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ISSN 1063 7850, Technical Physics Letters, 2015, Vol. .... Au4Zr NCs are formed in the course of implantation and decompose under the subsequent annealing ...
ISSN 10637850, Technical Physics Letters, 2015, Vol. 41, No. 6, pp. 543–546. © Pleiades Publishing, Ltd., 2015. Original Russian Text © O.N. Gorshkov, M.E. Shenina, A.P. Kasatkin, D.A. Pavlov, I.N. Antonov, A.I. Bobrov, D.O. Filatov, 2015, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2015, Vol. 41, No. 11, pp. 62–70.

Formation of Au4Zr Nanocrystals in Yttria Stabilized Zirconia in the Course of Implantation of Gold Ions O. N. Gorshkov, M. E. Shenina*, A. P. Kasatkin, D. A. Pavlov, I. N. Antonov, A. I. Bobrov, and D. O. Filatov Lobachevsky State University of Nizhni Novgorod, Nizhni Novgorod, Russia *email: [email protected] Received December 15, 2014

Abstract—Highresolution transmission microscopy and optical absorption spectroscopy have been used to examine how metallic nanocrystals (NCs) are formed in the course of implantation of Au ions into thin films of yttria stabilized zirconia (YSZ) and subsequent thermal annealing. It was found that, under certain implan tation conditions, Au4Zr NCs are formed in addition to Au NCs in the YSZ matrix. The conditions in which Au4Zr NCs are formed in the course of implantation and decompose under the subsequent annealing were determined. DOI: 10.1134/S1063785015060048

Nanocomposite materials and, in particular, arrays of metallic nanocrystals (MNCs) dispersed in dielec tric matrices have been attracting attention recently as promising materials for medicine [1–3], nonlinear optics [4], and electronics [5], as well as for develop ment of nonvolatile memory elements [6]. The mech anisms by which MNCs are formed in oxide matrices by the ionimplantation method are commonly stud ied using ions of chemically inactive metals, e.g., Au. So far, the formation mechanisms and properties of Au nanocrystals (NCs) in such matrices as SiO2 and Al2O3 have been studied most thoroughly [7]. The formation and properties of Au–Ag NCs in a SiO2 matrix have also been studied in rather great detail [8]. Phianites and, in particular, bulk single crystals of yttria stabilized zirconia (YSZ) are promising materi als for development of optoelectronic devices and fuel cells [9]. In addition, it has been shown that thin YSZ films can be used in metal–insulatormetal (MIM) structures exhibiting the resistive switching effect, which are promising for development of resistive non volatile memory units on their basis [10]. So far, the formation of NCs in the YSZ matrix has been studied for Zr [11, 12], Au [13], Ag [14, 15], Fe [16], and other metals [17]. In this study, we examined the specific features of the MNC formation in the course of implantation of Au ions into YSZ single crystals and YSZ/Si thin films and in the subsequent thermal annealing by highres olution transmission electron microscopy (HRTEM) and opticalabsorption spectroscopy. It was found for the first time that Au4Zr NCs are formed under certain conditions in the YSZ matrix, together with the Au NCs. The implantation and thermal annealing condi

tions were determined in which predominantly Au NCs are observed in the YSZ matrix. In the experiment on opticalabsorption spectros copy, we used singlecrystal YSZ wafers with 12 mol % Y2O3, produced at the Institute of General Physics, Russian Academy of Sciences [9]. To carry out struc tural studies, we formed MNCs in thin YSZ films with a thickness of ~80 nm, deposited by highfrequency magnetron sputtering on Si (100) substrates. The sam ples were irradiated with Au ions at doses of (0.5–4) × 1016 cm–2 on a Raduga3 setup operating in the pulsed mode: ion current density of ~12 μA cm–2, accelerat ing voltage of ~80 kV, pulse repetition frequency of 30 Hz, pulse width of ~200 μs, and average Au ion charge of +2.0. Successive postimplantation treat ment of the samples was performed in the course of 1 h in the temperature range of Ta = 400–1000°C with steps of 100°C in air in order to anneal out radiation defects in YSZ and to examine the evolution of param eters of the thusformed NCs under thermal treat ment. The opticalabsorption spectra were studied with a VARIAN Cary 6000i spectrophotometer at room temperature in the transmission configuration. Structural studies were performed and the elemental composition of the implanted layers was determined by crosssectional HRTEM with a Jeol JEM2100F transmission electron microscope. Figure 1 shows the opticalabsorption spectra of YSZ single crystals upon their irradiation with Au ions and postimplantation annealing. The spectra show peaks associated with the resonance plasmon absorp tion in the MNCs that appeared upon the implanta tion of Au ions [13]. Curve 1 in Fig. 1 is the opticalabsorption spectrum of an YSZ crystal implanted with Au ions at a dose Φ =

