Ultramicroscopy 111 (2011) 1381–1387

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Ultramicroscopy

0304-39

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/ultramic

High energy-resolution electron energy-loss spectroscopy study of thedielectric properties of bulk and nanoparticle LaB6 in the near-infrared region

Yohei Sato a,n, Masami Terauchi a, Masaki Mukai b, Toshikatsu Kaneyama b, Kenji Adachi c

a Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Sendai 980-8577, Japanb JEOL Ltd., 3-1-2, Musashino, Akishima, Tokyo 196-8558, Japanc Ichikawa Research Laboratories, Sumitomo Metal Mining Co. Ltd., 3-18-5, Ichikawa, Nakakokubun, Chiba 272-8588, Japan

a r t i c l e i n f o

Article history:

Received 10 August 2010

Received in revised form

6 May 2011

Accepted 8 May 2011Available online 14 May 2011

Keywords:

High energy-resolution electron

energy-loss spectroscopy

Surface plasmon

Optical properties of metal nanoparticle

Optical absorption in the near-infrared

region

Solar-heat shielding filter

91/$ - see front matter & 2011 Elsevier B.V. A

016/j.ultramic.2011.05.003

esponding author. Tel./fax: þ81 22 217 5374

ail address: y-sato@tagen.tohoku.ac.jp (Y. Sat

a b s t r a c t

The dielectric properties of LaB6 crystals and the plasmonic behavior of LaB6 nanoparticles, which have

been applied to solar heat-shielding filters, were studied by high energy-resolution electron energy-loss

spectroscopy (HR-EELS). An EELS spectrum of a LaB6 crystal showed a peak at 2.0 eV, which was

attributed to volume plasmon excitation of carrier electrons. EELS spectra of single LaB6 nanoparticles

showed peaks at 1.1–1.4 eV depending on the dielectric effect from the substrates. The peaks were

assigned to dipole oscillation excitations. These peak energies almost coincided with the peak energy of

optical absorption of a heat-shielding filter with LaB6 nanoparticles. On the other hand, those energies

were a smaller than a dipole oscillation energy predicted using the dielectric function of bulk LaB6

crystal. It is suggested that the lower energy than expected is due to an excitation at 1.2 eV, which was

observed for oxidized LaB6 area.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Stained glass windows, in which submicron-scale metal par-ticles of Au, Ag, and Cu are dispersed, are colored by theabsorption of photons in the near-infrared (NIR) and visibleregions (2–3 eV) [1–3]. The absorption originates from the scat-tering of photons by the submicron metal particles, which can bedescribed by Mie scattering theory [4]. It is known that thisoptical scattering property due to the metal particles is related tothe plasmonic behavior of the dispersed metal particles. For nano-scale metal particles, a high optical absorption efficiency, knownas localized surface plasmonic resonance (LSPR), has beenreported [5,6]. LSPR has been applied to bio-sensors [7,8], wave-guides [9], and photonic circuits [10,11]. The LSPR condition issensitive to the size [6,12] and shape of the nanoparticles [13–16]and their dielectric environment [7,17,18]. Thus, in order toinvestigate LSPR, it is necessary to identify the size, shape, andmaterials surrounding the nanoparticles. Near-field optical micro-scopy is one of the highest spatial resolution methods for opticalexperiments, but it may be difficult to determine the shapes ofindividual nanoparticles [19]. Transmission electron microscopy

ll rights reserved.

.

o).

(TEM) is better suited to investigate the size, shape, and environ-ment of nanoparticles.

Electron energy-loss spectroscopy (EELS) based on TEM with asub-nanometer electron probe is a powerful method for theinvestigation of the plasmonic properties of individual nanopar-ticles. Nelayah et al. [20] reported the spatial distribution ofsurface plasmon excitations of silver nanotriangles. They showedthat the excited surface modes were different for the corners,edges, and centers of the nanotriangles. N’Gom et al. [21]investigated surface plasmon excitations of silver nanowires andgold nanorods. The plasmon peak energies obtained from theindividual nanowires were almost the same with the opticalabsorption energy of the nanowires dispersed in a solution. Inthese studies, the observed plasmon peak intensity in NIR regionis observed on a large background intensity of the tail of a zero-loss peak. Thus, they applied to deconvolution method to improvethe energy resolution and the determination accuracy of theplasmon energy in the NIR region.

