Ni Nanoislands Research Articles

Tip-enhanced Raman imaging of plasmon-driven dimerization of 4-bromothiophenol on nickel-decorated gold nanoplate bimetallic nanostructures.

We used tip-enhanced Raman spectroscopy (TERS) to examine plasmon-driven dimerization of 4-bromothiophenol (4-BTP) into thiophenol (TP) and 4,4'-biphenyldithiol (4,4'-BPDT) on Au and Ni@AuNPs. TERS revealed that cross-coupling of these molecular reactants into 4,4'-BPDT occurred primarily on Ni nano islands rather than the surrounding Au on the surface of Ni@AuNPs.

Superconducting proximity effect in (7×7)R19.1∘ Ni nanoislands on Pb(111)

We have studied the proximity-induced superconductivity in $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.{1}^{\text{o}}$ Ni nanoislands by combing scanning tunneling microscopy/spectroscopy (STM/STS) with density-functional theory calculation. Through depositing Ni onto Pb(111) substrate at 80 K, the monolayer Ni nanoislands with the $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.{1}^{\text{o}}$ surface structure have been fabricated, where the termination of Ni atoms at the hexagonal close-packed site is energetically preferred and the filling of $3d$ orbitals from the charge transfer leads to the vanishing magnetic moment of Ni atoms. The topographic $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.{1}^{\text{o}}$ lattice as well as the asymmetric height contrast in atomic unit cell have been further corroborated by the STM simulations. With high spatial and energy resolution, tunneling conductance ($dI/dU$) spectra have resolved an isotropic superconducting gap with ${\mathrm{\ensuremath{\Delta}}}_{Ni(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})}\ensuremath{\approx}1.29$ meV, which is slightly larger than ${\mathrm{\ensuremath{\Delta}}}_{Pb}\ensuremath{\approx}1.25$ meV. The temperature dependence of ${\mathrm{\ensuremath{\Delta}}}_{Ni(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})}$ supports the substrate-induced superconducting proximity effect according to the same transition temperature ${T}_{c}\ensuremath{\approx}7.14$ K with the Pb(111). The line spectroscopy has spatially mapped out the small increase of ${\mathrm{\ensuremath{\Delta}}}_{Ni(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})}$, which could be explained by an enhanced electron-phonon interaction (${V}_{ep}$) under the framework of Bardeen-Cooper-Schrieffer theory as a manifestation of the hole doping of Pb(111) from the surface Ni atoms.

Fabrication and repair of GaN nanorods by plasma etching with self-assembled nickel nanomasks

High crystal quality GaN nanorod arrays were fabricated by inductively coupled plasma (ICP) etching using self-organized nickel (Ni) nano-islands mask on GaN film and subsequent repaired process including annealing in ammonia and KOH etching. The Ni nano-islands have been formed by rapid thermal annealing, whose density, shape, and dimensions were regulated by annealing temperature and Ni layer thickness. The structural and optical properties of the nanorods obtained from GaN epitaxial layers were comparatively studied by high-resolution X-ray diffraction (HRXRD), Raman spectroscopy and photoluminescence (PL). The results indicate that damage induced by plasma can be successfully healed by annealing in NH3 at 900 °C. The average diameter of the as-etched nanorod was effectively reduced and the plasma etch damage was removed after a wet treatment process in a KOH solution. It was found that the diameter of the GaN nanorod was continuously reduced and the PL intensity first increased, then reduced and finally increased as the KOH etching time sequentially increased.

Strain anisotropy and magnetic domain structures in multiferroic heterostructures: High-throughput finite-element and phase-field studies

Understanding magnetic domain structures and their responses to electric fields in multiferroic heterostructures is critical to the design of electric-field-driven spintronic devices. High-throughput finite-element and phase-field simulations are performed to probe the piezoelectric strain anisotropy and its relaxation, magnetic domain structures and their responses to applied voltages as function of the in-plane dimensions and thickness of the magnetic Ni nanoislands grown on a Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) membrane. The piezoelectric strain anisotropy is found to increase with the in-plane aspect ratio, but it can be significantly relaxed, as large as >80%, in nanoislands of thickness larger than >15 nm. Magnetic domain diagrams are established to identify the domain structures for Ni nanoislands of different lengths of in-plane major and minor axis, as well as thickness. When a voltage is applied to the multiferroic heterostructure, the single-domain magnetic domain can be switched by the piezoelectric strain, whereas the vortex domain is not switched. However, for a multiferroic heterostructure with a thick nanoisland wherein most of the piezoelectric strain is relaxed, the single-domain magnetic domain shows a weak response to the voltage and cannot be switched. The effect of thickness variation in real magnets on the switching is also discussed. The present results are expected to provide guidance to the understanding and design of multiferroic nanostructures for achieving electric-field-modulated magnetic properties.

