All entries for william tisdale

William A. Tisdale

Title: Charles & Hilda Roddey Career Development Assistant Professor
Department: Department of Chemical Engineering

Collaborators

V. Bulović, MIT EECS
J. Grossman, MIT DMSE
J. Kong, MIT EECS
S. Coe-Sullivan, QD Vision, Inc.
J. Steckel, QD Vision, Inc.

Postdoctoral Associates

F. Prins
Graduate Students
R. Hoffman, ChemE
M. Weidman, ChemE
A.J. Mork, Physical Chemistry
E.M.Y. Lee, ChemE (co-supervised with Prof. J. Grossman)
A. Goodman, Physical Chemistry

Graduate students

L. Poulikakos, MechE (ETH Zurich)

Support Staff

B. Balkwill

Publications

Y. Shirasaki, G.J. Supran, W.A. Tisdale, V. Bulović, “Origin of Efficiency Roll-Off in Colloidal Quantum-Dot Light-Emitting Diodes,” Phys. Rev. Lett. (2013).

G. M. Akselrod, B. J. Walker, W. A. Tisdale, M. G. Bawendi and V. Bulović, “Twenty-fold enhancement of molecular fluorescence by coupling to a J-aggregate critically coupled resonator,” ACS Nano, vol. 6, pp. 467-471 (2012).

W. A. Tisdale and X.-Y. Zhu, “Artificial atoms on semiconductor surfaces,” Proc. Natl. Acad. Sci. USA., vol. 108, pp. 965-970 (2011).

W. A. Tisdale, K. J. Williams, B. A. Timp, D. J. Norris, E. S. Aydil and X.-Y. Zhu, “Hot-Electron Transfer from Semiconductor Nanocrystals,” Science, vol. 328, pp. 1543-1547 (2010).

K. J. Williams, W. A. Tisdale, K. S. Leschkies, G. Haugstad, D. J. Norris, E. S. Aydil and X.-Y. Zhu, “Strong electronic coupling in two-dimensional assemblies of colloidal PbSe quantum dots,” ACS Nano, vol. 3, pp. 1532-1538 (2009).

M. Muntwiler, Q. Yang, W. A. Tisdale and X.-Y. Zhu, “Coulomb barrier for charge separation at an organic semiconductor interface,”Phys. Rev. Lett., vol. 101, pp. 196403 (2008).

W. A. Tisdale, M. Muntwiler, D. J. Norris, E. S. Aydil and X.-Y. Zhu, “Electron dynamics at the ZnO (10-10) surface,” J. Phys. Chem. C, vol. 112, pp. 14682-14692 (2008).

Spatially Resolved Exciton Dynamics in Quantum Dot Thin Films

Authors: F. Prins, A. J. Mork, W. A. Tisdale
Sponsorship: EFRC Center for Excitonics

The diffusion of excitonic energy from within the photosensitive layer of a photovoltaic cell towards the charge separation interface is a crucial step in solar energy generation^[1]. Depending on the material-type and its morphology, the length scale at which exciton diffusion occurs in nanostructured materials ranges from a few 10’s up to 100 nm^[2]. To date, exciton diffusion in nanostructured materials has been studied using time-resolved spectroscopy. However, since these measurements are done on bulk volumes of the nanosctrutured materials, they are unable to resolve the nanoscopic details of the influence of the sample morphology on the energy transfer.

Currently, we are developing near-field optical techniques to study the flow of excitonic energy in colloidal quantum-dot thin films with nanometer-scale spatial resolution (see figure 1 for a schematic). While the resolution of conventional optical microscopy is fundamentally limited by the diffraction limit of light, near-field microscopy makes use of highly localized fields at the apex of a metal-coated atomic force microscopy (AFM-) tip. Such tip-enhanced microscopy techniques can reach optical resolutions as high as 15 nm, enough to resolve individual nano-objects such as colloidal quantum dots^[3]. In addition to this unprecedented optical resolution, the AFM itself provides information about the morphology of the sample. The simultaneous acquisition of all these parameters allows us to correlate the dynamics of exciton diffusion with the local ordering and possible defects in the sample, information that is crucial to understand the limiting factors the diffusion length.

Figure 1: Artist’s impression of a tip-enhanced microscopy setup.

R. Lunt et al., “Practical roadmap and limits to nanostructured photovoltaics,” Advanced Materials, vol. 23, pp. 5712-5727, 2011. [↩]
B. Kippelen and J. L. Bredas, “Organic Photovoltaics,” Energy and Environmental Science, vol. 2, pp. 251-261, 2009. [↩]
A. Hartschuh, “Tip-Enhanced Near-Field Optical Microscopy,” Angewandte Chemie, vol. 47, pp. 8178-8191, 2008. [↩]

Heat Transport in Nanostructured Materials

Authors: A. J. Mork, F. Prins, W. A. Tisdale
Sponsorship: Eni S.p.A., MIT School of Engineering Charles E. Reed Faculty Initiative Fund, NSF GRFP

Figure 1: Time-resolved anti-Stokes Raman imaging schematic. A) A high-energy pulsed laser excites and locally heats the sample with a Gaussian excitation profile. B) After a variable delay time Δt, the Raman probe laser measures the anti-Stokes scattered signal at a known distance from the excitation source. By varying the position and time between the laser pulses, we will map the thermal properties of our samples.

