AFM Bias Induced Electrochemistry:
Redox Processes at the Solid Liquid Interface
by
Jim Mara B.Eng, CA, CTA, PDA
The thesis is submitted to University College Dublin
in part fulfilment of the requirements for the degree of
Master’s (MSc) NanoBio Science
School of Physics
UCD Conway Institute of
Biomolecular and Biomedical Research
Head of School: Prof. Padraig Dunne
Principal Supervisor: Dr. Brian Rodriguez
ii
Table of Contents 1 Abstract ......................................................................... 3
2 Abbreviations ................................................................ 4
3 Acknowledgment ........................................................... 5
4 Introduction .................................................................. 6
4.1 Atomic force microscope ............................................................................ 8
5 Materials...................................................................... 15
5.1 Highly Oriented Pyrolytic Graphite (HOPG) ............................................17
5.2 Silver Nitrate ............................................................................................. 23
6 Method ........................................................................ 26
7 Results ......................................................................... 28
8 Discussion ................................................................... 45
9 Conclusions ................................................................. 54
10 Future Work ................................................................ 57
11 References ................................................................... 59
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1 Abstract
Our experiments are aimed at investigating the nano-redox processes that occur in an
electrolyte between a negatively biased AFM tip and underlying substrate. We explore
the possibility of modifying substrates in a 0.01M AgNO3 electrolyte solution using an
Atomic Force Microscope (AFM) in contact mode. The voltage potential difference
between the AFM tip and substrate was pulsed in order to initiate redox processes at
the tip. All our work was geared towards gaining a better understanding and control
of nanofabrication methods in order to construct e.g. nano scale interconnects. The
motivation for conducting this work lies is in the field of nanowire based biosensors-
detection and quantification of biological and chemical species are critical to many
areas of health care and the life sciences
The approach we used to fabricate the nanostructures was contact based AFM voltage
induced electrochemical reduction of aqueous silver cations (‘Ag+’), accompanied by
sometime oxidative pit formation on primarily Highly Ordered Pyrolytic Graphite
(HOPG) substrates. We did this in a meniscus of aqueous silver nitrate. All substrates
were imaged using amplitude modulation-AFM (AM-AFM) pre and post bias in order
to confirm the presence of any nano-redox reaction. All biases were applied in AFM
contact mode.
We varied the duration and magnitude of the bias in order to quantify their effect on
the nano-redox process. We suggest reaction mechanisms for the nano-redox
processes studied, including the pit formation, and nano/micro deposits formed.
We conduct the majority of our experiments on HOPG, and study the effect of the
following experimental parameters: voltages between -5.0V to -3.75V; voltage pulse
durations between 2.5 to 30 seconds; AFM metal coated tips versus pure Si AFM tips,
and AFM bias contact forces between 4.5nN and 9.1nN. We see resulting Ag
deposition volumes of between 2,833.3 nm3 to 1,475,104.0 nm3. Our experiments also
produce sometime pit formation on the substrates of between 803.9 nm3 and
283,636.6 nm3 volume. Our experiments on gold substrates show similar trends but
at lower voltages and pulse durations.
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2 Abbreviations
Atomic Force Microscope (AFM)
Highly Ordered Pyrolytic Graphite (HOPG)
Nanowire (NW)
Scanning probe microscopy (SPM)
Amplitude Modulation-AFM (AM-AFM)
Noncontact Atomic Force Microscopy (NC-AFM)
Scanning Tunnelling Microscopy (STM)
Nano Electrochemical Lithography (EL)
Dip-pen nanolithography (DPN)
Full Width at Half Maximum (FWHM)
Overpotential Deposition (OPD)
Scanning probe microscopy (SPM)
Local anodic oxidation (LAO)
Electron beam lithography (EBL)
Nanoimprint lithography (NIL).
Electrochemical force microscopy (EcFM)
Electrochemical “dip-pen” nanolithography (E-DPN).
Transmission electron microscopy (TEM)
Scanning Electron Microscopy (SEM)
Chemical Vapour Deposition (CVD)
Silver Nitrate (AgNO3),
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3 Acknowledgment
I would like to express my gratitude to Dr. Brian Rodriguez for his patience and
insightful help over the course of this work.
I would like, in general, to thank all members of the UCD Nanoscale Function Group
who were always very generous with their time and knowledge, with special mention
for Craig Carville, Liam Collins and Bart Lukasz.
Also, my family and friends, especially Mam and Da, who always supported me even
when I least deserved it.
Finally, a word of mention for the unknown passer-by who smiled at me and looked
away at just the right moment.
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4 Introduction
The objective of our experiment was to explore the possibility of modifying surfaces
in an electrolyte solution using an Atomic Force Microscope (AFM) in contact mode.
The voltage potential difference between the AFM tip and substrate was pulsed in
order to initiate redox processes at the tip. All our work was geared towards gaining a
better understanding and control of, nanofabrication methods in order to construct
e.g. nano scale interconnects. Furthermore our aim was to fabricate the
nanostructures on a number of substrates namely: Highly Ordered Pyrolytic Graphite
(HOPG) and Gold.
The motivation for conducting this work lies is in the field of nanowire based
biosensors- detection and quantification of biological and chemical species are
critical to many areas of health care and the life sciences, from diagnosing disease to
the discovery and screening of new drug molecules. Central to detection is the
transduction of a signal associated with the selective recognition of a species of
interest.1 Nanostructures, such as nanowires and nanocrystals offer new and
sometimes unique opportunities to develop novel sensors. The diameters of these
nanostructures are comparable to those of the biological and chemical species being
sensed. Therefore, they represent excellent primary transducers for producing signals
that ultimately interface to macroscopic instruments. In particular, inorganic
nanowires (NWs) and nanocrystals exhibit highly reproducible electrical and optical
properties. The size-tuneable colours of semiconductor nanocrystals together with
their highly robust emission properties offer advantages over conventional organic
molecular dyes for labelling and optical-based detection of biological species. The
combination of tuneable conducting properties of semiconducting NWs and the
ability to bind analytes on their surface yields direct, label-free electrical readout,
which is exceptionally attractive for many applications.1
Figure 4.a (below) shows schematically the parts comprising a typical biosensor:
a) bio-receptors that specifically bind to the analyte; b) an interface architecture
where a specific biological event takes place and gives rise to a signal picked up by c)
the transducer element.
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The transducer signal (which could be anything from the in-coupling angle of a laser
beam to the current produced at an electrode) is converted to an electronic signal and
amplified by a detector circuit using the appropriate reference and sent for processing
by, e.g., d) computer software to be converted to a meaningful physical parameter
describing the process being investigated; finally, the resulting quantity has to be
presented through e) an interface to the human operator. Biosensors can be applied
to a large variety of samples including body fluids, food samples, cell cultures and can
be used to analyze environmental samples.2
Figure 4.a
Elements and selected components of a typical biosensor2
The approach we used to fabricate the nanostructures was AFM based voltage
induced electrochemical reduction of aqueous silver cations (‘Ag+’). We did this in a
meniscus of aqueous silver nitrate (AgNO3), on various substrates (see above) to form
solid silver nano structures on said substrate.
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4.1 Atomic force microscope
An AFM was originally a tool for imaging/studying samples: An AFM senses inter-
atomic forces occurring between a sharp probe tip and a sample surface to produce
images of sample surfaces such as ceramic materials, biological membranes, metals,
polymers, and semiconductors with subnanometer resolution. The images produced
are 3-D with resolution on the order of 0.1 to 1 nm. The AFM uses a microcantilever,
with a sharp probe tip on its lower surface, which is scanned over a sample surface.
Deflection of the cantilever, due to interatomic forces between the probe tip and the
sample, at each scan point is representative of the sample height. By plotting the
sample height versus the horizontal position of the probe, a 3-D image of the surface
can be obtained. The high image resolution of the AFM is due to the size of the probe
tip, which may be only a few atoms wide. This gives the AFM an advantage over
optical microscopes, which are limited by the wavelength of visible light, which is
approximately 400–700 nm.
Most commonly, the probe tip is dragged across the sample at a constant force, which
is referred to as contact mode imaging. Continuous lateral force on the sample from
the probe tip may cause damage to softer fragile samples. Tapping mode (amplitude
modulation) was developed to reduce lateral forces on such samples. A schematic
showing the typical instrumentation of an AFM is shown in Figure 4.b (below)3
For softer less stable samples, it is therefore very challenging to study their nano-
mechanical properties by applying tip forces directly to them.
For that reason, we used amplitude modulation-AFM (AM-AFM) for imaging the
substrates, which minimizes the lateral forces between the tip and the sample. Jim V.
