Download Charge Writing and Detection by Electric Scanning Probe Techniques

Charge Writing and Detection by
Electric Scanning Probe Techniques
AFM
Asylum Research
Nikolaus Knorr, Sony Deutschland GmbH, Materials Science Laboratory, Stuttgart, Germany
Stefan Vinzelberg, Asylum Research, Mannheim, Germany
Introduction
Understanding the generation mechanisms and dissipation behavior of electric charge on the surface and in the bulk of
insulating materials is of importance to the study of a variety of phenomena. These include triboelectrification and electrostatic
discharge (ESD), as well as to several current and future applications, such as cable insulation, laser printer and photocopier
xerography, charge based data storage, plastic material recycling, and electret field-effect transistors, microphones, and dust
filters. Remarkably, the charging and discharging mechanisms of insulators are often not understood in detail, and even today
the identity of the charge carriers remains obscure in many cases.1,2,3 A convenient tool for investigating these mechanisms is
the atomic force microscope (AFM) because the charging can be realized with the same probe used for the analysis, and electric
field sensitive AFM modes such as Electrostatic Force Microscopy (EFM) and Kelvin Probe Force Microscopy (KPFM), also known
as Scanning Kelvin Probe Microscopy (SKPM), provide highly resolved spatial information of the charge distribution with
extremely high sensitivity. Accordingly, these methods have been used since their invention for the study of charging and
discharging phenomena on insulators.4,5
Biased AFM-probe-induced charge writing has found
widespread interest, often in conjunction with data storage
or nanoxerography. Charge pattern and xerographically
assembled nanoparticles with feature sizes below 100 nm
have been generated.6 Triboelectric (frictional) and contact
charging phenomons are also well suited for investigation
by EFM and KPFM because both the morphology and the
charge distribution of the contacted surface are detected
simultaneously in high resolution. Triboelectricity is a common
phenomenon experienced any time two materials come into
contact. Tribocharging affects us frequently in our daily lives
due to the widespread use of highly insulating synthetic fibers
and plastics with low surface and volume charge dissipation.
Research on triboelectric charging mechanisms has recently
found renewed interest,3,7,8 and AFM studies have helped
in revealing new insights such as prevalent bipolar charging
characteristics on micrometer length scales and the link of
morphological features to the charge distribution.9 In this
application note, biased-probe induced and triboelectric
charging of thin insulating films is covered, with the focus on
practical aspects of the AFM techniques. An Asylum Research
MFP-3D™ AFM was used for all work described.
EFM and KPFM Modes for
Charge Detection
EFM and KPFM investigations are useful for high resolution
studies of any system where electric fields are present, such
as biased electrodes and devices, or for contact potential
differences of materials. Here, we are interested in the electric
fields generated by uncompensated charge carriers deposited
on, or injected into, insulators. The physical system under
1
study is that of a thin insulating sheet or film of thickness
d, with its free face charged by the biased AFM probe, by
rubbing, or by evaporation of water, and its opposing face in
good mechanical and electrical contact to a grounded back
electrode. For both EFM and KPFM scans, the so-called dualpass method is applied, with a first scan in AC mode recording
the topography and a second scan for electric field recording
along the same scan path in the non-contact mode but with
an adjusted lift height h added to the previously recorded
height information.
Typically, bare or metal covered (Au, Pt, or PtIr) AFM probes
made of strongly doped Si with medium-range force constants
are used for EFM and KPFM. Here, Pt-covered AFM probes
with relatively stiff cantilevers (40 N/m force constant) have
been used to reduce the distortion of the topographic scan
by the attraction between probe and charged surface, and
to increase the resolution and sensitivity of the EFM scans by
small EFM amplitude scanning (typically h = -10 nm, measured
with respect to the height of contact in the topographic scan).
