interface between Fe_5−xGeTe₂ and graphene. Magnetic and transport studies reveal that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond nonscalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.     

Peculiarities of the Current–Voltage Characteristic of n-GaP–p-(InSb)1 – x(Sn2)x Heterostructures

Technical Physics Letters - The current–voltage (I–V) characteristics of n-GaP–p-(InSb)1 – x(Sn2)x (0 ≤ x ≤ 0.05) heterostructures have been studied. At low...     

Spontaneous (Anti)meron Chains in the Domain Walls of van der Waals Ferromagnetic Fe5−xGeTe2

The promise of topologically vortex-like magnetic spin textures hinges on the intriguing physical properties and theories in fundamental research and their distinguished roles as high-efficiency information units in future spintronics. The exploration of such magnetic states with unique spin configurations has never ceased. In this study, the emergence of unconventional (anti)meron chains from a domain wall pair is directly observed at zero field in 2D ferromagnetic Fe_5−xGeTe₂, closely correlated with significant enhancement of the in-plane magnetization and weak van der Waals interactions. The simultaneous appearance of a large topological Hall effect is observed at the same temperature range as that of the abnormal magnetic transition. Moreover, the distinctive features of the (anti)meron chains and their collective dynamic behavior under external fields may provide concrete experimental evidence for the recent theoretical prediction of the magnetic-domain-wall topology and endorse a broader range of possibilities for electronics, spintronics, condensed matter physics, etc.     

Van der Waals Epitaxial Growth of Mosaic-Like 2D Platinum Ditelluride Layers for Room-Temperature Mid-Infrared Photodetection up to 10.6 µm

Mid-infrared (MIR) photodetection, covering diverse molecular vibrational regions and atmospheric transmission windows, is vital to civil and military purposes. Versatile use of MIR photodetectors is commonly dominated by HgCdTe alloys, InSb, and quantum superlattices, which are limited by strict operation demands, high-cost, and environmental toxicity. Despite the rapid advances of black phosphorus (BP)-based MIR photodetectors, these are subject to poor stability and large-area integration difficulty. Here, the van der Waals (vdW) epitaxial growth of a wafer-scale 2D platinum ditelluride (PtTe₂) layer is reported via a simple tellurium-vapor transformation approach. The 2D PtTe₂ layer possesses a unique mosaic-like crystal structure consisting of single-crystal domains with highly preferential [001] orientation along the normal direction, reducing the influence of interface defects and ensuring efficient out-of-plane carrier transportation. This characteristic, combined with the wide absorption of PtTe₂ and well-designed vertical device architecture, makes the PtTe₂/Si Schottky junction photodetector capable of sensing ultra-broadband light of up to 10.6 µm with a high specific detectivity. Also, the photodetector exhibits an excellent room-temperature infrared-imaging capability. This approach provides a new design concept for high-performance, room-temperature MIR photodetection based on 2D layered materials.     

Synthesis and characterization of 3D topological insulators: a case TlBi(S1?xSex)2

In this article, practical methods for synthesizing Tl-based ternary III-V-VI₂ chalcogenide TlBi(SSe_x)₂ are described in detail, along with characterization by x-ray diffraction and charge transport properties. The TlBi(SSe_x)₂ system is interesting because it shows a topological phase transition, where a topologically nontrivial phase changes to a trivial phase without changing the crystal structure qualitatively. In addition, Dirac semimetals whose bulk band structure shows a Dirac-like dispersion are considered to exist near the topological phase transition. The technique shown here is also generally applicable for other chalcogenide topological insulators, and will be useful for studying topological insulators and related materials.     

Enhanced linear magneto-resistance near the Dirac point in topological insulator Bi2(Te1−xSex)3 nanowires

Nano Research - We report the composition and back-gate voltage tuned transport properties of ternary compound Bi2(Te1−xSex)3 nanowires synthesized by chemical vapor deposition (CVD). It is...     

Photoconductivity in single crystals of (TlGaSe2)1−x(TlInS2)x alloys (0−0.45 ≤ x ≤0.55-1)

The spectral distribution of the photoconductivity in (TlGaSe₂)_1−x(TlInS₂)_x single crystals has been studied at 77 K and 300 K. At O ≤ x ≤ 4, Eg is observed to vary linearly with x. Eg(x) deviates from linearity at x = 0.6. This deviation is attributed to the effect of disorder in the composition. Over the range 0.6 to 2.2 eV pronounced impurity photoconductivity is detected at 77 K and 300 K. Deep impurity levels and their neighbourhood in this alloy are established to preserve their positions with the variation in the composition. The analysis of the obtained results indicates that the impurity centres are mainly connected with the cation neighbourhood.     

