Researches from Chicago have found a way to turn nanocrystals into conductive solids without ruining their optical activity.
Nanocrystalline get their behaviour not only from the materials they are made from, but their size and shape. Hence their opto-electronic properties can be tuned by making the crystals bigger or smaller.
When they are formed onto solids, whatever holds them together also affects optical and electronic properties.
While still in solution, nanocrystals have to be stabilised by chemically attaching molecules to them and, until now, the stabilising molecules that work have all been insulating, said the University of Chicago.
According to the University, these molecules cannot be completely removed before solidification, and end up as an insulating layer around each crystal that ruins the conductivity of the resulting solid.
Under the direction of chemist Dr Dmitri Talapin, and with the US Lawrence Berkeley National Laboratory, Chicago researches have developed stabilising molecules based on metal chalcogenide complexes (MCCs) that decompose into semiconducting layers around the crystals as they are formed into solids.
The MCCs concerned are centred on elements such as Cu, Ga, In, S, Sb, Se, Sn, Te, and Zn - all familiar in compound semiconductors.
Heating concentrated nanocrystals to around 180C is sufficient to decompose the MCCs into semiconductor, at the same time bonding the crystals into a solid.
So long as there is not too much MCC, the optic properties of the nanocrystals still dominate the solid, but conductivity is dramatically improved.
The University reports 1011 times the conductivity of solids based on hydrocarbon-coated nanocrystals, which is more conductive than solution-cast conductive polymers and graphene composites, but still less than some carbon nanotube-based mats.
Transistors fabricated with channels made from 4.5nm CdSe crystals stabilised with Sn2S6 MCC showed an on/off ratio of 105, and 3x102cm2/Vs electron mobility in the saturation regime.
Illumination of the solid, reports the University, increased conductivity by several orders of magnitude.
One proposed application is all-solution-processed thin-film solar cells.
The University of Chicago licensed the underlying technology for thermoelectric applications to Evident Technologies in February.
A vial of nanocrystals in solution