Characterization of Nanomaterials by Physical Methods

Much progress in nanoscience and nanotechnology has been made in the past few years thanks to the increased availability of sophisticated physical methods to characterize nanomaterials. These techniques include electron microscopy and scanning probe microscopies, in addition to standard techniques such as X-ray and neutron diffraction, X-ray scattering, and various spectroscopies. Characterization of nanomaterials includes the determination not only of size and shape, but also of the atomic and electronic structures and other important properties. In this article we describe some of the important methods employed for characterization of nanostructures, describing a few case studies for illustrative purposes. These case studies include characterizations of Au, ReO3, and GaN nanocrystals; ZnO, Ni, and Co nanowires; inorganic and carbon nanotubes; and two-dimensional graphene.

Pristine nanomaterials: synthesis, stability and applications

Capping-free and linker-free nanostructures/hybrids possess superior properties due to the presence of pristine surfaces and interfaces. In this review, various methods for synthesizing pristine nanomaterials are presented along with the general principles involved in their morphology control. In wet chemical synthesis, the interplay between various reaction parameters results in diverse morphology. The fundamental principles behind the evolution of morphology including nanoporous aggregates of metals and other inorganic materials, 2D nanocrystals of metals is elucidated by capping-free methods in aqueous medium. In addition, strategies leading to the attachment of bare noble metal nanoparticles to functional oxide supports/reduced graphene oxide has been demonstrated which can serve as a simple solution for obtaining thermally stable and efficient supported catalysts with free surfaces. Solution based synthesis of linker-free oxide-semiconductor hybrids and capping-free metal nanowires on substrates are also discussed in this context with ZnO/CdS and ultrathin Au nanowires as examples. A simple and rapid microwave-assisted method is highlighted for obtaining such hybrids which can be employed for high-yield production of similar materials.

Nanomaterials coated multiplexed fiber Bragg grating for multiparameter sensing

Fiber Bragg Grating (FBG) sensors have been extensively used for strain and temperature sensing. However, there is still a need to measure multiple environmental parameters with a single sensor system. We demonstrate a multiplexed FBG sensor with various nano materials (polyallylamine-amino-carbon-nanotube, carbon nanotubes, polyelectrolyte and metals) coated onto the surface of the core/cladding FBG for sensing multiple environmental parameters such as pH (64 pm/pH), protein concentration (5 pm/mu g/ml), temperature (15 pm/degrees C), humidity (31 pm/% RH), gas concentration (7 pm/1000 ppm), and light intensity (infrared: 33 pm/mW, visible: 12 pm/mW and UV: 1 pm/mW) utilizing the same FBG based platform.

TiO2-RGO hybrid nanomaterials for enhanced water splitting reaction

The production of H-2 via photocatalytic water splitting reaction has attracted a great attention as a clean and renewable energy for next generation. Despite tremendous efforts, the present challenge for materials scientist is to develop highly active photo catalysts for splitting of water at low cost. This article reports the synthesis of TiO2-reduced graphene oxide hybrid nanomaterials through ionothermal method using functionalized ionic liquid for the enhanced hydrogen generation via water splitting reaction. The structural and morphological properties of the samples were investigated by XFtD, Raman spectroscopy, TG-DTA, UV-vis spectroscopy and TEM. A substantial increase of H-2 evolution was observed for TiO2-reduced graphene oxide hybrid nanomaterials. This is due to the high migration efficiency of photo-induced electrons and the inhibition of charge carrier recombination due to the electronic interaction between TiO2 and reduced graphene oxide. i.e, reduced graphene oxide acts as an electron-acceptor which effectively hinders the electron hole pair recombination of TiO2. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Tungsten-based nanomaterials (WO3 & Bi2WO6): Modifications related to charge carrier transfer mechanisms and photocatalytic applications

