Thin Solid Films 518 (2010) 1737–1743

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Optical and photoelectrical studies of gold nanoparticle-decorated C60 films

N.L. Dmitruk a,⁎, O.Yu. Borkovskaya a, S.V. Mamykin a, D.O. Naumenko a, V. Meza-Laguna b,E.V. Basiuk (Golovataya-Dzhymbeeva) c,1, I. Puente Lee d

a Institute for Physics of Semiconductors, National Academy of Sciences of Ukraine, 45 Nauki Prospect, Kyiv 03028, Ukraineb Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México (UNAM), Circuito Exterior, Ciudad Universitaria, A. P. 70-186, C. P. 04510 México D.F., Mexicoc Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México (UNAM), Circuito exterior S/N Ciudad Universitaria,A. P. 70-186, C. P. 04510 México D.F., Mexicod Facultad de Química, UNAM, Circuito de la Investigación Científica, Ciudad Universitaria, 04510 México D.F., Mexico

⁎ Corresponding author. Tel.: +38 44 525 64 86; fax:E-mail address: dmitruk@isp.kiev.ua (N.L. Dmitruk).

1 Tel.: +52 55 5622 8602x1150; fax: +52 55 5622 8

0040-6090/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.tsf.2009.11.083

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 April 2009Received in revised form 27 November 2009Accepted 27 November 2009Available online 4 December 2009

Keywords:Fullerene C60

Thin filmsGold nanoparticlesOptical propertiesBarrier structuresPhotocurrent

Optical and photoelectrical studies were performed on octane-1,8-dithiol cross-linked fullerene films, withsupported gold nanoparticles (C60–DT–Au). According to high-resolution transmission electron microscopyobservations, the average size of obtained gold nanoparticles was about 5 nm, and the shape was spherical.The comparative investigation of optical properties of pristine and cross-linked with octane-1,8-dithiol C60

films, decorated with gold nanoparticles, found the difference in the extinction coefficient spectra, whichwas observed also in the photocurrent spectra of barrier heterostructure Au/C60/Si. The analysis of darkcurrent–voltage characteristics for Au/C60/Si heterostructures showed that the model for them includes thebarrier at the C60/Si interface and internal barriers in the C60 layer, caused by the trapping centers. Thehopping mechanism of the current transport in the C60 layer was supplemented with the Poole–Frenkelemission process on these centers, with the barrier height greater for the fullerene C60 film cross-linked withoctane-1,8-dithiol.

+38 44 525 83 42.

651.

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The optical properties of metal nanoparticles have been exten-sively studied by many research groups (see, for example, [1,2] andreferences therein), and found a lot of applications. For example, theselection of nanoparticles for achieving efficient contrast for biologicaland cell imaging applications, as well as for photothermal therapeuticapplications, is based on them [3]. Since those physical properties arestrongly dependent on the shape and size of nanoparticles, thetechnological applications of such structures are connected with thedetail characterization of these parameters from growth to deviceprocessing [4]. Another significant strategy on the way toward betterapplication of nanoparticles in nanotechnology is controlling thenanoparticle size, shape, and detailed modelling of their effects onfunctional properties [5]. In nanoscience and nanotechnology, for thedevelopment of plasmonic nanomaterials, chemical and biologicalsensors, the important task must be the combination of the abovementioned strategies: characterization and controlling the size andshape of metal nanoparticles.

It is well known that gold is one of the most studied materials dueto many advantages such as chemical stability, catalytic activity ofsmall size particle, and biocompatibility [3,5]. It was found that gold

