DCMPMS Seminars

Optical spectroscopic studies on low-dimensional semiconductors: Imidazolium Lead Iodide and transition metal dichalcogenides

by Dr. Dipankar Jana (Visiting Post-doctoral Fellow, DCMP&MS, T.I.F.R., Mumbai)

Wednesday, December 11, 2019 from to (Asia/Kolkata)
at AG80
Description
Low-dimensional semiconductors have received a great deal of attention because of the possibility of fine-tuning their properties for improved device performance.  Here we will discuss our studies on two such systems, the first an organic-inorganic hybrid Imidazolium Lead Iodide (C3H5N2)4PbI6 (ImPI) and the second being semiconductors from the inorganic transition metal dichalcogenide (TMD) family.

Organic-inorganic hybrid semiconductors like Methyl Ammonium Lead Iodide CH3NH3PbI3 (MAPI) were shown to have potential for photovoltaics, light emission and detection applications. However, their stability remains issue since electronic properties of MAPI is found to change with temperature, presence of moisture and light. To improve the stability researchers have tried to fine-tune the chemical structure by varying the organic cation.  We will present results of optical spectroscopic studies on ImPI, a new hybrid material which has been shown to have significantly improved thermal stability as compared to MAPI. We performed photoluminescence (PL) and photoluminescence excitation and time resolved PL spectroscopy measurements on ImPI and also on its parent compound PbI2. By comparison with calculations based on the effective mass theory, we suggest that in spite of its 3-dimensional (3D) structure, carriers in the ground state of ImPI behave as if they are in a 1-dimensional (1D) solid with carriers confined to face-sharing PbI64– octahedron chains. A large spectral shift between the absorption and the emission spectrum of ImPI suggests self trapping of excitons, consistent with the expected increase in carrier-phonon interaction in 1D systems. We have also studied the temporal decay of luminescence in ImPI and propose a model for self-trapped exciton recombination dynamics.  

The second part of the talk will be devoted to inorganic TMD semiconductors which can exist as stable 2-dimensional (2D) monolayers (ML). A setup built for deterministic transfer of exfoliated ML-TMDs, and for making heterostructure of ML-TMDs with hexagonal boron nitride (hBN) will be described. We have performed a systematic study of the influence of hBN on ML-TMDs through spectroscopic measurements. Our studies reproduce the remarkable improvement in the optical emission linewidth and intensity in hBN/ML-TMD/hBN/substrate type structure as compared to ML-TMD/substrate structure, reported earlier. These results will be discussed in terms of the effect of local dielectric function and optical cavity formation on two-dimensional excitons/trions in ML-TMDs.