- Email: [email protected]
Desorption of metal nanoclusters (2–40 nm) from nanodispersed targets of gold by swift heavy cluster (C8, 20 MeV) and atomic (fission fragments) projectiles
Nuclear Instruments and Methods in Physics Research B 187 (2002) 451–458 www.elsevier.com/locate/nimb
Desorption of metal nanoclusters (2–40 nm) from nanodispersed targets of gold by swift heavy cluster (C8, 20 MeV) and atomic (fission fragments) projectiles I. Baranov a,*, A. Brunelle b, S. Della-Negra b, D. Jacquet b, S. Kirillov a, Y. Le Beyec b, A. Novikov a, V. Obnorskii a, A. Pchelintsev a, K. Wien c, S. Yarmijchuk a a
c
V.G. Khlopin Radium Institute, 2nd Murinskij Avenue 28, 194021 St. Petersburg, Russia b Institut de Physique Nucl eaire, CNRS-IN2P3, 91406 Orsay Cedex, France Institut f€ ur Kernphysik der Technische Universit€at Darmstadt, Schloßgartenstraße 9, 64289 Darmstadt, Germany Received 24 April 2001; received in revised form 18 September 2001
Abstract New results on desorption of metal nanoclusters from nanodispersed targets by ions are obtained through irradiation with swift heavy cluster projectiles (20 MeV C8 ). It is shown that highly energetic cluster ions (ðdE=xÞel 2 104 eV/nm) are capable to desorb intact gold grains sized at 2–40 nm. Desorption of nanoclusters induced by 20 MeV C8 ions is compared to that induced by fission fragments (FFs) having approximately the same ðdE=dxÞel . Objects of comparison are size distributions and shapes, angular distributions and yields of the desorbed nanoclusters. The sizes of the bombarded metal grains are comparable to the range where various ‘cluster’ effects in the interaction of cluster projectiles with electrons of the target take place. However, no essential difference between the two desorption phenomena was observed. The fraction of desorbed clusters with distorted spherical shapes with sizes larger than 15 nm is somewhat higher for 20 MeV C8 ions than that for FFs. Results are discussed. 2002 Elsevier Science B.V. All rights reserved. PACS: 79.20.Rf; 61.82.Rx; 36.40.-c Keywords: Nanodispersed metal targets; Nanometer cluster desorption; Fission fragments; MeV cluster projectiles
1. Introduction It is known that when cluster ions impact the matter both enhancement and inhibition effects *
Corresponding author. Tel.: +7-812-247-5749/545-4370; fax: +7-812-247-8095/545-4370. E-mail address: [email protected] (I. Baranov).
arise compared to monatomic projectiles due to space-and-time-correlated movement of atomic cluster constituents [1,2]. One of enhanced effects observed in the electron stopping mode is track formation in bulk metal (Ti) being irradiated with 20 MeV C60 [3]: the size of the tracks (25 nm diameter, up to several 100 nm in length) is several times larger than of those created in the same
0168-583X/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 1 5 7 - 0
452
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
target by 1 GeV U ions with the same ðdE=dxÞel (40–50 keV/nm). This enhancement effect is, supposedly, mainly due to the stage of relaxation of primary electron excitation, when softer energy spectrum and shorter range of d-electrons provide for high localization of electron excitation and for more effective energy transfer to the atomic system. Other cluster effects are that electron emission from solids bombarded with swift gold clusters scales sublinearly with ðdE=dxÞel [4] and that the charge state of atomic constituents of an MeV carbon cluster after passing a foil is smaller than that of a single carbon atom [5]. These effects occur due to electrons, which are produced in the very first part of the track (some nanometers). At this stage the constituents of the cluster are not fully stripped and are moving very close to one another, mutually modifying the conditions in which each of them interacts with the electrons of the target. The question is if such enhancement or inhibition effects observed in bulk material or thin foils occur also when fast clusters impact and pass through nanometer grains – i.e. spatially limited aggregates of matter – and how they influence the desorption process of these grains. The ejection of metal nanoclusters by multicharged ions (MCI)-fission fragments (FFs) from nanodispersed metal targets was discovered in [6,7]. Earlier this phenomenon was studied and treated as ‘inelastic-electronic sputtering’ of nanodispersed metal layers by MCI [8]. Now it is known that when an MCI-FF passes through metal (Au) islets with sizes in the nanometer range (2–30 nm) spread on a substrate, the islets are desorbed as intact particles – nanoclusters [9]. The size of the desorbed grains–islets is one of the most important parameters affecting the characteristics of the desorption process. The yield of nanoclusters of gold desorbed by FFs, was shown recently to decrease with cluster size [10]. Angular distribution of the desorbed nanoclusters is stretched along the normal to the target surface and is the narrower the larger the clusters are [10]. It should be also noted that when changing the angle of incidence at least in the 0–45 range, the angular distribution of the desorbed particles remains unaffected [11]: supposedly the electron excitation energy that is deposited in the initial track first
distributes fast (1015 s) to all the volume of the islet, thus smoothing the temperature of the conductivity electron gas [8,12]. So on the time scale of energy transfer from hot electrons to atoms ð1014 –1013 Þ s the direction of the initial track is lost. The purpose of the present work was to prove if fast cluster projectiles (20 MeV C8 ) are capable of desorbing gold grains from nanodispersed targets (grain sizes 2–40 nm) as intact species and to study this process in comparison with desorption induced by atomic projectiles (MCI-FF). First, the selected bombarding ions have approximately the same stopping power in gold (20 keV/nm), and second, inside the grains the tracks of the individual cluster constituents still overlap – this can cause various ‘cluster’ effects when compared with desorption by atomic ions producing single tracks. The measured and compared quantities are sizes, shapes, angular distributions and yields of nanoclusters desorbed from the same nanodispersed targets of gold by two different types of projectiles. The measurements were performed as a function of the degree of the dispersity of the targets and of the sizes of the desorbed nanoclusters.