nanoparticles with plasmon-resonant absorption in the near-infraredcan be used to destroy cancerous tumors in mice. Such nanoparticlesexhibit strong optical scattering and absorption at visible and near-infrared wavelengths due to localized surface-plasmon resonance.This is a classical effect in which the light's electromagnetic fielddrives the collective oscillations of the nanoparticle's free electronsinto resonance (see, for example, [6]). It is no doubt that the usefulproperties of noble-metal nanoparticles are determined by theirgeometry. Jain et al. [3] found that themagnitude of extinction and therelative contribution of scattering to the extinction rapidly increase byincreasing the size of gold nanospheres from 20 to 80 nm. Goldnanospheres studied in this work [3] in the size range about 40 nmshowed an absorption cross-section 5 orders higher than conven-tional absorbing dyes, while the magnitude of light scattering by 80-nm gold nanospheres was 5 orders higher than the light emissionfrom strongly fluorescing dyes. However, the variation in the plasmonwavelength maximum of nanospheres, from 520 to 550 nm, was toolimited to be useful for in vivo applications [3]. The differentspectroscopic techniques such as surface-enhanced Raman spectros-copy, a variety of nonlinear scattering measurements and time-resolved measurements, have been applied [7] to determine theoptical properties of metal nanoparticles, but the extinction, absorp-tion, and scattering were always and are still the primary opticalproperties of interest.

In spite of the shapes and sizes, the metallic nanoparticles can becharacterized using electron and scanning probe microscopy, and in

Fig. 1. SEM images and EDS spectra for Si/C60Au (a, b) and Si/C60DT–Au (c) films.

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some cases, it is possible to determine the optical properties ofindividual nanoparticles [8,9]. A lot of complicating factors exists inunderstanding the nanoparticle optical properties. The presence of asupporting substrate, the solvent molecules around the particles, adistance between particles can have significant influence on theextinction spectra [7]. The absorption of light in different materials,doped with very few Au atoms, is caused by excitation of the abovementioned plasmons and plasmon polaritons. The wavelength of thesurface (localized) plasmons depends not only on size and shape, butalso on topology and the dielectric environment of gold clusters inglasses [10,11] and other media [12]. Since gold nanoclusters alreadyproved a considerable potential for the application in nanoelectronicdevices, the attachment of gold nanoclusters to fullerene moleculesalready appeared as a useful way to prepare nanohybrids withinteresting properties. A photoemission study of C60 adsorption on theAu(100) surface showed that the lowest unoccupiedmolecular orbitalof fullerene undergoes a strong hybridization with the Au d-bands[13]. Such nanohybridmaterials can be based onmetallic nanocenters,gold clusters commonly, attached to fullerene C60 molecules with theaid of compounds that contain sulfur atoms [14–16].

In our previous works [17,18] we developed an approach wherefullerene C60 films were exploited as molecular templates for metalcluster superstructure. The chemical bonding of metal nanoparticlesto fullerene support reduced considerably the undesirable coales-cence effects. We have used chemically cross-linked fullerenesupports with an aliphatic bifunctional amine and thiol as linkers, tobind and immobilize silver and gold nanoparticles. The stablenanocomposite films have been obtained, uniformly decorated withAg and Au nanoparticles. The measuring of optical properties in [19]showed that the diamine treatment changed recombination proper-ties of fullerene films. It was found that the decoration of C60 pristineor cross-linked with silver nanoparticles leads to decreasing inphotoluminescence intensity, and to band gap decreasing by about0.1 eV, but had a weak influence on electrical and photoelectricalproperties of Au/C60/Si barrier structures [19].

In the present work, we studied the optical and photoelectricalproperties of fullerene C60 films cross-linked with octane-1,8-dithioland decorated with gold nanometer-sized particles.

2. Experimental details

2.1. Materials and chemical procedures

Fullerene C60 powder from MER Corp. (99.5% purity), octane-1,8-dithiol (98% purity), hydrogen tetrachloroaureate (III) HAuCl4trihydrate (99.9% purity), 2-propanol from Aldrich, and citric acidfrom Baker were used as received.

The C60 fullerene films were deposited by sublimation methodonto silicon Si(100) wafers (size of ca 10×10 mm), as well as ontotransmission electron microscopy (TEM) grids, in a vacuum chamberat a pressure of 8.2×10−4 Pa and the C60 source temperature of250 °C, without heating the substrates.

