core shell nanostructures by oblique angle deposition

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Synthesis of SiOx Ag core-shell nanostructures by oblique angledeposition

Dhruv P. Singh, Rupali Nagar, and J. P. Singh a

Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

Received 15 January 2010; accepted 17 February 2010; published online 8 April 2010

We report synthesis of SiOx Ag core-shell nanostructures comprising of a uniform and patterned

shell of Ag nanoparticles 67

9 nm by oblique angle deposition OAD at room temperature. TheAg nanoparticles were observed to form hexagonal tessellation over the curved silica surface. Thedistribution of nanoparticles over silica spheres is explained in view of surface energy minimizationby Eulers characteristic for best coverage. The oblique angle of Ag vapor flux with respect to thesubstrate normal during growth was observed to be the control parameter in formation of SiOx Agcore-shell nanostructures. Usually, OAD has been used to grow columnar nanostructures byexploiting the shadowing effect of adatoms during deposition but the application in surfacemodification at nanoscale by controlling the competing effect of shadowing and surface diffusion isthe novelty in this work. 2010 American Institute of Physics. doi:10.1063/1.3366714

I. INTRODUCTION

Ordered growth of nanostructures has been an importantfocus of research due to its promising applications in thefields of photonics,1 growth of functional materials,2,3 and insensing applications.4,5 Various techniques exist to synthesizeuniformly ordered nanostructures of different materials.However, strength of a synthesis technique lies in its simplic-ity, better control, and reproducibility. In the past, obliqueangle deposition OAD Refs. 610 has emerged as apromising technique to fabricate uniform nanostructures hav-ing high aspect ratio with desired distribution but use of thistechnique in modifying the surfaces at nanoscale by growinglarge area ordered nanoparticles or patches is still not wellexplored. Recently, Pawar and co-workers have used OAD in

which atomic shadow cast by neighboring nanoparticleswithin a monolayer has been exploited to produce functionalpatchy particles.11 In OAD, film morphology is decided bydeposition rate and competing effects of shadowing and sur-face diffusion of adatoms over the surface.10 A proper controlover these growth parameters helps in tuning the film mor-phology and it widens the scope of OAD applications. Thisstudy is an attempt to use OAD to modify the surface atnanoscale by growing uniformly distributed silver nanopar-ticles over silica sphere. Such dielectric-metal core-shellnanostructures find potential applications in catalysis,12 pho-tonic crystals,13 modulation of optical properties,14 and pro-viding efficient drug delivery solutions for treatment and di-

agnosis of cancer like diseases.15

Many different synthesistechniques have been used thus far for the growth of nano-particles over spherical surfaces and to fabricate dielectric-metal core-shell nanostructures, such as seed plating,16

polyol process,17 surface functionalization,18 wet chemicalmethods,19 or by generating thermal stress in film.20 Al-though researchers have reported a uniform growth of nano-particles shell over core particle by these techniques but the

involved processes do have issues related to shell layer pu-rity, growth temperature, versatility, and simplicity in imple-

mentation.In this study, we report OAD process to grow SiOx Agcore-shell nanostructures with uniformly distributed pat-terned silver nanoparticles constituting the shell at room tem-perature by controlling the competing effect of shadowingand surface diffusion of adatoms over the surface. These sil-ver nanoparticles arrange themselves in order to minimizethe surface energy and follow Eulers characteristic for bestcoverage. Applicability for most of the materials with main-taining their purity makes the OAD a versatile and mostsuitable technique for growth of core-shell nanostructures.

II. EXPERIMENTAL DETAILSSilver nanoparticles were grown over silica spheres dis-

persed on Si 100 substrate by thermal evaporation of silverpowder using OAD method. A schematic of OAD techniqueis shown in Fig. 1. Before the deposition, Si substrates werecleaned in boiling piranha solution 98% concentratedH2SO4 and 30% concentrated H2O2 in the ratio of 4:1 mak-ing the surface of Si substrate hydrophilic. An aqueous col-loidal solution of silica spheres was prepared by dissolvingsilica powder in deionized water in the ratio of 1:10 by

a Author to whom correspondence should be addressed. Electronic mail:[email protected]. FIG. 1. Color online Schematic of OAD system.

