Researchers in India have succeeded in tuning the light emission properties of zinc oxide quantum dots – a result that could have implications for solid-state lighting and lasers, among other applications.
Tuning emission colours in ZnO QDs
Zinc oxide is a wide band gap (3.3 eV) semiconductor and is a promising material for use in light-emitting diodes (LEDs), laser diodes and phosphors for flat-panel displays. Recently, scientists have been investigating how to incorporate ZnO nanocrystals, or quantum dots (QDs), into various solvents, polymers and silicones to make encapsulated LEDs. This is because the colours emitted by the QD can be tuned when the QD interacts with the solvent it is in.
Divi Haranath of the National Physical Laboratory (CSIR) in New Delhi and colleagues have now tuned the emission wavelength of ZnO QDs in the 480–562 nm range by dispersing them in solvents with varying refractive indices. The researchers have observed a linear dependence of emission wavelength with refractive index term (n2–1/2n2+1) – something that could technologically useful for practical applications, says Haranath.
The team adopted a simple wet chemical route for synthesizing lithium and sodium-doped ZnO QDs. Using a Quantum Confined Atom model (
www.nanocrystals.com), the dopant atoms were confined within a tiny ZnO nanocrystal smaller than 5 nm. After ageing, the QDs were precipitated out using a solvent and a small amount of the nano-agglomerate re-dispersed in solvents with varying refractive indices.
According to Haranath and co-workers, the surface chemistry of these small particles in different solvents is critical in governing dispersion processes and growth kinetics of QDs. Both these factors ultimately dictate diverse photoluminescence peaks, Haranath told
nanotechweb.org. The PL emission also blue shifts depending on the change in dielectric constant and relative polarity of the medium, the size and shape of the dispersed QDs and the refractive index of the solvent.
Improving LED efficiency?
The result might be useful for drawing out more light from LEDs and making them even more energy efficient, added Haranath. "Eventually, we might expect LEDs with a wide colour spectrum with no back scattering, flexible design and lower cost. Solid-state lighting as well as laser applications might be the two important uses for these tuneable QDs."
The team is now extending its study to other QDs, such as titanium oxide, tin oxide and zinc oxysulphide for practical applications related to lighting.
The work was published in
Applied Physics Letters.
Source：
nanotechweb.org
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