Filamentary Superconductivity in Semiconducting Policrystalline ZrSe<Subscript>2</Subscript> Compound with Zr Vacancies

Z r S e ₂ is a band semiconductor studied long time ago. It has interesting electronic properties, and because its layer structure can be intercalated with different atoms to change some of the physical properties. In this investigation, we found that Zr deficiencies alter the semiconducting behavior and the compound can be turned into a superconductor. In this paper, we report our studies related to this discovery. The decreasing of the number of Zr atoms in small proportion according to the formula Zr _x Se₂, where x is varied from about 8.1 to 8.6 K, changing the semiconducting behavior to a superconductor with transition temperatures ranging between 7.8 and 8.5 K, is depending on the deficiencies. Outside of those ranges, the compound behaves as semiconducting with the properties already known. In our experiments, we found that this new superconductor has only a very small fraction of superconducting material determined by magnetic measurements with applied magnetic field of 10 Oe. Our conclusions is that superconductivity is filamentary. However, in one studied sample, the fraction was about 10.2 %, whereas in others is only about 1% or less. We determined the superconducting characteristics; the critical fieldsthat indicate a type 2 superonductor with Ginzburg-Landau κ parameter of the order about 2.7. The synthesis procedure is quite normal following the conventional solid state reaction. In this paper, included are the electronic characteristics, transition temperature, and evolution with temperature of the critical fields.     

Micromechanism study on electronic and magnetic properties of silicene regulated by oxygen

Silicene, a two-dimensional (2D) silicon counterpart of graphene with attractive electronic properties, has attracted increasing attention. Understanding of its interaction with oxygen is of fundamental importance for nano-electronics in silicon-based technology. Here, we have systematically studied the structural, electronic and magnetic properties of silicene with oxygen atoms adsorption by using an unbiased structure search method coupled with First-principles calculations. The results show that the most favorable oxygen adsorption site on silicene surface is bridge site and oxygen atoms tend to chemisorb on silicene. A detailed analysis of the electronic band structure and density of state (DOS) suggests that there is a band gap opening near Fermi level after oxygen adsorption, which lead to pristine silicene changing from a gapless semiconductor to a direct or indirect bandgap semiconductor. The important finding is that two and six oxygen atoms adsorbed silicene are more advantageous due to the relatively large direct band gaps at the K point. The calculated magnetic moments and spin density isosurfaces reveal that the total magnetic moments are mostly localized on silicene sheet. This finding provides new insights for further materials design based on two-dimensional silicon systems.     

Self-assembled lamellar MoS2, SnS2 and SiO2 semiconducting polymer nanocomposites

Lamellar nanocomposites based on semiconducting polymers incorporated into layered inorganic matrices are prepared by the co-assembly of organic and inorganic precursors. Semiconducting polymer-incorporated silica is prepared by introducing the semiconducting polymers into a tetrahydrofuran (THF)/water homogeneous sol solution containing silica precursor species and a surface-active agent. Semiconducting polymer-incorporated MoS(2) and SnS(2) are prepared by Li intercalation into the inorganic compound, exfoliation and restack in the presence of the semiconducting polymer. All lamellar nanocomposite films are organized in domains aligned parallel to the substrate surface plane. The incorporated polymers maintain their semiconducting properties, as evident from their optical absorption and photoluminescence spectra. The optoelectronic properties of the nanocomposites depend on the properties of both the inorganic host and the incorporated guest polymer as demonstrated by integrating the nanocomposite films into light-emitting diodes. Devices based on polymer-incorporated silica and polymer-incorporated MoS(2) show no diode behaviour and no light emission due to the insulating and metallic properties of the silica and MoS(2) hosts. In contrast, diode performance and electroluminescence are obtained from devices based on semiconducting polymer-incorporated semiconducting SnS(2), demonstrating that judicious selection of the composite components in combination with the optimization of material synthesis conditions allows new hierarchical structures to be tailored for electronic and optoelectronic applications.     

