Swift heavy ion irradiation of Pt nanocrystals embedded in SiO2

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NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 266 (2008) 3158–3161 www.elsevier.com/locate/nimb

Swift heavy ion irradiation of Pt nanocrystals embedded in SiO2 R. Giulian a,*, P. Kluth a, D.J. Sprouster a, L.L. Araujo a, A. Byrne b, M.C. Ridgway a a

Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australia b Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australia Available online 25 March 2008

Abstract Pt nanocrystals (NCs) formed by ion implantation in SiO2 were irradiated with 185 MeV Au ions in the fluence range from 2  1012 to 6  1014 cm 2. Transmission electron microscopy (TEM) shows that spherical Pt NCs with diameter smaller than a certain threshold ˚ ) do not change in shape, but decrease in size with increasing irradiation fluence until dissolved in the matrix. On the other hand, (70 A ˚ change from spheres (unirradiated) to prolate spheroids (2  1013 cm 2), finally achieving a rod-like shape with NCs larger than 70 A the major axis aligned parallel to the beam direction for fluences up to 1014 cm 2. The NC minor dimension decreases with increasing ˚ , while at the same time the major irradiation fluence, resulting in a nearly monodispersed distribution with mean value around 70 A dimension increases significantly resulting in the formation of rod-like NCs with aspect ratio as great as 10. Ó 2008 Elsevier B.V. All rights reserved. PACS: 61.72.Ww; 61.46.Hk; 61.80.Jh; 61.10.Eq Keywords: Pt nanocrystals; Swift heavy ion irradiation; TEM

1. Introduction Nanocrystals (NCs) have been the subject of intensive investigation in recent years. Due to their small size and consequently large surface-to-volume ratio, NCs have characteristics that are different from their bulk counterparts [1]. The NC properties are not only influenced by their size but also by their shape. Examples include the linearly polarized emission of semiconductor quantum rods [2,3], the anisotropic magnetic properties of elongated Co nanoparticles [4] and the enhanced catalytic activities of non-spherical Pt NCs [5]. Several techniques can be used to fabricate NCs. Ion implantation followed by thermal annealing is an example with great advantages like the control in depth and concentration of impurity atoms, as well as the protection offered to the NCs by the surrounding matrix [6]. Ion irradiation prior [7] or subsequent [8] to NC formation along with *

Corresponding author. Tel.: +61 2 6125 0037; fax: +61 2 6125 0511. E-mail address: [email protected] (R. Giulian).

0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.176

variations in implanted atom concentration and annealing conditions [9] can be used to tailor the size distribution, producing NCs with application-specific dimensions. Swift heavy ion irradiation (SHII) of metallic NCs embedded in SiO2 leads to a shape transformation characterized by elongation along the direction of irradiation, as demonstrated for Co [4], Au [10] and Ag [11] NCs. Several models have been developed to explain the complex interaction between SHII and matter. The Coulomb explosion [12], viscoelastic model [13], ion hammering effect [14] and thermal spike model [15] are examples of possible mechanisms behind the changes in shape observed for solid matter under SHII. The thermal spike model, which predicts the formation of a molten track around the ion path, is one plausible explanation for elongation of metallic NCs embedded in an amorphous matrix [4,11,16]. However, further work is needed given scant and sometimes conflicting results reported thus far [10,11]. This work seeks to demonstrate the effects of SHII on Pt NCs embedded in SiO2, providing experimental evidence of fundamental importance for the development of a reliable

R. Giulian et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3158–3161

and comprehensive theoretical model. Moreover, the NC nearly monodispersed dimension distribution achieved by SHII highlights one of the advantages of using this method as a tool in NC shape transformation. 2. Experimental

