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Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer
NOC-16289; No of Pages 5 Journal of Non-Crystalline Solids xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer S. Charnovych a,⁎, I.A. Szabó b, A.L. Tóth c, J. Volk c, M.L. Trunov d, S. Kökényesi a a
Department of Experimental Physics, University of Debrecen, Bem sq. 18/a, 4026, Debrecen, Hungary Department of Solid State Physics, University of Debrecen, Bem sq. 18/b,4026, Debrecen, Hungary c Research Institute for Technical Physics and Materials Science, Budapest, Hungary d Uzhgorod National University, Uzhgorod, Ukraine b
a r t i c l e
i n f o
Article history: Received 1 October 2012 Received in revised form 20 November 2012 Available online xxxx Keywords: Amorphous chalcogenides; Photo-induced effects; Gold nanostructures; Surface plasmon resonance
a b s t r a c t The influence of the localized surface plasmon fields on the light stimulated transformations in amorphous chalcogenide films was investigated. It was established that both in the gold nanoparticle array-chalcogenide film and in the gold film with nanohole array-chalcogenide film composites the plasmon fields, induced during the laser illumination of the structures, increase the efficiency of structural transformations, the related photodarkening or bleaching, as well as of the volume change, surface deformations. The results, obtained for structures with As20Se80 amorphous layer, can be applied for other selected chalcogenide layers and fabrication of locally driven information recording media. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Amorphous chalcogenide semiconductor layers exhibit well known photo-induced structural changes under the influence of photons with energy close to the band gap (Eg) energy [1–3]. Photo-induced expansion, surface deformation and lateral mass transport were detected by investigation of periodic surface relief formation in As1−xSex layers via in situ atomic-force microscope measurement [4]. It was shown [5] that holographic recording of surface relief in As20Se80 glasses has at least two components: a fast, but with a rather small change of the thickness, which can be related to stimulated volume expansion as observed by Tanaka [6] in As2S3 and a slower but a giant one associated with light induced lateral mass transport [7,8]. It was shown that the mass transport in amorphous chalcogenides due to the polarized illumination is controlled by anisotropic volume diffusion [9,10]. Stimulated darkening or expansion and mass transport also appear when chalcogenide layer is irradiated by e-beam and build in electric fields are created [11,12]. Consequently the electric field should influence the generation of electron-hole pairs, creation of defect states, mass transport and so the plasmon field of the metallic nanostructures also can be involved to the recording processes. Surface plasmons can be excited on the metallic surface by light at special matching conditions [13]. Localized surface plasmon resonance (LSPR) can be excited more easily in nanosized metallic nanoparticles [14] or even nanohole arrays in metallic layers [15]. Gold and silver are widely used for surface plasmon resonance (SPR) measurement, because the resonant conditions (absorption maximum ⁎ Corresponding author. E-mail address: [email protected] (S. Charnovych).
in a visible spectral range) are easily achieved and the technology of nanoparticles is well developed. Gold nanoparticles (GNP) on a silica glass substrate and nanohole arrays in a gold layer (GNH) with dimensions in the 5–100 nm range satisfy the conditions for SPR in green–red spectral region, where As–S(Se) glasses are the most sensitive to the illumination. It was shown previously that the plasmon resonance wavelength of the nanostructures can be controlled by changing the sizes of the nanostructures [16–18]. The electric field of an incoming p-polarized (vector E perpendicular to the interface) light can induce certain distribution of the surface charge density of the particle. There is a net electric field around the excited nanoparticle that is composed by superposition of an external applied field (electromagnetic wave) and the induced field of the particle. This net electric field can influence the photo-induced changes in chalcogenides as it was shown previously [19]. The aim of this work was further investigation of the effect of LSPR on photoinduced changes in As20Se80 + gold nanostructured system. For this investigation GNP and GNH were produced to create localized surface plasmons. The effect of these nanostructures on the transmission changes in chalcogenide layers due to the laser irradiation, as well as on the volume (roughness) change has been investigated.
