Volume 74, Issue 9

A focus of frontline interdisciplinary research today is the development of the conceptual framework and the experimental background of the science of nanostructured materials and the perspectives of its technological applications. We consider some current directions in the preparation, characterization, manipulation, and interrogation of nanomaterials, in conjunction with the modeling of the unique structure­dynamics­function relations of nanostructures and their assemblies. The implications of quantum size and shape effects on the energetics, nuclear­electronic level structure, electric-optical response and dynamics, reveal new unique physical phenomena that qualitatively differ from those of the bulk matter and provide avenues for the control of the function of nanostructures. Current applications in the realm of nanoelectronics, nanooptoelectronics, and information nanoprocessing are addressed, and other directions highlighted. Chemical sciences make a central contribution to this novel and exciting scientific­technological area.

The porous metal oxides are an important class of materials, because the surface area/volume ratio of a material is increased by many fold, making them very useful in surface-related applications. The mesoporous materials were discovered in the 1990s, and since then they have been excellent candidates for materials science research. These mesoporous materials are prepared by hydrolyzing the inorganic precursor (usually metal alkoxide) in an acid, basic, or neutral medium in the presence of an organic structure-directing agent, the surfactant, in a conventional method. Recently, we have demonstrated that the sonochemical technique can be employed for the synthesis of mesoporous metal oxides. The sonochemical method reduced the time period required for such synthesis by many fold, and also produced more stable structures. We got excellent results with silica, titania, yittria-stabilized zirconia (YSZ), and Fe 2 O 3 . We also used an inorganic precursor other than an alkoxide for the preparation of mesoporous metal oxides. In this article, we present some of the recent results on this topic.

Nanocrystalline nickel films of technological importance have been grown on various liquid nitrogen-cooled substrates by magnetron sputtering with and without a substrate bias. The atomic structural and chemical studies have unveiled variations in inter- and intragranular structures under the different process conditions. The origin and the development of the crystallization process with and without the substrate bias voltage have been inferred from the results.

In nanocatalysis, clusters deposited on solid, well-defined surfaces play an important role. For the detection of size effects it is, however, important to prepare samples consisting of deposited clusters of a single size, as their chemical properties change with the exact number of atoms in the cluster. In this paper, the experimental tools are presented to prepare such model systems. The existence of monodispersed clusters is confirmed by various experimental findings. First, the carbonyl formation of deposited Nin clusters shows no change in the nuclearity when comparing the size of the deposited clusters with one of the formed carbonyls. Second, scanning tunneling microscopy (STM) studies show that fragmentation of Sin clusters upon deposition can be excluded. In addition, the adsorption behavior of CO on deposited Pd atoms points to the existence of single atoms on the surface. Furthermore, CO oxidation results on Aun clusters confirm the existence of monodispersed clusters trapped on well-defined adsorption sites. Finally, we use Monte-Carlo simulations to define the range of clusters and defect densities, for which monodispersed clusters can be expected.

While there are many techniques for the production of nanophase materials, we have been using a hydrothermal process because it is a low-temperature method that can lead to energy savings. A recent innovation is the introduction of microwaves in the hydrothermal system, and we named this process the microwave-hydrothermal (M-H) process. M-H synthesis is a novel processing technology for the production of a variety of nanophase ceramic oxide and metal powders under closed-system conditions. This closed-system technology not only prevents pollution at its source, but also saves energy and, thus, could substantially reduce the cost of producing nanophase powders of all kinds. With several examples, the value of this technique is reviewed here. The M-H technique leads to (a) rapid heating to temperature of treatment, which can save energy and time; (b) increased reaction kinetics by one to two orders of magnitude, which also saves time and energy; (c) formation of novel phases; and (d) selective crystallization.

Nanotubes of HfS 2 , ZrS 2 , NbS 2 , and TaS 2 have been obtained by the reduction of the corresponding metal trisulfides in a stream of H 2 (mixed with an inert gas in some cases) at elevated temperatures. The nanotubes have been characterized by transmission electron microscopy (TEM) and other techniques. These disulfide nanotubes make an important addition to the growing family of nanotubes of inorganic layered materials.

Here we shall discuss the importance of grazing incidence X-ray scattering techniques in studying morphology of nanostructured materials confined in thin films and multilayers. In these studies, the shapes, sizes, and structures of nanostructured materials and their distribution in composites are investigated. These studies are important for understanding properties that may deviate considerably from the known bulk properties. We shall first outline basics of three X-ray scattering techniques, namely X-ray reflectivity, grazing incidence small-angle X-ray scattering, and grazing incidence diffraction, used for these studies. We shall then demonstrate the utility of these techniques using some known results.

