Silicene, the ultimate scaling of a silicon atomic sheet in a buckled honeycomb lattice, represents a monoelemental class of two-dimensional (2D) materials similar to graphene but with unique potential for a host of exotic electronic properties. Nonetheless, there is a lack of experimental studies largely due to the interplay between material degradation and process portability issues. This review highlights the state-of-the-art experimental progress and future opportunities in the synthesis, characterization, stabilization, processing and experimental device examples of monolayer silicene and its derivatives. The electrostatic characteristics of the Ag-removal silicene field-effect transistor exhibit ambipolar charge transport, corroborating with theoretical predictions on Dirac fermions and Dirac cone in the band structure. The electronic structure of silicene is expected to be sensitive to substrate interaction, surface chemistry, and spin-orbit coupling, holding great promise for a variety of novel applications, such as topological bits, quantum sensing, and energy devices. Moreover, the unique allotropic affinity of silicene with single-crystalline bulk silicon suggests a more direct path for the integration with or revolution to ubiquitous semiconductor technology. Both the materials and process aspects of silicene research also provide transferable knowledge to other Xenes like stanene, germanene, phosphorene, and so forth.

Electrostatic gating of two-dimensional (2D) materials with ionic liquids (ILs), leading to the accumulation of high surface charge carrier densities, has been often exploited in 2D devices. However, the intrinsic liquid nature of ILs, their sensitivity to humidity, and the stress induced in frozen liquids inhibit ILs from constituting an ideal platform for electrostatic gating. Here we report a lithium-ion solid electrolyte substrate, demonstrating its application in highperformance back-gated n-type MoS 2 and p-type WSe 2 transistors with sub-threshold values approaching the ideal limit of 60 mV/dec and complementary inverter amplifier gain of 34, the highest among comparable amplifiers. Remarkably, these outstanding values were obtained under 1 V power supply. Microscopic studies of the transistor channel using microwave impedance microscopy reveal a homogeneous channel formation, indicative of a smooth interface between the TMD and underlying electrolytic substrate. These results establish lithium-ion substrates as a promising alternative to ILs for advanced thin-film devices.

Here, two novel approaches for disassembling epitaxial silicene from the native substrate and transferring onto arbitrary target substrates are presented. From the processing perspective, the two methodologies open up a new route for handling silicene, and in general any epitaxial Xene, in view of establishing reliable process flows for the development of a Xenebased nanotechnology. Integration of silicene in a back-gated controlled device architecture is demonstrated and the built up of unique multi-stack heterostructures between silicene, but potentially every Xene, and other technological relevant materials, like transparent conductive oxides and transition metal dichalcogenides is shown.

Molybdenum trioxide (MoO3), an important transition metal oxide (TMO), has been extensively investigated over the past few decades due to its potential in existing and emerging technologies, including catalysis, energy and data storage, electrochromic devices, and sensors. Recently, the growing interest in two-dimensional (2D) materials, often rich in interesting properties and functionalities compared to their bulk counterparts, has led to the investigation of 2D MoO3. However, the realization of large-area true 2D (single to few atom layers thick) MoO3 is yet to be achieved. Here, we demonstrate a facile route to obtain wafer-scale monolayer amorphous MoO3 using 2D MoS2 as a starting material, followed by UV–ozone oxidation at a substrate temperature as low as 120 °C. This simple yet effective process yields smooth, continuous, uniform, and stable monolayer oxide with wafer-scale homogeneity, as confirmed by several characterization techniques, including atomic force microscopy, numerous spectroscopy methods, and scanning transmission electron microscopy. Furthermore, using the subnanometer MoO3 as the active layer sandwiched between two metal electrodes, we demonstrate the thinnest oxide-based nonvolatile resistive switching memory with a low voltage operation and a high ON/OFF ratio. These results (potentially extendable to other TMOs) will enable further exploration of subnanometer stoichiometric MoO3, extending the frontiers of ultrathin flexible oxide materials and devices.

To achieve large area growth of transition metal dichalcogenides of uniform monolayer thickness, we demonstrate metal–organic chemical vapor deposition (MOCVD) growth under low pressure followed by a high-temperature sulfurization process under atmospheric pressure (AP). Following sulfurization, the MOCVD-grown continuous MoS 2 film transforms into compact triangular crystals of uniform monolayer thickness as confirmed from the sharp distinct photoluminescence peak at 1.8 eV. Raman and X-ray photoelectron spectroscopies confirm that the structural disorders and chalcogen vacancies inherent to the as-grown MOCVD film are substantially healed and carbon/oxygen contaminations are heavily suppressed. The as-grown MOCVD film has a Mo/S ratio of 1:1.6 and an average defect length of ∼1.56 nm, which improve to 1:1.97 and ∼21 nm, respectively, upon sulfurization. The effect of temperature and duration of the sulfurization process on the morphology and stoichiometry of the grown film is investigated in detail. Compared to the APCVD growth, this two-step growth process shows more homogenous distribution of the triangular monolayer MoS 2 domains across the entire substrate, while demonstrating comparable electrical performance.

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We investigate the ability of different carrier gases to control defects and secondary nucleation in atmospheric pressure chemical vapor deposition (APCVD) growth of MoS 2 on Si/SiO 2 substrates. We observe that a reducing environment using H 2 gas is more favorable for achieving uniform two-dimensional (2D) growth. Compared to the growth in an inert environment, secondary nucleation on primary MoS 2 domains grown using H 2 as the carrier gas (H−MoS 2 ) is drastically reduced. We employ a phase-field model to understand the role of enhanced surface diffusion in H−MoS 2 , due to passivation of defects and dangling bonds, promoting compact secondary domain formation as opposed to dendritic secondary domains under an inert environment. Using X-ray photoelectron spectroscopy, we show that the Mo(VI) oxidation state (corresponding to MoO 3 ), which is prominent for MoS 2 grown under an inert atmosphere, is highly suppressed in H−MoS 2 , leading to more pristine MoS 2 . This explains the superior electrical performance of H−MoS 2 compared to those grown with other carrier gases. Our results offer a facile route to explore different growth environments to realize large-area true 2D films.

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