Secondary Electron Yield of Glassy Carbon Dust Grains

Transcription

1 WDS'08 Proceedings of Contributed Papers, Part II, 68 73, ISBN MATFYZPRESS Secondary Electron Yield of Glassy Carbon Dust Grains I. Richterová 1,2, D. Fujita 1, Z. Němeček 2, M. Beránek 2, and J. Šafránková2 1 National Institute for Materials Science, Sengen, Tsukuba, Ibaraki , Japan. 2 Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. Secondary electron emission from a micron spherical dust grain has been investigated locally by a narrow (less than 30 nm) electron beam at energies ranging from 0.6 to 10 kev using the secondary electron microscopy. For investigations, glassy carbon grains were chosen for their conductivity and smooth surface as well as carbon abundance in space and laboratory dusty plasmas. The energetic dependences of a secondary electron yield along the grain curved surface and from a planar graphite substrate are reported. We have observed the grain yield to increase quadratically as a function of an angle of incidence. A ratio of the integral grain yield to the bulk value for glassy carbon was found to be The established second crossover energy (900 ev for a clean surface) is in a good agreement with other single isolated dust grain experiments. Our preliminary measurements of a low-energy part of the secondary electron spectrum have revealed its Maxwellian shape (with the main maximum at 2.5 ev) with tiny subsidiary maxima in the ev range. Introduction A number of phenomena connected with dust grains within the solar system can be explained by their electrical charging. Dust grains in space can interact with the ambient environment through various processes as photoemission, electron and ion attachments, secondary or field emissions, grain sputtering, etc. occurring simultaneously, thus they cannot be studied separately. While the plasma parameters in the solar system are relatively well known, material properties determining dust grain charge should be accounted only approximately. Among them, the secondary electron yield and spectrum of secondary electrons are of a great importance in a broad range of plasma parameters. The secondary electron emission phenomenon is known for one century. The primary electrons impinging a target are scattered inside losing their energy and exciting matter electrons. Some of excited electrons could leave the surface and these so-called true secondaries have typically energies of a few ev. Their yield increases with the beam energy as more target electrons are excited. After reaching a maximum, it decreases because electrons are excited deeper from the surface. Many conducting samples exhibit a similar yield profile where its maximum value, δ max, and position, E max, are characteristic material constants. Among many published theoretical [e.g., Sternglass, 1957] as well as semi-empirical [e.g., Draine and Salpeter, 1979] profiles, the last one exhibits an excellent agreement with measurements in a wide energy range (at least up to 10 E max ). On the other hand, backscattered primary electrons have energies close to the beam energy and their yield increases with the primary beam energy and is rather constant above several hundreds of ev. Although no universal curve has been found, an energetic dependence of the secondary electron yield, σ, is similar to that of true secondaries. Contrary to planar samples, a highly curved surface of a dust grain causes the yield enhancement, since the secondary electron yield increases with an angle of incidence. The growth profile depends on a target material, nevertheless, the main trend can be roughly described as sec α [Bronstein and Fraiman, 1969] or as exp(const (1 cos α)) [Hachenberg and Brauer, 1959]. Moreover, a dust grain size is often comparable with a primary electron penetration depth when the grain boundary becomes reachable for almost all primary electrons. Draine and Salpeter 68

