White-Light Emission from Annealed ZnO:Si Nanocomposite Thin Films
aa r X i v : . [ c ond - m a t . m t r l - s c i ] J a n White-Light Emission from Annealed ZnO:Si Nanocomposite ThinFilms.
Shabnam a , Chhaya Ravi Kant a and P. Arun b ∗ a Department of Applied Sciences,Indira Gandhi Institute of Technology,Guru Gobind Singh Indraprastha University,Delhi 110 006, India. b Department of Physics & Electronics,S.G.T.B. Khalsa College,University of Delhi, Delhi - 110 007, IndiaOctober 8, 2018
Abstract
As grown ZnO:Si nanocomposites of different compositional ratios were fabricated by thermal evap-oration techniques. These films were subjected to post deposition annealing under high vacuum at atemperature of 250C o for 90min. The photoluminescence (PL) spectra of annealed samples have shownmarked improvements both in terms of intensity and broadening. For the first time in ZnO:Si nanocom-posite films we see huge UV, red and orange peaks at 310, 570 and 640nm. Structural and Raman analysisshow formation of a Zn-Si-O shell around ZnO nano clusters wherein on heating Zn SiO compound forms.The new emissions are due to Zn SiO which completes white light spectrum. Keywords
Nano-composites, Nanostructures, Photoluminescence, Oxides ∗ email:[email protected], Telephone:091 011 29258401, Fax: 091 011 27666220 Introduction
Ever since the extraction of blue light from Mg doped Gallium Nitride(GaN) was made possible, research hasbeen directed to yield cost effective white light emitting devices from a single chip. Owing to its structuralsimilarity with GaN and wide band gap of ∼ . ZnO:Si nanocomposites films were fabricated by thermally evaporating a mixture of powdered ZnO and n-Silicon at a vacuum of ∼ − Torr in a Hind High Vac (Bengaluru), Thermal evaporation coating unit,Model 12A4D. The deposition was carried out on microcopic glass substrates maintained at room temperature.Starting material was prepared by mixing ZnO and Silicon in the proportions of 1:1, 1:2, 1:3, 2:3 and 2:5 (byweight). These mixtures were then pelletized, to prevent its flying off the boats. As deposited nanocompositefilms were subjected to annealing under high vacuum of the order of ∼ − Torr and then allowed to coolnaturally under vacuum. Films of varying composition with thickness 600˚A are named as sample (a1), (b1),(c1), (d1) and (e1) in order of the increasing Silicon content in starting material. The vacuum annealedcounterparts are referred to as (a1v), (b1v), (c1v), (d1v) and (e1v).The structural studies of the surface is measured by Pananalytical PW3050/60 Grazing Incidence angleX-Ray Diffractometer (GIXD) and that of the bulk region by Philips PW 3020 X-Ray Diffractometer (XRD). X-Ray Photoelectron Spectroscopy (XPS) was performed with Perkin-Elmer X-ray Photo-electron Spectrometer(Model 1257) with Al K α (1486 eV) X-ray source. Photoluminescence (PL) scans were recored on FluorologJobin Yvon spectroscope (Model 3-11) using an excitation wavelength of 270nm. Renishaw’s “Invia Reflex”Raman spectroscope was used for measurements uisng Ar +2 . The surface morphology and texture of the asgrown nanocomposite films were studied using TECHNAI-20 G Transmission Electron Microscope (TEM).Below we enlist the results of the various analysis done on our samples.2
Results and Discussion
The structural changes in the films caused by heat treatment was examined using GIXD. To compare theeffect of heat-treatment, Figure 1(A) and 1(B) shows the GIXD scans of the samples after fabrication and postvacuum annealing. Similar to the pattern obtained for the as grown films, the GIXD scans of the annealedsamples show peaks at 2 θ =36 and 43 . These peaks are the (101) and (102) plane of ZnO and Zn respectively.A mixed response is seen on heat treatment with no variation in samples (b1v) and (e1v), however, there is amarked increase in peak intensities in (d1v) while it diminishes in (c1v).To investigate the increase in peak intensity of sample (d1v), we have calculated the grain size of the preand post annealed samples. The grain size as calculated from the ZnO peak of (d1) was 8.7nm while that ofsample (d1v) was found to be 9 . ± The surface and the film’s bulk are usually different [12] and hence it is necessary to investigate the film belowthe surface. For this we studied the samples using XPS not only at the surface but also beneath it by sputtering125˚A