MoTe2 is an exfoliable transition metal dichalcogenide (TMD) which crystallizes in three symmetries; the semiconducting trigonal-prismatic 2H−phase, the semimetallic 1T ′ monoclinic phase, and the semimetallic orthorhombic T d structure 1-4 . The 2H−phase displays a band gap of ∼ 1 eV 5 making it appealing for flexible and transparent optoelectronics. The T d−phase is predicted to possess unique topological properties 6-9 which might lead to topologically protected non-dissipative transport channels 9 . Recently, it was argued that it is possible to locally induce phasetransformations in TMDs 3,10,11,14 , through chemical doping 12 , local heating 13 , or electric-field 14,15 to achieve ohmic contacts or to induce useful functionalities such as electronic phase-change memory elements 11 . The combination of semiconducting and topological elements based upon the same compound, might produce a new generation of high performance, low dissipation optoelectronic elements. Here, we show that it is possible to engineer the phases of MoTe2 through W substitution by unveiling the phase-diagram of the Mo1−xWxTe2 solid solution which displays a semiconducting to semimetallic transition as a function of x. We find that only ∼ 8 % of W stabilizes the T d−phase at room temperature. Photoemission spectroscopy, indicates that this phase possesses a Fermi surface akin to that of WTe2 16 .The properties of semiconducting and of semimetallic MoTe 2 are of fundamental interest in their own right, but are also for their potential technological relevance. In the mono-or few-layer limit it is a direct-gap semiconductor, while the bulk has an indirect bandgap 5,17,18 of ∼ 1 eV. The size of the gap is similar to that of Si, making 2H−MoTe 2 particularly appealing for both purely electronic devices 19,20 and optoelectronic applications 21 . Moreover, the existence of different phases opens up the possibility for many novel devices and architectures. For example, controlled conversion of the 1T ′ −MoTe 2 phase to the 2H−phase, as recently reported 22 , could

Two-dimensional materials are subject to intrinsic and dynamic rippling that modulates their optoelectronic and electromechanical properties. Here, we directly visualize the dynamics of these processes within monolayer transition metal dichalcogenide MoS2 using femtosecond electron scattering techniques as a real-time probe with atomic-scale resolution. We show that optical excitation induces large-amplitude in-plane displacements and ultrafast wrinkling of the monolayer on nanometer length-scales, developing on picosecond time-scales. These deformations are associated with several percent peak strains that are fully reversible over tens of millions of cycles. Direct measurements of electron-phonon coupling times and the subsequent interfacial thermal heat flow between the monolayer and substrate are also obtained. These measurements, coupled with first-principles modeling, provide a new understanding of the dynamic structural processes that underlie the functionality of two-dimensional materials and open up new opportunities for ultrafast strain engineering using all-optical methods.

Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to an unexpected large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. This work establishes a new method for dynamic, nonequilibrium tuning of correlation-driven dispersive interactions and of the optomechanical functionality of TMDC quasi-two-dimensional materials.