Thickness dependence of refractive index and optical gap of PMMA layers prepared under electrical field

Transcription

1 J Mater Sci: Mater Electron (2008) 19: DOI /s z Thickness dependence of refractive index and ical gap of layers prepared under electrical field V. Švorčík Æ O. Lyutakov Æ I. Huttel Received: 26 April 2007 / Accepted: 29 May 2007 / Published online: 13 June 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Optical parameters (refractive index, dispersion energy, ical gap) of polymethylmethacrylate () layers prepared by spin coating and modified by electric field have been studied. Refractive index was measured using a refractometer, internal structure was investigated as a structural parameter (E d ) within the One Oscillator Model. Optical gap width (E g ) was assessed using Tauc Approximation from UV-Vis spectra. Surface morphology and roughness was investigated using an AFM. The electric field imposed during preparation of layers increases their refractive index. The highest increase in n (Dn = 0.042) was found for the thinnest layer (70 nm). layers have produced higher E g than non-oriented ones for all studied values of thickness. The electrical field applied at preparation of the oriented layer will not change its surface morphology and roughness. 1 Introduction V. Švorčík (&) O. Lyutakov I. Huttel Department of Solid State Engineering, Institute of Chemical Technology, Prague , Czech Republic vaclav.svorcik@vscht.cz Solid state physics and material science study dependence between structure and properties of the materials [1]. Opposite to crystalline solids, in the case of amorphous materials, the theory of the condensed matter in disordered state is not unique [2]. Various authors have published in this field their data, frequently with different conclusions [3 8]. This may be caused by various technologies to prepare samples [9], different methods to measure properties of the material [10] and by different approximation of results [11]. Therefore certain properties and structure of amorphous materials are not understood at the required level yet. Optical parameters (e.g. refractive index, n) of polymethylmethacrylate () depend on its molecular structure and they can be modified e.g., by external electric field [12 14]. The field causes that polar groups orientate [14], which is caused by b-relaxation, i.e., by turning of polar groups in the polymer [15]. These changes are reflected in increased refraction index in oriented samples [12 14]. The dispersion dependence of refraction index on wavelength, or energy (hm) can be approximated within the Single Oscillator Model proposed by Wemple and DiDomeniko [16] and some oscillator parameters can be evaluated nðmþ 2 1 ¼ E de 0 E ; ð1þ 0 2 ðhmþ2 where E 0 is the single oscillator energy and E d is the dispersion energy. Value of E d that can be obtained by extrapolating the linear part on plot (n 2-1) 1 vs. hv is a measure of internal ordering of the substance. Higher values of E d indicate higher ordering of the substance [17]. The Wemple and DiDomeniko model [16] can be used in the range of wavelengths, where the light absorption is relatively low. In the range of wavelengths where a sharp increase of absorption appears in the substance, the Tauc relation for dependence of absorbance on wavelength or light photon energy applies [18] aðmþ ¼ Aðhm E g hm Þx ; ð2þ

2 364 J Mater Sci: Mater Electron (2008) 19: where a is the absorption coefficient of the substance, E g is the substance ical gap, x is the parameter that gives the type of electron transition and factor A depends on the transition probability and can be assumed to be constant within the ical frequency range [19]. The band gap width (E g ) depends on many parameters, e.g. on crystalline materials, on theirs anisotropy, temperature, pressure, on effect of external electric and magnetic forces [1]. For amorphous inorganic substances, it has been found experimentally that absorption tail broadens with increased levels of thermal or structural disorder [20, 21]. The interpretation of this result differs. Optical band gap was determined [6, 20, 21] through the analysis of Tauc [17] and shift in the ical gap towards lower energies with higher levels of disorder was reported. Opposite to it, O Leary et al. [7, 8] developed a new model, which was characterized by two principal parameters, the mean energy gap E g and the o 2 (energy gap variance) [22, 23]. O Leary [3] employed this model to conclusion that the fundamental characteristics of the material (mean energy gap) does not depend on the material arrangement level. A change in material ical gap appears when absorption of photons with adequate energy changes in the material dependent on its arrangement. This work has studied the change of ical parameters of caused by electric field during preparation of micron and submicron films from solution. We have monitored the change in refraction index, a parameter characterizing internal ordering in the layer and ical gap on thickness of prepared layers. onto SiO 2 substrate. The layers were prepared from wt.