of film layer. Figure 2(A) shows this depth profiling, recorded for Zn-2p / of sample (c1v). Presence ofa single peak at around 1022eV suggests Zinc exists in the sample as ZnO. Contribution from elemental Zinc ifany is insignificant. However, a shift to higher binding energy along the film thickness is visually evident fromfigure 2(A) (and ploted in figure 3A). Before proceeding, it is worth mentioning that the existence of singleSilicon peak in XPS as is the case with Oxygen (figure 2B and 2C respectively) eliminates the possibility ofexistence of elemental Zinc and Silicon dioxide through out the film.In our previous report on as deposited films, we had observed that Zinc’s 2p / peak shift towards lowerenergy side along the film thickness. We believe this shift is related to the change in the neighboring environmentin terms of relative ZnO to Silicon abundance. However, here we see a shift in Zinc’s peak towards the higherenergy side as we move deeper into the film.Following the methodology we adopted in our earlier work [12], we determined the ratio of ZnO to Si alongthe thickness of the film. Figure 3(B) shows the linear decrease of this ratio with film depth. Decrease in ZnO:Siratio from 1.023 to 0.271 along the depth of the film can also be interpreted as an increase in the Silicon contentwith the depth of the film. Combining the results of Figure 3(A) and 3(B), figure 3(C) shows the variation ofZnO peak position with ZnO:Si ratio. The data indicates a shift in Zinc 2p / peak to the lower energy sidewith decreasing Silicon environment. Though the samples under study here and in our previous study weredifferent, the fact that Zinc 2p / peak moves to the lower energy side with decreasing Silicon environmentis consistent. Thus, structurally, morphologically and compositionally vacuum annealing has not effected oursample. We have noticed that Raman spectra reveals more information on the structural properties of our nano-composites than X-Ray diffraction [12]. Hence, to investigate the structural modification incorporated in oursamples on vacuum annealing we analysed the samples using Raman spectroscopy. The Raman spectra wastaken in standard back scattering geometry using Argon ion laser for excitation. Figure 4 compares the Ramanspectra of the as grown samples (a1-e1) with their vacuum annealed counterparts (a1v-e1v). The as grownfilms irrespective of the ZnO/Si starting ratio gave broad spectra in 300-600cm − range. The spectras were3ssentially three prominent unresolved peaks, namely at ∼ ,
440 and 565 cm − . The ∼ − peakcorresponds to the LA mode of amorphous Silicon [11, 12]. Similarly, the ∼ − and ∼ − peaksare attributed to Wurtzite ZnO bond vibrations and defect related bondings in ZnO respectively [17, 18].Visual examination show significant changes not only in peak sizes but also in their positions. For example,the plot of figure 5(A) shows variation in ratio of area enclosed by the 440cm − peak of the vacuum annealedsample to that of as grown with ZnO content. A decreasing trend is seen with increasing ZnO content. Webelieve that this decrease manifests due to decrease in ordering in the ZnO. Also, the ordering is easily brokenin samples with higher ZnO content on vacuum annealing. Even the peak position (440cm − ) of the annealedsamples as compared to those of as deposited peaks show a linear trend with the peak moving to a higherwave-number on annealing (fig 5B). That is, on vacuum annealing the peak position corresponding to bondingsof the Wurtzite structured ZnO shows increased wave-number where the increase is more substantial if the asgrown samples peak was at a higher wave number. This too must be indicative of increased disorder. Also,Yadav et al [19] have reported an increase in wavenumber with decreasing grain size. This would reason thatvacuum annealing has resulted in a decrease in grain size (except for sample a1v).This lack of ordering discussed above should reflect in the vacuum annealed sample’s ∼ − peak thatcorresponds to the defects in ZnO [20]. Plots of ∆ VA / ∆ AG (∆ represents peak area of vacuum annealed ‘VA’,and as grown ‘AG’) and I VA / I AG (peak intensities) with varying ZnO content shows linear trend. Figure 5(C)shows the variation in ratio of intensities with ZnO content. As expected, the linear trend with positive slopesuggests an increase in the defects