High‐Density Periodically Ordered Magnetic Cobalt Ferrite Nanodot Arrays by Template‐Assisted Pulsed Laser Deposition

A novel nanopatterning method using pulsed laser deposition through an ultrathin anodic aluminium oxide (AAO) membrane mask is proposed to synthesize well‐ordered nanodot arrays of magnetic CoFe₂O₄ that feature a wide range of applications like sensors, drug delivery, and data storage. This technique allows the adjustment of the array dimension from ～35 to ～300 nm in diameter and ～65 to ～500 nm in inter‐dot distance. The dot density can be as high as 0.21 Terabit in.^?2. The microstructure of the nanodots is characterized by SEM, TEM, and XRD and their magnetic properties are confirmed by well‐defined magnetic force microscopy contrasts and by hysteresis loops recorded by a superconducting quantum interference device. Moreover, the high stability of the AAO mask enables the epitaxial growth of nanodots at a temperature as high as 550 °C. The epitaxial dots demonstrate unique complex magnetic domains such as bubble and stripe domains, which are switchable by external magnetic fields. This patterning method creates opportunities for studying novel physics in oxide nanomagnets and may find applications in spintronic devices.     

Size effect on polarization switching kinetics of P(VDF-TrFE) copolymer nanodots

We demonstrate polarization switching kinetics of poly(vinylidene fluoride-ran-trifluoroethylene) P(VDF-TrFE) nanodots depending on their sizes. Ferroelectric copolymer P(VDF-TrFE) nanodots, ranging from 20 to 250 nm in lateral size, were deposited on a Nb-doped SrTiO₃ substrate by a position-controlled dip-pen nanolithography technique. Ferroelectric switching characteristics of P(VDF-TrFE) nanodots was investigated by piezoresponse force microscopy. Asymmetric switching behavior was observed during switching process, indicating that the threshold electric fields for polarization reversal were measured differently under the same conditions according to the switching polarity. Significantly, when the size of P(VDF-TrFE) nanodot approached around 250 nm, electric fields for polarization switching were compatible with those of the P(VDF-TrFE) thin film. We suggest that the electric fields required for polarization reversal of the P(VDF-TrFE) nanodots present asymmetric switching behaviors, and asymptotic behavior be observed as the nanodots increase in lateral size.     

Facile Preparation of PbTiO3 Nanodot Arrays: Combining Nanohybridization with Vapor Phase Reaction Sputtering

A facile route is presented for the fabrication of spherical PbTiO₃ (PTO) nanodot arrays on platinized silicon substrates using PbO vapor phase reaction sputtering on micellar monolayer films of polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) loaded with TiO₂ sol–gel precursor. Short exposure to PbO transforms the amorphous TiO₂ into polycrystalline PTO, while keeping the inherent size and periodicity of TiO₂ nanodots. HRTEM images show that the spherical PTO nanodots, with an average size and height of 63 nm and 40 nm, respectively, are fixed on the Pt supported by residual carbon. XPS narrow scan spectra of Ti 2p and O 1s strongly verify the evolution of chemical identity and the reduction of the Ti‐O binding energy from TiO₂ to PTO. The amplitude and phase images of piezoelectric force microscopy (PFM) confirm a multidomain structure attributed by the crystalline orientation of the PTO nanodots. Furthermore, the discrete PTO nanodots show remarkable switching properties due to the low strain field induced by the small lateral size, and the absence of domain pinning effects by grain boundary.     

