- Email: [email protected]
Phenomenological understanding of dewetting and embedding of noble metal nanoparticles in thin films induced by ion irradiation
Materials Chemistry and Physics 147 (2014) 920e924
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Phenomenological understanding of dewetting and embedding of noble metal nanoparticles in thin films induced by ion irradiation Jai Prakash a, b, *, A. Tripathi c, Sanjeev Gautam d, K.H. Chae d, Jonghan Song d, V. Rigato e, Jalaj Tripathi a, K. Asokan c a
Department of Chemistry, MMH College (Ch. Charan Singh University Meerut), Ghaiziabad 201001, India Chemical Physics of Materials, Universit e Libre de Bruxelles, Campus de la Plaine, CP 243, B-1050 Bruxelles, Belgium Inter University Accelerator Centre, Aruna Asif Ali Marg, New Delhi 110067, India d Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136e791, Republic of Korea e INFN Laboratori Nazionali di Legnaro, Via Romea. 4, 35020 Legnaro, Padova, Italy b c
h i g h l i g h t s Phenomenological interpretation of dewetting and embedding of metal NPs in thin film. Exploring fundamental thermodynamic principles under influence of ion irradiation. Ion induced surface/interface microstructural changes using SEM/X-TEM. Ion induced sputtering, thermal spike induced local melting. Thermodynamic driving forces relate to surface and interfacial energies.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2013 Received in revised form 12 May 2014 Accepted 15 June 2014 Available online 3 July 2014
The present experimental work provides the phenomenological approach to understand the dewetting in thin noble metal films with subsequent formation of nanoparticles (NPs) and embedding of NPs induced by ion irradiation. Au/polyethyleneterepthlate (PET) bilayers were irradiated with 150 keV Ar ions at varying fluences and were studied using scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (X-TEM). Thin Au film begins to dewet from the substrate after irradiation and subsequent irradiation results in spherical nanoparticles on the surface that at a fluence of 5 1016 ions/cm2 become embedded into the substrate. In addition to dewetting in thin films, synthesis and embedding of metal NPs by ion irradiation, the present article explores fundamental thermodynamic principles that govern these events systematically under the effect of irradiation. The results are explained on the basis of ion induced sputtering, thermal spike inducing local melting and of thermodynamic driving forces by minimization of the system free energy where contributions of surface and interfacial energies are considered with subsequent ion induced viscous flow in substrate. © 2014 Elsevier B.V. All rights reserved.
Keywords: Thin films Irradiation effects Nanostructures Interface Electron microscopy (SEM and TEM) Thermodynamic properties
1. Introduction Ion beam irradiation has become a unique tool for fabrication of nanostructures and nano-patterning in thin metal films [1e7]. The electrical, magnetic and optical properties of metal nanoparticles (NPs) have proven their potential applications in field of
Libre de * Corresponding author. Chemical Physics of Materials, Universite Bruxelles, Campus de la Plaine, CP 243, B-1050 Bruxelles, Belgium. E-mail addresses: [email protected], [email protected], jai.prakash@ ulb.ac.be (J. Prakash). http://dx.doi.org/10.1016/j.matchemphys.2014.06.038 0254-0584/© 2014 Elsevier B.V. All rights reserved.
nanoelectronics, magnetic memory devices and plasmonics [4e8]. Dewetting in thin films has been one of the useful processes for creating NPs on surfaces [9,10]. This phenomenon is a topic of interest in thin film technology [3,4,11,12] and also of both scientific as well as technological importance. Dewetting is generally unwanted in thin film growth, but it offers new possibilities for the generation of self organized structures on a sub-micron or even nm-scale [9,10,13]. It has been observed that ion induced dewetting of thin metal films deposited on a substrate leads to the formation of NPs [3,4], For example: Pt NPs were produced by 800 keV ion irradiation induced dewetting of thin Pt films on SiO2 substrate [3,12]. The
J. Prakash et al. / Materials Chemistry and Physics 147 (2014) 920e924
behavior of the NPs on the surface is important in fields of nanotechnology such as nanoelectronics, nanomagnetics and optoelectronics [3,4,11,12,14]. Metal NPs are thermodynamically unstable on the substrate surface due to their high surface energy and can either wet on the substrate or embed into the substrate leading to the formation of nanocomposites [15e18]. Zimmermann et al. [15] have demonstrated that burrowing of Co NPs into Cu and Ag surfaces take place when deposited at 600 K but, at room temperature, burrowing does not take place. It has been explained that embedding is driven by extremely large capillary forces on the particle and it occurs in the systems when NPs have a higher surface energies than the substrates [15,16]. Hu et al. [16] have observed the embedding of pre-prepared Pt NPs into SiO2 substrate under the 800 keV Kr ion irradiation and explained that embedding of NPs occurs because of the capillary driving forces (high surface energy of the Pt NPs than the Pt/SiO2 interface energy) and ion induced viscous flow in amorphous SiO2. Similarly, Klimmer et al. [17] have shown embedding of supported Au NPs in SiO2 under the influence of 200 keV Ar and Xe ion irradiation using cross-sectional TEM and explained these results theoretically taking thermodynamic driving forces and sputtering into account. Embedding of NPs into substrates has been shown useful to probe glass transition temperature of the polymer substrates [18e21] and to study ion induced viscous flow in amorphous substrates [16,17], which are also of fundamental and technological interest. So far, several studies on embedding of NPs have been carried out using pre-prepared NPs on a variety of substrates [3,12,15,18e23]. Satpati et al. [23] have studied the ion induced embedding and nanoscale mixing for Au nanoislands and thin Au films on silicon substrate and found that embedding as well as mixing occurred in case of nanoislands. This has been explained as the combined effect of capillary driving force, ion-induced viscous flow and thermal spike. Several studies have been carried out discretely on ion induced dewetting [3,12,24,25], nano-patterning of thin films [5,6] and embedding of NPs [16,17]. The present paper deals with the phenomenological understanding of the processed involved in ion irradiation of thin metal films on polymer substrate and subsequent formation of metal NPs on the surface followed by embedding of NPs. These events follow a systematic sequence and there is the need to understand the phenomenology. Recently, we have experimentally demonstrated the synthesis of Au NPs at the surface and embedded in carboneous matrix by 150 keV Ar ion induced dewetting and nanopatterning [4]. For better understanding of the involved processes that drive the physical evolution during modifications, growth of film/nanostructures at interface and more the embedded nanostructures, combined analyses of surface and interface microstructures using scanning electron microscopy (SEM) ad cross sectional transmission electron microscopy (X-TEM), have been carried out and discussed. Here, we propose an interpretation of the whole phenomenology in terms of sputtering, thermal spike inducing local melting and of thermodynamic driving forces by minimization of the system free energy where contributions of surface and interfacial energies are considered with subsequent ion induced viscous flow in substrate. 