Study of the conductive properties of metal-organic materials using Conductive AFM

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MSc Physics and Astronomy

Advanced Matter and Energy Physics

AMEP Lab Project

“Study of the conductive properties of metal-organic materials

using Conductive AFM”

Alexandra Zeltsi

Student ID Nr. UvA: 12287350

Student ID Nr. VU: 2656959

6 ECTS

1

st

Examiner: dr. Esther Alarcón Lladó

2

nd

Examiner: dr. Elizabeth von Hauff

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The present work constitutes the mandatory AMEP Lab Project which is part of the curriculum of the Master Program “Physics and Astronomy”, “Advanced Matter and Energy Physics Track” joint degree between the University of Amsterdam and the Vrije Universiteit Amsterdam. The one-month project has been executed during June 2019 at the research institute AMOLF in the group of “3D Photovoltaics” with group leader Dr. Esther Alarcόn Lladό. The project is equivalent to 6 ECTS in the curriculum.

The main purpose of this work was the study of the conductive properties of metal-organic materials. For that purpose, Conductive AFM has been used as the main characterization technique. Special focus has been done to [Cu(m-SPhCO2H)]n metal-organic material with the AFM providing wealth of information about its conductivity.

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Acknowledgements

For the prosecution of the study, I would like to express my sincere gratitude to my supervisor Dr. Esther Alarcόn Lladό group leader of “3D Photovoltaics” AMOLF for the opportunity, trust and her invaluable support. I would specially thank Mark Aarts, PhD candidate for his daily supervision and guidance, introduction to the Conductive AFM as well as comments and suggestions.

I would also like to thank Yorick Bleiji, PhD candidate for his assistance in obtaining the SEM images of the metal-organic materials. I extend my gratitude to all the members of “3D Photovoltaics” group for the advices and the encouragement I received.

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Abstract

The study of the conductive properties of metal-organic materials has been the subject of the present work. Samples of [Au(m-SPhCO2H)]n, [Au(o-SPhCO2H)]n and [Cu(mSPhCO2H)]n have been studied for their morphology using SEM. Conductive AFM has been used to analyze the conductive properties of [Cu(m-SPhCO2H)]n metal-organic material. AFM provided wealth of information about the conductivity of the material and revealed that for different parameters, its conductivity can vary between 4 orders of magnitude (10-2_-10-5_{) S/cm. This result is important because no higher conductivity than} 10-5 S/cm has been recorded in previous studies for this kind of materials.

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Introduction to metal-organic materials

Previous studies

Seeking of high-performance molecular materials for electronic applications is one of the most competitive arenas in contemporary materials science. In this regard, organic and organometallic materials reveal advantages over traditional inorganic solid materials which include processibility as well as tenability [1]. Because of their distinctive conducting properties, Copper(I) sulfide polymorphs have been studied in semiconductors [2]_,

photovoltaic cells [3] and other electronic devices [4]. The charge transporting properties of Copper (I) sulfide are, however, related to its synthetic procedures such as topotactic ion exchange [5]_{, depositions in gas phase (chemical vapor deposition)}[6] _{or solution phase}

(chemical bath deposition) [7].

Over the past few decades, metal-organic complexes of third-row transition metal ions such as O2 II, Ir III, Pt II, and Au I have been extensively studied with regard to their use in organic light-emitting diode devices[8] .However, there have been only a few reports on the applications of first-row transition metal complexes in organic optoelectronics [8e].While p-conjugated organic molecules such as pentacenes[9]_{and oligothiophenes}[10][11]_{have been}

extensively studied for the fabrication of field-effect transistors (FETs), related work involving metal-organic complexes has been sparse. Notably, however, Copper (II) phthalocyanines have been extensively studied in relation to FETs, and relatively high charge mobility values (0.01–0.02 cm2V1s1) have been attained. [12]

Τhe [Cu(SR)]_∞ polymers are inexpensive and easily prepared in high yields and purities which makes them good for developing applications of [Cu(SR)] _∞ in materials science.

[13]_{Charge mobilities of nanorod FETs fabricated from [Cu(SR)]}

∞ polymers cover a wide range of values (10-2–10-5 cm2V1s1) and are thus comparable to those of 10-2 cm2V1s1 reported for oligothiophene [10] and polythiophene compounds [11].

[14]_{Highly crystalline [Cu(SCH}

3)]∞ nanowires with lengths of the order of hundreds of micrometers or even up to a few millimeters have been synthesized. They are p-type conducting and have a hole mobility as high as 2 cm2V1S1, which is 102–105 times higher than previously reported values. This high-hole mobility may provide them with a promising future in electronic-device applications.

According to K.-H. Low et al. [15]an unprecedented copper 4-hydroxythiophenolate system with a hexagonal Cu3S3 unit with resemblance to graphene structure is presented. This material displayed conducting properties as high as 120 S cm-1 which is much higher than the organic polymers like emeraldine. [16] SEM and TEM images revealed that solid sample of CuHT contained micrometer sized and aggregated platy crystallites with a thickness of ca. 50nm. Finally, after O-acetylation of CuHT, the charge transporting properties switched from a conductor to a semiconductor.

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9 Measurements on Copper (I)-Thiophenolate-based Coordination Polymers such as [Cu(m-SPhCO2H)]n and [Cu(o-SPhCO2H)]n were done in iCeMS (Kyoto). When the mercaptobenzoic acids are used in ortho and meta positions, the structures of the resulting [Cu(SPhCO2H)]n coordination polymers are 1D and 2D, respectively. In 1D structure with Cu-S core, all Cu and S atoms are tribonding (Figure 1) while in the 2D structure Cu-S layers have hexagonal arrangement (Figure 2).

Figure 1: Structure of [Cu(o-SPhCO2H)]n (a) projection of the structure on the (ab) plane,

(b) representation of the Cu3S3 network. Hydrogen atoms are omitted for clarity. [17]

Figure 2: Structure of [Cu(m-SPhCO2H)]n (a) projection of the structure on the (bc) plane, (b)

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10 σ-measurements on the [Cu(m-SPhCO2H)]n have been done in iCeMS (Kyoto). The measurements have been executed at 30mV and 1MHz-1Hz frequency for temperatures: 30-60-90-120-150-180-150-120-90-60-30℃ in 2 cycles. The experiments revealed that the conductivity ranges from 10-7_{S/cm order of magnitude at 30}o_{C to 10}-5_{S/cm order of} magnitude at 180oC.

[19] _{Hybrid Au(I) compounds exhibit a large domain of applications such as electronic}

devices, contrast agents, sensors or photocatalysts. These applications are related to the ability of Au(I) to form aurophilic interactions, implying self-assembly and luminescence

[20]_{. Among the compounds, Au(I) thiolates are an important class of materials, due to their}

soft-soft interaction with thiolates and their potential to form self-assembled monolayers, to protect and functionalize gold clusters and nanoparticles and to generate oligomeric or polymeric species, used for a long time as antiarthritic agents. However, due to the fast precipitation and the insolubility of these solids, a little is known about their structure making the origin of the photoluminescence difficult to rationalized [17]_.

[21] _{Gold(I) thiophenolate-based coordination polymers [Au(SPhR)]n have been}

synthesized. Depending on the substituent and its position, different chain-like and lamellar structures are obtained. The photophysical properties of these hybrid materials have been analyzed in solid-state. Studies showed different origins of the charge transfers depending on the electrophilicity of the thiolate molecules coupled with the participation or not of the aurophilic interactions. Among those coordination polymers, some exhibit high quantum yield around 70 % at room temperature and in the solid state [21b], another one shows a rare switch ON of the emission with a thermally induced solid state phase change from amorphous to crystalline[21a].

