Meniscus confined electrochemical fabrication of copper nanostructures

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Master Physics and Astronomy

Science for Energy and Sustainability

Master Thesis 60 EC

Conducted between 05-11-2018 and 03-10-2019

Meniscus Confined Electrochemical

Deposition for 3D microprinting

by

Rosa Rougoor

UvA Student ID: 10610901

Supervisor

Dr. Esther Alarc´

on Llad´

o

Daily supervisor

MSc. Mark Aarts

Second Examiner

Prof. dr. Erik Garnett

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Abstract

By rationally designing materials on nano- and microscale, it is possible to tune material properties. These so-called metamaterials find their use in a lot of different fields, among which the manipulation of optical properties of materials used in photovoltaic applications. Fabrication of these materials is a hot-topic. In this work meniscus confined electrochemical deposition (MCED) of metals is experimentally investigated, a promising method for printing 3D structures at micro or nanoscale. It is shown how the pipettes used for MCED can be produced in-house and how the size of the pipette nozzle can be determined by an in-situ resistance measurement. The growth of copper wires was used as a test system to investigate the effect of potential and retraction speed on the growth process. For more negative potentials, the rate of electrochemical deposition is higher and therefore faster retraction speeds can be used. At one certain potential, there is a window of retraction speeds that enables stable growth. Within this window, the higher retraction speeds result in structures with curved edges as a result of higher evaporation rates. Lower retraction speeds result in more solid structures. Furthermore, high retraction speeds result in wires with a smaller radius. Next, the nucleation mechanism in a MCED system has been investigated, showing a transition from progressive to instantaneous nucleation mechanism. This indicates a system where electrochemical current is limited by mass transfer. Lastly, preliminary research has been done on the use of MCED for the fabrication of functional materials.

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1

Introduction to 3D micro printing

By rationally designing materials on nano- and microscale, it is possible to tune material properties. An example is a mechanical metamaterial that shows unusual deformation behaviour, that ordinary materials cannot show [1]. Next to mechanical properties, also optical properties of materials can be tuned by careful design. By changing the geometry of nanostructures for example, light absorption of a material can be enhanced. This opens up design possibilities to increase efficiencies of photovoltaic (PV) cells [2, 3]. Another example is the increase of light emission directivity in optoelectronic devices by careful design of materials on both nano- and microscale [4]. Since the use of these rationally designed materials is a hot topic in different fields, the fabrication of them is as well.

A 3D printing method is ideal for the fabrication of nano- or microscale structured materials, for multiple reasons. Firstly, it enables the fabrication of more complex structures because of the three spatial dimensions that can be exploited. Furthermore, it offers possibilities for the use of different materials in one structure and lastly, since it is a additive manufacturing method, only a minimal amount of material is used. So far, the vast majority of 3D printing methods use polymer based materials. However, this limits the applications that the fabricated structures can be used for. If one wants to produce functional end products with interesting optical and electronic properties that can withstand elevated temperatures for example for use in PV applications and electronic devices, 3D printing of metals and semiconductors is required [5].

A technique that is widely used to fabricate metals at room temperature [6], but also semiconducting materials at low temperatures [7–9] is electrochemical deposition. Also in this project, electrochemistry is used as fabrication method. However, the electrochemical deposition is not performed in a regular electrochemical cell. Instead, it is confined to the small meniscus formed between a micro pipette and a substrate once the filled pipette is brought in close proximity of the substrate. A schematic of the system is shown in figure 1.1, showing the pipette used as a printing nozzle that can move in all three dimensions. By matching the speed of the pipette nozzle with the rate of electrochemical deposition, this system enables 3D printing of metals on micro scale at room temperature in ambient conditions and with minimal material use.

Recently, meniscus confined electrochemical deposition (MCED) has already been proven to be able to print different structures ranging from nanowires, to overhanging and spiralling structures with sub-micron diameters to even intertwined structures through layer by layer deposition, all shown in figure 1.2 [10, 11]. Although these results prove that MCED is a promising technique for 3D printing on micron and sub-micron scale, the authors of these works do not go into details on how different parameters of the system influence the growth process. Multiphysics finite element modelling of the MCED system was done by Morsali and co-workers in order to understand what the effect was of nozzle size and nozzle speed on the growth process in MCED [12]. However, the part that is still missing is experimental research on the growth process in MCED as a function of different system parameters. This project is aiming to partly fill in that gap, by focusing on the effect of potential and retraction speed on the growth process of freestanding micro wires in a meniscus confined electrochemical system. Furthermore, there has been no research so far for the use of MCED to print functional material such as semiconductors. The first steps towards functional material fabrication by MCED are made in this project as well.

The next chapter starts with discussing the basic principles of electrochemical deposition in general and afterwards focuses on meniscus confined electrochemical deposition. After that, the third chapter discusses the experimental set-up used for growing free standing wires and the different techniques that are used for

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Figure 1.1: Schematic representation of the MCED system, showing the meniscus that is formed between the pipette nozzle and the substrate that is printed on by means of electrochemical deposition. The pipette can move in all three dimensions. By matching the growth rate of the electrochemical deposited material with the retraction speed of the pipette, it is possible to print continuously in all dimensions.

