Cite this: RSC Advances, 2013, 3,19501

Particle-free inkjet printing of nanostructured porousindium tin oxide thin films

Received 28th January 2013,Accepted 27th June 2013

DOI: 10.1039/c3ra40487k

www.rsc.org/advances

Mei Fang,a Andrey Aristov,b K. V. Rao,a Andrei V. Kabashin*b and Lyubov Belova*a

We report a simple, low-cost, single-step inkjet printing method for the fabrication of nanostructured,

highly transparent and conductive ITO films, which completely avoids the use of ITO particles in the

fabrication process. In our method, the inks are formed from a liquid solution presenting a properly

selected mixture of indium and tin acetates. After jet printing, the ink is decomposed during a subsequent

annealing step, in which the released CO2 gas bubbles control the ITO nucleation process to provide a

porous film texture. We show that the fabricated ITO films are highly crystalline, stoichiometric, and

nanoporous with controlled porosity. Electrical measurements show relatively low resistivity values for the

films (down to 0.029 V cm) comparable to those of the best ITO thin films fabricated by other methods.

Optical ellipsometry tests demonstrate a relatively high refractive index (1.5–1.7) and high transparency of

the films over a wide region of the spectrum ranging from 500 to 1700 nm. Since the method does not

require any pre-fabricated ITO particles, masks or templates, and enables the deposition of films on

substrates of various materials and shapes, it can be employed for fabrication of nanoporous ITO films for

a diversity of applications, including solar cell, bio- and chemical sensing, etc.

Introduction

Due to its optical transparency in a broad range of wavelengthsand good electrical conductivity, Indium Tin Oxide (ITO) hasbecome a material of choice for many applications, includinginformation displays (liquid crystal displays, plasma displays,touch panels etc.),1 organic light-emitting devices,2,3 antire-flective coatings,4 thin film transistors,5 waveguide compo-nents in plasmonic devices,6 novel lab-on-chip architecturesand various bioanalytical devices.7–9 When combined with thelarge surface area of the nanoporous film texture, ITO canfurther be used as an efficient active element in solar cells,10

gas,11,12 bio and chemical sensors,13,14 etc. The above-mentioned applications require the fabrication of high-qualityITO films by simple, low-cost and easily scalable methods. ITOthin films with low resistivity (typically y1024 V cm) and ahigh optical transmittance can be fabricated by a variety ofphysical vapour deposition techniques, including electronbeam or thermal evaporation,15–17 sputtering,18,19 pulsed laserdeposition,2,3 etc. Despite high quality of the produced films,the vapour deposition techniques appear to be relatively costlyand inefficient from the point of view of excessive materialconsumption. Alternatively, ITO films can be prepared bychemical methods, including sol–gel process,20 dip-coating,21

wet chemical routes,22,23 etc. In particular, sol–gel methods

make possible the fabrication of ITO films with mesoporousstructure24,25 that is needed for a variety of applications.10–14

However, chemical methods are less compatible with micro-electronics technology and cannot produce patterned thinfilms in a single deposition cycle. To further complicate thesituation, a rapidly growing consumption of indium sourcesworld-wide constantly raises their production cost that dictatesthe necessity of minimizing indium usage and waste in allfabrication pathways.26,27

Inkjet printing has recently emerged as a low-cost alter-native for the fabrication of many functional films/coating andnanostructures.28 This technology is a direct patterningprocess on virtually any substrate, with a minimum waste offunctional material.28–30 Inkjet printing has been used for thedeposition of ITO films by dispersing ITO micro- ornanoparticles in solvents in the presence of suitable surfac-tants.31–33 However, this particle suspension approachrequires the production of ITO particles in a separate processprior to the ink preparation, while high annealing tempera-tures are required after the patterning to remove thesurfactants and achieve a good contact among the ITOparticles, yielding to a good electrical conductivity.34

In this paper we report a novel inkjet printing-basedmethod for the preparation of ITO nanoporous films, whichdoes not use ITO particles and thus makes it possible topattern the films in a single step. Here, the main idea consistsin a particular preparation of precursor inks from solutions ofacetates, ink printing and a further decomposition of theacetates through a subsequent annealing step.

aDepartment of Materials Science and Engineering, KTH-Royal Institute of

Technology, Stockholm SE10044, Sweden. E-mail: lyubo@kth.sebAix Marseille University, CNRS, LP3 UMR 7341, Campus de Luminy, Case 917,