Recently, monochromators for conventional TEM have beendeveloped [22–25]. One important aim of those developments is torealize high energy-resolution EELS analyses of an energy resolu-tion better than 0.1 eV with a nanometer-sized electron probe. Theenergy resolution is much better than that of a cold-type field-emission gun (0.3–0.5 eV) and a Schottky-type gun (0.5–0.7 eV).Thus, monochromator-based TEMs enable us accurate investiga-tions of the plasmonic properties of single nanoparticles in the NIR

Y. Sato et al. / Ultramicroscopy 111 (2011) 1381–13871382

region without applying deconvolution process for experimentspectra.

Metal nanoparticles, which absorb NIR rays, have receivedconsiderable attention due to those practical functions [26–28].Transparent films, which are coated as paint-dispersed metalnanoparticles, absorb NIR rays with good efficiency [27,28]. Thesefilms, used as solar heat-shielding filters, can reduce the electricpower required for air conditioning [27], and are already inpractical use. These filters can also help to solve recent ecologicalproblems. There is a demand for improving NIR absorptionefficiency while transmitting visible light. To satisfy the demand,nanoparticles of lanthanum hexaboride (LaB6), metal-doped tung-sten oxide (MxWO3), and tin-doped indium oxide (In2O3:Sn, ITO)have been applied as solar heat-shielding filters [27–29]. Takedaet al. [28] confirmed that polyethylene films and glass substrates,which were coated with dispersed LaB6 nanoparticles, showed aremarkable optical absorption peak at 1.3 eV in transmittancemeasurements. An origin of the NIR absorption was believed to bedue to plasmon excitation of the nanoparticles. However, plas-monic behavior of individual single LaB6 nanoparticles in the NIRregion has not been reported.

In this study, EELS spectra of a bulk LaB6 crystal and singlenanoparticles were measured using a newly developed mono-chromator TEM [25]. The loss function of LaB6 crystal wasobtained from an EELS spectrum, and the dielectric functionwas derived from the loss function by Kramers–Kronig analysis(KKA). EELS spectra of single LaB6 nanoparticles on MgO (insu-lator) and on substrate of aggregated LaB6 nanoparticles (metal)were measured to examine not only the origin of the NIRabsorption of solar heat-shielding filters but also the dielectriceffect from the environment of nanoparticles. The spectra ofnanoparticles were analyzed using the derived dielectric func-tions of LaB6 crystal. The relation between the origin of the NIRabsorption and the EELS spectra of nanoparticles is discussed.

2. Material and methods

LaB6 nanoparticles were produced by breaking a LaB6 crystalinto pieces using a paint shaker mill with 0.3 mm zirconia beads[27,28]. It is known that the plasmon energies of nanoparticlesdepend on the dielectric properties of their substrate and/orsurrounding materials [30–32]. To confirm this effect, two speci-mens were prepared. One was prepared by dropping a suspensionof ethyl alcohol containing LaB6 nanoparticles onto a piece of MgOsingle crystal (insulator) pasted on a copper mesh for TEM

Fig. 1. (a) Electron probe image at the specimen position of about 1 nm in diameter an

of 90 meV.

experiment. The other was prepared by dropping the suspensiononto a microgrid for TEM experiment.

A newly developed transmission electron microscope with amonochromator was used for the present experiments [25]. Themonochromator is located between the extraction anode of theZrO/W emitter and the accelerating tube. The monochromatorconsists of two dodecapole-type Wien-filters, and a selectablewidth energy selection slit is inserted between the two filters. Thefirst filter forms a spectrum of the electron source onto the energyselection slit. Electrons passing through the slit go into the secondfilter. The second filter cancels the energy dispersion of the firstfilter, and forms an achromatic, stigmatic beam image of theelectron source at the entrance of the acceleration tube. Thecolumn of a JEM-2010FEF transmission electron microscope isutilized as the illumination lens system, the specimen goni-ometer, and imaging lens system, in which an omega-filter isequipped as an analyzer. In this experiment, imaging plates,which are made large in size to enable the one-step capture of awide energy range and a large dynamic range (1.0�10�14–1.0�10�10 C/cm2), were used to record the intensity of thespectrum. Fig. 1 shows (a) an electron probe image at the speci-men position and (b) a spectrum of the probe (zero-loss peak) atan accelerating voltage of 100 kV. The incident beam diameter,probe current, and energy resolution in this experiment wereabout 1 nm, 3 pA, and 90 meV, respectively. Total acquisition timeof each EELS spectrum was 6 s.

