UHV Evaporation Research Articles

Structural and magnetic properties of self-assembled cobalt on porous silicon; experimental and micromagnetic investigations

In this paper, we report on self-assembled Co nanoparticles deposited in and on porous silicon (PS) matrix by using UHV evaporation. Four samples were prepared by varying the Co deposited thickness (t = 3, 5, 7 and 10nm). All samples have been investigated by means of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Physical Properties Measurement System (PPMS). The increase of t has induced an increase of the nanoparticle diameter from 3nm to about 150nm. Referring to the magnetic characterizations, this increase has been followed by a single to multi-domain transition. Therefore, this has been evidenced by a switching from superparamagnetism to purely ferromagnetism accompanied by a change in the magnetic reversal dynamics. Thus, by performing micromagnetic calculation, we have shown that a transition from the uniform rotation to vortex state occurs at a critical diameter of about 55nm.

Adsorption, Desorption, and Reaction of 1-Octyl-3-methylimidazolium Tetrafluoroborate, [C₈C₁Im][BF₄], Ionic Liquid Multilayers on Cu(111).

Multilayers of 1-octyl-3-methylimidazolium tetrafluoroborate [C8C1Im][BF4] have been deposited on a Cu(111) surface by evaporation in UHV. XPS shows that [C8C1Im][BF4] adsorbs without decomposition for substrate temperatures < 300 K. XPS and UPS data indicate that ionic liquid (IL) deposition onto a 120 K Cu(111) surface results in the IL forming multilayers by a simultaneous-multilayer growth process. IL deposition onto a room temperature Cu(111) surface results in a different arrangement where at a coverage of one monolayer the IL forms droplets of about 100 Å height covering only about 1/10th of the surface. Multilayers deposited at 120 K convert to the room temperature arrangement upon heating. Further heating above room temperature causes the IL multilayer droplets to desorb leaving an IL monolayer of ≈6 Å thickness at ≈430 K. At higher temperatures, this monolayer reacts with the surface and BF3 is emitted, leaving products containing C, N, and some F on the surface. We propose a surface reaction where [BF4](-) ions react to form chemisorbed fluorine (Cu-F) and gaseous BF3, with the remaining [C8C1Im](+) decomposing on the Cu(111) in an unidentified manner.

From novel PtSn/Pt(110) surface alloys to SnOx/Pt(110) nano-oxides

UHV evaporation of Sn on (1 × 2) Pt(110), followed by UHV annealing, provides three different PtSn/Pt(110) surface alloys, each characterized by a specific low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) pattern. When annealed in a controlled oxygen background, the PtSn/Pt(110) surface alloys can be transformed into SnOx/Pt(110) nano-oxides. This paper reports the experimental phase diagram of the metallic and oxidized surface superstructures.

Growth of ultrathin epitaxial Fe/MgO spin injector on (0, 0, 1) (Ga, Mn)As

We have grown an ultrathin epitaxial Fe/MgO bilayer on (Ga, Mn)As by e-beam evaporation in UHV. The system structure has been investigated by high resolution transmission electron microscopy (TEM) experiments which show that the Fe and MgO films, covering completely the (Ga, Mn)As, grow with the epitaxial relationship Fe[100](001) ∥ MgO[110](001) ∥ (Ga,Mn)As[110](001). The magnetic reversal process, studied by the magneto-optical Kerr effect (MOKE) at room temperature, demonstrates that the iron is ferromagnetic and possesses a cubic anisotropy, confirming the epitaxy relationship found with TEM. Resistivity measurements across the barrier display a non-Ohmic behavior characterized by cubic conductance as a function of the applied voltage suggesting tunneling-dominated transport across the barrier.

