Nanostructure formation aided by self-organised Bi nanolines on Si(0 0 1)

We provide a mini review of recent theoretical investigations of nanostructure formation aided by self-organised Bi nanolines on the Si(0 0 1) surface. It is suggested that hydrogen-passivated single-domain Si(0 0 1) produced by the formation of defect-free, hundreds of nm long, and 1.2-1.5 nm wide Bi nanolines provides an appealing template with preferential sites for adsorption of other elements. Based on ab initio pseudopotential calculations it is suggested that using the Bi nanoline template it should be possible to grow the following structures: mixed Ge-Si dimer structures on the Si(0 0 1) terrace between two neighbouring Bi nanolines; small In nanoclusters along the nanoline; and line and cluster structures of Fe atoms with novel electronic and magnetic properties.     

Stress relief as the driving force for self-assembled bi nanolines

Bi nanolines self-assemble on Si(001) and are remarkable for their straightness and length-they are often more than 400 nm long, and a kink in a nanoline has never been observed. Through electronic structure calculations, we have found an energetically favorable structure for these nanolines that agrees with our scanning tunneling microscopy and photoemission experiments; the structure has an extremely unusual subsurface structure, comprising a double core of seven-membered rings of silicon. Our proposed structure explains all the observed features of the nanolines, and shows that surface stress resulting from the mismatch between the Bi and the Si substrates is responsible for their self-assembly. This has wider implications for the controlled growth of nanostructures on semiconductor surfaces.     

Growth of well-aligned Bi nanowire on Ag(1 1 1)

We report the fabrication of one-dimensional (1D) Bi nanowires grown on Ag(1 1 1) with average lateral width from 9 to 20 nm by physical vapor deposition in ultra high vacuum conditions. In situ low-temperature scanning tunneling microscopy analyses reveal that the preferred growth of 1D Bi nanostructures is driven by the highly anisotropic bonding in the crystallographic structure of the Bi(1 1 0) plane. The Bi nanowires grow along direction and align with the directions on Ag(1 1 1). The growth of the Bi nanowires proceeds in a bilayer growth mode resulting from the layer pairing in Bi(1 1 0) which saturates the dangling bonds and lowers the total energy.     

A theoretical study of Ge adsorption on Si(0 0 1) covered with Bi nanolines

The energetic stability and the equilibrium geometry of Ge adsorption on the Si(0 0 1) surface covered with Bi nanolines were examined by ab initio total energy calculations. We find that there is an energetic preference of Ge atoms lying on the Si(0 0 1) terraces, forming Si^down-Ge^up mixed dimers. Further investigations reveal a repulsive interaction between the mixed dimers and the Bi nanolines, suggesting that the formation of Si^down-Ge^up dimers can be tailored by the presence of the Bi nanolines.     

Growth of Ag nanostructures on TiO2(1 1 0)

We present a study of the growth of silver nanoparticles or clusters on a TiO₂(1 1 0) substrate in ultra-high vacuum. The growth is monitored in situ by ion and neutral scattering spectroscopy using He⁺ scattering and Auger spectroscopy. The scattering measurements show that only part of the surface is covered by Ag suggesting formation of clusters. Additionally an ex-situ study was performed by scanning electron microscopy and atomic force microscopy to determine the size distribution of these clusters. The average size distributions were found to range from about 5 to about 20 nm as a function of the evaporation flux. At the higher evaporation flux we observe formation of the smaller sized clusters.     

Interaction of Ag and O on W(1 1 0) studied by scanning tunneling microscopy

The structural changes of Ag films on W(1 1 0) upon coadsorption of oxygen have been studied by scanning tunneling microscopy. The exposure of one monolayer Ag to oxygen leads to a phase separation into an Ag bilayer and patches of O-covered W(1 1 0). The effective Ag island thickness increases linearly with oxygen exposure. For Ag submonolayer-islands the onset of the bilayer formation is delayed, the induction period increases with the available free W area. We conclude that the steps of the transport process are (1) dissociation of oxygen on W and on the Ag islands, (2) site exchange of atomic oxygen with Ag atoms predominantly at the island edges - while on W(1 1 0) the oxygen is immobile, (3) diffusion of the displaced Ag atoms to the island edges where they are incorporated into the monolayer and (4) initiation of Ag bilayer formation, once the W(1 1 0) is saturated with O. This indicates an unexpected activity of the Ag monolayer on W(1 1 0) towards oxygen dissociation. In case of a reversed deposition sequence, where submonolayer quantities of Ag are adsorbed on an oxygen-precovered W(1 1 0) surface, growth of Ag clusters is observed. The distribution of cluster size and cluster height depends critically on the spatial order within the predeposited oxygen overlayer - it is obvious that the oxygen overlayer on the W surface acts as a structured template for preferential Ag nucleation.     

