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We report the voltage regulation of electrodeposited elliptical magnetostrictive Ni nanodot arrays from single-domain to nonvolatile vortex state at room temperature. On the piezoelectric substrate, isolated elliptical Ni nanodots are fabricated between a pair of square electrodes, with the long axis parallel to the joint line of the electrodes. By applying a voltage to the surface electrodes pair, local stress is generated to induce the magnetization of the nanodots from single-domain to vortex state. The magnetization state of the nanodots is characterized by a magnetic force microscope. Even after the voltage is removed or applying another voltage, the nanodots maintain a stable vortex magnetization state, which shows that the vortex state after regulation is nonvolatile. These results are of great significance for the study of the low-energy-consumption regulation of the nano-dimensional magnetic material and vortex state-based nonvolatile memory.

The antiferromagnetic spintronics is attracting intensive attention due to the recent progress of antiferromagnetism. Especially, the topological magnetic structures, discovered in the antiferromagnetic materials, have opened an alternative path for energy efficient information processing, e.g., constructing logical devices. Herein, we theoretically extend the building block of spintronics devices by utilizing the antiferromagnetic-antimeron, which can be formed in ferromagnet nanodots with gain. A rotational mode of the antiferromagnetic-antimeron is observed under the injection of spin polarized current that originates from the excitation of spin waves propagating along the radial normal of the nanodots. The frequency of rotation (f) can be analytically calculated, expressed in the form of f ∝ k J α, where k is a proportionality coefficient, and J and α are the current density and Gilbert damping, respectively. The value of f is proportional to the current density (J), but inversely proportional to α. Finally, the logical gates based on the antiferromagnetic-antimeron are proposed for Boolean logic operation of OR, XOR, XNOR, and AND. Utilizing the unique dynamic behaviors of the antiferromagnet-antimeron under the injection of current sheds light on the development of the building block of spintronics devices.

We study the interplay of superconductivity, ferromagnetism, and the Kondo effect in a single system, using vertical geometry and planar magnetic tunnel junction devices, in which a thin CoFe layer is inserted in the middle of the MgO layer, forming a quantum dot like system. It is shown that the Kondo resonance peak at the zero bias coexists with a sharp Bardeen-Cooper-Schrieffer gap on double tunnel junctions, Al/MgO/CoFe nanodot/MgO/Al. It is also found that the competition between superconductivity and the Kondo effect is tunable with magnetic fields and the temperature. The coexistence of Kondo screening and superconductivity survives long range magnetic order in CoFe nanodots with a spin polarization of 0.2; however, it disappears when the CoFe layer becomes a continuous film with a spin polarization of 0.5. The competition between SC and the Kondo effect in the presence of magnetic ordering opens exciting possibilities to control information in nanomagnets.

HfO2 nanodots and epitaxial HfO2 thin films with ferroelectric properties were grown on yttria-stabilized zirconia substrates with indium tin oxide bottom electrodes via a pulsed laser deposition (PLD) method. The crystallinity and ferroelectric properties of the epitaxial HfO2 thin films were analyzed to compare ferroelectric properties resulting from crystallinity and the strained structure of HfO2 nanodots. Using the total number of PLD pulses as a control, it was possible to grow HfO2 nanodots with diameters of 7, 12, and 20 nm and heights of 4.9, 7.8, and 14.8 nm. Based on the d33 piezoelectric hysteresis loops and polarization switching phenomena, it was confirmed that the HfO2 nanodots exhibited good ferroelectric properties even for scaling of less than 7 nm in diameter. The observation of the piezoelectric d33 hysteresis loop of the HfO2 nanodots and epitaxial HfO2 thin films revealed that HfO2 nanodots had improved ferroelectric properties due to the size effect.

We report the development of an efficient imprinting process for the formation of metal (Au) nanodot arrays using a square-patterned medium substrate. Solid-state dewetting is induced by differences in the surface energy of the metal film and the interface energy between the substrate and the metal film. Thus, uniform metal nanodot arrays were transferred to the desired substrate by controlling the interfacial surface free energy between the metal film and the substrate. Optical extinction measurements showed an intense extinction peak at 550 nm, corresponding to the simulated result. Imprinting of the Au-nanodot arrays on the substrate enhanced the light trapping function and supported the electrical properties of a polymer electrode. In addition, the combination of a transparent conducting oxide-free device with the Au-nanodot arrays and a polymer electrode resulted in enhanced performance. This can be attributed to the fact that the Au-nanodot arrays allowed higher charge extraction, as confirmed by electrical analyses. Finally, a next-generation approach of imprinting metal nanodot arrays was introduced through the controlled solid-state dewetting mechanism in a specific area, which can be applicable not only in the development of optoelectronic devices but also in semiconductor processes requiring metal nanostructures.

