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Photoluminescence and optical transitions in C60 fullerene thin films deposited on glass, silicon and porous silicon
Thin Solid Films 690 (2019) 137566
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Photoluminescence and optical transitions in C60 fullerene thin films deposited on glass, silicon and porous silicon
T
⁎
A.I. Bayramov , N.T. Mamedov, T.D. Dzhafarov, Y.N. Aliyeva, Kh.N. Ahmadova, E.H. Alizade, S.Q. Asadullayeva, M.S. Sadigov, Sh.Kh. Ragimov Institute of Physics, Azerbaijan National Academy of Sciences, 131 Javid ave., Az1143 Baku, Azerbaijan
A R T I C LE I N FO
A B S T R A C T
Keywords: Fullerene Photoluminescence Spectroscopic ellipsometry Porous silicon Intra-molecular vibration Band gap
The C60/glass, C60/Si, C60/porous Si/Si as well as porous Si/Si thin film structures for comparison were prepared and studied by photoluminescence spectroscopy at room temperature. Along with a broad-band low-energy emission at 1.774 eV in the photon energy range of the nominaly forbidden optical transitions, the C60 films on glass substrate exhibit high-energy emissions at 2.115 eV and 2. 342 eV in the range of allowed dipole transititions. On the other hand, C60 films on Si or porous Si/Si display only the 1.774 eV emission band. The intensity of this emission in C60/porous Si/Si is 200 times higher than that in C60/Si and at least 20 times exceeds the one for the emission observed on porous Si/Si. Such a dramatic increase in emission intensity is indicative of the relaxation of the selection rules for nominally forbidden optical transitions and suggests strong interaction of C60 molecules with porous silicon walls. Molecular signatures of the C60 films are clearly manifested by the vibronic transitions with 60 meV energy separation (Ag-mode energy) between the irradiative levels, observed on 1.774 eV emission band in C60/porous Si/Si. The origin of the 2.115 and 2.342 eV emission bands is analyzed within a standard band insulator approach, together with the obtained ellipsometric data. The first corresponds to the band-to-band transitions at band gap. The appearance of the second can be related to the splitting of the valence band in the X point of the Brillouin zone of C60 film due to the crystal field. The strength of the crystal field splitting is then ~ 200 meV.
1. Introduction C60 thin films deposited on different substrates have been prepared and extensively studied for a long time since the discovery [[1]] of C60 buckyball molecules. Today, room temperature solid phase of C60 is known as phase centered cubic (fcc) pristine C60 [[2]] or, shortly, fullerite C60. The available vast experimental data on optical and electronic properties of fullerite C60 witness that along with emerged band properties, solid state manifestation of C60 retain clear-cut molecular fingerprints, such as vibronic transitions and Frenkel excitons [[3–5]]. Transformation of theoretical views on electronic spectrum of fcc C60 solids was going in parallel with accumulated experimental data on electronic structure and optical properties of C60 deposited on various substrates. Early views, together with then-available experimental data, were comprehensively described in a book by Dresselhaus et al. [[6]]. Nowadays, band structure calculations that predict a value of 2.15 eV [[7]] for the direct band gap (HOMO–LUMO gap in molecular terminology) in fullerite C60 is referred to as most accurate, because of ⁎
the quasiparticle approach employed and a fairly good agreement with PES (photoemission spectroscopy), inverse PES and angle resolved IPES data. Recently, however, somewhat smaller value (2.12 eV) of the band gap has been found from the first principles calculations using Tran and Blaha regular and non-regular modified Becke-Johnson (TB-mBJ) potential [[8]]. Interpretation of the available numerous experimental data on optical transitions at, below and above band gap remain rather controversial up to now. According to optical absorption and luminescence measurements [[9–14]], the distance between the occupied (HOMO) and unocuppied (LUMO) states is between 1.5 and 2.7 eV. Such a big uncertainty is largely associated with the overlapping of the spectral features of the molecular and purely band transitions at small values of absorption coefficient. Therefore, photoluminescence (PL) spectroscopy that is also used in the present work is reasonable to apply to C60 solids in pair with ellipsometric spectroscopy to single-out interband optical transitions. The last spectroscopy is rather insensitive to small absorption (forbidden or weakly allowed transitions inherent in a region below the band gap of C60 solids) but is very accurate in determination
Corresponding author. E-mail address: [email protected] (A.I. Bayramov).
https://doi.org/10.1016/j.tsf.2019.137566 Received 31 May 2018; Received in revised form 11 September 2019; Accepted 11 September 2019 Available online 12 September 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
Thin Solid Films 690 (2019) 137566
A.I. Bayramov, et al.
of the extinction coefficient in a region above the band gap with absorption level above 103 cm−1. Photoluminescence studies of C60 solids can remove the abovementioned uncertainty provided the balance between the radiative transitions of molecular nature and those of interband nature is either preserved or tipped towards interband transitions. The latter take place exclusively between band states. In the present work we show that C60 thin films on soda lime glass (SLG) substrate just meet this condition. In addition, C60/Si and C60/ porous Si/Si (C60/PS/Si) multilayer structures for which the molecular radiative transitions strongly dominate over interband radiative transitions are also studied for the sake of completeness.