2. Experimental technique To obtain the dependencies of the desorption characteristics versus the dispersity of the irradiated nanodispersed (islet) layer of gold and versus the mean size of desorbed clusters three islet targets of gold with various islet size distributions in the 2–40 nm range were used. The islets of gold were produced by vapour deposition in vacuo on thin-film substrates (1 lm Al covered with 20 nm of amorphous carbon), the temperature of the substrate being 250 C, and studied afterwards by means of a transmission electron microscope (TEM). Fig. 1(a) shows a TEM micrograph of islet layer on targets #3 (with the largest islets). TEM micrographs of islet layers were used to build lateral size distributions of islets and to measure parameters of the shape of the projection of islets. As a size parameter of an islet we used the so-called equivalent diameter – i.e. the diameter of the circle having the same area as the projection of the islet
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
453
Fig. 1. TEM images (a) of islet layer of the target #3 (islet size 2–40 nm), (b) of a collector with clusters desorbed from this target by FFs and (c) of a collector with clusters desorbed from this target by 20 MeV C8 ions.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4 area=p. As a shape parameter we used the so-called elongation, i.e. the ratio between the major and the minor axis of the projection of an islet [13]. TEM images were processed with UTHSCSA ImageTool v2.0 software [14]. Each target was irradiated in vacuo (1 106 Torr) with FFs from a 252 Cf source and with C8 cluster ions of 20 MeV from a tandem accelerator (Institute de Physique Nucleaire, Orsay, France). The FFs entered the targets from the backside and desorbed the gold islets at the target exit. After passing the target substrate, the energy loss ððdE= dxÞel Þ of the light as well as of the heavy group in the gold islets had almost the same value of about 23 keV/nm. The irradiation of the target by FFs
was limited to a spot of about 2.4 mm in diameter by means of an aperture between Cf source and the target. For FFs the angle of incidence was 35 15 against the surface normal. The C8 beam hit the target at the front side within a spot of 1:8 2:2 mm2 and at the angle of incidence of 50 against the surface normal (the divergence of the beam <1). The value of ðdE=dxÞel for 20 MeV C8 ions in gold was estimated by summing up the ðdE=dxÞel values of eight independent carbon atoms moving with 2.5 MeV energy [15], that results in 21 keV/nm. A collector technique developed in the work [10] was employed to detect and study desorption of nanoclusters. All stages of the experiment
454
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
and the data processing are described in detail in [10]. Clusters ejected from the target passed the drift range (10 mm) and were collected on a flat sectioned collector consisting of several (5–7 as depending on the experiment) TEM grids. TEM micrographs of the collector grids were used to determine the following characteristics: • p cluster size distribution (equivalent diameter ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4 S=p, where S is an area of the cluster projection), • cluster projection shape distribution (elongation, i.e. the ratio of the major and the minor axis of the projection of a cluster), • angular distribution of the clusters desorbed, • cluster absolute yields Y ¼ Nclus =ðNproj aÞ, where Nclus is the total number of the clusters desorbed from the target, Nproj is the total number of projectiles that passed through the target and a is the part of the target surface covered with gold islets (a ¼ 15–30%). While measuring the nanocluster yield the fluence of projectiles was 1–2 1011 projectile/cm2 , that provided the regime where the numbers of clusters desorbed and gathered on a collector are proportional to the fluence value. It was shown earlier that while irradiating islet targets of gold with FFs this proportionality remains at least up to 5 1011 FF/cm2 [16]. In experiments with C8 ions the projectile beam intensity was measured with an MCP-detector in a regime of the modulated beam (1:4 reducing of the intensity) before and after each irradiation. During the typical time of exposure (20 min) the intensity of the beam changed within 4%, that provided estimation of the average beam intensity and of the total number of projectiles that passed through the target with an accuracy 2%. The number of FFs per second through the target were deduced from the intensity of the source measured with an Au–Si detector taking into account geometrical parameters of irradiation (active spot diameter, diameter of the diaphragm, sourcediaphragm distance) and isotropic angular distribution of FFs, accuracy of the estimation being 12%.
3. Results 3.1. Size distributions of islets and clusters Figs. 1(b) and (c) show TEM micrographs of collectors with clusters desorbed from target #3 by FFs and 20 MeV C8 , respectively. Similar micrographs were obtained for targets with smaller islets (## 1 and 2). Figs. 2 and 3 show the size distributions of islets on targets ## 3 and 1 (Figs. (a)), and clusters that were desorbed from them by FFs (Figs. (b)) and C8 ions (Figs. (c)). Numeric characteristics of these size distributions (mean size, standard deviation) for all the three targets are compiled in Table 1. For two-humped cluster and islet size distributions (targets ## 2, 3) mean size and standard deviation (StD) are given for both size groups separately.
