Effect of high current density on the admittance response of interface states in ultrathin MIS tunnel junctions.

International audienceThe effect of a high current density on the measured admittance of ultrathin Metal-Insulator-Semiconductor (MIS) tunnel junctions is investigated to obtain a reliable energy distribution of the density, D-S(E), of defects localized at the semiconductor interface. The behavior of admittance Y(V, T, omega) and current density J(V, T) characteristics is illustrated by rectifying Hg//C12H25-Si junctions incorporating n-alkyl molecular layers (1.45 nm thick) covalently bonded to n-type Si(111). Modeling the forward bias admittance of a nonequilibrium tunnel junction reveals several regimes which can be observed either in C(omega approximate to 0) vs. (J) plots of the low frequency capacitance over six decades in current or in M ''(omega) plots of the electrical modulus over eight decades in frequency. At low current density, the response of interface states above mid-gap is unaffected and a good agreement is found between the interface states densities derived from the modeling of device response time tau(R)(V) and from the low-high frequency capacitance method valid for thick MIS devices; the low defect density near mid-gap (D-S 1 mA cm(-2)), the admittance depends strongly on both the density of localized states and the dc current density, so that the excess capacitance method overestimates D-S. For very high current densities > 10 mA cm(-2)), the observation of a linear C(omega approximate to 0) vs. (J) dependence could indicate some Fermi level pinning in a high interface density of states located near the Si conduction band. The temperature-independent excess capacitance C(omega approximate to 0) - C(1 MHz) observed at very small J, not predicted by the admittance model, is attributed to some dipolar relaxation in the molecular junction

Barrier height distribution and dipolar relaxation in metal-insulator-semiconductor junctions with molecular insulator: Ageing effects

International audienceElectrical transport through molecular monolayers being very sensitive to disorder effects, admittance and current density characteristics of Hg // C12H25 – n Si junctions incorporating covalently bonded n-alkyl molecular layers, were investigated at low temperature (150-300 K), in the as-grafted state and after ageing at the ambient. This comparison reveals local oxidation effects both at the submicron scale in the effective barrier height distribution and at the molecular scale in the dipolar relaxation. In the bias range dominated by thermionic emission and modified by the tunnel barrier attenuation, , where is the thickness of the molecular tunnel barrier and is the inverse attenuation length at zero applied bias, some excess current is attributed to a distribution of low barrier height patches. Complementary methods are used to analyze the current density J(V, T) characteristics of Metal-Insulator-Semiconductor tunnel diodes. Assuming a Gaussian distribution of barrier heights centered at provides an analytical expression of the effective barrier height, ; this allows fitting of the distribution standard deviation and tunnel parameter over a wide temperature range. In a more realistic modeling including the voltage dependence of barrier height and circular patch area, the so-called “pinch-off” effect is described by a distribution of parameter which combines interface potential modulation and patch area variations. An arbitrary distribution of values, fitted to low-temperature J(V) data, is equally well described by Gaussian or exponential functions. Ageing in air also increases the interface oxidation of Si substrate and affects the density of localized states near mid gap, which typically rises to the high 1011 eV-1.cm-2 range, as compared with < 1011 eV-1.cm-2 in the as-grafted state. The bias-independent relaxation observed near 1 kHz at low temperature may be attributed either to dipoles in the alkyl chain induced by the strong permanent dipoles of interface silicon oxide or to a local relaxation of water molecules trapped at the OML / silicon interface. The respective roles of SiO2 formation and water physisorption on the decrease of patch barrier height are also discussed

Synthesis of Functionalized Few-Layer Graphene Through Fast Electrochemical Expansion of Graphite

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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

Photoactive silicon surfaces functionalized with high-quality and redox-active platinum diimine complex monolayers

