Interlayer Twist Modulation On Spectral Characteristics Of Two-dimensional Materials

Two-dimensional materials including graphene have attracted immense interests for their exotic optical,thermal and electrical properties in nanoscale.Electronics and photo-electronics based on two-dimensional materials have also shown an extremely wide application prospect.The interlayer crystalline mismatch,in other words,the interlayer twist angle in van der Waals stacked multi-layer two-dimensional materials has a non-negligible influence on phonon and electron performance of two-dimensional materials,which would affect the electrical transport and opto-electrical response of nano devices.As a result,it is of great significance to unveil the interlayer twist tuning effects on properties of two-dimensional materials.In this dissertation,we first summarized the advances and experimental methods of twisted bilayer two-dimensional materials.Then we chose two representative two-dimensional materials,graphene and MoS₂,as our research subjects and fabricated bilayer homo-and hetero-structures with various twisted angles.Based on these as-fabricated samples,tuning effects of interlayer twist on their optical spectroscopy have been investigated.The main work and conclusions of this dissertation are as follows.(1)The fabrication method of twisted bilayer structure was explored and improved.Using the optimized atom-layer transfer method,twisted bilayer structures with various twisted angles were successfully obtained.Also,oxygen plasma treatment and vacuum thermal annealing were conducted to improve the quality and interlayer coupling of as-fabricated samples.(2)Interlayer difference in twisted bilayer MoS₂ homo-structures was studied.It was found that,compared with those of the top layer,the bottom layer MoS₂demonstrated a Raman peak blueshift,an intensity enhancement and a peak blueshift of photoluminescence emission,indicating there exists n-doping and more strain in the bottom layer.Moreover,to exclude substrate coupling,we fabricated suspended monolayer MoS₂ samples and conducted a systematic investigation on them,from which we verified the difference between the top and bottom layer of twisted bilayer MoS₂ homo-structures was resulted from different coupling with the substrate.(3)We systematically investigated the twist-angle tuning effect on Raman and photoluminescence spectra of graphene and MoS₂ bilayer twisted homo-structures.During the measurements,Raman 2D peak of graphene demonstrated a remarkable twist-angle dependence,while G peak showed an approximately 10-fold intensity enhancement at a twist-angle near 13°,which was attributed to occurrence of Van Hove singularity.Meanwhile,photoluminescence intensity and peak locations of MoS₂ varied with twist-angle.(4)Interlayer twist modulation on exciton absorption in graphene/MoS₂hetero-structures was experimentally observed.The photoluminescence emission of graphene/MoS₂ hetero-structures was found to be twist-angle dependent.As the twist-angle increased,photoluminescence emission demonstrated an intensity enhancement and a peak blueshift.In addition,through optical reflectance measurements,it was found that,in the vicinity to the wavelengths of 620 nm and670nm which were exactly the peak positions of photoluminescence emission of MoS₂,the reflectance of graphene/MoS₂ hetero-structures decreased linearly with increasing twist-angle.This suggested that interlayer crystalline mismatch could modulate exciton absorption in graphene/MoS₂ hetero-structures.Finally,we investigated the work function distribution across graphene/MoS₂ hetero-structures and confirmed that the exciton absorption variation was resulted from twist-angle dependent interlayer charge transfer.