When the mercaptobenzoic acids are used in ortho and para positions, the structures of the resulting [Au(SPhCO2H)]n coordination polymers are 1D and 2D, respectively (Figure 3).

Figure 3: Structure representations of [Au(p-SPhCO2H)]n. (a) View of gold-thiolate chains on the (ab) plane, (b) view of the network on the (bc) plane and (c) on the (ac) plane. Pink, yellow, red and gray spheres are gold, sulfur, oxygen and carbon atoms, respectively. Hydrogen atoms are omitted for clarity. Dotted red and blue bonds represent the aurophilic and hydrogen interactions,

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11 Au(I)-S layers formed by helices interconnected through zig-zag aurophilic interactions. Organic layers are connected through catemeric H-bonds.

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Characterization techniques

In this chapter the characterization techniques of the samples used are described. In order to investigate the properties of the metal-organic materials, Scanning Electron Microscopy (SEM) and Conductive Atomic Microscopy (C-AFM) have been used. SEM has been used to reveal the surface morphology of the materials, but for the conducting properties C-AFM has been applied.

Scanning Electron Microscope (SEM)

A scanning electron microscope (SEM) was used to characterize the morphology of the particles. A SEM produces images by scanning the sample with a focused electron beam, which interacts with the atoms in the sample. This interaction can lead to the creation of secondary electrons (SE), backscattered electrons (BSE), characteristic X-rays and others, such as cathodoluminescence, Auger electrons and transmitted electrons (Figure 4). During the characterization of the samples, only the secondary electrons (SE) are used for the characterization.

Detection of SE is the most used imaging mode. These SE are generated when the focused beam ejects weakly bound electrons to the atoms inside the sample. The generation of SE is a highly inelastic process. Typically the SE have energies in the order of 1-15 eV

[22]_{and the focused electron beam has typical energies in the order of 5-10 keV.}

While the SE are created in the whole interaction volume of the focused electron beam, the escape depth of the SE, however, is of the order of a few nanometers. Therefore only SE generated near the surface can be detected. These SE can be detected by two different type of detectors, the Everhart-Thornley detector (ETD) and the Through the Lens detector (TLD). In the used SEM apparatus, these are known as the field-free mode and the immersion mode, respectively. The ETD detector is located out of a scintillator which fluoresces when is hit by the SE. The signal is increased by a photo-multiplier tube located outside the SEM. In contrast to the ETD detector, the TLD detector is located inside the column. Due to its location, this detector will mostly detect the pure SE and it will limit the detecting of SE that interacted with either the sample itself or surrounding materials

[23]_{.Therefore this TLD detector is able to obtain ultra-high resolution images, with}

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13 Figure 4: Electron-matter interaction volume: the different types of signals which are generated[24]_.

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AFM: Characterization technique of conducting properties

AFM technique [26]

The atomic force microscope (AFM) is one kind of scanning probe microscopes (SPM). SPMs are designed to measure local properties, such as height, friction, magnetism, with a probe. To acquire an image, the SPM raster-scans the probe over a small area of the sample, measuring the local property simultaneously.

AFMs operate by measuring force between a probe and the sample. Normally, the probe is a sharp tip, which is a 3-6 μm tall pyramid with 15-40nm end radius. Although the lateral resolution of AFM is low (~30nm) due to the convolution, the vertical resolution can be up to 0.1nm.

To acquire the image resolution, AFMs can generally measure the vertical and lateral deflections of the cantilever by using the optical lever. The optical lever operates by reflecting a laser beam off the cantilever. The reflected laser beam strikes a position-sensitive photo-detector consisting of four-segment photo-detector. The differences between the segments of photo-detector of signals indicate the position of the laser spot on the detector and thus the angular deflections of the cantilever (Figure 6). Piezo-ceramics position the tip with high resolution. Piezoelectric ceramics are a class of materials that expand or contract when in the presence of a voltage gradient. They make it possible to create three-dimensional positioning devices of arbitrarily high precision.

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[27] _{Conductive Atomic Force Microscopy (C-AFM)}

For the conducting properties of the materials C-AFM has been applied. C-AFM is a secondary imaging mode that characterizes conductivity variations across medium to low-conducting and semilow-conducting materials. It is used to measure and map current in the 2 pA to 1µA range, while simultaneously collecting topographic information.

Conductive AFM (C-AFM), and the related modes Tunneling AFM (TUNA) and PeakForce TUNA™, are powerful current-sensing techniques that represent part of Bruker’s array of Nanoelectrical Characterization Modes.

TUNA and Conductive AFM use contact mode AFM and a conductive probe. In contact mode, AFMs use feedback to regulate the force on the sample. In feedback mode, the output signal is the DC bias, adjusted to maintain the electric current set-point. [26]The AFM not only measures the force on the sample but also regulates it, allowing acquisition of images at very low forces. The feedback loop consists of the tube scanner that controls the height of the tip; the cantilever and optical lever, which measures the local height of the sample; and a feedback circuit that attempts to keep the cantilever deflection constant by adjusting the voltage applied to the scanner. A well-constructed feedback loop is essential to microscope performance.

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16 With the tip at virtual ground, a selectable bias voltage is applied between the conductive tip and sample (see Figure 8). While scanning in Contact Mode, a linear amplifier with a range of 1pA to 1μA senses the current passing through the sample. By maintaining a constant force between tip and sample, simultaneous topographic and current images are generated, enabling the direct correlation of local topography with electrical properties.

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[28] _{C-AFM Modes of Operation}

Depending on the situation, forces that are measured in AFM include many kinds of force, such as mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces. The AFM can be operated in a number of modes, depending on the application. In general, possible imaging modes are divided into static (also called contact) modes and a variety of dynamic (non-contact or "tapping") modes where the cantilever is vibrated or oscillated at a given frequency.

Figure 9: Conductive Plot of the interatomic force as a function of distance. The regions of three AFM modes are indicated: contact-mode (or static mode) operating in the repulsive region, the Tapping™ and noncontact modes (dynamic modes) operating respectively in attractive/repulsive

and attractive regions [28](a)_.

[29] _{Contact Mode}

In contact mode the tip is in continuous physical contact with the sample surface. Usually the deflection of the cantilever is used as the primary feedback mechanism. The system interacts with the sample via the tip attached to the end of the small cantilever. During imaging, the laser beam is reflected off of the back of the cantilever and the piezo element in the scanner physically moves the probe in the X, Y and Z directions. As the tip scans across the surface, changes in topography of the sample cause changes in the deflection of the cantilever. These changes in the cantilever's deflection are detected by sensing movement of the laser beam on a photodetector. The resulting voltage output ranges from +10V to –10V, depending upon the position of the laser spot on the photodiodes. This position change is read by the feedback loop, which moves the sample in Z to restore the

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18 spot to its original position. The Ramp Mode allows to check the interaction between the cantilever and the sample surface.

[29]_{PeakForce Tapping Mode}

PeakForce Tapping Mode has long dominated the world of AFM. Its main advantage has been the lack of lateral forces that are inherent to contact imaging. Not only can PeakForce Tapping generate data that are equal and often better than TappingMode images, but these data can be obtained reliably by a new user using ScanAsyst.