Nanostructured cobalt on porous silicon substrate: Structure and magnetic behaviour

AbstractDuring an anodization process, porous silicon (PS) consisting of pores with a diameter of about 40 nm and a depth from 5 µm to 40 µm has been produced. To achieve oriented channels in this mesoporous range, a p+‐type Si wafer was electrochemically etched in an aqueous electrolyte of HF. We report the formation, after the anodization step, of a cobalt nanostructure in a porous silicon matrix. Co nanocrystals on and in a porous silicon layer have been prepared by the UHV evaporation technique and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). This technique was performed to show the chemical element distribution within the channels. It is found that the deposition condition is an important factor for obtaining nano‐structures. Initial deposition leads to Co particle penetration in silicon pores whereas subsequent deposition results only in an increase of the thickness at the surface with no further penetration. Additional experiments were carried out by using the magneto‐optical Kerr effect to obtain information about the magnetic properties. The first results show that the magnetic response for layers &lt;5 nm presents an important perpendicular component of magnetization whereas for thicker deposited layers (8 nm &lt; t &lt; 20 nm) the magnetic response seems to act as that of a thin film in which the squareness of the hysteresis loop decreases with increasing film thickness. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Magnetic nanostructures for local spin control in semiconductors

Abstract We present two approaches to achieve a pronounced remanent out-of-plane magnetization in ferromagnetic nanostructures: Ni nanomagnets with high aspect ratios and Fe/Tb multilayer structures with strong interface anisotropy were defined by electron beam lithography and fabricated by electrochemical deposition and UHV evaporation, respectively. Both technologies allow a local spin control in an underlying semiconductor via the magnetic fringe field.

Model reaction studies on vanadium oxide nanostructures on Pd(111)

Deuterium desorption and reaction between deuterium and oxygen to water has been studied on ultrathin vanadium oxide structures prepared on Pd(111). The palladium sample was part of a permeation source, thus enabling the supply of atomic deuterium to the sample surface via the bulk. Different vanadium oxide films have been prepared by e-beam evaporation in UHV under oxygen atmosphere. The structure of these films was determined using low energy electron diffraction and scanning tunneling microscopy. The mean translational energy of the desorption and reaction products has been measured with a time-of-flight spectrometer. The most stable phases for monolayer and submonolayer VOx are particular surface-V2O3 and VO phases at 523 and 700 K, respectively. Thicker films grow in the form of bulk V2O3. The mean translational energy of the desorbing deuterium species corresponds in all cases to the thermalized value. Apparent deviations from this energy distribution could be attributed to different adsorption/desorption and/or accommodation behaviors of molecular deuterium from the gas phase on the individual vanadium oxide films. The water reaction product shows a slightly hyperthermal mean translational energy, suggesting that higher energetic permeating deuterium contributes with higher probability to the water formation.

Study of the early stages of Cr/6H-SiC(0001) interface formation

Abstract The early stages of the Cr/6H-SiC(0 0 0 1) interface formation at room temperature were investigated using XPS, LEED and work function (WF) measurements. Upon stepwise Cr evaporation in UHV up to a thickness of 5–10 monolayers (ML) at RT, the binding energy of the XPS Cr 2p 3/2 core level peak shifted from 576.1 eV, at submonolayer coverage, to 574.7 eV (corresponding to metallic Cr) for the final Cr deposit, while the binding energies of the substrate XPS core level peaks remained stable. The WF exhibited a steep decrease of about 0.5 eV from the initial SiC substrate value, upon submonolayer coverage, but then increased gradually to saturation at a value of about 4.8 eV (polycrystalline Cr film with some chemisorbed oxygen). The growth of the ultrathin film was via 3D-cluster formation. The height of the Schottky barrier for the Cr/6H-SiC(0 0 0 1) contact was found by XPS to be 0.5 ± 0.1 eV. The results, generally, indicate the absence of any extended interfacial silicide-like interaction at RT.

