J.L. Bubendorff

Kinetics of scanned probe oxidation: Space-charge limited growth

This article proposes an enhanced oxidation model for scanning probe microscope (SPM) nanolithography that reproduces the power-of-time law reported for tip-induced anodic oxidation. It is shown that the space charge resulting from nonstoichiometric states strongly limits the oxidation rate. The direct relationship between the oxide thickness and time is provided by integration of the oxide rate equation. Measurements on SPM-induced oxides generated on a titanium surface are compared to theory. The predominant role of the space charge is corroborated by electrical measurements on oxide barriers that exhibit current fluctuations due to Coulombic effects.

Structural, optical and cathodoluminescence characteristics of sprayed undoped and fluorine-doped ZnO thin films

Undoped ZnO and F-doped ZnFxO(1−x) (x = 5 at%) films prepared by the spray pyrolysis (SP) technique have been studied using cathodoluminescence imaging and spectroscopy, x-ray diffraction and spectrophotometry. These films, deposited at the optimal substrate temperature (Ts = 450 °C), present a hexagonal close packed structure. The doping of this material with fluorine keeps their optical absorption threshold unchanged. At room temperature, the cathodoluminescence (CL) spectra of the undoped and the doped ZnO films exhibit the common near-ultra-violet (UV) band gap peak at λ = 382 nm but they differ as regards their visible emissions. The undoped ZnO emits an intensive blue-green light (λ = 520 nm) and a red emission (λ = 672 nm). The presence of fluorine gives rise to a new light emission corresponding to λ = 454 nm and the disappearance of the blue-green emission. The CL imaging of undoped ZnO films shows that the luminescence is located at defined sites giving rise to a grain-like structure inherent to the sample surface morphology. The presence of fluorine leads to a more homogeneous re-partition of the emitting centres.

Microstructure and cathodoluminescence study of sprayed Al and Sn doped ZnS thin films

Here we report on the study of ZnS and X-doped ZnS (with 4 at% of X = Al, Sn) thin films, prepared by spray pyrolysis technique using chloride precursors. Cathodoluminescence imaging and spectroscopy, x-ray diffraction, x-ray energy dispersive spectrometry and spectrophotometry have been used for their characterization. Deposited at their optimal substrate temperature (Ts = 773 K), these films are polycrystalline and consist of mixed hexagonal (α) and cubic (β) phases with a predominance of the cubic phase. Their growth is preferentially oriented along the (111)β direction and their optical bandgap always remains close to 3.56 eV regardless of the sample considered. The cathodoluminescence spectra of ZnS and Al–ZnS films are similar and are characterized by a blue emission peak at 407 nm (3.05 eV) and a broad blue–green one located at 524 nm (2.36 eV) due to the presence of chlorine. The insertion of Sn2+ ions in the ZnS material leads to the formation of the SnCl2 compound and to the disappearance of the blue–green emission associated with Cl ionized donors.

Overpotential driven perpendicular magnetization of electrodeposited ultrathin cobalt films

Depending on the overpotential applied during electrodeposition of ultrathin cobalt films (η=0.1–2.2 V), either in-plane or out-of-plane magnetization can be stabilized. The regime of high supersaturation in particular allows to engineer electrodeposited cobalt films on Au(111) that show perpendicular magnetization (in the thickness range from 2 to 8 atomic layers) exactly like their ultrahigh vacuum grown counterparts. The film topography depends on the overpotential as shown by atomic force microscopy: continuous films are obtained at low overpotential whereas high overpotential leads to the formation of islands. The perpendicular magnetization results from a subtle thickness dependent competition between the shape and the interface anisotropy.