EFM
Vertical force gradients generated by the electric fields
emanating from the charged sample and interacting with the
probe modify the harmonic potential of the cantilever spring
(Figure 1). Attractive force gradients weaken the restoring
cantilever force while repulsive force gradients cause the
Charge Writing and Detection by
Electric Scanning Probe Techniques
KPFM
Figure 1: In Electric Force Microscopy (EFM), the electric field generated
by charge on the sample surface interacts with the AFM probe at the
end of the vibrating cantilever beam (distortion of the electric field by
electrostatic induction at the probe and cantilever are not considered).
Just as for EFM, there are several different KPFM modes. In our
experiments, the cantilever is no longer driven mechanically
by the piezo in the second pass, but an AC voltage VAC sin(ωt)
is applied to the AFM chip with ω /2π at the resonance of the
cantilever. Any electric fields reaching the probe will exert a
force on the charges induced at the probe by the applied AC
voltages and excite a mechanical oscillation of the cantilever
at the resonance frequency. The oscillation is however nullified
by a feedback applying an additional DC voltage (VDC) to the
AFM chip, annihilating the external fields at the probe. In the
case of charge deposited on an insulating film,VDC is equal
to the surface potential (VSP ) generated by the charge at the
probe with respect to the grounded back electrode. Mapping
VDC (x,y) thus reflects the distribution of the surface potential
on the sample surface. The origin of the force on the probe can
opposite effect (Figure 2A). This results in a negative or positive
shift of the resonance frequency of the cantilever (Figure 2B). In
EFM mode, these resonance shifts are detected by measuring
the phase or the amplitude change of the cantilever oscillation
which is mechanically excited by a piezo element at or near
its resonance frequency. For small resonance shifts, the phase
signal is proportional to the resonance frequency shift. Larger
frequency shifts can be tracked by using a phase lock loop-type
feedback that adjusts the drive frequency such that the phase
remains at 90°. In this work, the phase signal (φ) was evaluated.
Attractive force gradients make the cantilever effectively
“softer,” reducing the cantilever resonant frequency and
increasing the phase. Conversely, repulsive force gradients
make the cantilever effectively “stiffer,” increasing the
resonant frequency and reducing the phase. A quantitative
interpretation of the EFM signal is difficult because it depends
in a complicated fashion upon the geometries of the probe,
sample, and charge pattern due to the long range nature of the
Coulomb force. For example, a point charge q on the surface of
the insulting film will induce mirror charges in the probe apex
(qp) and the back electrode, which will all be mirrored again ad
infinitum at the probe, dielectric, and electrode surfaces. For
proximate scanning of the charge, the dominant force on the
probe will be generated by the interaction of q and qp, yielding
an attractive force which is proportional to q 2 (Figure 3). For
strong or extended charge patterns, the fields reaching the cone
of the probe and the cantilever will also considerably affect
the cantilever oscillation. To obtain stronger and better defined
charge signals, usually a constant DC bias (Vp ) is applied to the
probe during scanning of the second pass. In this case, for a
point charge q, often only three forces need to be considered:
the forces between the charge in the biased probe (Qp ) and its
mirror charges in the dielectric and the electrode (Qm ), between
Qp and q, and between q and qp (Fig. 3).10 For Vp well above
the surface potential of q at the probe, Δ φ is approximately
proportional to q.
2
Figure 2: (A) The harmonic potential of the cantilever spring is modified
by an attractive or repulsive force gradient. (B) This modification leads
to a shift of the cantilever resonance frequency and an associated phase
shift which is detected by the AFM control electronics.
Charge Writing and Detection by
Electric Scanning Probe Techniques
derive from the KPFM-determined surface potential values at
the probe apex due to the long range nature of the Coulomb
field, especially for strong and concentrated charge patterns.
