It is demonstrated that the luminescence efficiency of monolayers composed of MoS2, WS2, and WSe2 is significantly limited by the substrate and can be improved by orders of magnitude through ...substrate engineering. The substrate affects the efficiency mainly through doping the monolayers and facilitating defect‐assisted nonradiative exciton recombinations, while the other substrate effects including straining and dielectric screening play minor roles. The doping may come from the substrate and substrate‐borne water moisture, the latter of which is much stronger than the former for MoS2 and WS2 but negligible for WSe2. Using proper substrates such as mica or hexagonal boron nitride can substantially mitigate the doping effect. The defect‐assisted recombination depends on the interaction between the defect in the monolayer and the substrate. Suspended monolayers, in which the substrate effects are eliminated, may have efficiency up to 40% at room temperatures. The result provides useful guidance for the rational design of atomic‐scale light emission devices.
Luminescence efficiency in transition metal dichalcogenide monolayers can be improved by orders of magnitude through engineering the interaction with substrates. The luminescence efficiency of the monolayers is mainly affected through doping and facilitating nonradiative exciton recombination. Since the doping effect significantly depends on the kind of substrate, choosing the right type of substrates is a crucial step toward maximizing efficiency.
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In string theories, one-loop scattering amplitudes are characterized by integrals over genus-one surfaces using the Kronecker-Eisenstein series. A recent methodology proposed a genus-one ...basis formed from products of these series of chain topologies. A prior work further deconstructed cyclic products of the Kronecker-Eisenstein series on this basis. Building on it, our study further employs advanced and comprehensive combinatorial techniques to decompose more general genus-one integrands including a product of an arbitrary number of cyclic products of Kronecker-Eisenstein series, supplemented by Mathematica codes. Our insights enhance the understanding of multiparticle amplitudes across various string theories and illuminate loop-level parallels with string tree-level amplitudes.
The ongoing pandemic of a new human coronavirus, SARS-CoV-2, has generated enormous global concern. We and others in China were involved in the initial genome sequencing of the virus. Herein, we ...describe what genomic data reveal about the emergence SARS-CoV-2 and discuss the gaps in our understanding of its origins.
A series of zirconium polyphenolate‐decorated‐(metallo)porphyrin metal–organic frameworks (MOFs), ZrPP‐n (n = 1, 2), featuring infinite ZrIV‐oxo chains linked via polyphenolate groups on four ...peripheries of eclipse‐arranged porphyrin macrocycles, are successfully constructed through a top–down process from simulation to synthesis. These are the unusual examples of Zr‐MOFs (or MOFs in general) based on phenolic porphyrins, instead of commonly known carboxylate‐based types. Representative ZrPP‐1 not only exhibits strong acid resistance (pH = 1, HCl) but also remains intact even when immersed in saturated NaOH solution (≈20 m), an exceptionally large range of pH resistance among MOFs. The metallation at the porphyrin core gives rise to materials with enhanced sorption and catalytic properties. In particular, ZrPP‐1‐Co, with precise and uniform distribution of active centers, exhibits not only high CO2 trapping capability (≈90 cm3 g−1 at 1 atm, 273 K, among the highest in Zr‐MOFs) but also high photocatalytic activity for reduction of CO2 into CO (≈14 mmol g−1 h−1) and high selectivity over CH4 (>96.4%) without any cocatalyst under visible‐light irradiation (λ > 420 nm). Given the strong chemical resistance under extreme alkali conditions, these catalysts can be recycled without appreciable loss of activity. The possible mechanism for photocatalytic reduction of CO2‐to‐CO over ZrPP‐1‐Co is also proposed.
Top–down fabrication of robust and porous materials based on infinite ZrIV‐polyphenolate chains linked via eclipsed‐arranged porphyrin macrocycles is presented. Among them, ZrPP‐1 retains its framework integrity when immersed in saturated NaOH solution as long as 1 week. Moreover, metallation at the porphyrin core gives rise to materials with enhanced CO2 trapping capability and high visible‐light‐driven CO2‐to‐CO photoreduction activity.