PeakForce Tapping avoids lateral forces by intermittently contacting the sample. It operates in a non-resonant mode. The PeakForce Tapping oscillation is performed at frequencies well below the cantilever resonance by an oscillating system that combines two benefits for imaging: direct force control (Contact mode) and avoidance of damaging lateral forces. As the tip approaches the sample surface, it will experience long-range van der Waals attraction until dF/dx > k, causing the cantilever to jump into contact with the sample. After contact, the short range repulsive forces dominate the interaction, leading to the peak point at the approaching curve (figure 10). When the tip begins to unload it goes through an adhesion minimum, usually caused by capillary meniscus and finally becomes free. PeakForce Tapping refers to the proprietary control method that uses the individual peak force points as triggering mechanisms to force the z-piezo to retract. The feedback algorithm recognized the local peak force even though the set-point is below the baseline. Operating below the baseline allows operation at very low forces, which in turn is crucial for obtaining high resolution data on soft samples.

Figure 10: Conductive plot of the interatomic force as a function of distance: the motion the cantilever driven by forces as it approaches the sample[29](a)_.

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[29] _{ScanAsyst of PeakForce Tapping Mode}

ScanAsyst uses PeakForce Tapping mechanism, which decouples cantilever response from resonance dynamics, to automatically adjust all critical imaging parameters. The peak force feedback directly controls the interaction force. Direct interaction force control enables the possibility of producing a uniformly optimized feedback loop for all the points of the inhomogeneous sample.

Using a patent pending image correlation algorithm, feedback oscillation is detected and eliminated in a matter of milliseconds. A real-time feedback loop constantly monitors and adjusts the gain to keep the data quality within a predefined noise level. ScanAsyst optimizes the gain according to current sample condition at different locations.

The ScanAsyst algorithm also optimizes the set-point to the minimum force required to track sample surfaces, controls the scan rate, and can automatically lower the z limit if necessary. This results in extremely high-quality images.

PeakForce Tapping only responds to short range interaction. The long range interactions (adhesive and electrostatic forces) are basically ignored for height control. Short range interactions are the key to high-resolution imaging. By consistently controlling the short range interaction forces, PeakForce Tapping enables image quality control with fewer artifacts linked to complication of the tip surface interactions and cantilever dynamics. Advantages

Peak Force Tapping is insensitive to the effects that geometries and therefore has no difficulty reaching for example the bottom of the trench.

Another type of sample where PeakForce Tapping solves problems commonly encountered in TappingMode are nested structures with steep and often high topographies. Images can be flawless and without streaking and tip parachuting. This is again a direct effect of not operating at resonance, which enables direct force control and thus not being affected by the quality Q-factor of the cantilever. Increasing the Q-factor of the micro-cantilever results in an increase in force sensitivity and a reduction in tapping force.

Examples for operation in changing environments include heating and cooling experiments and scanning in fluids. Imaging in fluids is a common and necessary operating mode for an AFM. Besides the obvious benefit of being able to study a sample under low force due to the lack of usually high capillary forces, a lot of studies simply require the sample being immersed in fluids. Examples include the study of biological specimens under physiological relevant conditions or the examination of electrochemical phenomena for corrosion or battery research, to name just a few. It is often interesting or necessary to image samples under fluid or at temperatures above or below room temperature.

ScanAsyst - PeakForce Tapping works well in these environments with several benefits. For one, it is not necessary to tune the cantilever at all, as the cantilever is operated at a fixed frequency. Consequently there is no re-tuning required when the temperature is

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20 changed or when changing from air to fluid operation. With PeakForce Tapping, the system is not being driven at the cantilever resonance, so it is not sensitive to changes in probe resonant frequency and Q. Any background changes caused by temperature or fluid level fluctuations that can influence operation are subtracted in real-time by the ScanAsyst software, allowing imaging forces as low as a few tens of pico-newtons.

[29]_{Point and Shoot}

After imaging a specific area of the sample it is needed to focus on specific points. Then, the Point and Shoot View from the NanoScope Toolbar is used. This tool allows to select specific points on an image for data collection. It marks the location with a different crosshair (+) each time in order to ramp each point and create a force curve for each point.

[29] _{C-AFM & I-V Spectra}

The conducting properties of a sample can be tested by recording the I-V Spectra.

[29] _{NanoScope software permits “IV” measurements (current, I, as a function of voltage,}

V) in a specified location on the sample. In particular, the tip is placed at a certain point of the image and the IV-spectrum is captured by instant contact between the sample and the tip. IV measurements are performed after setting the parameters in the Ramp Parameter list and executing Continuous Ramps. That is, repeated cycling measurements which continue until the user stops them. Finally, the acquired spectra can be captured and saved with the use of Chi760e software as it is proposed in the current work.

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Experimental Set up: C-AFM by Bruker

[30]_{For the execution of the experiments,} _{The Dimension® Icon® Atomic Force}

Microscope with ScanAsyst™ by Bruker in combination with NanoScope 9.3v2 and Chi760e software have been used. The experimental device is shown in Figure 11.

Figure 11: The Dimension® Icon® Atomic Force Microscope with ScanAsyst™ by Bruker- experimental device of Conductive AFM.

Dimension Hardware Description [31]

The main hardware parts of The Dimension application module are the Application

Module AFM Scanner and the Application Module AFM Sensors (Figure 12). Vital part

of the hardware is the Application Module Probe Holder which provides electrical connection from the tip to the application module sensor, while maintaining standard features for contact mode and tapping mode imaging. SSRM, TUNA, PeakForce TUNA and C-AFM share a common probe holder, shown in Figure 13. All Dimension application modules are provided with a tiny wire, the Probe Module Universal Connector (Universal Connector), shown in Figure 14, which is required when a module is in use.

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22 Figure 12: Dimension 3100 Application Module AFM Scanner and Application Module Sensors.

Figure 13: Dimension SSRM, TUNA and C-AFM Probe Holder.

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Experimental procedure

Sample preparation

We were given five samples of metal-organic materials dissolved in ethyl-acetate. The materials are the following (Figure 15):

1. [Au(m-SPhCO2H)]n 2. [Cu(o-SPhCO2H)]n 3. [Ag(o-SPhCO2H)]n 4. [Au(o-SPhCO2H)]n 5. [Cu(m-SPhCO2H)]n

Figure 15: Samples of (1) [Au(m-SPhCO2H)]n, (2) [Cu(o-SPhCO2H)]n,

(3) [Ag(o-SPhCO2H)]n, (4)[Au(o-SPhCO2H)]n, (5)[Cu(m-SPhCO2H)]n

dissolved in ethyl-acetate.

The goal was to study the conductive properties of these metal-organic materials, using the Conductive AFM on them. First and foremost, we had to see their morphology. In order to do that we used the scanning electron microscope (SEM) and made the samples suitable for SEM. We deposited the samples (1) [Au(m-SPhCO2H)]n, (4)[Au(o-SPhCO2H)] and (5)[Cu(m-SPhCO2H)]n on a Si substrate embedded with Au, using pipettes.

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24 For sample (4) [Au(o-SPhCO2H)], with the pipette at the volume of 10μL, we deposited 2 droplets. A spot like “coffee stain” was formed.

For sample (1) [Au(m-SPhCO2H)]n the pipette was set at a volume of 2 μL and we deposited just one droplet at the edge of the sample.

For sample number (5) [Cu(m-SPhCO2H)]n we deposited one droplet at the center of the substrate with the pipette of 2 μL volume. The samples were inserted in SEM setup in order to be morphologically characterized (Figure 16).

Figure 16: The metal-organic samples on the substrates on the cylindrical base, ready to be inserted in SEM.

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Morphological Characterization of the samples – SEM

The morphology of the metal-organic materials [Au(m-SPhCO2H)]n, [Au(o-SPhCO2H)]n and [Cu(m-SPhCO2H)]n was characterized by using scanning electron microscopy (SEM). Their SEM images with different magnifications are shown in the figures below. The used voltage is 5kV.

For Au[(m-SPhCO2H)]n we obtained the pictures of Figure 17. Rods and star-like

particles can be easily noticed.

