Biaxial strain tuning of the optical properties of single-layer transition metal dichalcogenides

Since their discovery single-layer semiconducting transition metal dichalcogenides have attracted much attention thanks to their outstanding optical and mechanical properties. Strain engineering in these two-dimensional materials aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. In this paper we demonstrate that biaxial strain, both tensile and compressive, can be applied and released in a timescale of a few seconds in a reproducible way on transition metal dichalcogenides monolayers deposited on polymeric substrates. We can control the amount of biaxial strain applied by letting the substrate expand or compress. To do this we change the substrate temperature and choose materials with a large thermal expansion coefficient. After the investigation of the substrate-dependent strain transfer, we performed micro-differential spectroscopy of four transition metal dichalcogenides monolayers (MoS2, MoSe2, WS2, WSe2) under the application of biaxial strain and measured their optical properties. For tensile strain we observe a redshift of the bandgap that reaches a value as large as 95 meV/% in the case of single-layer WS2 deposited on polypropylene. The observed bandgap shifts as a function of substrate extension/compression follow the order MoSe2 < MoS2 < WSe2 < WS2. Theoretical calculations of these four materials under biaxial strain predict the same trend for the material-dependent rates of the shift and reproduce well the features observed in the measured reflectance spectra.Comment: 10 pages, 5 figures, 2 tables, supporting informatio

Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.Comment: Main text (3 Figures) and Supp. Info. (23 Figures

Three-particle correlation from a Many-Body Perspective: Trions in a Carbon Nanotube

Trion states of three correlated particles (e.g., two electrons and one hole) are essential to understand the optical spectra of doped or gated nanostructures, like carbon nanotubes or transition-metal dichalcogenides. We develop a theoretical many-body description for such correlated states using an ab-initio approach. It can be regarded as an extension of the widely used $GW$ method and Bethe-Salpeter equation, thus allowing for a direct comparison with excitons. We apply this method to a semiconducting (8,0) carbon nanotube, and find that the lowest optically active trions are red-shifted by $\sim 130$ meV compared to the excitons, confirming experimental findings for similar tubes. Moreover, our method provides detailed insights in the physical nature of trion states. In the prototypical carbon nanotube we find a variety of different excitations, discuss the spectra, energy compositions, and correlated wave functions.Comment: main text, supplemen

Diversity of trion states and substrate effects in the optical properties of an MoS2_2 monolayer

The optical and electrical properties of atomically thin transition metal dichalcogenides critically depend on the underlying substrate. Here, the authors develop an abinitio many-body formalism to investigate the full spectrum of negative and positive trions in these layered semicondutors

Liquid Crystalline Behavior of Graphene Oxide in the Formation and Deformation of Tough Nanocomposite Hydrogels

In this paper, we report the formation and transformation of graphene oxide (GO) liquid crystalline (LC) structures in the synthesis and deformation of tough GO nanocomposite hydrogels. GO aqueous dispersions form a nematic LC phase, while the addition of poly­(<i>N</i>-vinylpyrrolidone) (PVP) and acrylamide (AAm), which are capable of forming hydrogen bonding with GO nanosheets, shifts the isotropic/nematic transition to a lower volume fraction of GO and enhances the formation of nematic droplets. During the gelation process, a phase separation of the polymers and GO nanosheets is accompanied by the directional assembly of GO nanosheets, forming large LC tactoids with a radial GO configuration. The shape of the large tactoids evolves from a sphere to a toroid as the tactoids increase in size. Interestingly, during cyclic uniaxial tensile deformation a reversible LC transition is observed in the very tough hydrogels. The isolated birefringent domains and the LC domains in the tactoids in the gels are highly oriented under a high tensile strain