Electron beam induced growth of silver nanowhiskers

Madis Umalas a,b, Sergei Vlassov a,b,n, Boris Polyakov c, Leonid M. Dorogin a,b, Rando Saar a,b,Ilmar Kink a, Rünno Lõhmus a,b, Ants Lõhmus a, Alexey E. Romanov a,d

a Institute of Physics, University of Tartu, Ravila 14c, 50412 Tartu, Estoniab Estonian Nanotechnology Competence Center, Ravila 14c, 50412 Tartu, Estoniac Institute of Solid State Physics, University of Latvia, Kengaraga 8, LV-1063, Riga, Latviad ITMO University, Kronverkskiy pr., 49, 197101 Saint Petersburg, Russia

a r t i c l e i n f o

Article history:Received 22 May 2014Received in revised form10 October 2014Accepted 12 October 2014Available online 24 October 2014

Keywords:A1. Growth modelsA2. StressesB1. MetalsNanostructuresNanomaterials

a b s t r a c t

In this paper we report an electron beam induced rapid (up to several tens of nm/s) growth of silvernanowhiskers from silver nanowire networks coated with TiO2 by sol–gel method. Different growthconditions are tested and it is demonstrated that growth is optimal for samples with the film thicknessin the range 50–200 nm and previously annealed at 400 1C for 5–10 min. Growth mechanism isattributed to cooperative effect of several factors including diffusion of Ag into TiO2 matrix duringannealing, electromigration of Ag atoms caused by strong electric field, and presence of mechanicalstresses at interfaces enhanced by thermal expansions due to local heating under e-beam illumination.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Spontaneous growth of one-dimensional (1D) solid structureswith the diameter ranging from several nm to several microns andthe length reaching sometimes several mm known as whiskers, wasoften observed on the metallic coatings with relatively low meltingpoint such as tin, cadmium, zinc in ambient conditions (see, e.g. [1,2]).

Besides the growth in ambient conditions, metal whiskers wereobserved to grow when exposed to specific environment. Pronouncedformation of silver whiskers was observed in gaseous sulphur contain-ing environment [3]. Aluminium whiskers grew with a high rate incertain narrow range of temperatures [4]. Whisker growth rate can berelatively high with the incubation period from several days toseveral years.

The problem of metal whisker growth was widely discussedstarting from 1940’ [5,6]. Nowadays, real-time observations of whis-kers growth can be done with advanced experimental facilities, e.g.using the focused ion beam (FIB) and scanning electron microscopy(SEM) techniques, see the results by Jadhav et al. [7] for microndiameter scale tin whiskers. Moreover, irradiation of certain speci-mens with electron beam during routine observation in electronmicroscopes can facilitate the growth of whiskers. E.g. Edmondson [8]demonstrated electron beam induced growth of silver whiskers fromzeolites chemically loaded with silver ions.

One of the characteristic features of the whisker growth istheir extrusion from the base material [9]. Presence of intrinsiccompressive mechanical stresses was named as one of the mainconditions and the driving force of spontaneous growth. Thispremise was exploited in various models of whisker growth. Theconcepts and models of whiskers growth were proposed usingdifferent approaches: dislocation based models [10–12], models ofpolycrystalline grain recrystallization [13], diffusion related mod-els [14–17]. Distribution of mechanical stresses in the coating wasmodelled and mapped to whiskers growth [18]. There were alsoattempts to link metal whiskers growth with the strong electricfield [19]. To the best of our knowledge, there is no general andcomprehensive model explaining growth of whiskers at givenconditions and in connection to their intrinsic structure.

Silver nanowires (AgNWs) have been the focus of many recentresearch projects of fabrication of transparent electrodes for solarcell, touch panel and flat-panel display applications [20]. TheAgNW random network is interesting due to its high electricalconductivity and optical transmittance [21,22]. The electrodes canbe readily prepared by simple and scalable methods such as spincoating, rod coating, and spraying from a AgNW-s dispersion[23,24]. AgNWs are also promising components in fabrication offlexible transparent electrodes due to high fatigue resistance ofNWs [21,25]. However, such electrodes can fail due to corrosionof AgNWs exposed to ambient conditions, which adversely affectsthe conductivity of AgNW network [22]. Another reason of theelectrode failure is that AgNWs are instable at elevated tempera-tures caused, e.g. by electrical current [26]. Coating of AgNWs

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Journal of Crystal Growth

http://dx.doi.org/10.1016/j.jcrysgro.2014.10.0210022-0248/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ37255941841.E-mail address: vlassovs@ut.ee (S. Vlassov).

