Review Article New Trends in Energy Harvesting from Earth ...downloads.hindawi.com/journals/amse/2014/252879.pdf · reported. In particular, we discuss the role of the rectenna system

Review ArticleNew Trends in Energy Harvesting from EarthLong-Wave Infrared Emission

Luciano Mescia1 and Alessandro Massaro2

1 Dipartimento di Ingegneria Elettrica e dellrsquoInformazione (DEI) Politecnico di Bari Via E Orabona 4 70125 Bari Italy2 Istituto Italiano di Tecnologia (IIT) Center for Biomolecurar Nanotechnologies (CBN) Via Barsanti 73010 Arnesano Italy

Correspondence should be addressed to Luciano Mescia lucianomesciapolibait

Received 12 June 2014 Accepted 18 July 2014 Published 11 August 2014

Academic Editor Andrea Chiappini

Copyright copy 2014 L Mescia and A Massaro This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

A review even if not exhaustive on the current technologies able to harvest energy from Earthrsquos thermal infrared emission isreported In particular we discuss the role of the rectenna system on transforming the thermal energy provided by the Sun andreemitted from the Earth in electricity The operating principles efficiency limits system design considerations and possibletechnological implementations are illustrated Peculiar features of THz and IR antennas such as physical properties and antennaparameters are providedMoreover somedesign guidelines for isolated antenna rectifying diode and antenna coupled to rectifyingdiode are exploited

1 Introduction

During the last 20 years the worldwide energy demands havebeen strongly increased and as a consequence the deleteriouseffects of hydrocarbon-based power such as global warningair pollution acid precipitation ozone depletion and forestdestruction are increasingly apparent In order to limit thesedrawbacks suitable actions aimed at reducing the depen-dence on the fossil fuels are needed and the search for cleanand renewable alternative energy resources is one of the mosturgent challenges to the sustainable development of humancivilization [1 2]

The Sun is themost powerful source of energy providing acontinuous stream of energy which warms us causes crops togrow via photosynthesis heats the land and sea differentiallyand so causes winds and consequently waves and of courserain leading to hydropower As a result several approachesand technologies to directly or indirectly harvest energy fromthe Sun have been successfully proposed and implementedIn particular in addition to the energy resources drivinghuman society today such as petroleum coal and nuclearplants renewable energy resources such as wind solarhydropower geothermal hydrogen and biomassbiofuelhave been positively used to give a strong contribution to

power generation without increasing environmental pollu-tion [3]

Photovoltaic (PV) conversion is the direct conversion ofsunlight into electricity without any heat engine to interferePhotovoltaic devices are rugged and simple in design andrequire very little maintenance and their construction asstand-alone systems provides outputs from microwatts tomegawatts With such a vast of properties the worldwidedemand for PV is increasing every year and industry esti-mates suggest as much as 18 billion watts per year couldship by 2020 As a result to meet the increased demandsfor solar-conversion technologies dramatic improvementsare required in terms of PV efficiency and costcomplexityreduction [4]

Solar cells convert sunlight directly into electricity Theyare made of semiconducting materials that can absorb thephotons from sunlight knocking electrons from atoms toproduce a flow of electricity The energy producing aspectof the PV module has two primary steps The first is asemiconductingmaterial such as silicon and the second is theconversion of the electricity into direct current through anarray of solar cells The first generations of solar cells used in90of todayrsquos cells use a single p-n junction to extract energyfrom sunlight photons and are manufactured from silicon

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2014 Article ID 252879 10 pageshttpdxdoiorg1011552014252879

2 Advances in Materials Science and Engineering

semiconductorsThey have about 30 efficiency but result ina price too high to compete with fossil fuels [5] The secondgenerations of solar cells called thin-film solar cells aremadefrom amorphous silicon or nonsilicon materials and exhibitlow production costs but result in much lower efficiency rates[5 6]The third generations of solar cells are beingmade fromvariety of new materials including solar inks solar dyes andconductive plastics Some new solar cells use plastic lensesor mirrors to concentrate sunlight onto a very small piece ofhigh efficiency PVmaterials [5 7] However the PVmaterialis more expensive and because the lenses must be pointed atthe Sun the use of concentrating collectors is limited to thesunniest parts of the country

As a quantum device in the semiconductor solar cellsonly sunlight of certain energies will work efficiently tocreate electricity So the efficiency of PV is fundamentallylimited by the fact that only photons with energy equal tothe band gap can be efficiently harvested For single-junctioncells the upper efficiency limit is sim30 and with complexmultijunction designs the theoretical efficiency plateau isaround 55 without excessive concentration of the incidentradiation Moreover more complex solar cells able to harvestenergy from a wider range of the electromagnetic spectrumwith higher efficiency have been proposed but they are tooexpensive for widespread use However another drawback ofPV-based technologies is the fact of being strongly dependenton daylight which in turn makes them sensitive to theweather conditions [8]

The energy created by the fusion reaction in the Sun isconverted in thermal radiation and transferred in the formof electromagnetic waves into the free space Solar radiationoccurs over a wide range of wavelengths nevertheless themain range of this radiation includes ultraviolet (120582 lt

04 120583m) of which the content is less than 9 visible (light04 120583m lt 120582 lt 07 120583m) where the content is approximately39 and the remaining 52 consists of infrared radiation(07 120583m lt 120582 lt 100 120583m) Approximately 30 of the solarradiation is scattered and reflected back to the space from theatmosphere and about 70 is absorbed by the atmosphereand by the surface of the Earth [9] By absorbing the incomingsolar radiation the Earth temperature rises and as a heatedobject mainly reemits electromagnetic radiation in the wave-length range from 8120583m to 14 120583m with a peak wavelengthof about 10120583m Due to the different spectral properties ofthe Sun and Earth emission they are classified as short-waveand long-wave infrared (LWIR) radiation respectively Thereemitted LWIR radiation energy is underutilized by currenttechnology