Nanoparticle Arrays on Surfaces Fabricated Using Anodic Alumina Films as Templates

High density nanoparticle arrays on surfaces have been created using a template‐assisted approach. Templates were produced by evaporating aluminum onto substrates and subsequently anodizing the aluminum to produce nanoporous alumina films. The resulting templates have a narrow distribution of pore sizes tunable from ～ 25 to ～ 70 nm. To demonstrate the flexibility of this approach for producing nanoparticle arrays on various substrates, templates have been fabricated on silicon oxide, silicon, and gold surfaces. In all cases, a final chemical etching step yielded pores that extended completely through the template to the underlying substrate. Because the templates remain in intimate contact with the substrate throughout processing, they may be used with either vacuum‐based or wet chemical deposition methods to direct the deposition of nanoparticles onto the underlying substrates. Here we have produced gold nanodot arrays using evaporation and gold nanorod arrays by electrodeposition. In each case, the diameter and height of the nanoparticles can be controlled using the confining dimensions of the templates, resulting in high density (～ 10¹⁰ cm^–2) arrays of nanoparticles over large areas (> 1 cm²).     

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.     

Ultrafast Synthesis of Ultrasmall Poly(Vinylpyrrolidone)‐Protected Bismuth Nanodots as a Multifunctional Theranostic Agent for In Vivo Dual‐Modal CT/Photothermal‐Imaging‐Guided Photothermal Therapy

To elaborately fabricate real‐time monitoring and therapeutic function into a biocompatible nanoplatform is a promising route in the cancer therapy field. However, the package of diagnosis and treatment into a single‐“element” nanoparticle remains challenge. Herein, ultrasmall poly(vinylpyrrolidone)‐protected bismuth nanodots (PVP‐Bi nanodots) are successfully synthesized through an ultrafacile strategy (1 min only under ambient conditions). The nanodots are easy to synthesize in both laboratory and large scale using low‐cost bismuth ingredients. PVP‐Bi nanodots with ultrasmall size show good biocompatibility. Due to the high X‐ray attenuation ability of Bi element, PVP‐Bi nanodots have prominent performance on X‐ray computed tomography (CT) imaging. Moreover, PVP‐Bi nanodots exhibit a high photothermal conversion efficiency (η = 30%) because of the strong near‐infrared absorbance, which can serve as nanotheranostic agent for photothermal imaging and cancer therapy. The subsequent PVP‐Bi‐nanodot‐mediated photothermal therapy (PTT) result shows highly efficient ablation of cancer cells both in vitro and in vivo. PVP‐Bi nanodots can be almost completely excreted from mice after 7 d. Blood biochemistry and histology analysis suggests that PVP‐Bi nanodots have negligible toxicity. All the positive results reveal that PVP‐Bi nanodots produced through the ultrafacile method are promising single‐“element” nanotheranostic platform for dual‐modal CT/photothermal‐imaging‐guided PTT.     

Spontaneous Outcropping of Self‐Assembled Insulating Nanodots in Solution‐Derived Metallic Ferromagnetic La0.7Sr0.3MnO3 Films

A new mechanism is proposed for the generation of self‐assembled nanodots at the surface of a film based on spontaneous outcropping of the secondary phase of a nanocomposite epitaxial film. Epitaxial self‐assembled Sr–La oxide insulating nanodots are formed through this mechanism at the surface of an epitaxial metallic ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) film grown on SrTiO₃ from chemical solutions. TEM analysis reveals that, underneath the La–Sr oxide (LSO) nanodots, the film switches from the compressive out‐of‐plane stress component to a tensile one. It is shown that the size and concentration of the nanodots can be tuned by means of growth kinetics and through modification of the La excess in the precursor chemical solution. The driving force for the nanodot formation can be attributed to a cooperative effect involving the minimization of the elastic strain energy and a thermodynamic instability of the LSMO phase against the formation of a Ruddelsden–Popper phase Sr₃Mn₄O₇ embedded in the film, and LSO surface nanodots. The mechanism can be described as a generalization of the classical Stranski–Krastanov growth mode involving phase separation. LSO islands induce an isotropic strain to the LSMO film underneath the island which decreases the magnetoelastic contribution to the magnetic anisotropy.     

