Highlights
Two-dimensional materials including TMDCs, hBN, graphene, non-layered compounds, black phosphorous, Xenes and other emerging materials with large lateral dimensions exceeding a hundred ...micrometres are summarised detailing their synthetic strategies.
Crystal quality optimisations and defect engineering are discussed for large-area two-dimensional materials synthesis.
Electronics and optoelectronics applications enabled by large-area two-dimensional materials are explored.
.
Large-area and high-quality two-dimensional crystals are the basis for the development of the next-generation electronic and optical devices. The synthesis of two-dimensional materials in wafer scales is the first critical step for future technology uptake by the industries; however, currently presented as a significant challenge. Substantial efforts have been devoted to producing atomically thin two-dimensional materials with large lateral dimensions, controllable and uniform thicknesses, large crystal domains and minimum defects. In this review, recent advances in synthetic routes to obtain high-quality two-dimensional crystals with lateral sizes exceeding a hundred micrometres are outlined. Applications of the achieved large-area two-dimensional crystals in electronics and optoelectronics are summarised, and advantages and disadvantages of each approach considering ease of the synthesis, defects, grain sizes and uniformity are discussed.
Two-dimensional (2D) crystals are promising materials for developing future nano-enabled technologies
. The cleavage of weak, interlayer van der Waals bonds in layered bulk crystals enables the ...production of high-quality 2D, atomically thin monolayers
. Nonetheless, as earth-abundant compounds, metal oxides are rarely accessible as pure and fully stoichiometric monolayers owing to their ion-stabilized 'lamellar' bulk structure
. Here, we report the discovery of a layered planar hexagonal phase of oxides from elements across the transition metals, post-transition metals, lanthanides and metalloids, derived from strictly controlled oxidation at the metal-gas interface. The highly crystalline monolayers, without the support of ionic dopants or vacancies, can easily be mechanically exfoliated by stamping them onto substrates. Monolayer and few-layered hexagonal TiO
are characterized as examples, showing p-type semiconducting properties with hole mobilities of up to 950 cm
V
s
at room temperature. The strategy can be readily extended to a variety of elements, possibly expanding the exploration of metal oxides in the 2D quantum regime.
Two-dimensional piezotronics will benefit from the emergence of new crystals featuring high piezoelectric coefficients. Gallium phosphate (GaPO
) is an archetypal piezoelectric material, which does ...not naturally crystallise in a stratified structure and hence cannot be exfoliated using conventional methods. Here, we report a low-temperature liquid metal-based two-dimensional printing and synthesis strategy to achieve this goal. We exfoliate and surface print the interfacial oxide layer of liquid gallium, followed by a vapour phase reaction. The method offers access to large-area, wide bandgap two-dimensional (2D) GaPO
nanosheets of unit cell thickness, while featuring lateral dimensions reaching centimetres. The unit cell thick nanosheets present a large effective out-of-plane piezoelectric coefficient of 7.5 ± 0.8 pm V
. The developed printing process is also suitable for the synthesis of free standing GaPO
nanosheets. The low temperature synthesis method is compatible with a variety of electronic device fabrication procedures, providing a route for the development of future 2D piezoelectric materials.
Silicon photonics has demonstrated great potential in ultrasensitive biochemical sensing. However, it is challenging for such sensors to detect small ions which are also of great importance in many ...biochemical processes. A silicon photonic ion sensor enabled by an ionic dopant–driven plasmonic material is introduced here. The sensor consists of a microring resonator (MRR) coupled with a 2D restacked layer of near‐infrared plasmonic molybdenum oxide. When the 2D plasmonic layer interacts with ions from the environment, a strong change in the refractive index results in a shift in the MRR resonance wavelength and simultaneously the alteration of plasmonic absorption leads to the modulation of MRR transmission power, hence generating dual sensing outputs which is unique to other optical ion sensors. Proof‐of‐concept via a pH sensing model is demonstrated, showing up to 7 orders improvement in sensitivity per unit area across the range from 1 to 13 compared to those of other optical pH sensors. This platform offers the unique potential for ultrasensitive and robust measurement of changes in ionic environment, generating new modalities for on‐chip chemical sensors in the micro/nanoscale.
A silicon photonic ion sensor enabled by a dopant‐driven 2D restacked layer of near‐infrared plasmonic molybdenum oxide is introduced. It has dual sensing outputs due to the refractive index and optical absorption changes of the 2D material in different ionic environments. The demonstrated pH sensing model shows up to 7 orders improvement in sensitivity per unit area.
