Derivation of the near-surface dielectric function of amorphous silicon from photoelectron loss spectra

International audienceThe near-surface dielectric function ε(ℏω) of hydrogenated amorphous silicon (a-Si:H) films has been derived from X-ray photoelectron energy-loss spectra, over the energy range 0-45 eV. Removal of low lying single-electron excitations is a prerequisite step to proceed to the derivation of the single plasmon energy loss function Im[−1/ε(ℏω)] due to collective electron oscillations. Several methods are compared to separate interband transitions from bulk or surface plasmons excitation. The shape of interband excitation loss in the range 1-10 eV can be described by a Henke function; alternatively, its removal using a sigmoid weighting function is a low-noise and reliable method. After deconvolution of multiple plasmon losses and self-consistent elimination of surface plasmon excitation, the single plasmon loss distribution allows recovery of optical (ellipsometry) data measured in the near-UV to visible range

Temperature dependence of current density and admittance in metal-insulator-semiconductor junctions with molecular insulator

International audienceElectrical transport in ultrathin Metal-insulator-semiconductor (MIS) tunnel junctions is analyzed using the temperature dependence of current density and admittance characteristics, as illustrated by Hg//C12H25 - n Si junctions incorporating n-alkyl molecular layers (1.45 nm thick) covalently bonded to Si(111). The voltage partition is obtained from J(V, T) characteristics, over eight decades in current. In the low forward bias regime (0.2-0.4 V) governed by thermionic emission, the observed linear T-dependence of the effective barrier height, qΦEFF(T) = qΦB+(kT)β0dT, provides the tunnel barrier attenuation, exp(-β0dT), with β0= 0.93 Å−1 and the thermionic emission barrier height, ΦB = 0.53 eV. In the high-forward-bias regime (0.5-2.0 V), the bias dependence of the tunnel barrier transparency, approximated by a modified Simmons model for a rectangular tunnel barrier, provides the tunnel barrier height, ΦT = 0.5 eV; the fitted prefactor value, G0 = 10−10 Ω−1, is four decades smaller than the theoretical Simmons prefactor for MIM structures. The density distribution of defects localized at the C12H25 - n Si interface is deduced from admittance data (low-high frequency method) and from a simulation of the response time τR(V) using Gomila's model for a non equilibrium tunnel junction. The low density of electrically active defects near mid-gap (DS < 2 × 1011 eV−1.cm−2) indicates a good passivation of dangling bonds at the dodecyl - n Si (111) interface

Fonctionnalisation de la surface de couches minces de carbone amorphe

Ce travail porte sur la fonctionnalisation de la surface de couches minces de carbone amorphe à l aide de molécules d alcènes linéaires. Ce type d interfaces trouve son domaine d application dans les capteurs chimiques ou biologiques, et les mémoires moléculaires. Le choix de substrats carbonés repose sur les propriétés de bio-compatibilité des couches minces de carbone et sur la robustesse attendue des liaisons carbone-carbone (covalentes et non polaires) formées à l interface molécule / substrat. Différentes couches de carbone, obtenues par pulvérisation ou par ablation laser, sont étudiées présentant des densités volumiques et des hybridations sp2 / sp3 variables. Les surfaces, toutes de faible rugosité, permettent une étude quantitative du taux de couverture moléculaire par photoémission XPS. Le principal apport expérimental de cette thèse est la réalisation d un système d évaporation de molécules dans un bâti ultravide permettant le greffage thermique dans des conditions de propreté optimales. Des caractérisations UPS et des mesures de transport électrique (dispositif à goutte de mercure) sur des interfaces modèles Si(111) alcane ont permis de qualifier la qualité du greffage obtenu en phase vapeur sous ultravide. Le greffage en phase vapeur sur ces surfaces de carbone peut être réalisé à 230C sans hydrogénation préalable de la surface. Au-dessus d une température de seuil, la cinétique de greffage est plus rapide sur les surfaces riches en carbone hybridé sp2. Le taux de couverture à saturation est comparable à celui obtenu sur la surface du Si(111):H. La robustesse de la monocouche obtenue a été établie par des recuits thermiques sous ultravide.The functionalization of amorphous carbon thin film surfaces has been performed using linear alkene molecules. This kind of interface is useful for chemical or biological sensors, and molecular memories. Carbon-based substrates have been selected for the reported biocompatibility of amorphous carbon thin films and the expected robustness of the covalent and non-polar C-C bonds formed at the molecule / substrate interface. Besides this original material, complementary thermally-assisted processes have been chosen: liquid phase grafting (ester, pyridine or ferrocene functionalities) and gas phase grafting (perfluorodecene). Different amorphous carbon films have been compared: sputtered a-C and pulsed-laser deposited a-C, with variable atom density and sp2 / sp3 hybridizations. We have developed an evaporation chamber for the gas phase grafting in a UHV system, which allows thermal grafting in a clean environment. The quality of the monolayer interface obtained in this thermal process has been assessed using UPS characterizations and electrical transport (with a mercury drop electrode) on model interfaces Si(111) alkane. We have shown that gas phase grafting of alkene molecules can be performed at 230C without hydrogenation and without any preparation of the amorphous carbon surfaces. Above a threshold grafting temperature the grafting kinetics is faster for sp2 rich surfaces. The saturated molecular coverage is comparable to that obtained on Si(111):H surfaces. X-ray reflectivity and XPS data show that a monolayer is assembled to the a-C surface; thermal annealing in UHV confirms the robustness of a perfluorodecene monolayer grafted on amorphous carbon.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Bulk and surface plasmon excitations in amorphous carbon films measured by core-level loss spectroscopy

