Session W34: Focus Session: Nano V: Nanoscale Materials and Properties II

In Quest of a Systematic Framework for Unifying and Defining Nanoscience

A \textit{central paradigm driven, Mendeleev-like nano-periodic system} has been cited as a critical missing link in the transformation of nanotechnology from an empirical to a highly predictive science. A systematic framework is proposed based on the same first principles underpinning ``central paradigms'' for chemistry/physics.\footnote{D.A. Tomalia, \textit{J. Nanopart. Res.} (2009), 11, 1251.} As such, a \textbf{Nanomaterials Classification Roadmap} considers \textit{structure controlled} nanoparticles defined by \textbf{Critical Nanoscale Design Parameters (CNDPs); }namely, \textbf{size, shape, surface chemistry, flexibility, architecture and elemental composition}. Classified as either \textbf{hard (H) (}inorganic) or \textbf{soft (S) (}organic)\textbf{ nano-element categories}$, $these nanoparticles (e.g., nano-clusters) generally manifest pervasive \textbf{atom mimicry }features.\footnote{S.N. Khanna, A.W. Castleman, et al., \textit{PNAS }(2006), 103 (49), 18405.} Many literature examples demonstrate chemical bonding/assembly of these nano-element categories to produce extensive libraries of \textbf{hard-hard [H}$_{n}$\textbf{:H}$_{n}$\textbf{], soft-soft [S}$_{n}$\textbf{-S}$_{n}$\textbf{]or hard-soft [H}$_{n}$\textbf{-S}$_{n}$\textbf{]} nano-element combinations, referred to as \textbf{nano-compounds}. Due to their quantized CNDP features, these nano-element/compounds exhibit many well-defined\textbf{ nano-periodic property patterns}. These property patterns are observed in their intrinsic physico-chemical properties (i.e., melting points, reactivity/self-assembly, sterics), as well as important functional/ performance properties (i.e., magnetic, photonic, and electronic behavior). The importance of these CNDP directed property patterns was recently demonstrated by publication of \textbf{first Mendeleev-like nano-periodic tables} by Percec, et al.\footnote{V. Percec, et al., \textit{J. Am. Chem. Soc}. (2009), 131, 17500.} Similarly, Mirkin, et al.\footnote{C.A. Mirkin, et al., \textit{Science }(2011), 334, 204.} recently reported six CNDP dependent nano-periodic rules for predicting hard-soft nano-element assemblies. These two independent reports appear to fulfill/validate this proposed nano-periodic concept. This lecture will overview this unifying \textbf{nano-periodic system }suitable for tuning optimal nanostructure/application properties, as well as predicting important risk/benefit/performance boundaries in the nanoscience field.   

DFT-based Modeling of Field-Dependent Control and Response of Nanomagnetic Molecules

Regardless of whether one is interested in characterizing, utilizing or controlling molecular-scale systems [1], one requisite to their understanding, design, and improvement is the ability to realistically model their response to electromagnetic fields. Since such responses are often collective their description requires an understanding of the interplay between bonding, spin, spin-orbit, vibrations, and electromagnetic fields. Inclusion of spin and magnetism influences the behaviors significantly. I provide an overview of a density-functional-based method (NRLMOL) for determining resonant tunneling of magnetization and Berry's phase oscillations in molecular magnets (primarily Mn$_{12}$-Acetate and derivatives) [2] and spin-electric effects in frustrated spin systems [Na$_{12}$Cu$_3$(AsW$_9$O$_{33}$)$_2\cdot$3H$_2$0] [3]. The complexities related to spin- and magnetically dependent transport are compared to those of a nonmagnetic case [4]. Direct comparisons to experiments will be made. Challenges and recent progress associated with incorporating these effects into a realistic description of the frequency and amplitude dependent field driven response of many-electron/spin nanosystems will be discussed.\\[4pt] [1] MRP and SN Khanna, PRB {\bf 60} 9566 (1999).\\[0pt] [2] AV Postnikov, J. Kortus \& MRP, PSSB {\bf 243} 2533 (2006).\\[0pt] [3] MF Islam, JF Nossa, CM Canali, \& MRP, PRB {\bf 82} 15546 (2010).\\[0pt] [4] N.A. Zimbovskaya, MRP, AS Blum, BR Ratna and R. Allen, JCP {\bf 130} 094702 (2009).   