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Au

5

(a) [121] [011] [000] [110] [110] [121] [011]

Absorption, 10−4 cm−1

3

2

3

2

Zr

1

1.5

0.5 1/nm

0

1

0

Au4Zr[111]

Surface

2.0 2.5 Photon energy, eV

5 nm (b)

3.0

Fig. 1. Optical absorption spectra of YSZ single crystals irradiated with (0) Zr and (1–5) Au ions at a dose Φ, 1016 cm–2: (1, 2) 0.5, (3) 1, (4) 2, and (5) 4. Annealing temperature Ta, °C: (0, 1, 3–5) no annealing and (2) 1000.

[222]

[533]

[111]

[442]

[000]

[351]

[111]

Au[314] [222]

5 × 1015 cm–2. It has been shown previously [13] that similar bands peaked at an energy of ~2.4 eV in the opticalabsorption spectra of YSZ irradiated with Au ions at small doses of this kind are due to the formation of Zr NCs from zirconium ions displaced from their crystal lattice sites. For comparison, Fig. 1 shows the opticalabsorption spectrum of an YSZ with Zr NCs formed by implantation of Zr ions into the YSZ (curve 0, [12]). Thus, the absorption band peaked at hν ~ 2.4 eV (Fig. 1, curve 1) is due to the plasmon absorption in Zr NCs. This peak disappears upon annealing at Ta = 400°C, which is due to the dissolution of Zr NCs in the course of annealing, with the subsequent recombina tion of excess Zr atoms separated from the NCs with Zr vacancies. At the same time, annealing in the range Ta = 700–1000°C results in that bands peaked at hν ~ 2.1 eV appear in the opticalabsorption spectra (Fig. 1, curve 2), these bands being associated with the forma tion of Au NCs [13]. As the dose of Au ions is raised, the spectral posi tion of the peak associated with the plasmon absorp tion in NCs is shifted from the peak of Au NCs to higher energies (Fig. 1, curves 3–5) and lies between the absorption peaks of Zr NCs (curve 1) and Au NCs (curve 2). This may indicate that not only the embed ded Au atoms, but also Zr atoms, are involved in the formation of MNCs at high doses. In this case, several variants of Zr atoms participation in the formation of MNCs are possible. In the first of these, two NC subsystems are formed: Au NCs and Zr NCs. In the second, a system of NCs is formed from intermetallic compounds [18]. This process is the most probable in massive cascades of collisions, with the concentrations of excess implanted Au ions and knocked on Zr atoms being at a maximum simulta neously.

Surface

[260] [171]

5 nm Fig. 2. Crosssectional HRTEM images of thinfilm YSZ/Si(100) structures irradiated with Au ions at a dose Φ = 4 × 1016 cm–2; (a) before annealing and (b) upon annealing at Ta = 800°C. The insets show the correspond ing patterns of electron nanodiffraction from individual NCs.

In the third, two MNC subsystems are formed: one constituted by Au NCs, and the other constituted by NCs of intermetallic compounds. To elucidate this question, we used the crosssectional HRTEM method in the mode of electron nanodiffraction on individual MNCs. This analysis was made on thin film YSZ/Si samples, with the singlecrystal Si sub strate serving as a reference for calibrating the spatial scale in TEM images and for determining the interpla nar spacings in the nanodiffraction patterns. The nan odiffraction data demonstrated that, among the NCs in the YSZ film irradiated with Au ions at a dose Φ = 4 × 1016 cm–2 (Fig. 2a), there are both Au NCs and NCs the interplanar spacings of which do not correspond to those in Zr and Au crystalline structures (Fig. 2a). The interpretation of the nanodiffraction data showed the presence in the NC film of the Au4Zr intermetallic compound [18] having an orthorhombic crystal lattice with the [111] orientation with respect to the electron beam. In films irradiated at the same dose and sub jected to postimplantation annealing at 800°C, only Au NCs were observed (Fig. 2b). These Au nanocrys

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FORMATION OF Au4Zr NANOCRYSTALS 1

Absorption, 10−4 cm−1

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Au4Zr

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1.50 5

0.75

0 1.4

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Au

1.8 2.0 2.2 2.4 Photon energy, eV

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Fig. 3. The effect of the thermal annealing on the optical absorption spectra of YSZ single crystals irradiated with Au ions at a dose Φ = 4 × 1016 cm–2 for 1 h. Annealing tem perature Ta, °C: (1) before annealing, (2) 400, (3) 600, and (4) 1000. Curves 5 and 6 represent the decomposition of curve 1 into components corresponding to peaks associ ated with the plasmon absorption of Au (hν = 2.1 eV) and Au4Zr (hν = 2.2 eV) NCs, respectively.

