Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://danp.sinp.msu.ru/LIIWM/Nimb256_mar07_515Mashkova.pdf Äàòà èçìåíåíèÿ: Sun Oct 21 20:34:06 2007 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 22:22:48 2012 Êîäèðîâêà:

N IM B
Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 256 (2007) 515­519 www.elsevier.com/locate/nimb

Ion-induced electron emission of glassy carbons
N.N. Andrianova a, A.M. Borisov a, E.S. Mashkova a,*, A.S. Nemov a, E.S. Parilis b, A.I. Sorokin c, Yu.S. Virgiliev c
a

Institute of Nuclear Physics, Moscow State University, Leninsky Gori, 119992 Moscow, Russia b California Institute of Technology, 200-36, Pasadena, CA 91125, USA c NIIgrafite, 111141 Moscow, Russia Available online 16 December 2006

Abstract The temperature dependence (þ200 °C< T 6 350 °C) of ion-induced electron emission yield, c, for glassy carbons with different heat treatment (from 850 to 2500 °C) under high-dose (1018­1019 ion/cm2) 30 keV Ar+ and N× normal incidence ion bombardment has been 2 measured. A step-like increase in the yield at certain annealing temperature Ta has been detected only for relatively ``high-temperature'' glassy carbons, when RHEED has shown diffraction patterns corresponding to a high degree RT-irradiation-caused disorder. It is analogous to that for the graphitized carbon-based materials, which is due to an increase in electron path length during the radiation damage annealing. For the ``low-temperature'' glassy carbons only a slow monotonic increase in c with the irradiation temperature has been observed. The obtained results are discussed in terms of both accumulation and annealing of the radiation damage, the presence of some fullerene-related structures in the glassy carbons and their role in the ion-induced electron emission. ñ 2007 Elsevier B.V. All rights reserved.
PACS: 34.50.Dy; 79.20.Rf Keywords: Glassy carbon; High-dose ion irradiation; Ion-induced electron emission; Fullerene-related structures

1. Introduction Carbon-based materials are broadly used in different areas of physics, technology and industry. The high hardness and thermal stability, extreme resistance to chemical attack and high impermeability make the commercial glassy carbons very perspective for using in metallurgy, electrochemistry and medicine. Wide utilization of carbon-based materials in fission reactors and fusion devices has necessitated studies of their physical characteristics and effects of radiation damage in dependence on conditions of neutron or ion irradiation. It is known that the glassy carbons are manufactured by carbonization of network polymers using successive stages of hardening, pyrolysis and high-temperature treatment [1]. As a contrast to

*

Corresponding author. Tel.: +7 495 9393 904; fax: +7 495 9390 896. E-mail address: esm@anna19.npi.msu.su (E.S. Mashkova).

polygranular and highly-oriented pyrolytic graphites, the glassy carbons are an example of non-graphitizing carbon. The structure of glassy carbon is very complex and has been a subject of many studies, see for example [1­5]. Different models for the structure of glassy carbon were proposed. According to widely accepted Jenkins­Kawamura model [1,2] glassy carbon is a globule-celled construction. The primary element of this construction is an isotropic globule (size 10­30 nm) having turbostratic structure with a pore inside. The globules are covered by a highly-oriented film of 10­15 nm thickness. The film forms curved and twisted carbon ribbons containing graphitic microcrystallites. Using this model, though widely accepted, it is difficult to explain the high impermeability of glassy carbons to gases and their low reactivity. Recently, a model for glassy carbon, which incorporates carbine-like chains, based on electronic properties considerations, has been proposed [3,4]. However, the authors pointed out that there were no direct experimental confirmations for the

0168-583X/$ - see front matter ñ 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.051


