Sanjay Mathur

Fabrication and electrical characterization of circuits based on individual tin oxide nanowires

Two-xa0and four-probe electrical measurements on individual tin oxide (SnO(2)) nanowires were performed to evaluate their conductivity and contact resistance. Electrical contacts between the nanowires and the microelectrodes were achieved with the help of an electron-xa0and ion-beam-assisted direct-write nanolithography process. High contact resistance values and the nonlinear current-bias (I-V) characteristics of some of these devices observed in two-probe measurements can be explained by the existence of back-to-back Schottky barriers arising from the platinum-nanowire contacts. The nanoscale devices described herein were characterized using impedance spectroscopy, enabling the development of an equivalent circuit. The proposed methodology of nanocontacting and measurements can be easily applied to other nanowires and nanometre-sized materials.

Portable microsensors based on individual SnO2 nanowires

Individual SnO(2) nanowires were integrated in suspended micromembrane-based bottom-up devices. Electrical contacts between the nanowires and the electrodes were achieved with the help of electron- and ion-beam-assisted direct-write nanolithography processes. The stability of these nanomaterials was evaluated as function of time and applied current, showing that stable and reliable devices were obtained. Furthermore, the possibility of modulating their temperature using the integrated microheater placed in the membrane was also demonstrated, enabling these devices to be used in gas sensing procedures. We present a methodology and general strategy for the fabrication and characterization of portable and reliable nanowire-based devices.

Water vapor detection with individual tin oxide nanowires

Individual tin oxide nanowires (NWs), contacted to platinum electrodes using focused ion beam assisted nanolithography, were used for detecting water vapor (1500-32u2009000xa0ppm) in different gaseous environments. Responses obtained in synthetic air (SA) and nitrogen atmospheres suggested differences in the sensing mechanism, which were related to changes in surface density of the adsorbed oxygen species in the two cases. A model describing the different behaviors has been proposed together with comparative evaluation of NW responses against sensors based on bulk tin oxide. The results obtained on tenxa0individual devices (tested >6 times) revealed the interfering effect of water in the detection of carbon monoxide and illustrated the intrinsic potential of nanowire-based devices as humidity sensors. Investigations were made on sensitivity, recovery time and device stability as well as surface-humidity interactions. This is the first step towards fundamental understanding of single-crystalline one-dimensional (1D) tin oxide nanostructures for sensor applications, which could lead to integration in real devices.

Kinetically controlled synthesis of metastable YAlO3 through molecular level design

Heterometal alkoxide [YAl(OPri)6(PriOH)]2 has been used as a molecular template for the controlled and selective synthesis of metastable yttrium aluminate (YAlO3, YAP) in nanocrystalline form. The results indicate that the strict control over the Y∶Al ratio (1∶1) in the precursor suppresses the formation of other Y∶Al compositions (Y3Al5O12, YAG and Y4Al2O9, YAM) that are thermodynamically favoured over YAP in the conventional synthesis.

Ferromagnetic Nanostructures by Atomic Layer Deposition: From Thin Films towards Core-shell Nanotubes

Nickel, cobalt and iron oxide nanotubes were obtained by atomic layer deposition (ALD) into the pores of alumina membranes. Initially, a metal oxide film was grown by the reaction of a precursor vapor of NiCp2 (nickelocene), CoCp2 (cobaltocene) or FeCp2 (ferrocene) with ozone, respectively. Subsequently, the metal oxide film was reduced in hydrogen atmosphere and converted to a metallic ferromagnetic phase with low-degree of surface roughness. In a similar manner, Fe3O4 films have also been obtained by the atomic layer deposition of Fe2O3 films based on the reaction of water and iron(III) tertbutoxide (Fe2(O t Bu)6), followed by a hydrogen reduction to Fe3O4 after the ALD process. By conformal coating of self-ordered Al2O3 membranes, arrays of magnetic nanotubes with diameters down to 30 nm and wall thicknesses of less than 3 nm have been achieved. The magnetic properties of the nanotube arrays as a function of wall thickness and tube diameter have been studied by SQUID magnetometry. Atomic layer deposition (ALD) was proven to be a very suitable method for the conformal deposition of magnetic thin films in pore structures of high aspect ratio, while offering high uniformity and precise tuning of the layer thickness and the magnetic properties.