in vacuum annealed ZnO:Si nanocomposites for samples with large ZnOcontent. There also data point of ‘d1v’ stands apart from the trendline. However, this indicates ‘d1v’ containsappreciable defects along with substantial ordering. The fact that sample ‘d1v’ shows a simultneous increasein crystallinity and defects appears contradictory at first glance, however, we have been able to show that asample with comparable amounts of defects co-existing with good ordering gives broadening in PL spectra[12]. Figure 6 gives a plot of relative presence of Wurtzite ZnO to ZnO with defects (∆438cm − / ∆565cm − )with varying ZnO content. This graph helps in predicting broadening of PL spectra. In an earlier work on asgrown samples (shown by filled circles in figure 7), it was observed that samples which had nearly equal areas(and hence ratio ∼
1) showed maximum broadening in PL [12]. It can be appreciated from the plot that thesample ‘d1v’ has nearly equal areas of the peaks resulting from defect free and defect related ZnO lattices.Moreover, sample ‘d1v’ lies on the minimum of the curve (visual aid showing the trend), so it is expected toshow maximum broadening in photoluminescence. The downward shift of the ‘a1v’ data point shows that thissample has also got comparable contributions from wurtzite and defect related ZnO structures. As per ourprediction, ‘a1v’ should also show broadening in PL spectra along with the other heat treated samples.Finally we now comment on the reduction in the ∼ − peak of annealed samples as compared tothat in the as deposited samples. In our ZnO:Si nanocomposites, we have ascribed this peak to LA modeof amorphous Silicon [11, 12]. Since the annealing of the samples has been done in the high vacuum of theorder of 10 − torr, as also by our XPS results we rule out the possibility of Silicon’s oxidation. Hence, webelieve this reduction is due to some improvement in ordering of Silicon. However, since XRD failed to give ananocrystalline peak of Silicon, we believe only short-range ordering of amorphous silicon has taken place. In our previous works we have shown that a broad spectra is achievable from ZnO:Si nanocomposite films.However, they had poor or no emission in the red wavelength region. To obtain further broadening and toimprove emission in the red wavelength region, we annealed our samples under high vacuum. Figure 7 comparesthe PL spectra of samples (a1) and (a1v). Broadening accompanied with a multifold increase in intensity onannealing can be easily appreciated. In the absence of prominent shoulders resulting from broadening of peakswe were not able to deconvolute the spectra using standard softwares. However, based on our knowledge fromstudy of asgrown samples we expect peaks at 365 and 420nm. These peaks correspond to band edge emissionfrom Wurtzite ZnO and the interfacial layer between ZnO grains and Si background. These two peaks in the4nnealed samples were unresolved and hence using Peakfit-4 we have placed a peak at 395nm (fig 8). In theprevious section, Raman analysis suggested a decrease in the grain size of ZnO accompanied with reductionin 438cm − peak implying decrease in Wurtzite ordering. A direct relationship between the Raman 438cm − peak and PL’s 365nm was established [12]. Thus one expects a reduced contribution from the band edgeemission peak of ZnO. However, since the unresolved peak remains significant, one can infer an improvementin contribution from the interface, i.e. the 420nm peak. Another peak of the blue region which appears inasgrown samples and presists even after annealing is the 470nm peak. We attribute this peak too to theinterface [10, 12].Increasing ZnO content in this study showed decreasing grain size, hence the 430nm peak seems to beenhanced for samples with smaller grain size. A question that would need answering would be “how candiminishing grain size contribute more to an emission process?” . To analyse this we took a sample (this samplewas used in our earlier study [13]) and heated it at 250 o C for intervals of 30, 60, 120min. The grain size wasfound to decrease with increased heating. We had proposed the formation of a “shell” around the increasinglycrystalline ZnO with Zn-Si-O shell material growing into the asgrown cluster, thus reducing the grain size. Infig 8 we plot the PL 430nm peak with increasing shell volume. Shell volume V shell