Since the incoming LWIR is an electromagnetic waveradiation at terahertz frequencies it can be collected bytuning an antenna in such a way that it is resonant atsuch frequencies This can be achieved by shrinking thedimensions of the antenna to the scale of the wavelength Tothis aim nanoantennas are an alternative approach used toscale themicrowave theory down to the IR regions of the elec-tromagnetic spectrum [10ndash12] These antennas can enhancethe interaction of IR waves with nanoscale matter providinga high electric field at the feeding point of the antenna [13]In particular this electric field generates a high-frequency

alternate current or voltage which can be rectified to obtainDC current The combination of a rectifying device at thefeed points of a receiving antenna is often known as arectenna [14ndash17] Accurate numerical modeling is neededfor nanoantenna performance prediction design and refine-ment as well as for obtaining some qualitative propertiesthat may help in the design of more complex antenna arrayThe identification of the optimal geometric parameters andthe frequency-dependent model of the permittivity of theconsidered materials is essential [15 18 19] Moreover thedesign of these novel antennas by using well-known printingtechniques allowing costs reduction and a quick prototypingapproach is another important aspect to consider To this aimin this paper an overview of the rectenna system is provideddetailing principles of operation antenna designs materialsand fabrication Moreover some recent technologies per-taining to both the nanoantennas and the rectifying diodesfabrication are also presented In particular the physicalproperties of nanoantennas the nanoantenna parametersand the computational considerations as well as importantaspects pertaining to the radiation efficiency directivitybandwidth polarization and impedance matching are illus-trated

2 The Rectenna Topology

Although nanoantennas capture infrared energy they needa rectifier to recover energy these devices which couplerectifiers to nanoantennas are also known as rectennasThe rectenna is a special type of antenna used to directlyconvert microwave energy into DC electricity The idea ofcollecting solar electromagnetic radiation with a rectennawas proposed three decades ago [20] but it has not yet beenfully achieved However this technology has been fruitfullyused in microwave energy harvesting for space solar powersatellite applications [21 22] wireless power transmission[23] low power electronics [24ndash26] and hybrid harvesters[27 28]

In the rectenna system the absorption of the incidentelectromagnetic radiation occurs at the resonant frequency ofthe antenna In particular when the resonantmode is exciteda cyclic plasma movement of free electrons is induced in themetal antenna The electrons freely flow along the antennagenerating alternating current at the same frequency as theresonance flowing toward the antenna feed point Howeverantennas do not provide a means of converting the collectedpower at high frequencies into DC power so this will need tobe accomplished by a transducer such as rectifier A typicalrectenna block diagram is present in Figure 1 It consists ofa nanoantenna a low-pass filter (LPF) a rectifying circuita LPF for DC path and a load The nanoantenna collectsthe IR incoming power the input low-pass filter providesmatching between the antenna and the rectifier as well assuppressing the unwanted higher harmonics rejected by therectifying circuit The rectifying circuit typically a dioderectifies the AC current induced in the antenna and the DCpass filter provides a DC path to the load by separating thehigh-frequency components from the DC signal [23]

Advances in Materials Science and Engineering 3

DC low pass filter

Rectifying circuit(diode)

LoadInput

low pass filter

IR an

tenn

a

Figure 1 Block diagram of rectenna

21 Isolated IR Antenna In recent years the use of nanoan-tennas has gained a great interest for solar energy harvest-ing [10 15 18 29] These antennas couple electromagneticradiation at very high frequencies THz and IR regimes inthe same way that RF antennas do at the correspondingwavelengths As a consequence several studies are currentlyfocused on translating the concepts of RF antennas into theoptical frequency regime

Because the size of nanoantennas is in the range froma few hundred nanometres to a few microns the techno-logical limits did not allow their realization until a fewyears ago However thanks to the development of electron-beam lithography and similar techniques the required levelof miniaturization for the realization and demonstration ofnanoantennas has been obtained [13 30 31] Nanoantennasexhibit potential advantages in terms of polarization tunabil-ity and rapid time response [10 29] In fact they have (i) avery small detection area they (ii) allow the electromagneticfield localization beyond the diffraction limit (iii) they veryefficiently release radiation from localized sources into the farfield (iv) they make possible the tailoring of the interactionof electromagnetic field at the nanoscale and (v) they canbe tuned to a specific wavelength Finally the nanoscaleantenna dimensions combined with the high electric fieldenhancement in the antenna gap enable a small devicefootprint making it compact enough to be monolithicallyintegrated with electronics and auxiliary optics

The guidelines for the nanoantenna design are quitesimilar to those used at RF frequencies but crucial differencesin their physical properties and scaling behavior occur Infact in contrast to perfectly conducting concept used at RFfrequencies at optical frequencies metals no longer behaveas perfect conductors and their interaction with electro-magnetic field is determined by the frequency-dependentcomplex dielectric function [32] In particular the Lorentz-Drude model is generally used to explain the dispersivebehavior of the metal [33 34]

120576

119903(120596) = 120576

119891

119903(120596) + 120576

119887

119903(120596) (1)

where 120576119891119903(120596) describes the free-electron effects (intraband)

and 120576119887119903(120596) describes the bound-electron effects (interband)

In particular the intraband contribution is described by theDrude model

120576

119891

119903(120596) = 1 minus

Ω

2

119901

120596 (120596 minus 119894Γ

0)

(2)

while the interband contribution is described by the modelresembling the Lorentz result for insulators

120576

119887

119903(120596) =

119896

sum

119895=1

119891

119895120596

2

119901

(120596

2

119895minus 120596

2) + 119894120596Γ

119895

(3)

where 120596

119901is the plasma frequency 119896 is the number of

oscillators with frequency 120596119895 strength 119891

119895 and lifetime 1Γ

119895

and Ω119901is the plasma frequency of the intraband transitions

with oscillator strength 119891

0and damping constant Γ

0 The

availability of this model also allows the calculation of theskin depth which at optical frequencies is comparable withthe dimensions of the antenna As a result the resonantlength of the antenna does not exactly scale linearly withthe incident frequency thus in order to better evaluate theantenna parameters an effective wavelength 120582eff should becalculated [10 35]

120582eff = 1198991 + 1198992120582

120582

119901

(4)

where 120582119901is the plasma wavelength of the metal and 119899

1and 1198992

are constant values depending on the geometry and dielectricparameters of the antenna

The radiation efficiency directivity and bandwidth of theantenna are critical parameters to take into account To thisaim to assess the overall antenna efficiency a figure of meritFoM is often defined in terms of half-power beam widthfractional bandwidth and peak gain [12 36] Moreover thedesign of the best antenna for a given application is a problemnot easy to solve because of contradicting requirementsIn fact a strong directivity a large bandwidth a smallsize and a large radiation resistance need to be combinedFurthermore the typical design strategies of the radio waveantenna engineering cannot be completely used withoutcareful considerations In fact at the THz and IR frequenciesthe metal losses became a constraint that antenna and circuitengineering have to take into account Rather a lot of theenergy in the surface modes is carried in the dielectric abovethe antenna Compared to RF regime the large losses and thefinite skin depth generate consequences as reduced radiationefficiency lower quality factor of the resonances deviatingradiation patterns and current distribution Finally thewell-known impedance matching circuits based on passivestub-like resonator structures have to be carefully designedsince the metal losses strongly reduce the overall radiationefficiency [10 32]

