This section focuses on the fundamental properties of nanostructured WO x and start with its various crystal structures and the conditions for phase transitions between these structures. The structures of nonstoichiometric WO x and WO 3 hydrates Nanostructured Tungsten Oxide -Properties, Synthesis, and Applications Metal oxides are the key ingredients for the development of many advanced functional materials and smart devices. Nanostructuring has emerged as one of the best tools to unlock their full potential. Tungsten oxides (WO x ) are unique materials that have been rigorously studied for their chromism, photocatalysis, and sensing capabilities. However, they exhibit further important properties and functionalities that have received relatively little attention in the past. This Feature Article presents a general review of nanostructured WO x , their properties, methods of synthesis, and a description of how they can be used in unique ways for different applications.

Two-dimensional (2D) oxides have a wide variety of applications in electronics and other technologies. However, many oxides are not easy to synthesize as 2D materials through conventional methods. We used nontoxic eutectic gallium-based alloys as a reaction solvent and co-alloyed desired metals into the melt. On the basis of thermodynamic considerations, we predicted the composition of the self-limiting interfacial oxide. We isolated the surface oxide as a 2D layer, either on substrates or in suspension. This enabled us to produce extremely thin subnanometer layers of HfO, AlO, and GdO The liquid metal-based reaction route can be used to create 2D materials that were previously inaccessible with preexisting methods. The work introduces room-temperature liquid metals as a reaction environment for the synthesis of oxide nanomaterials with low dimensionality.

Nitrogen dioxide (NO2) is a gas species that plays an important role in certain industrial, farming, and healthcare sectors. However, there are still significant challenges for NO2 sensing at low detection limits, especially in the presence of other interfering gases. The NO2 selectivity of current gas-sensing technologies is significantly traded-off with their sensitivity and reversibility as well as fabrication and operating costs. In this work, we present an important progress for selective and reversible NO2 sensing by demonstrating an economical sensing platform based on the charge transfer between physisorbed NO2 gas molecules and two-dimensional (2D) tin disulfide (SnS2) flakes at low operating temperatures. The device shows high sensitivity and superior selectivity to NO2 at operating temperatures of less than 160 °C, which are well below those of chemisorptive and ion conductive NO2 sensors with much poorer selectivity. At the same time, excellent reversibility of the sensor is demonstrated, which has rarely been observed in other 2D material counterparts. Such impressive features originate from the planar morphology of 2D SnS2 as well as unique physical affinity and favorable electronic band positions of this material that facilitate the NO2 physisorption and charge transfer at parts per billion levels. The 2D SnS2-based sensor provides a real solution for low-cost and selective NO2 gas sensing.

The properties and applications of molybdenum oxides are reviewed in depth. Molybdenum is found in various oxide stoichiometries, which have been employed for different high-value research and commercial applications. The great chemical and physical characteristics of molybdenum oxides make them versatile and highly tunable for incorporation in optical, electronic, catalytic, bio, and energy systems. Variations in the oxidation states allow manipulation of the crystal structure, morphology, oxygen vacancies, and dopants, to control and engineer electronic states. Despite this overwhelming functionality and potential, a definitive resource on molybdenum oxide is still unavailable. The aim here is to provide such a resource, while presenting an insightful outlook into future prospective applications for molybdenum oxides.

In the quest to discover the properties of planar semiconductors, two‐dimensional molybdenum trioxide and dichalcogenides have recently attracted a large amount of interest. This family, which includes molybdenum trioxide (MoO3), disulphide (MoS2), diselenide (MoSe2) and ditelluride (MoTe2), possesses many unique properties that make its compounds appealing for a wide range of applications. These properties can be thickness dependent and may be manipulated via a large number of physical and chemical processes. In this Feature Article, a comprehensive review is delivered of the fundamental properties, synthesis techniques and applications of layered and planar MoO3, MoS2, MoSe2, and MoTe2 along with their future prospects.

