Nanoscale Islands Research Articles

Chiral magnetization configurations in magnetic nanostructures in the presence of Dzyaloshinskii-Moriya interactions

Many low-dimensional systems, such as nanoscale islands, thin films, and multilayers, as well as bulk systems, such as multiferroics, are characterized by the lack of inversion symmetry, a fact that may give rise to a Dzyaloshinskii-Moriya (DM) interaction. For sufficient strength, the DM interaction will favor spiral spin configurations of definite chirality. In order to harness such systems for applications, it is important to understand the conditions under which these spiral spin configurations form and how they can be controlled via an external field. Here, we present exact solutions of the 1D magnetization profiles in such systems for arbitrary material parameters in closed form. Determining the energy per unit length exactly, we are able to present the critical strength of the DM interaction, at which spiral solutions are energetically favorable. These magnetization profiles, in general, take the form of a domain wall or soliton lattice, with all solitons having the same chirality, whose sign is dictated by DM interaction. Conversely, given an energetically favorable spiral solution, we determine quantitatively how the magnetization profile changes as a function of the applied field.

Emergent magnetic monopoles, disorder, and avalanches in artificial kagome spin ice (invited)

We study artificial spin ice with isolated elongated nanoscale islands arranged in a kagome lattice and solely interacting via long range dipolar fields. The artificial kagome spin ice displays a phenomenology similar to the microscopic pyrochlore system, where excitations at sub-Kelvin temperatures consist of emergent monopole quasiparticles that are connected via a solenoidal flux line, a classical and observable version of the Dirac string. We show that magnetization reversal in kagome spin ice is fundamentally different from the nucleation and extensive domain growth scenario expected for a generic 2D system. Here, the magnetization reverses in a strictly 1D fashion: After nucleation, a monopole-antimonopole dissociates along a 1D path, leaving a (Dirac) string of islands with reversed magnetization in its wake. Since the 2D artificial spin ice spontaneously decays into a 1D subsystem, magnetization reversal in kagome spin ice provides an example of dimensional reduction via frustration.

Enhancement in the Light Output Power of GaN-Based Light-Emitting Diodes with Nanotextured Indium Tin Oxide Layer Using Self-Assembled Cesium Chloride Nanospheres

In this study, enhanced light output power in GaN-based light-emitting diodes (LEDs) with a nanotextured indium tin oxide (ITO) transparent conductive layer was observed. Wafer-scale self-assembled cesium chloride nanospheres were formed on the ITO transparent conductive layer and served as the mask in a dry etching process. After the inductively coupled plasma (ICP) etching process, nanoscale islands were fabricated on the ITO layer. Compared with LEDs with a planar ITO layer, the light output power of LEDs with a nanotextured ITO layer was improved by 23.4%. Optoelectronic measurement showed that the performance of the fabricated LEDs was greatly enhanced.

Amyloid-Beta 1-42 Binds Preferentially to Nanoscale Electrostatic Domains in Cholesterol-Enriched DOPC Lipid Membrane

The plasma membrane is a complex structure, composed primarily of phospholipids and other macromolecules, such as proteins, sterols and steroids. It plays a vital role in cell motility and acts as a barrier for extracellular materials. Amyloid fibrils are linked to multiple neurodegenerative diseases, such as Alzheimer’s. Although fibril plaque formation is associated with biological membranes in vivo, the role of the lipid membrane in fibril formation and toxicity is not well understood. We investigated the interaction of model lipid membranes with Aβ 1-42 peptide. Using Atomic force microscopy, we demonstrated that binding of Aβ 1-42 peptide to DOPC bilayers with 20% cholesterol is non-uniform and resulted in the formation of nanoscale islands loaded with A-beta. We attribute this effect to the presence of electrostatic nanoscale domains induced by cholesterol in the DOPC bilayer. These domains were resolved by AFM imaging of the lipid bilayer and by frequency modulated Kelvin probe force microscopy in the monolayer samples prepared with the Langmuir-Blodgett monolayer technique.

Novel method for fabrication of nanoscale single-electron transistors: Electron beam induced deposition of Pt and atomic layer deposition of tunnel barriers

A novel method for fabricating metal-based single-electron transistors (SETs) is reported that combines a nanoscale island produced via electron beam induced deposition (EBID) of Pt metal with a tunnel barrier dielectric produced via atomic layer deposition (ALD) of alumina (Al2O3). A characteristic nonlinearity in Ids-Vds and Coulomb blockade oscillations of the SET are observed at 300 mK. The versatility of the EBID method provides a way to fabricate 3D structures that could be particularly useful in a number of charge sensing applications and which, combined with the accuracy and precise control that ALD provides for the SET’s tunnel barriers, greatly expands the choice of techniques suitable for SET fabrication.