Fig. 2. Size distributions of (a) islets on the target #3 (islet size 2–40 nm), (b) of clusters desorbed from this target by FFs and (c) of clusters desorbed from this target by 20 MeV C8 ions.
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
455
elongated they are. Elongation (the ratio of the major and the minor axis of the projection of a cluster) increases from 1:2 0:2 for clusters with 5 nm sizes to 1:4 0:3 for those with 21 nm sizes. The projection of most clusters observed on the collector is, however, circular as seen in Fig. 1(b) and (c). The corresponding elongation was found to be 1:1 0:1 for clusters, which had sizes up to 21 nm in case of FF induced desorption and up to 15 nm in case of C8 induced desorption. For larger clusters we observed that a certain fraction (30% for C8 and 12% for FF) exhibited irregular shapes similar to that of the target islets. Mean and maximum elongation of these large clusters was found to be 1.2 and 1.8, respectively. 3.3. Angular distributions of the desorbed clusters
Fig. 3. Size distributions of (a) islets on the target #1 (islet size 2–12 nm), (b) of clusters desorbed from this target by FFs and (c) of clusters desorbed from this target by 20 MeV C8 ions.
3.2. Islet and cluster shape One can see from Fig. 1(a) that islets on the target have a slightly elongated and sometimes irregular shape. The larger the islet size the more
Angular distributions were determined for clusters desorbed from all three targets by FFs and 20 MeV C8 . For each pair ‘target’–‘projectile’ angular distributions were determined for 2–4 size windows within the size distribution for the given statistic ensemble of clusters. For both types of projectiles the angular distributions are directed along the surface normal and well described by dn=dX ¼ gðhÞ expðkhÞ cosðhÞ function. The larger the cluster size the narrower the angular distribution. Parameter of the exponent k grows from 1.2 to 4.5 with the mean desorbed cluster size growth from 3 to 21 nm. Angular distributions were used to determine the mean polar angles of cluster detachment from the target. For each cluster size window the mean value of the cluster
Table 1 Characteristics of size distributions of islets on the targets and clusters desorbed by FFs and C8 ions Entire statistical ensemble
‘Heavy’ group
‘Light’ group
Mean (D) (nm)
StD (nm)
Mean (D) (nm)
Target 1 Coll. FF Coll. C8
5.7 6.6 6.7
2 2.5 2.5
One hump distribution One hump distribution One hump distribution
StD (nm)
Mean (D) (nm)
StD (nm)
Target 2 Coll. FF Coll. C8
7.0 11.0 10.5
4.1 5.1 3.8
8.9 12.1 11.2
3.8 4.2 2.9
2.3 2.3 3.4
1.0 0.9 0.9
Target 3 Coll. FF Coll. C8
10.3 14.5 14.4
7.7 7.2 11.1
15.4 17.9 14.6
7.4 7.1 13
1.5 3.4 4.2
2.9 3 1.9
456
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
clusters were chosen wider than in the measurements of the angular distributions. They covered either the whole islet and cluster size distribution (target #1) or separately the ‘light’ and the ‘heavy’ size groups distinguished within the two hump size distributions (targets ## 2, 3). For the target #2 the yields were measured only for the ‘heavy cluster group’ since for the ‘light’ group statistics occurred insufficient. For each cluster size window the mean value of the cluster size was determined. The absolute yields plotted versus the mean cluster size are presented in Fig. 5. Fig. 4. Dependence of mean polar angle of nanocluster desorption versus nanocluster size.
size was determined. Fig. 4 shows the results obtained for various ‘target’–‘projectile’–‘cluster size sub-range’ combinations in coordinates: ‘mean desorption polar angle’–‘mean size of the cluster desorbed’. 3.4. Cluster yield Absolute yields of desorbed clusters were measured for each target–projectile pair. The yield of clusters was normalized to the coverage of the targets with islets within chosen size windows. The size windows for target islets and for desorbed
Fig. 5. Absolute yields of nanoclusters of gold desorbed by FFs and by 20 MeV C8 ions in dependence of nanocluster size.