Few‐Layer Silicene Nanosheets with Superior Lithium‐Storage Properties

Silicene, a 2D silicon allotrope with unique low‐buckled structure, has attracted increasing attention in recent years due to its many superior properties. So far, epitaxial growth is one of the very limited ways to obtain high‐quality silicene, which severely impedes the research and application of silicene. Therefore, large‐scale synthesis of silicene is a great challenge, yet urgently desired. Herein, the first scalable preparation of free‐standing high‐quality silicene nanosheets via liquid oxidation and exfoliation of CaSi₂ is reported. This new synthesis strategy successfully induces mild oxidation of the (Si²ⁿ)^2n? layers in CaSi₂ into neutral Si²ⁿ layers without damage of pristine silicene structure and promotes the exfoliation of stacked silicene layers. The obtained silicene sheets are dispersible and ultrathin ones with monolayer or few‐layer thickness and exhibit excellent crystallinity. As a unique 2D layered silicon allotrope, the silicene nanosheets are further explored as new anodes for lithium‐ion batteries and exhibit a nearly theoretical capacity of 721 mAh g^?1 at 0.1 A g^?1 and an extraordinary cycling stability with no capacity decay after 1800 cycles in contrast to previous most silicon anodes showing rapid capacity decay, thus holding great promise for energy storage and beyond.     

Silicene on non-metallic substrates: Recent theoretical and experimental advances

Silicene, the silicon counterpart of graphene, has been successfully grown on metallic substrates such as Ag(111), ZrB₂(0001), and Ir(111) surfaces. However, characterization of its electronic structure is hampered by the metallic substrate. In addition, potential applications of silicene in nanoelectronic devices will require its growth on or integration with semiconducting and insulating substrates. We herein present a review of recent theoretical works regarding the interaction of silicene with non-metallic templates, distinguishing between the weak van-der-Waals-like interactions of silicene with, for example, layered metal (di)chalcogenides, and the stronger covalent bonding between silicene and, for example, ZnS surfaces. We then present a methodology to effectively compare the stability of diverse silicene structures using thermodynamics and molecular dynamics density functional theory calculations. Recent experimental results on the growth of silicene on MoS₂ are also reported and compared to the theoretical predictions.

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Lithiation/Delithiation Synthesis of Few Layer Silicene Nanosheets for Rechargeable Li–O2 Batteries

Silicene has recently received increasing interest due to its unique properties. However, the synthesis of silicene remains challenging, which limits its wide applications. In this work, a top‐down lithiation and delithiation process is developed to prepare few layer silicene‐like nanosheets from ball‐milled silicon nanopowders. It is found that delithiation solvent plays a critical role in the structure evolution of the final products. The use of isopropyl alcohol renders 2D silicene‐like products 30–100 nm in length and ≈2.4 nm in thickness. The electrochemical characterization analysis suggests that the product shows high performance for rechargeable Li–O₂ batteries with 73% energy efficiency and high stability. The top‐down synthesis strategy proposed in this work not only provides a new solution to the challenging preparation issue of few layer silicene but also demonstrates the feasibility of producing 2D materials from nonlayered starting structures.     

Controllable synthesis of MoS<Subscript>2</Subscript> nanostructures from monolayer flakes,few-layer pyramids to multilayer blocks by catalyst-assisted thermal evaporation

Molybdenum disulfide (MoS₂), as a representative of two-dimensional layered materials, has been extensively investigated due to their unique structure and interesting electronic and optical properties. However, the controllable synthesis of monolayer and pyramidal MoS₂ nanostructures needs improvement, and their growth mechanism requires more investigations. Here, uniform MoS₂ nanostructures from monolayer flakes, few-layer pyramids to multilayer blocks were successfully fabricated by a catalyst-assisted thermal-evaporation-based chemical vapor deposition method via simply adjusting the carrier Ar gas flow rate. After the comprehensive characterization on the obtained materials, their nucleation and growth mechanisms were proposed with specifically highlighting the influence of the carrier gas flow rate, which might also be of help to understand the synthesis processes of other two-dimensional semiconducting transition metal dichalcogenides by similar method.     

