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Nantenna

Figure 1. Spectral irradiance of wavelengths in the solar spectrum. The red shaded areashows the irradiance at sea level. There is less irradiance at sea level due to absorption of

light by the atmosphere.

A nantenna (nanoantenna) is ananoscopic rectifying antenna, anexperimental technology beingdeveloped to convert light to electricpower. The concept is based on therectenna (rectifying antenna), a deviceused in wireless power transmission. Arectenna is a specialized radio antennawhich is used to convert radio wavesinto direct current electricity. Light iscomposed of electromagnetic waveslike radio waves, but of much smallerwavelength. A nantenna is a very smallrectenna the size of a light wave,fabricated using nanotechnology,which acts as an "antenna" for light,converting light into electricity. It ishoped that arrays of nantennas couldbe an efficient means of convertingsunlight into electric power, producing solar power more efficiently than conventional solar cells. The idea was firstproposed by Robert L. Bailey in 1972. As of 2012, only a few nantenna devices have been built,[citation needed]

demonstrating only that energy conversion is possible. It is unknown if they will ever be as cost-effective asphotovoltaic cells.

A nantenna is an electromagnetic collector designed to absorb specific wavelengths that are proportional to the sizeof the nantenna. Currently, Idaho National Laboratories has designed a nantenna to absorb wavelengths in the rangeof 3–15 μm.[1] These wavelengths correspond to photon energies of 0.08 - 0.4 eV. Based on antenna theory, anantenna can absorb any wavelength of light efficiently provided that the size of the nantenna is optimized for thatspecific wavelength. Ideally, nantennas would be used to absorb light at wavelengths between 0.4 and 1.6 μmbecause these wavelengths have higher energy than far-infrared (longer wavelengths) and make up about 85% of thesolar radiation spectrum [2] (see Figure 1).

History of nantennasRobert Bailey, along with James C. Fletcher, received a patent in 1973 for an “electromagnetic wave converter”.[3]

The patented device was similar to modern day nantenna devices. Alvin M. Marks received a patent in 1984 for adevice explicitly stating the use of sub-micron antennas for the direct conversion of light power to electricalpower.[4] Marks’s device showed substantial improvements in efficiency over Bailey’s device. In 1996, Guang H.Lin was the first to report resonant light absorption by a fabricated nanostructure and rectification of light withfrequencies in the visible range. In 2002, ITN Energy Systems, Inc. published a report on their work on opticalantennas coupled with high frequency diodes. ITN set out to build a nantenna array with single digit efficiency.Although they were unsuccessful, the issues associated with building a high efficiency nantenna were betterunderstood. Research on nantennas is ongoing.

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Theory of nantennasThe theory behind nantennas is essentially the same for rectifying antennas. Incident light on the antenna causeselectrons in the antenna to move back and forth at the same frequency as the incoming light. This is caused by theoscillating electric field of the incoming electromagnetic wave. The movement of electrons is an alternating currentin the antenna circuit. To convert this into direct current, the AC must be rectified, which is typically done with somekind of diode. The resulting DC current can then be used to power an external load. The resonant frequency ofantennas (frequency which results in lowest impedance and thus highest efficiency) scales linearly with the physicaldimensions of the antenna according to simple microwave antenna theory. The wavelengths in the solar spectrumrange from approximately 0.3-2.0 μm. Thus, in order for a rectifying antenna to be an efficient electromagneticcollector in the solar spectrum, it needs to be on the order of hundreds of nm in size.

Figure 3. Image showing the skin effect at high frequencies. The darkregion, at the surface, indicates electron flow where the lighter region

(interior) indicates little to no electron flow.

Because of simplifications used in typical rectifyingantenna theory, there are several complications thatarise when discussing nantennas. At frequencies aboveinfrared, almost all of the current is carried near thesurface of the wire which reduces the effective crosssectional area of the wire, leading to an increase inresistance. This effect is also known as the “skin effect”.From a purely device perspective, the I-Vcharacteristics would appear to no longer be ohmic,even though Ohm’s law, in its generalized vector form,is still valid.

Another complication of scaling down is that diodesused in larger scale rectennas cannot operate at THzfrequencies without large loss in power. The large lossin power is a result of the junction capacitance (also known as parasitic capacitance) found in p-n junction diodesand Schottky diodes, which can only operate effectively at frequencies less than 5 THz. The ideal wavelengths of0.4–1.6 μm correspond to frequencies of approximately 190–750 THz, which is much larger than the capabilities oftypical diodes. Therefore, alternative diodes need to be used for efficient power conversion. In current nantennadevices, metal-insulator-metal (MIM) tunneling diodes are used. Unlike Schottky diodes, MIM diodes are notaffected by parasitic capacitances because they work on the basis of electron tunneling. Because of this, MIM diodeshave been shown to operate effectively at frequencies around 150 THz.

