Metallic Nanodots Research Articles

Hierarchical Directed Self‐Assembly of Diblock Copolymers for Modified Pattern Symmetry

Block copolymer lithography exploiting diblock copolymer thin films is promising for scalable manufacture of device‐oriented nanostructures. Nonetheless, its intrinsic limitation in the degree of freedom for pattern symmetry within hexagonal dot or parallel line array greatly diminishes the potential application fields. Here, we report multi‐level hierarchical self‐assembled nanopatterning of diblock copolymers for modified pattern symmetry. Sequential hierarchical integration of two layers of diblock copolymer films with judiciously chosen molecular weights and chemical composition creates nanopatterned morphology with modified pattern symmetry, including sparse linear cylinder or lamellar arrays. Internal structure of the hierarchically patterned morphology is characterized by grazing‐incidence small‐angle X‐ray scattering throughout the film thickness. Pattern transfer of the modified nanopattern generates linear metal nanodot array with uniform size and regular spacing as a typical example of functional nanopatterned structures.

Blue-emitting and amphibious metal (Cu, Ni, Pt, Pd) nanodots prepared within supramolecular polymeric micellesfor cellular imaging applications

We propose a new method for the preparation of blue-emitting and amphibious metal (Cu, Ni, Pt, Pd) nanodots using supramolecular polymeric micelle nanoreactors ​for cellular imaging applications.

Efficient Fabrication Process of Metal Nanodot Arrays Using Direct Nanoimprinting Method with a Polymer Mold

A new fabrication process of metal nanodot arrays using the thermal dewetting method was developed in this study. This process was comprised of three steps: thin Au film deposition on a quartz glass substrate, groove patterning by direct nanoimprinting, and self-organization of metal nanodot arrays by thermal dewetting. A new idea to utilize a polymer film mold for groove patterning by direct nanoimprinting was examined. The polymer film mold was prepared by hot-embossing groove patterns of a mother mold on a cyclo olefin polymer (COP) film. The mother mold was prepared from a silicon wafer. The polymer film mold was used for direct nanoimprinting on a metal film deposited on a quartz substrate. The experimental results revealed that the COP film mold can effectively form a micro groove pattern on the Au film despite the COP film mold being softer than the Au film. The micro groove on the Au film was also found to be effective in aligning the nanodots in lines. The micro groove patterning using the COP film mold was also confirmed to be useful in controlling the dot size and alignment during the thermal dewetting process.

Functional Optical Devices Based on Highly Ordered Metal Nanostructures Obtained Using Anodic Porous Alumina

The fabrication of an array of square metal nanodots using anodic porous alumina as an evaporation mask and its application to a substrate for surface-enhanced Raman scattering (SERS) measurement are described in detail. The nanodot shape could be controlled by changing the shape of the nanoholes in the anodic porous alumina. The obtained Au nanodot array could be applied to a substrate for SERS measurement with high sensitivity. It is expected that this process can be applied to fabricate not only highly ordered metal nanodot arrays but also functional optical devices.

Metal nanodot arrays fabricated via seed-mediated electroless plating with block copolymer thin film scaffolding

We present an alternative approach to fabricating hexagonally arranged nanodot arrays of various metals by seed-mediated electroless plating with a cylinder-forming block copolymer thin film, PEO-b-PMA(Az), as a scaffold. Metal ions were selectively incorporated into PEO cylinders, followed by their reduction to metal and the etching of the scaffold to obtain highly ordered seed arrays of Au, Pd, and Pt. Nanodot arrays of the target metals (Au, Ag, and Ni) were selectively grown on the seed with their highly ordered arrangement by electroless plating. We studied the fabrication processes’ suitability for control of the nanodot array size, as well as the plasmonic properties thereof.

Enhancement of near-field light generated by metal nanodot on semiconductor substrate for heat-assisted magnetic recording heat source

In the novel device for a heat-assisted magnetic recording heat source where a metal nanodot is formed on a semiconductor substrate, the possibility of enhancing the near-field light by selecting suitable combinations of the materials for the metal nanodot and semiconductor substrate was investigated through a numerical simulation. It was found that the enhancement factor of the light intensity at a certain resonance wavelength for the light polarized perpendicularly to the surface of the substrate can be improved by two methods. One is to use an alloy as the material for the metal nanodot and the other is to use a ternary mixed crystal as the material for the semiconductor substrate. Design examples were shown for both methods. The ratio of each element in the alloy or ternary mixed crystal should be determined considering the balance between the enhancement factor and some other factors such as stability, availability, and convenience.