is V shell ∝ R o − R n where, R o is the grain size of the as grown sample and R n is that of annealed samples. We find a linearco-relation, suggesting that the 430nm PL emission peak intensity is due to the “shell” volume or in turnamount of Zn-Si-O linkages present.The PL spectra was recorded without use of optical filters. This results in the presence of a huge peak at540nm which is called the second harmonic peak. As compared to the unheated samples PL spectra, the neckof this harmonics is quite broad (example fig 6). This broadening maybe due to existence of the 540nm greenpeak associated with oxygen vacancy in ZnO. While this peak was resolvable in as grown sample’s PL sectrafrom the second harmonic peak, we were not able to do the same in this study.The other two peaks lying at 570nm and 640nm with marked contributions have been observed for thefirst time in our samples. While 570nm peak could be ascribed to emission between energy levels caused byZn interstial defects [21] it does not explain the existence of the 640nm peak. Contribution of emission fromnano-Si can be ruled out as per our XRD and Raman analysis. We believe vacuum annealing has transformedsome of the ZnO and Si linkages present in the shell into Zn SiO . Zn SiO is known to emit strongly in red,orange and UV regions [22, 23]. In fact one can notice the UV emission at 310nm is strong whenever an intense570 and 640nm emission peak exists. We noticed that the sample (c1v), which has significant red and orangeemissions also show a huge 310nm peak. Discussion
Based on our previous studies and results from the present work, we now are in a position to explain theprocesses taking place in ZnO:Si nanocomposite films. While the Silicon matrix prevents ZnO grains fromagglomerating the unsaturated bonds of ZnO and Si at the interface form Zn-Si-O linkages. The interface thuscontributes to emissions at 420nm and 470nm. Due to the thermal evaporation technique used for fabriaction,ZnO clusters have defects. While the ordered bondings give rise to emissions at 350nm, those associated withdefects give emissions at 540nm. based on relative proportion of the two, the contribution of emission varies.We have shown comparable proportions give comparable emissions and has a broadening of PL in the blue-green region. Annealing results in migration of defects from the core of ZnO to the interface. This resultsin increased Zn-Si-O bondings that go on forming a shell which expands inwards decreasing ZnO grain size.Appropriate temperature of annealing leads to the formation of Zn SiO within the shell that leads to newemissions in UV and red regions. As stated the present work along with our initial results give a good insight of5he material process taking place and gives scope of understanding how to engineer samples for broad emissionwhite light LEDs based on ZnO:Si nanocomposites. Conclusion
As deposited nanocomposites of ZnO:Si of various compositional ratios were annealed under high vacuum.Structural and compositional studies carried out by X-Ray Diffraction and XPS were not able to detect ap-preciable variation. However, PL spectra have shown a significant improvement. Not only have the intensitiesincreased but appearance of three new peaks at 310, 570 and 620nm can be easily noticed. These peaks ap-peared along with earlier reported peaks in the blue-green region. Annealing has not only pushed the interfaceinto the ZnO grain but also transformed Zn-Si-O linkages present at interface into Zn SiO . We attributethe new peaks to Zn SiO existing in the shell surrounding the ZnO grains. This study further substantiatesour earlier claims correlating broadening in blue-green region to existence of appropiate ratio of two phase,namely crystalline and defect related ZnO. Sample (a1v) shows improved broadening as compared to its asgrown counterparts, due to appropriate mix of the two phases achieved after annealing in vacuum. Presenceof all the wavelengths completes our white-light spectrum. We thus successfully show that suitable ZnO:Sicompositional ratio with appropriate post deposition treatment is crucial for obtaining white light. This studyalong with more careful investigations should pave the way for future white light emitting devices Acknowledgment