Advantages of nantennasOne of the biggest claimed advantages of nantennas is their high theoretical efficiency. When compared to thetheoretical efficiency of single junction solar cells (30%), nantennas appear to have a significant advantage.However, the two efficiencies are calculated using different assumptions. The assumptions involved in the nantennacalculation are based on the application of the Carnot efficiency of solar collectors. The Carnot efficiency, η, is givenby

where Tcold is the temperature of the cooler body and Thot is the temperature of the warmer body. In order for there to be an efficient energy conversion, the temperature difference between the two bodies must be significant. R. L. Bailey claims that nantennas are not limited by Carnot efficiency, whereas photovoltaics are. However, he does not provide any argument for this claim. Furthermore, when the same assumptions used to obtain the 85% theoretical efficiency for nantennas are applied to single junction solar cells, the theoretical efficiency of single junction solar

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cells is also greater than 85%.The most apparent advantage nantennas have over semiconductor photovoltaics is that nantenna arrays can bedesigned to absorb any frequency of light. The resonant frequency of a nantenna can be selected by varying itslength. This is an advantage over semiconductor photovoltaics, because in order to absorb different wavelengths oflight, different band gaps are needed. In order to vary the band gap, the semiconductor must be alloyed or a differentsemiconductor must be used altogether.

Limitations and disadvantages of nantennasAs previously stated, one of the major limitations of nantennas is the frequency at which they operate. The highfrequency of light in the ideal range of wavelengths makes the use of typical Schottky diodes impractical. AlthoughMIM diodes show promising features for use in nantennas, more advances are necessary to operate efficiently athigher frequencies.Another disadvantage is that current nantennas are produced using electron beam (e-beam) lithography. This processis slow and relatively expensive because parallel processing is not possible with e-beam lithography. Typically,e-beam lithography is used only for research purposes when extremely fine resolutions are needed for minimumfeature size (typically, on the order of nanometers). However, photolithographic techniques have advanced to whereit is possible to have minimum feature sizes on the order of tens of nanometers, making it possible to producenantennas by means of photolithography.

Production of a nantennaAfter the proof of concept was completed, laboratory-scale silicon wafers were fabricated using standardsemiconductor integrated circuit fabrication techniques. E-beam lithography was used to fabricate the arrays of loopantenna metallic structures. The nantenna consists of three main parts: the ground plane, the optical resonance cavity,and the antenna. The antenna absorbs the electromagnetic wave, the ground plane acts to reflect the light backtowards the antenna, and the optical resonance cavity bends and concentrates the light back towards the antenna viathe ground plane.

Lithography methodIdaho National Labs used the following steps to fabricate their nantenna arrays. A metallic ground plane wasdeposited on a bare silicon wafer, followed by a sputter deposited amorphous silicon layer. The depth of thedeposited layer was about a quarter of a wavelength. A thin manganese film along with a gold frequency selectivesurface (to filter wanted frequency) was deposited to act as the antenna. Resist was applied and patterned via electronbeam lithography. The gold film was selectively etched and the resist was removed.

Roll-to-roll manufacturingIn moving up to a greater production scale, laboratory processing steps such as the use of electron beam lithographyare slow and expensive. Therefore a roll-to-roll manufacturing method was devised using a new manufacturingtechnique based on a master pattern. This master pattern in effect mechanically “stamps” the precision pattern ontoan inexpensive flexible substrate and thereby creates the metallic loop elements seen in the laboratory processingsteps. The master template fabricated by Idaho National Laboratories consists of approximately 10 billion antennaelements on an 8-inch round silicon wafer. Using this semi-automated process, Idaho National Labs has produced anumber of 4-inch square coupons. These coupons were combined to form a broad flexible sheet of nantenna arrays.

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Atomic Layer DepositionResearchers at the University of Connecticut are using a technique called selective area atomic layer deposition thatis capable of producing them reliably and at industrial scales. Research is ongoing to tune them to the optimalfrequencies for visible and infrared light.

Proof of principleThe proof of principle for nantennas started out with a 1 cm2 silicon substrate with the printed nantenna array fillingthe area. The device was tested using infrared light with a range of 3 to 15 µm. The peak emissivity is found to becentered at 6.5 µm and reaches an emissivity of 1. An emissivity of 1 means the nantenna absorbs all of the photonsof a specific wavelength (in this case, 6.5 µm) that are incident upon the device.[5] Comparing the experimentalspectrum to the modeled spectrum, the experimental results are in agreement with theoretical expectations (Figure5). In some areas, the nantenna had a lower emissivity than the theoretical expectations, but in other areas, namely ataround 3.5 µm, the device absorbed more light than expected.

Figure 5. Experimental and theoretical emissivity as a function of wavelength. Theexperimental spectrum was determined by heating the nantenna to 200 °C and comparing

the spectral radiance to blackbody emission at 200 °C.

After a proof of concept on a stiffsilicon substrate, the experiment wasreplicated on a flexible polymer-basedsubstrate. The target wavelength forthe flexible substrate was set to 10 µm.Initial tests show that the nantennadesign can be translated to a polymersubstrate, but further experiments areneeded to fully optimize thecharacteristics.