2. Experimental details High purity (99.99%) fullerene powder (C60) was deposited on soda lime glass substrate (C60/SLG) by sublimation technique in a vacuum of 1.33⊓10−4 Pa. For comparison, the same fullerene powder was also deposited directly on silicon (C60/Si) and porous silicon (C60/PS/Si) preliminarily fabricated on silicon substrate by anodic etching. The thicknesses of C60 films were measured during evaporation by using a deposition controller (Inficon, Leybold) and were in the range of 100–200 nm. PS layers with thickness of 10–20 μm were prepared on p-type Si substrate (with resistivity ρ = 10 Ω.cm) by anodic etching in HF: H2O solution under the white illumination. The average porosity, i.e. the void fraction in the porous layer was measured by gravimetric technique, using the eq. P = {(m1 − m2)/(m1 − m3)} 100% [17]. Here m1 is Si sample mass before the etching, m2 after etching and m3 after the removal of the porous layer by rapid dissolution of the completely porous layer in a 3% KOH solution. The porous silicon thickness (d) was determined using the equation d = (m1 − m2)/ρS, where ρ is the Si density (2.33 g/cm3) and S is the etched surface. The average porosity for PS layers and density of pores were found to be 70–75% and 3.4 1010 cm−2, respectively. X-Ray diffraction (XRD) analyses of the films were carried out using Bruker D2 Phaser (Germany) diffractometer in θ-2θ scan mode with Nifiltered CuKα radiation (λ = 1.54060 Ǻ) source. Topography analysis of the films were performed in Smart SPM 1000 AIST NT (Tokyo Instruments, Japan). Surface, cross-section and elemental analyses of the multi-layer structures were carried out using Scanning Electron Microscopy SEM S-4800 with Energy Dispersive Spectroscopy (EDS) system (Hitachi Ltd., Japan) of 1 nm resolution. Operating voltage of 10 kV and probing spot of 8–10 μm size were used. Photoluminescence (PL) measurements were performed using PL/ PLE/Raman spectrometer (Tokyo Instruments, Inc.). The emission of the samples were excited by 325 nm wavelength laser beams. Raman spectra were measured by Confocal PL/Raman microscope Nanofinder 30-NM01 (Tokyo Instruments, Inc.). The ellipsometric measurements in 220–1700 nm spectral range were performed using Woollam M2000 (USA) rotating compensator instrument. Incident light angles were varied between 55 and 75° with 5° step. WVASE32 computer program was used for the ellipsometric data fitting procedure. Experimental data were fitted (employing the Levenberg–Marquardt algorithm) to optical model using parameterized model dielectric functions simultaneously for all the data points measured in UV/VIS ranges. Gen-Osc model available in CompleteEase database was used to model C60/SLG structure dielectric function for the entire range of wavelengths at 55°, 60° and 65° incident light angles. The. structure of Gen-Osc was composed of three Gaussian and one Psemi-M0 oscillators. The free substrate was measured and fitted separately to obtain its optical parameters for inclusion into the. model. This approach allows indirectly relate the experimental data to electronic and structural properties of the samples.
Fig. 1. Raman shift of C60 film on SLG substrate.
All of the measurements were performed at room temperature.
3. Results and discussions 3.1. Samples structure, surface morphology and cross-section profile XRD patterns of the films did not reveal any noticeable reflexes. A weak reflex around 2θ~11° (most intensive line of C60 with high degree of crystallinity [[15]]) appears after relatively long time exposure of the samples to X-rays. This is caused, in the first place, by low thicknesses of the obtained films. Of course, incomplete crystallization of the films obtained at room temperature contributes as well. However, the results of the detailed Raman studies on the films show that the last factor is not decisive. The Raman spectrum of a C60 film excited with 532 nm laser is shown in Fig. 1. Close inspection of the obtained spectrum shows that the last reproduces practically all Raman active modes observed so far on the perfect examples of C60 solids [[6],[16]]. These modes lie above 260 cm−1 and are intra-molecular in nature. The mode with frequency 33 cm−1 is external (inter-molecular) mode and corresponds to librational motions [[6]]. It should be noted, that C60 -pristine samples might experience photo-transformation under visible or ultraviolet light irradiation during Raman or photoluminescence measurements. According to [[7]], the photransformation introduces a new mode at 116 cm-1, identified with an intermolecular vibrational mode. In general, samples containing large concentration of dimers or linear chains exhibit very reach structure with many peaks in the range 250–500 cm-1 [[8]]. In our case such peaks (if any) must have been so weak that they are hardly observable in Fig. 1. In other words, even if some polymerization of the studied samples did take place, its influence on Raman spectra was very weak. AFM image (not shown) of the fullerene C60 film deposited on SLG substrate at room temperature, obtained in non-contact mode shows the dense and regular character of the clusters. In Fig. 2, 2D surface (upper, left hand) and cross-section (upper, right-hand) Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) (lower, left-hand) images of C60/PS/Si multilayer structure are displayed. C60 and PS thicknesses of the structure were found as 109 nm and 12 μm, respectively. Lower right-hand picture shows carbon, silicon and oxygen distributions in the porous part of the structure. As it is clearly seen, the C60 molecules penetrate deep into PS closely to single part of the silicon. Moreover, the EDS analysis shows the presence of oxygen in PS along with the C60 molecules.
2
Thin Solid Films 690 (2019) 137566
A.I. Bayramov, et al.
Fig. 2. Surface and cross-section SEM images of C60/PS/Si structure and corresponding EDS spectra and Si, C, O distribution across depth.
