Research area: Single nanostructure optical microscopy
SINGLE NANOWIRE MICROSCOPY
Single particle emission microscopy represents a powerful way to unravel the optical and electrical properties of nanostructures. By going beyond the ensemble average, detailed measurements of single nanostructure physics are possible. This has led to quantitative estimates of fundamental nanowire properties such as their absorption cross sections, emission quantum yields, emission/excitation polarization anisotropies and even dielectric sensitivities. In parallel, using concerted transient absorption and time correlated single photon counting experiments we have begun to clarify the fundamental nature of carriers within nanowires, namely, whether they exist as free electrons/holes or as tightly bound excitons.
The true power of single particle emission techniques, however, lies in the possibility of observing unexpected phenomena. In the case of nanowires, this has led to the discovery of nanowire emission flickering, unusual power law kinetics, spectral shifting and even emission steering using external electric fields. We are therefore pursing detailed single nanowire studies to explain in a comprehensive fashion the origin of these responses. This is important since such behavior occurs in other systems and suggests that a universal explanation for unexpected physics at the single particle level exists.
SINGLE NANOWIRE ABSORPTION MICROSCOPY
A new area of investigation we are currently pursuing involves measuring the absorption spectrum of individual nanowires. This entails conducting sensitive direct absorption experiments through the spatial modulation of a NW’s position. The primary motivation for this research is that virtually all single molecule and single particle spectroscopies today are emission measurements. While these are convenient zero background techniques, the approach limits the field since only emissive species can be studied. Furthermore, single particle emission experiments are plagued by photobleaching effects, which limit the duration one can experimentally observe individual fluorophores. Our work therefore seeks to circumvent these issues by developing general, single particle, absorption spectroscopies that will open the door to new information about the individuality of single molecules, nanoparticles and other nanostructures. These studies will also reveal details about electronic interactions with the local chemical environment, leading to spectral variations as well as changes in relevant energy- and electron-transfer rates.
The following images show the first direct extinction spectra of single one-dimensional (1D) nanostructures obtained at room temperature using a spatial modulation approach. For these systems, ensemble averaging in conventional absorption spectroscopy has limited our understanding about the interplay between carrier confinement and electrostatic interactions with the local environment. By probing individual CdSe NWs, we have identified and assigned size-dependent transitions occurring across the visible. In turn, we have revealed the existence of room temperature 1D excitons in the narrowest NWs and have also observed the delicate balance between spatial confinement and dielectric contrast/confinement in these materials.
Future studies are aimed at probing the size-dependent absorption spectra of other nanostructures. Of particular interest are CdSe nanorods since these materials lie between the limiting cases of a (0D) quantum dot and a (1D) nanowire. The importance of this study is that within nanostructures a competition exists between quantum confinement and dielectric contrast/confinement. One causes electron/hole kinetic energies to increase while the other causes them to decrease due to enhancements of the electron-hole Coulomb interaction. We therefore seek to map out for the first time this competition between confinement and dielectric contrast/confinement as one transitions dimensionality from 1D to 0D.
SINGLE NANOSHEET MICROSCOPY
Our recent work has focused on examining the optical properties of individual 2D nanomaterials. Representative systems include graphene oxide, reduced graphene oxide and TiS2 nanosheets. Motivating these studies are thickness-dependent changes to 2D optical and electrical properties when materials are made single layer or few layer. In addition, intrasheet optical/electrical heterogeneities exist in chemically-derived 2D materials. As an example, the strong thickness dependent band gap of TiS2 means that any thickness variation within an ensemble will result in distributions of apparent optical gaps as well as excited state progressions. Furthermore, because of intrasheet chemical or thickness variations local structure will emerge in a given sheet’s optical response. We are therefore pursuing single sheet, spatially resolved absorption and emission spectroscopies to unravel these thickness and chemical environment dependencies.