We study the dynamic susceptibility of skyrmion clusters in Co/Pt nanodots using micromagnetic simulations. For this, we investigate the dynamic response of the skyrmions to a small magnetic field pulse applied parallel to the dot axis. Skyrmions exhibit a main resonance peak associated with the skyrmions' breathing mode, which is closely linked to the average radius of the skyrmions. Furthermore, we find that non-trivial secondary modes appear when the cluster size increases. These results can be used as a detection and characterization mechanism for skyrmion lattices in confined systems.

High crystallinity Si films containing silicide nanodots (NDs) were epitaxially grown on Si substrates at high temperature (∼750 °C), where the silicide phase of NDs (metallic α-FeSi2 or semiconductor β-FeSi2) was selectable by tuning the Fe deposition amount. The high crystallinity high-temperature-grown Si films with NDs exhibited lower thermal conductivity (5.4 W m−1 K−1) due to the phonon scattering at the ultrasmall ND interfaces than bulk Si-silicide nanocomposites that have ever been reported. In this ND system with extremely low thermal conductivity, due to the less point defects and high quality ND interface, the thermoelectric power factor (∼28 μW cm−1 K−2) was observed to be the same as the high value of Si films without NDs at room temperature, which is the highest value among Si-silicide bulk nanocomposites ever reported. The simultaneous achievement of a high power factor and low thermal conductivity in the high quality ND system will provide the key for design of high thermoelectric performance of Si-based nanostructured films.

We have systematically explored the magnetoelectric (ME) coupling effect of Co/BiFeO3 multiferroic heterostructured nanodot arrays, fabricated by the anodic aluminum oxide template method. Piezoresponse hysteresis loops of these nanodots demonstrate a significant enhancement of the ME coupling effect. More intriguingly, we have realized a magnetic domain transformation from an initial single-domain state to a vortex state by applying a regional or local voltage, and the single-domain state can be recovered by using an external in-plane magnetic field. Our results will guide the invention of high-density, energy-efficient, non-volatile multifunctional ME microdevices.We have systematically explored the magnetoelectric (ME) coupling effect of Co/BiFeO3 multiferroic heterostructured nanodot arrays, fabricated by the anodic aluminum oxide template method. Piezoresponse hysteresis loops of these nanodots demonstrate a significant enhancement of the ME coupling effect. More intriguingly, we have realized a magnetic domain transformation from an initial single-domain state to a vortex state by applying a regional or local voltage, and the single-domain state can be recovered by using an external in-plane magnetic field. Our results will guide the invention of high-density, energy-efficient, non-volatile multifunctional ME microdevices.

Spin-orbit torque-induced switching of an elliptical nanomagnet with an in-plane easy axis allows sub-ns and field-free operation. Since its properties crucially depend on the design of the nanomagnet such as the easy-axis direction, it is of high importance to systematically elucidate the dependence of performance on various parameters of the nanomagnet towards magnetoresistive random access memory applications. Here, we show a scheme to statistically evaluate the switching properties of in-plane nanomagnets in a short turnaround time. We use devices with an array of CoFeB/MgO nanomagnets formed on a cross-shaped Ta/W Hall bar, and the differential planar Hall resistance is measured to study the magnetization switching. Using the scheme, we investigate the easy-axis angle dependence of switching properties at zero magnetic fields for various current pulse widths from 100 ms to 1.7 ns. We show that the dependence of threshold switching current on the easy-axis direction significantly varies with the pulse width.

Through utilizing individual nanodots, as smallest memory units, to convert stress into readable electronic information, we report force-induced high-density data storage in the periodic nanostructure of ferroelectric polymer fabricated by nanoimprinting lithography. The nanostructure is ideal for the stress concentration and increasing the non-uniformity of strains, thus leading to strain gradients inversely proportional to the feature size. A force as low as 400 nN is applied to generate the internal electric field in response to strain gradients and switch polarization state of each memory unit. It can achieve storage density theoretically up to TB/inch2 with the up-to-date nanofabrication technology to miniaturize the unit size.

We present a ferromagnetic resonance (FMR) spin pumping experiment at room temperature in periodic arrays of permalloy nanodots of different radii deposited onto a 3D topological insulator Bi2Se3 film. We measure the dc voltage signal generated by spin-to-charge conversion of the pumped spin current due to the spin-orbit coupling in the bulk of Bi2Se3. In the nanostructured samples, two resonance peaks are observed, associated with Kittel and inhomogeneous edge modes, respectively. This more complex modal composition in comparison to continuous systems may provide additional advantages for development of prospective spintronic devices. We support our experimental results by theoretical calculations, which are based on micromagnetic modeling of the magnetization dynamics under FMR excitation in a nanodot. A numerical approach to the calculation of the spin-pumping voltage is proposed, and the efficiency of spin-to-charge conversion is estimated for two nanostructured samples with different dot sizes.

