Nanopatterned Films Research Articles

Optical Direction of Morphological Complexity of Periodic Semiconductor Nanoarchitectures Deposited Via Templateless Photeolectrodeposition

Electrodeposition of Se-Te alloy films under illumination spontaneously generates nanopatterned films with significant periodic order. The feature sizes, periodicities, anisotropies, and orientations of the nanoscale pattern are a function of the exact nature of the optical excitation. Isotropic morphologies consisting of ordered arrays of nanopores were generated using unpolarized illumination whereas linearly polarized light resulted in highly-anisotropic lamellar morphologies with the long axes of the patterns aligned along the E-field vector. The pattern periodicity was encoded by the illumination spectral profile. A single periodicity in single spatial direction was only generated even with the use of broadband and multimodal spectral profiles and multiple polarization inputs and the periodicity was found to be sensitive to all investigated tuning of such profiles. Structures with nonequal periodicities in the two orthogonal in-plane directions could also be generated and both periodicities could be independently controlled. Additoinally, the anisotropies, subwavelength feature size and periodic nature of the deposits enables the fabrication of optical elements with significant wavelength and polarization discrimination. The nanopatterning process occurred without the use of any type of physical or chemical templating agents: no photomask, patterned substrate nor surfactants/ligands were used to influence the morphology. Modeling of the growth using a combination of full-wave electromagnetic simulations of light absorption and scattering coupled with Monte Carlo simulations of mass addition successfully reproduced the experimentally observed morphologies and indicated that morphology development was a consequence of the fundamental light-matter interactions during growth.

Transparent Displays Utilizing Nanopatterned Quantum Dot Films

We report the realization of a transparent display using glass covered by a nanopatterned quantum dot (QD) film with good transmittance. The film was fabricated by nanoimprint lithography (NIL) and spin coating of colloidal QDs with specificexcitation maxima. The produced nanopatterned QD film was attached to transparent glass, enabling active image generation using a laser light source of a specific wavelength. Selective light emission was induced by strongly exciting the laser-exposed film surface, creating desired images, with color modulationenabled by controlling the QD layer (dozens of nanometers in size) via nanopatterning. The nanopatterned QD film used for image generation exhibits excellent transmittance (>80%), and can be used for transparent displays, with image realization in both bright and dark spaces. The fabricated displays have wide viewing anglesowing to their good light emission characteristics, and the fabrication through spin coating renders the fabrication process simple and applicable to large areas.

Dimension- and shape-dependent thermal transport in nano-patterned thin films investigated by scanning thermal microscopy

Scanning thermal microscopy (SThM) is a technique which is often used for the measurement of the thermal conductivity of materials at the nanometre scale. The impact of nano-scale feature size and shape on apparent thermal conductivity, as measured using SThM, has been investigated. To achieve this, our recently developed topography-free samples with 200 and 400 nm wide gold wires (50 nm thick) of length of 400–2500 nm were fabricated and their thermal resistance measured and analysed. This data was used in the development and validation of a rigorous but simple heat transfer model that describes a nanoscopic contact to an object with finite shape and size. This model, in combination with a recently proposed thermal resistance network, was then used to calculate the SThM probe signal obtained by measuring these features. These calculated values closely matched the experimental results obtained from the topography-free sample. By using the model to analyse the dimensional dependence of thermal resistance, we demonstrate that feature size and shape has a significant impact on measured thermal properties that can result in a misinterpretation of material thermal conductivity. In the case of a gold nanowire embedded within a silicon nitride matrix it is found that the apparent thermal conductivity of the wire appears to be depressed by a factor of twenty from the true value. These results clearly demonstrate the importance of knowing both probe-sample thermal interactions and feature dimensions as well as shape when using SThM to quantify material thermal properties. Finally, the new model is used to identify the heat flux sensitivity, as well as the effective contact size of the conventional SThM system used in this study.

Macroscale Films of Highly Ordered Periodic 3D Semiconductor Nanostructures Generated Via Lithography-Free Photoelectrodeposition

