This paper addresses the study of adventitious contamination layers on the technically important substrates by X-ray and UV photoemission (XPS and UPS) as well as near-edge X-ray absorption fine ...structure (NEXAFS) spectroscopies. This investigation draws attention with regard to its application in the electronic devices of lower dimension. The Au, Pt, Ag, Cu, H passivated and SiO2 coated Si (100) are used as substrates. The XPS studies reveal that the composition of the adventitious contamination layer on studied noble substrates is distinct from those of non-metallic (H passivated and SiO2 coated Si) substrates. The ratio of the composition is different for various noble metal substrates. Furthermore, chemisorbed oxygen layer is found to present on Pt, Ag and Cu substrates in contrast to Au one. Our UPS results expose that the valence electronic structure, as well as workfunction of noble metal surfaces, are modified due to the contamination layer onto the substrate surfaces. The C K-edge NEXAFS spectra from the contamination layers on several substrates are presented to exhibit that the composition of contamination layer is different on different substrates whereas angle dependent NEXAFS data suggests that adventitious contamination layers on all substrates are amorphous in nature.
•Composition of the adventitious contamination layer on noble metal substrates is different from passivated Si (100) wafers.•The ratio of the composition is different for various noble metal substrates.•Valence band and workfunction of noble metal surfaces are modified for the presence of contamination layer on the substrates.•Adventitious contamination layer is amorphous in nature.•Different C-K edge NEXAFS spectra is found from contamination layer onto the various substrates.
X-ray absorption spectra (XAS), the density of states (DOS) and the electron density distribution of the HOMO and LUMO for flat and twisted rubrene molecules have been calculated using density ...functional theory (DFT). The simulated XAS spectra are validated by experimental C K-edge near-edge X-ray absorption fine structure (NEXAFS) data. We demonstrate that the NEXAFS spectra of rubrene thin films of different thicknesses can be explained in terms of different combinations of spectral intensity from the twisted and the flat randomly oriented molecules. All the fine structure of the NEXAFS spectra is well reproduced and the energetic positions of the resonances agree within a window of ±0.3 eV with the calculated XAS. Our calculation reveals that the peak at lowest photon energy (α') of the NEXAFS spectra at the lower coverage of rubrene molecules appears only from the twisted molecules. Other peaks in the case of the flat molecules appear either from the backbone or the wings, whereas, for the twisted molecules, the backbone and the wings contribute somewhat equally. Lowering of the HOMO-LUMO gap, as well as redistribution of the electron density of both the frontier orbitals, is found to take place in the case of the twisted molecule. The redistribution explains the reduction in conductivity for the twisted molecule compared to the flat one despite the lower band gap for the former. This finding will further strengthen the progress of rubrene thin film based devices.
The performances of organic thin film transistor devices are crucially connected with electronic and structural properties at organic semiconductor/dielectric interfaces. In our studies, Silicon ...substrates with native SiO2 layer were made hydrophilic according to Radio Corporation of America (RCA) cleaning procedure (RCA treated SiO2, RCA/SiO2) and hydrophobic by following octadecyltrichlorosilane (OTS) modification method (OTS treated SiO2, OTS/SiO2). The surface morphology, molecular orientation and nature of interaction have been studied at rubrene/RCA treated SiO2 and rubrene/OTS treated SiO2 interfaces with increasing thickness of rubrene film from sub-monolayer to multilayer. The angle dependent near edge x-ray absorption fine structure (NEXAFS) spectroscopy and atomic force microscopy (AFM) were used to probe the evolution of molecular configuration and surface morphology of rubrene thin films respectively. The nature of interaction was investigated by X-ray photoemission spectroscopy (XPS) techniques. XPS and NEXAFS studies indicate that the rubrene molecules were physically adsorbed onto both substrates. Angle dependent NEXAFS data suggest that OTS molecules were in standing up configuration on SiO2 surface while rubrene molecules were randomly oriented on both RCA and OTS treated SiO2 substrates. On RCA/SiO2 substrate the percentage of rubrene molecules with twisted backbone was higher compared to that on OTS/SiO2 for 20nm thick films. The rubrene films were found to grow with different features onto two studied substrates due to their difference in surface energy. At lower coverage, the grain size was larger and nucleation density was smaller onto hydrophilic RCA/SiO2 surface, compared to hydrophobic OTS/SiO2. Hydrophobation of the dielectric substrate surface tends to hinder the diffusion of rubrene molecules resulting in smaller grains for hydrophobic substrate compared to hydrophilic one.
•Growth of rubrene films on two differently modified SiO2\Si substrates•Rubrene film was grown with different topographies on two substrates.•Rubrene molecules were randomly oriented on both substrates.•Rubrene molecules were physically adsorbed onto both substrates.
The contact between the electrode and the organic semiconductor is one of the most crucial factors in determining the organic device performance. The development and production technology of ...different organic devices require the understanding of different types of metal/organic semiconducting thin film interfaces. Comparisons about the electronic structures at Rubrene/Ag and Ag/Rubrene interfaces have been studied using photoemission spectroscopy. The Ag on rubrene interfaces is found to show more interesting and complex natures than its counterpart. The vacuum level (VL) was shifted about 0.51 eV from push back effect for deposition of 5 Å rubrene onto Ag film whereas the electronic features of silver was only suppressed and no energy shift was resulted. While the deposition of 5 Å Ag onto rubrene film leads to the diffusion of the Ag atoms, as a cluster with quantum size effect, inside the film. Angle dependent XPS measurement indicates that diffused metal clusters were present at entire probed depth of the film. Moreover these clusters dope the uppermost surface of the rubrene film which consequences a shift of the electronic states of thick organic film towards higher binding energy. The VL was found to shift about 0.31 eV toward higher binding energy whereas the shift was around 0.21 eV for the electronic states of rubrene layer.