Figure 17: SEM images of Au[(m-SPhCO2H)]n for different magnifications from 500μm to 10μm.

For Au[(m-SPhCO2H)]n we obtained the pictures of Figure 18. One can easily notice that

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26 why TLD mode for better resolution in sub-micro meter scale is used. The particles look like wires which are bent together, forming bundles. The diameter of these wires is more than 100nm.

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Figure 18: SEM images of Au[(o-SPhCO2H)]n for different magnifications from 400μm to 200nm.

For Cu(m-SPhCO2H)]n we obtained the pictures of Figure 19. It is clearly seen that the

particles form rods either independent or stack together. Different heights of the rods are easy to notice too.

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AFM experimental procedure

After the morphological characterization of the samples using SEM, we proceeded to the imaging and investigation of their conductive properties. We focus our study and data analysis on [Cu(m-SPhCO2H)]n because of its clearly visible rods.

First of all we had to find an appropriate way to use the conductive AFM for the material. We tried to connect the sample we made to the C-AFM setup, so as the current can flow through the material and we figured out an efficient way to do it. In particular, we made the following construction (Figure 20). We glued the sample which was previously inserted in SEM, on a piece of glass and connected it with a piece of wire, using Copper tape. The construction has been placed on the top a base (white part).

Figure 20: The experimental construction inserted in the Conductive AFM.

The whole construction was attached to a magnetic puck provided with the system and connected to the springs of the puck through the red wire. The good connection has always been checked using the multimeter. The stage with the sample on top, was connected to the Application Module sensor with 0 MΩ resistance.

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30 Figure 21: Electrical connection of the experimental construction and the Conductive AFM.

C-AFM requires an electrically conductive tip. The potential for tip wear in Contact Mode is another important consideration in selecting a tip. Standard C-AFM probes include PtIr-coated tips for Contact Mode (SCM-PIC) and CoCr PtIr-coated (magnetized) Si tips (MESP). Less conductive diamond coated tips (DCT-ESP) are an alternative if tip wear is a particular concern.

Next we mounted the tip on the C-AFM Probe Holder which was afterwards bonded the Application Module and Sensors (Figure 22).

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31 Figure 23: C-AFM with all the parts ready for measurements.

After preparing the set up for the measurement, NanoScope 9.3v2 software was activated. We chose the Electrical & Magnetic Experimental Category, the Application Module from the Experimental groups and finally the Conductive AFM (C-AFM) Experiment. Laser was aligned in order to have the maximum signal and then we Navigated so as to find surface appropriate for the measurement and execution of the imaging.

Imaging in Contact Mode was proved to be not appropriate for the samples because they were destroyed. These materials are metal-organic, soft and thus very sensitive. This is why we switched to PeakForce Tapping Mode for the imaging.

Execution of IV curves

After the image was finished, we used the Point and Shoot window. We placed the markers where we wanted to execute IV curves and executed a ramp on each of them. We set the Potentiostat Ch760e and chose the Cyclic Voltammetry technique for the IV curves.

In order to record the IV curves using AFM, the current I output of the experimental setup was connected to the Dimension® Icon® Atomic Force Microscope. The main parameters were set as following:

Table 1: The main parameters of Potentiostat Ch760e.

Parameter value

Initial potential E 0

High E/ Low E 2V/-2V or 1V/-1V

Scan rate high E/20

Sample interval Scan rate/10

Quiet time 0

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Calculations

AFM provides a 3D profile of surface on nanoscale by measuring the force exerted from the sharp probe to the surface at a very short distance (0.2-10nm). The probe is placed on the end of a spring-like cantilever. The amount of force between the probe and the sample depends on the distance between them and on the spring constant k (stiffness) of the cantilever. Thus, this force can be described by Hooke’s law:

F=-kx

Where k is the spring constant and x the cantilever deflection.

Figure 24: a)Spring depiction of cantilever b)SEM image of triangular SPM cantilever with probe(tip)[32]_.

Based on Hooke’s law, the total force exerted on the sample from the cantilever is expressed as: 𝑭[𝒏𝑵] = = 𝑺𝒑𝒓𝒊𝒏𝒈 𝒌𝒐𝒏𝒔𝒕𝒂𝒏𝒕 [𝑵 𝒎] 𝒙 𝑫𝒆𝒇𝒍𝒆𝒄𝒕𝒊𝒐𝒏 𝒔𝒆𝒕𝒑𝒐𝒊𝒏𝒕 [𝑽]𝒙 𝑫𝒆𝒇𝒍𝒆𝒄𝒕𝒊𝒐𝒏 𝑺𝒆𝒏𝒔𝒊𝒕𝒊𝒗𝒊𝒕𝒚 [ 𝒏𝒎 𝑽 ] = (𝒌)𝒙(𝒑)𝒙(𝑺) (1) The above equation has been used for the calculation of the total force.

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Vital Experimental parameters

Deflection set-point

[33]_{Set-point is a vital parameter that controls the scanning of the sample surface during}

the scanning probe microscopy. In Contact mode the point refers to the deflection set-point of the cantilever, and is the cantilever deflection (usually given in Volts or nanoAmps) at which the AFM will scan and is determined by the operator. Since the force the cantilever exerts on the sample is directly proportional to the deflection of the cantilever via Hooke’s Law, it is possible for the operator to control the amount of force being applied to the sample surface while imaging. Increasing the set-point will result in increasing the imaging force while reducing the set-point will result in reduced imaging forces. If the set-point is too low, then the tip may not be able to track the surface properly and could come out of feedback if not enough force is applied. However, if too much force is applied to the surface then the tip could damage the sample surface, or in time damage to the tip itself can result in image artifacts e.g. double tipping.

Deflection sensitivity

[34]_{The detector's sensitivity is calibrated to convert volts measured on the photodetector}

to nanometers of motion. The calibration is performed by measuring a force curve on an "infinitely stiff" surface. The "infinitely stiff" surface is chosen relative to the cantilever such that the cantilever does not indent the sample during the force curve measurement. Once the force curve of photodetector signal vs. piezo movement is collected, the slope of the repulsive portion of the wall is then calculated. This is the deflection sensitivity.

Spring constant

AFM cantilever material typically consists of either silicon or silicon nitride, where silicon nitride is reserved for softer cantilevers with lower spring constants. The dimensions of the cantilever are very important as they dictate its spring constant or stiffness. This stiffness is fundamental to governing the interaction between the AFM cantilever tip and the sample surface and can result in poor image quality if not chosen carefully. The relationship between the cantilever’s dimensions and spring constant, k, is defined by the equation:

𝒌 =

𝑬𝒘𝒕 𝟑

𝟒𝑳𝟑

(2)

where w = cantilever width; t = cantilever thickness; L = cantilever length

and E = Young’s modulus of the cantilever material. Nominal spring constant values are typically provided by the vendor when buying the probes, but there can be significant variation in the actual values.

(34)

34 Possible limitations in the stiffness of the probes when imaging soft samples maybe occur. Thus, the selection of the appropriate AFM tip to measure the conductivity is also of vital importance.

In the current work we used three types of AFM tips: RMN-25PT300B, PFTUNA and

AD-40-SS by ADAMA INNOVATIONS by Bruker for room-temperature measurements.

[35] _{RMN-25PT300B is a solid metal probe of 18 N/m spring constant and resonant}

frequency 20 kHz. RockyMountain Nanotechnology (RMN) probes are uniquely constructed from pure platinum and placed on a standard AFM probe sized ceramic substrate. Solid metal probes offer excellent conductivity and suffer no thin-film adhesion problems that occur with metal-coated silicon probes. These probes also have a tip radius (< 20 nm) which is difficult to routinely obtain by standard AFM probe processing methods. They are available in a range of spring constants.