Zoval et al17, note that for optimum imaging quality ‘noncontact atomic force
microscopy (NC-AFM) was employed in an attempt to overcome the noise problems
inherent to the Scanning Tunnelling Microscopy (STM) and repulsive mode AFM
imaging modes.’17
Previous work by other researchers has established that NC-AFM is superior to STM
or repulsive mode AFM for the investigation of many compliant surfaces and for
observing weakly adsorbed or laterally mobile molecular species.4
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Figure 4.b
Schematic of the instrumentation of an AFM.3
In AM-AFM the cantilever is oscillated at a drive frequency which is slightly lower
than the resonant frequency. As the lever approaches the sample, tip interactions
(either contact or non-contact) cause a change in the amplitude of oscillation. These
oscillation changes yield information about the sample topography, and the phase of
the oscillation gives information on the material. The attractive force between the
cantilever and the sample will cause higher amplitude, and the repulsive force will
cause lower amplitude. This mode is suitable for studying any silver nano-structure
we form as it results in a much lower lateral force on the sample than contact mode,
therefore less damage is done to the nanostructure.5
NC-AFM was employed in an attempt to overcome the noise problems inherent to the
STM and repulsive mode AFM imaging modes. Like the conventional repulsive mode
AFM, the operational principle of the NC-AFM (also called the “dynamic”, “attractive
mode”, and “ac” atomic force microscope) involves the detection and maintenance of
a small force which is exerted locally by the probe tip on the sample surface. In
contrast to the repulsive mode AFM experiment, however, the NC-AFM probe tip is
located in the attractive region of the tip-sample interaction potential and at greater
distances from the surface of 10-15 Å.
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The attractive tip-sample forces present in this separation regime are smaller than
those involved in repulsive mode AFM imaging (10-11-10-15 N vs 10-9 N), with the result
that the NC-AFM tip is a less perturbative probe of the surface topography. The
attractive force imparted by the tip to the sample is detected optically as a deflection
of the AFM cantilever toward the sample surface.17 Prior work has established that
NCAFM is superior to STM or repulsive mode AFM for the investigation of many
compliant surfaces6 and for observing weakly adsorbed or laterally mobile molecular
species.7
It was further recognized that scanning probe microscopy (SPM) allows not only
imaging, but also manipulation of matter. The tip-surface force interactions enabled
approaches such as molecular and atomic manipulation and nano-scratching, while
voltage control enabled polarization switching in ferroelectric materials and
electrochemical reaction based nano-oxidation. In recent years, much attention is
focused toward probing the local mechanism(s) of tip-induced processes. In these
processes, tip force- and bias-induced physical and chemical changes occur in
materials, and the simultaneously measured functional response provides
information on the thermodynamics and kinetics of the associated processes.8
Nano electrochemical lithography (EL) methods are based on the building of an
electrochemical cell, which, in the simplest configuration, consist of two conductive
surfaces (electrodes) separated by either a liquid or solid conductive phase
(electrolyte). By applying an appropriate bias, an electrochemical reaction, namely a
charge transfer process localized at the electrode/electrolyte interface, occurs. In
electrochemical lithography, the electrodes are, respectively, the substrate and a
conductive structure that could be the tip of a scanning probe microscope or the
protrusion of a stamp. For our work we used the conductive tip of the AFM. In
principle, electrochemical lithography offers a broad spectrum of possible
applications such as the local control of the reactions (electrochemical oxidation or
electrochemical reduction), to change the chemical nature of materials in confined
and specific places, local deposition of materials, and the fabrication of chemical
patterns. Moreover, unlike all of the other techniques for nanofabrication, in EL the
substrate can be directly exploited as the reactive layer.
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This capability allows us to overcome the concepts of bottom-up and top-down
nanofabrication and represents an example of the so-called “third way” for
nanofabrication.9
Yan Li et al reported in 2001 on a method focusing on electrochemical AFM “Dip-
Pen” Nanolithography. The “dip-pen” nanolithography (DPN) method uses an atomic
force microscope tip as a “nib” to directly deliver organic molecules onto suitable
substrate surfaces, such as Au.10
When an AFM is used in air to image a surface, the narrow gap between the tip and
surface behaves as a tiny capillary that condenses water from the air. “Dip-pen” AFM
lithography uses the water meniscus to transport organic molecules from tip to
surface. However, unlike in the previous AFM “dip-pen” methods where water is only
used as a solvent for the molecules, the researchers used this tiny water meniscus as a
nanometer-sized electrochemical cell in which metal salts could be dissolved, reduced
into metals electrochemically, and deposited on the surface, see Figure 4.c (below).
The DC voltage needed for metal deposition depends on the type of precursor salt and
the resistivity between the AFM tip and the surface.11
Figure 4.c
Schematic sketch of the E-DPN experimental setup.11
The above researchers, amongst others, endeavours and experimental techniques
informed our approach to fabricating nanostructures using an AFM on various
substrates. However there are a couple of peculiarities to our approach; unlike Yan
Li’s we are not using the AFM tip to deliver the silver cations into the electrochemical
cell: we are forming a meniscus of silver cations on the substrate, and in this way
delivering the reagents to the substrate and AFM tip site.
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Furthermore the general approach amongst researchers to date is to apply positive
voltages to the tip with respect to the substrate. Our approach is to apply a negative
bias to the tip and thus deliver the electrons via the tip. No matter the set up we are
still expecting nano redox reactions in the vicinity of the tip: between the biased tip
and the electrolyte and between the biased tip and substrate.
Nanometer-scale metal particles possess chemical and physical properties which
differ significantly from macroscopic metal phases. In recent years, the list of particle
size dependent properties has grown to include bond distances, the van der Waals
attractive force operating between particles, the surface plasmon resonance, the
melting point, the standard electrode potential, and the photoelectric yield. One or
more of these properties becomes size-dependent for metal particles having
dimensions below a critical threshold which is in the range from 2 to 10 nm,
depending on the particular property and metal considered.17
Janousek et al researched AFM local anodic oxidation on graphene12. Graphene is an
extensively studied material due to its remarkable electronic and mechanical
properties. Local anodic oxidation (LAO) by means of AFM with a conductive tip is
the method used by the researchers for device patterning in the nanometer scale at
laboratory conditions. It represents a clean alternative to up-to-date nano-
lithographic techniques such as electron beam lithography (EBL) and nanoimprint
lithography (NIL). Oxidation reaction between the tip and a conductive substrate
creates an oxide line, which acts as a potential barrier. This enabled the researchers
to make narrow constrictions of 20 nm in width.
They did this by applying LAO on single-layer graphene chemical vapour deposition
(CVD) grown on copper foil and graphene grown on SiC prepared by high
temperature annealing. Under optimum conditions for LAO the graphene oxide is
created. The thickness of the oxide line depends on various parameters e.g. a tip bias,
speed of the tip and ambient relative humidity. By applying negative bias to the
conductive AFM tip with respect to the conductive substrate, dissociated OH− ions
oxidize the substrate and thus create a pattern. The quality of oxide lines depends on
parameters as bias voltage applied to the tip, velocity of the tip, relative humidity and
set-point.
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The main difference between this work and the work presented in this thesis is that
we are utilizing a reduction reaction at the tip substrate interface as follows:
Ag+ + e- = Ag reduction of Ag+ and nucleation at the tip/substrate interface.
Janousek et al local anodic oxidation reaction on graphene12:
H2O + e- = OH- +H
OH- + Cx = CxO + H- oxidation of the Graphene substrate
The researchers analyzed the properties of LAO oxide lines for different negative
voltages applied on the tip from -3.5 V to -7 V with respect to the sample (Figure 4.d
below). The visible oxidation started at the bias -4 V. Note the authors did not
provide details of the AFM tip they were using which is critical to predicting the
electric field between the tip and substrate.
Figure 4.d
Influence of tip bias from -3.5 to -7.0 V. For bias voltage -3.5 V, the oxidation process
is not observable. Height profiles in the two sections are at the bottom. Tip velocity was
100 nm/s. (B) Oxide lines for different tip velocity from 25 to 2000 nm/s. Height
profiles in the two sections are at the bottom. Tip bias was -6 V12
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Further research in this area was conducted by Sergei Kalinin et al and in their 2014
paper13, they report on bias dependent mechanisms of irreversible cathodic and
anodic processes on a pure CeO2 film studied using a modified AFM. For a moderate
positive bias applied to the AFM tip an irreversible electrochemical reduction
reaction is found, associated with significant local volume expansion. Simultaneous
detection of tip height and current allows the onset of conductivity and the
electrochemical charge transfer process to be separated, further elucidating the
reaction mechanism. The researchers’ observations suggest that the desired
electrochemical reaction proceeds only after the surface activation energy has been
surpassed. For a tip positive bias the summarized observation are:
(a) At sufficiently low biases, the surface is reduced
(b) The onset of the electronic conduction reverses the electrochemical process
direction
(c) The process is strongly mediated by the water layer on the surface.
Further the researchers state that the detected current represents a sum of two
contributions, namely Faradaic current (electrical current mediated by ions as
opposed to electrical current purely mediated by electron movement) of
electrochemical reaction (changes in material below the tip) and, conductive
electronic and ionic currents. Here, the conductive electronic current is classical
electronic transport from the bottom electrode to the tip through the material. The
ionic current is associated with the electrochemical process on the bottom electrode,
ionic transport through the material, and the electrochemical process on the top
electrode without affecting the material.13
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5 Materials
Cantilevers:
HQ:DPE-XSC11 tip B14
Conductive Pt-coated tip (HQ:DPE-XSC11 tip B), manufacturer Nanoandmore.
The coating thickness is increased on this cantilever, which gives more freedom for
using it in contact electrical modes. The conducting Pt coating covers the entire
Silicon chip, cantilevers and tips. It provides high conductivity and enhances the laser
reflectivity.
Manufacturers’ Specifications:
Tip Shape Tip Height Tip Radius Full Cone Angle
Rotated (12-18 µm) <40 nm 40°
Cantilever
Shape Length Width Thickness Force Const. Res. Freq.
Beam 210 µm 30 µm 2.7 µm 1.1 - 5.6 N/m 60 - 100 kHz
Calibrated Values: 447.60 pN/nm
The tip used (tip B) was suitable for imaging the deposition because it is a relatively
soft cantilever and very responsive to forces encountered- as a result the agitation of
any Ag nucleation is minimized under the cantilever pressure. Also, the tips used had
a high resonant frequency which is needed for taping mode AFM.
Figure 5.a
SEM image of the DPE silicon tip14
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Si tip NanoWorld SSS-NCH AUD15
We also conducted testing with a used Si doped tip with no reflex coating
(NanoWorld SSS-NCH AUD), Veeco model.
Manufacturers’ Specifications:
Shape:
Polygon based pyramid
Tip Height Tip Radius Full Cone Angle
10 - 15 µm 2 nm 20°
Length Width Thickness Force Const. Res. Freq.
120 – 130µm 25 – 35µm 3.5 - 4.5µm 21 – 78N/m 250 – 390kHz
NanoWorld Pointprobe® NCH probes are designed for non-contact or tapping mode
imaging. This probe type combines high operation stability with outstanding
sensitivity and fast scanning ability.
The probe is made from monolithic silicon which is highly doped to dissipate static
charge. They are chemically inert and offer a high mechanical Q-factor for high
sensitivity.
The purpose of using Si probes was so that we could juxtapose results obtained from
conductive metal coated tips with those results obtained from conductive non-metal
coated tips. This was in order to determine if the tips metal coating dissolved under
the applied bias and was thus the source of our ‘deposition’, see Discussion 8 (below)
for further discussion on this point. An indication of tip deterioration during bias
application is a reduction in the intensity of the reflected laser into the photodiode (a
reduced SUM, or changes in the resonance frequency or tip deflection figure).
AFM used: MFP3D (Asylum Research)
Bias pulses were generated using a custom code developed in Igor Pro
(Wavetrics) and was used to control the bias output of the AFM controller.
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Substrates:
HOPG:
Windsor Scientific HOPG/ZYH/DS/2-1, 10mm x 10mm
Agar Scientific, HOPG 3.5 +/- 1.5 Mosaic Spread 10mm x 10mm x 2mm
Gold Substrate, Arrandee 11x11mm metal coated substrates
BioForce NanoSciences UV/Ozone ProCleaner used for tip and Gold
irradiation.
Silver nitrate solution volumetric, 0.01M AgNO3 (0.01N), Sigma-Aldrich
manufacturers.
Oscilloscope- Hamey Instruments, 50mHz, Analogue oscilloscope, HM 504-2
Lens Paper- Fischerbrand lens cleaning tissue 80 x 100mm, Fischer Scientific
Wicking Paper- Filter paper ‘Whatman’ Circular (90mm)
5.1 Highly Oriented Pyrolytic Graphite (HOPG)
We conducted the vast majority (81%) of our AFM electrochemical deposition
experiments on HOPG. HOPG is an allotrope of carbon and part of the graphite
family.
"Usual" graphite, especially natural one, exhibits quite imperfect structure due to an
abundance of defects and inclusions. A number of technologies have developed for
the preparation of perfect graphite samples to take advantage of its unique structure.
Of these, pyrolysis of organic compounds is the most common and effective. Pyrolytic
graphite is a graphite material with a high degree of preferred crystallographic
orientation of the c-axes perpendicular to the surface of the substrate.
It is manufactured by graphitization heat treatment of pyrolytic carbon or by
chemical vapour deposition at temperatures above 2500K.16
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Hot working of pyrolytic graphite by annealing under compressive stress at
approximately 3300K results in HOPG. Thus HOPG is a highly-ordered form of high-
purity pyrolytic graphite (impurity level is of the order of 10 ppm ash or better).16
HOPG is characterized by the highest degree of three-dimensional ordering. The
density, parameters of the crystal lattice, preferable orientation in a plane (0001) and
anisotropy of the physical properties of the HOPG are close to those for natural
graphite mineral. In particular, like mica, HOPG belongs to lamellar materials
because its crystal structure is characterized by an arrangement of carbon atoms in
stacked parallel layers – this two-dimensional and single-atom thick form of carbon
is called graphene. Graphite structure can be described as an alternate succession of
these identical staked planes. Carbon atoms within a single plane interact far more
strongly than with those from adjacent planes- this explains characteristic cleaving
behaviour of graphite.16
Furthermore, graphene is a planar, hexagonal arrangement of carbon atoms. The
lattice of graphene consists of two equivalent interpenetrating triangular carbon sub-
lattices A and B, see Figure 5.b (below). Each one contains one half of the carbon
atoms. Each atom within a single plane has three nearest neighbours: the sites of one
sub-lattice (A – marked by red) are at the centres of triangles defined by three nearest
neighbours of the other one (B – marked by blue).16
The lattice of graphene thus has two carbon atoms, designated A and B, per unit cell,
and is invariant under 120° rotation around any lattice site. The network of carbon
atoms is connected by the shortest bonds in a honeycomb like shape. However in
bulk HOPG, even in bi-layer graphene, A- and B-sites carbon atoms become in-
equivalent (including those on the surface): two coupled hexagonal lattices on the
neighbour graphene sheets are arranged according to Bernal ABAB stacking, where
every A-type atom in the upper (surface) layer is located directly above an A-type
atom in the adjacent lower layer, whereas B-type atoms do not lie directly below or
above an atom in the other layer, but sit over a void – a centre of a hexagon. Figure
5.b illustrates the assumed non-equivalent types of carbon atoms.16
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Thus in each layer the atoms form a grid of correct hexagons with distances between
atoms equal 0.1415 nm. The distance between layers is equal 0.3354 nm which results
in a theoretical calculated value of density ρ = 2.265 g/cm3.16
HOPG terminated with graphene layer is an excellent tool for using in scanning probe
microscopy as a substrate- this is an easily renewable material with an extremely
smooth surface. This is vital for any SPM measurements that require uniform, flat,
and clean substrates, for samples where elemental analysis is to be done.
Figure 5.b
Schematic representation of the structure of the bulk hexagonal graphite crystal. The
dashed lines show the axes of bulk unit cell. Side insets: top view of the basal plane
of graphite and schematic representation of the surface structure (carbon atoms) of
graphite most viewed by SPM, where every other atom is enhanced (right-side inset)
and viewed under ideal conditions, where every single atom is seen (left-side inset).16
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Similar to mica, HOPG specimens are layered polycrystals. Each bulk polycrystal
looks like mosaic of microscopic mono-crystal grains of different sizes. The structure
is columnar, the columns run vertically within the flat slab of the material, and the
grain boundaries can be seen on the lateral surfaces. The grains are slightly
disoriented with respect to each other. An angular spread of the c-axes of the
crystallites is of the order of 1 degree. The surface of specimen consists of many
randomly placed steps – result of the cleaving process: single atomic steps and steps
of several or dozens of atomic layers.16
Although the heights of multilayer hills and valleys are not calibrated, single steps
have the well defined height of 0.34 nm. To characterize the angle of deviation of the
grain's boundaries from the perpendicular axis of the columnar structure, a measure
of the parallelism of grains – perfectness of HOPG samples, a "mosaic spread" term is
used. The lower the mosaic spread, the more highly ordered the HOPG. The term
originates from X-ray crystallography.16
The disordering results in broadening of the (002) diffraction peak: the more
disordering, the wider the peak. Therefore, ‘perfectness’ of HOPG can be easily
related to a Full Width at Half Maximum (FWHM) of the Cu-Ka rocking curve
(radiation peak) measured in degrees – "mosaic spread angle". Thus, the smaller this
angle, the higher the quality of HOPG. The size of grains also varies with the mosaic
spread. The lower mosaic spread results in a freshly cleaved surface that exhibits the
smaller number of the steps due to the bigger size of grains. The higher the quality is-
the less the roughness of the surface. The lower level grade material is also more
"cleavable" allowing the bigger number of cleavings per sample.