International audienceNew platinum diimine dichloro (PtCl2) and dithiolene (Ptdithiolene) complexes have been synthesized and covalently bound to hydrogen-terminated, oxide-free silicon(111) surfaces using a two-step procedure. This immobilization route resulted in the formation of ca. 2.0-2.5 nm-thick densely packed and perfectly clean Pt complex monolayers with the expected structure of the grafted molecular chains. The surface coverages of the attached PtCl2 and Ptdithiolene were electrochemically estimated at 8.9 x 10(-11) and 7.5 x 10(-11) mol cm(-2), respectively, corresponding to 0.07 PtCl2 and 0.06 Ptdithiolene per surface silicon atom. The Pt complex-functionalized photoactive silicon surfaces showed under simulated sunlight a redox activity similar to that observed for the complexes in solution at a non-photoactive electrode. The two one-electron reduction processes centered on the bipyridine ligand were however observed at much less negative potentials, owing to the photovoltage generated at the silicon/monolayer interface. These modified photocathodes showed great promise for solar photoelectrocatalysis, as exemplified with the photoassisted electrocatalytic reduction of CO2. Based on the measured cathodic photocurrent densities, the Ptdithiolene-modified surface showed superior catalytic performance

Mechanical and thermodynamic properties of half-metallic ferromagnetism containing cobalt and titanium

International audienceWe have studied the structural, electronic, elastic, magnetic and Thermodynamic properties of Co-based full-Heusler alloys Co2TiSi and Co2TiGe. have been studied by first-principles full-potential linearized augmented plane wave (FPLAPW) method with the Generalized Gradient Approximation (GGA) based on density functional theory (DFT). This shows that the magnetic properties of the compound are dependent on electron concentration of main group element and all compunds are magnetic in their equilibrium L21 structure. The electronic structure report that, our compounds have half-metallic (HM) nature. The mechanical results show that these compounds are mechanically stable. and exhibit 100% spin polarization at the Fermi level where it can be shifted within the energy-gap. In addition, the quasi-harmonic Debye model is applied to determine the thermal properties of the alloy. © 2019 by authors

Identification of dipolar relaxation and interface states response in the low temperature admittance of MIS tunnel junctions with molecular monolayer insulator : Hg // C12H25 – n-Si(111)

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Electroless patterned assembly of metal nanoparticles on hydrogen-terminated silicon surfaces for applications in photoelectrocatalysis.

International audienceThe deposition of gold and platinum nanoparticles (NPs) on hydrogen-terminated Si(100) (Si(100)-H) surfaces has been performed by galvanic displacement using fluoride-free sub-millimolar metallic salt solutions. The scanning electron microscopy (SEM) images showed the formation of oblate hemispherical NPs, with an average diameter of ca. 40 nm and an average height of 20 ± 10 and 10 ± 5 nm for Au and Pt, respectively. Furthermore, the calculated number density was (6.0 ± 0.8) × 10(9) Au NPs cm(-2) and (6.6 ± 1.3) × 10(9) Pt NPs cm(-2) with a larger size distribution measured for Au NPs. The Au 4f and Pt 4f X-ray photoelectron spectra of the metallized surfaces were characterized by a principal component corresponding to either the metallic gold or platinum. However, two other components located at higher binding energies were also visible and ascribed to gold or platinum silicides. Using this fluoride-free deposition process and a "reagentless" UV photolithography technique, we have also demonstrated that it was possible to prepare metallic NP micropatterns. Following this approach, single metal (Au) and two metals (Au and Pt) patterns have been produced and characterized by energy-dispersive X-ray spectroscopy (EDS) which revealed the presence of the expected metal(s). Such metallic NP micropatterned surfaces were used as photocathodes for H(2) evolution from water as a proof-of-concept experiment. These electrodes exhibited much higher electrocatalytic performance than that of nonmetallized Si(100)-H, both in the absence of light and under illumination. The overpotential for hydrogen evolution was significantly decreased by ca. 450 mV with respect to Si(100)-H (measured for a current density of 0.1 mA cm(-2)) under identical illumination conditions

Sequential anodic oxidations of aliphatic amines in aqueous medium on pyrolyzed photoresist film surfaces for the covalent immobilization of cyclam derivatives

International audienceWe investigate here the derivatization of pyrolyzed photoresist film (PPF) surface by anodic oxidations of amino-containing linkers in aqueous medium. As an example of application, nitrogen macrocycles are then covalently immobilized by chemical reactions with the modified surface. The surface coverage is estimated by X-ray photoelectron spectroscopic analyses and cyclic voltammetry using redox probes, and the thickness of the film is measured by atomic force microscopy from scratching experiments. The surface coverage increases when two successive grafting processes are carried out and stops after the third electrolysis. The influence of the electrolyte medium on the thickness of the film is also highlighted