Probing the Si-Si dimer breaking of Si(100)2x1 surfaces upon molecule adsorption by optical spectroscopy

Effect of Water Adsorption on the Photoluminescence of Silicon Quantum Dots

The optical properties of silicon quantum dots (Si QDs) are strongly influenced by circumjacent surface-adsorbed molecules, which would highly affect their applications; however, water, as the ubiquitous environment, has not received enough attention yet. We employed the time-dependent density functional calculations to investigate the water effect of photoluminescence (PL) spectra for Si QDs. In contrast with the absorption spectra, PL spectra exhibit distinct characteristics. For Si32H38, PL presents the single maximum in the dry and humid environment, while the emission spectrum displays a dual-band fluorescence spectroscopy in the low-humidity environment. This phenomenon is also observed in the larger Si QDs. The distinct character in spectroscopy is dominated by the stretching of the Si-Si bond, which could be explained by the self-trapped exciton model. Our results shed light on the Si-water interaction that is important for the development of optical devices based on Si-coated surfaces.

Electrophilic surface sites as precondition for the chemisorption of pyrrole on GaAs(001) surfaces

We report how the presence of electrophilic surface sites influences the adsorption mechanism of pyrrole on GaAs(001) surfaces. For this purpose, we have investigated the adsorption behavior of pyrrole on different GaAs(001) reconstructions with different stoichiometries and thus different surface chemistries. The interfaces were characterized by x-ray photoelectron spectroscopy, scanning tunneling microscopy, and by reflectance anisotropy spectroscopy in a spectral range between 1.5 and 5 eV. On the As-rich c(4 × 4) reconstruction that exhibits only nucleophilic surface sites, pyrrole was found to physisorb on the surface without any significant modification of the structural and electronic properties of the surface. On the Ga-rich GaAs(001)-(4 × 2)/(6 × 6) reconstructions which exhibit nucleophilic as well as electrophilic surface sites, pyrrole was found to form stable covalent bonds mainly to the electrophilic (charge deficient) Ga atoms of the surface. These results clearly demonstrate that the existence of electrophilic surface sites is a crucial precondition for the chemisorption of pyrrole on GaAs(001) surfaces.

Unusual two-stage kinetics of ethylene adsorption on Si(100) unraveled by surface optical spectroscopy and Monte Carlo simulation

The adsorption of ethylene on a Si(100)-2×1 surface in an ultrahigh vacuum has been monitored at room temperature by use of real-time surface differential reflectance spectroscopy, which clearly demonstrated that the adsorption follows a two-stage process. About half a monolayer is obtained for 1 L, while the second stage is much slower, yielding the complete monolayer for an exposure of ∼400  L. The kinetics over the full range has been successfully reproduced by a Monte Carlo calculation. The key point of this two-stage adsorption kinetic lies in the reduced adsorption probability (by a factor of several hundreds) on the Si dimers, neighbors of dimers which have already reacted, with respect to the adsorption probability on isolated dimers. This new kind of adsorption kinetics, due to a repulsion between already adsorbed molecules and additional molecules impinging on the surface, makes it a textbook case for a "cooperative" adsorption process.

Optical differential reflectance spectroscopy of ultrathin epitaxial organic films

This Perspective does not have the ambition to entirely review the subject of optical spectroscopy on thin organic films. What we will try to achieve instead is to give an overview on optical reflectance spectroscopy of highly ordered organic thin films in the thickness range from submonolayers to several monolayers, as a tool to study the absorption behavior of such films. By doing so, we will emphasize the relations between the physical layer structure and the resulting optical properties. More specifically, we intend to show on the basis of particular examples what physical effects can be favorably examined by means of real-time optical spectroscopy, i.e., applied during the actual film growth, especially differential reflectance spectroscopy (DRS). Epitaxial organic films on inorganic substrates (insulators and conductors) will be in focus, and also the perspectives of investigating organic-organic heteroepitaxial layers will be addressed.

Adsorption of Phenylacetylene on Si(100)-2 × 1: Kinetics and Structure of the Adlayer

Direct adsorption of phenylacetylene on clean silicon surface Si(100)-2 x 1 is studied in ultrahigh vacuum (UHV). The combination of scanning tunnel microscopy (STM) and surface differential reflectance spectroscopy (SDRS) with Monte Carlo calculations are put together to draw a realistic kinetic model of the evolution of the surface coverage as a function of the molecular exposure. STM images of weakly covered surfaces provide evidence of two very distinct adsorption geometries for phenylacetylene, with slightly different initial sticking probabilities. One configuration is detected with STM as a bright spot that occupies two dangling bonds of a single dimer, whereas the other configuration occupies three dangling bonds of adjacent dimers. These data are used to implement a Monte Carlo model which further serves to design an accurate kinetic model. The resulting evolution toward saturation is compared to the optical data from surface differential reflectance spectroscopy (SDRS). SDRS is an in situ technique that monitors the exact proportion of affected adsorption sites and therefore gives access to the surface coverage which is evaluated at 0.65. We investigate the effect of surface temperature on this adsorption mechanism and show that it has no major effect either on kinetics or on structure, unless it passes the threshold of dissociation measured at ca. 200 degrees C. This offers a comprehensive image of the whole adsorption process of phenylacetylene from initial up to complete saturation.

Real-time coverage monitoring of initial oxidation processes on Si(001) by means of surface differential reflectance

Initial oxidation processes on Si(001) have been studied by means of surface differential reflectance (SDR). The time courses of the SDR spectra measured during thermal oxidation at 820 and 920 K allowed two different growth modes, Langmuir-type adsorption and two-dimensional island growth, to be distinguished. No photon energy dependence was observed in the time course of the SDR intensity at either temperature. On the other hand, different uptake curves were observed at different photon energies for oxidation at 300 K. The difference between the oxidation mechanisms at 300 K and at high temperatures was qualitatively apparent from SDR results, because significant photon energy dependence was observed only at 300 K. Possible assignments of the spectral components in the SDR spectra are discussed.