13288, Marseille cedex 9, France. E-mail: kabashin@lp3.univ-mrs.fr

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Results and discussion

Fabrication of ITO films

The whole procedure for the preparation of ITO films consistsof several stages summarized schematically in Fig. 1. As thefirst stage, we prepare printing inks from indium(III) acetate(99.99%, Alfa Aesar) and tin(IV) acetate (Sigma-Aldrich). Theacetates are typically dissolved in acetylacetone (¢99%,viscosity 0.8 mPa s at room temperature, Sigma-Aldrich) witha cation ratio of [Sn4+] : [In3+] = 1 : 9, which provided the beststoichiometry and eventually the best quality of formed films.Then, the solution is heated on a hotplate kept at 120 uC toincrease the solubility and chelate the acetates. 0.01 molhydrogen peroxide (30 wt% in H2O, Merck) is added drop-wisein 6 aliquots at 30 min intervals into the solution to preventphase separation due to a re-crystallization as a result of theevaporation of the solvents after printing. The final ink forprinting normally becomes reddish-brown in colour indicatingthe formation of metal acetylacetonate complexes, with cationconcentration ([Sn4+] + [In3+]) of 0.1 M. The viscosity of the inkwas in the range of 1–10 mPa s. During the second printingstage, the prepared ink is deposited on glass substrates via acustom-built printing station using Xaar 126 drop-on-demand(DOD) ink-jet printheads. The substrates are preliminarily wellcleaned by sonication in acetone for 10 min, followed by 10min sonication in isopropanol. They are subjected to N2 gasgun for drying and pre-heated to 80 uC before printing. Thesubstrates are positioned on a hotplate during printing andafter each printing pass the as-printed thin films are dried at

150 uC for 10 min to evaporate the solvents. Finally, after thefull printing cycle the dried films are annealed at differenttemperature ranging from 300 uC to 500 uC for 2 h, in O2 flowto achieve full burn-off of carbon from the organic compo-nents. In order to verify the validity of the proposed particle-free inkjet printing-based fabrication methodology, we applieda battery of techniques to structurally, electrically and opticallycharacterize the deposited films. Results of such characteriza-tions are given below.

Thermogravimetric analysis of the ink

Thermal behaviour of the precursor ink was studied by TGanalysis. TG analysis was carried out for both an as-preparedink in the liquid state and an ink dried on a hotplate at 150 uCfor 10 min. The main reduction of the weight for the liquid inktook place at about 135 uC, which roughly corresponded to theevaporation of the solvent. For the dried ink, the weight lossoccurred at temperatures ranging from 165 uC to 445 uC thatwas obviously due to burning of organic components.

During the ink preparation step, tin(IV) acetates(Sn(CH3COO)4) and indium(III) acetates (In(CH3COO)3) werechelated with acetylacetone (C5H8O2) and formed In(C5H7O2)3,Sn(C5H7O2)4, while acetic ions were dissolved in the solvents.After drying on the hotplate at 150 uC for 10 min, the solventswere evaporated together with the chelate complex, while onlyacetic ions left on the substrate. In the following TG analysistests, the metal acetylacetonate complexes and the acetic ionsstarted to burn at temperature over 160 uC to form oxides.Based on comparative analysis of molar masses (M) of thereactants and the resultants, the weight percent of the solidafter decomposition can be calculated as (2), where wt% = 23%is very close to the observed value from TG analysis.

wt% = M(In0.9Sn0.1O1.55){[M(In(C5H7O2)3) +M((C2H3O2)2) 6 3] 6 9 + [M(Sn(C5H7O2)4) +

M((C2H3O2)2) 6 4] 6 0.1}21 6 100% (1)

