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Desorption of gold nanoclusters (2–150 nm) by 1 GeV Pb ions
Nuclear Instruments and Methods in Physics Research B 230 (2005) 495–501 www.elsevier.com/locate/nimb
Desorption of gold nanoclusters (2–150 nm) by 1 GeV Pb ions I. Baranov a,*, S. Kirillov a, A. Novikov a, V. Obnorskii a, M. Toulemonde b, K. Wien c, S. Yarmiychuk a, V.A. Borodin d, A.E. Volkov d a
c
V.G. Khlopin Radium Institute, 2nd Murinskii avnue 28, 194021 St. Petersburg, Russia b CIRIL, Rue Claude Bloch BP 5133, 14070, Caen Cedex 5, France Institut fu¨r Kernphysik der Technische Universita¨t Darmstsdt, Schloßgartenstraße 9, 64289 Darmstadt, Germany d RCC Kurchatov Institute, Kurchatov Sq., 123182 Moskow, Russia
Abstract Nanodispersed targets of gold (grains sized at 2–150 nm) were irradiated with 956 MeV ions of Pb54+ ((dE/dx)e in gold 87 keV/nm). Ejected gold was gathered on collectors. Desorbed nanoclusters of gold were detected by means of TEM while the total matter transfer was measured by neutron activation analysis. For all the targets a part of ejected gold presents nanoclusters in the same size range as that of the grains on the corresponding targets. Desorption of nanoclusters with the size up to 90 nm was observed for the first time for atomic primary ions in the electronic stopping regime. The yield of the desorbed nanoclusters decreases from 22 to 1.4 cluster/ion with increasing the mean grain size from 6 to 30 nm. The total matter transfer measured for the target with the grain size 6–10 nm has a great value – 5 · 105 at./ion. Results are discussed. 2004 Elsevier B.V. All rights reserved. PACS: 79.20.Rf; 61.82.Rx; 36.40.c Keywords: Nanodispersed metal targets; Nanometer cluster desorption; Cluster yield
1. Introduction Already first studies have shown that very intense ejection of matter 103 to 104 at./ion – takes place while metal nanodispersed layers are irradiated with multiply charged ions (MCI) [1–3]. It *
Corresponding author. Tel.: +7 812 545 43 70; fax: +7 812 247 80 95. E-mail address: [email protected] (I. Baranov).
has been established in works [4,5] that the ejection of matter from metal islet layers under 252Cf fission fragment (FF) bombardment occurs due to the desorption of intact metal islets from the target surface. The primary process that causes all the following stages is a strong excitation of the electronic subsystem of the target by impinging ion. Desorption of nanoclusters occurs due to the size effect i.e. spatial confinement of the electronic excitation zone by the borders of nanograins. This
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.090
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I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 495–501
creates conditions of an effective energy transfer from the excited electrons to atoms. Up to now studies of the nanocluster desorption were limited by the electronic excitation energy of 25 keV/nm and the desorption was observed when the grain size was less than some threshold Dthr (for nanodispersed targets of gold and FFs Dthr is estimated as [6]). Hence employing swift heavy ions (SHI) with energies 5 MeV/nucleon one can create in the ion track great energy density and specific transient states whose relaxation can produce variety of new effects [7,8]. In this case one changes the energy deposited to the electronic subsystem of an islet, the spectrum of delta-electrons, the size of the region of primary excitation, the temperature of exited electrons in the conduction band of the metal islet at the thermalisation of hot electrons [9,10], in the leakage rate of hot electrons from the islet etc. In this article we present results of the first experiment on interaction of 956 MeV Pb ions with nanodispersed targets of gold when the ion energy loss (dE/dx)e is close to the maximum possible value for monatomic ions. The absolute yield of the nanocluster desorption is measured depend-
ing on the cluster size and compared to the yield of nanoclusters desorbed by FFs from similar targets [6]. For some targets the total transfer of the matter is measured.