Silver nanostructure arrays abundant in sub-5 nm gaps as highly Raman-enhancing substrates

Two types of highly Raman-enhancing arrays substrates were fabricated using anodic aluminum oxide (AAO) templates by controlling the AAO template temperature and evaporated silver thickness during e-beam evaporating: complex patterned Ag nanoparticle arrays abundant in sub-5 nm gaps (type I); hexagonal Ag nanopore arrays (type II). The surface enhanced Raman scattering (SERS) enhancement factors (EF) of both substrates are estimated experimentally to exceed 10⁵, especially that of type I reaches 10⁷ due to the existence of numerous sub-5 nm gaps. The simulation using finite-difference time-domain (FDTD) method confirmed that gap effect has significantly improved the substrates’ SERS activity.     

Formation of ordered arrays of Ag nanowires and nanodots on Si(5 5 7) surface

The ordered arrays of Ag nanowires and nanodots have been grown in ultra-high vacuum on the Si(5 5 7) surface containing regular steps of three bilayer height. Formation of Ag nanostructures have been studied by scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy at room temperature. It was shown that a sample exposure in the vacuum before Ag growth affects the shape of the forming Ag islands. This effect is caused by oxygen adsorption on the silicon surface from the residual atmosphere in the vacuum chamber. When Ag is deposited on the clean silicon surface the islands, overlapping several (1 1 1) neighboring terraces, form. The arrays of silver nanowires elongated along steps and silver nanodots, arranged in lines parallel to the steps, can be formed on the Si(5 5 7) surface depending on the amount of adsorbed oxygen.     

Growth of silver structures on silicon surfaces observed in vivo by scanning tunneling microscopy

Early stages of growth of silver thin films on oriented silicon surfaces Si(1 0 0)2 × 1 and Si(1 1 1)7 × 7 were studied directly during deposition at room temperature by the scanning tunneling microscopy. Single Ag atoms deposited on the Si(1 0 0)2 × 1 surface diffuse too fast on the surface to be imaged by the microscope. Nucleation on C-type defects of the Si(1 0 0)2 × 1 reconstruction has been observed. During further growth, the defects represent stable terminations of silver chains. Ag nanoclusters growing on the Si(1 1 1)7 × 7 surface have been studied as a system with low diffusivity at room temperature. On this surface, presence of effective interaction between Ag clusters and individual Ag atoms in neighboring cells of the reconstruction has been identified. The interaction results in lowering the barrier for Ag atom hopping to an adjacent unit cell occupied by an Ag cluster. Unique possibilities arising from scanning the surface directly during growth are demonstrated.     

Surface morphology of metal films deposited on mica at various temperatures observed by atomic force microscopy

Thin films of eight metals with a thickness of 150 nm were deposited on mica substrates by thermal evaporation at various temperatures in a high vacuum. The surface morphology of the metal films was observed by atomic force microscopy (AFM) and the images were characterized quantitatively by a roughness analysis and a bearing analysis (surface height analysis). The films of Au, Ag, Cu, and Al with the high melting points were prepared at homologous temperatures T/T_m = 0.22-0.32, 0.40, and 0.56. The films of In, Sn, Bi, and Pb with the low melting points were prepared at T/T_m = 0.55-0.70, where T and T_m are the absolute temperatures of the mica substrate and the melting point of the metal, respectively. The surface morphology of these metal films was studied based on a structure zone model. The film surfaces of Au, Ag, and Cu prepared at the low temperatures (T/T_m = 0.22-0.24) consist of small round grains with diameters of 30-60 nm and heights of 2-7 nm. The surface heights of these metal films distribute randomly around the surface height at 0 nm and the morphology is caused by self-shadowing during the deposition. The grain size becomes large due to surface diffusion of adatoms and the film surfaces have individual characteristic morphology and roughnesses as T increases. The surface of the Al film becomes very smooth as T increases and the atomically smooth surface is obtained at T/T_m = 0.56-0.67 (250-350 °C). On the other hand, the atomically smooth surface of the Au film is obtained at T/T_m = 0.56 (473 ± 3 °C). The films of In, Sn, Bi, and Pb prepared at T/T_m = 0.55-0.70 also show the individual characteristic surface morphology.     

Thin films of silver nanoparticles deposited in vacuum by pulsed laser ablation using a YAG:Nd laser

We report the deposition of thin films of silver (Ag) nanoparticles by pulsed laser ablation in vacuum using the third line (355 nm) of a YAG:Nd laser. The nanostructure and/or morphology of the films was investigated as a function of the number of ablation pulses, by means of transmission electron microscopy and atomic force microscopy. Our results show that films deposited with a small number of ablation pulses (500 or less), are not continuous, but formed of isolated nearly spherical Ag nanoparticles with diameters in the range from 1 nm to 8 nm. The effect of increasing the number of pulses by one order of magnitude (5000) is to increase the mean diameter of the globular nanoparticles and also the Ag areal density. Further increase of the number of pulses, up to 10,000, produces the formation of larger and anisotropic nanoparticles, and for 15,000 pulses, quasi-percolated Ag films are obtained. The presence of Ag nanoparticles in the films was also evidenced from the appearance of a strong optical absorption band associated with surface plasmon resonance. This band was widened and its peak shifted from 425 nm to 700 nm as the number of laser pulses was increased from 500 to 15,000.     