tals had the [314] orientation with respect to the elec tron beam. The composition found for the NCs in the YSZ matrix makes it possible to determine the frac tions of the Au and Au4Zr phases in the total relative volume of the NCs by analysis of the opticalabsorp tion spectra. For this purpose, we decomposed the opticalabsorption peak (Fig. 3, curve 1) into Lorent zian components peaked at ~2.1 and ~2.2 eV (Fig. 3, curves 5 and 6), which correspond to the plasmon absorption in Au and Au4Zr NCs. According to the Mie theory [19], the fractions of Au and Au4Zr NCs are proportional to the integral intensities of the corre sponding components. Analysis shows that the contri bution of the Au4Zr NCs to the total relative volume of NCs is ~86%, while that of Au NCs is ~14% at an Au dose Φ = 4 × 1016 cm–2. Average NC radius r0, relative NC volume NV, NC concentration N, and electron concentration N0 in the NC material obtained from opticalabsorption spectra in accordance with the Mie theory are listed in the table. Parameters of Au4Zr and Au NCs formed in YSZ single crys tals under irradiation with Au ions at a dose Φ = 4 × 1016 cm–2 Au4Zr

Au

N0, 1022 cm–3

3.3

3.1

r0, nm

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1.8

5.6

9.7

2.7

4.1

NV, 10 N,

–3

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The decrease in the intensity of the peak associated with plasmon absorption in MNCs and its shift to lower energies with increasing annealing tempera ture Ta (Fig. 3, curves 1–4) indicate that the fraction of Zr atoms in the MNC material decreases. This effect can be attributed to the decomposition of the Au4Zr compound in the course of thermal annealing, with subsequent diffusion of Zr atoms from deep in an NC to its surface, which serves as an effective drain for Zr atoms further diffusing into the YSZ matrix to recombine there with Zr vacancies generated in the course of implantation. The decomposition tempera ture of bulk Au4Zr is ~1220°C [20], which exceeds the annealing temperatures used in the present study. It should, however, be kept in mind that the thermody namic properties of the nanosize MNC material (such as the melting point, etc.) may strongly differ from those of bulk materials due to the influence exerted by the NC surface with a small curvature radius [21]. Thus, the formation of metallic nanocrystals in the course of implantation of Au atoms into YSZ single crystals and thin films and subsequent thermal anneal ing were studied by HRTEM and opticalabsorption spectroscopy. The data obtained by these methods demonstrated that NCs composed of the Au4Zr inter metallic compound crystallizing in the orthorhombic structure are formed in the YSZ matrix in addition to Au nanocrystals. The fraction of Au4Zr nanocrystals relative to the total volume of nanocrystals is ~86%, while that of Au nanocrystals is ~14% at an Au ion dose of 4 × 1016 cm–2. When the annealing tempera ture is raised to above 400°C, a shift of the peak asso ciated with the plasmon absorption of nanocrystals and a decrease in its intensity are observed, which is due to the decomposition of the Au4Zr compound. As a consequence, mostly Au NCs are observed in YSZ sam ples annealed at elevated temperatures (800–1000°C), with the Au NCs thus being more thermally stable than Au4Zr NCs. Acknowledgments. Authors are grateful to Yu.A. Dudin, leading engineer at the Physico Techni cal Research Institute (Nizhni Novgorod State Uni versity) for performing the ion implantation. This study was supported by the Ministry of Educa tion and Science of the Russian Federation under state assignment no. 3.2441.2014/K. The study used the equipment of the Center of Collective Use–Physics of SolidState Nanostructures Scientific Educational Center at Lobachevsky State University of Nizhni Novgorod. REFERENCES 1. 2. 3. 4. 5.

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14. Y. Saito, Y. Imamura, and A. Kitahara, Nucl. Instrum. Methods Phys. Res. B 206, 272. 15. A. Y. Ho, K. Mochiduki, and K. Saito, Jpn. J. Appl. Phys. 48, 035 508 (2009). 16. H. Nakajima, K. Itoh, H. Kaneko, and Y. Tamaura, J. Phys. Chem. Solids 68, 1946 (2007). 17. B. Savoini et al., Phys. Rev. B 57, 1339 (1998). 18. E. Stolz and K. Schubert, Zeitschr. Metallk. 53 (7), 433 (1962). 19. G. Mie, Ann. Phys. 25, 377 (1908). 20. Phase Diagrams of Binary Metal Systems Ed. by N. P. Lyakishev (Mashinostroenie, Moscow, 1996), p. 424 [in Russian]. 21. V. D. Borman et al., JETP Lett. 92, 166 (2010).

Translated by M. Tagirdzhanov

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