Self-assembled Nanodots Research Articles

New Magnetic Nanodot Memory with FePt Nanodots

A new magnetic nanodot (MND) memory with FePt nanodots was proposed. The FePt nanodots dispersed in SiO2 insulating film was successfully fabricated by self-assembled nanodot deposition (SAND). The size of the FePt nanodot can be controlled by SAND with a different target area ratio of the FePt pellets area in the SiO2 target. Thermal annealing converts the magnetic properties of the FePt nanodots from antiferromagnetic into high coercivity ferromagnetic without thermal agglomeration. An L10 face-centered tetragonal (fct) FePt MND film was successfully formed which acted as a charge retention layer. Furthermore, the fundamental characteristics of the MND memory were investigated using magnetic metal oxide semiconductor (MOS) capacitor devices.

Flux-pinning characteristics as a function of density of columnar defects comprised of self-assembled nanodots and nanorods in epitaxial YBa 2Cu 3O 7− δ films for coated conductor applications

Pure YBa 2Cu 3O 7− δ (YBCO) and YBCO films with extended columnar defects comprised of self-assembled columns of nanodots and nanorods of BaZrO 3 (BZO), CaZrO 3 (CZO) and 8 mol.% Y 2O 3 stabilized ZrO 2 (YSZ) were prepared by pulsed laser deposition on RABiTS templates of configuration CeO 2/YSZ/Y 2O 3/Ni–W. Effects on increasing the size and density of columnar defects by varying the total volume fraction BZO, CZO or YSZ incorporations within the YBCO film are reported. BZO volume fraction was varied between 0% and 8%, CZO between 0% and 2% and YSZ 0–2%. The critical transition temperature ( T c) was found to linearly decrease with increase in vol.% of BZO, CZO and YSZ, with the steepest decline for CZO additions. Nevertheless, YBCO films with these columnar defects exhibit significantly reduced J c dependence on applied field. At 77 K, best properties are obtained for 1–2 vol.% BZO incorporation in the form of columns of self-assembled nanodots and nanorods in fields of 0.1–1.5 T.

Assembly of Metallic Carbon Nanodots Aligned on a Vicinal Si(111)-7×7 Surface

We report the assembly of carbon nanodots aligned on the sharp step edges of Si(111)-7×7 reconstruction surface. The self-assembled and well-aligned pyramid-like carbon nanodot array performs metallic transport behavior on the p-type silicon substrate and can be erased from the step edges by flash heating at ∼900 °C. The carbon nanodot array has some potential applications for single electron transistors and bionanosensors.

Strong enhancement of flux pinning inYBa2Cu3O7−δ multilayers with columnar defects comprised of self-assembledBaZrO3 nanodots

Multilayer structures comprisingYBa2Cu3O7−δ (YBCO) films with columns of self-assembledBaZrO3 (BZO) nanodotswith interlayers of CeO2 or pure YBa2Cu3O7−δ were grown on rolling-assisted biaxially textured substrates (RABiTSs) usingpulsed laser deposition. A significant enhancement of the critical current density(Jc) was observed for the multilayers compared with a single layer ofYBCO + BZO.Jc variesas Jc∼H−α withα of 0.27 forsingle layer of YBCO + BZO and 0.34 for both multilayered films. Enhancement of pinning in the multilayers isattributed to the presence of columnar defects comprised of self-assembled nanodots ofBZO as well as planar CuO-type stacking defects arising as a result of interfacial reactionsin the multilayers.

Fabrication of Self-Assembled Au Nanodots and Their Applications to Ferroelectric Nanocapacitors

We report on the fabrication of self-assembled Au nanodots by high temperature annealing of Au thin films on SiO2, Pb(Zr,Ti)O3 (PZT) thin films and PbTiO3 nanodots and on their application to ferroelectric nanocapacitors as top electrodes. When Au films 5–20 nm thick were annealed at 500–900 °C, aggregation of Au thin films occurred, and isolated Au nanodots were formed. By annealing Au films 10 nm-thick at 500 °C for 10 min in air, Au nanodots with average lateral sizes of 82 and 122 nm were obtained on SiO2 and PZT, respectively. We have successfully measured piezoelectric hysteresis loops and current–voltage curves of nanocapacitors with Au nanodots with areas of (1.8–7.3)×104 nm2 as the top electrode using a scanning probe microscope. Au nanodots were also fabricated on PbTiO3 nanodots, demonstrating the possibility of fabricating ferroelectric nanocapacitors with self-assembled top electrodes and ferroelectrics.

