Toward a Mechanistic Understanding of Exciton-Mediated Hydrosilylation on Nanocrystalline Silicon

White-light initiated hydrosilylation of nanocrystalline porous silicon was found to be far more efficient (in terms of both kinetics and yield) in the presence of electron-accepting molecules with suitably high reduction potentials, particularly halocarbons. It is known that absorption of visible light by nanocrystalline silicon results in the formation of excitons (electron/hole pairs) and that this exciton can be harnessed to drive a hydrosilylation reaction with an alkene; the Si–C bond forms as a result of attack of the π-electrons of the alkene on the positively charged holes. In order to better understand the white-light initiated mechanism through which this reaction takes place, and to compare with UV-mediated photoemission on Si(111)–H, a series of electron acceptors were screened for their effect on surface alkene hydrosilylation. A very strong correlation between reduction potentials (<i>E</i>_red) of the oxidant and reaction efficiency was observed, with a minimum “turn-on” <i>E</i>_red required for an increase to take place. The oxidant appears to accept, or remove, the electron from the nanocrystallite-bound exciton, favoring attack by the alkene on the positively charged Si nanocrystallite, leading to Si–C bond formation. Radical reactions were discounted for a number of reasons, including lack of effect of radical traps, no apparent Si–Cl bond formation, lack of oxidation of the surfaces, and others. Unlike with other oxidants such as nitro-aromatics, halocarbons do not cause additional surface reactions and promote very clean, fast, and selective hydrosilylation chemistry

Elucidating the Surface Chemistry of Zinc Phosphide Nanoparticles Through Ligand Exchange

Zn₃P₂ nanoparticles, a potential earth abundant nanomaterial for photovoltaic applications, are prepared via a solution-based synthesis and end up capped with weakly bound phosphine ligands. These ligands are easily displaced from the nanoparticle surface, leading to an irreversible aggregation of particles. In this work, we elaborate the chemistry of Zn₃P₂ nanoparticles both to elucidate the surface functionalities present after synthesis, and to enable the production of stable solutions of Zn₃P₂ colloidal solutions. Three different types of anionic type ligands, formed from their neutral precursors of oleic acid, <i>n</i>-decylphosphonic acid, and 1-octadecanethiol, were shown to be effective in yielding soluble functionalized nanoparticles. The functionalized Zn₃P₂ nanoparticles were thoroughly characterized by electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction analyses, and FTIR spectroscopy. A combination of FTIR and multinuclear solution NMR spectroscopic studies on the starting agglomerated Zn₃P₂ nanoparticles and the functionalized particle solutions reveals that the particle surface is terminated by Zn–CH₃ and −PH_<i>x</i>­(SiMe₃)_3–<i>x</i> groups. Using oleic acid as the workhorse ligand, it was shown that addition of oleic acid to agglomerated nanoparticles led to a homogeneous dispersion of Zn₃P₂ nanoparticles, in toluene, along with production of CH₄ and C₁₇H₃₃­COO­SiMe₃ as byproducts, as determined by ¹H and ¹³C NMR spectroscopy. FTIR spectroscopy of the ligand-exchanged particles indicated oleate coordination, along with the appearance of what has been assigned as a P–H stretch. Similar reaction chemistry was observed during ligand exchange with <i>n</i>-decylphosphonic acid and 1-octadecanethiol. On the basis of these data, a general mechanism for ligand exchange chemistry on the Zn₃P₂ nanoparticle surface was proposed to enable both the production of zinc phosphide nanoparticle solutions and the determination of various routes to surface functionalization of this material

UV-Initiated Hydrosilylation on Hydrogen-Terminated Silicon (111): Rate Coefficient Increase of Two Orders of Magnitude in the Presence of Aromatic Electron Acceptors

UV-initiated (254 nm) hydrosilylation of hexadecene on Si(111)-H has been studied in the presence of various aliphatic and aromatic molecules (additives). Many of these additives cause an enhancement in the pseudo-first-order rate coefficient (<i>k</i>_obs) of hydrosilylation, some up to 200× faster than observed in neat hexadecene. It is proposed that these additives capture the photoejected electron from the surface, thereby increasing the probability of reaction of the alkene with the surface hole (h⁺), leading to Si–C bond formation. While the ability of these additives to increase <i>k</i>_obs is related to their reduction potential, aromatic additives are particularly efficient; we suspect this is due to the relatively strong physisorption of the aromatic molecules leading to a favorable geometry for electron transfer. The presence of these additives permits the use of a much lower intensity of UV light (∼30 μW/cm²), reducing the probability of photodegradation of the monolayer, and maximum coverage can be reached within minutes

Pixels of a typical junction and three associated terminal points showing the counting of adjacent skeleton pixels.

