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306 K. Kumar et al.

Fig. 1 Scanning ElectronMicroscope (SEM) (a and c)and Atomic Force Microscope(AFM) (b and d) imagesshowing the surfacemorphology of thin films of CsI.The film thickness of samplesshown in micrographs (a and b)is 690 nm, while those in

micrographs (c and d) arethicker and have thickness of880 nm

3 Results and discussion

3.1 Structural and morphological studies

Interesting information can be inferred by just examining thesurface morphology of our samples with both the ScanningElectron Microscope (SEM) and Atomic Force Microscope(AFM). The SEM micro-graphs of our films show largewhite grains present on a black background. The micro-graphs are similar to those reported by Senesi et al. [7] andsuggest some aging effects in the films. The grain size of thefilms were determined from the SEM micrographs, usingthe reference scale given by the electron microscope. Thevalue reported here are average sizes of 10 to 15 grains se-

lected randomly in the field of view of the micrograph. Thismethod gives the size of the grains lying on the surface. Thegrain size and density depends on the film thickness. In thethinner films the white grain density is poor with a large in-termediate dark region (Fig. 1a). However, with increasingfilm thickness, the grain size and density increased to suchan extent that the whole film is covered with these whitegrains (Fig. 1c).

The dark background also shows some morphologicalfeatures suggesting material is present in this region. We

Fig. 2 Chemical composition of the two distinctly visible phases of

our films were determined by EDX. The spectra ( a) suggests the whitegrains of Fig. 1(a and c) have chemical composition of CsI while ( b)shows that the dark contrast region of Fig. 1(a) is metallic cesium

have determined the chemical composition of the two dis-tinct regions using Energy Dispersive X-ray Analysis, EDX(Fig. 2). Comparison of the spectral peaks and lack of iodinepeaks in the dark region established the fact that the whitegrains are of cesium iodide (CsI), while the dark region isfilms of cesium metal. Also, notice the existence of silicon


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The effect of cesium metal clusters on the optical properties of cesium iodide thin films 307

and oxygen peaks contributed from the substrate; it has in-creased (proportion of these peak intensities in Fig. 2b) sug-gesting the dark background is thinner than regions wherethe white grains exist. Senesi et al. [7] did not report lackof iodine in the dark background. We believe they mighthave missed this since the films in their study were verythin (300 nm). Hence, the amount of cesium present would

have been too small to give a strong enough signal in EDX.It appears that as the film is being deposited, chemi-

cal dissociation takes place followed by iodine sublimat-ing from the film surface. Hence, the samples are multi-phased films of CsI and Cs. However, as the film thicknessincreases, disassociation is discouraged and we successfullyobtain uniform films of CsI. The whole surface is tightlypacked with white large grains of CsI if the grown films havea thickness greater than 780 nm.

Figure 1(b) and (d) shows some of the samples surfacesas seen using the AFM. As compared to SEM, AFM showsa larger grain density, which is uniformly distributed on the

surface for the same sample. Grains are even seen in theSEMs dark background regions. The disparity between thetwo images is basically due to the difference in image for-mation of the two methods. The dark contrast region of ce-sium showed no distinct morphology in SEM due to poorformation of secondary electrons. However, Van der Waalsinteraction makes the granularity of this region visible inAFM. On the other hand it is not possible to resolve whichgrains are of cesium metal and which are of cesium iodide.The cesium metal grains, however, seem to be smaller thanthe cesium iodide grains, since the average grain size asdetermined from AFM images is smaller than that deter-mined from SEM. The inset of Fig. 3 shows the variation

Fig. 3 Variation of average grain size (grains of both phases, cesiummetal and cesium iodide) with film thickness. The inset shows the re-lation of grain size of cesium iodide (determined from SEM) with av-erage grain size (determined from AFM), using which the variationof cesium iodide grain size with film thickness was found to follow3.527T 1416, where T is the film thickness in nm

of grains size as determined from AFM images with corre-sponding size determined from SEM micro-graphs. The lin-earity shows a correlation between the cesium iodide grainsize and the average grain size (considering both cesium andcesium iodide grains). Hence, we can infer the grain size ofCsI from the AFM images. Figure 3 shows that variation ofthe average grain size (as determined from AFM) increased

linearly for increasing film thickness. Using the equation ofa least square fit line of the two figures, we have an equationdescribing the variation of CsI grain size with film thickness:

CsI = 3.527T 1416 (1)

where T, is the film thickness in nm.X-Ray diffraction studies showed that the films of CsI

grown at room temperature were polycrystalline in naturewithout exception. The existence of cesium in elementalstate in the films whose thicknesses lie below 780 nm is alsoevident from the X-ray diffractograms of our samples shown

in Fig. 4. They show a prominent broad peak at 27.5o

thaton close examination can be deconvoluted into two peaks(Fig. 4). The first deconvoluted peak matches the peak po-sition listed in ASTM Card No. 42-1245 of pure cesium(2 = 27.4).

