sensors
Article
Visualized Multiprobe Electrical Impedance
Measurements with STM Tips Using Shear Force
Feedback Control
Luis Botaya, Xavier Coromina, Josep Samitier, Manel Puig-Vidal * and Jorge Otero *
Department of Electronics, Universitat de Barcelona, Barcelona 08028, Spain; lbotaya@el.ub.edu (L.B.);
xcoromina@el.ub.edu (X.C.); jsamitier@ub.edu (J.S.)
* Correspondence: manel.puig@ub.edu (M.P.-V.); jotero@el.ub.edu (J.O.); Tel.: +34-93-403-9161 (M.P.-V.)
Academic Editor: Stephane Evoy
Received: 16 March 2016; Accepted: 21 May 2016; Published: 25 May 2016
Abstract: Here we devise a multiprobe electrical measurement system based on quartz tuning forks
(QTFs) and metallic tips capable of having full 3D control over the position of the probes. The system
is based on the use of bent tungsten tips that are placed in mechanical contact (glue-free solution)
with a QTF sensor. Shear forces acting in the probe are measured to control the tip-sample distance
in the Z direction. Moreover, the tilting of the tip allows the visualization of the experiment under
the optical microscope, allowing the coordination of the probes in X and Y directions. Meanwhile,
the metallic tips are connected to a current–voltage amplifier circuit to measure the currents and
thus the impedance of the studied samples. We discuss here the different aspects that must be
addressed when conducting these multiprobe experiments, such as the amplitude of oscillation, shear
force distance control, and wire tilting. Different results obtained in the measurement of calibration
samples and microparticles are presented. They demonstrate the feasibility of the system to measure
the impedance of the samples with a full 3D control on the position of the nanotips.
Keywords: scanning tunneling microscope (STM) tip; scanning probe microscopy; multiprobe SPM;
quartz tuning forks; impedance measurement
1. Introduction
Since the invention of the scanning tunneling microscope (STM) in 1981 by Binning and Rohrer [1],
the investigation of the local electrical properties of a wide range of samples at the nanoscale
has been widely reported [2,3]. The main idea is to measure the electrical interactions between
a sharp metallic tip (nanometric tip radius) and the surface to determine the local properties of
the sample of study. The original microscopy technique was developed by using the exponential
decay of the tunneling current with the tip-sample distance. Controlling this current, it is possible to
keep the tip-sample distance constant while scanning the sample to acquire the surface topography.
The limitation of working with conducting samples was overcome with the development of the atomic
force microscope [4], where the nanometric tip is placed on a microfabricated cantilever. Then, the
bending of this cantilever is measured to determine the atomic forces between the tip and the sample
surface (which depend on the tip-sample distance and have an exponential decay as well [5]), thus
allowing for the reconstruction of the surface topography if the sample is scanned while keeping
the force interaction constant. Evolved from these, several techniques have appeared over the last
several decades to measure the nanoelectrical properties of the sample, where we can clearly identify
two different groups: wire-based and cantilever-based techniques. Wire-based techniques (STM [6],
electrochemical STM [7], scanning electrochemical microscopy (SECM) [8], scanning ion-conductance
microscopy (SICM) [9], etc.) are limited in that the tip-sample distance is controlled by electrical
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measurement meanings, thus introducing artifacts due to the coupling between the topography and
electrical properties of the surface. On the other hand, cantilever-based techniques (conductive atomic
force microscopy (C-AFM) [10], Kelvin probe microscopy (KPM) [11], electrostatic force microscopy
(EFM) [12], etc.) overcome the previous limitation by measuring the interaction forces (and then the
topography) independently from the electrical signals; nevertheless, the electrostatic forces that appear
between the tip and the sample at nanometric distances make the cantilever bend and collapse towards
the sample surface; thus, the techniques are limited to larger tip-sample distances or contact-mode
measurements. Simultaneous force and conductance measurements have been taken with cantilevered
tips taking into account those considerations [13]. Further, for working in liquid environments, wires
are easier to isolate electrically than are microfabricated cantilevers. Finally, both solutions (wire- and
cantilever-based) measure the tip-sample electrical properties in Z direction, but they cannot determine
the X and Y components (two probes would be needed for that).
Putting it all together, a system which overcomes the limitations of both solutions would
be desirable. In this manuscript, we present a system where two sharp wires are used for the
measurement by controlling the tip-sample distance independently from the electrical measurements.
This is accomplished by mechanically coupling the wires to quartz tuning fork (QTF) sensors to
measure the shear force between the tip and the sample. The two sensors are constantly visualized by
an optical microscope and can be positioned relatively between them with submicron accuracy; in this
way, 3D electrical measurements can be made (in X, Y, and Z directions), together with differential
measurements, which ease the data analysis by compensating parasitic effects (mainly the parasitic
currents which appear due to the wires and electrical connections of the sample). Additionally, it opens
the door for further applications where the optical images can give important information [14]. The use
of wires instead of cantilevers also eases working in liquid media by isolating the wires with some of
the well-known techniques used in electrochemical scanning tunneling microscopy (EC-STM) [15,16].
2. Shear Force Control on Metallic Sharp Probes
In recent years, quartz tuning forks have been used as nanoscopy sensors due to their advantages
under certain conditions over standard microcantilever sensors [17,18]. Some of the disadvantages of
these QTF sensors are being overcome, such as the lack of a confident value for the spring constant [19]
and the difficulty in making the tip radius comparable in size with the ones in commercial probes [20];
therefore, in the present work, QTF sensors were used to control the tip-sample distance. In this
way, the shift in the frequency of oscillation of the QTF and the electric current flowing through the
electrode are measured independently so that the former can be used to maintain the tip-sample
distance constant and the latter to measure the impedance of the sample. The QTF is electrically
excited, instead of mechanically, which enables a better quantitative control of the response of the
system. The tip is not glued onto the QTF, but the two are in tight mechanical contact, which allows
the use of relatively long metallic tips.
QTF devices are mostly used in electronics for precise oscillation circuits. The resonance
frequency depends on the effective mass spring constant of the device [21]. We used commercial QTFs
encapsulated in vacuum conditions (with a resonance frequency of 32,768 Hz). To use these QTFs
as distance nanosensors, they must be removed from their metallic capsules, and the wire must be
placed in mechanical contact with one of the prongs of the resonator, thus producing a variation in
the resonant frequency of the QTF. The nanosensor is oscillated parallel to the sample surface. When
a shear force appears, there is a shift in the resonant frequency of the nano-tool. Control of this shift
in the resonance frequency allows us to keep the tip-sample force constant and, hence, the tip at
a very short distance from the surface of the sample. It should be noted, however, that metallic tips
are heavier than fiber tips (the most common kind of tip used in QTF sensors for scanning probe
microscopies). When a metallic tip is glued to a QTF prong, its weight produces resonating modes that
make it extremely difficult to use as a nanosensor. We have avoided these problems by fixing the tip to
the QTF prong without glue.
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3. Multiprobe Experimental Setup
3. Multiprobe Experimental Setup
In order to work cooperatively under the optical microscope, the tungsten wires needed to be
orderconsiderations
to work cooperatively
under
opticalonmicroscope,
wires
tothe
be
tilted.InSome
were taken
intothe
account
that aspect.the
Astungsten
can be seen
in needed
Figure 1,
tilted.
Some
considerations
were
taken
into
account
on
that
aspect.
As
can
be
seen
in
Figure
1,
the
wires needed to be tilted at a minimum β angle from the horizontal axis to avoid touching the surface
wires
needed
be tilted at a they
minimum
β angle
the horizontal
avoid
touching
the
surface
with the
cone.toFurthermore,
were tilted
at afrom
minimum
angle β’ axis
fromtothe
vertical
axis to
make
the
with
the
cone.
Furthermore,
they
were
tilted
at
a
minimum
angle
β’
from
the
vertical
axis
to
make
the
tip end visible through the upright optical microscope. If a is the cone angle of the tip, the minimum
tip
visible
upright optical
microscope.
If a ismetallic
the conetips
angle
of the tip,
thecommercial
minimum
tilt end
angle
is a, through
and thethe
maximum
tilt angle
is 90°-a. The
selected
were
˝
tilt
angle isSTM
a, and
the(TT-ECM10,
maximum tiltfrom
angleBruker
is 90 -a.
The probes),
metallic tips
selected
commercial
tungsten
tips
AFM
which
had were
a cone
angle of tungsten
32° ± 6°
˝ ˘ 6˝ (measured
STM
tips
(TT-ECM10,
from
Bruker
AFM
probes),
which
had
a
cone
angle
of
32
(measured from 10 different probes); therefore, the wires should be tilted an angle between 38° and
from
10 different
probes);
therefore, the wires should be tilted an angle between 38˝ and 52˝ from the
52° from
the horizontal
axis.
horizontal axis.
Figure 1. Schematic representation of the geometries of the two tips. The wires should be tilted at a
Figure 1. Schematic representation of the geometries of the two tips. The wires should be tilted at
minimum angle β from the horizontal axis to ensure that the tip end interacts with the sample.
a minimum angle β from the horizontal axis to ensure that the tip end interacts with the sample.
Additionally, the wires should be tilted a minimum angle β’ from the vertical axes to ensure the tip
Additionally, the wires should be tilted a minimum angle β’ from the vertical axes to ensure the tip
visibility from the optical microscope.
visibility from the optical microscope.