2. Experimental technique Nanodispersed targets. Four nanodispersed gold islet targets with different size distributions of metal grains–islets within the size range of 2–150 nm were used in this work. Islet layers were deposited onto substrates (amorphous carbon) by vapor-deposition of gold in vacuum and were characterized by means of the transmission electron microscope (TEM) in accordance with the procedure described in works [4–6,11]. It is important to note that in order to obtain targets within as wide as possible size range of islets we had to deposit gold onto different targets at substantially different substrate temperature (Tsub = 20–400 C), what apparently lead to different adhesion of islets to substrates. TEM images of islet layers are shown in Fig. 1(a)– (d). Characteristics of the targets are compiled in Table 1 and Fig. 2(a)–(d).
Fig. 1. (a–d) TEM micrographs of islet layers of the nanodispersed gold targets #1, #2, #3, #4 and (e–h) TEM micrographs of collectors with nanoclusters desorbed from the these targets correspondingly.
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Table 1 Parameters of different nanodispersed targets of gold Target
#1
Islet size, d ± r (nm) 6.1 ± 4.3
#2
8.7 ± 6.4
The part of the target area covered by islets (%)
The density islets on the target (sm2)
Mean content of atom in grain (at.)
The energy for the sublimation of the islet completely (keV)
23
7 · 1011
9.62 · 103
36.08
344
2.86 · 104
107.25
502
27
11
4.01 · 10
10
3
Mean energy loss in the gold islet by ion of the Pba (keV)
#3
4.4 ± 4.7 24.3 ± 15.3
1,5 44
6.06 · 10 8.28 · 1010
7.0 · 10 6.0 · 105
26.3 2250
284 1424
#4
8.6 ± 6.0 91.9 ± 81.6
4 58
4.71 · 1010 8.08 · 109
6.0 · 104 2.97 · 107
225 111 375
490 5020
a
Mean energy loss was calculated by (dE/dx)e Æ 2/3Dcl.
Irradiation of the targets and collection of the ejected matter. The targets were irradiated with 956 MeV 208Pb54+ ions on the accelerator GANIL (Caen, France). The calculated stopping power of 208 Pb ions with energy 956 MeV in gold is (dE/ dx)e = 86.7 keV/nm [12]. The ion beam intensity was measured by means of a Faraday cup right before and after each irradiation. The projectile fluence hitting a target during an irradiation Nproj (1010–1011 cm2) was estimated with a precision of 15–20%. The ions impinged on the targets from the front side, angle of incidence being u = 45. The beam of primary ions was restricted by a diaphragm of 2 mm in diameter. Desorbed particles of gold were gathered on a flat sectioned collector consisting of several TEM grids, located on the surface is parallel to that of the target. The collector technique and the approach used in this work for evaluation of the angular distributions and yields of nanoclusters desorbed are described in detail in [6]. The number of desorbed nanoclusters larger than 2 nm in size was determined by means of TEM and yield of the gold nanocluster was evaluated. Y cl ¼ ðN clust cosðuÞÞ=ðN proj aÞ;
ð1Þ
where a is the coverage of a target with islets (a = 20–60%). For the targets with the mean islet size between 6 and 9 nm the total transfer of the matter regardless the type of ejected particles has been measured. The ejected matter was collected on 1 lm thick Al films. By means of neutron activation analysis the total number of gold atoms N tot at
ejected from a target was measured and the total yield of the transferred gold was evaluated. Y at ¼ ðN tot at cosðuÞÞ=ðN proj aÞ:
ð2Þ
Afterward this value was compared to that transferred in the form of nanoclusters. The matter transfer in the form of nanoclusters was calculated assuming them spherical particles with the atomic density equal to that of bulk gold nAu = 59.2 at./ nm3. inclust Y at ¼ N clust ðdN clust =dDÞ ðp D3 nAu =6Þ
cosðuÞÞ=ðN proj aÞ:
ð3Þ
Size distributions of desorbed nanoclusters dNclust/ dD (D – the nanocluster size) were built taking into account obtained angular distributions.