2 [1979] made one of first theoretical estimations of all size effects connected with the secondary emission from dust grains but their approach was very simplified. Later, single dust experiments were performed. Ziemann et al. [1995] determined the secondary electron yield of small drops falling through an electron beam, while Švestka et al. [1993] started a series of measurements of an equilibrium potential of single isolated dust grains [Pavlů et al., 2008]. Simultaneously, several attempts of modelling of the electron-dust interaction were reported [Ziemann et al., 1995; Chow et al., 1994; Richterová et al., 2006]. Despite these advances, several questions still remain open, mainly due to the fact that a grain equilibrium surface potential is a complex product of the yield and spectrum of secondary electrons. Since low-energy electrons cannot leave a grain charged to a higher positive potential, the spectrum plays an important role in understanding the grain charging. Following up numerous spectrum measurements performed on various metallic planar samples, a Maxwellian-like distribution of the temperature of a few ev became widely accepted [Bronstein and Fraiman, 1969; Hachenberg and Brauer, 1959]. Superimposed on this slowly varying electron energy distribution of secondary electrons is a fine structure which has been shown to reflect features in the empty density states above the vacuum level [Willis et al., 1974]. In the case of dust grains, the Maxwellian distribution was successfully used for glass and zinc grains [Richterová et al., 2007; Velyhan et al., 2001]. Nevertheless, fits of grain surface equilibrium potentials provide much better results using a distribution of secondary electrons suggested by Draine and Salpeter [1979], especially for heavy metal grains (gold and silver) or those from melamine-formaldehyde resin [Velyhan et al., 2001; Richterová et al., 2006]. The aim of the present paper is to engage another way of determination of the secondary electron yield and spectrum. Our independent direct yield measurements should allow us to verify, whether the model [Richterová et al., 2006] fitting grain equilibrium potential profiles gives appropriate parameters, and thus can be used as a reliable tool for dust grains of various shapes. Experimental Method For our investigations of the secondary emission process, we have used the advantages of scanning electron microscope (SEM). A narrow (few tens of nm) electron beam is scanned over the sample surface, while a collected current of true secondaries or backscattered electrons can be detected continuously. The measurements were performed by a Scanning Auger Microprobe PHI SAM650 that is equipped with a cylindrical mirror analyser (CMA). The CMA is proposed to detect Auger peaks and has a reduced sensitivity bellow 20 ev. Since the CMA generally exhibits a narrow angle of view, it is inadequate even for a backscattered electron yield calibration without knowing its spatial distribution. The advantage of the used SAM650 system is better UHV conditions a typical working pressure is 10 9 Pa, contrary to 10 5 Pa being common for SEMs. The corresponding monolayer formation times are 3 days and 30 s, respectively. moreover, the residual gas ionisation must be taken into account for SEMs. In our experiment, particular grains were in contact with a conducting polished plate that was placed upon a sample holder and the net current on the holder was measured externally. A small (0.1 mm) hole in the plate was used for precise measurements of the beam current because the system plate-holder can be considered as a Faraday cup. Inside it, the beam impact angle was set to 40 to suppress the amount of backscattered electrons leaving the cup. The working beam current was chosen to be A. While smaller primary beam currents can be hardly detected due to a noise, higher primary beam currents increase the spot size. The whole sample can be biased up to 30 V (at both polarities). Thus, the true secondaries and backscattered electrons exiting the grain/plate system can be distinguished. Note that the yield of an isolated grain would be a bit greater than that measured due to electrons that are emitted towards the plate, i.e., remain captured in our system. 69

3 RICHTEROVA ET AL.: ELECTRON ELECTRON YIELD OF GLASSY CARBON DUST GRAINS Figure 1. SEM images of typical carbon (left), silver (middle), and glass (right) grains. The size of each given grain is 4.9 µm, 1.2 µm, and 1 µm, respectively. The beam energy of the SAM650 system is tunable up to 15 kev, and the beam spot size is 30 nm above 1 kev. Such focused beam allows us to investigate the local yields and their spatial dependences across a sample. The lens system is optimised to the beam energy of 3 kev. Since secondary electron yield variations connected to the primary beam penetration through the grain are not observable at our set-up, larger grains are preferred because they provide a better spatial resolution. Dust Samples Fig. 1 shows high resolution SEM images of several types of grains investigated by Pavlu et al. [2008] we have obtained by JEOL JSM6500F. The variations of contrast along the grain surface in SEM images is connected with significant fluctuations of local secondary yield. One can see that silver (and similarly other metallic) grain surfaces consist of a 10 nm structures (Fig. 1 middle). On the other hand, well shaped glass grains are charged regardless of a substrate bias (Fig. 1 right). Thus, we chose conductive and smooth grains of glassy carbon for this study (Fig. 1 left). Besides diamond and graphite, carbon occurs in many allotropic forms ranging from nanotubes to amorphous carbon [Harris, 2005]. The glassy carbon is a vitreous form discovered in 1957 with a fulleren-like (i.e., closed tightly packed disordered sp2 ) structure. Consisting only of pores 7 nm, glassy carbon exhibits low mass density (1500 kgm 3 ) and high chemical resistance. In spite of it, adsorbed layers can influence non-negligibly surface emission properties [Bera nek et al., 2008]. Thus, the comparison to bulk data is rather ambiguous. Results The spatial and bias dependeces of the sample current (corresponding to a known primary beam current) have been measured inside the SAM650 system. Locations of investigated points along a grain are shown in Fig. 2 left. A 12 µm glassy carbon sphere laying on a graphite substrate was studied in this case. For each point, a corresponding angle of incidence, α, can be easily derived as α = arcsin(x/r) where R is the grain radius. We have chosen a set of 4 points for each equi-spaced α value what allows us to estimate in-situ the errors caused by thermal drifts of a sample holder and/or local surface defects. Around each point, a dark area of a diameter 0.5 µm appears due to a beam irradiation. When the electron beam scanning is stopped, a mean current density gets locally several orders higher (in the ratio of sizes of illuminated areas) and it can force the adsorbate to remove more efficiently (saturated within few minutes). The secondary electron yield of a treated surface is lower than before cleaning. Without an intensive electron bombardment, the yield grows again slowly due to migration of residual ambient adsorbate. The whole 500 nm area corresponds not only to the area bombarded by primary electrons ( 30 nm) but also to a range of leaving electrons reversed inside the matter. Local secondary electron yields as a function of an angle of incidence are summarised in Fig. 2 right for various electron beam energies. The yields are scaled by their mean value except 70