KPFM scans are, however, somewhat less difficult to interpret as
compared to EFM scans, because of the reduced parameter set
(no Vp ) and the fact that the measured electric field distribution
remains almost unaffected by the presence of the probe (the
concept of mirror charges in the probe is not applicable because
the electric fields are nullified by the applied voltage). In a
working approximation, charge densities which are constant on
an area large enough to be resolved by KPFM are proportional
to the measured surface potential.11 Under these assumptions,
the surface density σ of charges that are located on the surface
of the insulating film can be approximated by the capacitor
equation:
Figure 3: Center linecuts of EFM phase-scans of a positively charged
spot at x = 0 on a 40 nm PMMA film. For Vp = 0, no force acts on the
probe (φ ≈ 90°) for | x| > 150nm. Closer to the charge, φ is increased
due to attraction of q and qp. For Vp = -5V, φ is increased (φ ≈ 100°
for |x| > 250 nm), due to the attraction between Qp and Qm. Close to
the charge, φ is further increased due to the attraction of Qp and q and
q and qp. For Vp = +5V, φ is increased symmetrically (φ ≈ 98° for |x | >
250 nm), also due to the attraction between Qp and Qm , which have
however interchanged signs. Close to the charge, φ is reduced due to
the repulsion of Qp and q, which is however somewhat attenuated by
the attraction of q and qp. For smaller positive Vp , these opposing forces
result in camel-shaped phase scans.
be derived by considering that the energy of the capacitor (C )
formed by the probe and the electrode is given by:
E = ½ C V 2Eq. 1
The z component of the electrostatic force is F = ½ dC / dz V 2.
Because V = (VDC – VSP ) + VAC sin(ωt), the vertical electrostatic
force is the sum of three terms: a static term, one term
oscillating with a frequency of ω, and one with 2ω. The
ω-term is:
Fω = dC / dz ( VDC – VSP ) VAC sin(ωt)
Eq. 2
which is nullified by a feedback loop for VDC = VSP. For a proper
operation of the feedback loop, the insulating film has to be thin
enough (d < 50 µm for the usual type of AFM probes), so that
the probe-electrode capacity generates sufficient forces on the
probe for the regulation mechanism to work efficiently.
Uncharged insulators have a surface potential VSP close to
zero. However, measured VSP values can easily exhibit offsets of
several volts caused by fields emanating from electrostatically
charged objects close to the cantilever substrate. For example,
the plastic cap of the AFM probe holder can get charged easily.
To reduce the offsets, the probe holder may be discharged, for
example, with an antistatic gun or by wiping with a wet cloth.
Similar to the difficulties in interpreting EFM signals
quantitatively, surface charge densities are generally difficult to
σ(x,y) = εε0 VSP (x,y) d -1Eq. 3
where ε is the dielectric constant of the insulting film and ε0 the
electric permittivity of free space.12 Eq. 3 follows from the fact
that the charge induced in the probe/back-electrode capacitor
by the applied voltage has to match the charge at the insulator
surface in order to nullify the electric fields between the charge
and the probe. For a uniform space charge density ρ in the
insulating film, it follows from the Poisson equation that:
ρ = 2εε0 VSP d -2.13
The lateral resolution of KPFM and EFM probes can not be
determined easily. It is, however, generally advisable to use long
and slender probes for higher resolution.14 In addition, the use
of rather thin insulating films is beneficial because the range of
the electric fields of the charges at the surface is reduced by the
compensating fields of the mirror charges in the proximate back
electrode. It should, however, be noted that, for biased-probe
charge writing on very thin films (d < 50 nm), electric break
down can easily damage the probe.15
Biased-Probe Induced Charge Writing
Many different types of insulating material are suitable for AFM
charging experiments. Insulating sheets can be either glued to
an electrode substrate, or a metal film can be evaporated onto
the backside of the sheet. Alternatively, insulating films can
be deposited directly on an electrode sample, for example, by
thermal evaporation or by spin- or dip-casting from solution.
The latter methods have the advantage of typically providing
discharged and well-attached films of low surface roughness.