Journal of Crystal Growth 410 (2015) 63–68

network with thin oxide layer could potentially protect them fromthe corrosion [27]. Moreover, it was recently demonstrated thatTiO2 shells increase thermal stability of individual AgNWs [28].

In this paper we demonstrate the electron beam-inducedgrowth of silver nanowhiskers observed in real-time in SEM onthe surface of a TiO2 layer that covers AgNWs network. Growthconditions are systematically studied and possible growth mech-anisms are discussed.

2. Materials and methods

2.1. Preparation of TiO2-coated AgNWs network

A sol–gel route was utilized for titania (TiO2) coating synthesis.Titanium(IV) butoxide (Alfa Aesar 99%) was used as sol precursors forsubsequent coating of Ag NWs networks. Three different TiO2

precursors were prepared: with HCl as catalyst, with HNO3 as catalyst,and without any catalyst.

General preparation procedure was similar for all three cases. Thesol precursor was obtained by a partial hydrolysis and condensationof titanium(IV) butoxide with water, and using ethanol as the solvent.The mixture of water, ethanol and if applicable the catalyst was

added to titanium(IV) butoxide, and the sol was stirred for 1 h atroom temperature. The molar ratio of water to titanium(IV) butoxidewas 0.5 and the molar ratio of the catalyst to titanium(IV) butoxidewas 0.05. The ratio of titanium(IV) butoxide/ethanol was set to 34/66by weight. Water and ethanol were removed by rotator evaporator(Büchi R-114) to decelerate the hydrolysis and condensation reactionof the sol precursor, and the concentrated sol was dissolved inhexane (10 wt%). The solution was kept overnight before depositionof titania layers onto AgNW network. Since titanium(IV) butoxide issensitive to water, ethanol and hexane were dried using CaH2 anddistilled prior to utilization.

AgNWs network was prepared by depositing a droplet of AgNWs-containing ethanol solution (Blue nano, 1 mg/mL) onto silicon (100)substrates (10 mm�10 mm) by spin coating at 1000 r/min for 60 s.Samples with AgNW networks were dried at 120 1C for a few minutesto remove the solvent. TiO2 precursors were spun at 3000 r/min for60 s onto the AgNWs network to form TiO2 sol layer.

2.2. Thermal treatment

Ten samples were prepared with HCl as catalysts and twoannealing series were carried out at different temperatures and

Fig. 1. SEM image of AgNW network on silicon substrate covered with TiO2 layer annealed at (a) 200 1C, (b) 300 1C, (c) 400 1C and (d) 600 1C.

M. Umalas et al. / Journal of Crystal Growth 410 (2015) 63–6864

times to study the effect of temperature on Ag NWs, and tocrystallize TiO2 layer. Samples 1–5 were heated in a furnace(Nabertherm HTR70-150/13) for 30 min at 5 different tempera-tures: 200 1C, 300 1C, 400 1C, 500 1C, and 600 1C (see Table S1 insupporting file). Ramping rate was 1.7 1C/min. Samples 6–10 wereplaced in the furnace at 400 1C and held for 5, 10, 30, 60, and180 min. Experiments were repeated for samples prepared usingHNO3 as catalyst and then without any catalyst.

2.3. Characterization

The surface morphology, structural features and elemental dis-tribution in the samples before and after annealing were characterizedwith a SEM (FEI Helios 600) equipped with a focused ion beam (FIB)and Energy-dispersive X-ray detector (EDX, Oxford Instruments).Crystal phases of TiO2 were examined by the Raman microspectrom-eter (Renishaw in Via) using 514.5 nm excitation wavelength and byX-ray diffractometer (Rigaku SmartLab) in grazing-incidence mode(GIXRD) using Cu Kα radiation at 8.1 kW.

