Erik K. Richman

Hexagonal nanoporous germanium through surfactant-driven self-assembly of Zintl clusters

Surfactant templating is a method that has successfully been used to produce nanoporous inorganic structures from a wide range of oxide-based material. Co-assembly of inorganic precursor molecules with amphiphilic organic molecules is followed first by inorganic condensation to produce rigid amorphous frameworks and then, by template removal, to produce mesoporous solids. A range of periodic surfactant/semiconductor and surfactant/metal composites have also been produced by similar methods, but for virtually all the non-oxide semiconducting phases, the surfactant unfortunately cannot be removed to generate porous materials. Here we show that it is possible to use surfactant-driven self-organization of soluble Zintl clusters to produce periodic, nanoporous versions of classic semiconductors such as amorphous Ge or Ge/Si alloys. Specifically, we use derivatives of the anionic Ge94- cluster, a compound whose use in the synthesis of nanoscale materials is established. Moreover, because of the small size, high surface area, and flexible chemistry of these materials, we can tune optical properties in these nanoporous semiconductors through quantum confinement, by adsorption of surface species, or by altering the elemental composition of the inorganic framework. Because the semiconductor surface is exposed and accessible in these materials, they have the potential to interact with a range of species in ways that could eventually lead to new types of sensors or other novel nanostructured devices.

Vertically oriented hexagonal mesoporous films formed through nanometre-scale epitaxy.

Polymer- and surfactant-templated mesoporous inorganic materials offer a unique combination of controllable nanoscale architecture, materials variation and low-cost solution processing. Inorganic materials can be produced with a range of periodic pore structures, with feature size ranging from 2 to 30 nm, and from a diverse set of materials. Unfortunately in thin-film form, the pores of the ubiquitous hexagonal honeycomb phase tend to lie in the plane of the substrate making these materials unsuitable for applications where diffusion into the pores is required. Here, we show that nanometre-scale epitaxy on a patterned substrate can be used to form vertically oriented pores in honeycomb-structured films. We use the surface of cubic mesoporous films to form the pattern; as such, our method does not sacrifice the simple processing advantages of a self-assembled system. A precise lattice match between the hexagonal and cubic films is needed for vertical orientation, a condition that can be achieved using mixed templates or selective pore swelling. Pore orientation is characterized by a combination of microscopy and diffraction. Here, we present alignment data on oriented nanopores in the 10-15 nm range, but the method should be applicable across the 2-30 nm pore size range of these self-organized materials.

Ordered Mesoporous Silicon through Magnesium Reduction of Polymer Templated Silica Thin Films

This paper describes the process of making ordered mesoporous silicon (Si) thin films. The process begins with mesoporous silica (SiO 2) thin films that are produced via evaporation induced self-assembly (EISA) using sol-gel silica precursors with a diblock copolymer template. This results in a film with a cubic lattice of 15 nm diameter pores and 10 nm thick walls. The silicon is produced through reduction of the silica thin films in a magnesium (Mg) vapor at 675 degrees C. Magnesium reduction preserves the ordered pore-solid architecture but replaces the dense silica walls with 10-17 nm silicon crystallites. The resulting porous silicon films are characterized by a combination of low and high angle X-ray diffraction, combined with direct SEM imaging. The result is a straightforward route to the production of ordered nanoporous silicon.

Measurement of anisotropic fracture energies in periodic templated silica/polymer composite coatings

We report measurements of the fracture energies of hexagonal honeycomb structured silica/polymer composite films that were produced through an evaporation induced self-assembly process. These films exhibit large anisotropy with their hexagonal pore axes aligned with the dip-coating direction. The experimental strategy included depositing films onto a flexible Kapton substrate and then straining them, in situ, under a microscope. To study the effect of the anisotropic microstructure on the fracture energy, cracks were propagated both parallel and perpendicular to the cylindrical pore axis directions. For both cases, the geometries of the evolving crack patterns with loading were micrographically recorded and the desired energy release rates were calculated using a two-dimensional steady-state channeling crack model. The model was implemented using the ANSYS finite element program. The experimental observations showed significant inelastic film deformation prior to crack propagation. These deformations were...

Thermal conductivity of cubic and hexagonal mesoporous silica thin films

No. % Uncertainty tf (nm) Min Max Min Max kf (W/m.K) Uncertainty 1 Hexagonal P123 46 ± 5 320 7 10 3 5 0.18 ± 0.02 2 Hexagonal P123 48 ± 5 160 7 10 3 5 0.18 ± 0.01 3 Hexagonal P123 40 ± 5 300 7 10 3 5 0.22 ± 0.01 4 Hexagonal P123 43 ± 5 540 7 10 3 5 0.20 ± 0.01 5 Hexagonal P123 45 ± 5 130 7 10 3 5 0.18 ± 0.01 6 Cubic Brij76 21 ± 5 155 3 5 2 3 0.30 ± 0.04 7 Cubic Brij76 23 ± 5 150 3 5 2 3 0.29 ± 0.02 8 Cubic Brij76 23 ± 5 170 3 5 2 3 0.34 ± 0.03 9 Cubic P123 29 ± 5 185 8 10 3 5 0.28 ± 0.03 10 Cubic P123 23 ± 5 200 8 10 3 5 0.38 ± 0.02 11 Cubic P123 26 ± 5 85 8 10 3 5 0.27 ± 0.01 12 Cubic P123 25 ± 5 80 8 10 3 5 0.27 ± 0.01 13 Cubic KLE 27 ± 5 300 15 18 10 12 0.35 ± 0.01 14 Cubic KLE 30 ± 5 130 15 18 10 12 0.32 ± 0.04 Matrix phase: Temperature: