G. U. Kulkarni

Metal nanoparticles and their assemblies

Metal nanoparticles of varying sizes can be prepared by physical as well as chemical methods. They exhibit many fascinating properties, the size-dependent metal to nonmetal transition being an important one. Metal nanoparticles capped by thiols can be organized into ordered one-, two- and three-dimensional structures and these structures have potential applications in nanodevices. In this context, organization of arrays of metal nanoparticles with a fixed number of atoms assumes significance.

Size-Dependent Chemistry: Properties of Nanocrystals

Properties of materials determined by their size are indeed fascinating and form the basis of the emerging area of nanoscience. In this article, we examine the size dependent electronic structure and properties of nanocrystals of semiconductors and metals to illustrate this aspect. We then discuss the chemical reactivity of metal nanocrystals which is strongly dependent on the size not only because of the large surface area but also a result of the significantly different electronic structure of the small nanocrystals. Nanoscale catalysis of gold exemplifies this feature. Size also plays a role in the assembly of nanocrystals into crystalline arrays. While we owe the beginnings of size-dependent chemistry to the early studies of colloids, recent findings have added a new dimension to the subject.

Superlattices of metal and metal-semiconductor quantum dots obtained by layer-by-layer deposition of nanoparticle arrays

Superlattices formed by arrays of Pt or Au nanoparticles have been obtained by layer-by-layer deposition by using dithiols as cross-linkers. The superlattices have been characterized by X-ray diffraction, photoelectron spectroscopy, and scanning tunneling microscopy. The core-level intensities of the metal and of the dithiol in the X-ray photoelectron spectra show the expected increase with successive depositions. The formation of such structures has been confirmed by depositing Pt and Au layers alternatively. Layers of metal and CdS nanoparticles have been deposited alternatively to obtain heterostructures.

Ultrafast response humidity sensor using supramolecular nanofibre and its application in monitoring breath humidity and flow

Measuring humidity in dynamic situations calls for highly sensitive fast response sensors. Here we report, a humidity sensor fabricated using solution processed supramolecular nanofibres as active resistive sensing material. The nanofibres are built via self- assembly of donor and acceptor molecules (coronene tetracarboxylate and dodecyl methyl viologen respectively) involved in charge transfer interactions. The conductivity of the nanofibre varied sensitively over a wide range of relative humidity (RH) with unprecedented fast response and recovery times. Based on UV-vis, XRD and AFM measurements, it is found that the stacking distance in the nanofibre decreases slightly while the charge transfer band intensity increases, all observations implying enhanced charge transfer interaction and hence the conductivity. It is demonstrated to be as a novel breath sensor which can monitor the respiration rate. Using two humidity sensors, a breath flow sensor was made which could simultaneously measure RH and flow rate of exhaled nasal breath. The integrated device was used for monitoring RH in the exhaled breath from volunteers undergoing exercise and alcohol induced dehydration.

Room temperature hydrogen and hydrocarbon sensors based on single nanowires of metal oxides

Hydrogen sensing characteristics of single nanowires of ZnO, TiO2 and WO2.72 have been investigated by contact mode atomic force microscopy. All these nanostructures are able to sense hydrogen, but the WO2.72 nanowire (40 nm diam) exhibits the highest sensitivity of 22 for 1000 ppm at 298 K. The WO2.72 nanowire is also found to be good at sensing aliphatic hydrocarbons in the form of liquefied petroleum gas with a sensitivity of 15 for 1000 ppm at room temperature. A WO2.72 nanowire with a diameter of 40 nm shows better sensing characteristics than a nanowire of 16 nm diameter.

Functionalized Au22 Clusters: Synthesis, Characterization, and Patterning

We synthesized fluorescent, porphyrin-anchored, Au(22) clusters in a single step, starting from well-characterized Au(25) clusters protected with glutathione (-SG) by a combined core reduction/ligand exchange protocol, at a liquid-liquid interface. The prepared cluster was characterized by UV/vis, photoluminescence, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, elemental analysis, and matrix-assisted laser desorption ionization mass spectrometry. The absence of a 672 nm intraband transition of Au(25) and the simultaneous emergence of new characteristic peaks at 520 and 635 nm indicate the formation of the Au(22) core. An increase in the binding energy of 0.4 eV in Au 4f core-level peaks confirmed the presence of a reduced core size. Quantitative XPS confirmed the Au/S ratio. The presence of a free base, tetraphenylporphyrin (H(2)TPPOAS-), on the Au(22) core was confirmed by fluorimetric titrations with Cu(2+) and Zn(2+) ions. From all of these, the composition of the cluster was determined to be Au(22)[(-SG)(15)(-SAOPPTH(2))(2)], which was supported by mass spectrometry and elemental analysis. We utilized the fluorescence nature of these water-soluble clusters for the fabrication of fluorescent patterns by soft lithography. The patterns were studied using tapping-mode atomic force microscopy and confocal fluorescence imaging.

An experimental charge density study of aliphatic dicarboxylic acids

Abstract By employing reliable structural data on malonic, succinic, glutaric, adipic and pimelic acids, obtained by a single crystal X-ray crystallographic study of these systems (using a CCD detector), the total electron densities and the Laplacians at the critical points of various bonds including the dimeric hydrogen bonds have been obtained. The study shows interesting systematics in the charge densities and Laplacians in the series of dicarboxylic acids. Malonic and pimelic acids exhibit minimum charge density in the C–C, C–O and the O–H bonds while glutaric acid shows the maximum density. The CO and the C–H bonds exhibit the opposite trend. The C–O and the CO bonds in malonic and glutaric acids have unusual Laplacians. The estimated dipole moment in the solid state shows alternation, but there is no alternation in any of the structural parameters. The present study also indicates the role of interchain interactions.

Size-dependent changes in the electronic structure of metal clusters as investigated by scanning tunneling spectroscopy

Abstract Pd, Ag, Cd and Au clusters of varying sizes have been investigated by scanning tunneling spectroscopy under ultra-high vacuum. The conductance of the clusters decreases markedly when the cluster diameter is ≤1 nm. A plot of the density of states at the Fermi level against the cluster volume varies nearly linearly up to a cluster volume of 4 nm 3 (diameter ∼2 nm). Below a cluster diameter of 1 nm, an energy gap occurs, the value of which increases with the decrease in cluster size, reaching values up to 70 meV at small sizes. Clearly, the very small clusters tend to become non-metallic.

Highly conducting patterned Pd nanowires by direct-write electron beam lithography.

Palladium hexadecylthiolate is shown to serve as a negative-tone direct-write electron resist to produce nanopatterns down to 30 nm. The written patterns do not deviate much from the precursor in composition, while a post-treatment at 230 degrees C in air produced metallic Pd nanowires with residual carbon less than 10% and resistivity close to the bulk value, a desirable property of interconnects in nanocircuitry. The as-written patterns contain small nanocrystals (<5 nm) in a hydrocarbon matrix, which upon annealing aggregate to form well-connected networks of larger nanocrystals (5-15 nm), thus giving rise to metallic conductivity.

Mesoscale organization of metal nanocrystals

Nanocrystals of metals covered by alkanethiols organize themselves in two-dimensional arrays. We discuss such arrays of metal nanocrystals at length, with focus on the dependence of the structure and the stability of the arrays on the particle diameter and the distance between the particles. Three-dimensional superstructures of metal nanocrystals obtained by the use of alkanedithiols are examined. These ordered two- and three-dimensional structures of thiolized metal nanocrystals are good examples of mesoscale self-assembly. The association of metal nanocrystals to give rise to giant clusters with magic nuclearity provides an even more graphic demonstration of mesoscale self-assembly.