To perform the cross-linking of deposited fullerene films withdithiol, and the following decoration with gold nanoparticles, weemployed the procedures described previously in [17]. The samples(silicon and TEM grid-supported C60 films, preliminarily degassed invacuum) were placed into a Pyrex glass reactor along with a smalldrop of octane-1,8-dithiol, and the thiol addition reaction was carriedout at a pressure of ca. 133 Pa and 140 °C for 3 h. For the deposition ofgold nanoparticles onto dithiol-functionalized films, the fullerenesamples were placed into 10 ml of 2-propanol. Then two solutions, ofHAuCl4 in 2-propanol and of citric acid in 2-propanol, weresimultaneously added drop-wise while the system was vigorouslystirred at room temperature for 30 min. After finishing the depositionprocess, the samples were washed with 2-propanol, dried and storedunder vacuum at room temperature.

Along with the cross-linking of C60 films, and decorating themwith gold nanoparticles, we performed for some comparison the samereactions with pristine fullerene powder.

2.2. Optical and photoelectric characterization

A comparative investigation of Si-supported fullerene C60 films,namely pristine (C60/Si), cross-linked with octane-1,8-dithiol C60 films(C60–DT/Si), as well as the above mentioned films decorated with goldnanoparticles (C60–Au/Si and C60–DT–Au/Si, respectively), was carriedout at room temperature in air. For their optical characterization thereflectance spectra were measured in the wavelength range of λ=400–1000 nm at variable angles of incidence of p- and s-polarized light. Thefullerene layer thicknessvalueand its optical parameters (refractive indexn, and extinction coefficient k at complex refractive index ň=n−ik)were determined by fitting of experimental dependencies with

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theoretical ones, calculated within the framework of a one-layer modelfor pristine or functionalized C60 films. Parameters obtained allowed tocalculate the spectral dependencies of the absorption coefficient(α=4πk/λ) and the spectra of light transmittance in layered barrierstructures metal(Au)/fullerene/Si. The photodiode structures werefabricated by vacuum evaporation of a semitransparent gold electrodelayer (25.5 nm thickness) through an opaquemask (opening diameter of1.3 mm) onto pristine and functionalized C60 films, with ohmic contactdeposited all over the rear side of the n-Si substrate.

2.3. Analytical instruments

Transmissionelectronmicroscopyobservations of thegrid-supportedsampleswere carriedout ona JEOL2010 instrument, operating at 200 kV.Scanning electron microscopy (SEM) with energy dispersive X-rayspectroscopy (EDS) was carried out on a JEOL JSM-5900 instrumentoperating at 20 kV.

The spectral dependencies of the light reflectance for C60/Siheterostructures and the photocurrent of Au/C60/Si photodiodestructures were measured with the automated arrangement basedon IKS-12 spectrometer equipped with a device for angulardependence of reflectance measurement, with a Glan prism as apolarizer and with the calibrated Si photodetector.

3. Results and discussion

In our previous work [17] we demonstrated that the dithiol-functionalized, cross-linked fullerene C60 film can be employed as a

Fig. 2. TEM images of different magnifications (a,b) of gold nanoparticles deposited onto C60

fullerene powder present two different geometries: spherical (c) and cubic (d).

support for stable and homogeneous deposition of gold nanoparticlesdue to binding mechanism through a strong coordination attachmentbetween Au nanoclusters and sulfur donor atoms of the functionalizedfullerene, that was also supported by density functional theorycalculations [17]. Here, before studying optical and photoelectricalproperties of obtained gold nanoparticle-decorated fullerene films,the comparative morphology study of fullerene samples, by means ofscanning electron microscopy with energy dispersive X-ray spectros-copy, and transmission electron microscopy, was employed.

While SEM images (Fig. 1) present the formation of big goldclusters onto pristine fullerene film, where Au particles tend toundergo strong coalescence, the gold nanoparticles linked on dithiol-functionalized fullerene film are spread uniformly, without large-scale aggregation on the surface. EDS spectra of pristine and cross-linked fullerene films after deposition of gold particles, revealed thepeaks of gold at ca. 2.12 keV, 9.7 keV and 11.5 keV for both kinds offilms, and also a sharp peak at ca. 2 keV due to sulfur in the cross-linked fullerene films sample.

