Using a first-principles pseudopotential technique, we have investigated the adsorption of C 2 H 2 on the Si͑001͒ surface. We have found that, at low temperatures, the di-bond configuration is the most stable structure from the energetic point of view. According to our calculations C 2 H 2 adsorbs preferentially on the alternate dimer sites, corresponding to a coverage of 0.5 monolayer. Our calculated surface band structure suggests that the end-bridge configuration, recently pointed out as a more favorable configuration by firstprinciples calculations, presents a metallic character and thus is Peierls unstable. The di-adsorbed system is characterized by symmetric and slightly elongated Si-Si dimers, and by a symmetric CC bond with length close to the double carbon bond length of the ethylene molecule. Our total-energy calculations suggest that other metastable configurations, like the 1,2-hydrogen transfer, the p bridge and the tetra-model are also possible. Available high-resolution electron-energy-loss spectroscopy experimental data are reinterpreted to support the existence of the tetra-model.

Effect of the cluster size in modeling the H 2 desorption and dissociative adsorption on Si(001)Using a first-principles pseudopotential method we have studied the adsorption and dissociation of NH 3 , PH 3 , and AsH 3 on the Si͑001͒-͑2ϫ1͒ surface. Apart from the existence of a barrier for the adsorption of the precursor state for arsine, we observe that the global behavior for the chemisorption of the XH 3 molecules considered in this work is as follows: the gas phase XH 3 adsorbs molecularly to the electrophilic surface Si atom and then dissociates into XH 2 and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. The energy barrier, corresponding to a thermal activation, is much smaller than the usual growth temperature, indicating that all three molecules will be observed in their dissociated states at room temperature. All adsorbed systems are characterized by elongated Si-Si dimers that are ͑almost͒ symmetric in the dissociative case but asymmetric in the molecular case. According to our first-principles calculations, all XH 3 and XH 2 systems retain the pyramidal geometry observed for the gas molecules. Our calculated vibrational spectra further support the dissociative model for the XH 3 molecules considered here.

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Using a first-principles pseudopotential method we have studied the adsorption and dissociation of the common n-type dopant molecule PH 3 on the Si͑001͒ surface. We have found that for low phosphorus coverages ( 1 4 monolayer͒ phosphine adsorbs molecularly on one side of the Si-Si dimer and, at temperatures around 140 K, fully dissociates into PH 2 and H, with each component attached to one side of the dimer. For higher phosphorus coverages ( 1 2 monolayer͒ the interaction between adjacent dimers plays a decisive role in the dissociation process. For both coverages, the surface is characterized by an elongated dimer, symmetric for the dissociated case and asymmetric for the molecular case. The H-P-H angles and H-P bond lengths for the dissociative case are very close to those obtained for the PH 3 molecule. However, for the molecular case, while the H-P bond length is close to that observed for the PH 3 molecule, the H-P-H angle is ϳ8% bigger. Available experimental scanning tunneling microscopy image results are reinterpreted using theoretical images for the model provided in this work. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental work.

Using the first-principles pseudopotential-local-density-approximation approach, we have studied the dissociative molecular adsorption of NH 3 on Si(001)-(2ϫ1). We find that upon adsorption the Si dimer becomes symmetric and somewhat elongated. We also find that the Si-N bond is inclined at 10°with respect to the surface normal, the N-Si bond length is 1.75 Å, and the N-H bond length is 1.05 Å. Finally, our electronic structure calculations suggest that the adsorption of ammonia almost completely passivates the silicon surface.

With a view to contribute to the understanding the surface effects on optical properties process, and its hole in the electronic properties of the nanoparticles, CdS based nanoparticles are characterised by different experimental techniques and the experimental results compared to density functional theory calculations. Our results indicate that cubic CdS nanoparticles present a strong structural deformation, hexagonal reconstructed structures preserve their lattice behaviour. Both cubic and hexagonal CdS nanoparticles are S-terminated after relaxation, even when mildly Cd-rich nanoparticles are considered. A broad peak observed in our PL measurements is interpreted as an experimental evidence of the surface related peak observed around 1.8 eV in our calculated DOS for the hexagonal relaxed structure.