With the visibility from the top, it is possible to precisely position and coordinate the tips in X
With
the visibility
the top,
it is possible
to precisely
position and
coordinateand
the control
tips in
and Y directions
using from
the optical
microscope
information
throughout
the strategies
X
and Y directions
using
the optical
information
throughout
the and
strategies
andZ-axis
control
algorithms
previously
developed
in microscope
[22]. To control
the tip-sample
distance
thus the
of
algorithms
previously
developed
in
[22].
To
control
the
tip-sample
distance
and
thus
the
Z-axis
of
the measurement system, QTF sensors were used (AB38T model, Abracon Corp., Irvine, CA, USA).
the
measurement
QTF
sensors
used (AB38T
Corp.,
Irvine,
CA,
USA).
A common
way tosystem,
use these
sensors
forwere
tip-sample
distancemodel,
controlAbracon
is to glue
the tip
to them
with
an
A
common
way
to
use
these
sensors
for
tip-sample
distance
control
is
to
glue
the
tip
to
them
epoxy. However, in our application, larger wires than usual were needed, so the added mass towith
the
an
epoxy.
However,
in our
application,
larger wires
thandifficult
usual were
needed,
so the the
added
mass
to
QTF
resonator
is heavier
than
usual. Therefore,
it is very
to drive
electrically
sensor
with
the
is heavier
thanthe
usual.
Therefore,
it is which
very difficult
driveto
electrically
thewithout
sensor
thisQTF
hugeresonator
added mass
because
electrical
energy,
can betogiven
the device
with
this
huge
added
mass
because
the
electrical
energy,
which
can
be
given
to
the
device
without
damaging it, is very low. On the other hand, electrical excitation of the device was desirable because
damaging
it,the
is very
low.and
On the
other for
hand,
excitation
of measurements
the device was (when
desirable
it simplifies
design,
it allows
theelectrical
quantification
of the
thebecause
QTF is
it
simplifies
the design,
it allows
the quantification
of coupling
the measurements
(when
theand
QTFthe
is
driven
mechanically
by and
using
a ditherfor
piezo,
the mechanical
between the
dither
driven
mechanically
by
using
a
dither
piezo,
the
mechanical
coupling
between
the
dither
and
the
QTF
QTF should be known for quantification). Following the idea presented in [23] for using a glue-free
should
known
for quantification).
the idea
in [23]
for using
a glue-free
solution
solutionbefor
developing
the distanceFollowing
sensor based
on presented
optical fibers,
authors
presented
a glue-free
for
developing
the
distance
sensor
based
on
optical
fibers,
authors
presented
a
glue-free
solution
for
solution for metallic tips in a previous work [24]. Briefly, the metallic tip was placed in mechanical
metallic
tips
in
a
previous
work
[24].
Briefly,
the
metallic
tip
was
placed
in
mechanical
contact
with
contact with the QTF resonator by using a specifically designed holder with a 3D micropositioning
the
QTF In
resonator
by using
a specifically
designed
holderexperimentally
with a 3D micropositioning
In that
system.
that way,
the best
contact point
was found
by looking atsystem.
the frequency
way,
the
best
contact
point
was
found
experimentally
by
looking
at
the
frequency
response
and
then
response and then maintained by using a spring-based system. The designed head was adapted
so
maintained
by
using
a
spring-based
system.
The
designed
head
was
adapted
so
that
working
with
that working with tilted wires in multitip applications was possible, as shown in Figure 2.
tilted wires in multitip applications was possible, as shown in Figure 2.
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(a)
(b)
Figure 2. (a) 3D CAD design
the head. (b) Photograph of the designed head
the multitip system.
(a) of
(b) for
Figure 2. (a) 3D CAD design
of the head; (b) Photograph of the designed head
for the multitip system.
The wire and the QTF can be positioned independently and precisely by using micrometric screws.
The wire
and
the3DQTF
be positioned
preciselyhead
by using
screws.
Figure
2. (a)
CADcan
design
of the head. independently
(b) Photograph ofand
the designed
for themicrometric
multitip system.
Rubber strips are used to maintain the mechanical contact between the wire and the QTF without the
Rubber
strips
are
used
to
maintain
the
mechanical
contact
between
the
wire
and
the
QTF
without
The wire and the QTF can be positioned independently and precisely by using micrometric screws. the
need
glue.
need of
of any
any glue.
Rubber strips are used to maintain the mechanical contact between the wire and the QTF without the
need of any glue.
