Matthew P. McDonald

Single nanowire extinction spectroscopy.

Here we show the first direct extinction spectra of single one-dimensional (1D) semiconductor nanostructures obtained at room temperature utilizing a spatial modulation approach. (1) For these materials, ensemble averaging in conventional extinction spectroscopy has limited our understanding of the interplay between carrier confinement and their electrostatic interactions. (2-4) By probing individual CdSe nanowires (NWs), we have identified and assigned size-dependent exciton transitions occurring across the visible. In turn, we have revealed the existence of room temperature 1D excitons in the narrowest NWs.

Nanowire-functionalized cotton textiles.

We show the general functionalization of cotton fabrics using solution-synthesized CdSe and CdTe nanowires (NWs). Conformal coatings onto individual cotton fibers have been achieved through various physical and chemical approaches. Some involve the electrostatic attraction of NWs to cotton charged positively with a Van de Graaff generator or via 2,3-epoxypropyltrimethylammonium chloride treatments. Resulting NW-functionalized textiles consist of dense, conformal coatings and have been characterized for their UV-visible absorption as well as Raman activity. We demonstrate potential uses of these functionalized textiles through two proof-of-concept applications. The first entails barcoding cotton using the unique Raman signature of the NWs. We also demonstrate the surface-enhancement of their Raman signatures using codeposited Au. A second demonstration takes advantage of the photoconductive nature of semiconductor NWs to create cotton-based photodetectors. Apart from these illustrations, NW-functionalized cotton textiles may possess other uses in the realm of medical, anticounterfeiting, and photocatalytic applications.

Direct Measurement of Single CdSe Nanowire Extinction Polarization Anisotropies.

The origin of sizable absorption polarization anisotropies (ρabs) in one-dimensional (1D) semiconductor nanowires (NWs) has been debated. Invoked explanations employ either classical or quantum mechanical origins, where the classical approach suggests dielectric constant mismatches between the NW and its surrounding environment as the predominant source of observed polarization sensitivities. At the same time, the confinement-influenced mixing of states suggests a sizable contribution from polarization-sensitive transition selection rules. Sufficient evidence exists in the literature to support either claim. However, in all cases, these observations stem from excitation polarization anisotropy (ρexc) studies, which only indirectly measure ρabs. In this manuscript, we directly measure the band edge extinction polarization anisotropies (ρext) of individual CdSe NWs using single NW extinction spectroscopy. Observed polarization anisotropies possess distinct spectral features and wavelength dependencies that correlate well with theoretical transition selection rules derived from a six-band k·p theory used to model the electronic structure of CdSe NWs.

Light Induced Nanowire Assembly: The Electrostatic Alignment of Semiconductor Nanowires into Functional Macroscopic Yarns

The electrostatic alignment and directed assembly of semiconductor nanowires into macroscopic, centimeter-long yarns is demonstrated. Different morphologies can be produced, including longitudinally segmented/graded yarns or mixed composition fibers. Nanowire yarns display long range photoconductivities and open up exciting opportunities for potential use in future nanowire-based textiles or in solar photovoltaics.

Supercontinuum spatial modulation spectroscopy: Detection and noise limitations

Supercontinuum spatial modulation spectroscopy is a facile tool for conducting single molecule/particle extinction spectroscopy throughout the visible and near infrared (420-1100 nm). The techniques capabilities are benchmarked using individual Au nanoparticles (NPs) as a standard since they are well studied and display a prominent plasmon resonance in the visible. Extinction spectra of individual Au NPs with diameters (d) ranging from d ~ 8 to 40 nm are resolved with extinction cross sections (σ(ext)) of σ(ext) ~ 1 × 10(-13)-1 × 10(-11) cm(2). Corresponding signal-to-noise ratios range from ~30 to ~1400. The techniques limit of detection is determined to be 4.3 × 10(-14) cm(2) (4.3 nm(2)). To showcase supercontinuum spatial modulation spectroscopys broader applicability, extinction spectra are acquired for other model systems, such as individual single-walled carbon nanotubes (SWCNTs) and CdSe nanowires. We show for the first time extinction spectra of individual (8,3) and (6,5) SWCNTs. For both chiralities, their E11 [(8,3) 1.30 eV (952 nm); (6,5) 1.26 eV (986 nm)] and E22 [(8,3) 1.86 eV (667 nm); (6,5) 2.19 eV (567 nm)] excitonic resonances are seen with corresponding cross sections of σ(ext) ~ 10(-13) cm(2) μm(-1).

