Pritish Mukherjee

Evidence that blue luminescence of oxidized porous silicon originates from SiO2

We have analyzed red and blue luminescence from porous silicon as a function of oxidation parameters and feature dimension determined with an atomic force microscope. We have found correlation between blue luminescence intensity and the increase in feature size caused by oxidation. We have further shown that blue luminescence, is identical, with respect to spectrum and fast decay, to that of high microelectronic quality SiO2 grown on crystalline silicon using dry oxygen plus an organic chlorine compound. Thus, we conclude that blue luminescence originates from SiO2 film rather than from the silicon nanocrystals in the porous material. Intensity enhancement, as compared to SiO2 on crystalline wafers, comes from the gigantic surface area of porous silicon.

Dual‐laser ablation for particulate‐free film growth

A novel dual laser ablation process that leads to particulate‐free film growth is presented. A pulsed CO2 laser and an excimer (KrF) laser have been spatially overlapped on a Y2O3 target with a temporal delay between the pulses. The particulate density of the films grown by this method are at least three orders of magnitude smaller than the particulate density of a single excimer laser ablated film of similar thickness. In addition, a time‐of‐flight ion probe study indicates a sixfold enhancement of the plume species kinetic energies under dual‐laser ablation. The degree of the plume excitation is observed to depend strongly on the delay between the laser pulses.

Magnetic anisotropy and field switching in cobalt ferrite thin films deposited by pulsed laser ablation

We report the observation of contrasting magnetic behavior in cobalt ferrite (CFO) thin films deposited on single crystalline magnesium oxide (MgO) and strontium titanate (STO). Epitaxial films on MgO (100) with a lattice mismatch of 0.35% showed out-of-plane anisotropy whereas the films on STO (100) with a lattice mismatch of 7.4% displayed in-plane anisotropy. Stress anisotropy calculated from angle-dependent x-ray diffraction analysis confirmed that the change in anisotropy originates from the lattice mismatch. An additional low-field switching characteristic is observed in the M-H loops of the CFO films, which became prominent with lowering temperature as also evidenced from the rf transverse susceptibility measurements. The obtained results revealed that the low field switching is associated with the film-substrate interface.

Surface spin disorder and exchange-bias in hollow maghemite nanoparticles

We report a comparative study of the magnetic properties of polycrystalline hollow γ-Fe2O3 nanoparticles with two distinctly different average sizes of 9.2 ± 1.1 nm and 18.7 ± 1.5 nm. High-resolution transmission electron microscopy images reveal the presence of a shell with thickness of 2 nm and 4.5 nm for the 9.2 nm and 18.7 nm nanoparticles, respectively. The field-cooled hysteresis loops show interesting features of enhanced coercivity and horizontal and vertical shifts associated with the polarity of the cooling field for both types of nanoparticles. While the anomalously large horizontal shifts and open hysteresis loop in a field as high as 9 T observed for the 9.2 nm nanoparticles corresponds to a “minor loop” of the hysteresis loop, the loop shift observed for the 18.7 nm nanoparticles manifests an intrinsic “exchange bias” (EB). Relative to the 18.5 ± 3.2 nm solid nanoparticles, a much stronger EB effect is achieved in the 18.7 nm hollow nanoparticles. Our studies point to the importance of inner...

Mechanism and controlled growth of shape and size variant core/shell FeO/Fe3O4 nanoparticles

We report a novel synthesis approach for the growth of core/shell FeO/Fe3O4 nanoparticles with controlled shape and size. FeO particles were partially oxidized to form core/shell FeO/Fe3O4 structures, as evidenced from transmission electron microscopy, X-ray diffraction, and magnetometry analysis. We find that the molar ratios and concentrations of surfactants are the key parameters in controlling the particle size. The particles can grow in either isotropic or anisotropic shapes, depending upon a chemical reaction scheme that is controlled kinetically or thermodynamically. The competitive growth rates of {111} and {100} facets can be used to tune the final shape of nanoparticles to spherical, cubic, octahedral, octopod, and cuboctahedral geometries. FeO particles can also be oxidized chemically or thermally to form Fe3O4 nanoparticles. By following the same synthesis technique, it is possible to synthesize rods and triangles of Fe3O4 by introducing twinnings and defects into the crystal structure of the seed. The thermally activated first-order Verwey transition at ~120 K has been observed in all the synthesized FeO/Fe3O4 nanoparticles, indicating its independence from the particle shape. These core/shell nanoparticles exhibit a strong shift in field-cooled hysteresis loops accompanied by an increase in coercivity (the so-called exchange bias effect), but the low field-switching behavior appears to vary with the particle shape.