For our calculations with RMN-25PT300B we used the value of probe spring constant of 𝒌 = 𝟏𝟖𝑵

𝒎 and 𝒓𝒕𝒊𝒑 ≈ 𝟏𝟗𝒏𝒎 as the tip radius.

[36]_{AD-40-SS by Bruker is a Super Sharp Conductive Single Crystal Diamond Probe, 40}

N/m, 180 kHz, <5 nm ROC, 5-Pack. The Super sharp tip for highest resolution combined with high spring constant is suitable for higher force and harder sample applications including:

-Topography imaging in PeakForce Tapping, Tappingmode, and contact mode. -Electrical characterization with PeakForce TUNA, TUNA, CAFM. -Nanomechanics with PeakForce QNM, FASTForce Volume, contact resonance, nanoindentation.

The model has 𝑟𝑡𝑖𝑝 < 5𝑛𝑚. For our calculations we considered that 𝒓𝒕𝒊𝒑 ≈ 𝟒𝒏𝒎 and the spring constant was experimentally measured/calibrated.

[37]_{PFTUNA probes of 0.4 N/m spring constant and resonant frequency 40 kHz combine}

the low spring constant and high sensitivity of a nitride cantilever with a sharp, electrically conductive tip. When used with Bruker’s exclusive PeakForce TUNA mode, they enable an unprecedented level of high resolution electrical characterization on fragile samples. According to the manufacturers the radius of the tip is 𝑟𝑡𝑖𝑝(𝑛𝑜𝑚) = 25𝑛𝑚, so for our calculations we will consider that 𝑟_𝑡𝑖𝑝 = 25𝑛𝑚.

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35

Conductivity calculation

In Contact mode, current flows through the sample once the tip touches it. The conductivity of the sample at specific locations on the particles has been measured with the Conductive AFM. Point contact IV curves have been measured for many areas of the sample.

Then, the Resistance can be easily calculate because it corresponds to the inverse of the slope of the linear fitting of the IV curves, according to Ohm’s law. That is,

𝑰 =

𝑽 𝑹

→ 𝑹 =

𝑽 𝑰

=

𝟏 𝒃

[Ω]

(3)

[38]_{Τhe electrical resistance of a conductor (or wire) is proportionally greater the longer it is and} proportionally less the greater is its cross-sectional area. In other words, the resistance of a conductor is directly proportional to the length (L) of the conductor, that is: R ∝ L, and inversely proportional to its area (A), R ∝ 1/A.

𝑹 ∝

𝑳

𝑨 (4)

But as well as length and conductor area, we would also expect the electrical resistance of the conductor to depend upon the actual material from which it is made, because different conductive materials, copper, silver, aluminium, etc all have different physical and electrical properties. Thus we can convert the proportionality sign (∝) of the above equation into an equals sign simply by adding a “proportional constant” into the above equation giving:

𝑹 = 𝝆

𝑳

𝑨

[𝜴]

(5)

Where R is the resistance in ohms (Ω), L is the length in metres (m), A is the area in square

metres (m2_{), and where the proportional constant ρ (the Greek letter “rho”) is known}

as Resistivity.

The electrical resistivity of a particular conductor material is a measure of how strongly the material opposes the flow of electric current through it. This resistivity factor enables the resistance of different types of conductors to be compared to one another at a specified temperature according to their physical properties without regards to their lengths or cross-sectional areas. Thus the higher the resistivity value of ρ the more resistance and vice versa.

While both the electrical resistance (R) and resistivity (or specific resistance) ρ, are a function of the physical nature of the material being used, and of its physical shape and size expressed by its length (L), and its sectional area (A), Conductivity, or specific conductance relates to the ease at

which electric current con flow through a material. Conductance (G) is the reciprocal of

resistance (1/R) with the unit of conductance being the Siemens (S).

Conductivity, σ (Greek letter sigma), is the reciprocal of the resistivity. That is 1/ρ and is measured in siemens per metre (S/m). Since electrical conductivity σ = 1/ρ, the previous expression for electrical resistance, R can be rewritten as:

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36

𝑹 = 𝝆

𝑳 𝑨

[𝜴] and 𝝈 =

𝟏 𝝆

[

𝟏 𝜴

]

(6)

𝑹 =

𝑳 𝝈𝑨

[𝜴]

(7)

Then we can say that conductivity is the efficiency by which a conductor passes an electric current or signal without resistive loss. Therefore a material or conductor that has a high conductivity will have a low resistivity, and vice versa

𝝈 =

𝑳

𝑹𝑨

[

𝑺

𝒎

]

(8)

For our calculations we considered the height of the rod H [m] as the path length. Moreover, we considered the sectional area to be the area of the tip 𝑨 = 𝝅𝒓𝟐, where r is the radius of the tip.

Overall, the conductivity could be sufficiently approximated with the formula:

𝝈 =

𝑯𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒕𝒉𝒆 𝒓𝒐𝒅 𝑹𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆∗𝒕𝒊𝒑_{𝒂𝒓𝒆𝒂}

=

𝑯 𝑹∗𝑨

=

𝑯 𝑹∗𝝅𝒓𝟐

(9)

(37)

37

Measurements

Before imaging, the deflection sensitivity was calibrated. This is the sensitivity of the amplitude oscillation measurement. The cantilever was ramped over a very stiff surface taking care not to contain any particles. Accordingly, all downward piezoelectric motion was equal to cantilever bending. From the graph that came up, the inverse of the measured slope corresponded to the deflection sensitivity in [nm/V].

We started our measurements using the tip of RMN-25PT300B. The force exerted on the sample was found to be F=333.72 nN according to equation (1) and using the parameters of Table 2.

Table 2: Parameters for the calculation of the total Force.

Spring Constant k[N/m] Deflection sensitivity S [nm/V] Deflection setpoint P [V] Force F [nN] 18 185.4 0.1 333.72

With the AFM imaging process, we obtained the image of a single rod (Figure 22) with Height of H=692.937 nm close to the spot of measurement and Width of W=1.969 μm around the spot we executed the IV curve.

Figure 25: AFM image of [Cu(m-SPhCO2H)]n rod. The red cross specifies the spot of the

(38)

38 Setting the applied potential from Emin=-1V to Emax=1V, the following IV curve was recorded. -1 0 1 -2,0x10-9 -1,5x10-9 -1,0x10-9 -5,0x10-10 0,0 5,0x10-10 1,0x10-9 1,5x10-9 2,0x10-9 2,5x10-9 measurement on particle Curr en t ( Ampe re ) Potential (Volts)

Figure 26: IV curve of [Cu(m-SPhCO2H)]n rod for Emin=-1V to Emax=1V and Force=333.72 nN.

Changing the range of the applied potential from Emin=-2V to Emax=2V for the same applied force, the following IV curve was recorded.

Figure 27: IV curve of [Cu(m-SPhCO2H)]n rod for Emin=-2V to Emax=2V and Force=333.72 nN.

-2 -1 0 1 2 -4,0x10-9 -2,0x10-9 0,0 2,0x10-9 4,0x10-9 6,0x10-9 measurement on particle Curr en t ( Ampe re ) Potential (Volts)

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39 In the first case for Emin=-1V to Emax=1V linear behavior of the IV curve is obtained for the whole range. For the potential range of Emin=-2V to Emax=2V, the IV curve seems to exhibit linear behavior only at negative bias. The curves were fitted linearly to obtain the data of Table 3.

