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N IM B
Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 258 (2007) 109-115 www.elsevier.com/locate/nimb

Ion beam-induced electron emission from carbon-based materials
A.M. Borisov *, E.S. Mashkova
Institute of Nuclear Physics, Moscow State University, 119992 Moscow, Russia Available online 22 December 2006

Abstract In this paper, the regularities of ion beam-induced electron emission from modified surface layers of different carbon-based materials under high-fluence irradiation are outlined. The materials considered here are those whose bonding is sp2 - polygranular graphites, highly oriented pyrolytic graphites and glassy carbons. The results are discussed in terms of accumulation and annealing of the radiation damage and their effect on electron transport in carbon-based materials. Ó 2006 Elsevier B.V. All rights reserved.
PACS: 34.50.Dy; 79.20.Rf Keywords: Carbon based materials; High-fluence ion irradiation; Ion-induced electron emission; Fullerene-related structures

1. Introduction The electron emission from solids under atomic bombardment - electronic stopping related phenomenon - has been extensively studied for many decades [1-4]. Many processes of ion interaction with solids, for example, ion scattering, sputtering, ion-induced electron and photon emission are sensitive to a degree of solid order [5-7] The regularities of ion-induced electron emission were used to analyze both the order-disorder changes in semiconductors [7-9] and the transitions to steady-state conditions [10,11]. It is also known that electron emission yield for polycrystalline metals virtually does not depend on target temperature if in the studied temperature range there are no any structural phase transitions [2,3]. Systematic investigations of kinetic ion-electron emission of carbon-based materials were started in the Institute of Nuclear Physics of Moscow State University in early this millennium [12-17]. The angular and temperature dependences of ion-electron yield, c, developed surface topography, the surface layer structure changes for both the graphitizing materials (polygranular graphites, highly-oriented pyrolitic graphite (HOPG))
*

and non-graphitizing ones (glassy carbons) under high-fluence 10 keV's heavy atomic and molecular ions were studied. It has been found that the c(T)-dependence shows a step-like behavior typical for the defect annealing curves. This behavior was explained by the dependence of secondary electron path length k on changes in lattice structure. A direct experimental evidence was obtained that the electron interaction with amorphous solid differs from that with a crystal where, at least in an ideal crystal, the electrons are weakly scattered [18]. The aim of the present work is to systematize and outlook the problems of ion-induced electron emission, structure and topography of modified surface layers of different carbon-based materials under intensive ion bombardment. 2. Experimental The experiments were performed using the mass-monochromator of the Institute of Nuclear Physics, Moscow State University [6]. The 5-35 keV ion beam was produced in an arc source with a longitudinal magnetic field. The ions were separated and the beam was focused by a Siegbahn-type magnetic sector field. The angular spread of the ion beam at the focus of the instrument was +1œ. The targets were mounted in the collision chamber. The target

Corresponding author. Tel.: +7 495 9393 904; fax: +7 495 9390 896. E-mail address: borisov@anna19.npi.msu.su (A.M. Borisov).

0168-583X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.078


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holder allowed the variation of the angle h of ion incidence from 0œ to 89œ with an angular step of 0.5œ and the variation of the temperature from À180 to 1000 œC. The samples for the investigation were commercial carbon-based materials: USA-production polygranular graphite POCOAXF-5Q and produced by NIIGrafite (Moscow, Russia) polygranular graphite MPG-LT, MPG-8, highly-oriented pyrolytic graphite UPV-1T, glassy carbons SU-850, SU1000, SU-1300, SU-2000, SU-2500, marked by the temperature of heat treatment Ttr = 850, 1000, 1300, 2000, and 2500 œC. Polygranular graphites used are fine-grained ones and have typical crystallite dimensions La $ 80 nm and Lc $ 60 nm. HOPG is characterized by the highest degree of three-dimensional ordering [19,20]. The density, parameters of the crystal lattice, preferable orientation in a plane (0 0 1) and anisotropy of the physical properties of the HOPG are close to those for natural graphite mineral. In particular, HOPG crystal structure is characterized by an arrangement of carbon atoms in parallel layers, and in each layer the atoms form a grid of correct hexagons with distances between carbon atoms equal 0.1415 nm. The distance between layers is equals to 0.3354 nm that gives a theoretical value of density q = 2.265 g/cm3. HOPG is often used in research of anisotropy effects in carbon-based materials under neutron irradiation in nuclear reactors, and high-temperature plasma in fusion reactors. Glassy carbons are manufactured by carbonization of network polymers using successive stages of hardening, pyrolysis and high-temperature treatment [19]. As a contrast to polygranular and highly-oriented pyrolytic graphites, the glassy carbons are of non-graphitizing carbons. The X-ray analysis of the unirradiated samples both the graphitizing carbon-based materials and non-graphitiziting ones clearly demonstrates the difference of their structures. One can see from Fig. 1 that for glassy carbon the X-ray pattern is typical for amorphous solid. The halo maximum approximately corresponds to the reflex position for polycrystalline graphite. A halo profile asymmetry may be
(002)

