Nanodot Arrays Research Articles

Boosting Light Collection Efficiency of Optical Fibers Using Metallic Nanostructures

Optical fibers represent one of the most important photonic components with applications in numerous fields; however, their performance with respect to light collection at large incident angles is severely limited. Here, we show that arrays of metallic nanodots located on the end faces of regular step index fibers allow addressing incoupling angles up to 80° at high efficiencies, representing a regime in which bare fibers fail to operate. The experimental demonstrations including lithographic implementation and optical characterization supported by corresponding numerical simulations and a toy model reveal the feasibility and potential of the concept of nanostructure-enhanced fiber incoupling, which is compatible with other types of nanostructures and can lead to significant improvements, particularly in the fields of bioanalytics and quantum technologies.

Magnetic Properties of Epitaxially Grown SrRuO3 Nanodots.

We present the fabrication and exploration of arrays of nanodots of SrRuO3 with dot sizes between 500 and 15 nm. Down to the smallest dot size explored, the samples were found to be magnetic with a maximum Curie temperature TC achieved by dots of 30 nm diameter. This peak in TC is associated with a dot-size-induced relief of the epitaxial strain, as evidenced by scanning transmission electron microscopy.

Spin-orbit torque-induced switching of in-plane magnetized elliptic nanodot arrays with various easy-axis directions measured by differential planar Hall resistance

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.

Magnetoelectric couplings in high-density array of nanoscale Co/BiFeO3 multiferroic heterostructures

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.

In Situ Measurement for Diffusion-Adsorption Process of Cl− and SPS in Through-Silicon Via Using SERS Effect Produced by Cu Nanodot Arrays

In this study, we analyzed the diffusion behavior of additives, such as Cl− and bis–(3–sulfopropyl) disulfide (SPS), during the through-silicon via process with our surface-enhanced Raman spectroscopy (SERS) measurement system equipped with a model structure of a micro via. The via structure, made of poly(dimethyl siloxane) (PDMS), which has a transparent wall, was attached horizontally on the Cu nano-patterned substrate, an array of nanodots with 150-nm diameter and 300-nm pitch. This substrate provides the SERS effect with high uniformity and also works as one of the sidewalls of the via. The simultaneous diffusion of Cl− and SPS into the micro via on the Cu nanodot wall was observed by Raman microspectroscopy. The obtained SERS spectrum clearly indicated the diffusion of these two species. First, Cl− adsorbs on Cu, because it has a larger diffusion coefficient; SPS removes the pre-adsorbed Cl− to dominate the adsorption site of the surface a few minutes later. Pre-adsorbed SPS is sufficiently stable, and is not removed by a highly negative potential (for example, −200 mV vs. Ag/AgCl).

Decreasing in stiction force between two electrodes in vibration energy harvester by nano-dots array on SiO<sub>2</sub> electret

Decreasing in stiction force between two electrodes in vibration energy harvester by nano-dots array on SiO<sub>2</sub> electret

Close-packed silane nanodot arrays by capillary nanostamping coupled with heterocyclic silane ring opening.

We report the parallel generation of close-packed ordered silane nanodot arrays with nanodot diameters of few 100 nm and nearest-neighbor distances in the one-micron range. Capillary nanostamping of heterocyclic silanes coupled with ring-opening triggered by hydroxyl groups at the substrate surfaces yields nanodots consisting of silane monolayers with exposed terminal functional groups. Using spongy mesoporous silica stamps with methyl-terminated mesopore walls inert towards the heterocyclic silanes, we could manually perform multiple successive stamping cycles under ambient conditions without interruptions for ink refilling. Further functionalizations include the synthesis of polymer nanobrushes on the silane nanodots by surface-initiated atom-transfer radical polymerization. Proteins-of-interest fused to the HaloTag were site-specifically captured to silane nanodots functionalized by copper-free reactions with azide derivatives. Thus, bioorthogonal functionalization for bioanalytics with a spatial resolution in the one-micron range may be realized on solid supports compatible with fluorescence-based optical microscopy. The feature sizes of the silane nanodot arrays match well the length scales characteristic of a variety of biomolecular submicroscopic organizations in living cells, thus representing a compromise between miniaturization and the resolution limit of optical microscopy for sensitive high-throughput bioanalytics.

