Liquid-Metal-Assisted Deposition and Patterning of Molybdenum Dioxide at Low Temperature

Construction of a red phosphorus-molybdenum dioxide electron-rich interface for efficient photocatalytic reduction of carbon dioxide

Developing efficient catalysts to enhance photoreduction carbon dioxide (CO2) into hydrocarbon fuels is a great challenge. As metallic material, molybdenum dioxide (MoO2) has very high conductivity and charge density, which make it a promising candidate as photocatalyst. However, its photocatalytic activity is limited by the serious charge recombination. How to effectively make full use of the metallic MoO2 for photocatalytic CO2 reduction is still a critical issue. The potential effective way to solve this problem is to introduce appropriate auxiliary catalysts to construct electron-rich interfaces. In this study, red phosphorus (P) is dispersed on MoO2 nanoparticles to construct electron-rich interfaces which can serve as the active site for photocatalytic CO2 reduction. The results show that the reduction of CO2 by pure MoO2 only produces carbon monoxide (CO) and methane (CH4). However, with the aid of red P, the P-MoO2 photocatalyst can produce ethylene (C2H4) with the yield of 5.43 μmol h-1 g-1, and the CO and CH4 yields are also significantly improved. Experimental results and density functional theory (DFT) calculations indicate that photogenerated carriers can migrate from MoO2 to the interface, and the reduction of CO2 occurs at the interface. This study provides a significant insight for the design of efficient photocatalysts by using metallic photocatalysts.

Controlled Fabrication of Metallic MoO2 Nanosheets towards High-Performance p-Type 2D Transistors

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are extensively employed as channel materials in advanced electronic devices. The electrical contacts between electrodes and 2D semiconductors play a crucial role in the development of high-performance transistors. While numerous strategies for electrode interface engineering have been proposed to enhance the performance of n-type 2D transistors, upgrading p-type ones in a similar manner remains a challenge. In this work, significant improvements in a p-type WSe2 transistor are demonstrated by utilizing metallic MoO2 nanosheets as the electrode contact, which are controllably fabricated through physical vapor deposition and subsequent annealing. The MoO2 nanosheets exhibit an exceptional electrical conductivity of 8.4 × 104 S m‒1 and a breakdown current density of 3.3 × 106 A cm‒2. The work function of MoO2 nanosheets is determined to be ≈5.1 eV, making them suitable for contacting p-type 2D semiconductors. Employing MoO2 nanosheets as the electrode contact in WSe2 transistors results in a notable increase in the field-effect mobility to 92.0 cm2 V‒1 s‒1, which is one order of magnitude higher than the counterpart devices with conventional electrodes. This study not only introduces an intriguing 2D metal oxide to improve the electrical contact in p-type 2D transistors, but also offers an effective approach to fabricating all-2D devices.

Gallium-based liquid metals as reaction media for nanomaterials synthesis

Gallium-based liquid metals (LMs) and their alloys have gained prominence in the realm of flexible and stretchable electronics. Recent advances have expanded the interest to explore the electron-rich core and interface of LMs to synthesize various nanomaterials, where Ga-based LMs serve as versatile reaction media. In this paper, we delve into the latest developments within this burgeoning field. Our discussion begins by elucidating the unique attributes of LMs that render them suitable as reaction media, including their high metal solubility, low standard reduction potential, self-limiting oxidation and ultra-smooth and "layer" surface. We then provide a comprehensive categorized summary of utilizing these features to fabricate a variety of nanomaterials, including pure metallic materials (metal alloys, metal crystals, porous metals, high-entropy alloys and metallic single atoms), metal-inorganic compounds (2D metal oxides, 2D metallic inorganic compounds and 2D graphitic materials), as well as metal-organic composites (metal-organic frameworks). This paper concludes by discussing the current challenges in this field and exploring potential future directions. The versatility and unique properties of Ga-based LMs are poised to play a pivotal role in the future of nanomaterial science, paving the way for more efficient, sustainable, and innovative technological solutions.