connected with a presence of amorphous carbon [19]. This reason ranks peculiarly to the materials with typical dimensions of the graphite crystallites La 6 10 nm, Lc 6 3 nm. In this case, X-ray coherent scattering is due to a scattering both by the atoms of an amorphous carbon and single hexagonal layers. The target irradiation was preferably carried out with 30 keV Nþ and Ar+ ions. The total ion current was 0.1- 2 0.2 mA; the cross-section of the ion beam was 0.35 cm2. It has been found, that the stabilization of ion induced electron yield c with fluence occurs at F $ 1019 ion/cm2. One may suppose that such a situation gives evidence of reaching steady-state conditions, when an amount of the implanted species remains constant and character of surface topography is not more changed. All values of c presented below correspond to such steady-state conditions. The ion-induced electron emission yield, c, was determined from the ratio of the secondary electron current to the primary ion current. Before and after ion irradiation the samples were analysed by scanning electron microscopy (SEM). The crystalline structure of the surface layers was analysed by a RHEED with 50 keV energy of electrons and 50 lA beam current. 3. Results and discussion For all carbon-based materials except for low-temperature glassy carbons, a step-like increase of ion-induced electron emission yield c with increasing target temperature under high-fluence 10s keV ions (Nþ , Ar+) at a certain 2 annealing temperature Ta takes place, see Fig. 2 [12-17]. It appeared that the step-like c(T)-dependencies were due to the ion-induced structure transition in carbon-based materials from a high degree of disorder at T < Ta into a relatively ordered structure at T > Ta. The analysis of the obtained results shows that there is a relation between electron transport in solid and the radiation-induced structure changes.

5.0 4.8 4.6 4.4 4.2
glassy carbon SU-850 SU-2000

Intensity, a.u

SU-1300

, el./ion

4.0 3.8 3.6 3.4 3.2

Ta

HOPG Ta

polygranular graphite Ta (MPG-8)

polycrystalline copper

MPG-8

(100)

(101) (004) (110)

2.4 2.2 0 50 100 150 200 250 o Temperature T C 300 350

2

Fig. 1. Difractograms for graphite MPG-8 and glassy carbon SU-1300. Cu Ka1,2 X-ray is used.

Fig. 2. The temperature dependences of c for the carbon-based materials under 30 keV Nþ ion irradiation at normal incidence (h = 0). For 2 comparison c (T) for polycrystalline copper is presented.


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111

3.1. Appropriated theoretical model of kinetic ion-electron emission To analyze our results, we used the Parilis-Kishinevsky theory [3] of kinetic ion-electron emission based on the Firsov treatment of inelastic energy loss in elementary pair collision of bombarding ion (atom) with target atom [21] and quasimolecular Fano-Lichten model [22]. According to the Parilis-Kishinevsky theory the secondary electron yield c for not too grazing incidence is c ? qre kw= cos h; ð1Þ

yield c(h) virtually reaches its limiting value clim = qrewRe that does not depend on h. To demonstrate this behavior using some simple formulas, we can take an approximation for the velocity dependence of the cross-section as re ðtÞ $ t2 À t2 [15]. In this case its reduction along the path o l is re ðtl Þ ? re ðtÞð1 À l=Re Þ; where Re ? ðt2 À t2 Þ=k . 0 By substituting Eq. (4) into the Eq. (3) we found c ? clim k=ðRe cos hÞ?1 À k=ðRe cos hÞð1 À expðÀR  cos h=kÞÞ;
e

ð 4Þ

where q is the atomic density of the target, k is the secondary electron path length, w is the electron escape probability. The binary collision ionisation cross-section re is determined by the total energy deposited in the electron shells of both the target atoms and the projectiles and is equal to re ðtÞ ? 1:16a0 hJ À1 ?ðZ 1 þ Z 2 ÞðZ þZ
1=2 À1 2 2Þ 1=2 1

