Structural Phase Transitions by Design in Monolayer Alloys

Two-dimensional monolayer materials are a highly anomalous class of materials under vigorous exploration. Mo- and W-dichalcogenides are especially unusual two-dimensional materials because they exhibit at least three different monolayer crystal structures with strongly differing electronic properties. This intriguing yet poorly understood feature, which is not present in graphene, may support monolayer phase engineering, phase change memory and other applications. However, knowledge of the relevant phase boundaries and how to engineer them is lacking. Here we show using alloy models and state-of-the-art density functional theory calculations that alloyed MoTe₂–WTe₂ monolayers support structural phase transitions, with phase transition temperatures tunable over a large range from 0 to 933 K. We map temperature–composition phase diagrams of alloys between pure MoTe₂ and pure WTe₂, and benchmark our methods to analogous experiments on bulk materials. Our results suggest applications for two-dimensional materials as phase change materials that may provide scale, flexibility, and energy consumption advantages

Strain Engineering in Monolayer Materials Using Patterned Adatom Adsorption

We utilize reactive empirical bond order (REBO)-based interatomic potentials to explore the potential for the engineering of strain in monolayer materials using lithographically or otherwise patterned adatom adsorption. In the context of graphene, we discover that the monolayer strain results from a competition between the in-plane elasticity and out-of-plane relaxation deformations. For hydrogen adatoms on graphene, the strain outside the adsorption region vanishes due to out-of-plane relaxation deformations. Under some circumstances, an annular adsorption pattern generates homogeneous tensile strains of approximately 2% in graphene inside the adsorption region, approximately 30% of the strain in the adsorbed region. We find that an elliptical adsorption pattern produces strains of as large as 5%, close to the strain in the adsorbed region. Also, nonzero maximum shear strain (∼4%) can be introduced by the elliptical adsorption pattern. We find that an elastic plane stress model provides qualitative guidance for strain magnitudes and conditions under which strain-diminishing buckling can be avoided. We identify geometric conditions under which this effect has potential to be scaled to larger areas. Our results elucidate a method for strain engineering at the nanoscale in monolayer devices

Data Mining for New Two- and One-Dimensional Weakly Bonded Solids and Lattice-Commensurate Heterostructures

Layered materials held together by weak interactions including van der Waals forces, such as graphite, have attracted interest for both technological applications and fundamental physics in their layered form and as an isolated single-layer. Only a few dozen single-layer van der Waals solids have been subject to considerable research focus, although there are likely to be many more that could have superior properties. To identify a broad spectrum of layered materials, we present a novel data mining algorithm that determines the dimensionality of weakly bonded subcomponents based on the atomic positions of bulk, three-dimensional crystal structures. By applying this algorithm to the Materials Project database of over 50,000 inorganic crystals, we identify 1173 two-dimensional layered materials and 487 materials that consist of weakly bonded one-dimensional molecular chains. This is an order of magnitude increase in the number of identified materials with most materials not known as two- or one-dimensional materials. Moreover, we discover 98 weakly bonded heterostructures of two-dimensional and one-dimensional subcomponents that are found within bulk materials, opening new possibilities for much-studied assembly of van der Waals heterostructures. Chemical families of materials, band gaps, and point groups for the materials identified in this work are presented. Point group and piezoelectricity in layered materials are also evaluated in single-layer forms. Three hundred and twenty-five of these materials are expected to have piezoelectric monolayers with a variety of forms of the piezoelectric tensor. This work significantly extends the scope of potential low-dimensional weakly bonded solids to be investigated

Ultrafast Electronic and Structural Response of Monolayer MoS₂ under Intense Photoexcitation Conditions

We report on the dynamical response of single layer transition metal dichalcogenide MoS₂ to intense above-bandgap photoexcitation using the nonlinear-optical second order susceptibility as a direct probe of the electronic and structural dynamics. Excitation conditions corresponding to the order of one electron–hole pair per unit cell generate unexpected increases in the second harmonic from monolayer films, occurring on few picosecond time-scales. These large amplitude changes recover on tens of picosecond time-scales and are reversible at megahertz repetition rates with no photoinduced change in lattice symmetry observed despite the extreme excitation conditions

Effects of Uniaxial and Biaxial Strain on Few-Layered Terrace Structures of MoS₂ Grown by Vapor Transport

One of the most fascinating properties of molybdenum disulfide (MoS₂) is its ability to be subjected to large amounts of strain without experiencing degradation. The potential of MoS₂ mono- and few-layers in electronics, optoelectronics, and flexible devices requires the fundamental understanding of their properties as a function of strain. While previous reports have studied mechanically exfoliated flakes, tensile strain experiments on chemical vapor deposition (CVD)-grown few-layered MoS₂ have not been examined hitherto, although CVD is a state of the art synthesis technique with clear potential for scale-up processes. In this report, we used CVD-grown terrace MoS₂ layers to study how the number and size of the layers affected the physical properties under uniaxial and biaxial tensile strain. Interestingly, we observed significant shifts in both the Raman in-plane mode (as high as −5.2 cm^–1) and photoluminescence (PL) energy (as high as −88 meV) for the few-layered MoS₂ under ∼1.5% applied uniaxial tensile strain when compared to monolayers and few-layers of MoS₂ studied previously. We also observed slippage between the layers which resulted in a hysteresis of the Raman and PL spectra during further applications of strain. Through DFT calculations, we contended that this random layer slippage was due to defects present in CVD-grown materials. This work demonstrates that CVD-grown few-layered MoS₂ is a realistic, exciting material for tuning its properties under tensile strain