Polarity-Reversed Robust Carrier Mobility in Monolayer MoS₂ Nanoribbons

Using first-principles calculations and deformation potential theory, we investigate the intrinsic carrier mobility (μ) of monolayer MoS₂ sheet and nanoribbons. In contrast to the dramatic deterioration of μ in graphene upon forming nanoribbons, the magnitude of μ in armchair MoS₂ nanoribbons is comparable to its sheet counterpart, albeit oscillating with ribbon width. Surprisingly, a room-temperature transport polarity reversal is observed with μ of hole (h) and electron (e) being 200.52 (h) and 72.16 (e) cm² V^–1s^–1 in sheet, and 49.72 (h) and 190.89 (e) cm² V^–1 s^–1 in 4 nm nanoribbon. The high and robust μ and its polarity reversal are attributable to the different characteristics of edge states inherent in MoS₂ nanoribbons. Our study suggests that width reduction together with edge engineering provide a promising route for improving the transport properties of MoS₂ nanostructures

Mechanism of Interaction of Water above the Methylammonium Lead Iodide Perovskite Nanocluster: Size Effect and Water-Induced Defective States

Water is often viewed as detrimental to organic halide perovskite stability. However, evidence highlights its efficacy as a solvent during organic perovskite liquid synthesis. This paradox prompts an investigation into water’s influence on perovskite nanoclusters. Employing first principle calculations and ab initio molecular dynamics simulations, surprisingly, we discover some subsurface layers of methylammonium lead iodide (MAPbI3) nanoclusters exhibit stronger relaxation than surface layers. Moreover, a strong quantum confinement effect enhances the band gap of MAPbI3 as the nanocluster size decreases. Notably, the water molecules above MAPbI3 nanoclusters induce rich localized defect states, generating low-lying shallow states above the valence band for the small amounts of surface water molecules and band-like deep states across the whole gap for large nanoclusters. This work provides insights into water’s role in the electronic structure and structural evolution of perovskite nanoclusters, aiding the design of water-resistant layers to protect perovskite quantum dots from ambient humidity

Modulating Carrier Density and Transport Properties of MoS₂ by Organic Molecular Doping and Defect Engineering

Using first-principles calculations, we investigate the effect of molecular doping and sulfur vacancy on the electronic properties and charge modulation of monolayer MoS₂. It is found that tetrathiafulvalene and dimethyl-<i>p</i>-phenylenediamine molecules are effective donors, whereas tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) are effective acceptors, and all these molecules are able to shift the work function of MoS₂. For MoS₂ containing sulfur vacancies, these molecules are able to change the position of the defect levels within the band gap and modulate the carrier density around the defect center. Charge transfer analysis shows that TCNE and TCNQ induce a free-carrier depletion of the defect states, which is beneficial for the suppression of the nonradiative trionic decay and a higher excitonic efficiency due to a decrease in the screening of excitons. Furthermore, the effects of molecular adsorption on Seebeck coefficient of MoS₂ are also explored. Our work suggests that an enhanced excitonic efficiency of MoS₂ may be achieved via proper defect engineering and molecular doping arising from the charge density modulation and charge screening

Exotic Quartic Anharmonicity Induced by Rattling Effect in Layered Isostructural Compounds

Anharmonicity of phonons correlates with less dispersive potential surfaces and usually governs the thermal transport of low-dimensional materials. Here, we demonstrate the significant role of the so-called “rattling” action in affecting lattice anharmonicity, originating from the ease of freedom of confined but loose atoms in two-dimensional space. Based on calculations of X2Si2Te6 (X = Sb and Bi) within the Peierls–Boltzmann framework, the degree of high-order four-phonon scattering differs strikingly despite their isostructural feature. Upon switching on four-phonon scattering, a significant drop of thermal conductivity (κph) occurs in Bi2Si2Te6 up to 43.15% (71.62%) at 300 K (1000 K), while a moderate reduction occurs for Sb2Si2Te6. This arises from a stronger quartic anharmonicity of Bi2Si2Te6 than Sb2Si2Te6, dominated by the redistribution four-phonon process (λ + λ′ → λ″ + λ‴). We show that the strong quartic anharmonicity is more likely to occur in systems with flat phonon bands, large atoms, and rattling atomic units. These new insights provide perspectives in the design of materials with low κph through introducing rattling units in layered materials or interfaces

Ende der Talfahrt fuer die ostdeutsche Wirtschaft in Sicht?