Liquid Metal Interface for Two-Precursor Autogenous Deposition of Metal Telluride-Tellurium Networks

Liquid metal-electrolyte can offer electrochemically reducing interfaces for the self-deposition of low-dimensional nanomaterials. We show that implementing such interfaces from multiprecursors is a promising pathway for achieving nanostructured films with combinatory properties and functionalities. Here, we explored the liquid metal-driven interfacial growth of metal tellurides using eutectic gallium-indium (EGaIn) as the liquid metal and the cation pairs Ag+-HTeO2+ and Cu2+-HTeO2+ as the precursors. At the EGaIn-electrolyte interface, the precursors were reduced and self-deposited autogenously to form interconnected nanoparticle networks. The deposited materials consisted of metal telluride and tellurium with their relative abundance depending on the metal ion type (Ag+ and Cu2+) and the metal-to-tellurium ion ratios. When used as electrode modifiers, the synthesized materials increased the electroactive surface area of unmodified electrodes by over 10 times and demonstrated remarkable activity for model electrochemical reactions, including HexRu(III) responses and dopamine sensing. Our work reveals the promising potential of the liquid metal-templated deposition method for synthesizing complex material systems for electrochemical applications.

Recent progress in liquid metal printing and its applications

This paper focuses on the latest research printing technology and broad application for flexible liquid metal (LM) materials. Through the newest template printing method, centrifugal force assisted method, pen lithography technology, and laser method, the precision of liquid metal printing on the devices was improved to 10 nm. The development of novel liquid metal inks, such as PVA-LM ink and ethanol/PDMS/LM double emulsion ink, have further enhanced the recovery, rapid printing, high conductivity, and strain resistance. At the same time, liquid metals also show promise in the application of biochemical sensors, photocatalysts, composite materials, driving machines, and electrode materials. Liquid metals have been applied to biomedical, pressure/gas, and electrochemical sensors. The sensitivity, biostability, and electrochemical performance of these LM sensors were improved rapidly. They could continue to be used in healthy respiratory, heartbeat monitoring, and dopamine detection. Meanwhile, the applications of liquid metal droplets in catalytic-assisted MoS2 deposition, catalytic growth of two-dimensional (2D) lamellar, catalytic free radical polymerization, catalytic hydrogen absorption/dehydrogenation, photo/electrocatalysis, and other fields were also summarized. Through improving liquid metal composites, magnetic, thermal, electrical, and tensile enhancement alloys, and shape memory alloys with excellent properties could also be prepared. Finally, the applications of liquid metal in micro-motors, intelligent robot feet, nanorobots, self-actuation, and electrode materials were also summarized. This paper comprehensively summarizes the practical application of liquid metals in different fields, which helps understand LMs development trends, and lays a foundation for subsequent research.

Nanoheterostructure by Liquid Metal Sandwich-Based Interfacial Galvanic Replacement for Cancer Targeted Theranostics

Nanoheterostructures with exquisite interface and heterostructure design find numerous applications in catalysis, plasmonics, electronics, and biomedicine. In the current study, series core-shell metal or metal oxide-based heterogeneous nanocomposite have been successfully fabricated by employing sandwiched liquid metal (LM) layer (i.e., LM oxide/LM/LM oxide) as interfacial galvanic replacement reaction environment. A self-limiting thin oxide layer, which would naturally occur at the metal-air interface under ambient conditions, could be readily delaminated onto the surface of core metal (Fe, Bi, carbonyl iron, Zn, Mo) or metal oxide (Fe₃ O₄ , Fe₂ O₃ , MoO₃ , ZrO₂ , TiO₂ ) nano- or micro-particles by van der Waals (vdW) exfoliation. Further on, the sandwiched LM layer could be formed immediately and acted as the reaction site of galvanic replacement where metals (Au, Ag, and Cu) or metal oxide (MnO₂ ) with higher reduction potential could be deposited as shell structure. Such strategy provides facile and versatile approaches to design and fabricate nanoheterostructures. As a model, nanocomposite of Fe@Sandwiched-GaIn-Au (Fe@SW-GaIn-Au) is constructed and their application in targeted magnetic resonance imaging (MRI) guided photothermal tumor ablation and chemodynamic therapy (CDT), as well as the enhanced radiotherapy (RT) against tumors, have been systematically investigated.

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications

Despite over a decade of intense research efforts, the full potential of two-dimensional transition-metal dichalcogenides continues to be limited by major challenges. The lack of compatible and scalable dielectric materials and integration techniques restrict device performances and their commercial applications. Conventional dielectric integration techniques for bulk semiconductors are difficult to adapt for atomically thin two-dimensional materials. This review provides a brief introduction into various common and emerging dielectric synthesis and integration techniques and discusses their applicability for 2D transition metal dichalcogenides. Dielectric integration for various applications is reviewed in subsequent sections including nanoelectronics, optoelectronics, flexible electronics, valleytronics, biosensing, quantum information processing, and quantum sensing. For each application, we introduce basic device working principles, discuss the specific dielectric requirements, review current progress, present key challenges, and offer insights into future prospects and opportunities.

