Metallic Nanoislands Research Articles

Fabrication of Mesoporous Metal Chalcogenide Nanoflake Silica Thin Films and Spongy Mesoporous CdS and CdSe

Mesoporous silica metal oxide (ZnO and CdO) thin films have been used as metal ion precursors to produce the first examples of mesoporous silica metal sulfide (meso-SiO(2) @ZnS, meso-SiO(2) @CdS) or silica metal selenide (meso-SiO(2) @ZnSe, meso-SiO(2) @CdSe) thin films, in which the pore walls are made up of silica and metal sulfide or metal selenide nanoflakes, respectively. A gentle chemical etching with a dilute HF solution of the meso-SiO(2) @CdS (or meso-SiO(2) @CdSe) produces mesoporous cadmium sulfide (meso-CdS) (or cadmium selenide, meso-CdSe). Surface modified meso-CdS displays bright blue photoluminescence upon excitation with a UV light. The mesoporous silica metal oxides are formed as metal oxide nanoislands over the silica walls through a self-assembly process of a mixture of metal nitrate salt-two surfactants-silica source followed by calcination step. The reactions, between the H(2) S (or H(2) Se) gas and solid precursors, have been carried out at room temperature and monitored using spectroscopy and microscopy techniques. It has been found that these reactions are: 1) taking place through the diffusion of sulfur or selenium species from the top metal oxide layer to the silica metal oxide interface and 2) slow and can be stopped at any stage to obtain mesoporous silica metal oxide metal sulfide or silica metal oxide metal selenide intermediate thin films.

Surface X-ray diffraction analysis of Fe nanostructured films grown on c(2 × 2)-N/Cu(100)

We report the results of a Surface X-Ray Diffraction (SXRD) study of Fe nanostructured films deposited on c(2×2)-N/Cu(100) at room temperature (RT), with Fe coverage ΘFe=0.5ML and ΘFe=1ML. The c(2×2)-N/Cu(100) surface is an example of self-organised system, that can be used for growth of arrays of metal nano-islands and organic molecules assemblies. We chose two different values of N coverage, ΘN=0.3ML and ΘN=0.5ML, the second value corresponding to N saturation. We monitored the presence of surface diffraction peaks in hk scans and we performed Crystal Truncation Rods (CTR) analysis with ROD fitting programme. In the case of ΘN=0.5ML, i.e. at saturation coverage, the CTR could be fitted with one surface domain with p4gm(2×2) symmetry. In the surface cell adopted, N atoms occupy four-fold hollow sites, with Fe (intermixed with Cu) giving rise to a “clock” reconstruction previously observed on iron nitride films obtained by co-deposition and annealing. This result is an indirect confirmation of N surface segregation on top of the Fe films, occurring during the growth at RT. When subsaturation N coverage (ΘN=0.3ML) is used as a substrate for Fe deposition, the best results could be obtained with a model where two surface domains are present: the first one corresponds to a surface cell with Fe sitting in four-fold hollow sites on bare Cu areas, with possible interdiffusion in the second lattice. The second domain is assigned to growth of Fe on the N-covered square islands occurring once the bare Cu areas are fully covered. The SXRD analysis on N-covered surface domains shows that the mechanism of reconstruction and of N segregation on top layer is already active at RT for all N-coverage values.

Memristor device based on carbon nanotubes decorated with gold nanoislands

Memristors are passive nanoelectronics devices with applications in memories and switches which, although not containing magnetic components, have as distinct signature a hysteresis loop in the current-voltage dependence. Even if the prototype material for memristors is a binary oxide, we demonstrate that such devices can be easily implemented using carbon nanotubes decorated with metallic nanoislands, where the hysteretic action is a result of the thermionic emission of electrons trapped in the gold nanoislands.

Effective permittivity of the system of metal nanoislands in strong electric fields

The effect of the electric field on the value and sign of the low-frequency effective permittivity of metal island films is studied. It is shown that the permittivity in island metal films can be both positive and negative. The permittivity of some samples changed sign with the polarity of the applied electric field. Film polarization mechanisms are considered.

Investigation of Ag-poly(3,4-ethylenedioxythiophene):polystyrene sulfonate nanocomposite films prepared by a one-step aqueous method

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) hybrid films containing Ag nanoparticles were synthesized by a one-step aqueous method. The embedded Ag nanoparticles were produced using PEDOT as a reducing agent and PSS as a stabilizer. We have also demonstrated nanoparticle growth with increasing reaction time and silver nitrate content. Finally, as a nanocomposite-containing nanoparticle without surface-capping molecules and embedded in a conducting polymer matrix, the Ag NPs-PEDOT:PSS nanocomposite is interesting because it represents a type of material in which a metallic nano-island is embedded in the semiconducting matrix with a pure interface. One-step synthesized silver nanoparticles were found to induce a lowering of the work function and an increase of the highest occupied molecular orbital level in pristine PEDOT:PSS films by electron exchange with PEDOT, and also induced an inhibition of the electrostatic bond between PEDOT and PSS.