Electrodeposition of Se-Te alloy films under illumination spontaneously generates nanopatterned films with significant periodic order. The feature sizes, periodicities, anisotropies, and orientations of the nanoscale pattern are a function of the exact nature of the optical excitation. Isotropic morphologies consisting of ordered arrays of nanopores were generated using unpolarized illumination whereas linearly polarized light resulted in highly-anisotropic lamellar morphologies with the long axes of the patterns aligned along the E-field vector. The use of two non-orthogonal polarized sources simultaneously generated patterns oriented along the average E-field vector and with degrees of anisotropy related to the difference in orientation between the two input E-field vectors and the phase correlation between the sources. The pattern periodicity was encoded by the illumination spectral profile. A single periodicity in single spatial direction was only generated even with the use of broadband and multimodal spectral profiles and multiple polarization inputs and the periodicity was found to be sensitive to all investigated tuning of such profiles. The incidence of the illumination set the direction the material grew from the substrate, mimicking natural phototropism: grazing illumination resulted in growth at significant angle to the surface normal. Also, additional 3D morphological complexity could be directed by utilizing temporal changes in the illumination; for example, zig-zag structures resulted from oscillation of the illumination incidence back and forth across the surface normal and woodpile structures were generated by periodically switching between orthogonal polarization states. The nanopatterning process occurred without the use of any type of physical or chemical templating agents: no photomask, patterned substrate nor surfactants/ligands were used to influence the morphology. Modeling of the growth using a combination of full-wave electromagnetic simulations of light absorption and scattering coupled with Monte Carlo simulations of mass addition successfully reproduced the experimentally observed morphologies and indicated that morphology development was a consequence of the fundamental light-matter interactions during growth.

Nano-patterned polyelectrolyte multilayers assembled using block copolymer templates: The combined effect of ionic strength and nano-confinement

Polyelectrolyte multilayers gain their importance from their applicability to a wide variety of functional building blocks. The ability to create these multilayers as laterally nano-patterned films, which has only been scarcely investigated so far, augments the functionality of the multilayer and makes it valuable for applications that require nanoscale features or periodic arrangement, such as photonic devices, catalytic surfaces, and biomedical applications. In this study we reveal how the lateral confinement imposed by block copolymer nano-domains in thin film templates affects the assembly of the deposited polyelectrolyte layers at different ionic strengths, and how the combined effects of nano-confinement and ionic strength dictate the final structure of the multilayer. These fundamental insights provide the basis for successful construction of nano-patterned, functional coatings.

Pinning effects on flux flow instability in epitaxial Nb thin films

The flux flow properties of epitaxial niobium films with different pinning strengths are investigated by dc electrical resistance measurements and mapped to results derived within the framework of a theoretical model. The cases of weak random pinning in as-grown films, strong random pinning in Ga ion-irradiated films, and strong periodic pinning induced by a nanogroove array milled by a focused ion beam are investigated. The generic feature of the current–voltage curves of the films consists of instability jumps to the normal state at some instability current density j* as the vortex lattice reaches its critical velocity v*. While monotonically decreases for as-grown films, the irradiated films exhibit a non-monotonic dependence attaining a maximum in the low-field range. In the case of nanopatterned films, this broad maximum is accompanied by a much sharper maximum in both and , which we attribute to the commensurability effect when the spacing between the vortex rows coincides with the location of the grooves. We argue that the observed behavior of can be explained by the pinning effect on the vortex flow instability and support our claims by fitting the experimental data to theoretical expressions derived within a model accounting for the field dependence of the depinning current density.

Hybrid ZnO-organic semiconductor interfaces in photodetectors: A comparison of two near-infrared donor-acceptor copolymers

Hybrid organic-inorganic photodiode interfaces have gained significant interest due to their unique physical properties such as mechanical flexibility and high photosensitivity. Two diketopyrrolopyrrole (DPP)-based donor-acceptor copolymers with different backbone conformations are characterized in an inverted non-fullerene photodiode architecture using ZnO nano-patterned films as the electron transport layer. The DPP copolymer with a thienothiophene unit (PBDT-TTDPP) is more planar and rigid compared to the DPP system with a thiophene unit connecting the donor and acceptor moieties within the monomer (PBDT-TDPP). The hybrid interfaces were optimized by using poly(3-hexylthiophene) (P3HT) as the p-type layer for monitoring the critical thickness and morphology of the ZnO layer. The maximum photoresponsivity from a P3HT:ZnO photodiode was found to be 56 mA/W. The photoresponsivity of PBDT-TTDPP:ZnO photodiodes were found to be more than two orders of magnitude higher than PBDT-TDPP:ZnO photodiodes, which is attributed to an enhanced transport of carriers due to the planar backbone conformation of the PBDT-TTDPP copolymer. Capacitance-voltage measurements from hybrid Schottky barrier interfaces further shed light into the nature of photocarriers and device parameters. First-principles time-dependent density-functional theoretical calculations yield a higher absorptivity for the PBDT-TTDPP dimer compared to PBDT-TDPP.

