The authors detect vacancy clusters at Au nanoparticle surfaces using a combination of positron lifetime spectroscopy, 1- detector, and 2-detector measurements of Doppler broadening of annihilation radiation. Gold nanoparticles are formed by MeV implantation of gold ions into MgO (100) followed by annealing. Clusters of two Mg and two O vacancies (v{sub 4}) are attached to the gold nanoparticle surfaces within the projected range (R{sub p})

Henderson, D.

Ishibashi, S.

Mills, A.P.

Moxom, J.

Somieski, B.

Suzuki, R.

Ueda, A.

White, C.W.

Xu, J.

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“The submitted manuscript has been authored by a contractor of the U.S. Bovernment under contract No. DE- ACfXXlOOR22725. Accordingly, the U.S. Gov&ment retains a nonex- clusive, royalty-free license to >$ublish or reproduce the published form of this contribution, or allow others to do so, for US. Government purposes.” VACANCY CLlJSTERS AT NANOPARTlCLE SURFACES Jun Xu’, J. Moxom’, B. Somieski’, C. W. White’, A. P. Mills2, Jr., R. Suzuki3, S. Ishibashi3, A. Ueda4, and D. Henderson4 ‘Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, TN 378316142 2Bell Laboratories, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974 3Electrotechnical Laboratory, l-l -4 Umezono, Tsukuba, Jbaraki 305, Japan 4Physics Department, Fisk University, 1000 17th Ave. North, Nashville, TN 37208 Kevwords: nanoparticles, nanosurfaces, vacancy, beam positron lifetime spectroscopy, 2D-Doppler broadening coincidence ABSTRACT: We detect vacancy clusters at Au nanoparticle surfaces using a combination of positron lifetime spectroscopy, l- detector, and 2-detector measurements of Doppler broadening of annihilation radiation. Gold nanoparticles are formed by MeV implantation of gold ions into MgO (100) followed by annealing. Clusters of two Mg and two 0 vacancies (~4) are attached to the gold nanoparticle surfaces within the projected range (R,). Nanopatticles are important because they exhibit unique properties, including size effects, which determine colors of luminescence of nanocrystals; quantum mechanical tunneling, which causes “insulators” to lose their ability to isolate; and quantum confinement, which enables conductors to become more resistive. These novel properties will be important for new generations of semiconductor and optica communication devices that are being driven to nanoscale dimensions. Interestingly, the surface-to-bulk ratio is high as materials are reduced to the nanometer scale. Therefore, surface structure can dominate bulk properties, and understanding the surface defects of mmoparticles becomes more urgent, A new tool is needed to characterize nanoparticle surfaces. In this paper, we present advanced positron spectroscopy for revealing clusters of four atomic vacancies in the surfaces of colloidal gold particles embedded in crystalline MgO. Gold nanoparticles are formed in the near surface region of single crystals of MgO using a combination of ion implantation and thermal annealing. [ 1 ] Gold nanoparticles in magnesia are fabricated at the ORNL Surface Modification and Characterization Facility by ion implantation (1.1 MeV gold ions at doses of 1, 3, 6, and 10~10’~ Au ions/cm2), followed by sequential annealing. Au particles are detected in the range of 1.6-4 pm from the surface, as shown in Figure 1. These nanoparticles are not spherical, but approximately cubic or rectangular, which is believed to be due to MgO properties. The Au nano particles have sizes that are in 2- 18 nm. Figure 1 Cross section TEM image of Au nanoparticles embedded in MgO. Positron lifetime spectra are measured for both a deep MgO layer where Au implantation least affects the data and for the Au nanoparticle layer-[21 Both spectra were deconvoluted using Laplace inversion into the probability density functions (pdf) as a function of positron 2 lifetime. Figure 2 shows the pdf spectra for MgO layer (open circles) and Au nanoparticle layers (solid circles), respectively. Positron lifetime components for the MgO layer are (0.22fi.04) ns with (89+3%) contribution (vr in Fig. 2) and (0.59 ti.07) ns with (1 l&3%) contribution, while those for the Au nano layer are (0.4&0.08) ns at (90%) and (1.8 ti.3) ns at (7%). The positron lifetimes are calculated for different defects in MgO using Puska and co-workers’ insulator model in which the annihilation rates are determined by the positron density 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Positron Lifetime (ns) Figure 2. Probability density functions as a function of overlapping with the enhanced electron positron lifetime, resulting from Laplace inversion density that is proportional to the atomic (CONTIN) of lifetime spectra for MgO layer (open circles) polarizability of MgO [3]. Based on the and Au nanoparticles embedded in MgO (solid circles). Dwti (nm) near surface imperfections. (45 445 850 ,348 1927 After Au nanoparticle formation, the S parameter (solid circles) is greatly increased in the region between the surface and the projected range (R,). The increased S parameter indicates that a large number of vacancies are produced by ion implantation. The S parameter also increases for depths greater than Rr, suggesting that vacancies extend to the region beyond the reach of the Au ions due to damage by low-mass knock-on ions. It appears that there is a “valley” in the S parameter profile, I I which is coincident with the Au nanoparticle layer. At ', . 