Silicene and germanene freestanding layers are usually described as a honeycomb lattice formed by two hexagonal sub-lattices presenting a height difference, namely the layer buckling. In this work, first-principles calculations show that silicene and germanene can be rippled at 0 K with various wavelengths, without any compressive strain of the layer. For germanene, the height difference between two Ge atoms from the same sub-lattice can be as high as 4.7 for an undulation length of 81 . The deformations are related to slight (lower than 1.7°) bond angle modifications, and the energy cost is remarkably low, lying between 0.1 and 0.8 meV per atom. These undulations modify the electronic structure, opening a gap of 15 meV.

Molecular simulations have been employed in order to explore at the microscopic scale the adsdrption of bis-chlorinated aromatics (1,2- and 1,3-dichlorobenzene) in Faujasite, a large pore openings zeolite. Both, the purely siliceous and the sodium cation exchanged forms of zeolite structures have been respectively considered, aiming to clearly determine the role of the charge compensating cation in the adsorption phenomenon of the investigated aromatics. A suited combination of classical and electronic structure simulation tools provided a clear overall picture of the adsorption process, from both local and global points of view, well matching with the accessible experimental data. The adsorbate preferential locations, adsorbate/zeolite interaction nature and geometry as well as the adsorption energy were extracted from Density Functional Theory calculations. Furthermore, on the basis of classical force fields, the Gibbs ensemble Monte Carlo simulations allowed predicting the room temperature (298 K) adsorption isotherms for the investigated molecules in Faujasite, within the purely siliceous and Na+ exchanged form, with mobile extra-framework cations upon the adsorption process. Finally, we accomplished a' detailed analysis of the microscopic mechanism in play along the whole adsorption process, with a special highlight to the understanding of the interaction geometry of the molecule with the sodium cation, in function of its crystallographic site. Location of the charge compensating cation has been found to influence the nature of interaction with the adsorbate molecule. (C) 2017 Elsevier Inc. All rights reserved.

Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), and manipulation studies were performed on an ordered self-assembled monolayer (SAM) of N,N'-bis(1-hexylheptyl)perylene-3,4:9,10-bis(dicarboximide) molecules on epitaxial graphene on hexagonal silicon carbide - SiC(0001). Four novel aspects of the molecular SAM on graphene are presented. Molecules adsorb in both armchair and zig-zag configurations, giving rise to six orientations of the molecular layer with respect to the underlying substrate. The interaction between the molecules and the graphene surface shifts the LUMO towards the Fermi level, inducing a charge transfer and the opening of a band gap in the graphene, with the LUMO inside. This decouples the LUMO from the surface rendering it invisible in the dI/dV spectroscopy. The HOMO only becomes visible at short tip-surface distances, as its energy lies within the band gap of the SiC substrate. Finally, the observed molecular defects are very particular, being composed exclusively of molecular dimers. These molecular dimers have a stronger interaction with the graphene than other molecules.

Scanning tunneling microscopy (STM) studies of the fullerene C${}_{60}$ molecule adsorbed on the silicon carbide SiC(0001)-$3\ifmmode\times\else\texttimes\fi{}3$ surface, combined with density functional theory (DFT) calculations, show that chemisorption of individual C${}_{60}$ molecules occurs through the formation of one bond to one silicon adatom only in contrast to multiple bond formation on other semiconducting surfaces. We observe three stable adsorption sites with respect to the Si adatoms of the surface unit cell. Comprehensive DFT calculations give different adsorption energies for the three most abundant sites showing that van der Waals forces between the C${}_{60}$ molecule and the neighboring surface atoms need to be considered. The C${}_{60}$ molecules are observed to form small clusters even at low coverage indicating the presence of a mobile molecular precursor state and nonnegligible intermolecular interactions.

We present a density-functional theory (DFT) study combined with scanning tunneling microscopy (STM) experiments of the chemisorption of the N,N'-bis(1-hexylheptyl) perylene-3,4:9,10-bis(dicarboximide) molecule, noted here as DHH-PTCDI, on the SiC(0001)-3 x 3 surface. Five possible adsorption configurations have been investigated in which molecular adsorption occurs on two adjacent Si adatoms via different pairs of atoms of the molecule. We have calculated the energies, structures, density of states, local density of states, and a calculated STM image and show that chemisorption via two oxygen atoms located on the same side of the molecule on two adjacent Si adatoms is the most favorable, in agreement with the experimental STM images. A comparison between the PTCDI and the adsorption of another large organic molecule (phthalocyanine) on the SiC(0001) surface completes this work.

Density functional theory (DFT) calculations are used to investigate the stability on SiC(0001) surfaces of different chemical groups -NH2, -NO2, -CH3, -OH, -SH and -CN. The adsorption stability decreases in the order -NO2>−OH>−NH2>−SH>−CN>−CH3. The stability of the single molecule-substrate bond is strongly influenced by the polarizability, which in turn depends on different parameters such as the electronegativity, atomic size and chemical environment. In a further step, methyl (−ACH3) and phenyl (−AC6H5) substituted groups are also considered and similar behaviour is observed. The inductive effect of the -CH3 or -C6H5 groups modifies the polarization of the Si adatom-molecule bond and the steric hindrance due to their size influences the molecular orientation. These two parameters affect the calculated adsorption energy, and are more important for –C6H5 substituent. This study provides clear tendencies that can be applied to more complex systems. Comparison of the adsorption of two large molecules, H2Pc (metal-free phthalocyanine) and PTCDI (perylene tetracarboxylic diimide) on the SiC(0001) surface is presented as an example.