We designed and fabricated double stack FePt-nanodots (FePt-NDs) with different coercivities between the lower and upper NDs, in which the FePt-NDs were formed by exposing Pt/Fe bilayer stacks of different thicknesses to a remote H2 plasma. The double-stacked NDs show a clear change in their resistive state depending on the relative directions of magnetization between the upper and lower dots. From the high on/off current ratio between the high and low resistive states observed in the current-voltage characteristics, the FePt-NDs can be regarded as a spin-diode gate.

Carbon nanomaterials with variable bandgaps exhibit wide spectral absorption, and photoluminescent nanodots have attracted much interest. In this work, carbon nanodots (CNDs) are grafted onto the surface of TiO2 nanotubes to enhance the photocatalytic properties. The CNDs increase light absorption, trap and shuttle photo-generated electrons, and enhance the pollutant adsorptivity. In addition, the synergistic photothermal effect of the CNDs-based nanocomposite facilitates photocatalysis. The CNDs-based nanocomposites with improved photothermal performance and efficient photocatalytic characteristics have large potential in environment and energy applications.

International audience; Since a few years there have been many efforts for studying solid state dewetting that means the spontaneous agglomeration of a metastable film into an assembly of 3D crystallites [1–3]. Since the size of the so-formed 3D crystallites depends on the initial thickness of the film [3–6], solid state dewetting of ultra-thin films is a promising method for producing nanodots [7, 8]. Many studies thus focused on the effect of the film thickness on the dewetting morphologies. Generally thicker films dewet by void nucleation and opening process forming then well-organized 3D islands [1–3]. In contrast to this thinner films tend to form more complex structures [9–14]. For instance Kinetic Monte Carlo simulations have shown that a 3 ML-thick film dewets by void nucleation whereas a 1 ML-thick film dewets by forming a labyrinthine pattern of bilayer islands [10, 11]. In the literature the specific behavior of the thinner films has been attributed to barrierless void nucleation [15], spinodal dewetting (enhanced sensitivity to thickness or thermal fluctuations) [16], short range effect of the wetting potential [17, 18] or local stresses [14, 19]. Recently we investigated dewetting properties of Silicon-on-Insulator (SOI) samples capped by a chemically-prepared oxide layer. We thus reported that films thicker than 8 nm dewet in four steps and break down into self-organized 3D crystals whereas films thinner than 6 nm dewet by labyrinthine formation [9]. In this paper we show that this thin film effect is actually due to a subtle interplay between film deoxidation and film dewetting. Indeed the temperature at which a film dewets (T dew (h)) depends on its thickness h whereas the deoxidation temperature (T deox) is a constant. We thus show that if T dew (h) > T deox SOI dewetting occurs by void opening and leads to 3D crystals whereas if T dew (h) < T deox SOI dewetting leads to labyrinthine morphologies. In other words, the dewetting of the thicker films does not depend on the oxide cap (that decomposes before dewetting and thus only depends on the surface energy gain due to the disappearance of the Si surface with respect to the uncovered bare substrate) while the dewetting of the thinner films depends on the deoxidation (since only the clean deoxidized parts of the silicon film can dewet). We use LEEM to investigate in-situ and in real time the decomposition of the oxide as well as the dewetting of the SOI films. We define the deoxidation temperature T deox as the temperature at which LEEM enables to see void opening in the oxide cap layer with subsequent silicon film appearance. In a similar manner, the dewet-ting temperature T dew is defined as the temperature at which the amorphous SiO 2 substrate starts becoming visible by retraction of the silicon film. Obviously both temperatures T deox and T dew do not correspond to phase transitions and thus depend on the observation means. However since measured in the same experimental conditions they can be compared without any ambiguity. The SOI samples used in the experiments (12 nm Si on top of 25 nm SiO 2) were prepared as described elsewhere [20]. By means of repeated etching and oxidation cycles a stepwise thinning of the samples is possible to produce films of a certain thickness with a very high accuracy ending with a final oxidation step. Since this protective oxide layer is produced with the same wet chemical oxidation for each Si film thickness the thickness of the oxide is the same for all samples. Ellipsometry measurements have proven that the oxide thickness is constant with approximately 1 nm. FIG. 1. Temperature at which the initial deoxidation (black) and dewetting (red) was observed depending on the Si film thickness. Dewetting velocities are in the range of 1 to 8 nm s. The data point at 21.6 nm was obtained using a SOI sample of 22 nm Si on top of 150 nm SiO2. In Figure 1 are thus reported T deox and T dew as a function of the Silicon film thickness. As expected, the decomposition temperature of the capping oxide does not depend on the thickness of the silicon film and is rather constant at T deox =760 • C whereas T dew decreases as h n with n = 0.08 ± 0.01 comparable to [21, 22] where exponents n = 0.15 ± 0.01 have been reported. In the following we will report dewetting morphologies