Synthesis of large-scale atomic-layer SnS<Subscript>2</Subscript> through chemical vapor deposition

Two-dimensional layers of metal dichalcogenides have attracted much attention because of their ultrathin thickness and potential applications in electronics and optoelectronics.Monolayer SnS2,with a band gap of ～2.6 eV,has an octahedral lattice made of two atomic layers of sulfur and one atomic layer of tin.Till date,there have been limited reports on the growth of large-scale and high quality SnS2 atomic layers and the investigation of their properties as a semiconductor.Here,we report the chemical vapor deposition (CVD) growth of atomic-layer SnS2 with a large crystal size and uniformity.In addition,the number of layers can be changed from a monolayer to few layers and to bulk by changing the growth time.Scanning transmission electron microscopy was used to analyze the atomic structure and demonstrate the 2H stacking poly-type of different layers.The resultant SnS2 crystals is used as a photodetector with external quantum efficiency as high as 150％,suggesting promise for optoelectronic applications.     

MoS<Subscript>2</Subscript>/h-BN heterostructures: controlling MoS<Subscript>2</Subscript> crystal morphology by chemical vapor deposition

Tuning the properties of van der Waals heterostructures based on alternating layers of two-dimensional materials is an emerging field of research with implications for electronics and photonics. Hexagonal boron nitride (h-BN) is an attractive insulating substrate for two-dimensional materials as it may exert less influence on the layer’s properties than silica. In this work, MoS₂ layers were deposited by chemical vapor deposition (CVD) on thick h-BN flakes mechanically exfoliated deposited on Si/SiO₂ substrates. CVD affords the controllable, large-scale preparation of MoS₂ on h-BN alleviating shortcomings of manual mechanical assembly of such heterostructures. Electron microscopy revealed that in-plane and vertical to the substrate MoS₂ layers were grown at high yield, depending on the sample preparation conditions. Raman and photoluminescence spectroscopy were employed to assess the optical and electronic quality of MoS₂ grown on h-BN as well as the interactions between MoS₂ and the supporting substrate. Compared to silica, MoS₂ layers grown on h-BN are less prone to oxidation and are subjected to considerably weaker electronic perturbation.     

Silicene: Wet‐Chemical Exfoliation Synthesis and Biodegradable Tumor Nanomedicine

Silicon‐based biomaterials play an indispensable role in biomedical engineering; however, due to the lack of intrinsic functionalities of silicon, the applications of silicon‐based nanomaterials are largely limited to only serving as carriers for drug delivery systems. Meanwhile, the intrinsically poor biodegradation nature for silicon‐based biomaterials as typical inorganic materials also impedes their further in vivo biomedical use and clinical translation. Herein, by the rational design and wet chemical exfoliation synthesis of the 2D silicene nanosheets, traditional 0D nanoparticulate nanosystems are transformed into 2D material systems, silicene nanosheets (SNSs), which feature an intriguing physiochemical nature for photo‐triggered therapeutics and diagnostic imaging and greatly favorable biological effects of biocompatibility and biodegradation. In combination with DFT‐based molecular dynamics (MD) calculations, the underlying mechanism of silicene interactions with bio‐milieu and its degradation behavior are probed under specific simulated physiological conditions. This work introduces a new form of silicon‐based biomaterials with 2D structure featuring biodegradability, biocompatibility, and multifunctionality for theranostic nanomedicine, which is expected to promise high clinical potentials.     

Physics of Silicene Stripes

Silicene, a monolayer of silicon atoms tightly packed into a two-dimensional honeycomb lattice, is the challenging hypothetical reflection in the silicon realm of graphene, a one-atom thick graphite sheet, presently the hottest material in condensed matter physics. If existing, it would also reveal a cornucopia of new physics and potential applications. Here, we reveal the epitaxial growth of silicene stripes self-aligned in a massively parallel array on the anisotropic silver (110) surface. This crucial step in the silicene “gold rush” could give a new kick to silicon on the electronics road-map and open the most promising route towards wide-ranging applications. A hint of superconductivity in these silicene stripes poses intriguing questions related to the delicate interplay between paired correlated fermions, massless Dirac fermions and bosonic quasiparticles in low dimensions.     

