Mesoporous bismuth ferrite with amplified magnetoelectric coupling and electric field-induced ferrimagnetism.

Coupled ferromagnetic and ferroelectric materials, known as multiferroics, are an important class of materials that allow magnetism to be manipulated through the application of electric fields. Bismuth ferrite, BiFeO3, is the most-studied intrinsic magnetoelectric multiferroic because it maintains both ferroelectric and magnetic ordering to well above room temperature. Here we report the use of epitaxy-free wet chemical methods to create strained nanoporous BiFeO3. We find that the strained material shows large changes in saturation magnetization on application of an electric field, changing from 0.04 to 0.84 µb per Fe. For comparison, non-porous films produced using analogous methods change from just 0.002 to 0.01 µb per Fe on application of the same electric field. The results indicate that nanoscale architecture can complement strain-layer epitaxy as a tool to strain engineer magnetoelectric materials.

Homo- and heterometallic luminescent 2-D stilbene metal-organic frameworks.

Metal-organic frameworks (MOFs) can provide a matrix for the assembly of organic chromophores into well-defined geometries, allowing for tuning of the material properties and study of structure-property relationships. Here, we report on the effect of the coordinated metal ion on the luminescence properties of eight isostructural MOFs having the formula M(1)2M(2)L3(DMF)2 (M(1) = M(2) = Zn (1), Cd (2), Mn (3), Co (4); M(1) = Zn, M(2) = Cd (5), Mn (6), Co (7); M(1) = Co, M(2) = Mn (8); L = trans-4,4'-stilbene dicarboxylate), synthesized by reaction of the appropriate metal nitrate or mixtures of metal nitrates with LH2 in DMF. The crystal structures of 2, 3 and 5-8 were determined by X-ray diffraction to be composed of trinuclear metal clusters linked by stilbene dicarboxylate linkers in a paddlewheel geometry, extending to form a 2-D layered structure. In the mixed-metal cases, the larger metal ion was found to occupy the octahedral site in the cluster while the smaller ion occupies the tetrahedral positions, suggesting a selective, ligand-directed assembly process for the mixed-metal species. Variable temperature magnetic measurements for paramagnetic MOFs 3 and 6-8 were consistent with the site occupancies determined crystallographically, and indicated weak intra-cluster antiferromagnetic coupling for 3 and 8. Comparison between the crystal structures of 2, 3 and 5-8 and those reported for 1 and 4 in the literature reveal close resemblances between linker environments, with important intermolecular stilbene-stilbene geometries that are comparable in all cases. Interestingly, pale-colored 1-3 and 5-7 display very similar emission profiles upon excitation at λ(ex) = 350 nm, whereas dark-colored 4 and 8 do not exhibit detectable emission spectra. The bright, well-resolved luminescence of 1, 2 and 5 is ascribed to rigidification of the linker upon coordination to the d(10) metal ions, whereas the weaker emission observed for 3, 6 and 7 is presumably a result of quenching due to close proximity of the linker to one or more paramagnetic ions. Time-resolved measurements for 1, 2, 5 and 6 reveal biexponential emission decays, where the lifetime of the longer-lived state corresponds to observed variations in the nearest-neighbor cofacial stilbene-stilbene distances in their crystal structures. For 3, a monoexponential decay with shorter lifetime was determined, indicating significant paramagnetic quenching of its emissive state.

Selective sidewall wetting of polymer blocks in hydrogen silsesquioxane directed self-assembly of PS-b-PDMS.

We show the importance of sidewall chemistry for the graphoepitaxial alignment of PS-b-PDMS using prepatterns fabricated by electron beam lithography of hydrogen silsesquioxane (HSQ) and by deep ultraviolet (DUV) lithography on SiO(2) thin films. Density multiplication of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) within both prepatterns was achieved by using a room temperature dynamic solvent annealing environment. Selective tuning of PS and PDMS wetting on the HSQ template sidewalls was also achieved through careful functionalization of the template and substrate surface using either brush or a self-assembled trimethylsilyl monolayer. PDMS selectively wets HSQ sidewalls treated with a brush layer of PDMS, whiereas PS is found to selectively wet HSQ sidewalls treated with hexamethyldisilazane (HMDS) to produce a trimethylsilyl-terminated surface. The etch resistance of the aligned polymer was also evaluated to understand the implications of using block copolymer patterns which have high etch resistance, self-forming (PDMS) wetting layers at both interfaces. The results outlined in this work may have direct applications in nanolithography for continued device scaling toward the end-of-roadmap era.

