J. S. Thakur

Bandgap engineering by tuning particle size and crystallinity of SnO2-Fe2O3 nanocrystalline composite thin films

We report the structural and optical properties of xSnO2–yFe2O3 nanocrystalline composite thin films. SnO2 and Fe2O3 exhibit strong phase separation instability and their particle size and crystallinity are tunable by changing their composition and annealing temperature. The bandgap for these composites continuously increases from 2.3 to 3.89 eV. We discuss the increasing bandgap values in terms of the quantum confinement effect manifested by the decreasing size of Fe2O3 crystallites. The method provides a generic approach for the tuning of the bandgap in nanocomposite systems.

Temperature dependence of mobility and carrier density in InN films

We investigate the temperature dependence of Hall mobility μ and carrier density Ne for thin InN films grown by molecular-beam epitaxy and plasma source molecular-beam epitaxy over three orders-of-magnitude difference in their carrier density: for the low-density film Ne=5.8×1017∕cm3 and for the high-density film Ne=3.2×1020∕cm3. In both the films, for temperature up to 300 K, a large temperature-independent concentration of carriers is observed. For higher temperatures, however, carrier density increases with temperature. The characteristic behavior of the mobility for the low-density film is different from that of the high-density film, particularly for temperatures less than 300 K. The low-density film shows a peak behavior in the mobility around 250 K in contrast to the temperature-independent mobility observed for the high-density film for T<300K. We investigate theoretically the concentrations of donor, acceptor, and threading dislocations for both the films and also discussed various electron-scatt...

Raman scattering studies of magnetic Co-doped ZnO thin films

Structural, magnetization, and Raman spectroscopic investigations have been carried out on spin coated Zn1−xCoxO (0⩽x⩽0.1) films. We investigated the broadening of the E2low mode for different values of x in both air and vacuum annealed films, and found a direct correlation of Co ions incorporation at the Zn sites with ferromagnetic order. The magnitudes of the magnetic moment are directly related to the size of the disorder correlation radius. However, for x∼5% where the magnetic moment becomes negligible, the broadening of the E2low Raman mode also becomes negligible, suggesting rejection of the Co ions from the Zn sites. For a given x, Co ions are incorporated at the Zn sites with the same concentration in both the air and vacuum annealed films but the magnetic moment values are much smaller for the air annealed samples. This behavior is directly related to the presence of oxygen vacancies. We also discuss the effects of air and vacuum annealing on the linewidth of E2high and A1 (LO) modes. At higher d...

Raman spectroscopic differentiation of activated versus non-activated T lymphocytes: an in vitro study of an acute allograft rejection model.

Acute rejection (AR) remains a significant complication in renal transplant patients. Using serum creatinine for AR screening has proven problematic, and thus a noninvasive, highly sensitive and specific test is needed. T cells from human peripheral blood were analyzed using Raman spectroscopy. Fifty-one Mixed Lymphocyte Culture (MLC) activated T cells (ATC), 28 Mitomycin C inactivated T cells (ITC), and 35 resting T cells (RTC), were studied utilizing 785 and 514.5 nm wavelengths. Statistical analysis following subtraction of fluorescence used Students t test to quantify peak ratio differences and discriminant function analysis (DFA), with three distinct sectors assigned for grouping purposes: Sector I, ITC; Sector II, ATC; Sector III, RTC. Differences between ATC and non-activated T cells (ITC and RTC) were found at 1182 and 1195 cm-1 peak positions for both wavelengths. Significant differences in peak ratios for 785 and 514.5 nm wavelengths existed between ATC and RTC (p=0.001 and p=0.006, respectively) and ATC and ITC (p=0.001 and p=0.001, respectively), with a trend in differences observed between ITC and RTC (p=0.07 and p=0.08, respectively). Analysis of the DFA-derived sector distribution for the 785 and 514.5 nm wavelengths revealed a sensitivity of 95.7% and 89.3%, respectively, and a specificity of 100% and 93.8%, respectively. This data suggests that Raman spectroscopy can detect significant differences between activated and nonactivated T cells based upon cell-surface receptor expression, thereby establishing unique signatures that can aid in the development of a noninvasive AR screening tool with high sensitivity and specificity.

Room-temperature photoluminescence and resonance-enhanced Raman scattering in highly degenerate InN films

We report the results of room-temperature photoluminescence (PL) and resonance-enhanced Raman scattering studies on highly degenerate (carrier concentration >3×1020cm−3) InN films grown on c-plane sapphire substrates by plasma source molecular-beam epitaxy. Carrier concentration-dependent PL emission peak is observed in the 1.4–1.8 eV range. These films show strong resonance-enhanced first- and second-order Raman scattering under 785 nm (1.58 eV) excitation energy and not with 514.5 nm (2.41 eV) excitation, suggesting the existence of electronic states ∼1.5eV in these samples. The PL emission peak energies and their dependence on the carrier concentration are consistent with the observed optical absorption edges. These results are compared to the data on single crystalline, low degenerate InN film grown by molecular-beam epitaxy, which shows a band-gap energy of ∼0.6eV. The results imply a large shift in the optical absorption edges due to band filling effects in the highly degenerate InN samples.

