Electrochromic NanocrystalQuantum Dots

Prof. Philippe Guyot-Sionnest’s group (Univ. of Chigaco) :1. Electrochromic Nanocrystal Quantum Dots, Science 20012. 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

dNDoS

dE

Density of states (DoS)

dE

dN

E

E1/

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., 1986.

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

WorkingElectrode (Pt)

CounterElectrode(Pt) Voltage

Optics sensor

Light

Teflon(300μm)

6mm

CdSe Nanocrystal film (0.5μm)

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

5.4nm 7.0nm

Detailed electrochromic effects

Infrared Visible Light Ultraviolet Frequencies (Hz) 0.003 - 4 x 1014 4 - 7.5 x 1014 7.5 x 1014 - 3 x 1016 Wavelengths (nm) 1 mm - 750 750 - 400 400 - 10 Quantum energies (eV) 0.0012 - 1.65 1.65 - 3.1 3.1 - 124

Strong infrared (IR) intraband absorption

Complete bleach of the visible interband transition

Two electrons are injected into 1Se orbital of every nanocrystal

Infra-red Visible

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: 1Se1Pe

– Complete bleach of visible interband transition: two electons injection

– Reversibility: potentially to help understand the process

Thank you for your attention!