Theoretical and Experimental Study of Metallic Dot Agglomeration Induced by Thermal Dewetting

The authors previously developed a new fabrication method for a metal nanodot array, by combination of nanogroove grid patterning and thermal dewetting of metal deposited on a substrate. However, a comprehensive understanding of the thermal dewetting mechanism is necessary to improve the quality and control the variation of the metallic nanodot array. In this study, thermal dewetting-induced nanodot agglomeration mechanism is studied from a theoretical point of view. An analytical model is proposed, based on the total free energy of a dot and substrate system. The theoretical minimum and natural dot sizes show the same trend with an increase of contact angle. The theoretical model is validated by the experimental results.

Fabrication of plasmonic nanopillar arrays based on nanoforming

A new fabrication method for realizing ordered nanopillar array is reported in this paper. First, a metallic nanodot array is prepared by combining two techniques, viz., nano-grid patterning by Nanoplastic Forming (NPF) and dot agglomeration by thermal dewetting. These processes do not include complicated and expensive procedures such as electron beam lithography. Then Au-capped nanopillar arrays are produced by reactive ion etching (RIE). In the refractive index sensing application, it is shown that the sensitivity of the Au-capped nanopillar array is higher than that of the Au nanodot array. The plasmonic nanopillar arrays with Ag coating are used as SERS-active substrates, which exhibit good performance.

Stacking of nanoscale metal dot array using liquid transfer imprint lithography with roll press

The stacking of metal nanodot patterns is useful for plasmonic devices and metamaterials. Metal nanodot patterns are conventionally fabricated using a combination of lithography and a metal lift-off process, which is time consuming and complex. On the other hand, there is demand for an expanding the patterning area of the metal nanodot. Therefore, the fabrication method with high throughput is required. For more efficient fabrication, we have developed a novel stacking technique that combines ultraviolet nanoimprint lithography (UV-NIL), contact printing, and liquid transfer imprint lithography (LTIL). UV-NIL was used to obtain a replica mold with a nanoscale hole pattern, and the metal layers were deposited onto the mold by using a vacuum evaporator. Contact printing removed a metal layer on the mold surface, leaving a metal nanodot array in the holes of the replica mold. This metal layer was transferred by UV-NIL, and LTIL with a roll press was employed to reduce the thickness of the UV-curable resin; the excess resin was split from the mold in the liquid phase. The replica mold with the metal dots and an intermediate layer was then transferred to a polyester film. In this study, this process was repeated to obtain five stacked layers. The stack consisting of gold nanodots displayed a red plasmonic color, whereas the stack of silver nanodots displayed a blue plasmonic color. Scanning electron microscopy was used to confirm the stacked structures through tilted angle and cross-sectional view images. The advantages of this process are that nanodot patterns can be fabricated using a mold-based process and that the thickness of the stacking layer can be controlled by LTIL. This process can therefore be used for high-throughput fabrication of stacking metal devices such as plasmonic memory.

High-Resolution Metal Nanopatterning by Means of Switchable Block Copolymer Templates.

In this review, recent developments in the fabrication of hexagonal and parallel ordered arrays of metallic nanodomains on a substrate are described. We focus on the nanopatterning approach by means of switchable block copolymer thin films. This approach is highly advantageous, because it can lead to extremely regular patterns with metal subunits of only a few nanometers in diameter and center-to-center distances of tens of nanometers. Hence, the resulting 1D or 2D periodic arrays of metal nanodots and nanowires on silicon substrates can be fabricated with extremely high unit densities and on very large areas. The templated deposition of presynthesized metal nanoparticles on functional block copolymers is described in detail. Current challenges are discussed and an outlook for further developments is given.

Fabrication of Resistive Random Access Memory by Atomic Force Microscope Local Anodic Oxidation

The fabrication of gallium, zinc and nickel oxide nanodots for application of resistive random access memory (RRAM) was demonstrated using the atomic force microscopy (AFM) local anodic oxidation technique. Thin metal films were deposited on indium tin oxide conductive glass substrates. In the atmospheric environment, using AFM equipped with an Ag -coated probe can generate metal oxide nanodots locally on the metal films. These nanodots act as an insulator layer in a single unit cell of the RRAM. The voltage-biased method allows devices to reset from a low-resistance state (LRS) to a high-resistance state (HRS) at 0.9 V. These results show the ability of the AFM local anodic oxidation to produce 50 nm NiO nanodots on glass substrates for potentially high-density RRAMs. As we developed the characteristics of the structure, we found that a lateral NiO nanobelt RRAM performs very low power operation from such experimental manufacturing process. Using a current-biased method, the lateral device switches from a HRS to a LRS with a low writing voltage of 0.64 V.

