Use of atomic layer deposition to improve the stability of silver substrates for in situ,high‐temperature SERS measurements

A method to stabilize silver surface‐enhanced Raman spectroscopy (SERS) substrates for in situ, high‐temperature applications is demonstrated. Silver island films grown by thermal evaporation were coated with a thin layer (from 2.5 to 5 nm) of alumina by atomic layer deposition (ALD), which protects and stabilizes the SERS‐active substrate without eliminating the Raman enhancement. The temporal stability of the alumina‐coated silver island films was examined by measurement of the Raman intensity of rhodamine 6G molecules deposited onto bare and alumina‐coated silver substrates over the course of 34 days. The coated substrates showed almost no change in SERS enhancement, while the uncoated substrates exhibited a significant decrease in Raman intensity. To demonstrate the feasibility of the alumina‐coated silver substrate as a probe of adsorbates and reactions at elevated temperatures, an in situ SERS measurement of calcium nitrate tetrahydrate on bare and alumina‐coated silver was performed at temperatures ranging from 25 to 400 °C. ALD deposition of an ultrathin alumina layer significantly improved the thermal stability of the SERS substrate, thus enabling in situ detection of the dehydration of the calcium nitrate tetrahydrate at an elevated temperature. Despite some loss of Raman signal, the coated substrate exhibited greater thermal stability compared to the uncoated substrate. These experiments show that ALD can be used to synthesize stable SERS substrates capable of measuring adsorbates and processes at high temperature. Copyright © 2009 John Wiley & Sons, Ltd.     

Ferromagnetic alignment of iron nanostructures on the silicon surface

The phase composition, electronic structure, and magnetic properties of nanostructures formed upon deposition of iron on the surface of the Si(556) vicinal face coated by a submonolayer silver film with a √3×√3-Ag structure have been studied using high-energy-resolution photoelectron spectroscopy and analysis of magnetic linear dichroism in photoemission of Fe 3p electrons. The effective thickness of the deposited iron layer is varied from 1 to 25 ?. It has been shown that a 1- to 2-? Fe coverage leads to the formation of a metastable iron silicide thin layer with a CsCl-type structure on the surface of the sample. A further deposition of Fe (up to ≈7 ?) brings about the formation of chains consisting of nonmagnetic islands of the Fe—Si solid solution on this layer, which are oriented along the steps of the substrate. A ferromagnetic alignment of the system along the surface of the sample appears only at coverages of approximately 10 ?, when larger (≈100 nm) iron islands start to grow on the solid solution layer.     

Vacuum vapour deposition of phenylphosphonic acid on amorphous alumina

A study of the growth of phenylphosphonic acid (PPOA) on amorphous alumina thin films on top of polycrystalline aluminium foil is presented. The self-assembled monolayer (SAM) of the acid was grown in ultra high vacuum via vapour phase deposition, which allows us to investigate the molecular growth process in situ by photoelectron spectroscopy. The acid adlayer was deposited on the alumina surface at 300 K from an evaporator consisting of a ceramic tube heated by a tungsten wire. On adsorption of the PPOA acid on the alumina surface the anchoring properties of the phosphonic group lead to the formation of an ordered layer with an outward phenyl ring group and a phosphonate interface. The C 1s, P 2s, P 2p, Al 2p and O 1s core level spectra were analysed as a function of the deposition time. The acid adsorption occurs in two ranges: formation of an initial monolayer phase, followed by saturation coverage where the P 2s peak intensity is constant with deposition time. It was confirmed that the phenylphosphonic acid molecules react with the alumina surface forming P-O-Al bonds. It was shown that the SAM formation is driven by the evolution of acid molecular coverage on the alumina surface. The results on the vapour deposited PPOA SAM monolayer on the amorphous alumina surface are consistent with previous findings for a number of phosphonic acids SAMs prepared through immersion of the alumina surfaces in an acid solution.     