Table 3: Linear fitting parameters

Potential Range [V] slope b[A/V] Intercept α[A]

from -1 to 1 1.845*10-9_±8.196*10-12 _7.169*10-11_±4.743*10-12

from -2 to 0 2.277*10-9_±1.964*10-11 _2.815*10-10_±2.268*10-11

The Resistance was calculated behaving to equation (3) to finally obtain the value for Conductivity at the specific point, following equation (9) (Table 4).

Table 4: Parameters for the calculation of Conductivity Potential Range [V] Tip Radius [nm] Tip area [nm]2 Rod Height [nm] Slope b[A/V] Resistance [MΩ] Conductivity [S/m] Conductivity [S/cm] -1 to 1 2 1133.54 692.937 1.845*10-9 ₅₄₂ _1.128 _1.128*10-2 -2 to 2 2 1133.54 692.937 2.277*10-9 ₄₃₉ _1.392 _1.392*10-2

The linear behavior of the IV curves, reveals the inherent metallic character of the [Cu(m-SPhCO2H)]n. The conductivity per cm is found to be of the 10-2 order of magnitude in both cases.

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40 We continued our measurements using the tip of AD-40-SS. Before imaging, the

deflection sensitivity 𝑆 and the spring constant 𝑘 were calibrated. Force set-point was set at 𝒑=0.05V. In this case the Force exerted on the sample was found to be approximately F=113.18 nN, according to equation (1) and the parameters of Table 5.

Spring Constant k[N/m] Deflection sensitivity S [nm/V] Deflection setpoint P [V] Force F [nN] 13.995 161.74 0.05 113.18

With the AFM imaging process for scan size of 10μm, we obtained the image of a single rod with different heights (Figure 28).

Figure 28: AFM image of [Cu(m-SPhCO2H)]n rod for 10μm scan size. The black and blue

crosses specify the spots of the measurement of the current.

Applying potential from Emin=-2V to Emax=2V, the following IV curves were recorded for two different spots on the rod (black and blue respectively).

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41 -2 -1 0 1 2 -3,0x10-9 -2,0x10-9 -1,0x10-9 0,0 1,0x10-9 2,0x10-9 3,0x10-9 4,0x10-9 5,0x10-9 6,0x10-9 7,0x10-9 8,0x10-9 spot 1 spot 2 Curr en t(Amp er e) Potential(Volt)

Figure 29: IV curve of [Cu(m-SPhCO2H)]n rod for Emin=-2V to Emax=2V, Force =113.18nN and

10μm scan size.

For the spot 2 (blue cross and blue line) we notice two regions with different behavior. At negative bias the IV curve is linear, while at positive bias the current through the rod is zero. This means that the rod suddenly blocks the current at positive bias and it thus has zero conductivity. If we focus only on the linear region from Emin=-2V to Emax=0V, we can fit the curve linearly as below:

-2 -1 0 1 2 -2,0x10-9 -1,8x10-9 -1,6x10-9 -1,4x10-9 -1,2x10-9 -1,0x10-9 -8,0x10-10 -6,0x10-10 -4,0x10-10 -2,0x10-10 0,0 2,0x10-10 spot 2 linear fitting [Cu(m-SPhCO2H)]n Current vs Potential Curr en t(Amp er e) Potential(Volt)

Figure 30: IV curve of spot 2 for Emin=-2V to Emax=0V, Force=113.18nN and 10μm scan size.

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42 For the spot 1 (Black cross and line line) even though at positive bias the IV curve seems to follow an exponential growth alike to the Shockley diode behavior, the IV curve in the potential range from -2V to 0V is linear and can be fitted linearly as well (figure 31).

-2,0 -1,5 -1,0 -0,5 0,0 -2,5x10-9 -2,0x10-9 -1,5x10-9 -1,0x10-9 -5,0x10-10 0,0 spot 1 linear fitting Curr en t ( A) Potential (Volts)

Figure 31: IV curve of spot 1 for Emin=-2V to Emax=0V, Force=113.18nN and 10μm scan size.

Red line is the linear fitting line.

After fitting the IV curves, the data of Table 6 were obtained. Table 6: Linear fitting parameters

from -2 to 0 1.060*10-9_±1.514*10-11 _3.054*10-10_±1.887*10-11

from -2 to 0 1.292*10-9_±4.031*10-12 _7.881*10-11_±4.655*10-12

Table 7: Parameters for the calculation of Conductivity Potential Range [V] Tip Radius [nm] Tip area [nm]2 Rod Height [nm] Slope b[A/V] Resistance [MΩ] Conductivity [S/m] Conductivity [S/cm] -2 to 0 4 50.24 226.101 1.060*10-9 _943.40 _4.772 _4.772*10-2 -2 to 0 4 50.24 225.823 1.292*10-9 _773.99 _5.807 _5.807*10-2

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43 Focusing more than 10μm for scan size of 3μm, we obtained a new image of the single rod (Figure 32).

Figure 32: AFM image of [Cu(m-SPhCO2H)]n rod for 3μm scan size. The black and blue crosses

specify the spots of the measurement of the current.

The parameters for the force calculation (Table 8) remained the same as in the scan of 10 μm and result in force of 𝐅 = 𝟏𝟏𝟑. 𝟏𝟖 𝐧𝐍 as well.

Measurements of the current were executed at two spots of different height. The Height at the black spot on the left is H1= 256.983 nm and at the blue point on the right is H2=235.147 nm. For the applied potential from Emin=-2V to Emax=2V, the following IV curves were recorded at both spots (Figure 33).

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44 -2 -1 0 1 2 -2,0x10-11 -1,5x10-11 -1,0x10-11 -5,0x10-12 0,0 5,0x10-12 left right Curr en t ( Ampe re ) Potential (Volts)

Figure 33: IV curve of [Cu(m-SPhCO2H)]n rod for Emin=-2V to Emax=2V, Force=226.36nN and

3μm scan size.

Similar linear behavior of the IV curves is obtained for both spots. Interestingly, current flows through the rod only with negative values. After linearly fitting the IV curves, the data of Table 9 were obtained.

Table 9: Linear fitting parameters Spot of

measurement

Potential

Range [V] slope b[A/V] Intercept α[A]

Black from -2 to 2 3.615*10-12_±3.779*10-14 _-1.226*10-11_±4.364*10-14

Blue from -2 to 2 3.823*10-12_±3.765*10-14 _-1.171*10-11_±4.348*10-14

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45 Table 10: Parameters for the calculation of Conductivity

Spot of measurement Potential Range [V] Tip Radius [nm] Tip area [nm]2 Rod Height [nm] Slope b[A/V] Resistance [GΩ] Conductivity [S/m] Conductivity [S/cm] Black -2 to 2 4 50.24 256.983 3.615*10-12 _276.60 _1.849*10-2 _1.849*10-4 Blue -2 to 2 4 50.24 235.147 3.823*10-12 _261.58 _1.789*10-2 _1.789*10-4

For scan size of 10 μm, the IV curves show not only linear behavior but also exponential. For the linear region, the conductivity per cm is found to be of the 10-2 S/cm order of magnitude and the exponential behavior could resemble the Shockley diode. Surprisingly, the current seems to be blocked current at positive bias for one of the two spots(blue). However, the 3μm scan size at two different spots showed that current can flow through them. Independently of the spot of measurement across the rod, the current exhibits an almost linear behavior. The conductivity per cm, in this case, is found to be two orders of magnitude less (10-4 S/cm).

We continued our measurements using the tip of PFTUNA. After the calibration and the collection of the data for the calculation of the total force (Table 11), it was found to approximately F=113.18 nN, according to equation (1).

With the AFM imaging process for scan size of 10μm, we obtained the image of a single rod with average height of 𝑯 = 𝟔𝟒𝟏. 𝟎𝟗𝟐𝒏𝒎 (Figure 34).