All the other physical characteristics of graphite, including atom-to-atom distance,
that is an atomic property of carbon, are independent of its grade and remain the
same for all types of HOPG. Due to the anisotropic nature of HOPG such
characteristics as thermal conductivity and electrical resistivity are different in
different directions: along the basal plane and along the principal axis c
(perpendicular to the basal plane).16
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HOPG is a highly stable material. It remains stable at the temperatures up to 500°C
in air and up to two-three thousand degrees Celsius in a vacuum or inert
environment. It exhibits high chemical inertness to just about everything.16
Zoval, Jim V., et al also report that the graphite basal plane surface is
electrochemically ‘very inert’17. Furthermore the same group go on to outline how
silver nanocrystallites interact weakly with the graphite surface and are removed by
the sweeping action of the AFM probe tip from the imaging area.17 This effect has
been previously documented for gold particles on graphite by Schaefer et al.18
Silver micro and nanocrystallites which nucleate at defect sites are observed by STM
and AFM, and, it has been concluded that silver overpotential Deposition (OPD) on
graphite is initiated by nucleation exclusively at defects, such as step edges, on the
graphite surface. Further they report that the NC-AFM data presented for low-defect
density surfaces such as the graphite basal plane, STM and repulsive-mode AFM data
can provide a misleading view of nucleation by “ignoring” the presence of weakly
adsorbed metal nanocrystallites which are not associated with defects.17
It is worth bearing in mind that these researchers experiments were conducted within
the following parameters: voltage pulses having amplitudes of 100, 250, and 500 mV
vs. Ag0 and durations of 10 or 50 ms were applied to graphite surfaces immersed in
dilute (≈1.0 mM) aqueous silver nitrate. Also, the potentiostatic deposition of silver
was accomplished by using a silver wire reference electrode immersed directly in the
silver plating solution. In contrast our experiments were conducted in a meniscus of
AgNO3 using conductive AFM tips at higher voltage and longer durations (see Results
7 below for details).17
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Figure 5.c (below) demonstrates Zoval, Jim V., et al’s experimental set up.
Figure 5.c
Schematic diagram of the instrument employed by Zoval, Jim V., et al for pulsed
potentiostatic deposition of silver nanocrystallites.17
Zoval, Jim V., et al go on to note that the capacitance of a graphite basal plane surface
was 1.70 µF cm-2, which is in the normal range. Further in successive silver deposition
trials in which the graphite surface was cleaved prior to each experiment, the
apparent capacitance of the surface fluctuated by 10-20%, presumably due to
fluctuations in the defectiveness of the graphite surface which is exposed during
cleavage.17 This observation can have important implications for the consistency of
results in our experiments.
Further they highlight ‘isolated silver nuclei were never observed on atomically
smooth regions of the graphite surface in any of these experiments’.
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5.2 Silver Nitrate
It is worth remembering that humans have been experimenting with depositing silver
ions for generations and it is thus an ideal system to be studied on the nanoscale
using AFM electrochemical techniques. Indeed any macroscopic deposition must, at
least transiently, by obtained by nanoscopic deposition en route to the macro scale.17
Digital photography now replaces most of the chemical and physical applications that
chemical photography employed over its relatively short history of about 170 years.
The colour and form of a digital image after capture with a camera can be
manipulated through Photoshop, allowing a photographer an exacting control over
the finished image, with little if nothing left to chance and with the ability to
reproduce numerous identical copies.
In the early development of chemical photography, individual images were more
unique in their nature, as a silver image produces something that is far more difficult
to control and exhibits a more random nature from chemical reaction. This is
particularly the case when the print maker is controlling the chemistry and exposure.
Early photographic chemistry can exhibit enormous variations in reaction to colour,
sensitivity and stability, making the visual outcome unique and often unrepeatable.
By experimenting with some of these more antiquated processes, it is possible to
produce these on paper without the need for a suspension medium such as gelatine.
Thus, early efforts revolved around the use of chemistry as a printmaking medium
and together with others, such as ink and graphite. The development of digital
photography has therefore helped establish this chemical art form in its rightful place
within the context of printmaking; see Figure 5.d (below) for an early example of
early silver ion chemical photography.
Page 24 of 61
Figure 5.d
Early print making using silver ions.
To explore the fascinating possibilities, it is necessary to look back at the dawn of
photography and some of the experiments made by practitioners working at that
time. Robert Hunt who experimented with light sensitive substances, in his
“Researches on Light in its chemical reactions” published in 1844, points out many
interesting reactions of substances with light, including charcoal:
“If a stick of charcoal is placed in a bottle in which is some solution of nitrate of
silver, so that one half of the charcoal is in the solution, and the other half above it,
there will in a short time appear little spangles of silver upon the upper portion of
the charcoal, if it is exposed to diffused light. In full sunshine the effect is greatly
retarded. If the bottle is placed in a dimly illuminated place, there will in the course
of a few weeks, form in the solution around the charcoal, a series of the most
delicate thread-like crystallisations of the silver. After these have formed, if the
bottle is exposed to sunshine they are gradually re-dissolved into the fluid.”
Page 25 of 61
There is still much to be learnt by looking in depth at the work of these experimenters
and reactions they dismissed at that time, which did not follow the objective of
achieving a practical working photographic process. As these may now prove useful
when employed in combination with 21st century technology, such as scanning and
flash photography, to record precise moments of chemical reaction and
decomposition. Let us bear in mind that technological development and in this case
chemical discoveries, are not always in step with chronological progression.
Fox Talbot who developed the first real practical form of photography, employing a
negative and positive, used the word Calotype to describe his process, which is
derived from the Greek word Kalos, meaning beautiful. Looking at some of the early
examples, beautiful is surely a very apt word.
Page 26 of 61
6 Method
1. We fixed the HOPG substrate to a copper supporting plate using conductive silver
paint, and further soldered a conductive wire to the copper plate to facilitate
attaching of a crocodile clip. The crocodile clip was then routed through the AFM
BNC to ground the circuit.
2. We cleaved the HOPG with adhesive tape, and then pipette 100μL AgNO3 onto the
HOPG.
3. We then wicked away the AgNO3 leaving only a meniscus of AgNO3 on the
substrate, taking care not to touch the substrate surface with the wicking paper if
at all possible.
4. We absolutely minimized the time between pipetteing the AgNO3 onto the
substrate and eventual bias application. This is because the AgNO3 is
photosensitive (see materials 5.2 above). Similarly the AFM light was switched off
during bias and image acquisition. This is to reduce any static noise interference
from the AFM light.
5. All AFM tips were UV irradiated (using a UV/Ozone ProCleaner) for 10 minutes
prior to loading into the AFM cantilever holder- this was to clean the tips.
6. We first AC mode imaged the substrate surface (for subsequent juxtaposition
with post bias images).
7. In order to determine if there was any deposition, we AC mode imaged the
substrate (per and post bias).We applied small tip-substrate forces when imaging
and applying bias in order to avoid unnecessary agitation of any nucleated
particles. We did this by using small amplitudes, and small set points. It is
important to only use topographic images of the substrate for confirmation
purposes, as other images such as ‘phase’ are not a true reflection of any
deposition. See Results 7 (below) for details of set points used.
Page 27 of 61
8. All biases were applied using custom code for the MFP3D. The biases were
applied in a grid fashion (see Figure 7.dd below) at predetermined sites, for set
durations at specified voltages. The voltage was applied so that the AFM tip was
at a negative bias with respect to the substrate which was grounded. In order to
apply a negative bias to the AFM tip we attached a conductive wire underneath
the cantilever holder and this wire was then routed through a BNC output
channel, thus enabling us to apply a specified voltage to the tip. We applied all
biases in contact mode with the surface. The experiment was carried out in
ambient air with temperature of ≈ 298 +/- 2K.
9. For any experiments that we conducted using a gold substrate (Arrandee gold
11 x 11 mm), we prepared the surface as follows- Ozone irradiate for 10 minutes,
rinse in isopropanol, ethanol and then polish with lens paper (Fisherbrand lens
cleaning tissue range 0960F00021, not with Kim wipes- as these scratch the
substrate) , Milli-Q water and then desiccate using a dry nitrogen gun.
10. We also calibrated the cantilever using the Thermal method (in order to ascertain
the spring constant) and by performing a force distance curve on glass to get the
Invols (inverse optical lever sensitivity) of the cantilever.
11. All experiments were performed in air at room temperature. See Results 7
(below) for details of voltages applied, bias duration and set points used in the
different experiments.
12. AFM topography images were processed using Gwyddion (64 bit) freeware,
Nanotec Electrónica WSxM freeware, and Igor Pro software.
Page 28 of 61
7 Results
The nano-deposits generated during the contact bias application are shown below.
Figure 7.a (below) shows Ag deposition (bright silver feature in upper left) on the
HOPG surface following an applied bias of -3.75V for 5.5 seconds using a set point of
0.6V which corresponds to a tip-surface force of 6.8nN. The volume of this Ag
deposition is 1,475,104.0 nm3.
Figure 7.b (below) shows the cross section profile of line 1 from Figure 7.a.
Figure 7.c (below) shows the same surface prior to any bias in a meniscus of AgNO3
for comparison purposes. The step edges and basal planes of the freshly cleaved
HOPG are clearly visible.
There is no evidence of any pit associated with this deposition.
Figure 7.a
Ag deposition on HOPG after an applied bias of -3.75V for 5.5 seconds in contact
mode.
The deposition is non symmetric, and is not associated with a step edge, and is
generally conical in shape being wider at the base.
