University of Notre Dame

The research we conduct at Notre Dame has focused on imaging individual semiconductor nanostructures such as colloidal quantum dots and solution-based semiconductor nanowires. These are nanostructures that we chemically synthesize and which have dimensions on the order of 5 nm. Typical emission quantum yields of the former are ~30% while those of the latter are on the order of 0.1%. Transmission electron micrographs of both species are shown in the accompanying Figure 1.

One of our current research projects deals with better understanding the cause of a phenomenon referred to as fluorescence intermittency or “blinking” in colloidal quantum dots. This is a process whereby under continuous excitation, individual fluorophores undergo intermittent “on”/”off” emission cycling. The phenomenon has been observed in abovementioned colloidal quantum dots as well as in systems as diverse as single molecules, fluorescent proteins, light harvesting complexes, porous silicon and even in nanowires. We are therefore conducting single quantum dot imaging experiments to study their long timescale (ms-to-minute) blinking dynamics when exposed to different chemical environments and when influenced by external perturbations such as an electric field.

Another active research area has entailed better understanding the optical properties of individual semiconductor nanowires. Specifically we have been interested in understanding the origin of their strong absorption and emission polarization sensitivities. As a consequence, we have carried out a number of polarization anisotropy experiments wherein we have excited individual wires with linearly polarized light and have looked at their emission. An example of such an experiment can be found in the accompanying Figure where it is apparent that, for the tripod nanowire shown, different arms become bright when the incident light polarization vector becomes collinear with their growth axis. From an examination of this data, excitation polarization anisotropies on the order of r=0.9 can subsequently be extracted, showing that the wires exhibit sizable polarization sensitivities.

In tandem, we have found that both the emission intensity as well as spatial position of the nanowire emission can be modulated using external electric fields. This is somewhat surprising but can be seen in the accompanying Figure where Panel a shows the emission image of a single tripod nanowire under zero field conditions. Panels b and c, however, show the emission from the same wire when under the influence of an external bias. It can be seen that the emission localizes on the side of the wire closest to the positive electrode. Furthermore, there is an apparent doubling of its intensity in this region with a corresponding quenching of near identical magnitude at the opposite end of the wire. Reversing the polarity of the external bias in Panel c causes an identical localization/enhancement, however, now at the end closest to the new positive electrode. We have investigated the origin of this behavior and have attributed it to the presence of mobile carriers on or within the nanowire, which can respond to an external field. These free charges then appear to heavily influence the emission quantum yield of the wires. Given the potential usefulness of this phenomenon from an applications standpoint, we are carrying out additional experiments to deliberately add charges to the wires in order to investigate the effect this has on their emission properties.

All of the above (emission) images were acquired using a homebuilt single molecule imaging system built around a Nikon TE2000 inverted microscope. For exciting samples, we have used various diode lasers as well as the grating dispersed output of a supercontinuum white light source. For detecting any emitted light, we have used avalanche photodiodes in conjunction with a DVC1412 CCD camera, attached either directly to the microscope or onto the exit port of an imaging spectrometer. Frames or multi-frame tiff movies acquired were subsequently analyzed using Image J.

We have been very impressed with the performance of the DVC camera. In fact, despite having an EMCCD camera from another vendor, the DVC1412 is our default workhorse camera. It has proven to be robust and is virtually turnkey. In this regard, the operation of the camera is simple and this, in turn, allows it to be easily implemented into customized experiments. Furthermore, we have been very pleased with the technical support we have received on the software side of things. My overall experience with the DVC1412 can therefore be summarized as: fantastic hardware, robust, easy to use and with a knowledgeable technical support staff to back it up.

Sincerely,

Masaru K. Kuno
Associate Professor
Dept. of Chemistry and Biochemistry
University of Notre Dame

Figure 1. High resolution TEM image of two CdSe quantum dots (left) and of a single tripod-shaped CdSe nanowire (right).

Figure 2. Excitation polarization anisotropy of an individual tripod-shaped CdSe nanowire illustrated using still frames at fixed polarization angles relative to the wire. The graph below shows the emission intensity from individual arms when the incident light polarization vector is rotated continously.

Figure 3. (a) Tripod nanowire emission image in the absence of an external electric field. (b) Tripod emission image with an external field applied, resulting in localization of the emission on the side of the wire closest to the positive electrode. (c) Nanowire image with the polarity of the external field reversed.