M. Hohage

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.

Kinetic Monte Carlo simulation scheme for studying desorption processes

Abstract We introduce a discrete event kinetic Monte Carlo (KMC) simulation program that is capable of reproducing thermal desorption data. The KMC program is suitable for the analysis of both, desorption in 2D quasi-equilibrium and desorption influenced by kinetic effects. Whereas the KMC simulation exhibits a clear advantage over approaches using rate equations regarding kinetic influences, the well-known quasi-equilibrium desorption provides a crucial test for the KMC simulation. Under the appropriate circumstances, the KMC simulation is, indeed, in full agreement with the quasi-equilibrium analysis based on the Polanyi–Wigner equation.

Online measurement of the optical anisotropy during the growth of crystalline organic films

We report a reflectance difference spectroscopy (RDS) investigation of the growth of para-sexiphenyl (p-6P) on a TiO2(110) single crystal substrate at 100, 300, and 400K. The results demonstrate that RDS is a powerful technique to monitor organic thin film growth from the submonolayer regime to device relevant thicknesses. Based on the polarization dependence of the optical absorption at characteristic wavelengths, the orientation and the crystalline properties of the organic molecules can be directly determined from the RD spectrum with an extremely high sensitivity.

Selective protein and DNA adsorption on PLL-PEG films modulated by ionic strength.

We describe a soft thin film which selectively adsorbs DNA but averts the non-specific binding of proteins. Indium tin oxide (ITO) substrates were surface-modified with a poly(L-lysine)-g-poly(ethylene glycol) (PLL-PEG) film which carries an outer protein-repelling PEG layer and an underlying positively charged PLL layer that attracts DNA. Binding of DNA could be tuned by a factor of over 90 by varying the salt concentration. The dependence of DNA binding on ionic strength was described with a physicochemical model which led to the conclusion of an unexpectedly high enrichment of salt inside the PEG layer. In addition, the model led to an expanded definition of the Debye-Huckel type effective screening length parameter z. Our new findings on a film with dual passivation/attraction properties can find applications in biopolymer-specific coatings useful in bioseparation and biosensing. In addition, the physicochemical characterisation provides new insight into the interactions between biopolymers and polymer-coated interfaces.

A rotating-compensator based reflectance difference spectrometer for fast spectroscopic measurements

We present a new type of reflectance difference (RD) spectrometer for fast spectroscopic measurements based on a rotating-compensator (RC) design. The instrument uses a 1024 element Si photodiode linear array for simultaneous multiwavelength detection. High quality RD spectra covering a spectral range from 1.5 to 4.5 eV can be acquired within a few seconds. A detailed description of the working principle, the instrumentation, and the algorithms used for data collection and reduction is presented, followed by a discussion of errors introduced by lamp instability and optical imperfections of the compensator. Finally, to demonstrate the performance of the new RCRD spectrometer, we illustrate its application for the in situ, real-time monitoring of the initial stages of organic thin film growth of para-sexiphenyl (p-6P) on the Cu(110)-(2 x 1)O surface.

The influence of weak adsorbate–adsorbate interactions on desorption

We have studied the influence of weak lateral adsorbate–adsorbate interactions on submonolayer thermal desorption spectra using a comprehensively tested novel Kinetic Monte Carlo (KMC) simulation scheme. Both attractive and repulsive adsorbate–adsorbate interactions are investigated. It is shown, that the desorption order continuously changes from zero to first to second order like behavior when decreasing the adsorbate–adsorbate binding energy from positive (attractive) to negative (repulsive) values. The desorption of N2 on Cu(1 1 0)–(2×1)O and Xe on Pt(1 1 1) provide examples for systems crucially influenced by weak lateral interactions. Both systems are investigated by means of KMC simulations, which allow to quantitatively reproduce the experimental desorption spectra.

Growth and optical properties of Ag clusters deposited on poly(ethylene terephthalate)

The growth and concomitant evolution of the optical properties of Ag nano-clusters deposited on biaxially extruded poly(ethylene terephthalate) films is studied by reflectance difference spectroscopy. It is demonstrated by low energy ion scattering and simulated optical spectra that the clusters form a two-dimensional layer buried beneath the surface of the substrate. The experimental spectra are described by simulations in which different configurations of the host such as anisotropy, amorphization, and dilution are considered in an effective medium approach. The contribution of the anisotropic substrate is used to explain the resulting line shapes. We also discuss the role of the rate of change of the filling fraction with Ag coverage in the evolution of the spectra and the detection of the onset of coalescence by optical means.

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.

Revealing the buried interface: para-sexiphenyl thin films grown on TiO2(110).

The thickness dependent optical and electronic structure of para-sexiphenyl thin films grown on TiO(2)(110) at around 400 K reveals that the substrate is first wet by one monolayer of molecules lying with their long axis parallel to the [001] direction of the substrate, while the molecules in subsequent layers are almost standing upright. Whilst ultraviolet photoemission spectroscopy (UPS) is sensitive to the molecules in the outermost layer, reflection difference spectroscopy (RDS) shows that the molecules at the buried interface do not dewet and maintain the orientation of the original wetting monolayer.