Magnetic properties of FePt nanodots formed by a self-assembled nanodot deposition method

Fe 50 Pt 50 nanodots dispersed in a SiO2 film (Fe50Pt50 nanodot film) were formed by a self-assembled nanodot deposition (SAND) method in which Fe50Pt50 and SiO2 are cosputtered in a high vacuum rf magnetron sputtering equipment. Fe50Pt50 pellets are laid on a SiO2 target in a sputtering chamber to form the Fe50Pt50 nanodot film in the SAND method. The size and density of Fe50Pt50 nanodot were controlled by changing the ratio of the total area of Fe50Pt50 pellets to that of SiO2 target. The Fe50Pt50 nanodot size decreases and its density increases when the ratio decreases. As-deposited Fe50Pt50 nanodots self-assembled to a face-centered-cubic phase of single-crystal structure. The Fe50Pt50 nanodot films were annealed to evaluate the nanodot size controllability, the magnetic anisotropy, and the thermal stability. Fully ordered L10 face-centered-tetragonal Fe50Pt50 nanodots with high magnetocrystalline anisotropy (Ku≅8.7×107ergs∕cm3) were obtained by in situ annealing at 600°C for 1h in a high vacuum ambience. Furthermore, the Fe50Pt50 nanodot film with a monolayer of Fe50Pt50 nanodots was formed by annealing at 800°C due to the agglomeration of Fe50Pt50 nanodots in the SiO2 film.

Self-assembled Fe nanodots on Ge(001)

The initial nucleation of Fe onto the (2×1) reconstructed Ge(001) surface is studied. Uniformly sized two-dimensional Fe dots are shown to nucleate. They have an apparent corrugation height ∼1.1Å and lateral dimensions ∼12×8Å2. Further to their uniform size, due to their registry with respect to the substrate, they are shown to nucleate on a single equivalent surface site. It is suggested that their stability is due to either a “magic” number effect or adsorbate-substrate interactions. Tunneling I(V) spectra show the dot/Ge(001) contact to exhibit marked current rectifying behavior, which is extremely well confined to the contact area.

Distribution of the magnetic anisotropy energy of an array of self-ordered Co nanodots deposited on vicinalAu(111): X-ray magnetic circular dichroism measurements and theory

The magnetic anisotropy energy (MAE) of a self-assembled Co nanodot array on a Au vicinal surface is studied by x-ray magnetic circular dichroism. The mean dot diameter is $2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, i.e., 120 atoms, and each dot is aligned on a $3.8\ifmmode\times\else\texttimes\fi{}7.2\phantom{\rule{0.3em}{0ex}}{\mathrm{nm}}^{2}$ lattice. The hysteresis cycles have been determined for various temperatures below and above the blocking temperature. The cycles are well fitted by a rate equation model based on the Stoner-Wohlfarth Hamiltonian. Dynamical hysteresis cycles are calculated with this model for various MAE distributions. It is shown that the blocking temperature indicates the highest MAE in the assembly and that the coercive field indicates the mean MAE. In our self-ordered sample, we find a mean MAE of $26\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ per dot and a full width at half maximum of $53\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The determination of experimental distributions from cycles published in the literature generally shows negligible interactions among dots and a MAE distribution broader than the size distribution. The latter is interpreted in terms of atomic relaxations inside the Co dots.

Self-assembled L1 0 FePt thin films for high-density magnetic recording

A new technique, which utilizes the interlayer diffusion, for preparation of self-assembled nanodot magnetic structures has been proposed. L1 0-phase Pt/FeCu and Pt/FeAg films have been successfully synthesized by this technique. Both the coercivity of Pt/FeCu and Pt/FeAg films exhibited, respectively 4.1 and 8.0 kOe in perpendicular direction. Pt/Fe and Pt/FeAg films show positive values, while Pt/FeCu shows negative value in δ m plot. The results indicate that the exchange coupling between the grains has been decoupled in the self-assembled nanodot structure in Pt/FeCu film.

Heteroepitaxy of zinc-blende SiC nano-dots on Si substrate by organometallic ion beam

The self-assembled SiC nano-dots were fabricated on Si(111) substrate at low-temperatures using the organometallic ion beam deposition technique. The single precursor of methylsilicenium ions (SiCH 3 +) with the energy of 100 eV was deposited on Si(111) substrate at 500, 550 and 600 °C. The characteristics of the self-assembled SiC nano-dots were analyzed by reflection high-energy electron diffraction (RHEED), Raman spectroscopy and atomic force microscope (AFM). The RHEED patterns showed that the crystal structure of the SiC nano-dots formed on Si(111) substrate was zinc-blende SiC (3C-SiC) and it was heteroepitaxy. The self-assembled SiC nano-dots were like a dome in shape, and their sizes were the length of 200–300 nm and the height of 10–15 nm. Despite the low-temperature of 500 °C as SiC crystallization the heteroepitaxial SiC nano-dots were fabricated on Si(111) substrate using the organometallic ion beam.

Nanodots and other low-dimensional structures of titanium oxides

Atomically resolved scanning tunnelling microscope (STM) images of low-dimensionalstructures of titanium oxides grown on Ni(110) have been recorded. The overlayerswere prepared by vapour phase deposition of Ti followed by oxidation in1 × 10−7 mbarO2 at 800 K. Arrays of self-assembled TiO nanodots of about8 Å in diameter are observed, which appear to contain five Ti atoms. In addition, rutileTiO2(110) islands are observed, along with TiO(001) terraces and a previously reportedquasi-hexagonal overlayer.

Irradiation-free, columnar defects comprised of self-assemblednanodots and nanorods resulting in strongly enhanced flux-pinning inYBa2Cu3O7−δ films

The development of biaxially textured, second-generation, high-temperaturesuperconducting (HTS) wires is expected to enable most large-scale applicationsof HTS materials, in particular electric-power applications. For many potentialapplications, high critical currents in applied magnetic fields are required. It iswell known that columnar defects generated by irradiating high-temperaturesuperconducting materials with heavy ions significantly enhance the in-field criticalcurrent density. Hence, for over a decade scientists world-wide have sought meansto produce such columnar defects in HTS materials without the expense andcomplexity of ionizing radiation. Using a simple and practically scalable technique,we have succeeded in producing long, nearly continuous vortex pins along thec-axisin YBa2Cu3O7−δ (YBCO), in the form of self-assembled stacks ofBaZrO3 (BZO) nanodots and nanorods. The nanodots and nanorods have a diameter of∼2–3 nm and an areal density (‘matching field’) of 8–10 T for 2 vol.% incorporation ofBaZrO3. In addition, four misfit dislocations around each nanodot or nanorod arealigned and act as extended columnar defects. YBCO films with such defectsexhibit significantly enhanced pinning with less sensitivity to magnetic fieldsH. In particular, at intermediate field values, the current density,Jc, variesas Jc∼H−α,with α∼0.3 rather than the usual values 0.5–0.65. Similar results were also obtained forCaZrO3 (CZO) and YSZ incorporation in the form of nanodots and nanorods within YBCO,indicating the broad applicability of the developed process. The process could also be usedto incorporate self-assembled nanodots and nanorods within matrices of other materials fordifferent applications, such as magnetic materials.

Artificial control of ZnO nanodots by ion-beam nanopatterning

The use of focused ion-beam (FIB) nanopatterning for manipulating self-assembled ZnO nanodots is described. Highly aligned ZnO-nanodot arrays with various periodicities (e.g., 750, 190, and 100 nm) on FIB-nanopatterned SiO2∕Si substrates were prepared by metal-organic chemical-vapor deposition (MOCVD). The artificially assembled ZnO nanodots had an amorphous structure. Ga atoms incorporated into the surface areas of FIB-patterned nanoholes during FIB engraving were found to play an important role in the artificial control of ZnO, resulting in the production of ZnO nanodots on the FIB-nanopatterned areas. The nanodots evolved into single-crystalline dot clusters and rods with increasing MOCVD-growth time. In addition, microphotoluminescence measurements showed that the ZnO-nanodot arrays have low-dimensional quantum characteristics.

Nanostructured carbon-metal composite films

Diamondlike carbon–silver and diamondlike carbon–titanium nanocomposite films with thicknesses greater than one μm were deposited on Ti–6Al–4V alloy substrates using a multicomponent pulsed laser deposition process. Z-contrast scanning transmission electron microscopy (STEM-Z) and electron energy loss spectroscopy (EELS) were used to determine film microstructure and carbon bonding, respectively. Self-assembled silver nanodot arrays were observed in the diamondlike carbon–silver films, and alternating titanium carbide/diamondlike carbon nanolayers were observed in the diamondlike carbon–titanium films. Scratch testing demonstrated good adhesion of diamondlike carbon–metal nanocomposite films to the Ti–6Al–4V alloy. Nanoindentation testing of the diamondlike carbon-metal films demonstrated excellent hardness and modulus values, within the range of 35GPa and 350GPa, respectively. Diamondlike carbon-metal composite films have numerous potential applications, including use in machine tools, medical prostheses, and electronic devices.