Page 29 of 61
Figure 7.b
Cross-sectional profile of deposition for an applied bias of -3.75V for
5.5 seconds.
Figure 7.c
Surface of HOPG prior to any bias being applied. Image acquired in AM-AFM mode
Figure 7.d below shows Ag deposition on HOPG in a meniscus of AgNO3. The applied
voltage was -3.85V for 3.5seconds in contact mode with a set point of 0.8V (9.1nN
tip-HOPG contact force). The larger deposition (bottom middle) volume is
199,556.3nm3., the smaller deposition volume is 20,194.0nm3, total deposition of
219,750.3nm3;.
Figure 7.f shows the same surface of HOPG prior to any bias being applied. Image
acquired in AM-AFM mode, set point 0.8V
Page 30 of 61
Figure 7.d
Ag deposition on HOPG, bias applied -3.85V for 3.5 seconds in contact mode. Image
acquired in AM-AFM
Figure 7.e
Cross-sectional profile of deposition for an applied bias of -3.85V for 3.5 seconds.
Again the deposition is conical in shape and not closely associated with a step edge.
The deposition volume is markedly reduced from that obtained in Figure 7.a above.
The deposition in Figure 7.a was done at a higher voltage and for a longer duration.
Page 31 of 61
Figure 7.f
Surface of HOPG prior to any bias being applied. Image acquired in AM-AFM mode
Figure 7.g (below) shows Ag deposition on HOPG in a meniscus of AgNO3. The
applied voltage was -3.90V for 2.5 seconds in contact mode with a set point of 0.8V
(9.1nN tip-HOPG contact force).The deposition volume is 687,901.2 nm3; the smaller
deposition volume is 1.2 nm3.
Figure 7.g
Ag deposition on HOPG, bias applied -3.90V for 2.5 seconds in contact mode. Image
acquired in AM-AFM mode.
The bias was applied in contact mode at a single location (just below the beginning of
the finger of deposition). This finger of deposition was in contrast to the localised
deposition of Figure 7.a & Figure 7.b (above). It is possible the finger of deposition
was originally localised deposition that was spread out by the action of the AM-AFM
tip motion.
Page 32 of 61
Figure 7.h
Cross-sectional profile of deposition from Figure 7.g for an applied bias of -3.90V for
2.5 seconds.
Figure 7.i (below) shows Ag deposition on HOPG in a meniscus of AgNO3. The
applied voltage was -3.85V for 3.0 seconds in contact mode with a set point of 0.4V
(4.5nN tip-HOPG contact force).The deposition volume is 113,187.1 nm3.
Figure 7.i
Ag deposition on HOPG, bias applied -3.85V for 3.0 seconds in contact mode. Image
acquired in AM-AFM mode, set point 0.4V, free air amplitude 0.50V.
Again a finger like deposition is observed in contrast to localised deposition, even
though the bias was applied at a single spot.
Page 33 of 61
Figure 7.j
Cross-sectional profile of deposition for an applied bias of -3.85V for 3.0
seconds.
Figure 7.k
Surface of HOPG shown in Figure 7.i prior to any bias being applied. Image
acquired in AM-AFM mode, set point 0.35V, free air amplitude 0.44613V.
Figure 7.l (below) shows Ag deposition on HOPG in a meniscus of AgNO3. The
applied voltage was -3.80V for 3.0 seconds in contact mode with a set point of 0.4V
(4.5nN tip-HOPG contact force).The deposition volume is 72,617.3 nm3; the volume
of the pit is 200,864.4nm3.
Figure 7.l
Ag deposition on HOPG Bias applied -3.80V for 3.0 seconds in contact mode. Image
acquired in AM-AFM mode, set point 0.4V.
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It is clear from Figure 7.l above and Figure 7.m below that any deposition is now
accompanied by the formation of a pit. The deposition volume is 36.2% of the pit
volume.
Figure 7.m
Cross-sectional profile of deposition shown in Figure 7.l for an applied bias of -
3.80V for 3.0 seconds.
Figure 7.n
Surface of HOPG shown in Figure 7.l prior to any bias being applied. Image
acquired in AM-AFM mode, set point 0.39V, free air amplitude 0.50V.
Page 35 of 61
Figure 7.o (below) shows another site of Ag deposition on HOPG in a meniscus of
AgNO3. The applied voltage was -3.80V for 3.0 seconds in contact mode with a set
point of 0.4V (4.5nN tip-HOPG contact force).The deposition volume is 69,235.7nm3;
the volume of the pit is 283,636.6 nm3. The deposition volume is 24.4% of the pit
volume.
Figure 7.o
Ag deposition on HOPG Bias applied -3.80V for 3.0 seconds in contact mode. Image
acquired in AM-AFM mode, set point 0.4V.
Figure 7.p
Cross-sectional profile of deposition and pit shown in Figure 7.o for an applied bias
of -3.80V for 3.0 seconds.
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Figure 7.q (below) shows another site of Ag deposition on HOPG in a meniscus of
AgNO3. The applied voltage was -3.80V for 3.0 seconds in contact mode with a set
point of 0.4V (4.5nN tip-HOPG contact force). The deposition volume is
44,2983.4nm3; there is no associated pit, and the deposition is not along a step edge.
Figure 7.q
Ag deposition on HOPG, bias applied -3.80V for 3.0 seconds in contact mode. Image
acquired in AM-AFM mode, set point 0.4V.
Figure 7.r
Cross-sectional profile of deposition shown in Figure 7.q for an applied bias of -
3.80V for 3.0 seconds.
Figure 7.s (below) shows Ag deposition on HOPG in a meniscus of AgNO3 using a Si
tip. The applied voltage was -4.5V for 4.5 seconds in contact mode with a set point of
0.39V. The deposition volume is 6,865.2 nm3; with the associated pit volume being
30,868.9nm3. The deposition volume is 22.2% of the pit volume.
Page 37 of 61
Figure 7.s
Ag deposition on HOPG, bias applied -4.50V for 4.5 seconds in contact mode. Image
acquired in AM-AFM mode, set point 0.39V, free air amplitude 0.49337V, using a Si
tip.
Figure 7.t
Cross-sectional profile of deposition shown in Figure 7.s for an applied bias of -
4.50V for 4.5 seconds using a Si tip.
Figure 7.u
Surface of HOPG shown in Figure 7.s prior to any bias being applied. Image
acquired in AM-AFM mode, set point 0.35V, free air amplitude 0.45266V.
Page 38 of 61
Figure 7.v (below) below shows Ag deposition on HOPG in a meniscus of AgNO3
using a Si tip. The applied voltage was -5.0V for 3.0 seconds in contact mode with a
set point of 0.39V. The deposition volume is 671,390.0 nm3; with the associated pit
volume being 8,257.2 nm3. The deposition volume is 8,131% per the pit volume.
Figure 7.v
Ag deposition on HOPG, bias applied -5.0V for 5.0 seconds in contact mode. Image
acquired in AM-AFM mode, set point 0.39V.
Figure 7.w
Cross-sectional profile of deposition shown in Figure 7.v for an applied bias of -5.0V
for 5.0 seconds using a Si tip.
Figure 7.x (below) shows Ag deposition on HOPG in a meniscus of AgNO3 using a Si
tip. The applied voltage was -5.0V for 3.5 seconds in contact mode with a set point of
0.4V. The deposition volume is 3,658.6 nm3; with the associated pit volume being
803.9 nm3. The deposition volume is 455% per the pit volume.
Page 39 of 61
In addition to the bias site deposition decorating the atomically-smooth regions of
the graphite surface, it is clear from the AM-AFM image shown in Figure 7.x (below)
that nanoscopic silver particles/ribbons are also appearing along step edges on the
graphite surfaces. This deposition could be triggered by the photosensitive nature of
the AgNO3, see 5.2 (above) and or/coupled with the higher conductivity of the step
edges compared to basal planes (see above).
Figure 7.x
Ag deposition on HOPG, bias applied -5.0V for 3.5 seconds in contact mode. Image
acquired in AC mode, set point 0.4V.
Figure 7.y
Cross-sectional profile of deposition shown in Figure 7.x for an applied bias of -
5.0V for 3.5 seconds using a Si tip.
Page 40 of 61
Figure 7.z (below) shows Ag deposition on HOPG in a meniscus of AgNO3 using a Si
tip. The applied voltage was -5.0V for 4.0 seconds in contact mode with a set point of
0.4V. The deposition volume is 257,457.2 nm3; with the associated pit volume being
26,416.8nm3; the deposition volume being 974% per the pit volume.
Figure 7.z
Ag deposition on HOPG Bias applied -5.0V for 4.0 seconds in contact mode. Image
acquired in AM-AFM mode, set point 0.4V.
Figure 7.aa
Cross sectional profile of deposition shown in Figure 7.z for an applied bias of -5.0V
for 4.0 seconds using a Si tip
Page 41 of 61
Figure 7.bb (below) shows further Ag deposition on HOPG in a meniscus of AgNO3
using a Si tip. The applied voltage was -5.0V for 4.5 seconds in contact mode with a
set point of 0.4V. The deposition volume is 41,283.7 nm3; with the associated pit
volume being 37,044.1 nm3; the deposition volume being 111% per the pit volume.