Transformation of monolayer MoS<Subscript>2</Subscript> into multiphasic MoTe<Subscript>2</Subscript>: Chalcogen atom-exchange synthesis route

Molybdenum ditelluride (MoTe2),which is an important transition-metal dichalcogenide,has attracted considerable interest owing to its unique properties,such as its small bandgap and large Seebeck coefficient.However,the batch production of monolayer MoTe2 has been rarely reported.In this study,we demonstrate the synthesis of large-domain (edge length exceeding 30 μm),monolayer MoTe2 from chemical vapor deposition-grown monolayer MoS2 using a chalcogen atom-exchange synthesis route.An in-depth investigation of the tellurization process reveals that the substitution of S atoms by Te is prevalently initiated at the edges and grain boundaries of the monolayer MoS2,which differs from the homogeneous selenization of MoS2 flakes with the formation of alloyed Mo-S-Se hybrids.Moreover,we detect a large compressive strain (approximately-10％) in the transformed MoTe2 lattice,which possibly drives the phase transition from 2H to 1T'at the reaction temperature of 500 ℃.This phase change is substantiated by experimental facts and first-principles calculations.This work introduces a novel route for the templated synthesis of two-dimensional layered materials through atom substitutional chemistry and provides a new pathway for engineering the strain and thus the intriguing physics and chemistry.     

Crystal structure of [CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>][(UO<Subscript>2</Subscript>)(H<Subscript>2</Subscript>AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The crystal structure of a previously unknown compound [CH₃NH₃][(UO₂)(H₂AsO₄)₃] was solved by direct methods and refined to R ₁ = 0.038 for 3041 reflections with |F _hkl | >-4σ |F _hkl |. The compound crystallizes in the monoclinic system, space group P2₁/c, a = 8.980(1), b = 21.767(2), c = 7.867(1) Å, β = 115.919(5)°, V = 1383.1(3) ų, Z = 4. In the structure of the compound, pentagonal bipyramids of uranyl ions, sharing bridging atoms with tetrahedral [H₂AsO₄]^? anions, form strongly corrugated layered complexes [(UO₂)(H₂AsO₄)₃]^? arranged parallel to the (100) plane. The protonated methylamine molecules [CH₃NH₃]⁺ form unidimensional tapelike packings parallel to the c axis and linked by hydrophilic-hydro-phobic interactions. The topology of the layered uranyl arsenate complex [(UO₂)(H₂AsO₄)₃]^? is unusual for uranyl compounds and was not observed previously. A specific feature of this topology is the presence of monodentate arsenate “branches” arranged within the layer.     

Silicene nanoribbons on transition metal dichalcogenide substrates: Effects on electronic structure and ballistic transport

The idea of stacking multiple monolayers of different two-dimensional materials has become a global pursuit. In this work, a silicene armchair nanoribbon of width W and van der Waals-bonded to different transition-metal dichalcogenide (TMD) bilayer substrates MoX₂ and WX₂, where X = S, Se, Te is considered. The orbital resolved electronic structure and ballistic transport properties of these systems are simulated by employing van der Waals-corrected density functional theory and nonequilibrium Green’s functions. We find that the lattice mismatch with the underlying substrate determines the electronic structure, correlated with the silicene buckling distortion and ultimately with the contact resistance of the two-terminal system. The smallest lattice mismatch, obtained with the MoTe₂ substrate, results in the silicene ribbon properties coming close to those of a freestanding one. With the TMD bilayer acting as a dielectric layer, the electronic structure is tunable from a direct to an indirect semiconducting layer, and subsequently to a metallic electronic dispersion layer, with a moderate applied perpendicular electric field.

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Flexible and High‐Performance All‐2D Photodetector for Wearable Devices