3. Results Desorption of nanoclusters. TEM images of desorbed nanoclusters are shown in Fig. 1(e)–(h) for all the four targets. The overall size range of the nanoclusters observed on the collectors is from 2 nm (250 atoms) to 90 nm (3 · 107 atoms). In contrast to the target islets having irregular (uneven) shape (see Fig. 1), desorbed nanoclusters with size less than 90 nm have smooth and mostly round shape (excluding the largest ones). Size distributions of the clusters desorbed there from the targets ## 1–4 are shown in Fig. 2(a)–(d) and Table 2. In the size distribution of the nanoclusters desorbed from the target #2 (one size group) one can see two size groups: the main one
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I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 495–501 0.10
(a)
Target islets Desorbed clusters
0.05
0.00 0
20
40
60
80
100
120
(b)
Target islets Desorbed clusters
Normalized ∆ N /
∆ D, 1/nm
0.05
0.00 0
20
40
60
80
100
(c) 120
Target islets Desorbed clusters
0.15
0.10
0.05
0.00 0.4
0
10
20
30
40
50
60
70
80
90
100
110
Target islets Desorbed clusters
(d)
120
0.3
0.01 0.2
0.1
0.00 25
50
75
100
125
150
0.0
0
20
40
60
80
100
120
Cluster and Islet size (D), nm Fig. 2. Size distributions of islets on the target #1 (a), #2 (b), #3 (c), #4 (d) and of clusters desorbed from these targets by GeV Pbions.
characterized by the mean size of 11.7 nm and a new one of clusters smaller than 3 nm. Angular distributions and absolute yields of the desorbed nanoclusters. For all the targets angular distributions of the desorbed nanoclusters as well as their absolute yields have been measured. The
best fit for the angular distribution is given by the function dn/dX = g(h) exp(k · h) · cos(h). Parameter k increases from 1.7 to 8 with the mean nanocluster size increasing from 4 to 28.6 nm. Mean polar angle is presented in Fig. 3 and Table 2. Absolute nanocluster yields decrease from
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499
Table 2 Parameters of size-, angular-distributions, yield desorption and total matter transfer from different targets Target
Nanocluster size, d ± r (HM)
#1
4.7 ± 4.4
hhi Mean polar angle (deg)
Yield (cl/ion)
The matter transfer as clusters (at./ion)
The total matter transfer (at./ion)
hhi Mean polar angle for the total mass transfer (deg)
The mass transfer by cluster/the total mass transfer
30
22.16
1.31 · 105
4.9 · 105
33
0.26
5
4.6 · 105
27
0.22
a
a
1.03 · 10
#2
2.8 ± 1.9 11.7 ± 8.6
21
2.96
#3
3.5 ± 4.9 28.6 ± 10.7
34 14
22.36 1.4
– –
– –
– –
– –
#4
5.7 ± 5.2 58.3 ± 16.7
24 –
2.26 0.03
– –
– –
– –
– –
a
The nanocluster yield was calculated for all size distribution.
30
50
Fission Fragments [7]
956 MeV
208
25
956 MeV
Pb
40
Abs. yield, 1/ion
Mean polar angle (<θ>),
o
Fission Fragments [7]
30
208
Pb
20 15 10
20 5 0
10 3
6
9
12
15
18
21
24
27
2
4
6
Fig. 3. Dependence of the mean polar angle of the nanocluster desorption by GeV Pb-ions and fission fragments [6] versus nanocluster size.
22 cluster/ion for islets with mean size 6 nm to 1.4 cluster/ion for islets with mean size of 28 nm (Table 2). Absolute yield in function of the nanocluster size is presented in Fig. 4. Total yield of the ejected matter. Total yields of the ejected matter regardless the particle type (atoms, clusters) were determined for the targets #1 and 2. Taking all the factors like precision of the beam monitoring, precision of the neutron activation measurements of quantities of collected gold, precision of the evaluation of the total amount of gold removed from the target, that contribute to the accuracy of the yield measurements the latter is estimated as 30%. From the Table
8
10 12 14 16 18 20 22 24 26 28 30
Nanocluster size (d), nm
Nanocluster size (d), nm
Fig. 4. Absolute yields of nanoclusters desorbed GeV Pb-ions and fission fragments [6] versus the nanocluster size.
2 one can see that the total ejection yield normalized to the target area occupied by the islets, varies between 4.5 and 4.9 · 105 at./ion, that is a big value.