Electrical Control of the Magnetic State of Fe

Magneto-electric coupling offers a new pathway to information storage in magnetic memory devices. This phenomenon has been observed in various materials ranging from insulators to semiconductors. In bulk metallic systems, magneto-electric coupling has been disregarded as the electric field cannot enter bulk metals. In this work, we show that a substantial magneto-electric coupling exists in metallic Fe nano-islands grown on Cu(111). Using the electric field in the tunnel junction of a scanning tunneling microscope, the magnetic order parameter and the crystal structure of Fe was changed on the nanometer scale. This allows high density nonvolatile information storage by means of magneto-electric coupling in a simple metallic system.

Kinetic Monte Carlo Simulation of Metallic Nanoislands Grown by Physical Vapor Deposition

AbstractWe report kinetic Monte-Karlo (KMC) simulation of self-assembled synthesis of nanocrystals by physical vapor deposition (PVD), which is one of most flexible, efficient, and clean techniques to fabricate nanopatterns. In particular, self-assembled arrays of nanocrystals can be synthesized by PVD. However size, shape and density of self-assembled nanocrystals are highly sensitive to the process conditions such as duration of deposition, temperature, substrate material, etc. To efficiently synthesize nanocrystalline arrays by PVD, the process control factors should be understood in detail. KMC simulations of film deposition are an important tool for understanding the mechanisms of film deposition. In this paper, we report a KMC modeling that explicitly represents PVD synthesis of self-assembled nanocrystals. We study how varying critical process parameters such as deposition rate, duration, temperature, and substrate type affect the lateral 2D morphologies of self-assembled metallic islands on substrates, and compare our results with experimentally observed surface morphologies generated by PVD. Our simulations align well with experimental results reported in the literature.

Gold nano-islands on FTO as plasmonic nanostructures for biosensors

Metal nanoislands (NIs) deposited on transparent surfaces can be a convenient plasmonic material for bio/organic sensors, under the condition that a stable morphology is reached.Plasmonic materials suitable for the fabrication of low cost biosensors based on localized surface plasmon resonance (LSPR) UV–Vis spectroscopy, are fabricated by a simple methodology based on thermal evaporation of Au on commercially available, transparent fluorine-doped tin oxide (FTO) surfaces. The LSPR UV–Vis spectroscopy performed in transmittance mode reveals: (i) a small energy shift, Δλmax, of the LSPR band under immersion both in organic solvent, and significantly in aqueous media, and (ii) a sensible and reproducible Δλmax under formation of organic SAMs on the NIs surface. These data indicate that the Au NIs when deposited on FTO substrate exhibit (i) strong adhesion and a high stability, and (ii) a good sensitivity to molecular interaction. The samples also show that the LSPR bands recover the original feature after being exposed to different type of SAMs. Significantly, the absorption maximum, λmax of the Au-NIs LSPR spectra shows a red shift when SAMs incorporating single strands DNA are exposed to the complementary strands. The plasmonic system based on Au NI deposited on FTO surfaces because of (i) the inexpensive fabrication of stable NIs, (ii) the easy way to detect the molecular interaction occurring at their surface, and (iii) the sensitivity of their LSPR to molecular interaction represents a convenient platform for biosensors.

Study of Friction of Ag and Ni Nanoparticles: An Atomistic Approach

Manipulation of metal nanoparticles using atomic force microscopy is a promising new technique for probing tribological properties at the nanoscale. In spite of some advancements in experimental investigations, there is no unambiguous theoretical treatment of processes accompanying the movement of metallic nanoislands adsorbed on a flat surface, and additional research is required. In this paper, we describe computer experiments based on classical molecular dynamics in which the behavior of silver and nickel nanoparticles interacting with a graphene sheet and sheared with constant force is studied. Frictional force acting on the nanoislands is measured as a function of their size. It is shown that its average value grows approximately linearly with contact area, and slopes of linear fits are close to the experimentally observable ones. The dependence of the friction force value and of the shape of the measured friction curves on the type of metal atom is revealed, and its possible reasons originating from atomistic background are discussed.