The effect of iridium precursor on oxide-supported iridium catalysts prepared by atomic layer deposition

Alumina, silica and beta zeolite supported iridium catalysts were prepared by atomic layer deposition (ALD) from two different metal precursors, Ir(acac)₃ and Ir(thd)(COD). The use of Ir(thd)(COD) in ALD is reported for the first time. The aim was to investigate the effect of the precursor on catalyst surface species, chemical state and characteristics.Controllable ALD reaction was successful with both iridium precursors on alumina and with Ir(acac)₃ on β zeolite. On these catalysts, iridium particle sizes were very small (1-3 nm). Instead, some thermal decomposition of both precursors was observed during deposition on silica. At conditions, where no or very little decomposition of the precursors took place, the differences in the chemical state and characteristics of the as-prepared Ir/support samples were negligible, In ALD, Ir(acac)₃ is slightly more stable at high deposition temperatures (>200 °C) while Ir(thd)(COD) enables the utilization of larger temperature range since it vaporizes at lower temperature compared to Ir(acac)₃. The results thus indicate that Ir(thd)(COD) is a suitable new precursor for ALD.     

Self-assembled organic templates for the selective adsorption of gold nanoparticles into confined domains

A simple procedure for the fabrication of submicron-sized functional organic templates is demonstrated. Native silicon samples are partially coated in millimolar octadecyltrichlorosilane (OTS) solutions. After coating, atomic force microscopy (AFM) reveals islands with diameters ranging from 0.6 to 1 μm. The hydrocarbon chains of the self-assembled silane entities within these islands subsequently are chemically functionalized following a robust preparation scheme. X-ray photoelectron spectroscopy (XPS) and water contact angle measurements were used for characterization. After functionalization, alkylsiloxane islands provide a versatile means to direct the deposition of nanoscopic components. In particular, citrate-stabilized gold nanoparticles (d = 16 ± 2 nm) are shown to selectively adsorb onto aminated islands, whereas adsorption on areas between these islands is negligible.     

Study of ultrathin iron silicide films grown by solid phase epitaxy on the Si(001) surface

Ultrathin films of iron silicide have been grown by high-temperature annealing of 0.14-to O.5O-nm-thick Fe films deposited on the Si(001) surface at room temperature. It has been found that annealing leads to the formation of nanoislands of iron silicide on the surface, so that their type depends on the thickness of the Fe film. High-energy electron diffraction and atomic force microscopy measurements have revealed that the deposition of Fe films less than 0.32 nm thick on the Si(001) surface stimulates epitaxial growth of both three-dimensional β-FeSi₂ and two-dimensional γ-FeSi₂ islands. It has been found that, for Fe coverages of more than 0.32 nm thick, a complete transition to solide phase epitaxy is observed only for two-dimensional β-FeSi₂ islands. The effect of prolonged annealing at 850°C on the morphology of the surface of the iron silicide film has been investigated.     

Surface modification of cobalt-chromium-tungsten-nickel alloy using octadecyltrichlorosilanes

Cobalt-chromium (Co-Cr) alloys have been extensively used for medical implants because of their excellent mechanical properties, corrosion resistance, and biocompatibility. This first time study reports the formation and stability of self-assembled monolayers (SAMs) on a Co-Cr-W-Ni alloy. SAMs of octadecyltrichlorosilanes (OTS) were coated on sputtered Co-Cr-W-Ni alloy thin film and bulk Co-Cr-W-Ni alloy. OTS SAM coated alloy specimens were characterized using contact angle goniometry, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Contact angle analysis and FTIR suggested that ordered monolayers were coated on both sputtered and bulk alloy. XPS suggested the selective dissolution of cobalt from the alloy during the formation of OTS SAM. The bonding between the alloy and the OTS SAM was mainly attributed to Si-O-Cr and Si-O-W covalent bonds and a smaller contribution from Si-O-Co bonds. AFM images showed the distribution of islands of monolayers coated on the alloy. The height of monolayers in majority of the islands was closer to the theoretical length of fully extended OTS molecules oriented perpendicular to the surface. The stability of OTS SAM was investigated in tris-buffered saline at 37 °C for up to 7 days. Contact angle, FTIR, and XPS collectively confirmed that the monolayers remain ordered and bound to the alloy surface under this condition. This study shows that Co-Cr alloys can be surface modified using SAMs for potential biomedical applications.     