3.2. Photoluminescence and optical constants of C60 films on SLG substrate
spectrum spreads over the photon energy range from 1.5 to 2.7 eV nm and shows clearly resolvable sub-bands. Three overlapping bands apparently appear in the spectrum. The spectrum can be deconvoluted into three Gaussians (Fig. 3, broken curves) with the following parameters: first peak at 1.774 eV with half width 294 meV; second peak at 2.115 eV with half width 258 meV; third peak at 2.342 eV with half width 234 meV. Fig. 4. (broken curves) shows above Gaussians together with photon energy dependence of the optical constants, refractive index and extinction coefficient (Fig. 4, full curves), obtained ellipsometrically. It is easy to realize from Fig. 5 that the emissions represented by curves 2 and 3 are in the region of the allowed dipole transitions. The emission represented by curve 1 corresponds to vanishing extinction and can be associated only with the forbidden dipole transitions provided that this emission is caused by the transitions between eigen electronic states of C60 film. For the sake of convenience, the regions of the allowed and forbidden transitions are conditionally separated from each other by vertical broken line (Fig. 4). In such a case, the curve 2 is supposed to be emission due to bandto-band radiative transitions and the energy position (2.115 eV) of the maximum of this emission should correspond to the energy gap for direct optical transitions at band gap. Therefore, the band gap energy, indicated by the arrow with sign Eg in Fig. 5 is tentatively assumed 2.115 eV. Actually, asymmetric line-shape for the band-to-band transitions cannot be considered at this stage but will be discussed later. Finally, the emission described by curve 3 in Fig. 4 occurs in a highly absorptive region above the band gap and probably has to correspond to higher interband radiative transitions. Note that the difference between the energy position of the last
A typical photoluminescence spectrum of C60 film on SLG under excitation wavelength 325 nm is given in Fig. 3 (full curve). The
Fig. 3. PL of C60 film on SLG substrate under 325 nm excitation wavelength (broken curves are spectrum, deconvoluted into three Gaussians). 3
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vibronic transitions. Vibronic structures do not appear on curve 1 in Figs. 3 and 4, but are resolved for the emission of C60 film in C60/PS/Si structures. As it is seen in Fig. 5, the last emission exhibits well-developed vibronic structure with 60 meV separation between vibronic levels. This separation corresponds to the Raman peak at 493 cm −1 (Fig. 1) that is considered as Ag mode (in spectroscopic symmetry notation) [[16]]. Note that the emissions observed on C60/Si and C60/PS/Si structures and the emission described by curve 1 in Figs. 3 and 4 are identical, as verified by the comparison of their normalized spectra given in the insert of Fig. 5. Therefore, this emission really represents singlet Frenkel exciton radiative decay. The striking feature of C60 film in C60/PS/Si structures is very high PL intensity. The last is 200 times higher than that in C60/Si and up to 50 times exceeds the one for the emission observed on PS/Si (Fig. 5) (the PL spectrum of C60 film in C60/SLG structure (not shown in Fig. 5) and that of the one in C60/Si structure have comparable intensities.) Considering the sizes of C60 molecules of about 1.0 nm and pores of about 1.5 nm diameter in the sponge like structure of porous silicon, C60 may occupy the pores of PS. As it was shown from our EDS studies, (Fig. 2), the C60 molecules penetrate deep into PS and as a result, a strong interaction between C60 molecules and PS walls may be expected. Disappearance of emission bands at 550 nm and 590 nm in C60/ PS/Si structures can be explained by formation of molecular Frenkel excitons of singlet type, as well as by mono-molecular character of the C60 in mesapores of silicon [[11],[12],[14]]. The red shift of the 710 nm emission band to 730 nm is obviously caused by the vibronic correction in PS-C60 structure. On the other hand, the strong interaction between C60 molecules and PS walls may result in a change of molecular symmetry of C60. This change, in turn leads to the perturbation of the delocalized π- orbitals, which determines the electronic, as well as optical properties of the C60 molecule [[19]]. The perturbation may be sufficient to relax some of the selection rules for optical transitions within the excited-state structure of C60. As a result, the quantum yields of radiative transition are enhanced over a broad region and a significant increase of the emission intensity is observed in C60/PS/Si structures.
Fig. 4. Refractive index (n) and extinction coefficient (k) of C60 film on SLG substrate as functions of photon energy. The results of the deconvolution of the PL spectrum are shown by broken curves.
Fig. 5. PL spectra of C60/Si, PS/Si, and C60/PS/Si structures under 325 nm excitation wavelength. The insert displays the normalized PL spectra of C60/ SLG, C60/Si and C60/PS/Si structures.
3.4. Optical transitions at fundamental absorption edge of C60 films It has been proposed in Section 3.2 that emission described by curve 2 in Figs. 3 and 4 corresponds to band-to-band transitions at band gap (forbidden gap), and curve 3 in Figs. 3 and 4 to higher interband optical transitions. Using the relation α = 2πkE/hc (E-photon energy, h-Plank constant, c- speed of light) to convert the obtained extinction coefficient (k) into the absorption coefficient (α) and to plot α2E2 versus E at fundamental absorption edge we can examine whether the last plot is linear and follows the well- known law, α2E2 ~ (E-Eg) for direct optical transitions at band gap. The results of such an examination are shown in Fig. 6. Extrapolation of the linear section of the plot in Fig. 6 down to zero absorption gives the value of 2.115 eV for band gap (Eg) in C60 thin film. Definitely not by chance, the last energy exactly coincides with the energy position of the maximum of curve 2 on Figs. 3 and 4, as shown by vertical broken-dotted line in Fig. 6. Thus, photoluminescence and spectroscopic ellipsometry, both lead to the same value of the energy gap for direct transitions. The linear section of the plot in Fig. 6 corresponds to the absorption coefficient values between 103 and 104 cm−1, which is quite standard for absorption caused by direct allowed optical transitions. Makarova [[20]] has reported similar absorption values for direct optical transitions in C60 thin films. According to the insert in Fig. 6, the onset of optical absorption is around 1.9 eV or coincides with the one reported earlier from ellipsometric measurements by Kelly et al. [[21]]. It is natural to consider the range between 1.9 and 2.115 eV as the one occupied by tail- to- tail