2. Experimental details Au/PET bilayer samples were prepared by depositing thin Au film (thickness 15 nm) on PET substrates using electron beam evaporation technique in ultra-high vacuum chamber at base pressure of 105 Pa. The deposition rate was 0.3e0.5 Å s1 and film thickness was controlled using a quartz crystal thickness monitor. PET Substrates of thickness of about 350 mm used in the experiment were semi crystalline and opaque in nature with smooth surface (as received from Goodfellow Ltd, Cambridge, England). Ion beam
921
irradiation was performed with 150 keV Ar ions at the fluences varying from 5 1015 to 5 1016 ions/cm2. The nuclear energy loss of the ions at the Au/PET interface is̴ 90 eV Å1 as calculated by the SRIM (stopping and range of ions in matter) code. More experimental details and ion irradiation parameters were reported elsewhere [4]. X-TEM was carried out on pristine and irradiated samples at 200 kV using FEI, Tecnai F20 G2 TEM. 3. Results and discussion Fig. 1 shows cross sectional TEM images of unirradiated thin Au film (with EDAX analyses) and irradiated thin Au film on PET with 150 keV Ar ions at fluences of 5 1015, 1 1016, and 5 1016 ions/ cm2. From the cross-sectional TEM images, it is revealed that the unirradiated Au film is continuous while after irradiation, Au film begins to dewet from the substrate and at fluence of 5 1015 ions/ cm2 connected patterned nanostructures evolve on the surface as shown in Fig. 1(b). As the fluence increases to 1 1016 ions/cm2, spherical nanoparticles are formed at the surface that with subsequent irradiation become embedded into the substrate at a fluence of 5 1016 ions/cm2 as shown in Fig. 1(c) and (d) respectively. For the better understanding of these events with respect to the surface and interface microstructural changes, a comparable study has been made on the basis of surface and interface analysis studied with SEM and X-TEM respectively as shown in Fig. S1. Dewetting of Au film from polymer surface can also be understood in terms of the contact angle between the nanostructures and the substrate. It is known that if the contact angle (qcont) is less than 90 , the nanostructure will show wetting behavior while nonwetting or dewetting is found when qcont > 90 [26]. X-TEM images of the sample irradiated at 5 1016 ions/cm2 [inset of Fig. 1(b) right side] show that the estimated contact angle of the nanostructures to the substrate is (qcont) > 90 which confirms the dewetting of thin Au film on PET. Dewetting takes place by the creation of molten zones through the formation of craters due to thermal spike induced by Ar ions in thin Au film. Induced thermal spike is produced due to collision cascades, results in melting of Au film, and craters are formed as a result of the sputtered outflow of the Au atoms from the hot molten zones of the cascade [3,4,24]. The estimated maximum size of the induced molten zone leading to dewetting in Au film by Ar ion impact was found to be z 3 nm [4] using the following relation [3,4] :- r m ¼ (E d/pn0 3m)1/2, where, E0d is the deposited energy per unit length by ion along the ion path, n0 is the atomic density of metal and 3m (z3kTm) is the average energy of the atom at the melting temperature Tm. This result is consisted with dewetting of thin Pt films on silica, as a result of molten zone due to thermal spike through the formation of craters and holes [27,28] in the Pt film induced by keV ion irradiation [3,12,29]. During thermal spike, the higher surface energy of metal with respect to the substrate promotes the outflow of metal atoms from the molten zones and helps in dewetting from the substrate [12]. Similarly, in the present case, the surface energy of Au (for bulk Au is gAu ¼ 1.2e1.4 J m2) [23,30] is higher than that of the substrate (gPET ¼ 0.042 J m2) [31] thus suggesting dewetting during ion irradiation [12]. On increasing fluence to 1 1016 ions/ cm2, dewetting and sputtering of thin Au film leads to the formation of spherical Au NPs [(45 ± 20 nm) as estimated from SEM image (Fig. S1)]. Irradiation of these NPs leads to the embedding into the substrates [4]. Further irradiation of NPs promotes the formation of satellites i.e smaller NPs around the original NPs as similar as reported by Rizza et al. [2] in case of embedded Au NPs in amorphous SiO2. Due to the significant recoil implantations with Au atoms and cascade mixing at NP/polymer interface [4], smaller NPs precipitate into the matrix as can be observed in X-TEM image in Fig. 1(d). Similar satellite formation has been reported in case of
922
J. Prakash et al. / Materials Chemistry and Physics 147 (2014) 920e924
Fig. 1. X-TEM images of Au/PET systems (a) pristine with EDAX spectrum and after irradiation at fluence (b) 5 1015, (c) 1 1016, and (d) 5 1016 ions/cm2. [Figures in partial have been reproduced from the source (Ref.4): Jai Prakash et al., Synthesis of Au nanoparticles at the surface and embedded in carbonaceous matrix by 150 keV Ar ion irradiation, J. Phys. D: Appl. Phys 44, 125302 (2011) with permission from IOP publishing Ltd., Bristol].
Pt nanoparticles embedded into SiO2 substrate under the effect of heavy ion irradiation due to recoil implantation [16]. As discussed in the previous report [4], the local melting induced by thermal spike and crater formation are the primary mechanisms for the dewetting of thin Au film from the polymer substrate whereas, sputtering, interface mixing and polymer decomposition are responsible for NPs formation at the surface and embedded nanostructures. In addition to dewetting in thin films,
synthesis and embedding of metal NPs by ion irradiation engender a unique ion induced process in thin films exploring fundamental thermodynamic principles that govern NPs embedding into the substrates under the effect of irradiation. Because of the higher surface energies of NPs and thermodynamic instability on dielectric surfaces, the system tends to reduce the surface energy introducing a thermodynamic driving force for embedding of NPs [16]. It has been shown that embedding of NPs takes place when surface
J. Prakash et al. / Materials Chemistry and Physics 147 (2014) 920e924