Fabrication of ultrahigh resolution metal nanowires and nanodots through EUV interference lithography

We demonstrate the fabrication of high-resolution (sub-100nm) nanowires and nanodots using HSQ/PMMA bilayer resists for a negative-tone lift-off process. The high resolution patterning on HSQ layers was performed using EUV (extreme ultraviolet) interference lithography at the Swiss Light Source (SLS). The thicknesses of HSQ and of the sacrificial PMMA layers were optimized for different resolutions. We succeeded in transferring periodic HSQ nanoline structures with resolution as high as 16nm half-pitch into PMMA layers by O2 plasma etching and then into Cr/Au nanowires by metallization and lift-off. The duty cycles of the nanowires are varied by controlling the exposure doses of the irradiation light. HSQ nanodots and nanoholes are obtained by exposing the resist layer using a two-dimensional mask. After subsequent treatment, Cr/Au nanodots with sizes of 25nm–60nm are obtained.

High thermal performance of SnO2:F thin transparent heaters with scattered metal nanodots.

Facile production and novel transparent heaters consisting of fluorine-doped tin oxide (SnO2:F or FTO) thin films covered with three different scattered metal nanodots (Cr-nd, NiCr-nd and Ni-nd) prepared by plasma-enhanced sputtering system and electron cyclotron resonance-metal organic chemical vapor deposition are investigated. The heaters exhibit excellent optical transmittances of over 85% and superior saturated temperatures of more than 80 °C when a relatively low 12 V DC is supplied. The scattered metal nanodots FTO heaters successfully improve the specific power of bare FTO heater by 21, 15, and 12% for NiCr-nd FTO, Cr-nd FTO, and Ni-nd FTO, respectively. These results reveal that the FTO transparent heaters with scattered metal nanodots are the suitable heating materials that can be applied for various functional devices.

Probing the nanoscale Schottky barrier of metal/semiconductor interfaces of Pt/CdSe/Pt nanodumbbells by conductive-probe atomic force microscopy.

The electrical nature of the nanoscale contact between metal nanodots and semiconductor rods has drawn significant interest because of potential applications for metal-semiconductor hybrid nanostructures in energy conversion or heterogeneous catalysis. Here, we studied the nanoscale electrical character of the Pt/CdSe junction in Pt/CdSe/Pt nanodumbbells on connected Au islands by conductive-probe atomic force microscopy under ultra-high vacuum. Current-voltage plots measured in contact mode revealed Schottky barrier heights of individual nanojunctions of 0.41 ± 0.02 eV. The measured value of the Schottky barrier is significantly lower than that of planar thin-film diodes because of a reduction in the barrier width and enhanced tunneling probability at the interface.

Probing the Minimum Geometric Requirements for T-Cell Stimulation

The immune recognition process involves an elaborate arrangement of adhesion, costimulatory and signaling molecules organized into a stereotypic geometric structure known as the immunological synapse (IS). We have developed a versatile engineered platform to probe the minimum geometric requirements (in terms of spacing and stoichiometry) for T-cell stimulation. Arrays of metallic nanodots, ∼ 2-10 nm in size, to which a UCHT1 Fab antibody was bound, were created by nanolithography. These served as individual T-cell receptor (TCR) binding sites. The adhesion molecule ICAM-1 was either statically bound to a PEG-silane brush surrounding the nanodots, or it was allowed to move freely within the arrays in a supported lipid bilayer. T-cells were plated on arrays comprising individual TCR binding sites, small clusters and extended hexagonal close packed arrays, with spacings ranging from 40 nm and below to 1 μm. TCR signaling strength was monitored by measuring phosphorylated tyrosine (pY) intensity. T-cell adhesion was assessed by the number of cells bound to the arrays, as well as by the area of spread cells.Systematic variation of the spacing and cluster size of TCR binding sites in the arrays enabled determination of the minimum conditions that support T-cell signaling and the formation of the IS. TCR signaling increased with decreasing spacing below a threshold spacing ∼ 60 nm. For clusters with these spacings the formation of the stereotypic “bullseye” geometry that characterizes the immune synapse became evident, with ICAM-1 excluded to the periphery. In terms of stoichiometry, at least 4 TCR-binding sites (within ∼ 60 nm) were required for T-cell adhesion and spreading. Further, in the absence of ICAM-1 (or at low concentrations), a threshold density of agonist sites was found, above which the TCR apparently plays a dual role of immune activity and adhesion.

Growth mechanism of metal-oxide nanowires synthesized by electron beam evaporation: a self-catalytic vapor-liquid-solid process.