One important aspect that a rectenna has to verify isthat it should be able to concentrate the propagating free-space LWIR plane waves having a wide spectral bandwidthand incoming from a range of directions of incidenceAs a result the design of the isolated antenna plays animportant role for the overall rectenna efficiency Amongvarious types of antenna planar antennas are gaining pop-ularity owing to their low profile light weight and simplecoupling with rectifying element [12 16 17] In addition theyoffer versatility in terms of resonant frequency polarizationradiation pattern and impedance They are supported by asubstrate and considering that it is electrically thicker at THz

4 Advances in Materials Science and Engineering

frequencies a decreased efficiency occurs with respect to theirRF counterparts In order to overcome this drawback printedantennas having grounded substrates are generally preferredIn fact due to the image dipole generated by the groundinterface the antenna impedance is modified and substratethickness can be reduced to increase the efficiency Moreoverthe presence of the ground allows the radiation in onlyone direction On the contrary the radiation properties ofthese antennas become sensitive to substrate losses especiallywhen the substrate thickness increases and the substratepermittivity acting as a parasitic impedance causes a redshift of the resonant frequency As a result for a givensubstrate permittivity there is a particular substrate thicknessmaximizing the performance of the printed antennas Untilnow dipole [11 14 15 37] crossed dipole [12 38] bowtie[11 39 40] log-periodic [11 41] square-spiral [13 18] andArchimedean spiral [11 42] geometries have been proposedfor IR and THz antennas

Half-wave dipoles could be designed to have purelyreal input impedance thus no conjugate impedance matchoccurs Their very good directivity is attractive in terms ofenhanced sensitivity for detection The input impedancecurrent distribution radiation efficiency broadside gaineffective area and effective length depend on the arm sizefrequency and employed metal [11 14] In particular thephysical length is shorter but close to half the wavelengthand decreases by increasing the arm thickness Moreoverthe interelement distance affects input impedance and fieldenhancement in the feed-gap region [37] Unfortunately thiskind of antenna does not allow flexibility to increase oroptimize the electric field in the gap The only approach is tovary the gap size or increase the rods width

The bowtie antenna could be a good candidate to replacethe dipole antenna It is constituted of two triangles facingeach other tip to tip This configuration allows a simpledesign and broadband impedance and makes possible themodification of several antenna parameters In fact gapsize apex angle and antenna dimensions could be tuningto increase the captured electric field in the gap Moreoverbecause they represent the two-dimensional analogue of abiconical antenna they possess a broad bandwidth Anotheradvantage of bowtie antennas is the ability of building anarray by coupling many elements and combining the electricfield from each element at array feeding point where arectifier can be embedded In order to consider bowtieantennas for practical applications a finite gap between thefeed points and a finite size have to be used Generally theseconstraints result in limited bandwidth but no significanteffect on the radiation pattern or the impedance typicallyoccurs if the antenna is terminated with a bow-arm lengthof 2120582eff [11]

Due to their broad bandwidth spiral antennas have beenproposed to collect solar energy [13 18] They allow con-centrating the electric field in the gap between two metallicarms which constitutes an appropriate point to transportenergy needed to supply other circuitries These antennasare good resonators and it is expected to capture a largeelectric field at resonance Moreover the gain performanceof the spiral antenna can be easily improved by increasing

the number of arms Round spiral antennas are generallydesigned by using Archimedean spiral geometries whichhave linear growth rates and frequency independent radiativecharacteristics Moreover the frequency independency islimited to a wavelength band determined by the antenna sizeSpiral antennas can be constructed as planar structures andthey can radiate linearly or circularly polarized waves Theoptimal reception of a spiral antenna occurs when the spiralarm length equals approximately one wavelength whichcorresponds to a diameter of 119863 = 120582eff120587 for the circularspiral and a side length 119882 = 120582eff4 for the square spiralAccording to these relations square-spiral geometries havemore advantages in terms of size with respect to circularones because comparable antenna gain can be obtainedwhen the width of the square spiral is approximately 75of the diameter of the circular spiral antenna However themain drawback with this type of antenna is the difficulty inconfiguring an array Despite that equiangular spiral can bechosen as the array element since it allows (i) convenientconnection of DC lines at the tips of the spiral arms (ii)possible dual polarization and (iii) convenient feed point fordiode connection

Although the THz and IR antennas are usually synthe-sized by means of basic and somewhat simple elementsthe lack of guidelines for the synthesis process as well asthe absence of mature theory and design equations fornanoantennas makes the computational tools very usefulto fulfill complex or nonstandard design requirements Infact considering that the antenna design usually involves theoptimization of amultidimensional parameter space a carefulinvestigation of proper global optimization tools has to beperformed in order to reduce the severe computational limitsdue to the expensive discretizations due to the numericalmodeling To this aim efficient optimization tools basedon stochastic optimization techniques such as genetic algo-rithms and particle swarm optimization have been efficientlyemployed in the antenna synthesis [43]

22 IR Antenna Coupled to Rectifying Diode As mentionedabove a suitable choice of the antenna material as well as anaccurate design of the antenna has to be fulfilled to improvethe coupling efficiency of the free-space radiation into theantenna However a suitable rectifier has to be attached tothe antenna to obtain a DC signal As a consequence the RF-to-DC conversion efficiency of a rectenna is influenced bythe amount of power loss in the diodes by the impedancematch between the antenna and the rectifier and betweenthe rectifier and the load and also by the antenna efficiencyIn fact nonoptimized element design impedance mismatchbetween components and inefficient rectifying junctionscould contribute to unsuccessful collection of the incomingelectromagnetic energy

Figure 2 shows the equivalent circuit of the antennacoupled to rectifying diode The receiving antenna whenoperating at its resonant frequency can be modeled by avoltage source 119881open and an impedance in series 119885

119860= 119877

119860+

119895119883

119860 In particular119881open is the open circuit voltage occurring

at the end of the antenna when no load is connected and 119885119860

Advances in Materials Science and Engineering 5

LWIR

IR antenna Rectifying diode

RA XA

ZLVDRD(VD)CD

CgapVopen

Figure 2 Circuit model of IR antenna coupled to rectifying diode

is the antenna impedance where119883119860is the antenna reactance

and119877119860is the antenna resistancewhich is a combination of the

radiation resistance modelling the radiated power in serieswith the loss resistance and modelling the conductive anddielectric losses Moreover the capacitance 119862gap generatedby the air gap should be considered in order to rightly modeltwo-arm antenna

