Nonequilibrium self-assembly of metals on diblock copolymer templates

Selective Deposition of Copper on Self-Assembled Block Copolymer Surfaces via Physical Vapor Deposition

Block copolymer (BCP) self-assembly produces chemically and topographically patterned surfaces which are used to guide the formation of Cu nanostructures by exploiting differences in the mobility of vapor-deposited species on each microdomain. Cu metal films a few nm thick were deposited on three different BCP surfaces self-assembled from poly(styrene-b-methyl methacrylate) (PS-b-PMMA) and polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP). For PS-b-PMMA, the effects of chemical heterogeneity dominate over the effects of the 2 nm peak-to-valley topography, and sputtered Cu preferentially wets the PS block. PS-b-P2VP has greater chemical and topographical contrast and shows a wider process window for selective deposition. Cu grown by evaporation has less surface mobility, and shadowing effects are believed to dominate pattern formation. The hierarchical self-assembly process of thin metal films on BCP surfaces provides a route to fabricating heterogeneous metallic nanostructures.

Revealing the growth of copper on polystyrene-block-poly(ethylene oxide) diblock copolymer thin films with in situ GISAXS

Copper (Cu) as an excellent electrical conductor and the amphiphilic diblock copolymer polystyrene-block-poly(ethylene oxide) (PS-b-PEO) as a polymer electrolyte and ionic conductor can be combined with an active material in composite electrodes for polymer lithium-ion batteries (LIBs). As interfaces are a key issue in LIBs, sputter deposition of Cu contacts on PS-b-PEO thin films with high PEO fraction is investigated with in situ grazing-incidence small-angle X-ray scattering (GISAXS) to follow the formation of the Cu layer in real-time. We observe a hierarchical morphology of Cu clusters building larger Cu agglomerates. Two characteristic distances corresponding to the PS-b-PEO microphase separation and the Cu clusters are determined. A selective agglomeration of Cu clusters on the PS domains explains the origin of the persisting hierarchical morphology of the Cu layer even after a complete surface coverage is reached. The spheroidal shape of the Cu clusters growing within the first few nanometers of sputter deposition causes a highly porous Cu-polymer interface. Four growth stages are distinguished corresponding to different kinetics of the cluster growth of Cu on PS-b-PEO thin films: (I) nucleation, (II) diffusion-driven growth, (III) adsorption-driven growth, and (IV) grain growth of Cu clusters. Percolation is reached at an effective Cu layer thickness of 5.75 nm.

Correlating Nanostructure, Optical and Electronic Properties of Nanogranular Silver Layers during Polymer-Template-Assisted Sputter Deposition

Tailoring the optical and electronic properties of nanostructured polymer-metal composites demonstrates great potential for efficient fabrication of modern organic optical and electronic devices such as flexible sensors, transistors, diodes, or photovoltaics. Self-assembled polymer-metal nanocomposites offer an excellent perspective for creating hierarchical nanostructures on macroscopic scales by simple bottom-up processes. We investigate the growth processes of nanogranular silver (Ag) layers on diblock copolymer thin film templates during sputter deposition. The Ag growth is strongly driven by self-assembly and selective wetting on the lamella structure of polystyrene-block-poly(methyl methacrylate). We correlate the emerging nanoscale morphologies with collective optical and electronic properties and quantify the difference in Ag growth on the corresponding homopolymer thin films. Thus, we are able to determine the influence of the respective polymer template and observe substrate effects on the Ag cluster percolation threshold, which affects the insulator-to-metal transition (IMT). Optical spectroscopy in the UV-vis regime reveals localized surface plasmon resonance for the metal-polymer composite. Their maximum absorption is observed around the IMT due to the subsequent long-range electron conduction in percolated nanogranular Ag layers. Using X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy, we identify the oxidation of Ag at the acrylate side chains as an essential influencing factor driving the selective wetting behavior in the early growth stages. The results of polymer-templated cluster growth are corroborated by atomic force microscopy and field emission scanning electron microscopy.

