Optical Properties of Two-dimemsional Van Der Waals Crystals

Download or read book Optical Properties of Two-dimemsional Van Der Waals Crystals written by Liang Zhao and published by . This book was released on 2015 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Van der Waals (vdW) materials are layered structures bonded by the weak vdW force. As such, stable single atomic layers can be isolated either by mechanical exfoliation or chemical methods as chemical vapor deposition. Atomically thin vdW materials have emerged as new types of two-dimensional (2D) systems with unique electronic and optical properties that are distinct from that of their bulk counterparts. Studies of this new class of material are not only interesting fundamentally; they can potentially also lead to applications in next-generation electronics and optoelectronics devices. In this thesis, we investigate two prototypes of 2D vdW materials, graphene (a semimetal) and semiconducting transition metal dichalcogenides (TMD) based on optical spectroscopy. Electro-magnetic radiation ranging from the far-infrared (or terahertz (THz)) to the visible has been utilized to investigate two questions: (1) the excitonic effects in Mo/W dichalcogenides; and, (2) the free carrier response in graphene. For the first topic, exciton series in monolayer WSe2 and the effect of electric field on the excitons is studied. A exciton series of WSe2 is observed by a complimentary measurement of linear absorption and two-photon photoluminescense excitation (2PPLE). Strong exciton binding energy ($\sim$ 0.4 eV) and non-Rydberg series are observed arising from 2D screening of Coulomb interactions. Using field-effect transistor structures we apply electrostatic doping and/or perpendicular electric field to WSe2 monolayer through the gates. Trion peak is observed under doping, which further splits under high electric fields. This phenomenon can be explained by Rashba spin-orbit interaction induced spin sub-bands hybridization. For the second topic, the free carrier response in monolayer graphene is investigated using the Fourier transform infrared (FTIR) spectroscopy in steady state conditions and the optical pump-THz probe spectroscopy under non-equilibrium conditions. We observe the Drude response under both conditions. For the steady states study, we find the Drude scattering rate strongly dependent on the doping density, revealing different scattering mechanisms. Under low power photo excitation, we observe that mobilities remain in relatively high values and carrier multiplication is achievable.