Microscopical Study of Au Nanocrystals Self-Assembled on (100)Si and SiO2/(100)Si Substrates

ABSTRACTThis work reports on the structural characterization of Au nanocrystals directly prepared on the surface of (100)Si and 150 nm-thick SiO2 deposited (100)Si substrates, by a physical self-assembly method, consisting in the UHV evaporation of a thin Au film and its successive high temperature annealing. The morphology, orientation, and crystalline structure of Au nanocrystals were characterized by scanning and high-resolution transmission electron microscopy and X-ray diffraction, respectively. Experimental results show that the nature of the substrate strongly influences the process of Au nanocrystal formation upon heat treatment, by affecting the interaction of deposited Au with the underlying material. In the case of clean (100)Si substrates the Au strongly interacts with Si, so that Au nanoislands are obtained with a well defined epitaxial relationships with the substrate, i.e. [100]AuÐÐ[110]Si and [110]AuÐÐ[311]Si. The nanoisland shape is affected by faceting at the Au/Si interface, the Au nanocrystal being limited by the {111}, {311}, {711} and {-111} planes of Si. In the case of SiO2/(100)Si substrates spherical shaped Au nanoparticles with random crystal orientation are instead, produced.

Low temperature and low pressure CO oxidation on gold clusters supported on MgO(100)

CO oxidation has been investigated on Au/MgO(100) model catalysts at RT and low pressure. The gold particles prepared by UHV evaporation on clean MgO surfaces are characterized by HRTEM. The gold particles are FCC single crystals or multiple twins with five-fold symmetry. Infrared spectroscopy indicates that two types of adsorption sites are present which correspond to loosely and strongly bound CO. The equilibrium CO coverage for the strongly bound CO is smaller than 0.1 ML. CO titration experiments show that oxygen does not dissociate on the gold nanoparticles. The CO oxidation reaction is studied at RT by molecular beam methods. A steady state CO reaction probability up to 0.50 is measured, for the first time at low pressure, for gold particles with a mean size of 1.5 nm. A reaction mechanism is proposed in which CO adsorbed on low coordinated gold atoms reacts with oxygen adsorbed molecularly, possibly at the Au/MgO interface.

Reduction of Irreversible Protein Adsorption on Solid Surfaces by ProteinEngineering for IncreasedStability

The influence of protein stability on the adsorption and desorption behavior to surfaces with fundamentally different properties (negatively charged, positively charged, hydrophilic, and hydrophobic) was examined by surface plasmon resonance measurements. Three engineered variants of human carbonic anhydrase II were used that have unchanged surface properties but large differences in stability. The orientation and conformational state of the adsorbed protein could be elucidated by taking all of the following properties of the protein variants into account: stability, unfolding, adsorption, and desorption behavior. Regardless of the nature of the surface, there were correlation between (i) the protein stability and kinetics of adsorption, with an increased amplitude of the first kinetic phase of adsorption with increasing stability; (ii) the protein stability and the extent of maximally adsorbed protein to the actual surface, with an increased amount of adsorbed protein with increasing stability; (iii) the protein stability and the amount of protein desorbed upon washing with buffer, with an increased elutability of the adsorbed protein with increased stability. All of the above correlations could be explained by the rate of denaturation and the conformational state of the adsorbed protein. In conclusion, protein engineering for increased stability can be used as a strategy to decrease irreversible adsorption on surfaces at a liquid-solid interface.

A-Si Capping SIMS for shallow dopant profiles

Capping a sample surface using amorphous-Si (a-Si) prior to SIMS analysis is a common method for improving ultra-shallow boron depth profiling, however, details of the limitations of a-Si capping have not been thoroughly investigated. In this study we have investigated the problems and/or limitations of a-Si capping method when applied to SIMS. Capping Si layers were grown using Si evaporation in UHV (∼2×10 −8 Pa) at less than 100 °C during growth. Using this a-Si capping SIMS method we have successfully measured shallow depth profiles for ultra-shallow B and P.

Growth and magnetism of cobalt nanowires, and continuous films on nanofaceted NiO (110)

We have grown Co films by either magnetron sputtering or UHV evaporation on V-grooved NiO epitaxial layers. The morphology of NiO (1 1 0) surface is obtained thanks to the large atomic plane energy difference between (1 0 0) and (1 1 0) planes that favors spontaneous decomposition of nominal (1 1 0) NiO surface into elongated (1 0 0) nanofacets. The growth of Co on top of such a surface either leads to continuous films with conformal morphology in the case of sputtering, or to the formation of nanowires or self-aligned nanocrystals for evaporated Co layers.