The atomic force microscope as a mechano-electrochemical pen.

We demonstrate a method that allows the controlled writing of metallic patterns on the nanometer scale using the tip of an atomic force microscope (AFM) as a “mechano–electrochemical pen”. In contrast to previous experiments, no voltage is applied between the AFM tip and the sample surface. Instead, a passivated sample surface is activated locally due to lateral forces between the AFM tip and the sample surface. In this way, the area of tip–sample interaction is narrowly limited by the mechanical contact between tip and sample, and well-defined metallic patterns can be written reproducibly. Nanoscale structures and lines of copper were deposited, and the line widths ranged between 5 nm and 80 nm, depending on the deposition parameters. A procedure for the sequential writing of metallic nanostructures is introduced, based on the understanding of the passivation process. The mechanism of this mechano–electrochemical writing technique is investigated, and the processes of site-selective surface depassivation, deposition, dissolution and repassivation of electrochemically deposited nanoscale metallic islands are studied in detail.

Fabrication of discrete gallium nanoislands on the surface of a Si(001) substrate using afocused ion beam

A gallium focused ion beam (FIB) has been used to implant Ga at specificsites on the surface of undoped Si(001) substrates. Upon annealing at600 °C, discrete nanoscale surface islands form within the FIB patternedregions when the total Ga ion dose, or fluence, is greater than1.0 × 1016 ions cm − 2. The number of islands depends on the size of the irradiated regionand a single island can be achieved for a FIB milled region that is100 nm × 100 nm.The average sizes of the islands were found to range from 24.5 nm when exposed to a total ion dose of1.2 × 1016 ions cm − 2 to 45 nm fora dose of 3.0 × 1016 ions cm − 2. We have confirmed that these surface islands are metallic Ga by performing aselective chemical etch that removes the islands and by transmission electronmicroscopy characterization. These patterned Ga surface templates could serve asnucleation sites for the lateral arrangement of discrete quantum dot structures.

Single electron spintronics

Single electron electronics is now well developed, and allows the manipulation of electrons one-by-one as they tunnel on and off a nanoscale conducting island. In the past decade or so, there have been concerted efforts in several laboratories to construct single electron devices incorporating ferromagnetic components in order to introduce spin functionality. The use of ferromagnetic electrodes with a non-magnetic island can lead to spin accumulation on the island. On the other hand, making the dot also ferromagnetic introduces new physics such as tunnelling magnetoresistance enhancement in the cotunnelling regime and manifestations of the Kondo effect. Such nanoscale islands are also found to have long spin lifetimes. Conventional spintronics makes use of the average spin-polarization of a large ensemble of electrons: this new approach offers the prospect of accessing the quantum properties of the electron, and is a candidate approach to the construction of solid-state spin-based qubits.

Spin valve effect in single-atom contacts

Magnetic single-atom contacts have been controllably fabricated with a scanning tunnelling microscope. A voltage-dependent spin valve effect with conductance variations of ≈40% is reproducibly observed from contacts comprising a Cr-covered tip and Co and Cr atoms on ferromagnetic nanoscale islands on W(110) with opposite magnetization. The spin-dependent conductances are interpreted from first-principles calculations in terms of the orbital character of the relevant electronic states of the junction.

Multi-Order Dynamic Range DNA Sensor Using a Gold Decorated SWCNT Random Network

A novel electrical DNA biosensor is presented, which consists of gold (Au) nanoscale islands and a single-walled carbon nanotube (SWCNT) network on top of a concentric Au electrode array (also referred to as the CGi). The decorated Au islands on the SWCNT network provide ideal docking sites for ss-DNA probe (p-DNA) molecules. They also provide better adhesion between the SWCNT network and the chip substrate. In addition, the concentric electrode gives asymmetric current voltage characteristics in the solution and provides more flexible bias options to the electrodes. The sensor system is applied to a DNA sensor after functionalization with a 25 mer p-DNA (5'-HSC(6)-C(18)-GCCATTCTCACCGGATTCAGTCGTC-3'), hereafter called the [CGi+p-DNA]. The response of the DNA sensor has been measured in both real-time during hybridization with the complementary target ss-DNAs (t-DNA) and the static mode after the hybridization and washing steps. A wide dynamic range from the 100 fM to 1 μM has been achieved from the real-time mode and the static mode. Moreover, it is shown that the sensor system differentiates partially mismatched (single nucleotide polymorphism (SNP), half mismatch, noncomplementary) t-DNA, as well. The [CGi] sensor platform can be easily extended to target specific biological recognition elements such as aptamers or proteins.