4 /< > kev 5 kev 3 kev 2 kev 1 kev 0.6 kev Figure 2. Left: A SEM image of the studied 12 µm carbon grain as obtained by SAM650. Measured points and an area representing the substrate are sketched. Note that details are blurred comparing to Fig 1. Right: The relative secondary electron yield as a function of an incident angle for various beam energies and empirical profiles: sec α (dotted), exp(const (1 cos α)) (dashed), and a 0 + a 2 α 2 (full). The quadratic fit parameters are: a 0 = 0.49 and a 2 = the outer points. Those points stand only 100 nm from the grain boundary and are most sensitive to all drifts. Thus, their yields result to an arbitrary value between those expected and substrate (σ graphite σ grain (α = 0.5)). Despite the widely accepted curves [Bronstein and Fraiman, 1969; Hachenberg and Brauer, 1959], a quadratic growth of scaled yields with the angle of incidence describes better our data (Fig. 2 right). The same parabola (σ/ σ (α) = α 2 ) fits well all profiles for beam energies ranging from 1 kev. the profile measured for 600 ev seems to be flatter but we have noted a very low image contrast during the measurements at this energy that suggested a dispersed beam. We presume that the measured profile is flatter only apparently and use the same parabola for all beam energies in further estimations. Following this fit, we can assume a bulk glassy carbon secondary electron yield to be equal to σ 0 = σ grain (α = 0). The fit can be then written as σ(α) = σ 0 (1+1.68α 2 ). The resulting yields at a normal angle of incidence with respect to the beam energy are plotted in Fig. 3 left. Since we are not aware of published data on secondary electron yields of glassy carbon, the yield of the graphite substrate (i.e., measured at the same conditions) are given for comparison. Above 7 kev, both yield profiles tend to a value 0.19, whereas at lower beam energies the graphite yield reaches higher values (1.5 larger at 600 ev). Although maxima of both profiles are localised out of the working energy range, we can conclude that the graphite and glassy carbon yield profiles clearly do not follow the same universal curve. Applying a small positive bias voltage to the sample, low-energy secondary electrons are not able to leave it while beam parameters are changed negligibly. Varying the voltage, an integral spectrum of secondaries can be directly measured. An example of the preliminary results is shown in Fig. 3 right. This spectrum was measured at the grain top using the beam energy of 3 kev. Contrary to spectra measured in continuous regime, our data were taken for several bias voltages only. Thus, we refrain from differentiating the spectra numerically and present the integral form instead. 20 % of secondary electrons has energies above 30 ev and a low-energy part of the spectrum follows well the Maxwellian distribution with the temperature of 2.5 ev. A subsidiary electron population was found at energies of about ev. Discussion We have observed a decrease of the secondary electron yield of glassy carbon grains due to the electron beam irradiation and we have attributed it to a beam-induced desorption. In the case of a clean 12 µm grain and the beam energy ranging from 600 ev to 10 kev, we have found the quadratic angular profile of the secondary electron yield independently on the beam 71