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46

Figure 34: AFM image of [Cu(m-SPhCO2H)]n rod for 10μm scan size.

For the applied potential from Emin=-2V to Emax=2V the following IV curve was recorded. This IV curve exhibited time dependence because it was evolving in a different way for every cycle of measurement. For seven successive cycles, the time evolvement of IV curve was the following (Figure 35).

-2 -1 0 1 2 -1,0x10-9 0,0 1,0x10-9 2,0x10-9 3,0x10-9 1st_cycle 2nd_cycle 3rd_cycle 4th_cycle 5th_cycle 6th_cycle 7th_cycle Curr en t(Amp er e) Potential(Volt)

Figure 35: IV curves of [Cu(m-SPhCO2H)]n rod for Emin=-2V to Emax=2V, Force=7.393nN and

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47 As the number of cycles increases, more and more current at negative and positive bias seems to flow through the rod. The IVs at negative bias could be considered linear, while at positive they seem to follow exponential behavior. Isolating the IV curve of the 7th cycle and choosing the linear region at negative bias we linearly fitted the curve to obtain the following data (Table 12).

from -2 to 0 5.314*10-10_±6.903*10-12 _1.212*10-10_±7.970*10-12

Table 13: Parameters for the calculation of Conductivity Potential Range [V] Tip Radius [nm] Tip area [nm]2 Rod Height [nm] Slope b[A/V] Resistance [GΩ] Conductivity [S/m] Conductivity [S/cm] -2 to 0 25 1962.5 641.092 5.314*10-10 _1.88 _0.174 _1.74*10-3

The deflection set-point was increased from 𝒑𝟏= 𝟎. 𝟎𝟓𝑽 to 𝒑𝟐= 𝟎. 𝟏𝑽 and then decreased to 𝒑_𝟑= 𝟎. 𝟎𝟑𝑽. Different values for total Force came up (Table 14).

Spring Constant k[N/m] Deflection sensitivity S [nm/V] Deflection setpoint P [V] Force F [nN] 0.687 215.250 0.1 14.786 0.687 215.250 0.03 4.436

IV curves for both cases are shown in Figure 36, in comparison with the IV curve with the set-point at 𝒑_𝟏= 𝟎. 𝟎𝟓𝑽.

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48 -2 -1 0 1 2 -1,0x10-9 0,0 1,0x10-9 2,0x10-9 3,0x10-9 0.1 Volt 0.05 Volt 0.03 Volt Curr en t(Amp er e) Potential(Volt)

Figure 36: IV curves of [Cu(m-SPhCO2H)]n rod for set-points 𝑝₂ = 0.1𝑉, 𝑝₁= 0.05𝑉 and

𝑝3 = 0.03𝑉.

However, not much current seems to flow through the particle after the set-point variance. In order to check if the rod conducts after the current variance, we linearly fitted the curves (green and red) (Table 15).

from -2 to 2 3.975*10-12_±4.002*10-14 _-1.224*10-11_±4.621*10-14

from -2 to 2 3.769*10-12_±4.103*10-14 _-1.216*10-11_±4.737*10-14

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Total Force [N] Potential Range [V] Tip Radius [nm] Tip area [nm]2 Rod Height [nm] Slope b[A/V] Resistance [GΩ] Conductivity [S/m] Conductivity [S/cm] 14.786 -2 to 2 25 1962.5 641.092 3.975*10-12 _251.57 _1.299*10-3 _1.299*10-5 4.436 -2 to 2 25 1962.5 641.092 3.769*10-12 _265.32 _1.231*10-3 _1.231*10-5

For set-point at p1=0.05V the IV curve showed a unique behavior with different evolvement in time. Linear behavior occurred only at negative bias where the conductivity found to be of the order of magnitude of 10-3 S/cm. In contrast, both at lower and higher force set-points, the conductivity was found to be two orders of magnitude lower.

One more image has been recorded, using the same PFTUNA (Figure 37).

On the specific spot (red cross) the force was calculated (Table).

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50 First of all, we made a single measurement of the current, then we switched off the light and finally we switched it on again setting it manually to the full. We did measurements at all the three stages and obtained the following IV curves (Figure 38):

-2 -1 0 1 2 -6,0x10-10 -4,0x10-10 -2,0x10-10 0,0 2,0x10-10 4,0x10-10 initial dark light Curr en t ( Ampe re ) Potential (Volt)

Figure 38: IV curves of [Cu(m-SPhCO2H)]n rod for Force=3.208nN in three successive stages of

light supply.

By linear fitting the three IV curves, we obtain the Resistance and later the Conductivity: Table 18: Linear fitting parameters

Light supply Potential Range [V] slope b[A/V]

Initial from -2 to 2 2.335*10-10

Dark from -2 to 2 2.325*10-10

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Light supply Potential Range [V] Tip Radius [nm] Tip area [nm]2 Rod Height [nm] Slope b[A/V] Resistance [GΩ] Conductivity [S/m] Conductivity [S/cm] Initial -2 to 2 25 1962.5 423.132 2.335*10-10 _4.283 _5.034*10-2 _5.034*10-4 Dark -2 to 2 25 1962.5 423.132 2.325*10-10 _4.301 _9.273*10-2 _9.273*10-4 Light on -2 to 2 25 1962.5 423.132 2.274*10-10 _4.398 _4.902*10-2 _4.902*10-4

From the imaging we also measured the heights of the rods: 𝒉𝒍𝒆𝒇𝒕 𝒓𝒐𝒅= 516.134nm , 𝒉_{𝒓𝒊𝒈𝒉𝒕 𝒓𝒐𝒅} = 795.401nm, 𝒉_{𝒊𝒏 𝒃𝒆𝒕𝒘𝒆𝒆𝒏} = 478.959nm and recorded the IV curves at different spots.

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52 -2 -1 0 1 2 -6,0x10-10 -4,0x10-10 -2,0x10-10 0,0 2,0x10-10 4,0x10-10 b₇=2.35x10-10 b₆=2.41x10-10 b₅=2.50x10-10 b₄=2.41x10-10 b₃=2.33x10-10 b₂=2.35x10-10 Curr en t(Amp er e) Potential(Volt) b₁=2.34x10-10

Figure 40: IV curves of [Cu(m-SPhCO2H)]n rod for Force=3.208nN at seven different spots of

measurements.

By linear fitting the three IV curves, we obtain the Resistance and later the Conductivity:

Spot Potential Range [V] slope b[A/V]

1 from -2 to 2 2.34*10-10 2 from -2 to 2 2.35*10-10 3 from -2 to 2 2.33*10-10 4 from -2 to 2 2.41*10-10 5 from -2 to 2 2.50*10-10 6 from -2 to 2 2.41*10-10 7 from -2 to 2 2.35*10-10

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Spot Tip Radius [nm] Tip area [nm]2 Rod Height [nm] Resistance [GΩ] Conductivity [S/m] Conductivity [S/cm] 1 25 1962.5 478.959 4.274 5.710*10-2 _5.710*10-4 2 25 1962.5 478.959 4.255 5.736*10-2 _5.736*10-4 3 25 1962.5 516.134 4.292 6.128*10-2 _6.128*10-4 4 25 1962.5 516.134 4.149 6.339*10-2 _6.339*10-4 5 25 1962.5 795.401 4.000 1.013*10-2 _1.013*10-4 6 25 1962.5 795.401 4.149 9.769*10-2 _9.769*10-4 7 25 1962.5 516.134 4.255 6.181*10-2 _6.181*10-4

Two new rods were captured with heights 𝒉𝒍𝒆𝒇𝒕 𝒓𝒐𝒅= 𝟒𝟐𝟓. 𝟖𝟗𝟐𝐧𝐦 and 𝒉𝒓𝒊𝒈𝒉𝒕 𝒓𝒐𝒅 = 𝟏𝟎𝟑𝟏. 𝟖𝟓𝟐𝒏𝒎 (Figure 41).