Magneto-Impedance Biosensor With Enhanced Sensitivity for Highly Sensitive Detection of Nanomag-D Beads

A magnetoimpedance (MI) biosensor based on Co-based amorphous ribbon was designed and tested to detect functionalized Nanomag-D magnetic beads. While previous studies were focused mainly on exploring the MI change for biosensing, we show that the sensitivity of the biosensor can be enhanced when the change in ac magnetoresistance (MR) or magnetoreactance (MX) is used. The frequency at which the sensitivity of the sensor is optimized can be tuned. This is of potential interest in developing functional biosensors with improved sensitivity and tunable frequency.

Anisotropy effects in magnetic hyperthermia: A comparison between spherical and cubic exchange-coupled FeO/Fe3O4 nanoparticles

Spherical and cubic exchange-coupled FeO/Fe3O4 nanoparticles, with different FeO:Fe3O4 ratios, have been prepared by a thermal decomposition method to probe anisotropy effects on their heating efficiency. X-ray diffraction and transmission electron microscopy reveal that the nanoparticles are composed of FeO and Fe3O4 phases, with an average size of ∼20 nm. Magnetometry and transverse susceptibility measurements show that the effective anisotropy field is 1.5 times larger for the cubes than for the spheres, while the saturation magnetization is 1.5 times larger for the spheres than for the cubes. Hyperthermia experiments evidence higher values of the specific absorption rate (SAR) for the cubes as compared to the spheres (200 vs. 135 W/g at 600 Oe and 310 kHz). These observations point to an important fact that the saturation magnetization is not a sole factor in determining the SAR and the heating efficiency of the magnetic nanoparticles can be improved by tuning their effective anisotropy.

Detection of low-concentration superparamagnetic nanoparticles using an integrated radio frequency magnetic biosensor

Improving the sensitivity of existing biosensors for highly sensitive detection of magnetic nanoparticles as biomarkers in biological systems is an important and challenging task. Here, we propose a method of combining the magneto-resistance (MR), magneto-reactance (MX), and magneto-impedance (MI) effects to develop an integrated magnetic biosensor with tunable and enhanced sensitivity. A systematic study of the 7 nm Fe3O4 nanoparticle concentration dependence of MR, MX, and MI ratios of a soft ferromagnetic amorphous ribbon shows that these ratios first increase sharply with increase in particle concentration (0–124 nM) and then remain almost unchanged for higher concentrations (124 nM–1240 nM). The MX-based biosensor shows the highest sensitivity. With this biosensor, ∼2.1 × 1011 7 nm Fe3O4 nanoparticles can be detected over a detection area of 2.0 × 105 μm2, which is comparable to a superconducting quantum interference device biosensor that detects the presence of ∼1 × 108 11 nm Fe3O4 nanoparticles ove...

Synthesis and magnetic properties of gold coated iron oxide nanoparticles

We report on synthesis, structural, and magnetic properties of chemically synthesized iron oxide (Fe3O4) and Fe3O4@Au core-shell nanoparticles. Structural characterization was done using x-ray diffraction and transmission electron microscopy, and the magnetite phase of the core (∼6nm) and fcc Au shell (thickness of ∼1nm) were confirmed. Magnetization (M) versus temperature (T) data at H=200Oe for zero-field-cooled and field-cooled modes exhibited a superparamagnetic blocking temperature TB∼35K (40K) for parent (core-shell) system. Enhanced coercivity (Hc∼200Oe) at 5K along with nonsaturating M-H loops observed for Fe3O4@Au nanoparticles indicate the possible role of spin disorder at the Au–Fe3O4 interface and weak exchange coupling between surface and core spins. Analysis of ac susceptibility (χ′ and χ″) data shows that the interparticle interaction is reduced upon Au coating and the relaxation mechanism follows the Vogel–Fulcher law.

Two-photon and three-photon absorption coefficients of InSb

The two-photon absorption coefficient β of intrinsic InSb and the three-photon absorption coefficient γ of doped InSb were measured with high-intensity picosecond CO2 laser pulses. The picosecond duration permitted the determination of the multiphoton absorption coefficients without interference from carrier recombination and diffusion. Moreover, free-carrier absorption was made use of explicitly to enhance the absorption of the laser beam, permitting a more accurate measurement of these coefficients. The values of β and γ were found to be 2.5 cm/MW and 0.2 cm3/MW2, respectively. These values are compared with theoretical calculations in the literature.