3. Results

The structural characterisation of the spin-coated with AgNWs/ethanol solution samples revealed uniform network of ran-domly oriented Ag NWs with approximately 17% of surface cover-age (Fig. S1a in supporting file). After coating with TiO2 thestructure of Ag NWs networks remained unchanged. TiO2 filmappeared smooth and free of cracks (Fig. S1b in supporting file).

Typical SEM images of the samples annealed at differenttemperatures (for samples 1, 2, 3, and 5) are given in Fig. 1. Forthe sample annealed at 200 1C for 30 min, cracks formed in TiO2

layer at the places above Ag NWs (Fig. 1a). Moreover, AgNWs werepartly decomposed leaving wire-shaped patterns inside TiO2 layer.At 300 1C the decomposition of AgNW was more pronounced andsilver atoms diffused into the TiO2 layer resulting in fine particlesof silver formed onto the surface and inside the TiO2 layer (Fig. 1b).At 400 1C most of AgNW-s were decomposed (Fig. 1c). Theannealing at 500 1C and 600 1C resulted in complete decomposi-tion of all Ag NWs (Fig. 1d).

In the process of SEM characterization of the sample annealedat 400 1C an intensive spontaneous growth of Ag whiskers fromthe TiO2 layer was observed as soon as an electron beam wasfocused on the area of interest (see Video S1 and S2 in supportingfile). The growth was observed mostly from the areas whereAgNWs were clearly visible below the TiO2 film (Fig. 2). Somewhiskers grew also in vicinity of AgNWs and at the areas whereAgNWs were decomposed. Diameters of whiskers varied from afew nm to several hundred nm. Most of the whiskers had circularcross-section, while some whiskers possessed irregular geometry(Fig. 2b). At e-beam parameters 10 kV and 0.34 nA the growth ratewas up to several tens of nm/s. An average growth time untilgrowth termination was approximately 30 s (see Fig. S2 in sup-porting file). Variation of the electron beam parameters (voltagefrom 5 kV to 30 kV and current from 5.4 pA to 5.5 nA) had noconsiderable effect on the growth. Examination of the FIB-cuttrenches inside TiO2 layer revealed homogeneous distribution ofAg dots in the vicinity of AgNWs (see Fig. S3 in supporting file).

To study mechanism of Ag whiskers growth more precisely, thesecond annealing series at 400 1C for 5, 10, 30, 60 and 180 minwere carried out (samples 6 to 10 respectively). For the sampleannealed for 5 min some AgNWs were partly decomposed and Agdiffused into the TiO2 layer (Fig. 3a). No whiskers growth wasobserved. For the sample annealed for 10 min the growth of a fewsingle whiskers was observed (Fig. 3b). On the samples annealedfor 30 (Fig. 3c) and 60 min the whisker growth was intense. After

annealing for 180 min all AgNW-s were decomposed and nowhiskers were observed (Fig. 3d).

In addition, a dependence of the whiskers growth on TiO2 layerthickness was studied. It was found that the whiskers appearwhen the coating thickness ranges from 50 to 200 nm. For thinnerand thicker coatings no growth was observed.

To exclude the role of the HCl catalysts in whiskers formation,experiments were repeated on the samples with TiO2 coatingprepared using HNO3 as catalyst, and then with catalyst-free coating.In all cases results were similar and the whiskers growth wasobserved for the samples annealed to 400 1C for 30 min. To studythe effect of an aging on the whiskers growth, two samples were keptat ambient conditions for 7 weeks and were characterized in SEMonce per week. Every time the whiskers growth was observed.

Video S1. Ag whiskers growth from the sample annealed to 400 1C. A video clip isavailable online. Supplementary material related to this article can be found onlineat http://dx.doi.org/10.1016/j.jcrysgro.2014.10.021.

Video S2. Ag whiskers growth from the sample annealed to 400 1C. A video clip isavailable online. Supplementary material related to this article can be found onlineat http://dx.doi.org/10.1016/j.jcrysgro.2014.10.021.

