“The realization of radial superlattices at well defined positions cannot be accomplished by any other technology,” Christoph Deneke of the Max Planck Institute told nanotechweb.org. “It’s therefore expected to stimulate new fundamental as well as application-oriented research. What’s more, the observation that these radial superlattices can be functionalized as individual nanoreactors to synthesize hybrid materials at well defined positions, opens the path to create functional laterally modulated heterostructures.”
Deneke and colleagues made the rolled-up nanotubes by using molecular beam epitaxy to lay down a 2.8 nm thick layer of AlAs on a GaAs substrate. Then they added an inherently strained bilayer of 1.4 monolayers of InAs and 14 monolayers of GaAs. Finally, etching away the AlAs layer caused rolled-up nanotubes of InAs/GaAs to form.
“The release and roll-up of thin solid films is a powerful technique for creating novel self-assembled nano-objects at well defined places on a substrate surface,” said Deneke. “The technique therefore elegantly combines the top-down (lithography) and bottom-up (self-assembly) approaches, which is of vital importance for functionalizing single nanostructures in new nanodevices.”
The scientists believe that the noncrystalline layers in the nanotube walls formed during roll-up as the bilayer bonded to itself. The noncrystalline regions were typically a few monolayers thick.
One nanotube had a 10-period crystalline/noncrystalline wall structure - the scientists say that rolled-up nanotubes with such a high number of rotations represent an entirely new form of a radial superlattice.
The team also used a 0.4 mW laser beam to heat a small section of one of the rolled-up nanotubes. This removed As from the tube and oxidized it, forming a region of β-Ga2O3 and leaving the rest of the tube as Ga(In)As.
“[Such] laterally modulated heterostructures could possibly form the basis of a new generation of electronic and opto-electronic devices,” said Deneke. “In the short term we will explore the possibilities given by the local reaction process, i.e. we will try to find out what materials can be synthesized and what is the quality of the obtained material. In the long term we will try to use the nanoreaction products in new semiconductor devices.”
The researchers reported their work in Applied Physics Letters.