Yong Jun Seo

Effect of solution concentration on the functional properties of ZnO nanostructures: Role of Hexamethylenetetramine

In this research, ZnO nanorods have been successfully synthesized via wet chemical method. XRD results revealed the single phase nature with the wurtzite structure of the as prepared ZnO nanorods. By only varying the concentration of Hexamethylenetetramine (HMT) in the solution, morphology of ZnO changed from hexagonal facet nanorods to pencil like nanorods and size of nanorods also changed. The band gap of as-synthesized ZnO nanorods was found to increase with increasing the concentration of HMT in the solution. The narrow full-width at half-maximum (FWHM) of the UV emission of PL spectra indicated that the grown ZnO nanorods have high crystal quality and is well matched with the obtained XRD results. These results revealed that the concentration of Hexamethylenetetramine plays a vital role to control the properties of ZnO nanorods.

ELECTRICAL TRANSPORT PROPERTIES AND MAGNETORESISTANCE OF (La0.7Ca0.3MnO3)1-x/(CuFe2O4)x COMPOSITES

The (La0.7Ca0.3MnO3)1-x/(CuFe2O4)x (x = 0–0.15) composites were prepared by conventional solid state reaction method. We have investigated the structural, magnetic and electrical properties of (La0.7Ca0.3MnO3)1-x/(CuFe2O4)x composites using X-ray diffraction (XRD), scanning electron microscopy, field cooled DC magnetization and magnetoresistance (MR) measurements. The resistance measured as a function of temperature demonstrates that pure LCMO samples display metal to semiconductor transition. On the other hand, all the LCMO/CFO composites samples clearly present the electrical behavior of insulator/semiconductor. It indicates that resistivity of the samples increase systemically with the increase of CFO content. The MR was measured in the presence of 0.5 T field. At low temperatures, it is clearly observed that the MR effect is enhanced with x = 0.03 of CFO composition. In summary, the spin-polarized tunneling and the spin-dependent scattering may be helpful to the improved low-field magnetoresistance (LFMR) effect. These phenomena can be explained by the segregation of a new phase related to CFO at the grain boundaries or surfaces of the LCMO grains.

Ta Doped SnO 2 Transparent Conducting Films Prepared by PLD

Transparent and conducting thin films of Ta-doped were fabricated on a glass substrate by a pulse laser deposition(PLD) method. The structural, optical, and electrical properties of these films were investigated as a function of doping level, oxygen partial pressure, substrate temperature, and film thickness. XRD results revealed that all the deposited films were polycrystalline and the intensity of the (211) plane of decreased with an increase of Ta content. However, the orientation of the films changed from (211) to (110) with an increase in oxygen partial pressure (40 to 100 mTorr) and substrate temperature. The crystallinity of the films also increased with the substrate temperature. The electrical resistivity measurements showed that the resistivity of the films decreased with an increase in Ta doping, which exhibited the lowest resistivity () for 10 wt% Ta-doped film, and then increased further. However, the resistivity continuously decreased with the oxygen partial pressure and substrate temperature. The optical bandgap of the 10 wt% Ta-doped film increased (3.67 to 3.78 eV) with an increase in film thickness from 100-700 nm, and the figure of merit revealed an increasing trend with the film thickness.

Electrical Transport Properties and Magnetoresistance of (1-x)La 0.7 Sr 0.3 MnO 3 /xZnFe 2 O 4 Composites

Abstract The (1- x )La 0.7 Sr 0.3 MnO 3 (LSMO)/ x ZnFe 2 O 4 (ZFO) ( x = 0, 0.01, 0.03, 0.06 and 0.09) composites were prepared bya conventional solid-state reaction m ethod. We investigated the structural properties, m agnetic properties and electricaltransport properties of (1- x )LSMO/ x ZFO composites using X-ray diffraction (XRD), scanning electron microscopy (SEM),field-cooled dc magnetization and magnetoresistance (MR) measurements. The XRD and SEM results indicate that LSMOand ZFO coexist in the composites and the ZFO mostly segregates at the grain boundaries of LSMO, which agreed wellwith the results of the magnetic measurements. The resistivity of the samples increased by the increase of the ZFO dopinglevel. A clear metal-to-insulator (M-I) transition was observed at 360K in pure LSMO. The introduction of ZFO furtherdownshifted the transition temperature (350K-160K) while the transition disappeared in the sample ( x = 0.09) and itpresented insulating/semiconducting behavior in the measured temperature range (100K to 400K). The MR was measuredin the presence of the 10kOe field. Compared with pure LSMO, the enhancement of low-field magnetoresistance (LFMR)was observed in the composites. It was clearly observed that the magnetoresistance effect of x = 0.03 was enhanced at roomtemperature range. These phenomena can be explained using the double-exchange (DE) mechanism, the grain boundaryeffect and the intrinsic transport properties together.Key words - (1 x a )L

Magnetic and electrical transport properties on (La0.7Sr0.3MnO3)1−x/(CuFe2O4)x composites

The magnetic and electrical properties of (La0.7Sr0.3MnO3)1?x/(CuFe2O4)x have been investigated. The (La0.7Sr0.3MnO3)1-x/(CuFe2O4)x composites with x = 0, 0.01, 0.03, 0.06 and 0.09 wt.% were prepared by conventional solid state reaction method, respectively. The structures, morphology, magnetic and electrical properties of the samples have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and physical property measurement system (PPMS). The resistance measured as a function of temperature demonstrates that the pure LSMO samples display metal to semiconductor transition. On the other hand, the others indicated semiconductor/insulator behaviors. The MR was measured in an applied magnetic field of 0.5T. The results clearly show that the magnetoresistance (MR) effect is enhanced at a wide temperature range with CuFe2O4 composition. It may be helpful to improve the low-field magnetoresistance (LFMR) under the influence of the spin-polarized tunneling and the spin-dependent scattering.

