The team grew the crystals from solution, using the transformation between a zincblende-type cubic crystal structure and a wurtzite hexagonal structure to cause branching. By applying new techniques the scientists were able to tailor the shape and composition of each region of the nanostructure. This enabled them to tune the electrical properties of the crystals. The team also developed modelling techniques so that they could predict the properties of different morphologies.

"We had previously found a way to create tetrapods [a structure with four branching 'feet'] of a single semiconductor material - cadmium telluride - simply by varying the crystal phase," said Delia Milliron of Lawrence Berkeley National Laboratory. "It occurred to us that for electronic purposes we might be able to make branching structures from more than one kind of material - theoretically you could use anything with the two crystal phases. Once you have two materials, the branching possibilities increase enormously."

Milliron and colleagues assembled their structures from suspensions of cadmium, selenium, tellurium and sulphur. As a starting point they formed either a linear wurtzite rod or a branched tetrapod. Then they grew layers of a second semiconductor material epitaxially onto the "first-generation" nanostructure, in either a branched or a linear fashion.

According to the scientists, the terminal quantum rods and dots of this second material are coupled through the tuneable barrier defined by the first-generation structure. Using additional generations of growth would produce structures with even more complex interactions.

For example, the team grew CdSe extensions onto a CdS nanorod. The extensions acted as quantum rods separated by a barrier for electrons and holes, giving a structure that could be useful for the control of quantum coherence. To tune the coupling of the rods, the scientists could change the length of the original CdS nanorod or of the extensions, or use a different material to change the barrier height.

The team also made branched tetrapods containing a central CdSe tetrapod and terminal CdTe branches. The scientists say these structures were of interest for their unusual charge-separating properties, since they absorb light across the visible spectrum and then separate electrons and holes across their interfaces. As a result, they could have applications in solar cells.

The researchers reported their work in Nature.