A. Baruth

Crossover From Nanoscopic Intergranular Hopping to Conventional Charge Transport in Pyrite Thin Films

Pyrite FeS2 is receiving a resurgence of interest as a uniquely attractive thin film solar absorber based on abundant, low-cost, nontoxic elements. Here we address, via ex situ sulfidation synthesis, the long-standing problem of understanding conduction and doping in FeS2 films, an elusive prerequisite to successful solar cells. We find that an abrupt improvement in crystallinity at intermediate sulfidation temperatures is accompanied by unanticipated crossovers from intergranular hopping to conventional transport, and, remarkably, from hole-like to electron-like Hall coefficients. The hopping is found to occur between a small volume fraction of conductive nanoscopic sulfur-deficient grain cores (beneath our X-ray diffraction detection limits), embedded in nominally stoichiometric FeS2. In addition to placing constraints on the conditions under which useful properties can be obtained from FeS2 synthesized in diffusion-limited situations, these results also emphasize that FeS2 films are not universally p-type. Indeed, with no knowledge of the active transport mechanism we demonstrate that the Hall coefficient alone is insufficient to determine the sign of the carriers. These results elucidate the possible transport mechanisms in thin film FeS2 in addition to their influence on the deduced carrier type, an enabling advancement with respect to understanding and controlling doping in pyrite films.

Optimization of long-range order in solvent vapor annealed poly(styrene)-block-poly(lactide) thin films for nanolithography

Detailed experiments designed to optimize and understand the solvent vapor annealing of cylinder-forming poly(styrene)-block-poly(lactide) thin films for nanolithographic applications are reported. By combining climate-controlled solvent vapor annealing (including in situ probes of solvent concentration) with comparative small-angle X-ray scattering studies of solvent-swollen bulk polymers of identical composition, it is concluded that a narrow window of optimal solvent concentration occurs just on the ordered side of the order-disorder transition. In this window, the lateral correlation length of the hexagonally close-packed ordering, the defect density, and the cylinder orientation are simultaneously optimized, resulting in single-crystal-like ordering over 10 μm scales. The influences of polymer synthesis method, composition, molar mass, solvent vapor pressure, evaporation rate, and film thickness have all been assessed, confirming the generality of this behavior. Analogies to thermal annealing of elemental solids, in combination with an understanding of the effects of process parameters on annealing conditions, enable qualitative understanding of many of the key results and underscore the likely generality of the main conclusions. Pattern transfer via a Damascene-type approach verified the applicability for high-fidelity nanolithography, yielding large-area metal nanodot arrays with center-to-center spacing of 38 nm (diameter 19 nm). Finally, the predictive power of our findings was demonstrated by using small-angle X-ray scattering to predict optimal solvent annealing conditions for poly(styrene)-block-poly(lactide) films of low molar mass (18 kg mol(-1)). High-quality templates with cylinder center-to-center spacing of only 18 nm (diameter of 10 nm) were obtained. These comprehensive results have clear and important implications for optimization of pattern transfer templates and significantly advance the understanding of self-assembly in block copolymer thin films.

Magnetization dynamics triggered by surface acoustic waves

Investigations into fast magnetization switching are of both fundamental and technological interest. Here we present a low-power, remote method for strain driven magnetization switching. A surface acoustic wave propagates across an array of ferromagnetic elements, and the resultant strain switches the magnetization from the easy axis into the hard axis direction. Investigations as a function of applied magnetic field as well as unidirectional anisotropy (the exchange bias) reveal excellent agreement with prediction, confirming the viability of this method.

Domain overlap in antiferromagnetically coupled [Co/Pt]/NiO /[Co/Pt] multilayers

Antiferromagnetically coupled magnetic thin films with perpendicular anisotropy exhibit domain overlap regions originating from magnetostatic stray fields localized in the vicinity of the domain walls. Using high resolution magnetic force microscopy, the authors investigate these overlap regions in [Co∕Pt]∕NiO∕[Co∕Pt] multilayers with various strengths of the interlayer exchange coupling. They develop a simple model that provides a quantitative explanation of the formation of these regions and the relationship between the domain overlap width and the coupling strength. Their results are important for application of magnetic layered structures with perpendicular anisotropy in advanced magnetoresistive devices.

Size-Tuned ZnO Nanocrucible Arrays for Magnetic Nanodot Synthesis via Atomic Layer Deposition-Assisted Block Polymer Lithography

Low-temperature atomic layer deposition of conformal ZnO on a self-assembled block polymer lithographic template comprising well-ordered, vertically aligned cylindrical pores within a poly(styrene) (PS) matrix was used to produce nanocrucible templates with pore diameters tunable via ZnO thickness. Starting from a PS template with a hexagonal array of 30 nm diameter pores on a 45 nm pitch, the ZnO thickness was progressively increased to narrow the pore diameter to as low as 14 nm. Upon removal of the PS by heat treatment in air at 500 °C to form an array of size-tunable ZnO nanocrucibles, permalloy (Ni80Fe20) was evaporated at normal incidence, filling the pores and creating an overlayer. Argon ion beam milling was then used to etch back the overlayer (a Damascene-type process), leaving a well-ordered array of isolated ZnO nanocrucibles filled with permalloy nanodots. Microscopy and temperature-dependent magnetometry verified the diameter reduction with increasing ZnO thickness. The largest diameter (30 nm) dots exhibit a ferromagnetic multidomain/vortex state at 300 K, with relatively weakly temperature-dependent coercivity. Reducing the diameter leads to a crossover to a single-domain state and eventually superparamagnetism at sufficiently high temperature, in quantitative agreement with expectations. We argue that this approach could render this form of block polymer lithography compatible with high-temperature processing (as required for technologically important high perpendicular anisotropy ordered alloys, for instance), in addition to enabling separation-dependent studies to probe interdot magnetostatic interactions.