JOURNAL OF APPLIED PHYSICS 107, 074306 2010

0021-8979/2010/107 7 /074306/4/$30.00 2010 American Institute of Physics107, 074306-1

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weight, 10 ml of this colloidal solution was further diluted 25times in steps. This colloidal solution was dispersed overpiranha cleaned substrates. This concentration of colloidalsolution ensures a sparse dispersion of silica spheres over theSi substrate without stacking or forming clusters on the sur-face. For deposition by OAD, a substrate assembly was de-signed to hold the substrates such that the substrate normalmade different angles 0, 65, 75, 85, and 87 from the

direction of incident vapor flux. A very low deposition rate ofabout 0.10 sec1 was maintained for studying the nucle-ation and initial growth during OAD. During deposition, thepressure in OAD chamber was better than 3106 Torr.Film morphology and elemental composition analysis wereperformed using field emission scanning electron microscope

SEM, Jeol Ltd. and transmission electron microscope TEM, Tecnai G20 S-Twin at 200 keV .

III. RESULTS AND DISCUSSION

The low concentration of silica colloidal solution re-sulted in sparsely dispersed silica spheres over Si 100 sub-strate. A low deposition rate was found to result in a mosaicpattern throughout the substrate, whereas higher depositionrate resulted in the growth of slanted nanorods. A low depo-sition rate of 0.10 sec1 maintained for =0 was foundto be appropriate to understand the interdependence of thetwo effects that control the growth in OAD, namely shadoweffect and surface diffusion of adatoms. The deposition ratewill depend on the angular orientation of substrate with re-spect to the incoming vapor flux during growth, this is calledthe effective deposition rate hereafter. Figure 2 a shows aSEM micrograph of the growth of Ag nanoparticles on silicaspheres for =87. A region of sparsely dispersed SiOx

spheres with Ag nanoparticles covering the entire substrate isvisible. A magnified view of a single silica sphere in Fig.2 b shows the uniform coverage of Ag nanoparticles onSiOx spheres. The average diameter of Ag nanoparticlesforming silver shell over silica core was observed to be679 nm, while the interparticle separation is about145 nm as determined from Fig. 2 a . The Ag nanopar-ticles deposited over SiOx spheres seem to follow a hexago-nal pattern as shown in Fig. 2 b . The inset image shows thepresence of a pentagon observed on another SiOx sphere.Transmission electron microscopy TEM image in Fig. 3 ashows the silver particles distributed over silica spheres andthe spot energy dispersive x-ray EDX analysis shown in

Fig. 3 b confirms the presence of silver on the silica sphere.The Si and O peaks in EDX spectra are attributed to the Sisubstrate and silica sphere while the Cu peak is due to thesupporting TEM grid. The origin of sulfur peak is due to theuse of piranha solution, which leaves S residue on Si sub-strate after the cleaning step.21

During OAD, the silver vapor flux reaches the substrateat a very small grazing angle. Every silica sphere, due to itsfinite size, casts shadow on the substrate which then blocksthe incident silver vapor flux reaching its shadow region.Therefore, with respect to the direction of the incident flux,the rear side of the silica sphere receives very little or novapor flux at all. This region termed as the self-shadow re-

gion of the silica sphere and this would not allow silvernanoparticles to grow within this region. Contrary to this, auniform coverage of Ag nanoparticles over SiOx spheres isobserved. This suggest that the growth follows a differentmechanism for the above mentioned deposition conditions. Aschematic showing the self-shadow region of the silicasphere with respect to the incidence silver vapor flux isshown in Fig. 4 a . To understand this, the surface diffusioncoefficient D and diffusion length l of Ag adatoms overSiOx spheres were estimated as

22

D =1

2a0

2 exp

Ed

kT, 1

FIG. 2. Color online a SEM image of silica spheres decorated with Agnanoparticles. The arrow shows the direction of incident vapor flux. bMagnified SEM image showing the formation of hexagonal pattern of Agnanoparticles on the silica sphere. Inset showing the presence of pentagon inthe growth pattern. Scale bar in inset is of 100 nm length.