International audienc

Surface energy and hybridization studies of amorphous carbon surfaces

International audienceSurface properties of a large number of amorphous carbon (a-C) films have been investigated using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Dense a-C surfaces with variable sp3/(sp2 + sp3) average hybridization were grown using sputtering or pulsed laser deposition (PLD) and were further chemically modified by thermal annealing, ion bombardment or covalent grafting of organic monolayers. The average carbon hybridization, impurity level and mass density, were deduced from XPS and photoelectron energy loss spectroscopy (PEELS). The depth sensitivity of the dispersive (Lifshitz-van der Waals) interaction, estimated at 1-2 nm from the dependence of γLW on the grafted perflorodecene molecule coverage, is much better than XPS which probes a 3-5 nm depth. The observation of a non-monotonic behavior in the correlation between surface hybridization and electron donor component of surface energy reveals that the average carbon hybridization alone does not describe the entire surface energy physics. The role of π bond clustering in the polar interactions is thus considered and some implications on surface reactivity and mutual interactions with molecular or biomolecular species are discussed

Tunnel barrier parameters derivation from normalized differential conductance in Hg/organic monomolecular layer-Si junctions

International audienceThe shape of tunnel barrier junctions is derived from experimental current density versus bias, J(V), using the normalized differential conductance, NDC=d log J/d log V, to discriminate barrier height, ΦT, and barrier width, dT, effects. Parameterization of the Simmons model for a rectangular tunnel barrier, with NDC≈dTV/(ΦT-qV)1/2, provides physical (dT,ΦT) values for Hg||monomolecular layer--n Si(111) junctions incorporating functionalized n-alkyl layers covalently bonded to silicon

Covalent Grafting of Organic Layers on Sputtered Amorphous Carbon: Surface Preparation and Coverage Density

The covalent attachment of ester-, pyridine-, and ferrocene-functionalized organic layers to amorphous carbon surfaces, grown by graphite sputtering in Ar-H2 mixtures, is reported. The surface modification is based on a low temperature (160 C) liquid-phase process. Thermally assisted grafting of linear alkenes is investigated by means of X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and contact angle measurements. The beneficial role of surface oxygen removal by argon ion sputtering or thermal treatment prior to the ester grafting step is also discussed. Careful XPS data analysis provides a quantitative estimate of the density (0.2-4.3 × 1014 cm-2) of attached molecules as a function of surface preparation. The large packing density has been confirmed by the voltammetric response of a ferrocene-functionalized carbon surface

花精治療在兒童情緒上之應用

National audienceAu cours de cette allocution, quelques voies de recherche et développement en photonique intégrée à base de matériaux organiques seront développées. Plusieurs approches hybrides qui sont issues de domaines transverses de la physique et de la chimie pour la réalisation de Micro-Résonateurs (MRs) 2D, 2.5D et 3D sur polymères seront présentées. De telles voies parallèles soulignent l'intérêt de développer des technologies et procédés hybrides, comme les traitements plasmas, la micro-fluidique, la déposition de films biomoléculaires en biophysique, qui s'adaptent et se couplent aux processus classiques de couches minces déjà existants pour l'intégration optique

Ballistic Electron Emission Microscopy (BEEM): local probe of electronic properties at buried interfaces

International audienc