Core photoionization satellites in fullerence and related model systems

Gao

The Journal of Chemical Physics

et al. 2014

The core-hole excitation spectra-near-edge x-ray absorption spectroscopy (NEXAFS), x-ray emission spectroscopy (XES), and x-ray photoelectron spectroscopy (XPS) shake-up satellites have been simulated at the level of density functional theory for the azafullerene C 59 N and its derivatives (C 59 N) + , C 59 HN, (C 59 N) 2 , and C 59 N-C 60 , in which the XPS shake-up satellites were simulated using our developed equivalent core hole Kohn-Sham (ECH-KS) density functional theory approach [B. Gao, Z. Wu, and Y. Luo, J. Chem. Phys. 128, 234704 (2008)] which aims for the study of XPS shake-up satellites of large-scale molecules. Our calculated spectra are generally in good agreement with available experimental results that validates the use of the ECH-KS method in the present work. The nitrogen K-edge NEXAFS, XES, and XPS shake-up satellites spectra in general can be used as fingerprints to distinguish the azafullerene C 59 N and its different derivatives. Meanwhile, different carbon K-edge spectra could also provide detailed information of (local) electronic structures of different molecules. In particular, a peak (at around 284.5 eV) in the carbon K-edge NEXAFS spectrum of the heterodimer C 59 N-C 60 is confirmed to be related to the electron transfer from the C 59 N part to the C 60 part in this charge-transfer complex. © 2014 AIP Publishing LLC.

Section: 47mentioning

confidence: 99%

A comparative theoretical study on core-hole excitation spectra of azafullerene and its derivatives

Gao

The Journal of Chemical Physics

et al. 2014

“…C 1s→LUMO x-ray-absorption spectra for the indicated samples, at a resolution of 0.16 eV (Maxwell et al, , 1998. The inset is a Zϩ1 calculation (Enkvist et al, 1993;Lunell et al, 1994) of the LUMO wave function, which should be applicable to XAS (Wä stberg et al, 1994;Nyberg et al, 1999). The angle of incidence of the light is indicated for the monolayer systems, whereas no angle dependence has been observed for thick films.…”

Section: C 60 : Placing and Characterizing The Core-excited Statementioning

confidence: 99%

“…The fact that a Zϩ1 description captures the essential aspects of the 1s XAS data (Wä stberg et al, 1994;Nyberg et al, 1999) strongly suggests that this description is accurate in its main details. Calculations of similar quality within a Jahn-Teller picture have not been carried out at this time, presumably due to computational difficulties, but one surmises that the results, for example, of the LUMO probability distribution in the core-excited state should be similar in its major details to that found in a Zϩ1 model (Bagus and Schaeffer, 1972;Domcke and Cederbaum, 1977;Å gren et al, 1981;Enkvist et al, 1993;Å gren, 1994, 1999;Lunell et al, 1994).…”

Section: C 60 : Placing and Characterizing The Core-excited Statementioning

confidence: 99%

Charge-transfer dynamics studied using resonant core spectroscopies

2002

The authors review the use of core-level resonant photoemission and resonant Auger spectroscopy to study femtosecond charge-transfer dynamics. Starting from simple models of the relevant processes, they examine the rationale for this approach and illustrate the approximations and known subtleties for the inexperienced experimentalist. Detailed analysis of case studies of increasing complexity are taken up, as well as the connection to related approaches using both valence excitation and the core-level fluorescent channel.

ECS J. Solid State Sci. Technol.

“…A high resolution scan of the shake-up features for a layer (∼17 ML) of C 60 is shown in Figure 1. Lunell and Enkvist et al 32,34 were able to assign the observed peaks in an experimental spectrum to particular transitions in the Huckel molecular orbital structure, shown in Figure 1b, and the theoretically predicted peak center of mass are shown in red in Figure 1a. Almost all of the shake-up structures correspond to some transition to the 5t 1u * (LUMO) level, giving a clear picture of the valence band characteristics of C 60 .…”

Section: Resultsmentioning

confidence: 91%

LiF Doping of C₆₀Studied with X-ray Photoemission Shake-Up Analysis

Turak

Zgierski

Dharma‐wardana

2017

We report our investigation of the chemical doping mechanism induced by LiF interaction with fullerene thin films. High resolution Xray photoelectron spectroscopy of the C1s shake-up satellites and F1s main core level, supported by density functional calculations, suggest the formation of a charge transfer complex between covalent LiF monomers and dimers and C 60 . This interaction was observed in both LiF/C 60 and C 60 /LiF depositions, suggesting that some charge transfer complexation can occur in these systems even without dissociation. hydrogen storage materials, 5 and even antibacterial treatments. 6,7 Due to the rich electronic properties in the ground and excited states, C 60 is most widely used as an organic semiconductor. With the ability to act as both an electron acceptor and donor, it is a standard material in organic light emitting diodes, solar cells, transistors and Schottky diodes.The interface with the metal contacts play a crucial role in the effectiveness of organic semiconductors in device performance and stability.8 Introducing a thin buffer layer of LiF between an organic film and an Al cathode has been seen to dramatically enhance the energy-level alignment and stability of the interface.9-13 Additionally, alternating stacks of LiF and C 60 14 and LiF doped C 60 nanocomposite buffer layers 15,16 have been shown to have high conductivities and stability in OLEDs and OPVs. Several mechanisms have been proposed for these behaviors including chemical reactions, dipole alignment, tunneling, and LiF dissociation requiring the additional presence of Al. 17In this contribution, we present results showing that in the absence of Al, there is a charge transfer interaction between LiF and C 60 , leading to the formation of LiF••C 60 complexes without dissociation. Using the well-developed and described X-ray photoelectron shakeup structure of C 60 and the strong feature from F1s XPS core level, we see evidence of a direction independent interaction. The spectroscopic features can be described by the semi-covalent interaction of LiF monomers or dimers with C 60 using density functional theory. Coupled with XPS core level shift calculations, using approximate Madelung energy, the predicted Millikan charges and bond lengths give values that approximate the observed core level shifts. Additionally, the small transfer of charge to C 60 predicted by the modelling is consistent with the minimal impact on the shake-up satellites of the C1s of C 60 . ExperimentalThe samples were produced using a Kurt J. Lesker OLED cluster attached to an X-ray photoelectron spectroscopy tool. For deposition of C 60 , ∼5Å of LiF on 100 nm sputter deposited Au/Si was used z E-mail: turaka@mcmaster.ca as a substrate. For deposition of LiF, the substrate was 350Å of C 60 thermally deposited onto cleaned Si with native oxide. C 60 and LiF were thermally evaporated from home built crucible sources at an average rate of 1 Å/min and 2-3 Å/hr as measured by oscillating quartz crystal microbalance (Inficon XTM/2) respectively onto previ...