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N Lorente

Publications and source records attributed to N Lorente.

15 recordsLinked to original sources
PubMed2006

Electron transport via local polarons at interface atoms.

Electronic transport is profoundly modified in the presence of strong electron-vibration coupling. We show that in certain situations, the electron flow takes place only when vibrations are excited. By controlling the segregation of boron in semiconducting Si(111)-square root 3 x square root 3 R 30 degrees surfaces, we create a type of adatom with a dangling-bond state that is electronically decoupled from any other electronic state. However, probing this state with scanning tunnelling microscopy at 5 K yields high currents. These findings are rationalized by ab-initio calculations that show the formation of a local polaron in the transport process.

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PubMed2006

Spontaneous formation of triptycene supramolecules on surfaces.

In the limit of weak molecular interaction with an inorganic surface, noncovalent interactions between molecules dominate the nucleation and thin-film growth. Here, we report on the formation of three-dimensional triptycene clusters with a particularly stable structure. Once formed at the early stage of molecular adsorption, the clusters are stable for all temperatures until desorption. Furthermore, the clusters diffuse and nucleate as individual entities, therefore constituting building blocks for the later thin-film formation. High resolution scanning tunneling microscopy images indicate that the cluster is stabilized by C-H-pi interactions. The formation of such molecular structures at a surface is possible because the three-dimensional structure of the triptycene molecule leads to a very weak and mobile adsorption state. These results show that it is possible to investigate complex pathways in the formation of three-dimensional supramolecules at surfaces using a scanning tunneling microscope.

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PubMed2006

Inelastic spectroscopy identification of STM-induced benzene dehydrogenation.

Inelastic electron tunneling spectroscopy (IETS) performed with the scanning tunneling microscope (STM) has been deemed as the ultimate tool for identifying chemicals on the atomic scale. However, IETS-based chemical analysis is error-prone due to the numerous degrees of freedom of chemisorbed molecular systems. First-principles simulations of IETS are presented that, by quantitative comparison with the experimental spectra, permit one to determine the final products of an STM-induced reaction on chemisorbed benzene. Our simulations reveal that IETS possesses an enhanced sensitivity to atomic structure as compared to topographic imaging due to both its energy and space resolution.

Journal Article↗
PubMed2005

Adsorbate motions induced by inelastic-tunneling current: theoretical scenarios of two-electron processes.

We discuss how the excitation of high-frequency modes in adsorbed molecules may result in motion (e.g., rotation, translation, or dissociation) of the molecules. Our study is based on rate equations and considers one- and two-vibrational excitation processes, corresponding to linear and quadratic dependences of the reaction rate on the tunneling current in the case the scanning tunneling microscopy is used to excite the vibrations (inelastic tunneling). From the results reported in this paper it should be possible to obtain intramolecular transition rates directly from the experimental data, and gain some understanding on how these important quantities depend on the modes involved and on the substrate.

Journal Article↗
PubMed2005

Metallic and semimetallic silicon 100 nanowires.

Silicon nanowires grown along the 100 direction with a bulk Si core are studied with density-functional calculations. Two surface reconstructions prevail after exploration of a large fraction of the phase space of nanowire reconstructions. Despite their energetical equivalence, one of the reconstructions is found to be strongly metallic while the other one is semimetallic. This electronic-structure behavior is dictated by the particular surface states of each reconstruction. These results imply that doping is not required in order to obtain good conducting Si nanowires.

Journal Article↗
PubMed2003

Selectivity in vibrationally mediated single-molecule chemistry.

The selective excitation of molecular vibrations provides a means to directly influence the speed and outcome of chemical reactions. Such mode-selective chemistry has traditionally used laser pulses to prepare reactants in specific vibrational states to enhance reactivity or modify the distribution of product species. Inelastic tunnelling electrons may also excite molecular vibrations and have been used to that effect on adsorbed molecules, to cleave individual chemical bonds and induce molecular motion or dissociation. Here we demonstrate that inelastic tunnelling electrons can be tuned to induce selectively either the translation or desorption of individual ammonia molecules on a Cu(100) surface. We are able to select a particular reaction pathway by adjusting the electronic tunnelling current and energy during the reaction induction such that we activate either the stretching vibration of ammonia or the inversion of its pyramidal structure. Our results illustrate the ability of the scanning tunnelling microscope to probe single-molecule events in the limit of very low yield and very low power irradiation, which should allow the investigation of reaction pathways not readily amenable to study by more conventional approaches.

