Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS2), recently discovered unexpected properties of WTe2 are provoking strong interest in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that bulk MoTe2 exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics.

The self-ordering of nanoporous anodic aluminum oxide (AAO) in the course of the hard anodization (HA) of aluminum in sulfuric acid (H2SO4) solutions at anodization voltages ranging from 27 to 80 V was investigated. Direct H2SO4-HA yielded AAOs with hexagonal pore arrays having interpore distances D(int) ranging from 72 to 145 nm. However, the AAOs were mechanically unstable and cracks formed along the cell boundaries. Therefore, we modified the anodization procedure previously employed for oxalic acid HA (H2C2O4-HA) to suppress the development of cracks and to fabricate mechanically robust AAO films with D(int) values ranging from 78 to 114 nm. Image analyses based on scanning electron micrographs revealed that at a given anodization voltage the self-ordering of nanopores as well as D(int) depend on the current density (i.e., the electric field strength at the bottoms of the pores). Moreover, periodic oscillations of the pore diameter formed at anodization voltages in the range from 27 to 32 V, which are reminiscent of structures originating from the spontaneous growth of periodic fluctuations, such as topologies resulting from Rayleigh instabilities.

A perfect two-dimensional porous alumina photonic crystal with 500 nm interpore distance was fabricated on an area of 4 cm 2 via imprint methods and subsequent electrochemical anodization. By comparing measured reflectivity with theory, the refractive indices in the oxide layers were determined. The results indicate that the porous alumina structure is composed of a duplex oxide layer: an inner oxide layer consisting of pure alumina oxide of 50 nm in thickness, and an outer oxide layer of a nonuniform refractive index. We suggest that the nonuniform refractive index of the outer oxide arises from an inhomogeneous distribution of anion species concentrated in the intermediate part of the outer oxide.

Dense, ordered arrays of <100>-oriented Si nanorods with uniform aspect ratios up to 5:1 and a uniform diameter of 15 nm were fabricated by block copolymer lithography based on the inverse of the traditional cylindrical hole strategy and reactive ion etching. The reported approach combines control over diameter, orientation, and position of the nanorods and compatibility with complementary metal oxide semiconductor (CMOS) technology because no nonvolatile metals generating deep levels in silicon, such as gold or iron, are involved. The Si nanorod arrays exhibit the same degree of order as the block copolymer templates.

We present a methodology for the analysis of the grain morphology of self-ordered hexagonal lattices and for the quantitative comparison of the quality of their grain ordering based on the distances between nearest neighbors and their angular order. Two approaches to grain identification and evaluation are introduced: (i) color coding the relative angular orientation of hexagons containing a central entity and its six nearest neighbors, and (ii) incorporating triangles comprising three nearest neighbors into grains or repelling them from grains based on deviations of the side lengths and the internal angles of the triangles from those of an ideal equilateral triangle. A spreading algorithm with tolerance parameters allows single grains to be identified, which can thus be ranked according to their size. Hence, grain size distributions are accessible. For the practical evaluation of micrographs displaying self-ordered structures, we suggest using the size of the largest identified grain as a quality measure. Quantitative analyses of grain morphologies are key to the systematic and rational optimization of the fabrication of self-assembled materials.

A photonic crystal (PC) structure is described revealing a complete three-dimensional (3D) photonic band gap of about 25% if realized as a silicon/air structure. It is based on two systems of parallel circular pores being orthogonal to each other. The gap size depends on the degree of mutual penetration of the pore systems. A possible fabrication route is based on macroporous silicon (lattice constant a=0.5 μm), into which orthogonal pores are drilled, e.g., by focused-ion-beam etching. This yields a 3D photonic crystal with a complete band gap in the near infrared. The dispersion behavior of the PC is theoretically analyzed (band structure, density of states), varying the pore radii. We discuss the influence of pore shape variations and topological modifications on the size of the gap.

Three-dimensional (3D) photonic crystals with 3D complete photonic band gaps exhibit interesting optical properties. First they promise to allow the inhibition of spontaneous emission 1-4 for light frequencies within the gap. Moreover, the 3D confinement of light at defects eliminates scattering losses and can therefore lead to high Q microresonators and efficient photonic crystal waveguides. Indeed 3D photonic crystals have been fabricated and are known as Yablonovites, 5,6 Lincoln log structures, 7,8 or self-ordered inverse opals.9 However, the fabrication of structures with band gaps in the near infrared or visible spectral region is difficult (Yablonovite), tedious (Lincoln log), or the structures suffer from disorder (self-ordered opals). This limits the sizes of perfect structures to a few lattice constants. To achieve a more extended Yablonovite-like 3D photonic crystal Lourtioz et al.10,11 used a combination of photoelectrochemical macropore etching in silicon and subsequent drilling of two pore sets with a focused ion beam (FIB). The FIB drilling of the etched macroporous silicon is faster than of bulk material and the problem of redeposition of milled material is minimized at the same time. We apply this technique to fabricate an alternative 3D photonic crystal structure recently proposed by Hillebrand et al., 12 which consists only of two orthogonal interpenetrating pore sets in a high index material (Fig. 1). Each pore set of our structure consists of a pattern of two-dimensional (2D) hexagonally arranged pores forming a 3D structure of orthorhombic symmetry with the primitive lattice vectors a = ͑1 0 0͒, b = ͑1/2 ͱ 3/2 1/2͒, andThe resulting first Brillouin zone resembles the slightly distorted Brillouin zone of a fcc lattice (Fig. 2(a)). We performed band-structure calculations using silicon with a refractive index of 3.5 as the matrix material applying the MPB program developed by Johnson. 13 When the ratio of the pore radius r to lattice constant a of both pore sets is r / a = 0.38, a maximum complete 3D photonic band gap of ⌬ / center = 25.1% is obtained (Fig. 2(b)). The gap appears at low normalized frequencies in the band structure between the second and third bands and should therefore be insensitive to modest disorder.We realized the described structure by first creating a 2D hexagonal pattern of etch pits applying a KOH solution on the (100) surface of an n-type silicon wafer. The position of the etch pits and the lattice constant of a = 500 nm were lithographically defined. Afterwards the structured side of the wafer was immersed in hydrofluoric acid and a photoelectrochemical etch process 14 was used to create 50-m-deep macropores in the silicon wafer starting at the etch pits. The radius of the photoelectrochemically etched pores r etch = 190 nm was determined by the etch current. This pore pattern already forms a 2D photonic crystal 15-17 and the characteristic 2D band gaps were observed in reflection measurements confirming the given structure parameters a and r etch . The structure ...

We performed systematic adsorption studies using self-ordered nanoporous anodic aluminum oxide (AAO) in an extended range of mean pore diameters and with different pore topologies. These matrices were characterized by straight cylindrical pores having a narrow pore size distribution and no interconnections. Pronounced hysteresis loops between adsorption and desorption cycles were observed even in the case of pores closed at one end. These results are in contrast with macroscopic theoretical models and detailed numerical simulations of the adsorption in a single pore. Extensive measurements involving adsorption isotherms, reversal curves, and subloops carried out in closed-bottom pores suggest that the pores do not desorb independently from one another.