Heads with the integrated tungsten wires and QTF sensors were mounted into a micro- and
Heads with the integrated tungsten wires and QTF sensors were mounted into a micro- and
nanopositioning
system
under an tungsten
upright wires
optical
microscope
(Nikkon,
custominto
made
development
Heads with
the integrated
and
QTF sensors
were mounted
a microand
nanopositioning system under an upright optical microscope (Nikkon, custom made development
with nanopositioning
interchangeablesystem
objectives
5× to optical
50× magnification),
as shown
in made
Figure
3. Macro- and
underfrom
an upright
microscope (Nikkon,
custom
development
with interchangeable objectives from 5ˆ to 50ˆ magnification), as shown in Figure 3. Macro- and
with interchangeable
objectives fromwere
5× to accomplished
50× magnification),
shown in Figure
3. Macroand
micropositioning
of the end-effectors
withasmicrostepper
motors
(THORLABS
micropositioning
of the
end-effectors
were
accomplished
with
microstepper
motors
(THORLABS
MT3
micropositioning
of
the
end-effectors
were
accomplished
with
microstepper
motors
(THORLABS
MT3 and APT604 stages) with a resolution of 40 nm and a travel range of 12 mm. Nanopositioning
and APT604
stages)
with
a resolution
of 40 nm
and
travel
rangerange
of 12ofmm.
Nanopositioning
of the
MT3
andwas
APT604
stages)
withby
a resolution
of 40
nma and
a travel
12 mm.
of the
tips
accomplished
two piezoelectric
positioning
systems
(PI Nanopositioning
Nanocube) with a
tips was
accomplished
by two piezoelectric
positioning
systems (PI
Nanocube)
with a resolution
of the
tips was accomplished
by two piezoelectric
positioning
systems
(PI Nanocube)
with a of
resolution of 1 nm and a travel range of 100 μm. A digital controller (Dulcinea controller from
of range
1 nm and
a travel
of controller
100 μm. A(Dulcinea
digital controller
(Dulcinea
controller
from
1 nm resolution
and a travel
of 100
µm. Arange
digital
controller
from Nanotec
Electrónica)
Nanotec Electrónica) with 4 PI controllers, 2 lock-in amplifiers, and 16 analog I/0 channels was used
Nanotec
Electrónica)
with
4
PI
controllers,
2
lock-in
amplifiers,
and
16
analog
I/0
channels
was
used
with 4 PI controllers, 2 lock-in amplifiers, and 16 analog I/0 channels was used to drive the actuators,
to drive
the actuators,
measure
the
sensors,
and
execute
thecontrol
controlalgorithms.
algorithms.
to
drive
the
actuators,
measure
the
sensors,
and
execute
the
measure the sensors, and execute the control algorithms.
Figure
3. The
nanopositioning
stationwith
withthe
the end-effectors
end-effectors integrated.
Microstepper
motors
are used
Figure
3. The
nanopositioning
station
integrated.
Microstepper
motors
are used
for
coarse
positioning
and
piezoelectric
actuators
for
fine
positioning.
The
sample
and
the
endfor coarse positioning and piezoelectric actuators for fine positioning. The sample and the end-effectors
Figure
3. The are
nanopositioning
station
with
the microscope.
end-effectors integrated. Microstepper motors are used
effectors
byoptical
an
upright
optical
are visualized
byvisualized
an upright
microscope.
for coarse positioning and piezoelectric actuators for fine positioning. The sample and the endFor are
the visualized
measurement
of upright
the shearoptical
force in
the QTF sensors, a custom-made electronics [25] was
effectors
by an
microscope.
For
measurement
of the shear
forceamplifiers
in the QTF
sensors,
custom-made
electronics
[25] was
usedthe
together
with the integrated
lock-in
in the
digitalacontroller.
Briefly,
the electronics
usedFor
together
with
the
integrated
lock-in
amplifiers
in
the
digital
controller.
Briefly,
the
electronics
integrate
the parasitic capacitor
compensation,
which
allows
for control
of the amplitude
of vibration
the measurement
of the shear
force in the
QTF
sensors,
a custom-made
electronics
[25] was
integrate
thequality
parasitic
compensation,
whichFor
allows
for control
of the amplitude
vibration
and
the
of the sensors
independently.
the impedance
measurement,
a custom
I–V
used
together
with factor
thecapacitor
integrated
lock-in
amplifiers
in
the
digital
controller.
Briefly,
theofelectronics
and
the
quality
factor
of
the
sensors
independently.
For
the
impedance
measurement,
a
custom
I–V
converter
was
developed
based
on
the
AD549JH
amplifier
(Analog
Devices,
Norwood,
MA,
USA),
integrate the parasitic capacitor compensation, which allows for control of the amplitude of vibration
and a was
lock-in
amplifier (Anfatec
eLockIn204/2)
wasamplifier
used. A complete
scheme
of the
whole electronic
converter
developed
based
on
the
AD549JH
(Analog
Devices,
Norwood,
MA,
USA),
and the quality factor of the sensors independently. For the impedance measurement, a custom I–V
presented (Anfatec
in Figure 4.
and asystem
lock-inisamplifier
eLockIn204/2) was used. A complete scheme of the whole electronic
converter was developed based on the AD549JH amplifier (Analog Devices, Norwood, MA, USA),
system
is presented
in Figure
4. eLockIn204/2) was used. A complete scheme of the whole electronic
and
a lock-in
amplifier
(Anfatec
system is presented in Figure 4.