X-ray diffractograms also show two other peaks of vary-ing prominence, marked as (211) and (220) in Fig. 4. Thesepeaks match the peak positions listed for cesium iodide inASTM No. 06-0311. Thus, as our morphology studies sug-gested, our film have two phases, namely cesium metal thatexists in tetragonal phase (a= 3.3645 and c= 12.552 )and CsI in cubic phase (a = 4.568 ). The second of the de-

convoluted peaks can be ascribed to the (110) peak of CsI.This peak is shifted from the 2 = 27.59 position listed inthe ASTM card. A shift in the peak position when comparedto its position for single crystal of the sample indicates thatthe crystal is in a stressed state. A displacement of the X-raypeak to the right as compared to the peak position of singlecrystal indicates a decrease in d-spacing, implying compres-sive stress acting on the film, and vice versa the increase ind-spacing is indicative of tensile stress. The term residualstress (or simply stress) emphasizes the fact that the stressremains after all external forces are removed. These stressesacting in the film can cause important effects on the prop-

erties of the material. Hence, we shall try to understand therole of stress on our samples. The stress in the film is calcu-lated after evaluating the strain using the relation

d

d=dobs dASTM

dobs(2)

where dobs is the d-spacing measured for the thin film anddASTM is the corresponding peaks d-spacing of the singlecrystal as reported in the ASTM Card. The stress then canbe determined by multiplying the average strain d/d by


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308 K. Kumar et al.

Fig. 4 Some X-Raydiffractograms of CsI films. TheX-ray diffractograms areidentical in nature with Cs metalpeaks and CsI peaks

Fig. 5 variation in strain acting perpendicular to the planes of cesiumiodide crystals with increasing film thickness. Strain acting perpendic-ular to parallel planes (110) and (220) decreases with increasing filmthickness. However, it shows an increasing trend in the direction per-pendicular to (211) plane

the elastic constant of material. We have characterized ourfilms with respect to strain, since it is fundamental in nature.

Cesium iodides peak showed shifts indicating that thesamples are in a state of stress. The stress in our samplesessentially arises from the thin film state they are in. Thisis evident from the variation in strain acting on CsI withfilm thickness. The parallel planes of (hh0) show decreas-

ing tensile stress with increasing film thickness for filmswhose thicknesses lie between 550 to 780 nm. The (211)plane shows a slow increase in tensile stress with film thick-ness. The variation is so small that for all practical purposeswith respect to the stress on (hh0) plane we can consider thatthere is no variation in stress on the (211) plane with vary-ing film thickness. Interestingly, samples of thickness lowerthan 550 nm showed compressive stress. However, the datapoints of these samples did not fall on the linear trend ex-hibited in Fig. 5. Hence, we believe that while film thick-

Fig. 6 The variation in strain acting on CsI is seen to be influencedby the strain acting on the Cs dispersed in the film. The correlation of0.9645 shows the strong linear relation between the two

ness maybe a contributing factor, the stress acting on CsI isinfluenced by some other factors.

The Cs (101) peak is also displaced from its ASTMposition of 27.4, indicative of strain. Evaluation of thisshows that compressive stress acts on cesium polycrystallinegrains. The tensile and compressive stress acting on CsIshows a dependency on the stress acting on cesium grains(Fig. 6). As stated earlier, the compressive stress acting onCsI did not show any correlation with film thickness. Thissuggests that the stress acting on the cesium iodide unit cellis essentially due to the deforming stress acting on the ce-sium unit cell, or in other words, the mismatch in latticestructure between the two manifests as stress. Cesium in theelemental state exists in the tetragonal state, and one wouldexpect the stress applied on the CsI unit cell would deformits cubic structure to a tetragonal structure. As can be under-stood from Fig. 7, the tensile stress acting along the diagonal(perpendicular to 110 plane) would result in an expansion ofthe unit cell in the x and y direction. This would result ina compressive force acting along the z direction. In otherwords, the stress would result in an increase in lattice con-stant a and decrease in lattice constant c.


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The effect of cesium metal clusters on the optical properties of cesium iodide thin films 309

Fig. 7 The tensile stress actingon the (110) plane leads to anexpansion of the unit cell in thex and y axis that results in acompressive force acting on thez axis

Fig. 8 variation in lattice constants of CsI as a function of the strainacting perpendicular to the plane of (110). The cubic structured crys-tal (lattice constant 4.5679 , shown by a dashed line) deforms andbecomes tetragonal with a increasing and c decreasing with strainincreasing (0.002)

To test this we have estimated the value of lattice con-stants maintaining the Miller indices. The average value oflattice constant a was evaluated using the new inter-planardistances (d) for the (hh0) planes. Using this, the lattice con-stant c was calculated using the inter-planar distances forthe (211) plane. Figure 8 shows the variation of lattice con-stants with the stress acting perpendicular to the (110) plane.As can be seen, the lattice deformation only becomes evi-dent for large stress (strain 0.002). Thus, both small com-

pressive and tensile stress show lattice constants approxi-mately equal to that of the undeformed cubic unit cell. How-ever, with increasing tensile stress (or decreasing film thick-ness, Fig. 5), there is an increase in the lattice size in the xand y axis. As suggested above, this in turn leads to a com-pressive force in the z direction resulting in the decrease inlattice constant c. This proves that the mismatch in crys-tal structures of cesium and cesium iodide results in stressesthat tend to modify the cubic unit cell of CsI to tetragonalcells.