4. Discussion The results of this work first of all show that swift heavy ions having electronic stopping power in gold about 9 · 104 eV/nm, i.e. of the value close to the maximum possible value for monatomic ions, are capable to produce desorption of intact nanoclusters from nanodispersed targets of gold, of the same order the size range (2–150 nm) as that
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I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 495–501
of the target grains. The transformation of the size distribution for the target #2 (Fig. 2(b)) is observed for the first time. This fact is probably a result of two simultaneous processes: (1) higher probability to hit larger islets and consequently higher desorption probability of larger nanoclusters and (2) fragmentation of nanoclusters to two or more fragments whose probability is proportional to energy per atom, i.e. 1=R2cl . It is for the first time when desorption of nanoclusters with size up to 90 nm has been observer. This value cannot be considered as the desorption size threshold since the corresponding target has been prepared at a significantly higher substrate temperature 400 C, that is with a stronger islet–substrate adhesion. Comparison of the angular distribution and of the yields of nanoclusters (Figs. 3 and 4) desorbed by the Pb ions with that obtained earlier with FFs [6] as projectiles indicate: (1) the angular distribution are narrow and stretched along the surface normal, (2) the yield of nanocluster desorption depends on the electronic stopping power of the projectiles, e.g. for targets with mean grain size of about 6 nm, the nanocluster desorption yield is 22 per ion for (dE/dx)e = 87 keV/nm and 4 per ion for (dE/dx)e = 23 keV/nm. Total matter transfer measured for the target characterized by mean sizes 6 nm and 9 nm has a great value of 5 · 105 at./ion. The observed desorption from targets presents 30% (Table 2) of the total matter transfer. One can imagine that a fragmentation takes place of the desorbed nanoclusters into smaller ones that are difficult to detect by TEM. So far the process of desorption of nanoclusters by monatomic projectiles in electronic stopping regime was not considered theoretically. Here some qualitative suggestions are presented briefly. The desorption process is connected with the size effect of nanoparticles (<100 nm). It demonstrates itself in a higher temperature of the electron gas and slower electron leakage. After an MCI passes through a nanoparticle for the time from 5 · 1015 to 5 · 1014 s, the atom lattice may be represented as a frozen ensemble of atoms [13]. However, the delta-electrons and then thermalized electrons bump into these immovable atoms. As a
result of this for this time part of energy is transferred from the electrons to the lattice atoms like from electrons of an accelerator beam. By 1013 s electron–phonon (E–Ph) interactions start. By this time the temperature of the electronic gas decreases due to thermo-electronic emission, exchange with substrate electrons, transfer of a part of the energy to islet atoms. In electron–phonon interactions energy from the electronic subsystem will be transferred faster to shell atoms of the gold islet and even faster to carbon atoms substrate layer – as defects for thermalized electrons of the gold islet. This is why in a small volume energy is accumulated under the islet, pressure appears, and the impulse directed towards the substrate surface takes off the intact islet with some kinetic energy. The yields of nanoclusters more than one nanocluster per ion is probably a result of a surface shock wave induced on the pressure pulse in the islet substrate contact area.
Acknowledgment The work is accomplished with a financial support of the International Science and Technology Center (ISTC project #2390).
References [1] B.M. Aleksandrov, I.A. Baranov, A.S. Krivohatskii, G.A. Tutin, Sov. At. Energ. 33 (1972) 941. [2] H. Andersen, H. Knudsen, H. Petersen, J. Appl. Phys. 49 (1978) 5638. [3] I.A. Baranov, Yu.V. Martynenko, S.O. Tsepelevich, Yu.N. Yavlinskii, Sov. Phys. Usp. Fiz. Nauk. 31 (11) (1988) 1015. [4] I.A. Baranov, A.C. Novikov, V.V. Obnorskii, S.O. Tsepelevich, B.N. Kozlov, I.I. Pilyugin, Nucl. Instr. and Meth. B 65 (1992) 177. [5] I. Baranov, A. Novikov, V. Obnorskii, C.T. Reimann, Nucl. Instr. and Meth. B 146 (1998) 154. [6] I. Baranov, S. Kirillov, A. Novikov, V. Obnorsky, A. Pchelintsev, S. Yarmiychuk, Nucl. Instr. and Meth. B 183 (2001) 232. [7] H. Dammak, A. Dunlop, D. Lesueur, A. Brunelle, S. Della-Negra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135. [8] C. Trautmann, M. Toulemonde, K. Schwartz, J.M. Costantini, A. Mu¨ller, Nucl. Instr. and Meth. B 164–165 (2000) 365.