% solution of in chloroform. The films were prepared without or with the assistance of DC electrical field. The electrical field 7.5 kv cm -1 was applied during the film preparation on centrifuge. The surface topography was examined using atomic force microscopy (AFM, tapping mode), performed under ambient conditions on a Digital Instruments CP II set-up. Veeco oxide-sharpened silicon probes RTESPA-CP with the spring constant 40 N/m were chosen. Mean roughness (R a ) represents the arithmetic average of the deviations from the center plane of the sample. The film thickness was measured (standard error ±10%) by a profilometer Hommel 1,000 and Talystep. Refractive index of films was determined in the spectral range nm using refractometer Avaspec 2048 as the mean from 6 independent measurements. The dependence of refractive index (n) on wavelength (k) for films deposited on substrate was determined with the aid of software program AvaSoft Full 6.1, including code Spectra 3. The Wemple and DiDomeniko approximation was assessed with a refractometer and the structural parameter E d was calculated from it using equation (1). The absorption spectra UV-Vis were measured on pristine and modified layers in the spectral range nm using standard spectrophotometer (Varian Cary 50). Spectra were assessed from eq. (2) and the ical gap width E g of layers was assessed from the linear part of plot ((ahm) x vs. hm). Indirect transition cannot be excluded in these layers and therefore x = ½ was used in the calculation. 2 Experimental 2.1 Materials Present experiments were performed on in ical purity supplied by Goodfellow. The glass transition temperatures (T g = 112 C) of polymers determined by standard calorimetric method using DSC 2920 technique and from molecular weights of polymers (M w = , M n = ) were measured by gel permeatic chromatography GPC technique. In both cases the typical uncertainties are below ±5%. 2.2 Methods For refractometric measurements, nm thick polymer films were prepared by spin-coating method (1500 rpm) [12 14] onto a silicon (crystallographic orientation (1 0 0), resistance W cm, refractive index n = 3.505). For measurements of UV-Vis transmission spectra were prepared nm thick polymer layers 3 Results and discussion We are going to study and discuss the influence of thickness of layers prepared under electric field on change of their refractive index and on arrangement of polymer polar groups (dispersion energy). We are also going to study the possibility to interpret the ical band gap of the layers using UV-Vis spectra (Tauc model), again as dependent on layers thickness. 3.1 Refractive index and dispersion energy The dependence of difference between the refractive index (Dn) of layers, oriented/non-oriented in electric field on thickness of prepared layers is shown in Fig. 1. We can see there that electric field imposed to layers during their preparation increases their refractive index for all studied thickness. Then for layers thick approximately 1 lm or more, Dn remains constant. Fig. 1 shows dramatic increase of Dn for layers with thickness below 1 lm.

3 J Mater Sci: Mater Electron (2008) 19: ,04 2,0 This phenomenon is more pronounced in thin layers, which can be attributed e.g., to lower viscosity of the solution during coating, lower interaction among chains and therefore their easier orientation in the field, longer time for solvent evaporation. 0,03 Refractive index Dispersion energy 1,5 3.2 Surface morphology n 0,02 0,01 1,0 0,5 E d A surface morphology of 100 nm thick oriented and nonoriented layers on Si substrate investigated by atomic force microscopy is shown in Fig. 2. From Fig. 2 it is not obvious significant difference in surface morphology and roughness between non- and oriented samples. Both layers are very flat, they have very low R a. The electrical field applied at preparation of the oriented layer will not change its surface morphology and roughness. 