˚ in diameter. tion was observed for NCs larger than 70 A At the same time, smaller NCs decreased in size upon irradiation, remaining spherical until dissolved in the matrix. The NC minor (Dminor) and major (Dmajor) dimension distributions as a function of SHII fluence are plotted in Fig. 2. Results for samples containing Pt NCs with mean ˚ (130 A ˚ ) are shown on the left (right) coldiameter 78 A umn. The values of the mean dimensions (Dminor, Dmajor) of the distributions, the distribution maximum (peakm, peakM) and the half width at half maximum (HWHM) to each side of the maximum value are listed in Table 1, as well as the mean aspect ratio of the NCs. The HWHM measured from the distribution maximum to lower NC sizes (Lm, LM) and from the distribution maximum to higher NC sizes (Hm, HM) were determined separately to better demonstrate the high asymmetry of the distributions (as a result of the latter, the distribution maximum does not always coincide with the mean dimension of the NCs). As visible from Fig. 2 and Table 1, the Dmajor distributions increase in mean value and asymmetry with increasing SHII fluences up to 2  1014 cm 2 while at the same time the Dminor distribu˚ mean diameter tions decrease. For Pt NCs with 78 A (prior to irradiation), the evolution of Dminor and Dmajor

NC Dmean= 130 Å

NC Dmean= 78 Å

unirradiated

Pt nanocrystals (NCs) were formed in 2 lm amorphous SiO2 films, thermally grown on Si substrates, using ion implantation and thermal annealing. Pt ions of 4.5 MeV were implanted at liquid N2 temperature to a total fluence of 1  1017 cm 2, resulting in a peak atomic concentration of  3 at.%. The samples were then annealed in forming gas (95% N2 + 5% H2) for one hour at different temperatures to promote NC growth. Samples annealed at ˚ , while 1200 oC yielded NCs with mean diameter of 78 A o samples annealed at 1300 C yielded NCs with mean diam˚ . Subsequently, the samples were irradiated eter of 130 A with 185 MeV Au ions at room temperature with fluences varying from 2  1012 to 6  1014 cm 2. The shape and size evolution of the NCs were evaluated by means of transmission electron microscopy (TEM) with samples prepared in cross-sectional geometry using the small angle cleavage technique [17]. The analyses were performed using a Philips CM300 microscope operating at 300 kV. TEM dimension distributions were obtained through the direct measurement of the NC minor (Dminor) and major (Dmajor) dimensions from micrographs of representative samples. At least 500 NCs were measured in each sample to yield reliable statistics.

3159

Dminor

2x10 cm

Dmajor

13 -2

Fig. 1 shows TEM micrographs of Pt NCs (mean diam˚ ) before and after SHII. The NCs, spherical in eter 130 A shape prior to irradiation, elongate in the direction of the incident beam becoming ellipsoids, with aspect ratio 2 for fluences up to 2  1013 cm 2. For SHII fluences of 2  1014 cm 2, the NCs exhibit a rod-like shape. Elonga-

Normalized number of particles

3. Results and discussion

14

2x10 cm

-2 14

6x10 cm

-2

0 Fig. 1. TEM micrographs of Pt NC samples before and after SHII at different fluences, as indicated in each panel. The mean NC diameter prior ˚. to SHII was 130 A

100

200

300

100

200

300

Particle Dimension (Å) Fig. 2. NCs dimension distributions estimated from TEM micrographs for Pt NC samples irradiated with 185 MeV Au ions at different fluences.

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R. Giulian et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3158–3161

Table 1 Pt NC mean dimensions as a function of SHII fluence ˚) Minor dimension (A

˚) Major dimension (A

Sample

SHII fluence (cm 2)

Dminor

peakm

Lm

Hm

Dmajor

peakM

LM

3 at.% 1200 oC

Unirradiated 2  1013 2  1014 6  1014

77.9 53.9 52.3 48.7

61.4 44.8 47.5 41.1

12.5 14.5 14.8 16.0

15.5 21.4 21.6 26.0

77.9 61.5 73.8 65.5

61.4 49.3 50.1 45.4

12.5 13.7 15.4 15.6

15.5 18.8 21.4 22.0

1 1.1 1.4 1.3

3 at.% 1300 oC

Unirradiated 2  1013 2  1014 6  1014

129.5 96.9 66.2 65.1

119.7 90.1 65.5 66.7

29.5 21.2 9.2 6.4

38.3 27.7 10.8 7.2

129.5 131.4 210.7 143.7

119.7 111.1 100.1 97.9

29.5 37.7 35.8 52.6

38.3 57.5 55.5 113.5

1 1.4 3.2 2.2

ARmean HM

Dminor and Dmajor represent the statistical mean values of the distributions; peakm and peakM stand for the distribution maximum; Lm and LM represent the half width at half maximum (HWHM) relative to the lower NC sizes while Hm and HM represent the HWHM relative to the higher NC sizes. ARmean stand for the mean aspect ratio of the particles.