2. Experimental Experiments on photo-induced changes have been done on composite structures, which contained appropriate plasmonic element and a chalcogenide layer. Results were compared with data on a single chalcogenide layer. At the first step plasmonic elements – GNP or GNH – were created. Since the As20Se80 is highly sensitive just to the
0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
2
S. Charnovych et al. / Journal of Non-Crystalline Solids xxx (2012) xxx–xxx
action of red laser light the plasmon resonance wavelength of these elements was adjusted to the red spectral region. For creation of gold nanoparticles, first a 25 nm thick gold layer was deposited in vacuum on the silica glass substrate due to the Ostwald ripening. This sample was annealed for 4 h at 550 °C and GNP were created. The obtained samples of GNP were investigated by Scanning Electron Microscope (SEM) (Hitachi S-4300). The average size of the GNP was calculated from the SEM pictures (see. Fig. 1c) by a special program setup and it was about 120 nm. The plasmon resonance wavelength of these structures was near 630 nm. The nanoholes in the gold layer of the same thickness, deposited on the similar glass substrate, were prepared by e-beam lithography method (see Fig. 1d). The diameter of the holes was about 200 nm because it had been shown previously [18] that nanoholes with a size of about 200 nm in gold layer have plasmon resonance close to 630 nm. A 600 nm thick As20Se80 layer was thermally evaporated in vacuum on the above mentioned gold nanostructures. The schematic diagram of the sample structure with GNP and GNH and chalcogenide layer is presented on Fig. 1a and b. The sizes of the GNH rectangles were rather small (see Fig. 1b) to perform direct optical measurements in a GNHchalcogenide structure. So here mostly the surface changes were
investigated. The investigated structures were irradiated by red laser beam (λ=633 nm, output power P=7 mW) through a diaphragm with 1.2 mm diameter. The maximum light intensity at the surface was 600 mW/cm2. The optical transmission spectra of the samples have been measured with Shimadzu UV-3600 spectrophotometer. The change of the transmission on the irradiation time has been detected with power meter setup (Thorlabs PM100). The layer thickness and its changes have been measured by Ambios XP-1 profile meter. Optical transmission spectra have been used for calculation of the refractive index, absorption coefficient, and absorption edge change using Swanepoel method [20]. Surface relief gratings were recorded on the samples by two coherent p-polarized laser beams with equal intensity. The experimental set-up for transmission measurements by coherent laser beam was modified for in situ measurements of surface roughness change in atomic force microscope (AFM) (Veeco-diCaliber). Both the ChG/GNP and ChG/GNH samples were irradiated from the bottom side, while the scanning of the surface was done on the top of the samples. The samples were irradiated with the same setup as before but with lower intensity (3 mW). The process was done through a spot, whose diameter was 0.5 mm, so the laser intensity on the surface of the sample was 40 mW/cm 2. Each scan takes few minutes so the time
Fig. 1. a) and b) Schematic diagram of the sample structure with GNP and GNH and chalcogenide layer. c) SEM image of the created GNP structures. d) and e) 2D and 3D AFM images of the created GNH structures.
Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
S. Charnovych et al. / Journal of Non-Crystalline Solids xxx (2012) xxx–xxx
1.3
dependence of the roughness change was measured. We have investigated single layers without nanostructures and layers, which consist of nanostructures (GNP and GNH) too. A small shift in the series of scans made by AFM was detected so a Labview program setup was used for analyzing the results.
1.25
1.15 1.1 1.05
The influence of the plasmonic field of the metallic nanostructures on the optical parameter changes due to the red, i.e. near band gap (for the given chalcogenide) laser irradiation was investigated. The transmission spectra of the single chalcogenide layer and samples, which consisted chalcogenide glass and GNP were rather different and were influenced by laser irradiation. From the data of optical transmission spectra the changes of the optical parameters of the irradiated structures were calculated using Swenapoel method (see Table 1). As it was shown previously [19,21] photo-darkening effect was detected in both samples (in the pure chalcogenide and in chalcogenide/GNP too), while the rate of these process was faster in the system which included GNP. In our single layer the well known optical photo-darkening effect was detected, which saturated with time. At the same time in the system with GNP after the initial faster process of darkening an unusual effect of photo-bleaching was established: the transmission started to increase and saturated with time (see Fig. 2). For the better understanding of the influence of the plasmonic field of the GNP on the volume and surface changes in amorphous chalcogenides the creation of surface relief gratings was investigated (Fig. 3). Surface relief gratings were created at the same recording conditions in a single As20Se80 layer and in a chalcogenide+ GNP system. The period of the created gratings was 1500 nm. It was established that the height of the surface relief increased up to 62 nm for the single chalcogenide layer and to 81 nm for the composite system after the equal exposure, so it means that the local giant expansion occurs up to 10 and 14% of the initial film thickness respectively. To understand how the plasmonic field influences the surface change, in situ AFM investigations were done to measure the change of the roughness due to the uniform laser illumination with photon energy close to the Eg of the material. It was observed from in situ roughness monitoring that, while the roughness of the single chalcogenide layer did not change significantly (Δd ≈ 1 nm) for the sample with GNP a more significant change, flattening was detected (Δd ≈ 12 nm). The same measurement was done on the composite sample with GNH. Some kind of shift in the AFM scans was detected, so we used a special program for analyzing the results. It was shown that the roughness of the investigated samples with GNH before irradiation was 4 nm, after it became 2 nm, so the sample became smoother as in the previous case, while the roughness of the surface of the sample without GNH had not changed due to the laser irradiation. Photo-darkening effect, i.e.an increase of the absorption coefficient at the wavelength of illumination and the shift of the absorption edge towards longer wavelengths, as well as an increase of the refractive index were detected and calculated at the given conditions of illumination intensity in a single chalcogenide layer. At the same time
Table 1 Changes of the parameters of the samples (refractive index n, thickness d, coefficient of absorption α, absorption edge Eg) after the laser illumination till the saturation of the process. As20Se80
n d, nm α, *104 Eg, eV
2
1.2
I/I0 3. Results
3
As20Se80 +GNP
As Dep
Ill
As Dep
Ill
2.64 600 2.0 1.96
2.65(+0.01) 580 (−3.3%) 2.1 (+0.1, at 600 nm) 1.95 (−0.01)
2.84 600 4.0 1.90
2.86 560 4.0 1.90
(+0.02) (−6.6%) (+/−0, at 600 nm) (+/−0)
1.0 0.95 0.9 0
1
500
1000
1500
2000
2500
3000
Time,s Fig. 2. Time dependence of the optical transmission on the irradiation time in a single As20Se80 layer (1) and structures with GNP (2).