The rectifying diode is generally characterized by athreshold voltage a junction capacitance 119862

119863 and a non-

linear series resistance 119877119863 The junction capacitance has an

impact on diode switching time a fast diode should havesmall junction capacitance In fact the cutoff frequency 119891

119888

characterizing the frequency response of the diode effectivelydepends on both the diode resistance and capacitance asfollows

119891

119888=

1

2120587119877

119863119862

119863

(5)

So considering that the resistance 119877119863mainly depends on

the fabrication process the cutoff frequency can be tunedby adjusting the capacitance 119862

119863 However the presence of

the antenna resistance modifies the overall device responseso that the cutoff frequency of the device is evaluated by thefollowing relation

119891

119888=

119877

119860+ 119877

119863

2120587119877

119860119877

119863(119862gap + 119862119863)

(6)

Moreover the threshold voltage is a very important factorto consider especially when low power levels have to beharvested So for rectification purpose a low-cutoff voltagediode has to be selected

The voltage 119881open can be expressed as

119881open = 2119864119894radic119877

119860119860eff119885

0

(7)

where 119864119894is the incident electric field 119860eff is the effective area

of the antenna and1198850is the intrinsic impedance of free space

Moreover 119860eff is defined as

119860eff =120582

2

119866

4120587

(8)

where 119866 is the antenna gain and 120582 is the free-space wave-length The amplitude of the incident electric field can becalculated as

119864

119894= radic2119885

0119875

119894

(9)

where 119875119894is the incident power density In particular consid-

ering the thermal radiation emitted by the Earth the incidentpower can be expressed in terms of the radiation emittedby a black body at the temperature 119879 per unit of area andwavelength [44]

119875

119894(120582 119879) =

2120587119888

2

ℎ

120582

5

1

exp (ℎ119888120582120581119879) minus 1 (10)

where 119888 is the speed of light119879 is the temperature expressed inKelvin degree 119875

119894(120582 119879)119889120582 is the amount of the power emitted

in the wavelength range from 120582 to 120582 + 119889120582 per unit of areaunit of time and unit of solid angle and ℎ and 120581 are the Plankand Boltzmann constant respectively Using (7)ndash(9) the finalexpression of the open circuit voltage is

119881open =radic

2119877

119860120582

2

119866

120587

119875

119894

(11)

Considering the equivalent circuit illustrated in Figure 2 thepower delivered to the load 119885

119871is given by the following

equation

119875

119871=

1

2

119877

119863

(119877

119860+ 119877

119863)

2

+ (119883

119860+ 119883

119863)

2

1003816

1003816

1003816

1003816

1003816

119881open1003816

1003816

1003816

1003816

1003816

2

(12)

or using (11)

119875

119871(120582 119879) =

1

120587

119877

119860119877

119863120582

2

119866

(119877

119860+ 119877

119863)

2

+ (119883

119860+ 119883

119863)

2119875

119894(120582 119879) (13)

Considering the frequency dependence of the incident powerdensity the total received power over a range of frequenciesis given by

119875

119871tot = int1205822

1205821

119875

119871119889120582 = int

1205822

1205821

1

120587

119877

119860119877

119863120582

2

119866119875

119894

(119877

119860+ 119877

119863)

2

+ (119883

119860+ 119883

119863)

2119889120582

(14)

where 1205821and 120582

2are the starting and stopping wavelengths

The RF-to-DC conversion efficiency of the rectenna isusually defined as the ratio between the power delivered tothe load (harvest DC power) and the amount of the powerthat the receiving antenna could inject in a perfectly matchedcircuit

120578 =

119875DC119875

119894tot=

119875DC

int

1205821

1205821

119860eff119875119894119889120582 (15)

The nonlinear nature of diodes complicates the analyticalevaluation of the conversion efficiency In fact for most rec-tifier circuits the 119875DC depend on input power 119875

119871tot operatingfrequency impedance matching and diode properties Inparticular a good model to estimate 120578 is

120578 =

119881

119863119868out

(1119879RF) int119879RF

0

Vin (119905) 119868DC (119905) 119889119905 (16)

6 Advances in Materials Science and Engineering

where 119879RF is the period of the input RF signal Vin is the inputvoltage to the rectifier 119868out is the current flowing throughthe load terminals 119868DC(119905) is the current flowing through thediode terminals and 119881

119863is the DC voltage

This circuit model illustrated in Figure 2 gives quitedetailed information on how the THz and IR solar rectennaworks including the parameters affecting its performanceHowever the main limitation of this circuit is that a goodRF-to-DC conversion efficiency is given for a well-definedoperation point characterized by a specific input power levelcentral frequency and load impedance Outside these oper-ating parameters the energy conversion efficiency stronglydecreases In fact rectenna structure well works for anoptimal input power level and becomes inefficient at anotherpower level This problem is very huge since harvestingsystems generally are required to operate at variable work-load conditions to dynamically track voltage levels whileconserving energy In order to overcome these limitationsspecific design procedures have to be fulfilled in terms ofload and power matching Typical solutions are based on theuse of maximum power point tracking voltage boost stage[45] a dynamic switching conversion scheme based on activecontrol for harvesting energy [46] and modified Greinacherrectifier [47]

23 Rectifying Element There are a number of issues relatedto the development of a rectenna Firstly the antennaelements need to be extremely small Another difficultyis making diodes with small physical size small turn-onvoltage and efficient operation at THz and IR frequenciesable to rectify the received signals to DC as a usable outputMoreover to efficiently convert electromagnetic energy andto take full advantage of the enhanced electric field in thecenter gap of the antenna the diode should be coplanar andcoupled to the antenna As a result the development of diodetechnology is the key challenge to demonstrate the feasibilityof rectenna to convert the thermal Earth radiation in DCcurrent

Low power Schottky diodes are used for rectificationand detection in the low frequency regime up to 5 THz[48] In fact due to their ultrafast transport mechanismthey are scalable to very high frequencies by reducing theirphysical contact area The most important advantages of theSchottky diode are the lower forward resistance and lowernoise generation However the fabrication of large arraysrequires challenging efforts and additional engineering issuesare needed for their coupling with antennas