Heterogeneous photocatalysis is an ideal green energy technology for the purification of wastewater. Although titania dominates as the reference photocatalyst, its wide band gap is a bottleneck for extended utility. Thus, search for non-TiO2 based nanomaterials has become an active area of research in recent years. In this regard, visible light absorbing polycrystalline WO3 (2.4-2.8 eV) and Bi2WO6 (2.8 eV) with versatile structure-electronic properties has gained considerable interest to promote the photocatalytic reactions. These materials are also explored in selective functional group transformation in organic reactions, because of low reduction and oxidation potential of WO3 CB and Bi2WO6 VB, respectively. In this focused review, various strategies such as foreign ion doping, noble metal deposition and heterostructuring with other semiconductors designed for efficient photocatalysis is discussed. These modifications not only extend the optical response to longer wavelengths, but also prolong the life-time of the charge carriers and strengthen the photocatalyst stability. The changes in the surface-bulk properties and the charge carrier transfer dynamics associated with each modification correlating to the high activity are emphasized. The presence of oxidizing agents, surface modification with Cu2+ ions and synthesis of exposed facets to promote the degradation rate is highlighted. In depth study on these nanomaterials is likely to sustain interest in wastewater remediation and envisaged to signify in various green energy applications. (C) 2015 Elsevier B.V. All rights reserved.

Investigation of the Internal Heterostructure of Highly Luminescent Quantum Dot-Quantum Well Nanocrystals

In this paper, we report on the growth and characterization of quantum dot−quantum well nanostructures with photoluminescence (PL) that is tunable over the visible range. The material exhibits a PL efficiency as high as 60% and is prepared by reacting ZnS nanocrystals in turn with precursors for CdSe and ZnS in an attempt to form a simple “ZnS/CdSe/ZnS quantum-well structure”. Through the use of synchrotron radiation-based photoelectron spectroscopy in conjunction with detailed overall compositional analysis and correlation with the size of the final composite nanostructure, the internal structure of the composite nanocrystals is shown to consist of a graded alloy core whose composition gradually changes from ZnS at the very center to CdSe at the onset of a CdSe layer. The outer shell is ZnS with a sharp interface, probably reflecting the relative thermodynamic stabilities of the parent binary phases. These contrasting aspects of the internal structure are discussed in terms of the various reactivities and are shown to be crucial for understanding the optical properties of such complex heterostructured nanomaterials.

Extraordinary synergy in the mechanical properties of polymer matrix composites reinforced with 2 nanocarbons

One of the applications of nanomaterials is as reinforcements in composites, wherein small additions of nanomaterials lead to large enhancements in mechanical properties. There have been extensive studies in the literature on composites where a polymer matrix is reinforced by a single nanomaterial such as carbon nanotubes. In this article, we examine the significant synergistic effects observed when 2 different types of nanocarbons are incorporated in a polymer matrix. Thus, binary combinations of nanodiamond, few-layer graphene, and single-walled nanotubes have been used to reinforce polyvinyl alcohol. The mechanical properties of the resulting composites, evaluated by the nanoindentation technique, show extraordinary synergy, improving the stiffness and hardness by as much as 400% compared to those obtained with single nanocarbon reinforcements. These results suggest a way of designing advanced materials with extraordinary mechanical properties by incorporating small amounts of 2 nanomaterials such as graphene plus nanodiamond or nanodiamond plus carbon nanotube.

Supramolecular gels `in action'

n recent years, self-assembly has emerged as a powerful tool for the construction of functional nanostructures. Myriad applications of these nanoscale architectures, especially the supramolecular gels derived from low molecular mass compounds, in fields such as optoelectronics, light harvesting, organic–inorganic hybrid materials, tissue engineering and regenerative medicine are being envisaged. This review attempts to present a succinct overview of the current state of research on functional nano-scale systems—the design, synthesis and applications of self-assembled nanomaterials engineered to carry out precise functions, with an emphasis on supramolecular gel phase materials.

