Effect of quantum confinement on the dielectric function of PbSe

Monolayers of lead selenide nanocrystals of a few nanometers in height have been made by electrodeposition on a Au(111) substrate. These layers show a thickness-dependent dielectric function, which was determined using spectroscopic ellipsometry. The experimental results are compared with electronic structure calculations of the imaginary part of the dielectric function of PbSe nanocrystals. We demonstrate that the size-dependent variation of the dielectric function is affected by quantum confinement at well-identifiable points in the Brillouin zone, different from the position of the band-gap transition.     

Surface functionalization of titanium dioxide nanoparticles with alkanephosphonic acids for transparent nanocomposites

The surface functionalization of rutile titanium dioxide nanoparticles with 1-decylphosphonic acid and diethyl undec-10-enyl phosphonate in a two-stage process, involving a change in reaction medium, is described. Similarly, 1-decylphosphonic acid and diethyl 1-decylphosphonate were employed as surface modifiers. The nanoparticles coated in two successive steps formed stable, transparent dispersions in toluene. Surface functionalization was monitored using thermogravimetric analysis (TGA), which showed enhanced surface coverage after the second capping step. Incorporation of C=C-terminal surface coupling molecules in the second stage was directly proved using FTIR. Dynamic light scattering measurements showed that the dual-functionalized particles possessed a uniform size of around 13 nm. Particle dimensions were further analyzed using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Transparent nanocomposites were formed by introducing the functionalized nanoparticles into a poly(benzyl acrylate) matrix. The refractive index of poly(benzyl acrylate) composites, measured by spectroscopic ellipsometry, increased from 1.57 for the pure polymer to 1.63 for 14.0 vol.% TiO₂ at λ = 586 nm. Nanocomposite films with particle weight percentages of up to 30% (9.5 vol.%) showed a high light transmittance of around 90% at wavelengths above λ = 400 nm.     

Simultaneous monitoring of protein adsorption at the solid–liquid interface from sessile solution droplets by ellipsometry and axisymmetric drop shape analysis by profile

In this paper two in situ techniques are combined to simultaneously examine protein adsorption at the solid–liquid interface from sessile solution droplets. With axisymmetric drop shape analysis by profile (ADSA-P) the change in solid–liquid interfacial tension is determined, while ellipsometry is employed to measure the amount of protein adsorbed from the same solution droplet at the solid–liquid interface. Three proteins (human serum albumin (HSA), immunoglobulin G (IgG) and fibrinogen (Fb)) were dissolved to a concentration of 0.05 mg ml⁻¹ in PBS (pH 7) and sessile droplets were placed for 2 h on a 47.8 nm thick gold coating on glass. The gold coated glass was positioned onto a quartz prism with immersion oil. The prism was aligned in a rotating analyser ellipsometer and the optical beam was thus allowed to be reflected at the hydrophobic gold surface. The ADSA-P set-up was built in 90° cross-beamed set-up around the prism. By combining the results for the adsorbed amounts and the interfacial tension changes over the two hour adsorption period, two stages in the adsorption process could be distinguished. In the first stage, the adsorbed amounts increase in correspondence with the interfacial tension changes, indicating that the interfacial tension changes are caused by adsorption, whereas in the second stage interfacial tension changes continue despite the adsorbed amounts being constant. Consequently, the second stage must be associated with conformational changes of the adsorbed proteins. For HSA and Fb, the conformational contribution to the interfacial tension changes (7.8 and 5.3 mJ m⁻², respectively) were approximately 2-fold the adsorption contribution, while for IgG both were equal around 3 mJ m⁻².     

Ellipsometric identification of collective optical properties of silver nanocrystal arrays

The optical properties of silver nanocrystal arrays are investigated using spectroscopic ellipsometry in combination with polarized reflection measurements. Analysis of the ellipsometry and reflectometry spectra in terms of the "thin island film" theory enables a transparent identification of the contribution of collective effects to the optical response. Negligible image charge effects imply that only dipole contributions have to be considered. The interactions between the hexagonally ordered silver nanocrystals give rise to an effective modification of the spherical response to oblate entities with different polarizabilities parallel and perpendicular to the substrate, expressed in terms of corresponding depolarization factors. The effect of nanocrystal ordering, nearest-neighbor distance, size distribution, surrounding ambient, and the optical properties of the single nanocrystals on the optical response are analyzed. The extent of plasmon resonance peak splitting as a function of surface coverage is discussed.     

Measurement of the sputter yield after mild ion erosion of a pristine Cu(001) surface

Using the STM technique we have determined the sputter yield on a pristine Cu(001) surface after mild (fluence less than 0.044 ions per surface atom) bombardment of the pristine surface with 800 eV Ar⁺ions at normal incidence. The experiments have been performed at substrate temperatures ranging from 200 to 350 K. Making use of the positional correlation of adatoms and surface vacancies, at 200 K and 325 K, we concluded that about 1/3 of the surface adatoms originate from interstitials arriving at the surface and they give a direct indication of the buried bulk vacancies. A careful analysis of the different areas for surface vacancies and adatom then allowed a quantitative evaluation of the sputter yield at 1.2 Cu atoms per 800 eV Ar⁺ ion.     

In situ ellipsometry study of atomic hydrogen etching of extreme ultraviolet induced carbon layers

Atomic hydrogen based etching is generally considered an efficient method for the removal of carbon films resulting from photo-induced hydrocarbon dissociation, as occurs in extreme ultraviolet (EUV) photolithography environments. The etch rate of atomic hydrogen for three different kinds of carbon films was determined, namely for EUV-induced carbon, hot filament evaporated carbon and e-beam evaporated carbon. The etching process was monitored in situ by spectroscopic ellipsometry. The etch rate was found to depend on the type of carbon (polymer or graphite-like), on the layer thickness, and on the temperature. The EUV-induced carbon shows the highest etch rate, with a value of ∼0.2 nm/min at a sample temperature of 60 °C. The more graphite-like carbon layers showed an etch rate that was about 10 times lower at this temperature. An activation energy of 0.45 eV was found for etching of the EUV-induced carbon layer.     

Subsurface miscibility of metal overlayers with V, Nb and Ta substrates

The high solubility and diffusivity of oxygen in Nb, Ta and V are responsible for the difficulty in the preparation of a clean, well-ordered (1 0 0) surfaces of these early transition metals. The deposition and subsequent annealing of a metal overlayer on Nb(1 0 0) are a convenient route for the preparation of flat surfaces with the Nb lattice constant and a metallic character. Such well-ordered, stable, inert and easily reproducible Nb(1 0 0)-like surfaces can be produced due to the suppression of oxygen surface segregation by a suitable layer blocking sub-surface oxygen diffusion. For example, a Nb(1 0 0)-like surface can be obtained by annealing thin Au or Pd films deposited on Nb(1 0 0), since this results in a Au–Nb or Pd–Nb alloy situated just below the surface which preserves the structure of the Nb-lattice and effectively suppresses the oxygen segregation toward the surface. In contrast, Ag and Cu layers do not show this property as these metals do not form a bulk alloy with Nb. The presence or absence of bulk alloying and its relation to surface oxygen contamination is a general phenomena observed for many metallic layers deposited on V, Nb and Ta substrates. A discussion of results reported in the literature is given for these adsorbate systems.