Nanoporous templates based
We use nanoporous Si (PS) and anodized aluminium oxide (Al2O3) as template materials. These templates differ with respect to typical length scales (2-10nm vs. 10-200nm) and chemical functionality that can be further manipulated. Different dyes (Rhodamine 6G and Malachite green) and molecules (magnetic and electronic degrees of freedom) are used to probe the pore/cavity wall functionality.
For initial doping experiments transition metal (Cu,Fe,Pt,Ni,Co) porphyrines have been templated from the liquid phase. We observe photo luminescence shifts and optical nonlinearities. The preparation of these systems is now well established and the involved processes can be simultaneously used to functionalize and template the systems. In a second step we use Sol-Gel Ag particles for a further enhancement of the local electric fields.
Nanoporous Si is a primary host or template material for our studies as they allow a precise controle of morphology and chemical functionality of the pore/cavity walls. The achieved length scales are also compartible with our range of molecular magnets. The template is prepared by chemical etching and characterized using BET, PL, Raman, etc. From the absorption/desorption of N2 gas the porosity and pore diameters are determined routinely. Pore diameters of 5-12 nm are most interesting for the present applications. The pore size is controlled by the composition of the electrolyte (e.g. HF/propanol ratio) and the current density.
Porous Si prepared by
chemical etching, The bar corresponds to
a length scale of 100nm.
Oxidation of the porous Si allows to change the basic chemical functionality of the host material and to make it hydrophilic.
Oxidized porous Si,
The bar corresponds to a length scale of 100nm.
Templates from Anodized Aluminium Oxide (AAO) and nanowire preparation
Hongdan Yan showing a nanostructured template made using priniciples of self-organization.
In a similar way metallic aluminum can be anodized leading to more regular, self organized poores in the length scale from 15-150nm diameter. These templates can be used to growth wires or immobilize molecules for use in live science. Examples that we show below are related to magnetic materials that are electrodeposited in the pores.
Further investigations are performed with the magnetic wires in the pores (aniostropic magnetization/relaxation and transport) and on the nanomaterials disentangled from the matrix. Some examples are shown below.
AAO with pore diameters of 30,
70 and 100 nm (left to right).
Pore diameter, distance and height/thickness are controlled via the etching process and I(U) characteristics. Templates can be prepared one-sided with a metallic back or transmissive, i.e. completely oxidized.
prepared by sol-gel and assembled in the pores. The template is then
Electrochemically prepared Fe and Co wires
disentangled from the
template. The diameters are 80nm
and 40nm, respectively.
Presently we prepare Au wires for local electric field enhancement and a decoration on top of the wires to add chemical functionality.
Functionalization of nanoporous and templated systems
The loading of Molecular Magnets is presently performed in the liquid phase using dipping techniques. Magnetic nano particles are prepared with a Sol-Gel and a following annealing process. The assemblies are then investigated as prepared or with the host removed in a second etching process. In this way nano rods of CoFe2O4 have been prepared.
Effect of HF concentration on physical and electronic properties
of electrochemically formed nano-porous silicon, P. Kumar, P.
Lemmens, M. K. Ghosh, F. Ludwig, and M. Schilling,
Iron/Nickel nanowire growth in Anodic Aluminum Oxide templates:
Transfer of length scales and periodicity, H.D. Yan, P. Lemmens,
H. Dierke, S.C. White, F. Ludwig, M. Schilling,
Tuning the pore wall morphology of mesoporous silicon from
branchy to smooth, tubular by chemical treatment, P. Kumar, T.
Hoffman, P. Huber, P. Scheib, and P. Lemmens,
p.lemmens at tu-bs.de, last change: 06.03.2009
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