XRD results

Since the heating rate in TG analysis is relatively high, 20 uCmin21, the actual annealing cycle for the printed patterns toburn off the organic components can in principle be done inoxygen at a temperature somewhat lower than 445 uC but withlonger annealing time. Fig. 2 shows XRD patterns for 4-passprinted thin films which were annealed at 300 uC, 350 uC, 400uC, 450 uC and 500 uC for 2 h. Diffraction peaks are matchedwith In2O3 (Joint Committee on Powder Diffraction Standards,JCPDS No. 00-006-0416, body-centred cubic with the latticeparameter of a = 10.118 Å). Our tests showed that thefabricated films are nanocrystalline, which is evidenced by aseries of characteristic ITO-related peaks, as shown in Fig. 2.XRD peak positions (222) and (400) were used to refine thelattice parameter a of prepared thin films via Celref3 software.For films annealed at temperature lower than 450 uC the latticeparameter a is equal to 10.111 ¡ 0.005 Å, which is slightlysmaller than a = 10.118 Å for cubic indium oxide. Since theSn4+ has smaller Pauling radii (71 pm) than the In3+ ion (81pm), substitutional incorporations of Sn ions into In- sites incubic In2O3 structure contract the lattice.35 For the film

Fig. 1 Schematic representation of all stages of the proposed particle-free inkjetprinting procedure for the preparation of ITO films.

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annealed at 500 uC, a = 10.117 ¡ 0.005 Å which is very close tothe value for bulk In2O3 lattice. It should be noted that ourtests revealed an extra peak at 2h around 27.5u, which matches(111) diffraction peak of calcium (JCPDS No. 01-089-3683). Itprobably comes from the substrate since the films are ratherthin (tens of nanometer in thickness) and the X-rays canpenetrate through them. Thus, our tests unambiguously showthat the proposed particle-free inkjet printing procedure,followed by annealing of printed precursors at temperatureshigher than 300 uC, leads to the formation of nanostructuredITO films.

FIB/SEM microscopy study

Fig. 3 shows typical surface morphology and cross sectionimage of the prepared thin films. The specific thicknessmeasured from cross-section image shown in Fig. 3 inset isy40 nm for a 2-pass-printed thin film. Similar measurementswere carried out for films printed with single and 4-passes,with thicknesses of y15 nm and y80 nm, respectively. Our

analysis showed that continuous and uniform ITO thin filmswere fabricated on the glass substrates as a result of thedescribed in situ preparation technique. Since the precursorswere printed on the substrate and then a reaction withformation of ITO was triggered, the nucleation of ITOpreferentially started at the interface of the substrate and thesolid precursors during the annealing, resulting in an excellentadhesion of the film to the substrate. Besides, the patterningof the liquid ink on solid substrates guarantees a relativeuniformity of solid precursors after the evaporation of thesolvents. In should be noted that we did not observe anyartefacts related to aggregation of particles as it often happensin cases of suspension inks. It is important that both top viewand section images reveal the presence of branched ITOcrystals among ITO nanoscale grains. These branched crystalsare likely to contribute to a better electrical transport throughthe thin film due to smaller amount of grain boundaries. It isalso remarkable that the films have essentially nanoporousstructure. We suggest that the formation of the porousstructure takes place during the annealing of the films: aburning of organic components during precursor decomposi-tion leads to the production of CO2 or CO gases, whosebubbles diffuse to the ITO films during their growth creating aporous structure throughout the film.36 Due to such nano-porous structure with significantly enhanced surface area,these films could be attractive for new functional applicationssuch as solar cells and sensors.25

I–V measurements

Resistances (R) of the prepared ITO thin films at roomtemperature were measured from Current–Voltage (I–V)characteristics via a four probe method. Since the filmthickness (h) is much smaller than the space between probes,their resistivity (r) can be calculated according to:37

r~p

ln (2)hR (2)

The thickness of the prepared ITO thin films was measuredfrom FIB cross section SEM images (similar to the exampleshown in Fig. 3). We estimated that each single pass addedabout 20 nm to the film thickness. Fig. 4(a) shows the roomtemperature resistivity of ITO films having different thick-nesses as a function of the annealing temperature. The datashows that higher annealing temperatures result in the lowerresistivity. It is likely that higher temperature results information of higher purity ITO. Besides, the improved crystalstructure at higher annealing temperature could also benefitconductivity. The thicker films, however, have slightly higherresistivity. The measured thickness from the FIB cross sectionimages for 1, 2 and 4 print passes films was y15 nm, 40 nmand 80 nm, indicating higher porosity of thicker films, whichmost likely is the reason for higher resistivity. The obtainedlowest resistivity was 0.074 V cm, from 2 passes printed thinfilm annealed at 500 uC. This value is higher than the reportedvalue of y1024 V cm for ITOs prepared by PLD.3,19 We suggestthe reasons are the nanoscale grain structure and thenanoporous structure which hampers electron transportthrough the film.23 The annealing process in oxygen which

Fig. 2 XRD intensity patterns of ITO thin films annealed at different tempera-tures. Insets: Magnified diffraction peaks from (222) and (440) crystal planes.The data has been shifted along the y-axis for clarity.

