Novel Devices ♦ news digest
Nanowires fabricated using the new techniques developed by Silvija Gradečak and her team can have varying widths, profiles and composition along their lengths, as illustrated here, where different colours are used to indicate compositional variations (Image courtesy of the Gradečak laboratory)
Nanowires are grown by using “seed” particles, metal nanoparticles that determine the size and composition of the nanowire. By adjusting the amount of gases used in growing the nanowires, Gradečak and her team were able to control the size and composition of the seed particles and the nanowires as they grew. “We’re able to control both of these properties simultaneously,” she says.
While the researchers carried out their nanowire- growth experiments with InN and InGaN, they say the same technique could be applied to a variety of different materials.
The scientists observed the nanowires using electron microscopy and made adjustments to the growth process based on the results. Electron tomography also enabled them to reconstruct the three-dimensional shape of individual nanoscale wires.
The team has also published work regarding the use of electron-microscopy cathodoluminescence to observe what wavelengths of light are emitted from different regions of individual nanowires.
Precisely structured nanowires could facilitate a new generation of semiconductor devices, says Gradečak. Such control of nanowire geometry and composition could enable devices with better functionality than conventional thin-film devices made of the same materials, she says.
One likely application of the materials developed by Gradečak and her team is in LED light bulbs, which have far greater durability and are more energy-efficient than other lighting alternatives. The most important colours of light to produce from LEDs are in the blue and ultraviolet range; ZnO and GaN nanowires produced by the MIT group can potentially produce these colours very efficiently and at low cost, she says.
While LED light bulbs are available today, they are relatively expensive. “For everyday applications,
the high cost is a barrier,” Gradečak says. One big advantage of this new approach is that it could enable the use of much less expensive substrate materials, a major part of the cost of such devices, which today typically use sapphire or SiC substrates. The nanowire devices have the potential to be more efficient as well, she says.
Such nanowires could also find applications in solar-energy collectors for lower-cost solar panels. Being able to control the shape and composition of the wires as they grow could make it possible to produce very efficient collectors.
The individual wires form defect-free single crystals, reducing the energy lost due to flaws in the structure of conventional solar cells. Also, by controlling the exact dimensions of the nanowires, it’s possible to control which wavelengths of light they are “tuned” to, either for producing light in an LED or for collecting light in a solar panel.
Complex structures made of nanowires with varying diameters could also be useful in new thermoelectric devices to capture waste heat and turn it into useful electric power. By varying the composition and diameter of the wires along their length, it’s possible to produce wires that conduct electricity well but heat poorly. This combination is hard to achieve in most materials, but is key to efficient thermoelectric generating systems.
The nanowires can be produced using tools already in use by the semiconductor industry, so the devices should be relatively easy to gear up for mass production, the team says.
Zhong Lin Wang, the Regents’ Professor and Hightower Chair in Materials Science and Engineering at the Georgia Institute of Technology, says that being able to control the structure and composition of nanowires is vitally important for controlling their nanoscale properties. The fine- tuning in the growth behaviour of these materials opens up the possibilities for fabricating new optoelectronic devices that are likely to have superior performance.
More details of this work have been published in the paper, “ Controlled Modulation of Diameter and Composition along Individual III–V Nitride Nanowires” by Sung Keun Lim et al, in Nanoletters, published online on 7 February 2012. DOI: 10.1021/
March 2012
www.compoundsemiconductor.net 135
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136