Nanoscale optical probes using carbon nanotubes that facilitate sub-wavelength, sub-diffraction limit, and spatial resolution for near-field scanning optical microscopes are disclosed in U.S. Patent 7,623,746. Boston College researchers developed a nanoscale optical probe for use with a near-field scanning optical microscope that includes an inner conductor with a top end, a bottom end, and a body; a dielectric material which surrounds the inner conductor; and an outer conductor which surrounds the dielectric material. The inner conductor is longer at a tip surface of the probe than the dielectric material and the outer conductor. The nanoscale optical probe can achieve resolutions of less than about 10 nanometers (nm) in a transverse direction. In the art for nanoscale optical probes, the Boston College probe extends the range of measurements possible compared with conventional near-field scanning optical microscopy techniques.
Professor Michael J. Naughton, Krzysztof J. Kempa and Zhifeng Ren created a magnifying element for use with a near-field scanning optical microscope. The magnifying probe is made of a film with a top surface, a bottom surface and cylindrical channels and an array of nanoscale optical probes penetrating the film through cylindrical channels. Each nanoscale optical probe has an inner nanowire with a top end, a bottom end, and a body; a dielectric material which surrounds the inner nanowire; and an outer metal material which surrounds the dielectric material. Because the width of the inner conductor may be significantly smaller than the wavelength of visible light, which is in the range of about 300 nm to about 700 nm, the nanoscale optical probe may be used to image objects with spatial resolution well under this range.
The inventors developed a method of fabricating a nanoscale optical probe which involves growing a carbon nanotube (CNT) on the optical fiber; depositing a dielectric material over the carbon nanotube; and depositing an outer metal material over the dielectric material. They also fabricated a nanoscale optical probe by electrodepositing a catalytic transition metal on an AFM-type tip; growing a carbon nanotube (CNT) on the optical fiber; depositing a dielectric material over the carbon nanotube; and depositing an outer metal material over the dielectric material.
Plasma enhanced chemical vapor deposition (PECVD) is used grow the carbon nanotube. A dielectric photovoltaic material having both electrical conductivity and transparency (for example silicon- and non-silicon-based materials) is deposited over the carbon nanotube via (for example, via PECVD, sputtering, or evaporation). Typically, the dielectric material is coated to yield a thickness of about 10 nm to about 200 nm. An outer metal (for example, aluminum) is then deposited (via CVD, sputtering or evaporation) over the dielectric material, forming a nanoscale optical probe having a coplanar waveguide configuration. If desired, the outer metal may be removed from the bottom surface of the probe (via focused ion beam or wet etch), thus exposing the photovoltaic material and the carbon nanotube, yielding a nano-optical antenna at the substrate surface of the probe.
Near-field scanning optical microscopy (NSOM) is a type of microscopy where a sub-wavelength light source, usually a fiber tip with an aperture smaller than 100 nm, is used as a scanning probe over a sample. Near-field scanning optical microscopy is one in a family of scanned probe techniques that includes scanning tunneling microscopy and atomic force microscopy (AFM) where an image is obtained by raster scanning a probe across a surface collecting data at an array of points during the scan. In order to achieve an optical resolution better than the diffraction limit, the scanning probe has to be brought within the near-field region (that part of the radiated field nearest to the antenna, where the radiation pattern depends on the distance from the antenna). NSOM is based upon the detection of non-propagating evanescent waves in the near-field region. The probe is scanned over a surface of the sample at a height above the surface of a few nanometers and allows optical imaging with spatial resolution beyond the diffraction limit.
The scanning probe can either detect in the near-field directly, by means of the sub-wavelength size aperture (collection mode), or by using the probe as a waveguide with a sub-wavelength scattering source and detecting the evanescent waves as they propagate into the far-field (transmission mode). The achievable optical resolution of NSOM is mainly determined by the aperture size of the scanning probe and the probe-surface gap. NSOM may, in theory, be combined with any spectroscopic technique to gather spectra from small regions of a sample. Infrared (IR), Raman, visible, and V, as well as NSOM fluorescence, photoluminescence, photoconductance, and magnetooptical (MOKE) spectroscopies have been investigated.