The quality of the contact of the film to the sample is crucial
because for loose contacts, charge will also accumulate on the
side of the sheet facing the electrode. Frequently used systems
are oxide layers grown on doped Si wafers and thin (d < 100
nm) spin-coated films of poly(methyl methacrylate) (PMMA)
which can be strongly and persistently charged by comparatively
low voltages (<50 V) under ambient conditions. Charging of
insulators has been found to be highly variable in different
laboratories and for different experimenters due to, amongst
3
Charge Writing and Detection by
Electric Scanning Probe Techniques
Charge Spot Writing
Charge spots were written in Figure 6 by square voltage pulses
of up to 80 V on a 12 µm thick float glass slide and on a 1.4 µm
thick spin-cast film of poly(1-trimethylsilyl-1-propyne) (PTMSP),
which is a high-free-volume glassy polymer of intrinsic porosity
(inner surface: 550 m2/g).17 First, the probe was brought into
contact with the polymer film by using contact mode AFM
feedback with a controlled, low load force (typically 25 nN).
Next, a square voltage write pulse was applied by triggering
an external function generator connected to an external high
voltage amplifier, and then the tip was retracted before moving
to the next spot location. Usually, no modification in the
topography at the contact position due to indentation of the tip
is observed, because of the low load forces and relatively low
voltages (<100 V) used.
Figure 7 summarizes the performance of some functions of
AFM based charge writing which are relevant for data storage.
All charge spots shown have been written and scanned in
ambient air. Figure 7A indicates the high reproducibility of the
method over a 90 x 90 µm2 scan area, 7B demonstrates the
targeted reversing in polarity (erasing) of single bits, 7C their
rewriteability, 7D illustrates the insensitivity of charging to the
applied load and that writing from a lift height is possible, and
7E demonstrates high resolution scanning of charge spots which
are as small as 30nm (full width half max).
Figure 4: MFP-3D AFM operated in a glovebox, which is in turn located
inside of an acoustic isolation chamber.
other things, its strong dependence on the humidity. Therefore,
for quantitative charging experiments it is advisable to work
under a well-defined atmosphere by performing the AFM
experiments in a glove box (Figure 4) or in a humidity cell. A low
humidity will also reduce the charge dissipation rate.16
AFM charge writing can be accomplished by applying a constant
bias or voltage pulses to the continuously scanning probe, or
point-wise by applying a voltage pulse to the probe at a fixed
location. In Figure 5, the surface of a 160 nm PMMA film has
been charged continuously while scanning in contact mode
by application of +50 V bias voltages using MicroAngelo,™
the built-in tool in the MFP software. To avoid damage to the
AFM electronics, bias voltages above ±10 V were applied to the
back electrode, electrically connected by a metal clamp which
touches the electrode at a point where the insulating film has
been removed. In this case the cantilever substrate was held at
ground. For ±10 V, the authors verified that charge writing is
symmetric for the two alternatives of bias application (cantilever
substrate versus back electrode). In AFM charge writing of
insulators, a probe which is positively biased relative to the back
electrode usually yields positive charging and surface potential
values at the sample surface and vice versa. To avoid confusion,
voltages stated in the following text always refer to potentials
applied to the cantilever substrate relative to the back electrode.
4
Charge Spot Writing Charging Mechanism
The surface potential peak heights and widths of the charge
spots are not correlated to the applied pulse voltages in a simple
manner, one reason being that the size of the charged surface
is often too small to be resolved by the probe, yielding average
values of the surface-potentials above the charge pattern and
the neighbouring uncharged areas.16 It has however been shown
that the KPFM peak volumes, which are proportional to the total
Figure 5: Surface potential scan of a charge pattern written on PMMA
in ambient air in contact mode using the Asylum Research MicroAngelo
built-in tool software. Scan size: 8 µm x 8 µm. Color scale: 1 V.
Charge Writing and Detection by
Electric Scanning Probe Techniques
Figure 6: Surface potential scans of charge spots written in 2% RH on
(A) glass, and (B) PTMSP, by 20 V, 40 V, 60 V, 80 V 0.5s in length pulses.