FIG. 3. a TEM image showing the Ag nanoparticles distributed on thesilica spheres. b EDX spectrum shows the presence of silver along withsilicon and oxygen contributions from Si substrate and silica sphere.

074306-2 Singh, Nagar, and Singh J. Appl. Phys. 107, 074306 2010



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l =2D

expEa

kT

1/2

, 2

where a0 is the jump distance between the adjacent adsorp-tion sites and is equal to the lattice constant of surface ma-terial, Ed is the surface diffusion energy, Ea is the surfaceadsorption energy, is the vibration frequency of an adatomon the silica surface, and k is the Boltzmann constant. Usingthe values for these parameters a0 =7.09 for silica, =3.51010 sec1, Ea =0.37 eV, Ed=0.1 eV 23 and consid-ering T as the room temperature 300 K , the value ofsurface diffusion D is obtained as 184.7 m2 sec1. There-fore, silver has a large surface diffusion on silica surface, andunder the above deposition conditions, the average value oflcovered by silver adatoms over the silica surface is about 130nm. This value of diffusion length is sufficiently large toensure the movement of silver adatoms from their primaryincident sites falling in the exposed region into the self-shadow regions of the silica spheres. We now consider theeffect of deposition rate on the observed growth of silvernanoparticles over the silica spheres. For the deposition of

conventional silver film on Si substrate, i.e., for =0, thedeposition rate was determined to be about 0.10 sec1.Using the cosine law of evaporation, the effective depositionrate for the sample placed at 87 tilt angle is estimated as0.006 sec1. This ultralow deposition rate greatly reducesthe probability of adatoms to get buried underneath the con-tinuously incoming vapor flux of silver atoms. The large sur-face diffusion length along with the ultra low deposition rateaids in establishing a dynamics in which the adatoms diffuseover the surface of the sphere rather than resulting in theformation of the nuclei only in the exposed region and sub-sequently growing to large size nanostructures. A higherdeposition rate, however, disturbs the above dynamics and

leads to the growth of columnar rodlike structures. This as-pect has been experimentally verified but is not discussed inthe present study. The diffused silver adatoms distribute overthe surface of silica sphere and seem to arrange themselvesin order to minimize the surface energy. This phenomenonhas been mathematically formulated as Eulers characteristicfor the best coverage of spherical surface. These nanopar-ticles observe particular patterns of arrangement which when

joined with neighboring nanoparticles results in a tessellationof polygons over the sphere. These polygons follow Eulerscharacteristics which is given by FE+V=2, where F, E,and V are the number of faces, edges, and vertices,respectively.24 This characteristic allows the presence of

maximum number of hexagons along with few pentagonsand heptagons. Presence of these pentagons or heptagonsmay be viewed as the deformations in the hexagonal pattern,enabling it to fit on the sphere for providing maximum cov-erage of the surface and thus minimizing energy of thesystem.24 One such pentagon over the SiOx sphere is shownin Fig. 2 b . The schematic of atomic distribution following

Eulers rule and formation of hexagons along with a penta-gon is shown in Fig. 4 b . These uniformly distributed silveradatoms act as the nucleation centers and grow into nanopar-ticles with time by continuously receiving silver atoms eitherdirectly from the incident flux or the ones that have diffusedon the surface. In OAD, as the vapor flux angle measuredwith respect to the substrate normal is decreased, the effec-tive deposition rate increases. For higher deposition rates, thedominance of surface diffusion over shadow effect no longerexists. The probability of adatoms interacting among them-selves and forming clusters becomes higher as compared tothe case of lower deposition rate where the adatom kinetics isindependent of such interactions. Thus, increase in interac-tion probability among the silver adatoms at lower tilt angles constrains the nucleation but promotes their columnargrowth predominantly in the exposed regions of the silicaspheres. This can be observed from Fig. 5 as the depositionangle is varied from =87 to 65. It may be mentioned thatthe motion of adatoms on a spherical surface is not governedby their tilt angle .25 Therefore, in case of normal deposi-tion, i.e., when =0, it is possible to observe the growth ofcore-shell structures if an ultra low deposition rate 0.006 sec1 can be maintained. However, such an ultralow deposition rates cannot be achieved at normal incidentflux using a conventional thermal evaporation set up. OAD,

therefore, provides a simple technique to reduce the effectivedeposition rate in a controlled manner.