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 495–501 [9] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362. [10] E. Volkov, V.A. Borodin, Nucl. Instr. and Meth. B 107 (1996) 172. [11] I. Baranov, A. Brunelle, S. Della-Negra, D. Jacquet, S. Kirillov, Y. Le Beyec, A. Novikov, V. Obnorsky, A.
501
Pchelintsev, K. Wien, S. Yarmiychuk, Nucl. Instr. and Meth. B 187 (2002) 451. [12] J. Ziegler, J. Biersack, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1990. [13] A.E. Volkov, V.A. Borodin, Nucl. Instr. and Meth. B 146 (1998) 137.
Desorption of gold nanoclusters (2–150 nm) by 1 GeV Pb ions I. Baranov a,*, S. Kirillov a, A. Novikov a, V. Obnorskii a, M. Toulemonde b, K. Wien c, S. Yarmiychuk a, V.A. Borodin d, A.E. Volkov d a
c
V.G. Khlopin Radium Institute, 2nd Murinskii avnue 28, 194021 St. Petersburg, Russia b CIRIL, Rue Claude Bloch BP 5133, 14070, Caen Cedex 5, France Institut fu¨r Kernphysik der Technische Universita¨t Darmstsdt, Schloßgartenstraße 9, 64289 Darmstadt, Germany d RCC Kurchatov Institute, Kurchatov Sq., 123182 Moskow, Russia
Abstract Nanodispersed targets of gold (grains sized at 2–150 nm) were irradiated with 956 MeV ions of Pb54+ ((dE/dx)e in gold 87 keV/nm). Ejected gold was gathered on collectors. Desorbed nanoclusters of gold were detected by means of TEM while the total matter transfer was measured by neutron activation analysis. For all the targets a part of ejected gold presents nanoclusters in the same size range as that of the grains on the corresponding targets. Desorption of nanoclusters with the size up to 90 nm was observed for the first time for atomic primary ions in the electronic stopping regime. The yield of the desorbed nanoclusters decreases from 22 to 1.4 cluster/ion with increasing the mean grain size from 6 to 30 nm. The total matter transfer measured for the target with the grain size 6–10 nm has a great value – 5 · 105 at./ion. Results are discussed. 2004 Elsevier B.V. All rights reserved. PACS: 79.20.Rf; 61.82.Rx; 36.40.c Keywords: Nanodispersed metal targets; Nanometer cluster desorption; Cluster yield
1. Introduction Already first studies have shown that very intense ejection of matter 103 to 104 at./ion – takes place while metal nanodispersed layers are irradiated with multiply charged ions (MCI) [1–3]. It *
Corresponding author. Tel.: +7 812 545 43 70; fax: +7 812 247 80 95. E-mail address: [email protected] (I. Baranov).
has been established in works [4,5] that the ejection of matter from metal islet layers under 252Cf fission fragment (FF) bombardment occurs due to the desorption of intact metal islets from the target surface. The primary process that causes all the following stages is a strong excitation of the electronic subsystem of the target by impinging ion. Desorption of nanoclusters occurs due to the size effect i.e. spatial confinement of the electronic excitation zone by the borders of nanograins. This
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.090
496
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 495–501
creates conditions of an effective energy transfer from the excited electrons to atoms. Up to now studies of the nanocluster desorption were limited by the electronic excitation energy of 25 keV/nm and the desorption was observed when the grain size was less than some threshold Dthr (for nanodispersed targets of gold and FFs Dthr is estimated as [6]). Hence employing swift heavy ions (SHI) with energies 5 MeV/nucleon one can create in the ion track great energy density and specific transient states whose relaxation can produce variety of new effects [7,8]. In this case one changes the energy deposited to the electronic subsystem of an islet, the spectrum of delta-electrons, the size of the region of primary excitation, the temperature of exited electrons in the conduction band of the metal islet at the thermalisation of hot electrons [9,10], in the leakage rate of hot electrons from the islet etc. In this article we present results of the first experiment on interaction of 956 MeV Pb ions with nanodispersed targets of gold when the ion energy loss (dE/dx)e is close to the maximum possible value for monatomic ions. The absolute yield of the nanocluster desorption is measured depend-
ing on the cluster size and compared to the yield of nanoclusters desorbed by FFs from similar targets [6]. For some targets the total transfer of the matter is measured.