Direct observation of slow morphological transformations and wetting behavior of pulsed laser deposited sub-monolayer gold on (0 0 0 1) sapphire in atmosphere

Using high-resolution atomic force microscope we observed in ambient atmosphere the slow morphological transitions of the incipient adlayer of gold grown on (0 0 0 1) sapphire substrate by pulsed laser deposition. The equivalent average uniform thickness of the gold deposition was about 0.55 Å, which is about one-fourth of its monolayer. A dynamic simulation revealed that about 10% of the gold was implanted into the substrate up to the depth of about 3.3 nm and the top monolayer of the sapphire surface was almost completely depleted of oxygen atoms due to the preferential sputtering by the plume particles. The gold adlayer transformed into a labile phase which enhanced the surface roughness and had a preferred orientation of a wavy structure during 24 h of the deposition. The auto-correlation function of this wavy structure in labile metastable phase revealed two-fold symmetry and provided a preferential size of about 4 nm (peak to peak) with a mean separation of 8 nm. At the end of about 6 days this phase was found to completely transform into an apparently de-wetted phase of beads with average in-plane diameter of ∼20 nm and height of ∼7 nm having large size distribution. Each bead was seen to have coating of a concentric corona layer, which might be that of the condensed moisture or other gaseous species from atmosphere because subjecting these samples to vacuum removed this layer. These observations shed light on the dynamics of the pulsed laser deposited metastable gold adlayer in the incipient stage of its growth on sapphire and their wetting or de-wetting mechanisms in ambient atmosphere.     

Growth process of nanostructured silver films pulsed laser ablated in high-pressure inert gas

The growth process of silver thin films deposited by pulsed laser ablation in a controlled inert gas atmosphere was investigated. A pure silver target was ablated in Ar atmosphere, at pressures ranging between 10 and 100 Pa, higher than usually adopted for thin film deposition, at different numbers of laser shots. All of the other experimental conditions such as the laser (KrF, wavelength 248 nm), the fluence of 2.0 J cm⁻², the target to substrate distance of 35 mm, and the temperature (295 K) of the substrates were kept fixed. The morphological properties of the films were investigated by transmission and scanning electron microscopies (TEM, SEM). Film formation results from coalescence on the substrate of near-spherical silver clusters landing as isolated particles with size in the few nanometers range. From a visual inspection of TEM pictures of the films deposited under different conditions, well-separated stages of film growth are identified.     

Ag nanostructures on Au(7 8 8): A self-assembled superlattice of metallic quantum resonators

We have studied by scanning tunneling microscopy (STM) the effect of the reconstruction of a stepped Au(1 1 1) surface on the growth of silver sub-monolayer deposition. For narrow terraces, the reconstruction is disturbed and its pattern changes, Ag growth is therefore influenced. Thus growth of Ag on Au(7 8 8) vicinal surface can be controlled and leads to the formation of a highly ordered superlattice of nanostructures. Moreover, we show by tunneling conductance images that Ag islands exhibit electronic confinement effects of the Shockley surface state. Due to the homogeneity of their shapes and sizes, all the nanostructures of the self-assembled superlattice should exhibit similar electronic properties.     

Nucleation control for ZnO nanorods grown by catalyst-driven molecular beam epitaxy

We report a study of the annealing temperature and time on Ag catalyst size and density for subsequent growth of ZnO nanorods by catalyst-driven molecular beam epitaxy (MBE). Two different substrates (SiO₂ and SiN_X) for the Ag deposition were used and the thickness of the Ag held constant at 25 Å. Annealing between 600 and 800 °C produced Ag cluster sizes in the range 8-30 nm diameter on SiO₂ and 10-65 nm on SiN_X with a cluster density from 100 to 2500 mm⁻² for SiO₂ and 30 to 1900 mm⁻² for SiN_X. ZnO nanorods grown on these clusters show single-crystal, wurtzite-phase nature and strong band-edge photoluminescence at 380 nm. The nanorods can also be grown selectively on lithographically-patterned dielectric stripes with Ag clusters formed on top by e-beam evaporation and annealing.     

