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 ¼ 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 ¼ 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.

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A new approach for the determination of activation energy and attempt frequency for the diffusion of single adatoms on a surface is proposed and demonstrated for Pt adatom diffusion on Pt(111). The method is based on a combination of low temperature scanning tunneling microscopy (STM) measurements and kinetic Monte Carlo simulations. The method involves only a minimum of assumptions and is independent of classical nucleation theory. This independence allows one to analyze the applicability of the latter. In addition, it is found that the measurement of the mean square displacement of individual adatoms is influenced by the STM tip. PACS numbers: 81.15.Hi, 61.16.Ch, 68.35.Fx, 68.55.Jk Probably the most fundamental diffusion process in molecular beam epitaxy is the migration of the deposited atoms on the substrate, which enables the formation of deposit islands. The basic quantities characterizing adatom migration are the activation energy E d and the attempt frequency n 0 . Besides the field emission microscopy approach [1] there are two methods to determine these parameters: the measurement of the temperature dependence of the mean square displacement adatoms by field ion microscopy (FIM method) [2] and the analysis of the temperature dependence of the island number density by the rate equation approach of the nucleation theory (nucleation theory method) [3][4][5]. While the FIM method involves only basic diffusion theory, the nucleation theory is rather sophisticated and its application always involves assumptions and approximations of the physical situation. Both methods have been applied successfully. In addition, Monte Carlo simulations of simple models were used to test and improve its formulation [6,7]. However, to the authors' best knowledge it has not yet been tried to test experimentally the applicability of the nucleation theory without presupposing its validity. Such a test of the nucleation theory might allow direct insight into the nature and relevance of the approximations to the physical situation involved in its application.In this Letter we report a new method for determination of E d and n 0 in the temperature range just above the onset of adatom migration, based on scanning tunneling microscopy (STM) measurements and kinetic Monte Carlo (MC) simulations, involving only a minimum of assumptions. This method is independent of nucleation theory and enables us to perform a test of the nucleation theory method for the Pt adatom migration on Pt(111).The experiments were performed in a variable temperature STM system with a base pressure of 5 3 10 211 mbar described elsewhere [8]. The sample crystal was carefully prepared and cleaned before each deposition experiment according to standard procedures [8]. In order to remove gas atoms adsorbed during the initial cool down to 20 K, the sample is flashed prior to the low temperature deposition. After that, the density of impurities was less than 10 26 Å 22 as checked by STM imaging. Pt deposition was performed by electron bombardment heating of a thoro...

Whereas on the clean Pt(111) surface Pt grows in a multilayer (3D) mode at 300 and 400 K, preadsorption of an ordered p(2x 2) oxygen overlayer leads to a high quality layer-by-layer (2D) growth with the oxygen Aoating on the film. In contrast to other surfactants oxygen can be completely removed from the film surface at the growth temperature.It is demonstrated that the presence of oxygen reduces the barrier height for the motion of Pt adatoms across step edges. This facilitates the interlayer mass transport and thus the layer-by-layer growth.PACS numbers: 68.55.Jk, 61.16.Ch, 68.55.Ce, 68.55.Ln Recently surfactants have attracted considerable interest in both heteroand homoepitaxial growth by increasing the smoothness of the deposited films [1][2][3][4].

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