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 Thickness (µm) Fig. 1 Dependence of difference in refractive index (Dn) and dispersion energy (DE d ) of layers, oriented and non-oriented in electric field, on thickness of prepared layers The highest increase n = was measured for the thinnest layer (70 nm). Thinner layers cannot be prepared in this way because of their bad homogeneity and reproducibility. Fig. 1 shows also the dependence of difference dispersion energy (DE d ) of layers oriented in electric field, on thickness of prepared layers. The results of Fig. 1 can be interpreted in the way that E d is for all oriented layers higher that for non-oriented ones. We can see also that DE d increases with decreasing layer thickness. Significant increase of dispersion energy appears for thickness below 1 lm, and dramatic increase for layers below 250 nm. It has been reasoned that higher E d values correspond to higher arrangement in the substance [17]. We can assume that orientation of a polar group in the polymer chain appears because of effect of electric field during preparation of the layers and this way transfers in a more ordered state (the refractive index also increases), compared with a layer prepared without a field. 0,0 3.3 Optical band gap UV-Vis spectra of oriented and non-oriented layers have been also assessed with the goal to assess E g according to Tauc s model. The UV-Vis spectrum of an oriented and non- oriented layer 70 nm thick is presented in Fig. 3 for the sake of transparency. It is evident that a sharp increase of light absorption occurs below 220 nm, which corresponds to p fi p * transitions of carbonyl groups in macromolecules [24]. shows lesser width of the absorption tail and higher absorbance in this area. The decrease of absorption tail width seems to be a consequence of a higher level of ordering in oriented chains. The light absorption is in proportion to the imaginary component of the refractive index in this material, the relation between the real and imaginary parts of the refractive index is given by the relation Kramers Krourevska [25]. This relation implies that increased light absorption (increased imaginary part of the refractive index) in in area below 220 nm causes increased refractive index in the range k > 220 nm, which correlates with published results [12, 13]. UV-Vis spectra of oriented and non-oriented layers have been analysed through the Tauc model (see eq. (2)) with the goal to acquire the ical band gap magnitude (E g ). Just Tauc plots assessed based on UV-Vis Fig. 2 AFM images of 100 nm non- and oriented layer prepared by spin-coating on Si substrate. R a is the surface roughness in nm

4 366 J Mater Sci: Mater Electron (2008) 19: ,55 0,50 0, ,40 0,35 8 Absorbance 0,30 0,25 (αhv) 2 6 0,20 4 0,15 0,10 2 0, Wavelength (nm) Fig. 3 UV-Vis spectrum of oriented and non-oriented layers, 70 nm thick 0 4,8 5,2 5,6 6,0 6,4 6,8 Energy (ev) Fig. 4 Tauc plots assessed based on UV-Vis spectra from plot (ahm) 2 vs. hm for oriented and non-oriented layer, 70 nm thick spectra from the plot (ahm) 2 vs. hm for oriented and nonoriented layers 70nm thick are presented in Fig. 4 for the sake of transparency. The analysis shows that a nonoriented layer has E g = 5.75 ev, an oriented layer E g = 6.07 ev. The dependence of E g for oriented and non-oriented films on layer thickness is shown in Fig. 5. Because of high demands on preparation of samples, this dependence has been measured 5-times just in samples 15 lm, 500 and 70 nm thick, which monitor the studied thickness interval. We can infer from Fig. 5 that the ical gap remains constant for thickness in range ,000 nm for oriented and non-oriented samples. Dramatic increase of E g for oriented and non-oriented layers appears with layers thickness decreasing below 500 nm. We can see also that oriented layers have shown higher values of E g than non-oriented ones for all studied values of thickness. Below 500 nm, as we can see in Fig. 5, the difference between E g for the oriented and non-oriented layers increases with decreasing thickness. A similar change of E g in polymers was published under doping with metal halogenides [26], after ions implantation in polyimide [27], and after implanting electrons or protons in PP, PTFE, PET, PI [28]. Correlation with Fig. 1 shows that a higher level of order in the material causes higher values of the ical gap. Optical band gap (ev) 6,1 6,0 5,9 5,8 5,7 5,6 5,5 5,4 5,3 6,0 5,8 5,6 5,4 0,0 0,2 0,4 0,6 0,8 1,0 1, Thickness (µm) Fig. 5 Dependence of ical band gap (E g ) for oriented and nonoriented films on thickness of layers. The insert shows this plot in detail for the low thickness