We are thankful to Dr.D.K.Pandaya at Indian Institute of Technology, Delhi for GIXD measurements. Theresources utilized at University Information Resource Center, Guru Gobind Singh Indraprasta University isgratefully acknowledged. We also would like to express our gratitude to Dr. Kamla Sanan and Dr. MaheshSharma (both at National Physical Lab., Delhi) for carrying out the photoluminescence and XPS studiesrespectively. Author CRK is thankful to University Grants Commission (India) for financial assistance interms of research award, F.No-33-27/2007(SR). One of the authors PA is grateful to Department of Scienceand Technology (India) for funding present work with research project SR/NM/NS-28/2010. .6 igure Captions
1. GIXD scans of (A) as grown and (B) vacuum annealed samples.2. XPS depth profile scans of (A) 2p / peaks of Zinc (B) Silicon and (C) Oxygen of sample (c1v).3. (A) Variation of Peak position of Zn 2p / with depth (B) Fraction of Zinc in bonding to amount ofSilicon present along the thickness and (C) Peak position of Zinc in bonding with Oxygen to its fractionof presence.4. Raman spectra of (A) as grown samples (a1), (b1), (c1), (d1), (e1) and (B) vacuum annealed samples(a1v), (b1v), (c1v), (d1v) and (e1v). Also seen are the deconvoluted peaks assigned to amorphous silicon(310cm − ), wutzite structure of ZnO (438cm − ) and with defect related peak of ZnO (570cm − ).5. (A) Relative change in Intensity of 438 cm − peak in vacuum annealed samples to that in as grownsamples with respect to zno content, (B) Variation of peak position in the 438 cm − peak in vacuumannealed samples with respect to that in the as grown samples and (C) Relative change in Intensity of560 cm − peak in vacuum annealed samples to that in as grown samples with respect to ZnO content.6. Relative presence of defedt related ZnO to wurtzite ZnO (Area 438 cm − /Area 565 cm − from RamanSpectra) for varying ZnO content in film for (A) vacuum annealed (B)as deposited films.7. PL spectra of samples (a1) and (a1v). (Counts of (a1) have been scaled by 3 (i.e.X3) to compare thespectra.)8. PL of samples (a1v), (b1v), (c1v) and (d1v).9. Plot of shell volume with respect to intensity of 430nm observed in photoluminescence (method describedin the text). 7 eferences [1] C. Jagadish and S. J. Pearton, “Zinc Oxide Bulk, Thin Films and Nanostructures, Elsevier Ltd.” , 2006.[2] U. Ozgur, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho and M.Morkoc, J. Appl. Phys., (2005) 041301 (and the references therein).[3] A.K.Das, P.Misra and L.M.Kukreja, J.Phys. D:Appl. Phys., (2009) 165405.[4] B.Yang, A.Kumar, H.Zhang, P.Feng, R.S.Katiyar and Z.Wang, J.Phys. D:Appl. Phys., (2009) 045415.[5] W.J.Shen, J.Wang, Q.Y. Wang, Y.Duan and Y.P. Zheng, J.Phys. D:Appl. Phys., (2006) 269.[6] P. Klason, P. Steegstra, O. Nur, Q-H. Hu, M. M. Rahman, M.Willander and R. Turan, Proceedings: “ENS2007, Paris: France (2007)”.[7] ZHAO Bo, LI-Q-S, Qi H-X and ZHANG N, Chin Phys.Lett., (2006) 1299.[8] R. G. Singh, Fouran Singh, V. Aggarwal and R. M. Mehra, J.Phys. D: Appl. Phys., (2007) 3090.[9] U. Pal, J. Garcia Serrano, N. Koshizaki and T. Sasaki, Mater. Sci. Eng. B, (2004) 24.[10] Yu-Yun Peng, Tsung-Eong Hseih and Chia-Hung Hsu, Nanotechnology, (2006) 174.[11] S.Siddiqui, C.R.Kant, P.Arun and N.C.Mehra. Phys. Lett. A (2008) 7068.[12] Shabnam, C.R.Kant and P.Arun. Size and Defect related Broadening of Photoluminescence Spectra inZnO:Si Nanocomposite Films. communicated available at arXiv:1007.2142.[13] Shabnam, C.R.Kant and P.Arun. Mater. Res. Bull. (2010) 13068.[14] Z.D.Sha, Y.Yan, W.X.Qin, X.M.Wu and L.J.Zhuge, J.Phys. D:Appl.Phys. (2006) 3240.[15] H.S.kan, J.S. Kang, S.Sik Pang, E.S.Shim and S.Y.Lee, Mater. Sci. and Engg. B (2003) 313.[16] “ Elements of X-Ray Diffraction”, B.D.Cullity (London,1959)[17] Ramon Cusco, Esther Alarcon-Llado, Jordi Ibanez, Luis Artus, Juan Jimenez, Buguo Wang, and MichaelJ. Callahan, Phys. Rev. B (2007) 165202.[18] K.A.Alim, V.A.Fonoberov and A.A.Balandin, J.Appl.Phys., (2005) 124313.[19] H.K.Yadav, V.Gupta, K.Sreenivas, S.P.Singh, B.Sundrakannan and R.S.Katiyar, Phys. Rev. Lett., (2006) 085502.[20] C.X.Xu, X.W.Sun, B.J.Chen, P.Shum, S.Li and X.Hu, J.Appl.Phys., (2004) 661.[21] N.Bano, I.Hussain, O.Nur, M.Willander, P.Klason and A.Henry, Semicond.Sci.Technol., (2004) 125015.[22] Q.Zhuang, X.Feng, Z.Yang, J.Kang and X.Yuan, Appl. Phys. Lett., (2008) 091902.[23] X.Feng, X.Yuan, T.Sekiguchi, W.Lin and J.Kang, J.Phys.Chem. B, (2005) 15786.