4. Discussion The main result of the present work is the establishing that highly energetic cluster ions (20 MeV C8 , ðdE=xÞel 2 104 eV/nm) are capable to desorb gold grains from nanodispersed (islet) targets of gold, size of metal grains–islets being 2–40 nm. The largest clusters desorbed by cluster as well as by atomic projectiles had a size of 40 nm, they contained 2106 atoms, their mass was 48 amu. But this should not be considered as an upper size/mass limit – larger islets were not present on the targets. Another important result is that comparing the two employed desorption methods – desorption induced by 20 MeV – C8 cluster ions and by fission fragments – at the energy loss of 20 keV/nm in both cases, it turned out that there is no essential difference between the two desorption phenomena: (1) Clusters desorbed by both types of projectiles from the same targets have almost the same size distribution defined by that of islets on the target. These size distributions are in fact slightly shifted towards larger sizes, as demonstrated by the mean sizes in Table 1 (see also Figs. 2 and 3). This shift is probably due to the geometrical desorption cross-section being proportional to the square of the islet diameter. Larger islets occupy larger part of the target, and the projectiles hit them more frequently. (2) Both for C8 and FF bombardment the desorption of clusters in the studied size range is accompanied with changes in the shape of the particles, this change probably being due to melt-
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
ing. In most cases the islets of irregular shape are transformed into particles of a regular round shape. According to [9] the three-dimensional shape of the desorbed clusters is close to ideally spherical, i.e. they are desorbed as liquid metal drops. Starting from the size of the desorbed 15–20 nm nanoclusters a small fraction of the particles with the irregular shape similar to that of the target islets appears (30% for C8 and 12% for FF). It is probable that in these cases the energy deposited into the islets by projectiles was sufficient to desorb but not to melt them. (3) The character of the detachment of the nanoclusters from the target is also the same. For cluster as well as for atomic projectiles, the angular distributions of the nanoclusters are narrow and stretched along the surface normal. Fig. 4 shows that regardless of the projectile type all the experimental points ‘mean desorption angle’–‘mean desorbed cluster size’ are more or less evenly distributed along a single curve. The angular distribution in both cases is defined solely by the size of desorbed nanoclusters: the larger the cluster size, the narrower the angular distribution. The mean polar angle hhi decreases from 45 to 22 when the mean nanocluster size hdi increases from 3.5 to 26.5 nm. (4) Fig. 5 demonstrates that the yields of clusters desorbed by FFs and C8 are close both with respect to values and to the character of the dependence versus the cluster size. For both types of the projectiles the yield of clusters desorbed decreases rapidly from 3 to 1 while the cluster size increase from 4 to 8 nm. In the 8–18 nm size range the yield of clusters decreases more slowly from 1 to 0.1. Yields > 1 for the nanoclusters with sizes <8 nm indicate that some part of the target islets of such size can be desorbed without passing through them of a projectile. This effect was also observed in the works [10,17]. And there seems to be no essential difference between forward experiment like in [10] and in the case of FF-irradiation of the present work and backward experiment like in [17] and in the case of C8 irradiation of the present work. Some ideas were proposed in [8,10,17,18] to explain it, e.g. excited electrons come from the impinged islet to the neighboring ones either geometrically through
457
vacuum or tunneling through a conducting substrate, islets are removed by a shock wave etc. However, no direct experiment has been carried out yet. The only thing one can conclude now, that the ‘additional’ clusters are not products of fragmentation of the larger ones. If it were so one would observe an increase of the yield of small clusters from the target where two size groups coexist (target #3) compared to the target with only one size group (target #1). Actually we observe the yields to be very close (compare the two first points for FFs in Fig. 5). For clusters larger than 8 nm the absolute yield for desorption by cluster projectiles becomes slightly lower than that for clusters of the same size that were desorbed by FFs. But this difference falls within the error bars (see Fig. 5). Acknowledgements This work was supported by a grant from the International Science and Technology Center (ISTC project # 902-98). References [1] Y. Le Beyec, Int. J. Mass Spectrom. Ion Proc. 174 (1998) 101. [2] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Ch. Schoppmann, Nucl. Instr. and Meth. B 125 (1997) 207. [3] H. Dammak, A. Dunlop, D. Lesueur, A. Brunlle, S. DellaNegra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135. [4] K. Baudin, A. Brunelle, S. Della-Negra, J. Depauw, Y. Le Beyec, E.S. Parilis, Nucl. Instr. and Meth. B 117 (1996) 47. [5] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Phys. Rev. A 59 (1999) 4456. [6] 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. [7] V.-T. Nguyen, K. Wien, I.A. Baranov, A.C. Novikov, V.V. Obnorskii, Rapid. Commun. Mass Spect. 10 (1996) 1463. [8] I.A. Baranov, V.V. Obnorskii, S.O. Tsepelevich, Nucl. Instr. and Meth. B 35 (1988) 140. [9] I. Baranov, A. Novikov, V. Obnorskii, C.T. Reimann, Nucl. Instr. and Meth. B 146 (1998) 154. [10] I. Baranov, S. Kirillov, A. Novikov, V. Obnorsky, A. Pchelintsev, S. Yarmijchuk, Nucl. Instr. and Meth. B 183 (2001) 1232. [11] I.A. Baranov, S.O. Tsepelevich, Voprosy Atomnoy Nauki i Tekhniki, Ser. FRP i RM Vol. 1 (39) (1987) 75.
458
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
[12] I.A. Baranov, Yu.V. Martynenko, S.O. Tsepelevich, Yu.N. Yavlinskii, Sov. Phys. Usp. Fiz. Nauk. 31 (11) (1988) 1015. [13] Gregory A. Baxes, Digital Processing, Wiley, New York, 1994. [14] The program is available from the Internet by anonymous FTP from maxrad6.uthscsa.edu. [15] J.F. Ziegler, J.P. Biersack, U. Litmark, in: J.F. Ziegler (Ed.), Stopping Power and Ranges of Ions in Matter,
Vol. 1, Pergamon, New York, 1985, We used the version TRIM 90. [16] V.V. Obnorskii, Ph.D. Thesis (in Russian), V.G. Khlopin Radium Institute, Leningrad, 1986. [17] H.H. Andersen, H. Knudsen, P. Moller Petersen, J. Appl. Phys. 49 (1978) 5638. [18] I.A. Baranov, V.V. Obnorskii, Radiat. Eff. 79 (1983) 1.