it is known [21] that there is a transition from darkening to bleaching even in a single chalcogenide layer under illumination with laser intensities above a certain threshold. In comparison to the results of illumination in a single chalcogenide layer, in the system with GNP a faster photo-darkening effect, an unusual photo-bleaching process, a larger increase of the refractive index was detected and calculated, while the absorption coefficient and the absorption edge had not changed after the bleaching. The reason was the local decrease of the thickness in the investigated samples due to laser irradiation. It was in accordance with expected local contraction–expansion effects. According to our previous measurements [10] the direction of the photo-induced volume change depends on the intensity of the illumination, so in structures with GNP we reduced it at lower intensities. 4. Discussion According to the previously developed theory (10) and to the experimental results of this work the increase of the light intensity above a certain threshold leads to the decrease of the local expansion and even transition to the local contraction in the illuminated spot. This effect relates to the bulk (volume) diffusion, in which a value linearly depends on the light intensity. In its turn, the volume diffusion relates to the efficiency of local charge carrier generation and transport, i.e. we can influence the stimulated structural transformations at the first stage of the excitation process. The laser irradiation, which affects not only the amorphous chalcogenide, but excites the plasmonic field of the GNP, as well, causes a decrease of viscosity, which increases the mobility of the atoms of the material. It can be noted that the surface of the illuminated region becomes much smoother than that before the illumination. This smoother surface is assumed to occur as a result of photoinduced fluidity under surface tensions [22]. The electric field of an incoming p-polarized light can induce surface plasmons in the GNP and GNH. Around the metallic nanoparticle a net electric field appears, which is composed of superposition of an electromagnetic external field and the induced field of the GNP. This net electric field can result the photo-induced changes in chalcogenides by influencing the defect bonds as it was shown previously [19,21]. So in our case the presence of the plasmon field of the GNP should affect the system like the increase of the irradiation intensity, and contraction should appear at lower threshold intensities. At the same illumination conditions the value of contraction has to be larger for system where increased net electric field appears, in comparison with the system without it. It was shown in our measurements that the thickness change for the system with GNP was larger as in the system without
Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
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S. Charnovych et al. / Journal of Non-Crystalline Solids xxx (2012) xxx–xxx
Fig. 3. AFM images of the holographic surface relief gratings in a pure ChG (a, b) and ChG/GNP composite (c, d).