Eutectic gallium‐indium (EGaIn) liquid metal droplets have been considered as a suitable platform for producing customized 3D composites with functional nanomaterials owing to their soft and highly ...reductive surface. Herein, the synthesis of a 3D plasmonic oxide framework (POF) is reported by incorporating the ultra‐thin angstrom‐scale‐porous hexagonal molybdenum oxide (h‐MoO3) onto the spherical EGaIn nanodroplets through ultrasonication. Simultaneously, a large number of oxygen vacancies form in h‐MoO3, boosting its free charge carrier concentration and therefore generating a broad surface plasmon resonance across the whole visible light spectrum. The plasmonic chemical sensing properties of the POF is investigated by the surface‐enhanced Raman scattering detection of rhodamine 6G (R6G) at 532 nm, in which the minimum detectable concentration is 10−8 m and the enhancement factor reached up to 6.14 × 106. The extended optical absorption of the POF also allowed the efficient degradation of the R6G dye under the excitation of ultraviolet‐filtered simulated solar light. Furthermore, the POF exhibits remarkable photocurrent responses towards the entire visible light region with the maximum response of ≈1588 A W−1 at 455 nm. This work demonstrates the great potential of the liquid metal‐based POFs for high‐performance sensing, catalytic, and optoelectronic devices.
A liquid metal‐based 3D plasmonic oxide framework (POF) is developed, which consists of eutectic EGaIn nanodroplets coated with sub‐stoichiometric ultra‐thin hexagonal MoO3–x. The POF shows a broadband surface plasmon resonance across the visible and near‐infrared region. Together with the ultra‐high surface active area, the 3D POF demonstrates excellent performances in chemical sensing, photocatalytic, and optoelectronic applications.
Functional materials coated on optical fibers have demonstrated great potential for optical gas sensing applications. However, their sensitivity is typically limited to the sub‐parts per million ...(sub‐ppm) range. Here, for the first time a 2D near‐infrared plasmonic tungsten oxide (WOx) enabled ultrasensitive fiber optics gas sensor on a side‐polished D‐shape single mode optical fiber is presented. The plasmon resonance wavelength range of 2D WOx is matched with a conventional telecommunications wavelength of 1550 nm for driving the optical fiber, therefore inducing a strong light–matter interaction. Upon the surface adsorption of gas molecules, free electrons in the 2D WOx body are redistributed changing the plasmon resonance properties and hence the transmission through the optical fiber. The sensor is selectively responsive to NO2 at concentrations down to 44 parts per billion (ppb) with a limit of detection of 8 ppb at a relatively low elevated temperature. Such an excellent sensing performance is significantly improved over the previously reported fiber optics NO2 sensors, which suggests the integration of 2D plasmonic degenerated semiconductors as a viable approach to develop high‐performance fiber optics gas sensors.
A 2D near‐infrared plasmonic tungsten oxide enabled ultrasensitive fiber optics gas sensor on a side‐polished D‐shape single mode optical fiber is presented. The sensor is selectively responsive to NO2 at concentrations down to 44 parts per billion (ppb) with a limit of detection of 8 ppb.
Heterostructures assembled from atomically thin materials have led to a new paradigm in the development of the next‐generation high‐performing functional devices. However, the construction of the ...ultrathin van der Waals (vdW) heterostructures is challenging and/or limited to materials with layered crystal structures. Herein, liquid metal vdW transfer method is used to construct large area heterostructures of atomically thin metal oxides of p‐SnO/n‐In2O3 with ease. The heterostructure exhibits both outstanding photodetectivity of 5 × 109 Jones and photoresponsivity of 1047 A W−1 with fast response time of ≤1 ms under illumination of the 280 nm light. Such excellent performances are due to the formation of the narrow bandgap of the staggered gap at the p–n junction produced by the high‐quality SnO/In2O3 heterostructure. The facile production of high‐quality vdW heterostructures using the liquid metal–based method therefore provides a promising pathway for realizing future optoelectronic devices.
This work illustrates a framework for the development of van der Waals (vdW) heterostructures using atomically thin surface oxides of the low melting point liquid metals. vdW heterostructures that are made from printing surface oxides of liquid indium (In2O3) and tin (SnO) on top of each other have efficient and fast response features for photodetection.