Effect of modification of S-terminated Ge(1 0 0) surface on ALD HfO2 gate stack

When S-termination on a Ge(1 0 0) surface was desorbed at an elevated temperature and an atomic layer deposition (ALD) HfO₂ film was deposited, interfacial thickness was less than 1 nm. As a result, the equivalent oxide thickness (EOT) of the stack on the initially S-terminated surface was thinner than that deposited on the O₃-oxidized surface, while HfO₂ film thickness was almost identical on both surfaces. Nevertheless, the HfO₂ stack on the initially S-terminated surface exhibited improved leakage current characteristics due to an increase in barrier height. Its thinner but robust interface will contribute to the scaling down of gate oxide integrity.     

An XPS study of CrOx on a thin alumina film and in alumina supported catalysts

We have investigated chromium layers evaporated onto a thin alumina film at room temperature. The oxidation and reduction behavior of this model catalyst was compared to atomic layer deposition (ALD) and impregnated alumina supported catalysts using X-ray photoelectron spectroscopy (XPS) with a detailed analysis method utilizing asymmetric peak shapes to represent both metallic and oxidic states. The ALD and impregnated catalysts were measured after calcination in air and after reduction with several gases at 850 K. Both catalysts show Cr³⁺ and Cr⁶⁺ species after calcination and mostly Cr³⁺ after reduction. The chromium layers deposited in vacuum show initially small partial oxidation due to the interaction with the oxygen terminated alumina film. These model catalysts can be oxidized in vacuum to Cr³⁺ species but not to higher oxidation states. The model catalysts were also subjected to calcination and reduction treatments after deposition in vacuum. Under these conditions the model systems exhibit similar oxidation/reduction behavior as the supported catalysts. Photoreduction of Cr⁶⁺ during the measurements was also studied and found to be very slow having a negligible effect on the results.     

Atomic layer deposition of iridium(III) acetylacetonate on alumina, silica-alumina, and silica supports

The deposition of noble metal particles and films by atomic layer deposition (ALD) has recently gained interest in the fields of catalysis and microelectronics. However, there is little information on the mechanisms governing the reactions of noble metals with high surface area supports. In this work, iridium(III) acetylacetonate was deposited from gas phase onto alumina, silica-alumina, and silica supports to gain insight into the reaction mechanisms. According to elemental analysis and infrared spectroscopy, ligand exchange reaction between iridium acetylacetonate and surface OH groups took place on all substrate surfaces, but the iridium deposition on alumina and silica-alumina appeared to be hindered by sterical effects of the acetylacetonate ligands. Part of the iridium on silica was in metallic form. To reduce the content of iridium, reactive sites of the support surfaces were blocked with H-acetylacetonate (2,4-pentanedione). The blocking reduced the iridium content by more than 90% on alumina but by only 30-50% on silica-alumina. The attempted blocking had almost no effect on silica as expected. According to the results of this work, ALD can provide a feasible method for preparing iridium catalyst with reasonable iridium loadings.     

Tunnelling spectroscopy on silver islands and large deposited silver clusters on Ge(001)

Morphology and electronic properties of silver islands and deposited silver clusters on Ge(001) have been studied using scanning tunnelling microscopy (STM) and spectroscopy (STS) at low temperatures. Already the clean surface bears an interesting electronic structure, which is obvious from the STS. The tunnelling spectra exhibit strong peaks associated with dangling bond-derived surface states and an antibonding σ-state of the surface dimer. For silver islands of only few monolayers in height, complex spectra are interpreted to be dominated by metal–semiconductor interface states. These islands show energy gaps which are not observed for larger ones beyond 1 nm in height. Spectra of the larger islands contain a series of distinct peaks originating from lateral and three-dimensional electron confinement, respectively. Silver clusters – preformed in the gas phase using a cluster source – have been fabricated, size selected and deposited onto germanium(001). In tunnelling spectra dips at the Fermi level are accompanied by two maxima. These characteristics seem to be almost independent from the cluster size. Additional weak structures are found at higher bias voltages, which are understood in terms of quantized states. PACS 36.40.Cg; 61.46.+w; 73.20.At     