energy of NPs is greater than the interfacial energy between NPs and substrate [16]. The interfacial energy between NPs and substrate can be calculated as follows [16]: gNP/S ¼ gSgNP. cosqcont, where, qcon is the contact angle between the NP and substrate, gNP is surface energy of NP, gS is the surface energy of the substrate and gNP/S is the interfacial energy. Similarly, Kovacs et al. [32,33] have shown that the complete embedding of metal NP into polymer is expected if gNP > gpolymer þ gNP/polymer [18]. In the present case, the interfacial energy between Au NP and PET was calculated using the expression [16]: gAuNP/PET ¼ gPETgAuNP. cosqcont, where, gAuNP/PET is the interfacial energy between Au NPs and Polymer, gPET is the surface energy of the PET and gAu NP is the surface energy of Au NP, qcont average contact angle of Au NPs with the PET surface. For PET, gPET ¼ 0.042 J m2 [31]. The surface energy of free metal NPs is found to be greater than that of bulk metal film and for Au NPs gAu 2 NP ¼ 8.78 J m , taken for NPs size 10e25 nm [18,34]. The estimated average contact angle between Au NPs and PET is qcont ~130 ±5 [average contact angles of more than 15 NPs as shown in X-TEM Fig. 1(c). Most of the particles are on the surface and some are partially embedded]. The estimated interfacial energy gAu NP/PET is 5.68 J m2 which is less than the reported surface energy of Au NP (gAu NP ¼ 8.78 J m2). This calculated interfacial energy (gAu NP/ 2 PET ¼ 5.68 J m ) is comparable to the reported values of 6.3, 2 8.7 J m for Au NPs and 6 J m2 for Pd NPs with polymers [18]. Similarly, embedding of Au NPs in polystyrene (PS) substrate has been observed and interfacial energy has been estimated theoretically as 8.7 J m2 [18]. The embedding of Pt NPs in SiO2 is observed because of higher surface energy of Pt NPs as compared to Pt/SiO2 interface energy, but embedding is not observed in case of Al2O3 because interfacial energy of Pt/Al2O3 is greater than the surface energy of Pt NPs [16]. This indicates that thermodynamic driving force favors the embedding of Au NPs in the PET under the influence of ion irradiation. The free energy of embedded NP is not the same as but lower than that of the exposed NP at substrate surface [16,17] and it should be substituted by the interface energy ginterface of particle and the surrounding material [17]. After complete embedding of the NP into the substrate, increasing ion fluence does not result in further embedding of the NP. However further increase of ion fluence promotes the formation of smaller NPs around the embedded NPs [2,17] and elongation of NPs [16] as evidently observed in X-TEM images of Fig. 1(d). As discussed above, irradiation induced embedding of metal NPs in SiO2 has been reported and explained by means of thermodynamic driving (capillary) forces and viscous flow [16,17,23]. Hu et al. [16] have reported the embedding of Pt NPs in SiO2 due to capillary driving forces and ion induced viscous flow of amorphous SiO2 explaining that ion induced viscosity arises from localized defects rather than from the creation of molten zone. On the other hand, Satpati et al. [23] have demonstrated that 1.5 MeV Au ion induced embedding of Au nanoislands in amorphous SiO2 takes place due to ion induced capillary force, viscous flow and thermal spike. Ion induced viscous flow is the ion induced stress relaxation in the locally mesoscopic regions, centered around individual ion tracks [35,36]. In the present case, the PET substrate is a semi-crystalline polymer [37]. However, it has been demonstrated that due to nuclear energy deposition through cascade collision along the ion track, nonthermodynamic relaxation processes and densification due to dehydrogenation occur creating amorphous carbon rich material [38,39]. The Au NPs thus, embed into carbon rich polymer surface by the combined effect of thermodynamic capillary driving forces, ion-induced cascade/interface mixing and ion-induced viscous flow of polymer. Fig. 2 shows the schematic representation of the ion induced dewetting and nanopatterning of thin metal film on polymer substrates followed by embedding of metal NPs into the substrate.
923
Fig. 2. A schematic representation of ion induced synthesis and embedding of metal NPs in polymer substrate by ion beam irradiation of metal/polymer bilayer systems. Various ion induced phenomenon leading to the dewetting, nanostructuring, and embedding of NPs have been shown with increasing ion fluences. In addition, thermodynamically driving forces related to surface and interfaces energies of metal, metal NPs and polymer respectively, governing dewetting and embedding of NPs have also been shown.
Schematically, we suggest how ion induced dewetting of thin metal film leading to NPs formation and embedding, occurs along with the thermodynamic principles that govern dewetting and embedding phenomena. When metal/polymer system is bombarded with ion beam, ion induced phenomena such as cascade collision, thermal spike leading to formation of molten zones and sputtering of the thin metal film occur. With increasing ion fluence, dewetting of the film and sputtering leads to the formation of partially connected nanostructures where higher surface energy of the metal favors the phenomenon. Additional irradiation induces, sputtering, interface mixing leading to the formation of spherical metal NPs at the surface and embedded NPs into the matrix accompanied by ion induced viscous flow and thermodynamic driving forces. Eventually, embedded NPs surrounded with smaller NPs are formed into ion modified substrate. 4. Conclusion The present work provides a phenomenological interpretation of the processes involved in ion beam irradiation of thin metal films on polymer substrates including dewetting of thin film and subsequent formation of spherical nanoparticles that at a proper fluence eventually become embedded into the substrate. Ion beam irradiation of thin Au film on PET substrates by 150 keV Ar ions was studied. Results are explained by dewetting and sputtering of Au film through molten zones due to ion induced thermal spike, ion induced viscous flow of polymer, and thermodynamic capillary driving forces related to the surface and interface energies of the Au NPs and polymer. Acknowledgment Author (JP) would like to acknowledge the CSIR, New Delhi for providing Senior Research Fellowship (CSIR-SRF-2011). Authors also like to thank Dr. D. Kanjilal and Dr. Pravin Kumar for providing LEIBF for ion beam irradiation. JP would like to thank Dr. D. K.
924
J. Prakash et al. / Materials Chemistry and Physics 147 (2014) 920e924
Avasthi, for scientific discussion. Authors are thankful to Dr. F. Singh, Mr Saif, Mrs. Sulania, Dr. GBVS Lakshmi, Ms. Srashti, and Mr. Udai for fruitful discussion and their help during the experiment and characterizations at IUAC, New Delhi.
[18]
[19]
Appendix A. Supplementary data
[20]
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2014.06.038.