Molecular Dynamics Study of the Formation of a Self-Assembled Monolayer on Gold

Molecular dynamics simulations have been used to study the formation of nanoscale islands of self-assembled monolayers (SAMs) starting from alkanethiol molecules initially lying down in a disordered physisorbed layer on gold. These islands form when tens of alkane thiols stand up together within tens of ns after chemisorption begins. The alkane chains in these islands are found to be tilted, and the tilt direction precesses around the center of the island. This precession, together with the packing of the sulfur atoms, signals the formation of a SAM island, occurring prior to the tilting and orientation ordering of the chains.

Dealloying Studies of Cu3Pd Single Crystal Surfaces

Nano-porous metals formed by dealloying have application potential in a variety of fields such as catalysts, actuators, biomedical sensors and fuel cells. Several aspects of formation of porous surface during dealloying have not completely been understood. The Cu-Pd system is interesting as a model system for corrosion as well as a potential catalyst material. The initial dealloying and selective dissolution of single-crystals Cu3Pd (111) and Cu3Pd (100) in 0.1 M H2SO4 in the potential range of 400-800mV has been studied. With in-situ X-ray diffraction (Synchrotron Light), we observed the epitaxial Pd layer peak to appear by increasing the potential close to the critical potential. Ex-Situ atomic force microscopy (AFM) showed the formation of nanoscale islands of Pd (20-50 nm). Our aim in this project is to study the initial steps of dealloying of well-defined Cu-Pd surfaces on the atomic scale and to compare the results to the Cu-Au system.

Analysis of write-head synchronization and adjacent track erasure in bit patterned media using a statistical model

An analysis of the performance of a bit patterned media data storage system composed of nanoscale islands with variations in position and magnetic properties has been carried out. The statistical model of write errors includes adjacent track erasure and error rates are computed according to the down-track synchronization of the write head switching position and cross-track head position variations. Two-dimensional maps of bit error rates reveal that distributions of position and anisotropy have a severe impact on the performance of the system. Results show that head field cross-track gradients needs to be tightly controlled to minimize the effects of adjacent track erasure.

Effect of ammonia gas etching on growth of vertically aligned carbon nanotubes/nanofibers

The etching effect of ammonia (NH 3) on the growth of vertically aligned nanotubes/nanofibers (CNTs) was investigated by direct-current plasma enhanced chemical vapor deposition (DC-PECVD). NH 3 gas etches Ni catalyst layer to form nanoscale islands while NH 3 plasma etches the deposited amorphous carbon. Based on the etching effect of NH 3 gas on Ni catalyst, the differences of growing bundles of CNTs and single strand CNTs were discussed; specifically, the amount of optimal NH 3 gas etching is different between bundles of CNTs and single strand CNTs. In contrast to the CNT carpet growth, the single strand CNT growth requires shorter etching time (5 min) than large catalytic patterns (10 min) since nano dots already form catalyst islands for CNT growth. Through removing the plasma pretreatment process, the damage from being exposed at high temperature substrate occurring during the plasma generation time is minimized. High resolution transmission electron microscopy (HTEM) shows fishbone structure of CNTs grown by PECVD.

Magnetic dipolar ordering on geometrically frustrated brick-shaped lattices

The magnetic ordering of frustrated arrays of nanoscale islands can be strongly influenced by the array patterns. We theoretically present three kinds of artificial geometrically frustrated systems with different brick-shaped geometries, defined as brick-shaped lattices, and investigate their magnetic dipolar ordering at the ground state using the Monte Carlo method. The simulated results show that the magnetic ordering of three brick-shaped frustrated lattices depends strongly on the strength of dipolar interactions, depending on the lattice spacing. It is found that the long-range dipolar interactions tend to suppress the long-range ordered state and induce the short-range quasi-ice state at each vertex of the lattices. In addition, the correlations for neighboring spin pairs are closely related to not only the dipolar coupling strength but also the geometry of the lattices.

