F. Willig

Quantum Chemical Calculations of the Influence of Anchor-Cum-Spacer Groups on Femtosecond Electron Transfer Times in Dye-Sensitized Semiconductor Nanocrystals

Electronic properties of dye-sensitized semiconductor nanocrystals, consisting of perylene (Pe) chromophores attached to 2 nm TiO2 nanocrystals via different anchor-cum-spacer groups, have been studied theoretically using density functional theory (DFT) cluster calculations. Approximate effective electronic coupling strengths for the heterogeneous electron-transfer interaction have been extracted from the calculated electronic structures and are used to estimate femtosecond electron-transfer times theoretically. Results are presented for perylenes attached to the TiO2 via formic acid (Pe-COOH), propionic acid (Pe-CH2-CH2-COOH), and acrylic acid (Pe-CH [Formula: see text] CH-COOH). The calculated electron transfer times are between 5 and 10 fs with the formic acid and the conjugated acrylic acid bridges and about 35 fs with the saturated propionic acid bridge. The calculated electron injection times are of the same order of magnitude as the corresponding experimental values and qualitatively follow the experimental trend with respect to the influence of the different substitutions on the injection times.

Apparatus for investigating metalorganic chemical vapor deposition-grown semiconductors with ultrahigh-vacuum based techniques

An apparatus is described here in detail for the transfer of a sample from a metalorganic chemical vapor deposition (MOCVD) reactor to an ultrahigh-vacuum (UHV) chamber without introducing any contamination. The surface of the sample does not change during transfer as is borne out by the identical reflectance difference (RD) spectrum measured first in the MOCVD reactor, i.e., in situ, and afterwards again in the UHV chamber. Making use of the earlier apparatus a semiconductor can be grown in the MOCVD reactor and can afterwards be investigated with any desired tool of surface science, in particular also those that require UHV. All the data collected in UHV can be identified with the RD spectrum measured already in the MOCVD reactor. Several examples are presented here for data collection in UHV on III–V semiconductors grown in the MOCVD reactor. They illustrate the ease and reliability of the here described apparatus for contamination-free sample transfer. Signals are presented in particular for the genui...

Theory of ultrafast photoinduced heterogeneous electron transfer: Decay of vibrational coherence into a finite electronic–vibrational quasicontinuum

Photo-induced electron transfer from a surface attached dye molecule to the band levels of a semiconductor is modeled via an electronic–vibronic quasicontinuum. The description enables one to obtain a fairly accurate expression for the decay of the excited molecular state, including initial vibronic coherences. The model accounts for (a) the effect of a finite band width, (b) variations in reorganization energy and electronic coupling, (c) various energetic positions for the injecting level, (d) different initial vibrational wave packets in the excited state, and (e) two vibrational modes participating in the electron transfer process. Most cases are studied numerically and can be reasonably well understood from the obtained decay expression.

Ultrafast dynamics of light-induced electron injection from a molecular donor into the wide conduction band of a semiconductor as acceptor

Abstract Coherent vibrational wave packet motion is shown here to survive the heterogeneous light-induced electron transfer reaction and to continue even in the product state M + . Thus, the reaction does not start from a thermally equilibrated occupation of the vibrational modes in the donor molecule M*, in contrast to the hitherto standard model applied in photoelectrochemistry. Whereas the rise of the transient absorption signals of the product states are probing the forward electron injection reaction their decay does not simply probe the recombination reaction to the ground state dye and is complicated due to several additional effects. It is postulated here that the injection of hot electrons is in general faster than the capture of hot electrons by an adsorbed molecule.

RDS, LEED and STM of the P-rich and Ga-rich surfaces of GaP(100)

Abstract Reflectance difference spectroscopy was measured in the metal organic chemical vapor deposition reactor and also in UHV at 20 K. It revealed a characteristic negative peak at the low energy side that was indicative of the specific surface reconstruction. This peak disappeared completely if the sample was kept within a narrow intermediate temperature range. At 20 K the negative peak appeared at 2.4 eV for the Ga-terminated (2×4)-reconstructed surface and at 2.6 eV for the P-terminated (2×1)/(2×2)-reconstructed surface. RDS for the two different surface reconstructions displayed strong structures also in the range of the bulk transitions. A characteristic zig-zag pattern was observed in the STM image of the P-terminated surface.

Ultrafast heterogeneous electron transfer reactions: comparative theoretical studies on time- and frequency-domain data.

Recent theoretical studies on linear absorption spectra of dye-semiconductor systems [perylene attached to nanostructured TiO2, L. Wang et al., J. Phys. Chem. B 109, 9589 (2005)] are extended here in different respects. Since the systems show ultrafast photoinduced heterogeneous electron transfer the time-dependent formulation used to compute the absorbance is also applied to calculate the temporal evolution of the sub-100 fs charge injection dynamics after a 10 fs laser-pulse excitation. These studies complement our recent absorption spectra fit for two perylene bridge-anchor group TiO2 systems. Moreover, the time-dependent formulation of the absorbance is confronted with a frequency-domain description. The latter underlines the central importance of the self-energy caused by the coupling of the dye levels to the semiconductor band continuum. The used model is further applied to study the effect of different parameters such as (1) the dependence on the reorganization energies of the involved intramolecular transitions, (2) the effect of changing the transfer integral which couples the excited dye state with the band continuum, and (3) the effect of the concrete form of the semiconductor band density of states. Emphasis is also put on the case where the charge injection level of the dye is near or somewhat below the band edge. This nicely demonstrates the change from a structureless absorption to a well-resolved vibrational progression including characteristic shifts of the absorption lines which are a direct measure for the dye-semiconductor coupling.

Bridge mediated ultrafast heterogeneous electron transfer

Bridge mediated photoinduced ultrafast heterogeneous electron transfer (ET) from a molecularly anchored chromophore to a semiconductor surface is modelled theoretically. The continuum levels of the semiconductor substrate are taken into account in the numerical calculations via a polynomial expansion. Electron transfer for the direct injection case in the strong coupling limit is studied and compared with cases where intermediate bridging states are successively introduced to weaken the effective electronic coupling. The role of vibronic coherences in the strong electronic coupling limit as well as in off-resonant bridge mediated electron transfer is also discussed.

Angle Resolved Photoemission Spectroscopy of the InP(001) surface

Abstract The P-rich (2×1)/(2×2) and the In-rich (2×4) reconstructions of InP(001) are studied by Angle Resolved Photoemission Spectroscopy (ARPES). Experimental results for the In-rich (2×4) InP(001) surface show two bound surface states and one surface resonance, located at −1.6 eV binding energy. This agrees well with ab-initio calculations. For the P-rich (2×1)/(2×2) InP(001) surface, neither theoretical nor experimental work regarding the electronic surface structure exists until now. We identify two possible surface states showing no discernible dispersion along the Γ – J and Γ – J ′ directions of the surface Brillouin zone (SBZ). The energetically highest state, located close to the valence band maximum (VBM), is only observed along the [110] direction and is most likely a P-dimer-bond state. The second state, located at −4.4 eV binding energy, is assigned to a surface resonance.

Atomic structure and composition of the P-rich InP(001) surfaces

Abstract The microscopic structure of the P-rich (2×1)-like surfaces of InP(001) is investigated by soft X-ray photoemission spectroscopy (SXPS) and scanning tunneling microscopy (STM). The samples were grown by metal organic vapor phase epitaxy (MOVPE) and then transferred under ultra high vacuum (UHV) conditions to UHV analysis chambers. STM images show a P-rich as-grown surface after transfer and another less P-rich surface after prolonged annealing at 350°C. The In4d emission line is not affected by the change in surface reconstruction and shows a small surface component shifted by 0.48 eV towards higher binding energies. The line shape of the P2p core level, on the other hand, changes: On the as-grown surface, one surface component which is shifted by 0.98 eV towards higher binding energies is found after transfer, whereas two surface components shifted to higher and lower binding energies, respectively, appear after annealing at 350°C. These results are consistent with the structure models derived from STM for the different P-rich (2×1)-like surfaces.

Theory of ultrafast heterogeneous electron transfer: Contributions of direct charge transfer excitations to the absorbance

Absorption spectra related to heterogeneous electron transfer are analyzed with the focus on direct charge transfer transition from the surface attached molecule into the semiconductor band states. The computations are based on a model of reduced dimensionality with a single intramolecular vibrational coordinate but a complete account for the continuum of conduction band states. The applicability of this model to perylene on TiO2 has been demonstrated in a series of earlier papers. Here, based on a time-dependent formulation, the absorbance is calculated with the inclusion of charge transfer excitations. A broad parameter set inspired by the perylene TiO2 systems is considered. In particular, the description generalizes the Fano effect to heterogeneous electron transfer reactions. Preliminary simulations of measured spectra are presented for perylene-catechol attached to TiO2.