Figure 7.bb
Ag deposition on HOPG, bias applied -5.0V for 4.5 seconds in contact mode. Image
acquired in AM-AFM mode, set point 0.4V.
Figure 7.cc
Cross-sectional profile of deposition shown in Figure 7.bb for an applied bias of -
5.0V for 4.5 seconds using a Si tip.
Figure 7.dd (below) shows the typical grid scenario that we employed for bias site
selection for testing the bias/duration/nucleation parameters.
Figure 7.dd
Typical grid scenario used for testing the bias/duration/nucleation
parameters. The Nucleation shown corresponds to Figure 7.o (above)
Page 42 of 61
Figure 7.ee (below) shows Ag deposition on a gold substrate in a meniscus of AgNO3
using a Si tip. The applied voltage was -2.5V for 3.0 seconds in contact mode with a
set point of 0.39V. The deposition volume is 9.25 x 10-2 m3; with no associated pit.
Figure 7.ee
Ag deposition on Gold, Bias applied -2.5V for 3.0 seconds in contact mode. Image
acquired in AM-AFM mode, set point 0.39V.
Figure 7.ff
Cross-sectional profile of deposition shown in Figure 7.ee for an applied bias of -
2.5V for 3.0 seconds using a Si tip.
Page 43 of 61
HOPG Substrate Results
Experiment
No.
Tip Voltage, V Duration, s Set Point, V Tip Force,
nN
Deposit
Volume, nm3
Pitt Volume, nm3
1 HQ:DPE -5.00 30.00 0.8 9.1 88,539.1 119,192.8
2 HQ:DPE -3.75 5.50 0.6 6.8 1,475,104.0 n/a
3 HQ:DPE -3.80 5.50 0.65 7.4 2,833.3 n/a
4 HQ:DPE -3.85 5.5 0.64 7.2 205,089.6 n/a
5 HQ:DPE -3.85 3.5 0.80 9.1 219,750.3 n/a
6 HQ:DPE -3.85 3.5 0.80 9.1 135,985.9 n/a
7 HQ:DPE -3.90 2.5 0.80 9.1 687,902.4 n/a
8 HQ:DPE -3.82 3.4 0.40 4.5 27,038.2 n/a
9 HQ:DPE -3.85 3.0 0.40 4.5 113,187.1 n/a
10 HQ:DPE -3.80 3.0 0.40 4.5 72,617.3 200,864.4
11 HQ:DPE -3.80 3.0 0.40 4.5 69,235.7 283,636.6
12 HQ:DPE -3.80 3.0 0.40 4.5 44,298.4 n/a
13 Si, SSS -4.5 4.5 0.39 * 6,865.2 30,868.9
14 Si, SSS -5.0 3.0 0.39 * 671,390.0 8,257.2
15 Si, SSS -5.0 3.5 0.40 * 3,658.6 803.9
16 Si, SSS -5.0 4.0 0.40 * 257,457.2 26,416.8
17 Si, SSS -5.0 4.5 0.40 * 41,283.7 37,044.1
Page 44 of 61
Gold Substrate Results
Experiment
No.
Tip Voltage, V Duration, s Set Point, V Tip Force,
nN
Deposit
Volume, nm3
Pitt Volume, nm3
18 Si, SSS -2.5 3.00 0.39 * 92,529,623.7 n/a
19 HQ:DPE -1.0 1.50 0.47 5.3 7,208,942.9 23,294.6
20 HQ:DPE -1.25 0.75 0.36 4.1 3,219,836.0 n/a
21 HQ:DPE -1.0 1.75 0.066 0.7 3,704,274.9 n/a
* Tip Force not calculated, this is because the Si tips Invols data was not collected at the time of experiment. The Si tips are far stiffer than the HQ:DPE tips, (see
Materials 5 (above)) so the tip-substrate force will be far higher for experiments using Si tips than HQ:DPE tips.
Page 45 of 61
8 Discussion
Jiang, Yan et al in their 2008 paper19 investigate convex and concave nanodots they
created on HOPG in ambient air by applying a voltage pulse between a metal-coated
AFM tip and the sample surface. Using a linear scan with a positive substrate bias,
nanoscale lines were also etched on the HOPG surface. Depending on the amplitude
and duration of the voltage pulse, the nanostructures were either convex or concave.
The depth of the concave structure sharply increased with the amplitude and
duration of the voltage pulse, while the height of the convexity stayed at a low level
and varied in a small range with the voltage lower than a threshold value. Under
negative substrate bias or in a vacuum, no change occurred on the HOPG surface in
the experimental range.
The formation of the nanostructures was ascribed by the authors to the primary
dissociative adsorption of water and oxygen in air induced by the intensive hole
concentration and the subsequent defect-assisted oxidation of graphite (with the
proviso that in our experiments we also have additional dissociated NO3- ions
available to play a role).24 The external electric field can induce a reaction of the
carbon surface with absorbed gases, which has been used in the fabrication of carbon-
based nanostructures20. Pits with minimum diameter down to 2 nm were produced by
applying positive voltage pulses of 3–8 V to the HOPG for 10–100 μs (note the
substrate is positively biased with respect to the tip, so electrons are flowing from the
tip to substrate which mirrors our experimental set up).21 When the HOPG and the tip
are immersed in liquid, convex structures on the HOPG surface can be created by
using electrical methods22
Jiang, Yan et al19 report that both concave and convex topography nanostructures
were produced in ambient air. The convex structures were created on the surface at
low electric voltage or short pulse duration. On increasing the amplitude or duration
of the voltage, ‘the convex profile will convert into concave morphology’.
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The authors propose that the convex profiles are attributed to concentrated holes
inducing water and oxygen dissociative adsorption onto the graphite layers while the
concave ones are formed by defect-assisted oxidation of the carbon layers under
enhanced electric field.24 We see evidence of pit formation in our experiments
(experiment no. 1, 10, 11, 13 through 17, & 19), which probably follow the mechanism
as outlined by Jiang, Yan et al. When pulses with short duration (<=1,000 ms) were
applied, nanodots with convex profiles were formed, However, where the voltage
pulses with longer durations were applied, concave dots were produced. The depth of
nanodots formed increased apparently with the duration. It is shown that the pulse
duration has a strong influence on the formation of the nanostructures.
When lower voltage pulses were applied for the same duration of 10s, the
nanostructures were not observed at the corresponding sites. This indicates that the
formation of nanodots occurs only at voltages larger than a threshold value.24 In
Jiang, Yan et al’s case, the threshold voltage is estimated to be in the range of 5–6 V,
and is dependent on the etching speed and air humidity.20Convex etching lines were
formed at the voltage lower than a specific threshold. On increasing the voltage over
the threshold, the etching lines became concave. It is dependent on the etching speed
and air humidity.20
The authors go on to suggest that the reaction between the water or oxygen and the
carbon layers most likely causes the formation of the nanostructures. The authors
further go on to outline the mechanism for nanodot and nanopit formation: under
the applied voltage one of the oxygen-free23 electron pairs of a water molecule close to
the HOPG surface can interact with the hole and a C–O bond is formed. The
formation of C–O bonds may cause the carbon lattice to produce a strain of the
topmost layer of HOPG and form a protrusion. This can be described as the first
stage. At the following stage, on increasing the amplitude or duration of the applied
voltage, more C–O bonds are formed and the carbon lattice strain increases. When
the strain reaches a limitation, some C–C bonds on the top area of the protrusions
begin to fracture and a small pit is produced. The whole process is shown in Figure
8.a (below).24
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Figure 8.a
Schematic diagrams of nanostructures on the HOPG surface produced in ambient
air by the AFM tip under an external electric field.24
Whilst our experiments were not conducted under the same conditions as Jiang, Yan
et al.: our experiments were conducted in a meniscus of AgNO3 and not solely in
ambient air (i.e. H2O meniscus/electrochemical cell) which would affect the reactions
occurring at the substrate surface, also our applied electric field was stationary- no
scan-speed. We did observe pit formation (experiments 1, 10, 11, 13, 14, 15, 16, 17 &
19). Pits were preferentially formed in our experiments that were conducted with Si
tips, but not exclusively (experiments 1, 10, 11 & 19, see results tables above). It is
possible that our pit formation follows a similar path as outlined above ‘defect
induced oxidation of graphite facilitates the formation of concave structures’24.
Furthermore the stiffer Si tips would make a better contact with the HOPG surface
thus lowering the impedance and facilitating the oxidation of the HOPG. This could
be further investigated by repeating our experiments whilst at the same time
measuring voltage and current.