ð 5Þ

which at normal and near normal incidence depends mainly on k. At oblique and grazing ion incidence c is determined by the ratio k/(Re cosh). 3.2. Polygranular graphites

t arctan?0:6ðt À t0 Þ10À7 ;

ð2Þ

where J is the ionisation energy, t is the projectile velocity, t0 is the threshold velocity, a0 is the Bohr radius, Z1 and Z2 are the atomic numbers of the projectile and target atom respectively. It should be noted that the theory was developed for polycrystalline metals that were usually considered as randomly packed solids and the problem of radiation damage influence has not been discussed. As far as we know, the effects of radiation damage on electron emission under ion bombardment were discussed for the semiconductor crystals in connection with disappearing of the electron emission anisotropy under high-fluence ion bombardment [3,7,23]. As to graphite, their physical properties are drastically altered by radiation damage, see also [20,24]. We suppose that the changes in crystal perfection and size of the crystallites in carbon-based materials may influence mainly the electron path length k. Let Re(to) be the mean path length at which an ion is slowed down to the threshold velocity to due to energy loss along its path l, and Re cosh is the depth at which it still retains the power to ionise the target atoms. The decrease of the ion velocity follows a simple law t2 À t2 ? kl, where tl is the velocity at the distance l and l 1=2 1=2 k ? 2:48pqa0 e2 Z 1 Z 2 =?ðM 1 þ M 2 ÞðZ 1 þ Z 2 Þ2=3 [3]. Then c may be expressed as Z Re ðt0 Þ c? qre ðtl Þw expðÀl cos h=kÞdl:
0

ð3Þ

The analysis of the angular dependence of this integral shows that at oblique incidence, above a certain angle hc determined by the inequality Re coshc 6 k, the electron

The dependence of electron emission yield on target temperature at normal and near normal 30 keV Nþ and 2 30 keV Ar+ ion incidence manifests a step-like increase [12-15], see also Fig. 2. This temperature behaviour of c is similar to the typical defect annealing dependencies, cf. [24]. The analysis of c(T)-dependencies shows that they are transformed with ion incident angle increase. It has been found that in contrary to the normal ion incidence, when c is virtually constant at T > Ta and c(T > Ta) > c(T < Ta), an increase of h results in essential changes of c(T), see Fig. 2 in [15]. Namely, a relative rise of c is observed in the middle temperature (MT) range (RT Ta) are different both from the RHEED pattern before irradiation and from the ones taken when ion irradiation has been produced at T < Ta. Namely, some slightly smeared polycrystalline diffraction rings are observed.


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To compare qualitatively obtained experimental results with theoretical predictions, the formula (5) may be presented as [14] clim k=ðRe cos hÞ at h 6 hc c? ; ð6Þ clim at h > hc where hc is determined by the inequality Re coshc 6 k. Fig. 3 demonstrates both experimental dependences c cosh, where c correspond to the target temperatures 25, 100 and 300 œC and the dependences calculated according to formula (6). One can see that the behaviour of the experimental curves is similar to that of the calculated ones. After the break at relatively large h the curves for RT and elevated temperatures (HT) virtually merge. The intermediated curve (MT) before the break passes between the RT and HT curves, then intersects them and, at grazing ion incidence, cMT > cRT ffi cHT. A comparison of the experimental obtained and calculated angular dependences shows that the jump of c at T = Ta (see Fig. 2) is due to the increase in secondary electron path length k, if Re is assumed constant. The secondary electrons before their escape from solid experience flux attenuation due to multiple scattering on both the lattice and impurity atoms, the implanted particles, and the radiation defects. The electron path length k = (qr)À1 is determined by the electron flux attenuation cross section r. The polycrystalline metals, as well as the