them. The explanation of the enhancement relates to the initial stage of photoinduced effects, i.e. to the generation of electron-hole pairs, creation or modification of defects (dangling bonds, change of the coordination of chalcogen). The local electric field of the metallic nanoparticles enhances the electron processes, influences the rate, the final value of the photoinduced changes, which consists of small initial shifts of atoms, change of the interchain distances (increase of the free volume), as well as the further diffusion and mass transport (giant local expansion–contraction). Creation of the surface holographic structures can be ascribed not only to the electron-hole creation, bond braking and rearrangement, but to the lateral mass transport as well as it was described and investigated in [4]. As it was mentioned previously the mass transport effect that causes giant surface change in amorphous chalcogenide due to the near band gap illumination is controlled by volume diffusion, which depends on the intensity of the irradiation. So with increasing the laser intensity the height of the surface grating relief will increase too at the same recording conditions (polarization, recording scheme and time). From our results of atomic force microscopy it can be concluded that at the same recording conditions we obtained higher surface relief gratings on the sample, which contained metallic nanoparticles. It can be associated with the plasmonic field of the GNP, which enhances the processes that influence the rate of the mass transport, surface changes. The main question: do and how do the plasmon fields influence the photoinduced structural changes in amorphous chalcogenides may be answered positively after these experiments. Besides the optical effect of darkening and change of the refractive index [2] this influence relates to the lateral mass transport too. So we can use our previous considerations about the mechanism of surface relief (bumps and valleys) formation under non-uniform, periodical illumination [22] and flattening of the recorded gratings under uniform irradiation [9] to our experiments on surface smoothing. The rates of flattening and the diffusion coefficient depend on the light intensity and local surface curvature similarly to the recording process. These follow from equation written for a periodical distribution of the excitation (9): ðxÞ þ σ ðxÞΔω ðxÞ; μ k ðxÞ ¼ μ 0k þ qk IðxÞ þ K ðxÞγω
k ¼ P; C:
ð1Þ
Here k denotes pnictide (P) or chalcogenide (C) atom, μ0k is the bulk chemical potential of the atoms without illumination; the term qkI gives the increase of the chemical potential due to deformation of chemical bonds around the electron-hole pair or defect (in a specific volume qk) gives an account for capillary excited by light with intensity I, Kγ ω forces caused by the curvature of the surface profile, γ is the surface is the atomic volume of P and C tension; and σ is the built in stress, ω atoms. Of course, in the presented experiments besides some non-uniformity of the excitation of the chalcogenide layer due to the distribution of sizes and fill factor of gold nanoparticles or holes and in appropriate distribution of the plasmon fields, the excitation can be considered as uniform in comparison with periodical grating. So the driving force ðxÞ and of the flattering relates first to all of the terms K ðxÞγω ðxÞ in Eq. (1), and the rate, as well as the final value of σ ðxÞΔω smoothening is increased in the localized plasmon field, since the wavelength and intensity (I) of the illumination influence both the processes. The SPR in investigated composites can be used for measuring the refractive index changes of the chalcogenide glasses as a sensing medium [23]. The same can be concluded from our first modeling of such structures [24].
5. Conclusions Conclusion can be made from the above analyzed experimental results on the change of the optical transmission, on the local volume change and on the related effect of the surface roughness change, that the plasmon fields, generated during the illumination of the ChG/GNP or ChG/GNH composite structures enhance the structural transformation processes. The enhancement relates to the first basic stage of the photo-induced transformations i.e. generation of electron-hole pairs and the subsequent changes in defects and atomic displacements. Besides the enhancement of changes of optical parameters the plasmon fields enhance mass transport processes in illuminated chalcogenides/gold nanocomposite, which can be applied for more efficient surface modification in such structures. Further model calculations are under development which will support the selection of the best GNP/ChG nanocomposites, for example for the recording
Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
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media with higher sensitivity and resolution at nano-scale, as well as with increased non-linear optical parameters. Acknowledgments This work was supported by the TAMOP 4.2.1./B-09/1/KONV2010-0007 and TAMOP 4.2.2./B-10/1-2010-0024 projects, which is co-financed by the European Union and European Social Fund. References [1] A.V. Kolobov, Photo-induced Metastability in Amorphous Semiconductors, WileyVCH, Weinheim, Germany, 2003. [2] K. Tanaka, K. Shimakawa, Amorphous Chalcogenide Semiconductors and Related Materials, Springer, New York, Dordrecht Heidelberg, London, 2011. [3] J. Singh, K. Shimakawa, Advances in Amorphous Semiconductors, Taylor & Francis, London, New York, 2003.. [4] M. Trunov, P. Lytvyn, V. Takats, I. Charnovich, S. Kokenyesi, J. Optoelectron. Adv. Mater. 11 (2009) 1959. [5] M.L. Trunov, P.M. Nagy, V. Takats, P.M. Lytvyn, S. Kokenyesi, E. Kalman, J. Non-Cryst. Solids 355 (2009) 1993. [6] H. Hisakuni, K. Tanaka, Appl. Phys. Lett. 65 (1994) 1993. [7] M.L. Trunov, JETP Lett. 86 (2007) 313. [8] A. Saliminia, T.V. Galstian, A. Villeneuve, Phys. Rev. Lett. 85 (2000) 4112.