emission and that of the emission described by curve 2 in Fig. 4 is 227 meV. The last energy exceeds by 32 meV the highest energy (195 meV [[18]]) observed for the normal modes in C60 films. In other words, above difference is too large to consider curves 2 and 3 as vibronic structures of the spectrum and, in turn, supports the proposed scenario for optical transitions at and above band gap of the obtained C60 films. The pholuminescence observed below the energy gap is very well correlating with the previous data for C60 bulk materials. The main difference is related to emissions observed in the present work for photon energies at and above energy gap, which appears only under 325 nm wavelength excitation. To the best of our knowledge, there have not been reports on photoluminescence of C60 thin films under this excitation. All of the previous PL studies were performed under the excitation wavelength above 500 nm. 3.3. Photoluminescence of C60 films in C60/Si and C60/PS/Si structures According to the numerously verified and commonly accepted view [[3],[4]], the emission band described by curve 1 in Figs. 3 and 4 can be accounted for the radiative decay of Frenkel excitons. These excitons are localized on individual C60 molecules and the corresponding transitions incorporate vibrational excitations of a single C60 molecule. The resultant transitions are therefore not purely electronic and called 4
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radiative transitions involving vibrational energy levels of singlet Frenkel exciton. The Ag-vibration mode of 60 meV is most active in the formation of vibronic structure of the emission of C60/PS/Si. A dramatic increase of the emission intensity in C60/PS/Si as compared to C60/glass and C60/Si structures is very likely to result from the further relaxation of the selection rules for almost forbidden optical transitions due to the strong interaction of C60 molecules with PS walls. References [1] E.A. Rohlfing, D.M. Cox, A.J. Kaldor, Production and characterization of supersonic carbon cluster beams, J. Chem. Phys. 81 (7) (1984) 3322–3330. [2] W.I. David, R.M. Ibberson, J.C. Matthewman, K. Prassides, T.J. Dennis, J.P. Hare, H.W. Kroto, R. Taylor, D.R. Walton, Crystal structure and bonding of ordered C60, Nature 353 (1991) 147–149. [3] J. Feldmann, R. Fischer, W. Guss, E. Gobel, S. Schmitt-Rink, W. Kratschmer, White luminescence from solid C60, Europhys. Lett. 20 (1992) 553–558. [4] R.W. Lof, M.A. van Veenendaal, B. Koopmans, H.T. Jonkman, G.A. Sawatzky, Band gap, excitons, and coulomb interaction in solid C60, Phys. Rev. Lett. 68 (1992) 3924–3927. [5] S. Saito, A. Oshiyama, Cohesive mechanism and energy band of solid C60, Phys. Rev. Lett. 66 (1991) 2637–2640. [6] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, 1995. [7] A.M. Rao, P. Zhou, K.-A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.-T. Lee, X.X. Bi, P.C. Eklund, D.S. Cornett, M.A. Duncan, I.J. Amster, Photoinduced polymerization of solid C60 films, Science 259 (1993) 955–957. [8] B. Sundquist, Polymeric fullerene phases formed under pressure, structure and bonding, Fullerene-Based Mater. 109 (2004) 85–126. [9] E.L. Shirley, S.G. Louie, Electron excitations in solid C60. Energy gap, band dispersions, and EN’ects of orientational disorder, Phys. Rev. Lett. 71 (1993) 133–136. [10] S. Jalali-Asadabadi, E. Ghasemikhan, T. Ouahrani, B. Nourozi, M. Bayat-Bayatani, S. Javanbakht, H.A. Rahnamaye Allabad, I. Ahmad, J. Nematollahi, M. YazdaniKachoeli, Electronic structure of crystalline buckyballs: fcc-C60, J. Electron. Mater. 45 (2016) 339–345. [11] Y. Wang, Photophyslcal properties of fullerenes and fullerene/N,N-Diethylanilhe charge-transfer complexes, J. Phys. Chem. 96 (1992) 764–767. [12] S.P. Sibley, S.M. Argentine, A.H. Francis, A photoluminescence study of C60 and C70, Chem. Phys. Lett. 188 (1992) 187–193. [13] H. Byrne, W. Maser, W. Rühle, A. Mittelbach, S. Roth, Nonlinear luminescence phenomena in fullerene crystallites, J. Appl. Phys. A 56 (1993) 235–239. [14] E. Shin, L. Park, M. Lee, D. Kim, Y.D. Suh, S.I. Yang, S.M. Jin, S.K. Kim, Temperature-dependent photoluminescence study of C60 and C70, Chem. Phys. Lett. 209 (1993) 427–433. [15] D. Faiman, S. Goren, E.A. Katz, M. Koltun, N. Melnik, A. Shames, S. Shtutina, Structure and optical properties of C6o thin films, Thin Solid Films 295 (1997) 283–286. [16] H. Kuzmany, R. Pfeiffer, M. Hulman, C. Kramberger, Raman spectroscopy of fullerenes and fullerene–nanotube composites, Phil. Trans. R. Soc. London A362 (2004) 2375–2406. [17] T.D. Dzhafarov, C. Oruc, S. Aydin, Humidity-voltaic characteristics of Au-porous silicon interfaces, J. Phys.D: Appl. Phys. 37 (2004) 404–409. [18] M.K. Nissen, S.M. Wilson, M.L.W. Thewalt, Highly structured singlet oxygen photoluminescence from crystalline C60, Phys. Rev. Lett. 69 (1992) 2423–2427. [19] B. Hamilton, J.S. Rimmer, M. Anderson, D. Leigh, White light emission from C60 molecules confined in molecular cage materials, Adv. Mater. 5 (1993) 583–585. [20] T.L. Makarova, Electrical and optical properties of pristine and polymerized fullerenes, Semiconductors 35 (3) (2001) 243–278. [21] M.K. Kelly, P. Etchegoin, D. Fuchs, W. Krätschmer, K. Fostiropoulos, Optical transitions of C60 films in the visible and ultraviolet from spectroscopic ellipsometry, Phys. Rev. B 46 (1992) 4963–4968. [22] M. Knupfer, J. Fink, Frenkel and charge-transfer excitons in C60, Phys. Rev. B 60 (1999) 10731. [23] F. Bechstedt, M. Fiedler, L.J. Sham, Excitonic effects in linear and nonlinear optical properties of C60, Phys. Rev. B 59 (1999) 1857.
Fig. 6. Photon energy dependence of the squared value of absorption coefficient (α) multiplied by the squared value of photon energy (E), together with curve 2 in Figs. 3 and 4. Insert: absorption coefficient as a function of photon energy, together with curve 3 in Figs. 3 and 4.