A promising alternative is the unipolar nanodiodesknown as self-switching devices (SSDs) [49 50] Thesedevices are based on an asymmetric nanochannel whichresults in a nonlinear diode-like current-voltage character-istic but without using any doped junctions or any tunnelingbarriers Their threshold voltage only depends on the geom-etry and zero-threshold detectors can be easily fabricatedMoreover a single fabrication step needs for the fabrication ofarrays a large number of SSDs connected in parallelThe SSDhas been demonstrated in a variety of materials including

two-dimensional electron gases (2DEGs) in GaAs [50] andInGaAs [51] silicon on insulator [52] and both organic [53]and metal-oxide [54] thin films

Antenna-coupled microbolometer detectors have beendemonstrated in the infrared at wavelengths near 10120583mTheoperation principle of these devices is based on change ofthe bolometer resistance with an increase in the temperatureIn particular their advantages are the room temperatureoperation as well as their tunability for wavelength andpolarization response [31 55 56]

The diodes can be classified in low voltage tunnel typediodes and ultralow-voltage diodes Examples of low voltagetunnel diodes are studied in [57] where different tunnelingjunction dimensionalities exhibit different turn-on character-istics The most popular rectifier in THz and IR rectennasis the metal-insulator-metal (MIM) diode It is a thin-filmdevice in which the electrons tunnel through the insulatorlayer from the first metal layer to the second metal [15]The main advantages of these diodes are small size CMOScompatibility and ability to offer full functionality withoutcooling and applied bias The rectification is based on theelectron tunneling process occurring through the insulatorlayer The study of inorganic (the insulator used can bethe nickel oxide) and organic (the insulator used can bepolyaniline thiol) MIM tunnel junctions has been discussedin [58] This study was oriented on solarthermal energyconversion efficiency by converting waste heat to electricalenergy using rectenna discussing the implementation of self-assembled monolayers (SAMs) as alkanethiol SAMs For asuccessful rectification the I-V characteristics of a MIMdiode should be nonlinear and asymmetrical with no externalbias appliedMoreover the insulator layer should be very thinto allow sufficiently large electrical current and to ensure theoccurrence of the tunneling effect To this aim MIM diodesfabricated with dissimilar metals on both sides of the insu-lator layer result in higher efficiency energy conversion thanwith similar metals When operating in higher frequenciesgreater optimization of the device is required to address lowimpedance and high nonlinearityMoreover in order to allowthe rectification at THz frequencies the diode area has to bevery small

Ultralow-voltage diodes are ballistic with geometricalasymmetry and are characterized by a low capacitance Two-dimensional ballistic nanodevices could be able to rectifyan electric signal if the device has a taper-type nonuniformcross section [59] Tapered profiles can be also considered inAuSiO

2or AuSi plasmon waveguide for nanoscale focusing

of light at 830 nm [60] and also for midinfrared energy[61] Considering innovative materials graphene could beimplemented for rectennas improving performances orientedon THz resonator material and rectifying 106 120583m radiationcorresponding to an operating frequency of 28 THz [62]The ballistic rectifier can be also manufactured by means ofGaAs-AlGaAs heterostructures in asymmetricmicrojunctionconfiguration [63]

24 Technological Aspects andMaterials As discussed in pre-vious sections significant progress in improving the overall

Advances in Materials Science and Engineering 7

rectenna efficiency can be obtained through a careful designto efficiently match the broadband arbitrarily polarizednature of the radiation energy reemitted by the Earth Inaddition the introduction of innovative layouts andmaterialscould provide a broadband high conversion efficiency low-cost solution supporting conventional photovoltaic solarcells Moreover a little added cost by integrating the plas-monic emitter with the cell could significantly increase theefficiency of photovoltaic PV cells [64] In this direction CP1polymer material can be used for both IR transmissive andelectrically conductive materials for MEMS based thermaldevice in satellites [65] other polyimides tend to be expen-sive absorb toomuch solar energy have lower UV resistanceand are not as transparent as CP1 degrading more rapidly inthe space environment Planar metal-insulator-metal (MIM)diodes cannot provide a sufficiently low RC time constantto rectify visible light but could be easily integrated insolar rectennas [66] Thermal infrared light represents anextreme challenge to harvest efficiently using planar MIMdiodes their large RC time orients the diodes on visiblelight frequency rectification they can work at low terahertzfrequencies but for thermal infrared frequencies of sim30 THzand higher they cannot respond efficiently Radiative coolingdevices should ideally work with a substrate blocking solarradiation but it is transparent around 8ndash13 120583m An innovativenew type of material for radiative cooling applications is thepolyethylene foils pigmented with nanocrystalline TiO

2[67]

providing high IR transmittance and high solar reflectanceTitania nanoparticles are also suitable for high-resonantenergy photons allowing a broad solar spectral absorption[68] from the visible and near-infrared domain Consideringnanocomposite materials which are made by a polymer withthe introduction of nanofillers improving optical and physicalproperties [69] NIR reflectance efficiency for solar thermalcontrol interface films was found for PMMAZnO nanopar-ticles [70] Also dye-sensitized solar cells could utilize nano-materials such as semiconductor nanowires nanoconesnanotubes and nanofibers which could be prepared bychemical vapour deposition (CVD) colloidal lithographytemplate-guided deposition or electrospinning technique[71] Optoelectronic emissive energy harvester is commonlyimplemented by rectenna In particular concerning tech-nology the antennas could be fabricated by high-resolutionelectron-beam lithography and metal lift-off on double-side-polished silicon substrates using polymethyl methacrylate aselectron-sensitive polymer and by thermally evaporated gold[61]

The fabrication of THz and IR antenna requires reliableand reproducible structuring techniques able to accuratelydefine critical antenna dimensions such as gap size and armlength Various top-down and bottom-up nanofabricationapproaches have been applied to experimentally realize thesekinds of antennas In particular top-down approaches suchas electron-beam lithography (EBL) [72 73] and focused-ionbeam (FIB)milling [74] are capable of fabricating large arraysof nearly identical nanostructures with defined orientationand distances On the other hand bottom-up approachestake advantage of chemical synthesis and self-assembly ofnanoparticles in solution but they often require precise size

selection and nanopositioning as well as assembly strategiesto create nontrivial structures