We also performed the studies of obtained gold nanoparticles bymeans of TEM, applying for comparison of different kinds of C60supports: powder and thin film, both functionalized by octane-1,8-dithiol linker, where a similar deposition technique was employed.Interestingly, while a variety of shapes of gold nanoparticles,apparently cubic and spherical, was distinguished on powder sample,with an average particle size around 10–15 nm (Fig. 2), in the case ofdithiol cross-linked C60 film we found only spherical-shape nanopar-ticles of less than 5 nm in diameter, where the nanoparticles werewell separated from each other (Fig. 3).

powder functionalized by octane-1,8-dithiol. The obtained nanoparticles, supported on

Fig. 3. TEM images of spherical gold nanoparticles deposited on C60–DT films.

Fig. 4. Optical parameters, n (a) and k (b) for C60 (1), C60–DT (2), C60–Au (3), and C60–DT–Au (4) layers on the Si substrate.

Fig. 5. Spectra of the light absorption coefficients α in coordinates (α·hν)2 vs hν for C60

(1), C60–DT (2), C60–Au (3), and C60–DT–Au (4) layers on the Si substrate.

Table 1Parameters of pristine and treated C60 layers.

Sample Thickness, nm Band gap, eV

C60 (1) 80.0±0.4 2.34±0.02C60–DT (2) 82.3±0.4 2.33±0.02C60–Au (3) 81.7±0.5 2.33±0.02C60–DT–Au (4) 90.6±0.5 2.32±0.02

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For obtained fullerene films with the immobilized spherical goldnanoparticles of the controlled size and shape, we performed opticaland photoelectrical studies.

Spectra of optical constants, n and k, for pristine and functionalizedC60 films in the spectral range of 400–1100 nmare shown in Fig. 4 (a, b).Themore essential difference is observed for the k value. Below 480 nmthe presence of octane-1,8-dithiol molecules produces a slight increasein extinction spectrum, which is further enhanced when goldnanoparticles are deposited. Some increase of k in the range ofλ≈550–740 nm, and the appearance of absorption (k) in the range ofλ≈740–1000 nm for C60–DT–Au film, are found as well. The authorsof [20] observed that for gold nanoparticles of spherical geometry theabsorption spectrum shows a wider structure from 300 nm to 800 nm,showing a shoulder at about 520 nm.

Spectra of the light absorption coefficients α=4πk/λ in coordi-nates (α·hν)2 on hν (corresponding to so-called direct band-to-bandtransition) and their linear approximations for these films are shownin Fig. 5. The values of intercepts of these lines on the hν axis, treatedas the band gap values, are presented in Table 1 together with thedetermined thicknesses of the C60 layers. These results are obtained inthe framework of the one-layer model for functionalized C60 films.Since the observed decrease of the C60 band gap value due tocorresponding treatments does not exceed the experimental errors,the influence of the hybridization effects [21], if they occur, is toosmall and/or takes place only in a thin upper layer of the C60 film.

The determined parameters of the investigated filmswere used forthe calculation of spectra of the light transmittance into the Sisubstrate through the C60 (Fig. 6a) and Au/C60 (Fig. 6b) layers. Aulayer with thickness of 25.5 nm was evaporated onto the C60 surface

to obtain Au/C60/Si diode structures for the studies of photoelectricand electric properties.

The corresponding spectra of the short-circuit photocurrent,expressed as the external quantum efficiency Qext, (that is, thenumber of photocurrent carriers generated in Au/C60/Si structure byone photon of the incident light) normalized to the area of the diodesAu/C60/Si with pristine and functionalized C60 layers, are shown inFig. 7. Corresponding spectra of internal quantum efficiency Qint (that

Fig. 6. Spectra of light transmittance: (a) into Si substrate through the C60 (1), C60–DT(2), C60–Au (3), and C60–DT–Au (4) layers; (b) through the Au/C60 layers (1–4) andthrough the Au layers (1′–4′) for Au/C60/Si (1, 1′), Au/C60–DT/Si (2, 2′), Au/C60–Au/Si(3, 3′), and Au/C60–DT–Au/Si (4, 4′) structures on Si.

Fig. 8. Spectra of internal quantum efficiency, normalized to the area of the diode, forAu/C60/Si (1), Au/C60–DT/Si (2), Au/C60–Au/Si (3), and Au/C60–DT–Au/Si (4) structures.