Fig. 3 SEM images of inkjet-printed ITO films. Top inset: a magnified SEM imagerevealing a nano-porous texture of the films. Lower inset: FIB cross-section of anITO thin film prepared with 2 print passes on glass substrate.

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facilitates burn off of the organic precursors also results in alow concentration of oxygen vacancies in the thin film. Toincrease the amount of oxygen vacancies, the ITO filmsannealed in O2 at 500 uC were post heat treated in N2 at 500uC for two hours. Fig. 4(b) shows resistivity of 1 pass, 2 passesand 4 passes printed ITO thin films before and after N2

annealing. The resistivity decreased by more than half after theannealing in N2 with the lowest resistivity of the 2 passesprinted ITO thin film being 0.029 V cm.

Optical properties

We carried out a series of ellipsometric measurements intransmission and reflection geometry (see details of measure-ments methodology in Experimental section). In the transmis-sion geometry, unfocused light from (diameter 2 mm) of anellipsometer (Woollam M-2000) was directed into ITO samplesat zero angle, while the ellipsometer recorded spectraldistribution of transmitted intensity in the range of 350–1800 nm. In reflection geometry, light was reflected from theITO films at different angles, while the ellipsometer recordedamplitude (tan(Y)) and phase (D) characteristics of reflectedlight. Then, the values of tan(Y) and phase (D) from reflectionmeasurements were substituted into the dispersion model (apart of WoollamCompleteEase software) to determine theindex of refraction and the absorption coefficient of thin films(see details in Methods section).

Fig. 5 shows transmission spectra of various ITO samples.One can see that all samples exhibited a very high transpar-ency in the visible and IR range exceeding 90%, while in theUV range they strongly absorbed light due to interbandabsorption.38 The edge of absorption was at 350 nm, which

is consistent with the presence of Sn dopants in ITO matrix.39

As shown in Fig. 5(a), the transmittance of 1-pass printedsamples was almost independent on annealing temperature.Spectral dependences of the absorption coefficient (k) andrefractive index (n) and for various ITO films are shown inFig. 5(b, c). It is evident that refractive index experienced agradual decrease while moving from UV to IR range. Themagnitude of n was maximal at l = 400 nm ranging from 1.9 to2.1 depending on the annealing temperature. In the IR range ndecreases down to values ranging from 1.5 to 1.7 (at l = 1700nm). In contrast, light absorption k was almost negligible inUV-visible range and experienced a slight increase in the near-IR (l . 1000 nm), but the value of k was still very small for allsamples (k , 0.09). Refractive index and absorption coefficientof ITO films did not strongly depend on nature of ambient gas

Fig. 4 (a) Dependence of resistivity of ITO thin films of different thicknesses onannealing temperature; (b) resistivity of thin films as a function of film thicknessbefore (&) and after ( ) annealing in N2.

Fig. 5 Spectral dependences of 1- pass printed ITO samples annealed in air atdifferent temperatures (300, 350, 400, 450 uC): (a) transmission spectra fromITO films, normalized to the spectrum from the substrate; spectral dependencesfor the refractive index n (b) and absorption coefficient (c).

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(air, oxygen + argon) in the furnace where annealing took place(within statistic error). However, n and k demonstrated agradual increase, as the annealing temperature increases from300 to 450 degrees (Fig. 5(b, c)), which is consistent withresistivity measurements (Fig. 4). It should be noted that afurther increase of the annealing temperature caused unpre-dictable dependence of optical characteristics, associated withthermal destruction of ITO films.