Fabrication and tribological characterization of deformation-resistant nano-textured surfaces produced by two-photon lithography and atomic layer deposition

Abstract In this study, we demonstrated a novel strategy for fabricating deformation-resistant nano-textured surfaces. Ordered arrays of IP-DIP photoresist nanodots of various diameters were first fabricated using two-photon lithography. IP-DIP/Al2O3 core-shell nanostructures (CSNs) were then formed by conformally coating the nanodots with Al2O3 of different thicknesses via atomic layer deposition (ALD). The CSNs showed 85% lower coefficients of friction than the bare IP-DIP nanodots and exhibited minimal nanostructure deformation at contact pressures greater than 20 GPa. The IP-DIP/Al2O3 CSNs also have lower adhesion forces than the bare IP-DIP nanodots. The excellent deformation resistance and superior tribological properties of these CSNs demonstrate that the combination of two-photon lithography and ALD is a very promising solution to tribological issues in miniaturized systems.

2D Metal Chalcogenide Nanopatterns by Block Copolymer Lithography

AbstractNanoscale structure engineering is in high demand for various applications of 2D transition metal dichalcogenides (TMDs). An edge‐exposed 2D polycrystalline MoS2 nanomesh thin film is demonstrated via block copolymer (BCP) nanopatterning. Molybdenum nanomesh structure is formed by direct metal deposition of hexagonal cylinder BCP nanotemplate and the following lift‐off process. Subsequent sulfurization of the molybdenum nanomesh creates MoS2 nanomesh thin films without any degradative etching step. The approach is applicable to not only other metal sulfides and oxides but also other nanoscale structures of TMD thin films including nanodot and nanowire array by means of various BCP nanotemplate shapes. As the edge site of MoS2 is highly active for NO2 sensing, the edge‐exposed MoS2 nanomesh demonstrates sevenfold enhancement of sensitivity for NO2 molecules compared to uniform thin film as well as superior reversibility even under 80% relative humidity environment. This structure engineering method could greatly strengthen the potential application of 2D TMD materials with the optimal customized nanoscale structures.

Directed Assembly of Au-Tipped 1D Inorganic Nanostructures via Nanolithographic Docking.

Controlled assembly of nanostructures is a key challenge in nanotechnology. In this work, we introduce an approach for the controlled assembly of 1D nanodumbbells-Au-tipped semiconductor nanorods-into arbitrary 2D higher architectures, by their chemical docking to nanopatterned functionalities. We realized the docking functionalities via nanoimprinted metallic nanodots functionalized with an organic monolayer, whose terminal thiol groups chemically bind the nanodumbbell tips. We demonstrated that the functional nanopattern encodes the nanodumbbell assembly and can be designed to deterministically position nanodumbbells in two possible modes. In the single-docking mode, the nanodot arrays are designed with a spacing that exceeds the nanodumbbell length, restricting each nanodumbbell to dock with one edge and physically connect with its free edge to one of the neighboring nanodumbbells. Alternatively, in the double-docking mode, the nanodots are spaced to exactly fit the nanodumbbell length, allowing nanodumbbell docking with both edges. We found that the docking kinetics can be described by a random attachment model, and verified that for the used docking chemistry, nanodumbbells that are docked to the same dot do not interact with each other. Our work demonstrates the possibility for massively parallel positioning of sub-100 nm 1D semiconductor nanostructures, and can potentially enable their future integration into functional nanodevices and nanosystems.