4 Optical studies

The existence of metal clusters embedded or dispersed in aninsulating background in our films whose thicknesses wereless than 780 nm is evident from their UVvisible absorp-tion spectra. Such samples give peaks (generally broad) inthe visible region of the spectra. The increased absorption

corresponds to the resonant frequency of the surface plas-mons. The Surface Plasmon Resonance (SPR) effect arisesdue to the collective oscillations of free electrons in metal.This essentially entails the localized excitation of electronsand is observed in metal nanoparticles whose dimensions arefar smaller than the incident wavelength of light. The local-ization of the electron excitation is achieved in nanoparticleswhen they are isolated from each other either by dielectricsor voids. In turn the resonant wavelength of SPR dependson the nanoparticles size, shape and its dielectric constantalong with that of its surrounding. The inset of Fig. 9 showsthe peaks of SPR in two of our samples which confirms our

EDX analysis and proves the existence of two phases (ce-sium metal in cesium iodide).

The UVvisible spectra of our films also show the sig-nature of a thickness dependence. Only the samples whosethickness is less than 780 nm show SPR peaks in the visibleregion of the spectrum. The peak position and Full-Width atHalf-Maxima (FWHM) of the SPR are indicative of the ce-sium cluster size. Assuming the nanocrystals to have spher-ical shape, the nanocrystal size/diameter is estimated using

D =hvF

2E1/2(3)

where h is Plancks constant, E1/2 is the full-width at half-maximum of the absorption peak and vF is the Fermi ve-locity of electrons. Liang-Fu Lou [12] gives the value of theFermi velocity to be around 1 108 m/s. Using this formulawe calculated the cesium grain size for different samples.The values are listed in Table 1.

The grain size of cesium polycrystalline grains were alsocalculated from the Full-Width at Half-Maximum (FWHM)of the deconvoluted X-ray diffraction peaks using the Scher-rer relation [13],

D=0.9B cos (4)

where D is the grain size (in ), B is the FWHM of theparticular peak, (in radians) is the Bragg angle and (1.5405 ) is the wavelength of the X-rays. Table 1 com-pares the grain sizes of cesium metal cluster obtained fromSPR and XRD results. The results from XRD would havemore error since it would depend on the accuracy of the de-convolution of the peaks. The results indicate that the metalnanoparticles are 3040 nm in size.


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310 K. Kumar et al.

Table 1 Table compares the grain size of cesium clusters as obtainedfrom deconvoluted X-ray peaks, SPR peaks and the surrounding CsIgrains (from (1))

Thickness (nm) SPR (nm) XRD (nm) CsI (from (1))

584.4 37.8 29.75 645

612.8 39.9 34.7 745

692.4 30.8 68.0 1026

724.5 29.1 39.1 1140

Fig. 9 The absorbance spectra (inset) shows classical SPR peaks dueto metal cesium clusters scattered in the films. The SPRs peak posi-tion decreases linearly with increasing grain size (the grain sizes weredetermined using SPR peaks)

Figure 9 shows the correlation of the Cs cluster size withpeak position. The graph shows a red shift in SPR peak posi-tion with decreasing grain size. Link [14] and Liu [15] haveshown that the SPR peaks of metal nanoparticles show a redshift when they are embedded in a host with higher per-mittivity. A careful examination of Fig. 9 with inferencesof this manuscript shows that the less strained films withnear-cubic structure have metal nanoparticles of30 nm di-ameter. However, the strained samples with tetragonal lat-tice have metal nanoparticles of40 nm diameter with theirSPR peaks at 300 nm. This implies that the lattice defor-mities may lead to a lowering of the dielectric constant ofthe cesium iodide host. A search of the literature does notgive any experimental confirmation to this idea. However,Shukla et al. [16] theoretically show that an increasing lat-tice constant of cubic CsI would result in a decreased dielec-tric constant. These results, hence, could be useful to furthertheoretical investigations to understand the dependency ofthe crystal structure on the dielectric constant and in turn

the optical properties of cesium iodide in the nanoparticlestate.

5 Conclusions

The manuscript discusses the morphological and optical

properties of cesium iodide thin films. Results show the for-mation of nanocrystalline films of CsI which have increasingcesium metal nanoclusters presence for lower thicknesses.These nanoclusters of cesium not only influence the opti-cal properties directly, but also indirectly, by influencing thehost dielectric constant. These experimental results give in-sight into the optical properties of CsI and its dependence onits lattice constant.

Acknowledgements The help in completing the spectroscopic anddiffraction analysis by Mr. Dinesh Rishi (USIC), Mr. Padmakshan andMr. Rohtash (Department of Geology, Delhi University) is gratefullyacknowledged. Author K.K. wishes to acknowledge the financial as-

sistance from U.G.C.(India) in terms of Minor Project 6-1(222)/2008(MRP/NRCB).

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