In another experiment, we compared optical properties of 1,2 and 4 passes printed ITO films, which were annealed at thesame temperature (450 uC). As shown in Fig. 6(a), single-passprinted films exhibited highest transmission, followed by 2-and 4- passes printed films. The higher transmission could beattributed to lower absorption coefficient, which was propor-tional to the number of printing passes (Fig. 6(c)).Nevertheless, as shown in Fig. 6(b), the refractive index ofdeposited ITO films was almost independent on the number ofpasses. The above-stated optical measurement data andobtained tendencies are consistent with properties of indust-rially important ITO samples known in the literature.40

However, there are some particular features. First, refractiveindex appears to be rather high in the whole range ofmeasurements (350–1700 nm). As shown in Fig. 5(b) and6(b), index of refraction is higher than 1.9 in the UV range andhigher than 1.5 in the IR range (for l = 1000) nm for allsamples of ITO. It is important than n is essentially higherthan 1, that prevents the use of newly fabricated samples asreflective mirrors. It is interesting that values of n are higher inthe IR range than in the case of other ITO films prepared by avariety of methods.40 This property can be useful for someapplications, including display panels, wave guiding, bio- andchemical sensing. Here, as potentially promising architectureswe foresee the employment of ITO structures together withnovel advanced nanoplasmonic and metamaterial-basedstructures. In particular, combined metal/ITO structures canbe used for photonic band gap engineering,6 while theemployment of ITO sublayer together with plasmonic meta-material-based sensors41,42 and other advanced sensor geo-metries (e.g., Si-based Surface Plasmon Resonanceminiaturized biosensor)43,44 can facilitate light coupling andlead to ultimate miniaturization of sensor devices.

Another striking property of the ITO films consists in theirlow absorption in the IR. Indeed, for all fabricated andannealed films (even those consisted of 2 and 4 printingpasses), the absorption coefficient k was less than 0.01 that islower than for most ITO samples known from the literature.40

In particular, relatively low k values were reported by Laux45 forITO layers prepared by plasma ion-assisted evaporation, butthe recorded absorption (k = 0.2–0.3 around l = 1600–1700 nm)was still higher than in our case. Such a high transparency ofour ITO films is a pleasant surprise taking into accountpotential applications in display panels, optoelectronics,biosensing. It should be noted that properties of ITO filmsare known to critically depend on parameters of post-fabrication annealing process. Therefore, we cannot excludethat a proper annealing procedure can significantly tuneoptical properties of our films. Finally, the dispersion modelused in ellipsometry makes possible the estimation ofelectrical parameters (resistivity) based on optical measure-

ments of optical ellipsometry parameters. In our case, theestimated resistivity of the single-pass samples was of theorder of y0.02–0.5 V cm, which is in good correlation withresults of direct electrical measurements (see Fig. 4).

To estimate the porosity of the fabricated ITO films, we usedthe approach described in ref. 38. Here, this parameter isdetermined from the comparison of the experimentallymeasured dielectric constant (it is implied that this dielectricconstant eeff relates to the effective porous medium composedof ITO nanocrystals and air cavities) and the one of theindividual nanoparticles estimated with a help of a freeelectron gas model (this model is known to provide a fairlygood description of properties of ITO films38,46,47). For highlydoped ITO films, the free electron gas model gives thefollowing expression for the dielectric function:38,46,47

Fig. 6 Spectral dependences of 1-, 2- and 4- passes printed ITO films annealedin air under 450 Deg. C: (a) Transmission spectra from ITO films, normalized tothe spectrum from the substrate; Spectral dependences for n (b) and k (c).

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eito(v)~e?{v2

p

v: vzi

t0

� �’ (3)

Where v and vp (v2p~

nee2

mee0) are light and plasma frequencies,

e‘ is a high frequency dielectric constant, which is equal to 4 inour conditions,23,46 t0 is relaxation time related to ionized impurity

scattering (t0~me

rnee2), e is the elementary charge, me and ne are

the effective electron mass and charge carrier concentration, r isthe resistivity of ITO nanoparticles. The plasma frequency vp canbe obtained experimentally from a characteristic descent oftransmission/reflection spectra in the infrared part of thespectrum.46,47 In our case, the plasma frequency was equal to0.15 eV, as it was determined from spectral dependence for thetransmission and its extrapolation by Drude-defined extinctionusing Woollam CompleteEase software. Substituting this value forvp to eqn (3) and using other typical ITO parameters, we canestimate the dielectric constant of densely packed nanoparticle-based films as eito = 3.99 at l = 630 nm. The porosity can then beestimated by the Bruggeman formula,48,46 which links thedielectric constant of the effective medium eeff with that of itsconstituents (eito and eair, where eair = 1 is the dielectric constant ofair):

feito(v){eeff (v)

eito(v)z2eeff (v)z(1{f )

eair(v){eeff (v)

eair(v)z2eeff (v)~0,

where f is the volume fraction (or filling factor) occupied by theITO nanoparticles. Substituting experimentally obtained eeff = 3.75and values for eito and eair, one can easily obtain that the fillingfactor was about 85%. It means that the porosity of our films wasabout 15%, which is in fairly good agreement with previousstudies.