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2016, 16,
16, 757
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of 99
55 of
Figure 4.
and
control
system.
The The
shearshear
forceforce
between
the tips
and
theand
surface
Figure
4. Scheme
Schemeofofthe
theelectronics
electronics
and
control
system.
between
the
tips
the
are
measured
through
custom-designed
electronics,
and
the
digital
controller
uses
them
to
control
surface are measured through custom-designed electronics, and the digital controller uses themthe
to
tip-sample
distance (PIdistance
control)(PI
by control)
adjusting
Z-axis of
theZ-axis
piezoelectric
actuators. The
impedance
control
the tip-sample
bythe
adjusting
the
of the piezoelectric
actuators.
The
between thebetween
two tipsthe
is measured
by an I–V and
amplifier.
impedance
two tips isindependently
measured independently
by aanlock-in
I–V and
a lock-in amplifier.
4. Results
Resultsand
andDiscussion
Discussion
4.
All the
the experiments
experimentspresented
presentedininthis
thiswork
workwere
werecarried
carried
ambient
conditions,
All
outout
in in
ambient
conditions,
withwith
an
7
7
an excitation
frequency
5 kHz
and
using
a fixed
gain
forthe
theI–V
I–Vconverter.
converter.Tungsten
Tungstenwires
wires were
were
excitation
frequency
of 5ofkHz
and
using
a fixed
1010gain
for
tilted at
at approximately 45 degrees with respect to the X plane.
tilted
The
experimentsconsisted
consistedofof
measuring
impedance
between
the probes
two probes
they
The experiments
measuring
thethe
impedance
between
the two
whenwhen
they were
were placed
at nanometric
distances
sample
surface
(distanceswere
werethe
thesample
sample impedance
impedance
placed
at nanometric
distances
fromfrom
the the
sample
surface
(distances
dominates, as
as discussed
discussed in
in the
the next
next section).
section). Complex
Complex impedance
impedance has
has contributions
contributions from
from resistive,
resistive,
dominates,
capacitive, and inductive
inductive contributions.
contributions. For
For the
the calibration
calibration experiments,
experiments, purely
purely resistive
resistive or
or capacitive
capacitive
capacitive,
samples were used.
samples
4.1. Calibration
Calibration Experiments
Experiments
4.1.
The first
first experiment
experiment to
to test
test the
the ability
ability of
of the
the system
system to
to measure
measure the
the sample’s
sample’s impedance
impedance was
was to
to
The
determine
the
optimal
working
setpoint
(tip-sample
distance).
When
the
metallic
tip
approaches
the
determine the optimal working setpoint (tip-sample distance). When the metallic tip approaches the
sample, the
sample
(which
depends
on on
the the
tip-sample
distance
and
sample,
the impedance
impedancebetween
betweenthe
thetip
tipand
andthe
the
sample
(which
depends
tip-sample
distance
the dielectric
constant
of the medium
in between)
can be measured.
Thismeasured.
impedanceThis
has
and
the dielectric
constant
of the medium
in increases
between)and
increases
and can be
a
mainly
non-linear
capacitive
behavior
[26].
To
determine
the
adequate
tip-sample
distance
where
the
impedance has a mainly non-linear capacitive behavior [26]. To determine the adequate tip-sample
sample impedance
becameimpedance
dominant, became
a series of
I–V curves
were of
recorded
while
the recorded
tip was moved
distance
where the sample
dominant,
a series
I–V curves
were
while
toward
the
sample.
The
tip-sample
distance
was
determined
from
an
amplitude
vs.
Z-curve
made
the tip was moved toward the sample. The tip-sample distance was determined from an amplitude
prior
to
the
measurements
(the
tip
is
moved
toward
the
sample
surface
while
recording
the
changes
in
vs. Z-curve made prior to the measurements (the tip is moved toward the sample surface while
the amplitude
oscillation).
this case, the
sample was aIngold
to awas
resistor.
Figure
recording
the of
changes
in theInamplitude
of oscillation).
thistrace
case,connected
the sample
a gold
trace5
shows
how
the
non-linear
behavior
(due
to
the
tip-sample
capacitance)
was
progressively
reduced
by
connected to a resistor. Figure 5 shows how the non-linear behavior (due to the tip-sample
moving
the
tip
toward
the
sample
up
to
a
certain
distance
(150
nm),
where
only
the
linear
(resistive)
capacitance) was progressively reduced by moving the tip toward the sample up to a certain distance
behavior
the sample
was
observed.
Thisbehavior
distanceof
was
for the
measurements
in
(150
nm), of
where
only the
linear
(resistive)
theused
sample
wassubsequent
observed. This
distance was
this
work.
used for the subsequent measurements in this work.