Synthesis and evolution of crystalline garnet phases in Y3Sc5-xGaxO12

Mixed-metal oxides with the composition Y3Sc x Ga5− x O12 (xu2009=u20092.0, 2.1, 2.2, 2.25, 2.3, 2.4, 2.5, and 3.0) have been prepared by an aqueous sol–gel method. The effects of scandium substitution on the garnet phase formation were studied by IR spectroscopy and X-ray powder diffraction (XRD). The XRD data indicate that single-phase Y3Sc x Ga5− x O12 ceramic samples were obtained for xu2009=u20092.0, 2.1, 2.2, 2.25, 2.3, and 2.4. The results also show that the formation of Y3Sc x Ga5− x O12 garnets depends on the molar ratio of scandium and gallium in the investigated composition, and consequently on the mean cationic radius at the Al3+ sites. The variation of lattice parameters for the Y3Sc x Ga5− x O12 phases with different x is reported.

One-dimensional semiconductor nanostructures: growth, characterization, and device applications

Molecule-based CVD is applied for the development of 1D semiconducting nanowires. By virtue of the chemical design of the metal-organic precursors, it is possible to achieve the required supersaturation ratio of phase-constituting elements in the gas phase, which allows to grow anisotropic structures with precisely controlled dimension and composition. [Ge(C5H5)2] with labile Ge-C bonds was thermolysed at 300 °C to grow single crystalline Ge nanowires (NWs). For tin oxide nanostructures, [Sn(OBut)4] with relatively strong and preformed Sn-O bonds was employed to synthesize anisotropic rutile phase. Determination of I-V characteristics of Ge NWs in different environments indicate surface passivation, possibly through hydrogen. Radial dimension of SnO2 NWs was varied in the range 30-1000 nm by choosing appropriate size of catalyst particles. Photo-conductance studies on different NW samples revealed a significant blue shift with shrinking wire diameters. Tin oxide nanowires were coated with vanadium oxide by CVD of [VO(OPri)3] on as-grown tin oxide nanowires. Composite SnO2/VOx 1D nanostructures showed a shift to higher wavelength in photo-response peak, when compared to pure SnO2 NWs. We also demonstrate the integration of single NW on pre-patterned electrodes for evaluating sensing and electrical properties on individual nanoobjects.

Chemical Nanotechnology: from Molecules to Applications

The article reviews the potential of bottom-up chemical methodologies in synthesizing nanoscopic materials, and evaluates the influence of chemistry involved in materials processing on functional and structural properties of ceramics. Application of molecular sources in preparation of nanomaterials allows an intrinsic chemical encodingl in the initial stages of molecule-to- material transformation which plays an important role in tuning the properties of nanomaterials. Salient examples describing the precursor-structure-property-application relationship have been discussed to support the above arguments. Concomitant development in the novel synthesis procedures for nanomaterials and instrumentation techniques, has brought nanoscience to the brink of being able to monitor, move and manipulate individual nanostructures. Despite of several existing preparative and analytical techniques, controlled synthesis and nanoscaled operations represent one of the most challenging avenues in demonstrating the potential of nanotechnology. Physical and chemical properties of nanostructures are generally surface-dominated because the distorted atomic symmetries, complicated charge compensations and uncontrollable dangling bonds can strongly influence the electronic structures of the surface states but are generally difficult to regulate (1). Chemical nanotechnology is the science and technology of nano-scaled objects in terms of materials engineering, and given its wide spread applications is important from both fundamental and technological perspectives (2-11). Controlled fabrication of nanostructures with defined chemical composition, size, morphology and location is the most crucial step underlying the technology transfer from laboratory results to industrial production which requires a detailed understanding of the growth mechanism and identification of the optimal processing parameters. Comprehending growth pathways is therefore the basis for up-scaled production of high-quality nanostructures for technological applications. We present herein an overview of molecule-to-material concept based on examples taken from work done in our laboratories and discuss the role of chemistry, molecule-structure-property relationship in different material systems in order to elaborate the strength of chemical processing of nanoscaled materials.