3. Results

3.1. EELS measurement of LaB6 crystal

An EELS spectrum obtained from LaB6 crystal is shown inFig. 2. Three dominant peak structures indicated by thick verticallines are observed at 2.0, 19.5, and 28 eV. It has been reportedthat the optical reflectance spectrum of LaB6 crystal shows asharp decrease at 2.0 eV called a plasma edge, which appears atplasmon energy of metal [29,33,34]. Thus, the sharp peak at2.0 eV in Fig. 2 can be assigned to a collective oscillation (volumeplasmon) of carrier electrons. Since LaB6, which has a cubicstructure with a lattice constant of 0.415 nm, has one conductionelectron per unit cell [35], a carrier electron density is calculatedto be 14.0 electrons/nm3. Thus, the corresponding volume plas-mon energy for free carrier electrons (Drude model) with thedensity is calculated to be 4.4 eV. This value is larger than theexperimental value of 2.0 eV. This discrepancy suggests that theplasmon energy is lowered by a screening effect due to interband

d (b) a spectrum of the probe (zero-loss peak) with a full width at half maximum

Fig. 2. EELS spectrum of a LaB6 crystal. The peaks at 2.0 and 19.5 eV are assigned

to volume plasmon excitation of carrier electrons and whole valence electrons,

respectively. A peak at 28 eV corresponds to O2,3-edge of La. Small structures

indicated by thin lines are due to interband transitions.

Fig. 3. EELS spectra of LaB6 crystals were measured from thicknesses (a) larger

than 50 nm, (b) 30 nm, and (c) 10 nm. Thicknesses of 10 and 30 nm were

determined by comparing experimental CBED patterns and simulated ones.

Y. Sato et al. / Ultramicroscopy 111 (2011) 1381–1387 1383

transitions, which is apparently recognized after deriving thedielectric function (see later). The density of whole valenceelectrons of LaB6 is calculated to be 278 electron/nm3, sinceLaB6 has 20 valence electrons (La: 6s2, B: 2s2 2p1) per unit cell.Then, volume plasmon energy corresponding to the wholevalence electrons is calculated to be 19.6 eV. Thus, the peakobserved at 19.5 eV should be assigned to a volume plasmonexcitation of whole valence electrons. The peak at 28 eV isassigned to O2,3-edge (5d-conduction band) of lanthanum[33,36]. Other small structures, indicated by thin vertical lines,are due to interband transitions, which are observed in opticalexperiment [33].

Fig. 3 shows EELS spectra obtained from a LaB6 crystalfragment measured from areas of various thicknesses. The thick-nesses were (a) lager than 50 nm, (b) 26 nm, and (c) 8 nm. Thethicknesses of (b) and (c) were determined by comparing experi-mental convergent-beam electron diffraction (CBED) patterns andsimulated patterns, which were calculated using the software‘‘MBFIT’’ [37,38]. Spectrum (a), which should correspond to bulkspectrum, shows a peak and a shoulder structure at 2.0 and1.8 eV, respectively. Spectrum (b) of 26 nm shows a peak at 1.8 eV(thick line) with a faint shoulder structure at 2.0 eV (thin line). Inaddition, a small peak is seen at 1.2 eV. Spectrum (c) of 10 nmshows two peaks at 1.2 and 1.7 eV. The peak at 2.0 eV in (a) andthe faint shoulder structure at 2.0 eV in (b) should be assigned tovolume plasmon excitations of carrier electrons. The peaks at1.8 eV in (a) and (b) and at 1.7 eV in (c) can be assigned to surfaceplasmon excitations of the carrier electrons for a flat boundarybetween LaB6 bulk and vacuum. The observed surface plasmonenergy is a little larger than EP=

ffiffiffi2p

, which is expected for a freeelectron model (EP is the volume plasmon energy, 2.0 eV). Thisdiscrepancy should be also due to the effect of interband transi-tions as stated in previous paragraph. As the thickness of LaB6

crystal decreases, the intensity of the volume plasmon peak at2.0 eV decreases, and the surface plasmon peak becomes domi-nant. This is due to Begrenzung’s effect [39,40], which is adamping effect of volume plasmons arising from couplingbetween volume and surface oscillations.