The crossover of preferred orientation in heteroepitaxial ZnO/MgO(0 0 1) films

Orientational crossover in heteroepitaxial ZnO/MgO(0 0 1) thin films deposited by radio frequency sputtering was examined by X-ray diffraction and scanning electron microscopy. At the initial stage of growth, highly strained c-domains with the (0 0 0 2) planes aligned to the surface normal were observed. As the film thickness was increased, the growth behavior then changed gradually to strain free a-domains with (1 0 1̄ 0) planes. Both the a- and c-domains showed a ±15° rotated in-plane domain configuration with respect to the MgO substrate. Nano-scale ZnO islands were formed on the sufficiently thick ZnO thin film at the later stages of growth. The orientational crossover might have been caused by interplay between the strain and surface energy.

Scanning tunnelling microscope investigation of the TiSi2 nano-islands on Sr/Si(100) surface

For the investigation of the interface stability of SrTiO3/Sr/Si(100) system during high temperature annealing process, we have grown 1—2 atom layer SrTiO3 ultra-thin film on Sr/Si(100)-2×1 substrate using pulsed laser deposition technique. After annealing, we found that nano-scale islands appear in the surface. These nano-islands show metallic property by scanning tunneling microscopy, and the STM image shows bias voltage dependence of these nano-islands. Oxygen in the oxide reacts with silicon and forms volatile silicon monoxide during vacuum annealing, while Ti atoms in the oxide react with silicon, forming C-54 TiSi2 islands.

Enhanced UV Photon-response of Tin Nano Cluster Loaded- laser Irradiated ZnO Thin film detector

ABSTRACTZinc Oxide (ZnO), II-VI compound semiconductor, is a promising material for ultraviolet (UV) photon sensor applications due to its attractive properties such as good photoconductivity, ease processing at low temperatures and excellent radiation hardness. The rf magnetron sputtering is a suitable deposition technique due to better control over stoichiometry and deposition of uniform film. Studies have shown that the presence of surface defects in ZnO and subsequently their passivation are crucial for enhanced photo-response characteristics, and to obtain the fast response speed. Worldwide efforts are continuing to develop good quality ZnO thin films with novel design structures for realization of an efficient UV photon sensor. In the present work, UV photon sensor is fabricated using a ZnO thin films deposited by rf magnetron sputtering on the corning glass substrate. Photo-response, (Ion/Ioff) of as-grown ZnO film of thickness 100 nm is found to be 3×103 with response time of 90 ms for UV intensity of 140 μW/cm2 (λ = 365 nm). With irradiation on ZnO thin film by pulsed Nd:YAG laser (forth harmonics 266 nm), the sensitivity of the UV sensor is found to enhance. The photo-response increases after laser irradiation to 4x104 with a fast response speed of 35 ms and attributed to the change in surface states and the native defects in the ZnO thin film. Further, enhancement in the ultraviolet (UV) photo-response (8×104) of detector was observed after integrating the nano-scale islands of Sn metal on the surface of laser irradiated ZnO thin film.

Electrical study of trapped charges in nanoscale Ge islands by Kelvin probe force microscopy for nonvolatile memory applications

Isolated Germanium nanoisland on top of silicon dioxide (SiO2) layer has been studied by Kelvin probe force microscopy (KPFM) at room temperature. Different surface potentials between Ge island and SiO2 dielectric layer were directly visualized from the KPFM image. The image contrast greatly increased after electron injection by applying a negative bias of −7 V. The dissipation of injected electrons was evaluated by measuring the surface potential variation due to the leakage of these injected charges. The long retention time of local charges in Ge dot is promising for applications in nonvolatile memories.

Sandwich-type gated mechanical break junctions

We introduce a new device architecture for the independent mechanical and electrostatictuning of nanoscale charge transport. In contrast to previous gated mechanicalbreak junctions with suspended source–drain electrodes, the devices presentedhere prevent an electromechanical tuning of the electrode gap by the gate. Thissignificant improvement originates from a direct deposition of the source and the drainelectrodes on the gate dielectric. The plasma-enhanced native oxide on the aluminumgate electrode enables measurements at gate voltages up to 1.8 V at cryogenictemperatures. Throughout the bending-controlled tuning of the source–drain distance, theelectrical continuity of the gate electrode is maintained. A nanoscale island in theCoulomb blockade regime serves as a first experimental test system for the devices, inwhich the mechanical and electrical control of charge transport is demonstrated.