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is, the number of current carriers generated by one photon of the lighttransmitted into Si) (Fig. 8) were calculated dividing Qext (λ) spectraby spectra of the light transmittance T2 into Si through the Au/C60layers (Fig. 6b).

The observed Qint values greater than unity are due to the planarcontribution of the carriers generated by light outside the diode area(planar component of photocurrent). The efficiency of this photocur-rent component is determined by the transmission of light into Sithrough the C60 layer T3 (Fig. 6a). Besides, the contribution of thephotocurrent, generated in the C60 layer can be seen in the spectralrange of λ≤530 nm as the shoulder of Qext (λ) dependencies andmaximum of Qint (λ) ones. In the last dependencies this contributionis intensified by the ratio of T1/T2, where T1 — is the transmittance of

Fig. 7. Spectra of external quantum efficiency, normalized to the area of the diode, forAu/C60/Si (1), Au/C60–DT/Si (2), Au/C60–Au/Si (3), and Au/C60–DT–Au/Si (4) structures.

light into the C60 layer through the Au film, shown in Fig. 6b, similarlyas it was mentioned in [19]. Although the values of photocurrent(Qext) for diodes of the same structure may differ, the dependenciesshown in Fig. 8 are typical for the investigated structures.

For interpretation of the photocurrent spectra, the barriercharacteristics of investigated structures were determined from thedark current–voltage (I–V) dependencies, measured in the directcurrent regime. The dark current–voltage characteristics for thesediode structures are presented in Fig. 9. They show the evidence forthe barrier existence on the C60/n-Si interface (with forward directioncorresponding to VN0 at Au electrode) causing the collection ofphotocarriers generated in Si as the main source of photocurrent.

The analysis of dark I–V characteristics showed that forward I–Vcharacteristics agree well with theoretical ones calculated in theframework of model taking into account emission, recombination,tunnel and ohmic (shunt) components of the current flow. In thatcase, similarly as in [22], the current component, interpreted astunnel, is the dominant one in a voltage range up to ∼2 V. The seriesresistance Rs value is caused mainly by the C60 layer resistance, sincethe resistance of the Si substrate layer together with ohmic contact toit is lesser by two orders of value. So, the specific resistance,determined for all the investigated fullerene films, placed betweenthe Au contact layer and Si substrate, ranges from 106 to 107 Ωcm,that agrees well with known data [23]. The ohmic character of the C60layer resistance suggests the hopping mechanism of the current flowfor this voltage range at room temperature. At the same time, a moreaccurate fitting of the experimental and theoretical characteristicswas obtained under the assumption of the Poole–Frenkel mechanismof the current transport in the C60 layer, which suggested the field-

Fig. 9. Dark I–V characteristics for Au/C60/Si (1), Au/C60–DT/Si (2), Au/C60–Au/Si (3),and Au/C60–DT–Au/Si (4) structures.

Fig. 11. Spectra of ratios of the external quantum efficiencies for diodes of structures withdifferent C60 treatment (a, b) and for diodes of the same structure (c): a) Au/C60–Au/Si toAu/C60/Si (1,2), Au/C60–DT/Si toAu/C60/Si (3,4); b)Au/C60–DT–Au/Si toAu/C60–DT/Si (1-3),Au/C60–DT–Au/Si toAu/C60/Si (4); c)Au/C60/Si (1), Au/C60–DT/Si (2), Au/C60–Au/Si (3), andAu/C60–DT–Au/Si (4).

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enhanced thermal excitation of electrons trapped with local centersinto the conduction band [24]. Corresponding I–V dependence wasexpressed as:

I = C U−Un

� �exp

2aT

ffiffiffiffiffiffiffiffiffiffiffiffiffiU−U

n

r !⋅ exp

qUnkT

� �−1

� �+

VRsh

ð1Þ

whereU=V− IRs, a = qk

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiq

4πεε0d

r, d is the thickness of the investigated

fullerene films, Rsh is the shunt resistance, n is the ideality factor, whichdetermines the part of voltage

V−IRs

napplied to the space charge region

in Si at theC60/Si interface, andC is the proportionality constant, value ofwhich depends on the barrier height at the C60/Si interface, on theinternal barriers heights, and the density of the trapping centers in C60.