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.
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.
Density functional theory (DFT) computations were performed to study the change in electronic and optical properties of free-standing graphene monolayer by the adsorption of PTCDI molecule. It is ...revealed that the K point symmetry of the graphene is broken due to this molecular adsorption. As a consequence, the semi-metallic graphene monolayer turns into a conventional semiconductor at room temperature with conduction and valence bands separated by a gap of ~ 0.21 eV. The LUMO state of PTCDI molecule acts as a localized state in the electronic structure of the graphene/PTCDI system and induces a bandgap in the semi-metallic band structure of graphene. From our calculations, it is also explored that the localized gap state induced conduction gap is created due to the charge transfer mediated molecular sites specific van der waals (vdW) interaction across the graphene/PTCDI interface. The most interesting findings in this system are that the electrons follow the rules of the ordinary two dimensional (2D) materials to participate in the charge conduction process. The effective masses for holes as well as electrons are found to marginally change due to the PTCDI adsorption. Furthermore, the study of their optical properties for both directions of polarization exposes that the optical response of the semiconducting graphene/PTCDI system is highly anisotropic in nature. It is exposed that the sample has birefringence characteristics also. Moreover, the adsorption of a PTCDI molecule leads to notable modifications in the dielectric constant, reflectivity, absorption coefficients, and electron energy loss spectra of pristine graphene. The findings in this study may pave the way towards the development of new graphene-based molecular electronic and optoelectronic devices.
Display omitted
•The semi-metallic graphene turns into a conventional semiconductor with a gap of ~ 0.21 eV due to adsorption of PTCDI.•The governing formalism for the conduction mechanism of both types of charge carriers is predicted.•The optical response of the semiconducting graphene/PTCDI system is highly anisotropic in nature.•This molecular adsorption leads to prominent modifications in various optical response functions of pristine graphene.•This sample has birefringence characteristics.
•F-LUMO, a hybridized state near EF confirms partial charge transfer.•Non-significant role of partial charge transfer in VL shift over push back effect.•Pyrrole sites affected for partial charge ...transfer from Cu and Ag substrates.•Negligible effect on pyrrole cites for Pt and Au substrates.
The performances of organic electronic devices are significantly associated with their energy level alignment at organic semiconductor/metal–electrode interfaces. The electronic character of an organic semiconducting molecular over-layer on a metal surface can vary from semiconducting to metallic, depending on the nature of the molecular orbitals with respect to the Fermi level of the electrode. The general tendency of extrapolating established models for single crystal substrates to ‘real’ device substrates is highly misleading. Hence, the importance of metal specific interaction, former lowest unoccupied molecular orbital (F-LUMO) and vacuum level (VL) shift have been investigated as a function of thickness of the deposited films by means of photoelectron spectroscopy (XPS and UPS) to understand the interface between CuPc and Cu, Ag, Pt and Au foils sequentially. The XPS data provides the signature of affectability of pyrrole sites of CuPc molecules for partial charge transfer from Cu and Ag substrates while a negligible effect on pyrrole cites resulted for Pt and Au substrates. Furthermore, the appearance of F-LUMO, a hybridized state close to the Fermi level gives confirmatory information about partial charge transfer. Contrary to the general belief that vacuum level shift caused by charge transfer can partially or totally cancel that for push back effect, our observation indicates that the partial charge transfer does not play significant role in the shift of vacuum level. The entire thickness dependent electronic energy level alignment of CuPc films on all noble metal substrates is explained in terms of a combined effect of partial charge transfer and photoemission final state relaxation energy. A systematic variation of HOMO-FWHM was observed with CuPc thickness due to continuous change in molecular orientation.
Graphene/ferromagnet hybrid heterostructures are important building blocks of spintronics due to the unique ability of graphene to transport spin current over unprecedented distances and possible ...increase in its spin-orbit coupling due to proximity and hybridization. Here, we present magnetization dynamics over a femtosecond to nanosecond timescale by employing an all-optical time-resolved magneto-optical Kerr effect technique in single-layer graphene (SLG)/CoFeB thin films with varying CoFeB thickness and compared them with reference CoFeB thin films without an SLG underlayer. Gilbert damping variation with CoFeB thickness is modelled to extract spin-mixing conductance for the SLG/CoFeB interface and isolate the two-magnon scattering contribution from spin pumping. In SLG/CoFeB, we have established an inverse relationship between ultrafast demagnetization time (
τ
m
) and the Gilbert damping parameter (
α
) induced by interfacial spin accumulation and pure spin-current transport
via
a spin pumping mechanism. This systematic study of ultrafast demagnetization in SLG/CoFeB heterostructures and its connection with magnetic damping can help to design graphene-based ultrahigh-speed spintronic devices.
We present magnetization dynamics in SLG/CoFeB by employing a femtosecond laser-based time-resolved magneto-optical Kerr effect technique and establish an inverse relationship between ultrafast demagnetization time and the Gilbert damping parameter.
PDF