516

N.N. Andrianova et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 515­519

proposed structure. More correct data on the nanostructure of glassy carbon were obtained using high-resolution transmission electron microscopy (HRTEM) and computer simulations [5], see also [6]. As a result a model has been proposed for the structure, which contains a high proportion of fullerene-related structures. A difference in the microstructures of high-temperature glassy carbons and low-temperature ones has been established. The microstructure of low-temperature 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. In our previous works [7­10] where the irradiated surface layers of graphitizing carbon-based materials, namely, polygranular graphites and highly-oriented pyrolytical graphite, were analysed, the temperature dependences of electron emission yield c were measured under 30 keV N× 2 and Ar+ ion irradiation at high fluences F P 1018 ion/ cm2. 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 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 [11]. Our first investigations with different glassy carbons under 30 keV N× ion irradiation have 2 shown that the c(T)-behavior analogous to corresponding behavior for polygranular graphites and HOPG exists only for relatively high-temperature glassy carbons [12]. The aim of the present work is to study the temperature dependence of ion-induced electron emission yield, structure and topography of modified surface layers of different glassy carbons under continuous high-dose 30 keV N× and 2 Ar+ ion bombardment. The Ar+ ion irradiation was used to avoid the chemical effects, which may be observed under nitrogen ion bombardment [13]. 2. Experimental The experiment was performed using the mass-monochromator of the Institute of Nuclear Physics, Moscow State University [14]. The 5­35 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 Siegbahntype magnetic sector field. The angular spread of the ion beam at the focus of the instrument was ±1°. The target holder allowed variation of the angle of ion incidence from 0° to 89° with an angular step of 0.5° and variation of temperature from þ200 °C to 1000 °C. The samples for this investigation were the glassy carbons SU-850, SU-1300, SU-2000 and SU-2500 (NIIgrafite production, Moscow, Russia), distinguished by the temperature of heat treatment, namely, 850 °C, 1300 °C, 2000 °C and 2500 °C. Sample

thickness was 3 mm, the width 15 mm and the length was 80 mm. The sample density is 1.46­1.52 g/cm3. Besides that, a glassy carbon ``SU-1000'' manufactured by heat treatment of SU-850 using the NIIgrafite method, was also investigated. The samples were washed using ethanol, and then annealed in vacuum. The target irradiation was carried out with 30 keV N× and Ar+ ions. The total ion current was 2 0.1­0.2 mA; the cross-section of the ion beam was 0.35 cm2. Ion fluences were F =1018­1019 ion/cm2. The ion-induced electron emission yield, c, was determined from the ratio of the electron current to the primary ion current. The temperature scans under continuous ion irradiation c(T) were preceded by measurements of the fluence dependence c(F) at room temperature (RT) until the c-stabilization occurred. Usually the electron emission yield went down during irradiation till the c-stabilization at steady-state conditions at F P 1018 ion/cm2. Total fluence during temperature scan was $1019 ion/cm2. Before and after ion irradiation the samples were analysed by scanning electron microscopy (SEM) using LEO 1430-vp. The crystalline structure of the surface layers was analysed by the method of reflection high energy electron diffraction (RHEED) in EMR-102 (Russian model) operated at 50 kV and electron beam current 50 lA. 3. Results and discussion The measurements of the temperature dependence of electron emission yield c show that c(T) depends both on heat treatment temperature Ttr and type of the bombarding ion. The studies of ion-electron emission under N× ion 2 bombardment show that there are two types of c(T)-dependences [12]. One type was observed for low-temperature glassy carbon (SU-850, Ttr = 850 °C) ­ a monotonic increase of c with T; the other type was observed for relatively high-temperature glassy carbons (Ttr P 1300 °C) ­ a non-monotonic step-like electron emission yield increase similar to c(T)-dependence, observed early for polygranular graphites and HOPG [7­10]. In the present study, to trace the transition from a monotonic increase to a steplike behavior, the c(T)-dependence for SU-1000 has also been measured, see Fig. 1. One can see that in this case the c(T) is an intermediate one between those for SU-850 and SU-1300. The measurements of c(T)-dependences under 30 keV Ar+ ion bombardment show a similar behavior, see Fig. 2. The differences are only in absolute values of c and in the treatment temperature Ttr, for which a transition from monotonic to a step-like increase has been observed. Indeed, for Ar+ irradiation c(T) was monotonic and coincided for SU-850 and SU-1000. Distinct step-like c(T) was observed for SU-2000 and SU-2500. The mentioned above differences between the interaction of the glassy carbons with argon and nitrogen ions could be caused by synthesis of amorphous carbon nitride under nitrogen bombardment [13]. The comparison with data for polygranular graphites and HOPG, obtained at analogous irradiation conditions