Preparation of Anodic Aluminum Oxide Masks with Size-Controlled Pores for 2D Plasmonic Nanodot Arrays

Anodic aluminum oxide (AAO) layers with nano-sized pores have been used as shadow masks for the fabrication of two-dimensional (2D) metal nanodot arrays (NDAs). However, the localized surface plasmon resonance (LSPR) of size-controlled NDAs fabricated using AAO masks has not been much studied. In this study, we report on the improved preparation method and utilization of an ultrathin AAO mask for the fabrication of 2D plasmonic metal NDAs. The greatest challenge in preparing an AAO mask is to control the pore diameter and to make it reproducible. AAO masks with size-controlled pores were reproducibly prepared using a two-step chemical wet etching method. Ag NDAs with different dot diameters (42, 60, and 80 nm) and Ag, Cu, and Au NDAs with dot a diameter of 80 ± 5 nm were fabricated on indium tin oxide glass substrates using AAO masks. The wavelengths corresponding to LSPR of 2D metal NDAs were investigated using ultraviolet–visible spectroscopy. Our results show that AAO masks with tunable pores can be used as shadow masks for the fabrication of 2D plasmonic NDAs.

Piezoelectrics: Large Local‐Compressive Stress‐Induced Improvements in Piezoelectric Characteristics of Lead Zirconate Titanate Thin Films on a Ni Nanodots Array (Adv. Electron. Mater. 7/2018)

As described by Yong Soo Cho and co-workers in article number 1800081, a nanodot array on a regular substrate induces extra local compressive stress and promising crystal orientation during in situ deposition of piezoelectric thin films. Uniformity of the nanodot array is secured with the aid of a magnetic field. The ideal growth orientation results in a significant improvement of effective piezoelectric coefficient.

The magnetic properties of CoFeB and CoFeB/Ag nanodot arrays fabricated by a template transfer imprinting method

In this study, a practical method for fabricating size-tunable and periodic CoFeB and CoFeB/Ag nanodot arrays on glass substrate was developed using a template transfer imprinting method, which consisted of nanoimprint lithography and vacuum magnetron sputtering. It was found that the coercivity could be distinctly enhanced by introducing periodic CoFeB nanodot arrays. Also, the coercivity of the CoFeB/Ag nanodot arrays could be flexibly tuned over a wide range by tailoring the thickness of the Ag layer, due to the interfacial magneto-electric coupling effect. Investigations such as those conducted in this study will provide promising references for the future fabrications of high performance nanostructured magnetic materials, which can be potentially utilized in many fields.

Fabrication of Highly Packed Plasmonic Nanolens Array Using Polymer Nanoimprinted Nanodots for an Enhanced Fluorescence Substrate.

A simple and cost-effective fabrication method for plasmonic nanolens arrays (PNA) with a narrow gap has been proposed for fabricating enhanced fluorescence substrates, in which the fluorophores interacting with the enhanced electromagnetic field generated by localized surface plasmons provide a higher fluorescence signal. The PNA was fabricated by the sequential depositions of the SiO2 and Ag layers on a UV-nanoimprinted nanodot array with a pitch of 500 nm, a diameter of 250 nm, and a height of 100 nm. During the deposition processes, the shape of the nanodots changed to that of nanolenses, and the gap between the nanolenses was decreased via sidewall deposition. To examine the feasibility of the fabricated PNA for enhanced fluorescence application, a streptavidin-Cy5 (SA-Cy5) conjugate dissolved in a saline buffer solution was spotted on the PNA, and the fluorescence signals of the SA-Cy5 were measured and compared with those on a bare glass substrate. The enhancement factor was affected by the gap between the nanolenses, and the maximum enhancement factor of ~128 was obtained from the PNA with a SiO2 layer thickness of 150 nm and an Ag layer thickness of 100 nm. Finally, an electromagnetic field analysis was used to examine the fluorescence signal enhancement, and was conducted using rigorous coupled wave analysis.

