Vera Abramova

Nanoporous silicon oxide memory.

Oxide-based two-terminal resistive random access memory (RRAM) is considered one of the most promising candidates for next-generation nonvolatile memory. We introduce here a new RRAM memory structure employing a nanoporous (NP) silicon oxide (SiOx) material which enables unipolar switching through its internal vertical nanogap. Through the control of the stochastic filament formation at low voltage, the NP SiOx memory exhibited an extremely low electroforming voltage (∼ 1.6 V) and outstanding performance metrics. These include multibit storage ability (up to 9-bits), a high ON-OFF ratio (up to 10(7) A), a long high-temperature lifetime (≥ 10(4) s at 100 °C), excellent cycling endurance (≥ 10(5)), sub-50 ns switching speeds, and low power consumption (∼ 6 × 10(-5) W/bit). Also provided is the room temperature processability for versatile fabrication without any compliance current being needed during electroforming or switching operations. Taken together, these metrics in NP SiOx RRAM provide a route toward easily accessed nonvolatile memory applications.

Double stacking faults in convectively assembled crystals of colloidal spheres.

Using microradian X-ray diffraction, we investigated the crystal structure of convectively assembled colloidal photonic crystals over macroscopic (0.5 mm) distances. Through adaptation of Wilsons theory for X-ray diffraction, we show that certain types of line defects that are often observed in scanning electron microscopy images of the surface of these crystals are actually planar defects at 70.5 degrees angles with the substrate. The defects consist of two parallel hexagonal close-packed planes in otherwise face-centered cubic crystals. Our measurements indicate that these stacking faults cause at least 10% of stacking disorder, which has to be reduced to fabricate high-quality colloidal photonic crystals.

Meniscus-Mask Lithography for Narrow Graphene Nanoribbons

Described here is a planar top-down method for the fabrication of precisely positioned very narrow (sub-10 nm), high aspect ratio (>2000) graphene nanoribbons (GNRs) from graphene sheets, which we call meniscus-mask lithography (MML). The method does not require demanding high-resolution lithography tools. The mechanism involves masking by atmospheric water adsorbed at the edge of the lithography pattern written on top of the target material. The GNR electronic properties depend on the graphene etching method, with argon reactive ion etching yielding remarkably consistent results. The influence of the most common substrates (Si/SiO2 and boron nitride) on the electronic properties of GNRs is demonstrated. The technique is also shown to be applicable for fabrication of narrow metallic wires, underscoring the generality of MML for narrow features on diverse materials.

Meniscus-mask lithography for fabrication of narrow nanowires.

We demonstrate the efficiency of meniscus-mask lithography (MML) for fabrication of precisely positioned nanowires in a variety of materials. Si, SiO2, Au, Cr, W, Ti, TiO2, and Al nanowires are fabricated and characterized. The average widths, depending on the materials, range from 6 to 16 nm. A broad range of materials and etching processes are used and the generality of approach suggests the applicability of MML to a majority of materials used in modern planar technology. High reproducibility of the MML method is shown and some fabrication issues specific to MML are addressed. Crossbar structures produced by MML demonstrate that junctions of nanowires could be fabricated as well, providing the building blocks required for fabrication of nanowire structures of varied planar geometry.

Revealing stacking sequences in inverse opals by microradian X-ray diffraction

We present the results of the structural analysis of inverse opal photonic crystals by microradian X-ray diffraction. Inverse opals based on different oxide materials (TiO2, SiO2 and Fe2O3) were fabricated by templating polystyrene colloidal crystal films grown by the vertical deposition technique. Our results suggest that most inverse opal films possess dominating twinned face-centered cubic structure accompanied by some fragments of hexagonal close-packed and random hexagonal close-packed structures. The studied samples possessed individual structures with different ratios of the above fragments. By fitting the results of the angular-dependent X-ray diffraction by the Wilson model we estimate the stacking probability α in the studied samples to be ~0.7–0.8. Microradian X-ray diffraction therefore provides detailed structural information on opal-based photonic crystals and can be applied to opaque inverse opals or the samples with a periodicity <300 nm, whose structure cannot be investigated by conventional optical methods.

Synthesis of high-quality inverse opals based on magnetic complex oxides: yttrium iron garnet (Y3Fe5O12) and bismuth ferrite (BiFeO3)

Magnetophotonic crystals (MPCs) are periodic structures that are made of a magnetic material or have a magnetic defect introduced in a periodic non-magnetic matrix, and possess interesting optical and magneto-optical properties. Of particular interest are three-dimensional (3D) MPCs made of magnetic complex oxides, such as yttrium iron garnet (Y3Fe5O12, YIG) and bismuth ferrite (BiFeO3, BFO). In this paper we report for the first time the synthesis of 3D MPCs with a face-centered cubic (fcc) inverse opal structure based on these materials. The samples were prepared by a sol–gel method that involves infiltration of polystyrene colloidal crystals with liquid precursors followed by a high-temperature annealing. The developed procedure yields high-quality single-phase YIG and BFO inverse opals with high filling fractions of magnetic materials. Optical measurements were performed on large (characteristic size ∼ 100 μm) single crystal domains of inverse opals, and angle-dependent reflectance peaks caused by the Bragg diffraction of light in highly ordered YIG and BFO MPCs were observed. Also, since BFO has a high refractive index (>2.8) and a low extinction coefficient at λ > 550 nm, BFO MPCs with the fcc structure have a potential for the realization of a complete photonic band gap in the visible and near infrared regions.

Luminescent photonic crystals

Angular distribution of photoluminescence of trivalent rare earth ions embedded inside three-dimensional opal and inverse opal photonic crystals is experimentally shown to be strongly modulated for the emission frequencies near the first and higher photonic stop-bands. Numerical simulations of fractional density of optical states also predict highly directional emission from photonic crystals with an ideal structure. Experimental results are shown to be in a good agreement with calculated angular dependencies of luminescence. However, better agreement between experimental and theoretical data requires smoothing of calculated dependencies, which can be attributed to the presence of structural imperfections in real photonic crystal samples.