Investigation of magnetic properties of Fe3O4 nanoparticles using temperature dependent magnetic hyperthermia in ferrofluids

Rate of heat generated by magnetic nanoparticles in a ferrofluid is affected by their magnetic properties, temperature, and viscosity of the carrier liquid. We have investigated temperature dependent magnetic hyperthermia in ferrofluids, consisting of dextran coated superparamagnetic Fe3O4 nanoparticles, subjected to external magnetic fields of various frequencies (188–375 kHz) and amplitudes (140–235 Oe). Transmission electron microscopy measurements show that the nanoparticles are polydispersed with a mean diameter of 13.8 ± 3.1 nm. The fitting of experimental dc magnetization data to a standard Langevin function incorporating particle size distribution yields a mean diameter of 10.6 ± 1.2 nm, and a reduced saturation magnetization (∼65 emu/g) compared to the bulk value of Fe3O4 (∼95 emu/g). This is due to the presence of a finite surface layer (∼1 nm thickness) of non-aligned spins surrounding the ferromagnetically aligned Fe3O4 core. We found the specific absorption rate, measured as power absorbed per gram of iron oxide nanoparticles, decreases monotonically with increasing temperature for all values of magnetic field and frequency. Using the size distribution of magnetic nanoparticles estimated from the magnetization measurements, we have fitted the specific absorption rate versus temperature data using a linear response theory and relaxation dissipation mechanisms to determine the value of magnetic anisotropy constant (28 ± 2 kJ/m3) of Fe3O4 nanoparticles.

Evaluation of pancreatic cancer with Raman spectroscopy in a mouse model.

Objectives: Detection of neoplastic changes using optical spectroscopy has been an active area of research in recent times. Raman spectroscopy is a vibrational spectroscopic technique that can be used to diagnose various tumors with high sensitivity and specificity. We evaluated the ability of Raman spectroscopy to differentiate normal pancreatic tissue from malignant tumors in a mouse model. Methods: We collected 920 spectra, 475 from 31 normal pancreatic tissue and 445 from 29 tumor nodules using a 785-nm near-infrared laser excitation. Discriminant function analysis was used for classification of normal and tumor samples. Results: Using principal component analysis, we were able to highlight subtle chemical differences in normal and malignant tissue. Using histopathology as the gold standard, Raman analysis gave sensitivities between 91% and 96% and specificities between 88% and 96%. Conclusions: Raman spectroscopy along with discriminant function analysis is a useful method to detect cancerous changes in the pancreas. Pancreatic tumors were characterized by increased collagen content and decreased DNA, RNA, and lipids components compared with normal pancreatic tissue.

Disorder effects on infrared reflection spectra of InN films

The effect of plasmon and LO-phonon damping on the optical measurements of InN films is discussed. Phonon and plasmon damping dramatically modifies the spectral features of the optical spectra and destabilizes the coupled modes of the system. Phonon damping affects the optical properties in a qualitatively different way from plasmon damping. Increased phonon damping leads to a merging of the coupled modes for a certain range of carrier density. Plasmon damping broadens the spectral linewidths of both of the coupled modes when plasmon energies are of the order of LO-phonon energies. However, when plasmon energies are larger than LO-phonon energies, increasing plasmon damping makes the higher-energy mode completely degenerate with plasmon, and may even have lower energies than plasmon. In weakly damped situations, we also discuss the low-energy optical transmission region that forms in between the coupled modes. Finally, the effect of plasmon and LO-phonon coupling on the analyses of the experimental infrar...

Differentiation of alloreactive versus CD3/CD28 stimulated T‐lymphocytes using Raman spectroscopy: A greater specificity for noninvasive acute renal allograft rejection detection

Acute rejection (AR) remains problematic in renal transplantation. As a marker, serum creatinine is limited, warranting a more effective screening tool. Raman spectroscopy (RS) can detect T‐cell activation with high sensitivity. In this study we explore its specificity. Seventy‐five inactivated, 40 alloantigen‐activated, and 75 CD3/CD28‐activated T cells were analyzed using RS. CD3/CD28‐activated peak magnitudes (PM) were 4.3% to 23.9% lower than inactivated PM at positions: 903, 1031, 1069, 1093, 1155, 1326, and 1449 cm−1, with a difference in peak ratio (PR) observed at the 1182:1195 cm−1 position (0.91 ± 0.06 vs. 1.2 ± 0.01, respectively: P = 0.006). Differences in CD3/CD28‐ and alloantigen‐activated PM were observed at: 903, 1031, 1093, 1155, 1326, and 1449 cm−1, with no PR differences at the 1182:1195 cm−1 position (0.91 ± 0.06 vs. 0.86 ± 0.09: P = 0.8). Spectral signature separation of CD3/CD28—and alloantigen‐activated groups was 100% specific and sensitive. We conclude that RS can differentiate T cells activated by different stimuli with high sensitivity and specificity.

Propagation of a shear-horizontal surface acoustic mode in a periodically grooved AlN/Al2O3 microstructure

We investigate the shear-horizontal surface acoustic waves (SH-SAWs) generated on an AlN/Al2O3 microstructure by laser-micromachined grooves on the AlN film. In the absence of grooves, the AlN/Al2O3 device shows resonance for only a lower velocity SAW mode. However, when grooves of periodicity smaller than half the wavelength of the surface acoustic wave are micromachined, a higher velocity resonance due to a SH-SAW mode is observed in the device. It is found that our SH-SAW mode remains undamped and is able to propagate across the device when loaded with a biofluidic mass. We have also measured the mass sensitivity of the SH-SAW mode under biofludic load in terms of frequency shift. Measurements show that this mode has a very high mass sensitivity of the order 1.35 ng/ml, which suggests that the device can be applied for the detection of a small concentration of antigens in a biofluid. In this article, the fabrication and grooving techniques of the device are also addressed.