Selective discharge of electrostatic charges on electrets using a patterned hydrogel stamp

Article abstract of DOI:10.1002/anie.201000766

(Figure Presented) Paint with water: A topographically patterned agarose hydrogel stamp transfers and prints water onto uniformly charged PMMA electrets for selectively discharging electrostatic charges. The high-resolution electrostatic charge patterns produced by this simple approach can induce the reduction of metal ions and allow the fabrication of micro- and nanoscale metallic arrays.

Full text of DOI:10.1002/anie.201000766

Angewandte
Chemie
DOI: 10.1002/anie.201000766
Electrets
Selective Discharge of Electrostatic Charges on Electrets Using a
Patterned Hydrogel Stamp**
Xinlei Ma, Dan Zhao, Mianqi Xue, Hai Wang, and Tingbing Cao*
[
1]
Electrets, materials that can permanently store charge, have
bulk and thin-film electrets without needing to be supported
on conductive substrates. Since many dielectrics can be
charged through a simple contact electrification process that
does not involve electric field, the selective discharge of
electrostatic charges should be an enhanced way to achieve
high-resolution charge patterns on electrets.
[2]
a long history in engineering and condensed-matter physics.
Recently, the field has drawn growing attention from the field
of chemistry, especially after the hot debate between two
[2–5]
renowned chemists, George Whitesides
and Allen
[6–9]
Bard,
on the mechanism of contact electrification of
dielectrics. Although the argument of charge transfer on
tribocharging, either electrons or ions, is controversial and has
not yet reached a consensus, the thorough investigation of the
electrostatic phenomenon from both sides will still pave the
way for other chemists and stimulate the exploitations for new
application of electrets.
Herein, we propose another charge patterning approach
based on the concept of selective discharge of electrostatic
charges. Different from the academic TSD process, this new
strategy stems from the practical observation that electro-
static charges are more stable in dry winter than in sultry
summer conditions because humid air helps to dissipate
electrostatic charges by keeping surfaces moist and increasing
the conductivity. Herein, we use a topographically patterned
hydrogel stamp to expose electrets purposely and selectively
to water for discharging, hence leaving the electrostatic
charges in the noncontacted area to achieve high-resolution
charge patterns; by applying “single electrode” electrochem-
Patterning of electrostatic charges, as widely used in
conventional xerography for many years, is attracting consid-
erable interest because it is associated with strong electro-
static fields to control the behavior of nanoscale electronic
[
10]
and mechanical devices, guide the assembly of nanomate-
[
11]
[12]
rials, or modulate the properties of biological systems.
[6]
High-resolution charge patterns can be produced either by
injection of electrons into materials using electron beam
ical reductions in the charged area, we can also obtain a
variety of metallic micro- or nanostructures on electrets.
Figure 1a,b shows the procedure to fabricate the hydrogel
stamp. Firstly, we prepared a PDMS stamp with topography
preliminarily determined by photolithography. Then, agarose
was molded against the PDMS relief structure to yield a
stamp with an inverted bas-relief. Agarose has good mechan-
ical stability combined with fast internal diffusion caused by a
tunable water content of 20–98%; hence, hydrogel stamps
[
13]
[14]
lithography,
conducting atomic force microscopy,
and
[
15]
electrical microcontact printing, or by printing of positive
or negative ionic charges with electrohydrodynamic jet-
[
16,17]
ting.
One common ground for these advanced techniques
is to selectively charge the electrets either by the inputting of
electrons or ions, or by inducing and maintaining macroscopic
electric dipoles by using a strong electric field.
[
18]
[19]
In our previous study, we proposed a much different
approach in patterning of charge that involves the selective
discharge of the electrostatic charges. We used a topograph-
ically patterned poly(dimethylsiloxane) (PDMS) stamp to
transfer and print heat energy onto uniformly charged
electrets, and the heat can neutralize the charges or release
the dipoles through a thermally stimulated discharge or
have been widely used for the etching of metals, printing of
[
20]
bacteria,
and fabrication of polymer microstructures by
[21]
soaking of appropriate solutions.
Herein, the agarose
hydrogel stamp was chosen for transferring and printing
water onto electrets for the selective discharge of electrostatic
charges.
Figure 1c,d shows a typical process of charging electrets:
A thin layer (100 nm) of poly(methylmethacrylate) (PMMA)
film supported on a silicon wafer is uniformly charged under
[1]
depolarization (TSD) process to form patterns of charges.
Hot microcontact printing (mCP), a simple and inexpensive
procedure, is capable of patterning of electrostatic charges on
À1
an electric field of 10 kVcm by using aluminum foil as an
electrode to apply electric potential. After applying pulsed
voltage for 20 seconds, the PMMA electrets are uniformly
charged, and the mechanism of this charging process is
dominated to be the transport of electrons as explained by
[
*] X. Ma, D. Zhao, M. Xue, Prof. T. Cao
Department of Chemistry, Renmin University of China
Beijing 100872 (China)
[15]
Jacobs and Whitesides.
Fax: (+86)10-6251-6444
When uniformly charged electrets are contacted with the
patterned hydrogel stamp as shown in Figure 1e, the electro-
static charges (either electrons or ions) on the contacted part
are removed through the diffusion from hydrogel stamp,
whereas the charges on the untouched area remain, which
results in high-resolution charge patterns. The patterns of
charge, mainly electrons on PMMA electrets by this charging
method, can reduce metal ions through electrostatic electro-
E-mail: tcao@chem.ruc.edu.cn
Dr. H. Wang
National Institute of Metrology
Beijing 100013 (China)
[
**] This research was supported by the National Science Foundation of
China under grant Nos. 20674096, 20733001, and 50773092.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000766.
Angew. Chem. Int. Ed. 2010, 49, 5537 –5540
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
Figure 1. The procedure of electrostatic charge patterning using a
hydrogel stamp. See text for details.
Figure 2. a) AFM and b–e) KFM images showing the topography and
surface potential differences on a PMMA thin film after selective