Assigning Electronic States in Carbon Nanodots

Luminescent carbon nanodots exhibit enormous potential to be used as functional building blocks in energy conversion devices. Their intrinsic heterogeneity, however, hinders a thorough characterization and subsequently their applicability. In this work, the spectroscopic and electrochemical properties of prototype carbon nanodots were thoroughly studied in solution. Spectro‐electrochemical experiments revealed information on the spectral features of several oxidized and reduced species of our carbon nanodots. Therefore, these experiments were complemented with pulse radiolysis where the spectral signature of the one‐electron oxidized and reduced forms were identified. Both, electrochemically and radiolytically reduced and oxidized products show high reactivity in solution which correlates with an open‐shell character, detected by paramagnetic resonance spectroscopy. The herein presented studies help to understand the complex nature of carbon nanodots and serve to overcome difficulties in device fabrication.     

Fabrication of a Macroporous Microwell Array for Surface‐Enhanced Raman Scattering

Here, a colloidal templating procedure for generating high‐density arrays of gold macroporous microwells, which act as discrete sites for surface‐enhanced Raman scattering (SERS), is reported. Development of such a novel array with discrete macroporous sites requires multiple fabrication steps. First, selective wet‐chemical etching of the distal face of a coherent optical fiber bundle produces a microwell array. The microwells are then selectively filled with a macroporous structure by electroless template synthesis using self‐assembled nanospheres. The fabricated arrays are structured at both the micrometer and nanometer scale on etched imaging bundles. Confocal Raman microscopy is used to detect a benzenethiol monolayer adsorbed on the macroporous gold and to map the spatial distribution of the SERS signal. The Raman enhancement factor of the modified wells is investigated and an average enhancement factor of 4 × 10⁴ is measured. This demonstrates that such nanostructured wells can enhance the local electromagnetic field and lead to a platform of ordered SERS‐active micrometer‐sized spots defined by the initial shape of the etched optical fibers. Since the fabrication steps keep the initial architecture of the optical fiber bundle, such ordered SERS‐active platforms fabricated onto an imaging waveguide open new applications in remote SERS imaging, plasmonic devices, and integrated electro‐optical sensor arrays.     

3D Microfluidic Surface‐Enhanced Raman Spectroscopy (SERS) Chips Fabricated by All‐Femtosecond‐Laser‐Processing for Real‐Time Sensing of Toxic Substances

A novel all‐femtosecond‐laser‐processing technique is proposed for the fabrication of 2D periodic metal nanostructures inside 3D glass microfluidic channels, which have applications to real‐time surface‐enhanced Raman spectroscopy (SERS). In the present study, 3D glass microfluidic channels are fabricated by femtosecond‐laser‐assisted wet etching. This is followed by the space‐selective formation of Cu‐Ag layered thin films inside the microfluidic structure via femtosecond laser direct writing ablation and electroless metal plating. The Cu‐Ag films are subsequently nanostructured by irradiation with linearly polarized beams to form periodic surface structures. This work demonstrates that a double exposure to laser beams having orthogonal polarization directions can generate arrays of layered Cu‐Ag nanodots with dimensions as small as 25% of the laser wavelength. The resulting SERS microchip is able to detect Rhodamine 6G, exhibiting an enhancement factor of 7.3 × 10⁸ in conjunction with a relative standard deviation of 8.88%. This 3D microfluidic chip is also found to be capable of the real‐time SERS detection of Cd²⁺ ions at concentrations as low as 10 ppb in the presence of crystal violet. This technique shows significant promise for the fabrication of high performance microfluidic SERS platforms for the real‐time sensing of toxic substances with ultrahigh sensitivity.     