N.N. Andrianova et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 515­519

517
, el./ion (Ar )
3.6
+

5.2

30 keV N2 , normal incidence
SU-850

+

, el./ion (N2 )
5.1 5.0

+

4.8

SU-1300

, electron/ion

4.9

4.4

SU-2000 SU-1000 SU-2500 MPG-8

4.8 4.7 4.6

SU-1300

SU-850 SU-1000
3.5

4.0
Ta

Ta

3.6

4.5 4.4

SU-2500

N

+ 2 +

SU-2000

Ar

3.4

POCO MPG-8
3.3 0.68 0.70 0.72 0.74 0.76

3.2 0 50 100 150 200 250 300 350

4.3

T, C
Fig. 1. The temperature dependence of c for glassy carbons and for MPG8 graphite under 30 keV N× ion irradiation at normal incidence (h =0°). 2 Data for MPG-8 were taken from [8], data for SU-850, 1300, 2000, 2500 were taken from [12].

o

4.2

0.66

parameter , nm
Fig. 3. Dependence of c for glassy carbons and polycrystalline graphites on parameter c for the initial samples.

30 kev Ar , normal incidence
3.8 3.6
SU-1000 SU-850

+

3.4
SU-1300

3.2 3.0
SU-2500 SU-2000

Ta

2.8 2.6 -200

-100

0

100

200

300

T, C
Fig. 2. The temperature dependences of c for glassy carbons under 30 keV Ar+ ion irradiation at normal incidence.

o

[7­10] shows that for glassy carbons the annealing temperatures Ta are lower and the absolute values of c at T > Ta are essentially larger. Fig. 3 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 [1]. One can see that c for high-temperature glassy carbons tends to the value 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 the RT irradiated low-temperature glassy carbons and the high-temperature ones show an essential difference in the diffraction patterns. For example, for SU-850 irradiated by N× ions (when c(T) 2 is monotonic) the diffraction pattern is close to the one

before irradiation (see Fig. 4(a)) and contains two weakly contrasted diffuse halos. For SU-2000 the initial pattern is transformed into a structureless halo, see Fig. 4(b). In other words, a high-dose ion irradiation at T < Ta results in disordering of high-temperature glassy carbons and virtually does not influence the structure of low-temperature ones. 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. For high-temperature glassy carbons SEM patterns show shallow etch pits with pentagon or hexagon forms. Typical example is presented in Fig. 5(a) for SU-2000. The surface topography of low-temperature glassy carbons also shows polygonal patterns ­ more deep pits were observed with the ridge-like walls. A typical example is presented in Fig. 5(b) for SU-850. Comparison of the RHEED and SEM patterns shows absence of qualitative differences between low-temperature and high-temperature glassy carbons irradiated at elevated temperatures. Indeed, 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, see an example in Fig. 4(c). The rings are more smeared than those for the polycrystalline graphites, [8]. In this case one need to take into account the developed surface submicron relief, which contains ridges with steep walls, see Fig. 5(c). In contrast to the samples of high-temperature glassy carbons irradiated at RT, when an amorphous phase is sputtered (Fig. 5(a) and (b)), at elevated temperatures, a selective sputtering of two-phase system (fullerene-related nanoparticles and amorphous carbon [6]) is occurred. In this case one may suppose that an amorphous phase is sputtered more efficiently [15]. The fast electrons experience diffraction on the ridge tops. A ring system similar to that for polygranular graphites testifies about ion-induced ordering of all studied glassy carbons irradiated at elevated temperature. The ring blurring may

, electron/ion


518

N.N. Andrianova et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 515­519

Fig. 4. RHEED patterns from low-temperature and high-temperature glassy carbons before and after 30 keV N× ion irradiation at normal 2 incidence: (a) SU-850 before irradiation; (b) SU-2000, ion irradiation at RT; (c) SU-850, ion irradiation at T = 300 °C.

