Researchers in the US are the first to use epitaxy to make nanometre-sized single crystals. Epitaxy is a standard process used in semiconductor fabrication and therefore the breakthrough could lead to the production of nanostructured thin films for a wide variety of applications, including solar cells.

Most nanostructures made from inorganic materials are either amorphous or polycrystalline, and scientists have struggled to make nanostructures from single crystals that grow in a well defined way with respect to a substrate. Such crystals could be used in a host of applications in which excellent charge transport over extremely small distances is called for. Single crystals are ideal for these applications because they don't contain grain boundaries between the crystallites. These boundaries can act as trap or scattering sites for electrons and thus degrade the ability of a nanostructure to transport charge.

Now, Ulrich Wiesner and colleagues at Cornell University have developed a way of making single-crystal silicon or nickel monosilicide nanostructures with the help of a block copolymer self-assembly technique. The researchers say that, as far as they know, nobody had ever succeeded in combining polymer self-assembly with inorganic single-crystal epitaxy until now.

Self-assembly technique

To make their nanostructures, the team first create a hexagonal array of tiny pores on a silicon substrate. This is done using a block copolymer self-assembly technique that involves depositing a thin film of block copolymers and other materials. The sample is heated and some of the compounds evaporate, leaving a hexagonal array of tiny holes separated by about 30 nm.

The team then fill the pores of the template with an amorphous inorganic material such as nickel monosilicide or silicon. This material is converted into a single crystal by melting it for about 10 ns using short laser pulses. Upon cooling, the melt solidifies from below, so creating a single-crystal nanostructure growing into the template. This part of the process is known as epitaxy. The block copolymer template is then removed, leaving an array of tiny single-crystal pillars that can be as tall as 10 nm.

The laser employed by Wiesner's team was a 40 ns XeCl excimer pulsed laser with a wavelength of 308 nm. Arrays of either isolated nanopillars or interconnected 3D nanostructures were produced depending on the template's thickness.

Complex, nanocrystalline shapes

The researchers say that their experiment proves that single-crystal nanostructures can be fabricated using a simple laser processing step. The new method could be used to make a variety of complex, nanocrystalline shapes in the future. These could either be used for fundamental studies on nanocrystals or directly in applications.

"It will now be fun to find out how far we can push this process," said. "For example, a particularly exciting aspect is to use our approach for limiting the contact between two inorganic materials that have a lattice mismatch to nanoscopic surface areas. This may allow us to grow single crystals of materials like germanium on single crystals of silicon – a very old and long-standing problem in the semiconductor industry."

The work was published in Science 330 214.