We demonstrate microwave-assisted magnetization switching of a perpendicular magnetic nanodot in a microwave stray field from a spin-torque oscillator (STO). The switching field decreases when the STO is operated by applying a current. The switching field reduction is almost the same as that in a microwave magnetic field generated by a signal generator despite the fluctuations of the STO oscillation. The switching field distribution, however, is broader when the STO is used. We also examine the magnetization switching process in the nanosecond region by applying a nanosecond-order pulse current to the STO and measuring the STO signal waveform. The onset of the STO oscillation and subsequent assisted switching occur within a few nanoseconds.

A micromagnetic and elastodynamic finite element model is used to compare the 180° out-of-plane magnetic switching behavior of CoFeB and Terfenol-D nanodots with perpendicular magnetic easy axes. The systems simulated here consist of 50 nm diameter nanodots on top of a 100 nm-thick PZT (Pby[ZrxTi1-x]O3) thin film, which is attached to a Si substrate. This allows voltage pulses to induce strain-mediated magnetic switching in a magnetic field free environment. Coherent and incoherent switching behaviors are observed in both CoFeB and Terfenol nanodots, with incoherent flipping associated with larger or faster applied switching voltages. The energy to flip a Terfenol-D memory element is an ultralow 22 aJ, which is 3–4 orders more efficient than spin-transfer-torque. Consecutive switching is also demonstrated by applying sequential 2.8 V voltage pulses to a CoFeB nanodot system with switching times as low as 0.2 ns.

To realize the desired zero-dimensional behavior of a quantum dot ensemble, the ability to fabricate quantum dots with a high packing density and a high degree of size, shape, and spacing uniformity is crucial. Here we report highly ordered InAs nanodot arrays grown by molecular-beam epitaxy on nonlithographically nanopatterned GaAs. Approximately 20 billion dots are grown in a 1cm2 area with the smallest size dispersion ever reported and forming a lateral superlattice in hexagonal dense packing form. These techniques presage a pathway to controlled growth of periodic quantum dot superstructures, which offer macroscopic spatial coherence in the interaction of quantum dots with radiation.

We produced single-crystalline Fe nanodot arrays grown in situ on a W(110) substrate in an ultrahigh vacuum system. An alumina shadow mask with perfectly ordered holes was used for Fe deposition. Polycrystalline Fe nanodots could be crystallized to single-domain nanodots by thermal annealing. After annealing, Fe wets tungsten substrate with one monolayer, but Fe islands neither coalesce nor form an alloy with the substrate.

A silicon substrate, prestructured by a focused ion beam, is used as a nano-electrode template to perform a selective electrodeposition of cobalt nanodots. It is found that the shape of the magnetic nanostructures determines to a large extent their magnetic properties. Using magnetic force microscopy, a transition from in-plane to out-of-plane single domain magnetization is observed when the diameter of the 400nm height dots is reduced from 400 to 200nm. Additional ferromagnetic resonance measurements confirm that the magnetic anisotropy is imposed mainly by the shape of the cylindrical dots.

Ordered nanodot arrays of titanium oxide (TiO2) were prepared from an epitaxial Al/TiN bilayered film on a sapphire substrate by electrochemical anodization of the TiN layer using a nanoporous anodic aluminum oxide (AAO) film as the template. The nanodots with an average diameter of about 60 nm can faithfully duplicate the size, shape, and hexagonal pore pattern of the AAO nanopores. The phase development of the isolated TiO2 nanodots is very much different from TiO2 thin films and powders. After high temperature annealing, the nanodots are polycrystalline and consist of a mixed phase of anatase and rutile instead of single rutile phase. We expect that TiO2 nanodots with a single phase of anatase can be realized as long as the size of the nanodots is smaller than the critical nuclei size for rutile formation.