Scan size: 10 µm x 5 µm. Color scale: 5 V.
amount of surface-near charge, approximately follow a square
law in the applied pulse voltage, offset by a threshold voltage
Vth. The pulse width dependencies of KPFM peak volumes,
heights, and widths can be approximated by power laws of
positive exponents smaller than one.15 Figure 8 shows results for
glass and PTMSP (including data from Figure 6), and for charge
written on a 1.0 µm thick polyvinyl acetate (PVAc) spin-cast film.
Vth is a key parameter shedding light on the charging
mechanism. Vth is increased for more hydrophobic materials of
low water permeability and water uptake capability. Based on
this and other observations, such as the comparably weak effect
of the probe load on the amount of injected charge (Figure 7),
an electrochemical charging mechanism of ionic charge carriers
mediated by the field-adsorbed water meniscus at the biased
probe has been proposed.15 Supposedly, either water ions are
injected from the water meniscus into the insulating film (Figure 9)
or, for materials with easily dissociable surface groups, ions from
the insulating material are removed to the water phase driven by
the high electrochemical potential of the biased water meniscus.
According to the former mechanism, Vth will be reduced for
polymers with a high degree of free volume or high water swellability, because the penetrating ions may keep a larger fraction
of their hydration shell when entering the polymer. The validity
of the proposed charge injection mechanism is supported by
the fact that charging is strong and achieved at low applied
voltages for dried amorphous materials with a comparably high
degree of water permeability such as SiO2, Al2O3, PMMA, PVAc,
polycarbonate (PC), PVAc, polyacrylamide (PAAm), poly(acrylic
acid), and PTMSP.
IV. Triboelectric Charging
Pronounced triboelectric bipolar charge patterns can be
generated easily by rubbing an insulating film with almost any
other material. Strong VSP signals in the few voltage range
can be achieved by using comparatively thick films (d = 1 µm)
according to Eq. 3. It is advisable to either use pointed rubbers,
Figure 7: KPFM study on different aspects of charge spot writing for data storage. (A) 25 x 25 array of spots written on a large scale by (5 V / 0.5s)
2
4
pulses. (B) 9 x 9 array of charge spots (-7 V / 0.5s) with a 3 x 3 array erased by (7 V / 0.5 s) pulses. (C) Spots rewritten by 1, 10 , and 10 (-30 V /
+30 V, 10 4 Hz) square wave cycles, from bottom to top. For higher cycling numbers, peak widths are increased. This spot broadening can be reduced
by the use of asymmetric pulses of shifted bias or pulse width. (D) From bottom to top: charge spots written in contact with load forces of 1 µN and
10 nN (+30 V / 5 ms), and with lift heights of h = 100 nm, 150 nm, and 200 nm by (+40 V / 10s) pulses. (E) Charge spots written (+15 V, +16 V,
+18 V, +20 V / 1s, from bottom to top) and scanned with a high aspect ratio (~7:1) AFM probe. (A,B) on 20 nm PMMA; (C,D) on 40 nm PMMA;
(E) on 30 nm COC.
5
Charge Writing and Detection by
Electric Scanning Probe Techniques
Figure 8: Peak volumes, heights and widths of the KPFM charge peaks, as a function of applied voltage pulse height and width. Charge spots written
on PTMSP ( g ), glass ( ), and PVAc ( ), all in 2% RH. Pulse widths in (A) are 0.05s for PVAc and 0.5s for glass and PTMSP. Pulse heights in (B) are
+80 V for glass; +80 V and -60 V for PTMSP; and ±50 V for PVAc (closed symbols for negative, open symbols for positive voltages). Lines indicate
power law fits in (B) and square law fits in (A) with Vth = -3 V for glass; ±19 V for PTMSP; and -10 V and +24 for PVAc.
