Sumona Sinha

Oxidation of rubrene thin films: an electronic structure study.

The performances of organic semiconductor devices are crucially linked with their stability at the ambient atmosphere. The evolution of electronic structures of 20 nm thick rubrene films exposed to ambient environment with time has been studied by UV and X-ray photoemission spectroscopy (UPS and XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). XPS, NEXAFS data, and DFT calculated values suggest the formation of rubrene-epoxide and rubrene-endoperoxide through reaction of tetracene backbone with oxygen of ambient environment. Angle dependent XPS measurement indicates that the entire probed depth of the films reacts with oxygen by spending only about 120 min in ambient environment. The HOMO peak of pristine rubrene films almost disappears by exposure of 120 min to ambient environment. The evolution of the valence band (occupied states) and NEXAFS (unoccupied states) spectra indicates that the films become more insulating with exposure as the HOMO-LUMO gap increases on oxidation. Oxygen induced chemical reaction completely destroys the delocalized nature of the electron distribution in the tetracene backbone of rubrene. The results are relevant to the performance and reliability of rubrene based devices in the environment.

Thickness dependent electronic structure and morphology of rubrene thin films on metal, semiconductor, and dielectric substrates

The evolution of the electronic structure and morphology of rubrene thin films on noble-metal, semiconductor and dielectric substrates have been investigated as a function of thickness of deposited films by using photoelectron spectroscopy and atomic force microscopy. The clean polycrystalline Au and Ag were used as noble-metals, whereas, H passivated and SiO2 coated Si (100) were used as semiconductors and dielectric substrates. Discussion and comparison on interface dipole, energy level alignment, and surface morphology for the four cases are presented. The formation of dipole at metallic interfaces is found to occur due to push back effect. S parameter obtained from the variation of barrier height with the change of work function of the contacting metal indicates moderately weak interaction between rubrene and the metal substrates. The thickness dependent energy level alignment of the physisorbed rubrene films on different substrates is explained by a dielectric model in terms of electrostatic screenin...

The effect of gate dielectric modification and film deposition temperature on the field effect mobility of copper (II) phthalocyanine thin-film transistors

The change of the field effect mobility of copper (II) phthalocyanine (CuPc) ultra-thin films with the change of dielectric surface and deposition temperature has been systematically investigated. CuPc films are prepared on bare and modified SiO2 substrates at different deposition temperatures. The crystalline structure and morphology of the films have been characterized by x-ray diffraction and atomic force microscopy. The observed device parameters show that the dielectric surface modification reduces the off-state mobile charge carrier density and interfacial trap density. Dielectric surfaces with lower or comparable surface energy to that of CuPc are found to produce devices with higher mobility. The dependence of mobility on deposition temperature shows a strong correlation with the behaviour of the dielectric layers at various temperatures and the diffusion of CuPc molecules.

Crystalline growth of rubrene film enhanced by vertical ordering in cadmium arachidate multilayer substrate.

The growth of highly crystalline rubrene thin films for organic field effect transistor (OFET) application remains a challenge. Here, we report on the vapor-deposited growth of rubrene films on the substrates made of cadmium arachidate (CdA) multilayers deposited onto SiO2/Si(100) via the Langmuir-Blodgett technique. The CdA films, containing 2n+1 layers, with integer n ranging from 0 to 4, are surface-terminated identically by the methyl group but exhibit the thickness-dependent morphology. The morphology and structure of both CdA and rubrene films are characterized by X-ray reflectivity (XRR), X-ray diffraction (XRD), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM). Crystalline rubrene films, evidenced by XRD and marked by platelet features in AFM images, become observable when grown onto the CdA layer thicker than 5L. XRD data show that vertical ordering, that is, ordering along surface normal, of CdA multilayer substrates exerts a strong influence in promoting the crystalline growth of rubrene films. This promoted growth is not due to the surface energy of CdA layer but derived from the additional interaction localized between rubrene and CdA island sidewall and presumably strengthened by a close dimensional match between the a-axis of rubrene lattice and the layer spacing of CdA multilayer. The best OFET mobility is recorded for 9L CdA substrate and reaches 6.7 × 10(-2) cm(2) V(-1) s(-1), presumably limited by the roughness of the interface between CdA and rubrene films.

Pseudopotential of gold fitted to elastic data

The pseudopotential of gold using Harrison’s modified point‐ion form has been calculated by fitting with the experimental elastic data and is compared with the pseudopotential curve obtained from the Fermi surface data. This pseudopotential has been used to calculate the third‐order elastic constants of gold.

Improvement of rubrene thin film transistor with self assembled monolayer of octadecyltrichlorosilane (OTS)

We have fabricated organic thin film transistors (OTFTs) using rubrene as an active layer with untreated SiO2 and OTS modified SiO2 gate dielectrics. The effect of the OTS treatment on the electric characteristics of OTFT was investigated. As a result of OTS modification, we can improve the mobility to 0.12×10−2 cm2/V.S and IOn/IOff ratio to 0.98×105.