Jonathan G. C. Veinot

Chemical Insight into the Origin of Red and Blue Photoluminescence Arising from Freestanding Silicon Nanocrystals

Silicon nanocrystals (Si NCs) are attractive functional materials. They are compatible with standard electronics and communications platforms and are biocompatible. Numerous methods have been developed to realize size-controlled Si NC synthesis. While these procedures produce Si NCs that appear identical, their optical responses can differ dramatically. Si NCs prepared using high-temperature methods routinely exhibit photoluminescence agreeing with the effective mass approximation (EMA), while those prepared via solution methods exhibit blue emission that is somewhat independent of particle size. Despite many proposals, a definitive explanation for this difference has been elusive for no less than a decade. This apparent dichotomy brings into question our understanding of Si NC properties and potentially limits the scope of their application. The present contribution takes a substantial step forward toward identifying the origin of the blue emission that is not expected based upon EMA predictions. It describes a detailed comparison of Si NCs obtained from three of the most widely cited procedures as well as the conversion of red-emitting Si NCs to blue emitters upon exposure to nitrogen-containing reagents. Analysis of the evidence is consistent with the hypothesis that the presence of trace nitrogen and oxygen even at the parts per million level in Si NCs gives rise to the blue emission.

Size vs Surface: Tuning the Photoluminescence of Freestanding Silicon Nanocrystals Across the Visible Spectrum via Surface Groups

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3-4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies.

Widespread nanoparticle-assay interference: implications for nanotoxicity testing.

The evaluation of engineered nanomaterial safety has been hindered by conflicting reports demonstrating differential degrees of toxicity with the same nanoparticles. The unique properties of these materials increase the likelihood that they will interfere with analytical techniques, which may contribute to this phenomenon. We tested the potential for: 1) nanoparticle intrinsic fluorescence/absorbance, 2) interactions between nanoparticles and assay components, and 3) the effects of adding both nanoparticles and analytes to an assay, to interfere with the accurate assessment of toxicity. Silicon, cadmium selenide, titanium dioxide, and helical rosette nanotubes each affected at least one of the six assays tested, resulting in either substantial over- or under-estimations of toxicity. Simulation of realistic assay conditions revealed that interference could not be predicted solely by interactions between nanoparticles and assay components. Moreover, the nature and degree of interference cannot be predicted solely based on our current understanding of nanomaterial behaviour. A literature survey indicated that ca. 95% of papers from 2010 using biochemical techniques to assess nanotoxicity did not account for potential interference of nanoparticles, and this number had not substantially improved in 2012. We provide guidance on avoiding and/or controlling for such interference to improve the accuracy of nanotoxicity assessments.

Cytotoxicity of surface-functionalized silicon and germanium nanoparticles: the dominant role of surface charges.

Although it is frequently hypothesized that surface (like surface charge) and physical characteristics (like particle size) play important roles in cellular interactions of nanoparticles (NPs), a systematic study probing this issue is missing. Hence, a comparative cytotoxicity study, quantifying nine different cellular endpoints, was performed with a broad series of monodisperse, well characterized silicon (Si) and germanium (Ge) NPs with various surface functionalizations. Human colonic adenocarcinoma Caco-2 and rat alveolar macrophage NR8383 cells were used to clarify the toxicity of this series of NPs. The surface coatings on the NPs appeared to dominate the cytotoxicity: the cationic NPs exhibited cytotoxicity, whereas the carboxylic acid-terminated and hydrophilic PEG- or dextran-terminated NPs did not. Within the cationic Si NPs, smaller Si NPs were more toxic than bigger ones. Manganese-doped (1% Mn) Si NPs did not show any added toxicity, which favors their further development for bioimaging. Iron-doped (1% Fe) Si NPs showed some added toxicity, which may be due to the leaching of Fe(3+) ions from the core. A silica coating seemed to impart toxicity, in line with the reported toxicity of silica. Intracellular mitochondria seem to be the target for the toxic NPs since a dose-, surface charge- and size-dependent imbalance of the mitochondrial membrane potential was observed. Such an imbalance led to a series of other cellular events for cationic NPs, like decreased mitochondrial membrane potential (ΔΨm) and ATP production, induction of ROS generation, increased cytoplasmic Ca(2+) content, production of TNF-α and enhanced caspase-3 activity. Taken together, the results explain the toxicity of Si NPs/Ge NPs largely by their surface characteristics, provide insight into the mode of action underlying the observed cytotoxicity, and give directions on synthesizing biocompatible Si and Ge NPs, as this is crucial for bioimaging and other applications in for example the field of medicine.

An investigation into near-UV hydrosilylation of freestanding silicon nanocrystals.

