TY  - THES
T1  - Nanostructured diluted magnetic semiconductors within mesoporous silica
A1  - Brieler,Felix
Y1  - 2012/04/11
N2  - In this work the successful formation of several diluted magnetic semiconductors inside various mesoporous host matrices is shown. For that purpose, different high quality mesoporous silicas were synthesised. Because of the possibility to obtain nanostructured quantum wires inside the respective pore systems, hexagonally ordered structures of the MCM-41 and SBA-15 kind were chosen as hosts for the intra-pore formation of the DMS nanoparticles. Later on it became obvious though, that the formation of nanowires inside the pore systems was not successful. Instead, particles with an elongated spherical or ellipsoidal shape were formed. With powder X ray diffraction the hexagonal order of the silica was proved. The diffraction patterns of the pristine host structures could all be indexed according to a hexagonal phase. The peaks at diffraction angles 2 theta greater than 2° indicated a high degree of long range order. This was further substantiated by TEM analysis. Over several hundreds of nanometres a perfect hexagonal arrangement of the pores was clearly visible. Physisorption showed the high specific surface areas of the respective materials. Depending on the pore size, surface areas of over 1000 m2/g could be obtained. From the sorption experiments the pore diameters of the host compounds were calculated using the well established algorithm of Barrett, Joyner and Halenda. All host materials had narrow pore size distributions with well-defined maxima at 3, 6 and 9 nm. With these highly ordered and pore size defined materials available, reasonable host structures for the intra-pore synthesis of nanoparticles can be synthesised. The following investigation, regarding the dependence of the physical properties of the nanostructures on their size, could now be carried out within a very defined size range.
The successful incorporation of the respective DMS compounds was shown by X ray diffraction, TEM and physisorption. The diffraction patterns revealed the preservation of the mesoporous host matrices even after several incorporation/conversion cycles. The absence of peaks at higher angles 2 theta indicated that no bulk material had formed and that the formation of the DMS compounds had taken place preferentially inside the respective pore systems. This was also apparent from the TEM pictures: the hexagonal arrangement of the pores was still clearly visible and no bigger particles were lying on the outer surface. Furthermore it could be seen from the TEM pictures with a cross sectional view of the host/guest compounds that the particles inside the pores are of an elongated spherical or ellipsoidal shape. The sorption studies also revealed the remaining mesoporosity of the samples and showed that the specific surface area was decreasing with each impregnation/conversion cycle. The remaining pore diameter remained nearly the same as in the pristine mesoporous material but the total number of freely accessible pores was found to be decreasing with every impregnation/conversion step. This was an indication for a blocking of the entrances of the pores with the nanoparticles which were formed inside the pore system.Raman spectroscopy proved the good crystallinity of the nanoparticles and showed a red-shift of the peak positions with decreasing particle diameter due to quantum confinement. By analysing the lineshape of the LO-phonon with the model of Campbell and Fauchet the shape of the nanoparticles was – as already indicated in the TEM pictures – found to be of an elongated spherical or ellipsoidal shape.
X-ray absorption spectroscopy revealed that no binary sulphides were formed inside the pore systems and the nanoparticles were really doped with randomly distributed manganese, resulting in a A1-xMnxS kind of structure. The local structure around the localised Mn2+ ions was found to be of a distorted "MnS" geometry. EPR measurements also showed, that a A1-xMnxS structure was formed and that the crystal structure is wurtzite for x greater than 0.01 for all samples.
PL measurements were carried out to investigate the optical properties of the nanostructured DMS particles. The internal Mn transitions were analysed with the Tanabe-Sugano-model, which revealed that the crystal structure of the nanoparticles was comparable to that of bulk crystals. The band gap related feature in the PLE spectra showed an increasing blue-shift with decreasing particle size due to quantum confinement. Also an increased band gap bowing was found for the nanoparticles of Cd1-xMnxS and Cd1-xMnxSe, whereas no bowing was observed for the Zn1-xMnxS particles. This effect could be explained by the dependence of the p-d exchange parameter and the susceptibility on the reduced dimensions in nanoparticles and was found to be consistent to literature. 
The magnetic behaviour of the DMS compounds was investigated with the analysis of the EPR data. It was found that the susceptibility and the Curie-Weiss parameter are strongly affected by the reduced lateral dimensions of the nanoparticles (less than 10 nm). Here, a better agreement with theoretical values was obtained, if a wire-like structure of the nanoparticles was assumed. However a perfect agreement was not observed, hence, it can still be presumed that the particles are of an ellipsoidal shape. The microscopic coupling between the Mn-ions though was not found to be altered. The observed macroscopic modifications could be explained by to geometrical restrictions in the nanoparticles: the number of neighbours in the various shells around a manganese ion in the surface region is considerably reduced compared to a manganese ion in the bulk of the structure. This effect was found to become increasingly important with decreasing lateral dimensions of the nanostructure. 
Furthermore a suppression of the paramagnetic to antiferromagnetic phase transition was found for MnS nanoparticles with lateral dimensions less than 6 nm. While the long range antiferromagnetic coupling between the Mn2+ spins was found to be suppressed, the local coupling of a Mn2+ spin to the neighbouring spins remained unaffected by the reduced dimensions of the MnS nanoparticles.

KW  - nanostrukturierte Halbleiter
KW  - mesoporöses Silica
CY  - Gießen
PB  - Universitätsbibliothek
AD  - Otto-Behaghel-Str. 8, 35394 Gießen
UR  - http://geb.uni-giessen.de/geb/volltexte/2012/8692
ER  -