GaN nanodot fabrication by implant source growth

By using implant source growth, an ion beam synthesis technique where implanted ions are used as a growth source for nanostructures, we have fabricated self-assembled GaN nanodots on SiO2. The fabrication method consists of implanting Ga ions into a 60nm thick thermally grown SiO2 layer by using a focused ion beam system followed by annealing under an NH3 flux. The morphology, crystal structure, and optical properties of the GaN nanodots are also shown and discussed.

Novel Method for Site-Controlled Surface Nanodot Fabrication by Ion Beam Synthesis

By using a Ga FIB system to spatially control the implantation of Ga into SiO(2) followed by vacuum annealing, we have fabricated self-assembled surface Ga nanodots with a high degree of control of nucleation location. The morphology of the Ga nanodots is closely related to Ga dose, showing a critical dose needed for nucleation that results in Ga nanodot formation just below the surface, while at higher doses Ga nanodots form on the surface as metallic Ga droplets. Possible applications include defining nucleation sites for subsequent growth, use as Ga source for GaN or GaAs quantum dots, or as catalyst for nanowire growth.

New frontiers in thin film growth and nanomaterials

This article reviews recent developments in thin film growth and formation of three-dimensional epitaxial nanostructures. First, we present a unified standard model for thin film epitaxy, where single-crystal films with small and large lattice misfits are grown by a new paradigm of domain matching epitaxy (DME). We define epitaxy as having a fixed orientation rather than the same orientation with respect to the substrate. The DME involves matching of integral multiples of lattice planes (diffracting as well as nondiffracting) between the film and the substrate, and this matching could be different in different directions. The idea of matching planes is derived from the basic fact that during thin film growth, lattice relaxation involves generation of dislocations whose Burgers vectors correspond to missing or extra planes, rather than lattice constants. In the DME framework, the conventional lattice matching epitaxy (LME) becomes a special case where matching of lattice constants results from matching of lattice planes with a relatively small misfit of less than 7 to 8 pct. In large lattice mismatch systems, we show that epitaxial growth of thin films is possible by matching of domains where integral multiples of lattice planes match across the interface. We illustrate this concept with atomic-level details in the TiN/Si(100) with 3/4 matching, the AlN/Si(100) with 4/5 matching, and the ZnO/α-Al2O3(0001) with 6/7 matching of lattice planes across the film/substrate interface. By varying the domain size, which is equal to the integral multiple of lattice planes, in a periodic fashion, it is possible to accommodate additional misfit beyond the perfect domain matching. Thus, we can potentially design epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces as long as there is wetting or finite interatomic interaction across the interface and cores of dislocations do not overlap. In-situ X-ray diffraction studies on initial stages of growth of ZnO films on sapphire correctly identify a compressive stress and a rapid relaxation within one to two monolayers, consistent with the DME framework and the fact that the critical thickness is less than a monolayer. The DME examples ranging from the Ge-Si/Si(100) system with 49/50 matching (2 pct strain) to metal/Si systems with 1/2 matching (50 pct strain) are tabulated, strategies for growing strain-free films by engineering the misfit to be confined near the interface are presented, and the potential for epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces is discussed. In the second part, we discuss the formation of epitaxial nanodots/nanocrystals in crystalline matrices such as MgO and TiN. The formation Ni nanocrystals inside MgO involves lattice misfit ranging from 3.0 to 31.3 pct, and the formation of Ni nanocrystals in TiN has a misfit of about 17 pct. To form epitaxial nanodots, we use DME framework to grow nanodots inside crystalline matrices. Growth characteristics and crystallography of nanodots are controlled to enhance the overall properties of nanostructured materials. We illustrate this for nanostructured magnetic materials where coercivity and blocking temperature can be considerably enhanced by controlling the orientation for magnetic memory and storage applications, among others. In the last part, the DME principles are applied to grow self-assembled epitaxial nanodots using pulsed laser deposition. By controlling the clustering kinetics, it is possible to obtain a uniform distribution of epitaxial nanodots and overcome thermodynamically driven Ostwald ripening. This process allows the formation of epitaxial nanostructures via three-dimensional self-assembly, the “holy grail” of nanostructured materials processing, leading to unique and novel properties.