EBL could be a convenient way to systematicallyinvestigate dimensions spacing and geometrical effects ina controlled manner Recently electron-beam induced depo-sition has been applied to build complex nanostructures [72]Moreover this technique could be applied to engineeringof the dielectric properties of the antenna environmentConsidering the high versatility of the direct patterningapproach the FIB milling has been successfully applied in arealization of a number of optical antennas Therefore thistechnique ensures a very good resolution and can be adoptedto almost any type of material However considering thatboth EBL and FIB are very slow and expensive they do notsupport large-scale manufacturing Possible alternatives arenanoimprint lithography (NIL) [75] and roll-to-roll (R2R)processing [76]The particular advantage ofNIL compared toother lithography techniques is the ability to fabricate large-area and complex 3D micronanostructures with low-costand high throughput The most important variety of NILprocess types demonstrating a sub-10 nm resolution is thehot embossing lithography (HEL) or thermal nanoimprintlithography (TNIL) and the UV-based nanoimprint lithogra-phy (UV-NIL) However in recent years a variety of new pro-cesses have been proposed and investigated such as reverseNIL soft UV-NIL laser assisted direct imprint (LADI)sub-10 nm NIL chemical nanoimprint and electrical field-assisted NIL [77] For conventional NIL processes the mostimportant problem is that it cannot significantly improve thethroughput in the patterning of large-area product with lowcost because it is not a continuous process To overcome thislimitation roller-type nanoimprint lithography (RNIL) [7879] has been developed and due to the continuous processsimple system construction high throughput low cost andlow energy consuming this technology is becoming themost potential manufacturing method for industrializationof nanoimprinting process However in future NIL mightbecome the ideal technique for low-cost highly reproduciblerealization of antenna arrays covering large areas

3 Conclusion

Theprogress and the challenges of rectenna to harvest energyfrom Earth long-wave infrared emission have been reviewedThe rectenna system can be made from different conduct-ing metals and dielectric materials a variety of broadbandantennas and a number of rectifying devices The use ofbroadband antennas for collection of long-wave infraredEarthrsquos energy has a big potential advantage As a result theaccurate design of the antenna is a key topic to improve theelectricity generation efficiency of the overall system Thestudy of IR and THz antennas is still in its initial stage andextensive research needs to be performed to improve thematching efficiency due to the mismatch between antennaand rectifier impedance as well as to produce maximumelectric field enhancement at the feeding point of the antennaMoreover further research activities have to be fulfilled toidentify the suitable materials and technology for the designand fabrication of efficient THz rectifiers

8 Advances in Materials Science and Engineering

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

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[2] A Arigliano P Caricato A Grieco and E Guerriero ldquoProduc-ing storing using and selling renewable energy the best mixfor the small medium industryrdquo Computers in Industry vol 65no 3 pp 408ndash418 2014

[3] A Khaligh and O G Onar Energy Harvesting Solar Wind andOcean Energy Conversion Systems CRC Press 2010

[4] G N Tiwari and S Dubey Fundamentals of PhotovoltaicModules andTheir Applications RSC Publishing 2010

[5] V V Tyagi N A A Rahim and J A L Selvaraj ldquoProgressin solar PV technology research and achievementrdquo Renewableand Sustainable Energy Reviews vol 20 pp 443ndash461 2013

[6] C Becker D Amkreutz T Sontheimer et al ldquoPolycrystallinesilicon thin-film solar cells status and perspectivesrdquo SolarEnergy Materials and Solar Cells vol 119 pp 112ndash123 2013

[7] Z Abdin M A Alim R Saidur et al ldquoSolar energy harvestingwith the application of nanotechnologyrdquo Renewable and Sus-tainable Energy Reviews vol 26 pp 837ndash852 2013

[8] A Goetzberger and V U Hoffmann Photovoltaic Solar EnergyGeneration Springer Berlin Germany 2005

[9] G L Stephens J Li M Wild et al ldquoAn update on Earthrsquosenergy balance in light of the latest global observationsrdquoNatureGeoscience vol 5 no 10 pp 691ndash696 2012

[10] P Bharadwaj B Deutsch and L Novotny ldquoOptical AntennasrdquoAdvances in Optics and Photonics vol 1 no 3 pp 438ndash4832009

[11] F J Gonzalez and G D Boreman ldquoComparison of dipolebowtie spiral and log-periodic IR antennasrdquo Infrared Physics ampTechnology vol 46 no 5 pp 418ndash428 2005

[12] I Kocakarin and K Yegin ldquoGlass superstrate nanoantennas forinfrared energy harvesting applicationsrdquo International Journalof Antennas and Propagation vol 2013 Article ID 245960 7pages 2013

[13] D K Kotter S D Novack W D Slafer and P J PinheroldquoTheory and manufacturing processes of solar nanoantennaelectromagnetic collectorsrdquo Journal of Solar Energy Engineeringvol 132 no 1 Article ID 011014 9 pages 2010

[14] Z Ma and G A E Vandenbosch ldquoOptimal solar energyharvesting efficiency of nano-rectenna systemsrdquo Solar Energyvol 88 pp 163ndash174 2013

[15] A M A Sabaawi C C Tsimenidis and B S Sharif ldquoAnalysisand modeling of infrared solar rectennasrdquo IEEE Journal onSelected Topics in Quantum Electronics vol 19 no 3 Article ID9000208 2013

[16] S Shrestha S Noh andDChoi ldquoComparative study of antennadesigns for RF energy harvestingrdquo International Journal ofAntennas and Propagation vol 2013 Article ID 385260 10pages 2013

[17] S Shrestha S R Lee and D-Y Choi ldquoA new fractal-basedminiaturized sual band patch antenna for RF energy harvest-ingrdquo International Journal of Antennas and Propagation vol2014 Article ID 805052 9 pages 2014

[18] M Gallo L Mescia O Losito M Bozzetti and F PrudenzanoldquoDesign of optical antenna for solar energy collectionrdquo Energyvol 39 no 1 pp 27ndash32 2012

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[20] R L Bailey ldquoProposed ne w concept for a solar-energy con-verterrdquo Journal of Engineering for Gas Turbines and Power vol94 no 2 pp 73ndash77 1972

[21] R Wang D Ye S Dong et al ldquoOptimal matched rectifyingsurface for space solar power satellite applicationsrdquo IEEE Trans-actions on Microwave Theory and Techniques vol 62 pp 1080ndash1089 2014

[22] A Takacs H Aubert S Fredon L Despoisse and H Blon-deaux ldquoMicrowave power harvesting for satellite health mon-itoringrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 62 pp 1090ndash1098 2014

[23] Z Ma and G A E Vandenbosch ldquoWideband harmonicrejection filtenna forwireless power transferrdquo IEEETransactionson Antennas and Propagation vol 62 no 1 pp 371ndash377 2014

[24] U Alvarado A Juanicorena I Adin B Sedano I Gutierrezand J De No ldquoEnergy harvesting technologies for low-powerelectronicsrdquo European Transactions on Telecommunications vol23 no 8 pp 728ndash741 2012

