Matthias Morkel

Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction

By combining ultraviolet and x-ray photoelectron spectroscopy with inverse photoemission spectroscopy, we find that the conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction is flat (0.0±0.2 eV). Furthermore, we observe a valence-band offset of 0.8±0.2 eV. The electronic level alignment is dominated by (1) an unusually large surface band gap of the CuInSe2 thin film (1.4 eV), (2) by a reduced surface band gap of the CdS overlayer (2.2 eV) due to intermixing effects, and (3) by a general influence of the intermixing on the chemical state near the interface.

Ultrahigh vacuum and high-pressure coadsorption of CO and H2 on Pd(111): A combined SFG, TDS, and LEED study

Sum frequency generation (SFG) vibrational spectroscopy was carried out in conjunction with thermal desorption spectroscopy, low-energy electron diffraction, and Auger electron spectroscopy to examine the coadsorption of CO and H2 on Pd(111). Sequential dosing as well as various CO/H2 mixtures was utilized to study intermolecular interactions between CO and H2. Preadsorbed CO effectively prevented the dissociative adsorption of hydrogen for CO coverages ⩾0.33 ML. While preadsorbed hydrogen was able to hinder CO adsorption at low temperature (100 K), hydrogen was replaced from the surface by CO at 150 K. When 1:1 mixtures of CO/H2 were used at 100 K, hydrogen selectively hindered CO adsorption on on-top sites, while above ∼125 K no blocking of CO adsorption was observed. The observations are explained in terms of mutual site blocking, of a CO–H phase separation, and of a CO-assisted hydrogen dissolution in the Pd bulk. The temperature-dependent site blocking effect of hydrogen is attributed to the ability ...

Sum Frequency Generation Study of CO Adsorption on Palladium Model Catalysts

In the present contribution CO adsorption on palladium aggregates was examined with IR-visible sum frequency generation vibrational spectroscopy. Supported Pd nanoparticles of different mean size and surface roughness were prepared by electron beam evaporation on an ordered Al2O3 thin film formed on NiAl(110). SFG spectra were acquired for a wide range of CO pressures (from 1 × 10—7 to 200 mbar) at 190 to 300 K. At low pressure, evidence for a particle structure dependence of the CO adsorption geometry was found, with on-top CO sites significantly occupied on defective particles. On the other hand, under a high pressure regime, discrepancies between rough and smooth well-faceted particles were mostly reduced, with the relative fraction of on-top CO nearly equal for both cases. As a reference model, adsorption on well-defined Pd(111) single crystals was monitored as a function of CO pressure up to 1 bar and contrasted to the work done on aggregates. Under ultrahigh vacuum the SFG spectra on large Pd particles closely resembled the Pd(111) response. However, for pressures higher than 1 mbar at 190 K twofold bridge bonded sites were no longer populated on the Pd(111) surface while the bridge bonded occupancy was significant on Pd aggregates for all pressures studied.