Transition‐metal dichalcogenides (TMDCs) are an important class of two‐dimensional (2D) layered materials for electronic and optoelectronic applications, due to their ultimate body thickness, sizable ...and tunable bandgap, and decent theoretical room‐temperature mobility. So far, however, all TMDCs show much lower mobility experimentally because of the collective effects by foreign impurities, which has become one of the most important limitations for their device applications. Here, taking MoS2 as an example, the key factors that bring down the mobility in TMDC transistors, including phonons, charged impurities, defects, and charge traps, are reviewed. A theoretical model that quantitatively captures the scaling of mobility with temperature, carrier density, and thickness is introduced. By fitting the available mobility data from literature over the past few years, one obtains the density of impurities and traps for a wide range of transistor structures. It shows that interface engineering can effectively reduce the impurities, leading to improved device performances. For few‐layer TMDCs, the lopsided carrier distribution is analytically modeled to elucidate the experimental increase of mobility with the number of layers. From our analysis, it is clear that the charge transport in TMDC samples is a very complex problem that must be handled carefully.
Transition‐metal dichalcogenides (TMDCs) are widely investigated for enhanced characteristics for electronics among next generation semiconductors. The understanding of charge transport in TMDCs is significant for further device applications. Through carefully analyzing the reported high performance MoS2 devices, this review provides a systematic theoretical and experimental path to optimize the device structure and improve device performance.
High-entropy alloys are a novel class of complex materials discovered near the center of phase diagram, and show exceptional performances over the traditional alloys. The ability of high-entropy ...alloys to be designed unique combinations of mechanical and functional properties in unlimited space of alloy compositions is encouraging. The unconventional chemical structures hold promise for achieving unprecedented functional performances, making it a potential functional material in the field of soft-magnetic, anti-radiation, catalyst, photothermal conversion materials, etc. Here, the recent progress in understanding the salient features of high-entropy alloys is reviewed. The functional performances and underlying mechanisms are carefully discussed.
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Actuators that can convert environmental stimuli into mechanical work are widely used in intelligent systems, robots, and micromechanics. To produce robust and sensitive actuators of different ...scales, efforts are devoted to developing effective actuating schemes and functional materials for actuator design. Carbon‐based nanomaterials have emerged as preferred candidates for different actuating systems because of their low cost, ease of processing, mechanical strength, and excellent physical/chemical properties. Especially, due to their excellent photothermal activity, which includes both optical absorption and thermal conductivities, carbon‐based materials have shown great potential for use in photothermal actuators. Herein, the recent advances in photothermal actuators based on various carbon allotropes, including graphite, carbon nanotubes, amorphous carbon, graphene and its derivatives, are reviewed. Different photothermal actuating schemes, including photothermal effect–induced expansion, desorption, phase change, surface tension gradient creation, and actuation under magnetic levitation, are summarized, and the light‐to‐heat and heat‐to‐work conversion mechanisms are discussed. Carbon‐based photothermal actuators that feature high light‐to‐work conversion efficiency, mechanical robustness, and noncontact manipulation hold great promise for future autonomous systems.
This review highlights the recent advances in carbon‐based photothermal actuators. Physical properties and light‐to‐heat conversion mechanisms of various carbon‐based functional materials are summarized. Photothermal actuating schemes such as photothermal expansion, desorption, phase change, surface tension effect, and magnetic susceptibility are reviewed. The current challenges and future perspectives of this field are also discussed.
Recently, sensors that can imitate human skin have received extensive attention. Capacitive sensors have a simple structure, low loss, no temperature drift, and other excellent properties, and can be ...applied in the fields of robotics, human–machine interactions, medical care, and health monitoring. Polymer matrices are commonly employed in flexible capacitive sensors because of their high flexibility. However, their volume is almost unchanged when pressure is applied, and they are inherently viscoelastic. These shortcomings severely lead to high hysteresis and limit the improvement in sensitivity. Therefore, considerable efforts have been applied to improve the sensing performance by designing different microstructures of materials. Herein, two types of sensors based on the applied forces are discussed, including pressure sensors and strain sensors. Currently, five types of microstructures are commonly used in pressure sensors, while four are used in strain sensors. The advantages, disadvantages, and practical values of the different structures are systematically elaborated. Finally, future perspectives of microstructures for capacitive sensors are discussed, with the aim of providing a guide for designing advanced flexible and stretchable capacitive sensors via ingenious human‐made microstructures.
The advantages, disadvantages, and practical applications of several popular microstructures that are widely employed in capacitive sensors are summarized. A microstructured dielectric layer or electrode can improve sensor sensitivity, reduce hysteresis, and endow the rigid electronic device with excellent elastic stretchability, which is an essential part of next‐generation wearable devices and soft robots.
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–1 s–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.
By combining a high‐κ dielectric substrate and a high density of charge carriers, Coulomb impurities in MoS2 can be effectively screened, leading to an unprecedented room‐temperature mobility of ≈150 ...cm2 V−1 s−1 and room‐temperature phonon‐limited transport in a monolayer MoS2 transistor for the first time.
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