Heterogeneous fluorescence intermittency in single layer reduced graphene oxide

We provide, for the first time, direct experimental evidence for heterogeneous blinking in reduced graphene oxide (rGO) during photolysis. The spatially resolved intermittency originates from regions within individual rGO sheets and shows 1/f-like power spectral density. We describe the evolution of rGO blinking using the multiple recombination center (MRC) model that captures common features of nanoscale blinking. Our results illustrate the universal nature of blinking and suggest a common microscopic origin for the effect.

Dimensional crossover in semiconductor nanostructures

Recent advances in semiconductor nanostructure syntheses provide unprecedented control over electronic quantum confinement and have led to extensive investigations of their size- and shape-dependent optical/electrical properties. Notably, spectroscopic measurements show that optical bandgaps of one-dimensional CdSe nanowires are substantially (approximately 100 meV) lower than their zero-dimensional counterparts for equivalent diameters spanning 5–10 nm. But what, exactly, dictates the dimensional crossover of a semiconductors electronic structure? Here we probe the one-dimensional to zero-dimensional transition of CdSe using single nanowire/nanorod absorption spectroscopy. We find that carrier electrostatic interactions play a fundamental role in establishing dimensional crossover. Moreover, the critical length at which this transition occurs is governed by the aspect ratio-dependent interplay between carrier confinement and dielectric contrast/confinement energies.

Spectroscopy and Microscopy of Graphene Oxide and Reduced Graphene Oxide

Graphene oxide (GO) is an important material that provides a scalable approach for obtaining chemically derived graphene. Its optical and electrical properties are largely determined by the presence of oxygen-containing functionalities, which decorate its basal plane. This chemical derivatization results in useful properties such as the existence of a band gap as well as emission spanning both the visible and near infrared. Notably, GO’s optical and electrical properties can be altered through reduction, which proceeds through the removal of these oxygen-containing functional groups. However, widely variable behavior has been observed regarding the evolution of GO’s optical response during reduction. These discrepancies arise from the different reduction methods being used and, in part, from the fact that nearly all prior measurements have been ensemble studies. Consequently, detailed mechanistic studies of GO reduction are needed which can transcend the limitations of ensemble averaging.

Visualizing Single-Cell Secretion Dynamics with Single-Protein Sensitivity

Cellular secretion of proteins into the extracellular environment is an essential mediator of critical biological mechanisms, including cell-to-cell communication, immunological response, targeted delivery, and differentiation. Here, we report a novel methodology that allows for the real-time detection and imaging of single unlabeled proteins that are secreted from individual living cells. This is accomplished via interferometric detection of scattered light (iSCAT) and is demonstrated with Laz388 cells, an Epstein-Barr virus (EBV)-transformed B cell line. We find that single Laz388 cells actively secrete IgG antibodies at a rate of the order of 100 molecules per second. Intriguingly, we also find that other proteins and particles spanning ca. 100 kDa-1 MDa are secreted from the Laz388 cells in tandem with IgG antibody release, likely arising from EBV-related viral proteins. The technique is general and, as we show, can also be applied to studying the lysate of a single cell. Our results establish label-free iSCAT imaging as a powerful tool for studying the real-time exchange between cells and their immediate environment with single-protein sensitivity.

Fluorescence intermittency originates from reclustering in two-dimensional organic semiconductors

Fluorescence intermittency or blinking is observed in nearly all nanoscale fluorophores. It is characterized by universal power-law distributions in on- and off-times as well as 1/f behaviour in corresponding emission power spectral densities. Blinking, previously seen in confined zero- and one-dimensional systems has recently been documented in two-dimensional reduced graphene oxide. Here we show that unexpected blinking during graphene oxide-to-reduced graphene oxide photoreduction is attributed, in large part, to the redistribution of carbon sp2 domains. This reclustering generates fluctuations in the number/size of emissive graphenic nanoclusters wherein multiscale modelling captures essential experimental aspects of reduced graphene oxides absorption/emission trajectories, while simultaneously connecting them to the underlying photochemistry responsible for graphene oxides reduction. These simulations thus establish causality between currently unexplained, long timescale emission intermittency in a quantum mechanical fluorophore and identifiable chemical reactions that ultimately lead to switching between on and off states.