M. Umalas et al. / Journal of Crystal Growth 410 (2015) 63–68 65

Fig. 2. Electron beam induced growth of Ag whiskers from the TiO2 coated AgNWs network annealed at 400 1C for 30 min. Typical whiskers (a) and irregular whiskers (b).

Fig. 3. SEM image of AgNW network on silicon substrate covered with TiO2 layer annealed at 400 1C for various times (a) 5 min, (b) 10 min (c) 30 min and (d) 180 min.

M. Umalas et al. / Journal of Crystal Growth 410 (2015) 63–6866

The samples 1 to 5 were analysed using Raman and X-ray spec-troscopies (Fig. 4). Raman spectra of the samples annealed totemperatures below 400 1C did not show any bands of crystallinephases of TiO2 [29], indicating that the TiO2 layer is amorphous(Fig. 2a). The sample annealed at 400 1C showed low intensity and abroad band at 144 cm�1 corresponding to Eg Raman mode of TiO2

anatase phase [30]. For the samples annealed at 500 1C and 600 1Cpeaks were observed at 144 (Eg), 197 (Eg), 399 (Eg) and 639 (Eg)cm�1, which correspond to anatase phase of TiO2. A weak band at110 cm�1 might be assigned to TiO2 crystallites in contact with Agnanoparticles or solution of Ag in TiO2 [31,32].

Fig. 2b represents the GIXRD patterns of the samples annealedat different temperatures. Below 400 1C only silver reflexes areseen at 2θ values of 38.31, 44.11, 64.41 and 77.51 [33,34]. No TiO2

reflexes are seen indicating absence of crystalline phase. Startingfrom 400 1C reflexes at 25.41, 38.41, 48.21, 53.91, 55.21, 62.81, 68.91,70.51, and 77.51 start to appear corresponding to TiO2 anatasephase [35,36] indicating crystallisation of TiO2. According toDebye–Scherrer’s formula [31] an average TiO2 crystallite sizeincreased from 15 nm for 400 1C to 20 nm for 600 1C. Silverreflexes are still present even after annealing the sample at600 1C for 30 min, indicating that silver particles are distributedin TiO2 layer after decomposition of AgNWs.

4. Discussion

In order to understand the mechanism of nanowhiskers growth,let us list the reasons possibly responsible for their formation. Theyfall into two main categories: (1) intrinsic physical/chemical factors, and (2) e-beam induced factors. The first one can include:(a) thermal expansion of the coating versus that of the AgNWs andthe substrate, and (b) diffusion of Ag in the porous coating structureand along its grain boundaries. The latter can be enhanced by suchphenomena as: (a) local heating of the samples during SEMobservation, (b) electric field induced migration of Ag.

Thermal expansion effects can play a significant role in thenanowhisker growth due to difference in thermal expansion coeffi-cients of Ag (αVE56�10�6 K�1 [37]) and TiO2 (αVE17�10�6 K�1

[38]). Moreover, melting of nanoscale Ag might happen at the temp-eratures starting from 400 1C [39] and lead to even larger expansionup to 10% (density of solid silver ρAgE10.57 g/cm3 and liquid silverρAgE9.346 g/cm3, [40]). Therefore, Ag/TiO2 thermal stress can partly

justify nanowhisker growth. At the same time Si substrate has evensmaller expansion coefficients (αVE7.8�10�6 K�1 for crystallinesilicon [41]) that can induce large compressive stress in the AgNWcontaining TiO2 coating under local heating. Coating/substrate inter-face could also promote slight shrinkage of the solidified coating andstructural defects formation during cooling. On the other side, thestress relief is been held until observation in SEM.

Dependence of whiskers growth on TiO2 layer thickness issupposedly closely related to mechanical stresses generated inthree-phase structure: SiO2, silver and TiO2. Mechanical stresses inour opinion play an important role in whisker growth. For coatingthickness below 50 nm cracks form along the whole length of thenanowires during annealing and nanowires become exposed(uncoated). As we know, no whiskers growth is observed fromuncoated Ag nanowires. For TiO2 layer thickness above 200 nm thewhole film starts to crack and mechanical stresses are supposedlyreleased below minimum required for growth initiation.