2. Experimental technique Nanodispersed targets. Four nanodispersed gold islet targets with different size distributions of metal grains–islets within the size range of 2–150 nm were used in this work. Islet layers were deposited onto substrates (amorphous carbon) by vapor-deposition of gold in vacuum and were characterized by means of the transmission electron microscope (TEM) in accordance with the procedure described in works [4–6,11]. It is important to note that in order to obtain targets within as wide as possible size range of islets we had to deposit gold onto different targets at substantially different substrate temperature (Tsub = 20–400 C), what apparently lead to different adhesion of islets to substrates. TEM images of islet layers are shown in Fig. 1(a)– (d). Characteristics of the targets are compiled in Table 1 and Fig. 2(a)–(d).
Fig. 1. (a–d) TEM micrographs of islet layers of the nanodispersed gold targets #1, #2, #3, #4 and (e–h) TEM micrographs of collectors with nanoclusters desorbed from the these targets correspondingly.
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497
Table 1 Parameters of different nanodispersed targets of gold Target
#1
Islet size, d ± r (nm) 6.1 ± 4.3
#2
8.7 ± 6.4
The part of the target area covered by islets (%)
The density islets on the target (sm2)
Mean content of atom in grain (at.)
The energy for the sublimation of the islet completely (keV)
23
7 · 1011
9.62 · 103
36.08
344
2.86 · 104
107.25
502
27
11
4.01 · 10
10
3
Mean energy loss in the gold islet by ion of the Pba (keV)
#3
4.4 ± 4.7 24.3 ± 15.3
1,5 44
6.06 · 10 8.28 · 1010
7.0 · 10 6.0 · 105
26.3 2250
284 1424
#4
8.6 ± 6.0 91.9 ± 81.6
4 58
4.71 · 1010 8.08 · 109
6.0 · 104 2.97 · 107
225 111 375
490 5020
a
Mean energy loss was calculated by (dE/dx)e Æ 2/3Dcl.
Irradiation of the targets and collection of the ejected matter. The targets were irradiated with 956 MeV 208Pb54+ ions on the accelerator GANIL (Caen, France). The calculated stopping power of 208 Pb ions with energy 956 MeV in gold is (dE/ dx)e = 86.7 keV/nm [12]. The ion beam intensity was measured by means of a Faraday cup right before and after each irradiation. The projectile fluence hitting a target during an irradiation Nproj (1010–1011 cm2) was estimated with a precision of 15–20%. The ions impinged on the targets from the front side, angle of incidence being u = 45. The beam of primary ions was restricted by a diaphragm of 2 mm in diameter. Desorbed particles of gold were gathered on a flat sectioned collector consisting of several TEM grids, located on the surface is parallel to that of the target. The collector technique and the approach used in this work for evaluation of the angular distributions and yields of nanoclusters desorbed are described in detail in [6]. The number of desorbed nanoclusters larger than 2 nm in size was determined by means of TEM and yield of the gold nanocluster was evaluated. Y cl ¼ ðN clust cosðuÞÞ=ðN proj aÞ;
ð1Þ
where a is the coverage of a target with islets (a = 20–60%). For the targets with the mean islet size between 6 and 9 nm the total transfer of the matter regardless the type of ejected particles has been measured. The ejected matter was collected on 1 lm thick Al films. By means of neutron activation analysis the total number of gold atoms N tot at
ejected from a target was measured and the total yield of the transferred gold was evaluated. Y at ¼ ðN tot at cosðuÞÞ=ðN proj aÞ:
ð2Þ
Afterward this value was compared to that transferred in the form of nanoclusters. The matter transfer in the form of nanoclusters was calculated assuming them spherical particles with the atomic density equal to that of bulk gold nAu = 59.2 at./ nm3. inclust Y at ¼ N clust ðdN clust =dDÞ ðp D3 nAu =6Þ
cosðuÞÞ=ðN proj aÞ:
ð3Þ
Size distributions of desorbed nanoclusters dNclust/ dD (D – the nanocluster size) were built taking into account obtained angular distributions.
