Janus crystals represent an exciting class of 2D materials with different atomic species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an electric field and leads to a wealth of novel properties, such as large Rashba spin–orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe2 and via plasma stripping followed thermal annealing of MoS2. However, the high processing temperatures prevent growth of other Janus materials and their heterostructures. Here, a room‐temperature technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low‐energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room‐temperature method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.

Newly discovered 2D Janus transition metal dichalcogenides layers have gained much attention from theory perspective owing to their unique atomic structure and exotic materials properties, but little to no experimental data are available on these materials. Here, our experimental and theoretical studies establish the vibrational and optical behavior of 2D Janus S-W-Se and S-Mo-Se monolayers under high pressures for the first time. CVDgrown classical TMD monolayers were first transferred onto vdW mica substrates and converted to 2D Janus sheets by surface plasma technique, and then integrated into 500µm size diamond anvil cell for high-pressure studies. Our results show that 2D Janus layers do not undergo phase transition up to 15 GPa, and in this pressure regime, their vibrational modes exhibit a non-monotonic response to the applied pressures (d/dP). Interestingly, these 2D Janus monolayers exhibit unique blue-shift in PL upon compression which is in contrast to many other traditional semiconductor materials. Overall theoretical simulations offer in-depth insights and reveal that the overall optical response is a result of competition between the ab-plane (blue-shift) and c-axis (red-shift) compression. Overall findings shed the very first light on how 2D Janus monolayers respond under extreme pressures and expand the fundamental understanding of these materials.Results mark the first high pressure studies on 2D Janus monolayers. Pressure studies show that 2D Janus monolayers S-Mo-Se and S-W-Se do not undergo phase transition up to 15GPa, but their vibrational modes exhibit a non-monotonic response to pressure. Janus S-W-Se monolayer undergo anomalous blue-shifting behavior and direct to indirect bandgap transition.

The family of layered BiTeX (X = Cl, Br, I) compounds are intrinsic Janus semiconductors with giant Rashba-splitting and many exotic surface and bulk physical properties. To date, studies on these materials required mechanical exfoliation from bulk crystals which yielded thick sheets in nonscalable sizes. Here, we report epitaxial synthesis of Janus BiTeCl and BiTeBr sheets through a nanoconversion technique that can produce few triple layers of Rashba semiconductors (<10 nm) on sapphire substrates. The process starts with van der Waals epitaxy of Bi2Te3 sheets on sapphire and converts these sheets to BiTeCl or BiTeBr layers at high temperatures in the presence of chemically reactive BiCl3/BiBr3 inorganic vapor. Systematic Raman, XRD, SEM, EDX, and other studies show that highly crystalline BiTeCl and BiTeBr sheets can be produced on demand. Atomic level growth mechanism is also proposed and discussed to offer further insights into growth process steps. Overall, this work marks the direct deposition of 2D Janus Rashba materials and offers pathways to synthesize other Janus compounds belonging to MXY family members.

We report the synthesis of layered anisotropic semiconductor GeSe and GeSe2 nanomaterials through low temperature and atmospheric pressure chemical vapor deposition using halide based precursors. The crystal phase is controlled by simply changing selenium vapor pressure.

While MAX phases offer an exotic combination of ceramic and metallic properties, rendering them a unique class of materials, their applications remain virtually hypothetical. To overcome this shortcoming, a sol-gel...