The ultra-high vacuum electron microscope tucked away in a lab down a nondescript aisle on the ground floor of the IBM Thomas J Watson Research Center (itself tucked away in the woods of Westchester County, NY) holds many clues that help scientists unlock the physics that takes place at nanoscale dimensions. Understanding how materials behave at such small sizes opens up the scientific community’s imagination for future new electronic devices. The electron microscope works a little like a farm, but instead of growing your favorite vegetable, the objects that grow are nanowires: extremely narrow but long crystals made of semiconducting materials, each with its own particular electronic properties.
The growth starts with tiny “seeds,” made up of catalytic metal droplets, that the scientists sprinkle onto a flat “field” of silicon. When the right ingredients are supplied – heat and special gases – each seed starts to grow a nanowire. But unlike in a real farm, where growth starts underground, here the seed droplets stay at the tips of their nanowires, making sure that growth only happens at the tips. The result is a forest of long, narrow crystals that grow straight upwards. In a new twist of experimentation, the team has shown that when they switch on an electric field, the droplets can be pulled sideways or stretched vertically. This little “dance” or “stretching” move forces the growing crystals to change their direction in response. Electric field control of nanowire growth is a new frontier, opening the doors to build customized nanostructures that can be integrated into new types of electronic devices.
IBM scientists, led by Dr. Frances Ross, in collaboration with the University of Cambridge, University of Pennsylvania, and Technical University of Denmark, published their results, “Controlling nanowire growth through electric field-induced deformation of the catalyst droplet” in the latest issue of Nature Communicationsthis week (DOI: 10.1038/NCOMMS12271).
To control the elegant droplet-mediated process that grows nanowires, the team has already tried out many simple tricks: changing the temperature, pressure, mix of gases, and catalyst materials during growth. “What we wanted to do here was try turning a new knob to see what kind of structure we would get. The knob we added is an electric field which we created by applying a voltage to the sample during growth. When we switched the field on and off, we could see each droplet deform and the nanowire growth then change to follow it,” said Ross, materials scientist, IBM Research.
This is the reason that the team carried out their growth experiments in the microscope: they could immediately see the nanowires move when turning on the electric field. The microscope magnifies the growing nanowire by 50,000 times and records 30 images each second, providing plenty of data to analyze.
“Electric fields seemed worth a try because we knew that the catalyst droplets would behave like any other metal in an electric field and get pulled in the direction of the field,” Ross said. “What was especially intriguing in these experiments was the way that the changed position of the droplet affected how growth took place at the nanowire tip.”
An interesting by-product of the research was being able to measure the surface tension of the liquid droplet. Surface tension is the skin that keeps droplets, such as water droplets on glass, in their spherical shapes. An accurate value for surface tension is a fundamental requirement for developing computer models to predict nanowire growth.
“We are always looking for the best way to grow crystals with particular properties. We know what we can get by changing temperature or pressure: interesting, useful nanowires, but always growing vertically. With the electric field, we finally have a way to force a wire to grow sideways or at an angle, so that we can form a three dimensional structure,” added Ross.
Applications for “dancing” nanowires
Modern electronic devices use an ever-increasing portfolio of materials in the quest to improve computing power and data capacity, and implement new functionalities. Angled or kinked nanowires could expand the materials repertoire, especially if they can be fabricated reliably. They could be useful as interconnects, where a device needs an electrical connection between different components in a circuit. They may allow for new types of IoT sensors or be used as probes. For example, a V-shaped probe could be poked into a living cell to monitor a cell’s tiny electrical signals. Other nanowires shaped like the letters “T” or “X” also have interesting applications. Placing these “letters” in a magnetic field and measuring current flow by applying voltages to different legs can help test fundamental theories of physics. These theories are abstruse, because they govern the behavior of special excitations in semiconducting materials. But they will perhaps also be practically relevant: the excitations may provide the means to store information in quantum computers in a way that avoids some of the limitations of current designs. Nanowires grown with dancing, stretching droplets may be the first step along this path.