Raza, Ali (2025) SYNTHESIS AND CHARACTERIZATION OF 2D FEW LAYERED WS2 FLAKES DOPED WITH ORGANIC MOLECULES. [Tesi di dottorato]

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Abstract

Two-dimensional materials and organic semiconductors are gaining significant interest in next-generation electronic and sensing technologies. Their integration combines the tunable properties of organics with the structural robustness and charge-transport efficiency of 2D materials. This dissertation focused on organic semiconductors, particularly α- sexithiophene (6T), deposited on WS2 flakes, while picene and pentacene were investigated separately for their distinct photo-optical dynamics. The 2D-WS2 flakes were synthesized using a solution-based CVD method, yielding few-layered structures with excellent crystallinity and micro-scale dimensions. Morphological analysis by AFM revealed the growth of high-quality triangular WS2 flakes with uniform and consistent thickness, confirming the formation of few-layered structures. The deposition of 6T onto WS2 surfaces led to the growth of anisotropic nanostructures, forming a well-aligned nanoneedle pattern. The preferential alignment of 6T nanoneedles on WS2 flakes is guided by van der Waals (vdW) interactions and the hexagonal symmetry of the 2D lattice, promoting anisotropic growth along specific crystallographic directions to minimize interfacial energy. Statistical analysis demonstrated a consistent trend in the ratios and angular orientation emphasizing the reliability and uniformity of the synthesis process. While the electrical measurements revealed the n-type semiconducting behavior of WS2, the findings underscored a trade-off between higher mobility in air and enhanced stability in vacuum, albeit with reduced mobility. These results offer critical insights into optimizing WS2-based devices in terms of their environmental sensitivity and performance consistency. Furthermore, Raman and PL analyses elucidated the interaction between WS2 and 6T, revealing the emergence of new spectral features in the high-wavenumber region after 6T deposition. These features indicate strong vibrational coupling and phonon scattering effects, suggesting an efficient charge transfer within the interlayers of both materials. Expanding the study, picene and pentacene were fabricated using supersonic molecular beam deposition and characterized via AFM, and two-photon photoemission measurements provided insights into their electronic dynamics, revealing distinct charge carrier lifetimes. Picene exhibited fast decay processes, whereas pentacene exhibited slower relaxation dynamics, highlighting differences in their electronic coupling and triplet state mechanisms. Eventually, these investigations bridge the gap in understanding hybrid organic-2D systems, delivering a comprehensive analysis of their structural, electronic and optical properties.

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