Morphology and magnetic properties of island-like Co and Ni films obtained by de-wetting

The morphological, structural, and magnetic properties of Co and Ni films of different thicknesses grown by RF sputtering on a Si–SiO substrate and submitted to controlled diffusion of atoms on the substrate (de-wetting) are studied through X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy, and alternating-gradient magnetometry. For both metals, de-wetting treatment leads to the growth of non-percolating, metallic nanoislands characterized by a distribution of sizes and aspect ratios. XRD spectra reveal a polycrystalline multi-component structure evolving by effect of de-wetting and directly affecting the magnetic properties of films. The magnetic response after de-wetting is consistent with the formation of a nanogranular magnetic phase characterized by a complex, thickness-dependent magnetic behavior originating from the simultaneous presence of superparamagnetic and blocked-particle contributions. At intermediate film thickness (around 10 nm), a notable enhancement in magnetic coercivity is observed for both metals with respect to the values measured in precursor films and in their bulk counterparts.

Nanoisland‐Based Random Activation of Fluorescence for Visualizing Endocytotic Internalization of Adenovirus

Super-resolution imaging is performed on metallic nanoislands by plasmonic-based activation of random near-field hot spots. Nanoislands can be synthesized to create small hot spots that spatially distinguish molecular events on the nanometer scale. Enhanced resolution is experimentally confirmed with fluorescent nanobeads, and is extended to imaging the transport of an adenovirus across a live cell membrane.

Direct transfer of gold nanoislands from a MoS2 stamp to a Si–H surface

A printing technique is proposed for the transfer of metallic nanoislands between two semiconductor surfaces in UHV. For the preparation of the stamp, a systematic study of the growth conditions of small, flat triangular shape Au nanoislands at the top surface of microfabricated MoS2 pillars is presented. Those pillars are organized in a stamping matrix to increase the transfer rate. Up to 10% of Au nanoislands can be transferred to a H-Si(100) surface. The atomic scale quality of the interface between the Au nanoislands and the semiconductor surface is characterized by transmission electron microscopy cross-sectional imaging. This MoS2 stamping technique is extended to other surfaces such as mica, SiO2, and graphite. It permits to handle well shaped Au nanoislands on surfaces where a direct growth of flat nanoislands is not possible. This printing of well defined triangular Au nanoislands offers the unique possibility to construct ultraclean interconnecting nanopad systems by scanning tunneling microscope on atomically clean and electronically suitable substrates, manipulating those nanopads one at a time. The apex of the triangular shaped island is suitable for nanocontact to a surface atomic scale conducting wire.

Silicon surface transformations: From initial phases of pore formation to nanoscopic metal–insulator–semiconductor junctions

The functioning and fabrication of nanoemitter solar energy converting structures is described. Metal nanoisland formation on a nanoporous anodic oxide template is investigated in model systems where Pt is deposited onto H-terminated single crystal Si electrodes. The formation of an oxide layer, concurrent with the noble metal electrodeposition (Pt, Ir) is revealed by synchrotron radiation photoelectron spectroscopy (SRPES) and TEM. Scanning tunnelling spectroscopy (STS) on the metal islands and at the adjacent oxide covered surface shows rectifying behaviour at the nanoscopic silicon-oxide-metal contact. In the divalent dissolution regime in fluoride containing solution, initial pores form at re-entrant sites on the surface. Interestingly, the adsorption of enzymes such as the reverse transcriptase (RT) of the avian myeloblastosis virus (AMV) also occurs preferably at similar sites on a step-bunched Si surface. The results are discussed based on highly local nanoscopic electrical fields in the semiconductor and in the adjacent electrolytic double layer.

Metal-in-metal localized surface plasmon resonance

Anomalous strong resonances in silver and gold nanoporous thin films which conduct arefound to arise from isolated metal nano-islands separated from the surrounding percolatingmetal network by a thin loop of insulator. This observed resonant optical response ismodelled. The observed peak position is in agreement with the observed averagedimensions of the silver core and insulator shell. As the insulating ring thickness shrinks,the resonance moves to longer wavelengths and strengthens. This structure is the Babinet’sprinciple counterpart of dielectric core–metal shell nanoparticles embedded in dielectric.Like for the latter, tuning of resonant absorption is possible, but here the matrix reflectsrather than transmits, and tuning to longer wavelengths is more practical. A new class ofmetal mirror occurring as a single thin layer is identified using the same resonancesin dense metal mirrors. Narrow band deep localized dips in reflectance result.