Block Copolymer Directed Nanostructured Surfaces as Templates for Confined Surface Reactions

Despite advances in nanomaterials synthesis, the bottom-up preparation of nanopatterned films as templates for spatially confined surface reactions remains a challenge. We report an approach to fabricating nanoscale thin film surface structures with periodicities on the order of 20 nm and with the capacity to localize reactions with small molecules and nanoparticles. A block copolymer (BCP) of polystyrene-block-poly[(allyl glycidyl ether)-co-(ethylene oxide)] (PS-b-P(AGE-co-EO)) is used to prepare periodically ordered, reactive thin films. As proof-of-principle demonstrations of the versatility of the chemical functionalization, a small organic molecule, an amino acid, and ultrasmall silica nanoparticles are selectively attached via thiol–ene click chemistry to the exposed P(AGE-co-EO) domains of the BCP thin film. Our approach employing click chemistry on the spatially confined reactive surfaces of a BCP thin film overcomes solvent incompatibilities typically encountered when synthetic polymers are funct...

Resonance-Based Detection of Magnetic Nanoparticles and Microbeads Using Nanopatterned Ferromagnets

Biosensing with ferromagnet-based magnetoresistive devices has been dominated by electrical detection of particle-induced changes to the devices' static magnetic configuration. There are however potential advantages to be gained from using field dependent, high frequency magnetization dynamics for magnetic particle detection. Here we demonstrate the use of nano-confined ferromagnetic resonances in periodically patterned magnetic films for the detection of adsorbed magnetic particles with diameters ranging from 6 nm to 4 $\mu$m. The nanopatterned films contain arrays of holes which can act as preferential adsorption sites for small particles. Hole-localized particles act in unison to shift the resonant frequencies of the various modes of the patterned layer with shift polarities determined by the localization of each mode within the nanopattern's repeating unit cell. The same polarity shifts are observed for a large range of coverages, even when hole-localized particles are covered by quasi-continuous particle sheets. For large particles however, preferential adsorption no longer occurs, leading to resonance shifts with polarities which are independent of the mode localization. Analogous shifts are seen in continuous layers where, for small particles, the shift of the layer's fundamental mode is typically about 10 times less than in patterned systems and induced by relatively weak fields emanating beyond the particle in the direction of the static applied field. This highlights the importance of having confined modes consistently positioned with respect to nearby particles.

In situ Electrochemical Impedance Spectroscopy of Electrostatically Driven Selective Gold Nanoparticle Adsorption on Block Copolymer Lamellae.

Electrostatic attraction between charged nanoparticles and oppositely charged nanopatterned polymeric films enables tailored structuring of functional nanoscopic surfaces. The bottom-up fabrication of organic/inorganic composites for example bears promising potential toward cheap fabrication of catalysts, optical sensors, and the manufacture of miniaturized electric circuitry. However, only little is known about the time-dependent adsorption behavior and the electronic or ionic charge transfer in the film bulk and at interfaces during nanoparticle assembly via electrostatic interactions. In situ electrochemical impedance spectroscopy (EIS) in combination with a microfluidic system for fast and reproducible liquid delivery was thus applied to monitor the selective deposition of negatively charged gold nanoparticles on top of positively charged poly(2-vinylpyridinium) (qP2VP) domains of phase separated lamellar poly(styrene)-block-poly(2-vinylpyridinium) (PS-b-qP2VP) diblock copolymer thin films. The acquired impedance data delivered information with respect to interfacial charge alteration, ionic diffusion, and the charge dependent nanoparticle adsorption kinetics, considering this yet unexplored system. We demonstrate that the selective adsorption of negatively charged gold nanoparticles (AuNPs) on positively charged qP2VP domains of lamellar PS-b-qP2VP thin films can indeed be tracked by EIS. Moreover, we show that the nanoparticle adsorption kinetics and the nanoparticle packing density are functions of the charge density in the qP2VP domains.

Area-selective monolayer deposition of iron(II) α-octabutoxyphthalocyanine complex on nano-patterned TiO2 thin films on a glass substrate

Iron(II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine complex (Fe(OBu)8Pc) was synthesized and deposited on nano-cratered titanium dioxide thin films on a glass substrate. The TiO2 thin films had been selectively modified with bifunctional 2-aminoethyl dihydrogen phosphate, which binds to the nano-cratered TiO2 surface from its phosphate group, but leaves the glass substrate unmodified, creating hence a nano-patterned amino-functionalized surface. n-Butyl amine was used as a model ligand to examine the ability of Fe(OBu)8Pc to bind with amino groups in a solution state. According to 1H nuclear magnetic resonance and UV–Vis measurements, two separate amino groups can bind to the axial sites of Fe(OBu)8Pc using coordination bonds. Fe(OBu)8Pc complex molecules form a monolayer on an amino-functionalized TiO2 thin films when using THF as a solvent in the deposition step. The X-ray photoelectron spectroscopy and scanning electron microscopy analysis results indicate that a monolayer of Fe(OBu)8Pc complexes lie parallel to the carrier surface replicating the nanostructure while having free coordination sites for sensing applications and leaving the glass substrate on the floors of the craters available for further functionalization.

Block Copolymer-Assisted Nanopatterning of Porous Lead Titanate Thin Films for Advanced Electronics

Chemical self-assembly of functional ferroelectric PbTiO3 using amphiphilic block copolymers is used to prepare low-cost nanopatterned porous thin films with nanostructure control down to ∼22 nm of lateral size. The approach developed in this work is highly effective in patterning PbTiO3 and avoids costly high-resolution lithography techniques and harmful etching processes. Nanopatterned films presenting a honeycomb-like arrangement of pores with diameters between 80 and 140 nm are obtained. The ordering and thickness of the films are discussed as a function of the precursors concentration and withdrawal rate used for films dip-coating. The nanopatterned porous films with thickness of 22 nm exhibit tetragonal perovskite crystallographic phase and ferroelectric response at the nanoscale. These films display vertical open porosity from the film surface to the substrate, leading to nearly 50% of the substrate surface being accessible to further functionalization toward low-cost, sustainable, multifunctional ...

Interplay of flux guiding and Hall effect in Nb films with nanogrooves

The interplay between vortex guiding and the Hall effect in superconducting Nb films with periodically arranged nanogrooves is studied via four-probe measurements in standard and Hall configurations and accompanying theoretical modeling. The nanogrooves are milled by focused ion beam and induce a symmetric pinning potential of the washboard type. The resistivity tensor of the films is determined in the limit of small current densities at temperatures close to the critical temperature for the fundamental matching configuration of the vortex lattice with respect to the pinning nanolandscape. The angle between the current direction with respect to the grooves is set at seven fixed values between 0° and 90°. A sign change is observed in the temperature dependence of the Hall resistivity of as-grown films in a narrow temperature range near Tc. By contrast, for all nanopatterned films is non-zero in a broader temperature range below Tc, allowing us to discriminate between two contributions in , namely one contribution originating from the guided vortex motion and the other one caused by the Hall anomaly just as in as-grown Nb films. All four measured resistivity components are successfully fitted to analytical expressions derived within the framework of a stochastic model of competing isotropic and anisotropic pinning. This provides evidence of the model validity for the description of the resistive response of superconductor thin films with washboard pinning nanolandscapes.

Micro-Shaping of Nanopatterned Surfaces by Electron Beam Irradiation.

We show that planar nanopatterned thin films on standard polycarbonate (PC) compact discs (CD) can be micro-shaped in a non-contact manner via direct e-beam exposure. The shape of the film can be controlled by proper selection of the e-beam parameters. As an example of application, we demonstrate a two-dimensional (2D) array of micro-lenses/reservoirs conformally covered by an Al 2D nanohole array (NHA) film on a PC CD substrate. It is also shown that such a curvilinear Al NHA layer can be easily transferred onto a flexible polymeric support. The presented technique provides a new tool for creating lab-on-CD architectures and developing multifunctional (flexible) non-planar nanostructured films and surfaces.

Cu diffusion as an alternative method for nanopatterned CuTCNQ film growth

In this paper we show by means of ‘in situ’ x-ray diffraction studies that CuTCNQ formation from Cu(solid)–TCNQ(solid tetracyanoquinodimethane) goes through Cu diffusion at room temperature. The film quality depends on the TCNQ evaporation rate. At low evaporation rate we get a single phase-I CuTCNQ film very well crystallized and well oriented. The film has a CuTCNQ(0 2 0) orientation. The film is formed by CuTCNQ nanorods of a very homogeneous size. The film homogeneity has also been seen by atomic force microscopy (AFM). The electronic properties of the film have been measured by x-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS). Thus, the Cu-diffusion method has arisen as a very simple, clean and efficient method to grow localized CuTCNQ nanorods on Cu, opening up new insights for technological applications.

