Laser Controls Nanoparticle Growth
Spontaneous self-assembly has been a primary approach to the production of nanoparticles. But researchers at Southampton University in the UK and Moscow State University in Russia, led by Southampton physics professor Nikolay I. Zheludev, have found that they can control particle growth with the application of low-intensity radiation. The process could lead to significant improvements in the performance of electronic and photonic devices based on gallium-nitride and gallium-arsenide quantum dots.
Illuminating the substrate with a low-power laser enables researchers to control the growth of gallium nanoparticles. Atomic force microscopy images of the nanoparticles in an area under no irradiation (above) and the same area under irradiation (below) demonstrate the effects on size uniformity and on distribution. Courtesy of Kevin F. MacDonald.
The discovery came about while the researchers were studying the nonlinear optical properties of gallium, said Kevin F. MacDonald, a member of the Southampton University team. As they monitored the reflected laser radiation from a gallium layer that they were depositing on the end-face of an optical fiber, they found that the low-power infrared pulses in the fiber promoted the formation of uniform nanoparticles on the end of the core.
The scientists decided to investigate a setup in which 1-µs pulses of 1550-nm radiation from a 17-µW laser diode illuminated the fiber substrate during vacuum deposition. They found that the laser induced the formation of uniform clusters that spread across the surface of the substrate, increasing their aspect ratio and filling factor as they did so.
They built a test system around a turbomolecular pump that produced a vacuum of approximately 10-6 mbar when the gallium source is in operation. The laser source was a fiber-coupled pulsed laser diode from Mitsubishi, and the fibers were Corning SMF-28 single-mode fibers for 1310- and 1550-nm wavelengths. The custom-made gallium source offered a deposition rate of approximately 0.3 nm/min. A cold finger cooled the fiber to 100 K.
Because the particles interact more strongly with the laser radiation as they spread, their growth can be controlled by nonthermal laser-induced desorption. The researchers believe that the size, shape and spatial distribution of the particles also can be controlled by changing deposition conditions, such as the atomic beam flux and the substrate temperature, and by changing the laser excitation parameters, such as wavelength and power.
Optical control of nanoparticle growth has several advantages over spontaneous self-assembly. For example, the latter tends to produce nanoparticles of varying diameters, which obscures the size-dependent properties that are essential to many applications.
The manipulation of the shape, size or size distribution of nanoparticles has been demonstrated before, but it has required high-power lasers and typically is performed after the particles have been formed. In contrast, the new process employs simple diode lasers and enables the production of nanostructures with a wide range of characteristics because it acts during their formation.
Another advantage is its simplicity. "Technically, there is no reason why the technique could not be made available immediately," MacDonald said. "Atomic-beam deposition systems with cryogenically cooled substrate holders are readily available. The only additional requirement, therefore, is an appropriate laser source and a few fiber optic components."
He noted, however, that more research is required to determine how parameters such as the illumination wavelength and the substrate temperature affect the nanostructures. Although the team has patented the technique, there are no plans to develop it commercially.
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