Sameer V. Mahajan

Carbon nanotube–nanocrystal heterostructures fabricated by electrophoretic deposition

Alternating layer, carbon nanotubes-nanocrystal composite films, comprising multi-walled carbon nanotubes (MWCNTs) and iron oxide (Fe(3)O(4)) nanocrystals, have been fabricated via electrophoretic deposition (EPD) on stainless steel and gold substrates. Low field-high current and high field-low current EPD schemes were integrated to produce the composite films. The low field-high current EPD approach produced porous mats from an aqueous suspension of the MWCNTs, while the high field-low current EPD approach produced tightly packed nanocrystal films from a dispersion of the nanocrystals in hexane. Large electric fields applied during the nanocrystal EPD and strong van der Waals interactions among the nanocrystals facilitated the formation of tightly packed nanocrystal films atop the MWCNT mats to create CNT mat-nanocrystal film composites. The surface coverage and homogeneity of the nanocrystal films improved with repeated deposition of the nanocrystals on the same mat. The assembly of nanotube mats on top of the CNT mat-nanocrystal film composite confirmed the feasibility of multilayered CNT mat-nanocrystal film heterostructures suitable for a range of devices. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques were employed to characterize the surface coverage, homogeneity, and topology of these composite films.

Understanding the growth of Eu2O3 nanocrystal films made via electrophoretic deposition

Eu(2)O(3) nanocrystals, surface-functionalized with oleic acid, were assembled into transparent thin films via electrophoretic deposition (EPD). Suspended in a non-polar solvent (hexane), the nanocrystals were cast into stable films on both the cathode and the anode. We characterized the nanocrystal films using optical microscopy, energy dispersive spectroscopy and photoluminescence spectroscopy. Scanning electron microscopy and atomic force microscopy provided information regarding the morphology, topology and surface coverage of the films. These homogeneous, densely packed films were composed predominantly of agglomerates (approximately 15 nm) of the Eu(2)O(3) nanocrystals rather than of individual nanocrystals. Nonetheless, the films possessed low root mean square (RMS) roughness (approximately 1.4 nm). High transparency of the film in the visible region was facilitated by the dense packing and the small diameter of the agglomerates, which reduced transmission losses due to scattering. The effect of EPD process parameters (applied voltage and nanocrystal concentration) on the growth uniformity and the thickness of the films was examined via surface contact profilometry. We discovered a correlation among the said EPD process parameters, the overall quality and thickness of these transparent films, which provided insight into the mechanisms of the nanocrystal deposition process.

Field Dependence of the Spin Relaxation Within a Film of Iron Oxide Nanocrystals Formed via Electrophoretic Deposition.

The thermal relaxation of macrospins in a strongly interacting thin film of spinel-phase iron oxide nanocrystals (NCs) is probed by vibrating sample magnetometry (VSM). Thin films are fabricated by depositing FeO/Fe3O4 core–shell NCs by electrophoretic deposition (EPD), followed by sintering at 400°C. Sintering transforms the core–shell structure to a uniform spinel phase, which effectively increases the magnetic moment per NC. Atomic force microscopy (AFM) confirms a large packing density and a reduced inter-particle separation in comparison with colloidal assemblies. At an applied field of 25 Oe, the superparamagnetic blocking temperature is TBSP ≈ 348 K, which is much larger than the Néel-Brown approximation of TBSP ≈ 210 K. The enhanced value of TBSP is attributed to strong dipole–dipole interactions and local exchange coupling between NCs. The field dependence of the blocking temperature, TBSP(H), is characterized by a monotonically decreasing function, which is in agreement with recent theoretical models of interacting macrospins.

Stepwise assembly of europium sesquioxide nanocrystals and nanoneedles

The stepwise self-assembly of europium sesquioxide nanocrystals into larger, anisotropic europium hydroxychloride nanostructures is observed. This involves the thermally assisted growth of 4.0 nm nanocrystals into elongated structures, called nanoneedles, and the subsequent assembly of those nanoneedles into larger, oriented bundles, called nanospindles. High-resolution transmission electron microscopy provides the size distribution, shape and atomic spacing of the nanostructures, whereas selective area electron diffraction and x-ray diffraction measurements identify their crystallinity. The optical properties, attributed to the environment of the host lattice for the europium ions, are investigated through photoluminescence measurements.

Electrophoretic Deposition of Star Polymer-Europium Chalcogenide Nanocomposite Films

The electrophoretic deposition of polystyrene/divinylbenzene (PS/DVB) star polymer-europium sulphide (EuS) nanocomposite films from a colloidal suspension is reported. Liquid suspension, containing both the PS/DVB star polymer and EuS nanocrystals were prepared by separately injecting dichloromethane (DCM) based solutions of EuS nanocrystals and of the star polymers, respectively, into a stratified liquid combination of hexane and DCM. Scanning electron microscopy illustrates images of the dependence of surface morphology on nanocrystal concentration of the PS/DVB-EuS star polymer film. These polymer-encased nanocrystal films may be a more practical option for the fabrication of magneto-optical thin film devices, such as optical switches, optical isolators, and optical memories.

Stepwise Assembly of Eu2O3 Nanocrystals and Nanoneedles

We report the stepwise assembly of europium oxide (Eu2O3) nanocrystals into one dimensional europium hydroxychloride (Eu(OH)2Cl) nanostructures. Synthesized via a simple colloidal chemistry procedure, the Eu2O3 nanocrystals organized into anisotropic nanoneedles and nanospindles via solution‐phase heat treatment. Transmission electron microscopy (TEM) revealed the shape and crystalline arrangement of the nanostructures.