A nonlithographic second-order self-assembly process for synthesizing uniform and ordered arrays of nanorods and nanodots is presented and applied to the fabrication of ZnO nanorod arrays. Nucleation sites were defined by patterning Au nanodot catalysts with a self-organized array of nanopores formed in anodized aluminum oxide (AAO). The self-assembled vertically aligned ZnO nanorods grown on GaN exhibit hexagonal facets, and have a uniform diameter of 60 nm and a mean length of 400 nm. The growth technique is simple, robust, and offers a direct control over array and single nanorod configurations. The growth temperature is significantly lower than normal, and yet, the resultant defect level is much lower than normal.

In this study, a distributed charge storage with GeO2 nanodots is demonstrated. The mean size and aerial density of the nanodots embedded in SiO2 are estimated to be about 5.5 nm and 4.3×1011 cm−2, respectively. The composition of the dots is also confirmed to be GeO2 by x-ray absorption near-edge structure analyses. A significant memory effect is observed through the electrical measurements. Under the low voltage operation of 5 V, the memory window is estimated to ∼0.45 V. Also, a physical model is proposed to demonstrate the charge storage effect through the interfacial traps of GeO2 nanodots.

We show that strain distribution on the surface of an isotropic spacer layer induced by an embedded island of large base-to-height aspect ratio deviates significantly from the description of the point force dipole model in the regime of small spacer layer thickness. In this regime, the strain profile displays several local maxima above the embedded island. The regions with local strain maxima serve as nucleation centers for growth of surface islands under appropriate growth conditions, resulting in locally well-organized surface islands above the embedded island. Our theoretical results are in excellent agreement with recent experiments for Ge islands embedded in Si.

Amplitude modulated electrostatic lithography using atomic force microscopy (AFM) on 20–50 nm thin polymer films is discussed. Electric bias of AFM tip increases the distance over which the surface influences the oscillation amplitude of an AFM cantilever, providing a process window to control tip-film separation. Arrays of nanodots, as small as 10–50 nm wide by 1–10 nm high are created via a localized Joule heating of a small fraction of polymer above the glass transition temperature, followed by electrostatic attraction of the polarized viscoelastic polymer melt toward the AFM tip in the strong (108–109 V/m) nonuniform electric field.

Selective formation of ZnO nanodots was accomplished by metalorganic chemical vapor deposition on nanopatterned SiO2/Si substrates. Self-organized ZnO nanodots were selectively formed in nanopatterned lines of Si created by etching of SiO2 with focused ion beam (FIB), whereas any nanodots were hardly observed on the SiO2 surface in the vicinity of the FIB-sputtered Si areas. The mechanism of the selective formation of ZnO nanodots on FIB-nanopatterned lines is mainly attributed to the effective migration of Zn adatoms diffusing on the SiO2 surface into the Si lines followed by the nucleation at surface atomic steps and kinks created by Ga+ ion sputtering. Cathodoluminescence measurements confirmed that the emission originated from the selectively grown ZnO nanodots.

Crystalline titanium nanodots have been formed in the subsurface layer of single-crystal SiO2, i.e., a Ti-based metal quantum dot composite, by subplantation of 9 keV Ti+ ions. Transmission electron microscopy images show that the Ti nanodots have a single, uniform size distribution of ∼3–4 nm, they are single crystals of mainly the Ti bcc β-phase, and their position in the subsurface is controllable through the ion energy. The unique features of subplantation for promoting the precipitation/clustering of crystalline Ti nanodots are discussed. These results confirm previous findings based on the linear optical properties of Ti in SiO2.

A double-step annealing profile has been used to synthesize germanium nanodots embedded in silicon oxide matrix with low defects, good crystallinity, good size distribution, and shape. A significant reduction in the photoluminescence was observed for samples annealed at temperature higher than 900 °C. The improved crystallinity of the nanodots synthesized via the double-step annealing process was investigated using Raman spectroscopy and transmission electron microscopy diffraction patterns. A mechanism of growth at different annealing temperature profiles is proposed. The optimum annealing profile was 1000 °C for 300 s followed by 700 °C for 60 s. The mean diameter of the dots at such annealing condition was found to be 9.5±1.6 nm with an area density of ∼5×1011 cm−2.

Self-assembled NiSi quantum-dot arrays have been grown on relaxed epitaxial Si0.7Ge0.3 on(001)Si. The formation of the one-dimensional ordered structure is attributed to the nucleation of NiSi nanodots on the surface undulations induced by step bunching on the surface of SiGe film owing to the miscut of the wafers from normal to the (001)Si direction. The two-dimensional pseudohexagonal structure was achieved under the influence of repulsive stress between nanodots. Since the periodicity of surface bunching can be tuned with appropriate vicinality and misfit, the undulated templates promise to facilitate the growth of ordered silicide quantum dots with selected periodicity and size.