Reactive sputter deposition of pyrite structure transition metal disulfide thin films: Microstructure, transport, and magnetism

Transition metal disulfides crystallizing in the pyrite structure (e.g., TMS2, with TM = Fe, Co, Ni, and Cu) are a class of materials that display a remarkably diverse array of functional properties. These properties include highly spin-polarized ferromagnetism (in Co1−xFexS2), superconductivity (in CuS2), an antiferromagnetic Mott insulating ground state (in NiS2), and semiconduction with close to optimal parameters for solar absorber applications (in FeS2). Exploitation of these properties in heterostructured devices requires the development of reliable and reproducible methods for the deposition of high quality pyrite structure thin films. In this manuscript, we report on the suitability of reactive sputter deposition from metallic targets in an Ar/H2S environment as a method to achieve exactly this. Optimization of deposition temperature, Ar/H2S pressure ratio, and total working gas pressure, assisted by plasma optical emission spectroscopy, reveals significant windows over which deposition of single-...

Magnetoelectric effects in ferromagnetic cobalt/ferroelectric copolymer multilayer films

Interactions between a ferromagnet, cobalt, and a ferroelectric copolymer, poly(vinylidene fluoride with trifluoroethylene) in thin film heterostructures result in a 5% change in the ferroelectric polarization on application of a perpendicular 6 kG magnetic field corresponding to a magnetoelectric coupling coefficient of α=5.45 V/cm Oe. The effect disappears on magnetic saturation, ruling out conventional strain coupling. A simple model posits that the ferroelectric film develops in-plane strain gradients, a consequence of the coupling to strain gradients present at the domain walls in the multidomain Co layer, resulting in the measured polarization change via the flexoelectric effect.

Nanoscale rings from silicon-containing triblock terpolymers

Nanoscopic ring arrays of various materials show promise for both technological applications and fundamental studies. In this work we report the preparation of sub-50 nm diameter ring arrays from metallic thin films using a block polymer lithographic pattern transfer approach. We prepared a triblock terpolymer that adopts a core-shell cylindrical morphology where the shell is an oxidizable polydimethylsiloxane block. Solvent annealed thin films of this terpolymer produce cylindrical features oriented perpendicular to the substrate surface. The polydimethylsiloxane shell is then converted into SiOx rings by an oxygen reactive ion etch. The resultant hard mask pattern is then transferred into Au, Ni80Fe20, and Ni80Cr20 thin films via Ar ion beam milling, demonstrating the generality of the approach.

Domain size and structure in exchange coupled [Co/Pt]/NiO/[Co/Pt] multilayers

We investigate the competing effects of interlayer exchange coupling and magnetostatic coupling in the magnetic heterostructure ([Co/Pt]/NiO/[Co/Pt]) with perpendicular magnetic anisotropy (PMA). This particular heterostructure is unique among coupled materials with PMA in directly exhibiting both ferromagnetic and antiferromagnetic coupling, oscillating between the two as a function of spacer layer thickness. By systematically tuning the coupling interactions via a wedge-shaped NiO spacer layer, we explore the energetics that dictate magnetic domain formation using high resolution magnetic force microscopy coupled with the magneto-optical Kerr effect. This technique probes the microscopic and macroscopic magnetic behavior as a continuous function of thickness and the interlayer exchange coupling, including the regions where interlayer coupling goes through zero. We see significant changes in domain structure based on the sign of coupling, and also show that magnetic domain size is directly related to the magnitude of the interlayer exchange coupling energy, which generally dominates over the magnetostatic interactions. When magnetostatic interactions become comparable to the interlayer exchange coupling, a delicate interplay between the differing energy contributions is apparent and energy scales are extracted. The results are of intense interest to the magnetic recording industry and also illustrate a relatively new avenue of undiscovered physics, primarily dealing with the delicate balance of energies in the formation of magnetic domains for coupled systems with PMA, defining limits on domain size as well as the interplay between roughness, domains and magnetic coupling.

High-precision solvent vapor annealing for block copolymer thin films

Despite its efficacy in producing well-ordered, periodic nanostructures, the intricate role multiple parameters play in solvent vapor annealing has not been fully established. In solvent vapor annealing a thin polymer film is exposed to a vapor of solvent(s) thus forming a swollen and mobile layer to direct the self-assembly process at the nanoscale. Recent developments in both theory and experiments have directly identified critical parameters that govern this process, but controlling them in any systematic way has proven non-trivial. These identified parameters include vapor pressure, solvent concentration in the film, and the solvent evaporation rate. To explore their role, a purpose-built solvent vapor annealing chamber was designed and constructed. The all-metal chamber is designed to be inert to solvent exposure. Computer-controlled, pneumatically actuated valves allow for precision timing in the introduction and withdrawal of solvent vapor from the film. The mass flow controller-regulated inlet, chamber pressure gauges, in situ spectral reflectance-based thickness monitoring, and low flow micrometer relief valve give real-time monitoring and control during the annealing and evaporation phases with unprecedented precision and accuracy. The reliable and repeatable alignment of polylactide cylinders formed from polystyrene-b-polylactide, where cylinders stand perpendicular to the substrate and span the thickness of the film, provides one illustrative example.