Influence of Precursor Design on the Growth of Nanomaterials

Chemical processing of inorganic materials demands an understanding of the precursor chemistry at the molecular level. Although chemical compounds imitating atomic composition or bonding features of solid phases are efficient templates for a controlled evolution of nano-matter , the intrinsic advantages of this approach, such as atomic level mixing of the constituents and phase-selective synthesis, rely on the chemistry involved in the transformation of molecules to materials. Therefore, a better understanding of principles underlying chemical processing is necessary to enable a rational synthesis of materials. We have deposited MgAl 2 O 4 thin films by the chemical vapor deposition of two Mg-Al alkoxide precursors, [MgAl 2 (OPr i ) 8 ] and [MgAl 2 (OBu t ) 8 ], which reveal that precursor attributes such as vapor pressure and ligand elimination mechanisms influence the microstructure and material properties of the spinel films. Under similar growth conditions, [MgAl 2 (OPr i ) 8 ] produces rough and poorly crystalline spinel films, whereas crystalline deposits and dense microstructure were obtained in the case of [MgAl 2 (OBu t ) 8 ]. The films were characterized by XRD, SEM, TEM, XPS and nano-indentation studies.

One-dimensional oxide nanostructures : growth, applications and devices

Zirconia coatings consisting of a mixture of coarse and fine grained zirconia powders prepared by spraying of suspensions and subsequent thermal treatment at limited temperatures (up to 500°C) are poor in adherence and in intrinsic mechanical strength. We have shown elsewhere that mechanical properties of these coatings can be improved clearly by adding a small amount of nanoscaled zirconia. Here, the structural and the chemical development of this coating material and of the nanoparticles is examined to gain information about the underlying bonding mechanisms. The applied temperature is relatively low in comparison to the usual onset temperature of accelerated sintering. Nevertheless, the results show that diffusion controlled material transport mechanisms play their role in bonding. The condensation of surface OH groups may participate in bonding, too. [20] T. Sugimoto, in Monodisperse Particles, Elsevier, Amsterdam, Netherlands 2001. [21] N. Nitin, L. E. W. LaConte, O. Zurkiya, X. Hu, G. Bao, J. Biol. Inorg. Chem. 2004, 9, 706. [22] Z. Liu, D. Zhang, S. Han, C. Li, B. Lei, W. Lu, J. Fang, C. Zhou, J. Am. Chem. Soc, 2005, 127, 6. [23] J. Wang, Q. Chen, C. Zeng, B. Hou, Adv. Mater. 2004, 16, 137. [24] J. R. Morber, Y. Ding, M. S. Haluska, Y. Li, J. P. Liu, Z. L. Wang, R. L. Snyder, Phys. Rev. B 2006, 110, 21672. [25] J. Bachmann, J. Jing, M. Knez, S. Barth, H. Shen, S. Mathur, U. Goesele, K. Nielsch, J. Am. Chem. Soc. 2007, 129, 9554. [26] (a) S. Mathur, M. Veith, V. Sivakov, H. Shen, V. Huch, U. Hartmann, H. B. Gao, Chem. Vap. Depos. 2002, 8, 277; (b) S. Mathur, V. Sivakov, H. Shen, S. Barth, C. Cavelius, A. Nilsson, P. Kuhn, Thin Solid Films 2006, 502, 88; (c) S. Mathur, H. Shen, V. Sivakov, U. Werner, Chem. Mater. 2004, 16, 2449. [27] D. M. Lind, S. D. Berry, G. Chern, H. Mathias, L. R. Testardi, Phys. Rev. B 1992, 45, 1838. [28] (a) S. Mathur, S. Barth, U. Werner, F. Hernandez-Ramirez, A. RomanoRodriguez, Adv. Mater. 2008, in print; (b) S. Mathur, S. Barth, Z. Phys. Chem. 2008, 222, 307. [29] M. Bohra, N. Venkataramani, S. Prasad, N. Kumar, D. S. Misra, S. C. Sahoo, R. Krishnan, J. Magn. Magn. Mater. 2007, 310, 2242. [30] (a) A. Vorniero, M. Ferroni, E. Comini, G. Fagkia, G. Sberveglieri, Nano Mechanisms of Bonding Effected by Nanoparticles in Zirconia Coatings Applied by Spraying of Suspensions J. Adam, M. Aslan, R. Drumm, M. Veith