[21]
References [1] K.H. Heining, T. Muller, B. Schmidt, M. Strobel, W. Moller, Interfaces under ion irradiation: growth and taming of nanostructures, Appl. Phys. A 77 (2003) 17e25. [2] G. Rizza, H. Cheverry, T. Gacoin, A. Lamasson, S. Henry, Ion beam irradiation of embedded nanoparticles: toward an in situ control of size and spatial distribution, J. Appl. Phys. 101 (2007) 14321. [3] X. Hu, G.D. Cahill, R.S. Averback, Nanoscale pattern formation in Pt thin films due to ion-beam-induced dewetting, Appl. Phys. Lett. 76 (2000) 3215e3217. [4] J. Prakash, A. Tripathi, V. Rigato, J.C. Pivin, J. Tripathi, K.H. Chae, S. Gautam, P. Kumar, K. Asokan, D.K. Avasthi, Synthesis of Au nanoparticles at the surface and embedded in carbonceeous matrix by 150 keV Ar ion irradiaion, J. Phys. D Appl. Phys. 44 (2011) 125302e125309. [5] Jai Prakash, Tripathi, G.B.V.S. Lakshmi, V. Rigato, Jalaj Tripathi, D.K. Avasthi, Synthesis of Ag nanoparticles on polymer surface: 150 keV Ar ion irradiation of Ag-PVC bilayer, Adv. Mat. Lett. 4 (2013) 408e412. [6] U.B. Singh, D.C. Agarwal, S.A. Khan, S. Mohapatra, A. Tripathi, D.K. Avasthi, A study on the formation of Ag nanoparticles on the surface and catcher by ion beam irradiation of Ag thin films, J. Phys. D Appl. Phys. 45 (2012) 445304. [7] Jai Prakash, Tripathi, J.C. Pivin, Jalaj Tripathi, A.K. Chawla, R. Chandra, S.S. Kim, K. Asokan, D.K. Avasthi, Study on synthesis of magnetic nanocomposite (NiTeflon) by swift heavy ion beam mixing, Adv. Mat. Lett. 2 (2011) 71e75. [8] Yasuhiko Kojima, Takahisa Kato, Nanoparticle formation in Au thin films by electron-beam-induced dewetting, Nanotechnology 19 (2008) 255605. [9] S. Amirthapandian, F. Schuchart, D. Garmatter, W. Bolse, In situ investigation of ion-induced dewetting of a thin iron-oxide film on silicon by high resolution scanning electron microscopy, J. Appl. Phys. 112 (2012) 103522e103528. [10] Alexander B. Tesler, Ben M. Maoz, Yishay Feldman, Alexander Vaskevich, Israel Rubinstein, Solid-state thermal dewetting of just-percolated gold films evaporated on glass: development of the morphology and optical properties, J. Phys. Chem. C 117 (2013) 11337e11346. [11] A. Stannard, Dewetting-mediated pattern formation in nanoparticle assemblies, J. Phys. Cond. Matter 23 (2011) 83001. [12] X. Hu, D.G. Cahill, R.S. Averback, R.C. Birtcher, In-situ transmission electron microscopy study of irradiation induced dewetting of ultrathin Pt films, J. Appl. Phys. 93 (2003) 165e169. [13] T. Bolse, H. Paulus, W. Bolse, Swift heavy ion induced dewetting of metal oxide thin films on silicon, Nucl. Inst. Meth. B 245 (2006) 264e268. [14] V. Torrsi, F. Rffino, A. Licciardello, M.G. Grimaldi, G. Marletta, Memory effects in annealed hybrid gold nanoparticles/block copolymer bilayers, Nanoscale Res. Lett. 6 (2011) 167. [15] C.G. Zimmermann, M. Yeadon, K. Nordlund, J.M. Gibson, R.S Averback, Burrowing of Co nanoparticles on clean Cu and Ag surfaces, Phys. Rev. Lett. 83 (1999) 1163e1166. [16] X. Hu, D.G. Cahill, R.S. Averback, Burrowing of Pt nanoparticles into SiO2 during ion-beam irradiation, J. Appl. Phys. 92 (2002) 3995e4000. [17] A. Klimmer, P. Ziemann, J. Biskupek, U. Kaiser, M. Flesch, Size-dependent effect of ion bombardment on Au nanoparticles on top of various substrates:
[22]
[23] [24] [25] [26] [27] [28]
[29]
[30] [31]
[32]
[33]
[34] [35]
[36]
[37] [38]
[39]
thermodynamically dominated capillary forces versus sputtering, Phys. Rev. B 79 (2009) 155427. Ranjan D. Deshmukh, R.J. Composto, Direct observation of nanoparticle embedding into the surface of a polymer melt, Langmuir 23 (2007) 13169e13173. J.H. Teichroeb, J.A. Forrest, Direct imaging of nanoparticle embedding to probe viscoelasticity of polymer surfaces, Phys. Rev. Lett. 91 (2003) 16104. V. Zaporojtchenko, T. Strunskus, J. Erichsen, F. Faupel, Embedding of noble metal nanoclasters into polymers as a potential probe of the surface glass transition, Macromolecules 34 (2001) 1125e1127. V.M. Rudoy, O.V. Dement'eva, I.V. Yaminskii, V.M. Sukhov, M.E. Kartseva, V.A. Ogarev, Metal nanoparticles on polymer surfaces: 1. A new method of determining glass transition temperature of the surface layer, Colloid J. 64 (2002) 746e754. K.R. Shull, A.J. Kellock, Metal particle adsorption and diffusion in a model polymer/metal composite system, J. Polym. Sci. B. Polym. Phys. 33 (1995) 1417e1422. B. Satpati, P.V. Satyam, T. Som, B.N. Dev, Ion-beam-induced embedded nanostructures and nanoscale mixing, J. Appl. Phys. 96 (2004) 5212e5216. Xuan Meng, Tamaki Shibayama, Ruixuan Yu, Shinya Takayanagi, Seiichi Watanabe, J. Mat. Sci. 48 (2013) 920. P. Matheswaran, K.M. Abhirami, B. Gokul, R. Sathyamurthy, Jai Prakash, K. Asokan, D. Kanjilal, Appl. Surf. Sci. 258 (2012) 8558. N. Taranets, Y. Naidich, Wettability of aluminum nitride by molten metals, Powder Metall. Metal Ceram. 35 (1996) 282e285. S.E. Donnelly, R.C. Birtcher, Radiation damage from single heavy ion impacts on metal surfaces, Proc. SPIE 3413 (1998) 174e182. S.E. Donnelly, R.C. Birtcher, K. Nordlund, Single ion induced spike effect on thin metal films:observation and simulations, in: E. Nnystautas (Ed.), Engineering Thin Films and Nanostructures by Ion Beams, Taylor and Francis, New York, 2005, p. 7. X. Hu, D.G. Cahill, R.S. Averback, Dewetting and nanopattern formation of thin Pt films on SiO2 induced by ion beam irradiation, J. Appl. Phys. 89 (2001) 7777e7783. L. Vitos, A.V. Ruban, H.L. Sdriver, J. Kollar, The surface energy of metals, Surf. Sci. 411 (1998) 186e202. R.L.W. Smithson, D.J. Mcclure, D.F. Evans, Effects of polymer substrate surface energy on nucleation and growth of evaporated gold films, Thin Solid Films 307 (1997) 110e112. G.J. Kovacs, P.S. Vincett, Formation and thermodynamic stability of a novel class of useful materials:Close-packed monolayers of submicron monodisperse spheres just below a polymer surface, J. Colloidal Interface Sci. 90 (1982) 335e351. G.J. Kovacs, P.S. Vincett, Subsurface particle monolayer and film formation in softenable substrates: techniques and thermodynamic criteria, Thin Solid Films 111 (1984) 65e81. K.K. Nanda, A. Maisels, F.E. Druis, H. Fissan, S. Stappert, Higher surface energy of free nanoparticles, Phys. Rev. Lett. 91 (2003) 106102. M.L. Brongersma, E. Snoeks, A. Polman, Temperature dependence of MeV heavy ion irradiation-induced viscous flow in SiO2, Appl. Phys. Lett. 71 (1997) 1628e1630. S.G. Mayr, Y. Ashkenazy, K. Albe, R.S. Averback, Mechanisms of radiationinduced viscous flow: role of point defects, Phys. Rev. Lett. 90 (2003) 55505. G.W. Ehrenstein, R.P. Theriault, Polymeric Materials: Structures, Properties, Applications, Hanser Verlag, 2001, pp. 67e78. Y.Q. Wang, R.E. Giedd, L.B. Bridwell, Ion induced structures and electrical conduction in implanted polymer films, Nucl. Inst. Meth. B79 (1993) 659e663. L. Calcagno, G. Foti, Density enhancement in ion implanted polymers, Nucl. Inst. Meth. B19/20 (1987) 895e898.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Phenomenological understanding of dewetting and embedding of noble metal nanoparticles in thin films induced by ion irradiation Jai Prakash a, b, *, A. Tripathi c, Sanjeev Gautam d, K.H. Chae d, Jonghan Song d, V. Rigato e, Jalaj Tripathi a, K. Asokan c a
