Abstract:

This talk presents an in-depth exploration of the physics of crystal vibration and electronic excitations in two dimensional semiconductors. These semiconductors are unique materials and are as thin as a single or double molecules. Electrons in a conventional solid possess properties like crystal momentum and spin but they are independent of each other for most solids. Some two-dimensional materials like Molybdenum disulfide (MoS2) possess the unique property of highly coupled crystal momentum and spin. Since crystal momentum of an electron in solid has valleys, this coupling gives rise to specific selection rules for interband transitions whereby electronic spin and hence valleys can be precisely controlled by polarization (helical polarization) and energy of an excitation laser. This control of quantum states of electrons has given rise to the emerging field of valleytronics, a highly interesting pathway to realize technologically useful quantum systems. In this talk, I will present basic physics including electronic and crystal vibration physics and how this physics is inter-related to crystal symmetries. We will explain ways to initialize and measure electronic configurations of these valley-spin states of MoS2 using helicity resolved photoluminescence and Raman spectroscopy. In addition, we use helicity resolved Raman spectroscopy to access vibrational state of the crystal called phonons. The method is utilized to probe the underlying symmetry of the crystal. One of the main challenges for valleytronic systems is that magneto-optical control of valley-spin quantum states is only effective under cryogenic temperatures. Additionally, technological application requires external control over the electronic structure. In this talk, we will discuss the use of externally applied strain as a mechanism which can markedly improve initialization efficiency of a valley spin system. In addition, strain can be used to vary this control from an OFF to a partially ON condition. We investigate the effect of uniaxial and biaxial strain on the electronic and phononic structure and modification of the electron and phonon scattering selection rules in the presence of strain. The effect is used to investigate the character of electron-phonon interaction in the system and finally localized biaxial strain is used to realize brighter photon emitter using these two-dimensional materials. This work opens up new ways to control valley spin systems and provides valuable insight into physics and underlying crystal structure modifications.

List of Publications:      

1. Ali, Shahzad Akhtar, Bilal Ahmed, Ahsan Javed, et al. " Room temperature polarization-resolved Raman and Photoluminescence in uniaxially strained layered MoS2." Applied Physics Letters 125,no.23 (2024).
2. Ali, Shahzad Akhtar, Abdullah Irfan, Aishani Mazumder,et al. "Helicity-selective Raman scattering from in-plane anisotropic α-MoO3 " Applied Physics Letters 119, no. 19 (2021).
3. Javed, Ahsan, Shahzad Akhtar Ali, Muhammad Asif, and Rafi Ullah. "Structural and electronic properties of monolayers: Enhancing computational accuracy " Materials Letters (2024): 137661
4. ul Ain, Qurat, Sumera Siddique, Shahzad Akhtar Ali, Hasan Ahmed, Mohsin Khan, Murtaza Saleem, Habib ur Rehman, Ata Ulhaq, and Ammar A. Khan. "Columnar liquid crystals as stability enhancing hole transport layers for Ruddlesden–Popper perovskite optoelectronics." Molecular Systems Design & Engineering 7, no. 7 (2022): 767-779.
5. Ain, Qurat Ul, Sehrish Iqbal, Shahzad Akhtar Ali, Murtaza Saleem, Habib Ur Rehman, Ata ulHaq, and Ammar A. Khan. "Engineering fibre morphology in self-assembled physical gels of a prototypical discotic liquid crystal." Liquid Crystals 48, no. 6 (2021): 888-901
6. Ikhlaq, Uzma, Samiullah Sami, Maria Khalil, Farman Ullah, Shahid M. Ramay, Shahzad Akhtar Ali, and Murtaza Saleem. "Structural, electronic, and optical properties of Zn-doped V2O5 thin films." Physica Scripta 99, no. 9 (2024): 095981.
7. Ilyas, Hamna, Shamim Aslam, Muhammad Tauseef Qureshi, Noor Alam, Ghazala Yunus, Hanan Ragab, Shahzad Akhtar Ali, and Murtaza Saleem. "Study of electronic, thermoelectric and optical properties of environment friendly Mg doped CeO2 for energy harvesting devices." Chemical Physics Letters 793 (2022): 139464.
8. Muzaffar, M. Usman, Ghulam Hussain, Nawazish A. Khan, Ubaid Ur Rehman, Shahzad Akhtar Ali, and H. M. Rehan Afzal. "Effect of Charge Reservoir Layer on the Structural and Transport Properties of CuTlBa 2− Y Sr Y-1223 (Y= 0− 0.25) Superconductor." Journal of Superconductivity and Novel Magnetism 30 (2017): 2053-2058.
9. Ain, Qurat Ul, Mohsin Raza, Shahzad Akhtar Ali, and Ammar Ahmed Khan. "Perovskite Devices Stability? Effect of Ruddleson Popper Perovskites with Liquid Crystal Based Novel Hole Transporters." In APS April Meeting Abstracts, vol. 2021, pp. H17-009. 2021.
10. Shahzad Akhtar Ali , Ahsan Javed and Ata ulHaq “Characterization of Lattice Vibrations and Crystallographic Orientation of monolayer MoS2 Under Uniaxial Strain Using Raman Spectroscopy”  Under Review
11. Shahzad Akhtar Ali, Ahsan Javed, Afsar Bano and Ata ulHaq “Mapping Bi-axial Strain-Driven Single-Photon Emitters in Layered MoS₂”  Under Review
12. Afsar Bano, Shahzad Akhtar Ali and Ata ulHaq “Fabrication of Topological Defect Based Polarized Optical Diffraction Gratings”  Under Review