Role of Acid Treatment of the Carbon Support in the Growth of Atomic-Layer-Deposited Pt Nanoparticles for PEMFC Fabrication

Ultrathin films or particles of atomic layer deposition (ALD) on high surface can improve the activity and durability of catalyst fields, so depending on the surface state, the ALD growth mechanism on porous materials should be systematically investigated and optimized to improve their characteristics of catalysts. Herein, a Pt catalyst used in polymer electrode membrane fuel cell (PEMFC) applications is synthesized through fluidized-bed-reactor ALD on carbon black whose surface is modified through treatment with citric acid. The functional groups, analyzed through X-ray photoelectron spectroscopy (XPS), are found to be maximized after 60 min of acid treatment with stirring. Compared with bare carbon (untreated), the acid-treated carbon presents rich oxidized functional groups and abundant defects but lower surface areas and pore volumes. After ALD Pt deposition, highly dense, uniform, and well-dispersed Pt nanoparticles (NPs) are observed on the carbon black subjected to acid treatment, because of the favorable surface modifications for ALD growth resulting from the acid treatment. The ALD-Pt NPs on the acid-treated carbon exhibit larger electrochemical active surface areas, improved oxygen reduction reactions, and PEMFC performances, when compared with that of NPs on bare carbon with similar Pt weight loading.     

Flow boiling heat transfer of alumina nanofluids in single microchannels and the roles of nanoparticles

This study investigates flow boiling heat transfer of aqueous alumina nanofluids in single microchannels with particular focuses on the critical heat flux (CHF) and the potential dual roles played by nanoparticles, i.e., (i) modification of the heating surface through particle deposition and (ii) modification of bubble dynamics through particles suspended in the liquid phase. Low concentrations of nanofluids (0.001–0.1 vol.%) are formulated by the two-step method and the average alumina particle size is ~25 nm. Two sets of experiments are performed: (a) flow boiling of formed nanofluids in single microchannels where the effect of heating surface modification by nanoparticle deposition is apparent and (b) bubble formation in a quiescent pool of alumina nanofluids under adiabatic conditions where the role of suspended nanoparticles in the liquid phase is revealed. The flow boiling experiments reveal a modest increase in CHF by nanofluids, being higher at higher nanoparticle concentrations and higher inlet subcoolings. The bubble formation experiments show that suspended nanoparticles in the liquid phase alone can significantly affect bubble dynamics. Further discussion reveals that both roles are likely co-existent in a typical boiling system. Properly surface-promoted nanoparticles could minimize particle deposition hence little modification of the heating surface, but could still contribute to the modification in heat transfer through the second mechanism, which is potentially promising for microchannel applications.     

Fabrication and characterization of Cu–CdSe–Cu nanowire heterojunctions

The Cu–CdSe–Cu nanowire heterojunctions were fabricated by sequential electrochemical deposition of layers of Cu metal and CdSe semiconductor within the nano-pores of anodic alumina membrane templates. X-ray diffraction reveals the cubic phase for Cu and hexagonal phase for CdSe in the electrodeposited Cu–CdSe–Cu nanowire heterojunctions. The composition of the nanowire heterojunction segments is characterized by energy dispersive X-ray spectroscopy. The morphological study of nanowire heterojunctions has been made using scanning electron microscope and high resolution transmission microscopy. The nanowire heterojunctions grown in 100 and 300 nm nano-pore size templates have been found to have optical band gaps of 1.92 and 1.75 eV, respectively. The absorption spectra of 100 nm nanowire heterojunctions show a blue shift of 0.18 eV. The collective nonlinear current–voltage (I–V) characteristics of the 300 and 100 nm nanowire heterojunctions show their rectifying and asymmetric behaviour, respectively.     