We present a study of the photochemical hydrosilylation of freestanding silicon nanocrystals (Si-NCs) using a near-UV source. The impact of reaction with alkenes and alkynes was studied using in situ photoluminescence (PL) spectroscopy, allowing measurement of both changes in intensity and PL maxima during the reaction. Understanding this behavior is important for the utilization of these materials in a number of applications where hydrosilylation is a leading method to functionalize Si-NCs. Changes in the PL were studied and shown arise from the influence of oxidation as well as the Si-C bond formation. Hydrosilylation with a range of conjugated alkynyl species was studied to understand how the introduction of these species to the NC surface can quench the PL from Si-NCs. These results were explained in context of the free-radical and exciton-mediated mechanisms for photochemical hydrosilylation proposed for Si-NCs. Materials in this study were characterized by Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), selected electron area diffraction (SAED), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA) and dynamic light scattering (DLS).

Silicon Nanocrystals and Silicon-Polymer Hybrids: Synthesis, Surface Engineering, and Applications

Silicon nanocrystals (Si-NCs) are emerging as an attractive class of quantum dots owing to the natural abundance of silicon in the Earths crust, their low toxicity compared to many Group II-VI and III-V based quantum dots, compatibility with the existing semiconductor industry infrastructure, and their unique optoelectronic properties. Despite these favorable qualities, Si-NCs have not received the same attention as Group II-VI and III-V quantum dots, because of their lower emission quantum yields, difficulties associated with synthesizing monodisperse particles, and oxidative instability. Recent advancements indicate the surface chemistry of Si-NCs plays a key role in determining many of their properties. This Review summarizes new reports related to engineering Si-NC surfaces, synthesis of Si-NC/polymer hybrids, and their applications in sensing, diodes, catalysis, and batteries.

Iron/iron oxide nanoparticles: a versatile support for catalytic metals and their application in Suzuki–Miyaura cross-coupling reactions

Iron/iron oxide core-shell nanoparticles (Fe@Fe(x)O(y)) are a versatile support for immobilizing catalytic metals. Fe@Fe(x)O(y) decorated with Pd (Fe@Fe(x)O(y)/Pd) exhibits high catalytic activity toward Suzuki-Miyaura cross-coupling reactions in aqueous solution at room temperature in air.

Influence of surface functionalization and particle size on the aggregation kinetics of engineered nanoparticles

In an effort to minimize the impact on the environment or improve the properties of choice, most engineered nanoparticles used for commercial applications are surface functionalized. The release of these functionalized engineered nanoparticles (FENPs) into the environment can be either deliberate or accidental. Scientific research to date has tended to focus on evaluating the toxicity of FENPs, with less attention being given to exposure assessments or to the study of their general behavior in natural environments. We have therefore investigated the effects of environmental parameters such as pH, NaCl concentration, and natural organic matter concentration on the aggregation kinetics of FENPs with time resolved dynamic light scattering, using functionalized gold nanoparticles (FAuNPs) as a representative of these particles. We also investigated the effects of average particle size, the type of surface capping agent, and particle concentration on FAuNP aggregation kinetics. Our results show that the physico-chemical properties of the capping agent have a greater influence on the aggregation behavior of FAuNPs than either their core composition or their particle size.

Synthesis and Photoluminescent Properties of Size-Controlled Germanium Nanocrystals from Phenyl Trichlorogermane-Derived Polymers

We report the preparation of luminescent oxide-embedded germanium nanocrystals (Ge-NC/GeO2) by the reductive thermal processing of polymers derived from phenyl trichlorogermane (PTG, C6H5GeCl3). Sol-gel processing of PTG yields air-stable polymers with a Ge:O ratio of 1:1.5, (C6H5GeO1.5)n, that thermally decompose to yield a germanium rich oxide (GRO) network. Thermal disproportionation of the GRO results in nucleation and initial growth of oxide-embedded Ge-NC, and subsequent reaction of the GeO2 matrix with the reducing atmosphere results in additional nanocrystal growth. This synthetic method affords quantitative yields of composite powders in large quantities and allows for Ge-NC size control through variations of the peak thermal processing temperature and reaction time. Freestanding germanium nanocrystals (FS-Ge-NC) are readily liberated from Ge-NC/GeO2 composite powders by straightfoward dissolution of the oxide matrix in warm water. Composites and FS-Ge-NC were characterized using thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), and photoluminescence (PL) spectroscopy.

Progress in high work function TCO OLED anode alternatives and OLED nanopixelation

As organic light-emitting diodes (OLEDs) increase in sophistication and our understanding of building block-structure-luminous response mechanism increases, the remarkable properties of these heterostructures raise intriguing possibilities for future optoelectronics. In this contribution, we address two complimentary areas of interest in OLED science and engineering: (i) development and application of new transparent conducting oxide (TCO) materials for OLED anodes; (ii) effective OLED patterning strategies for nanofabrication.