5 amorphous carbon graphite measured data maxwellian fit F SE E e / kev E SE / ev Figure 3. Left: The secondary electron yield of glassy carbon (the grain top at the normal incidence angle) and the graphite substrate with respect to the beam energy. Right: An energy spectrum of secondaries at the grain top and the beam energy of 3 kev. Fit parameters for true secondaries were found: T SE = 2.5 ev and their portion is 80 %. energy. Note that the grain and bulk angular dependences might differ for higher angles of incidence because the grain surface is curved and thus closer to a primary electron scattering area inside the grain. Further, a portion of emitted electrons hitting the substrate is dismissed. Thus, it would be better in further experiments to tilt the sample in such way that the beam hits at oblique incidences always a grain top (related to a substrate plane). Knowing the angular yield profile, we can derive an integral grain yield as σ = π/2 0 σ(α) sin(2α)dα = σ 0 ( (π 2 /8 0.5)) =. 2.23σ 0. Although the secondary yield maximum is out of the used energy range, we are able to determine the second crossover point, E 2, as the beam energy where σ 0 = 0.45, i.e., E 2 = (900 ± 100) ev (Fig. 3 left). Beránek et al. [2008] performed a single dust experiment with the same glassy carbon grains. They studied an influence of the electric field on the second crossover point and found it to be (1500 ± 100) ev for untreated grains at the zero-field limit. However, they reported it decreases down to 870 ev for one grain after 25-hour 4 kev electron bombardment. Contrary to our case, their relaxation times are much longer because they used lower electron beam current intensity ( A at 4 µm 2, i.e., ten orders of magnitude less than we do). The measurements of secondary electron spectra allow us to distinguish a ratio of true secondary and backscattered electrons. In the present case (Fig. 3 right), we found the backscattered electrons consist 20 % of all secondary electrons at a beam energy of 3 kev. We can combine this result with the previously measured secondary electron yield (Fig. 3 left) and compute the backscattered electron yield as %=0.05. These numbers as well as the temperature of true secondaries (2.5 ev) are of expected values for carbon [Bronstein and Fraiman, 1969]. Ueno et al. [1988] studied characteristic peaks in secondary electron spectra of graphite and glassy carbon by a CMA calibrated down to 2 ev. They reported the first maximum of glassy carbon at 3 ev which is close to our value. They detected numerous subsidiary maxima for graphite. Since glassy carbon does not have periodic crystal structure, the subsidiary maxima are smeared out comparing to graphite but they are still apparent at energies of ev. Their spectra are in a good agreement with our measurements. Conclusion We have studied the spatial distribution of the secondary yield from a spherical 12 µm glassy carbon grain laying on a conductive substrate by a well focused beam in the energy range of kev. The grain integral yield is enhanced by a factor of 2.23 related to a planar target. The resulting second crossover point (900 ev) is, within measuring error, in a good agreement with Beránek et al. [2008], where it was determined by a different method. An applied bias voltage revealed that true secondaries exhibit the Maxwellian distribution 72

6 with the temperature of 2.5 ev for the beam energy of 3 kev at a normal angle of incidence. Although the presented method seems to be promising, single isolated dust experiments stay as an essential way for dust grains of dimensions comparable to the primary beam penetration depth. Acknowledgments. The presented work was partly supported by the Research plan MSM that is financed by the Ministry of Education of the Czech Republic and partly by the Czech Grant Agency under Contracts 202/08/0063 and 202/08/H057 and partly by the Grant Agency of Charles University (GAUK ). References Beránek, M., Richterová, I., Němeček, Z., Pavlů, J., and Šafránková, J., Influence of the electric field on secondary electron emission yield, in WDS 08 Proceedings of Contributed Papers: Part II Physics of Plasmas and Ionized Media, edited by J. Šafránková, this issue, Matfyzpress, Prague, Bronstein, I. and Fraiman, B., Secondary Electron Emission (in Russian), Nauka, Moskva, Chow, V., Mendis, D., and Rosenberg, M., Secondary emission from small dust grains at high electron energies, IEEE Trans. Plasma Sci., 22, , Draine, B. and Salpeter, E., On the physics of dust grains in hot gas, Astrophys. J., 231, 77 94, Hachenberg, O. and Brauer, W., Secondary electron emission from solids, Advances in electronics and electron physics, 11, 413, Harris, P. J. F., New perspectives on the structure of graphitic carbons, Crit. Rev. Solid State Mater. Sci., 30, , Pavlů, J., Richterová, I., Němeček, Z., Šafránková, J., and Čermák, I., Interaction between single dust grains and ions or electrons: laboratory measurements and their consequences for the dust dynamics, Faraday Discuss., 137, , Richterová, I., Pavlů, J., Němeček, Z., and Šafránková, J., Model of secondary emission and its application on the charging of gold dust grains, Phys. Rev. B, 74, Richterová, I., Pavlů, J., Němeček, Z., Šafránková, J., and Beránek, M., Secondary emission from glass grains: Comparison of the model and experiment, IEEE Trans. Plasma Sci., 35, , Sternglass, E., Theory of secondary electron emission under electron bombardment, Scientific Paper P9, Westinghouse Research Laboratories, Pittsburgh 35, copy, Švestka, J., Čermák, I., and Grün, E., Electric charging and electrostatic fragmentation of dust particles in laboratory, Adv. Sp. Res., 13, , Ueno, K., Kumihashi, T., Saiki, K., and Koma, A., Characteristic secondary electron emission from graphite and glassy carbon surfaces, Jpn. J. Appl. Phys., 27, L759 L761, Velyhan, A., Němeček, Z., and Šafránková, J., Secondary electron emisson from small metallic grains, in WDS 01 Proceedings of Contributed Papers: Part II Physics of Plasmas and Ionized Media, edited by J. Šafránková, pp , Matfyzpress, Willis, R. F., Fitton, B., and Painter, G. S., Secondary-electron emission spectroscopy and the observation of high-energy excited states in graphite: Theory and experiment, Phys. Rev. B, 9, , Ziemann, P., Liu, P., Kittelson, D., and McMurry, P., Electron impact charging properties of size-selected, submicrometer organic particles, J. Phys. Chem., 99, ,