Mechanism of Elongation of Gold or Silver Nanoparticles in

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Mechanism of Elongation of Gold or Silver Nanoparticles in Silica by Irradiation with. Swift Heavy Ions, Koichi Awazu1 Xiamin Wang1 Makoto Fujimaki1 Junji Tominaga1 Shinji Fujii2 Hirohiko. Aiba2 Yoshimichi Ohki2 Tetsuro Komatsubara3, CAN FOR National Institute of Advanced Science and Technology 1 1 1 Higashi Tsukuba. 305 8562 Japan 2Waseda University Shinjuku Tokyo 169 8555 Japan 3Tandem. Accelerator Complex University of Tsukuba Tsukuba 305 8577 Japan. It has been reported that elongated Au nanoparticles oriented parallel to one another can be. synthesized in SiO2 by ion irradiation Our aim was to elucidate the mechanism of this. elongation We prepared Au and Ag nanoparticles with a diameter of 20 nm in an SiO2 matrix. It was found that Au nanoparticles showed greater elongated with a higher flux of ion beam. and with thicker SiO2 films In contrast Ag nanoparticles split into two or more shorter. nanorods aligned end to end in the direction parallel to the ion beam These experimental. results are discussed in the framework of a thermal spike model of Au and Ag nanorods. embedded in SiO2 The lattice temperature exceeds the melting temperatures of SiO2 Au and. Ag for 100 ns after one 110 MeV Br10 ion has passed through the middle of an Au or Ag. Author to whom correspondence should be addressed Electronic mail k awazu aist go jp. PACS 81 16 c 61 80 Jh 62 20 Fe 81 40 Lm 68 60 Bs, Key words swift heavy ion nanofabrication Au nanoparticles silica glass. 1 Introduction, Well defined Au nanoparticles and nanorods are desirable for their optical properties The. size and shape of nanocrystals affect the position of the plasmon bands which in turn have. been widely used in surface enhanced spectroscopy that includes both Raman and. fluorescence Gold nanoparticles or nanorods can be deposited randomly on a substrate 1. but it is difficult to synthesize Au nanorods that are oriented parallel to each other and. perpendicular to the substrate 2 3 The goal of the experiment from the viewpoint of. application to nanophotonics was to reveal new optical properties of Au nanorods We also. aimed to resolve the interesting phenomenon of elongation of Au nanoparticles in SiO2 To. theoretically understand the mechanism of elongation we employed the thermal spike. model 4 since it explains many phenomena caused by ion bombardment However several. problems with thermal spike models have been pointed out 5 For example temperature. changes in the order of femto seconds cannot be defined It is also impossible to ignore the. pressure dependence of the different physical parameters of the lattice Such questions remain. controversial so we will not focus on these matters in the present report. 2 Experimental procedures, A thermally oxidized silicon wafer was prepared as the SiO2 substrate The thickness of the.
SiO2 layer on the silicon was 2 m Five nm thick Au or Ag films were deposited on the. substrate by evaporation of high purity Au or Ag grains After deposition both the thin Au. and Ag films were heated at 300 C for 10 min to form nanoparticles The nanoparticles were. then embedded in an SiO2 layer deposited by radio frequency magnetron sputtering of a silica. target in an Ar atmosphere The thickness of the top layer of SiO2 was set to 200 nm or 1 m. by selecting deposition time The direction of propagation of the ion beam was perpendicular. to the upper surface of the SiO2 The 12 unit double Pelletron tandem accelerator at the. University of Tsukuba was used to irradiate the assemblies with 110 MeV Br10 ions Cross. sections of the pristine and irradiated samples were examined by transmission electric. microscopy TEM using a Hitachi H 9500 300 kV instrument Specimens for TEM. observation with a thickness of 100 nm were produced using a focused beam of 20 keV Ga. ions from a Hitachi FB 2100 instrument, The temperature evolutions of the particles when irradiated by swift heavy ions. are simulated using the thermal spike model 4 7 8 We extended this model to include two. materials to accommodate the current particle case First the swift ion deposits its energy in. the electron subsystem according to the radial distribution given by the Katz model 9 The. hot electrons then diffuse their energy by means of electron electron scattering and electron. phonon coupling To simulate this heat diffusing process numerically we need to know all. the thermal parameters For the electron subsystems specific heats and thermal conductivities. we assumed constant values for the insulator SiO2 as given in 4 and temperature dependent. values for the metal Au and Ag as in 8 For the lattice subsystems we used the. thermodynamic parameters of bulk materials as an approximation SiO2 s specific heat is. fitted to the data given by 10 11 and the specific heats of Au and Ag are from 12 Their. thermal conductivities are from the data recommended in Touloukian s handbook 13 In. addition the electron lattice coupling parameter of SiO2 is derived from the experimentally. fitted mean energy diffusion length given by 4 The coupling parameter of silver is given. theoretically by the quasi free electron gas model 8. 