Strain engineering of 2D semiconductors

Resumen

Strain engineering is a powerful tool that can be used to tune the optical and electronic properties of two-dimensional semiconductors such as TMDCs through the deformation of these materials. In this thesis I provide a whole description of the necessary details to perform optical measurements in function of the strain on two- dimensional materials like MoS2, MoSe2, WS2 or WSe2 by using experimental methods like micro-reflectance spectroscopy, photoluminescence and Raman spectroscopy. In the course of the thesis manuscript, I describe all the details that are needed for the calibration steps that will be useful for knowing the applied strain in the different experimental setups assembled in this work. Both for the method used for the application of uniaxial strain (by using the three-point flexural method on a polymer with a rectangular shape) and for the biaxial strain, thermal (through the thermal expansion of a polymer substrate) or mechanical (by bending a cruciform polymer substrate). In addition, I present a comparison to experimentally verify, as many theoretical works predict, that biaxial strain provides a more efficient way to tune the optical properties of MoS2 as compared with the uniaxial strain. In fact, we find that biaxial strain tunability or gauge factor is 2.3 times larger than the uniaxial strain gauge factor. I also show the behavior of the interlayer exciton in MoS2 flakes under the effect of the application of biaxial strain by using the thermal expansion method, finding that the gauge factor for this unconventional excitonic state is larger than the obtained for the A and B excitons. Moreover, the use of this method to apply strain will be crucial for the fabrication of microheaters that present a negligible thermal drift and a low time response. Some two-dimensional materials are air-sensitive materials and an inert atmosphere is necessary to manipulate them. That is the reason why I include an appendix in this thesis where I explore the use of a glove- less anaerobic chamber, a tool much more simple and convenient than a regular glove box for the effortless exfoliation and manipulation of the two-dimensional materials. Then I test the efficiency by comparing the stability of some air-sensitive materials such as black phosphorous or perovskites inside and outside the glove-less anaerobic chamber. Finally, I present another appendix where I show a method to fabricate large regions of atomically thin MoS2 layers through the sulfuration of MoO3. Areas of up to 300 × 300 μm2 with 2-4 layers in thickness are obtained. In addition, this layers are characterized by various techniques such as Raman spectroscopy, X-ray diffraction, transmission electron microscopy and electronic transport measurements, also finding a noticeable p-type behavior.