transitions and/or transitions to charge-transfer (CT) exciton states adjacent to the conduction band edge. In any case, the corresponding emission will be broadened by as much as ~200 meV and the real lineshape of the emission related to band-to-band transitions will be masked by this broadening. Note that CT-excitons result from interaction of the two charges localized on different C60 molecules and, thus, are the result of intermolecular interaction. Existence of CT-excitons in C60 films was reported in a number of works [[22],[23]]. Both quasi-particle approach [[9]] and TB -mBJ [[10]] calculations for C60 solid lead to a standard direct band insulator with energy gap above 2 eV at the X point of the Brillouin zone. The band gap value of 2.115 eV obtained in the present work is in a good agreement with the reported theoretical results [[9],[10]]. Another important result is related to the splitting of the five-fold degenerated state of the isolated C60 molecule at the same point of the Brillouin zone by the crystal field. The energy separation of 227 meV between the transitions at band gap and the higher interband transitions that correspond the maximum of the absorption coefficient in the insert of Fig. 6 is within the range of crystal field effect in C60 solid [[10]]. 4. Conclusions Using dual experimental approach based on application of both the photoluminescence spectroscopy and spectroscopic ellipsometry it has been shown that optical transitions above 2 eV in C60 thin film deposited on glass substrate are well described within the model of a standard direct band insulator with band gap energy and crystal field splitting of 2.115 eV and ~ 200 meV, respectively. It has been further shown that emission band that appears below 2 eV in C60/glass structure appears as well in C60/Si and C60/PS/Si structures and is caused by
5
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Photoluminescence and optical transitions in C60 fullerene thin films deposited on glass, silicon and porous silicon
T
⁎
A.I. Bayramov , N.T. Mamedov, T.D. Dzhafarov, Y.N. Aliyeva, Kh.N. Ahmadova, E.H. Alizade, S.Q. Asadullayeva, M.S. Sadigov, Sh.Kh. Ragimov Institute of Physics, Azerbaijan National Academy of Sciences, 131 Javid ave., Az1143 Baku, Azerbaijan
A R T I C LE I N FO
A B S T R A C T
Keywords: Fullerene Photoluminescence Spectroscopic ellipsometry Porous silicon Intra-molecular vibration Band gap
The C60/glass, C60/Si, C60/porous Si/Si as well as porous Si/Si thin film structures for comparison were prepared and studied by photoluminescence spectroscopy at room temperature. Along with a broad-band low-energy emission at 1.774 eV in the photon energy range of the nominaly forbidden optical transitions, the C60 films on glass substrate exhibit high-energy emissions at 2.115 eV and 2. 342 eV in the range of allowed dipole transititions. On the other hand, C60 films on Si or porous Si/Si display only the 1.774 eV emission band. The intensity of this emission in C60/porous Si/Si is 200 times higher than that in C60/Si and at least 20 times exceeds the one for the emission observed on porous Si/Si. Such a dramatic increase in emission intensity is indicative of the relaxation of the selection rules for nominally forbidden optical transitions and suggests strong interaction of C60 molecules with porous silicon walls. Molecular signatures of the C60 films are clearly manifested by the vibronic transitions with 60 meV energy separation (Ag-mode energy) between the irradiative levels, observed on 1.774 eV emission band in C60/porous Si/Si. The origin of the 2.115 and 2.342 eV emission bands is analyzed within a standard band insulator approach, together with the obtained ellipsometric data. The first corresponds to the band-to-band transitions at band gap. The appearance of the second can be related to the splitting of the valence band in the X point of the Brillouin zone of C60 film due to the crystal field. The strength of the crystal field splitting is then ~ 200 meV.
1. Introduction C60 thin films deposited on different substrates have been prepared and extensively studied for a long time since the discovery [[1]] of C60 buckyball molecules. Today, room temperature solid phase of C60 is known as phase centered cubic (fcc) pristine C60 [[2]] or, shortly, fullerite C60. The available vast experimental data on optical and electronic properties of fullerite C60 witness that along with emerged band properties, solid state manifestation of C60 retain clear-cut molecular fingerprints, such as vibronic transitions and Frenkel excitons [[3–5]]. Transformation of theoretical views on electronic spectrum of fcc C60 solids was going in parallel with accumulated experimental data on electronic structure and optical properties of C60 deposited on various substrates. Early views, together with then-available experimental data, were comprehensively described in a book by Dresselhaus et al. [[6]]. Nowadays, band structure calculations that predict a value of 2.15 eV [[7]] for the direct band gap (HOMO–LUMO gap in molecular terminology) in fullerite C60 is referred to as most accurate, because of ⁎
the quasiparticle approach employed and a fairly good agreement with PES (photoemission spectroscopy), inverse PES and angle resolved IPES data. Recently, however, somewhat smaller value (2.12 eV) of the band gap has been found from the first principles calculations using Tran and Blaha regular and non-regular modified Becke-Johnson (TB-mBJ) potential [[8]]. Interpretation of the available numerous experimental data on optical transitions at, below and above band gap remain rather controversial up to now. According to optical absorption and luminescence measurements [[9–14]], the distance between the occupied (HOMO) and unocuppied (LUMO) states is between 1.5 and 2.7 eV. Such a big uncertainty is largely associated with the overlapping of the spectral features of the molecular and purely band transitions at small values of absorption coefficient. Therefore, photoluminescence (PL) spectroscopy that is also used in the present work is reasonable to apply to C60 solids in pair with ellipsometric spectroscopy to single-out interband optical transitions. The last spectroscopy is rather insensitive to small absorption (forbidden or weakly allowed transitions inherent in a region below the band gap of C60 solids) but is very accurate in determination
Corresponding author. E-mail address: [email protected] (A.I. Bayramov).
https://doi.org/10.1016/j.tsf.2019.137566 Received 31 May 2018; Received in revised form 11 September 2019; Accepted 11 September 2019 Available online 12 September 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
Thin Solid Films 690 (2019) 137566
A.I. Bayramov, et al.
of the extinction coefficient in a region above the band gap with absorption level above 103 cm−1. Photoluminescence studies of C60 solids can remove the abovementioned uncertainty provided the balance between the radiative transitions of molecular nature and those of interband nature is either preserved or tipped towards interband transitions. The latter take place exclusively between band states. In the present work we show that C60 thin films on soda lime glass (SLG) substrate just meet this condition. In addition, C60/Si and C60/ porous Si/Si (C60/PS/Si) multilayer structures for which the molecular radiative transitions strongly dominate over interband radiative transitions are also studied for the sake of completeness.