Ellipsometric study of Atomic Layer Deposited TiO2 thin films

Atomic layer deposition was used to obtain TiO2 thin films on Si (100) and fused quartz, using a novel metal organic precursor. The films were grown at 400 degrees C, varying the amount of oxygen used as the reactive gas. X-ray diffraction showed the films to be crystalline, with a mixture of anatase and rutile phases. To investigate their optical properties, ellipsometric measurements were made in the UV-Vis-NIR range (300-1700 nm). Spectral distribution of various optical constants like refractive index (n), absorption index (k), transmittance (T), reflectance (R), absorption (A) were calculated by employing Bruggemann's effective medium approximation (BEMA) and Maxwell-Garnet effective medium approximation, in conjunction with the Cauchy and Forouhi-Bloomer (FB) dispersion relations. A layered optical model has been proposed which gives the thickness, elemental and molecular composition, amorphicity and roughness (morphology) of the TiO2 film surface and and the film/substrate interface, as a function of oxygen flow rate The spectral distribution of the optical band gap (E-g(opt)), complex dielectric constants (epsilon' and epsilon''), and optical conductivity (sigma(opt)), has also been determined.

Electrical Percolation studies of Polystyrene-multi wall carbon nanotubes Composites

Composites of Polystyrene-multi wall carbon nanotubes (PS-MWNTs) were prepared with loading up to 7 wt% of MWNTs by simple solvent mixing and drying technique. MWNTs with high aspect ratio similar to 4000 were used to make the polymer composites. A very high degree of dispersion of MWNTs was achieved by ultrasonication technique. As a result of high dispersion and high aspect ratio of the MWNTs electrical percolation was observed at rather low weight fraction similar to 0.0021. Characterization of the as prepared PS-MWNTs composites was done by Electron microscopy (EM), X-ray diffraction technique (XRD) and Thermogravimetery analysis (TGA).

Exciton Plasmon Coupling in Hybrid Semiconductor - Metal Nanoparticle Monolayers

Hybrid semiconductor-metal nanoparticles monolayer of Cadmium Sclenide and gold nanoparticles has been prepared, using Langmuir – Blodgett technique. The near field photoluminescence spectra from such monolayer films, shows red shift similar to 75 meV with respect to CdSe QDs monolayer film and splitting similar to 57 meV. The composite spectra are much broader similar to 330 meV compared to the corresponding emission spectra of CdSe monolayer similar to 165 meV. The possible explanation for the observed features are provided in terms of exciton - Plasmon interaction.

Microstructural stability and creep in nanocrystals

Microstructural stability is an important consideration during high temperature deformation and processing of nanomaterials. We will address issues relating to triple junctions in limiting grain growth during creep as well as densification. Although early studies on deformation have considered diffusion creep as a possible rate controlling deformation mechanism in nanocrystals, a critical inspection of available data indicates that there is no strong evidence for conventional diffusion creep in such materials. The possibility of diffusion creep by rapid diffusion along triple junctions will be analyzed, and interface controlled diffusion creep will also be discussed critically. It is shown that the critical grain size for dislocation activity is similar to that for occurrence of conventional diffusion creep.

Use of the liquid-liquid interface for generating ultrathin nanocrystalline films of metals, chalcogenides, and oxides

The air-water interface has traditionally been employed to prepare particle assemblies and films of metals and semiconductors. The interface between water and an organic liquid, however, has not been investigated sufficiently for possible use in preparing nanocrystals and thin films of materials. In this article, we demonstrate the use of the liquid-liquid interface as a medium for preparing ultrathin films of metals, chalcogenides and oxides. The method involves the reaction at the interface between a metal-organic compound in the organic layer and an appropriate reagent for reduction, sulfidation, etc. in the aqueous layer. Some of the materials discussed are nanocrystalline films of gold, CuS, CuSe, CuO, and Cu(OH)(2) formed at the liquid-liquid interface. The results reported in this article should demonstrate the versatility and potential of the liquid-liquid interface for preparing nanomaterials and ultrathin films and encourage further research in this area. (c) 2005 Elsevier Inc. All rights reserved.