Richard A. Register

Polymer crystallization in 25-nm spheres

Crystallization within the discrete spheres of a block copolymer mesophase was studied by time-resolved x-ray scattering. The cubic packing of microdomains, established by self-assembly in the melt, is preserved throughout crystallization by strong interblock segregation even though the amorphous matrix block is well above its glass transition temperature. Homogeneous nucleation within each sphere yields isothermal crystallizations which follow first-order kinetics, contrasting with the sigmoidal kinetics normally exhibited in the quiescent crystallization of bulk polymers.

Dense arrays of ordered GaAs nanostructures by selective area growth on substrates patterned by block copolymer lithography

GaAs has been selectively grown in a hexagonally ordered array of nanometer-scale holes with a density as high as ∼1011/cm2 by metalorganic chemical vapor deposition. This array of holes was created using block copolymer lithography, in which a thin layer of diblock copolymer was used as an etching mask to make dense holes in a 15-nm-thick SiNx film. These selectively grown nanoscale features are estimated to be 23 nm in diameter with narrow lateral size and height distributions as characterized by field-emission scanning electron microscopy and tapping mode atomic force microscopy. The narrow size distribution and uniform spatial position of the nanoscale dots we report offer potential advantages over self-assembled dots grown by the Stranski–Krastanow mode.

Efficient organic electroluminescent devices using single-layer doped polymer thin films with bipolar carrier transport abilities

Detailed studies of electroluminescent devices made from single-layer doped polymer blend thin films having bipolar carrier transport abilities are presented. The active organic layer consists of the hole-transport polymer poly(N-vinylcarbazole) (PVK) containing dispersed electron-transport molecules, as well as different fluorescent small molecules or polymers as emitting centers to vary the emission color. Both the photoluminescence and electroluminescence (EL) properties are extensively studied. In photoluminescence, very efficient transfer of energy can occur from the host to very dilute (/spl sim/1 wt.%) amounts of emitting materials. When covered with a metal layer, the intensity of photoluminescence from blend thin films was found to be dependent on the type of metal coverage. The optical and electrical properties of materials and devices were systematically studied to understand the operating mechanisms and to optimize the devices. In EL, excitons appear to be formed at doped emitting centers, rather than in the host. We show that in an optimized device, a relatively high external quantum efficiency (>1%, backside emission only) and a low operating voltage (<10 V for over 100 cd/m/sup 2/) can be easily achieved by this class of devices. It was also found air-stable Ag is as good as reactive Mg-Ag alloy for the cathode contact in devices using PVK containing dispersed electron-transport oxadiazole molecules.

NANOLITHOGRAPHIC TEMPLATES FROM DIBLOCK COPOLYMER THIN FILMS

We describe a technique for creating a thin polystyrene film containing a periodic array of cylindrical holes, with a hole size of approximately 13 nm and a lattice constant of 27 nm. The starting material is a polystyrene‐polybutadiene diblock copolymer, which self‐assembles into a hexagonally packed array of polybutadiene cylinders embedded in a polystyrene matrix. A technique described previously is used to orient the cylinders normal to the plane of the film. The polybutadiene domains are then removed by reaction with ozone, which attacks the double bonds in the polybutadiene backbone. Films of this type could potentially be used as templates for nanolithography on a scale not readily accessed by other techniques.

Flexible piezoelectric PMN-PT nanowire-based nanocomposite and device.

Piezoelectric nanocomposites represent a unique class of materials that synergize the advantageous features of polymers and piezoelectric nanostructures and have attracted extensive attention for the applications of energy harvesting and self-powered sensing recently. Currently, most of the piezoelectric nanocomposites were synthesized using piezoelectric nanostructures with relatively low piezoelectric constants, resulting in lower output currents and lower output voltages. Here, we report a synthesis of piezoelectric (1 - x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) nanowire-based nanocomposite with significantly improved performances for energy harvesting and self-powered sensing. With the high piezoelectric constant (d33) and the unique hierarchical structure of the PMN-PT nanowires, the PMN-PT nanowire-based nanocomposite demonstrated an output voltage up to 7.8 V and an output current up to 2.29 μA (current density of 4.58 μA/cm(2)); this output voltage is more than double that of other reported piezoelectric nanocomposites, and the output current is at least 6 times greater. The PMN-PT nanowire-based nanocomposite also showed a linear relationship of output voltage versus strain with a high sensitivity. The enhanced performance and the flexibility of the PMN-PT nanowire-based nanocomposite make it a promising building block for energy harvesting and self-powered sensing applications.