3. Results Desorption of nanoclusters. TEM images of desorbed nanoclusters are shown in Fig. 1(e)–(h) for all the four targets. The overall size range of the nanoclusters observed on the collectors is from 2 nm (250 atoms) to 90 nm (3 · 107 atoms). In contrast to the target islets having irregular (uneven) shape (see Fig. 1), desorbed nanoclusters with size less than 90 nm have smooth and mostly round shape (excluding the largest ones). Size distributions of the clusters desorbed there from the targets ## 1–4 are shown in Fig. 2(a)–(d) and Table 2. In the size distribution of the nanoclusters desorbed from the target #2 (one size group) one can see two size groups: the main one
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I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 495–501 0.10
(a)
Target islets Desorbed clusters
0.05
0.00 0
20
40
60
80
100
120
(b)
Target islets Desorbed clusters
Normalized ∆ N /
∆ D, 1/nm
0.05
0.00 0
20
40
60
80
100
(c) 120
Target islets Desorbed clusters
0.15
0.10
0.05
0.00 0.4
0
10
20
30
40
50
60
70
80
90
100
110
Target islets Desorbed clusters
(d)
120
0.3
0.01 0.2
0.1
0.00 25
50
75
100
125
150
0.0
0
20
40
60
80
100
120
Cluster and Islet size (D), nm Fig. 2. Size distributions of islets on the target #1 (a), #2 (b), #3 (c), #4 (d) and of clusters desorbed from these targets by GeV Pbions.
characterized by the mean size of 11.7 nm and a new one of clusters smaller than 3 nm. Angular distributions and absolute yields of the desorbed nanoclusters. For all the targets angular distributions of the desorbed nanoclusters as well as their absolute yields have been measured. The
best fit for the angular distribution is given by the function dn/dX = g(h) exp(k · h) · cos(h). Parameter k increases from 1.7 to 8 with the mean nanocluster size increasing from 4 to 28.6 nm. Mean polar angle is presented in Fig. 3 and Table 2. Absolute nanocluster yields decrease from
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499
Table 2 Parameters of size-, angular-distributions, yield desorption and total matter transfer from different targets Target
Nanocluster size, d ± r (HM)
#1
4.7 ± 4.4
hhi Mean polar angle (deg)
Yield (cl/ion)
The matter transfer as clusters (at./ion)
The total matter transfer (at./ion)
hhi Mean polar angle for the total mass transfer (deg)
The mass transfer by cluster/the total mass transfer
30
22.16
1.31 · 105
4.9 · 105
33
0.26
5
4.6 · 105
27
0.22
a
a
1.03 · 10
#2
2.8 ± 1.9 11.7 ± 8.6
21
2.96
#3
3.5 ± 4.9 28.6 ± 10.7
34 14
22.36 1.4
– –
– –
– –
– –
#4
5.7 ± 5.2 58.3 ± 16.7
24 –
2.26 0.03
– –
– –
– –
– –
a
The nanocluster yield was calculated for all size distribution.
30
50
Fission Fragments [7]
956 MeV
208
25
956 MeV
Pb
40
Abs. yield, 1/ion
Mean polar angle (<θ>),
o
Fission Fragments [7]
30
208
Pb
20 15 10
20 5 0
10 3
6
9
12
15
18
21
24
27
2
4
6
Fig. 3. Dependence of the mean polar angle of the nanocluster desorption by GeV Pb-ions and fission fragments [6] versus nanocluster size.