Fig. 5. SEM micrographs after ion irradiation of SU-850 and SU-2000: (a) 30 keV N× ­ SU-2000, RT, the tilt is 0; (b) 30 keV N× ­ SU-850, RT, 2 2 the tilt is 30°; (c) 30 keV Ar+ ­ SU-850, T = 300 °C, the tilt is 30°. In the inset the tilt is 0°.

be due to very small size of the nanoparticles. For SU-850 irradiated by 30 keV N× ions and for SU-850, SU-1000 2 irradiated by 30 keV Ar+ ions the ion-induced ordering of the material structure displays itself in the monotonic increasing of c(T). It should be noted that a possibility of radiation-induced transformation of carbon nanoparticle structure was also confirmed by a study using transmission electron microscope with 300 keV intensive electronic beam [6]. 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 ioninduced 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 [6]. 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 in Introduction was observed for polygranular graphites in passing from amorphous phase at T < Ta to polycrystalline one at T > Ta. [8]. According to [5] the heat treatment results in appearance of fullerenes similar in size to C60, or even some multilayered giant fullerenes and it is promoted as in the case of polygranular graphites to disordering under ion bombardment at rather low-temperatures (T < Ta). The presence of fullerenes (i.e. closed nanoparticles) in the high-temperature glassy carbons rises with the decrease


N.N. Andrianova et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 515­519

519

of the parameter c (for non-irradiated samples) and causes a decrease in the ion-induced electron yield down to the values typical for the polycrystalline graphite, see Fig. 3. Indeed, the ionizing collisions with fullerenes are known to be different from those in atomic collisions [16]. A fullerene C60, as a target for ion bombardment, consists of a cen° ° tral circle with a radius of 2.5 A, an area of 19.6 A2, ° 2 and containing 10 atoms in projection, 0.51 atoms per A ° a ring, about 1 A thick, of approximately the same area, ° 2 and the projected density 2.65 atoms/A2, contain° 18.9 A ing the remaining 50 atoms, with 40 of them, 2/3 of all ° atoms, projected in a 0.5 A-thick peripheral ring with ° 2. Such geometry favors shielding of the rear 4.24 atoms/A atoms by the front ones. The distance in the projection direction between the atoms, located in this very periphery ° is as small as d = 1.4­2.8 A, i.e. less than the distance between the carbon layers c/2, so the shielding in this region is comparable to that for polycrystalline graphite. The interaction of the projectile ion with carbon atoms in the outer ring is similar to grazing impact on a surface, when the small impact parameters in consecutive binary collisions are blocked. It would diminish the ionizing collisions rate and the ion-induced electron yield. This effect is much more pronounced for the compact convoluted fullerenes than for the curved sheets. With increasing relative amount of the fullerenes with increasing treatment temperature Ttr the atomic shielding becomes more efficient reducing the electron yield. This effect could explain the 7% reduction for argon and 14% for nitrogen in c in the direction from SU-850 towards SU- 2500 and MPG-8, at elevated temperatures, see Figs. 1­3. 4. Conclusion The temperature dependences of ion-induced electron emission yield c under 30 keV N× and Ar+ ion impacts at 2 normal incidence under dynamically steady-state conditions have been measured for glassy carbons with treatment temperature 850, 1000, 1300, 2000, 2500 °C. A step-like increase of c at an annealing temperature Ta, analogous to the c-behavior for polygranular graphites and HOPG, has been found for glassy carbons with relatively high treatment temperature Ttr. For glassy carbons Ta are smaller than for the polygranular graphites and HOPG. For low-temperature glassy carbons c monotonically increases with the temperature during irradiation. The temperature Ttr dividing monotonic and step-like c(T)-dependences is higher for argon than for nitrogen ion irradiation. The RHEED shows a diffuse halo for the high-temperature glassy carbons irradiated at RT and at T > Ta the diffraction rings are more smearing than for unirradiated polycrystalline graphites. For low-temperature glassy carbons irradiated at RT the diffraction patterns are close to the pattern before irradiation. At elevated temperatures the diffraction patterns are similar to those for high-temperature glassy carbons.