Measures of order for nearly hexagonal lattices

Motivated by hexagonal patterns with defects in natural and laboratory systems, we compare quantitative measures of order for nearly hexagonal, planar lattices. These include a spectral measure of order based on the Fourier transform, a geometric measure of order using the Delaunay triangulation, and topological measures of order introduced in this paper. The topological measures are based on a tool from topological data analysis called persistent homology. We contrast these measures of order by comparing their sensitivity to perturbations of Bravais lattices. We then study the imperfect hexagonal arrangements of nanodots produced by numerical simulations of partial differential equations that model the surface of a binary alloy undergoing erosion by a broad ion beam. These numerical experiments further distinguish the various measures of hexagonal order and highlight the role of various model parameters in the formation and elimination of defects. Finally, we quantify the dependence of order on prepatterning the surface to suggest experimental protocols that could lead to improved order in nanodot arrays.

Dynamics of Magnetic Skyrmions in Nanodots

Bloch and Neel magnetic skyrmions have been studied in systems of confined geometry (nanodots, a linear array of nanodots). The spectra of low- and high-frequency excitation modes of a skyrmion state have been calculated. It has been shown that skyrmion spectrum asymmetry, namely, the characteristic difference between the frequencies of the azimuthal modes of the azimuthal skyrmion modes rotating clockwise and counterclockwise, is associated with asymmetry in the magnetization profiles of high-frequency spin waves propagating on the background of a skyrmion state in a nanodot. The low-frequency spectrum contains the only gyrotropic mode localized near the center of a nanodot. The gyrotropic frequency depends on the material parameters of a nanodot and the size of a skyrmion. The eigenfrequency of the gyrotropic mode of an isolated skyrmion in a nanodot in ultrathin films (L ~ 1 nm) does not depend on the internal structure of a skyrmion and is the same for Bloch and Neel skyrmions. The interaction of skyrmions, in particular, in a linear chain of nanodots with the ground skyrmion state, leads to distinctions in low-frequency spectra. The structure of a skyrmion (of Bloch or Neel type) is exhibited as a shift of dispersion curves and a difference between the frequencies of ferromagnetic resonance in a system of interacting skyrmions.

Large Local‐Compressive Stress‐Induced Improvements in Piezoelectric Characteristics of Lead Zirconate Titanate Thin Films on a Ni Nanodots Array

AbstractInterfacial stress present in ferroelectric thin films is known to affect largely their piezoelectric properties by modifying crystal orientation and domain structure. Here, a nonconventional way is proposed to substantially improve the polarization and piezoelectric characteristics of Pb(Zr,Ti)O3 thin films by modifying the surface of the typical Si substrate into an embossed structure with Ni nanodots. Uniform Ni nanodot arrays are successfully produced by an external magnetic field through the consolidation process of thin Ni layer. The exceptionally large thermal expansion mismatch of ≈94% between the thin films and the Ni nanodots is believed to induce a large local‐compressive stress around the nanodot region and thus the intensified orientation toward c‐axis. For this demonstration, the in situ sputtering processing enabled by combining heavily 12 mol% Nb‐doping with a bottom electrode structure of Ir/TiW is used. As a highlight of the improvement, a significant increase of ≈33% in effective piezoelectric coefficient is observed for the 90 nm Ni nanodot case. An apparent shift of the polarization–electric field curve suggests the existence of internal field, as an evidence of the in situ domain formation.

Modification of the magnetization dynamics of a NiFe nanodot due to thermal spin injection

An array of NiFe nanodots has been prepared on a Cu/CoFeAl film. Since a thermal spin current is expected to be excited owing to a large spin-dependent Seebeck coefficient for the CoFeAl, we investigate the magnetization dynamics of the NiFe dots under the temperature gradient along the vertical direction. By using vector network analyzer measurements, we have demonstrated that the temperature gradient produces modulations of the frequency of ferromagnetic resonance and the linewidth of the resonance spectra. The observed parabolic dependences are well explained by the damping-like and field-like components of spin transfer torque.