Strain Anisotropy and Magnetic Domain Structures in Multiferroic Heterostructures: High-Throughput Finite-Element and Phase-Field Studies

Understanding magnetic domain structures and their responses to electric fields in multiferroic heterostructures is critical to the design of electric-field-driven spintronic devices. High-throughput finite-element and phase-field simulations are performed to probe the piezoelectric strain anisotropy and its relaxation, magnetic domain structures and their responses to applied voltages as function of the in-plane dimensions and thickness of the magnetic Ni nanoislands grown on a Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) membrane. The piezoelectric strain anisotropy is found to increase with the in-plane aspect ratio, but it can be significantly relaxed, as large as >80%, in nanoislands of thickness larger than >15 nm. Magnetic domain diagrams are established to identify the domain structures for Ni nanoislands of different lengths of in-plane major and minor axis, as well as thickness. For Ni nanoislands with a circular in-plane geometry, an analytical function describing the relationship between the critical thickness hcritical and the in-plane diameter is obtained based on high-throughput calculations. When a voltage is applied to the multiferroic heterostructure, the single-domain magnetic domain can be switched by the piezoelectric strain, whereas the vortex domain is not switched. However, for a multiferroic heterostructure with a thick nanoisland wherein most of the piezoelectric strain is relaxed, the single-domain magnetic domain shows a weak response to the voltage and cannot be switched by the voltage. The present results are expected to provide guidance to the understanding and design of multiferroic nanostructures for achieving electric-field-modulated magnetic properties.

Carrier Localization Effects in InGaN/GaN Multiple-Quantum-Wells LED Nanowires: Luminescence Quantum Efficiency Improvement and "Negative" Thermal Activation Energy.

Two-dimensional InGaN/GaN multiple-quantum-wells (MQW) LED structure was nanotextured into quasi-one-dimensional nanowires (NWs) with different average diameters with a combination approach of Ni nanoislands as mask + dry etching. Such nanotexturing bring out several appealing effects including deeper localization of carriers and significant improvement in quantum efficiency (e.g., from 4.76% of the planar MQW structure to 12.5% of the 160 nm MQW NWs) of light emission in the whole interested temperature range from 4 K to 300 K. With the aid of localized-state ensemble (LSE) luminescence model, the photoluminescence spectra of the samples are quantitatively interpreted in the entire temperature range. In terms of distinctive temperature dependence of photoluminescence from these samples, a concept of “negative” thermal activation energy is tentatively proposed for the MQW NWs samples. These findings could lead to a deeper insight into the physical nature of localization and luminescence mechanism of excitons in InGaN/GaN nanowires.

Fabrication of nanorod InGaN/GaN multiple quantum wells with self-assembled Ni nano-island masks

The evolution of nano-sized Ni metal islands deposited by electron-beam evaporation on InGaN/GaN multiple quantum wells (MQWs) surface as a function of rapid thermal annealing (RTA) temperature and initial deposited Ni film thickness is reported. It is shown that the dimension and density of self-assembled Ni nano-islands depend critically on the annealing temperature and deposited Ni film thickness. The formation of the islands is described in terms of Ostwald ripening and coarsening mechanisms. Subsequently, the nano-masks are successfully applied to fabricate nanorod InGaN/GaN MQWs by using inductively coupled-plasma (ICP) etching. Uniform etching rate has been obtained by comparing the nanorod height etched for different times. Photoluminescence (PL) investigation shows the nanorod MQWs with optimized light output efficiency could be acquired under particular ICP and RF etching power. Strain relaxation and dislocation reduction effect would contribute to enhanced light output of nanorod InGaN/GaN MQWs compared with the as-grown plane MQWs.

Arrays of GaN nano-pillars fabricated by nickel nano-island mask

Uniformly distributed GaN nano-pillars have been successfully fabricated by inductively coupled plasma etching using self-organized Nickel nano-islands mask on GaN film. Ni nano-islands have been formed through the annealing under the ammonia. The size and distribution of GaN nano-pillars can be controlled by the thickness of Ni film and the NH3 annealing time. The optimum diameter and the density of GaN pillars by analyzing the top-view SEM images are about 350nm and 2.6×108cm−2, respectively. The photoluminescence (PL) measurements showed the improved optical properties and the strain relaxation in GaN nano-pillars compared to the bulk materials.