Multi-walled as well as single-walled carbon nanotubes are conveniently prepared by the pyrolysis of organometallic precursors, such as metallocenes and phthalocyanines, in a reducing atmosphere. Pyrolysis of organometallics alone or in mixture with hydrocarbons also yields aligned nanotube bundles with useful field emission properties. By pyrolyzing organometallics in the presence of thiophene, Y-junction nanotubes are obtained in large quantities. The junction nanotubes have a good potential in nanoelectronics. Carbon nano-tubes prepared from organometallics are useful for preparing nanowires and nanotubes of materials such as BN, GaN, SiC, and Si 3 N 4 .

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.

Organized molecular assemblies have been one of the intensely pursued areas of contemporary chemistry. Among the various methodologies used to make organized monolayer structures, self-assembled monolayers (SAMs) have been attractive to many materials chemists owing to the simplicity of the preparative method and high stability. Advances in various techniques and their application in the study of SAMs have significantly improved our understanding of these molecular systems. These studies have been further intensified since the successful preparation of stable metal clusters protected with monolayers. This article reviews the structure, temperature-induced phase transitions, and associated dynamics of monolayers, principally in the context of our own work in this area. Alkanethiols on Au(111) and Ag(111) are taken as archetypal systems to discuss the properties of 2D SAMs; studies from our laboratory have been on evaporated thin films. Alkanethiols on Au and Ag cluster surfaces are taken as examples of 3D SAMs. Although our principal focus will be on alkanethiols, we will touch upon a few other adsorbate systems as well.

The formation of molecular films of 2,9,16,23-tetraamino metal phthalocyanines [TAM(II)Pc; M (II) = Co, Cu, and TAM(III)Pc; M = Fe] by spontaneous adsorption on gold and silver surfaces is described. The properties of these films have been investigated by cyclic voltammetry, impedance, and FT-Raman spectroscopy. The charge associated with Co(II) and Co(I) redox couple in voltammetric data leads to a coverage of (0.35 ± 0.05) x 10 -10 mol cm -2 , suggesting that the tetraamino cobalt phthalocyanine is adsorbed as a monolayer with an almost complete coverage. The blocking behavior of the films toward oxygen and Fe(CN) 6 3-/4- redox couple have been followed by cyclic voltammetry and impedance measurements. This leads to an estimate of the coverage of about 85% in the case of copper and the iron analogs. FT-Raman studies show characteristic bands around 236 cm -1 revealing the interaction between the metal substrate and the nitrogen of the -NH 2 group on the phthalocyanine molecules.

Nanotechnology is witnessing impressive advances on many different fronts. One of the key areas with important commercial implications concerns the assembly of nanoparticles to form thin films and superstructures by what is commonly known as the "bottom-up" approach. This paper covers some of the more recent developments in this fascinating field with particular emphasis on the work from the author's laboratory on assembly of nanoparticles using electrostatic interactions. The use of electrostatic interactions enables extension of the assembly protocols to the immobilization of biomacromolecules such as proteins/enzymes and DNA with exciting application potential.

The possibility of modifying existing materials through technology has become the recipe for preparation of advanced materials. Nonequilibrium processing of silicon through MeV ion irradiation will be shown here to yield interesting properties. We propose localization of vibrational energy following an ion irradiation process and their transport (nonlinear transport of energy) through linear chains of a single-crystalline lattice. The localization of energy can involve 3­4 atoms, and, hence, nanometer-sized entities evolve, distinguishable from the remaining periodic lattice owing to their unique interatomic distances. The energy required to produce these structures is supplied by a single high-energy heavy ion, incident normal to a suitable crystal face so as to lose energy by the electronic energy loss mechanism. These entities can be trapped at a desired location that leads to silicon nanostructures with modified band-gaps.

We review our recent contributions to the use of solvothermal methods for the preparation of different oxide and chalcogenide nanoparticles. We have prepared sub 10-nm g-Fe 2 O 3 , ZnFe 2 O 4 , and CoFe 2 O 4 particles by the decomposition of the corresponding cupferron complexes in the presence of n-octylamine or n-dodecylamine in solvothermal toluene. Similarly, dodecanethiol-capped chalcogenide nanoparticles of CdSe have been prepared by reacting cadmium stearates with H 2 Se under solvothermal conditions. The H 2 Se is generated in situ by the reduction of Se by tetralin. Using this latter technique, we have also been able to prepare PbSe and PbI 2 in toluene under solvothermal conditions, albeit in bulk (rather than nanocrystalline) form. In the preparation of PbI 2 , HI is prepared by the in situ reduction of I 2 by tetralin.