Emerging novel applications at the forefront of innovation horizon raise new requirements including good flexibility and unprecedented properties for the photoelectronic industry. On account of diversity in transport and photoelectric properties, 2D layered materials have proven as competent building blocks toward next‐generation photodetectors. Herein, an all‐2D Bi₂Te₃‐SnS‐Bi₂Te₃ photodetector is fabricated with pulsed‐laser deposition. It is sensitive to broadband wavelength from ultraviolet (370 nm) to near‐infrared (808 nm). In addition, it exhibits great durability to bend, with intact photoresponse after 100 bend cycles. Upon 370 nm illumination, it achieves a high responsivity of 115 A W^?1, a large external quantum efficiency of 3.9 × 10⁴%, and a superior detectivity of 4.1 × 10¹¹ Jones. They are among the best figures‐of‐merit of state‐of‐the‐art 2D photodetectors. The synergistic effect of SnS's strong light–matter interaction, efficient carrier separation of Bi₂Te₃–SnS interface, expedite carrier injection across Bi₂Te₃–SnS interface, and excellent carrier collection of Bi₂Te₃ topological insulator electrodes accounts for the superior photodetection properties. In summary, this work depicts a facile all‐in‐one fabrication strategy toward a Bi₂Te₃‐SnS‐Bi₂Te₃ photodetector. More importantly, it reveals a novel all‐2D concept for construction of flexible, broadband, and high‐performance photoelectronic devices by integrating 2D layered metallic electrodes and 2D layered semiconducting channels.     

Ab initio study of magnetism at the TiO<Subscript>2</Subscript>/LaAlO<Subscript>3</Subscript> interface

In this article, we study the possible relation between the electronic and magnetic structures of the TiO₂/LaAlO₃ interface and the unexpected magnetism found in undoped TiO₂ films grown on LaAlO₃. We concentrate on the role played by structural relaxation and interfacial oxygen vacancies. LaAlO₃ has a layered structure along the (001) direction with alternating LaO and AlO₂ planes, with nominal charges of +1 and −1, respectively. As a consequence of that, an oxygen-deficient TiO₂ film with anatase structure will grow preferently on the AlO₂ surface layer. We have therefore performed ab initio calculations for superlattices with TiO₂/AlO₂ interfaces with interfacial oxygen vacancies. Our main results are that vacancies lead to a change in the valence state of neighbor Ti atoms but not necessarily to a magnetic solution and that the appearance of magnetism depends also on structural details, such as second neighbor positions. These results are obtained using both the local spin density approximation (LSDA) and LSDA + U approximations.     

Synthesis and Magnetic Properties of Mn<Subscript>12</Subscript>-Based Single Molecular Magnets with Benzene and Pentafluorobenzene Carboxylate Ligands

We report on the synthesis and magnetic properties of Mn₁₂-based single molecular magnets (SMMs) with benzene and pentafluorobenzene carboxylate ligands. The changes in ligand structure are shown to have a decisive effect on the magnetic properties of the complexes produced. The compound with benzene demonstrates an unusual magnetic behavior, namely, temperature dependencies of magnetization taken under the zero-field-cooled and field-cooled conditions are split below 10 K and furthermore remnant magnetization and coercive force remain nonzero in this temperature range. In contrast, the compound with pentafluorobenzene displays the customary signatures of the blocking temperature at 3 K. The effect of ligand substitution was theoretically studied within the local density approximation taking into account on-site Coloumb repulsion. Calculation results confirm that the electronic structure and the magnetic exchange interactions between different Mn atoms strongly depend on the type of ligand.     

Evidence of silicene in honeycomb structures of silicon on Ag(111)

In the search for evidence of silicene, a two-dimensional honeycomb lattice of silicon, it is important to obtain a complete picture for the evolution of Si structures on Ag(111), which is believed to be the most suitable substrate for growth of silicene so far. In this work we report the finding and evolution of several monolayer superstructures of silicon on Ag(111), depending on the coverage and temperature. Combined with first-principles calculations, the detailed structures of these phases have been illuminated. These structures were found to share common building blocks of silicon rings, and they evolve from a fragment of silicene to a complete monolayer silicene and multilayer silicene. Our results elucidate how silicene forms on Ag(111) surface and provides methods to synthesize high-quality and large-scale silicene.     

Photoluminescence of Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> films formed by the molecular layer deposition method

Ta₂O₅ films of different thicknesses (20–100 nm) synthesized by the molecular layer deposition method on p-type silicon substrates and thermally oxidized silicon substrates have been studied by the methods of high-frequency capacitance–voltage characteristics and photoluminescence. A hole-conduction channel is found to form in the Si–Ta₂O₅–field electrode system. A model of the electronic structure of Ta₂O₅ films is proposed based on an analysis of the measured PL spectra and performed electrical investigations.