Controlling the intrinsic optical and electronic properties of a single molecule adsorbed on a surface requires electronic decoupling of some molecular orbitals from the surface states. Scanning tunneling microscopy experiments and density functional theory calculations are used to study a perylene molecule derivative (DHH-PTCDI), adsorbed on the clean 3 × 3 reconstructed wide band gap silicon carbide surface (SiC(0001)-3 × 3). We find that the LUMO of the adsorbed molecule is invisible in I(V) spectra due to the absence of any surface or bulk states and that the HOMO has a very low saturation current in I(z) spectra. These results present a paradox that the molecular orbitals are electronically isolated from the surface of the wide band gap semiconductor even though strong chemical bonds are formed.

The reversible hopping of a bistable atom on the Si(100)-(2×1):H surface is activated nonlocally by hole injection into Si-Si bond surface states with a low temperature (5 K) scanning tunneling microscope. In the contact region, at short distances (<1.5  nm) between the hole injection site and the bistable atom, the hopping yield of the bistable atom exhibits remarkable variations as a function of the hole injection site. It is explained by the density of state distribution along the silicon bond network that shows charge-transfer pathways between the injection sites and the bistable atom.

We have investigated from a theoretical point of view modifications of the 4,4(')-diacetyl-p-terphenyl molecule chemisorbed on Si(001) induced by the scanning tunneling microscope (STM). In previous experiments, these modifications were observed to occur preferentially at the end of the molecule after a +4.0 V voltage pulse and at the center after a +4.5 V voltage pulse. In the framework of ab initio simulations, we have realized a systematic energetic study of the dissociative chemisorption of one, two, or three phenyl rings of the substituted p-terphenyl molecule. Charge densities were then calculated for the investigated configurations and compared to the STM topographies. Before manipulation with the STM tip, the substituted p-terphenyl molecule is preferentially adsorbed without phenyl ring dissociation, allowing a partial rotation of the central phenyl ring. Our results show that the STM induced modifications observed at the end of the molecule might originate from the dissociation of two phenyl rings (one central and one external ring), while the modifications occurring at the central part of the molecule can be interpreted as a dissociation of the two external rings.

The adsorption of individual metal-free phthalocyanine molecules on the 6H-SiC(0001)3×3 surface was studied using the scanning tunneling microscope supported by density functional theory calculations. Phthalocyanine molecules were found to be chemisorbed through a reaction of two conjugated imide groups with two silicon adatoms. This type of anchoring opens numerous perspectives for the organic functionalization of a biocompatible wide band gap semiconductor.

We present an ab initio study of the adsorption of the 1,4(')-paratriphenyldimethylacetone molecule (trima) involving three phenyl rings and two COCH3 groups at the ends to anchor the molecule to the Si(001) surface. A preferential adsorption via the oxygen atom is found, confirming earlier near-edge x-ray fine-structure studies. Considering the trima molecule adsorbed on the surface, we propose a classification of the investigated configurations in two families which present similar characteristics in the external ring displacements and very different behavior concerning the central ring rotation. This central ring rotation is particularly investigated. Our systematic study allows further understanding of the experimental data: the phenyl-ring-underlying dimer tilt interaction is expected to play a major role in the configurational modifications observed in scanning tunneling microscopy experiments. The asymmetry of the adsorbed trima molecule with respect to the dimer rows largely stabilizes the type-I configurations in which the three phenyl rings present the same orientation, the tilt of the dimer beneath favoring a larger amplitude of the central ring rotation.

We report the study of both occupied and unoccupied electronic states of biphenyl adsorbed on Si(100) by using synchrotron radiation ultraviolet photoemission spectroscopy, x-ray photoemission spectroscopy, and near-edge x-ray absorption fine structure spectroscopy. The results are compared with calculations of the occupied densities of states. Evidence is given of two main effects: (i) a down shift in energy of the second highest occupied molecular orbital of the strongly chemisorbed configuration at room temperature compared to the weakly chemisorbed configuration at low temperatures and (ii) an energy splitting of some of the occupied and unoccupied states due to the different interactions of the two phenyl rings of the molecule. The electronic structure of biphenyl on Si(100) appears to be an appropriate test for state of the art experimental and theoretical methods.

At low temperature (5 K), a single biphenyl molecule adsorbed on a Si(100) surface behaves as a bistable device which can be reversibly switched by electronic excitation with the scanning tunneling microscope tip. Density functional theory suggests that the biphenyl molecule is adsorbed with one dissociated hydrogen atom bonded to a neighbor surface silicon atom. By desorbing this hydrogen atom with the STM tip, the interaction of the molecule with the surface is modified such that it becomes transformed into a multistable device with four stable states having switching yields increased by almost 2 orders of magnitude.

