Electrochromic Nanocrystal Quantum Dots Prof. Philippe Guyot-Sionnest’s group (Univ. of Chigaco) : 1. Electrochromic Nanocrystal Quantum Dots, Science Electrochromic semiconductor nanocrystal films, APL 2002 Yingqi Jiang

Video from Discovery Channel! —Fabrication of QD with Colloidal chemistry method!

Quantum dots (QDs) definition Quantum dots are nanometre-scale semiconductor crystals. – Nanometer scale: each dimension<100nm Exactly to say, <one electron Fermi wavelength – Semiconductor crystal: potential excellent optical and electronic properties – Quisi-0 dimension: quantum confinement, discrete energy levels——“artificial atoms”

Structure vs. Energy Brad Gussin, John Romankiewicz Bulk Quantum Well Quantum Wire Quantum Dot Density of states (DoS) 1 δ(E)

Fabrication Methods 3 primary methods (to form 3-D confinements) – Colloidal chemistry Precipitate QDs from solutions Size variations by reaction dynamics – Epitaxy Stranski-Krastanov growth (MBE, MOCVD) Size and shape fluctuations, ordering – Lithography Etch (EBL) pillars in heterostructures Slow, contamination, low density, defect formation A. Scherer and H.G. Craighead. Appl. Phys. Lett., Colloidal chemistry method is cheapest and fastest! T. Fukui et al. Appl. Phys. Lett. May, 1991 Victor I. Klimov, Los Alamos Science, 2003

QD applications Fluorescent biological markers Photovoltaic materials (solar cells) Lasers of very precise, chosen wavelengths – More efficient, higher material gain, lower threshold – Less thermal dependence, spectral broadening Quantum computing – Represent binary information by charge configuration Potentially more! “smart bombs”,…

Fabrication of electrochromic QDs 1. CdSe nanocrystals capped with trioctylphosphine oxide 2. CdSe nanocrystals are cleaned and dried under vacuum, and then dissolved in 9:1 (V/V) hexane:octane mixture. 3. A drop of the CdSe nanocrystal solution is placed on the surface of the working electrode and allowed to dry slowly to form an optically clear film 4. Test in a spectroelectrochemical cell The optical properties change in response to application of an electric current.

Basic observation Size tunable absorption (smaller  larger distance of discrete energies) Electrochromic tunable absorption 5.4nm7.0nm

Detailed electrochromic effects Strong infrared (IR) intraband absorption Complete bleach of the visible interband transition Two electrons are injected into 1S e orbital of every nanocrystal Infra-redVisible

Reversibility Efficient and reversible electron injection simply by adjusting the applied potential. Changes in the visible and IR follow each other very closely, indicating that they are the results of the same electron injection and ejection process. Charging and discharging of the nanocrystal films occur at about the same fast rate Detailed investigations of the factors that influence the electron injection and ejection processes.

Summary Essential reviews of QDs – Definition: nanoscale semiconductor crystal – Mechanism: 0-D structure – Fabrications: Colloidal chemistry is cheapest and fastest – Applications: promising optical and electrical applications Electrochromic properties of colloidal nanocrystals – Charge tunable by electron injection – Strong IR absorption: 1S e  1P e – Complete bleach of visible interband transition: two electons injection – Reversibility: potentially to help understand the process

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