do not differ significantly given the majority of particles are smaller than the threshold diameter required for shape transformation and thus remaining spherical. The small differences between Dminor and Dmajor are due to the few par˚ ; otherwise, the distributions ticles larger than 60–70 A reflect the abundance of spherical particles with mean diam˚ . On the other hand, samples containing NCs eter 50 A ˚ (prior to irradiation) show prowith mean diameter 130 A nounced differences between the evolution of Dminor and Dmajor with increasing SHII fluence. While the NCs Dmajor distribution increases in mean value and asymmetry, Dminor decreases upon irradiation, achieving a very narrow distribution for SHII fluences 1014 cm 2 and above. For SHII fluences above 2  1014 cm 2, the aspect ratio of the elongated NCs starts to decrease mainly due to fragmentation along the major axis, similar to what is observed for annealed nanowires [18] and known as Rayleigh instability [19]. This fragmentation can be observed in Fig. 3, which shows selected TEM micrographs of Pt NCs irradi-

ated with 4  1014 cm 2. The comparison between the observed phenomenon for the elongated NCs and the Rayleigh instability is used here in an illustrative way. Surrounding the elongated NCs, very small Pt clusters were observed, originated from NC fragmentation and dissolution. Due to their small dimensions, their contribution to the NC size distribution was negligible from the TEM point of view. The evolution of Dminor and Dmajor as a function of SHII fluence is shown in Fig. 4 where the presence of a threshold diameter required for elongation is readily apparent. For ˚ , the NC Dminor and Dmajor dimensions smaller than 70 A have approximately the same values, characteristic of spherical particles. For higher values of Dmajor the curves evolve until saturation of Dminor for fluences above 1014 cm 2. Note that upon irradiation Dminor decreases to approximately the same threshold diameter required for ˚ , resulting in a very narrow Dminor distrielongation – 70 A bution. At the same time, Dmajor increases significantly for SHII fluences up to 2  1014 cm 2, resulting in the formation of rods with aspect ratio as high as 10.

140

unirradiated 13 -2 2x10 cm

120

1x10 cm

14

-2

14

-2

14

-2

Dminor (Å)

2x10 cm

100

6x10 cm

80 60 40 20 50

100

150

200

250

300

Dmajor (Å) Fig. 3. TEM micrographs of selected Pt NCs irradiated with 185 MeV Au ions to a fluence of 4  1014 cm 2.

Fig. 4. Dminor as a function of Dmajor for Pt NC samples irradiated with 185 MeV Au ions at different fluences.

R. Giulian et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3158–3161

4. Conclusions Shape transformations were observed for embedded Pt NCs irradiated with 185 MeV Au ions. The results obtained show that spherical particles with diameter larger ˚ elongate along the irradiating beam direction, than 70 A achieving a rod-like shape with a very narrow Dminor distribution for fluences 1014 cm 2. Smaller particles decrease in size upon irradiation, remaining spherical until dissolved in the matrix. SHII fluences above 2  1014 cm 2 cause fragmentation and dissolution along the NC major axis until the particles become spherical again and undergo the same dissolution process observed for NCs smaller than the threshold diameter. The underlying mechanisms responsible for NCs shape transformation remain under investigation. Acknowledgement The authors thank the Australian Research Council for financial support. References [1] E. Roduner, Chem. Soc. Rev. 35 (2006) 583. [2] H. Htoon, J.A. Hollingworth, A.V. Malko, R. Dickerson, V.I. Klimov, Appl. Phys. Lett. 82 (2003) 4776.

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