<p>Highlighted are pixels representing (i) terminal points at the end of the line or branch, each adjacent to only 1 pixel, (ii) contiguous points along the line, each with 2 neighbour pixels, and (iii) junction points where three or more branches meet, having 3 or more neighbour pixels. Similar to minesweeper games, the number of adjacent pixels determines the value of each skeleton pixel.</p

Automated Defect and Correlation Length Analysis of Block Copolymer Thin Film Nanopatterns

<div><p>Line patterns produced by lamellae- and cylinder-forming block copolymer (BCP) thin films are of widespread interest for their potential to enable nanoscale patterning over large areas. In order for such patterning methods to effectively integrate with current technologies, the resulting patterns need to have low defect densities, and be produced in a short timescale. To understand whether a given polymer or annealing method might potentially meet such challenges, it is necessary to examine the evolution of defects. Unfortunately, few tools are readily available to researchers, particularly those engaged in the synthesis and design of new polymeric systems with the potential for patterning, to measure defects in such line patterns. To this end, we present an image analysis tool, which we have developed and made available, to measure the characteristics of such patterns in an automated fashion. Additionally we apply the tool to six cylinder-forming polystyrene-<i>block</i>-poly(2-vinylpyridine) polymers thermally annealed to explore the relationship between the size of each polymer and measured characteristics including line period, line-width, defect density, line-edge roughness (LER), line-width roughness (LWR), and correlation length. Finally, we explore the line-edge roughness, line-width roughness, defect density, and correlation length as a function of the image area sampled to determine each in a more rigorous fashion.</p></div

Table of topological defect components typically found in BCP thin film nanopatterns.

<p>Shown are each major type of component defect, as exists in either the positive (e.g. P2VP) phase or the negative (e.g. PS) phase. For each, 3-branch junctions, terminal points, and dots, examples are given with defects highlighted by a magenta dot. This analysis is done relative to an ideal striped pattern without any interrupting features, save for the edge of the image.</p

Correlation lengths and orientation maps for six SEM images of metallized PS-<i>b</i>-P2VP (50k-<i>b</i>-16.5k, 44k-<i>b</i>-18.5k, and 32.5k-<i>b</i>-12k) patterns with different degrees of thermal annealing.

<p>SEMs are shown in false colour to display the angle of each wire as used in the calculation of the correlation functions, shown right. The raw correlation data is shown in red, the smoothed data is blue, and the calculated correlation length (κ) is marked with a green line and noted on each plot. Beside each image is a blue circle whose radius is equal to the correlation length, as the correlation length is often given as a measure of average grain size. Each image is shown cropped here to ~2 μm wide. The scale bar is 1 μm. (See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0133088#pone.0133088.s006" target="_blank">S5 Fig</a> for full images). The labels (A-F) correspond to the same labelled images in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0133088#pone.0133088.g014" target="_blank">Fig 14</a>.</p

Process for determining line width and period directly from binary patterns.

<p>(A) Unmodified binary image of platinized PS(50k)-<i>b</i>-P2VP(16.5k) and (B) simplified binary image; (C) fit of particle area as a function of perimeter for the unmodified image and (D) fit for the simplified image. (E) Demonstration that a fit of 18 nm for line-width is reasonable for the filtered greyscale image, (F) the thresholded binary image, and (G) a profile of the filtered image. (H) Line diagram showing the relationship between particle area, perimeter, and length.</p

Diagrams depicting measurement of line-edge roughness and line-width roughness.

<p>(A) Sketch to conceptually demonstrate line edge roughness, where the variation in edge position of the line (shown in rose with black edge) varies with respect to the ideal (shown overlaid in blue) or, in this case, the average edge position. Each individual displacement is measured with respect to the average, and the LER calculated as 3 times the standard deviation. (B) Sketch of line-width roughness, which is the variation in line-width. The sketch is adapted from the bulges and pinches shown in the SEM image below. (C) SEM image of block copolymer templated Pt nanowires on a Si wafer, using PS(44k)-<i>b</i>-P2VP(18.5k), annealed at 200°C for 20 minutes.</p

Solution-Processed Zinc Phosphide (α-Zn₃P₂) Colloidal Semiconducting Nanocrystals for Thin Film Photovoltaic Applications

Zinc phosphide (Zn₃P₂) is a promising earth-abundant material for thin film photovoltaic applications, due to strong optical absorption and near ideal band gap. In this work, crystalline zinc phosphide nanoparticles are synthesized using dimethylzinc and tri-<i>n</i>-octylphosphine as precursors. Transmission electron microscopy and X-ray diffraction data show that these nanoparticles have an average diameter of ∼8 nm and adopt the crystalline structure of tetragonal α-Zn₃P₂. The optical band gap is found to increase by 0.5 eV relative to bulk Zn₃P₂, while there is an asymmetric shift in the conduction and valence band levels. Utilizing layer-by-layer deposition of Zn₃P₂ nanoparticle films, heterojunction devices consisting of ITO/ZnO/Zn₃P₂/MoO₃/Ag are fabricated and tested for photovoltaic performance. The devices are found to exhibit excellent rectification behavior (rectification ratio of 600) and strong photosensitivity (on/off ratio of ∼10²). X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy analyses reveal the presence of a thin 1.5 nm phosphorus shell passivating the surface of the Zn₃P₂ nanoparticles. This shell is believed to form during the nanoparticle synthesis