Once the
the minimum
minimum tip-sample
tip-sample distance
distance was
was determined,
determined, aa series
series of
of purely
purely capacitive
capacitive samples
samples
Once
were
measured
(gold
traces
connected
to
discrete
capacitors
of
a
known
value).
The
tip-sample
distance
were measured (gold traces connected to discrete capacitors of a known value). The tip-sample
was
maintained
below
150
nm
so
that
the
sample
capacitance
(linear)
dominated
over
the
tip-sample
distance was maintained below 150 nm so that the sample capacitance (linear) dominated over the
capacitancecapacitance
(non-linear).
The amplitude
of the signal
at 5signal
kHz was
a sweep
´5 V and−55 V
V.
tip-sample
(non-linear).
The amplitude
of the
at 5 kHz
was between
a sweep between
Results
for
the
positive
sweep
(negative
amplitudes
produced
symmetrical
results)
are
shown
in
and 5 V. Results for the positive sweep (negative amplitudes produced symmetrical results) are
Figure 6.
shown
in Figure 6.
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of 99
Figure
Series
I–V
curves
the
sample
recorded
while
the
Figure
I–V
curves
overover
the resistor
sample
recorded
while changing
the tip-sample
distance.
Figure 5.5.
5.Series
Seriesofof
of
I–V
curves
over
the resistor
resistor
sample
recorded
while changing
changing
the tip-sample
tip-sample
distance.
The
due
to
capacitance
is
for
greater
The
non-linear
behaviorbehavior
due to the
capacitance
is observed
for distances
greater
than
distance.
The non-linear
non-linear
behavior
duetip-sample
to the
the tip-sample
tip-sample
capacitance
is observed
observed
for distances
distances
greater
than
150
nm.
150
nm.
than 150 nm.
18
18
Measured
Measured Value
Value
Error
Error
Linear
Linear Fit
Fit of
of Measured
Measured Value
Value
measured(pF)
(pF)
CCmeasured
Error(%)
(%)
Error
16
16
14
14
12
12
10
10
88
66
44
22
22
(a)
(a)
44
66
88
10
10
12
12
14
14
16
16
C
Cvalue
value (pF)
(pF)
(b)
(b)
Figure
6.
Capacitors
measurements.
kHz
over
known-value
capacitors
by
Figure 6.
6. Capacitors
Capacitors measurements.
measurements. (a)
(a) Curves
Curves obtained
obtained at
at 555 kHz
kHz over
over known-value
known-value capacitors
capacitors by
by
Figure
(a)
Curves
obtained
at
varying
the
voltage
amplitude
and
measuring
the
current.
(b)
Error
in
the
measurements.
The
I–V
varying
the
voltage
amplitude
and
measuring
the
current.
(b)
Error
in
the
measurements.
The
I–V
varying the voltage amplitude and measuring the current. (b) Error in the measurements. The I–V
curves
display
the
expected
linear
behavior.
The
values are
great
with
curvesdisplay
displaythe
the
expected
linear
behavior.
The measured
measured
are in
inaccordance
great accordance
accordance
with
curves
expected
linear
behavior.
The measured
valuesvalues
are in great
with nominal
2
nominal
values,
with
errors
lower
than
6%.
Linear
fit
equation
is
0.07
+
1.045x
with
adjusted
nominal
values,
errors
than fit
6%.
Linear is
fit0.07
equation
is with
0.07 adjusted
+ 1.045x R
with
adjusted R
R2
2 of 0.9997.
values,
with
errorswith
lower
than lower
6%. Linear
equation
+ 1.045x
of
0.9997.
of 0.9997.
Capacitors
were measured
precisely prior
to the
measurements with
with an impedance
analyzer,
Capacitors
Capacitors were
were measured
measured precisely
precisely prior
prior to
to the
the measurements
measurements with an
an impedance
impedance analyzer,
analyzer,
which
gave
the
values
used
as
“reference”
(2.2
pF,
3.6
pF,
5.3
pF,
6.8
pF,
8
pF,
10.5
pF,
12.3
pF, and
which
gave
the
values
used
as
“reference”
(2.2
pF,
3.6
pF,
5.3
pF,
6.8
pF,
8
pF,
10.5
pF,
12.3
which gave the values used as “reference” (2.2 pF, 3.6 pF, 5.3 pF, 6.8 pF, 8 pF, 10.5 pF, 12.3 pF,
pF, and
and
15.8
pF).
Thus,
results
obtained
with
the
developed
system
displayed
aa maximum
difference
of 6%
15.8
pF).
Thus,
results
obtained
with
the
developed
system
displayed
maximum
difference
15.8 pF). Thus, results obtained with the developed system displayed a maximum difference of
of 6%
6%
with
these previous
“reference” values,
so we
think that
the results
demonstrated that
the system
is
with
with these
these previous
previous “reference”
“reference” values,
values, so
so we
we think
think that
that the
the results
results demonstrated
demonstrated that
that the
the system
system is
is
capable
of
measuring
a
purely
capacitive
sample
with
ample
accuracy.
capable
of
measuring
a
purely
capacitive
sample
with
ample
accuracy.
capable of measuring a purely capacitive sample with ample accuracy.