Computed dc transport in compressed arrays of metallic quantum dots exhibits a voltage-induced phase transition at low temperatures. The transition is seen in the temperature dependence of the conductance at different voltages: from a variable hopping dependence at low voltage to an ohmic, activated behavior at higher voltages. The computations also exhibit the transition as a break in the current versus voltage plots at low temperatures where, at higher voltages, the plot is linear. At higher temperatures, the conductance is ohmic. A many-electron basis is used. The same transition is seen in the surface potential contours.

A simple fabrication of gate-controlled nanodevices made of the Co nanodots on tungsten (W)-nanowire templates by using conventional photolithography and a thin film technique is reported. The combined multiple grain nanobridge allows the observation of the Coulomb gap up to a temperature as high as 200 K and shows nonlinear current–voltage characteristics up to 250 K. The combination of Co dots with straight W-nanowire templates opens possibilities of reproducible blockade devices.

Ordered nanodot arrays of titanium oxides were prepared from TiN/Al films on the silicon substrate by electrochemical anodization of a TiN layer using a nanoporous anodic aluminum oxide film as the template. The microstructure of the nanodot arrays was studied by transmission electron microscopy and scanning electron microscopy, and the chemical composition of nanodots was analyzed by electron energy loss spectroscopy. The as-prepared nanodots are basically composed of amorphous TiOx with a hexagonal arrangement and an average diameter of about 60 nm. Using this approach, it is expected that nanodot arrays of various oxide semiconductors can be achieved.

A technique for reproducible fabrication of nanometer-scale silver dots, wires, characters, and figures on a Si(111) surface in ultrahigh vacuum using scanning tunneling microscopy and their electronic properties are presented. This method allows patterns to be formed at will by the computer-controlled application of voltage pulses between a silver-coated tip and the surface. Scanning tunneling spectroscopy shows unambiguous metallic behavior on the nanodots and depletion layer formation in the neighboring region. The dominant mechanism of nanodot formation can be attributed to the spontaneous formation of a point contact due to field-enhanced diffusion of silver atoms to the tip apex.

Grazing-incidence small-angle scattering (GI-SAXS) technique was applied to self-organized Ge islands capped with Si. The average size and height as well as the average distance between Ge islands were obtained. The present results demonstrate that GI-SAXS is a useful tool to examine the structure of self-organized semiconducting nanodots even if they are covered with a cap layer, and a simple kinematical approach can be safely used under some conditions.

Highly ordered arrays of nanosized GaAs-based dots were successfully prepared on GaAs (001) substrates by molecular-beam epitaxy using selected area growth. Selected area growth employed alumina nanochannel array (NCA) templates formed by anodic oxidation, bonded to the GaAs substrates. Homogeneous GaAs dots, as well as compositionally modulated heterostructures within the nanosized dots, were demonstrated. In the latter case, multilayer InGaAs/GaAs heterostructured nanodot arrays were fabricated. Dot growth occurred only as defined by the template mask, resulting in a hexagonal lattice of dots with 100 nm period spacing, with dots retaining the circular lateral shape of the pores as determined by the NCA template pore size; dot diameters were adjustable from 45 to 85 nm for a lattice period of 100 nm. Cathodoluminescence spectra from an InGaAs/GaAs 10×10 dot array clearly showed an emission peak at 920 nm (5 K), confirming the formation of a high-quality InGaAs/GaAs quantum dot array.

From frequency-dependent ac susceptibility studies of (Cu,T1)BaSrCa2Cu3Oy superconducting thin films, with and without nanodot-induced artificial pinning centers, we estimated the activation energy of flux jumps. The result was that, in the film with nanodots, the pinning potential is several times higher, leading to a probability of thermally activated flux jumps several orders of magnitude lower than in the film without artificial pinning centers. We suggest that our no cost straightforward method for creating extended defects can be successfully employed for the reduction of thermal noise in superconducting electronic devices.

The production of self-organized, spatially dense nanodots on a MgO(100) surface by Ar+ bombardment at elevated sample temperatures (700 and 1000 °C) and ion energies in the range 0.5–5 keV is reported. The results show that at elevated temperatures, the size of the nanodots can be modified by varying the ion energy in a well-defined manner. The nanodot diameter is found to increase with increasing temperature and decrease with increasing ion flux and increasing ion energy. The dot diameter is several tens of nanometers and the height is several nanometers. These results can be interpreted in terms of a recently developed theoretical model.

Atomic structure of FeSi2 or CoSi2 grown on the Si(100) 2×n surface has been investigated by scanning tunneling microscopy. After annealing the Fe or Co covered Si(100) 2×n substrate at ∼800 °C, an ordered adatom vacancy array appears on the nominal 1×1 surface of the formed FeSi2 or CoSi2 islands, which has not been observed for silicide on the Si(100)–2×1. Upon further annealing to ∼1100 °C, the vacancies coalesce into striped domains along one of the 〈011〉 directions. These nanostructured features are a result of the Ni impurities, and can be a promising template for fabricating nanodot arrays.

