COMMUNICATION (1 of 8)deployed in the form of van der Waals (vdW) heterostructures that enable fascinating coupled properties from stacked individual layers of 2D sheets which can be exploited in several applications, [2,6,7] including tunneling transistors, [8] quantum hall systems, [9] electrochemical hydrogen evolution reaction, [10] optoelectronics, [4,11] and electronics. [3,12] The p-n junctions are the building blocks of the semiconductor industry, in which the p-n junction heterostructures made from ultrathin materials are of great interest in specialized electronics, optoelectronics, and photonics due to their intriguing coupled properties of the different crystals. [5][6][7]13] Several methods for exfoliation and/or deposition exist such as chemical vapor deposition (CVD), [7] pulsed laser deposition (PLD), [14] molecular beam epitaxy (MBE), [15] pick-and-lift vdW technique, [16] and mechanical exfoliation. [17] These conventional approaches are time consuming and require complicated fabrication processes, [18] yet resulting in devices with small effective areas. [19] Liquid metals are emerging materials which can be used in microfluidics components, [20] sensors, [21] electrodes, [22] phototransistors, [23] flexible and stretchable devices, [24] disease treatment, [22] biomedical field, [22] and in synthesis of low-dimensional materials. [25] Liquid metals have been shown to form a naturally occurring atomically thin layer of oxide at their interface with air, [26][27][28][29] and using liquid metal as a reaction solvent can give access to a sizable portion of oxide elements including oxides which are intrinsically nonlayered crystals. [26] The exfoliated oxides can be converted to sulfides and phosphates. [30] Combination of these atomically thin layers should provide a vast number of vdW heterostructures that are yet to be explored.In this work, atomically thin oxide skin of low melting point liquid metals of tin and indium including p-type tin oxide (SnO) [27] and n-type indium oxide (In 2 O 3 ) [31] are stacked to produce large-area heterostructures with a high degree of homogeneity. Indeed, the p-n vdW heterojunctions feature current rectification properties with exceptionally fast photoresponse times and high sensitivity for UV light. The demonstrated liquid metal synthesis framework offers the possibility of synthesizing and exploring a range of tailored heterostructures for applications in next-generation optoelectronic and photodetection devices.

Intriguing physical and chemical properties of atomically thin semiconductors provide avenues for the development of the next-generation electronics, optoelectronics, and sensing applications. However, many materials are intrinsically nonlayered and therefore difficult to obtain in two dimensions (2D) due to the presence of strong in-plane bonds. Here, we adopted liquid metal synthetic strategies to produce 2D gallium sulfide (Ga2S3), which is an intrinsically nonlayered material. The obtained monoclinic α-Ga2S3 has a relatively high field-effect mobility of 3.5 cm2 V–1 s–1 and features a p-type material with a bandgap of 2.1 eV. Photodetectors that are made based on these synthesized 2D Ga2S3 exhibit relatively strong photodetectivity of 1010 jones and photoresponsivity of 240 A W–1 in visible wavelengths. The 2D Ga2S3 is also found to be suitable for sensing of nitrogen dioxide (NO2) gas at low evaluated temperatures. Excellent electronic, optical, and gas sensing performance demonstrated in this work offers great promises for synthesizing high quality 2D materials based on the liquid metal framework.

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.

Ultrathin transparent conductive oxides (TCOs) are emerging candidates for next-generation transparent electronics. Indium oxide (In2O3) incorporated with post-transition-metal ions (e.g., Sn) has been widely studied due to their excellent optical transparency and electrical conductivity. However, their electron transport properties are deteriorated at the ultrathin two-dimensional (2D) morphology compared to that of intrinsic In2O3. Here, we explore the domain of transition-metal dopants in ultrathin In2O3 with the thicknesses down to the single-unit-cell limit, which is realized in a large area using a low-temperature liquid metal printing technique. Zn dopant is selected as a representative to incorporate into the In2O3 rhombohedral crystal framework, which results in the gradual transition of the host to quasimetallic. While the optical transmittance is maintained above 98%, an electron field-effect mobility of up to 87 cm2 V–1 s–1 and a considerable sub-kΩ–1 cm–1 ranged electrical conductivity are achieved when the Zn doping level is optimized, which are in a combination significantly improved compared to those of reported ultrathin TCOs. This work presents various opportunities for developing high-performance flexible transparent electronics based on emerging ultrathin TCO candidates.