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[9] Yu. Kaganovskii, M.L. Trunov, D.L. Beke, S. Kökényesi, Mater. Lett. 66 (2011) 159. [10] Yu. Kaganovskii, D.L. Beke, S. Charnovych, S. Kokenyesi, M.L. Trunov, J. Appl. Phys. 063502 (2011) 1414. [11] J.R. Neilson, A. Kovalskiy, M. Vlček, H. Jain, F. Miller, J. Non-Cryst. Solids 353 (2007) 1427. [12] M. Trunov, P. Lytvyn, P.M. Nagy, Cs. Cserhati, I. Charnovich, S. Kokenyesi, Proc. SPIE 7598 (2010) 75981H. [13] R.B.M. Schasfoort, A.J. Tudos, Handbook of Surface Plasmon Resonance, RSC Publishing, Cambridge, 1998. [14] H.E. Fendler, J. Adv. Mater. 16 (2004) 1685. [15] J. Parsons, E. Hendry, C.P. Burrows, B. Auguié, J.R. Sambles, W.L. Barne, Phys. Rev. B 79 (2009) 073412. [16] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature 391 (1998) 667. [17] S. Eustis, M.A. El-Sayed, Chem. Soc. Rev. 35 (2006) 209. [18] T.A. El-Brolossy, T. Abdallah, M.B. Mohamed, S. Abdallah, K. Easawi, S. Negm, H. Talaat, Eur. Phys. J. Spec. Top. 153 (2008) 361. [19] S. Charnovych, S. Kokenyesi, Gy. Glodán, A. Csik, Thin Solid Films 519 (2011) 4309. [20] W. Swanepoel, J. Phys. E Sci. Instrum. 16 (1983) 1224. [21] S. Charnovych, J. Optoelectron. Adv. Mater. 13 (2011) 412. [22] Yu. Kaganovskii, D.L. Beke, S. Kökényesi, Appl. Phys. Lett. 97 (2010) 061906-1. [23] A.A. Popescu, R. Savastru, D. Savastru, S. Miclos, J. Optoelectron. Adv. Mater. 6 (2011) 1245. [24] S. Charnovych, N. Dmitruk, I. Voynarovych, N. Yurkovich, S. Kokenyesi, Plasmonics 7 (2012) 341.
Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer S. Charnovych a,⁎, I.A. Szabó b, A.L. Tóth c, J. Volk c, M.L. Trunov d, S. Kökényesi a a
Department of Experimental Physics, University of Debrecen, Bem sq. 18/a, 4026, Debrecen, Hungary Department of Solid State Physics, University of Debrecen, Bem sq. 18/b,4026, Debrecen, Hungary c Research Institute for Technical Physics and Materials Science, Budapest, Hungary d Uzhgorod National University, Uzhgorod, Ukraine b
a r t i c l e
i n f o
Article history: Received 1 October 2012 Received in revised form 20 November 2012 Available online xxxx Keywords: Amorphous chalcogenides; Photo-induced effects; Gold nanostructures; Surface plasmon resonance
a b s t r a c t The influence of the localized surface plasmon fields on the light stimulated transformations in amorphous chalcogenide films was investigated. It was established that both in the gold nanoparticle array-chalcogenide film and in the gold film with nanohole array-chalcogenide film composites the plasmon fields, induced during the laser illumination of the structures, increase the efficiency of structural transformations, the related photodarkening or bleaching, as well as of the volume change, surface deformations. The results, obtained for structures with As20Se80 amorphous layer, can be applied for other selected chalcogenide layers and fabrication of locally driven information recording media. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Amorphous chalcogenide semiconductor layers exhibit well known photo-induced structural changes under the influence of photons with energy close to the band gap (Eg) energy [1–3]. Photo-induced expansion, surface deformation and lateral mass transport were detected by investigation of periodic surface relief formation in As1−xSex layers via in situ atomic-force microscope measurement [4]. It was shown [5] that holographic recording of surface relief in As20Se80 glasses has at least two components: a fast, but with a rather small change of the thickness, which can be related to stimulated volume expansion as observed by Tanaka [6] in As2S3 and a slower but a giant one associated with light induced lateral mass transport [7,8]. It was shown that the mass transport in amorphous chalcogenides due to the polarized illumination is controlled by anisotropic volume diffusion [9,10]. Stimulated darkening or expansion and mass transport also appear when chalcogenide layer is irradiated by e-beam and build in electric fields are created [11,12]. Consequently the electric field should influence the generation of electron-hole pairs, creation of defect states, mass transport and so the plasmon field of the metallic nanostructures also can be involved to the recording processes. Surface plasmons can be excited on the metallic surface by light at special matching conditions [13]. Localized surface plasmon resonance (LSPR) can be excited more easily in nanosized metallic nanoparticles [14] or even nanohole arrays in metallic layers [15]. Gold and silver are widely used for surface plasmon resonance (SPR) measurement, because the resonant conditions (absorption maximum ⁎ Corresponding author. E-mail address: [email protected] (S. Charnovych).
