Scientists create 'nano LEDs'
Chemists at the National Institute of Standards and Technology (NIST) have stumbled upon a way of producing light from nanowires. While they were refining a method for producing horizontally-grown wires across a substrate, Babak Nikoobakht and Andrew Herzing electrically charged an array of junctions between two materials and caused illumination to occur. The pair hope to further refine the technique so that these nano LEDs can be applied in the development of light sources and detectors useful in photonic devices or lab-on-a-chip platforms.
Nanowires are generally grown using a process called chemical vapor deposition, where molecules are deposited from a gas onto a base material. Most techniques result in nanowires that stand vertically from the substrate, but the fact that only one end touches the base material means that a wire doesn't share the characteristics of the material it sprouts from. Vertical growth also produces tightly packed forests of nanowires, making the task of locating and repositioning superior quality, individual wires more difficult.
Nikoobakht and Herzing's technique is reported to address such issues. They have developed a “surface-directed” method for growing nanowires horizontally across the base material. A gold catalyst is heated to 900 degrees Celsius (1,652 degrees Fahrenheit) to convert it to a growth site and medium for zinc oxide molecules to crystallize. As the zinc oxide crystal grows, it pushes the gold nanoparticle along the surface of a gallium nitride substrate to form a nanowire.
The NIST chemists recently increased the thickness of the gold catalyst from eight to about 20 nanometers and noticed that the change caused the nanowires to grow a secondary shark-fin-like structure called a nanowall. The structure's zinc oxide portion was found to be packed with electrons while the gallium nitride portion was less fortunate. When electricity was sent through the new structure, electrons flowed across the interface between the two materials and light was produced.
The chemists hope that future experiments using better geometry and material designs may lead to the nano LEDs being used in nanogenerators or lab-on-a-chip systems.
Detailed information on the techniques outlined above can be found in the published paper, Formation of planar arrays of one-dimensional p-n heterojunctions using surface-directed growth of nanowires and nanowalls.