5
•
so that the entire width of rubbed track can be scanned (the
maximum scan range of the MFP-3D scanner is 90 µm x 90 µm),
or to scan the edge of broader rubbed tracks with the cantilever
facing the untouched surface, in order to have a pristine surface
area on the scan which provides a surface potential reference. In
several literature studies, the AFM probe itself has been used for
the rubbing, which is pointed indeed and offers high control of
the applied load.5,18 Also, the scanning can be performed easily
after the charging without the need for relocating the rubbed
surface area. Relocation can be tedious if the rubbed track is
small and if the rubbing did not affect the morphology of the
film to such an extent that the scratched can be resolved with
the camera. Tracks which are extended in one dimension can be
located by single line EFM or KPFM scanning perpendicular to
the rubbing direction and subsequent translation of the sample
with the micrometer screws in the same direction by the distance
of the scanning range. Alternatively, the rubbed surface can be
6
marked by a scratch or ink close by. For the examples shown
here, as well as in Ref [9], rubbing was performed by hand-held
pointed hard cotton swabs (HUBY-340 BB013, curvature radius
∼100 μm), which were covered with polymer films before use by
soaking in viscous polymer solutions and subsequent drying. The
use of low-cost swabs covered with freshly prepared polymer
films guarantees that the polymer surfaces on the swab are
discharged before use and allows for single-use experiments
with pristine surfaces.
Important external parameters for tribocharging are the ambient
pressure, the temperature and the relative humidity (RH). Most
charging phenomena can conveniently be observed at room
temperature and atmospheric pressure. The effect of RH on
the charging behavior is multifaceted because adsorbed water
can be the source of the triboelectric charge (for example by
dissociation of the water molecules into positive and negative
water ions), but it can also act as a sink of charge, because
Charge Writing and Detection by
Electric Scanning Probe Techniques
and increased separation from the surface, leaving behind the
relatively stronger bound and near-surface negatively charged
water ions. This explanation finds support in the observation that
water droplets evaporating on hydrophobic surfaces generate
a pronounced bipolar charge pattern of a negatively charged
dewetted circumferential boundary and a strongly positively
charged core (Figure 12). Presumably, negative water ions are
preferentially left behind on the surface by the receding threephase line due to their higher affinity than the positive water
ions to the polymer/water interface, leading to an accumulation
of excess positive ions in the shrinking evaporating droplet.
Summary
Figure 9: Possible charging mechanism for a positively biased probe:
I) Ambient water is field-adsorbed. II) Water is dissociated (6H2O → O2 +
+
+
4H3O + 4e–). III) Positive water ions are injected (for example, H3 O ).
ionic charge carriers have much higher mobilities on the surface
and in the volume of most materials at increased RH.16 At low
RH (<10%), strong charge patterns can be generated when at
least one rubbing partner is made of a somewhat hydrophilic
material and the charge patterns are generally fairly durable
over many days. Examples are given in Figures 10 and 11 for
PAAm, PVAc, and PMMA films rubbed by cyclic olefin copolymer
(COC), PMMA, and PAAm. Some interesting observations are:
rubbed tracks are generally charged bipolarly, at onsets of tracks
material from the rubber is deposited and surfaces are strongly
charged, nanoscopic strongly charged particles are frequently
observed along the rubbed track (Figure 11), and stick-slip
friction is reflected in the charge pattern.
The EFM and KPFM methods have been discussed, including
interpretation of the measured variables with regard to
uncompensated charge deposited on insulating dielectric films.
Practical hints have been given for the controlled generation of
charge patterns by triboelectrification and biased-probe induced
charging, as well as methods for quantification of the measured
signals for the latter. Possible charging mechanisms have been
discussed, which may, interestingly, be caused by dissociation
and ion separation of adsorbed water in both cases.
At higher RH, however, the charge patterns are often rapidly
dissipated, especially for materials with a high degree of water
uptake capability or water permeability. Rubbing combinations
of two hydrophobic materials, such as Teflon, polystyrene,
polydimethylsiloxane (PDMS), cyclo olefin polymers (COP),
polyvinylchloride (PVC), silicone rubber, polyethylene (PE),
polypropylene (PP) will generate much weaker charge pattern at
low humidity, but stronger ones at higher humidity (RH > 50%),
and charge patterns on such materials are stable even in higher
humidity.
