Resumen de Uniaxial strain engineering based on TMDs and 2D anisotropic materials Hao
Strain engineering has been proven to be an efficient tool to tune the optical, physical, vibrational and electrical properties of 2D nanomaterials, which offers a huge opportunity to apply 2D nanomaterials in stretchable devices and other related fields. Relative studies are quite abundant while there are still challenges remained. For most strain engineering-related researches based on 2D materials, a lack of important mechanic details could be found, which is urged to be explained in detail. In addition, most reported works focus on only the effect of tensile strain on 2D isotropic materials, while for 2D anisotropic materials, which lends an additional degree of freedom to tune the optical and physical properties along different directions under strain, is pretty less explored. On the other hand, studies about compressive strain engineering on 2D materials are also really scarce due to being restricted by a lack of a facile and efficient straining mechanic apparatus. These barriers have motivated our research interests and propelled us to proceed a comprehensive study on 2D materials-based strain engineering.
In this thesis, we introduce two different straining systems for various needs and also calibrate the applied strain as in agreement with mechanic continuum formulas, further we study the strain-tunable exciton and Raman modes tunability of several 2D TMDs and anisotropic materials under tensile and compressive strain, which paves a way to have a comprehensive investigation of the effect of uniaxial strain on 2D materials. Apart from this, we also study the polarized optoelectrical property of ZrSe3 phototransistors and also the electrical property of PC-based flexible ZrSe3 strainsensor, leading to a new potential to flexible optoelectronics field.
Based on our previous work, with the assistance of the homebuilt three-points bending setup, we develop angle-dependent differential reflectance spectrum tests of 2D anisotropic ZrSe3 under uniaxial tensile strain by transferring flakes on a disk-like polymer substrate in Chapter 3. The obtained differential reflectance spectra show excitonic features that blueshift upon uniaxial tension, which is strongly dependent on the crystalline direction that strain is applied along. This strain-tunable anisotropy is further verified by ab initio calculation.
On the basis of the work above, in Chapter 4, we further studied the effect of strain on both exciton and Raman modes of another 2D anisotropic material ReS2 with the assistance of the home-built three-points bending apparatus. In the aspect of ReS2, we carry out the differential reflectance spectrum test and polarized Raman test on ReS2 flakes under uniaxial strain and demonstrate a strain-tunable anisotropy on both the exciton shift and the Raman mode shift in few-layered ReS2 nanoflakes, which also shows a huge dependence on the direction which the strain is being applied on.
In order to study the effect of compressive strain on 2D materials, in Chapter 5, we modify the homebuilt three-points bending system into a four-points bending setup by adding an extra cylinder. The applied strain is calibrated in a straightforward way which is in a good agreement with calculated strain obtained from the mechanic continuum formula. Further we carry out the micro-differential reflectance spectrum test on few-layered MoS2 flakes to study the effect of compressive strain on the exciton regulation. Besides, we observe an enhancement on the excitonic shift results from adamantane encapsulation. Further we study the statistical flake to flake variation of the compression gauge factor and maximum compression strain with 21 different MoS2 flakes.
In addition, we also probe into the effect of compression on anisotropic black phosphorus flake. As Chapter 6 shows, we explore the regulation of A1g, B2g, and A2g vibrational modes of black phosphorus under both compressive and tensile strain with our four-points bending apparatus. A strong anisotropic blue/red shift of these three modes under compressive/tensile strain is observed while with various polarized features, which is further verified resulting from the strains regulation on bond angle and bond length between phosphorus atoms.
Finally, we present an appendix chapter where we work on the electrical and optoelectrical properties investigation of ZrSe3 devices. We fabricate phototransistors based on few-layer ZrSe3 crystals and characterize their optoelectronic response. We also characterize the spectral behavior of the photoresponse by polarization-resolved photocurrent spectroscopy. In addition, we further study the electrical property of PC-based ZrSe3 flexible device, finding a polarized tunability on the conductivity of ZrSe3 under both uniaxial tensile and compressive strain.
