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Recent studies have demonstrated that tellurene is a van der Waals (vdW) two-dimensional material with potential optoelectronic and thermoelectric applications as a result of its pseudo-one-dimensional structure and properties. Here, we report on the pressure induced anomalous phase transition of tellurium nanoribbons. The observation of clean phase transitions was made possible with high quality single crystalline Te nanoribbons that are synthesized by hydrothermal reaction growth. The results show that phase transition has a large pressure hysteresis and multiple competing phases: during compression, the phase transition is sudden and takes place from trigonal to orthorhombic phase at 6.5 GPa. Orthorhombic phase remains stable up to higher pressures (15 GPa). In contrast, phase transition is not sudden during decompression, but orthorhombic and trigonal phases co-exist between 6.9 to 3.4 GPa. Grüneisen parameter calculations further confirm the presence of co-existing phases and suggest hysteretic phase change behavior. Finally, orthorhombic to trigonal phase transition occurs at 3.4 GPa which means overall pressure hysteresis is around 3.1 GPa. 

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 MoSe₂and 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.