Growth and magnetism of cobalt nanowires, and continuous films on nanofaceted NiO (1 1 0)

Abstract We have grown Co films by either magnetron sputtering or UHV evaporation on V-grooved NiO epitaxial layers. The morphology of NiO (1 1 0) surface is obtained thanks to the large atomic plane energy difference between (1 0 0) and (1 1 0) planes that favors spontaneous decomposition of nominal (1 1 0) NiO surface into elongated (1 0 0) nanofacets. The growth of Co on top of such a surface either leads to continuous films with conformal morphology in the case of sputtering, or to the formation of nanowires or self-aligned nanocrystals for evaporated Co layers.

Antireflection foils with multi-layer converter for ultracold neutron detectors

For experiments at the ultracold neutron (UCN) source of the new high-luminosity neutron source, Forschungsreaktor München II (FRM II), highly efficient detectors are needed. The desired type of detectors utilizes 6Li as the neutron converter based on the huge cross-section reaction 6Li(n, α)t and detection of the 2.06 MeV α-particles or the 2.73 MeV t-particles. The high reflectance of UCN from 6Li with its positive optical potential must be compensated by a material with negative optical potential. Instead of the expensive 62Ni material, natural Ti can be chosen. It was demonstrated that neither 6Li metal nor Ti metal can be deposited in a high-vacuum evaporation apparatus without creating a positive optical potential and thus increasing the reflectivity for the UCN due to oxygen impurities coming from the H 2O partial pressure in the high-vacuum system. To overcome these problems, a new UHV evaporation apparatus was developed and built which is capable of reaching a vacuum in the 10 −10 Pa range. Such a good vacuum can be obtained only when annealing the vacuum system up to 470 K. At such a high annealing temperature the silicon detectors for the α- and t particles might suffer degradation of their energy resolution. Therefore, the multi-layer system of 6Li and natTi is not deposited directly on the silicon detector but a thin rolled Ti foil is applied as backing. A test deposition of 200 double layers of 6LiF/ 62Ni on a thin Ti foil is described and a report is given about the setup of the new UHV evaporation apparatus.

Study of the early stages of Cu/6H–SiC(0 0 0 -1) interface formation

The early stages of the Cu/SiC interface formation at room temperature and the influence of annealing were investigated on the carbon-face of the single crystal n-type 6H–SiC, by X-ray photoelectron and auger electron spectroscopy (XPS/XAES), low energy electron diffraction and work function (WF) measurements. Upon stepwise copper evaporation in UHV up to a thickness of 5–10 monolayers (ML) at RT, the binding energy of the XPS Cu2p 3/2 core level peak shifted from 933.6±0.1 eV, below 0.1 ML coverage, to 932.7 eV for the final metallic Cu deposit. The WF, following an initial steep decrease from the clean SiC substrate value of 4.4 down to 4.0 eV up to 0.1 ML, increased then gradually to the metallic Cu value of ∼4.75 eV after approximately 5 ML of Cu. The growth of the film was initially via 2D-cluster formation, and exhibited a 3D character beyond 1–2 ML of Cu. The height of the Schottky barrier for the Cu/6H–SiC(0 0 0 -1) contact was found by XPS to be 1.5±0.1 eV. Annealing of the contact up to 520 K caused small changes in the Cu and SiC XPS peak intensities accompanied by a 0.3 eV increase of the WF, indicating small structural changes within the film and gave rise to a slight change of the Schottky barrier height to 1.6±0.1 eV. The results do not indicate any significant silicide formation at 520 K.