SIGLEIAB-90-0DD0-101200 AU 533 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

NIR Schottky Photodetectors Based on Individual Single-Crystalline GeSe Nanosheet

We have synthesized high-quality, micrometer-sized, single-crystal GeSe nanosheets using vapor transport and deposition techniques. Photoresponse is investigated based on mechanically exfoliated GeSe nanosheet combined with Au contacts under a global laser irradiation scheme. The nonlinearship, asymmetric, and unsaturated characteristics of the <i>I</i>–<i>V</i> curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects-induced in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination (maximum photoconductive gain ∼5.3 × 10² % at 4 V) at a wavelength of 808 nm. The significant photoresponse and good responsitivity (∼3.5 A W^–1) suggests its potential applications as photodetectors

Surface-Mediated Chemical Dissolution of Two-Dimensional Nanomaterials toward Hole Creation

Chemically engineered holes on two-dimensional (2D) nanomaterials may significantly increase the number of edge sites to tune their intrinsic properties to achieve promising performance. Here, we report a general and mild approach to the convenient creation of holes on atomically thin nanosheets for engineering bandgaps and enhancing properties of 2D materials. Through surface blocking, controlled dissolution, and chemical stabilization, WO₃ nanosheets are readily treated to create holes in the presence of bovine serum albumin (BSA) via the reaction of WO₃ with OH^– ions at pH 8. Arising from the increased bandgaps and more edge sites as demonstrated experimentally and theoretically, the resulting holey WO₃ nanosheets exhibit enhanced photocurrents and much better performance during selective adsorption and photocatalytic degradation compared with those of bulky WO₃ and nonporous nanosheets. Also, this approach is further extended to the convenient creation of holes on more 2D nanomaterials such as MoS₂ and C₃N₄ nanosheets, which are facilely made in aqueous solutions of diluted H₂O₂ and HCl, respectively. Overall, this work not only demonstrates a surface-mediated chemical dissolution strategy for generating holes on various ultrathin nanosheets but also provides new opportunities to exploit exotic properties and novel applications of geometrically constructed 2D nanomaterials

High-Yield Exfoliation of Ultrathin Two-Dimensional Ternary Chalcogenide Nanosheets for Highly Sensitive and Selective Fluorescence DNA Sensors

High-yield preparation of ultrathin two-dimensional (2D) nanosheets is of great importance for the further exploration of their unique properties and promising applications. Herein, for the first time, the high-yield and scalable production of ultrathin 2D ternary chalcogenide nanosheets, including Ta₂NiS₅ and Ta₂NiSe₅, in solution is achieved by exfoliating their layered microflakes. The size of resulting Ta₂NiS₅ and Ta₂NiS₅ nanosheets ranges from tens of nanometers to few micrometers. Importantly, the production yield of single-layer Ta₂NiS₅ nanosheets is very high, ca. 86%. As a proof-of-concept application, the single-layer Ta₂NiS₅ is used as a novel fluorescence sensing platform for the detection of DNA with excellent selectivity and high sensitivity (with detection limit of 50 pM). These solution-processable, high-yield, large-amount ternary chalcogenide nanosheets may also have potential applications in electrocatalysis, supercapacitors, and electronic devices

Highly Efficient Mass Production of Boron Nitride Nanosheets via a Borate Nitridation Method

Boron nitride nanosheets (BNNSs) have attracted intensive attention because of their fantastic properties, including excellent electrical insulating ability, splendid thermal conductivity, and outstanding oxidation resistance. However, facing the rising demand for versatile applications, the cost-effective mass production of BNNSs, similar to graphene, remains a huge challenge. Here, we provide a highly effective strategy for BNNS synthesis via a borate nitridation method utilizing solid borate precursors, producing gram-scale yields with efficiencies up to 88%. Combined with density functional theory (DFT) calculations, a vapor–solid–solid (VSS) mechanism was proposed in which ammonia vapor reacts with the solid borates, producing solid BNNSs at the vapor–solid interfaces. The strategy proposed herein, together with the diversity of borate compounds, allows numerous choices for the facile mass production of BNNSs at low cost. In addition, the remarkably enhanced thermal conductivity in composite materials demonstrated good quality and huge potential for these BNNSs in thermal management. This work reveals a cost-efficient method for the large-scale production of BNNSs, which should promote practical applications in various fields

Protein Induces Layer-by-Layer Exfoliation of Transition Metal Dichalcogenides

Here, we report a general and facile method for effective layer-by-layer exfoliation of transition metal dichalcogenides (TMDs) and graphite in water by using protein, bovine serum albumin (BSA) to produce single-layer nanosheets, which cannot be achieved using other commonly used bio- and synthetic polymers. Besides serving as an effective exfoliating agent, BSA can also function as a strong stabilizing agent against reaggregation of single-layer nanosheets for greatly improving their biocompatibility in biomedical applications. With significantly increased surface area, single-layer MoS₂ nanosheets also exhibit a much higher binding capacity to pesticides and a much larger specific capacitance. The protein exfoliation process is carefully investigated with various control experiments and density functional theory simulations. It is interesting to find that the nonpolar groups of protein can firmly bind to TMD layers or graphene to expose polar groups in water, facilitating the effective exfoliation of single-layer nanosheets in aqueous solution. The present work will enable to optimize the fabrication of various 2D materials at high yield and large scale, and bring more opportunities to investigate the unique properties of 2D materials and exploit their novel applications