Large-scale parallel arrays of silicon nanowires via block copolymer directed self-assembly.

Extending the resolution and spatial proximity of lithographic patterning below critical dimensions of 20 nm remains a key challenge with very-large-scale integration, especially if the persistent scaling of silicon electronic devices is sustained. One approach, which relies upon the directed self-assembly of block copolymers by chemical-epitaxy, is capable of achieving high density 1 : 1 patterning with critical dimensions approaching 5 nm. Herein, we outline an integration-favourable strategy for fabricating high areal density arrays of aligned silicon nanowires by directed self-assembly of a PS-b-PMMA block copolymer nanopatterns with a L(0) (pitch) of 42 nm, on chemically pre-patterned surfaces. Parallel arrays (5 × 10(6) wires per cm) of uni-directional and isolated silicon nanowires on insulator substrates with critical dimension ranging from 15 to 19 nm were fabricated by using precision plasma etch processes; with each stage monitored by electron microscopy. This step-by-step approach provides detailed information on interfacial oxide formation at the device silicon layer, the polystyrene profile during plasma etching, final critical dimension uniformity and line edge roughness variation nanowire during processing. The resulting silicon-nanowire array devices exhibit Schottky-type behaviour and a clear field-effect. The measured values for resistivity and specific contact resistance were ((2.6 ± 1.2) × 10(5)Ωcm) and ((240 ± 80) Ωcm(2)) respectively. These values are typical for intrinsic (un-doped) silicon when contacted by high work function metal albeit counterintuitive as the resistivity of the starting wafer (∼10 Ωcm) is 4 orders of magnitude lower. In essence, the nanowires are so small and consist of so few atoms, that statistically, at the original doping level each nanowire contains less than a single dopant atom and consequently exhibits the electrical behaviour of the un-doped host material. Moreover this indicates that the processing successfully avoided unintentional doping. Therefore our approach permits tuning of the device steps to contact the nanowires functionality through careful selection of the initial bulk starting material and/or by means of post processing steps e.g. thermal annealing of metal contacts to produce high performance devices. We envision that such a controllable process, combined with the precision patterning of the aligned block copolymer nanopatterns, could prolong the scaling of nanoelectronics and potentially enable the fabrication of dense, parallel arrays of multi-gate field effect transistors.

Thiol-ene click reaction as a general route to functional trialkoxysilanes for surface coating applications.

Functionalized trialkoxysilanes are widely used to modify the surface properties of materials and devices. It will be shown that the photoinitiated radical-based thiol-ene "click" reaction provides a simple and efficient route to diverse trialkoxysilanes. A total of 15 trialkoxysilanes were synthesized by reacting either alkenes with 3-mercaptopropyltrialkoxysilane or thiols with allyltrialkoxysilanes in the presence of a photoinitiator. The functionalized trialkoxysilanes were obtained in quantitative to near-quantitative yields with high purity. The photochemical reactions can be run neat in standard borosilicate glassware using a low power 15-W blacklight. A wide range of functional groups is tolerated in this approach, and even complex alkenes click with the silane precursors. To demonstrate that these silanes can be used as surface coating agents, several were reacted with iron oxide superparamagnetic nanoparticles and the loadings quantified. The photoinitiated thiol-ene reaction thus offers a facile and efficient method for preparing surface-active functional trialkoxysilanes.

Surface-directed dewetting of a block copolymer for fabricating highly uniform nanostructured microdroplets and concentric nanorings.