Experimental

Structural characterization

A thermogravimetric analysis system (TG, Perkin-Elmer TGS-2)was employed to examine the precursor ink by monitoringweight changes under an increase of temperature, whilesamples were placed in oxygen gas. The heating rate was 20uC min21. Grazing incidence X-ray diffraction intensity spectrawere examined by a Siemens D5000 system to study phase andstructural properties of the annealed thin films. The incidentangle of X-ray was 1.5u, and the scan ranged from 20u ¡ 2h ¡=80u with a step of 0.01u, while the scan speed was fixed at 2 s/step. The topography, cross section and thickness of the filmswere investigated by a DualBeam combined focused ion beam/scanning electron microscope (FIB/SEM, FEI Nova 600Nanolab). In order to protect the surface of the films againstFIB damage during sectioning, thin strips of Pt were locallydeposited in situ prior to cutting. To characterize the electricaltransport properties, I–V curves were collected on the preparedthin films via a home-designed set up using a standard four-probe method.

Optical characterization

The samples were characterized in a Woollam M-2000ellipsometer, which monitors phase-polarization propertiesof light reflected from a thin film in order to determine itsoptical parameters. The polarization state of light incident ona sample may be decomposed into s and p components (the sand p components correspond to oscillations of electric fieldperpendicular and parallel to the plane of incidence, respec-tively). After the reflection, the normalized amplitudes of the sand p components are denoted as rs and rp, respectively. Theellipsometer measures the complex reflectance ratio (acomplex quantity), which is the ratio of rp over rs and consistsof amplitude and phase components:

r~rp

rs

~ tan YeiD (4)

where tan(Y) is the ratio of rp and rs amplitudes uponreflection, and D is the phase shift (difference of phases of rp

and rs components). Notice that for simplicity of mathematicaltreatments the latter term of the equation is presented incomplex number form. Since ellipsometry is based onmeasurements of the ratio (or difference) of two values (ratherthan the absolute value of either), it is very robust, accurate,and reproducible. Measured parameters can then be sub-stituted into Woollam CompleteEase software, which iscapable of simulating, with a high precision, of opticalproperties of the films using an advanced dispersion model.The high precision of simulations in such model is due to apossibility of simultaneous assessment of metal-like anddielectric-like behaviours using Drude’s and oscillator models,respectively. Thus, using spectral dependences of Y and D,one can routinely determine the index of refraction and theextinction coefficient of thin films, as well as estimate variousparameters such as the thickness, the roughness, andcomposition of films. Furthermore, this technique makespossible the estimation of the film’s electrical properties(resistivity) based on the measured optical data.

Conclusions

We introduced a novel fast, inexpensive, easily scalablemethod for fabrication of highly transparent conductive filmsof indium-tin-oxide, based on inkjet printing from acetateliquid precursors. The method does not require any masks orclean room facilities and makes possible a single-steppatterning of ITO films on virtually any substrate includingsurfaces with pre-formed functional elements or devices. Thefabricated nanostructured ITO films have nano-porous struc-ture, low resistivity (down to the 0.029 V cm) and are highlytransparent in a broad spectral band ranging from 450 to 1700nm, which makes them very promising candidates for a widerange of applications, from solar cells and displays tobiosensors and lab-on-chip architectures. We believe that ourapproach based on the preparation of indium and tin acetateprecursors can be generalized to other thin film depositiontechniques.

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Acknowledgements

Mei Fang is supported by Chinese Scholarship Council for herstay in Sweden. The work was partially supported by theSwedish Research Council, the French National Agency ofResearch (ANR) grant BIONANOPLASMON, by the Ministry ofEducation and Science of the Russian Federation (contracts16.513.12.3010 and 11.519.11.3017), and by the EuropeanCommunity via project NanoPV (FP7 NMP-2009, contract246331).

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