Maintaining the Force of F=3.280nN, the IV curves have been recorded and linear fitted (Figure 42):

(54)

54 -2 -1 0 1 2 -6,0x10-10 -4,0x10-10 -2,0x10-10 0,0 2,0x10-10 4,0x10-10 b₃=2.34x10-10 b₂=2.20x10-10 b₁=2.17x10-10 point 1 left point 2 left point 1 right Curr en t(Amp er e) Potential(Volt)

Figure 42: IV curves of [Cu(m-SPhCO2H)]n rod for Force=3.208nN at three different spots of

measurements.

By linear fitting the three IV curves, we obtain the Resistance and later the Conductivity:

1 from -2 to 2 2.17*10-10

2 from -2 to 2 2.20*10-10

3 from -2 to 2 2.34*10-10

Table 24: Parameters for the calculation of Conductivity Spot Tip Radius

[nm] Tip area [nm]2 Rod Height [nm] Resistance [GΩ] Conductivity [S/m] Conductivity [S/cm] 1 25 1962.5 425.892 4.608 4.710*10-2 _4.710*10-4 2 25 1962.5 425.892 4.545 4.775*10-2 _4.775*10-4 3 25 1962.5 1031.852 4.274 1.230*10-2 _1.230*10-4

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55 One more measurement captured two rods with heights 𝒉_{𝒍𝒆𝒇𝒕 𝒓𝒐𝒅}= 𝟒𝟎𝟎. 𝟑𝟐𝟐𝐧𝐦 and 𝒉_{𝒓𝒊𝒈𝒉𝒕 𝒓𝒐𝒅} = 𝟗𝟎𝟔. 𝟐𝟓𝟕𝒏𝒎 (Figure 43).

-2 -1 0 1 2 -4,0x10-10 -2,0x10-10 0,0 2,0x10-10 4,0x10-10 b 2=2.05x10 -10 b 1=2.05x10 -10 left right Curr en t(Amp er e) Potential(Volt)

Figure 44: IV curves of [Cu(m-SPhCO2H)]n rod for Force=3.208nN at two different spots of

(56)

56 Table 25: Parameters for the calculation of the total Force.

By linear fitting the IV curves, we obtain the Resistance and later the Conductivity: Table 26: Linear fitting parameters

1 from -2 to 2 2.05*10-10

2 from -2 to 2 2.05*10-10

[nm] Tip area [nm]2 Rod Height [nm] Resistance [GΩ] Conductivity [S/m] Conductivity [S/cm] 1 25 1962.5 400.322 4.878 4.182*10-2 _4.182*10-4 2 25 1962.5 906.257 4.878 9.467*10-2 _9.467*10-4

Continuing our study, we did the imaging for a different rod (Figure 45) applying force of 𝐅 = 𝟏. 𝟖𝟖𝟎 𝐧𝐍.

(57)

57 IV Curves for 9 different spots on the rod are shown below.

-2 -1 0 1 2 -3x10-10 -2x10-10 -1x10-10 0 1x10-10 2x10-10 b_9=1.02x10-10 b_8=1.16x10-10 b_7=1.29x10-10 b_6=1.31x10-10 b_5=1.30x10-10 b_4=1.30x10-10 b_3=1.26x10-10 b_2=1.25x10-10 b_1=1.30x10-10 1 2 3 4 5 6 7 8 9 Curr en t(Amp er e) Potential(Volt)

Figure 46: IV curves of [Cu(m-SPhCO2H)]n rod for Force=1.880nN at nine different spots of

measurements.

For the calculation of the conductivity the maximum height (max height= 711.170nm) of the rod has been used in all cases.

By linear fitting the IV curves, we obtain the Resistance and later the Conductivity: Table 29: Linear fitting parameters

1 from -2 to 2 2.05*10-10 2 from -2 to 2 1.25*10-10 3 from -2 to 2 1.26*10-10 4 from -2 to 2 1.30*10-10 5 from -2 to 2 1.30*10-10 6 from -2 to 2 1.31*10-10 7 from -2 to 2 1.29*10-10

(58)

58

8 from -2 to 2 1.16*10-10

9 from -2 to 2 1.02*10-10

[nm] Tip area [nm]2 Rod Height [nm] Resistance [GΩ] Conductivity [S/m] Conductivity [S/cm] 1 25 1962.5 711.17 7.692 4.110*10-2 _4.110*10-4 2 25 1962.5 711.17 8.000 4.530*10-2 _4.530*10-4 3 25 1962.5 711.17 7.937 4.566*10-2 _4.566*10-4 4 25 1962.5 711.17 7.692 4.711*10-2 _4.711*10-4 5 25 1962.5 711.17 7.692 4.711*10-2 _4.711*10-4 6 25 1962.5 711.17 7.634 4.747*10-2 _4.747*10-4 7 25 1962.5 711.17 7.752 4.675*10-2 _4.675*10-4 8 25 1962.5 711.17 8.621 4.203*10-2 _4.203*10-4 9 25 1962.5 711.17 9.804 3.696*10-2 _3.696*10-4

Additionally, on the 8th cross we varied the value of the set-point from 0.01V to 0.2V and received the following responses:

-2 -1 0 1 2 -3x10-10 -2x10-10 -1x10-10 0 1x10-10 2x10-10 b_6=1.05x10-10 b_5=1.10x10-10 b_4=1.12x10-10 b_3=1.12x10-10 b_2=1.16x10-10 b_1=1.10x10-10 0.01V 0.05V 0.06V 0.08V 0.1V 0.2V Curr en t(Amp er e) Potential(Volt)

Figure 47: IV curves of [Cu(m-SPhCO2H)]n rod for different values of force on the 8th spot of

(59)

59 The maximum conductivity occurs for set-point at value of 0.05V(slope b2=1.16*10-10 [A/V] ) and thus Force= 𝟏. 𝟖𝟖𝟎 𝐧𝐍. The corresponding value for the conductivity is 4.203*10-4 _S/cm.

On the 9th cross for two different set-points, almost the same IV curve was recorded.

-2 -1 0 1 2 -2x10-10 -1x10-10 0 1x10-10 2x10-10 b_2=1.02x10-10 b_1=1.04x10-10 0.01V 0.05V Curr en t(Amp er e) Potential(Volt)

Figure 48: IV curves of [Cu(m-SPhCO2H)]n rod for different values of force on the 9th spot of

measurements.

The above IV curves show also a linear behavior. The conductivity per cm is found to be of the 10-4 order of magnitude, as it can be calculated from the slope. The result is the same independently of spot of the rod which we executed the measurement and the voltage applied.

Study of the conductive properties of metal-organic materials using Conductive AFM

MSc Physics and Astronomy

Advanced Matter and Energy Physics

AMEP Lab Project

“Study of the conductive properties of metal-organic materials

using Conductive AFM”

Alexandra Zeltsi

Student ID Nr. UvA: 12287350

Student ID Nr. VU: 2656959

6 ECTS

1

Examiner: dr. Esther Alarcón Lladó

2

Examiner: dr. Elizabeth von Hauff

Contents

Calculations

𝒌 =

𝑰 =

→ 𝑹 =

=

[Ω]

𝑹 ∝

𝑹 = 𝝆

[𝜴]

𝑹 = 𝝆

[𝜴] and 𝝈 =

[

]

𝑹 =

[𝜴]

𝝈 =

[

]

𝝈 =

=

=