Arrays of Inorganic Nanodots and Nanowires Using Nanotemplates Based on Switchable Block Copolymer Supramolecular Assemblies

Here, a novel and simple route to fabricate highly dense arrays of palladium nanodots and nanowires with sub‐30 nm periodicity using nanoporous templates fabricated from supramolecular assemblies of a block copolymer, polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) and a low molecular weight additive, 2‐(4′‐hydroxybenzeneazo) benzoic acid (HABA) is demonstrated. The palladium nanoparticles, which are directly deposited in the nanoporous templates from an aqueous solution, selectively migrate in the pores mainly due to their preferential attraction to the P4VP block covering the pore wall. The polymer template is then removed by oxygen plasma etching or pyrolysis in air resulting in palladium nanostructures whose large scale morphology mirrors that of the original template. The method adopted in this work is general and versatile so that it could easily be extended for patterning a variety of metallic materials into dot and wire arrays.     

Electronic structures and transport properties of BN nanodot superlattices of armchair graphene nanoribbons

The electronic and transport properties of embedded boron nitride(BN) nanodot superlattices of armchair graphene nanoribbons are studied by first-principles calculations.The band structure of the graphene superlattice strongly depends on the geometric shape and size of the BN nanodot,as well as the concentration of nanodots.The conduction bands and valence bands near the Fermi level are nearly symmetric,which is induced by electron-hole symmetry.When B and N atoms in the graphene superlattices with a triangular BN nanodot are exchanged,the valance bands and conduction bands are inverted with respect to the Fermi level due to electron-hole symmetry.In addition,the hybridization ofπorbitals from C and redundant B atoms or N atoms leads to a localized band appearing near the Fermi level.Our results also show a series of resonant peaks appearing in the conductance.This strongly depends on the distance of the two BN nanodots and on the shape of the BN nanodot. Controlling these parameters might allow the modulation of the electronic response of the systems.     

Hierarchical Porosity in Self‐Assembled Polymers: Post‐Modification of Block Copolymer–Phenolic Resin Complexes by Pyrolysis Allows the Control of Micro‐ and Mesoporosity

It is shown that self‐assembled hierarchical porosity in organic polymers can be obtained in a facile manner based on pyrolyzed block‐copolymer–phenolic resin nanocomposites and that a given starting composition can be post‐modified in a wide range from monomodal mesoporous materials to hierarchical micro‐mesoporous materials with a high density of pores and large surface area per volume unit (up to 500–600 m² g^–1). For that purpose, self‐assembled cured composites are used where phenolic resin is templated by a diblock copolymer poly(4‐vinylpyridine)‐block‐polystyrene (P4VP‐b‐PS). Mild pyrolysis conditions lead only to monomodal mesoscale porosity, as essentially only the PS block is removed (length scale of tens of nanometers), whereas during more severe conditions under prolonged isothermal pyrolysis at 420 °C the P4VP chains within the phenolic matrix are also removed, leading to additional microporosity (sub‐nanometer length scale). The porosity is analyzed using transmission electron microscopy (TEM), small‐angle X‐ray scattering, electron microscopy tomography (3D‐TEM), positron annihilation lifetime spectroscopy (PALS), and surface‐area Brunauer–Emmett–Teller (BET) measurements. Furthermore, the relative amount of micro‐ and mesopores can be tuned in situ by post modification. As controlled pyrolysis leaves phenolic hydroxyl groups at the pore walls and the thermoset resin‐based materials can be easily molded into a desired shape, it is expected that such materials could be useful for sensors, separation materials, filters, and templates for catalysis.     

Nanodot‐Enhanced High‐Efficiency Pure‐White Organic Light‐Emitting Diodes with Mixed‐Host Structures