On the other hand, the peak at 1.2 eV in Fig. 3(b) and (c) couldnot be assigned on the previous optical studies, which reportedthat bulk LaB6 has no intense interband transitions at energiesbelow 2 eV [29,33,34]. The peak intensity at 1.2 eV increased with

decreasing LaB6 thickness. Thus, the peak at 1.2 eV is not aproperty of the bulk LaB6 crystal, but should be due to a surfacelayer. One possibility is that the peaks at 1.2 and 1.7 eV might besurface plasmons of the asymmetric and the symmetric mode fora thinner plate [41]. The splitting of these surface mode energiesshould become larger as decreasing the thickness of the speci-men. However, this tendency is not observed in Fig. 3. Thus, itmight not be an appropriate assignment for this case. The originof this peak would be discussed later in this paper.

3.2. EELS measurement of LaB6 nanoparticles

Fig. 4 shows TEM images of LaB6 nanoparticles ((a) and (b)) onMgO (insulator), and a LaB6 nanoparticle (c) attached on a largeaggregate of LaB6 particles (dielectrics). The diameters of thenanoparticles were (a) 25 nm, (b) 38 nm, and (c) 18 nm. EELSspectra shown in Fig. 5(a) and (b) were obtained from center andedge regions of the nanoparticles indicated by black dots in thecorresponding TEM images. The spectrum of Fig. 5(c) was mea-sured from the center region of particle (c). The peak energiesobtained from the center and the edge of particle (a) are 1.4 and1.3 eV, respectively. In addition, a weak spectral structure mayexists at 1.7 eV, where a surface plasmon excitation for a flatsurface is expected. Peaks obtained from the center and the edgeof particle (b) were observed at 1.1 and 1.05 eV, respectively. Thespectrum of particle (c) had a peak at 1.2 eV, with additionalstructures at 1.0 eV (arrow head) and 1.7 eV.

Each spectrum of nanoparticle shows a peak in an energyrange from 1.1 to 1.4 eV, which is almost the same with opticalscattering peak of a heat-shielding filter of LaB6 nanoparticles[28]. It should be noted that each spectrum does not show a peakat 2.0 eV, where a volume plasmon excitation was observed in thespectrum of the LaB6 crystal of Fig. 3(a). It is due to Begrenzung’seffect, as stated on spectra in Fig. 3. It is also noted that the peakintensities in Fig. 5(a) and (b) are apparently larger in edgeregions, where a dipole mode of plasma oscillation of a metalparticle should be excited more strongly than other oscillationmodes. This indicates that the heat-shielding property should be

Fig. 4. TEM images of particles (a), (b), and (c). Black dots indicate electron probe positions, where EELS spectra shown in Fig. 5 were observed.

Y. Sato et al. / Ultramicroscopy 111 (2011) 1381–13871384

due to dipole oscillation mode. The peak energy in (b) is lowerthan that of (a). It may be due to a larger dielectric effect from theMgO substrate because the contact area between particle (b) andthe substrate in Fig. 4(b) seems larger than that for particle (a).The spectrum of the particle (c), which putted on a differentsubstrate from that of (a) and (b), shows an additional structure atabout 1 eV. This additional structure could be due to a coupledoscillation mode between the nanoparticle and the substrate ofLaB6 particles [31]. The coupling mode has been observed inexperimental [31] and theoretical studies [32] of surface plasmonbehavior of Al nanoparticles embedded on Al (metal) and Al2O3

(insulator) substrates. These studies also reported that decreasingintensity of volume plasmon of Al nanoparticle due to Begren-zung’s effect, and that modulation of surface plasmon modedepending on the dielectric constant of surrounding material.Those results are consistent with our results.