Wide bandgap semiconducting oxides are emerging as potential 2D materials for transparent electronics and optoelectronics. This fuels the quest for discovering new 2D metal oxides with ultrahigh ...transparency and high mobility. While the former can be achieved by reducing the thickness of oxide films to only a few nanometers, the latter is more commonly realized by intentional doping. This article reports a one‐step synthesis of few‐unit‐cell‐thick and laterally large antimony‐doped indium oxide (IAO). The doping process occurs spontaneously when the oxide is grown on the surface of a molten Sb–In alloy and 2D IAO nanosheets can be easily printed onto desired substrates. With thicknesses at the atomic scale, these materials exhibit excellent transparency exceeding 98% across the visible and near‐infrared range. Field‐effect transistors based on low‐doped IAO nanosheets reveal a high electron mobility of ≈40 cm2 V−1 s−1. Additionally, a notable photoresponse is observed in 2D IAO‐based photodetectors under ultraviolet (UV) radiation. Photoresponsivities of low‐doped and highly doped IAO at a wavelength of 285 nm are found to be 1.2 × 103 and 0.7 × 103 A W−1, respectively, identifying these materials as promising candidates for the fabrication of high‐performance optoelectronics in the UV region.
2D crystalline antimony‐doped indium oxide nanosheets with few‐atom thicknesses and laterally large dimensions are synthesized utilizing a single‐step, scalable liquid metal printing technique. The work proposes a viable pathway for realizing ultrathin transparent semiconducting oxides with enhanced electronic and optical properties, providing a fascinating semiconducting platform for next‐generation optoelectronics, neuromorphic devices, and beyond.
Nitrogen-doped carbon catalysts prepared from amino-functionalized metal–organic frameworks amino-MIL-101(Al) were investigated for the oxygen-reduction reaction (ORR) with special emphasis on ...elucidating the role of different nitrogen species (e.g., pyridinic, pyrrolic, and quaternary N) as active catalytic sites. Careful optimization of pyrolysis temperature of the amino-MIL-101(Al) leveraged the synthesis of the catalysts with or without quaternary N functionalities. This allowed us to investigate the type(s) of N species responsible for the ORR catalysis and thus address the conflicting results reported so far regarding the pyridinic and/or quaternary N as active sites for ORR catalysis via four-electron transfer (4e–) pathways. Our findings suggest that the total nitrogen content in the catalysts does not influence the ORR, while the quaternary N sites exclusively catalyze the reduction of O2 via the 4e– transfer pathway in both alkaline and acidic electrolytes. Catalysts containing only pyridinic and pyrrolic N were observed to be ineffective for the ORR. The experimental results were further supported by computational simulation using the gradient–correlated density functional theory which revealed that the dissociative O2 adsorption (i.e., binding and cleavage of OO bonds) is more favorable to quaternary N. Furthermore, calculations based on the relative surface potential energy, dipole moment, binding energy, and electron density indicate that the most stable structure of O2 chemisorption sites could only be achieved on the quaternary N carbon.
2D metal sulphides (MSs) have attracted enormous amounts of attention in developing high‐performance gas sensors. 2D noble metal sulphides and their derivatives, however, have been less studied due ...to their predominant nonlayered crystal structures for inefficient exfoliation, despite their surface and peculiar optoelectronic properties. Herein, we successfully synthesize 2D palladium sulphate (PdSO4) from palladium sulphide (PdS) bulk crystals by liquid‐phase exfoliation, in which the presence of oxygen species in the exfoliation solvent plays a key role in the sulphate transformation. Ultrathin 2D PdSO4 planar nanosheets, with thicknesses of ≈3 nm and submicrometer lateral dimensions, exhibit a broad absorption across the visible spectrum, a narrow bandgap of ≈1.35 eV, and a nanosecond scaled long exciton lifetime, which are all suitable for the visible‐light‐driven optoelectronic gas sensing applications. The 2D PdSO4‐based sensor demonstrates a reversible, selective, and sensitive response toward ppb‐leveled NO2 gas at blue light irradiation, featuring a response factor of ≈3.28% for 160 ppb NO2, a low limit of detection of 1.84 ppb, and a > 3 times response factor enhancement over other gases. Herein, the possibility of realizing 2D ultrathin noble metal sulphide compounds from their nonlayered crystal structures and strong potentials in developing high‐performance chemical sensors is explored.
2D palladium sulphate (PdSO4) nanosheets have successfully been delaminated from palladium sulphide bulk crystals utilizing liquid‐phase exfoliation featuring sub‐micron lateral dimensions, nano‐scale thicknesses, and a narrow bandgap. Upon blue light irradiation, 2D PdSO4‐based sensor exhibits reversible, selective, and sensitive responses toward nitrogen dioxide (NO2) gas with a response factor of 3.28% for 160 parts per billion (ppb).
PDF
