(By: TUDelft/TNW Webredactie Communication)

Directly connecting semiconductor nanocrystals in a new way allows networks with better electronic properties to be created. The created lead selenide networks are promising and could be used for future solar cells or transistors, for example. Last week, researchers under the supervision of TU Delft published their results in the online edition of the journal Nature Communications.

Lead selenide
The research, led by professor Laurens Siebbeles, concerns two-dimensional networks of nanocrystals in the form of lead selenide (PbSe) quantum dots (a kind of artificial atoms).
Networks of PbSe quantum dots are not unknown. They have been produced for some time now. The difference lies in the way in which quantum dots are chemically linked to each other. Up to now, they have been linked via carbon-based molecules. The researchers of TU Delft, FOM, Utrecht University and the French CNRS, succeeded in creating differently combined two-dimensional networks of PbSe quantum dots. The dots are no longer linked via carbon-based molecules, but via direct chemical bonds between the atoms. This change has a promising positive effect on the properties of the network.

Extremely mobile charges
The most important improved property is the fact that charge carriers (such as electrons) can move through the network at high speed. The mobility is increased by a factor of at least ten compared with the mobility in PbSe networks with carbon-based molecules. The mobility is an important parameter in, for example, the performance that such a material would have when applied in a solar cell. This improved mobility is also good news for other applications, such as infrared detectors or transistors. Moreover, a possible application would require further research, according to Siebbeles and principal researcher dr. Wiel Evers.   

Charge multiplication
There is another reason why the finding is promising with regard to future solar cells. In nanocrystals, such as PbSe, the absorption of a single photon can lead to multiple excited electrons - two for the price of one. According to very first measurements, charge multiplication in the new networks already occurs at a lower photon energy level than in previous networks with carbon-based molecules.

The Foundation for Fundamental Research on Matter (FOM) is the main financing agent of this research. At TU Delft researchers of the Chemical Engineering department of the Faculty of Applied Sciences and the Kavli Institute of Nanoscience were involved in this research. 

Further information

• Article: "High charge mobility in two-dimensional percolative networks of PbSe quantum dots connected by atomic bonds" (published 24 September 2015): DOI 10.1038/ncomms9195

• Press release FOM (in Dutch), February 13, 2015: Twee voor de prijs van één: 'bonuselektronen' in germanium nanokristallen kunnen leiden tot betere zonnecellen