Studies on Two-Dimensional PTCDA Crystals: Growth by Physical Vapor Assembly and Absorption-Emission Spectroscopy

Abstract

Since the successful mechanical exfoliation of graphene using the tape method by Professor Andre Geim in 2004,1 extensive research has been conducted on two-dimensional layered materials. Particularly, research on graphene, a type of inorganic semiconductor, and transition metal dichalcogenides (TMDs) has gained significant attention. These materials exhibit high charge mobility and optical transparency. Notably, TMDs, with their two-dimensional layered crystal structure, offer the advantage of easy tuning of various parameters, such as band gaps, by adjusting the layers. However, the industrial development of these materials faces challenges due to complex processes involving high temperature and pressure conditions. On the other hand, organic semiconductors, such as those in the category of π-conjugated molecules, allow for low-temperature processing, possess strong optical sensitivity, and exhibit high absorption in the visible light range. Despite these advantages, their industrial development is hindered by a limiting the electron mobility. To overcome these limitations, recent research has focused on the development of hybrid structures combining inorganic and organic semiconductors. However, limitations exist in achieving high crystallinity at the two-dimensional level and in understanding charge transfer at interfaces. In this study, the fabrication and characterization of a two-dimensional organic-inorganic heterojunction structure with high crystallinity and a single-layer thickness were pursued beyond the scope of previous research. 3,4,9,10- perylenetetracarboxylic dianhydride (PTCDA), a representative organic semiconductor, is known to form a layered structure on graphene.2 In this study, PTCDA is used as the organic component. The energy alignment between PTCDA and TMDs is reported to exhibit a type II band alignment. By employing various TMDs with different wave functions, the charge transfer at the interface between PTCDA and TMDs are modulated. Specifically, MoS2, WSe2, and WS2 are used as the inorganic materials for graphene-based systems. To observe charge transfer at the interface, both organic and inorganic materials are fabricated as monolayers to minimize interlayer interactions. Furthermore, the crystallinity of PTCDA is investigated depending on the type of inorganic material. The study is also controlling the number of layers to observe changes in the absorption-emission spectrum and investigate new charge transfer phenomena resulting from the energy band structure changes in the organic-inorganic hybrid structure. To achieve these objectives, the research plan encompasses three major steps. Firstly, the construction of a physical vapor assembly (PVA) apparatus for stable operation is established. Secondly, by adjusting the variables of the deposition process, the fabrication of samples with the desired thickness and high crystallinity are pursued. Lastly, the observation of charge transfer at the interface and the effects of different inorganic materials are conducted using absorption-emission spectroscopy. The investigation into two-dimensional PTCDA crystals grown through physical vapor assembly and studied using absorption-emission spectroscopy holds promise for the development of low-dimensional organic-inorganic heterojunction semiconductors and offers potential applications in optoelectronic devices.