Large area dense nanoscale patterning of arbitrary surfaces

We demonstrate a large-area fabrication of hexagonally ordered metal dot arrays with an area density of ∼1011/cm2. We produced 20 nm dots with a 40 nm period by combining block copolymer nanolithography and a trilayer resist technique. A self-assembled spherical-phase block copolymer top layer spontaneously generated the pattern, acting as a template. The pattern was first transferred to a silicon nitride middle layer by reactive ion etch, producing holes. The nitride layer was then used as a mask to further etch into a polyimide bottom layer. The metal dots were produced by an electron beam evaporation followed by a lift-off process. Our method provides a viable route for highly dense nanoscale patterning of different materials on arbitrary surfaces.

Lithography with a mask of block copolymer microstructures

Dense, periodic arrays of holes and troughs have been fabricated in silicon, silicon nitride, and germanium. The holes are approximately 20 nanometers (nm) wide, 20 nm deep, spaced 40 nm apart, and uniformly patterned with 3×1012 holes on a three inch wafer. To access this length scale, self-assembling resists were synthesized to produce either a layer of hexagonally ordered polyisoprene (PI) spheres or parallel cylinders of polybutadiene (PB) in a polystyrene (PS) matrix. The PI spheres or PB cylinders were then degraded and removed with ozone to produce a PS mask for pattern transfer by fluorine-based reactive ion etching. A PS mask of spherical voids was used to fabricate a lattice of holes and a mask of cylindrical voids was used to produce parallel troughs. This technique accesses a length scale difficult to produce by conventional lithography and opens a route for the patterning of surfaces via self-assembly.

Effect of carbazole–oxadiazole excited-state complexes on the efficiency of dye-doped light-emitting diodes

Interactions between hole-transporting carbazole groups and electron-transporting 1,3,4-oxadiazole groups were studied by photoluminescence and electroluminescence (EL) spectroscopy, in blends of poly(N-vinylcarbazole) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK:PBD) and in random copolymers with carbazole and oxadiazole groups attached as side chains. Different excited-state complexes form in the blends, which exhibit exciplexes, and in the copolymers, which manifest electroplexes, due to topological constraints on the position of carbazole and oxadiazole units in the polymer. Both types of complex red-shift the EL spectra of the matrices compared with pure PVK homopolymer, although the shift is significantly greater for the electroplex. The presence of these complexes has a profound effect on the external quantum efficiency of dye-doped organic light-emitting diodes employing the blends or copolymers as matrices, as it strongly affects the efficiency of Forster energy transfer from the matr...

INTEGRATED THREE-COLOR ORGANIC LIGHT-EMITTING DEVICES

We report a demonstration of the integration of individual polymer‐based light emitting devices of three different colors on the same substrate. Orange, green, and blue color devices are sequentially fabricated on the same indium–tin oxide (ITO) coated glass substrate coated with a patterned insulator on the ITO, by the spin coating of polymer thin films, the vacuum deposition of top metal contacts, and the patterning of polymer thin film by plasma etching, using the top metal contacts as the self‐aligned etching mask. The devices exhibit no degradation of device characteristics due to the integration processing compared to discrete devices on separate substrates. This demonstration shows a new path towards the fabrication of high performance low‐cost full‐color organic flat panel displays.

Aluminum nanowire polarizing grids: Fabrication and analysis

We have produced metal wire grids with 33nm periodicity, using a thin film of a self-assembling diblock copolymer as a template. These grids, supported on fused quartz wafers, function as transmission polarizers for visible and near-ultraviolet lights. Their polarization efficiency is measured to be near 50% in the visible. Quantitative comparison with a new theoretical analysis of such wire grids indicates that they should perform well into the far UV. This analysis also explains the reversal in polarization direction at shorter wavelengths which we observe in our specimens.