Annealing temperature was shown to be another critical factor forwhiskers growth. Edmondson et al. [8] reported e-beam inducedgrowth of Ag nanowires from zeolite samples chemically loaded withAg ions and heated to 260 1C to promote introduction of argentousions into the zeolite pores by the process of salt occlusion. Weobserve whiskers growth for samples heated to 400 1C, whichcorresponds to a phase transition, formation of crystallites andincrease of porosity in TiO2. Zeolites naturally have pores of nan-ometer size. Therefore it can be proposed, that presence of nano-pores in material is one of the necessary growth conditions, which incase of TiO2 is fulfilled during phase transformation at 400 1C.Diffusion of Ag can be largely enhanced in a porous polycrystallinematrix [42]. This is supported by our observation of Ag clusters athundreds of nm distance from AgNWs. It cannot however serve as amain or the only driving force for nanowhisker growth.

This naturally leads us to the factor of e-beam as a necessarycondition for nanowhisker growth along with the above describedphenomena. Electron beam in SEM may cause local heating of thesample, which can be enhanced by poor thermal conductivity ofTiO2. However, the peak temperatures would not likely exceed theannealing temperature.

Electric field can induce a rapid migration of Ag in metal oxidematrix as was reported in [43]. This migration may be directed bythe permanent thermal stress and accelerate stress relaxation.Moreover, metal whiskers growth can be promoted by the presenceof strong electric field as it was theoretically predicted in [18].

Fig. 4. Raman spectra (a) and GIXRD patterns (b) of the Ag NWs network on silicon substrate covered with TiO2 layer and annealed at different temperatures.

M. Umalas et al. / Journal of Crystal Growth 410 (2015) 63–68 67

Therefore, growth of Ag whiskers experimentally demonstratedin present work cannot be explained unambiguously and requiresfurther experimental and theoretical elaboration.

5. Conclusion

We demonstrated electron beam induced rapid growth of silvernanowhiskers from silver nanowire networks coated with TiO2 bysol–gel method. We found that the whisker growth was optimalafter the samples were annealed at 400 1C for 30 min and thethickness of TiO2 layer was within the range of 50 to 200 nm. TheSEM characterization showed that the diameter of the Ag whiskersvaries from few nm to several hundred nm, the growth rate wasup to several tens of nm/s and the whiskers may grow from a fewnm to several micrometres in length before the growth terminates.No considerable dependence of whiskers growth on electron beamparameters (voltage and current) was established.

Acknowledgments

The work was supported by ETF grants 8420, 9007, EstonianNanotechnology Competence Centre (EU29996), IUT2-25, ERDF “TRI-BOFILM” 3.2.1101.12-0028, “IRGLASS” 3.2.1101.12-0027 and “Nano-Com” 3.2.1101.12-0010. The authors are also grateful for partialsupport by COST Action MP1303, the Estonian Science Foundation(grant JD162), and European Union through the European RegionalDevelopment Fund (Centre of Excellence “Mesosystems: Theory andApplications”, TK114). The support from ITMO International ResearchLaboratory Program is acknowledged.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.jcrysgro.2014.10.021.

References

[1] H.L. Cobb, Cadmium whiskers,, Monthly Rev. Am. Electroplaters Soc. 33 (1946)28.

[2] K.G. Compton, A. Mendizza, S.M. Arnold, Filamentary growth on metalsurfaces-whiskers, Corrosion 7 (1951) 327.

[3] B.H. Chudnovsky, Degradation of power contacts in industrial atmosphere:silver corrosion and whiskers, in: Proc. 48th IEEE Home Conf. on Electr.Contacts, 2002, pp. 140.

[4] M. Chen, Y. Yue, Y. Ju, Growth of metal and metal oxide nanowires driven bythe stress-induced migration, J. Appl. Phys. 111 (2012) 104305.

[5] G.T. Galyon, Annotated tin whisker bibliography and anthology, IEEE Trans.Electron. Packag. Manuf 28 (2005) 94.

[6] C. Herring, J.K. Galt, Elastic and plastic properties of very small metal speci-mens, Phys. Rev. 85 (1952) 1060.