[25] K W Lui A Vilches and C Toumazou ldquoUltra-efficientmicrowave harvesting system for battery-less micropowermicrocontroller platformrdquo IETMicrowaves Antennas and Prop-agation vol 5 no 7 pp 811ndash817 2011

[26] J Masuch M Delgado-Restituto D Milosevic and P BaltusldquoCo-integration of an RF energy harvester into a 24 GHztransceiverrdquo IEEE Journal of Solid-State Circuits vol 48 no 7pp 1565ndash1574 2013

[27] A Collado and A Georgiadis ldquoConformal hybrid solar andelectromagnetic (EM) energy harvesting rectennardquo IEEE Trans-actions on Circuits and Systems I Regular Papers vol 60 no 8pp 2225ndash2234 2013

[28] T Peter T A Rahman S W Cheung R Nilavalan HF Abutarboush and A Vilches ldquoA novel transparent UWBantenna for photovoltaic solar panel integration and RF energyharvestingrdquo IEEE Transactions on Antennas and Propagationvol 62 pp 1844ndash1853 2014

[29] J Alda J M Rico-Garcıa J M Lopez-Alonso and G BoremanldquoOptical antennas for nano-photonic applicationsrdquo Nanotech-nology vol 16 no 5 pp S230ndashS234 2005

[30] M Bareiss B N Tiwari A Hochmeister et al ldquoNano antennaarray for terahertz detectionrdquo IEEE Transactions on MicrowaveTheory and Techniques vol 59 no 10 pp 2751ndash2757 2011

[31] M A Gritz I Puscasu D Spencer and G D BoremanldquoFabrication of an infrared antenna-coupled microbolometerlinear array using chrome as amaskrdquo Journal of Vacuum Scienceand Technology B vol 21 no 6 pp 2608ndash2611 2003

[32] P Biagioni J-SHuang andBHecht ldquoNanoantennas for visibleand infrared radiationrdquo Reports on Progress in Physics vol 75no 2 Article ID 024402 2012

[33] A D Rakic A B Djurisic J M Elazar and M L MajewskildquoOptical properties ofmetallic films for vertical-cavity optoelec-tronic devicesrdquo Applied Optics vol 37 no 22 pp 5271ndash52831998

[34] R Qiang R L Chen and J Chen ldquoModeling electricalproperties of gold films at infrared frequency using FDTD

Advances in Materials Science and Engineering 9

methodrdquo International Journal of Infrared andMillimeterWavesvol 25 no 8 pp 1263ndash1270 2004

[35] L Novotny ldquoEffective wavelength scaling for optical antennasrdquoPhysical Review Letters vol 98 Article ID 266802 2007

[36] S Ladan N Ghassemi A Ghiotto and KWu ldquoHighly efficientcompact rectenna for wireless energy harvesting applicationrdquoIEEE Microwave Magazine vol 14 no 1 pp 117ndash122 2013

[37] A Locatelli ldquoAnalysis of the optical properties of wire antennaswith displaced terminalsrdquo Optics Express vol 18 no 9 pp9504ndash9510 2010

[38] J L Stokes Y Yu Z H Yuan et al ldquoAnalysis and design ofa cross dipole nanoantenna for fluorescence-sensing applica-tionsrdquo Journal of the Optical Society of America B vol 31 pp302ndash310 2014

[39] E Briones J Alda and F J Gonzalez ldquoConversion efficiency ofbroad-band rectennas for solar energy harvesting applicationsrdquoOptics Express vol 21 no 3 pp A412ndashA418 2013

[40] P M Krenz B Tiwari G P Szakmany et al ldquoResponseincrease of IR antenna-coupled thermocouple using impedancematchingrdquo IEEE Journal of Quantum Electronics vol 48 no 5pp 659ndash664 2012

[41] A D Semenov H Richter H W Hubers et al ldquoTerahertzperformance of integrated lens antennas with a hot-electronbolometerrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 55 pp 239ndash247 2007

[42] S Cherednichenko A Hammar S Bevilacqua V DrakinskiyJ Stake and A Kalabukhov ldquoA room temperature bolometerfor terahertz coherent and incoherent detectionrdquo IEEE Trans-actions on Terahertz Science and Technology vol 1 no 2 pp395ndash402 2011

[43] P Bia D Caratelli L Mescia and J Gielis ldquoElectromag-netic characterization of supershaped lens antennas for high-frequency applicationsrdquo in Proceedings of the 43rd EuropeanMicrowave Conference pp 1679ndash1682 2013

[44] M Planck ldquoUber das Gesetz der Energieverteilung imNormal-spektrumrdquo Annalen der Physik vol 4 pp 553ndash558 1901

[45] V Marian B Allard C Vollaire and J Verdier ldquoStrategy formicrowave energy harvesting from ambient field or a feedingsourcerdquo IEEE Transactions on Power Electronics vol 27 no 11pp 4481ndash4491 2012

[46] A Costanzo A Romani D Masotti N Arbizzani and VRizzoli ldquoRFbaseband co-design of switching receivers formultiband microwave energy harvestingrdquo Sensors and Actua-tors A vol 179 pp 158ndash168 2012

[47] U Olgun C Chen and J L Volakis ldquoInvestigation of rectennaarray configurations for enhanced RF power harvestingrdquo IEEEAntennas andWireless Propagation Letters vol 10 pp 262ndash2652011

[48] H Kazemi K Shinohara G Nagy et al ldquoFirst THz and IR char-acterization of nanometer-scaled antenna-coupled InGaAsInPSchottky-diode detectors for room temperature infrared imag-ingrdquo in Infrared Technology and Applications XXXIII 65421Jvol 6542 of Proceedings of SPIE Orlando Fla USA April 2007

[49] C Balocco S R Kasjoo L Q Zhang Y Alimi and A MSong ldquoLow-frequency noise of unipolar nanorectifiersrdquoAppliedPhysics Letters vol 99 no 11 Article ID 113511 2011

[50] C Balocco S R Kasjoo X F Lu et al ldquoRoom-temperatureoperation of a unipolar nanodiode at terahertz frequenciesrdquoApplied Physics Letters vol 98 no 22 Article ID 223501 2011

[51] C Balocco M Halsall N Q Vinh and AM Song ldquoTHz oper-ation of asymmetric-nanochannel devicesrdquo Journal of PhysicsCondensed Matter vol 20 no 38 Article ID 384203 2008

