Topological insulators and topological semimetals are both new classes of quantum materials, which are characterized by surface states induced by the topology of the bulk band structure. Topological ...Dirac or Weyl semimetals show linear dispersion around nodes, termed the Dirac or Weyl points, as the three-dimensional analog of graphene. We review the basic concepts and compare these topological states of matter from the materials perspective with a special focus on Weyl semimetals. The TaAs family is the ideal materials class to introduce the signatures of Weyl points in a pedagogical way, from Fermi arcs to the chiral magnetotransport properties, followed by hunting for the type-II Weyl semimetals in WTe
2
, MoTe
2
, and related compounds. Many materials are members of big families, and topological properties can be tuned. As one example, we introduce the multifunctional topological materials, Heusler compounds, in which both topological insulators and magnetic Weyl semimetals can be found. Instead of a comprehensive review, this article is expected to serve as a helpful introduction and summary by taking a snapshot of the quickly expanding field.
Two-dimensional (2D) transition metal dichalcogenide (TMD) alloy-based nanomaterials are emerging class of hydrogen evolution reaction (HER) catalysts. However, the primary challenges of 2D TMDs is ...to tune and maximize the concentration of edge-exposed layers with abundant active sites to realize efficient HER catalysis. In this work, we report a simple precursor solution aging-combined hydrothermal route to synthesize high-density edge-exposed WxMoS2 nanosheets. It is demonstrated that progressive aging of the molytungsten precursor solution facilitates the formation of high-density of edge-exposed WxMoS2 layers, strongly influenced by the pH and aging time of precursor solution. In particular, the pH value of solution, which changes with aging time, is the most important factor in converting the morphology of WxMoS2 from 2D nanosheets to self-agglomerated edge-exposed layers and vice versa. Depending on the structural changes, the HER performance of WxMoS2 is rationally modulated, showing the highest activity (η10: 0.209 VRHE and Tafel slope: 46 mV dec−1) when the precursor aging increases by 7 days, but interestingly, further aging again leads to poor HER performance. Accordingly, the systematic study of precursor aging facilitates optimal synthetic conditions that maximize the density of edge-exposed WxMoS2 layers suitable for designing cost-effective and earth-abundant catalysts for industrial applications.
Display omitted
•High-density edge-exposed WxMoS2 layers are achieved by precursor solution aging.•Solution pH changes with aging time and affects the number of edge-exposed layers.•Self-controlled solution pH is a crucial factor in tuning the WxMoS2 nanostructure.•WxMoS2 catalysts formed under intermediate aging conditions show high HER activity.•A surface area-conductivity correlation is unravelled for WxMoS2/C hybrid catalyst.
Biosensors are very important for detecting target molecules with high accuracy, selectivity, and signal-to-noise ratio. Biosensors developed using biomolecules such as enzymes or nucleic acids which ...were used as the probes for detecting the target molecules were studied widely due to their advantages. For example, enzymes can react with certain molecules rapidly and selectively, and nucleic acids can bind to their complementary sequences delicately in nanoscale. In addition, biomolecules can be immobilized and conjugated with other materials by surface modification through the recombination or introduction of chemical linkers. However, these biosensors have some essential limitations because of instability and low signal strength derived from the detector biomolecules. Functional nanomaterials offer a solution to overcome these limitations of biomolecules by hybridization with or replacing the biomolecules. Functional nanomaterials can give advantages for developing biosensors including the increment of electrochemical signals, retention of activity of biomolecules for a long-term period, and extension of investigating tools by using its unique plasmonic and optical properties. Up to now, various nanomaterials were synthesized and reported, from widely used gold nanoparticles to novel nanomaterials that are either carbon-based or transition-metal dichalcogenide (TMD)-based. These nanomaterials were utilized either by themselves or by hybridization with other nanomaterials to develop highly sensitive biosensors. In this review, highly sensitive biosensors developed from excellent novel nanomaterials are discussed through a selective overview of recently reported researches. We also suggest creative breakthroughs for the development of next-generation biosensors using the novel nanomaterials for detecting harmful target molecules with high sensitivity.
The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D ...materials to harbor a variety of active molecular components to move the current silicon‐based semiconductor technology forward to a post‐Moore era focused on molecule‐based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface‐to‐volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli‐responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components—as active units triggered by different external inputs—onto pivotal i2DMs to assess their role in the expanding field of molecule‐programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.
Molecular engineering of stimuli‐responsive inorganic 2D materials (i2DMs) is showcased by covalently bonding ad hoc active molecular components in i2DMs. Selected examples of their implementation as molecular switches triggered by different inputs provide insight into their suitability for the development of a new generation of post‐Moore nanoelectronics with molecule‐programmable responsiveness.
Phototransistors based on multilayer MoS2 crystals are demonstrated with a wider spectral response and higher photoresponsivity than single‐layer MoS2 phototransistors. Multilayer MoS2 ...phototransistors further exhibit high room temperature mobilities (>70 cm2V−1s−1), near‐ideal subthreshold swings (∼70 mV decade−1), low operating gate biases (<5 V), and negligible shifts in the threshold voltages during illumination.
Solution‐processed, low cost thin films of layered semiconductors such as transition metal dichalcogenides (TMDs) are potential candidates for future printed electronics. Here, n‐type ...electrolyte‐gated transistors (EGTs) based on porous WS2 nanosheet networks as the semiconductor are demonstrated. The WS2 nanosheets are liquid phase exfoliated to form aqueous/surfactant stabilized inks, and deposited at low temperatures (T < 120 °C) in ambient atmosphere by airbrushing. No solvent exchange, further additives, or complicated processing steps are required. While the EGTs are primarily n‐type (electron accumulation), some hole transport is also observable. The EGTs show current modulations > 104 with low hysteresis, channel width‐normalized on‐conductances of up to 0.27 µS µm−1 and estimated electron mobilities around 0.01 cm2 V−1 s−1. In addition, the WS2 nanosheet networks exhibit relatively high volumetric capacitance values of 30 F cm−3. Charge transport within the network depends significantly on the applied lateral electric field and is thermally activated, which supports the notion that hopping between nanosheets is a major limiting factor for these networks and their future application.
Electrolyte‐gated transistors based on air‐brushed WS2 nanosheet networks as an n‐type semiconductor are presented. The transistors exhibit current modulations of >104 with electron mobilities around 0.01 cm2 V−1 s−1 and high volumetric capacitance of 30 F cm−3. Charge transport within the network is found to depend significantly on the lateral electric field and to be thermally activated.
We present a comprehensive optical study of the excitonic Zeeman effects in transition metal dichalcogenide monolayers, which are discussed comparatively for selected materials: MoSe2, WSe2 and WS2. ...We introduce a simple semi-phenomenological description of the magnetic field evolution of individual electronic states in fundamental sub-bands by considering three additive components: valley, spin and orbital terms. We corroborate the validity of the proposed description by inspecting the Zeeman-like splitting of neutral and charged excitonic resonances in absorption-type spectra. The values of all three terms are estimated based on the experimental data, demonstrating the significance of the valley term for a consistent description of magnetic field evolution of optical resonances, particularly those corresponding to charged states. The established model is further exploited for discussion of magneto-luminescence data. We propose an interpretation of the observed large g-factor values of low energy emission lines, due to so-called bound/localized excitons in tungsten based compounds, based on the brightening mechanisms of dark excitonic states.
PDF