Defect structure of xY2O3 · (1 − x)TiO2 (x = 0.5–0.58) solid solutions

X-ray diffraction characterization with monochromatic synchrotron X-rays has demonstrated that xY₂O₃ · (1 ? x)TiO₂ (x = 0.5–0.58) solid solutions consist of a fluorite-like (Fm3m) disordered phase and a nanoscale (?40–1000 nm) pyrochlore-like (Fd3m) ordered phase of the same composition, coherent with the disordered phase. In the composition range of the solid solutions (0.5 ≤ x ≤ 0.58), the lattice parameter of the fluorite-like phase follows Vegard’s law. The formation of nanodomains with different degrees of order is shown to be caused by the internal strain due to the high density of structural defects in their unit cells. The materials obtained in this study possess enhanced sorption capacity and can be used as catalysts, catalyst supports, gas sensors, etc.     

Superstructures exhibited by oxides of the aurivillius family, (Bi2O2)2+ (An−1BnO3n+1)2−

Bi₅Ti₃FeO₁₅ and Bi₇Ti₃Fe₃O₂₁ which are n=4 and n=6 members of the family of oxides of the general formula (Bi₂O₂)²⁺(A_n−1B_nO_3n+1)²⁻ show unusual superstructures, possibly due to cation ordering.     

Ionic conductivity in the ternary system (ZrO2)1−0.08x−0.12y–(Y2O3)0.08x–(CaO)0.12y

The total electrical conductivity of the samples in the ternarysystem (ZrO₂)_{1-0.08x-0.12y} –(Y₂O₃)_0.08x –(CaO)_0.12y was measured by adirect current four-probe method in the temperature range 773 to1673 K. It was found that partial replacement of Y₂O₃ by CaO in thesystem ZrO₂–Y₂O₃ may enhance the electrical conductivity at highertemperatures. At lower temperatures, however, doping CaO as the thirdcomponent into the system ZrO₂–Y₂O₃ depresses the conductivity. Theobserved mixed dopant effect was then analyzed by considering thecombined effect of both parameters appeared in the traditionalArrhenius equation, the activation energy, E, and the preexponentialfactor, ₀, on the temperature-dependence of the measured conductivity.     

Growth of perfect-crystal Si-Si1−x Gex-(Ge2)1−x (InP)x structures from the liquid phase

Structures comprising Si-Si_1−x Ge_x-(Ge₂)_1−x (InP)_x with an intermediate Si_1−x Ge_x buffer layer were grown on silicon substrates. Morphological examinations, scanning patterns and diffraction spectra, and also the electrophysical and luminescence properties of the heterostructures were used to show that the crystal perfection of these structures depends on the choice of liquid-phase epitaxy conditions. Pis’ma Zh. Tekh. Fiz. 25, 37–40 (December 26, 1999)     

Structure and Diamagnetic Properties of (Pb0.6SnyCu0.4 − y)Sr2(Y1 − xCax)Cu2Oz

The effect of Sn doping in (Pb_0.6Sn _y Cu_{0.4 ? y} )Sr₂(Y_{1 ? x} Ca _x )Cu₂O _z with 0 ≤ y ≤ 0.3 and 0 ≤ x ≤ 0.7 was investigated. It was established that a nearly pure 1212 phase can be obtained at 0 ≤ y ≤ 0.1 and 0 ≤ x ≤ 0.3. The obtained XRD patterns as well as the results of the EDX and ICP-AES analyses showed that Sn substitution is possible in the (Pb,Cu)-1212 phase. Superconductivity was observed at 0.4 ≤ x ≤ 0.7. The onset of the diamagnetic transitions varied from 10 to 30 K. The influence of the strong Pb deficiency on the superconducting properties of the samples was discussed.     

Thermodynamic properties of (TeO2) n (WO3)1 − n glasses in the range 0–650 K

The heat capacity of (TeO₂) _n (WO₃)_{1 ? n} tellurite glasses with n = 0.75, 0.78, 0.85, and 0.90 has been determined using precision adiabatic calorimetry (6–350 K) and dynamic scanning calorimetry (320–650 K), and the thermodynamic characteristics of their glassy state and devitrification have been evaluated. The experimental heat capacity data have been used to calculate the standard thermodynamic functions of the glassy and supercooled liquid states at temperatures from T → 0 to 650 K: heat capacity C _p ⁰ (T), enthalpy H ⁰(T) — H ⁰(0), entropy S ⁰(T), and Gibbs function G ⁰(T) — H ⁰(0). The character of structural heterodynamicity of the tellurite glasses has been assessed by processing the low-temperature heat capacity data using the multifractal formulation of the Debye theory of heat capacity of solids. The composition dependences of the devitrification temperature and 298.15-K thermodynamic functions have been obtained, and the 298.15-K C _p ⁰ of tellurium dioxide has been estimated. The thermal and thermodynamic properties of the (TeO₂) _n (WO₃)_{1 ? n} tellurite glasses have been compared with those of (TeO₂) _n (ZnO)_{1 ? n} glasses.     