Structural study of TiO2 thin films by micro-Raman spectroscopy

The Raman spectroscopy method was used for structural characterization of TiO₂ thin films prepared by atomic layer deposition (ALD) and pulsed laser deposition (PLD) on fused silica and single-crystal silicon and sapphire substrates. Using ALD, anatase thin films were grown on silica and silicon substrates at temperatures 125–425 °C. At higher deposition temperatures, mixed anatase and rutile phases grew on these substrates. Post-growth annealing resulted in anatase-to-rutile phase transitions at 750 °C in the case of pure anatase films. The films that contained chlorine residues and were amorphous in their as-grown stage transformed into anatase phase at 400 °C and retained this phase even after annealing at 900 °C. On single crystal sapphire substrates, phase-pure rutile films were obtained by ALD at 425 °C and higher temperatures without additional annealing. Thin films that predominantly contained brookite phase were grown by PLD on silica substrates using rutile as a starting material.     

Growth of thin films of TiN on MgO(100) monitored by high-pressure RHEED

Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the initial growth of titanium nitride (TiN) thin films on single-crystal (100) MgO substrates by pulsed laser deposition (PLD). This is the first RHEED study where the growth of TiN films is produced by PLD directly from a TiN target. At the initial stage of the growth (average thickness ∼2.4 nm) the formation of islands is observed. During the continuous growth the islands merge into a smooth surface as indicated by the RHEED, atomic force microscopy and field emission scanning electron microscopy. These observations are in good agreement with the three-dimensional Volmer–Weber growth type, by which three-dimensional crystallites are formed and later cause a continuous surface roughening. This leads to an exponential decrease in the intensity of the specular spot in the RHEED pattern as well.     

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

Peculiarities of the initial stage of growth of niobium-based nanostructures on a Si(111)-7 × 7 surface

The initial stage of growth of nanoislands prepared by thermal deposition of niobium on the reconstructed surface of Si(111)-7 × 7 in ultrahigh vacuum is experimentally investigated. The morphological and electrophysical properties of niobium-based nanostructures are studied by means of low-temperature scanning tunneling microscopy and spectroscopy. It is found that upon the deposition of niobium on a substrate at room temperature, clusters and nanoislands are formed on the silicon surface, having a characteristic lateral size of 10 nm with the metallic type of tunneling conductivity at low temperatures. Upon the deposition of niobium on a heated substrate, quasi-one-dimensional (1D) and quasi-two-dimensional (2D) structures with typical lateral dimensions of up to 200 nm and three-dimensional pyramidal islands with semiconducting type of tunneling conductivity at low temperatures are formed.     

Zirconium-assisted reaction in low temperature atomic layer deposition using Bis(ethyl-methyl-amino)silane and water

The reaction of Bis(ethyl-methyl-amino)silane (BEMAS) and water in atomic layer deposition (ALD) became possible when Zr-containing species were adsorbed on the vacant sites of the surface after a pulse and purge of BEMAS. The growth rates of the Si(Zr)O_x films were 0.8-0.9 nm/cycle in the temperature range of 185-325 °C. This phenomenon probably originates from the highly reactive hydroxyl species generated by Zr atoms. From this point of view, transition metals make reactant gas molecules to be highly activated in the ALD processes of transition metal oxides and nitrides, which might be an important factor that determines the ALD characteristics.     

The effects of thermal annealing on the structure and the electrical transport properties of ultrathin gadolinia-doped ceria films grown by pulsed laser deposition

Ultrathin crystalline films of 10 mol% gadolinia-doped ceria (CGO10) are grown on MgO (100) substrates by pulsed laser deposition at a moderate temperature of 400°C. As-deposited CGO10 layers of approximately 4 nm, 14 nm, and 22 nm thickness consist of fine grains with dimensions ≤∼11 nm. The films show high density within the thickness probed in the X-ray reflectivity experiments. Thermally activated grain growth, density decrease, and film surface roughening, which may result in the formation of incoherent CGO10 islands by dewetting below a critical film thickness, are observed upon heat treatment at 400°C and 800°C. The effect of the grain coarsening on the electrical characteristics of the layers is investigated and discussed in the context of a variation of the number density of grain boundaries. The results are evaluated with regard to the use of ultrathin CGO10 films as seeding templates for the moderate temperature growth of thick solid electrolyte films with improved oxygen transport properties.