Coating of gallium-based liquid metal particles with molybdenum oxide and oxysulfide for electronic band structure modulation

Liquid metal (LM) droplets are now used in many applications including catalysis, sensing, and flexible electronics. Consequently, the introduction of methods for on-demand alternating electronic properties of LMs is necessary. The active surface of LMs provides a unique environment for spontaneous chemical reactions that enable the formation of thin layers of functional materials for such modulations. Here, we showed the deposition of n-type MoO_x and MoO_xS_y semiconductors on the surface of EGaIn LM droplets under mechanical agitation to successfully modulate their electronic structures. The "liquid solution"-"liquid metal" interaction resulted in the formation of oxide and oxysulfide layers on the surface of LM droplets. The comprehensive study of electronic and optical properties revealed a decrease in the band gap of the droplets after surface decoration with MoO_x and MoO_xS_y, leading to deeper n-type doping of the materials. This method provides a facile procedure for engineering the electronic band structure of LM-based composites when they are necessary for various applications.

Formation of inorganic liquid gallium particle-manganese oxide composites

Gallium (Ga) is a low melting point post-transition metal that, under mild mechanical agitation, can form micron and submicron-sized particles with combined fluid-like and metallic properties. In this work, an inorganic network of Ga liquid metal particles was synthesised via spontaneous formation of manganese (Mn) oxide species on their liquid metallic surfaces forming an all-inorganic composite. The micron-sized Ga particles formed by sonication were connected together by Mn oxide nanostructures spontaneously established from the reduction of a Mn salt in aqueous solution slightly above the melting point of Ga. The formed Mn oxide nanostructures were found to coalesce from the surface of the Ga particles into a continuous inorganic network. The morphology of the composites could be altered by varying the Mn salt concentration and by performing post-treatment annealing. The composites presented a shell of various Mn oxide nanostructures including wrinkled sheets, rods and nanoneedles, around spherical liquid Ga particles, and a liquid metal core. The photoelectric and optical properties of the composites were thoroughly characterised, which revealed decreasing bandgaps and valence band edge characteristics as a function of increased Mn oxide coverage. The photoluminescence properties of the composites could be also engineered by increasing the Mn oxide coverage. The all-inorganic liquid Ga composite could be formed via a straightforward reduction reaction of a Mn-rich salt at the surface of liquid Ga particles with tunable surface properties for future optoelectronic applications.

Smart Eutectic Gallium-Indium: From Properties to Applications

Eutectic gallium-indium (EGaIn), a liquid metal with a melting point close to or below room temperature, has attracted extensive attention in recent years due to its excellent properties such as fluidity, high conductivity, thermal conductivity, stretchability, self-healing capability, biocompatibility, and recyclability. These features of EGaIn can be adjusted by changing the experimental condition, and various composite materials with extended properties can be further obtained by mixing EGaIn with other materials. In this review, not only the are unique properties of EGaIn introduced, but also the working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The applications of EGaIn in various fields, such as flexible electronics (sensors, antennas, electronic circuits), molecular electronics (molecular memory, opto-electronic switches, or reconfigurable junctions), energy catalysis (heat management, motors, generators, batteries), biomedical science (drug delivery, tumor therapy, bioimaging and neural interfaces) are reviewed. Finally, a critical discussion of the main challenges for the development of EGaIn-based techniques are discussed, and the potential applications in new fields are prospected.

Liquid Metal-Templated Tin-Doped Tellurium Films for Flexible Asymmetric Pseudocapacitors

Liquid metals can be surface activated to generate a controlled galvanic potential by immersing them in aqueous solutions. This creates energized liquid-liquid interfaces that can promote interfacial chemical reactions. Here we utilize this interfacial phenomenon of liquid metals to deposit thin films of tin-doped tellurium onto rigid and flexible substrates. This is accomplished by exposing liquid metals to a precursor solution of Sn²⁺ and HTeO₂⁺ ions. The ability to paint liquid metals onto substrates enables us to fabricate supercapacitor electrodes of liquid metal films with an intimately connected surface layer of tin-doped tellurium. The tin-doped tellurium exhibits a pseudocapacitive behavior in 1.0 M Na₂SO₄ electrolyte and records a specific capacitance of 184.06 F·g^-1 (5.74 mF·cm^-2) at a scan rate of 10 mV·s^-1. Flexible supercapacitor electrodes are also fabricated by painting liquid metals onto polypropylene sheets and subsequently depositing tin-doped tellurium thin films. These flexible electrodes show outstanding mechanical stability even when experiencing a complete 180° bend as well as exhibit high power and energy densities of 160 W·cm^-3 and 31 mWh·cm^-3, respectively. Overall, this study demonstrates the attractive features of liquid metals in creating energy storage devices and exemplifies their use as media for synthesizing electrochemically active materials.