3 Results and discussion, We examined the elongation of Au nanoparticles as a function of the thickness of SiO2. on Au nanoparticles and as a function of ion flux Figures 1 show cross sectional TEM views. of Au nanoparticles generated at 300 C and subsequently embedded in SiO2 The thicknesses. of the SiO2 layer for Au nanoparticles in a b and d are 200 nm and for c it is 1 m. Figure 1 a presents Au nanoparticles in SiO2 before irradiation Particle size is estimated. from the TEM view at 20 nm The Au particles in the pristine film seems to be touching each. other in Figures 1 a But from plane view of scanning electronic microscope SEM we. confirmed that Au particles were separated Figures 1 b c and d show Au nanoparticles. embedded in SiO2 after irradiation with 110 MeV Br10 ions at a fluence of 1 1014 cm 2 Ion. fluxes are 8 2 1010 cm 2 sec 1 for b and c and 1 5 0 2 1011 cm 2 sec 1 for d In the. pictures the direction of ion propagation is from top to bottom as shown by the arrow A. comparison between b and d shows that Au nanoparticles under 1 m thick SiO2 were. much more elongated than those under 200 nm thick SiO2 A comparison between b and c. reveals that Au nanoparticles are much more elongated at higher ion fluxes. Here we discuss the dependence of SiO2 thickness Kla munzer et al proposed the. effect of matrix hammering introduced the particles creep 14 Penninkhof et al have reported. that in colloids consisting of a Au core and a silica shell the Au core showed a large. elongation along the ion beam direction provided the silica shell is thick enough 40nm. 15 Our present results are consistent with the report by Penninkhof et al The model. proposed by them to explain the elongation of the Au core is an indirect deformation scenario. in which the in plane strain generated by ion tracks in the silica shell imposes a stress on the. metal core 15 With the Au being relatively soft under ion irradiation this in plane stress. may then cause the Au core to flow in the out of plane direction i e along the direction of the. ion beam by Newtonian viscous flow 15 This argument seems consistent with the fact that. larger elongation is found for colloids with a thicker silica shell About the flux dependence. we assumed that at high flux they did not have the time to cool between successive impacts. on the same area, Silver nanoparticles embedded in SiO2 are shown in Figure 2 a The thickness of the. top layer of SiO2 is 200 nm Ag nanoparticles of a uniform diameter of 20 nm are present in. the SiO2 After irradiation with 110 MeV Br10 ions at a fluence of 1 1014 cm 2 and at a flux. of 8 2 1010 cm 2 sec 1 Ag nanoparticles split into two or more shorter nanorods aligned. end to end in the direction parallel to the ion beam. Penninkhof et al reported the behavior of Ag cores embedded in silica with 30MeV Si. ions irradiation and Ag cores did not show elongation 15 The Ag behavior in the present. work is different from the previous report, Lattice temperature for Au and Ag nanorods with radii of 5 nm embedded in SiO2 were. calculated and are shown in Figures 3 a and b respectively Here we used nanorods not. nanoparticles because calculation of temperature on nanorods is simpler than that on. nanoparticles and Au shape is close to nanorods after irradiation These show the evolution of. the lattice temperature against time at various distances from the ion axis For a time of less. than 10 13 s the temperature characterizes the energy imparted to the atoms In Figure 3 a. the temperatures of the Au nanorod monitored at 1 nm and 3 5 nm from the center are shown. by black squares and red circles respectively 300 fs 3 10 13 s after the ion impact both. temperatures exceed the melting point of Au 1337 K which is shown by a black dashed line. The maximum temperatures at both distances also reach the melting point of SiO2 1992 K. which is shown by a dashed horizontal line A point 6 5 nm from center