discharging by a hydrogel stamp; f) SEM image showing the adsorp-
tion of silica microbeads on the obtained charge patterns.
[
6]
chemistry to produce micro- or nanostructures of metals, as
shown in Figure 1 f.
It is not necessary that the electrets are supported on a
conductive Si substrate to be charged by an electric field as
shown in Figure 1c,d, because many other charging methods,
such as a simple contact electrification process, could be
applied in this step for uniformly charging the electrets (see
the Supporting Information). Herein, we chose Si wafer
supported PMMA films as electrets to characterize the high-
resolution charge patterns by Kelvin probe force microscopy
demonstrate one of the applications of the as-prepared
charge patterns.
The charge patterns measured by KFM or scanning
electric potential microscopy (SEPM) demonstrate great
progress in the study of electrostatics on dielectrics and
[8]
other solids. However, as described by Liu and Bard, the net
charge on a dielectric surface is the sum of the electronic and
the ionic charge, and the nature of the charge cannot be
distinguished by simply measuring the total surface charge or
surface potential. They developed a chemical approach, so-
called “single-electrode” electrochemistry or electrostatic
electrochemistry, to selectively detect electrons (rather than
ions) on the surface of dielectrics. By carrying out chemical
redox reactions such as metal deposition, ion reduction and
chemiluminescence on the rubbed teflon, they identified
electrons as the main components in contact electrification.
Herein, we utilize their developments to fabricate metallic
micro- or nanostructures by reducing metal ions on the charge
patterns (mainly electrons).
(
KFM), a tool to measure electrostatic potential through an
electric cycle. Atomic force microscopy (AFM) images (Fig-
ure 2a) show the surface topography of PMMA film after the
selectively discharging, and the flat surface morphology
indicates that there are no observable differences in top-
ography before and after the hydrogel stamping operation.
The KFM images (Figure 2b–e) show a set of surface
potential changes after the selective discharging process.
From these images, we clearly find that the technique can
generate various electrostatic charge patterns on PMMA thin
film electrets, and the different geometries and length scales
are determined by the features of the as-used agarose
hydrogel stamp with resolution around 150 nm. Figure 2 f
shows a SEM image of silica microbeads adsorbed on the
charged area of electrets, and this phenomenon, as widely
applied in photocopier and nanoxerography, is used to
Figure 3 shows SEM images of different metallic micro-
and nanostructures on PMMA by reducing metal ions with
patterns of charge. Although the charge density in PMMA
5
538
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PMMA, whereas on the hydrogel directly contacted area
where charge should be dissipated, there are still some
noticeable Ag nanoparticles. For nickel and copper, the
microstructures look much more regular, and the nano-
particles on the discharged area are nearly negligible. We
+
attribute this phenomenon to the reduction of Ag being
2
+
2+
much easier than that of Cu and Ni (the reduction
+
potential of Ag is much lower), therefore the residual
charges in PMMA after hydrogel discharging can induce the
+
2+
2+
reduction of Ag rather than Cu and Ni .
To confirm the reduction of metal (e.g. copper) on the
patterns of electrostatic charge, we used the auger electron
spectroscopy (AES) and X-ray photoelectron spectroscopy
(XPS) to analyze the as-prepared microstructures. As shown
in Figure 4a, an AES depth profile was measured to analyze
Figure 3. SEM images of different metal micro- and nanostructures
reduced by the patterns of electrostatic charge. The figures on the
right (b, d, f, h) are magnified views of the corresponding left of set
+
2+
Figure 4. a,b) Auger electron spectroscopy and c) X-ray photoelectron
spectroscopy analysis of the as-prepared copper microbeads.
images (a, c, e, g). a–d) Ag (reduced from Ag ); e,f) Ni (from Ni );
2
+
g,h) Cu (from Cu ).
+
using the electric field charging method should be higher than
that charged by contact electrification, the total number of
charges or electrons on electrets is still rather small. To obtain
observable and regular metallic microstructures on the
dielectric, we intentionally dissolved the metal ion salts in
ethanol rather than in water. The aqueous solution of metal
salts can leave some stains on PMMA during evaporation.
Especially when a small volume of solution is dropped onto
charge patterns, the coffee-ring-like stains will inevitably be
confused with as-reduced metallic microstructures. Herein,
the PMMA electrets supported on silicon were uniformly
dipped into ethanol solution; since ethanol can ensure the
ionization of these metal salts without dissipating the charges
too quickly (pKawater = 14, pKaethanol = 16), metal ions can be
easily reduced into metallic microstructures over a large area.
Figure 3a–d shows the reduction of Ag from AgNO3
solution in ethanol, and Figure 3e–h shows the reduction of
Ni and Cu from their salts in ethanol. From these Figures, we
can clearly find that Ag microstructures corresponding to the
features on agarose stamp can be formed on charged area on
the copper microbeads by sputtering the sample with Ar .
Following the sputtering time (from bottom to top as in
Figure 4a), the auger spectra are shifted towards higher
kinetic energy from 918.0 eV to 919.8 eV (the values of
characteristic auger spectra peaks are shown in Figure 4b,
[22]
2
+
which means the transition from Cu to Cu. We demonstrate
the phenomenon with a cartoon image in the inset in
Figure 4b, which indicates that the inner core of the micro-
structure is made of as-reduced Cu and the outer ring remains
in its oxidized state. This hypothesis was further verified by
XPS results (Figure 4c), which shows a shift of the Cu(2p )
3
/2
peaks from 935.3 eV to 932.9 eV and a decrease of shake-up
peaks. The electrons on the surface of the electrets can
reduce Cu to Cu in the inner core, while the embedded
[
23]
2
+
[2,8]
electrons
in the electrets attract cupric ions from the
ethanolic solution through a xerographic effect and achieve
the outer ring of the as-prepared metallic microstructures.
In conclusion, we have demonstrated a very simple and
inexpensive procedure to transfer and print “water” or
humidity to induce the charge dissipation on solid surfaces.
Angew. Chem. Int. Ed. 2010, 49, 5537 –5540
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5539