Templated Electrochemical Synthesis of Fe–Pt Nanopatterns for High-Density Memory Applications

We demonstrate an electrochemical deposition process suitable to form arrays of L10 Fe–Pt nanodots with hard magnetic properties via electron-beam lithography, template deposition, and thermal annealing. Synthesis parameters are selected by growing single and multilayer blanket films and investigating the effect of thermal annealing on the various films. We find that the fastest ordering transformation is achieved in eight-layer Fe–Pt multilayers with a sublayer thickness of 2.5 nm, while a single Fe–Pt layer or four- and twenty-layer multilayers exhibit a more sluggish ordering process due to the imperfect layering (twenty-layer) or to the limited mean free path upon annealing (single- and four-layer); the coercivity of the eight-layer Fe–Pt multilayer after annealing at 450 °C reached 6.6 kOe, whereas the single Fe–Pt layer showed only 1.0 kOe. TEM imaging and diffraction show that the L10 Fe–Pt nanodot arrays are single crystal with a (111) orientation, suggesting a facile L10 ordering when using a mul...

Molecular-Level Analysis of Surface Species for Electrochemical Deposition Processes Using Density Functional Theory Calculations and Surface Enhanced Raman Microscopy with Plasmonic Sensors

Electrochemical deposition processes have been widely applied to form functional thin films and micro/nano structures featuring their controllability and capability to fabricate uniform deposits to wide, non-flat surfaces. For achieving further precise control, we have attempted molecular-level analysis of the deposition processes and reaction mechanisms through theoretical and experimental approaches. In this paper, we will introduce these approaches with some of our resent results. For the theoretical analysis to quantitatively understand the reaction mechanisms, we have applied density functional theory (DFT) calculations focusing on the surface species such as additives, reaction intermediates, reducing agents (for electroless deposition), and so on[1]. Effects of solvation and catalytic activity of the surfaces, etc., were also investigated and behaviours of these species, which are unique to the deposition at solid/liquid interfaces, were elucidated. While these theoretical analysis could predict rigorous mechanisms of the deposition reaction processes, acquiring such molecular-level data through experimental approach is quite challenging since the amount of these surface species are trace and they are “buried” under the electrolyte. For this, we focused on surface enhanced Raman scattering (SERS) equipped with confocal microscopy and developed plasmonic sensors with patterned nanostructures to achieve very high enhancement effect, and we applied them to analyze the electrochemical deposition processes such as the behaviour of additives. For the plasmonic sensors, we have developed “reflection” type and “transmission” type; the “reflection” type sensors consist of metals active for plasmonic enhancement such as Ag, Au, and Cu with centrifuge-grooved nanostructures or nanodot arrays for maximizing the enhancement, which can be directly used as the substrates for the deposition[2]. The “transmission” type sensors, which are single or array of the microlens coated with discrete nanoparticles of the plasmon-active metals, can be applied to various processes regardless of the substrate materials[3]. By using these sensors, we analyzed behaviours of the additives and reductants at the deposition sites. In addition, we have also developed a system for in situ analysis of the processes inside the microscopic pores such as through silicon via (TSV) [4]. A model-pore was fabricated using the sensor (to be a part of “side-wall” of the pore) with the metal nanodot array covered with the structure made of poly(dimethylsiloxane) (PDMS). It was confirmed that Raman signal of the additives could be detected in situ in high sensitivity without interference of the signals originated from PDMS, and their behaviour was investigated in detail. Furthermore, attempts on measuring local temperature as well as pH in the vicinity of the electrode were made by using the SERS measurements, which will be able to provide new insights for precise analysis of the behaviours of surface species during the deposition processes. This work was financially supported in part by “Development of Systems and Technology for Advanced Measurement and Analysis,” Japan Science and Technology Agency, and Grant-in-Aid for Scientific Research, MEXT, Japan. [1] For example, T. Homma, Electrochemistry, 83, 680 (2015). [2] B. Jiang, M. Kunimoto, M. Yanagisawa, T. Homma, J. Electrochem. Soc., 160, D366 (2013). [3] M. Yanagisawa, M. Saito, M. Kunimoto, T. Homma, Appl. Phys. Express, 9 , 122002 (2016). [4] T. Homma, A. Kato, M. Kunimoto, M. Yanagisawa, Electrochem. Commn., submitted.