Alexander Slesarev

Layer-by-Layer Removal of Graphene for Device Patterning

Reactions of graphene with zinc enable etching of a single graphene layer. The patterning of graphene is useful in fabricating electronic devices, but existing methods do not allow control of the number of layers of graphene that are removed. We show that sputter-coating graphene and graphene-like materials with zinc and dissolving the latter with dilute acid removes one graphene layer and leaves the lower layers intact. The method works with the four different types of graphene and graphene-like materials: graphene oxide, chemically converted graphene, chemical vapor–deposited graphene, and micromechanically cleaved (“clear-tape”) graphene. On the basis of our data, the top graphene layer is damaged by the sputtering process, and the acid treatment removes the damaged layer of carbon. When used with predesigned zinc patterns, this method can be viewed as lithography that etches the sample with single-atomic-layer resolution.

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.

Photonic Crystal Formed by the Imaginary Part of the Refractive Index

2010 WILEY-VCH Verlag Gmb Photonic crystals (PhCs) are widely studied photonic structures that provide unprecedented control over the propagation of light. The large majority of PhCs are formed by a periodic modulation of the refractive index, i.e., a modulation of the real part of the dielectric constant. Additional functionality can be created by including absorbing features into the structure, thus creating PhCs out of materials with complex dielectric index. This is particularly interesting when the photonic resonance created by the refractive index contrast overlaps with the absorption feature, hence creating a ‘‘resonantly absorbing PhC’’. An example of such a resonantly absorbing structure is that of a semiconductor saturable absorber mirror (SESAM) used in mode-locked lasers where quantum wells are incorporated into a multilayer Bragg stack. Similar structures referred to as Resonantly Absorbing Bragg Reflectors (RABR) have been used to demonstrate optical switching, optical storage, and nonlinear optical conversion. The examples above are based on Bragg mirrors or 1D photonic crystals. An extension to 2D and 3D structures has been proposed and demonstrated by backfilling the voids of a conventional photonic crystal with resonantly absorbing materials such as quantum dots and metal. Most of these structures are based on refractive index modulation with absorption providing additional features. In contrast, the structure we propose and demonstrate here, an extension of the 1D case approaches of Prineas et al. and Kozhekin et al., is formed exclusively by the absorbing feature, hence it is a true ‘‘imaginary refractive index’’ structure. A refractive index contrast naturally exists near the absorption feature, as required by the Kramers–Kronig relationship, but away from this feature, the refractive index contrast is practically zero. In order to demonstrate this effect experimentally, we created a template using holographic lithography with a diffractive optical element (DOE) (Fig. 1), which generates a 2D photonic lattice of SU-8 polymer disks (Fig. 2a). The disks are doped with a high concentration of the organic dye Rhodamine B (RhB) that has an absorption peak around 564 nm, so the absorption of the lattice is strongly dependent on wavelength. Subsequent filling of the voids with the same SU-8 polymer, but without dye doping, gives rise to an imaginary index photonic lattice (Fig. 2b). The diffraction pattern of the structure before back-filling is given by the dielectric modulation that exists for all wavelengths as in a conventional photonic lattice and is much like a rainbow (Fig. 3a). By contrast, only yellow/green light diffraction around 564 nm can be observed once the structure has been back-filled (Fig. 3b–d). The wavelength dependent diffraction clearly shows that the structure only acts as a PhC in the vicinity of the absorption window. Out of this window, the structure behaves as a uniform polymer layer. This new type of photonic crystal offers intriguing properties for further study in the field of saturable light absorption and emission control. The PhC may also be applied for optical switching and optical logic operation. The imaginary index structure can be treated as a 2D grating with wavelength-dependent phase and intensity modulation. According to diffraction theory, the intensity distribution I created by the grating is given by the Fourier transform of the transmission function of the grating t:

Cs(I) and Sr(II) Sorption onto Graphene Oxide

ABSTRACT This article demonstrates the efficacy of graphene oxide (GO) for Cs(I) and Sr(II) removal from aqueous solutions in the presence of competing cations. The interaction mechanisms of Cs(I) and Sr(II) with GO were studied at varying pHs, ionic strengths, and solution compositions. Thermal treatment was studied as a possible approach to minimize the volume of secondary radioactive waste, and cumulative pre-concentration factors were recorded for both cations.

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.

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.

Correction to Improved Synthesis of Graphene Oxide

In the Experimental Procedures section, the order of addition of reagents in the description of the improved method is incorrect. The correct procedure is as follows: for the improved method, KMnO4 (18.0 g, 6 wt equiv) was slowly added in 6 equal portions to a 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) and graphite flakes (3.0 g, 1 wt equiv), producing a slight exotherm that should not exceed 35−40 °C. The reaction was then heated to 50 °C and stirred for 12 h. The reaction was cooled to room temperature and poured onto ice (400 mL) containing 30% H2O2 (3 mL). This correction is important because, in the instances where there would be different addition order, the KMnO4 might be in high concentration in the acid mixture, and this can become explosive.

Rapid method for the purification of graphene oxide

A rapid and facile purification method for graphene oxide (GO) is important for its production above the gram scale. Such a method would allow for the development of GOs large-scale industrial applications. Out of several protocols in this study, including centrifugation, filtration, precipitation and decantation, filtration using a gas-press proved to be the most effective. Gas-press filtration using filter beds of Celite, perlite, glass wool, ceramic tape, or woven glass fibre allowed for adequate purification of 1 g of crude large-flake (∼30 μm flake diameter) GO in less than 60 min using a lab-scale set-up. The present technique could be easily scaled-up, it generates minimal waste, and can be tuned by changing the dimensions of the equipment, pressure, and filter bed. This would allow a user to obtain a higher work-up efficiency. The quickly purified product is called efficiently purified GO or EGO.