New frontiers in thin film growth and nanomaterials

This article reviews recent developments in thin film growth and formation of three-dimensional epitaxial nanostructures. First, we present a unified standard model for thin film epitaxy, where single-crystal films with small and large lattice misfits are grown by a new paradigm of domain matching epitaxy (DME). We define epitaxy as having a fixed orientation rather than the same orientation with respect to the substrate. The DME involves matching of integral multiples of lattice planes (diffracting as well as nondiffracting) between the film and the substrate, and this matching could be different in different directions. The idea of matching planes is derived from the basic fact that during thin film growth, lattice relaxation involves generation of dislocations whose Burgers vectors correspond to missing or extra planes, rather than lattice constants. In the DME framework, the conventional lattice matching epitaxy (LME) becomes a special case where matching of lattice constants results from matching of lattice planes with a relatively small misfit of less than 7 to 8 pct. In large lattice mismatch systems, we show that epitaxial growth of thin films is possible by matching of domains where integral multiples of lattice planes match across the interface. We illustrate this concept with atomic-level details in the TiN/Si(100) with 3/4 matching, the AlN/Si(100) with 4/5 matching, and the ZnO/α-Al2O3(0001) with 6/7 matching of lattice planes across the film/substrate interface. By varying the domain size, which is equal to the integral multiple of lattice planes, in a periodic fashion, it is possible to accommodate additional misfit beyond the perfect domain matching. Thus, we can potentially design epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces as long as there is wetting or finite interatomic interaction across the interface and cores of dislocations do not overlap. In-situ X-ray diffraction studies on initial stages of growth of ZnO films on sapphire correctly identify a compressive stress and a rapid relaxation within one to two monolayers, consistent with the DME framework and the fact that the critical thickness is less than a monolayer. The DME examples ranging from the Ge-Si/Si(100) system with 49/50 matching (2 pct strain) to metal/Si systems with 1/2 matching (50 pct strain) are tabulated, strategies for growing strain-free films by engineering the misfit to be confined near the interface are presented, and the potential for epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces is discussed. In the second part, we discuss the formation of epitaxial nanodots/nanocrystals in crystalline matrices such as MgO and TiN. The formation Ni nanocrystals inside MgO involves lattice misfit ranging from 3.0 to 31.3 pct, and the formation of Ni nanocrystals in TiN has a misfit of about 17 pct. To form epitaxial nanodots, we use DME framework to grow nanodots inside crystalline matrices. Growth characteristics and crystallography of nanodots are controlled to enhance the overall properties of nanostructured materials. We illustrate this for nanostructured magnetic materials where coercivity and blocking temperature can be considerably enhanced by controlling the orientation for magnetic memory and storage applications, among others. In the last part, the DME principles are applied to grow self-assembled epitaxial nanodots using pulsed laser deposition. By controlling the clustering kinetics, it is possible to obtain a uniform distribution of epitaxial nanodots and overcome thermodynamically driven Ostwald ripening. This process allows the formation of epitaxial nanostructures via three-dimensional self-assembly, the “holy grail” of nanostructured materials processing, leading to unique and novel properties.

Controlled growth of GaN nanowires by pulsed metalorganic chemical vapor deposition

Controlled and reproducible growth of GaN nanowires is demonstrated by pulsed low-pressure metalorganic chemical vapor deposition. Using self-assembled Ni nanodots as nucleation sites on (0001) sapphire substrates we obtain nanowires of wurtzite-phase GaN with hexagonal cross sections, diameters of about 100nm, and well-controlled length. The nanowires are highly oriented and perpendicular to the growth surface. The wires have excellent structural and optical properties, as determined by x-ray diffraction, cathodoluminescence, and Raman scattering. The x-ray measurements show that the nanowires are under a complex strain state consistent with a superposition of hydrostatic and biaxial components.

Self-assembled FePt nanodot arrays with mono-dispersion and -orientation

For self-assembled nanodots, the ultimate dream is to simultaneously achieve tunable uniformity in size, spatial distribution, chemical composition, and crystallographic orientation. By utilizing the Volmer–Weber growth mode in thin film epitaxy, we have grown self-assembled two-dimensional arrays of FePt alloy nanodots that are uniform in size, chemical composition, and are all crystallgraphically aligned. These dot assemblies are ferromagnetic at room temperature and can be easily transferred onto other templates without destroying the size and orientation uniformity.