Park, Jin Gyu, et al.24 report in their 2007 paper that sub-100 nm holes were made on
HOPG surfaces using a metal-coated AFM tip and carbon nanotubes. The hole-
formation mechanism is related to the chemical reaction of graphite with adsorbed
water and tunnelling electrons from the tip to substrate. The authors suggest that
chemical reactions between HOPG and tunnelling electrons are a more important
mechanism than field-emission electrons. The substrate (HOPG) was always
maintained at a higher voltage than the AFM tip (this is the same configuration as our
experiments). However the authors applied −10 V pulses to the metal-coated tip with
a 50 ms pulse width (50% duty ratio). With 1000 repetition times, a hole was
fabricated on the HOPG surface.24
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The authors go on to contend that ‘hole diameter depended on the applied voltage,
contact force, number of pulses, and humidity; there was a threshold voltage to make
a hole and any amount over this voltage created a larger hole proportional to the
voltage amplitude.’
Further the authors submit that ‘the surface experienced too much damage with high
current (several μA) over the threshold voltage. Therefore higher contact force in this
voltage range gives higher current; hence the size cannot be controlled.’ The authors
suggest several mechanisms for hole fabrication such as mechanical contact between
tip and sample, heating, electro-migration, electronic contact, field-induced
electrochemical etching, and field-induced evaporation. However, the mechanical
effect can be ruled out based upon previous tests and other reports25. Our contact
force is below the HOPG yield strength (see ensuing discussion below on this point).
The local heating effect was controversial due to the high thermal conductivity of
graphite and metals.26 If the tip apex has low thermal conductivity, then the local heat
can build up to 1000K which is enough to dissociate graphite (C) to gas (CO2).26
Again this experimental set-up was similar to our configuration in that the substrate
(HOPG) was always maintained at a higher potential than the AFM tip, however the
voltages were larger (10V) and applied for shorter durations 50 ms pulse width (50%
duty ratio) with 1000 repetitions. However there are enough similarities between the
studies for the authors suggestion that the possible machining mechanism can be
ascribed to the chemical reaction of graphite with tunnelling electrons to be
applicable in our work, but as the authors note ‘further research is needed in a more
controlled atmosphere’ before we can make firm conclusions in this regard.
Xu (2003)27 reported on the nano-indentation of HOPG surface and pure elastic
deformation up to a maximum load of ≈610 μN.28 Fraxedas et al.29 (2002) concluded
that the plastic yield threshold was not reached when forces as large as 16 μN are
applied,29 with the penetration at plastic yield larger than 25 nm. Compared with the
above references, the constant contact force ≈4 nN in our work (with the HQ-DPE
tips) shall be negligible in playing a part in patterning or modification of the HOPG
sample surface.
Page 49 of 61
Zoval, Jim V. et al report on voltage pulses employed to electrochemically deposit
silver nanocrystallites on atomically smooth graphite basal plane surfaces.17 Voltage
pulses with amplitudes of 100, 250, and 500 mV vs Ag0 and durations of 10 or 50 ms
were applied to graphite surfaces immersed in dilute (≈1.0 mM) aqueous silver
nitrate. Whilst the experimental set up is not identical to ours the studies are similar
enough for us to gain insight from their findings. Zoval’s experimental set up is
shown in Figure 5.c (above).
Further the authors state that any nucleated silver nanocrystals interact weakly with
the graphite surface and are removed by the sweeping action of the probe tip from the
imaging area. This effect has been previously documented for gold particles on
graphite by Schaefer et al18 (see Introduction 1 above).
Further the authors state that silver micro and nanocrystallites which nucleate at
defect sites are observed by STM and AFM, and, consequently, it had been
concluded30 that silver over potential deposition (OPD) on graphite is initiated by
nucleation exclusively at defects, such as step edges, on the graphite surface. The NC-
AFM data presented by the authors suggest that on low-defect density surfaces such
as the graphite basal plane, STM and repulsive-mode AFM data can provide a
misleading view of nucleation by “ignoring” the presence of weakly adsorbed metal
nanocrystallites which are not associated with defects.17 The researchers also note
that in successive silver deposition trials in which the graphite surface was cleaved
prior to each experiment, the apparent capacitance of the surface fluctuated by 10-
20%, presumably due to fluctuations in the areal density of defects on the graphite
surface which is exposed during fresh cleavage.
The researchers note that the diameters of the silver particles associated with defects
such as step edges were smaller by 20-50% compared with silver particles which were
present on nearby basal plane regions of the same graphite surface.17 Further they
elucidate that the size disparity probably derives from the fact that the diffusional
transport of Ag+ to growing nuclei arrayed along step edges had a cylindrical
symmetry, whereas nuclei on the basal plane, which were farther removed on average
from nearest neighbours, experienced a more efficient hemispherical diffusional flux
leading to faster growth and a larger terminal radius, see Figure 7.z above for graphic
nucleation/deposition similarities in our work.
Page 50 of 61
However we are not seeing these results replicated entirely in our experiments: it is
worth bearing in mind our experimental parameters were different in that we
generally applied higher voltages for longer durations in a meniscus (as opposed to
bath) which may lead to a masking of the Zoval trends observed at smaller time
frames and lower voltages.
Significantly the researchers note that at values of coulometric loading (quantity of
matter transformed during an electrolysis reaction by measuring the amount of
electricity in coulombs consumed or produced) 31, QAg greater than ≈15 μC cm-2, a
branching occurs in which an ever increasing fraction of the deposition charge is
consumed with the deposition of silver onto micron-scale crystallites instead of onto
nanocrystallites on the surface. The results of our experiments are generally micron
sized deposits with some smaller associated nano-deposits in the general vicinity
(and sometime nano-pits (see above)). Probably our results are predominantly
micron sized because the voltages and durations we employed are higher than in
Zoval’s experiments, and our coulometric loading was therefore higher and favoured
eventual micron sized deposits. We would need to measure and monitor the induced
current during deposition to confirm this.
Zoval et al conclude that the silver electrodeposition mechanism is the following:
within ≈5 ms of the application of a large potentiostatic pulse to the graphite surface,
critical silver nuclei are established both at defect sites on the surface and at high
areal density on the defect-free graphite basal plane. The number density of these
silver nuclei does not increase appreciably with time (up to ≈50 ms). Following their
formation and for the ensuing 15 ms, critical nuclei grow at a rate limited by the
hemispherical (for deposition not located on step edges) diffusive flux of Ag+ to each
nucleation site. At approximately 15ms, the rate of growth of most silver
nanocrystallites slows dramatically, and further silver deposition is concentrated at a
small fraction of crystallites which increase very rapidly in size-attaining micron-scale
dimensions within ≈20-40ms. Again Zoval conducted his experiments in a bath of
AgNO3, but the tendency toward micron scale nucleation at a higher bias and
duration concur with our findings.
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The recent development of electrochemical force microscopy (EcFM)32 and its ability
to probe local bias- and time-resolved ion dynamics and electrochemical processes at
the solid–liquid interface would be invaluable for delving further into this
phenomenon.
Li, Yan et al11 report on the deposition of several metals and semiconductors on Si
surfaces at room temperature using the electrochemical “dip-pen” nanolithography
(E-DPN).technique, see Figure 4.c (above) for experimental set-up.
In a typical experiment, an ultra-sharp silicon cantilever coated with H2PtCl6 was
scanned on a cleaned P-type Si (100) surface with a positive DC bias applied on the
tip. During this lithographic process, H2PtCl6 dissolved in the water meniscus is
electrochemically reduced from Pt(IV) to Pt(0) metal at the cathodic silicon surface
and deposits as Pt nano-features according to the following equation:
PtCl62- + 4e → Pt + 6Cl-
The authors state that the height and width of the fabricated features, using the E-
DPN technique, depend on several factors, including the humidity, scan speed, and
applied voltage. They show that by varying these factors, they can change the height
and width of the created features. Using a similar process they authors succeeded in
creating features made of Au, Ge, Ag, Cu, Pd, etc. Further they state that in principle,
any metal or semiconductor that can be electrochemically deposited from an aqueous
solution of salts could be delivered to a surface with precise control of position to
form features with nanometer dimensions using E-DPN.11
See Figure 8.b (below) for an example of a silver line drawn by E-DPN method, with
AgNO3 solution as the ink, on a Si substrate.
Page 52 of 61
Figure 8.b
A silver line drawn by E-DPN method with AgNO3 solution as the ink. Experimental
conditions: relative humidity: 42%, voltage: 4V, scan speed: 20nm/s.
Although Li, Yan et al’s experimental set up is different to ours in that they work with
an AFM tip at a positive potential with respect to the substrate, they use a capillary
instead of meniscus to deliver the ionic reactants, and the working substrate was Si;
their work on biased deposition of Ag+ from AgNO3 bears relevance to our
investigations and offers corroborative evidence that the nucleation in our
experiments is biased induced electrochemical deposition of Ag.