2. Experimental details High purity (99.99%) fullerene powder (C60) was deposited on soda lime glass substrate (C60/SLG) by sublimation technique in a vacuum of 1.33⊓10−4 Pa. For comparison, the same fullerene powder was also deposited directly on silicon (C60/Si) and porous silicon (C60/PS/Si) preliminarily fabricated on silicon substrate by anodic etching. The thicknesses of C60 films were measured during evaporation by using a deposition controller (Inficon, Leybold) and were in the range of 100–200 nm. PS layers with thickness of 10–20 μm were prepared on p-type Si substrate (with resistivity ρ = 10 Ω.cm) by anodic etching in HF: H2O solution under the white illumination. The average porosity, i.e. the void fraction in the porous layer was measured by gravimetric technique, using the eq. P = {(m1 − m2)/(m1 − m3)} 100% [17]. Here m1 is Si sample mass before the etching, m2 after etching and m3 after the removal of the porous layer by rapid dissolution of the completely porous layer in a 3% KOH solution. The porous silicon thickness (d) was determined using the equation d = (m1 − m2)/ρS, where ρ is the Si density (2.33 g/cm3) and S is the etched surface. The average porosity for PS layers and density of pores were found to be 70–75% and 3.4 1010 cm−2, respectively. X-Ray diffraction (XRD) analyses of the films were carried out using Bruker D2 Phaser (Germany) diffractometer in θ-2θ scan mode with Nifiltered CuKα radiation (λ = 1.54060 Ǻ) source. Topography analysis of the films were performed in Smart SPM 1000 AIST NT (Tokyo Instruments, Japan). Surface, cross-section and elemental analyses of the multi-layer structures were carried out using Scanning Electron Microscopy SEM S-4800 with Energy Dispersive Spectroscopy (EDS) system (Hitachi Ltd., Japan) of 1 nm resolution. Operating voltage of 10 kV and probing spot of 8–10 μm size were used. Photoluminescence (PL) measurements were performed using PL/ PLE/Raman spectrometer (Tokyo Instruments, Inc.). The emission of the samples were excited by 325 nm wavelength laser beams. Raman spectra were measured by Confocal PL/Raman microscope Nanofinder 30-NM01 (Tokyo Instruments, Inc.). The ellipsometric measurements in 220–1700 nm spectral range were performed using Woollam M2000 (USA) rotating compensator instrument. Incident light angles were varied between 55 and 75° with 5° step. WVASE32 computer program was used for the ellipsometric data fitting procedure. Experimental data were fitted (employing the Levenberg–Marquardt algorithm) to optical model using parameterized model dielectric functions simultaneously for all the data points measured in UV/VIS ranges. Gen-Osc model available in CompleteEase database was used to model C60/SLG structure dielectric function for the entire range of wavelengths at 55°, 60° and 65° incident light angles. The. structure of Gen-Osc was composed of three Gaussian and one Psemi-M0 oscillators. The free substrate was measured and fitted separately to obtain its optical parameters for inclusion into the. model. This approach allows indirectly relate the experimental data to electronic and structural properties of the samples.
Fig. 1. Raman shift of C60 film on SLG substrate.
All of the measurements were performed at room temperature.
3. Results and discussions 3.1. Samples structure, surface morphology and cross-section profile XRD patterns of the films did not reveal any noticeable reflexes. A weak reflex around 2θ~11° (most intensive line of C60 with high degree of crystallinity [[15]]) appears after relatively long time exposure of the samples to X-rays. This is caused, in the first place, by low thicknesses of the obtained films. Of course, incomplete crystallization of the films obtained at room temperature contributes as well. However, the results of the detailed Raman studies on the films show that the last factor is not decisive. The Raman spectrum of a C60 film excited with 532 nm laser is shown in Fig. 1. Close inspection of the obtained spectrum shows that the last reproduces practically all Raman active modes observed so far on the perfect examples of C60 solids [[6],[16]]. These modes lie above 260 cm−1 and are intra-molecular in nature. The mode with frequency 33 cm−1 is external (inter-molecular) mode and corresponds to librational motions [[6]]. It should be noted, that C60 -pristine samples might experience photo-transformation under visible or ultraviolet light irradiation during Raman or photoluminescence measurements. According to [[7]], the photransformation introduces a new mode at 116 cm-1, identified with an intermolecular vibrational mode. In general, samples containing large concentration of dimers or linear chains exhibit very reach structure with many peaks in the range 250–500 cm-1 [[8]]. In our case such peaks (if any) must have been so weak that they are hardly observable in Fig. 1. In other words, even if some polymerization of the studied samples did take place, its influence on Raman spectra was very weak. AFM image (not shown) of the fullerene C60 film deposited on SLG substrate at room temperature, obtained in non-contact mode shows the dense and regular character of the clusters. In Fig. 2, 2D surface (upper, left hand) and cross-section (upper, right-hand) Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) (lower, left-hand) images of C60/PS/Si multilayer structure are displayed. C60 and PS thicknesses of the structure were found as 109 nm and 12 μm, respectively. Lower right-hand picture shows carbon, silicon and oxygen distributions in the porous part of the structure. As it is clearly seen, the C60 molecules penetrate deep into PS closely to single part of the silicon. Moreover, the EDS analysis shows the presence of oxygen in PS along with the C60 molecules.
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Fig. 2. Surface and cross-section SEM images of C60/PS/Si structure and corresponding EDS spectra and Si, C, O distribution across depth.
3.2. Photoluminescence and optical constants of C60 films on SLG substrate