Department of Chemistry, MMH College (Ch. Charan Singh University Meerut), Ghaiziabad 201001, India Chemical Physics of Materials, Universit e Libre de Bruxelles, Campus de la Plaine, CP 243, B-1050 Bruxelles, Belgium Inter University Accelerator Centre, Aruna Asif Ali Marg, New Delhi 110067, India d Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136e791, Republic of Korea e INFN Laboratori Nazionali di Legnaro, Via Romea. 4, 35020 Legnaro, Padova, Italy b c
h i g h l i g h t s Phenomenological interpretation of dewetting and embedding of metal NPs in thin film. Exploring fundamental thermodynamic principles under influence of ion irradiation. Ion induced surface/interface microstructural changes using SEM/X-TEM. Ion induced sputtering, thermal spike induced local melting. Thermodynamic driving forces relate to surface and interfacial energies.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2013 Received in revised form 12 May 2014 Accepted 15 June 2014 Available online 3 July 2014
The present experimental work provides the phenomenological approach to understand the dewetting in thin noble metal films with subsequent formation of nanoparticles (NPs) and embedding of NPs induced by ion irradiation. Au/polyethyleneterepthlate (PET) bilayers were irradiated with 150 keV Ar ions at varying fluences and were studied using scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (X-TEM). Thin Au film begins to dewet from the substrate after irradiation and subsequent irradiation results in spherical nanoparticles on the surface that at a fluence of 5 1016 ions/cm2 become embedded into the substrate. In addition to dewetting in thin films, synthesis and embedding of metal NPs by ion irradiation, the present article explores fundamental thermodynamic principles that govern these events systematically under the effect of irradiation. The results are explained on the basis of ion induced sputtering, thermal spike inducing local melting and of thermodynamic driving forces by minimization of the system free energy where contributions of surface and interfacial energies are considered with subsequent ion induced viscous flow in substrate. © 2014 Elsevier B.V. All rights reserved.
Keywords: Thin films Irradiation effects Nanostructures Interface Electron microscopy (SEM and TEM) Thermodynamic properties
1. Introduction Ion beam irradiation has become a unique tool for fabrication of nanostructures and nano-patterning in thin metal films [1e7]. The electrical, magnetic and optical properties of metal nanoparticles (NPs) have proven their potential applications in field of
Libre de * Corresponding author. Chemical Physics of Materials, Universite Bruxelles, Campus de la Plaine, CP 243, B-1050 Bruxelles, Belgium. E-mail addresses: [email protected], [email protected], jai.prakash@ ulb.ac.be (J. Prakash). http://dx.doi.org/10.1016/j.matchemphys.2014.06.038 0254-0584/© 2014 Elsevier B.V. All rights reserved.
nanoelectronics, magnetic memory devices and plasmonics [4e8]. Dewetting in thin films has been one of the useful processes for creating NPs on surfaces [9,10]. This phenomenon is a topic of interest in thin film technology [3,4,11,12] and also of both scientific as well as technological importance. Dewetting is generally unwanted in thin film growth, but it offers new possibilities for the generation of self organized structures on a sub-micron or even nm-scale [9,10,13]. It has been observed that ion induced dewetting of thin metal films deposited on a substrate leads to the formation of NPs [3,4], For example: Pt NPs were produced by 800 keV ion irradiation induced dewetting of thin Pt films on SiO2 substrate [3,12]. The
J. Prakash et al. / Materials Chemistry and Physics 147 (2014) 920e924
behavior of the NPs on the surface is important in fields of nanotechnology such as nanoelectronics, nanomagnetics and optoelectronics [3,4,11,12,14]. Metal NPs are thermodynamically unstable on the substrate surface due to their high surface energy and can either wet on the substrate or embed into the substrate leading to the formation of nanocomposites [15e18]. Zimmermann et al. [15] have demonstrated that burrowing of Co NPs into Cu and Ag surfaces take place when deposited at 600 K but, at room temperature, burrowing does not take place. It has been explained that embedding is driven by extremely large capillary forces on the particle and it occurs in the systems when NPs have a higher surface energies than the substrates [15,16]. Hu et al. [16] have observed the embedding of pre-prepared Pt NPs into SiO2 substrate under the 800 keV Kr ion irradiation and explained that embedding of NPs occurs because of the capillary driving forces (high surface energy of the Pt NPs than the Pt/SiO2 interface energy) and ion induced viscous flow in amorphous SiO2. Similarly, Klimmer et al. [17] have shown embedding of supported Au NPs in SiO2 under the influence of 200 keV Ar and Xe ion irradiation using cross-sectional TEM and explained these results theoretically taking thermodynamic driving forces and sputtering into account. Embedding of NPs into substrates has been shown useful to probe glass transition temperature of the polymer substrates [18e21] and to study ion induced viscous flow in amorphous substrates [16,17], which are also of fundamental and technological interest. So far, several studies on embedding of NPs have been carried out using pre-prepared NPs on a variety of substrates [3,12,15,18e23]. Satpati et al. [23] have studied the ion induced embedding and nanoscale mixing for Au nanoislands and thin Au films on silicon substrate and found that embedding as well as mixing occurred in case of nanoislands. This has been explained as the combined effect of capillary driving force, ion-induced viscous flow and thermal spike. Several studies have been carried out discretely on ion induced dewetting [3,12,24,25], nano-patterning of thin films [5,6] and embedding of NPs [16,17]. The present paper deals with the phenomenological understanding of the processed involved in ion irradiation of thin metal films on polymer substrate and subsequent formation of metal NPs on the surface followed by embedding of NPs. These events follow a systematic sequence and there is the need to understand the phenomenology. Recently, we have experimentally demonstrated the synthesis of Au NPs at the surface and embedded in carboneous matrix by 150 keV Ar ion induced dewetting and nanopatterning [4]. For better understanding of the involved processes that drive the physical evolution during modifications, growth of film/nanostructures at interface and more the embedded nanostructures, combined analyses of surface and interface microstructures using scanning electron microscopy (SEM) ad cross sectional transmission electron microscopy (X-TEM), have been carried out and discussed. Here, we propose an interpretation of the whole phenomenology in terms of sputtering, thermal spike inducing local melting and of thermodynamic driving forces by minimization of the system free energy where contributions of surface and interfacial energies are considered with subsequent ion induced viscous flow in substrate. 