Desorption of metal nanoclusters (2–40 nm) from nanodispersed targets of gold by swift heavy cluster (C8, 20 MeV) and atomic (fission fragments) projectiles I. Baranov a,*, A. Brunelle b, S. Della-Negra b, D. Jacquet b, S. Kirillov a, Y. Le Beyec b, A. Novikov a, V. Obnorskii a, A. Pchelintsev a, K. Wien c, S. Yarmijchuk a a
c
V.G. Khlopin Radium Institute, 2nd Murinskij Avenue 28, 194021 St. Petersburg, Russia b Institut de Physique Nucl eaire, CNRS-IN2P3, 91406 Orsay Cedex, France Institut f€ ur Kernphysik der Technische Universit€at Darmstadt, Schloßgartenstraße 9, 64289 Darmstadt, Germany Received 24 April 2001; received in revised form 18 September 2001
Abstract New results on desorption of metal nanoclusters from nanodispersed targets by ions are obtained through irradiation with swift heavy cluster projectiles (20 MeV C8 ). It is shown that highly energetic cluster ions (ðdE=xÞel 2 104 eV/nm) are capable to desorb intact gold grains sized at 2–40 nm. Desorption of nanoclusters induced by 20 MeV C8 ions is compared to that induced by fission fragments (FFs) having approximately the same ðdE=dxÞel . Objects of comparison are size distributions and shapes, angular distributions and yields of the desorbed nanoclusters. The sizes of the bombarded metal grains are comparable to the range where various ‘cluster’ effects in the interaction of cluster projectiles with electrons of the target take place. However, no essential difference between the two desorption phenomena was observed. The fraction of desorbed clusters with distorted spherical shapes with sizes larger than 15 nm is somewhat higher for 20 MeV C8 ions than that for FFs. Results are discussed. 2002 Elsevier Science B.V. All rights reserved. PACS: 79.20.Rf; 61.82.Rx; 36.40.-c Keywords: Nanodispersed metal targets; Nanometer cluster desorption; Fission fragments; MeV cluster projectiles
1. Introduction It is known that when cluster ions impact the matter both enhancement and inhibition effects *
Corresponding author. Tel.: +7-812-247-5749/545-4370; fax: +7-812-247-8095/545-4370. E-mail address: [email protected] (I. Baranov).
arise compared to monatomic projectiles due to space-and-time-correlated movement of atomic cluster constituents [1,2]. One of enhanced effects observed in the electron stopping mode is track formation in bulk metal (Ti) being irradiated with 20 MeV C60 [3]: the size of the tracks (25 nm diameter, up to several 100 nm in length) is several times larger than of those created in the same
0168-583X/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 1 5 7 - 0
452
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
target by 1 GeV U ions with the same ðdE=dxÞel (40–50 keV/nm). This enhancement effect is, supposedly, mainly due to the stage of relaxation of primary electron excitation, when softer energy spectrum and shorter range of d-electrons provide for high localization of electron excitation and for more effective energy transfer to the atomic system. Other cluster effects are that electron emission from solids bombarded with swift gold clusters scales sublinearly with ðdE=dxÞel [4] and that the charge state of atomic constituents of an MeV carbon cluster after passing a foil is smaller than that of a single carbon atom [5]. These effects occur due to electrons, which are produced in the very first part of the track (some nanometers). At this stage the constituents of the cluster are not fully stripped and are moving very close to one another, mutually modifying the conditions in which each of them interacts with the electrons of the target. The question is if such enhancement or inhibition effects observed in bulk material or thin foils occur also when fast clusters impact and pass through nanometer grains – i.e. spatially limited aggregates of matter – and how they influence the desorption process of these grains. The ejection of metal nanoclusters by multicharged ions (MCI)-fission fragments (FFs) from nanodispersed metal targets was discovered in [6,7]. Earlier this phenomenon was studied and treated as ‘inelastic-electronic sputtering’ of nanodispersed metal layers by MCI [8]. Now it is known that when an MCI-FF passes through metal (Au) islets with sizes in the nanometer range (2–30 nm) spread on a substrate, the islets are desorbed as intact particles – nanoclusters [9]. The size of the desorbed grains–islets is one of the most important parameters affecting the characteristics of the desorption process. The yield of nanoclusters of gold desorbed by FFs, was shown recently to decrease with cluster size [10]. Angular distribution of the desorbed nanoclusters is stretched along the normal to the target surface and is the narrower the larger the clusters are [10]. It should be also noted that when changing the angle of incidence at least in the 0–45 range, the angular distribution of the desorbed particles remains unaffected [11]: supposedly the electron excitation energy that is deposited in the initial track first
distributes fast (1015 s) to all the volume of the islet, thus smoothing the temperature of the conductivity electron gas [8,12]. So on the time scale of energy transfer from hot electrons to atoms ð1014 –1013 Þ s the direction of the initial track is lost. The purpose of the present work was to prove if fast cluster projectiles (20 MeV C8 ) are capable of desorbing gold grains from nanodispersed targets (grain sizes 2–40 nm) as intact species and to study this process in comparison with desorption induced by atomic projectiles (MCI-FF). First, the selected bombarding ions have approximately the same stopping power in gold (20 keV/nm), and second, inside the grains the tracks of the individual cluster constituents still overlap – this can cause various ‘cluster’ effects when compared with desorption by atomic ions producing single tracks. The measured and compared quantities are sizes, shapes, angular distributions and yields of nanoclusters desorbed from the same nanodispersed targets of gold by two different types of projectiles. The measurements were performed as a function of the degree of the dispersity of the targets and of the sizes of the desorbed nanoclusters.