spectrum spreads over the photon energy range from 1.5 to 2.7 eV nm and shows clearly resolvable sub-bands. Three overlapping bands apparently appear in the spectrum. The spectrum can be deconvoluted into three Gaussians (Fig. 3, broken curves) with the following parameters: first peak at 1.774 eV with half width 294 meV; second peak at 2.115 eV with half width 258 meV; third peak at 2.342 eV with half width 234 meV. Fig. 4. (broken curves) shows above Gaussians together with photon energy dependence of the optical constants, refractive index and extinction coefficient (Fig. 4, full curves), obtained ellipsometrically. It is easy to realize from Fig. 5 that the emissions represented by curves 2 and 3 are in the region of the allowed dipole transitions. The emission represented by curve 1 corresponds to vanishing extinction and can be associated only with the forbidden dipole transitions provided that this emission is caused by the transitions between eigen electronic states of C60 film. For the sake of convenience, the regions of the allowed and forbidden transitions are conditionally separated from each other by vertical broken line (Fig. 4). In such a case, the curve 2 is supposed to be emission due to bandto-band radiative transitions and the energy position (2.115 eV) of the maximum of this emission should correspond to the energy gap for direct optical transitions at band gap. Therefore, the band gap energy, indicated by the arrow with sign Eg in Fig. 5 is tentatively assumed 2.115 eV. Actually, asymmetric line-shape for the band-to-band transitions cannot be considered at this stage but will be discussed later. Finally, the emission described by curve 3 in Fig. 4 occurs in a highly absorptive region above the band gap and probably has to correspond to higher interband radiative transitions. Note that the difference between the energy position of the last
A typical photoluminescence spectrum of C60 film on SLG under excitation wavelength 325 nm is given in Fig. 3 (full curve). The
Fig. 3. PL of C60 film on SLG substrate under 325 nm excitation wavelength (broken curves are spectrum, deconvoluted into three Gaussians). 3
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vibronic transitions. Vibronic structures do not appear on curve 1 in Figs. 3 and 4, but are resolved for the emission of C60 film in C60/PS/Si structures. As it is seen in Fig. 5, the last emission exhibits well-developed vibronic structure with 60 meV separation between vibronic levels. This separation corresponds to the Raman peak at 493 cm −1 (Fig. 1) that is considered as Ag mode (in spectroscopic symmetry notation) [[16]]. Note that the emissions observed on C60/Si and C60/PS/Si structures and the emission described by curve 1 in Figs. 3 and 4 are identical, as verified by the comparison of their normalized spectra given in the insert of Fig. 5. Therefore, this emission really represents singlet Frenkel exciton radiative decay. The striking feature of C60 film in C60/PS/Si structures is very high PL intensity. The last is 200 times higher than that in C60/Si and up to 50 times exceeds the one for the emission observed on PS/Si (Fig. 5) (the PL spectrum of C60 film in C60/SLG structure (not shown in Fig. 5) and that of the one in C60/Si structure have comparable intensities.) Considering the sizes of C60 molecules of about 1.0 nm and pores of about 1.5 nm diameter in the sponge like structure of porous silicon, C60 may occupy the pores of PS. As it was shown from our EDS studies, (Fig. 2), the C60 molecules penetrate deep into PS and as a result, a strong interaction between C60 molecules and PS walls may be expected. Disappearance of emission bands at 550 nm and 590 nm in C60/ PS/Si structures can be explained by formation of molecular Frenkel excitons of singlet type, as well as by mono-molecular character of the C60 in mesapores of silicon [[11],[12],[14]]. The red shift of the 710 nm emission band to 730 nm is obviously caused by the vibronic correction in PS-C60 structure. On the other hand, the strong interaction between C60 molecules and PS walls may result in a change of molecular symmetry of C60. This change, in turn leads to the perturbation of the delocalized π- orbitals, which determines the electronic, as well as optical properties of the C60 molecule [[19]]. The perturbation may be sufficient to relax some of the selection rules for optical transitions within the excited-state structure of C60. As a result, the quantum yields of radiative transition are enhanced over a broad region and a significant increase of the emission intensity is observed in C60/PS/Si structures.
Fig. 4. Refractive index (n) and extinction coefficient (k) of C60 film on SLG substrate as functions of photon energy. The results of the deconvolution of the PL spectrum are shown by broken curves.
Fig. 5. PL spectra of C60/Si, PS/Si, and C60/PS/Si structures under 325 nm excitation wavelength. The insert displays the normalized PL spectra of C60/ SLG, C60/Si and C60/PS/Si structures.
3.4. Optical transitions at fundamental absorption edge of C60 films It has been proposed in Section 3.2 that emission described by curve 2 in Figs. 3 and 4 corresponds to band-to-band transitions at band gap (forbidden gap), and curve 3 in Figs. 3 and 4 to higher interband optical transitions. Using the relation α = 2πkE/hc (E-photon energy, h-Plank constant, c- speed of light) to convert the obtained extinction coefficient (k) into the absorption coefficient (α) and to plot α2E2 versus E at fundamental absorption edge we can examine whether the last plot is linear and follows the well- known law, α2E2 ~ (E-Eg) for direct optical transitions at band gap. The results of such an examination are shown in Fig. 6. Extrapolation of the linear section of the plot in Fig. 6 down to zero absorption gives the value of 2.115 eV for band gap (Eg) in C60 thin film. Definitely not by chance, the last energy exactly coincides with the energy position of the maximum of curve 2 on Figs. 3 and 4, as shown by vertical broken-dotted line in Fig. 6. Thus, photoluminescence and spectroscopic ellipsometry, both lead to the same value of the energy gap for direct transitions. The linear section of the plot in Fig. 6 corresponds to the absorption coefficient values between 103 and 104 cm−1, which is quite standard for absorption caused by direct allowed optical transitions. Makarova [[20]] has reported similar absorption values for direct optical transitions in C60 thin films. According to the insert in Fig. 6, the onset of optical absorption is around 1.9 eV or coincides with the one reported earlier from ellipsometric measurements by Kelly et al. [[21]]. It is natural to consider the range between 1.9 and 2.115 eV as the one occupied by tail- to- tail
emission and that of the emission described by curve 2 in Fig. 4 is 227 meV. The last energy exceeds by 32 meV the highest energy (195 meV [[18]]) observed for the normal modes in C60 films. In other words, above difference is too large to consider curves 2 and 3 as vibronic structures of the spectrum and, in turn, supports the proposed scenario for optical transitions at and above band gap of the obtained C60 films. The pholuminescence observed below the energy gap is very well correlating with the previous data for C60 bulk materials. The main difference is related to emissions observed in the present work for photon energies at and above energy gap, which appears only under 325 nm wavelength excitation. To the best of our knowledge, there have not been reports on photoluminescence of C60 thin films under this excitation. All of the previous PL studies were performed under the excitation wavelength above 500 nm. 3.3. Photoluminescence of C60 films in C60/Si and C60/PS/Si structures According to the numerously verified and commonly accepted view [[3],[4]], the emission band described by curve 1 in Figs. 3 and 4 can be accounted for the radiative decay of Frenkel excitons. These excitons are localized on individual C60 molecules and the corresponding transitions incorporate vibrational excitations of a single C60 molecule. The resultant transitions are therefore not purely electronic and called 4