A straightforward and cheap method for creating extended defects, strong pinning centers, in superconducting thin films is proposed. Clearly, by very short time (3–5 s) rf sputtering at suitable substrate temperatures, we deposited Ag nanodots on SrTiO3 substrates prior to the growth of superconducting thin films. The nanodots were studied by atomic force microscopy. Due to the lattice mismatch and/or chemical poisoning, on top of the nanodots the superconducting phase does not form, creating in this way extended and effective pinning centers which increase the critical current density of the film. The method was applied to (Cu, Tl)BaSrCa2Cu3Oy films grown by amorphous phase epitaxy. Thin films grown in similar conditions, with and without nanodots, were characterized by x-ray diffraction and ac susceptibility. The results show that the nanodots increased the critical current density more than one order of magnitude.

We report on the near-field optical patterning of photochromic sol–gel films with subwavelength resolution. The sample containing functionalized azobenzene species is locally illuminated in the visible absorption band of these photochromes through the aperture of a metallized tapered optical fiber. The surface topography imaged by in situ shear-force microscopy reveals that, due to repeated photoisomerization cycles of the azobenzene molecules, photoinduced matter migration occurs under the tip leading to the formation of a surface relief. The shape of this structure is characteristic of the electromagnetic field distribution and strongly depends on the tip-to-sample distance. In near-field illumination conditions, protrusions of lateral dimension as small as 60 nm (≈λ/10) are currently produced. When repeating this process, compact arrays of nanodots are optically inscribed.

We report on the bimodal distribution and long-range ordering of GeAs nanodots obtained in strain-relaxed epitaxial Si0.5Ge0.5 alloy layers after arsenic implantation and rapid thermal annealing. GeAs dots of two different average sizes around 15 and 55 nm are found after high temperature rapid thermal annealing. The larger dots are of elliptical shape and located at the surface region; they are distributed preferably along 〈110〉 directions which correlates well with the observed cross-hatch pattern. The origin of the bimodal precipitate distribution as well as of the long-range ordering effect of the GeAs nanodots is discussed in terms of strain-induced nucleation and diffusion-limited growth.

Periodic magnetic nanodot arrays have been produced on an area as large as 1 cm×1 cm by direct nanolithography using interferometric laser radiation. The dots are formed by the local annealing of sputtered amorphous Co–C films in regions where the laser intensity is highest. At room temperature the dots exhibit ferromagnetic order and are embedded in a paramagnetic matrix. The onset of room-temperature ferromagnetism is caused by nanoscale chemical and morphological changes during dot formation and reflects the phase separation of magnetic Co-rich clusters. The present single-step nanolithography is potentially an efficient method for fabrication of patterned magnetic arrays.

We have investigated the processing and properties of La2/3Ca1/3MnO3 self-assembled nanodots formed in a nonmagnetic alumina matrix, which were produced by a pulsed-laser deposition process. The size of the nanodots was found to be in the range of 10–15 nm using high-resolution transmission electron microcopy. The average interlayer separation between two dots has been found to be 2–5 nm, which is sufficient to decouple the magnetic grains. The decoupling of the grains is supported by the zero-field-cooled and field-cooled magnetization (M) data. The coercivity of the La2/3Ca1/3MnO3 nanodots has been measured using magnetization measurements as a function of field (H) at different temperatures above and below the blocking temperature of the samples. The coercivity is found to vary from 600 Oe at 10 K to 400 and 200 Oe at 20 and 50 K, respectively. Above the blocking temperature, the sample is found to transform to a superparamagnetic magnetic state, resulting in the disappearance of any hysteresis in the ...

Complete oxidation of the (√3×√3)R30° Sn/Pt(111) surface alloy or submonolayer amounts of Sn adatoms on Pt(111) under ultrahigh vacuum conditions, forms a highly ordered, lateral superlattice of SnOx islands on the Pt(111) substrate. The island superstructure exhibits a sharp (5×5) low energy electron diffraction pattern. Scanning tunneling microscopy images show islands arranged in a hexagonal lattice, uniformly distributed over the whole sample. This island array is thermally stable up to 1050 K. The coincidence of the island periodicity with a multiple of the supporting substrate, and the same hexagonal symmetry of islands and substrate, suggests a strong island–substrate interaction. We propose that the island formation results from the breakup of a strained SnOx adlayer.