A relatively high‐efficiency, fluorescent pure‐white organic light‐emitting diode was fabricated using a polysilicic acid (PSA) nanodot‐embedded polymeric hole‐transporting layer (HTL). The diode employed a mixed host in the single emissive layer, which comprised 0.5 wt % yellow 5,6,11,12‐tetra‐phenylnaphthacene doped in the mixed host of 50 % 2‐(N,N‐diphenyl‐amino)‐6‐[4‐(N,N‐diphenylamino)styryl]naphthalene and 50 % N,N′‐bis‐(1‐naphthyl)‐N,N′‐diphenyl‐1,10‐biphenyl‐4‐4′‐diamine. By incorporating 7 wt % 3 nm PSA nanodot into the HTL of poly(3,4‐ethylene‐dioxythiophene)‐poly‐(styrenesulfonate), the efficiency at 100 cd m^–2 was increased from 13.5 lm W^–1 (14.7 cd A^–1; EQE: 7.2 %) to 17.1 lm W^–1 (17.6 cd A^–1; EQE: 8.3 %). The marked efficiency improvement may be attributed to the introduction of the PSA nanodot, leading to a better carrier‐injection‐balance.     

Conjugated Polymer Nanodots as Ultrastable Long‐Term Trackers to Understand Mesenchymal Stem Cell Therapy in Skin Regeneration

Stem cell–based therapies hold great promise in providing desirable solutions for diseases that cannot be effectively cured by conventional therapies. To maximize the therapeutic potentials, advanced cell tracking probes are essential to understand the fate of transplanted stem cells without impairing their properties. Herein, conjugated polymer (CP) nanodots are introduced as noninvasive fluorescent trackers with high brightness and low cytotoxicity for tracking of mesenchymal stem cells (MSCs) to reveal their in vivo behaviors. As compared to the most widely used commercial quantum dot tracker, CP nanodots show significantly better long‐term tracking ability without compromising the features of MSCs in terms of proliferation, migration, differentiation, and secretome. Fluorescence imaging of tissue sections from full‐thickness skin wound–bearing mice transplanted with CP nanodot‐labeled MSCs suggests that paracrine signaling of the MSCs residing in the regenerated dermis is the predominant contribution to promote skin regeneration, accompanied with a small fraction of endothelial differentiation. The promising results indicate that CP nanodots could be used as next generation of fluorescent trackers to reveal the currently ambiguous mechanisms in stem cell therapies through a facile and effective approach.     

Magnetoresistive Memory with Ultralow Critical Current for Magnetization Switching

The data writing and thermal stability of information storage are studied theoretically for a magnetic random access memory (MRAM) composed of a magnetic tunnel junction or multilayer exhibiting giant magnetoresistance. The theoretical analysis focuses on the magnetization switching in the “free” layer of a MRAM cell, which is induced by a spin‐polarized current imposing a spin‐transfer torque (STT) on the magnetization. It is shown that the writing current in such an STT‐MRAM reduces dramatically near a spin reorientation transition (SRT) driven by lattice strains and/or surface magnetic anisotropy and even tends to zero under certain conditions. In particular, at the size‐driven SRT in the perpendicular‐anisotropy CoFeB‐MgO tunnel junctions, the critical current densities for magnetization reorientations between the parallel and antiparallel states are expected to fall to low values of about 1.3 × 10⁵ and ?3.3 × 10⁴ A cm^?2. Remarkably, STT‐MRAMs may combine low writing current with very high thermal stability of information storage (retention over 10 years) even at a high density ≈500 Gbit inch^?2.     

Self‐Assembly of Nanostructured,Complex, Multication Films via Spontaneous Phase Separation and Strain‐Driven Ordering

Spontaneous self‐assembly of a multication nanophase in another multication matrix phase is a promising bottom‐up approach to fabricate novel, nanocomposite structures for a range of applications. In an effort to understand the mechanisms for such self‐assembly, complimentary experimental and theoretical studies are reported to first understand and then control or guide the self‐assembly of insulating BaZrO₃ (BZO) nanodots within REBa₂Cu₃O_7–δ (RE = rare earth elements including Y, REBCO) superconducting films. The strain field developed around BZO nanodots embedded in the REBCO matrix is a key driving force dictating the self‐assembly of BZO nanodots along REBCO c‐axis. The size selection and spatial ordering of BZO self‐assembly are simulated using thermodynamic and kinetic models. The BZO self‐assembly is controllable by tuning the interphase strain field. REBCO superconducting films with BZO defect arrays self‐assembled to align in both vertical (REBCO c‐axis) and horizontal (REBCO ab‐planes) directions result in the maximized pinning and J_c performance for all field angles with smaller angular J_c anisotropy. The work has broad implications for the fabrication of controlled self‐assembled nanostructures for a range of applications via strain‐tuning.     