in a visible spectral range) are easily achieved and the technology of nanoparticles is well developed. Gold nanoparticles (GNP) on a silica glass substrate and nanohole arrays in a gold layer (GNH) with dimensions in the 5–100 nm range satisfy the conditions for SPR in green–red spectral region, where As–S(Se) glasses are the most sensitive to the illumination. It was shown previously that the plasmon resonance wavelength of the nanostructures can be controlled by changing the sizes of the nanostructures [16–18]. The electric field of an incoming p-polarized (vector E perpendicular to the interface) light can induce certain distribution of the surface charge density of the particle. There is a net electric field around the excited nanoparticle that is composed by superposition of an external applied field (electromagnetic wave) and the induced field of the particle. This net electric field can influence the photo-induced changes in chalcogenides as it was shown previously [19]. The aim of this work was further investigation of the effect of LSPR on photoinduced changes in As20Se80 + gold nanostructured system. For this investigation GNP and GNH were produced to create localized surface plasmons. The effect of these nanostructures on the transmission changes in chalcogenide layers due to the laser irradiation, as well as on the volume (roughness) change has been investigated.
2. Experimental Experiments on photo-induced changes have been done on composite structures, which contained appropriate plasmonic element and a chalcogenide layer. Results were compared with data on a single chalcogenide layer. At the first step plasmonic elements – GNP or GNH – were created. Since the As20Se80 is highly sensitive just to the
0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
2
S. Charnovych et al. / Journal of Non-Crystalline Solids xxx (2012) xxx–xxx
action of red laser light the plasmon resonance wavelength of these elements was adjusted to the red spectral region. For creation of gold nanoparticles, first a 25 nm thick gold layer was deposited in vacuum on the silica glass substrate due to the Ostwald ripening. This sample was annealed for 4 h at 550 °C and GNP were created. The obtained samples of GNP were investigated by Scanning Electron Microscope (SEM) (Hitachi S-4300). The average size of the GNP was calculated from the SEM pictures (see. Fig. 1c) by a special program setup and it was about 120 nm. The plasmon resonance wavelength of these structures was near 630 nm. The nanoholes in the gold layer of the same thickness, deposited on the similar glass substrate, were prepared by e-beam lithography method (see Fig. 1d). The diameter of the holes was about 200 nm because it had been shown previously [18] that nanoholes with a size of about 200 nm in gold layer have plasmon resonance close to 630 nm. A 600 nm thick As20Se80 layer was thermally evaporated in vacuum on the above mentioned gold nanostructures. The schematic diagram of the sample structure with GNP and GNH and chalcogenide layer is presented on Fig. 1a and b. The sizes of the GNH rectangles were rather small (see Fig. 1b) to perform direct optical measurements in a GNHchalcogenide structure. So here mostly the surface changes were
investigated. The investigated structures were irradiated by red laser beam (λ=633 nm, output power P=7 mW) through a diaphragm with 1.2 mm diameter. The maximum light intensity at the surface was 600 mW/cm2. The optical transmission spectra of the samples have been measured with Shimadzu UV-3600 spectrophotometer. The change of the transmission on the irradiation time has been detected with power meter setup (Thorlabs PM100). The layer thickness and its changes have been measured by Ambios XP-1 profile meter. Optical transmission spectra have been used for calculation of the refractive index, absorption coefficient, and absorption edge change using Swanepoel method [20]. Surface relief gratings were recorded on the samples by two coherent p-polarized laser beams with equal intensity. The experimental set-up for transmission measurements by coherent laser beam was modified for in situ measurements of surface roughness change in atomic force microscope (AFM) (Veeco-diCaliber). Both the ChG/GNP and ChG/GNH samples were irradiated from the bottom side, while the scanning of the surface was done on the top of the samples. The samples were irradiated with the same setup as before but with lower intensity (3 mW). The process was done through a spot, whose diameter was 0.5 mm, so the laser intensity on the surface of the sample was 40 mW/cm 2. Each scan takes few minutes so the time
Fig. 1. a) and b) Schematic diagram of the sample structure with GNP and GNH and chalcogenide layer. c) SEM image of the created GNP structures. d) and e) 2D and 3D AFM images of the created GNH structures.
Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
S. Charnovych et al. / Journal of Non-Crystalline Solids xxx (2012) xxx–xxx
1.3
dependence of the roughness change was measured. We have investigated single layers without nanostructures and layers, which consist of nanostructures (GNP and GNH) too. A small shift in the series of scans made by AFM was detected so a Labview program setup was used for analyzing the results.
