A team of researchers, led by Kyle Shen and Darrell Schlom at Cornell University and Ivan Božović at Brookhaven National Lab, grew ultrathin films of LaNiO3 using a technique called molecular-beam epitaxy. They discovered that the material goes from being a metal to an insulator when it contains just two Ni atoms within its thickness. They also observed how the electronic states evolve through this metal-insulator transition using angle-resolved photoemission spectroscopy (ARPES).

“Our results tell us that, in the thinnest films, the electrons in the material arrange themselves to form a collective ordered state at the nanoscale, and it is this ordering that drives the insulating behaviour of this unusual metallic material,” says team member Phil King (now at the University of St Andews in the UK). “It is not just a gradual change in the material’s properties that we see, or a smooth change in its conductivity as you might expect to see in a conventional material, but a sudden crossover that comes from controlling the interactions between its electrons, with an exotic self-organisation driving the dramatic changes we observe.”

Insulator at two planes thick

The Cornell-BNL team obtained its results by making the LaNiO3 extremely thin. The material comprises alternating planes of nickel and oxygen atoms and lanthanum and oxygen atoms respectively. When there are three or more of the nickel-containing planes, the material is a metal. However, when the material is just two planes thick, it becomes an insulator, explains Shen. “To actually see this, we grew the compound using MBE, which allowed us to lay down single atom-thick planes, and layer these on top of each other with atomic precision. We alternated La-O and Ni-O layers and, in this way, drove the crossover from ultrathin insulator (at just a few atoms thick) to metal,” he told nanotechweb.org.

By starting from the ultrathin insulator and disrupting the collective state of the electrons – for example, by using an electrical gate voltage – the researchers hope that it could be possible to transform the material back into being a metal. Such a transformation could help make a very good nanoscale switch/transistor, says King. “These nickelates are also perfect for integrating with other 2D metal oxides, which are creating much excitement at the moment because they could be ideal for developing new electronic platforms with novel functionalities not present in today’s semiconductors.”

The team says that it is now busy trying to better understand the microscopic electronic interactions in LaNiO3 that ultimately allow for the metal-to-insulator transition. “We are also looking into ways to control this transition,” revealed King.

The present work is detailed in Nature Nanotechnology doi:10.1038/nnano.2014.59.