Through a combination of nanoimprint lithography and block copolymer self-assembly, a highly regular dewetting process of a symmetric diblock copolymer occurs whereby the hierarchal formation of microdroplets and concentric nanorings emerges. The process is driven by the unique chemical properties and geometrical layout of the underlying patterned silsesquioxane micrometer-sized templates. Given the presence of nonpreferential substrate-polymer interactions, directed dewetting was utilized to produce uniform arrays of microsized droplets of microphase separated polystyrene-block-poly(methyl methylacrylate) (PS-b-PMMA), following thermal annealing at 180 °C. Microdroplets with diameters greater than 400 nm exhibited a hexagonal close-packed arrangement of nanodots on the surface with polydomain ordering. At the droplet periphery, the polydomain ordering was severely disrupted because of a higher in-plane radius of curvature. By reducing the droplet size, the in-plane radius of curvature of the microdroplet becomes significant and the PMMA cylinders adopt parallel structures in this confined geometry. Continuous scaling of the droplet results in the generation of isolated, freestanding, self-aligned, and self-supported oblique nanorings (long axis ∼250-350 nm), which form as interstitial droplets between the larger microdroplets. Optical and magnetic-based nanostructures may benefit from such hierarchal organization and self-supporting/aligned nanoring templates by combining more than one lithography technique with different resolution capabilities.

Graphoepitaxial assembly of asymmetric ternary blends of block copolymers and homopolymers.

Ternary blends of cylinder-forming polystyrene-block-poly(methyl methacrylate) block copolymers and polystyrene and poly(methyl methacrylate) homopolymers were assembled in trench features of constant width. Increasing the fraction of homopolymer in the blend increased the spacing and size of block copolymer domains, which were oriented perpendicular to the substrate to form a hexagonal lattice within the trench. The number of rows of cylinders within the trench was controlled by the blend composition. Depending on the domain size and spacing, the hexagonal lattice was stretched or compressed perpendicular to the trench walls but not perturbed parallel to the walls, indicating a decoupling of the perturbation in the perpendicular and parallel directions. The row spacing was uniform across the trench as a function of position from the trench wall. The results are compared with an analytical model and with Monte Carlo simulations.

Self-assembled templates for the generation of arrays of 1-dimensional nanostructures: from molecules to devices.

Self-assembled nanoscale porous architectures, such as mesoporous silica (MPS) films, block copolymer films (BCP) and porous anodic aluminas (PAAs), are ideal hosts for templating one dimensional (1D) nano-entities for a wide range of electronic, photonic, magnetic and environmental applications. All three of these templates can provide scalable and tunable pore diameters below 20 nm [1-3]. Recently, research has progressed towards controlling the pore direction, orientation and long-range order of these nanostructures through so-called directed self-assembly (DSA). Significantly, the introduction of a wide range of top-down chemically and physically pre-patterning substrates has facilitated the DSA of nanostructures into functional device arrays. The following review begins with an overview of the fundamental aspects of self-assembly and ordering processes during the formation of PAAs, BCPs and MPS films. Special attention is given to the different ways of directing self-assembly, concentrating on properties such as uni-directional alignment, precision placement and registry of the self-assembled structures to hierarchal or top-down architectures. Finally, to distinguish this review from other articles we focus on research where nanostructures have been utilised in part to fabricate arrays of functioning devices below the sub 50 nm threshold, by subtractive transfer and additive methods. Where possible, we attempt to compare and contrast the different templating approaches and highlight the strengths and/or limitations that will be important for their potential integration into downstream processes.

Chemical interactions and their role in the microphase separation of block copolymer thin films.