Economics of nantennas

Nantennas (just the nano-antenna part,not the rectifier and other components)are cheaper than photovoltaics. Whilematerials and processing ofphotovoltaics are dear (currently thecost for complete photovoltaic modules is in the order of 430 USD / m² in 2011 and declining.[6]), Steven Novackestimates the current cost of the nantenna material itself as around 5 - 11 USD / m2 in 2008.[7] With properprocessing techniques and different material selection, he estimates that the overall cost of processing, once properlyscaled up, will not cost much more. His prototype was a 30 x 61 cm of plastic, which contained only 0.60 USD ofgold in 2008, with the possibility of downgrading to a material such as aluminum, copper, or silver.[8] The prototypeused a silicon substrate due to familiar processing techniques, but any substrate could theoretically be used as long asthe ground plane material adheres properly.

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Future research and goalsIn an interview on National Public Radio's Talk of the Nation, Dr. Novack claimed that nantennas could one day beused to power cars, charge cell phones, and even cool homes. Novack claimed the last of these will work by bothabsorbing the infrared heat available in the room and producing electricity which could be used to further cool theroom. (He did not discuss whether or not this would violate the second law of thermodynamics.)Currently, the largest problem is not with the antenna device, but with the rectifier. As previously stated, present-daydiodes are unable to efficiently rectify at frequencies which correspond to high-infrared and visible light. Therefore,a rectifier must be designed that can properly turn the absorbed light into usable energy. Researchers currently hopeto create a rectifier which can convert around 50% of the antenna's absorption into energy. Another focus of researchwill be how to properly upscale the process to mass-market production. New materials will need to be chosen andtested that will easily comply with a roll-to-roll manufacturing process.

References[1] Novack, Steven D., et al. “Solar Nantenna Electromagnetic Collectors.” American Society of Mechanical Engineers (Aug. 2008): 1–7. Idaho

National Laboratory. 15 Feb. 2009 <http://www.inl.gov/pdfs/nantenna.pdf>.[2] Berland, B. “Photovoltaic Technologies Beyond the Horizon: Optical Rectenna Solar Cell.” National Renewable Energy Laboratory. National

Renewable Energy Laboratory. 13 Apr. 2009 <http://www.nrel.gov/docs/fy03osti/33263.pdf>.[3] http:/ / patft. uspto. gov/ netacgi/ nph-Parser?Sect1=PTO1& Sect2=HITOFF& d=PALL& p=1& u=%2Fnetahtml%2FPTO%2Fsrchnum.

htm& r=1& f=G& l=50& s1=3760257. PN. & OS=PN/ 3760257& RS=PN/ 3760257[4] http:/ / patft. uspto. gov/ netacgi/ nph-Parser?Sect1=PTO1& Sect2=HITOFF& d=PALL& p=1& u=%2Fnetahtml%2FPTO%2Fsrchnum.

htm& r=1& f=G& l=50& s1=4445050. PN. & OS=PN/ 4445050& RS=PN/ 4445050[5] Robinson, Keith. Spectroscopy: The Key to the Stars. New York: Springer, 2007. Springer Link. University of Illinois Urbana-Champaign. 20

Apr. 2009 <http://www.springerlink.com/content/p3878194r0p70370/?p=900b261891484572a965aca5acb7d079&pi=0>.[6] Solarbuzz PV module pricing survey, May 2011 <http://solarbuzz.com/facts-and-figures/retail-price-environment/module-prices>[7] “ Nanoheating (http:/ / www. npr. org/ templates/ story/ story. php?storyId=93872974)”, Talk of the Nation. National Public Radio. 22 Aug.

2008. Transcript. NPR. 15 Feb. 2009.[8] Green, Hank. “ Nano-Antennas for Solar, Lighting, and Climate Control (http:/ / www. ecogeek. org/ content/ view/ 1357/ )”, Ecogeek. 7 Feb.

2008. 15 Feb. 2009. Interview with Dr. Novack.

Article Sources and Contributors 6

Article Sources and ContributorsNantenna  Source: http://en.wikipedia.org/w/index.php?oldid=558319800  Contributors: Ajefizzix, BadriTiwari, Chetvorno, DrStrangelove666, Dratman, Electron9, Explicit, Gagarine, Heron,Jojalozzo, Kolbasz, Mdurante, Nantennagroup, Nikevich, NuclearWarfare, Od Mishehu, Opticron, Sbyrnes321, Shawn in Montreal, Tyhou, Uwezi, Verkhovensky, Virtualerian, Whitepaw,Yowuza, 15 anonymous edits

Image Sources, Licenses and ContributorsImage:Radiation Spectrum.png  Source: http://en.wikipedia.org/w/index.php?title=File:Radiation_Spectrum.png  License: Public Domain  Contributors: Wikimedia Commons. Originaluploader was Nantennagroup at en.wikipediaImage:Skin Effect.png  Source: http://en.wikipedia.org/w/index.php?title=File:Skin_Effect.png  License: Public Domain  Contributors: Nantennagroup (talk). Original uploader wasNantennagroup at en.wikipediaFile:Nantenna Results.png  Source: http://en.wikipedia.org/w/index.php?title=File:Nantenna_Results.png  License: Public Domain  Contributors: Idaho National Labs. Original uploader wasNantennagroup at en.wikipedia

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