Thermal management in nanoscale materials dramatically affects the efficiency of novel energy generation technologies, and engineering devices with appropriate heat transport properties is requisite for their success. We are using Raman spectroscopy to measure and visualize heat transport in nanostructured materials, such as organic/polymeric or quantum dot thin films, which may be used for photovoltaic or thermoelectric devices. Raman spectroscopy measures the inelastic scatting of light off a sample, and the resulting spectrum contains a chemical fingerprint of the molecules involved in heat transport as well as a measure of local temperature via vibrational excitations. In particular, through pulsed local heating of a sample and measuring the temperature of a nearby point a known distance and time away with Raman spectroscopy, we can visualize thermal diffusion and unveil the vibrational pathways involved^[1]^[2].

Our unique setup allows for spatial, temporal, and spectral resolution of our Raman signal, which allows us to extract data related to the mechanism and efficiency of heat diffusion on the length scales over which it occurs. We have built a Time-resolved anti-Stokes Raman imaging microscope (see Figure 1), allowing us to collect point-by-point vibrational occupation information through laser scanning of our Raman probe beam relative to a heat pump. With this setup, we will be able to change the composition or architecture of our samples in known ways and measure the resulting changes in heat transport efficiency. In this way, our studies will inform engineering of higher quality devices by improving thermal management within the nanostructured films.

T. Beechem, J. Serrano. “Raman Thermometry of Microdevices: Choosing a Method to Minimize Error,” Spectroscopy, vol. 26, no. 11, pp 36-44, November 2011. [↩]
T. Beechem, S. Graham. “Temperature Measurement of Microdevices using Thermoreflectance and Raman Thermometry,” in BioNanoFluidic MEMS edited by P.J. Hesketh. Springer, 2008. [↩]

Effect of Morphology in Quantum Dot Electronics

Authors: M. C. Weidman, F. Prins, W. A. Tisdale
Sponsorship: MIT Department of Chemical Engineering, NSF GRFP

Quantum dots (QDs) are exciting materials because their bandgap is dependent on the dot size. The use of QDs allows for precise control over the wavelengths of light that are emitted or absorbed in applications such as light-emitting diodes or photovoltaic devices. Producing efficient devices made from QD films requires controlling the rates of charge transport and exciton diffusion among the QDs. Charge transport occurs via tunneling between neighboring QDs, so the interparticle spacing is a critical parameter^[1] while the rate of exciton diffusion is a function of the center-to-center distance between QDs. Therefore the morphology of a QD film strongly impacts its electronic properties^[2]. Whether the goal is to increase or decrease charge transport/exciton diffusion, it is important to be able to quantify the interparticle spacing and center-to-center distances. Figure 1a shows a transmission electron microscopy (TEM) image of lead sulfide QDs. Automated analysis of TEM images allows for the determination of the mean diameter and standard deviation, which in this case is 6.3 ± 0.4 nm. The top inset is an ideal hexagonally close packed lattice, where the different colors indicate shells of dots at the same distance from the reference dot (shown in black). The distance between the centers of every dot is calculated to give the center-to-center distance distribution in Figure 1b. The colored vertical lines in Figure 1b correspond to the shells of the same color in the ideal hexagonally close packed lattice. The first peak at 9.6 ± 0.5 nm represents the average center-to-center distance of the first neighbors in the QD film. Thus the distance between neighboring dots is calculated to be 3.3 ± 0.6 nm. Using these techniques, it is possible to better understand QD electrical properties by correlating them with film morphology.

Figure 1: a) TEM image of highly monodisperse lead sulfide QDs capped with oleic acid ligands. The dots have a mean diameter of 6.3 nm (σ=6.0%) and show self-assembly into a hexagonal closely packed lattice system. An ideal hexagonally close packed lattice is shown in the top inset, with color used to indicate different shells of dots. The bottom inset is high-resolution TEM on a single QD to highlight its crystallinity. b) Center-to-center distance distribution of the main TEM image in Figure 1a, confirming the high degree of ordering in the film. The first major peak indicates an average center-to-center distance between nearest neighbors of 9.6 ± 0.5 nm.

Y. Liu, M. Gibbs, J. Puthussery, S. Gaik, R. Ihly, H. W. Hillhouse, and M. Law, “Dependence of carrier mobility on nanocrystal size and ligand length in PbSe nanocrystal solids,” Nano Lett., vol. 10, pp. 1960-9, May 12 2010. [↩]
D. V. Talapin, J.-S. Lee, M. V. Kovalenko, and E. V. Shevchenko, “Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications,” Chemical Reviews, vol. 110, pp. 389-458, 2010. [↩]

MTL Annual Research Report 2013