We demonstrate how to tailor the spatial distribution of gold nanoparticles (Au-NPs) of different sizes within polystyrene (PS) thin, supported, film hosts, thereby enabling the connection between ...the spatial distribution of Au-NPs within the polymer film and the optical properties to be determined. The real, n, and imaginary parts, k, of the complex refractive indices N = n(λ)+ik(λ) of the nanocomposite films were measured as a function of wavelength, λ, using multivariable angle spectroscopic ellipsometry. The surface plasmon response of films containing nearly homogeneous Au-NP distributions were well described by predictions based on classical Mie theory and the Drude model. The optical spectra of samples containing inhomogeneous nanoparticle distributions manifest features associated with differences in the size and interparticle spacings as well as the proximity and organization of nanoparticles at the substrate and free surface.
A lot of interesting and sophisticated examples of nanoparticle (NP) self-assembly (SA) are known. From both fundamental and technological standpoints, this field requires advancements in three ...principle directions: (a) understanding the mechanism and driving forces of three-dimensional (3D) SA with both nano- and microlevels of organization; (b) understanding disassembly/deconstruction processes; and (c) finding synthetic methods of assembly into continuous superstructures without insulating barriers. From this perspective, we investigated the formation of well-known star-like PbS superstructures and found a number of previously unknown or overlooked aspects that can advance the knowledge of NP self-assembly in these three directions. The primary one is that the formation of large seemingly monocrystalline PbS superstructures with multiple levels of octahedral symmetry can be explained only by SA of small octahedral NPs. We found five distinct periods in the formation PbS hyperbranched stars: (1) nucleation of early PbS NPs with an average diameter of 31 nm; (2) assembly into 100–500 nm octahedral mesocrystals; (3) assembly into 1000–2500 nm hyperbranched stars; (4) assembly and ionic recrystallization into six-arm rods accompanied by disappearance of fine nanoscale structure; (5) deconstruction into rods and cuboctahedral NPs. The switches in assembly patterns between the periods occur due to variable dominance of pattern-determining forces that include van der Waals and electrostatic (charge–charge, dipole–dipole, and polarization) interactions. The superstructure deconstruction is triggered by chemical changes in the deep eutectic solvent (DES) used as the media. PbS superstructures can be excellent models for fundamental studies of nanoscale organization and SA manufacturing of (opto)electronics and energy-harvesting devices which require organization of PbS components at multiple scales.
Regioregular poly(3-hexylthiophene) (RR-P3HT) is a widely used donor material for bulk heterojunction polymer solar cells. While much is known about the structure and properties of RR-P3HT films, ...important questions regarding hole mobilities in this material remain unresolved. Measurements of the out-of-plane hole mobilities, μ, of RR-P3HT films have been restricted to films in the thickness regime on the order of micrometers, beyond that generally used in solar cells, where the film thicknesses are typically 100 to 200 nm. Studies of in-plane carrier mobilities have been conducted in thinner films, in the thickness range 100–200 nm. However, the in-plane and out-of-plane hole mobilities in RR-P3HT can be significantly different. We show here that the out-of-plane hole mobilities in neat RR-P3HT films increase by an order of magnitude, from 10–4 cm2/V·s, for a 80 nm thick film, to a value of 10–3 cm2/V·s for films thicker than 700 nm. Through a combination of morphological characterization and simulations, we show that the thickness dependent mobilities are not only associated with the differences between the average morphologies of thick films and thin films, but specifically associated with changes in the local morphology of films as a function of distance from the interfaces.
The segmental dynamics of a low glass transition temperature T g polymer under different conditions of morphological confinement are shown to manifest the influence of the frustration of the local ...“packing” of chain segments at interfaces of varying curvatures, unique to the morphology. Of particular interest are the polyisoprene (PI) segmental dynamics in polystyrene-b-polyisoprene (PS-b-PI) micelles and in layered onion-like structures in high T g, “frozen,” polystyrene (PS) hosts. At temperatures close to the T g of PI domains, the segmental relaxation times of PI chains in the micellar structures, τmicelle, are more than 1 order of magnitude shorter than those of PI chains in the onion-like structures, τonion. These rates are appreciably faster than those of homopolymer PI and of PI in the neat copolymers possessing lamellar and gyroid phases, τhomo ∼ 0.5τlam ∼ 0.5τgyroid, respectively. At high temperatures τhomo ∼ τlam ∼ τgyroid ∼ τonion ∼ τmicelle. These observations are readily reconciled with trends in the local T g of the PI phase in each sample.
The influence of morphology on the dc conductivity σdc, the charge carrier density n and the out-of-plane mobilities μ were investigated in conjugated polymer films of poly(3-hexylthiophene) (P3HT) ...using impedance spectroscopy (IS). IS was used for the first time to discern this information from P3HT films. Values of μ, which were found to be film-thickness dependent, increasing with increasing film thickness, h, for films of thickness h > 700 nm, are in excellent agreement with those measured using time-of-flight (ToF) and the method of charge extraction by linearly increasing voltage (CELIV). Both σdc and n are shown to decrease appreciably with increasing h. The thickness dependent trends in μ, σdc and n are consistent with changes in the morphology of these films. Conductive atomic force microscopy (CAFM) provided corroborating information, showing an appreciable dependence of carrier transport on the morphology of P3HT.