[7] Nitin Jadhav, Eric Buchovecky, Eric Chason, Allan Bower, Real-time SEM/FIBstudies of whisker growth and surface modification. J. Manage. 62 (2010) 30.

[8] Edmondson, et al., Adv. Mater. 13 (2001) 1608–1611.[9] S.E. Koonce, S.M. Arnold, Growth of metal whiskers, J. Appl. Phys. 24 (1953)

365.[10] M.O. Peach, Mechanism of growth of whiskers on cadmium, J. Appl. Phys. 23

(1952) 1401–1403.[11] J. Eshelby, A tentative theory of metallic whisker growth, Phys. Rev 91 (1953)

755–756.[12] U. Lindborg, A model for the spontaneous growth zinc, cadmium and tin

whiskers, Acta Metall. 24 (1976) 181–186.[13] J. Smetana, Theory of tin whisker growth: the end game, IEEE Trans. Electron.

Packag. Manuf 30 (2007) 11.[14] K.N. Tu, J.C.M. Li, Spontaneous whisker growth on lead-free solder finishes,

Mater. Sci. Eng., A 409 (2005) 131.[15] H.P. Howard, J. Cheng, P.T. Vianco, J.C.M. Li, Interface flow mechanism for tin

whisker growth, Acta Mater. 59 (2011) 1957.

[16] M. Chen, Y. Yue, Y. Ju, Growth of metal and metal oxide nanowires driven bythe stress-induced migration, J. Appl. Phys. 111 (2012) 104305.

[17] L. Yuan, Y. Wang, R. Mema, G. Zhou, Driving force and growth mechanism forspontaneous oxide nanowire formation during the thermal oxidation ofmetals, Acta Mater. 59 (2011) 2491.

[18] J. Zhao, P. Su, M. Ding, Microstructure-based stress modeling of tin whiskergrowth, IEEE Trans. Electron. Packag. Manuf. 29 (2006) 265–273.

[19] V. Karpov, Nucleation and Growth of Metal Whiskers, eprint arXiv:1309.3822,2013.

[20] Daniel Langley, G.a. el Giusti1, C.´eline Mayousse, Caroline Celle, Daniel Bellet,Jean-Pierre Simonat, Flexible transparent conductive materials based on silvernanowire networks: a review, Nanotechnology 24 (2013) 452001.

[21] Dong-Seok Leem, Angharad Edwards, Mark Faist, Jenny Nelson, Donal D.C. Bradley, John C. de Mello, Efficient organic solar cells with solution-processed silver nanowire electrodes, Adv. Mater. 23 (2011) 4371–4375.

[22] Liangbing Hu, Han Sun Kim, Jung-Yong Lee, Peter Peumans, Y.i. Cui, Scalablecoating and properties of transparent, flexible, silver nanowire electrodes, ACSNano 4 (2010) 2955–2963.

[23] Areum Kim, Yulim Won, Kyoohee Woo, Chul-Hong Kim, Jooho Moon, Highlytransparent low resistance ZnO/Ag nanowire/ZnO composite electrode forthin film solar cells, ACS Nano 7 (2013) 1081–1091.

[24] Y.C. Lu, K.S. Chou, Tailoring of silver wires and their performance astransparent conductive coatings, Nanotechnology 21 (2010) 215707.

[25] Sergei Vlassov, Boris Polyakov, Leonid M. Dorogin, Mikk Antsov, Magnus Mets,Madis Umalas, Rando Saar, Rünno Lõhmus, Ilmar Kink, Elasticity and yieldstrength of pentagonal silver nanowires: in situ bending tests, Mater. Chem.Phys. (2013) 1026–1031. http://dx.doi.org/10.1016/j.matchemphys.2013.10.042.

[26] Hadi Hosseinzadeh Khaligh, A. Goldthorpe Irene, Failure of silver nanowiretransparent electrodesunder current flow, Nanoscale Res. Lett. 8 (2013) 235.

[27] Zilberberg, et al., Adv. Funct. Mater. 24 (2014) 1671.[28] Parthiban Ramasamy, Dong-Min Seo, Sang-Ho Kima, Jinkwon Kim, Effects of

TiO2 shells on optical and thermal properties of silver nanowires, J. Mater.Chem. 22 (2012) 11651–11657.