2. Experimental technique To obtain the dependencies of the desorption characteristics versus the dispersity of the irradiated nanodispersed (islet) layer of gold and versus the mean size of desorbed clusters three islet targets of gold with various islet size distributions in the 2–40 nm range were used. The islets of gold were produced by vapour deposition in vacuo on thin-film substrates (1 lm Al covered with 20 nm of amorphous carbon), the temperature of the substrate being 250 C, and studied afterwards by means of a transmission electron microscope (TEM). Fig. 1(a) shows a TEM micrograph of islet layer on targets #3 (with the largest islets). TEM micrographs of islet layers were used to build lateral size distributions of islets and to measure parameters of the shape of the projection of islets. As a size parameter of an islet we used the so-called equivalent diameter – i.e. the diameter of the circle having the same area as the projection of the islet
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
453
Fig. 1. TEM images (a) of islet layer of the target #3 (islet size 2–40 nm), (b) of a collector with clusters desorbed from this target by FFs and (c) of a collector with clusters desorbed from this target by 20 MeV C8 ions.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4 area=p. As a shape parameter we used the so-called elongation, i.e. the ratio between the major and the minor axis of the projection of an islet [13]. TEM images were processed with UTHSCSA ImageTool v2.0 software [14]. Each target was irradiated in vacuo (1 106 Torr) with FFs from a 252 Cf source and with C8 cluster ions of 20 MeV from a tandem accelerator (Institute de Physique Nucleaire, Orsay, France). The FFs entered the targets from the backside and desorbed the gold islets at the target exit. After passing the target substrate, the energy loss ððdE= dxÞel Þ of the light as well as of the heavy group in the gold islets had almost the same value of about 23 keV/nm. The irradiation of the target by FFs
was limited to a spot of about 2.4 mm in diameter by means of an aperture between Cf source and the target. For FFs the angle of incidence was 35 15 against the surface normal. The C8 beam hit the target at the front side within a spot of 1:8 2:2 mm2 and at the angle of incidence of 50 against the surface normal (the divergence of the beam <1). The value of ðdE=dxÞel for 20 MeV C8 ions in gold was estimated by summing up the ðdE=dxÞel values of eight independent carbon atoms moving with 2.5 MeV energy [15], that results in 21 keV/nm. A collector technique developed in the work [10] was employed to detect and study desorption of nanoclusters. All stages of the experiment
454
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
and the data processing are described in detail in [10]. Clusters ejected from the target passed the drift range (10 mm) and were collected on a flat sectioned collector consisting of several (5–7 as depending on the experiment) TEM grids. TEM micrographs of the collector grids were used to determine the following characteristics: • p cluster size distribution (equivalent diameter ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4 S=p, where S is an area of the cluster projection), • cluster projection shape distribution (elongation, i.e. the ratio of the major and the minor axis of the projection of a cluster), • angular distribution of the clusters desorbed, • cluster absolute yields Y ¼ Nclus =ðNproj aÞ, where Nclus is the total number of the clusters desorbed from the target, Nproj is the total number of projectiles that passed through the target and a is the part of the target surface covered with gold islets (a ¼ 15–30%). While measuring the nanocluster yield the fluence of projectiles was 1–2 1011 projectile/cm2 , that provided the regime where the numbers of clusters desorbed and gathered on a collector are proportional to the fluence value. It was shown earlier that while irradiating islet targets of gold with FFs this proportionality remains at least up to 5 1011 FF/cm2 [16]. In experiments with C8 ions the projectile beam intensity was measured with an MCP-detector in a regime of the modulated beam (1:4 reducing of the intensity) before and after each irradiation. During the typical time of exposure (20 min) the intensity of the beam changed within 4%, that provided estimation of the average beam intensity and of the total number of projectiles that passed through the target with an accuracy 2%. The number of FFs per second through the target were deduced from the intensity of the source measured with an Au–Si detector taking into account geometrical parameters of irradiation (active spot diameter, diameter of the diaphragm, sourcediaphragm distance) and isotropic angular distribution of FFs, accuracy of the estimation being 12%.
3. Results 3.1. Size distributions of islets and clusters Figs. 1(b) and (c) show TEM micrographs of collectors with clusters desorbed from target #3 by FFs and 20 MeV C8 , respectively. Similar micrographs were obtained for targets with smaller islets (## 1 and 2). Figs. 2 and 3 show the size distributions of islets on targets ## 3 and 1 (Figs. (a)), and clusters that were desorbed from them by FFs (Figs. (b)) and C8 ions (Figs. (c)). Numeric characteristics of these size distributions (mean size, standard deviation) for all the three targets are compiled in Table 1. For two-humped cluster and islet size distributions (targets ## 2, 3) mean size and standard deviation (StD) are given for both size groups separately.