4.2. Microparticle Impedance Measurement
4.2.
4.2. Microparticle
Microparticle Impedance
Impedance Measurement
Measurement
For the sample preparation, aluminum microparticles (of sizes ranging from 10 to 100 µm,
For
For the
the sample
sample preparation,
preparation, aluminum
aluminum microparticles
microparticles (of
(of sizes
sizes ranging
ranging from
from 10
10 to
to 100
100 μm,
μm,
purchased from Pomenton Inc., Maerne, Italy) where diluted in ethanol. A drop of the solution was
purchased
purchased from
from Pomenton
Pomenton Inc.,
Inc., Maerne,
Maerne, Italy)
Italy) where
where diluted
diluted in
in ethanol.
ethanol. A
A drop
drop of
of the
the solution
solution was
was
deposited onto freshly cleaved mica and allowed to evaporate naturally. Then, the two tungsten
deposited
onto
freshly
cleaved
mica
and
allowed
to
evaporate
naturally.
Then,
the
two
tungsten
deposited onto freshly cleaved mica and allowed to evaporate naturally. Then, the two tungsten
wires were place over the sample. The left tip was placed over a microparticle, and the right tip
wires
wires were
were place
place over
over the
the sample.
sample. The
The left
left tip
tip was
was placed
placed over
over aa microparticle,
microparticle, and
and the
the right
right tip
tip was
was
was then placed at three different positions: over the same microparticle (Figure 7a), over a different
then
placed
at
three
different
positions:
over
the
same
microparticle
(Figure
7a),
over
a
different
then placed at three different positions: over the same microparticle (Figure 7a), over a different
microparticle in contact with the first one (Figure 7b), and over a different microparticle that was not
microparticle
microparticle in
in contact
contact with
with the
the first
first one
one (Figure
(Figure 7b),
7b), and
and over
over aa different
different microparticle
microparticle that
that was
was not
not
in contact with the first one (Figure 7c).
in
in contact
contact with
with the
the first
first one
one (Figure
(Figure 7c).
7c).
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(a)
(a)
(b)
(b)
(c)
(c)
Figure 7. Microparticle measurement experiment as visualized with the optical microscope at
Figure
Microparticle measurement
experimentvisualized
as visualized the
with the microscope
optical microscope
at
Figure
7.7.Microparticle
experiment
at distances
distances
of 22 μm, 45 measurement
μm, and 56 μm
betweenas
the tips. (a) with
Both tipsoptical
over the same microparticle;
distances
of
22
μm,
45
μm,
and
56
μm
between
the
tips.
(a)
Both
tips
over
the
same
microparticle;
of
µm,
45 µm,
and 56 µminbetween
Both(c)
tips
over
themicroparticles
same microparticle;
(b)22
tips
over
microparticles
contact the
withtips.
each(a)
other;
tips
over
that are(b)
nottips
in
(b)
tips
over microparticles
inwith
contact
with
each
other;
(c)microparticles
tips over microparticles
that
are not
in
over
microparticles
in
contact
each
other;
(c)
tips
over
that
are
not
in
contact
with
contact with each other.
contact
with each other.
each other.
Current–voltage curves on the three measurements are shown in Figure 8. When both tips are
Current–voltage curves on the three measurements are shown in Figure 8. When both tips are
over Current–voltage
the same microparticle,
an the
impedance
of 30.4 MΩ is
can assume
is are
the
curves on
three measurements
aremeasured,
shown in which
Figure one
8. When
both tips
over the same microparticle, an impedance of 30.4 MΩ is measured, which one can assume is the
impedance
of microparticle,
a single microparticle.
When
two MΩ
microparticles
contact
over
the same
an impedance
of 30.4
is measured,inwhich
oneare
canmeasured,
assume is the
impedance of a single microparticle. When two microparticles in contact are measured, the
impedance of
measured
is higher (104.1
MΩ)two
than
it is for a single
microparticle;
impedance
is more
impedance
a single microparticle.