2. Experimental details Au/PET bilayer samples were prepared by depositing thin Au film (thickness 15 nm) on PET substrates using electron beam evaporation technique in ultra-high vacuum chamber at base pressure of 105 Pa. The deposition rate was 0.3e0.5 Å s1 and film thickness was controlled using a quartz crystal thickness monitor. PET Substrates of thickness of about 350 mm used in the experiment were semi crystalline and opaque in nature with smooth surface (as received from Goodfellow Ltd, Cambridge, England). Ion beam
921
irradiation was performed with 150 keV Ar ions at the fluences varying from 5 1015 to 5 1016 ions/cm2. The nuclear energy loss of the ions at the Au/PET interface is̴ 90 eV Å1 as calculated by the SRIM (stopping and range of ions in matter) code. More experimental details and ion irradiation parameters were reported elsewhere [4]. X-TEM was carried out on pristine and irradiated samples at 200 kV using FEI, Tecnai F20 G2 TEM. 3. Results and discussion Fig. 1 shows cross sectional TEM images of unirradiated thin Au film (with EDAX analyses) and irradiated thin Au film on PET with 150 keV Ar ions at fluences of 5 1015, 1 1016, and 5 1016 ions/ cm2. From the cross-sectional TEM images, it is revealed that the unirradiated Au film is continuous while after irradiation, Au film begins to dewet from the substrate and at fluence of 5 1015 ions/ cm2 connected patterned nanostructures evolve on the surface as shown in Fig. 1(b). As the fluence increases to 1 1016 ions/cm2, spherical nanoparticles are formed at the surface that with subsequent irradiation become embedded into the substrate at a fluence of 5 1016 ions/cm2 as shown in Fig. 1(c) and (d) respectively. For the better understanding of these events with respect to the surface and interface microstructural changes, a comparable study has been made on the basis of surface and interface analysis studied with SEM and X-TEM respectively as shown in Fig. S1. Dewetting of Au film from polymer surface can also be understood in terms of the contact angle between the nanostructures and the substrate. It is known that if the contact angle (qcont) is less than 90 , the nanostructure will show wetting behavior while nonwetting or dewetting is found when qcont > 90 [26]. X-TEM images of the sample irradiated at 5 1016 ions/cm2 [inset of Fig. 1(b) right side] show that the estimated contact angle of the nanostructures to the substrate is (qcont) > 90 which confirms the dewetting of thin Au film on PET. Dewetting takes place by the creation of molten zones through the formation of craters due to thermal spike induced by Ar ions in thin Au film. Induced thermal spike is produced due to collision cascades, results in melting of Au film, and craters are formed as a result of the sputtered outflow of the Au atoms from the hot molten zones of the cascade [3,4,24]. The estimated maximum size of the induced molten zone leading to dewetting in Au film by Ar ion impact was found to be z 3 nm [4] using the following relation [3,4] :- r m ¼ (E d/pn0 3m)1/2, where, E0d is the deposited energy per unit length by ion along the ion path, n0 is the atomic density of metal and 3m (z3kTm) is the average energy of the atom at the melting temperature Tm. This result is consisted with dewetting of thin Pt films on silica, as a result of molten zone due to thermal spike through the formation of craters and holes [27,28] in the Pt film induced by keV ion irradiation [3,12,29]. During thermal spike, the higher surface energy of metal with respect to the substrate promotes the outflow of metal atoms from the molten zones and helps in dewetting from the substrate [12]. Similarly, in the present case, the surface energy of Au (for bulk Au is gAu ¼ 1.2e1.4 J m2) [23,30] is higher than that of the substrate (gPET ¼ 0.042 J m2) [31] thus suggesting dewetting during ion irradiation [12]. On increasing fluence to 1 1016 ions/ cm2, dewetting and sputtering of thin Au film leads to the formation of spherical Au NPs [(45 ± 20 nm) as estimated from SEM image (Fig. S1)]. Irradiation of these NPs leads to the embedding into the substrates [4]. Further irradiation of NPs promotes the formation of satellites i.e smaller NPs around the original NPs as similar as reported by Rizza et al. [2] in case of embedded Au NPs in amorphous SiO2. Due to the significant recoil implantations with Au atoms and cascade mixing at NP/polymer interface [4], smaller NPs precipitate into the matrix as can be observed in X-TEM image in Fig. 1(d). Similar satellite formation has been reported in case of
922
J. Prakash et al. / Materials Chemistry and Physics 147 (2014) 920e924
Fig. 1. X-TEM images of Au/PET systems (a) pristine with EDAX spectrum and after irradiation at fluence (b) 5 1015, (c) 1 1016, and (d) 5 1016 ions/cm2. [Figures in partial have been reproduced from the source (Ref.4): Jai Prakash et al., Synthesis of Au nanoparticles at the surface and embedded in carbonaceous matrix by 150 keV Ar ion irradiation, J. Phys. D: Appl. Phys 44, 125302 (2011) with permission from IOP publishing Ltd., Bristol].
Pt nanoparticles embedded into SiO2 substrate under the effect of heavy ion irradiation due to recoil implantation [16]. As discussed in the previous report [4], the local melting induced by thermal spike and crater formation are the primary mechanisms for the dewetting of thin Au film from the polymer substrate whereas, sputtering, interface mixing and polymer decomposition are responsible for NPs formation at the surface and embedded nanostructures. In addition to dewetting in thin films,
synthesis and embedding of metal NPs by ion irradiation engender a unique ion induced process in thin films exploring fundamental thermodynamic principles that govern NPs embedding into the substrates under the effect of irradiation. Because of the higher surface energies of NPs and thermodynamic instability on dielectric surfaces, the system tends to reduce the surface energy introducing a thermodynamic driving force for embedding of NPs [16]. It has been shown that embedding of NPs takes place when surface
J. Prakash et al. / Materials Chemistry and Physics 147 (2014) 920e924