On the mechanism of enhanced photocatalytic activity of composite TiO2/carbon nanofilms

We fabricated and analyzed well-defined model samples consisting of anatase and graphitic carbon films with and without modifying the interface between them by a thin SiO2 space layer. The study was performed in the search for the origin of the enhanced photocatalytic activity of composite TiO2--carbon systems observed previously by us, but also reported in number of publications. We found that the films with a TiO2/C interface show noticeably lower photoluminescence intensity and shorter carrier life times compared to single TiO2 films with the same thickness and composition. The stronger non-radiative recombination was mainly assigned to charge carrier leakage (transfer) at the interface between TiO2 nanocrystallites and the carbon film.   

Light-sensitive gold nanoparticles designed for solar energy use

Light-sensitive organic ligands are incorporated with gold nanoparticles (AuNPs) to utilize solar energy. The physical properties of the ligand-AuNP systems are mainly modulated by the ligand/AuNP ratio. Hydrodynamic size measurement, laser Doppler electrophoresis and transmission electron microscopy are used for physical characterization. The interconnectivity of the AuNPs by dual-functional ligands also has a great impact on the physical properties. In addition, fs-THz spectroscopy is applied to evaluate electron activation of the designed AuNPs. Electron beams of different energy levels are applied to change the surface energy of AuNPs, which strongly affects the absorption energy band. This study contributes to the basic understanding on the nanoparticle technology for solar energy use.   

Factors controlling thermodynamic properties at the nanoscale: Ab initio study of Pt nanoparticles

We analyze via density-functional-theory calculations how factors such as size, shape, and hydrogen passivation influence the bond lengths, vibrational density of states (VDOS), and thermodynamic quantities of 0.8-1.7 nm diameter Pt nanoparticles (NPs), whose shape was previously characterized via extended X-ray absorption fine structure spectroscopy (EXAFS) [1]. For a given shape, unsupported NPs display increasingly broader bond-length distributions with decreasing size. Since the VDOS is remarkably non-Debye-like (even for the largest NPs), the VDOS and the thermal properties are not correlated as they are in the bulk. Generally, the fundamental vibrational frequency of a NP is associated with the shape and decreases with increasing size, as in macroscopic systems. Not surprisingly, we find that the frequency of this fundamental mode largely characterizes the thermal properties. We demonstrate that the qualitative difference between the atomic mean-square-displacement and the corresponding mean bond-projected bond-length fluctuations should be taken into account when interpreting the Debye-Waller factor of NPs measured by X-ray (or neutron) scattering or EXAFS. We find that in H-passivated Pt NPs, H desorption with increasing temperature explains the appearance of negative thermal expansion.\\[4pt] [1] B. Roldan Cuenya, et al. (2011), preprint available   

Nucleation in the Anatase-to-Rutile Transformation of Nanocrystalline Anatase

We use molecular dynamics (MD) simulations to investigate the anatase-to-rutile transformation of titania nanocrystals in vacuum. By comparing energies of various Wulff-shaped nanocrystals, we find that rutile becomes favored over anatase past a critical size that is significantly smaller than that predicted by thermodynamic models based on surface energies, indicating that edges play a profound role in the energetics of nanocrystals. We develop a local order parameter to distinguish anatase from rutile and intermediate anatase (112) twins at the resolution of a single TiO$_{2}$ unit and we apply it in direct MD simulations of spherical and Wulff-shaped anatase nanocrystals, as well as nanocrystal aggregates. To further characterize the transformation, we simulate X-ray diffraction of the nanoparticles. The anatase-to-rutile transformation originates at surfaces and interfaces, where alternating anatase (112) twin planes form. Rutile nuclei form via transformation of anatase (112) twins and they grow rapidly when they reach a critical size that can be as small as 10 TiO$_{2}$ units. Rutile nucleii tend to have planar structures bounded by (101) surfaces and the ease with which they form is dependent on the structure of the nanocrystal.   