MoS2 oxidative etching caught in the act: formation of single (MoO3) n molecules

We report the presence of sub-nm MoO x clusters formed on basal planes of the 2H MoS2 crystals during thermal oxidative etching in air at a temperature of 370 °C. Using high resolution non-contact atomic force microscopy (AFM) we provide a histogram of their preferred heights. The AFM results combined with density functional theory (DFT) simulations show remarkably well that the MoO x clusters are predominantly single MoO3 molecules and their dimers at the sulfur vacancies. Additional Raman spectroscopy, and energy and wavelength dispersive X-ray spectroscopies as well as Kelvin probe AFM investigations confirmed the presence of the MoO3/MoO x species covering the MoS2 surface only sparsely. The X-ray absorption near edge spectroscopy data confirm the MoO3 stoichiometry. Taken together, our results show that oxidative etching and removal of Mo atoms at the atomic level follow predominantly via formation of single MoO3 molecules. Such findings confirm the previously only proposed oxidative etching stoichiometry.

Emerging Role of Liquid Metals in Sensing

Low melting point metals and alloys are the group of materials that combine metallic and liquid properties, simultaneously. The fascinating characteristics of liquid metals (LMs) including softness and high electrical and thermal conductivity, as well as their unique interfacial chemistry, have started to dominate various research disciplines. Utilization of LMs as responsive interfaces, enabling sensing in a flexible and versatile manner, is one of the most promising traits demonstrated for LMs. In the context of LMs-enabled sensors, gallium (Ga) and its alloys have emerged as multipurpose functional materials with many compelling physical and chemical properties. Responsiveness to different stimuli and easy-to-functionalize interfaces of Ga-based LMs make them ideal candidates for a variety of sensing applications. However, despite the vast capabilities of Ga-based LMs in sensing, applications of these materials for developing different sensors have not been fully explored. In the present review, we provide a comprehensive overview regarding the applications of Ga-based LMs in a wide range of sensing approaches that cover different physical and chemical sensors. The unique features of Ga-based LMs, which make them promising materials for sensing, are discussed in subsections followed by relevant case studies. Finally, challenges as well as the prospected future and developing motifs are highlighted for each type of LM-based sensors.

Liquid metals: an ideal platform for the synthesis of two-dimensional materials

The surfaces of liquid metals can serve as a platform to synthesise two-dimensional materials. By exploiting the self-limiting Cabrera-Mott oxidation reaction that takes place at the surface of liquid metals exposed to ambient air, an ultrathin oxide layer can be synthesised and isolated. Several synthesis approaches based on this phenomenon have been developed in recent years, resulting in a diverse family of functional 2D materials that covers a significant fraction of the periodic table. These straightforward and inherently scalable techniques may enable the fabrication of novel devices and thus harbour significant application potential. This review provides a brief introduction to liquid metals and their alloys, followed by detailed guidance on each developed synthesis technique, post-growth processing methods, integration processes, as well as potential applications of the developed materials.

Recent advances in the fabrication of 2D metal oxides

Atomically thin two-dimensional (2D) metal oxides exhibit unique optical, electrical, magnetic, and chemical properties, rendering them a bright application prospect in high-performance smart devices. Given the large variety of both layered and non-layered 2D metal oxides, the controllable synthesis is the critical prerequisite for enabling the exploration of their great potentials. In this review, recent progress in the synthesis of 2D metal oxides is summarized and categorized. Particularly, a brief overview of categories and crystal structures of 2D metal oxides is firstly introduced, followed by a critical discussion of various synthesis methods regarding the growth mechanisms, advantages, and limitations. Finally, the existing challenges are presented to provide possible future research directions regarding the synthesis of 2D metal oxides. This work can provide useful guidance on developing innovative approaches for producing both 2D layered and non-layered nanostructures and assist with the acceleration of the research of 2D metal oxides.