energy of NPs is greater than the interfacial energy between NPs and substrate [16]. The interfacial energy between NPs and substrate can be calculated as follows [16]: gNP/S ¼ gSgNP. cosqcont, where, qcon is the contact angle between the NP and substrate, gNP is surface energy of NP, gS is the surface energy of the substrate and gNP/S is the interfacial energy. Similarly, Kovacs et al. [32,33] have shown that the complete embedding of metal NP into polymer is expected if gNP > gpolymer þ gNP/polymer [18]. In the present case, the interfacial energy between Au NP and PET was calculated using the expression [16]: gAuNP/PET ¼ gPETgAuNP. cosqcont, where, gAuNP/PET is the interfacial energy between Au NPs and Polymer, gPET is the surface energy of the PET and gAu NP is the surface energy of Au NP, qcont average contact angle of Au NPs with the PET surface. For PET, gPET ¼ 0.042 J m2 [31]. The surface energy of free metal NPs is found to be greater than that of bulk metal film and for Au NPs gAu 2 NP ¼ 8.78 J m , taken for NPs size 10e25 nm [18,34]. The estimated average contact angle between Au NPs and PET is qcont ~130 ±5 [average contact angles of more than 15 NPs as shown in X-TEM Fig. 1(c). Most of the particles are on the surface and some are partially embedded]. The estimated interfacial energy gAu NP/PET is 5.68 J m2 which is less than the reported surface energy of Au NP (gAu NP ¼ 8.78 J m2). This calculated interfacial energy (gAu NP/ 2 PET ¼ 5.68 J m ) is comparable to the reported values of 6.3, 2 8.7 J m for Au NPs and 6 J m2 for Pd NPs with polymers [18]. Similarly, embedding of Au NPs in polystyrene (PS) substrate has been observed and interfacial energy has been estimated theoretically as 8.7 J m2 [18]. The embedding of Pt NPs in SiO2 is observed because of higher surface energy of Pt NPs as compared to Pt/SiO2 interface energy, but embedding is not observed in case of Al2O3 because interfacial energy of Pt/Al2O3 is greater than the surface energy of Pt NPs [16]. This indicates that thermodynamic driving force favors the embedding of Au NPs in the PET under the influence of ion irradiation. The free energy of embedded NP is not the same as but lower than that of the exposed NP at substrate surface [16,17] and it should be substituted by the interface energy ginterface of particle and the surrounding material [17]. After complete embedding of the NP into the substrate, increasing ion fluence does not result in further embedding of the NP. However further increase of ion fluence promotes the formation of smaller NPs around the embedded NPs [2,17] and elongation of NPs [16] as evidently observed in X-TEM images of Fig. 1(d). As discussed above, irradiation induced embedding of metal NPs in SiO2 has been reported and explained by means of thermodynamic driving (capillary) forces and viscous flow [16,17,23]. Hu et al. [16] have reported the embedding of Pt NPs in SiO2 due to capillary driving forces and ion induced viscous flow of amorphous SiO2 explaining that ion induced viscosity arises from localized defects rather than from the creation of molten zone. On the other hand, Satpati et al. [23] have demonstrated that 1.5 MeV Au ion induced embedding of Au nanoislands in amorphous SiO2 takes place due to ion induced capillary force, viscous flow and thermal spike. Ion induced viscous flow is the ion induced stress relaxation in the locally mesoscopic regions, centered around individual ion tracks [35,36]. In the present case, the PET substrate is a semi-crystalline polymer [37]. However, it has been demonstrated that due to nuclear energy deposition through cascade collision along the ion track, nonthermodynamic relaxation processes and densification due to dehydrogenation occur creating amorphous carbon rich material [38,39]. The Au NPs thus, embed into carbon rich polymer surface by the combined effect of thermodynamic capillary driving forces, ion-induced cascade/interface mixing and ion-induced viscous flow of polymer. Fig. 2 shows the schematic representation of the ion induced dewetting and nanopatterning of thin metal film on polymer substrates followed by embedding of metal NPs into the substrate.
923
Fig. 2. A schematic representation of ion induced synthesis and embedding of metal NPs in polymer substrate by ion beam irradiation of metal/polymer bilayer systems. Various ion induced phenomenon leading to the dewetting, nanostructuring, and embedding of NPs have been shown with increasing ion fluences. In addition, thermodynamically driving forces related to surface and interfaces energies of metal, metal NPs and polymer respectively, governing dewetting and embedding of NPs have also been shown.
Schematically, we suggest how ion induced dewetting of thin metal film leading to NPs formation and embedding, occurs along with the thermodynamic principles that govern dewetting and embedding phenomena. When metal/polymer system is bombarded with ion beam, ion induced phenomena such as cascade collision, thermal spike leading to formation of molten zones and sputtering of the thin metal film occur. With increasing ion fluence, dewetting of the film and sputtering leads to the formation of partially connected nanostructures where higher surface energy of the metal favors the phenomenon. Additional irradiation induces, sputtering, interface mixing leading to the formation of spherical metal NPs at the surface and embedded NPs into the matrix accompanied by ion induced viscous flow and thermodynamic driving forces. Eventually, embedded NPs surrounded with smaller NPs are formed into ion modified substrate. 4. Conclusion The present work provides a phenomenological interpretation of the processes involved in ion beam irradiation of thin metal films on polymer substrates including dewetting of thin film and subsequent formation of spherical nanoparticles that at a proper fluence eventually become embedded into the substrate. Ion beam irradiation of thin Au film on PET substrates by 150 keV Ar ions was studied. Results are explained by dewetting and sputtering of Au film through molten zones due to ion induced thermal spike, ion induced viscous flow of polymer, and thermodynamic capillary driving forces related to the surface and interface energies of the Au NPs and polymer. Acknowledgment Author (JP) would like to acknowledge the CSIR, New Delhi for providing Senior Research Fellowship (CSIR-SRF-2011). Authors also like to thank Dr. D. Kanjilal and Dr. Pravin Kumar for providing LEIBF for ion beam irradiation. JP would like to thank Dr. D. K.
924
J. Prakash et al. / Materials Chemistry and Physics 147 (2014) 920e924
Avasthi, for scientific discussion. Authors are thankful to Dr. F. Singh, Mr Saif, Mrs. Sulania, Dr. GBVS Lakshmi, Ms. Srashti, and Mr. Udai for fruitful discussion and their help during the experiment and characterizations at IUAC, New Delhi.
[18]
[19]
Appendix A. Supplementary data
[20]
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2014.06.038.