Fabrication and Properties of La-Manganite/SnO2 Composites

Composites with compositions La0.7Ca0.3MnO3(LCMO) and La0.7Sr0.3MnO3(LSMO)/SnO2 were prepared by a standard ceramic technique. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between La based maganite and SnO2 grains, and that SnO2 segregates mostly at the grain boundaries of La based maganite. The variation in resistivity with temperature has been studied and shows a semiconducting behavior, furthermore the composites exhibit metallic percolation. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 0.5 Tesla. The spin-polarized tunneling and the spin-polarized tunneling may be attributed to the enhanced low-field magnetoresistance (LFMR) effect.

Electrical and Magnetic Properties in [La 0.7 (Ca 1-x Sr x ) 0.3 MnO 3 )] 0.99 /(BaTiO 3 ) 0.01 Composites

Perovskite manganites such as RE1�x AxMnO3 (RE = rare earth, A = Ca, Sr, Ba) have been the subject of intense research in the last few years, ever since the discovery that these systems demonstrate colossal magnetoresistance (CMR). The CMR is usually explained with the double-exchange (DE) mechanism, and CMR materials have potential applications for magnetic switching, recording devices, and more. However, the intrinsic CMR effect is usually found under the conditions of a magnetic field of several Teslas and a narrow temperature range near the Curie temperature (Tc). This magnetic field and temperature range make practical applications impossible. Recently, another type of MR, called the low-field magnetoresistance (LFMR), has also been a research focus. This MR is typically found in polycrystalline half-metallic ferromagnets, and is associated with the spin-dependent charge transport across grain boundaries. Composites with compositions (La0.7(Ca1�x Srx)0.3MnO3))0.99/(BaTiO3)0.01 ((LCSMO)0.99/(BTO)0.01) were prepared with different Sr doping levels x by a standard ceramic technique, and their electrical transport and magnetoresistance (MR) properties were investigated. The structure and morphology of the composites were studied by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). BTO peaks could not be found in the XRD pattern because the amount of BTO in the composites was too small. As the content of x decreased, the crystal structure changed from orthorhombic to rhombohedral. This change can be explained by the fact that the crystal structure of pure LCMO is orthorhombic and the crystal structure of pure LSMO is rhombohedral. The SEM results indicate that LCSMO and BTO coexist in the composites and BTO mostly segregates at the grain boundaries of LCSMO, which are in accordance with the results of the magnetic measurements. The resistivity of all the composites was measured in the range of 90-400K at 0T, 0.5T magnetic field. The result indicates that the MR of the composites increases systematically as the Ca concentration increases, although the transition temperature Tc shifts to a lower range.

Electrical and Optical Properties of Ga-doped SnO 2 Thin Films Via Pulsed Laser Deposition

Ga₂O₃-doped SnO₂ thin films were grown by using pulsed laser deposition (PLD) technique on glass substrate. The optical and electrical properties of these films were investigated for different doping concentrations, oxygen partial pressures, substrate temperatures, and film thickness. The films were deposited at different substrate temperatures (room temperature to 600℃). The best opto-electrical properties is shown by the film deposited at substrate temperature of 300℃ with oxygen partial pressure of 80 m Torr and the gallium concentration of 2 wt%. The as obtained lowest resistivity is 9.57 × 10?³ Ω ㎝ with the average transmission of 80% in the visible region and an optical band gap (indirect allowed) of 4.26 eV.

Influence of CoO Phase on Structural, Magnetic and Electrical Properties of La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3

The composites of (1-x)La0.7Ca0.3MnO3/xCoO and (1-x)La0.7Sr0.3MnO3/xCoO (x =0 ~ 0.09) were fabricated by conventional solid state reaction method, and their electrical transport and magnetoresistance (MR%) properties were investigated by physical property measurement system (PPMS). The result of x-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicated that no new phase appeared in the composites except manganites matrix and CoO phases. CoO is mainly distributed at the grain boundaries and surfaces of the matrix. The resistivity LCMO and LSMO based composites were measured in the range 90-320K and 90-400K, respectively. The applied magnetic field is 0T and 0.5T. The observed variation of MR with varying Ca and Sr concentration has been qualitatively investigated.

Preparation of Pt Films on GaAs by 2-step Electroless Plating

Electroless plating is influenced by kinds of parameters including concentrations of electrolyte, plating time, temperature and so on. In this study, the Pt thin films were prepared on GaAs substrate by a 2-step electroless plating depending method. The small Pt catalytic particles by using Pt I bath exhibited islands-morphology dispersed throughout the substrate surface at 65℃, as function as a sensitized thin film, and then a thicker Pt film grew upon the sensitized layer by the second Pt Ⅱ bath. As the growth of Pt film is strongly influenced by the plating time and temperature, the plating time of Pt Ⅱ bath varied from 5 min to 40 min at 60~80℃ after Pt I bath at 60~80℃ for 5 min. It is found that the film grows with the increasing plating time and temperature. The resistivity value of Pt deposited layer was characterized to study the growth mechanism of 2-step plating.