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We have fabricated organic thin-film transistors (OTFTs) using a microcontact printing technique (μCP) that employs thin polydimethylsiloxane stamps on a rigid silicon substrate in order to reduce macroscopic distortions. Systematic variation of the printing pressure, printing time, and concentration of eicosanethiol, the “molecular ink” in the μCP process, permits the fabrication of devices with smaller channel lengths (Leff) than nominally defined by the stamp. Interdigitated Ti/Au electrode structures with Leff down to 100 nm could be fabricated which, after additional surface treatment and vacuum deposition of αα′-dihexylquaterthiophene, yield OTFTs with excellent characteristics.

We have studied the surface phonon modes of the reconstructed Si(111)-(7×7) surface by polarized Raman spectroscopy. Six surface vibration modes are observed in the frequency range between 62.5 and 420.0 cm −1 . The mode frequencies agree very well with reported calculation results. This enables their attribution to calculated eigenmodes, whose elongation patterns are dominated by specific atomic sites: the two most characteristic novel fingerprints of the (7×7) reconstruction are sharp Raman peaks from localized adatom vibrations, located at 250.9 cm −1 , and collective vibrations of the adatoms and first-and second-layer atoms, located at 420.0 cm −1 . While the sharp localized adatom vibration peak is a substantial refinement of an earlier broad spectral structure from electron energy-loss spectroscopy, no spectroscopic features were reported before in the collective-vibration frequency region. Furthermore, we observe in-plane wagging vibrations in the range from 110 to 140 cm −1 , and finally the backfolded acoustic Rayleigh wave at 62.5 cm −1 , which coincides with helium atom scattering data. Moreover, the Raman peak intensities of the surface phonons show a mode-specific dependence on the polarization directions of incident and scattered light. From this polarization dependence the relevant symmetry components in the Raman scattering process (A 1 and/or E symmetry) are deduced for each mode.

Ordered submonolayers of adsorbate atoms on semiconductor surfaces constitute a playground for electronic correlation effects, which are tightly connected to the local atomic arrangement and the corresponding vibration eigenmodes. We report on a study of the vibration eigenmodes of Au-covered Si(111) surfaces with (5 × 2) reconstruction using polarized Raman spectroscopy and first-principles calculations. Upon Au coverage, the vibration eigenmodes of the clean reconstructed Si(111)-(7 × 7) surface are quenched and replaced by new eigenmodes, determined by the Au-(5 × 2) reconstruction. Several polarization-dependent surface eigenmodes emerge in the spectral range from 25 to 120 cm −1 , with the strongest ones at 29, 51, and 106 cm −1. In our first-principles calculations we have determined the vibration frequencies, the corresponding elongation patterns, and the Raman intensities for two different structure models currently discussed in the literature. The best agreement with the experimental results is achieved for a model with 0.7 monolayer coverage and seven Au atoms per unit cell, proposed by S. G. Kwon and M. H. Kang [Phys. Rev. Lett. 113, 086101 (2014)].

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