Local Ionic Environment around Polyvalent Nucleic-Acid Functionalized Gold Nanoparticles

Polyvalent oligonucleotide-functionalized gold nanoparticles (DNA-AuNPs) are remarkably stable in a cellular environment against degradation by nucleases, a property that was recently attributed to the local high concentration of mono- and divalent ions (Ref 1). In order to evaluate this hypothesis, we investigated the composition of the ion cloud around spherical nanoparticles that are functionalized by stiff, highly charged polyelectrolyte chains by means of classical density functional theory and molecular dynamics simulations. We developed a cell model that includes ligands explicitly and both applies over the entire relevant parameter space and is in excellent quantitative agreement with simulations (Ref 2). The ion distribution around the DNA-AuNPs as a function of DNA grafting densities and bulk ionic concentrations, as well as different sizes of nanoparticles and chains, is studied. For small particles with high DNA surface densities, we find strongly enhanced local salt concentrations, a pronounced localization of divalent ions near the surface of the nanoparticle, and a large radial component of the electric field between the ligands. Therefore, we conclude that enzyme activity in general may be heavily influenced by the local environment around DNA-AuNPs.   

Mechanical properties of Ge nanowire

Nanowires possess unique properties owing to their low dimensionality and high surface-to-volume ratio. Although numerous calculations exist for the electronic properties of nanowires, the mechanical properties have not been addressed to the same extent. Here, we present real-space pseudopotential calculations for the mechanical properties of Ge nanowires. In particular, we examine three different orientations of Ge nanowires, with the axis along the [111], [110], and [100] directions. We present calculations for the elastic properties as a function of wire diameter. We find that Young's modulus is decreased as the surface-to-volume ratio increases, except for the [110] orientation, which shows the opposite trend. In addition, we will discuss the band structure under strain for each nanowire system.   

Chemisorption on Palladium and Silver Clusters

We continue our interest on the chemisorption of different atomic and molecular species on small clusters of metallic elements, by examining the interactions of H, O and F atoms with Pd$_{n}$ and Ag$_{n}$ clusters (n = 6 thru 12). The hybrid ab initio methods of quantum chemistry (particularly the DFT-B3LYP model) are used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bond-lengths, ionization potentials, electron affinities and HOMO-LUMO gaps for the clusters of the two different metals. Of particular interest are the comparisons of binding strengths at the three important types of sites: edge (E) sites, hollow sites (H) site and on-top (T) sites. Effects of crystal symmetries corresponding to the bulk structures for the two metals will also be investigated. Our theoretical results will be compared with the experimental studies where they are available. Implications for the molecular dissociation of the H$_{2}$ and O$_{2}$ species will be considered.   

Blinking in nanoscale systems: a universal theoretical framework

Fluctuations of fluorescence intensity (blinking) is observed in many different kinds of optically active nanoscale objects. These fluctuations with extremely long-term correlations manifest on timescales longer than seconds and were observed in the emission of colloidal and self-assembled quantum dots, nanorods, nanowires, and some organic dyes. We suggest the idea of a universal physical mechanism underlying the blinking phenomenon. Here we show that the features of this universal mechanism can be captured phenomenologically by the multiple recombination center model (MRC) we proposed in a recent work to explaining single colloidal QD intermittency. Within the framework of the MRC model we qualitatively explain all the important features of fluorescence intensity fluctuations for a broad spectrum of nanoscale emitters.