IV. CONCLUSION

The present work demonstrates the potential of OADtoward modifying the surface at nanoscale by growingdielectric-metal core-shell nanostructures at low temperature.We have synthesized SiOx Ag core-shell nanostructures us-ing OAD at room temperature. The silver nanoparticles inshell are distributed uniformly over the silica spheres as gov-erned by Eulers characteristic for the best coverage. The lowdeposition rate and high surface diffusion coefficient of Ag

FIG. 4. Color online a Schematic of silica sphere receiving the silvervapor flux at an angle with respect to the substrate normal. b Schematicof uniform nanoparticles distribution over a spherical surface following Eu-lers characteristic. For the best coverage, nanoparticles arrange themselvesin hexagons along with pentagon patterns on the sphere. FIG. 5. SEM images of samples grown at tilt angles =87, 85, 75, and

65, respectively. The columnar growth of silver is clearly visible for =75 and 65.




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adatoms on silica surface are found to be the controllingparameters in deciding the patterned growth of Ag nanopar-ticles over the silica sphere.

ACKNOWLEDGMENTS

The authors DPS and RN kindly acknowledge CSIR,India for the research fellowship. This research work wassupported by research under Grant No. SR/FTP/PS-41/

2005 from DST, India. We are thankful to Dr. V.N. Singh forTEM measurements.

1K. Busch, G. V. Freymann, S. Linden, S. F. Mingaleev, L. Tkeshelashvili,and M. Wegener, Phys. Rep. 444, 101 2007 .

2Y. Lei, W. Cai, and G. Wilde, Prog. Mater. Sci. 52, 465 2007 .3Y. Huang, X. Duan, Q. Wei, and C. M. Lieber, Science 291, 630 2001 .4X. J. Huang and Y. K. Choi, Sens. Actuators B 122, 659 2007 .5J. Yuan, K. Wang, and X. Xia, Adv. Funct. Mater. 15, 803 2005 .6Y. P. Zhao, S. B. Chaney, and Z. Y. Zhang, J. Appl. Phys. 100, 063527 2006 .7J. P. Singh, T. Karabacak, D. X. Ye, D. L. Liu, C. Picu, T. M. Lu, and G.C. Wang, J. Vac. Sci. Technol. B 23, 2114 2005 .

8C. M. Zhou and D. Gall, Appl. Phys. Lett. 90, 093103 2007 .9D. X. Ye, Y. P. Zhao, G. R. Yang, Y. G. Zhao, G. C. Wang, and T. M. Lu,

Nanotechnology 13, 615 2002 .

10K. Robbie, J. C. Sit, and M. J. Brett, J. Vac. Sci. Technol. B 16, 1115 1998 .

11A. B. Pawar and I. Kretzschmar, Langmuir 24, 355 2008 .12S. Phadtare, A. Kumar, V. P. Vinod, C. Dash, D. V. Palaskar, M. Rao, P. G.

Shukla, S. Sivaram, and M. Sastry, Chem. Mater. 15, 1944 2003 .13Y. Lu, Y. Yin, Z. Y. Li, and Y. Xia, Nano Lett. 2, 785 2002 .14R. D. Averitt, S. L. Westcott, and N. J. Halas, J. Opt. Soc. Am. B 16, 1824 1999 .

15A. G. Skirtach, C. Dejugnat, D. Braun, A. S. Susha, A. L. Rogach, W. J.Parak, H. Mohwald, and G. B. Sukhorukov, Nano Lett. 5, 1371 2005 .

16M. Zhu, G. Qian, Z. Hong, Z. Wang, X. Fan, and M. Wang, J. Phys. Chem.Solids 66, 748 2005 .