Efficiency Improvement of GaN-Based Light-Emitting Diode Prepared on GaN Nanorod Template

We demonstrated the formation of GaN-based nanorod (NR) structure by using self-assemble Ni nanoislands as the etching mask. It was found that crystal quality of the GaN epilayer prepared on an NR GaN template was significantly better than that prepared with a conventional low-temperature GaN nucleation layer. With the NR GaN template, it was found that 20-mA light-emitting diode (LED) output power can be enhanced by 39.8%, as compared to the conventional LED.

Detection of label-free cancer biomarkers using nickel nanoislands and quartz crystal microbalance

We present a technique for the label-free detection and recognition of cancer biomarkers using metal nanoislands intended to be integrated in a novel type of nanobiosensor. His-tagged (scFv)-F7N1N2 is the antibody fragment which is directly immobilized, by coordinative bonds, onto ~5 nm nickel islands, then deposited on the surface of a quartz crystal of a quartz crystal microbalance (QCM) to validate the technique. Biomarker GTPase RhoA was investigated because it has been found to be overexpressed in various tumors and because we have recently isolated and characterized a new conformational scFv which selectively recognizes the active form of RhoA. We implemented a surface chemistry involving an antibiofouling coating of polyethylene glycol silane (PEG-silane) (<2 nm thick) and Ni nanoislands to reach a label-free detection of the active antigen conformation of RhoA, at various concentrations. The methodology proposed here proves the viability of the concept by using Ni nanoislands as an anchoring surface layer enabling the detection of a specific conformation of a protein, identified as a potential cancer biomarker. Hence, this novel methodology can be transferred to a nanobiosensor to detect, at lower time consumption and with high sensitivity, specific biomolecules.

Fabrication of high-aspect ratio silicon nanopillars and nanocones using deep reactive ion etching

Bosch type processes have been employed to fabricate nanostructured Si surfaces. Nanopillars and nanocones in Si have been fabricated using different techniques for Ni micromasking. Plasma redeposition of Ni was found to be responsible for Si pillar formation with diameters varying in the submicron range. A possibility to produce tilted nanopillars with tilt angles up to as high as 25° has been demonstrated. In other method, previously deposited and annealed thin Ni films were employed. Smaller Ni nanoislands were obtained, and the formation of Si nanocones was demonstrated using longer passivation steps. In this case, reflection coefficients as low as 1.2% were obtained for the optimized etching process time.

Effects of plasma power on the growth of carbon nanotubes in the plasma enhanced chemical vapor deposition method

Effects of plasma power on the growth of the multi-wall carbon nanotubes (CNTs) are reported. CNTs were grown on the silicon wafers by plasma enhanced chemical vapor deposition (PECVD) method using a mixture of acetylene and hydrogen at the temperature of 650 °C. Plasma powers ranging from zero to 35 W were applied on the samples and the effects of different magnitudes of the plasma power on the growth direction of the CNTs were investigated. Regular vertically aligned nanotubes were obtained at plasma power of 25 W. In order to set on the plasma during the growth, electrical force was applied on the carbon ions. Nickel layer was used as a catalyst, and prior to the nanotubes growth step, it was treated by hydrogen plasma bombardment in order to obtain the Ni nano-islands. In this step, as the plasma power on the Ni layer was increased, the grain size of nickel nano-particles decreased, and hence, nanotubes of smaller diameter were obtained later on. At the last step some anomalous structures of agglomerated CNTs were obtained by controlling the plasma power. Samples were analyzed by scanning tunneling microscopy (STM) and scanning electron microscopy (SEM).

Self-organized growth of Ni nanoparticles on a cobalt-oxide thin film induced by a buried misfit dislocation network

We report on a clear tendency toward organized growth, at room temperature, of Ni nanoislands deposited on a $\mathrm{CoO}(001)$ nanopatterned surface. The $9.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ square surface patterning consists in a periodic displacement field induced at the surface by an underlying interfacial dislocations network that is buried at the interface between the $5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick $\mathrm{CoO}(001)$ thin film and the $\mathrm{Ag}(001)$ substrate. Grazing incidence small and wide angle x-ray scattering (GISAXS and GIXD) performed in situ, during growth, reveal that the nucleation of Ni particles is driven by the underlaying dislocation distribution. The tendency for organization is confirmed by scanning tunneling microscopy (STM), which also reveals a quite narrow size distribution of the Ni nanoparticles around $5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ width and $0.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ height.

