The VO2 polymorphs, i.e., VO2(A), VO2(B), VO2(M1) and VO2(R), have a wide spectrum of functionalities useful for many potential applications in information and energy technologies. However, synthesis ...of phase pure materials, especially in thin film forms, has been a challenging task due to the fact that the VO2 polymorphs are closely related to each other in a thermodynamic framework. Here, we report epitaxial stabilization of the VO2 polymorphs to synthesize high quality single crystalline thin films and study the phase stability of these metastable materials. We selectively deposit all the phases on various perovskite substrates with different crystallographic orientations. By investigating the phase instability, phonon modes and transport behaviours, not only do we find distinctively contrasting physical properties of the VO2 polymorphs, but that the polymorphs can be on the verge of phase transitions when heated as low as ~400 °C. Our successful epitaxy of both VO2(A) and VO2(B) phases, which are rarely studied due to the lack of phase pure materials, will open the door to the fundamental studies of VO2 polymorphs for potential applications in advanced electronic and energy devices.
Laser-induced graphene (LIG) is a multifunctional graphene foam that is commonly direct-written with an infrared laser into a carbon-based precursor material. Here, a visible 405 nm laser is used to ...directly convert polyimide into LIG. This enabled the formation of LIG with a spatial resolution of ∼12 μm and a thickness of <5 μm. The spatial resolution enabled by the relatively smaller focused spot size of the 405 nm laser represents a >60% reduction in LIG feature sizes reported in prior publications. This process occurs in situ in an SEM chamber, thus allowing direct observation of LIG formation. The reduced size of the LIG features enables the direct-write formation of flexible electronics that are not visible to the unaided eye. A humidity sensor is demonstrated which could detect human breath with a response time of 250 ms. With the growing interest in LIG for flexible electronics and sensors, finer features can greatly expand its utility.
Hydrogen production through facile photocatalytic water splitting is regarded as a promising strategy to solve global energy problems. Transition‐metal carbides (MXenes) have recently drawn attention ...as potential co‐catalyst candidates for photocatalysts. Here, we report niobium pentoxide/carbon/niobium carbide (MXene) hybrid materials (Nb2O5/C/Nb2C) as photocatalysts for hydrogen evolution from water splitting. The Nb2O5/C/Nb2C composites were synthesized by one‐step CO2 oxidation of Nb2CTx. Nb2O5 grew homogeneously on Nb2C after mild oxidation, during which some amorphous carbon was also formed. With an optimized oxidation time of 1.0 h, Nb2O5/C/Nb2C showed the highest hydrogen generation rate (7.81 μmol h−1 gcat−1), a value that was four times higher than that of pure Nb2O5. The enhanced performance of Nb2O5/C/Nb2C was attributed to intimate contact between Nb2O5 and conductive Nb2C and the separation of photogenerated charge carriers at the Nb2O5/Nb2C interface; the results presented herein show that transition‐metal carbide are promising co‐catalysts for photocatalytic hydrogen production.
Shedding light on carbides: Niobium pentoxide/carbon/niobium carbide hybrid materials (Nb2O5/C/Nb2C) are prepared as photocatalysts for hydrogen evolution. The Nb2O5/C/Nb2C sample with an oxidation time of 1.0 h shows a hydrogen generation rate that is four times higher than that of pure Nb2O5. The enhanced performance is attributed to the separation of photogenerated carriers and the formation of a Schottky junction.
Understanding the fundamental factors that drive ion solvation structure and transport is key to design high-performance, stable battery electrolytes. Reversible ion solvation and desolvation are ...critical to the interfacial charge-transfer process across the solid–liquid interface as well as the resulting stability of the solid electrolyte interphase. Herein, we report the study of Li+ salt solvation structure in aprotic solution in the immediate vicinity (∼20 nm) of the solid electrode–liquid interface using surface-enhanced Raman spectroscopy (SERS) from a gold nanoparticle (Au NP) monolayer. The plasmonic coupling between Au NPs produces strong electromagnetic field enhancement in the gap region, leading to a 5 orders of magnitude increase in Raman intensity for electrolyte components and their mixtures namely, lithium hexafluorophosphate, fluoroethylene carbonate, ethylene carbonate, and diethyl carbonate. Further, we estimate and compare the lithium-ion solvation number derived from SERS, standard Raman spectroscopy, and Fourier transform infrared spectroscopy experiments to monitor and ascertain the changes in the solvation shell diameter in the confined nanogap region where there is maximum enhancement of the electric field. Our findings provide a multimodal spectroscopic approach to gain fundamental insights into the molecular structure of the electrolyte at the solid–liquid interface.
We present a facile method to grow millimeter-size, hexagon-shaped, monolayer, single-crystal graphene domains on commercial metal foils. After a brief in situ treatment, namely, melting and ...subsequent resolidification of copper at atmospheric pressure, a smooth surface is obtained, resulting in the low nucleation density necessary for the growth of large-size single-crystal graphene domains. Comparison with other pretreatment methods reveals the importance of copper surface morphology and the critical role of the melting–resolidification pretreatment. The effect of important growth process parameters is also studied to determine their roles in achieving low nucleation density. Insight into the growth mechanism has thus been gained. Raman spectroscopy and selected area electron diffraction confirm that the synthesized millimeter-size graphene domains are high-quality monolayer single crystals with zigzag edge terminations.
The formation of semiconductor heterojunctions and their high-density integration are foundations of modern electronics and optoelectronics. To enable two-dimensional crystalline semiconductors as ...building blocks in next-generation electronics, developing methods to deterministically form lateral heterojunctions is crucial. Here we demonstrate an approach for the formation of lithographically patterned arrays of lateral semiconducting heterojunctions within a single two-dimensional crystal. Electron beam lithography is used to pattern MoSe2 monolayer crystals with SiO2, and the exposed locations are selectively and totally converted to MoS2 using pulsed laser vaporization of sulfur to form MoSe2/MoS2 heterojunctions in predefined patterns. The junctions and conversion process are studied by Raman and photoluminescence spectroscopy, atomically resolved scanning transmission electron microscopy and device characterization. This demonstration of lateral heterojunction arrays within a monolayer crystal is an essential step for the integration of two-dimensional semiconductor building blocks with different electronic and optoelectronic properties for high-density, ultrathin devices.
Realizing the commercialization of high-performance and robust perovskite solar cells urgently requires the development of economically scalable processing techniques. Here we report a ...high-throughput ultrasonic spray-coating (USC) process capable of fabricating perovskite film-based solar cells on glass substrates with a power conversion efficiency (PCE) as high as 13%. Perovskite films with high uniformity, crystallinity, and surface coverage are obtained in a single step. Moreover, we report USC processing on TiO2/ITO-coated polyethylene terephthalate (PET) substrates to realize flexible perovskite solar cells with a PCE as high as 8.1% that are robust under mechanical stress. In this case, a photonic curing technique was used to achieve a highly conductive TiO2 layer on flexible PET substrates for the first time. The high device performance and reliability obtained by this combination of USC processing with optical curing appear very promising for roll-to-roll manufacturing of high-efficiency, flexible perovskite solar cells.
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