polygranular graphites with relatively large-size grains may be considered as some sets of single crystals. The cross-section rcr for the single crystal is known to be smaller than the cross-section ram of electron flux attenuation in randomly packed amorphous solids [18]. It should be noted that a similar situation was discussed in connection with the interpretation of track formation in amorphous metals under swift heavy ion bombardment [25,26]. As it was pointed above, the radiation damage annealing in graphite results in transition of disordered surface layer at near RT ion irradiation into a relatively ordered structure at T > Ta. It results in turn in an increase of c at T = Ta due to a decrease of electron flux attenuation cross-section in the graphite lattice. 3.3. HOPG The dependence of electron emission yield c on HOPG target temperature during irradiation at normal and near normal ion incidence on the (0 0 1) plane of UPV-1T shows a step-like increase at some temperature Ta (see Fig. 2) analogous to the c(T)-behaviour observed for polygranular graphites. The difference is in the values of Ta. For HOPG Ta is smaller than for polygranular graphites. The analysis of c(T) at oblique and grazing ion incidence shows that the dependences change with ion incident angle increase both for HOPG and polygranular graphites. However, it has been found that the character of these changes for HOPG is in contrast to that for polygranular graphites, see Fig. 2 in [16]. The transformations of c(h) and c(T) dependences at h > 30œ and T > Ta may be connected to the k decrease relative to the value for the polycrystalline phase of the irradiated HOPG surface layer. The initial HOPG has a near-perfect structure, which consists of tightly bonded sheets of carbon atoms in a hexagonal lattice network, and some strongly different physical properties in the c-axis and a-axis directions [19,20]. For instance, the electrical conductivity perpendicular to the layered carbon sheets (c-axis direction) is $1000 times lower compared to that in the parallel direction (a-axis). One may suppose that at elevated temperatures and as h rises, the transition of the irradiated HOPG surface from the polycrystalline phase at h < 30œ to a near-perfect graphite crystal structure is due to a continued ordering that results in a considerable decrease of electron path length kII in the c-direction. Moreover, the experimental evidence is now found, that k? along the graphite layers in HOPG is twice as much as kII [27]. One may suppose that the corresponding electron path length kcr is less than kpoly for the polycrystal and the difference increases as h rises as kpoly is a value averaged over many randomly placed crystallites. The mere fact that kcr at h > 30œ becomes less than kpoly and the difference Dk = kpoly À kcr increases as h rises, results from the behaviour of the c cosh-dependence on h at elevated temperatures, see the curve corresponding to T = 340 œC in Fig. 4. It was observed that c cosh monotonically decreases as h rises.

a

3.5
30 keV Ar - POCO-AXF-5Q
+

3.0 cos

2.5

25 ?C 100 ?C
2.0

300 ?C

1.5

b

3.5

lim

= 9.5, = 9 nm = 12, = 8.2 nm = 9.5, = 8 nm

3.0



lim

cos



lim

2.5

2.0

1.5 0 15 30 45 60 75 90 Angle of incidence , deg.

Fig. 3. The angular dependences of c cosh: (a) experiment 30 keV Ar+ - POCO-AXF-5Q and (b) calculated data.


A.M. Borisov, E.S. Mashkova / Nucl. Instr. and Meth. in Phys. Res. B 258 (2007) 109-115
4.5 = 5 nm 4.0 cos 3.5 3.0 2.5 2.0 = 2 nm 1.5 1.0 0 20 40 60 80 Angle of incidence , deg. RT = 3 nm = 4 nm

113

30keV N2 - HOPG
T= 340 ?C

+

Fig. 4. The angular dependence of ccosh (experiment 30 keV Nþ - 2 HOPG) and calculated data at different k.

As mentioned above, if the path length k is constant c cosh = qrkw is also constant at h 6 hc. Such situations were seen approximately both for polygranular graphites at all irradiated temperatures (see Fig. 5 in [14]) and for the HOPG at RT, see Fig. 4. 3.4. Glassy carbon The high hardness and thermal stability, extreme resistance to chemical attack and high impermeability make the commercial glassy carbons promising for use in metallurgy, electrochemistry and medicine. The structure of glassy carbon is very complex and has been a subject of many studies [28,29]. The data on the nanostructure of glassy carbon were obtained using high-resolution transmission electron microscopy (HRTEM) and computer simulations [30]. As a result a model has been proposed for the structure, which contains a high proportion of fullerenerelated structures. A difference in the microstructures of high-temperature glassy carbons and low-temperature ones has been established. The microstructure of low-tempera, el./ion (N 2 )
+

, el./ion (Ar )

+

5.1 5.0 4.9 4.8 4.7 4.6 4.5 4.4 4.3 4.2 MPG-8 0.68 0.70 0.72 0.74 parameter c, nm 0.76 SU-2500 POCO
N

3.6

SU-850 SU-1000 SU-1300 3.5

SU-2000

Ar

+ 2 +

3.4

3.3

0.66

Fig. 5. Dependence of c for glassy carbons and polycrystalline graphites on parameter c for the initial samples.