High quality GaN nanowires synthesized from Ga2O3 with graphite powder using VPE method

Synthesis of single-crystalline GaN nanowires on C-Al2O3 substrates in a vapor phase epitaxy process by the help of a Ni catalyst was realized. The GaN nanowires were grown at 1000–1100°C using a mixed powder of Ga2O3 and graphite. GaN nanowires were found to have a single-crystalline hexagonal structure in a high-resolution transmission electron microscopy and X-ray scattering measurements in spite of atmospheric pressure growth. Diameters of the grown nanowires range from 60 to 120nm, which are comparable to the diameters (10–80nm) of hydrothermally prepared Ni nanoislands acting as a seed. This fact indicates that the diameter of GaN nanowires can be effectively tuned by controlling that of Ni catalysts.

Effect of Ni, Pd and Ni–Pd nano-islands on morphology and structure of multi-wall carbon nanotubes

In this research, the effect of Ni, Pd and Ni–Pd catalysts have studied on morphology and structure of synthesized multi-wall carbon nanotubes (MWCNTs). Initially, thin films of Ni (with two thicknesses of 10 and 20 nm), Pd/Ni (5/10 nm) and Pd (10 nm) were deposited as catalysts on SiO 2 (60 nm)/Si(1 0 0) substrates, using dc magnetron sputtering technique. The deposited films were annealed at 900 °C in ammonia environment for 45 min, in order to obtain nano-structured catalyst on the surface. Using scanning electron microscopy (SEM), the average size of Ni nano-islands (synthesized by the 10 and 20 nm Ni films), Pd and Ni–Pd nano-islands were measured about 55, 110, 45 and 50 nm, respectively. According to X-ray photoelectron spectroscopy analysis (XPS), the ratio of Ni/Pd on the surface was about 3 for the bilayer sample. The CNTs were synthesized on the nano-island catalysts at 940 °C in CH 4 ambient using a thermal chemical vapor deposition method. The results revealed that average diameter of the CNTs were about 70, 110, 120 nm for Ni, Ni–Pd and Pd catalysts, respectively. Raman spectra of the MWCNTs showed that intensity ratio of two main peaks located in the range of 1550–1600 and 1250–1450 cm −1 (as a quality factor for the CNTs) for Ni, Pd and Ni–Pd catalysts were 1.42, 0.91 and 0.85, respectively. Therefore, based on our data analysis, although addition of Pd to Ni catalyst caused a considerable reduction in the quality of the grown MWCNTs as compared to the pure Ni catalyst, but it resulted in an enhancement in the methane decomposition rate. For the pure Pd catalyst samples, both a slow methane decomposition rate as compared with Ni–Pd catalyst samples and a poor quality of CNTs were observed as compared with the Ni catalyst, under similar experimental conditions.

The evolution of Ni nanoislands on the rutile TiO 2(110) surface with coverage, heating and oxygen treatment

Scanning tunnelling microscopy (STM) and reflection high-energy electron diffraction (RHEED) have been used to examine in situ the morphology and growth of nickel nanoislands on the rutile TiO 2(110)-p(1×1) surface. Such analysis revealed growth at temperatures between 295 and 495 K via the Volmer–Weber (V–W) mode of three-dimensional islands, in agreement with thermodynamic expectations. The Ni islands grew first as small crystalline domes with no preferential alignment with the TiO 2(110) surface, but with increasing Ni dose their orientation evolved, until almost perfect monocrystalline (110) Ni∥(110) TiO 2 , [ 1 ̄ 10] Ni ∥[001] TiO 2 orientation-relations were established. This arrangement has one of the lowest lattice mismatches if an average of the two orthogonal surface directions [001] TiO 2 and [110] TiO 2 is considered. The epitaxial relation persists upon annealing in ultra-high vacuum (UHV) to 1065 K and the heat treatment causes the particles to coarsen and grow in size. When the crystalline islands of nickel are oxidised, the lattice parameters are modified, consistent with the formation of NiO. A new epitaxial relation is established between the overlayer and the substrate: (001) layer∥(110) TiO 2 , [1 1 ̄ 0] layer ∥[001] TiO 2 . This produces a large strain parallel to the [1 1 ̄ 0] TiO 2 direction and a very small strain along [001] TiO 2 (+0.13% mismatch). UHV annealing of the oxidised overlayer reduces the crystallites to islands with lattice parameters corresponding to nickel metal, but the original crystallography is not recovered. Instead, the nanoislands of reduced nickel maintain the same overlayer orientation as for the oxide. A model is presented in which the change in epitaxial relation is attributed to the formation of an interfacial NiO layer between the reduced Ni islands and TiO 2 substrate. The results demonstrate that the oxidation/reduction treatment of the substrate and overlayer can be used to modify the microstructure of the metal/oxide system, and have implications for the use of nickel as a supported metal catalyst.