Carbon nanotubes with platinum nano-islands as glucose biofuel cell electrodes

A novel method using intense pulsed light (IPL) for the metal nano-island formation on carbon nanotube (CNT) was introduced. The IPL-induced photothermal dewetting process improved platinum (Pt) catalyst utilization by transforming nano-islands from Pt film on CNT and increasing the surface area for the subsequent sputtering. The irradiation of high intensity of light on the Pt film causes surface-energy-driven diffusion of Pt atoms and forms the array of nano-islands on CNT. The thickness of Pt film can change the size of nano-islands. Cyclic voltammetry showed a dramatically improved glucose oxidation at the IPL morphology modified Pt–CNT electrode compared to the Pt sputtered CNT electrode without IPL irradiation. The power densities of glucose/air biofuel cell based on the morphology modified Pt–CNT electrode and the as-sputtered Pt–CNT electrode were 0.768μW/cm2 and 0.178μW/cm2, respectively. The biofuel cell based on morphology modified Pt–CNT electrode showed highly stable output in long-term performance. The power density dropped 14.1% in 30 days. Efforts are underway to improve the interface transfer to achieve higher potential and current output.

Transferring metallic nano-island on hydrogen passivated silicon surface for nano-electronics

In a planar configuration, precise positioning of ultra-flat metallic nano-islands on semiconductor surface opens a way to construct nanostructures for atomic scale interconnects. Regular triangular Au nano-islands have been grown on atomically flat MoS2 substrates and manipulated by STM to form nanometer gap metal-pads connector for single molecule electronics study. The direct assembly of regular shaped metal nano-islands on H-Si(100) is not achievable. Here we present how to transfer Au triangle nano-islands from MoS2 onto H-Si(100) in a clean manner. In this experiment, clean MoS2 substrates are patterned as array of MoS2 pillars with height of 8 μm. The Au triangle nano-islands are grown on top of the pillars. Successful printing transfer of these Au nano-islands from the MoS2 pillars to the H-Si(100) is demonstrated.

Magnetism of a cobalt-gold planar nanostructure on the silicon surface

The structural and magnetic properties of a cobalt nanolayer placed between a silicon substrate and a protective gold layer are studied. At a cobalt layer mass thickness of 1–2 nm, a nanoisland structure is shown to form. This thickness range is characterized by a local maximum of the magnetooptical Kerr effect and enhanced nonlinear optical and magnetic nonlinear optical responses at the second-harmonic frequency. This enhancement can be caused by the excitation of local surface plasmons and an increase in the local fields at the probing-radiation and second-harmonic frequencies in metallic nanoislands. The surface-sensitive nonlinear magnetooptical Kerr effect at the second-harmonic frequency is maximal at a cobalt layer thickness of about 2 nm, which corresponds to the characteristic scale of magnetization formation in the near-surface layer in cobalt.

Electronic Properties of Self-Organized Nanostructures: Theoretical Modeling on the Basis of the Scanning Tunneling Microscopy Characterization

An adequate modeling of the self-organized nanostructures is possible only on the basis of the modern characterization of those nanostructures. We discuss the recent results of modeling of self-organized semiconductor quantum dots and quantum rings as well as metal nanoislands based on the Scanning Tunneling Microscopy characterization. The examples given in the present review demonstrate a high level of complexity needed for a theoretical model to adequately represent the specific features of a nanosystem as determined by the Scanning Tunneling Microscopy.

Förster resonance energy transfer (FRET) with a donor–acceptor system adsorbed on silver or gold nanoisland films

We observed Förster resonance energy transfer (FRET) with a covalently linked donor-acceptor pair D-A consisting of two naphthalene groups acting as the donors and a benzofurazan group acting as the acceptor and adsorbed onto Ag or Au nanoisland films. The use of metal nanoisland films, which caused a strong enhancement of the Raman signal, permitted description of the adsorption mechanism onto the two metals. The intense fluorescence response of molecular adsorbates and the different behavior of the antenna on Ag and Au nanoislands are partly explained in terms of the radiating plasmon model.

Anomalously high low-frequency effective permittivity in a system of metal nanoislands

The low-frequency effective permittivity in a system of Co, W, and FeNi metal nanoislands in an electric field is investigated. The permittivity of films is determined from analyzing the dependences of the susceptance of the films on the temperature and the electric field. It is found that the effective permittivity of the structures under investigation has an anomalously high positive value (107–108) and that an increase in the electric field leads to a decrease in the effective permittivity and an increase in the electrical conductivity of the films. The nature of the high effective permittivity of the structures is governed by the island structure of the metal films and associated with the polarization of dipoles formed by pairs of negatively and positively charged metal nanoislands.