ture glassy carbon consists of tightly curled single carbon layers and nanopores of 1 nm in diameter. For high-temperature glassy carbons there are larger pores bounded by faceted or curved walls containing from two to four layer planes. This resembles a rather imperfect multilayer giant fullerene or a regular fullerene as well. The studies of ion-electron emission under Nþ ion bom2 bardment show that there are two types of c(T)-dependences, see Fig. 2 [31]. One type was observed for lowtemperature glassy carbon (SU-850, Ttr = 850 œC) - a monotonic increase of c with T; the other type was observed for relatively high-temperature glassy carbons (Ttr P 1300 œC) - a non-monotonic step-like electron emission yield increase similar to c(T)-dependence, observed for polygranular graphites and HOPG. A comparison with data for polygranular graphites and HOPG, obtained at analogous irradiation conditions shows that for glassy carbons the absolute values of c at T > Ta are essentially larger. Fig. 5 demonstrates a correlation between the electron yield at T > Ta and the parameter c, which describes the degree of order in the atomic structure and equals to the double distance between carbon layers, its increase meaning a diminution in the packing order [19]. One can see that c for high-temperature glassy carbons tends to c for polycrystalline graphites. At T < Ta the electron yields for high-temperature glassy carbons are close to those for polygranular graphites and HOPG for the same ions. The RHEED confirms that such correlation is caused by changes in the material (polygranular graphite, HOPG, high-temperature glassy carbon) degree of order. The RHEED studies of both low-temperature glassy carbons and the high-temperature ones irradiated at RT show an essential difference in the diffraction patterns. For example, for SU-850 irradiated by Nþ ions (when c(T) is monotonic) 2 the diffraction pattern is close to one before irradiation and contains two weakly contrasted diffuse halos. For SU-2000 the initial pattern is transformed into a structureless halo. In other words, a high-fluence ion irradiation at T < Ta results in disordering of high-temperature glassy carbons and virtually does not influence the structure of low-temperature ones. At elevated temperatures, when radiation damage is annealed, the RHEED patterns differ from the corresponding patterns for both non-irradiated samples and irradiated ones at RT. Namely, a system of three rings is observed. The rings are more smeared than those for the polycrystalline graphites. A ring system similar to that for polygranular graphites testifies about ion-induced ordering of all studied glassy carbons irradiated at elevated temperature. The complex topography of the irradiated surfaces of both low-temperature glassy carbons and high-temperature ones at and near RT also displays some marked differences, see Fig. 6 [32]. As it has been pointed above the RHEED patterns for irradiated low-temperature glassy carbons and the monotonic behavior of c(T) do not show any disordering of the material. Really, RHEED shows an increasing of


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ion-induced ordering as irradiation temperature rises. We suppose that this is caused by an increase of critical value mam of dpa necessary for surface layer amorphisation, due to smaller fullerene-related structure fragments, including nanotube-like fragments in the low-temperature glassy carbons as compared to the high-temperature ones. It should be noted, that the transport of electrons in the carbon nanotubes is close to ballistic with no or very small impedance [28]. This situation causes an increase of secondary electron path length k and therefore tremendous increase of c against a case of high-temperature glassy carbons at T < Ta, when an amorphisation of surface layer is occurred. The same effect as we pointed above was observed for polygranular graphites in passing from amorphous phase at T < Ta to polycrystalline one at T > Ta. According to [30] the heat treatment results in appearance of fullerenes similar in size to C60, or even some multilayered giant fullerenes and it promotes as in the case of polygranular graphites to disordering under ion bombardment at rather low temperatures (T < Ta).

4. Conclusion The regularities of ion beam-induced electron emission from modified surface layers of different carbon-based materials under a high-fluence irradiation are outlined. The materials considered here are those whose bonding is sp2, i.e. polygranular graphites, highly oriented pyrolytic graphite and glassy carbons. The results are discussed in terms of accumulation and annealing of the radiation damage and its effect on electron transport in carbon-based materials. Decrease of secondary electron path length under ion-induced disordering of crystal lattice is the direct experimental confirmation of the difference of electron interaction with amorphous solids and crystals. Ion-induced electron emission may be used as the effective tool for nanostructure studies. Further points of experimental interest include: ž Peculiarities of the electron transport in graphite and graphen structures of fullerene- related materials. ž Molecular effect with respect to accumulation and annealing radiation damage and their influence on secondary electron yield. ž Use of glassy carbon with different treatment temperature for the study of density effects in ion-solid interaction.

Fig. 6. SEM micrographs after ion irradiation of SU-1300 by 30 keV Nþ : 2 (a) RT, tilt is 30œ, (b) T = 350 œC, tilt is 30œ and (c) T = 350 œC, tilt is 0œ.

Acknowledgement This work has been supported by the National project ``Formation of the system of the innovation education at M.V. Lomonosov Moscow State University''.


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