Hydrotrope-Driven Self-Assembly in CTAB/ n-Hexanol/Water/Heptane Reverse Micellar System

Self-organization of nanoparticles into one-dimensional (1D) nanochains leads to new unpredicted physiochemical properties, which are further exploited to develop photonic or electronic devices. Thus, the controlled fabrication of 1D nanochains requires nanotemplate, which acts as building blocks for the self-assembly of nanoparticles. To address this issue, we designed a hydrotrope (sodium salicylate)-based CTAB/ n-hexanol/water/heptane reverse micellar system. Hydrotrope, herein, modulates electrostatic interactions between reverse micellar droplets and paves the way for the formation of self-assembled structures. Small-angle X-ray scattering studies were performed on the CTAB/heptane reverse micellar system by varying hydrotrope concentrations and water-to-surfactant ratios (W _x)_. The aqueous content of the reverse micellar pool is determined from the W _x value, where W _x = [H₂O]/[CTAB] and [CTAB] = 0.05 M. SAXS studies were performed for CTAB/heptane reverse micellar systems at three different W _x values, that is, 6, 12, and 16 and represented by W₆, W₁₂, and W₁₆, respectively. All SAXS profiles were modeled with a spherical form factor and a Baxter sticky hard sphere structure factor. The interaction between droplets was predicted in terms of stickiness parameter. The effect of W _x on the formation of self-assembled structures and forces governing the assembly has been discussed in detail. For the W₆ system, the electrostatic repulsion between reverse micellar droplets decreases, resulting in the formation of the 1D chain-like assembly of nanodroplets. In the case of the W₁₂ system, the dual feature of the hydrotrope has been observed, it increases the size of the reverse micellar system and reduces electrostatic repulsion between droplets because of which the formation of chain-like assemblies cannot be determined with accuracy. For the W₁₆ system, the decrease in micellar size with the increase in the hydrotrope concentration has been observed. Thus, our reverse micellar templates may provide a comprehensive method for the fabrication of high aspect ratio 1D nanochains of a variety of materials and harnessing their collective properties for magnetic, catalytic, and opto-electronic applications.

Characterizations of Rapid Sintered Nanosilver Joint for Attaching Power Chips

Sintering of nanosilver paste has been extensively studied as a lead-free die-attach solution for bonding semiconductor power chips, such as the power insulated gated bipolar transistor (IGBT). However, for the traditional method of bonding IGBT chips, an external pressure of a few MPa is reported necessary for the sintering time of ~1 h. In order to shorten the processing duration time, we developed a rapid way to sinter nanosilver paste for bonding IGBT chips in less than 5 min using pulsed current. In this way, we firstly dried as-printed paste at about 100 °C to get rid of many volatile solvents because they may result in defects or voids during the out-gassing from the paste. Then, the pre-dried paste was further heated by pulse current ranging from 1.2 kA to 2.4 kA for several seconds. The whole procedure was less than 3 min and did not require any gas protection. We could obtain robust sintered joint with shear strength of 30–35 MPa for bonding 1200-V, 25-A IGBT and superior thermal properties. Static and dynamic electrical performance of the as-bonded IGBT assemblies was also characterized to verify the feasibility of this rapid sintering method. The results indicate that the electrical performance is comparable or even partially better than that of commercial IGBT modules. The microstructure evolution of the rapid sintered joints was also studied by scanning electron microscopy (SEM). This work may benefit the wide usage of nanosilver paste for rapid bonding IGBT chips in the future.

Uniform metal nanostructures with long-range order via three-step hierarchical self-assembly

Large-scale nanopatterning is a major issue in nanoscience and nanotechnology, but conventional top-down approaches are challenging because of instrumentation and process complexity while often lacking the desired spatial resolution. We present a hierarchical bottom-up nanopatterning routine using exclusively self-assembly processes: By combining crystal surface reconstruction, microphase separation of copolymers, and selective metal diffusion, we produce monodisperse metal nanostructures in highly regular arrays covering areas of square centimeters. In situ grazing incidence small-angle x-ray scattering during Fe nanostructure formation evidences an outstanding structural order in the self-assembling system and hints at the possibility of sculpting nanostructures using external process parameters. Thus, we demonstrate that bottom-up nanopatterning is a competitive alternative to top-down routines, achieving comparable pattern regularity, feature size, and patterned areas with considerably reduced effort. Intriguing assets of the proposed fabrication approach include the option for in situ investigations during pattern formation, the possibility of customizing the nanostructure morphology, the capacity to pattern arbitrarily large areas with ultrahigh structure densities unachievable by top-down approaches, and the potential to address the nanostructures individually. Numerous applications of self-assembled nanostructure patterns can be envisioned, for example, in high-density magnetic data storage, in functional nanostructured materials for photonics or catalysis, or in surface plasmon resonance-based sensing.

Fractal patterning of nanoparticles on polymer films and their SERS capabilities

We demonstrate control, via electro-hydrodynamic (EHD) induced polymer instabilities and nanoparticle mobility, of hierarchical fractal arrangements of gold nanoparticles on patterned thin polymer films. The induced changes in the film curvature enhance fractal formation for high and not for low mobility nanoparticles. The high mobility nanoparticles cluster in circular fractal networks on the rims of a hexagonally ordered array of EHD-induced polymer peaks. These arrangements exhibit plasmonic properties for surface-enhanced Raman scattering (SERS) spectroscopy.

Effect of Au nanoparticle spatial distribution on the stability of thin polymer films

The stability of thin poly(methyl-methacrylate) (PMMA) films of low molecular weight on a solid substrate is controlled by the areal coverage of gold nanoparticles (NPs) present at the air-polymer interface. As the polymer becomes liquid the Au NPs are free to diffuse, coalesce, and aggregate while the polymer film can change its morphology through viscous flow. These processes lead at the same time to the formation of a fractal network of Au NPs and to the development of spinodal instabilities of the free surface of the polymer films. For thinner films a single wavelength is observed, while for thicker films two wavelengths compete. With continued heating the aggregation process results in a decrease in coverage, the networks evolve into disordered particle assemblies, while the polymer films flatten again. The disordering occurs first on the smallest scales and coincides (in thicker films) with the disappearance of the smaller wavelength. The subsequent disordering on larger scales causes the films to flatten.

Cobalt nanoparticles growth on a block copolymer thin film: a time-resolved GISAXS study

Cobalt sputter deposition on a nanostructured polystyrene-block-poly(ethylene oxide), P(S-b-EO), template is followed in real time with grazing incidence small-angle X-ray scattering (GISAXS). The polymer template consists of highly oriented parallel crystalline poly(ethylene oxide) (PEO) domains that are sandwiched between two polystyrene (PS) domains. In-situ GISAXS shows that cobalt atoms selectively decorate the PS domains of the microphase-separated polymer film and then aggregate to form surface metal nanopatterns. The polymer template is acting as a directing agent where cobalt metal nanowires are formed. At high metal load, the characteristic selectivity of the template is lost, and a uniform metal layer forms on the polymer surface. During the early stage of cobalt metal deposition, a highly asymmetric nanoparticles agglomeration is dominating structure formation. The cobalt nanoparticles mobility in combination with the high tendency of the nanoparticles to coalescence and to form immobile large-sized particles at the PS domains are discussed as mechanisms of structure formation.

Domain patterns in a diblock copolymer-diblock copolymer mixture with oscillatory particles

We investigate the orientational order transition of striped patterns in microphase structures of diblock copolymer-diblock copolymer mixtures in the presence of periodic oscillatory particles. Under certain conditions, although the macrophase separation of a system is almost isotropic, microphase separation of one diblock copolymer takes place and becomes anisotropic gradually. By changing the oscillatory frequency and amplitude, the orientational order transition of a striped microphase structure from the state parallel to the oscillatory direction to the state perpendicular to the oscillatory direction is observed. We also find that the order transition occurs when we change the initial composition ratio. Furthermore, we examine the domain size and the orientational order parameter of microstructure in the process of orientational order transition. The results may provide guidance for experimentalists. This model system can also give a simple way to realize orientational order transition of soft materials by changing the oscillatory field.