Fabrication of large scale arrays of metallic nanodots by means of high resolution e-beam lithography

We present the fabrication of large nanodot arrays with very small single dot sizes made of different metals and on different substrates using e-beam lithography and lift-off techniques. Nanodot arrays have a high potential for bioanalytical applications aiming towards single molecule detection. In addition, they can be used as well defined models of heterogeneous catalysts. For this purpose an independent control of the dot size and the distance between the dots is necessary. This is a limitation for many fabrication techniques, which can be overcome by e-beam lithography. However, aiming for the dot size of few tens of nanometers, we observed a strong influence of the beam focusing and astigmatism on the quality of the fabricated nanodot arrays. A significant improvement, i.e. reduction of defect density as well as better control of the dot size, was achieved by modification of the fine focus and stigmation system of the used e-beam writer. Using lift-off technique, we successfully fabricated platinum dot arrays on glassy carbon with target dot diameter ranging from 25 to 35 nm. By applying additional annealing step, we could fabricate nanodot arrays made of gold on SiO₂ with very small dot sizes down to 6 nm and pitch of 100 nm. Furthermore, the generation of large area arrays of nanosized pillars was demonstrated using the same exposure strategy in a negative tone HSQ resist.     

Facile Generation of L10‐FePt Nanodot Arrays from a Nanopatterned Metallopolymer Blend of Iron and Platinum Homopolymers

Hard ferromagnetic (L1₀ phase) FePt alloy nanoparticles (NPs) with extremely high magnetocrystalline anisotropy are considered to be one of the most promising candidates for the next generation of ultrahigh‐density data storage system. The question of how to generate ordered patterns of L1₀‐FePt NPs and how to transform the technology for practical applications represents a key current challenge. Here the direct synthesis of L1₀ phase FePt NPs by pyrolysis of Fe‐containing and Pt‐containing metallopolymer blend without post‐annealing treatment is reported. Rapid single‐step fabrication of large‐area nanodot arrays (periodicity of 500 nm) of L1₀‐ordered FePt NPs can also be achieved by employing the metallopolymer blend, which possesses excellent solubility in most organic solvents and good solution processability, as the precursor through nanoimprint lithography (NIL). Magnetic force microscopy (MFM) imaging of the nanodot pattern indicates that the patterned L1₀ phase FePt NPs are capable of exhibiting decent magnetic response, which suggests a great potential to be utilized directly in the fabrication of bit patterned media (BPM) for the next generation of magnetic recording technology.     

Sub‐Nanometer Level Size Tuning of a Monodisperse Nanoparticle Array Via Block Copolymer Lithography

The fabrication and catalytic application of a size‐tunable monodisperse nanoparticle array enabled by block copolymer lithography is demonstrated. Highly uniform vertical cylinder nanodomains are achieved in poly(styrene‐block‐4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer thin‐films by solvent annealing. The prominent diffusion of the anionic metal complexes into the protonated P4VP cylinder nanodomains occurs through specific electrostatic interactions in a weakly acidic aqueous solution. This well‐defined diffusion with nanoscale confinement enables preparation of the laterally ordered monodisperse nanoparticle array with sub‐nanometer level precise size tuning. The controlled growth of monodisperse nanoparticle arrays is proven by their catalytic use for vertical carbon nanotube (CNT) growth via plasma enhanced chemical vapor deposition (PECVD). Since the size of the catalyst particles is the decisive parameter for the diameters and wall‐numbers of CNTs, the highly selective growth of double‐walled or triple‐walled CNTs could be accomplished using monodisperse nanoparticle arrays.