[29] Sanjeev K. Guptaa, Jitendra Singh, K. Anbalagan, Prateek Kothari, RaviRaj Bhatia, Pratima K. Mishrab, V. Manjuladevic, Raj K. Guptac, J. Akhtar,Synthesis, phase to phase deposition and characterization of rutile nanocrys-talline titanium dioxide (TiO2) thin films, Appl. Surf. Sci. 264 (2013) 737–742.

[30] R. Pärna, U. Joost, E. Nõmmiste, T. Käämbre, A. Kikas, I. Kuusik, M. Hirsimäki,I. Kink, V. Kisand, Effect of cobalt doping and annealing on properties of titaniathin films prepared by sol–gel process, Appl. Surf. Sci. 257 (2011) 6897–6907.

[31] F.X. Liu, M. Tang, L. Liu, S. Lu, Z.Y. Chen, R. Ji, Enhanced optical properties ofAg–TiO2 (Rutile) hybrid nanopowder, Phys. Status Solidi A 179 (2000)437–443.

[32] M.i.n. Wu, Beifang Yang, Y.a.n. Lv, Zhengping Fu, Jiao Xu, Ting Guo,Yongxun Zhao, Efficient one-pot synthesis of Ag nanoparticles loaded on N-doped multiphase TiO2 hollow nanorod arrays with enhanced photocatalyticactivity, Appl. Surf. Sci. 256 (2010) 7125–7130.

[33] Mohd Abdul Majeed Khan, Sushil Kumar, Maqusood Ahamed, SalmanA. Alrokayan, Mohammad Saleh AlSalhi, Structural and thermal studies ofsilver nanoparticles and electrical transport study of their thin films, Nanos-cale Res. Lett. 6 (2011) 434.

[34] Haibo Mao, Jinyang Feng, Xiao Ma, C.a.n. Wu, Xiujian Zhao, One-dimensionalsilver nanowires synthesized by self-seeding polyol process, J. Nanopart Res.14 (2012) 887.

[35] Mukhtar H. Ahmed, Tia E. Keyes, John. A. Byrne, W. Charles, A. Blackledge,W. Jeremy, Hamilton adsorption and photocatalytic degradation of humanserum albumin on TiO2 and Ag–TiO2 films, J. Photochem. Photobiol., A 222(2011) 123–131.

[36] Mukhtar H. Ahmed, Tia E. Keyesa, John A. Byrne, The photocatalytic inactiva-tion effect of Ag–TiO2 on -amyloid peptide (1–42), J. Photochem. Photobiol., A254 (2013) 1–11.

[37] G.K. White, J.G. Collins, Thermal expansion of copper, silver, and gold at lowtemperatures, J. Low Temp. Phys. 7 (1972) 43–75.

[38] D.R. Hummer, P.J. Heaney, J.E. Post, Thermal expansion of anatase and rutilebetween 300and 575 K using synchrotron powder, Powder Diffr. 22 (2007)352–357.

[39] Parthiban Ramasamy, Dong-Min Seo, Sang-Ho Kim, Jinkwon Kim, Effects ofTiO2 shells on optical and thermal properties of silver nanowires, J. Mater.Chem. 22 (2012) 11651.

[40] A.D. Kirshenbaum, J.A. Cahill, A.V. Grosse, The density of liquid silver from itsmelting point to its normal boiling point 2450 1K, J. Inorg. Nucl. Chem. 24(1962) 333–336.

[41] H. Watanabe, N. Yamada, M. Okaji, Linear Thermal Expansion Coefficient ofSilicon from 293 to 1000 K, Int. J. Thermophys. 25 (2004) 221–236.

[42] J.D. McBrayer, R.M. Swanson, T.W. Sigmon, J. Bravman, Observation of rapidfield aided diffusion of silver in metaloxide semiconductor structures, Appl.Phys. Lett. 43 (1983) 653.

[43] M. He, T.-M. Lu, Metal-dielectric interfaces in gigascale electronics, SpringerSer. Mater. Sci. 157 (2012) 1–9.

M. Umalas et al. / Journal of Crystal Growth 410 (2015) 63–6868