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PubMed2001

Symmetry selection rules for vibrationally inelastic tunneling.

A combined experimental and theoretical study is presented for the C-D stretch mode excitation of acetylene isotopes, C2HD and C2D2, on Cu(100) via inelastic electron tunneling (IET) in a scanning tunneling microscope junction. The calculated IET images using density functional theory show that the measured signal from C2D2 derives from the antisymmetric stretch mode. Selection rules are derived and involve the constraint imposed by the IET image on the symmetry characters of the vibrational mode and the adsorbate-induced electron states at the Fermi level.

Journal Article↗
PubMed2000

Theory of single molecule vibrational spectroscopy and microscopy.

We have carried out a density functional study of vibrationally inelastic tunneling in the scanning tunneling microscope of acetylene on copper. Our approach is based on a many-body generalization of the Tersoff-Hamann theory. We explain why only the carbon-hydrogen stretch modes are observed in terms of inelastic and elastic contributions to the tunneling conductance. The inelastic tunneling is found to be efficient and highly localized in space without any resonant interaction and to be governed by a vibration-induced change in tunneling amplitude.

Journal Article↗
PubMed2000

Theoretical aspects of tunneling-current-induced bond excitation and breaking at surfaces.

We have performed a density functional study of the electronic structure, images and vibrationally inelastic tunneling in the scanning tunneling microscope and vibrational damping by excitation of electron-hole pairs of CO chemisorbed on the (111) and (100) faces of Cu. We find that the 2 pi* molecular orbital of CO turns into a broad resonance with parameters that differ significantly from those suggested by inverse and two-photon photoemission measurements. The calculated vibrational damping rate for the internal stretch mode and relative changes in tunneling conductance across vibrational thresholds are in agreement with experiment. The non-adiabatic electron-vibration coupling is well described by the Newn-Anderson model for the 2 pi*-derived resonance whereas this model is not able to describe the non-adiabatic coupling between the tunneling electrons and the vibration. We believe that this model misses an important mechanism for vibrational excitation in tunneling that involves the change of tunneling amplitude by deformation of the tails of the one-electron wavefunctions with vibrational coordinate.

Journal Article↗
PubMed1995

[The effect of a program of care for the diabetic on control of the disease].

OBJECTIVE: To study the association between the application of a diabetic care programme and modifications in glycated haemoglobin and the Body Mass Index. DESIGN: Descriptive study. SETTING: Two health centres. PATIENTS: 199 patients were studied, the total number of type II diabetics found in three general medicine practices. INTERVENTION: Patients followed, at different levels, a programme of diabetic care. Variation of glycated haemoglobin and the Body Mass Index were measured in each patient during the 1991-1992 period. These were then related to their continuation in the programme and whether they were included in this programme at the moment of the examination. RESULTS: The average drop in glycated haemoglobin was -15.3% (12.2 +/- 18.4) (p < 0.001). Average variation of the Body Mass Index was 0.2% (-0.6 +/- 1.00), not significant. Average drop in glycated haemoglobin in those included was -16% (12.5 +/- 19.5) (p < 0.001) and in those not included, -12.6% (5.3 +/- 19.9) (p < 0.001), no significant difference. The correlation between the degree of continuation in the programme and the variation of glycated haemoglobin had a r = -0.04 (p = 0.7) and between the degree of continuation and variation in the Body Mass Index, a r of 0.15 (p = 0.06). When the initial value of glycated haemoglobin was equal to or above 7.5, its average drop was -22.3% (17.8 +/- 26.9) (p < 0.001). CONCLUSIONS: The drop in glycated haemoglobin is significant and more important, the greater the initial value. But it has not been possible to link it with the level of continuance in the programme nor with whether patients were included in the programme. The Body Mass Index does not vary.

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