Synthesis and properties of dielectric (HfO2)1 − x (Sc2O3) x films

(HfO₂)_{1 ? x} (Sc₂O₃) _x films have been grown by chemical vapor deposition (CVD) using the volatile complexes hafnium 2,2,6,6-tetramethyl-3,5-heptanedionate (Hf(thd)₄) and scandium 2,2,6,6-tetramethyl-3,5-heptanedionate (Sc(thd)₃) as precursors. The composition and crystal structure of the films containing 1 to 36 at % Sc have been determined. The results demonstrate that, in the composition range 9 to 14 at % scandium, the films are nanocrystalline and consist of an orthorhombic three-component phase, which has not been reported previously. Using Al/(HfO₂)_{1 ? x} (Sc₂O₃) _x /Si test structures, we have determined the dielectric permittivity of the films and the leakage current through the insulator as functions of scandium concentration. The permittivity of the films with the orthorhombic structure reaches k = 42–44, with a leakage current density no higher than ～10^?8 A/cm².     

Study of coercive force for yZnFe2O4–(1 − y)CoFe2O4 magnetic nanoparticles system

The coercive forces are taken out from the loops of magnetisation cycle for yZnFe₂O₄–(1 ? y)CoFe₂O₄ mixed magnetic nanoparticles system (y is the volume fraction of ZnFe₂O₄ particles in yZnFe₂O₄–(1 ? y)CoFe₂O₄ magnetic nanoparticles mixed system). The results show that the coercive force of the system is increasing with the y decreasing and is interpreted by using particles chain model. The number of chains of particles under magnetic field is obtained by comparing the coercive force of the experiment and the theory of chain model. In yZnFe₂O₄–(1 ? y)CoFe₂O₄ magnetic nanoparticles mixed system, the coercive force comes from ferrimagnetic CoFe₂O₄ nanoparticles, and paramagnetic ZnFe₂O₄ nanoparticles make the chain length of the mixed magnetic system shorter than single CoFe₂O₄ nanoparticles system.     

Phase structure and microwave dielectric properties of Mn-doped (1−x)ZrTi2O6–xZnNb2O6(x = 0.13–0.53) ceramics

(1?x)ZrTi₂O₆–xZnNb₂O₆(x = 0.13–0.53) ceramics doped with 0.7 wt% MnCO₃ were prepared by solid-state reaction and they were all sintered well in air at 1,270 °C. XRD analysis suggested there were a two-phase region for ZrTi₂O₆–TiO₂(x = 0.20–0.33) and a single-phase region for ZrTi₂O₆(x = 0.40–0.53) because (Zn_1/3Nb_2/3)⁴⁺ preferentially substituted Ti⁴⁺ sites, and thereafter Zr⁴⁺ sites. The addition of Mn, working as acceptors to receive the electrons left by the lost oxygen, raised the Q × f value dramatically. For 0.7 wt% MnCO₃ doped (1?x)ZrTi₂O₆–xZnNb₂O₆ ceramics, with the increase of x, the ε_r and the τ_f dropped from 53.4 to 41.0 and from +95 ppm/°C to ?35 ppm/°C respectively, and the Q × f value increased from 26,300 GHz to 48,100 GHz. At last, 0.69ZrTi₂O₆–0.31ZnNb₂O₆+0.7 wt % MnCO₃ ceramics showed excellent dielectric properties: ε_r = 45.3, Q × f = 43,300 GHz and τ_f = ?0.5 ppm/°C. Another advantage is that the sintering atmosphere was in air, which is benefit to the commercial application.     

Investigation of Cs(H2PO4)1 − x (HSO4) x (x = 0.15–0.3) superprotonic phase stability

A detailed investigation of the highly conductive Cs(H₂PO₄)_1?x (HSO₄) _x (x = 0.15–0.3) proton electrolyte, its structural properties, and ageing behavior was carried out using X-ray diffraction, DSC, and impedance and NMR spectroscopy. The high conductivity of electrolytes (～2 × 10^?2 S/cm) remains stable during long-term ageing at 180–200°C due to stabilization of the high temperature phase to lower temperatures. The room temperature ¹H MAS NMR spectrum of (CsH₂PO₄)_1?x (CsHSO₄) _x demonstrates the predominantly highly mobile protons present in these materials with the residual low-mobile protons, which agrees with the XRD data. According to XRD and ¹H NMR data, the cubic phase of Cs(H₂PO₄)_{1 ? x} (HSO₄) _x (x = 0.15–0.3) that stabilizes at room temperature gradually transforms to a low-temperature monoclinic one. The kinetics of the phase transformation for mixed salt depends markedly on the relative air humidity. A possible stabilization mechanism of the Cs(H₂PO₄)_{1 ? x} (HSO₄) _x superionic phase with high proton mobility at low temperatures is discussed.