The thermodynamics of self-assembling systems are discussed in terms of the chemical interactions and the intermolecular forces between species. It is clear that there are both theoretical and practical limitations on the dimensions and the structural regularity of these systems. These considerations are made with reference to the microphase separation that occurs in block copolymer (BCP) systems. BCP systems self-assemble via a thermodynamic driven process where chemical dis-affinity between the blocks driving them part is balanced by a restorative force deriving from the chemical bond between the blocks. These systems are attracting much interest because of their possible role in nanoelectronic fabrication. This form of self-assembly can obtain highly regular nanopatterns in certain circumstances where the orientation and alignment of chemically distinct blocks can be guided through molecular interactions between the polymer and the surrounding interfaces. However, for this to be possible, great care must be taken to properly engineer the interactions between the surfaces and the polymer blocks. The optimum methods of structure directing are chemical pre-patterning (defining regions on the substrate of different chemistry) and graphoepitaxy (topographical alignment) but both centre on generating alignment through favourable chemical interactions. As in all self-assembling systems, the problems of defect formation must be considered and the origin of defects in these systems is explored. It is argued that in these nanostructures equilibrium defects are relatively few and largely originate from kinetic effects arising during film growth. Many defects also arise from the confinement of the systems when they are 'directed' by topography. The potential applications of these materials in electronics are discussed.

Study on the combined effects of solvent evaporation and polymer flow upon block copolymer self-assembly and alignment on topographic patterns.

Microphase separation of a polystyrene-block-polyisoprene-block-polystyrene triblock copolymer thin film under confined conditions (i.e., graphoepitaxy) results in ordered periodic arrays of polystyrene cylinders aligned parallel to the channel side-wall and base in a polyisoprene matrix. Polymer orientation and translational ordering with respect to the topographic substrate were elucidated by atomic force microscopy (AFM) while film thickness and polymer profile within the channel were monitored by cross-sectional transmission electron microscopy (TEM) as a function of time over a 6 h annealing period at 120 degrees C. Upon thermal annealing, the polymer film simultaneously undergoes three processes: microphase separation, evaporation of trapped solvent, and mass transport of polymer from the mesas into the channels. A significant volume of solvent is trapped within the polymer film upon spin coating arising from the increased polymer/substrate interfacial area due to the topographic pattern. Mass transport of polymer during this process results in nonuniform films, where subtle changes in the film thickness within the channel have profound effects on the microphase separation process. The initially disordered structure within the film underwent an orientation transition via an intermediate formation of perpendicular cylinders (nonequilibrium) to a parallel (equilibrium) orientation with respect to the channel base. Herein, we present a time-resolved study of the cylinder reorientation process detailing how changing film thickness during the annealing process dramatically affects both the local and lateral orientation of the observed structure. Finally, a brief mathematical model is provided to evaluate spin coating over a complex topography following a classical asymptotic approximation of the Navier-Stokes equations for the as-deposited films.

Pore directionality and correlation lengths of mesoporous silica channels aligned by physical epitaxy.

Herein we report on the alignment of mesoporous silica, a potential host for sub-10 nm nanostructures, by controlling its deposition within patterned substrates. In-depth characterization of the correlation lengths (length of a linear porous channel), defects of the porous network (delamination), and how the silica mesopores register to the micrometer-sized substrate pattern was achieved by means of novel focused ion beam (FIB) sectioning and in situ SEM imaging, which to our knowledge has not previously been reported for such a system. Our findings establish that, under confinement, directed deposition of the sol within channeled substrates, where the cross-sectional aspect ratio of the channels approaches unity, induces alignment of the mesopores along the length of the channels. The pore correlation length was found to extend beyond the micrometer scale, with high pore uniformity from channel to channel observed with infrequent delamination defects. Such information on pore correlation lengths and defect densities is critical for subsequent nanowire growth within the mesoporous channels, contact layout (electrode deposition etc.), and possible device architectures.

Propagation of polarized light through two- and three-layer anisotropic stacks.

The extended Jones formulation is used to investigate propagation at nonnormal incidence through two- and three-layer systems of birefringent material in which the optic axes of the individual layers are in the plane of the layers. Such systems are equivalent to two optical elements in series-an equivalent retardation plate and a polarization rotator. Analytical solutions are obtained for the equivalent retardation and rotation. The major finding is that, in general, there are two nonnormal incidence directions for which the retardation vanishes; therefore these two directions are optic axes of the composite system. These simple layered systems therefore behave in a manner similar to biaxial crystals. Moreover, the results illustrate the fact that even if the optic axes of individual layers in composite systems are in the plane of the layers, the optic axes of the system are, in general, out of this plane.