Growth of nano-crystalline metal dots on the Si(1 1 1)-7 × 7 surface saturated with C2H5OH

Metal atom on the Si(1 1 1)-7 × 7 surface undergoes migration by hopping among Si-adatom and Si-rest atom. If the hopping migration is prohibited, how change the deposited metals? In this paper, we studied the deposition of metals on the Si(1 1 1)-7 × 7 surface saturated with C₂H₅OH, on which the whole Si-rest atoms are changed to Si-H so that the hoping migration of metals will be prohibited. We found the growth of ca. 5 nm of crystalline dots by the deposition of Sn, Zn and Ag. Interestingly, Ag dots undergo layer-by-layer growth so that the surface is covered with 5 nm size dots with uniform height. When the hopping migration is prohibited, growth of dots is controlled by the kinetics of precursor state atoms instead of the lattice energy relating to lattice matching or strain.     

Gold nanostructures on chemically reinforced PDMS microwell arrays

This paper describes a facile strategy for fabricating arrays of two- and three-dimensional gold nanostructures using PDMS-infiltrated polystyrene (PS) colloidal crystals. PDMS molding of colloidal crystal, gold vapor deposition, and subsequent calcination of PS produced gold thin layers over hexagonal PDMS microwell arrays with hemispherical air-voids of approximately 140 nm on glass substrates. Vapor deposition of perfluoroalkylsilane thin layers improved the thermal stability of the colloidal template over 100 °C, providing a route to preparation of hollow architectures with gold thin layers supported by PDMS nanostructures. Surface modification of the PDMS using poly(allylamine hydrochloride) induced two-dimensional colloidal crystals of PS and PMMA spheres through electrostatic interactions. Particle aggregation of 13 nm gold nanoparticles in the PDMS microwells demonstrated a surface plasmon resonance band red-shifted to 810 nm, in comparison with that on the flat surface at 720 nm.     

Formation and stabilization of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template

We demonstrate the growth of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template by in situ scanning tunneling microscopy (STM). These clusters form near a dimer row at one side of the half-unit cell (HUC); and with three different equivalent orientations. A cluster model comprising three top layer Si atoms bonded to six Fe atoms at the next layer in the 7 × 7 faulted-half template is proposed. The optimized cluster structure determined by first-principles total-energy calculation shows an inward-shifting of the three center Fe atoms. The clusters and the nearby center-adatoms of the next HUCs appear with a significantly reduced height below bias voltages 0.4 V in high resolution empty-state STM images, suggesting an energy gap opening near the Fermi level at these localized cluster and adatom sites. We explain the stabilization of the clusters on the 7 × 7 template using the gain in electronic energy as the driving force for cluster formation.     

Initial stages of hydroxide formation and its reduction on Co(0 0 0 1) studied by in situ STM and XPS in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy (STM) has been applied to study the initial stages of hydroxide formation and its reduction on Co(0 0 0 1) in 0.1 M NaOH. XPS investigations give chemical information about the adlayer composition after oxidation and at the different reduction stages. In the subpotential range of oxidation at E<−0.55 V (SHE) the formation of a Co(OH)₂(0 0 0 1) superstructure is observed. It shows a hexagonal symmetry with an average periodicity of P=1.25±0.20 nm. The coincidence cell of the observed structure consists of 16 unit cells Co(OH)₂(0 0 0 1) showing an average lattice parameter of a=0.33±0.05 nm and thus the Co(OH)₂ monolayer forms a 5 × 5 superstructure with respect to the underlying metallic Co(0 0 0 1) substrate. XPS results clearly prove the presence of hydroxide and exclude the formation of CoO in the subpotential range.At the very beginning of the reduction process small two-dimensional metal clusters and islands can be observed. It is assumed that they are crystallization nuclei for metal formation. They enlarge and grow together with other islands or larger terraces. During this reduction process two-dimensional adatomic arrays consisting of OH⁻-Co²⁺- OH⁻ trimers appear on the surface. Some of these trimers accumulate at step edges, and finally decorate them. This decoration builds up an energy barrier for further metal incorporation and prevents further growth of the terraces with remaining metal clusters on their surfaces. The reduction of the Co(OH)₂ layer is found to be not completed which is confirmed by XPS results.     

Magnetic properties and giant magnetoresistance in electrodeposited Co–Ag granular films

Small cobalt particles embedded in a silver matrix have been prepared using the electrodeposition technique. The size of the clusters is controlled by the deposition potential and the Co growth time. Structural, magnetic and magneto-transport properties of Co–Ag samples have been investigated as a function of the Co concentration between 2 and 40 at% cobalt. Superparamagnetic behavior is evidenced for the low contents of cobalt while long-range magnetic order appears at higher Co concentrations. The particles size has been determined from magnetic properties and from the X-ray diffraction technique, and varies between 3.5 and 9 nm. Magnetoresistance passes through a maximum as a function of the cobalt concentration. A maximum of ∼4% GMR is obtained at room temperature while GMR reaches a value of 14% at 10 K.