1.25
1.15 1.1 1.05
The influence of the plasmonic field of the metallic nanostructures on the optical parameter changes due to the red, i.e. near band gap (for the given chalcogenide) laser irradiation was investigated. The transmission spectra of the single chalcogenide layer and samples, which consisted chalcogenide glass and GNP were rather different and were influenced by laser irradiation. From the data of optical transmission spectra the changes of the optical parameters of the irradiated structures were calculated using Swenapoel method (see Table 1). As it was shown previously [19,21] photo-darkening effect was detected in both samples (in the pure chalcogenide and in chalcogenide/GNP too), while the rate of these process was faster in the system which included GNP. In our single layer the well known optical photo-darkening effect was detected, which saturated with time. At the same time in the system with GNP after the initial faster process of darkening an unusual effect of photo-bleaching was established: the transmission started to increase and saturated with time (see Fig. 2). For the better understanding of the influence of the plasmonic field of the GNP on the volume and surface changes in amorphous chalcogenides the creation of surface relief gratings was investigated (Fig. 3). Surface relief gratings were created at the same recording conditions in a single As20Se80 layer and in a chalcogenide+ GNP system. The period of the created gratings was 1500 nm. It was established that the height of the surface relief increased up to 62 nm for the single chalcogenide layer and to 81 nm for the composite system after the equal exposure, so it means that the local giant expansion occurs up to 10 and 14% of the initial film thickness respectively. To understand how the plasmonic field influences the surface change, in situ AFM investigations were done to measure the change of the roughness due to the uniform laser illumination with photon energy close to the Eg of the material. It was observed from in situ roughness monitoring that, while the roughness of the single chalcogenide layer did not change significantly (Δd ≈ 1 nm) for the sample with GNP a more significant change, flattening was detected (Δd ≈ 12 nm). The same measurement was done on the composite sample with GNH. Some kind of shift in the AFM scans was detected, so we used a special program for analyzing the results. It was shown that the roughness of the investigated samples with GNH before irradiation was 4 nm, after it became 2 nm, so the sample became smoother as in the previous case, while the roughness of the surface of the sample without GNH had not changed due to the laser irradiation. Photo-darkening effect, i.e.an increase of the absorption coefficient at the wavelength of illumination and the shift of the absorption edge towards longer wavelengths, as well as an increase of the refractive index were detected and calculated at the given conditions of illumination intensity in a single chalcogenide layer. At the same time
Table 1 Changes of the parameters of the samples (refractive index n, thickness d, coefficient of absorption α, absorption edge Eg) after the laser illumination till the saturation of the process. As20Se80
n d, nm α, *104 Eg, eV
2
1.2
I/I0 3. Results
3
As20Se80 +GNP
As Dep
Ill
As Dep
Ill
2.64 600 2.0 1.96
2.65(+0.01) 580 (−3.3%) 2.1 (+0.1, at 600 nm) 1.95 (−0.01)
2.84 600 4.0 1.90
2.86 560 4.0 1.90
(+0.02) (−6.6%) (+/−0, at 600 nm) (+/−0)
1.0 0.95 0.9 0
1
500
1000
1500
2000
2500
3000
Time,s Fig. 2. Time dependence of the optical transmission on the irradiation time in a single As20Se80 layer (1) and structures with GNP (2).
it is known [21] that there is a transition from darkening to bleaching even in a single chalcogenide layer under illumination with laser intensities above a certain threshold. In comparison to the results of illumination in a single chalcogenide layer, in the system with GNP a faster photo-darkening effect, an unusual photo-bleaching process, a larger increase of the refractive index was detected and calculated, while the absorption coefficient and the absorption edge had not changed after the bleaching. The reason was the local decrease of the thickness in the investigated samples due to laser irradiation. It was in accordance with expected local contraction–expansion effects. According to our previous measurements [10] the direction of the photo-induced volume change depends on the intensity of the illumination, so in structures with GNP we reduced it at lower intensities. 4. Discussion According to the previously developed theory (10) and to the experimental results of this work the increase of the light intensity above a certain threshold leads to the decrease of the local expansion and even transition to the local contraction in the illuminated spot. This effect relates to the bulk (volume) diffusion, in which a value linearly depends on the light intensity. In its turn, the volume diffusion relates to the efficiency of local charge carrier generation and transport, i.e. we can influence the stimulated structural transformations at the first stage of the excitation process. The laser irradiation, which affects not only the amorphous chalcogenide, but excites the plasmonic field of the GNP, as well, causes a decrease of viscosity, which increases the mobility of the atoms of the material. It can be noted that the surface of the illuminated region becomes much smoother than that before the illumination. This smoother surface is assumed to occur as a result of photoinduced fluidity under surface tensions [22]. The electric field of an incoming p-polarized light can induce surface plasmons in the GNP and GNH. Around the metallic nanoparticle a net electric field appears, which is composed of superposition of an electromagnetic external field and the induced field of the GNP. This net electric field can result the photo-induced changes in chalcogenides by influencing the defect bonds as it was shown previously [19,21]. So in our case the presence of the plasmon field of the GNP should affect the system like the increase of the irradiation intensity, and contraction should appear at lower threshold intensities. At the same illumination conditions the value of contraction has to be larger for system where increased net electric field appears, in comparison with the system without it. It was shown in our measurements that the thickness change for the system with GNP was larger as in the system without
Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
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S. Charnovych et al. / Journal of Non-Crystalline Solids xxx (2012) xxx–xxx
Fig. 3. AFM images of the holographic surface relief gratings in a pure ChG (a, b) and ChG/GNP composite (c, d).