XPS investigations on the interactions of 1,6-hexanedithiol/Au(1 1 1) layers with metallic and ionic silver species

Abstract Surfaces of densely packed aliphatic (di)thiol/gold self-assembled monolayers (SAMs) with one thiolate–gold bond per molecule are prepared and used as models for functionalized polymer/metal interfaces. In detail, the penetration of metallic or ionic silver species through 1-hexanethiol/Au(1 1 1) and 1,6-hexanedithiol/Au(1 1 1) layers is investigated with X-ray photoelectron spectroscopy (XPS). These organic films are readily penetrated by silver species as well during Ag vapor deposition as during immersion in silver sulfate solution. As a driving force for the layer penetration strong Ag–thiolate interactions are discussed. Experimental proof for such bonding is provided by several findings. Ag–thiolate bond fission is observed to occur at higher temperatures than Au–thiolate decomposition. Furthermore, the deposition of metallic silver via UHV evaporation reveals the formation of Ag–thiolates at the SAM surface in the case of 1,6-hexanedithiol/Au(1 1 1) SAMs. Finally, the characterization of the dithiol/gold system indicates the chemisorption of hydrated Ag+ ions at the interface between the SAM and a silver sulfate electrolyte.

Photoemission study of the electronic properties of in situ prepared copper phthalocyanine (CuPc) thin films exposed to oxygen and hydrogen

The electronic properties of space charge layer of the copper phthalocyanine (CuPc) thin films in situ prepared by an UHV evaporation method and, subsequently, exposed to molecular oxygen (O 2) and molecular hydrogen (H 2) have been studied using the high resolution photoemission yield spectroscopy (PYS) technique. The influence of interaction of these active gases on the work function ϕ and ionisation energy Φ was determined, together with the changes of effective density of filled electronic states localised in the band gap below the Fermi level and in the upper part of the valence band. In order to direct comparison to the commonly used procedure for UPS and XPS techniques, the changes of macroscopic parameters of the space charge layer were expressed by the surface Fermi level position in the band gap E F– E v. The observed changes of surface Fermi level position upon adsorption/desorption of molecular oxygen O 2 and hydrogen H 2 were attributed to decrease/increase of the induced charge in near surface region.

A study of the conjugated Ooct-OPV5 oligomer/gold interface using photoelectron spectroscopy

The electronic structure of the interface between polycrystalline Au and a five-ring n-octyloxy substituted phenylenevinylene oligomer was studied by XPS and UPS, as a model system for the respective polymeric film. High purity oligomer films of up to 25 nm thickness were prepared on the sputter-cleaned metal surface by stepwise evaporation in UHV. The C 1s XPS peak after the first deposition step consisted of three components with binding energies (BE) 288.4, 286.7 and 284.4 eV, the first related to the interface and the latter two to oligomer functional groups. During growth, both C 1s and O 1s in the film exhibited an upward BE shift up to 0.52±0.05 eV , from which the band bending at the interface was evaluated. The UP spectra of the film exhibited characteristic peaks at 2.4 and 3.8 eV (delocalized electrons from π-bands) and at 6.5 and 8.5 eV (electrons in the benzene rings). The work function was ∼ 4.0±0.1 eV . The HOMO position in the spectrum was used in conjunction with the band bending, in order to yield the band line-up at the interface.

Preparation of highly ordered quinquethiophene thin films on TiO2 substrates

To understand the intrinsic electrical properties of conjugated polymers in general and of oligothiophenes in particular, ultra-pure and highly ordered thin films or single crystals are required. In this context, studies on thin films of quinquethiophene (α5T) prepared by evaporation in UHV were performed. As substrates insulating and conducting oxides such as different single crystal surfaces of TiO2 were chosen. The latter are particularly interesting because TiO2 can be doped to be n-conducting. These substrates may therefore be used for hybrid inorganic–organic thin film pn-diodes. Here, we report on SFM-measurements under ambient conditions, which show that the deposition of α5T from a Knudsen cell under UHV conditions leads to cluster growth on TiO2(011) and to ordered layers on undoped TiO2(001). On doped TiO2(001) the growth mode is largely influenced by the surface roughness, i.e. the presence of pinning centers. It could be shown that the α5T molecules on TiO2(001) are oriented with their long axis mainly perpendicular to the substrate plane and form two complete monolayers (Frank–Van der Merwe growth mode) before the growth mode changes to a simultaneous multilayer growth, in line with earlier experiments using other oxidic substrates.