The dependencies of C vs n for different diodes of the investigatedstructures are shown in Fig. 10. Twodifferent dependencies can be seen:one is for diodes with C60 or C60–Au layers and the other one for thestructures with C60–DT and C60–DT–Au layers. So, a greater internalbarrier height value (or lesser density of the trapping centers) can besuggested for the Au/C60–DT structure with respect to the Au/C60 one,causedbydithiol functionalization of C60films. Itwill benoted thatmorecorrected discrimination of the mechanisms of the current flow in suchheterostructures requires further investigation.

To evaluate the difference in Qext (λ) spectra for the structureswith pristine and functionalized C60 films the spectral dependenciesof their ratios are shown in Fig. 11 (a, b). The results for two differentdiodes of every structure are presented here. To distinguish thepeculiarities caused by functionalization of C60 films from thoseassociated with the unhomogeneities over the area of one plate withset of diodes, the ratios of Qext for diodes of the same structure arepresented in Fig. 11c. It is seen that the deposition of goldnanoparticles onto the C60 films, cross-linked with octane-1,8-dithiol,results in increase of photocurrent in the range of λ≈600–900 nm(Fig. 11b), while the deposition of Au nanoparticles onto pristine C60,or the cross-linking of C60 with octane-1,8-dithiol itself (Fig. 11a),decreases Qext in this spectral range. The spectral range of thisphotocurrent enhancement coincides with the one of the extinctioncoefficient increase (Fig. 4), calculated within the range of one-layermodel. So, the light transmittance into Si, calculated both for the diodearea (T2), and under the C60 layer (T3), is lesser for the C60–DT–Au filmthan for C60–DT and C60 in this spectral range. Therefore, such increaseof the photocurrent may be caused either by photogeneration ofcurrent carriers in the C60–DT–Au layer, or by the nontaken intoaccount increasing of light transmittance into Si due to a surfaceplasmon resonance excitation in the Au nanoparticles. Distinguishingbetween these effects requires further investigations.

Fig. 10. The dependencies of C vs n, obtained from Eq. (1), for different diodes of theinvestigated structures.

4. Conclusions

The comparative investigation of optical properties of pristine andcross-linked with octane-1,8-dithiol C60 films decorated with goldnanoparticles of spherical shape, found out the difference in theextinction coefficient spectra, which is more essential in the case ofC60–DT–Au films. These peculiarities of k (λ) spectra influencedifferently the photocurrent spectra of the barrier heterostructureAu/C60/Si. Thus, an increase of absorptance for the C60–DT and C60–DT–Au films with respect to the C60 and C60–Au ones in the spectralrange of λb600 nm, manifests itself in a decrease of the photocurrentof corresponding barrier structures. It seems that this smaller value ofphotocurrent in the case of C60–DT–Au films results from a lessertransmittance of light into Si, both under the diode area (Fig. 6b), andthrough the C60–DT–Au layer outside the diode (Fig. 6a). At the sametime, an additional region of the light absorptance with maximum atλ∼800 nm for C60–DT–Au films coincides with the region of the

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photocurrent increase. The last effect is probably caused by the goldnanoparticles deposited on the C60 film cross-linked with octane-1,8-dithiol, and respectively, by an additional photogeneration of thecurrent carriers in Si due to a surface plasmon excitation in Au nano-particles. The analysis of dark I–V characteristics for the Au/C60/Siheterostructures has shown that the more likely model for themincludes besides the barrier at the C60/Si interface, also internalbarriers in the C60 layer, caused by the trapping centers. So, thehopping mechanism of the current transport in the C60 layer issupplemented with the Poole–Frenkel emission process on thesecenters, the barrier height for which is greater in the case of dithiolcross-linking of the C60 film.

Acknowledgements

Financial support from the National Autonomous University ofMexico (grant DGAPA IN103009) and from the National Council ofScience and Technology of Mexico (grant CONACYT-56420) is greatlyappreciated.

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