22 cluster/ion for islets with mean size 6 nm to 1.4 cluster/ion for islets with mean size of 28 nm (Table 2). Absolute yield in function of the nanocluster size is presented in Fig. 4. Total yield of the ejected matter. Total yields of the ejected matter regardless the particle type (atoms, clusters) were determined for the targets #1 and 2. Taking all the factors like precision of the beam monitoring, precision of the neutron activation measurements of quantities of collected gold, precision of the evaluation of the total amount of gold removed from the target, that contribute to the accuracy of the yield measurements the latter is estimated as 30%. From the Table
8
10 12 14 16 18 20 22 24 26 28 30
Nanocluster size (d), nm
Nanocluster size (d), nm
Fig. 4. Absolute yields of nanoclusters desorbed GeV Pb-ions and fission fragments [6] versus the nanocluster size.
2 one can see that the total ejection yield normalized to the target area occupied by the islets, varies between 4.5 and 4.9 · 105 at./ion, that is a big value.
4. Discussion The results of this work first of all show that swift heavy ions having electronic stopping power in gold about 9 · 104 eV/nm, i.e. of the value close to the maximum possible value for monatomic ions, are capable to produce desorption of intact nanoclusters from nanodispersed targets of gold, of the same order the size range (2–150 nm) as that
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of the target grains. The transformation of the size distribution for the target #2 (Fig. 2(b)) is observed for the first time. This fact is probably a result of two simultaneous processes: (1) higher probability to hit larger islets and consequently higher desorption probability of larger nanoclusters and (2) fragmentation of nanoclusters to two or more fragments whose probability is proportional to energy per atom, i.e. 1=R2cl . It is for the first time when desorption of nanoclusters with size up to 90 nm has been observer. This value cannot be considered as the desorption size threshold since the corresponding target has been prepared at a significantly higher substrate temperature 400 C, that is with a stronger islet–substrate adhesion. Comparison of the angular distribution and of the yields of nanoclusters (Figs. 3 and 4) desorbed by the Pb ions with that obtained earlier with FFs [6] as projectiles indicate: (1) the angular distribution are narrow and stretched along the surface normal, (2) the yield of nanocluster desorption depends on the electronic stopping power of the projectiles, e.g. for targets with mean grain size of about 6 nm, the nanocluster desorption yield is 22 per ion for (dE/dx)e = 87 keV/nm and 4 per ion for (dE/dx)e = 23 keV/nm. Total matter transfer measured for the target characterized by mean sizes 6 nm and 9 nm has a great value of 5 · 105 at./ion. The observed desorption from targets presents 30% (Table 2) of the total matter transfer. One can imagine that a fragmentation takes place of the desorbed nanoclusters into smaller ones that are difficult to detect by TEM. So far the process of desorption of nanoclusters by monatomic projectiles in electronic stopping regime was not considered theoretically. Here some qualitative suggestions are presented briefly. The desorption process is connected with the size effect of nanoparticles (<100 nm). It demonstrates itself in a higher temperature of the electron gas and slower electron leakage. After an MCI passes through a nanoparticle for the time from 5 · 1015 to 5 · 1014 s, the atom lattice may be represented as a frozen ensemble of atoms [13]. However, the delta-electrons and then thermalized electrons bump into these immovable atoms. As a
result of this for this time part of energy is transferred from the electrons to the lattice atoms like from electrons of an accelerator beam. By 1013 s electron–phonon (E–Ph) interactions start. By this time the temperature of the electronic gas decreases due to thermo-electronic emission, exchange with substrate electrons, transfer of a part of the energy to islet atoms. In electron–phonon interactions energy from the electronic subsystem will be transferred faster to shell atoms of the gold islet and even faster to carbon atoms substrate layer – as defects for thermalized electrons of the gold islet. This is why in a small volume energy is accumulated under the islet, pressure appears, and the impulse directed towards the substrate surface takes off the intact islet with some kinetic energy. The yields of nanoclusters more than one nanocluster per ion is probably a result of a surface shock wave induced on the pressure pulse in the islet substrate contact area.
Acknowledgment The work is accomplished with a financial support of the International Science and Technology Center (ISTC project #2390).
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