CHARACTERIZATION OF ORGANIC GLASSES BY NEXAFS SPECTROSCOPY

Abstract

The orientational order and morphology of organic thin films play an important role in defining their physical properties. The development of general techniques to define and control molecular orientation at the nanoscale may lead to improved performance of organic electronic devices. Physical vapor deposition (PVD) is a common route of thin film fabrication for organic electronic devices. This research work deals with controlling the molecular orientation by using electric field and substrate temperature during the film growth by PVD. Recently, PVD has been shown to produce organic glasses with enhanced kinetic and thermodynamic stability. For many organic glass formers, the substrate temperature during the deposition process plays an important role in determining the stability, density, and molecular orientation in these organic glasses. The main objective of this work is to understand the structure of organic glasses prepared at different substrate temperatures during deposition and to investigate the parameters behind the stability of these glasses. The primary investigating tool used is Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. During the research multiple experiments have been designed by using NEXAFS spectroscopy to understand the factors behind the stability of organic glasses when prepared at different substrate temperatures. This work used the orientation sensitivity (linear dichroism) in NEXAFS spectroscopy to map a 3-D orientation of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) glasses deposited at variable substrate temperature. NEXAFS spectroscopy was also used to examine differences in the internal molecular conformation of various forms of TPD (vapor deposited thin films, powder, etc.). This work gave an understanding of how increased density is related with closer molecular packing. Spectroscopic simulations performed with DFT calculations attribute these spectroscopic shifts to the internal conformation of the TPD molecules in the films.