Richard Denk

Exciton-dominated optical response of ultra-narrow graphene nanoribbons

Narrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Unlike graphene--which shows a wavelength-independent absorbance for visible light--the electronic bandgap, and therefore the optical response, of graphene nanoribbons changes with ribbon width. Here we report on the optical properties of armchair graphene nanoribbons of width N=7 grown on metal surfaces. Reflectance difference spectroscopy in combination with ab initio calculations show that ultranarrow graphene nanoribbons have fully anisotropic optical properties dominated by excitonic effects that sensitively depend on the exact atomic structure. For N=7 armchair graphene nanoribbons, the optical response is dominated by absorption features at 2.1, 2.3 and 4.2 eV, in excellent agreement with ab initio calculations, which also reveal an absorbance of more than twice the one of graphene for linearly polarized light in the visible range of wavelengths.

Reflectance anisotropy spectroscopy as a tool for mechanical characterization of metallic thin films

In the present work reflectance anisotropy spectroscopy (RAS) is evaluated as a new tool for the mechanical characterization of metallic thin films on viscoelastic substrates. Cu and Cu–Zn thin films of thicknesses in the range from 50 to 1000 nm were sputter-deposited onto a viscoelastic polyimide substrate and subjected to uniaxial tensile loading. The changes in the mechanical, electrical and optical response of the films upon loading were monitored by simultaneous acquisition of total strain, electrical resistance and the RA-signal. The RA-spectrum of pure copper reveals a feature at a photon energy of ~4.0 eV that linearly increases with strain at the beginning of loading (elastic regime) and saturates at later stages (plastic regime). Post-mortem SEM studies of samples loaded to different strain values confirmed that this saturation corresponds to the onset of plastic deformation, defined by the appearance of slip lines. Concurrent measurements of the electrical resistance confirmed the absence of cracking at the onset of the 4.0 eV RA-signal saturation. Therefore we claim that the RAS technique can be employed for yield point determination. Besides the applicability of the RAS technique for pure metals, chemical sensitivity of RAS in terms of peak position was observed in the case of Cu–Zn thin films.

Layer resolved evolution of the optical properties of α-sexithiophene thin films.

We report a combined reflectance difference spectroscopy and scanning tunneling microscopy study of ultrathin α-sexithiophene (6T) films deposited on the Cu(110)-(2×1)O surface. The correlation between the layer resolved crystalline structure and the corresponding optical spectra data reveals a highly sensitive dependence of the excitonic optical properties on the layer thickness and crystalline structure of the 6T film.