of an Au nanorod of. radius 5 nm is located within the SiO2 matrix The temperature there at this point increases. dramatically 5 fs 5 10 15 s after ion impact and exceeds the melting point of SiO2 for a. period between 20 fs and 10 ps after the ion impact The lattice temperature 10 nm from the. center of the nanoparticle does not exceed the melting point of SiO2 SiO2 as well as Au is. melted for 10 ps in the region within 10 nm of the center of the nanoparticle In other words. the periphery of particles rather undergo an overheating with respect to the core because of. the low thermal conductivity of the surrounding matrix. Here we should explain why the silica temperature increases before that of the Au. nanoparticle even though the incident ion energy is first deposited in the Au nanoparticle and. only then coupled to the silica matrix The electrons along the ion s path respond. instantaneously to the penetrating ion resulting in a rise in electronic temperature These hot. electrons within Au then diffuse their energy rapidly to electrons in the surrounding silica and. cause their temperature to be raised correspondingly Because the electron lattice coupling. constant of silica is greater than that of Au although its conductivity is smaller we observed. in the simulation that the silica lattice temperature increases ahead of the inner Au particle. and thus will feed the heat back to the particle by phonon phonon interactions. In the experiment however Au nanoparticles 10 nm in radius are elongated by irradiation. from the experimental results as depicted in Figure 1 The radius in the experiments does not. exactly match those obtained in the calculation due to the inherent limitations of the current. spike model as well as uncertainty of the input parameters. We found from Figures 1 b and d that the elongation of Au nanoparticles depends on ion. flux This may imply that cooling time is not sufficient after initial ion bombardment at higher. ion flux It was found that Au nanoparticles are elongated but that Ag nanoparticles are split. into two or more Figures 3 a and b based on the thermal spike mode reveal no obvious. difference in the lattice temperatures of Au and Ag nanoparticles in SiO2. In the present study we found that the elongation of Au nanoparticles embedded in SiO2 was. influenced by ion flux and SiO2 thickness Silver nanoparticles are elongated and split into. two or more under irradiation We concluded that the elongation mechanisms must consist of. combination of the particle creep under the effect of the matrix hammering thermal spike. mechanical effects driven by stresses around the ion tracks. Acknowledgement This study was financially supported by the Budget for Nuclear Research. of the Ministry of Education Culture Sports Science and Technology based on screening. and counseling by the Atomic Energy Commission, Figure 1 Cross sectional TEM image of Au nanoparticles embedded in SiO2 a before ion.
irradiation b d irradiated with 110 MeV Br10 at a fluence of 1 1014 cm 2 Thicknesses. of SiO2 are 200 nm a b d and 1 m c Ion fluxes are 8 2 1010 cm 2 sec 1 b c. and 1 5 0 2 1011 cm 2 sec 1 d, Figure 2 Cross sectional TEM image of Ag nanoparticles embedded in SiO2 a followed by. 110 MeV Br10 ion bombardment at a fluence of 1 1014 cm 2 and at a flux of 8 2 1010. Figure 3 The calculated lattice temperature versus time at distances of 1 3 5 6 5 10 20 and. 100 nm from the ion path a and b 5 nm radius Au and Ag nanoparticles embedded in. SiO2 respectively The sample is at 300 K The melting temperatures of SiO2 Au and Ag are. shown in the Figures,1 N Halas MRS Bull 30 2005 362. 2 S Roorda T V Dillen A Polman C Graf A V Blaaderen and B J Kooi Adv Mater. 16 2004 235, 3 Y K Mishra D K Avasthi P K Kulriya F Singh D Kabiraj A Tripathi J C Pivin. and I S Bayer A Biswas Appl Phys Lett 90 2007 073110. 4 M Toulemonde E Paumier J M Costantini Ch Dufour A Meftah and F Studer Nucl. Instrum and Methods B116 1996 37, 5 S Klaum nzer Matematisk fysiske Meddelelser 52 2006 293. 6 S Eustis and M A El Sayed Chem Soc Rev 35 2006 209. 7 M Toulemonde C Dufour A Meftah and E Paumier Nucl Instrum and Methods. 1 Mechanism of Elongation of Gold or Silver Nanoparticles in Silica by Irradiation with Swift Heavy Ions Koichi Awazu1 Xiamin Wang1 Makoto Fujimaki1 Junji Tominaga1 Shinji Fujii2 Hirohiko Aiba2 Yoshimichi Ohki2 Tetsuro Komatsubara3 1CAN FOR National Institute of Advanced Science and Technology 1 1 1 Higashi Tsukuba 305 8562 Japan 2Waseda University Shinjuku Tokyo 169 8555 Japan

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