The cantilever deflection is related to the force applied to the sample as follows:33
F = k. Invols. ∆V
F = 447.60 pN/nm . Defl InvOLS 25.28 nm/V . Set point 400.0mV
F = 4.5 nN (force applied during imaging)
Using Hook’s law (F= -kx), the amplitude of the cantilever during imaging is given by:
Amplitude = 4.5 nN / 447.60 pN/nm = 10.05nm
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In AC mode imaging it is important to manipulate the Set Point and Integral Gain
(‘IG’) in order to improve the image quality; the IG determines how fast the AFM
responds to perturbations. The set point determines the force applied to the tip (and
hence sample), it is this force/value which the feedback loop maintains. In order to
optimize the IG and set point a systematic approach should be adopted - first adjust
the set point to an appropriate level for your objective (a lower set-point ≡ bigger
force applied to sample). Initially engage the surface with a high set point and then
adjust downward; then set the IG to a level which induces noise in the image quality,
now set the IG to a level just below the noise producing level. In this way the IG
amplification is maximised thus improving image quality.
During amplitude mode operation, the cantilever is oscillated near its resonance
frequency, ωo, resulting in a ‘free air’ amplitude, Ao. When the cantilever is placed in
close proximity to the surface, the tip – surface/deposition interactions result in
decreased cantilever oscillation amplitude from the ‘free air’ amplitude. The
cantilever oscillates at amplitude, A, after interacting with the surface. The AFM
image is acquired during raster scanning over the sample by measuring the necessary
adjustments to the vertical displacement of the scanner, via a feedback loop, to
maintain the constant value of set point ratio s = A/Ao. The feedback loop provides
precise control over the cantilever amplitude, and thus, the force between the tip and
surface.34 The force which the tip exerts on the sample is described by Hooks Law,
F = -k.x ; by using a cantilever with smaller length, the cantilever displacement is
reduced and so the force exerted on the sample is also reduced. Note shorter
cantilevers oscillate at higher frequencies and this affects the feedback loop and scan
speed.
Similar to conventional repulsive mode AFM, the operational principle of the NC-
AFM (also called the “dynamic”, “attractive mode”, and “ac” atomic force microscope)
involves the detection and maintenance of a small force which is exerted locally by
the probe tip on the sample surface. In contrast to the repulsive mode AFM
experiment, however, the NC-AFM probe tip is located in the attractive region of the
tip-sample interaction potential and at greater distances from the surface of 10-15 Å.
The attractive tip-sample forces present in this separation regime are smaller than
those involved in repulsive mode AFM imaging (10-11-10-15 N vs 10-9 N), with the
result that the NC-AFM tip is a less perturbative probe of the surface topography.
Page 54 of 61
The attractive force imparted by the tip to the sample is detected optically as a
deflection of the AFM cantilever toward the sample surface.17
It is also worth remembering that any Ag deposition dimensions obtained from cross
sectional analysis shown in Results 7 (above) are affected by tip convolution. The
apparent diameters obtained from NC-AFM image data are exaggerated by
convolution with the geometry of the probe tip. Since several different probe tips
were employed for our measurements, and the exact dimensions of each were not
measured, it is therefore impossible for us to deconvolve the tip contribution from the
NC-AFM image data. Other studies suggest that the apparent particle diameter is
larger than the true particle diameter by about twice the nominal tip radius. 17
9 Conclusions
It is clear from viewing our experimental results that we are inducing nano and micro
scale modifications on top of and into the HOPG surface (deposition and pits).
Our experimental set-up is not identical to any of the prior works that we studied, but
we can draw insight from the findings of Jiang, Yan et al. It is probable our stationary
electric field induces anion adsorption on the HOPG surface, weakening the HOPG
lattice structure. Thereafter this weakened lattice structure suffers further anion
adsorption and is simultaneously oxidized (forming C-O bonds). This process
increases the strain on the HOPG lattice eventually causing C-C bonds to fracture,
and the onset of pit formation.
Whilst our results do not consistently show pit formation, this may be due to a
varying AFM-HOPG contact, and also varying basal plane conductivity at each freshly
cleaved HOPG layer (change in areal density of defects per fresh layer).
Further it is possible that we are consistently producing pits but that depending on
the variable geometry of the pit and surrounding electrolyte; we may in fact
subsequently ‘fill’ in these pits with reduced/nucleated Ag. It is also important to bear
in mind that the AFM tip-HOPG contact is not an entirely static environment. The
AFM constantly tries to maintain the contact force we set, but as a pit forms under
the AFM tip the feedback loop must respond to this changed tip-surface dynamic. All
of these factors are an influence on the eventual pit-deposition structure under the
applied bias.
Page 55 of 61
Park, Jin Gyu, et al’s observations are also relevant; they note that hole diameter
depended on the applied voltage, contact force, number of pulses, and humidity.
There was a threshold voltage to make a hole and any amount over this voltage
created a larger hole proportional to the voltage amplitude. We see similar trends in
that longer duration voltage pulses created pits of larger volume (contrast pits formed
in experiments (1 vs. 10 vs. 11) and (13 vs. 16 vs. 17). Also the general trend follows the
magnitude of the voltage (higher voltage ≈ bigger volume pit).
We are presuming our deposits are Ag, and proceed according to the following
reaction:
Ag+ + e- = Ag
The electrons being supplied by the AFM tip and Ag+ from the silver nitrate solution.
The composition of our deposits could be confirmed by spectroscopic analysis (see
Section 10, Future Work, below).
We believe our deposits follow the mechanism outlined by Zoval et al. Zoval’s main
contention is that micron deposits succeed and are initiated by primary nano deposits
(nano deposits in the time frame ≈5 ms to ≈50 ms). Our experiments were run over
durations 3.8 to 5.0seconds and generally showed only micron scale deposits. Whilst
our experiments did not always follow the trend of longer durations resulting in
larger deposition volumes, this may be because the initial biased induced nucleation
absorbed/used up the majority of the available Ag+ ions in the surrounding meniscus;
Zoval conducted his experiments in a bath of AgNO3: we conducted our work in a
meniscus.
Li, Yan et al’s findings also corroborate Zoval and our findings, in that humidity,
voltage and scan speed (or lack thereof in our experiments) determines the quantum
of any deposition.
Page 56 of 61
We can conclude that the deposition is not metal dissolving from the AFM tip
because we get nucleation and pits when we use a pure Si tip (no metal coating). The
vastly increased quantity of deposition for the gold substrate versus the HOPG
substrate is a reflection of the inertness of HOPG versus Gold (high valence).
Furthermore, the HOPG has a lower areal density of defect sites which are conducive
for initial nucleation.
Also, the graphite basal plane surface is electrochemically very inert, and strongly
coordinating stabilizers are not involved in the silver electro-synthesis procedure.
Therefore the graphite-supported silver particles obtained by the voltage pulse
method have immediate applications for investigations of the intrinsic
electrochemical reactivity of silver particles over a wide range of particle diameters
The fabrication results show the AFM bias induced electrochemistry is a low-cost,
flexible and promising nanofabrication method which can modify a surface above and
below the surface layer.
These nano and microscopic deposits possess chemical and physical properties which
differ significantly from macroscopic metal phases. The list of particle size dependent
properties includes amongst others bond distances, the van der Waals attractive force
operating between particles, the surface plasmon resonance, the melting point, the
standard electrode potential, and the photoelectric yield. The variety of applications
which these properties can be put to work in is only limited by the imagination. In
our case, we are specifically aiming the results at biosensor applications, but this is a
small window into the endless possibilities.
Whilst the future is in the small, it is about to get a lot bigger.
Page 57 of 61
10 Future Work
Zoval et al17 report that it was sometimes possible to dislodge silver nanocrystallites
from the graphite surface by ultrasonication. The resulting suspension of particles
was then drop-coated onto a carbon-coated gold TEM grid for analysis. The TEM
data allowed formal identification of the silver nanocrystallites. We should use this
technique in future work to confirm the chemical identity of our deposits.
Furthermore, future work should include a spectroscopic analysis of any material
nucleated on the surface in order to confirm its elemental composition.
We should measure and document the current during the experiment. It is generally
accepted the deposition current is indicative of an instantaneous nucleation and
three-dimensional growth mode of deposition.35
Gunawardena et al.36 have shown that experimental data for several metals (including
silver) are consistent with the expression
Where:
D is the diffusion coefficient for the soluble form of the metal,
C* is the concentration of the metal in solution in units of mol cm-3,
M is the atomic weight of the metal (or the formula weight of the soluble metal
complex),
N is the total number of metal nuclei present on the electrode surface,
F is the density of the metal.17
Using this equation we can become more predictive and judgmental of our
experimental results.
We should SEM image the surface and tip pre and post experiment. Any nucleation
or deformation of the tip would result in a change to the applied electric field between
the tip and substrate. SEM analysis of the surface pre and post nucleation would
reveal the contribution of surface defects to the nucleation process.
Page 58 of 61
The z component of the tip deflection should also be recorded during bias dwell time.
Any change to the tip deflection is indicative of nucleation growth under/around the
tip. This information can be used in conjunction with the onset of tip-surface current
flow to elucidate the initiation condition of any nucleation.
The photo-sensitivity of the AgNO3 must be wavelength dependent. Therefore, if we
can isolate this wavelength, and then send a focused laser at this wavelength into the
AgNO3 solution on the HOPG, we may be able to achieve laser guided nucleation of
Ag on a HOPG or any substrate for that matter. The actuators of an AFM should allow
for the control of the laser beam.
Page 59 of 61
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