Triboelectric Charging Mechanism
The mechanism of triboelectric charging is under debate.
To explain the complexity of the observations, assumedly
several different mechanisms contribute to the charging, their
prevalence depending on the type of materials involved and the
conditions under which the triboelectrification is generated. In
Ref. 9, an insulator/insulator triboelectric charging mechanism
by separation of dissociated water ion pairs stemming from
adsorbed water has been proposed: the positive water ions,
i.e. hydrated protons, are preferentially dragged along the
surface together with an excess of adsorbed water in front of
a conformal rubbing contact due to their lower adhesion to
Figure 10: Gallery of various surface potential AFM images illustrating
the charge patterns on polymer films generated by rubbing polymer
covered cotton swabs across the surface (top row: PAAm rubbed by
COC). The rubbing direction is from top to bottom. Image size is 90 µm
x 90 µm; the lower right shows a complete trace of PVAc rubbed by
PAAm (110 µm x 162 µm composed of two separate scans). The color
scale varies slightly from image to image but in all images, the potential
signal covers a range of a few volts.
7
References
1. H
.J. Wintle, IEEE Trans. Dielectr. Electr. Insul. 6, 1 (1999);
IEEE Trans. Dielectr. Electr. Insul. 10, 826 (2003).
2. G
.C. Montanari and P.H.F. Morshuis,
IEEE Trans. Dielectr. Electr. Insul. 12, 754 (2005).
3. L .S. McCarty and G.M. Whitesides,
Angew. Chem., Int. Ed. 47, 2188 (2008).
4. J .E. Stern, B.D. Terris, H.J. Mamin, and D. Rugar,
Appl. Phys. Lett. 53, 2717 (1988).
5. B
.D. Terris, J.E. Stern, D. Rugar, and H.J. Mamin,
Phys. Rev. Lett. 63, 2669 (1989).
6. H
.O. Jacobs, P. Leuchtmann, O.J. Homan, and A. Stemmer,
J. Appl. Phys. 84, 1168 (1998).
Figure 11: (A) PMMA rubbed by COC at 2% RH. The surface potential
data have been overlaid as color on the 3D topography derived from the
height data.
7. M
.D. Hogue, E.R. Mucciolo, C.I. Calle, and C.R. Buhler,
J. Electrostat. 63, 179 (2005).
8. C. Liu and A.J. Bard, Nature Materials 7, 505 (2008).
9. N. Knorr, AIP Advances 1, 022119 (2011).
10. J . Lambert, C. Guthmann, and M. Saint-Jeanc,
J. Appl. Phys. 93, 5369 (2003).
11. E . Palleau, L. Ressier, L. Borowik, and T. Melin,
Nanotechnology 21, 225706 (2010).
12. L . Bürgi, T. Richards, M. Chiesa, R.H. Friend, and
H. Sirringhaus, Synthetic Metals 146, 297 (2004).
13. M
. Chiesa and R. Garcia, Appl. Phys. Lett. 96, 263112
(2010).
14. H
.O. Jacobs, P. Leuchtmann, O.J. Homan, and A. Stemmer,
J. Appl. Phys. 84, 1168 (1998).
15. N
. Knorr, S. Rosselli, T. Miteva, and G. Nelles,
J. Appl. Phys. 105, 114111 (2009).
16. N
. Knorr, S. Rosselli, and G. Nelles, J. Appl. Phys. 107,
054106 (2010).
17. K
. Nagai, T. Masuda, T. Nakagawa, B.D. Freeman,
and I. Pinnau, Prog. Polym. Sci. 26, 721 (2001).
Figure 12: Surface potential scan of bipolar charge pattern generated
by evaporated droplets of deionized water on a 1 µm thick COC film.
Scan size: 90 µm x 90 µm. Color scale: -0.5 V to 0.2 V. At the positively
charged spots VSP reaches up to 5 V.
18. H
. Sun, H. Chu, J. Wang, L. Ding, and Y. Li,
Appl. Phys. Lett. 96, 083112 (2010).
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