Understanding the evolution of the Sb/Si(111) interface is of great interest in the formation of devices of nanodimensions. We have undertaken in situ (∼10 -11 torr) studies of Sb adsorption (at room temperature) and its desorption on the 7 X 7 reconstructed Si(111) surface, by complementary techniques such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and electron energy loss spectroscopy (EELS). For room-temperature (RT) Sb adsorption, the overlayer grows in the Frank van der Merwe mode, forming an interface state of δ(7 X 7) in the submonolayer Sb coverage regime. Adsorption of 1.0 monolayer (ML) Sb at RT shows an abrupt shift of 0.8 eV in the peak position of the Sb 3d 5/2 transition owing to band-bending caused by a metallic (7 X 7) to a semiconducting (1 X 1) surface phase transformation. Changes observed in full width at half-maximum (fwhm) and Sb 3d 3/2 and 3d 5/2 branching ratio are discussed. Thermal annealing experiments provide evidence for agglomeration of Sb islands, before the multilayer and monolayer desorption. During this desorption process, we have observed two novel surface phases of (5 X 5) at 0.4 ML and (5√3 X 5√3­R30°) at 0.2 ML, stable at higher temperatures.

Close-packed metal surfaces and heteroepitaxial systems frequently display a structure consisting of regularly spaced misfit dislocations, with a network of domain walls separating face-centered cubic (fcc) and hexagonal close-packed (hcp) domains. These structures can serve as templates for growing regularly spaced arrays of nanoislands. We present a theoretical investigation of the factors controlling the size and shape of the domains, using Pt(111) as a model system. Upon varying the chemical potential, the surface structure changes from being unreconstructed to the honeycomb, wavy triangles, "bright stars", or Moiré patterns observed experimentally on Pt(111) and other systems. For the particular case of Pt(111), isotropically contracted star-like patterns are favored over uniaxially contracted stripes.

In this paper, we summarize our results on the effect of changing size and shape of semiconductor and metallic nanocrystals. The change in size (from 3.5 to 1.1 nm) and shape (from spherical to rod) of CdSe nanocrystals is found to affect both their optical and nonradiative properties. In gold quasi-spherical nanocrystals, results on the dependence of the electron-phonon relaxation of the surrounding medium are presented. For gold nanorods, results and conclusions on the mechanism of their self-assembly are presented. Enhanced Raman scattering on their surfaces is studied and compared with that on spherical particle surfaces.

Semiconductor­metal nanocomposites provide a simple and convenient way to tailor the properties of photocatalysts. Modification of semiconductor surface improves charge separation and promotes interfacial charge-transfer processes in nanocomposite systems. Charge accumulation in the metal layer results in Fermi-level equilibration raising the quasi-Fermi level of the composite close to the conduction band level of the oxide semiconductor. Phototransformations of such compositesincluding morphological changes, interfacial charge-transfer processes and photocurrent generation of TiO 2 -capped gold colloidsare presented in this review article.

In this paper, various mesoscopic structures made of cobalt nanocrystals are presented. It is demonstrated that the evaporation time and the application of an external magnetic field during the evaporation process play a major role in the shape of mesostructure obtained. Their magnetic properties change with the shape of the mesostructure.

Single-wall carbon nanotubes (SWNTs) are fascinating systems exhibiting many novel physical properties. In this paper, we give a brief review of the structural, electronic, vibrational, and mechanical properties of carbon nanotubes. In situ resonance Raman scattering of SWNTs investigated under electrochemical biasing demonstrates that the intensity of the radial breathing mode varies significantly in a nonmonotonic manner as a function of the cathodic bias voltage, but does not change appreciably under anodic bias. These results can be quantitatively understood in terms of the changes in the energy gaps between the 1D van Hove singularities in the electron density of states, arising possibly due to the alterations in the overlap integral of π bonds between the π-orbitals of the adjacent carbon atoms. In the second part of this paper, we review our high-pressure X-ray diffraction results, which show that the triangular lattice of the carbon nanotube bundles continues to persist up to ∼10 GPa. The lattice is seen to relax just before the phase transformation, which is observed at ∼10 GPa. Further, our results display the reversibility of the 2D lattice symmetry even after compression up to 13 GPa well beyond the 5 GPa value observed recently. These experimental results explicitly validate the predicted remarkable mechanical resilience of the nanotubes.