This letter describes the memory effect of an AlGaAs/GaAs heterojunction field-effect transistor that contains InAs nanodots in the barrier layer. The device experiences a shift of threshold gate voltage, as a function of the amount of the electrons trapped in the nanodots. These trapped electrons can be injected by applying a positive gate voltage and be erased by a visible light illumination at negative gate bias. Although the shift of the threshold gate voltage volatilizes with the time after the memory programing operation, a considerable part of the shift is retained even after 100 h at room temperature.

Alternating layers of Ge quantum dots embedded in either Al2O3 or AlN matrices were deposited on sapphire substrates by pulsed-laser deposition. The characteristics of the dots are shown to be independent of the surrounding matrix. The dots size (73, 130, 160, and 260 A±5%) was controlled by the laser energy density and deposition time, and was characterized by high-resolution transmission electron microscopy. The dots were single crystalline with no apparent GeOx interfacial layers. Transmission spectroscopy at room temperature and 77 K was used to probe the above-band-edge absorption of the Ge nanodots. The spectral positions of both E1/E1+Δ1 and E2 transitions were found to shift to higher energy in the absorption spectra with decreasing nanodot sizes. This indicates that strong quantum-confinement effect permits the optical properties of Ge dots to be modified in a controlled manner.

Using a platinum tip of a scanning tunneling microscope, ultrasmall nanodots of about 1 to 2 nm in diameter are reproducibly created on the Si(111)7×7 surface at room temperature by applying appropriate voltage pulses between tip and sample. Dynamic processes of the nanodot formation have been monitored by means of the displacement of the tip and the current through the tip. The experimental data suggest that a nanometer-scale bridge is formed between tip and sample, and that the created nanodot is a part of the nanobridge left on the surface. We have also observed relaxation of the tip apex and the created nanodots after the nanobridge is broken.

The authors show that 100 nm unilamellar thiol-tagged vesicles bind discretely and specifically to Au nanodots formed on a Si surface. An array of such dots, consisting of 20 nm Au-Si three-dimensional islands, is formed by self-assembly on terraces of small-angle-miscut Si(111) after Au deposition. Consequently, both the formation of the nanopattern and the subsequent attachment of the vesicles are self-organized and occur without the need for any "top-down" lithographic processes. This approach has the potential to provide the basis of a low-cost, high-density nanoarray for use in proteomics and drug discovery. © 2007 American Institute of Physics.

Carrier transport mechanism on Ge nanodot arrays formed on SiO2 monolayer covering over the Si surface is investigated by microscopic four-point-probe measurements combined with core-level photoemission spectroscopy and scanning tunneling microscopy. Different conduction natures are found depending on whether or not the nanodots and the substrate are directly connected by subnanometer-sized voids penetrating the SiO2 layer. In the presence of the voids, conductivity is regulated by the dot-size through quantum-size effect.

The magnetization reversal for perpendicularly magnetized L10-FePt (001) nanodots with different interdot distances was studied by magnetic domain observation. We show the results for two kinds of dot arrays: (i) the dots with physical percolation leading to direct exchange coupling, and (ii) the dots which are fully isolated experiencing only dipolar interaction. For the physically percolated dot array, ferromagnetic domains were observed in which domain expanded with magnetic field. On the other hand, the array with the isolated FePt dots also exhibited domain like features resembling to the percolated dots, indicating the existence of superferromagnetism in the array of FePt nanodots.

Ultrahigh density periodically ordered magnetic yttrium iron garnet (Y3Fe5O12, YIG) nanodot arrays have been prepared by pulsed laser deposition through an ultrathin alumina mask (UTAM). UTAM having periodically ordered circularly shaped holes with 350 nm in diameter, 450 nm in inter-pore distance, and 700 nm in height has been prepared on silicon substrate. Furthermore, the microstructure and magnetic properties of YIG nanodot arrays have been characterized. Nanodot arrays with a sharp distribution in diameter centered at 340 nm with standard deviation of 10 nm have been fabricated. Moreover, typical hysteresis loops and ferromagnetic resonance spectra in in-plane and out-of-plane revealed that this unique structure greatly influences the magnetics properties of YIG. First, coercivity of YIG nanodot arrays in in-plane was increased about from 15 Oe of YIG films to 500 Oe. Then, the degree of uniformity about nanodot height decided that two or more resonance peaks in out-of-plane were detected in the spectra. The peak-to-peak linewidth values were about 94 Oe and 40 Oe in the parallel and perpendicular directions, respectively, which indicated that the values were larger by the two-magnon scattering. Consequently, this pattering method creates opportunities for studying physics in oxide nanomagnets and may be applied in spin-wave devices.