Films made of semiconductor nanocrystals are
seen as a promising new material for a wide range of applications. Nanocrystals
could be used in electronic or photonic circuits, detectors for biomolecules,
or the glowing pixels on high-resolution display screens. They also hold
promise for more efficient solar cells.
The size of a semiconductor nanocrystal
determines its electrical and optical properties. But it’s very hard to control
the placement of nanocrystals on a surface in order to make structurally
uniform films. Typical nanocrystal films also have cracks that limit their
usefulness and make it impossible to measure the fundamental properties of
these materials.
Now, researchers at Massachusetts Institute
of Technology (MIT) say they have found ways of making defect-free patterns of
nanocrystal films where the shape and position of the films are controlled with
nanoscale resolution, potentially opening up a significant area for research and
possible new applications.
“We’ve been trying to understand how
electrons move in arrays of these nanocrystals,” which has been difficult with
limited control over the formation of the arrays, says physicist Marc Kastner,
the Donner Professor of Science, dean of MIT’s School of Science and senior
author of a paper published online in Nano Letters.
The work builds on research by Moungi
Bawendi, the Lester Wolfe Professor of Chemistry at MIT and a co-author of this
paper, who was one of the first researchers to precisely control nanocrystal
production. Such control made it possible, among other things, to produce
materials that glow, or fluoresce, in a range of different colors based on
their sizes—even though they are all made of the same material.
In the initial phases of the new work,
postdoctoral researcher Tamar Mentzel produced nanoscale patterns that emit invisible infrared
light. But working on such systems is tedious, since each fine-tuning has to be
checked using time-consuming electron microscopy. So when Mentzel succeeded in
getting semiconductor nanocrystal patterns to glow with visible light, making
them visible through an optical microscope, it meant that the team could
greatly speed the development of the new technology. “Even though the nanoscale
patterns are below the resolution limit of the optical microscope, the
nanocrystals act as a light source, rendering them visible,” Mentzel says.
The electrical conductivity of the
researchers’ defect-free films is roughly 180 times greater than that of the
cracked films made by conventional methods. In addition, the process developed
by the MIT team has already made it possible to create patterns on a silicon
surface that are just 30 nm across—about the size of the finest features
possible with present manufacturing techniques.
The process is unique in producing such
tiny patterns of defect-free films, Mentzel says. “The trick was to get the
film to be uniform, and to stick” to the silicon dioxide substrate, Kastner
adds. That was achieved by leaving a thin layer of polymer to coat the surface
before depositing the layer of nanocrystals on top of it. The researchers
conjecture that tiny organic molecules on the surface of the nanocrystals help them
bind to the polymer layer.
Such nanocrystal patterns could have many
applications, Kastner says. Because these nanocrystals can be tuned not only to
emit but also to absorb a wide spectrum of colors of light, they could enable a
new kind of broad-spectrum solar cell, he says.
But Kastner and Mentzel’s personal interest
has more to do with basic physics: Since the minuscule crystals behave almost
like oversized atoms, the researchers aim to use the arrays to study
fundamental processes of solids, Mentzel says. The success of this technique
has already enabled new research on how electrons move in the films.
Such materials could also be used to
develop sensitive detectors for tiny amounts of certain biological molecules,
either as screening systems for toxins or as medical testing devices, the
researchers say.