34 38 42 46 I n t en s i t y ( A r b . U n i t s ) θ (b1) (c1) (d1) (e1) 34 38 42 46 θ (b1v)(c1v)(d1v)(e1v) Figure 1:
GIXD scans of (A) as grown and (B) vacuum annealed samples. I n t en s i t y ( A . U . ) Binding Energy (eV) (A)
100 102 104 106 I n t en s i t y ( A . U . ) Binding Energy (eV) (B)
529 531 533 535 I n t en s i t y ( A . U . ) Binding Energy (eV) (C)
Figure 2:
XPS depth profile scans of (A) / peaks of Zinc (B) Silicon and (C) Oxygen of sample (c1v).
0 200 400 600
XPS Z n - / B i nd i ng E ne r g y ( e V ) Layer depth (in Angstrom) (A) Z n O C on t en t i n f il m Layer depth (in Angstrom) (B)
XPS Z n - / B i nd i ng E ne r g y ( e V ) ZnO Content (C)
Figure 3: (A) Variation of Peak position of Zn / with depth (B) Fraction of Zinc in bonding to amountof Silicon present along the thickness and (C) Peak position of Zinc in bonding with Oxygen to its fraction ofpresence.
250 350 450 550 650 (a1) I n t en s i t y ( A r b . U n i t s )
250 350 450 550 650 (b1)
250 350 450 550 650 (c1)
250 350 450 550 650 (d1)
250 350 450 550 650 (e1)
250 350 450 550 650 I n t en s i t y ( A . U . ) Wavenumber (cm -1 ) (a1v)
250 350 450 550
Wavenumber (cm -1 ) (b1v)
250 350 450 550 650
Wavenumber (cm -1 ) (c1v)
250 350 450 550 650
Wavenumber (cm -1 ) (d1v)
250 350 450 550 650
Wavenumber (cm -1 ) (e1v) Figure 4:
Raman spectra of (A) as grown samples (a1), (b1), (c1), (d1), (e1) and (B) vacuum annealedsamples (a1v), (b1v), (c1v), (d1v) and (e1v). Also seen are the deconvoluted peaks assigned to amorphoussilicon ( − ), wutzite structure of ZnO ( − ) and with defect related peak of ZnO ( − ). A r ea ( V a c uu m A nnea l ed / A s G r o w n ) ZnO Content (A)
420 440 460 480 400 420 440 460 480 P ea k P o s . i n A nnea l ed S a m p l e s Peak Pos. in As Grown Samples (B) R e l a t i v e I n t en s i t y ( VA / A G ) ZnO Content (C)
Figure 5: (A) Relative change in Intensity of 438 cm − peak in vacuum annealed samples to that in as grownsamples with respect to zno content, (B) Variation of peak position in the 438 cm − peak in vacuum annealedsamples with respect to that in the as grown samples and (C) Relative change in Intensity of 560 cm − peakin vacuum annealed samples to that in as grown samples with respect to ZnO content.
10 30 50 0.2 0.3 0.4 0.5 0.6 A r ea c m - / A r ea c m - Starting Zn/Si(a1)(a1v) (e1) & (e1v) out of scale
Figure 6:
Relative presence of defedt related ZnO to wurtzite ZnO (Area 438 cm − /Area 565 cm − from RamanSpectra) for varying ZnO content in film for (A) vacuum annealed (B)as deposited films.
300 400 500 600 (a1v) (a1)
700 800 I n t en s i t y ( A . U . ) Wavelength (nm)
Figure 7:
PL spectra of samples (a1) and (a1v). (Counts of (a1) have been scaled by 3 (i.e.X3) to compare thespectra.)
300 500 700 I n t en s i t y ( A . U . ) (d1v)
300 500 700 I n t en s i t y ( A . U . ) (c1v)
300 500 700 I n t en s i t y ( A . U . ) (b1v)
300 500 700 I n t en s i t y ( A . U . ) Wavelength (nm) (a1v)
Figure 8: