• Ultrafast Fiber Optics

• Spatiotemporal Solitons

• Semiconductor Nanocrystals

Lead-salt Semiconductor Quantum Dots

The electronic structure of quantum dots is analogous to a particle-in-a-box model. In an infinitely large (bulk) semiconductor crystal, electrons that are excited from the valence to conduction band form an electron-hole pair called an exciton. Like a hydrogen atom, this exciton will have a characteristic size called the exciton Bohr radius determined by the effective masses of the bands and the dielectric constant of the crystal. As the size of the crystal decreases below the exciton Bohr radius, the exciton is squeezed, increasing its kinetic energy-making an effective particle-in-a-box. If it is squeezed very tightly, its kinetic confinement energy will dominate the Coulombic energy of the exciton, causing its properties to be almost entirely determined simply by confinement.

fig.1 STEM image of close-packed PbSe quantum dots (right) and close-up of an individual dot with atomic resolution (right).

As the box gets smaller, the energy levels increase in separation. This can be measured experimentally by looking at their optical absorption or emission peaks. An increase in the separation of energy levels is seen as a blue-shift of their spectra. This is only a single example of the effects of quantum confinement. For a more through account, see our list of references below.

fig.2 Absorption (left) and emission (right) spectra of a variety of sizes of PbS QDs. The sizes range from ~3 nm (black) to ~ 7 nm (olive).

Charge transfer to TiO2 for the production of solar cells

We recently chemically bound PbS quantum dots to titanium dioxide nanoparticles to study charge transfer into the TiO2. In order to determine whether their energy levels were properly aligned, cyclic voltammetry measurements were used to determine the absolute energy levels of PbS and PbSe vs. vacuum. This indicated that only PbS QDs smaller than 4 nm could be used for efficient electron transfer. We then used simple fluorescent transient measurements to determine the surprisingly slow electron transfer time, on the order of 100s of nanoseconds. Finally, these QDs were incorporated into a working Graetzel-type solar cell.

fig.3 Energy levels measured by CV (left), photoluminescence lifetime of QD (center) and QD solar cell I-V curve (right).

Using Electrochemical Luminescence to Probe Nanocrystal Surfaces

Because photoluminescence (PL) can only probe optically allowed core energy levels, we recently used electrochemical luminescence (EL) to more directly probe the surface of quantum dots. In this process, an electron is donated into one quantum dot, and a hole into a separate dot. These dots move around in solution until colliding, which allows either the electron or hole to transfer to the other dot, followed by (sometimes) radiative emission. Because there is no strong photoluminescent background, even very rare emission (on the single photon level) can be detected from this process. Therefore, if the charge transfer is to surface states, these could be optically detected. We discovered that the amount of EL can be dramatically tuned over many orders of magnitude by changing the surface passivation. Additionally, when well passivated, the EL overlaps with the PL, indicating a lack of surface traps in our QDs.

fig.4 The EL process (left) and the overlapping spectra of PL and EL (right).

Fabricating Quantum Dot Ionic Liquids

Ionic liquids are both non-volatile and conductive, making them attractive for device fabrication. Synthesizing a quantum dot ionic liquid is challenging because a change in surface chemistry is required. But after fabrication, the liquid demonstrates all the normal useful properties of ionic liquids, while also surprisingly and dramatically increasing the stability of the quantum dots to oxygen exposure. Future work will attempt to incorporate these into electroluminescent devices or optical amplifiers.

fig.5 QD ionic liquid fabrication procedure.

Current and Future Research Areas

The following are things that we are potentially interested in, and in some cases are actively working on.

• Morphology (shape) dependence of electronic structure
• Charge transfer to other materials (explanation of apparently slow transfer rates?)
• Infrared electroluminescent devices
• Optical Amplifiers
• Second harmonic generation as a surface charge transfer probe
• Exploiting dielectric effects to shift energy levels

References

1. J. J. Choi, Y.-F. Lim, M. B. Santiago-Berrios, M. Oh, B.-R. Hyun, L. Sun, A. C. Bartnik, A. Goedhart, G. G. Malliaras, H. D. Abruma, F. W. Wise and T. Hanrath, "PbSe Nanocrystal Excitonic Solar Cells". Nano Letters (2009)
2. L. Sun, L. Bao, A. C. Bartnik, Y.-W. Zhong, J. C. Reed, D.-W. Pang, H. D. Abruna, G. G. Malliaras, and F. W. Wise., "Electrogenerated Chemiluminescence from PbS Quantum Dots". Nano Letters 9, 789 (2009)
3. B.-R. Hyun, Y.-W. Zhong, A. C. Bartnik, L. Sun, H. D. Abruna, F. W. Wise, J. D. Goodreau, J. R. Matthews, T. M. Leslie, and N. F. Borrelli., "Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles". ACS Nano 2, 2206 (2008)
4. A. C. Bartnik, A. Kigel, E. Lifshitz, F. W. Wise, "Electronic structure of PbSe/PbS core-shell quantum dots". Phys. Rev. B 75, 245424 (2007)
5. J. M. Harbold, H. Du, T. D. Krauss, K.-S. Cho, C. B. Murray, and F. W. Wise, "Time-resolved intraband relaxation of strongly-confined electrons and holes in colloidal PbSe nanocrystals". Phys. Rev. B 72, 195312 (2005)
6. Daniel R. Larson, Warren R. Zipfel, Rebecca M. Williams, Stephen W. Clark, Marcel P. Bruchez, Frank W. Wise, Watt W. Webb, "Water-Soluble Quantum Dots for Multiphoton Fluorescence Imaging in Vivo". Science 300, 1434 (2003)
7. H. Du, C. Chen, R. Krishnan, T. D. Krauss, J. M. Harbold, F. W. Wise, M. G. Thomas, and J. Silcox, "Optical Properties of Colloidal PbSe Nanocrystals". Nano. Lett. 2, 1321 (2002)
8. F. W. Wise, "Lead-salt quantum dots: the limit of strong quantum confinement". Acc. Chem. Res. 33, 773 (2000)
9. I. Kang and F. W. Wise, "Electronic structure and optical properties of PbS and PbSe quantum dots". J. Opt. Soc. B 14, 1632 (1997)
10. T. D. Krauss, F. W. Wise, and D. B. Tanner, "Observation of coupled vibrational modes of a semiconductor nanocrystal". Phys. Rev. Lett. 76, 1376 (1996)