Optical studies of nanocrystal system

Dates

Start date: 1 January 2009
End date: 31 January 2012

Summary

Nanocrystals (NC), also known as quantum dots, has very unique physical properties which allow it to change bandgap Eg by adjusting the size. These materials are of huge technological interest since many of their electrical and thermodynamic properties show strong size dependence and can therefore be controlled through careful manufacturing processes. The interest of researchers in these materials continues today with researchers focusing on the study of their optical and electronic behaviour. Nanocrystals have the potential to be a good material for lasing devices due to their broadly tuneable emission in the NIR, good radiative quantum efficiencies, and long excited state lifetime. Studies show that quantum dots may absorb and emit light in a very narrow spectral range which if controlled, for instance by an applied magnetic field, may soon find application in the construction more efficient and precisely controllable semiconductor lasers. High degeneracy of this material at ground state make NC very hard to lasing. To overcome this problem a series of studies has to be run in order to reduce the degeneracy via splitting of the relevant energy levels. This will be approached via the use of external magnetic effects (e.g. Zeeman) or internal effects achieved via doping with magnetic ion impurities.

Funding

University Kuala Lumpur – Majlis Amanah Rakyat (MARA), Malaysia.

Details

Optical studies of NC systems require measurement of absorption spectra, photoluminescence and lifetime photoluminescence decay. NCs optical properties will be controlled by environment for temperature dependent (3 to 300 Kelvin) and external magnetic field dependent (-7 to 7 Tesla).  Other optical studies have been done with organic donor-acceptor (TTF and TCNQ) to understand charge transfer mechanism in NC systems.

Results

The absorption and photoluminescence are found to display different temperature dependent behavior though both redshift as temperature is reduced. This results in a temperature dependent Stokes shift which increases from ~75 meV with reducing temperature from 300 K until saturating at ~130 meV below ~150 K prior to a small reduction to 125 meV upon cooling from 25 K to 3 K. The PL lifetime is found to monoexponential at 3 K with a lifetime of t1 = 6.5 us.
Measurement of the photoluminescence (PL) of PbS NCs with TTF indicates that the highest occupied molecular orbital (HOMO) of TTF is closely aligned with PbS NC 1sh energy level. Whilst in the case of PbS NC in presence of the acceptor molecule (TCNQ) the direct quenching of the PL of PbS NC is observed. The absorption spectra of PbS:TTF and PbS:TCNQ mixture are taken to observe the indirect quenching from ground state of PbS to TCNQ charge transfer mechanism.

(Click to enlarge image)

Output

Muhammad N. Nordin, Richard. J. Curry, Konstantinos N. Bourdakos; “Charge transfer in hybrid organic-inorganic PbS nanocrystal system”; Phys. Chem. Chem. Phy.; 12; 1-7.
Isabelle Etchart,  Arnaud Huignard,  Mathieu Berard,  Muhammad N. Nordin,  Ignacio Hernandez, Richard J. Curry,  William P. Gillin  and Anthony K. Cheetham “Oxide phosphors for efficient light upconversion: Yb3+ and Er3+ co-doped Ln2BaZnO5 (Ln ¼ Y, Gd)”; Jour, of Mat, Chem.; 20; 3989-3994.
Wijittra Wichiansee, Muhammad N. Nordin, Mark Green, Richard J. Curry; “Synthesis and Optical Characterisation of Infra-red emitting HgS Quantum Dots” Journal of Material Chemistry. DOI:10.1039/c1jm10363f.
Muhammad N. Nordin, Juerong Li, Steve. K. Clowes, Richard. J. Curry, Konstantinos; “Temperature dependent on optical properties of PbS nanocrystals” Physical Review B,

Investigator

• Richard Curry