5 J Mater Sci: Mater Electron (2008) 19: Therefore this phenomenon will be more pronounced in polymer layers with small thickness which turn (orientate) more easily and this way get in a more arranged state. 4 Conclusion The results can be summarized as follows: electric field imposed to layers during their preparation increases their refractivity index. The magnitude of Dn of the layers keeps constant for layers thick 1 lm and more, dramatic increase in Dn is evident for layers below 1 lm. The highest increase (Dn = 0.042) was found in the thinnest layers (70 nm), dispersion energy E d increases with decreasing layer thickness, significantly in layers less than 1 lm thick, and dramatic increase appears for layers under 250 nm, no significant differences in surface morphology and roughness between non- and oriented layers were observed, sharp increase of light absorption occurs in (both oriented and non-oriented) under 220 nm which corresponds to p fi p * transitions of COOCH 3 structures, UV-Vis spectra have been assessed through the Tauc model. From the plot (ahm) 2 vs. hm for oriented and nonoriented layers 70 nm thick, we can infer that the non-oriented layer gives E g = 5.75 ev, and the oriented layer E g = 6.07 ev, the value of E g keeps constant for thickness range ,000 nm for both oriented and non-oriented samples. Dramatic increase of E g occurs when the layer thickness drops below 500 nm. The oriented layers presented higher values of E g than non-oriented samples for all studied values of thickness. The difference in E g between oriented and non-oriented layers increases with decreasing thickness under 500 nm, a higher level of order in the material causes higher values of ical gap. Therefore this phenomenon will be more pronounced in thinner polymer layers which orientate more easily and this way get in a more ordered state. Acknowledgements This work was supported by the Czech Grant Agency within the project No , by GA ASCR within the project KAN and by the Czech Ministry of Education within Research Programs No. MSM and LC References 1. J.M. Ziman, in Principles of the Theory of Solids (Cambridge University Press, Cambridge, 1979) 2. G.D. Cody, J. Non-Cryst. Sol. 141, 3 (1992) 3. S.K. O Leary, Appl. Phys. Lett. 72, 1332 (1998) 4. G.D. Cody, T. Tiedje, B. Abeles, B. Brooks, Y. Goldstein, Phys. Rev. Lett. 47, 1480 (1981) 5. P.D. Persans, A.F. Ruppert, S.S. Chan, G.D. Cody, Sol. State Commun. 51, 203 (1984) 6. P.D. Persans, A.F. Ruppert, G.D. Cody, B.G. Brooks, Sol. State Commun. 54, 461 (1985) 7. S.K. O Leary, S. Zukotynski, J.M. Perz, Phys. Rev. B. 51, 4143 (1995) 8. S.K. O Leary, S. Zukotynski, J.M. Perz, Phys. Rev. B. 52, 7795 (1995) 9. M.T.K. Soh, N. Savvides, P.J. Martin, C.A. Musca, Thin Solid Films 515, 2284 (2006) 10. A. Soldera, E. Monterrat, Polymer 43, 6027 (2002) 11. M. Campoy-Quiles, P.G. Etchegoin, D.D.C. Bradley, Synt. Metals 155, 279 (2005) 12. V. Švorčík, M. Prajer, I. Huttel, V. Hnatowicz, J. Plešek, Mater. Lett. 59, 280 (2005) 13. V. Švorcík, I. Huttel, P. Paláček, Mater. Lett. 61, 953 (2007) 14. O. Lyutakov, I. Huttel, V. Švorčík, J. Mater. Sci. Mater. Electron. 18, 457 (2007) 15. L.C.E. Struik, in Physical Aging in Amorphous Polymer and Other Materials (Elseiver, New York, 1978) 16. S.H. Wemple, M. DiDomenico, Phys. Rev. Lett. 23, 1156 (1969) 17. S.H. Wemple, M. DiDomenico, Phys. Rev. B. 3, 1338 (1971) 18. J. Tauc, R. Grigorovici, A. Vanku, Phys. Stat. Sol. 15, 627 (1966) 19. J. Tauc, in Amorphous and Liquid Semiconductors (Springer, Heidelborg, 1974) 20. R.A. Street, T.M. Searle, I.G. Austin, R.S. Sussmann, J. Phys. C. 7, 1582 (1974) 21. T. Tiedje, J.M. Cebulka, Phys. Rev. B. 28, 7075 (1983) 22. M. Pelton, S.K. O Leary, F. Gaspari, S. Zukotynski, J. Appl. Phys. 83, 1029 (1998) 23. H. Fritzsche, Appl. Phys. Lett. 65, 2824 (1994) 24. M. Hammama, M.K. El-Mansyb, S.M. El-Bashirb M.G. El-Shaarawyb, Desalination 209, 244 (2007) 25. E. Bahaa, A. Saleh, M.C. Teih, in Fundamentals of Photonics (John Willey& Sons, Inc., New York, 1991) 26. H.M. Zidan, M. Abu-Elnader, Physica B. 355, 308 (2005) 27. V. Švorčík, I. Miček, V. Rybka, V. Hnatowicz, J. Mater. Res. 12, 1661 (1997) 28. R. Mishra, S.P. Tripathy, D. Sinha, K.K. Dwivedi, S. Ghosh, D.T. Khathing, M. Muller, D. Fink, W.H. Chung, Nucl. Instrum. Meth. B. 168, 59 (2000)