Memory effects in annealed hybrid gold nanoparticles/block copolymer bilayers

We report on the use of the self-organization process of sputtered gold nanoparticles on a self-assembled block copolymer film deposited by horizontal precipitation Langmuir-Blodgett (HP-LB) method. The morphology and the phase-separation of a film of poly-n-butylacrylate-block-polyacrylic acid (PnBuA-b-PAA) were studied at the nanometric scale by using atomic force microscopy (AFM) and Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS). The templating capability of the PnBuA-b-PAA phase-separated film was studied by sputtering gold nanoparticles (NPs), forming a film of nanometric thickness. The effect of the polymer chain mobility onto the organization of gold nanoparticle layer was assessed by heating the obtained hybrid PnBuA-b-PAA/Au NPs bilayer at T >Tg. The nanoparticles' distribution onto the different copolymer domains was found strongly affected by the annealing treatment, showing a peculiar memory effect, which modifies the AFM phase response of the Au NPs layer onto the polar domains, without affecting their surfacial composition. The effect is discussed in terms of the peculiar morphological features induced by enhanced mobility of polymer chains on the Au NPs layer.

Nanostrand formation of block copolymers at the air/water interface

Langmuir-Blodgett monolayers consisting of a network of nanostrands have occasionally been reported in the literature, but are often coexistent with other morphologies, which is not useful for potential applications. With the use of PS-P4VP/PDP, a polystyrene-poly(4-vinyl pyridine) diblock copolymer of 12 mol % VP content mixed with 3-pentadecylphenol, it is shown that the disordered nanostrand network morphology can be obtained reproducibly and uniformly over large surface areas by spreading chloroform solutions of relatively high copolymer concentration. Use of a more slowly evaporating spreading solvent, 1,1,2,2-tetrachloroethane, and a low subphase temperature, 8-9 °C, results in much more densely aligned nanostrands. Poorly spreading solvents such as nitrobenzene produce the well-known fingerprint pattern often observed in spin- or dip-coated thin films of block copolymers. A mechanism for nanostrand network formation is proposed that involves the momentary formation of a fingerprint morphology in spreading drops followed by its breakup at the level of the mobile P4VP/PDP stripes as spreading continues, leaving P4VP-anchored PS nanostrands floating on the water surface.

Novel mechanical properties in lamellar phases of liquid-crystalline diblock copolymers

Structural properties of flexible nematic diblock copolymers in the lamellar phase are investigated using a mean-field model. We address two complementary questions on the mechanics of the system: 1) How does the nematic order affect the elasticity of the one-dimensional solid? 2) What effect does the block copolymer microstructure has on the orientation of the nematic director? In the limit when the microstructure does not influence the nematic director orientation we predict a soft lamellar compression mode. When the microstructure does influence the nematic director orientation, small compressions lead to conventional elasticity, until a critical strain is reached, where there is then a transition to a softer response. On the other hand, we show that an identifiable lamellar symmetry provides a direction along which the nematic director prefers to align. Our model provides avenues to explore nonlinear properties of flexible diblock copolymers in which the monomers on both sides have mesogenic side groups.

Polymer-template-assisted growth of gold nanowires using a novel flow-stream technique

By utilizing a fluidic device, a gold nanoparticle dispersion is cast onto a nanostructured polymer template using solution subjected to hydrodynamic flow. With in situ grazing incidence small-angle X-ray scattering (GISAXS), the progressive gold deposition from a stream of gold solution onto the polymer template of a diblock copolymer with parallel cylinder morphology arranged into powder-like domains is investigated. The continuously flowing solution causes a systematic increase in the X-ray contrast between both of the microphase-separated blocks of the block copolymer film, indicating flow-induced selective gold immobilization on one block. Both in situ GISAXS data and atomic force microscopy of the metal-deposited polymer film prove the 1D coalescence of nanoparticles into continuous nanowires. With additional gold nanoparticle upload by the continuous flow-stream method, the selectivity of the nanoparticle deposition diminishes as a result of the formation of a pseudo uniform gold layer. Consequently, this flow-stream deposition technique introduces an easy alternative method to the vapor deposition technique for surface gold nanopatterning.

In situ GISAXS investigation of gold sputtering onto a polymer template

Microphase-separation structures in mixed diblock-triblock copolymer thin films are used for the incorporation of gold atoms inside the polymer matrix via sputtering of gold. Polystyrene (PS) spheres are arranged in a liquidlike type with a well defined nearest neighbor distance inside a polyisoprene matrix acting as a template for directing the gold atoms. Sputtering conditions are selected with a very low sputtering rate to avoid clustering in the atmosphere so that gold reaches the polymer surface in its atomic state. Due to the mobility of the gold atoms and the selective interaction with the PS parts of the microphase separation structure, gold is accumulated inside the polymer film in the PS spheres, as probed in situ with grazing incidence small-angle X-ray scattering (GISAXS). Nominally 4.3 A of gold is deposited, which by diffusion is spread out vertically over a thickness of 280 nm. UV-vis spectroscopy reveals a small blue shift for the gold sputtered polymer film. Atomic force microscopy proves the absence of gold clusters on the film surface. For low sputtering rate, GISAXS proves good sensitivity for gold migration inside the polymer film and opens new possibilities for studying polymer-metal interaction.

Regular arrays of monodisperse platinum/erbium disilicide core-shell nanowires and nanoparticles on Si(001) via a self-assembled template

We have developed a process for fabricating monodisperse noble metal/rare earth disilicide core-shell nanoparticles and nanowires in regular arrays on Si(001) with a density of 5 x 10(10) / cm2, and over areas > 1 mm2. Pt deposited via physical vapor deposition on a self-assembled rare earth disilicide nanowire template combined with reactive ion etching produces arrays of nanostructures. SEM images demonstrate the ability to select nanowires or nanoparticles as a function of Pt coverage. Statistical analysis of images of Pt nanoparticle arrays yield a mean feature size of 8 nm with a size variation of +/- 0.9 nm and interparticle spacing of approximately 15 nm.

Synthesis of Single Crystal Manganese Oxide Octahedral Molecular Sieve (OMS) Nanostructures with Tunable Tunnels and Shapes

A new and facile route is reported to manipulate the self-assembly synthesis of hierarchically ordered Rb-OMS-2 and pyrolusite with an interesting flowerlike morphology by a direct and mild reaction between rubidium chromateand manganese sulfate without any organic templates. The crystal forms, morphologies, and tunnel sizes of the obtained OMS materials can be controlled. A mechanism for the growth of manganese dioxides with flowerlike architectures was proposed. The obtained products exhibit potential for use in catalysis and other applications.

Micrometer-long gold nanowires fabricated using block copolymer templates

Micrometer-long gold nanowires were fabricated via self-assembly. Diblock copolymer films served as templates for the selective adsorption of 10 nm gold nanoparticles from solution to form well-defined nanostructures. An oxygen plasma treatment induced aggregation of the nanoparticles and the formation of continuous gold nanostructures. The electrical continuity of the nanostructures was observed using scanning electron microscopy.

Kinetic self-assembly of metals on copolymer templates

In this work, we seek to understand some of the fundamental processes that govern self-assembly at the nanoscale in the context of the formation of metallic structures on patterned copolymer templates. To this end, we focus on recent experiments where morphologies resulting from the evaporation-deposition of different metals on PS- b -PMMA phase-separated templates were studied. We show that the different morphologies obtained can be understood in terms of the relative importance of the energetics and kinetics. We then focus on a particular morphology: micron-long wirelike states obtained by the evaporation deposition of silver on the template. We show the existence of "nontrivial" correlations between adjacent wires that can be understood based on a purely kinetic mechanism. We also compare these correlations quantitatively to those obtained from simulations done with the relevant experimental parameters and find them in good agreement.

Nanosphere embedding into polymer surfaces: a viscoelastic contact mechanics analysis

Teichroeb and Forrest [Phys. Rev. Lett. 91, 016104 (2003)] image gold nanosphere embedment into a polystyrene surface and imply the existence of a liquid surface layer. We use a viscoelastic contact mechanics model of their results to give a contrary interpretation. The surface interactions between gold and polystyrene and the indentation depth determine the loads on the nanospheres. Using bulk properties, quantitative agreement between the model and the data is obtained, implying little or no depression in the glass temperature or existence of a liquid layer at the polystyrene surface.