Luminescent spectroscopy and imaging of textured sprayed Er-doped ZnO films in the near ultraviolet and visible regions

The textural dependence of the thermally sprayed Er-doped ZnO films on Er concentration is reported here. The [002] preferred growth orientation of hexagonal phase is obtained at the lower concentrations, while the [100] and [101] directions additionally raised from nearly 5at.% Er content onwards. The cathodoluminescent characteristics of the samples in the near ultraviolet (UV) and visible region depict a complete extinction of the visible emitted bands (λ=445, 526, and 665nm) at 1at.% Er content. Their deactivation below this concentration is explained by a compensation of oxygen defects in the material due to the oxygen-rich medium of the deposition bath. Their reactivation beyond this particular concentration is ascribed to the increase of the Er+3 ion shells whose internal radiative transitions lead to a recovering of these visible emitted bands. The radiative mechanism of the transitions from the (F9∕24) excited states to the (I15∕24) ground state, responsible for the λ=665nm emission, is predomina...

Anion promoted Ni-underpotential deposition on Au(111)

In situ scanning tunneling microscopy (STM) and cyclic voltammetry show that a Ni monolayer can be deposited from a sulfamate solution on Au(111) at positive potentials with respect to the Nernst potential of the NiNi2+ couple. This process is specific to H2NSO−3 anions since it could not be observed in the presence of sulfates. High resolution STM images suggest that the Ni layer builds up on the surface due to a complexation of Ni2+ by the sulfamate adlayer on the gold surface.

Room-temperature ferromagnetism in single crystal Fe1.7Ge thin films of high thermal stability grown on Ge(111)

We report on the epitaxial growth of ultrathin ferromagnetic Fe1.7Ge layers on Ge(111) wafer. These single crystal intermetallic layers adopt the InNi2 (B82) crystallographic structure. They are ferromagnetic with a Curie temperature well above room temperature. The interface between the ferromagnet layer and the Ge wafer is of high perfection. Interestingly, the annealing of the sample up to 300°C alters neither the crystallographic structure, nor the interface quality, nor the magnetic properties but leads to a nearly perfect smoothening of the germanide layer surface. This high thermal robustness should open the way for the growth of fully epitaxial iron germanide/Ge hybrid structures.

Real time atomic force microscopy imaging during nanogap formation by electromigration

We present real time atomic force microscopy imaging during nanogap fabrication by feedback controlled electromigration of a gold nanowire. The correlated measurements of electrical resistance and atomic force microscopy reveal that the major structural changes appear at the early stage of the process. Moreover, despite important morphological changes, the resistance of the nanowire shows a weak increase of just a few ohms. The detailed analysis of the atomic force microscopy images clearly shows that the electromigration process is strongly influenced by the initial microstructure of the nanowire.

Cathodoluminescence Properties of Zno Thin Films

The industry has a great need for high performance materials to feature well defined. These needs have prompted the development of methods of study and control of gradually more sophisticated based on the radiation-matter interaction. To identify the properties of materials, we must make a spectral analysis on the emitted photon using a spectrometer combined with a detection system. The analysis of the photon can take place directly in the form of an electrical current, as well as the spectrum received by the measurement system is represented by a function I (λ).

Tuning the morphology of silver nanostructures photochemically coated on glass substrates: an effective approach to large-scale functional surfaces

This paper reports on a simple and environmentally friendly photochemical process capable of generating nano-layers (8-22 nm) of silver nanostructures directly onto glass surfaces. This approach opens the way to large-scale functionalized surfaces with plasmonic properties through a single light-induced processing. Thus, Ag nanostructures top-coated were obtained through photo-reduction, at room temperature, of a photosensitive formulation containing a metal precursor, free from extra toxic stabilizers or reducing agents. The reactive formulation was confined between two glass slides and exposed to a continuous near-UV source. In this way, stable silver nano-layers can be generated directly on the substrate with a very good control of the morphology of as-synthesized nanostructures that allows tailoring the optical properties of the coated layers. The position and width of the corresponding surface plasmon resonance bands can be adjusted over a broad spectral window. By extension, this low-cost and easy-to-apply process can also be used to coat ultra thin layers of metal nanostructures on a variety of substrates. The possibility of controlling of nanostructures shape should achieve valuable developments in many fields, as diverse as plasmonics, surface enhanced Raman scattering, nano-electronic circuitry, or medical devices.