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radiative transitions involving vibrational energy levels of singlet Frenkel exciton. The Ag-vibration mode of 60 meV is most active in the formation of vibronic structure of the emission of C60/PS/Si. A dramatic increase of the emission intensity in C60/PS/Si as compared to C60/glass and C60/Si structures is very likely to result from the further relaxation of the selection rules for almost forbidden optical transitions due to the strong interaction of C60 molecules with PS walls. References [1] E.A. Rohlfing, D.M. Cox, A.J. Kaldor, Production and characterization of supersonic carbon cluster beams, J. Chem. Phys. 81 (7) (1984) 3322–3330. [2] W.I. David, R.M. Ibberson, J.C. Matthewman, K. Prassides, T.J. Dennis, J.P. Hare, H.W. Kroto, R. Taylor, D.R. Walton, Crystal structure and bonding of ordered C60, Nature 353 (1991) 147–149. [3] J. Feldmann, R. Fischer, W. Guss, E. Gobel, S. Schmitt-Rink, W. Kratschmer, White luminescence from solid C60, Europhys. Lett. 20 (1992) 553–558. [4] R.W. Lof, M.A. van Veenendaal, B. Koopmans, H.T. Jonkman, G.A. Sawatzky, Band gap, excitons, and coulomb interaction in solid C60, Phys. Rev. Lett. 68 (1992) 3924–3927. [5] S. Saito, A. Oshiyama, Cohesive mechanism and energy band of solid C60, Phys. Rev. Lett. 66 (1991) 2637–2640. [6] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, 1995. [7] A.M. Rao, P. Zhou, K.-A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.-T. Lee, X.X. Bi, P.C. Eklund, D.S. Cornett, M.A. Duncan, I.J. Amster, Photoinduced polymerization of solid C60 films, Science 259 (1993) 955–957. [8] B. Sundquist, Polymeric fullerene phases formed under pressure, structure and bonding, Fullerene-Based Mater. 109 (2004) 85–126. [9] E.L. Shirley, S.G. Louie, Electron excitations in solid C60. Energy gap, band dispersions, and EN’ects of orientational disorder, Phys. Rev. Lett. 71 (1993) 133–136. [10] S. Jalali-Asadabadi, E. Ghasemikhan, T. Ouahrani, B. Nourozi, M. Bayat-Bayatani, S. Javanbakht, H.A. Rahnamaye Allabad, I. Ahmad, J. Nematollahi, M. YazdaniKachoeli, Electronic structure of crystalline buckyballs: fcc-C60, J. Electron. Mater. 45 (2016) 339–345. [11] Y. Wang, Photophyslcal properties of fullerenes and fullerene/N,N-Diethylanilhe charge-transfer complexes, J. Phys. Chem. 96 (1992) 764–767. [12] S.P. Sibley, S.M. Argentine, A.H. Francis, A photoluminescence study of C60 and C70, Chem. Phys. Lett. 188 (1992) 187–193. [13] H. Byrne, W. Maser, W. Rühle, A. Mittelbach, S. Roth, Nonlinear luminescence phenomena in fullerene crystallites, J. Appl. Phys. A 56 (1993) 235–239. [14] E. Shin, L. Park, M. Lee, D. Kim, Y.D. Suh, S.I. Yang, S.M. Jin, S.K. Kim, Temperature-dependent photoluminescence study of C60 and C70, Chem. Phys. Lett. 209 (1993) 427–433. [15] D. Faiman, S. Goren, E.A. Katz, M. Koltun, N. Melnik, A. Shames, S. Shtutina, Structure and optical properties of C6o thin films, Thin Solid Films 295 (1997) 283–286. [16] H. Kuzmany, R. Pfeiffer, M. Hulman, C. Kramberger, Raman spectroscopy of fullerenes and fullerene–nanotube composites, Phil. Trans. R. Soc. London A362 (2004) 2375–2406. [17] T.D. Dzhafarov, C. Oruc, S. Aydin, Humidity-voltaic characteristics of Au-porous silicon interfaces, J. Phys.D: Appl. Phys. 37 (2004) 404–409. [18] M.K. Nissen, S.M. Wilson, M.L.W. Thewalt, Highly structured singlet oxygen photoluminescence from crystalline C60, Phys. Rev. Lett. 69 (1992) 2423–2427. [19] B. Hamilton, J.S. Rimmer, M. Anderson, D. Leigh, White light emission from C60 molecules confined in molecular cage materials, Adv. Mater. 5 (1993) 583–585. [20] T.L. Makarova, Electrical and optical properties of pristine and polymerized fullerenes, Semiconductors 35 (3) (2001) 243–278. [21] M.K. Kelly, P. Etchegoin, D. Fuchs, W. Krätschmer, K. Fostiropoulos, Optical transitions of C60 films in the visible and ultraviolet from spectroscopic ellipsometry, Phys. Rev. B 46 (1992) 4963–4968. [22] M. Knupfer, J. Fink, Frenkel and charge-transfer excitons in C60, Phys. Rev. B 60 (1999) 10731. [23] F. Bechstedt, M. Fiedler, L.J. Sham, Excitonic effects in linear and nonlinear optical properties of C60, Phys. Rev. B 59 (1999) 1857.
Fig. 6. Photon energy dependence of the squared value of absorption coefficient (α) multiplied by the squared value of photon energy (E), together with curve 2 in Figs. 3 and 4. Insert: absorption coefficient as a function of photon energy, together with curve 3 in Figs. 3 and 4.
transitions and/or transitions to charge-transfer (CT) exciton states adjacent to the conduction band edge. In any case, the corresponding emission will be broadened by as much as ~200 meV and the real lineshape of the emission related to band-to-band transitions will be masked by this broadening. Note that CT-excitons result from interaction of the two charges localized on different C60 molecules and, thus, are the result of intermolecular interaction. Existence of CT-excitons in C60 films was reported in a number of works [[22],[23]]. Both quasi-particle approach [[9]] and TB -mBJ [[10]] calculations for C60 solid lead to a standard direct band insulator with energy gap above 2 eV at the X point of the Brillouin zone. The band gap value of 2.115 eV obtained in the present work is in a good agreement with the reported theoretical results [[9],[10]]. Another important result is related to the splitting of the five-fold degenerated state of the isolated C60 molecule at the same point of the Brillouin zone by the crystal field. The energy separation of 227 meV between the transitions at band gap and the higher interband transitions that correspond the maximum of the absorption coefficient in the insert of Fig. 6 is within the range of crystal field effect in C60 solid [[10]]. 4. Conclusions Using dual experimental approach based on application of both the photoluminescence spectroscopy and spectroscopic ellipsometry it has been shown that optical transitions above 2 eV in C60 thin film deposited on glass substrate are well described within the model of a standard direct band insulator with band gap energy and crystal field splitting of 2.115 eV and ~ 200 meV, respectively. It has been further shown that emission band that appears below 2 eV in C60/glass structure appears as well in C60/Si and C60/PS/Si structures and is caused by
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