Louis J. Terminello

First experimental results from IBM/TENN/TULANE/LLNL/LBL undulator beamline at the advanced light source

The IBM/TENN/TULANE/LLNL/LBL Beamline 8.0 at the advanced light source combining a 5.0 cm, 89 period undulator with a high‐throughput, high‐resolution spherical grating monochromator, provides a powerful excitation source over a spectral range of 70–1200 eV for surface physics and material science research. The beamline progress and the first experimental results obtained with a fluorescence end station on graphite and titanium oxides are presented here. The dispersive features in K emission spectra of graphite excited near threshold, and found a clear relationship between them and graphite band structure are observed. The monochromator is operated at a resolving power of roughly 2000, while the spectrometer has a resolving power of 400 for these fluorescence experiments.

Atom-resolved electronic spectra for Alq3 from theory and experiment

The electronic structure of Alq3 is investigated using density functional theory-based calculations, photoemission and near-edge x-ray absorption fine structure. The distinct features of the observed spectra are understood in terms of contributions from the different atoms and molecular orbitals. Fingerprints of the molecular bonding and of the individual atoms are identified. These results are meant to be a reference for the monitoring of chemical processes that Alq3 may undergo during fabrication or degradation of light-emitting devices, and for the understanding of the effects of ligand or metal substitution.

Near‐edge x‐ray absorption of carbon materials for determining bond hybridization in mixed sp2/sp3 bonded materials

Near‐edge x‐ray absorption fine structure (NEXAFS) measurements were performed on a variety of carbon materials, covering a range of hybrid bonding character from pure sp3 type to pure sp2 type. Diamond, chemical vapor deposited (CVD) diamond films of varying quality. Diamond‐like carbon (DLC) films, and graphite were examined with this technique and these measurements were compared with Raman spectroscopy results and scanning electron microscopy images of carbon film morphology. For the mixed sp2 and sp3 bonded DLC materials, NEXAFS does not suffer from the large Raman cross‐section difference between sp2 and sp3 type bonds, thus allowing unambiguous characterization of carbon thin films with a broader range of sp2/sp3 bonding ratios than possible with Raman spectroscopy alone. This capability was used to determine the transition point where the sequential‐CVD carbon film growth technique produces predominately sp3 or sp2 bonded material.

Observation of intermixing at the buried CdS/Cu(In, Ga)Se2 thin film solar cell heterojunction

A combination of x-ray emission spectroscopy and x-ray photoelectron spectroscopy using high brightness synchrotron radiation has been employed to investigate the electronic and chemical structure of the buried CdS/Cu(In, Ga)Se2 interface, which is the active interface in highly efficient thin film solar cells. In contrast to the conventional model of an abrupt interface, intermixing processes involving the elements S, Se, and In have been identified. The results shed light on the electronic structure and interface formation processes of semiconductor heterojunctions and demonstrate a powerful tool for investigating buried interfaces in general.

Characterization of nanocrystalline diamond films by core‐level photoabsorption

Core‐level photoabsorption has been used to determine the sp2 and sp3 bonding content of nanocrystalline diamond thin films grown using C60 or CH4 precursors. The C(1s) absorption spectra show clear bulk diamond excitonic and sp3 features with little evidence of sp2 bonding, while the Raman spectra measured from these same films are ambiguous and indeterminate. This result can be attributed to the local structure (near‐neighbor bonding) sensitivity of core‐level photoabsorption that is insensitive to domain size, unlike Raman spectroscopy.

Strong quantum-confinement effects in the conduction band of germanium nanocrystals

Quantum-confinement effects in the conduction band of deposited germanium nanocrystals are measured to be greater than in similar-sized silicon nanocrystals. The germanium particles are condensed out of the gas phase and their electronic properties are determined with x-ray absorption spectroscopy. The conduction band edge shifts range from 0.2 eV for 2.7 nm particles up to 1.1 eV for 1.2 nm particles.

Photo‐oxidation of electroluminescent polymers studied by core‐level photoabsorption spectroscopy

The C 1s and O 1s core‐level photoabsorption spectra of poly[2‐methoxy,5‐(2′‐ethyl‐ hexoxy)‐1,4‐phenylene vinylene] (MEH‐PPV) before and after exposure O2 and broadband visible light were recorded to determine the degradation pathway for this polymer. The change in the O 1s spectra as a function of exposure demonstrates that the O adds to the polymer chain to form a carbonyl group. Exposure to only O2 or only light causes no change in the C 1s or O 1s spectra. In the C 1s spectra, the change in the dependence on the photon angle of incidence after exposure demonstrates that O attacks the polymer at the double bond in the vinyl group thereby altering the extended conjugation of the polymer.

Determination of the Exciton Binding Energy in CdSe Quantum Dots

The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using X-ray spectroscopy. Using X-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e., without an exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.

Unfilled orbitals of C60 and C70 from carbon K-shell X-ray absorption fine structure

Abstract The K-shell near-edge X-ray absorption fine structure spectra of solid C 60 and C 70 have been recorded with high spectral resolution. The spectra exhibit sharp resonances associated with transitions to unfilled π * molecular orbitals and, at higher energy, broader resonances associated with σ * -like orbitals. The observed energy splitting of the π * resonances can be accounted for by published extended Huckel calculations. Our data clearly reveal that the fullerence films resemble molecular solids with considerably stronger intra-molecular than inter-molecular interactions.

Core-level photoabsorption study of defects and metastable bonding configurations in boron nitride

A comprehensive study of different local bonding environments in boron nitride -bulk and thin filmshas been performed by core level photoabsorption. Several new features not present in crystalline reference samples are found in the absorption spectra of the thin films. These are identified as nitrogen vacancies in the hexagonal bonding of BN, nitrogen interstitials, boron clustering, sp -like metastable phases and sp phases. Quantitative information on the concentration and distribution of point defects is easily extracted from the photoabsorption data and is discussed with regard to formation of riew phases, the B :N ratio in the films, and compared with a random model of defect formation. Information on the stability of the new bonding environments is gained by annealing the thin films. Modification of the orientation of the sp hexagonal planes is attained by ion bombardment and annealing, and is monitored by angle resolved photoabsorption.