When
microparticles
in contact
are measured,
the impedance
impedance measured is higher (104.1 MΩ) than it is for a single microparticle; impedance is more
than
double
due
to
the
effects
of
the
pseudo-spherical
geometry
of
the
particles
and
the
non-ideal
measured is higher (104.1 MΩ) than it is for a single microparticle; impedance is more than double
than double due to the effects of the pseudo-spherical geometry of the particles and the non-ideal
ohmic
contact
between
them. Finally, when
the tipsofare
overand
two
different
particles
are
due
to the
effects
of the pseudo-spherical
geometry
theplaced
particles
the
non-ideal
ohmic that
contact
ohmic contact between them. Finally, when the tips are placed over two different particles that are
not in contact
eachwhen
other,
the
impedance
almost one
orderthat
of magnitude
(876.8 MΩ);
between
them.with
Finally,
the
tips
are placedincreases
over twotodifferent
particles
are not in contact
with
not in contact with each other, the impedance increases to almost one order of magnitude (876.8 MΩ);
thus,other,
the capacitor
formed
by the to
two
particles
and the
dielectric (876.8
(mica MΩ);
and air)
between
them
each
the impedance
increases
almost
one order
of magnitude
thus,
the capacitor
thus, the capacitor formed by the two particles and the dielectric (mica and air) between them
is measured.
formed
by the two particles and the dielectric (mica and air) between them is measured.
is measured.
Figure
Current–voltage curves
curves with
with one
one tip
tip over
over aa microparticle
and the
Figure 8.
8. Current–voltage
microparticle and
the second
second tip
tip at
at different
different
Figure
8. Current–voltage
curves
with increased
one tip over
a microparticle
and the the
second
tip
atasdifferent
distances.
As
expected,
the
impedance
with
the
distance
between
tips,
and
distances. As expected, the impedance increased with the distance between the tips, and as the
the tips
tips
distances.
As expected,
the impedance
increased
with
the
distance
between
the
tips,with
and each
as theother,
tips
contacted
the
same
microparticle
(a
distance
of
22
µm),
two
microparticles
in
contact
contacted the same microparticle (a distance of 22 μm), two microparticles in contact with each other,
contacted
the 45
same microparticle
(a distance of 22inμm),
two microparticles
in contact
withof
each other,
(a
(a distance
distance of
of 45 µm)
μm) and
and two
two microparticles
microparticles not
not incontact
contactwith
witheach
eachother
other(a
(adistance
distance56
56 ofµm).
μm).
(a distance of 45 μm) and two microparticles not in contact with each other (a distance 56 of μm).
5. Conclusions
Conclusions
5. Conclusions
Here we report the
the architecture,
architecture, design,
design, and
and validation
validation of aa novel,
novel, customized
customized multitip
multitip system
system
Here we report the architecture, design, and validation of a novel, customized multitip system
based on a QTF with a glue-free metallic tip working in shear mode for impedance measurements.
measurements.
based on a QTF with a glue-free metallic tip working in shear mode for impedance measurements.
Both tips
as as
force
andand
current
nanosensors.
The considerations
on theon
wire
tipswork
worksimultaneously
simultaneously
force
current
nanosensors.
The considerations
thetilting
wire
Both tips work simultaneously as force and current nanosensors. The considerations on the wire
and
tip-sample
distance have
beenhave
presented,
well as calibration
experiments
purely resistive
tilting
and tip-sample
distance
been as
presented,
as well as
calibrationover
experiments
over
tilting and tip-sample distance have been presented, as well as calibration experiments over
and
capacitive
Finally, samples.
an experiment
with
microparticles
has microparticles
been presented.has
purely
resistivesamples.
and capacitive
Finally,
analuminum
experiment
with aluminum
purely resistive and capacitive samples. Finally, an experiment with aluminum microparticles has
system presented here overcomes most of the limitations of the existing solutions offered in
beenThe
presented.
been presented.
the literature.
The
tip-sample
distance
can be
controlled
independently
from thesolutions
measured
current,
The system
presented
here
overcomes
most
of the limitations
of the existing
offered
in
The system presented here overcomes most of the limitations of the existing solutions offered in
and
differential
measurements
can be taken
to the two-sensors
configuration.
Additionally,
the
the literature.
The
tip-sample distance
can bedue
controlled
independently
from the measured
current,
the literature. The tip-sample distance can be controlled independently from the measured current,
use
wires instead
of cantilevers,
which
are
easy
to electrically
isolate,
opens theAdditionally,
door for future
and of
differential
measurements
can be
taken
due
to the
two-sensors
configuration.
the
and differential measurements can be taken due to the two-sensors configuration. Additionally, the
applications
in
liquid
media.
use of wires instead of cantilevers, which are easy to electrically isolate, opens the door for future
use of wires instead of cantilevers, which are easy to electrically isolate, opens the door for future
applications in liquid media.
applications in liquid media.
Sensors 2016, 16, 757
8 of 9
Acknowledgments: Authors want to acknowledge Dra. Elena Xuriguera for assistance in the preparation of the
sample of microparticles and Ángel María Benito for assistance in the holder design. This work was supported in
part by the Spanish Ministerio de Economía y Competitividad through projects TEC2009-10114 and TEC2010-16844.
Author Contributions: J.O. and M.P. conceived the idea and designed the experiments presented in this
manuscript. L.B. and X.C. developed the hardware and software parts of the system. L.B. performed the
experiments. J.O. and L.B. wrote the paper. J.S. provided advice on the revisions of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).