Communications
Electrostatic charges on many dielectrics, particularly for
those widely explored electrets fabricated by contact elec-
trification, can be uniquely patterned by using this method
Reduction of metal ions: The PMMA electrets with patterned
charge were immersed into ethanolic solution containing different
metal ion salts for 20 s, and then washed with ethanol. The solutions of
Ni(CH COO) ·4H O, CuSO ·5H O, and AgNO in ethanol were
3
2
2
4
2
3
(
a demonstration of the patterning method on contact
diluted to half of their saturated capacity.
electrification system is shown in Figure S1 in the Supporting
Information). Since high-resolution charge patterns are
extremely useful in data storage, nanocomponent optoelec-
tronics and self-assembly systems, the method of patterning
electrostatic charges through selectively discharging shows a
prominent role in exploring new application of electrets.
It is worthy of note that the mechanism of discharging
electrostatics by water is not explained in detail herein. As
one of the main controversial factors for contact electrifica-
tion, the role of water during charging and discharging of
electrets remains unclear and debatable. Both electrons and
Received: February 8, 2010
Published online: July 2, 2010
Keywords: electrets · electron microscopy ·
.
electrostatic chemistry · gels · lithography
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2
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Adsorption of nanoparticles: The charged PMMA film was
immersed into a 0.5% w/v suspension of silica microspheres (500 nm)
in ethanol for 30 s and quickly rinsed with ethanol to remove the
loosed attached part.
[
5
540
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Angew. Chem. Int. Ed. 2010, 49, 5537 –5540