Showing posts with label nanowire. Show all posts
Showing posts with label nanowire. Show all posts
Danish Scientists Improve Nanowire Manufacturing Techniques
Danish nanophysicists have developed a new method for manufacturing nanowires. The discovery has great potential applications in nanoelectronics and highly efficient solar cells.
It is PhD student Peter Krogstrup, Nano-Science Center, the Niels Bohr Institute at the University of Copenhagen, who developed the method during his dissertation. "We have changed the recipe for producing nanowires. This means that we can produce nanowires that contain two different semiconductors, namely gallium indium arsenide and indium arsenide. It is a big breakthrough, because for first time on a nanoscale, we can combine the good characteristics of the two materials, thus gaining new possibilities for the electronics of the future", explains Peter Krogstrup.
Today only approximately 1 % of the world’s electricity comes from solar energy. This is because it is difficult to convert solar energy into electricity. It is a great advantage for the researchers to be able to combine different semiconductors in the same nanowire. Different materials capture energy from the sun in different and quite specific absorption areas. When we manufacture nanowires of gallium indium arsenide and indium arsenide, which each have their own absorption area, they can collectively capture energy from a much wider area. We can therefore utilize more solar energy, if we produce nanowires from the two superconductors and use them for solar cells, explains Peter Krogstrup.
The nanowires of gallium indium arsenide and indium arsenide also have great potential in nanoelectronics. They can be used in the new OLED displays and LEDs. But it requires sharp transitions between the two materials in the nanowire.
A nanowire made of the two semiconductors gallium indium arsenide (GaInAs) and indium arsenide (InAs) with gold (Au) as a catalyst. To the right a schematic illustration of the new cultivation method, where the semiconductor materials can move both from the top of the gold droplet and from the underside. Credit: University of Copenhagen
The cultivation of nanowires takes place in a vacuum chamber. The researchers lay a gold droplet on a thin disc comprising of the semiconductor and the nanowire grows up from below. In the transition between the two semiconductor materials in the gold droplet there was previously a mixing between the materials in the gold droplet and there was a soft transition between the materials. With the new method both of the materials can go from the top of the gold droplet or from the underside of the gold droplet. When the material comes from the underside, there is no mixing of the semiconductor materials. There is therefore a sharp transition on the atomic level between the gallium indium arsenide and indium arsenide.
- This sharp transition between the two semiconductors is necessary for the current – in the form of electrons, to be able to travel with high efficiency between the two materials. If the transition is soft, the electrons can easily get caught in the border area. The new mixed nanowire can be beneficial for many areas of nano research around the world, says Peter Krogstrup, who has been working at the Danish III-V Nanolab, operated in collaboration between the University of Copenhagen and the Technical University of Denmark.
A new collaboration between the company SunFlake A/S and The Danish National Advanced Technology Foundation has been kicked off this week. SunFlake A/S uses nanowires to develop prototypes of solar cells and they can also benefit from the new method in their continuing work. The nanophysicists’ discovery has just been published in the prestigious scientific journal Nano Letters.
PhD student Peter Krogstrup, Nano-Science Center (www.nano.ku.dk), Niels Bohr Instituttet (www.nbi.ku.dk), Københavns Universiet på mail: krogstrup@fys.ku.dk or mobile: +45 26 71 51 91
Communication Officer Gitte Frandsen, Nano-Science Center (www.nano.ku.dk), Københavns Universitet på mail: gf@nano.ku.dk or mobile: +45 28 75 04 58
Link to paper in Nano Letters: Junctions in Axial III−V Heterostructure Nanowires Obtained via an Interchange of Group III Elements, Nano Letters http://pubs.acs.org/doi/full/10.1021/nl901348d
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