Enhancement of light collection through flexible polymeric films patterned using self-assembled photonic crystals

Flexible nanopatterned polymeric thin films are fabricated by a simple and cost-effective soft imprint lithography technique. The use of low-cost self-assembled photonic crystals as structured masters is the highlight of this work. Two types of polymeric films, with and without a dye, are patterned. Both have shown very good anti-reflection properties and the dyed films have also shown a polarization-independent enhancement in the absorption for the normal incidence of light. The structures with smaller features have shown better anti-reflection properties. Angle-dependent reflection measurements show an improved reduction in reflection at larger incidence angles. A maximum reduction in reflection of 11% is obtained from the patterned film in comparison to the bare glass substrate when the light is incident at an angle of 60°. This is very useful in photovoltaic devices as wide-angle anti-reflection coating. The reflection calculated using the effective medium approximation supports the experimental results.

The effect of shear-thickening on liquid transfer from an idealized gravure cell

Gravure printing is an economical roll-to-roll processing technique with potential to revolutionize the fabrication of nano-patterned thin films at high throughput. In the present study, we investigated the impact of shear-thickening on the liquid transfer from an idealized gravure cell by a combination of experiments and numerical computations. We chose as a model system fumed silica nanoparticles dispersed in polypropylene glycol; these dispersions exhibit shear and extensional thickening as verified by steady shear and filament stretching extensional rheometry. Model gravure printing experiments were conducted using a linear motor to pick out the fluid vertically up from a truncated conical shaped idealized gravure cell cavity; the cell size is large enough that gravity is important, and therefore experiments were also conducted to pickout the fluid vertically down from the cavity-on-top. The amount liquid transfer from the cavity was studied with varying stretch velocities and dispersion concentrations. The filament profile evolution during the pickout process was examined using a high speed camera. Beyond a critical stretch rate, shear-thickening of the fluid, manifested by the formation of long stable filaments, exacerbates gravitational drainage during pickout. Beyond a second critical stretch rate, shear-thinning induces conical profile evolutions that result in pickout insensitive to stretch rate. All of these observations were qualitatively predicted by finite element computations using a generalized Newtonian fluid model, where the shear rheology was modeled explicitly. We showed that under the influence of gravity, wetting/de-wetting may be a critical phenomenon in determining pickout at low stretch rates.

Photonic quasicrystal nanopatterned silicon thin film for photovoltaic applications

In this paper, the authors numerically studied the optical properties of a silicon photonic quasicrystal (PQC) nanohole array for photovoltaic applications. With the same active layer thickness, the ultimate efficiency of a solar cell integrated with an optimized PQC nanohole array can be enhanced by 9.01% and 1.40% compared to that with an ordered square lattice of a nanohole array and a random nanohole array, respectively. The absorptance enhancement is mainly due to the higher-order rotational symmetry in PQC structures, which leads to the presence of additional resonant modes, the broadening of existing modes and the reduction of surface reflectance. The angular response for both transverse-electric and transverse-magnetic modes are also analyzed in detail.

Two-dimensional skyrmion lattice in a nanopatterned magnetic film

We study the possibility of a two-dimensional (2D) skyrmion crystal stabilization in a magnetic film with perpendicular anisotropy in the absence of Dzyaloshinskii-Moriya interaction by creating the regular array of blind holes or stubs. By micromagnetic simulation we demonstrate that skyrmions can be stable in the patterned films with the parameters of ordinary materials such as CoPt, FePt, or FePd. The skyrmion lattices can be initialized in the system by simple magnetization in the uniform external magnetic field. At the zero external field the skyrmion helicity depends on the geometry of the blind hole or stub but also can be tuned by applying the field. The suggested method makes it possible to create dense enough (with the period less than 100 nm) skyrmion lattices which are important to carry out transport measurements.

Calcination-free micromolding in capillaries for nanopatterning of inorganic upconversion luminescent layers on flexible plastic sheets

We report calcination-free micromolding in capillaries for the nanopatterning of inorganic upconversion luminescent layers on flexible plastic sheets. We prepared Er3+- and Yb3+-codoped NaYF4 nanoparticles modified with a cationic polymer to improve dispersion stability of aqueous dispersion of the NaYF4 nanoparticles. We controlled the line width and density of nanoparticles in the NaYF4 nanoparticle films on flexible plastic sheets by adjusting the concentrations of the NaYF4 nanoparticle dispersion and the channel sizes of silicone stamps, resulting in the formation of nanopatterned NaYF4 nanoparticle films at the line width of the nanoparticle size (50nm). Visible upconversion luminescence and near-infrared fluorescence appeared from the NaYF4 nanoparticle films excited with a near-infrared laser beam. These results demonstrate that calcination free micromolding in capillaries using aqueous dispersion of NaYF4 nanoparticles modified with a cationic polymer allows for the nanopatterning of inorganic upconversion luminescent layers on flexible plastic sheets.