Fig. 2. Size distributions of (a) islets on the target #3 (islet size 2–40 nm), (b) of clusters desorbed from this target by FFs and (c) of clusters desorbed from this target by 20 MeV C8 ions.
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
455
elongated they are. Elongation (the ratio of the major and the minor axis of the projection of a cluster) increases from 1:2 0:2 for clusters with 5 nm sizes to 1:4 0:3 for those with 21 nm sizes. The projection of most clusters observed on the collector is, however, circular as seen in Fig. 1(b) and (c). The corresponding elongation was found to be 1:1 0:1 for clusters, which had sizes up to 21 nm in case of FF induced desorption and up to 15 nm in case of C8 induced desorption. For larger clusters we observed that a certain fraction (30% for C8 and 12% for FF) exhibited irregular shapes similar to that of the target islets. Mean and maximum elongation of these large clusters was found to be 1.2 and 1.8, respectively. 3.3. Angular distributions of the desorbed clusters
Fig. 3. Size distributions of (a) islets on the target #1 (islet size 2–12 nm), (b) of clusters desorbed from this target by FFs and (c) of clusters desorbed from this target by 20 MeV C8 ions.
3.2. Islet and cluster shape One can see from Fig. 1(a) that islets on the target have a slightly elongated and sometimes irregular shape. The larger the islet size the more
Angular distributions were determined for clusters desorbed from all three targets by FFs and 20 MeV C8 . For each pair ‘target’–‘projectile’ angular distributions were determined for 2–4 size windows within the size distribution for the given statistic ensemble of clusters. For both types of projectiles the angular distributions are directed along the surface normal and well described by dn=dX ¼ gðhÞ expðkhÞ cosðhÞ function. The larger the cluster size the narrower the angular distribution. Parameter of the exponent k grows from 1.2 to 4.5 with the mean desorbed cluster size growth from 3 to 21 nm. Angular distributions were used to determine the mean polar angles of cluster detachment from the target. For each cluster size window the mean value of the cluster
Table 1 Characteristics of size distributions of islets on the targets and clusters desorbed by FFs and C8 ions Entire statistical ensemble
‘Heavy’ group
‘Light’ group
Mean (D) (nm)
StD (nm)
Mean (D) (nm)
Target 1 Coll. FF Coll. C8
5.7 6.6 6.7
2 2.5 2.5
One hump distribution One hump distribution One hump distribution
StD (nm)
Mean (D) (nm)
StD (nm)
Target 2 Coll. FF Coll. C8
7.0 11.0 10.5
4.1 5.1 3.8
8.9 12.1 11.2
3.8 4.2 2.9
2.3 2.3 3.4
1.0 0.9 0.9
Target 3 Coll. FF Coll. C8
10.3 14.5 14.4
7.7 7.2 11.1
15.4 17.9 14.6
7.4 7.1 13
1.5 3.4 4.2
2.9 3 1.9
456
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
clusters were chosen wider than in the measurements of the angular distributions. They covered either the whole islet and cluster size distribution (target #1) or separately the ‘light’ and the ‘heavy’ size groups distinguished within the two hump size distributions (targets ## 2, 3). For the target #2 the yields were measured only for the ‘heavy cluster group’ since for the ‘light’ group statistics occurred insufficient. For each cluster size window the mean value of the cluster size was determined. The absolute yields plotted versus the mean cluster size are presented in Fig. 5. Fig. 4. Dependence of mean polar angle of nanocluster desorption versus nanocluster size.
size was determined. Fig. 4 shows the results obtained for various ‘target’–‘projectile’–‘cluster size sub-range’ combinations in coordinates: ‘mean desorption polar angle’–‘mean size of the cluster desorbed’. 3.4. Cluster yield Absolute yields of desorbed clusters were measured for each target–projectile pair. The yield of clusters was normalized to the coverage of the targets with islets within chosen size windows. The size windows for target islets and for desorbed
Fig. 5. Absolute yields of nanoclusters of gold desorbed by FFs and by 20 MeV C8 ions in dependence of nanocluster size.