It is generally believed that metal nanoparticles strongly quench the singlet-excited states of chromophores when attached to nanoparticle surfaces, through an energy-transfer mechanism, which limits their application in optoelectronic devices and photonic materials. Recent studies of fluorophore-linked metal nanoparticles reveal that there is a dramatic suppression in the quenching of the singlet-excited state of these molecules and they possess unusual photophysical properties. A summary of our work on the photophysical and excited-state properties of chromophore-functionalized gold nanoparticles is presented in this article. Pyrene-capped gold nanoparticles showed normal fluorescence in nonpolar solvents and an intermolecular excimer formation at higher loadings. The quenching of the emission, observed in pyrene-labeled gold nanoparticles in polar solvents, is attributed to the formation of pyrene radical cation through a photoinduced electron-transfer process. We have also functionalized gold nanoparticles using a thiol derivative of fullerene. The quenching of fluorescence and decreased yields of triplet-excited state, observed in these systems, are attributed to an energy-transfer process.

TiO 2 thin films with high volume fraction (∼50­70 %) of CdTe nanoparticles were prepared by radio frequency (rf) magnetron sputtering from a composite TiO 2 :CdTe target. With increase in substrate temperature Ts from room temperature (RT ∼300 K) to 373 K, a transition from an ordered structure exhibiting metallic-type conduction to a disordered structure exhibiting nonmetallic-type conduction was observed for annealed nanocrystalline CdTe:TiO 2 films. The annealed RT-deposited films showed a large coalescence of distinct islands (size ∼0.3­0.7 µm) mainly of Cd and CdTe, and as result, a 3D network was realized. For metallic regime films, electrical conduction is essentially due to electrical percolation through Cd/CdTe crystallites embedded in an amorphous TiO 2 matrix. However, the annealed high Ts films consisted of noncoalescent, small islands (size ∼0.15­0.3 µm) of Cd and CdTe embedded in amorphous TiO 2 matrix. Here, the conduction is essentially by hopping mechanism via thermally activated tunneling.

The need for cheaper, faster, and more accurate measurements has been a driving force for the development of new sensing devices. As is well known, the electrical conductivity of conjugated polymers can be reliably regulated over a wide range through interactions with electron acceptors and donors. This, together with the fast optical dynamics (either in the ground or excited states) of most conjugated polymers, has made conjugated polymers very attractive as transducer-active materials. On the other hand, the unusual electronic, mechanical, and thermal properties of carbon nanotubes have also led to their potential use in a wide range of devices, including sensors. In particular, the ability of carbon nanotubes and their derivatives to operate as gas and glucose sensors has been recently demonstrated. This article provides a status review on the research and development of sensors and sensor arrays based on conjugated polymers and carbon nanotubes. The unique features characteristic of most reported sensing transduction modes related to conjugated polymers and carbon nanotubes are discussed, along with their pros and cons.

Tailored sensoric, electronic, photoelectrochemical, and bioelectrocatalytic functions can be designed by organized molecular or biomolecular nanoparticle hybrid configurations on surfaces. Layered receptor-cross-linked Au nanoparticle assemblies on electrodes act as specific sensors of tunable sensitivities. Layered DNA-cross-linked CdS nanoparticles on electrode supports reveal organized assemblies of controlled electronic and photoelectrochemical properties. Au nanoparticle-FAD semisynthetic cofactor units are reconstituted into apo-glucose oxidase (GOx) and assembled onto electrodes. The resulting enzymes reveal effective electrical contacting with the electrodes, and exhibit bioelectrocatalytic functions toward the oxidation of glucose to gluconic acid. Magneto-switchable electrocatalysis and bioelectrocatalysis are accomplished by the surface modification of magnetic particles with redox-relay units. By the attraction of the modified magnetic particles to the electrode support, or their retraction from the electrode, by means of an external magnet, the electrochemical functions of the magnetic particle-tethered relays can be switched between "ON" and "OFF" states, respectively. The magneto-switchable redox functionalities of the modified particles activate electrocatalytic transformations, such as a biocatalytic chemoluminescence cascade that leads to magneto-switchable light emission or the activation of bioelectrocatalytic processes.