17J. M. Lee, D. W. Kim, Y. H. Lee, and S. G. Oh, Chem. Lett. 34, 928 2005 .

18C. Graf and A. van Blaaderen, Langmuir 18, 524 2002 .19S. Tang, Y. Tang, S. Zhu, H. Lu, and X. Meng, J. Solid State Chem. 180,

2871 2007 .20X. N. Zhang, C. R. Li, Z. Zhang, and Z. X. Cao, Appl. Phys. Lett. 85,

3570 2004 .21K. A. Reinhardt and W. Kern, Handbook of Silicon Wafer Cleaning Tech-

nology, 2nd ed. William Andrew, Norwich, New York, 2008 , pp. 2326.22M. Ohring, Materials Science of Thin Film, 2nd ed. Academic, Califor-

nia, 2002 , pp. 376388.23P. T. Stroud, J. Phys. C 4, 577 1971 .24E. B. Saff and A. B. J. Kuijlaars, Math. Intell. 19, 5 1997 .25

L. Abelmann and C. Lodder, Thin Solid Films 305, 1 1997 .


http://dx.doi.org/10.1016/j.physrep.2007.02.011http://dx.doi.org/10.1016/j.pmatsci.2006.07.002http://dx.doi.org/10.1126/science.291.5504.630http://dx.doi.org/10.1016/j.snb.2006.06.022http://dx.doi.org/10.1002/adfm.200400321http://dx.doi.org/10.1063/1.2349549http://dx.doi.org/10.1116/1.2052747http://dx.doi.org/10.1063/1.2709929http://dx.doi.org/10.1088/0957-4484/13/5/314http://dx.doi.org/10.1116/1.590019http://dx.doi.org/10.1021/la703005zhttp://dx.doi.org/10.1021/cm020784ahttp://dx.doi.org/10.1021/nl025598ihttp://dx.doi.org/10.1364/JOSAB.16.001824http://dx.doi.org/10.1021/nl050693nhttp://dx.doi.org/10.1016/j.jpcs.2004.09.013http://dx.doi.org/10.1016/j.jpcs.2004.09.013http://dx.doi.org/10.1246/cl.2005.928http://dx.doi.org/10.1021/la011093ghttp://dx.doi.org/10.1016/j.jssc.2007.08.022http://dx.doi.org/10.1063/1.1807953http://dx.doi.org/10.1088/0022-3719/4/5/007http://dx.doi.org/10.1007/BF03024331http://dx.doi.org/10.1016/S0040-6090(97)00095-3http://dx.doi.org/10.1016/S0040-6090(97)00095-3http://dx.doi.org/10.1007/BF03024331http://dx.doi.org/10.1088/0022-3719/4/5/007http://dx.doi.org/10.1063/1.1807953http://dx.doi.org/10.1016/j.jssc.2007.08.022http://dx.doi.org/10.1021/la011093ghttp://dx.doi.org/10.1246/cl.2005.928http://dx.doi.org/10.1016/j.jpcs.2004.09.013http://dx.doi.org/10.1016/j.jpcs.2004.09.013http://dx.doi.org/10.1021/nl050693nhttp://dx.doi.org/10.1364/JOSAB.16.001824http://dx.doi.org/10.1021/nl025598ihttp://dx.doi.org/10.1021/cm020784ahttp://dx.doi.org/10.1021/la703005zhttp://dx.doi.org/10.1116/1.590019http://dx.doi.org/10.1088/0957-4484/13/5/314http://dx.doi.org/10.1063/1.2709929http://dx.doi.org/10.1116/1.2052747http://dx.doi.org/10.1063/1.2349549http://dx.doi.org/10.1002/adfm.200400321http://dx.doi.org/10.1016/j.snb.2006.06.022http://dx.doi.org/10.1126/science.291.5504.630http://dx.doi.org/10.1016/j.pmatsci.2006.07.002http://dx.doi.org/10.1016/j.physrep.2007.02.011

core shell nanostructures by oblique angle deposition

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