them. The explanation of the enhancement relates to the initial stage of photoinduced effects, i.e. to the generation of electron-hole pairs, creation or modification of defects (dangling bonds, change of the coordination of chalcogen). The local electric field of the metallic nanoparticles enhances the electron processes, influences the rate, the final value of the photoinduced changes, which consists of small initial shifts of atoms, change of the interchain distances (increase of the free volume), as well as the further diffusion and mass transport (giant local expansion–contraction). Creation of the surface holographic structures can be ascribed not only to the electron-hole creation, bond braking and rearrangement, but to the lateral mass transport as well as it was described and investigated in [4]. As it was mentioned previously the mass transport effect that causes giant surface change in amorphous chalcogenide due to the near band gap illumination is controlled by volume diffusion, which depends on the intensity of the irradiation. So with increasing the laser intensity the height of the surface grating relief will increase too at the same recording conditions (polarization, recording scheme and time). From our results of atomic force microscopy it can be concluded that at the same recording conditions we obtained higher surface relief gratings on the sample, which contained metallic nanoparticles. It can be associated with the plasmonic field of the GNP, which enhances the processes that influence the rate of the mass transport, surface changes. The main question: do and how do the plasmon fields influence the photoinduced structural changes in amorphous chalcogenides may be answered positively after these experiments. Besides the optical effect of darkening and change of the refractive index [2] this influence relates to the lateral mass transport too. So we can use our previous considerations about the mechanism of surface relief (bumps and valleys) formation under non-uniform, periodical illumination [22] and flattening of the recorded gratings under uniform irradiation [9] to our experiments on surface smoothing. The rates of flattening and the diffusion coefficient depend on the light intensity and local surface curvature similarly to the recording process. These follow from equation written for a periodical distribution of the excitation (9): ðxÞ þ σ ðxÞΔω ðxÞ; μ k ðxÞ ¼ μ 0k þ qk IðxÞ þ K ðxÞγω
k ¼ P; C:
ð1Þ
Here k denotes pnictide (P) or chalcogenide (C) atom, μ0k is the bulk chemical potential of the atoms without illumination; the term qkI gives the increase of the chemical potential due to deformation of chemical bonds around the electron-hole pair or defect (in a specific volume qk) gives an account for capillary excited by light with intensity I, Kγ ω forces caused by the curvature of the surface profile, γ is the surface is the atomic volume of P and C tension; and σ is the built in stress, ω atoms. Of course, in the presented experiments besides some non-uniformity of the excitation of the chalcogenide layer due to the distribution of sizes and fill factor of gold nanoparticles or holes and in appropriate distribution of the plasmon fields, the excitation can be considered as uniform in comparison with periodical grating. So the driving force ðxÞ and of the flattering relates first to all of the terms K ðxÞγω ðxÞ in Eq. (1), and the rate, as well as the final value of σ ðxÞΔω smoothening is increased in the localized plasmon field, since the wavelength and intensity (I) of the illumination influence both the processes. The SPR in investigated composites can be used for measuring the refractive index changes of the chalcogenide glasses as a sensing medium [23]. The same can be concluded from our first modeling of such structures [24].
5. Conclusions Conclusion can be made from the above analyzed experimental results on the change of the optical transmission, on the local volume change and on the related effect of the surface roughness change, that the plasmon fields, generated during the illumination of the ChG/GNP or ChG/GNH composite structures enhance the structural transformation processes. The enhancement relates to the first basic stage of the photo-induced transformations i.e. generation of electron-hole pairs and the subsequent changes in defects and atomic displacements. Besides the enhancement of changes of optical parameters the plasmon fields enhance mass transport processes in illuminated chalcogenides/gold nanocomposite, which can be applied for more efficient surface modification in such structures. Further model calculations are under development which will support the selection of the best GNP/ChG nanocomposites, for example for the recording
Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038
S. Charnovych et al. / Journal of Non-Crystalline Solids xxx (2012) xxx–xxx
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Please cite this article as: S. Charnovych, et al., Plasmon assisted photoinduced surface changes in amorphous chalcogenide layer, J. Non-Cryst. Solids (2012), http://dx.doi.org/10.1016/j.jnoncrysol.2012.11.038