We report the growth mechanism of metal oxide nanostructures synthesized by electron beam evaporation. The condensed electron beam can easily decompose metal oxide sources that have a high melting point, thereby creating a self-catalytic metal nanodot for the vapor-liquid-solid process. The metal oxide nanostructures can be grown at a temperature just above the melting point of the self-catalyst by dissolving oxygen. The morphology of nanostructures, such as density and uniformity, strongly depends on the surface energy and surface migration energy of the substrate. The density of the self-catalytic metal nanodots increased with decreasing surface energies of the substrate due to the perfect wetting phenomenon of the catalytic materials on the high surface energy substrate. However, the surfaces with extremely low surface energy had difficulty producing the high density of self-catalyst nanodot, due to positive line tension, which increases the contact angle to >180°. Moreover, substrates with low surface migration energy, such as single layer graphene, make nanodots agglomerate to produce a less-uniform distribution compared to those produced on multi-layer graphene with high surface migration energy.

Large-area nanosquare arrays from shear-aligned block copolymer thin films.

While block copolymer lithography has been broadly applied as a bottom-up patterning technique, only a few nanopattern symmetries, such as hexagonally packed dots or parallel stripes, can be produced by spontaneous self-assembly of simple diblock copolymers; even a simple square packing has heretofore required more intricate macromolecular architectures or nanoscale substrate prepatterning. In this study, we demonstrate that square, rectangular, and rhombic arrays can be created via shear-alignment of distinct layers of cylinder-forming block copolymers, coupled with cross-linking of the layers using ultraviolet light. Furthermore, these block copolymer arrays can in turn be used as templates to fabricate dense, substrate-supported arrays of nanostructures comprising a wide variety of elements: deep (>50 nm) nanowells, nanoposts, and thin metal nanodots (3 nm thick, 35 nm pitch) are all demonstrated.

Simulation on near-field light generated by metal nano-dot on GaAs substrate for heat source of heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is promising for achieving more than 1 Tb/inch2 recording density. A near-field transducer (NFT), which forms a hot spot of 10–100 nm in diameter on a recording medium, is necessary in HAMR. In this study, localized surface plasmons generated by a metal nano-dot in a novel device for a heat source of heat-assisted magnetic recording were analyzed using a simple model in which a metal hemisphere was formed on a GaAs substrate and a quasi-electrostatic approximation. The scattering and absorption efficiencies as well as the enhancement factor were investigated for several kinds of metal. As a result, their dependence on the wavelength and the polarization direction of the incident light was clarified.

Optimization of long-range order in solvent vapor annealed poly(styrene)-block-poly(lactide) thin films for nanolithography.

Detailed experiments designed to optimize and understand the solvent vapor annealing of cylinder-forming poly(styrene)-block-poly(lactide) thin films for nanolithographic applications are reported. By combining climate-controlled solvent vapor annealing (including in situ probes of solvent concentration) with comparative small-angle X-ray scattering studies of solvent-swollen bulk polymers of identical composition, it is concluded that a narrow window of optimal solvent concentration occurs just on the ordered side of the order-disorder transition. In this window, the lateral correlation length of the hexagonally close-packed ordering, the defect density, and the cylinder orientation are simultaneously optimized, resulting in single-crystal-like ordering over 10 μm scales. The influences of polymer synthesis method, composition, molar mass, solvent vapor pressure, evaporation rate, and film thickness have all been assessed, confirming the generality of this behavior. Analogies to thermal annealing of elemental solids, in combination with an understanding of the effects of process parameters on annealing conditions, enable qualitative understanding of many of the key results and underscore the likely generality of the main conclusions. Pattern transfer via a Damascene-type approach verified the applicability for high-fidelity nanolithography, yielding large-area metal nanodot arrays with center-to-center spacing of 38 nm (diameter 19 nm). Finally, the predictive power of our findings was demonstrated by using small-angle X-ray scattering to predict optimal solvent annealing conditions for poly(styrene)-block-poly(lactide) films of low molar mass (18 kg mol(-1)). High-quality templates with cylinder center-to-center spacing of only 18 nm (diameter of 10 nm) were obtained. These comprehensive results have clear and important implications for optimization of pattern transfer templates and significantly advance the understanding of self-assembly in block copolymer thin films.

Enhancement of hole injection and electroluminescence by ordered Ag nanodot array on indium tin oxide anode in organic light emitting diode

We report the enhancement of hole injection and electroluminescence (EL) in an organic light emitting diode (OLED) with an ordered Ag nanodot array on indium-tin-oxide (ITO) anode. Until now, most researches have focused on the improved performance of OLEDs by plasmonic effects of metal nanoparticles due to the difficulty in fabricating metal nanodot arrays. A well-ordered Ag nanodot array is fabricated on the ITO anode of OLED using the nanoporous alumina as an evaporation mask. The OLED device with Ag nanodot arrays on the ITO anode shows higher current density and EL enhancement than the one without any nano-structure. These results suggest that the Ag nanodot array with the plasmonic effect has potential as one of attractive approaches to enhance the hole injection and EL in the application of the OLEDs.