Diluted Magnetic Semiconductor Nanomaterials Fabrication by a Chemical Vapor Deposition Method

Well-aligned DMS Zn1-xMnxO nanowires and nanorods were successfully synthesized in-situ using a chemical vapor deposition method. Low-dimensional nanostructures such as bowls and cages were deposited on a silicon surface downstream of the tube furnace. Variation of reaction temperature and Mn doping level were investigated on structure and properties of the as-grown nanomaterials. The as-grown nanowires and naorods are single crystalline wurtzite structure and possess a growth direction along the c axis. At 850 ºC, the most optimistic condition for ZnO crystal growth was obtained. At high reaction temperature 950ºC, Mn2+ were substitutionally doped into ZnO lattice, resulting in room-temperature ferromagnetic coupling with a saturated magnetization of 0.25emu/g. The ferromagnetic interaction is weakened, however, by larger concentration of Mn, due to the antiferromagnetic coupling of direct superexchange interaction between Mn2+. The well-aligned DMS 1-dimensional Mn doped ZnO nanostructures have great potential for application in spintronic nanodevices

Development of Nanostructured Glucose Biosensor

With the development of nanotechnology and nanomaterials, biosensors incorporated with novel nanomaterials and nanostructures have shown significant potential in point-of-care medical devices because of their rapid interaction with target analytes and their miniaturized systems. Nanomaterials and nanostructures with special chemical, physical and biological characteristics are able to enhance biosensors’ performance in terms of sensitivity and selectivity. Therefore, my study focused on development of special nanostructures used for advanced glucose biosensor. Monitoring of blood glucose level is essential for diabetes management. However, current methods require people with diabetes to have blood test with 5-8 times per day. Compared to other methods, optical and magnetic techniques have a potential in developing minimally invasive or non-invasive, and continuous glucose monitoring nanostructured biosensors. Consequently, this thesis presented nanostructured optical and magnetic glucose biosensors by incorporating novel nanomaterials and fabricating nanostructures for the next generation of glucose biosensor in the tears. The glucose biorecognition biomolecule used in the biosensors was Concanavalin A (Con A). Con A is a lectin protein that has strong affinity to glucose. Fluorescence resonance energy transfer (FRET) technique was applied to develop optical glucose biosensors. FRET biosensor is a distance-dependent biosensor. The fluorescence emission of a donor molecule could be used to excite acceptor when the distance between donor and acceptor is close enough (\u3c 20 nm). Three different types of nanostructures were developed and used as the donors of the glucose FRET biosensors. The first type of sensor is a ZnO/quantum dots-based glucose biosensors. Hybrid ZnO nanorod array with decoration of CdSe/ZnS quantum dots were prepared and coated on silicone hydrogel which is a common materials of contact lens. The patterned nanostructured FRET sensor could quickly measure rats’ tear glucose in an extremely small amount (2 µL) of diluted tear sample. The second type of biosensor is based on upconversion nanomaterials. Upconversion NaGdF4: Yb, Er nanoparticles with diameter of about 40±5 nm have been prepared by polyol process and coated on silicone hydrogel to directly sense the tear glucose level on the rats’ eye surface. The results show that the upconversion nanomaterials based lens sensor is able to quickly measure glucose in rats’ blood samples. The third type of sensor utilizes the unique optical properties of carbon nanomaterial, fluorescent carbon dots and graphene oxide nanosheets. The carbon dots with tunable fluorescence were developed by a microwave-assisted process. The carbon dots are used as a fluorescence donor in the biosensor, the chitosan coated graphene oxide acts as the fluorescence acceptor to quench the emission of carbon quantum dots. In the presence of glucose, the emission of carbon quantum dots could be restored as a function of the concentration of glucose. Two linear relationships of the restored emission of the sensor and the concentration of glucose were observed, in the range of 0.2 mM to 1 mM, and 1 mM to 10 mM, respectively. On the other hand, a magnetoresistive (MR) nanostructured glucose biosensor has been developed by exploiting hybrid graphene nanosheets decorated with FeCo magnetic nanopartciles. The Fe3O4/silica core/shell nanoparticles are used as the magnetic label of glucose, which could bind onto the surface of FeCo/graphene nanocomposited sensor. The binding of magnetic label onto the hybrid graphene nanosheets can result in the change of the magnetoresistance. The MR signal as a function of the glucose level of diluted rat blood samples is measured in a range of 2 mM to 10 mM. In summary, novel nanomaterials and nanostructures with special fluorescent and magnetoresistive properties are fabricated for developing nanostructured glucose biosensors, which could bring alternative approaches for convenient management diabetes

Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures

The occurrence of inhomogeneous spin-density distribution in multilayered ferromagnetic diluted magnetic semiconductor nanostructures leads to strong dependence of the spin-polarized transport properties on these systems. The spin-dependent mobility, conductivity and resistivity in (Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a function of temperature, scaled by the average magnetization of the diluted magnetic semiconductor layers. An increase of the resistivity near the transition temperature is obtained. We observed that the spin-polarized transport properties changes strongly among the three materials.Comment: 3 pages, 4 figure

Solvothermal synthesis derived Co-Ga codoped ZnO diluted magnetic degenerated semiconductor nanocrystals

Authors kindly acknowledge to the Estonian Research Council ( PUT1096 , IUT2-25 , PUT735 ), the Estonian Centre of Excellence in Research project “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics (TK141), and the financial support of HZB. We are grateful to the staff of BESSY II for the assistance and co-operation during the synchrotron-based measurements.Here we are reporting solvothermal synthesis derived diluted magnetic and plasmonic Co-Ga co-doped ZnO nanocrystals with high magnetization values (from 1.02 to 4.88 emu/g) at room temperature. Co-Ga co-doped ZnO nanocrystals show up to 2 fold increase in saturation magnetization compared to Co doped ZnO nanocrystals at the same Co concentration, with the observed room temperature magnetization higher than previously reported values for multifunctional magnetic and plasmonic nanocrystals, and the effect of Ga suggesting some role of the correspondingly introduced itinerant charge. While at the lowest Ga content the nanoparticles appear homogeneously doped, we note that already a moderate Ga content of several percent triggers a fraction of Co to segregate in metallic form in the bulk of the nanoparticles. However, the amount of segregated Co is not sufficient to account for the total effect, whereas a dominating contribution to the observed magnetism has to be related to itinerant charge mediated exchange interactions.Eesti Teadusagentuur IUT2-25,PUT1096,PUT735; Estonian Centre of Excellence in Research TK141; HZB; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART