WDS'06 Proceedings of Contributed Papers, Part II, 53–58, 2006.
ISBN 80-86732-85-1 © MATFYZPRESS
Influence of Charging Conditions on Dust Grain’s Field
Ion Emission
M. Jeřáb, I. Richterová, J. Pavlů, J. Šafránková, and Z. Němeček
Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.
Abstract. The research of charging processes of various types of dust grains is
important for understanding of the role of dust in dusty plasmas in a space as well
as in applications. One of the very important charging process is field ion emission.
The field ion emission can limit the positive charge on the dust grain because the
positive ions leave a grain surface due to a strong electric field. We analyzed many
discharging characteristics of an Au grain of 0.57 µm of radius measured under
different conditions (the ion beam energy, ion species, total dose of ion bombarding)
and found that the discharging current is determined not only by the surface field
intensity but it strongly depends on the structure of the grain surface and on the
history of measurements. We have found that the duration of charging (or total
dose of the ions impinging the grain) is one of the important parameters. The dose
of bombardment determines the number of implanted ions. Since their diffusion
toward the surface is very slow (hours for 5 keV ions), the history of the grain
should be considered in interpretation of the results.
Introduction
Laboratory simulations of dust grain charging properties are a way to understand the role
of dust grains in complex plasma in space as well as in technical applications [Langmuir et al.,
1924]. The dust grain immersed in plasma environments is charged by various processes like
collections of ions and electrons from the ambient plasma, photoemission, secondary electron
emission, and also minor charging processes that are believed to be negligible or interesting
only under specific conditions: thermo-emission and electric field emission. Although many
papers about field ion emission have been published [Dawson and Peng, 1979; Forbes, 1981;
Forbes et al., 1996; Kreuzer et al., 1992; Tzou and Müller., 1970; Müller et al., 1973], later
process remained probably as one of poorly understood discharging processes.
Field ion emission (FIE) is the effect when a positive charge leaves a surface due to a high
positive surface potential. The emission of positive ions is realized by three main processes:
field desorption, field ionization, and field evaporation [Good and Müller, 1988; Gomer, 1961].
The field desorption is the effect when adsorbed atoms or molecules are desorbed, then ionized
in the strong surface electric field and pushed away due to a repulsive electric force. Similar to
the field desorption is the field evaporation, however, leaving atoms are not adsorbed gas but
the atoms of the bulk material. Last of mentioned processes is field ionization. In this process,
atoms of surrounding gas get to the critical distance from the surface where they are ionized
and repulsed by a high electric field.
Figure 1. Scanning electron microscope (SEM) images of used Au dust grains.
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JEŘÁB ET AL.: INFLUENCE OF CHARGING CONDITIONS ON DUST GRAIN’S FIE
The process of the field desorption proceeds in two phases. Adsorbed gas atoms leave
the surface and they are ionized at (or slightly beyond) so called ’critical distance’ where the
probability of a tunnel effect has its maximum and then they are pushed away by a repulsive
electric force. The positive charge from the surface is leaving in the form of ionized atoms of
adsorbed gas by this process.
In the last 10 years, many laboratory simulations in which different kinds of charging
processes were studied have been carried out, however, only a few of them deal with field
ion emission [e.g., Sternovský et al., 2001; Pavlů et al., 2004, 2006]. Pavlů et al. [2006] and
Jeřáb et al. [2005] performed laboratory experiments where impacts of energetic ions led to
deposition of a positive charge on the spherical grain. In these experiments, accumulated
charge is spontaneously released and outgoing FIE current is measurable when some value of
the electric field at the grain surface is reached. This value was found to be of the order of
109 Vm−1 and depends on the grain material, surface treatment and, probably, on the other
factors.
The discharging currents are usually attributed to the field ionization of a surrounding gas
or field evaporation of the grain material. However, Jeřáb et al. [2005] found that the main
discharging process is field desorption of beam ions deposited on the grain surface. The authors
charged a small gold spherical grain with the He+ ion beam and found that the maximum field
intensity at the surface that can be reached is of the order of 109 Vm−1 . They concluded that
this field intensity is sufficient for slight bonding of the atoms on the surface and thus, the
grain is covered by a layer of adsorbed He atoms. The field desorption of these atoms then
limits the attainable surface electric field. As a continuation, we prepared a detailed study of
discharging currents from spherical Au grains (Figure 1) exposed with different high-energy ion
beams. Results of the study are presented in this paper.
Experimental set-up and measurement technique
A schematic view of our experimental set-up is shown in Figure 2. A detailed description of
the apparatus as well as measurement procedures have been presented in Čermák et al. [1995];
Žilavý et al. [1998]; Sternovský et al. [2001]; Čermák et al. [2004]. Briefly, our experimental
setup is based on a 3D electrodynamic quadrupole [Paul and Steinweel, 1956] where a single
dust grain is trapped for a long time (typically tens of hours). The trapped grain is illuminated
by a red laser beam and a grain oscillation is acquired by zooming the view of a grain by lens
system, by amplifying the light spot and then by detecting its movement on a position sensitive
detector (PIN diode in our case). The signal from the PIN diode is further processed in order
to separate the movement in axial and radial directions. Finally, we measure the frequency of
grain oscillations and calculate the Q/m ratio from the relation [Paul and Steinweel, 1956]
Q
fac fz
= π 2 r02 · ef · c(fz , fac )
m
Vac
(1)
where fz is the grain’s frequency of motion in the axial direction, r0 , the inner radius of a
ef , the quadrupole AC supply voltage, f , the supply
middle ring quadrupole electrode, Vac
ac
voltage frequency, and c(fz , fac ) the correction function described in Čermák et al. [1995].
Our experiment is performed in ultra-high vacuum conditions with the pressure ≈ 10−7 Pa.
To provide reproducible measurement conditions and long-term investigations, the system is
equipped with an electrical cooling system (not displayed in Figure 2) which allows us to stabilize the amplitude of grain oscillations in each direction to increase the signal-to-noise ratio.
Čermák et al. [2004] developed this system to control the vibrational temperature of trapped
grain and to reduce its possible heating through un-harmonic effects.
The trapped grain is exposed to ion or electron beams from a particular gun system. Beam
currents are monitored by Faraday cups and regulated via a feedback. The energy of guns can
be adjusted in ranges 100 eV-10 keV and 100 eV-5 keV for electron and ion guns, respectively.
54
JEŘÁB ET AL.: INFLUENCE OF CHARGING CONDITIONS ON DUST GRAIN’S FIE
dust particles
IG
telescope
EG
LS
SP
QPS
SE
quadrupole
FC
SG
C
las
e
r
Figure 2. Experimental set-up: SG-signal generator, QPS-quadrupole power supply, SEsampling electronics, FC-Faraday cups, IG-ion gun, EG-electron gun, LS-lens system, SP-signal
processing, C-counter.
Results and discussion
Our measurements presented and discussed in this section were carried out on one Au
spherical dust grain of 0.57 µm of radius. The mass of used dust grain was 1.48 · 10−14 kg
(method of Pavlů et al. [2004]). Both values were measured several time during all presented
experiments (shown in Figure 3 as shadowed areas) to confirm that the effect of the grain
sputtering can be neglected. The total time of measurements was about 200 hours but only
about 9 hours were devoted to the grain bombardment (mostly by He+ ions). Pavlů et al. [2007]
presented the study of an influence of the ion bombardment on the dust grain and found that
the time required to a measurable sputtering of the dust grain was about 20 hours for Ar+ ions
under similar experimental conditions.
Measurements of FIE discharging characteristics are performed in several steps. The
trapped dust grain is charged by ions of the high energy (typically units of keV). Then, the
surface potential of the dust grain reaches equilibrium value for a given energy and current
density of the ion beam. In this equilibrium point, the total current charging the dust grain
(realized by the ion beam on one side and background electrons and FIE on the other side)
is zero. The time spent in this dynamic equilibrium point we call “time of treatment”. After
it, the ion beam is switched off and the dust grain begins its spontaneous discharging due to
FIE and we measure a gradual time evolution of the Q/m ratio. The procedure is shown in
Figure 3 where the measured Q/m ratio is re-calculated into the grain surface potential, φ. The
proportionality constant is the grain specific capacitance that was determined by the method of
Pavlů et al. [2004]. The measurements needed for this determination were carried out during a
“technological interval” that is shown as shadowed area in Figure 3. Measured data are filtered
and we calculate the time derivative of these data, i.e., the discharging current. Finally, we
analyze the computed discharging current as a function of the dust grain surface potential (so
called discharging characteristics).
The presented experiments were led by an idea that the discharging current can be a
function of the grain material, charging ions, and surface electric field. However, Figure 4
shows that, although the profiles of the discharging currents are similar, they can differ by an
order of magnitude for the same surface potential. Our explanation can be found in Figure 5
55
JEŘÁB ET AL.: INFLUENCE OF CHARGING CONDITIONS ON DUST GRAIN’S FIE
0
5
10
15
20
25
30
35
1400
1200
1200
1000
1000
/ V
1400
800
800
600
600
400
400
200
200
0
0
0
Measurement time
5
10
15
20
25
30
35
/ hours
Figure 3. Illustrative plot of FIE measurements in time. Figure shows time sequence of
measured charge-to-mass ratio. The peaks corresponds to charging by ion beam and next
decrease of Q/m ratio corresponds to the FIE discharging. The gray area marks technological
measurements where the capacity or mass of the grain are measured. Horizontal lines mark
1000 V and 900 V of a surface potential level used for qualitative analysis of measurements.
where the currents for two values of surface potentials 900 V and 1000 V (dotted vertical lines
in Figure 4, horizontal lines in Figure 3) are plotted as a function of the time since prolonged
3-hour treatment. The currents for the same surface potential decrease with a time constant
of several hours. A possible explanation of this effect can be connected with an amount of ions
implanted to the grain that is a function of the time of treatment. These ions slowly diffuse
to the surface in course of the discharging and the discharging current is then limited by the
number of atoms reaching the surface due to diffusion.
In order to check this idea, we have applied ions of several masses (H+ , He+ , Ar+ ) because
the implantation depth as well as the diffusion coefficients vary with ion species. These effects
are clearly seen from Figure 6 where discharging currents are plotted for different ions. In
the figure, the dependence of discharging currents (for already mentioned two surface potential
levels) on the time of treatment (time spent in dynamic equilibrium point where the field ion
emission current is balanced by the current of impacting ions and background electrons) is
shown. In spite of a large spread of measuring points, the increase of the discharging current
with the time of treatment is clear for all ion species except He+ . This can be explained by the
fact that the measurements with He+ were carried out at a very beginning of the experiment, in
a short time interval after prolonged (3 hours) treatment and the measurements with H+ and
Ar+ species were carried out at the end of the experiment. The effect of the prolonged treatment
affects almost all measurements where the He+ ions have been used for dust grain charging and
600
10
10
3
2
I
-
/ e /s
10
4
10
10
800
1000
1200
Treatment:
1.
~3 hours
2.
~1 minute
3.
~1 minute
4.
~1 minute
5.
~3.1 hours
6.
~1 minute
7.
~1 minute
8.
~1 minute
9.
~1 minute
10
10
10
10.
~1 minute
11.
~20 seconds
12.
~1 minute
1
10
0
10
600
800
1000
4
3
2
1
0
1200
/ V
Figure 4. Time sequence of measured FIE discharging characteristics of one Au dust grain of
0.57 µm of radius (from 1 to 12). The dust grain was charging by the He+ ion beam with the
energy of Ek = 5 keV and current density of j = 4 · 10−3 Am−2 . The curves differ in time of
treatment.
56
JEŘÁB ET AL.: INFLUENCE OF CHARGING CONDITIONS ON DUST GRAIN’S FIE
0
10
20
30
40
50
60
70
80
80
after first 3 hours treatment
=900 V
70
70
=1000 V
after second 3 hours treatment
60
60
=900 V
I / e/s
=1000 V
50
50
40
40
30
30
20
20
10
10
0
0
0
10
20
30
T
ime
40
50
60
70
/ hours
Figure 5. Dependences of the emission current as a function of time since 3-hour long treatment. Points are obtained from a sequence of measured FIE (see Figure 3 and 4). The value
of currents has been take from each of measurement on same levels of surface potential of the
grain (φ =900 V and φ =1000 V).
0
5
10
15
20
25
60
0
60
5
25
0
90
5
10
15
20
25
30
35
40
35
=1000 V
30
30
25
25
90
=900 V
=1000 V
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
30
30
20
20
10
10
20
20
15
15
10
10
5
5
0
0
0
5
ime
T
10
15
20
of treatment / minutes
a
25
I / e/s
40
I / e/s
I / e/s
40
20
=900 V
35
50
=1000 V
15
40
=900 V
50
10
40
0
0
5
ime
T
10
15
20
o f treatment / minutes
b
25
0
0
5
10
15
20
25
ime treatment
T
of
30
35
40
/ minutes
c
Figure 6. Dependences of FIE emission currents on the time of treatment for He+ (a), H+
(b), Ar+ (c) ions.
the spread of experimental points can be explained by diffusion of implanted He+ ions discussed
above (Figure 5). Thus, we suppose that the spread is connected with a “memory” of the grain.
Conclusion
We have analyzed many discharging characteristics of one dust grain of 0.57 µm of radius
measured under different charging conditions (ion species, time of treatment). We have found
that the problem of charging of dust grains by energetic ions and their discharging by the ion
field emission effect is very complex. A spontaneous discharging of the gold grain charged by
energetic ions is observable at surface field intensities of the order of 109 V m−1 . This intensity
is too low for field evaporation of the bulk material and the discharging current depends on the
ion species used for charging. These facts confirm our earlier suggestions [Jeřáb et al., 2005]
that the principal discharging mechanism is field desorption (and consecutive ionization) of
beam atoms. Moreover, we have found that the duration of charging (or total dose of the ions
impinging the grain) is another important parameter. This dose of bombardment determines
the number of implanted ions. Since their diffusion toward the surface is very slow (several
hours for 5 keV ions), the history of the grain should be considered in interpretation of the
results.
57
JEŘÁB ET AL.: INFLUENCE OF CHARGING CONDITIONS ON DUST GRAIN’S FIE
Acknowledgments. This work is a part of the research plan MSM 0021620834 that is financed
by the Ministry of Education of the Czech Republic and was partly supported by the Czech Grant
Agency under contracts 202/03/H162 and 202/04/0912 and by Vacuum Praha corporation. The authors
thank Prof. Vı́tek and Mr. Penguin for their kind attention.
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