The SEM micrographs taken after ion irradiation at RT show for high-temperature glassy carbons some shallow etch pits with pentagon and hexagon forms, while for low-temperature ones there are also deeper polygonal pits. At elevated temperature for all studied glassy carbons some ridges with steep walls are developed and the electron diffraction may result from the ridge tops. It is proposed that the presence of two types of c(T)dependences ­ monotonic increase with T for low-temperature glassy carbons and step-like at some annealing Ta for high-temperature glassy carbons are due to different fullerene-related structures of low- and high-temperature glassy carbons resulting in much larger number of dpa necessary for disordering of low-temperature glassy carbons. The decrease of the ion-induced electron emission yield with increasing treatment temperature could be explained an increase in relative amount of the fullerenes compared to the curved carbon sheets. The fullerenes diminish the number of ionizing collisions due to shielding of the rear atoms in the projectile beam, while the carbon sheets enhance the electron transport. Acknowledgements This work has been supported by the National project ``Formation of the system of the innovation education at M.V. Lomonosov Moscow State University''. The authors are grateful to E.A. Pitirimova for performing RHEED, to M.A. Timofeev for SEM-analysis and L.A. Pesin for helpful discussion. References
[1] A.S. Fialkov, Uglerod mezhsloevye soedineniya i kompozity na ego osnove (Carbon and carbon based intercalation compounds and composites), Aspect Press, Moscow, 1997 (in Russian). [2] G.M. Jenkins, K. Kawamura, Nature 231 (1971) 175. [3] L.A. Pesin, J. Mater. Sci. 37 (2002) 1. [4] L.A. Pesin, E.M. Baitinger, Carbon 40 (2002) 295. [5] P.J. Harris, Philos. Mag. 84 (2004) 3159. [6] P.J. Harris, Carbon Nanotubes and Related Structures, Cambridge University Press, Cambridge, 1999. [7] A.M. Borisov, W. Eckstein, E.S. Mashkova, J. Nucl. Mater. 304 (2002) 15. [8] A.M. Borisov, E.S. Mashkova, A.S. Nemov, Vacuum 73 (2004) 65. [9] A.M. Borisov, E.S. Mashkova, A.S. Nemov, E.S. Parilis, Nucl. Instr. and Meth. B 230 (2005) 443. [10] A.M. Borisov, E.S. Mashkova, A.S. Nemov, E.S. Parilis, Vacuum 80 (2005) 295. [11] J.M. Ziman, Principles of the Theory of Solids, University Press, Cambridge, 1964. [12] A.M. Borisov, Yu.S. Virgiliev, E.S. Mashkova, A.S. Nemov, A.I. Sorokin, Fizika i khimiya obrabotki materialov 1 (2005) 27 (in Russian). [13] L.D. Bogomolova, A.M. Borisov, N.A. Krasil'nikova, E.S. Mashkova, A.S. Nemov, V.V. Tarasova, Radiat. Eff. Def. Solids 157 (2002) 493. [14] E.S. Mashkova, V.A. Molchanov, Medium-Energy Ion Reflection from Solids, North-Holland, Amsterdam, 1985. [15] V.I. Shulga, Nucl. Instr. and Meth. B 174 (2001) 423. [16] E.S. Parilis, Nucl. Instr. and Meth. B 88 (1994) 21.