The research activities on porous silicon described here cover one aspect of our systematic approach to fundamentally relate nanostructuring of thermoelectric materials to microscopic thermoelectric transport. Structuring materials on the nanometer length scale has long been established as a vital strategy to reduce the lattice thermal conductivity, one of the key parameters that define the thermoelectric performance. Gaining a more detailed understanding of the microscopic mechanisms of this reduction in lattice thermal conductivity is a major objective of this project. This is achieved by studying the modified phonon dispersion and phonon lifetimes upon nanostructuring of model systems such as silicon. It is also imperative to ascertain the influence of nanostructuring on other important transport parameters like the electrical conductivity, the Seebeck coefficient and the charge carrier densities.
The focus of our research activities is on mesoporous silicon (Fig.1 and Ref. [1, 2]), here regarded as a model system, which, however, simultaneously satisfies important criteria for industrial applications, like abundance and non-toxicity. Whereas bulk Si performs fairly poorly as thermoelectric material (ZT < 0.01), nanostructuring of Si offers the possibility to significantly impair heat flow, thus leading to superior thermoelectric properties. Mesoporous silicon is one realization of nanostructured silicon that inherently has a high potential for a thermoelectric organic-inorganic hybrid material.
The synthesis of mesoporous silicon by means of electrochemical etching in hydrofluoric acid in combination with a post-synthesis treatment  allows synthesizing macroscopic quantities of this single crystalline material at tunable porosity, pore size and surface morphology for parametric studies on structure, dynamics, and thermoelectric transport. Its single crystalline structure renders it particularly suited for scattering studies on elementary phononic and electronic excitations.
Comprehensive characterization of structured Si samples follows the dual approach of correlating microscopic and macroscopic material properties. On one side, X-ray and neutron scattering techniques as well as X-ray spectroscopy shall be employed to reveal peculiarities of structure, phonon dispersion and phonon lifetimes in nanostructured Si. Complementing these studies, the macroscopic functionality is investigated by exploiting the macroscopic measurement techniques within the laboratory infrastructure operated by the Working Group EM-AMCT.
Research Highlight: Low energy phonons in mesoporous silicon
In a recent study  neutron and X-ray scattering experiments were used to investigate structural and dynamical properties of electrochemically etched, porous silicon membranes with 8 nm wide tubular pores. In particular, inelastic cold neutron scattering techniques allowed measuring the phonon dispersion of the nanostructured, single crystalline samples in the energy range up to 4 meV. In porous silicon, a modified dispersion relation (Fig. 2) was found leading to systematically smaller sound velocities and altered elastic properties when compared to bulk silicon. The importance of these findings for nanostructured silicon as thermoelectric material of interest is outlined in detail in Ref.  predicting a subtle but sizable effect on thermal conductivity and the figure-of-merit ZT.
Fig. 2: Phonon dispersion of porous Si (pSi) and bulk Si (bSi) in the linear Debye regime. From left to right, symbols represent L/T , L/T , and L/T phonons. Dashed lines illustrate the linear dispersion relation close to the center of the Brillouin zone. For bSi the slopes of the dispersion relations correspond directly to the literature values for the expected sound velocities. Bulk data were used as a reference to remove uncertainties originating from any instrumental offsets. The slopes of the dispersion relations for the nanostructured sample were obtained by linear least squares fitting and provide the sound velocities in pSi. Shown data are corrected for the instrumental offset. Reprinted from Ref  with permission from ELSEVIER.
- Measuring and relating phonon life times in nanostructured silicon to thermal transport properties of nanostructured silicon.
- Synthesizing inorganic-organic hybrids based on nanostructured silicon and conducting polymers.
- Characterizing thermoelectric transport in inorganic-organic hybrids.
- Laboratory for Thermoelectric Material Synthesis and Compaction (part of HEMF)
- Laboratory for Thermoelectric Material Characterization and Transport Measurements
- BESSY II
- BER II
 A.V. Kityk, T. Hofmann, K. Knorr, Liquid-Vapor Coexistence at a Mesoporous Substrate, Phys. Rev. Lett. 100 (2008) 036105.
 T. Hofmann, P. Kumar, M. Enderle and D. Wallacher, Growth of Highly Oriented Deuterium Crystals in Silicon Nanochannels, Phys. Rev. Lett. 110 (2013) 065505.
 P. Kumar, T. Hofmann, K. Knorr, P. Huber, P. Scheib, P. Lemmens, Tuning the pore wall morphology of mesoporous silicon from branchy to smooth, tubular by chemical treatment, J. Appl. Phys., 103/2 (2008) 024303.
 T. Hofmann, D. Wallacher, R. Toft-Petersen, B. Ryll, M. Reehuis, K. Habicht, Phonons in mesoporous silicon: The influence of nanostructuring on the dispersion in the Debye regime, Microporous Mesoporous Mater. 243 (2017) 263-270.