[52] G Farhi E Saracco J Beerens D Morris S A Charlebois andJ-P Raskin ldquoElectrical characteristics and simulations of self-switching-diodes in SOI technologyrdquo Solid-State Electronicsvol 51 no 9 pp 1245ndash1249 2007

[53] L A Majewski C Balocco R King S Whitelegg and AM Song ldquoFast polymer nanorectifiers for inductively coupledRFID tagsrdquoMaterials Science and Engineering B vol 147 no 2-3 pp 289ndash292 2008

[54] J Kettle R M Perks and R T Hoyle ldquoFabrication of highlytransparent self-switching diodes using single layer indium tinoxiderdquo Electronics Letters vol 45 no 1 pp 79ndash81 2009

[55] A Hammar S Cherednichenko S Bevilacqua V Drakin-skiy and J Stake ldquoTerahertz direct detection in YBa

2Cu3O7

microbolometersrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 2 pp 390ndash394 2011

[56] B S Karasik A V Sergeev and D E Prober ldquoNanobolometersfor THz photon detectionrdquo IEEE Transactions on TerahertzScience and Technology vol 1 no 1 pp 97ndash111 2011

[57] S Agarwal and E Yablonovitch ldquoUsing dimensionality toachieve a sharp tunneling FET (TFET) turn-onrdquo in Proceedingsof the 69th Device Research Conference (DRC rsquo11) pp 199ndash200Santa Barbara Calif USA June 2011

[58] S Bhansali S Krishnan E Stefanakos and D Y GoswamildquoTunnel junction based rectennamdasha key to ultrahigh efficiencysolarthermal energy conversionrdquo in Proceedings of the Interna-tional Conference on Physics of Emerging Functional Materials(PEFM rsquo10) pp 79ndash83 Mumbai India September 2010

[59] D Dragoman and M Dragoman ldquoGeometrically inducedrectification in two-dimensional ballistic nanodevicesrdquo Journalof Physics D Applied Physics vol 46 no 5 Article ID 0553062013

[60] H Choo M-K Kim M Staffaroni et al ldquoNanofocusing ina metal-insulator-metal gap plasmon waveguide with a three-dimensional linear taperrdquo Nature Photonics vol 6 no 12 pp838ndash844 2012

[61] M Schnell P Alonso-Gonzalez L Arzubiaga et al ldquoNanofo-cusing of mid-infrared energy with tapered transmission linesrdquoNature Photonics vol 5 no 5 pp 283ndash287 2011

[62] Z Zhu S Joshi S Grover andGModdel ldquoGraphene geometricdiodes for terahertz rectennasrdquo Journal of Physics D AppliedPhysics vol 46 no 18 Article ID 185101 2013

[63] A M Song A Lorke A Kriele J P Kotthaus W WegscheiderandM Bichler ldquoNonlinear electron transport in an asymmetricmicrojunction a ballistic rectifierrdquo Physical Review Letters vol80 no 17 pp 3831ndash3834 1998

[64] S Krishnan Y Goswami and E Stefanakos ldquoNanoscaaleRectenna for thermal energy conversion to electricityrdquo Technol-ogy amp Innovation vol 14 no 2 pp 103ndash113 2012

[65] M A Darrin R Osiander J Lehtonen D Farrar D Douglasand T Swanson ldquoNovel micro electro mechanical systems(MEMS) packaging for the skin of the satelliterdquo in Proceeding ofthe IEEE Aerospace Conference vol 4 pp 2486ndash2492 Big SkyMont USA March 2004

[66] G Moddel ldquoChapter 1 Will rectenna solar cells be practicalrdquoinRectenna Solar Cells GModdel and S Grover Eds pp 3ndash24Springer New York NY USA 2013

[67] Y Mastai Y Diamant S T Aruna and A Zaban ldquoTiO2

nanocrystalline pigmented polyethylene foils for radiative cool-ing applications synthesis and characterizationrdquo Langmuir vol17 no 22 pp 7118ndash7123 2001

10 Advances in Materials Science and Engineering

[68] B E Hardin E T Hoke P B Armstrong et al ldquoIncreased lightharvesting in dye-sensitized solar cells with energy relay dyesrdquoNature Photonics vol 3 no 11 p 667 2009

[69] A Massaro F Spano M Missori et al ldquoFlexible nanocompos-ites with all-optical tactile sensing capabilityrdquo RSC Advancesvol 4 no 6 pp 2820ndash2825 2014

[70] S Soumya A Mohamed P Paul L Mohan and K Anan-thakumar ldquoNear IR reflectance characteristics of PMMAZnOnanocomposites for solar thermal control interface filmsrdquo SolarEnergy Materials amp Solar Cells vol 125 pp 102ndash112 2014

[71] M Yu Y Long B Sun and Z Fan ldquoRecent advances insolar cells based on one-dimensional nanostructure arraysrdquoNanoscale vol 4 no 9 pp 2783ndash2796 2012

[72] W F van Dorp and C W Hagen ldquoA critical literature review offocused electron beam induced depositionrdquo Journal of AppliedPhysics vol 104 no 8 Article ID 081301 2008

[73] A Weber-Bargioni A Schwartzberg M Schmidt et alldquoFunctional plasmonic antenna scanning probes fabricated byinduced-deposition mask lithographyrdquo Nanotechnology vol 21Article ID 065306 2010

[74] J Orlo M Utlaut and L Swanson High Resolution FocusedIon Beams FIB and Applications Kluwer AcademicPlenumPublishers 2002

[75] S Y Chou P R Krauss and P J Renstrom ldquoNanoimprintlithographyrdquo Journal of Vacuum Science and Technology B vol14 no 6 pp 4129ndash4133 1996

[76] K Jain ldquoFlexible electronics and displays high-resolution roll-to-roll projection lithography and photoablation processingtechnologies for high-throughput productionrdquo Proceedings ofthe IEEE vol 93 no 8 pp 1500ndash1510 2005

[77] M D Stewart and C G Willson ldquoImprint materials fornanoscale devicesrdquo MRS Bulletin vol 30 no 12 pp 947ndash9512005

[78] C Y Chang S Y Yang and J L Sheh ldquoA roller embossingprocess for rapid fabrication of microlens arrays on glasssubstratesrdquoMicrosystemTechnologies vol 12 no 8 pp 754ndash7592006

[79] S Youn M Ogiwara H Goto M Takahashi and R MaedaldquoPrototype development of a roller imprint system and its appli-cation to large area polymer replication for a microstructuredoptical devicerdquo Journal of Materials Processing Technology vol202 no 1ndash3 pp 76ndash85 2008

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