The PMMA opal film was infiltrated with SiO 2 using a homemade CVD setup operating at atmospheric pressure and room temperature with SiCl 4 and H 2 O as precursors [12]. The CVD process is based on the hydrolysis of silicon tetrachloride (SiCl 4 ) on the hydrophilic surface of the spheres; these had been previously wetted with water vapor. SiCl 4 and water are both separately bubbled by a N 2 flow that sweeps the vapor phases to the reactor where the sample is placed. Controlled filling fractions can be achieved by adjusting the N 2 -flow rate and time.Patterning of the PMMA/SiO 2 composite was performed by EBL, using a Hitachi S-800 and a LEO 1455 scanning electron microscope equipped with a Raith Elphy Plus EBL control unit, at an accelerating voltage of 25 kV and exposure doses between 100 and 850 lC cm ±2 . The samples were developed for 40 s in methyl isobutyl ketone and then placed in isopropanol for 10 s to stop the developing process.The optical characterization was performed with a FTIR spectrometer, IFS 66 from Bruker with an attached IR Scope II microscope. 15 and 36 Cassegrain objectives were used to focus and collect the light from the patterned motifs. The incident and collected light cover external angles from 5 to 15 (15 objective) and 20 to 57 (36 objective) from normal incidence with respect to the (111) family of planes.HRSEM was used to observe the alterations in the opal structure. Before examination, samples had been cleaved and sputtered with a thin film of gold. Block copolymer thin films are currently of great interest as contact masks for inexpensive, large-area lithography. Films of the order of 50 nm thickness, containing a single layer of spherical or cylindrical microdomains formed by one block in a matrix of the other, have been successfully used to pattern semiconductors, [1±3] fabricate ultradense arrays of metal [4,5] and III±V semiconductor quantum dots, [6] and condense and isolate magnetic storage media. [7,8] Progress has been impeded by the lack of a versatile technique for inducing long-range order and orientation of the microdomains in a predetermined direction. For example, the striped patterns [9] formed by either cylinders or edge-on lamellaeÐwhich, if aligned, could serve as precursors to arrays of metal nanowiresÐinstead form curved, wormlike patterns with no long-range order. Techniques capable of aligning these striped patterns over areas of several lm 2 include electric fields, [10] graphoepitaxy (where the substrate is topographically prepatterned at the micrometer length scale [11,12] ), and directional crystallization of a suitable solvent. [13] Ordering over even larger areas is potentially achievable by prepatterning the substrate with the desired nanometer-scale pattern, [14,15] and then replicating this pattern in the block copolymer film; however, the aim of COMMUNICATIONS 1736

We have studied the ordering dynamics of a two-dimensional system which consists of a single layer of spherical block copolymer microdomains in a thin film. We follow the annealing process after a quench from the disordered state by electron microscopy and time-lapse atomic force microscopy. At late times the orientational correlation length ξ6 of the microdomain lattice exhibits a t 1/4 temporal power law. We compare the time evolution of the densities of dislocations, disclinations and the dislocation orientational correlation function. While most disclinations condense into dislocations and most dislocations condense into "grain boundaries", the dynamics appear to involve the interplay of disclinations and lines of dislocations. Rather than isolated grains shrinking, the most frequently observed process is the collapse of a smaller grain which resides on the boundary of two larger grains.

We study the coarsening dynamics of two-dimensional hexagonal patterns formed by single microdomain layers of block copolymers, using numerical simulations. Our study is focused on the temporal evolution of the orientational correlation length, the interactions between topological defects, and the mechanisms of coarsening. We find no free disclinations in the system; rather, they are located on large-angle grain boundaries, commonly where such boundaries bifurcate. The correlation lengths determined from the scattering function, from the density of dislocations, and from the density of disclinations exhibit similar behavior and grow with time according to a power law. The orientational correlation length also grows following a power law, but with a higher exponent than the other correlation lengths. The orientational correlation length grows via annihilation of dislocations, through preferential annihilation of small-angle grain boundaries due to poor screening of the strain field around dislocations located on small-angle grain boundaries. Consequently, the patterns are characterized by large-angle grain boundaries. The most commonly observed mechanism of coarsening is the collapse of smaller grains residing on the boundary of two larger grains delimited by large-angle grain boundaries. Simulations agree remarkably well with experimental results recently obtained.