4. Discussion The main result of the present work is the establishing that highly energetic cluster ions (20 MeV C8 , ðdE=xÞel 2 104 eV/nm) are capable to desorb gold grains from nanodispersed (islet) targets of gold, size of metal grains–islets being 2–40 nm. The largest clusters desorbed by cluster as well as by atomic projectiles had a size of 40 nm, they contained 2106 atoms, their mass was 48 amu. But this should not be considered as an upper size/mass limit – larger islets were not present on the targets. Another important result is that comparing the two employed desorption methods – desorption induced by 20 MeV – C8 cluster ions and by fission fragments – at the energy loss of 20 keV/nm in both cases, it turned out that there is no essential difference between the two desorption phenomena: (1) Clusters desorbed by both types of projectiles from the same targets have almost the same size distribution defined by that of islets on the target. These size distributions are in fact slightly shifted towards larger sizes, as demonstrated by the mean sizes in Table 1 (see also Figs. 2 and 3). This shift is probably due to the geometrical desorption cross-section being proportional to the square of the islet diameter. Larger islets occupy larger part of the target, and the projectiles hit them more frequently. (2) Both for C8 and FF bombardment the desorption of clusters in the studied size range is accompanied with changes in the shape of the particles, this change probably being due to melt-
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
ing. In most cases the islets of irregular shape are transformed into particles of a regular round shape. According to [9] the three-dimensional shape of the desorbed clusters is close to ideally spherical, i.e. they are desorbed as liquid metal drops. Starting from the size of the desorbed 15–20 nm nanoclusters a small fraction of the particles with the irregular shape similar to that of the target islets appears (30% for C8 and 12% for FF). It is probable that in these cases the energy deposited into the islets by projectiles was sufficient to desorb but not to melt them. (3) The character of the detachment of the nanoclusters from the target is also the same. For cluster as well as for atomic projectiles, the angular distributions of the nanoclusters are narrow and stretched along the surface normal. Fig. 4 shows that regardless of the projectile type all the experimental points ‘mean desorption angle’–‘mean desorbed cluster size’ are more or less evenly distributed along a single curve. The angular distribution in both cases is defined solely by the size of desorbed nanoclusters: the larger the cluster size, the narrower the angular distribution. The mean polar angle hhi decreases from 45 to 22 when the mean nanocluster size hdi increases from 3.5 to 26.5 nm. (4) Fig. 5 demonstrates that the yields of clusters desorbed by FFs and C8 are close both with respect to values and to the character of the dependence versus the cluster size. For both types of the projectiles the yield of clusters desorbed decreases rapidly from 3 to 1 while the cluster size increase from 4 to 8 nm. In the 8–18 nm size range the yield of clusters decreases more slowly from 1 to 0.1. Yields > 1 for the nanoclusters with sizes <8 nm indicate that some part of the target islets of such size can be desorbed without passing through them of a projectile. This effect was also observed in the works [10,17]. And there seems to be no essential difference between forward experiment like in [10] and in the case of FF-irradiation of the present work and backward experiment like in [17] and in the case of C8 irradiation of the present work. Some ideas were proposed in [8,10,17,18] to explain it, e.g. excited electrons come from the impinged islet to the neighboring ones either geometrically through
457
vacuum or tunneling through a conducting substrate, islets are removed by a shock wave etc. However, no direct experiment has been carried out yet. The only thing one can conclude now, that the ‘additional’ clusters are not products of fragmentation of the larger ones. If it were so one would observe an increase of the yield of small clusters from the target where two size groups coexist (target #3) compared to the target with only one size group (target #1). Actually we observe the yields to be very close (compare the two first points for FFs in Fig. 5). For clusters larger than 8 nm the absolute yield for desorption by cluster projectiles becomes slightly lower than that for clusters of the same size that were desorbed by FFs. But this difference falls within the error bars (see Fig. 5). Acknowledgements This work was supported by a grant from the International Science and Technology Center (ISTC project # 902-98). References [1] Y. Le Beyec, Int. J. Mass Spectrom. Ion Proc. 174 (1998) 101. [2] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Ch. Schoppmann, Nucl. Instr. and Meth. B 125 (1997) 207. [3] H. Dammak, A. Dunlop, D. Lesueur, A. Brunlle, S. DellaNegra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135. [4] K. Baudin, A. Brunelle, S. Della-Negra, J. Depauw, Y. Le Beyec, E.S. Parilis, Nucl. Instr. and Meth. B 117 (1996) 47. [5] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Phys. Rev. A 59 (1999) 4456. [6] 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. [7] V.-T. Nguyen, K. Wien, I.A. Baranov, A.C. Novikov, V.V. Obnorskii, Rapid. Commun. Mass Spect. 10 (1996) 1463. [8] I.A. Baranov, V.V. Obnorskii, S.O. Tsepelevich, Nucl. Instr. and Meth. B 35 (1988) 140. [9] I. Baranov, A. Novikov, V. Obnorskii, C.T. Reimann, Nucl. Instr. and Meth. B 146 (1998) 154. [10] I. Baranov, S. Kirillov, A. Novikov, V. Obnorsky, A. Pchelintsev, S. Yarmijchuk, Nucl. Instr. and Meth. B 183 (2001) 1232. [11] I.A. Baranov, S.O. Tsepelevich, Voprosy Atomnoy Nauki i Tekhniki, Ser. FRP i RM Vol. 1 (39) (1987) 75.
458
I. Baranov et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 451–458
[12] I.A. Baranov, Yu.V. Martynenko, S.O. Tsepelevich, Yu.N. Yavlinskii, Sov. Phys. Usp. Fiz. Nauk. 31 (11) (1988) 1015. [13] Gregory A. Baxes, Digital Processing, Wiley, New York, 1994. [14] The program is available from the Internet by anonymous FTP from maxrad6.uthscsa.edu. [15] J.F. Ziegler, J.P. Biersack, U. Litmark, in: J.F. Ziegler (Ed.), Stopping Power and Ranges of Ions in Matter,
Vol. 1, Pergamon, New York, 1985, We used the version TRIM 90. [16] V.V. Obnorskii, Ph.D. Thesis (in Russian), V.G. Khlopin Radium Institute, Leningrad, 1986. [17] H.H. Andersen, H. Knudsen, P. Moller Petersen, J. Appl. Phys. 49 (1978) 5638. [18] I.A. Baranov, V.V. Obnorskii, Radiat. Eff. 79 (1983) 1.