We study the formation and organization of micelles, of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer chains, in thin film hosts composed of (1) blends of long and short chain ...polystyrenes, PSL and PSS, of degrees of polymerization P L and P S, respectively (P S < P L), and (2) blends of PSL and a homopolymer with which it is compatible, tetramethyl bisphenol-A polycarbonate (TMPC), of degree of polymerization P TMPC (P TMPC < P L). The role of competing entropic and enthalpic interactions on the block copolymer micelle formation and organization is examined. We show that the average size of the micelle cores, D core, decreased with increasing weight fraction of the shorter chain PS component in PS(P S = 125)/PS(P L = 15400) mixtures, for P L≫N PS, N PS is the degree of polymerization of the PS block (the micelle corona). D core also decreased with increasing weight fraction of TMPC in TMPC(P TMPC = 122)/PS(P L = 15400) mixtures. The values of D core in the TMPC/PSL hosts were smaller than those in PSS/PSL mixtures, for the same TMPC, or PSS, weight fractions (P S ≈ P TMPC). This is due to more extensive mixing between the TMPC host chains, compared to the PSS chains, and the micelle corona. Furthermore, we show that the size and the organization of the micelles within the films may be controlled independently, through changing the relative fractions of PSS or TMPC and the value of P L. The fraction of TMPC, or PSS, mixed with the corona decreased as P L decreased from P L = 15400 to smaller values.
We show that polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer chains aggregate to form micelles, composed of an inner P2VP core and an outer PS corona, as well as adsorb onto the ...substrate, forming brush layers, in supported thin film PS/PS-b-P2VP mixtures. The degrees of polymerization of the chains that comprised the corona and core were N PS and N P2VP, respectively. The diameter of the micelle cores, D core, increased with increasing degree of polymerization, P, of the PS host and became constant for large values of P. A decrease in the number density of micelles, n micelle, accompanied the increase in D core; n micelle reached a plateau in the same range of values of P where D core became constant. The organization of the micelles suggests the existence of attractive micelle–micelle interactions in the large P regime. Moreover, in this regime, the micelles preferentially migrated to the free surface. The morphology of this system is compared to thin film PS melts containing brush-coated nanoparticles. One fundamental difference between the two systems is that the micellar system has the ability to adjust the number of chains per micelle, and hence the number of micelles, in order to mediate the free chain/brush (micelle corona) interactions. This has important consequences on the location of the wet-brush to dry-brush transition.
Multistage Planar Thermoelectric Microcoolers Gross, A. J.; Gi Suk Hwang; Baoling Huang ...
Journal of microelectromechanical systems,
10/2011, Letnik:
20, Številka:
5
Journal Article
Recenzirano
Many types of microsystems and microelectromechanical systems (MEMS) devices exhibit improved performance characteristics when operated below room temperature. However, designers rarely pair such ...devices with integrated cooling solutions because they add complexity to the system and often have power consumption which far exceeds that of the microsystem itself. We report the design, fabrication, and testing of both one- and six-stage thermoelectric (TE) microcoolers that target MEMS applications through optimization for low-power operation. Both coolers use thin-film Bi 2 Te 3 and Sb 2 Te 3 as the n-and p-type TE materials, respectively, and operate in a planar configuration. The six-stage cooler has demonstrated a ΔT = 22.3 °C at a power consumption of 24.8 mW, while the one-stage cooler has demonstrated a ΔT = 17.9 °C at a lower power consumption of 12.4 mW.
We recently showed that a suspension of micrometer-sized polystyrene (PS) particles in a PDMS liquid, mixed with small (1 wt %) amounts of a nanocage, sulfonated polyhedral oligomeric silsesquioxane ...(s-POSS), exhibited significant electrorheological (ER) behavior. This behavior was associated with the formation of a thin adsorbed layer of s-POSS onto the surfaces of PS and the subsequent formation of polarization-induced aggregates, or structures, responsible for the ER effect in an applied electric, E, field. Current theory suggests that the ER effect would largely be determined by the dielectric and conductive properties of the conductive layer of core/shell particles in ER suspensions. We show here that sulfonated-PS (s-PS)/PDMS suspensions exhibit further increases in the yield stress of over 200%, with the addition of s-POSS. The yield stress of this system, moreover, scales as τ y ∝ E 2. The dielectric relaxation studies reveal the existence of a new relaxation peak in the s-POSS/s-PS/PDMS system that is absent in the s-POSS/PS/PDMS suspension. The relative sizes of these peaks are sensitive to the concentration of s-POSS and are associated with changes in the ER behavior. The properties of this class of ER fluids are not appropriately rationalized in terms of current theories.
A study of the poly(vinyl methyl ether) (PVME) segmental dynamics of bulk miscible blends of polystyrene (PS) and PVME reveals that while at high temperatures, T, there is evidence of a single ...α-relaxation process, at lower T, two separate dominant relaxation processes, associated with the change in structure of the blend with decreasing T, emerge. One relaxation process decreases with a much stronger dependence on T and “freezes” at a temperature comparable to the glass transition temperature, T g, of the blend measured using differential scanning calorimetry. The other exhibits a weaker T dependence and persists at much lower T, becoming Arrhenius (the so-called α′-process) at sufficiently low T. In thin PVME/PS films confined between aluminum substrates, a new relaxation process, αint, associated with PVME chains that preferentially segregate to the substrates, emerges. These observations are considered in light of the influence of spatial compositional heterogeneities on blend dynamics.
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