appreciable free carrier concentration. [5][6][7] The same strategy can potentially be implemented in selected 2D semiconductors. Another concern is the damping losses that should be kept low for applications such as optical communications, in which a long propagation of waves is necessary. [ 7 ] Reducing such damping losses requires that the product of the effective electron mass and the free charge mobility must be large in the 2D material. As a result, fi nding favorable materials that satisfy the aforementioned conditions are necessary for advancing the fi eld of 2D plasmonics.The creation of stable 2D semiconducting oxides of tungsten and molybdenum is possible, as we demonstrated previously. [ 8,9 ] In a recent topical feature article, Gregorieva and Geim have separated out these oxides as a unique group of 2D materials and predicted their signifi cant role in the future of planar structures. [ 10 ] The impact of these two metal oxides can be extended into the plasmonic realm, and, in fact, plasmon resonances in the one-dimensional (1D) morphologies of these two oxides have recently been demonstrated. Manthiram and Alivisatos reported plasmon resonances in 1D sub-stoichiometric semiconducting tungsten oxide, [ 6 ] while Huang et al. have shown the generation of plasmon resonances in 1D tubular reduced molybdenum oxide suspensions. [ 5 ] Advantageously both tungsten and molybdenum oxides can be ultra-doped and have also large dielectric constants, which both are important factors for obtaining plasmon resonances in the near IR and visible regions. [ 2 ] In 1D sub-stoichiometric tungsten and molybdenum oxides, the plasmon resonances are a function of two modest depolarization factors along the cross section of the 1D structure ( Figure 1 a -Supporting Information, Section S1 for the equations). However, the existence of one large depolarization factor reduces the wavelength of the plasmon resonances in 2D structures of similar stoichiometry.Accordingly, here, we explore tunable plasmonics in substoichiometric 2D molybdenum oxide nanofl akes in the visible range. The unique properties of 2D molybdenum oxide such as the possibility of high level ionic intercalation (hence ultradoping), large permittivity and the effect of the depolarization factor in 2D fl akes are used for demonstrating tunable plasmon resonance in this range. We investigate the effect of intercalating ions and changing the lateral dimensions of the fl akes on the plasmon resonance peaks of a reduced quasi-metallic form of molybdenum oxide.Molybdenum trioxide (MoO 3 ) is a stable n -type semiconductor under a wide range of conditions with a bandgap of ca. 3.2 eV, which is capable of adsorbing energy from a small portion of the visible light spectrum. [ 5,11 ] The most frequently 2D materials exhibit certain physical and chemical properties that are fundamentally different from their bulk counterparts. [ 1,2 ] The electronic and optical properties seen in the selected 2D materials may lead to signifi cantly altered plasmon dispersion relationsh...

We demonstrate that the energy bandgap of layered, high-dielectric α-MoO(3) can be reduced to values viable for the fabrication of 2D electronic devices. This is achieved through embedding Coulomb charges within the high dielectric media, advantageously limiting charge scattering. As a result, devices with α-MoO(3) of ∼11 nm thickness and carrier mobilities larger than 1100 cm(2) V(-1) s(-1) are obtained.

Two-dimensional (2D) transition metal dichalcogenide semiconductors offer unique electronic and optical properties, which are significantly different from their bulk counterparts. It is known that the electronic structure of 2D MoS2, which is the most popular member of the family, depends on the number of layers. Its electronic structure alters dramatically at near atomically thin morphologies, producing strong photoluminescence (PL). Developing processes for controlling the 2D MoS2 PL is essential to efficiently harness many of its optical capabilities. So far, it has been shown that this PL can be electrically or mechanically gated. Here, we introduce an electrochemical approach to actively control the PL of liquid-phase-exfoliated 2D MoS2 nanoflakes by manipulating the amount of intercalated ions including Li(+), Na(+), and K(+) into and out of the 2D crystal structure. These ions are selected as they are crucial components in many bioprocesses. We show that this controlled intercalation allows for large PL modulations. The introduced electrochemically controlled PL will find significant applications in future chemical and bio-optical sensors as well as optical modulators/switches.