[21]
References [1] K.H. Heining, T. Muller, B. Schmidt, M. Strobel, W. Moller, Interfaces under ion irradiation: growth and taming of nanostructures, Appl. Phys. A 77 (2003) 17e25. [2] G. Rizza, H. Cheverry, T. Gacoin, A. Lamasson, S. Henry, Ion beam irradiation of embedded nanoparticles: toward an in situ control of size and spatial distribution, J. Appl. Phys. 101 (2007) 14321. [3] X. Hu, G.D. Cahill, R.S. Averback, Nanoscale pattern formation in Pt thin films due to ion-beam-induced dewetting, Appl. Phys. Lett. 76 (2000) 3215e3217. [4] J. Prakash, A. Tripathi, V. Rigato, J.C. Pivin, J. Tripathi, K.H. Chae, S. Gautam, P. Kumar, K. Asokan, D.K. Avasthi, Synthesis of Au nanoparticles at the surface and embedded in carbonceeous matrix by 150 keV Ar ion irradiaion, J. Phys. D Appl. Phys. 44 (2011) 125302e125309. [5] Jai Prakash, Tripathi, G.B.V.S. Lakshmi, V. Rigato, Jalaj Tripathi, D.K. Avasthi, Synthesis of Ag nanoparticles on polymer surface: 150 keV Ar ion irradiation of Ag-PVC bilayer, Adv. Mat. Lett. 4 (2013) 408e412. [6] U.B. Singh, D.C. Agarwal, S.A. Khan, S. Mohapatra, A. Tripathi, D.K. Avasthi, A study on the formation of Ag nanoparticles on the surface and catcher by ion beam irradiation of Ag thin films, J. Phys. D Appl. Phys. 45 (2012) 445304. [7] Jai Prakash, Tripathi, J.C. Pivin, Jalaj Tripathi, A.K. Chawla, R. Chandra, S.S. Kim, K. Asokan, D.K. Avasthi, Study on synthesis of magnetic nanocomposite (NiTeflon) by swift heavy ion beam mixing, Adv. Mat. Lett. 2 (2011) 71e75. [8] Yasuhiko Kojima, Takahisa Kato, Nanoparticle formation in Au thin films by electron-beam-induced dewetting, Nanotechnology 19 (2008) 255605. [9] S. Amirthapandian, F. Schuchart, D. Garmatter, W. Bolse, In situ investigation of ion-induced dewetting of a thin iron-oxide film on silicon by high resolution scanning electron microscopy, J. Appl. Phys. 112 (2012) 103522e103528. [10] Alexander B. Tesler, Ben M. Maoz, Yishay Feldman, Alexander Vaskevich, Israel Rubinstein, Solid-state thermal dewetting of just-percolated gold films evaporated on glass: development of the morphology and optical properties, J. Phys. Chem. C 117 (2013) 11337e11346. [11] A. Stannard, Dewetting-mediated pattern formation in nanoparticle assemblies, J. Phys. Cond. Matter 23 (2011) 83001. [12] X. Hu, D.G. Cahill, R.S. Averback, R.C. Birtcher, In-situ transmission electron microscopy study of irradiation induced dewetting of ultrathin Pt films, J. Appl. Phys. 93 (2003) 165e169. [13] T. Bolse, H. Paulus, W. Bolse, Swift heavy ion induced dewetting of metal oxide thin films on silicon, Nucl. Inst. Meth. B 245 (2006) 264e268. [14] V. Torrsi, F. Rffino, A. Licciardello, M.G. Grimaldi, G. Marletta, Memory effects in annealed hybrid gold nanoparticles/block copolymer bilayers, Nanoscale Res. Lett. 6 (2011) 167. [15] C.G. Zimmermann, M. Yeadon, K. Nordlund, J.M. Gibson, R.S Averback, Burrowing of Co nanoparticles on clean Cu and Ag surfaces, Phys. Rev. Lett. 83 (1999) 1163e1166. [16] X. Hu, D.G. Cahill, R.S. Averback, Burrowing of Pt nanoparticles into SiO2 during ion-beam irradiation, J. Appl. Phys. 92 (2002) 3995e4000. [17] A. Klimmer, P. Ziemann, J. Biskupek, U. Kaiser, M. Flesch, Size-dependent effect of ion bombardment on Au nanoparticles on top of various substrates:
[22]
[23] [24] [25] [26] [27] [28]
[29]
[30] [31]
[32]
[33]
[34] [35]
[36]
[37] [38]
[39]
thermodynamically dominated capillary forces versus sputtering, Phys. Rev. B 79 (2009) 155427. Ranjan D. Deshmukh, R.J. Composto, Direct observation of nanoparticle embedding into the surface of a polymer melt, Langmuir 23 (2007) 13169e13173. J.H. Teichroeb, J.A. Forrest, Direct imaging of nanoparticle embedding to probe viscoelasticity of polymer surfaces, Phys. Rev. Lett. 91 (2003) 16104. V. Zaporojtchenko, T. Strunskus, J. Erichsen, F. Faupel, Embedding of noble metal nanoclasters into polymers as a potential probe of the surface glass transition, Macromolecules 34 (2001) 1125e1127. V.M. Rudoy, O.V. Dement'eva, I.V. Yaminskii, V.M. Sukhov, M.E. Kartseva, V.A. Ogarev, Metal nanoparticles on polymer surfaces: 1. A new method of determining glass transition temperature of the surface layer, Colloid J. 64 (2002) 746e754. K.R. Shull, A.J. Kellock, Metal particle adsorption and diffusion in a model polymer/metal composite system, J. Polym. Sci. B. Polym. Phys. 33 (1995) 1417e1422. B. Satpati, P.V. Satyam, T. Som, B.N. Dev, Ion-beam-induced embedded nanostructures and nanoscale mixing, J. Appl. Phys. 96 (2004) 5212e5216. Xuan Meng, Tamaki Shibayama, Ruixuan Yu, Shinya Takayanagi, Seiichi Watanabe, J. Mat. Sci. 48 (2013) 920. P. Matheswaran, K.M. Abhirami, B. Gokul, R. Sathyamurthy, Jai Prakash, K. Asokan, D. Kanjilal, Appl. Surf. Sci. 258 (2012) 8558. N. Taranets, Y. Naidich, Wettability of aluminum nitride by molten metals, Powder Metall. Metal Ceram. 35 (1996) 282e285. S.E. Donnelly, R.C. Birtcher, Radiation damage from single heavy ion impacts on metal surfaces, Proc. SPIE 3413 (1998) 174e182. S.E. Donnelly, R.C. Birtcher, K. Nordlund, Single ion induced spike effect on thin metal films:observation and simulations, in: E. Nnystautas (Ed.), Engineering Thin Films and Nanostructures by Ion Beams, Taylor and Francis, New York, 2005, p. 7. X. Hu, D.G. Cahill, R.S. Averback, Dewetting and nanopattern formation of thin Pt films on SiO2 induced by ion beam irradiation, J. Appl. Phys. 89 (2001) 7777e7783. L. Vitos, A.V. Ruban, H.L. Sdriver, J. Kollar, The surface energy of metals, Surf. Sci. 411 (1998) 186e202. R.L.W. Smithson, D.J. Mcclure, D.F. Evans, Effects of polymer substrate surface energy on nucleation and growth of evaporated gold films, Thin Solid Films 307 (1997) 110e112. G.J. Kovacs, P.S. Vincett, Formation and thermodynamic stability of a novel class of useful materials:Close-packed monolayers of submicron monodisperse spheres just below a polymer surface, J. Colloidal Interface Sci. 90 (1982) 335e351. G.J. Kovacs, P.S. Vincett, Subsurface particle monolayer and film formation in softenable substrates: techniques and thermodynamic criteria, Thin Solid Films 111 (1984) 65e81. K.K. Nanda, A. Maisels, F.E. Druis, H. Fissan, S. Stappert, Higher surface energy of free nanoparticles, Phys. Rev. Lett. 91 (2003) 106102. M.L. Brongersma, E. Snoeks, A. Polman, Temperature dependence of MeV heavy ion irradiation-induced viscous flow in SiO2, Appl. Phys. Lett. 71 (1997) 1628e1630. S.G. Mayr, Y. Ashkenazy, K. Albe, R.S. Averback, Mechanisms of radiationinduced viscous flow: role of point defects, Phys. Rev. Lett. 90 (2003) 55505. G.W. Ehrenstein, R.P. Theriault, Polymeric Materials: Structures, Properties, Applications, Hanser Verlag, 2001, pp. 67e78. Y.Q. Wang, R.E. Giedd, L.B. Bridwell, Ion induced structures and electrical conduction in implanted polymer films, Nucl. Inst. Meth. B79 (1993) 659e663. L. Calcagno, G. Foti, Density enhancement in ion implanted polymers, Nucl. Inst. Meth. B19/20 (1987) 895e898.