The wavelength of surface plasmon oscillation mode in NIRenergy region, which can be estimated approximately as lEpv/4op�102 nm (v: velocity of incident electron, op: plasmonfrequency) [42], is much larger than the size of these nanoparti-cles. This indicates that multi-pole oscillation modes hardly exist.This follows the results of Fig. 5, which the observed surfaceplasmon was predominantly in the dipole mode corresponding tothe optical excitation.

4. Discussion

The dielectric function of LaB6 was derived from the EELSspectrum shown in Fig. 2 by Kramers–Kronig analysis (KKA).

A spectrum due to single-scattering was obtained from theexperimental spectrum by removing the contribution of the directbeam using a Lorentz fit, and then removing multiple-scatteringeffects using a Fourier-log deconvolution method. The loss functionwas obtained by applying the sum rule for metal, Re[1/e(0)]¼0. Inapplying the sum rule to obtain the loss function, and in the KKA,energy integrations were performed up to 400 eV. Intensities above50 eV were obtained by extrapolation using E�a (E: loss energy)intensity dependence. Fig. 6 shows (a) the loss function derivedand (b) the real and imaginary parts of the dielectric function ofLaB6 crystal from 0 to 40 eV. The real part of the dielectric functionof a Drude model, which has the same carrier electron density withthat of LaB6, is also shown for comparison. The imaginary part ofthe dielectric function, e2, shows peak structures at 3.5, 5.1, 6.8, 8.3,10.7, and 12.0 eV, whose structures have been reported in opticalconductivity oe2 [43]. The electronic excitation peak at 3.5 eVindicated by an arrow is assigned to transitions from B-2p to La-5d[43]. It can be explained from the dielectric functions that theoscillator strength of interband transitions distributed from 3 to10 eV lower the condition of e1�0 from 4.4 eV of the Drude modelto about 2 eV. This means that the restoring force for plasmaoscillation is screened by the interband transitions and the plasmaoscillation frequency is lowered [44]. The dielectric function from0.5 to 2.2 eV can be approximately represented by a Drude-likemodel [45,46], where e1 is given by

e1 ¼ e1�O2p=ðo

2þG2Þ: ð1Þ

The parameters eN, Op, and G are the background dielectricconstant, the effective plasmon frequency op/O(mn/m0) (m0 is the

Fig. 5. EELS spectra of particles (a), (b), and (c) in Fig. 4. Spectra (a) and (b) were measured from center and edge regions of the corresponding particles.

Y. Sato et al. / Ultramicroscopy 111 (2011) 1381–1387 1385

rest mass of the electron, mn is effective mass), and a dampingterm, respectively. A screening effect of the interband transition isincluded in the parameter eN. By fitting Eq. (1) to the dielectricfunction shown in Fig. 6, the parameters were determined asfollows: eN¼8.6, Op¼5.3, and G¼0.26. The effective mass of thecarrier electrons calculated from these parameters was 0.83m0.This estimation of the effective mass obtained is consistent with areported value derived from an optical experiment [33].

In case of nanoparticles, the dielectric response to externalfield is different from that of bulk, because the boundary condi-tion at the surface should be taken into account. The heat-shielding property of LaB6 nanoparticles is due to scattering ofelectromagnetic (EM) waves by the particles. Scattering of a planeEM wave by a metal sphere can be described by Mie theory [4].When the EM wavelength is much longer than the size of themetal sphere, the scattering intensity is expressed as follows:

9Fscattering92pðn2�1Þ

ðn2þ2Þ

��������2

, ð2Þ

where n is the refractive index. Using the above Drude-like model[44,45], the refractive index can be represented asn2 ¼ e1 ¼ e1�O2

p= o2þG2� �

. The maximum scattering intensityis obtained for the condition of n2

þ2¼0(e1¼�2)

omax ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiO2

p

e1þ2�G2

s: ð3Þ

The energy _omax corresponds to the most effective energy ofthe heat-shielding property of the metal particle.