Nanofabrication is witnessing a rapid trend towards using self-assembled templates as a cost-effective method of generating densely patterned surfaces. In particular, templating with block copolymer (BCP) thin films, until recently an area of essentially academic interest, has become increasingly popular in the semiconductor industry. BCPs are macromolecules composed of two (a ªdiblock copolymerº, diBCP) or more chemically distinct, covalently connected, polymer chains, which are typically immiscible in bulk. Molecular connectivity prohibits macroscopic phase separation; instead, BCPs ªmi-crophase separateº, forming nanoscale domains. In diBCPs where one block is much shorter than the other, the minority blocks self-assemble into spheres within a matrix of the majority block; if the length disparity is less pronounced, the nanodomains are cylindrical or lamellar.[1] The self-assembled polymeric patterns obtained in this fashion can be used as templates for lithography. [2] This economical and versatile patterning technique, capable of creating arrays of dielectric, [3] metallic, [4] quantum, [5] or magnetic [6,7] dots spaced only a few tens of nanometers apart, is fully compatible with silicon semiconductor processing [8] and is being applied to the fabrication of devices including magnetic hard drives [9] and, very recently, nanocrystal flash memories.[10] The Achilles' heel of this fabrication technique is lack of addressability, which limits data storage density to well below the theoretical maximum of one bit per dot. While the arrays typically display excellent short-range order, only limited long-range order can be achieved by traditional self-assembly because of the existence of topological defects. Recent research efforts have been directed at guiding the self-assembly process to induce long-range in-plane order in the array of BCP nanodomains that define the templated structures. The high degree of isotropy imposed by the hexagonal packing of the spherical nanodomains complicates the alignment problem: electric fields, although highly successful for in-plane alignment of cylinder-forming BCPs, [11] which define the templates for stripe arrays, have not been profitably applied to arrays of BCP spheres. Good alignment, however, can be achieved by using straight, microfabricated substrate features (mesas [12] or troughs [13,14] ) to impose a preferential lattice orientation (graphoepitaxy). While the alignment persists everywhere along such a straight feature, it extends for at most a few micrometers in the direction normal to the feature. The degree of alignment obtained by graphoepitaxy can be significantly improved by increasing the mobility of the polymer chains via thermal or solvent [15] annealing, yet these methods are (by themselves) ineffective in imposing a predefined lattice direction. We demonstrate below that a previously developed shearing technique, [16] when applied to thin films of sphereforming BCPs, results in BCP dot-array templates with excellent long-range order. The process involves s...

Phonon heat transport in mesoscopic systems is investigated using methods analogous to the Landauer description of electrical conductance. A "universal heat conductance" expression that depends on the properties of the conducting pathway only through the mode cutoff frequencies is derived. Corrections due to reflections at the junction between the thermal body and the conducting bridge are found to be small except at very low temperatures where only the lowest few bridge modes are excited. Various non-equilibrium phonon distributions are studied: a narrow band distribution leads to clear steps in the cooling curve, analogous to the quantized resistance values in narrow wires, but a thermal distribution is too broad to show such features.

The use of block copolymer (BCP) thin films as self-assembled templates has become increasingly popular as an economical nanofabrication technique, with new applications and techniques constantly being developed. This bottom-up approach to nanofabrication is extremely versatile; BCP films have been used as sacrificial contact masks for thin-film lithography to produce nanometer-scale periodic patterns in a wide variety of materials, [1][2][3] which have been investigated in the data storage sector for high-density magnetic [4,5] and nano-crystal FLASH memories. [6,7] While the nanoscale domains (spheres, cylinders, etc.) typically organize into grains of micron or smaller size, with no overall orientation, several methods have been developed to direct the domain orientation, or simply increase the grain size, in BCP films. In the first category, epitaxy [8,9] can direct the alignment over large areas, but requires substrates pre-patterned at the nanometer scale; graphoepitaxy [10][11][12] creates needle-like grains very long in one dimension, but at most a few microns wide; electric fields [13] generate alignment parallel to the field direction over squaremicron areas; polarized light can create arbitrary orientation patterns in liquid-crystalline block copolymers with photoalignable side groups; [14,15] and shear can align BCP cylinders [16] and spheres [17,18] over square-centimeter areas, with an orientation specified by the shear direction. In the second category, uniform solvent annealing [19] can increase BCP grain size to a few microns without imposing a preferential direction to the pattern; by creating a moving gradient in solvent concentration, "zone-casting" from solution can produce macroscopically-aligned specimens. [20] Similarly, uniform thermal annealing [21,22] increases the grain size without imposing a preferential direction, but the grain size grows only as the 1 ⁄4 power of annealing time; the effect of a moving temperature gradient on a block copolymer thin film has not been reported previously.Hashimoto et al. [23] showed that sweeping a strong temperature gradient through a bulk specimen of a lamellar BCP, such that the material is heated above its order-disorder transition temperature, can produce strong alignment; as the material cools in the gradient, ordered lamellae grow from the disordered phase, with the lamellar planes aligned normal to the direction of the moving gradient. Though directional solidification has not been reported for neat BCP thin films, directional crystallization of a small-molecule solvent from a BCP solution has been reported by Thomas and co-workers; [24,25] crystallization of the solvent concentrates the BCP and drives it to order, similar to what zone-casting [20] achieves through solvent evaporation. When the solvent is directionally crystallized on a cold substrate, an aligned BCP film is left on the surface of the frozen solvent, but with a defect density higher than obtained with the methods discussed in the preceding paragraph. Our present study demonstr...

We report the observation of two-dimensional melting in a monolayer film of a sphere-forming diblock copolymer. By annealing in a well-controlled temperature gradient we obtain a complete record of the transition from a low-temperature hexatic phase to a high-temperature liquid in a single experiment. We investigate the temperature dependence of the orientational and translational correlation lengths, as well as of the topological defect density. All evidence suggests that the melting transition is first-order, but correlations in the liquid phase indicate the existence of an underlying second-order transition preempted by the first-order freezing.

The magnetic interaction of a pair of bound magnetic polarons (BMP) in diluted magnetic semiconductors (DMS) is analyzed via a generalized Hubbardtype Hamiltonian for two carriers in the presence of effective magnetic fields arising from the magnetic polarization of their respective polarons. For the case where the magnetic fields at the two sites have equal magnitude but are allowed to have arbitrary directions, it is shown that the energy of the two polarons is minimized for a ferromagnetic configuration of the carrier spins (in contrast to the case of hydrogenic centers in nonmagnetic semiconductors) if polaron fields are strong enough. A modified Heisenberg-type Hamiltonian is constructed to describe the low energy states of the resulting system.