Carbon system plays a twofold role in the SiGe, inducing both high stress fields and strong chemical effects. Our Monte Carlo simulations, based on a novel algorithm enabling C-insertion and equilibration, shed light on the stress field and composition of C-induced Ge islands on Si(100), a prototypical case where these two effects operate. It is shown that the dots do not contain C under any conditions of temperature and coverage, but have a gradual composition profile from SiGe at the bottom to Ge at the apex. The average compressive stress in the islands is considerably reduced, compared to the pure Ge/Si case. At low Ge coverage, the terrace around the dots is enriched with Si-C dimers. At high Ge contents, Ge wets the surface and covers the pre-deposited C geometries. We predict enhancement of Ge content in the islands upon C incorporation.

INTRODUCTION Scientific report According to project objectives, namely to grow and to evaluate the structural, electrical and optical properties of nanostructures in the Si-Ge-C system and to gain insight into fundamental physics of these structures, and to improve the luminescence efficiency of these structures, and according to the work plan for the second year (months 12 to 24), the following work packages have been continuously followed or initiated according to the work plan and results were obtained as described in detail in the following report: Workpackage I (continued): Fabrication and analysis of quantum dot structures Task 1: Nucleation mechanism and in-situ structural analysis of Ge islands Task 2: Formation of Ge dots on prepatterned substrates Task 3: Structural analysis of Ge and C-induced Ge dots Task 4: Optical properties of 0-dimensional structures in the SiGeC system Workpackage II (continued): Si/SiGe and Si/SiGeC heterostructures for intersubband photon emission. Forseen milestones for the second year are after 18 months (T2): Mastering of quantum dot structures on pre-patterned substrates Evaluation of the impact of interstitial C on the structural and optical properties; Understanding of ordering phenomena in C-induced Ge quantum dot structures. The scientific progress of the network has been considerable, as detailed in the individual work package reports. A series of publications in first-rate journals also demonstrates the obtained results. The following list shall give an overview over the scientific progress in the individual work packages (WP): WPI, Task 1: a detailed theoretical study of the nucleation mechanism especially of C-induced islands has been performed. In-situ studies using, e.g., scanning tunnelling microscopy (STM) of Ge islands on Si (001) under various growth conditions, as well as on the effect of overgrowth of such islands, have been performed (I. WPI, Task 2: very small Ge islands on prepatterned substrates have actually been achieved, island related photoluminescence has been observed, the effect of elastic relaxation due to small mesas on the island properties has been investigated (M.W. Dashiell et al. WPI, Task 3: the structural properties of many series of samples have been investigated by various x-ray scattering techniques, by transmission electron microscopy (TEM), atomic force microscopy (AFM), and STM. The effect of different growth procedures on the properties of the islands has been investigated, and growth conditions suited to obtain desired island properties have been singled out. WPI, Task 4: also the optical properties of various sample series have been investigated using photoluminescence …

We present a theoretical study of the adsorption and diffusion mechanisms of Pb on the Si(111)-(7 x 7) surface in the initial stages of Pb chemisorption within the DAS model. In the framework of the crystalline extension of the extended Huckel theory, two adsorption mechanisms are studied: the adsorption of Pb atoms on Si dangling bonds and Pb adsorption with the formation of Pb atom clusters. The second mechanism is found to be more favorable than the first, but the two mechanisms could possibly coexist. Trimers and tetramers of Pb atoms are more likely than pairs. This tendency to agglomeration, in agreement with STM observations for room-temperature deposition, is explained by the strong interaction between Pb atoms which is emphasized in our calculations. The faulted half of the unit cell is preferred by about 0.1 eV. The diffusion paths for Pb on Si(111)-(7 x 7) are then investigated: jumps of Pb atoms between unfaulted and faulted half unit cells are more difficult than jumps inside the same half unit cell. Considering the diffusion of a Pb pair between an adatom and a restatom, we show that the interaction between Pb atoms leads to a significant decrease in the barrier energy as compared to that for the diffusion of a single Pb atom. An interpretation of experimental data obtained for room-temperature deposition at low coverage is proposed. (C) 1998 Elsevier Science B.V. All rights reserved.

In a recent theoretical study of the adsorption and diffusion mechanisms of Pb on the Si(111)-(7 x 7) surface in the initial stages of the Db chemisorption, two adsorption mechanisms were especially considered: (i) adsorption of Db atoms on Si dangling bonds; (ii) Db adsorption with formation of Db clusters. The second mechanism was found to be more favorable than the first one, but the two mechanisms may coexist. The tendency to agglomeration was explained by strong interaction between the Pb atoms. In this paper, we focus on an interpretation of the photoemission spectra observed for the Pb/Si(111)-(7 x 7) system. Densities of states performed on models based on these two mechanisms allow us to propose an interpretation for the two surface states experimentally observed in the projected band structure of Pb/Si(111)-(7 x 7). (C) 1998 Elsevier Science B.V. All rights reserved.