On the other hand, the surface plasmon energy os of anisolated ideal metal sphere is determined by the following

condition [47]:

e1 ¼�ðlþ1Þ=l, ð4Þ

where e1 is given by Eq. (1). The index l represents the oscilla-tion mode. A flat surface corresponds to l¼N (e1¼0). The dipolemode l¼1 (e1¼�2), which was experimentally assigned fora peak in NIR of each spectrum in Fig. 5, is represented as

os ¼omax ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiO2

p=ðe1þ2Þ�G2q

. Thus, the dipole oscillation

energy of a metal nanoparticle obtained by EELS corresponds tothe energy of most effective heat-shielding property of theparticle. Therefore, the dominant peaks of nanoparticles inFig. 5(a) and (b) correspond to optical scattering dipole modefor heat-shielding filters.

Fig. 7 shows the loss functions Im[�1/(eþ2)] for surfacedipole mode [48], which is calculated using the dielectric functionof LaB6 shown in Fig. 6. The loss function for bulk mode Im[�1/e]calculated by the same way is also shown for comparison. Thepeak energy of Im[�1/(eþ2)] (dipole mode) is 1.8 eV, whereweak structures were observed in Fig. 5. However, the dominantpeak energies of 1.1–1.4 eV are apparently lower than thissimulated energy.

The surface plasmon energy of the nanoparticles dependsupon the shape of the nanoparticle and a surrounding dielectriceffect due to the substrate. On the other hand, TEM observationshowed that the shapes of the nanoparticles did not significantlydeviate from a sphere. Cerenkov radiation, which occurs in a thickinsulator under the condition of e14c2/v2, is unlikely, since LaB6

is a metal. By taking into account the results of thin specimen areaof LaB6 shown in Fig. 3, it should be thought that the lower peakenergies of 1.1–1.4 eV than the calculated one is due to an effect

Fig. 6. (a) Loss function Im[�1/e] and (b) dielectric function e¼e1þ ie2 of LaB6.

The dashed line in (b) indicates the real part of the dielectric function calculated

by the Drude model.

Fig. 7. Loss functions of the bulk and surface dipole modes of a nanoparticle. The

simulation of the dipole mode was calculated using the dielectric function of LaB6

shown in Fig. 6.

Y. Sato et al. / Ultramicroscopy 111 (2011) 1381–13871386

of an excitation at 1.2 eV in Fig. 3. From EELS experiments at theedge region of the specimen, not only B K-edge and La N-edge butalso O K-edge intensities were observed. This indicates oxidationof the surface of LaB6 crystal. Possible oxides of La2O3 and B2O3,which are insulators, do not have electronic excitations at about1.2 eV. X-ray diffraction measurements of LaB6 particles reporteda presence of peaks corresponding to LaO [28]. The La2þ ion hasan electronic excitation at 1.1 eV from 5d to 4f [49], whose energyis close to the peak energy of 1.2 eV. Thus, in order to evaluate theoptical NIR scattering energy accurately, the surface dielectricresponse of LaB6 nanoparticles covered with an oxidized shellshould be examined. Of course, other effects of the shape devia-tion from a perfect sphere and the interaction with the substratehave also to be considered in detail.

5. Conclusions

High energy-resolution EELS measurements were applied forinvestigating a plasmon oscillation excitation in the NIR region ofsingle LaB6 nanoparticles, which have been used for heat-shield-ing filters. It has been presented that EELS spectrum of eachnanoparticle shows a peak (mainly a dipole oscillation excitation),whose energy almost equal to the peak energy of 1.3 eV of filtersincluding LaB6 nanoparticles. It has been also apparent that thesimulation of spectra using a dielectric function of bulk LaB6 is notenough. For a more accurate understanding of this practicalresult, it is necessary to take into account the oxide layer coveringthe particle. In order to understand the heat-shielding property ofLaB6 particles, a simulation, which includes effects of a shapedeviation from sphere, a dielectric effect from the substrate, andalso the oxide layer of the particle, should be conducted.

Acknowledgments

The authors thank Mr. F. Sato for his skillful technical assis-tance. The present work was partly supported by a Grant-in-Aidfor Scientific Research in Priority Area ‘New Materials ScienceUsing Regulated Nano Space-Strategy in Ubiquitous Elements’ bythe Ministry of Education, Culture, Sports, Science, and Technol-ogy of Japan (No. 19051002), and the KAZATO ResearchFoundation.

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