1 ' I t I 'I 0 5 10 15 53 Es 30 Positron Energy (keV) this depth, some positrons apparently annihilate with the electrons associated with the Au. We interpret the data as closeness of the measured lifetimes to the calculated values, the 0.41 ns measured from MgO annealed after implantation indicates that the positrons are predominantly trapped in clusters consisting two Mg vacancies and two 0 vacancies. We denote these specific four vacancy clusters as “~4”. Measurements of Doppler broadening of annihilation radiation were conducted using the variable-energy slow positron beam at ORNL. Figure 3 shows the S parameter measured as a function of positron energy, corresponding to depth shown in the top scale, for MgO samples that contain Au nanoparticles. For as-grown MgO, the S parameter (open circles) is mostly low, except for the indicating that v4 clusters are formed on Au nanoparticle surfaces. In this case positrons remain Figure 3 Sparameters as a function of trapped in the v4 sites which apparently yield a lifetime positron energy for MgO samples with (solid indistinguishable from the undecorated v4 lifetime. circles) and without (open circles) However, when the trapped positrons annihilate with embedded Au nanoparticles. near-by-Au electrons, the S parameter is expected to be lower than that of the defected MgO bulk To confirm vacancy clusters at nanoparticle surfaces, we have conducted two detector coincidence measurements of Doppler broadening annihilation radiation, a system similar to that developed by Asoka-Kumar and Lynn.[4] Figure 4(left) shows our two dimensional data, while Fig. 4(right) shows the normalized ratio as a function of photon energy for Au nanoparticle (solid circles) and MgO layers (open circles). In the high-energy range (< 513 keV), the spectrum for Au nanoparticle layer shows two enhanced bands at 514.3 keV and 5 18.5 keV. These bands are dramatically different from that of the MgO layers, where normalized intensity is flat above the silicon. The difference is largely attributed to the surrounding Au atoms of the defects since the Au nanoparticle is different for the two environments. To confirm that the difference is due to the Au atoms, we measure the Doppler broadening spectrum for a pure Au film, shown as the solid line in Fig. 4(right). The Si-normalized spectrum for Au film also shows two main enhanced bands: 514.3 keV and 518.5 keV. This is consistent with the features of Au films observed in Myler and co- workers’ work [5], in which Au annihilation line was also normalized by that of perfect Si. There are a high degree of similarity between the Au films structures and the Au nanoparticle layer. This supports our interpretation that the positrons, trapped at the v4 sites (as we know it from lifetime spectra), annihilate with the electrons associated with Au nanoparticles. Of course, the defects’ environment includes the MgO matrix, which also contributes to the energy deviation spectrum. We conclude that vacancy sites where positrons are trapped are located in the Au nano surfaces. F Y 511 El DWI 1.8 . ..a..-Au nanm 1.6 0.8 0 2 Ey451 1 keV 6 8 Figure 4 (left) Two-dimensional spectrum of annihilation radiation of positrons injected into a p-S (ZOO), with 8 Q- cm. The diagonalfeature indicates the condition of E,+E2=1.022 MeV; (right) Normalized annihilation lines as a function of photon energy for Au nanoparticle layer (solid circles), MgO layer (open circles), and Au film (solid line). In conclusion, we combine both positron lifetime spectroscopy, which tells the existence of clusters of four atomic vacancies in Au nanoparticles embedded in MgO, and 2D-DBAR measurements, which reveals that these clusters are located on the surfaces of Au nanoparticles. The Au implants are not detectable until &ey form nanoparticles that are associated with vacancy defects. Our study also demonstrates that a positron probe can be used to specify defects of nanosurfaces, which is important as nanoscale fabrication becomes more prevalent. [l] A. Ueda, R. Mu, Y.S Tung, M. Wu, W.E.‘Collins, D. 0. Henderson, C. W. White, R. A. Zuhr, J. D. Budai, A. Meldrum, P. W. Wang, and X. Li, Nucl. Znstru, Meth. Phys. Res. B, 141,261 (1998). [2] Jun Xu, A. P. Mills, Jr., A. Ueda, D. Henderson, R. Suzuki, and S. Ishibashi, Phys. Rev. Lett. 83,4586, (1999). [3] M.J.Puska, S Makinen, M. Manninen,and R.M. Nieminen, Phys.Rev.B 39,7666 (1989). [4] P. Asoka-Kumar, M. Alatalo, V. J. Ghosh, A. C. Kruseman, B. Nielsen, and K. G. Lynn, Phys. Rev. Lett. 77,2097 (1996). [5] U. Myler, R. D. Goldberg, A. P. Knights, D. W. Lawther, and P. J. Simpson, Appl. Phys. I&t. 69,333(1996). Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC05- OOOR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. 

Vacancy Clusters at Nanoparticle Surfaces

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Vacancy Clusters at Nanoparticle Surfaces

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