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Figure 1.2: Scanning electron microscope images of a variety of structures printed with MCED. (a) shows a nanowire with a diameter smaller than 200 nm, (b) shows how MCED can be used to interconnect chips in electrical devices by printing overhanging structures and (c) shows a spiralling structure, which is another prove apart from the structures shown in (b), that MCED can print in all three dimensions [10]. Lastly, (d) shows a recently published result of five intertwined wires, fabricated by means of layer by layer meniscus confined electrochemical deposition [11].

characterisation of the resulting wires. The fabrication and characterisation of the pipettes is the subject of the fourth chapter after which the different experiments and results are the topic of chapter five. Then, before concluding and looking out to future possibilities in the final chapter, the results are shown of preliminary research for the use of MCED for the production of semiconducting materials.

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2

Electrochemical deposition

Electrochemistry is a multi-faceted field of science where both electrical and chemical effects play a role. It can be used to investigate the electrical and chemical properties of the different parts in your system through electrical control and measurements. An electrochemical system can also be used to fabricate materials from solution by electrochemical deposition [13]. This chapter focuses first on features of general electrochemical deposition. After that, the specifics of the electrochemical system in MCED are discussed.

2.1 General electrochemical deposition

2.1.1 Electrode processes

Electrochemistry involves reactions happening at the interface between an electrode and electrolyte. Two types of electrochemical reactions can happen. The first one, where electrons are transferred from the electrode to the ions in the electrolyte, is called a reduction reaction. The second one, where electrons transfer from ions in solution to the electrode, an oxidation reaction. In figure 2.1 a schematic shows a simplified version of the process that happens during electrochemical reduction of a molecule in solution at an electrode. As the potential is forced to more negative potentials, meaning that more electrons are pumped in the electrode, the energy of the electrons in the electrode rises. At some point the energy is high enough for the electrons to transfer from the electrode to the lowest unoccupied molecular orbital (LUMO) of the ions in the electrolyte [13]. During electrochemical deposition, the electrochemical reactions result in a solid material deposited on the electrode. An example of the electrochemical deposition of copper is given later on in this chapter.

Figure 2.1: A simplified schematic representation of the process of reduction at the electrode-electrolyte interface, where electrons can be transferred from the electrode to the ions in the electrolyte as the energy level of the electrons in the electrode is high enough. The energy level of the electrons in the electrode can be changed by applying a potential. Adapted from Bard & Faulkner (2010) [13]

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surface and a diffuse layer of ions in solution forms to screen the potential of the electrode. This layer is called the electric double layer (EDL) and has a typical thickness ranging from 0.1 nm to several 10 nm [14]. This can change the electron transfer, but nevertheless figure 2.1 provides a fair description of the macroscopic process. Because the electron transfer is a tunneling process, the current density resulting from the electron kinetics happening at an electrode is exponentially dependent on the overpotential under the condition that enough ions are available at the interface. The overpotential is defined as the difference between the actual electrode potential and the equilibrium potential of the redox couple present at the electrode [13].

In order to run a current, there should be a closed circuit. Therefore, as a reduction reaction occurs at one electrode, there should be another electrode at which an oxidation reactions happens. The electrode where the reaction of interest is happening, is called the working electrode. The electrode where the counter reactions happens, is called the counter electrode. In regular electrochemistry, often a reference electrode is used. This is an electrode with a stable and known potential with respect to the solution, and approaching ideal nonpolarisability. The potential of the working electrode is measured relative to this reference electrode [13]. In this project however, a two electrode system is used consisting only of a working and a counter electrode. The potential measured or applied at the working electrode, is therefore always relative to the counter electrode. Since at this counter electrode, also the counter reaction is happening, the potential can be less well defined than in a three electrode system. However, since the working electrode is small (in the order of 10 µm2_{), the currents that will run are small (in the order of a few nA). The small currents in}

combination with a counter electrode with a large surface area, the current density at the counter electrode will be extremely small and the potential of the counter electrode will not be affected too much.

2.1.2 Electrochemical techniques

At what potential electrochemical deposition can happen and at what rate, highly depends on the combina-tion of electrolyte and the electrode material [13]. Different electrochemical techniques can be used in order to obtain information on the electrochemical system that is used, two of which are discussed here.

Chrono Amperometry experiment

In a chrono amperometry (CA) experiment, a constant potential is applied for a certain amount of time, while the current is measured. The different stages that can typically be recognised in a CA curve during electrochemical nucleation and growth are shown in figure 2.2 [15]. On the left side it shows a schematic picture of what happens at the electrode and on the right side the corresponding part in the CA curve are indicated with a red arrow. Stage I is the state before the bias potential is applied, so the system is at open-circuit potential (i.e. the current is zero). Stage II starts upon the application of bias potential. At this stage the first nuclei are formed on the substrate. The emergence and growth of nuclei increases the active surface area available for reduction, causing an increase in current. At that active area ions are depleted, because they are used in the reduction reaction. In stage III the current is limited by the diffusion of ions through the diffusion zones around the nuclei. The diffusion zones of separate nuclei grow with nuclei size and start overlapping, causing increasing competition of nuclei for diffusing ions and thereby a decreasing current. Eventually the diffusion zones cover the full electrode and the reaction is limited by the mass transfer from ions in the bulk solution through the 2D control area of overlapped diffusion zones. This results in a constant current.

Cyclic Voltammetry experiment

In a cyclic voltammetry (CV) experiment, the potential of the working electrode is swept across a potential range. At the same time the current is measured. By convention, reduction currents are plotted as negative and oxidation currents as positive currents (by IUPAC convention, in America opposite signs are used) [16]. In figure 2.3, a CV is shown for system consisting of a CuSO4 solution, a glassy carbon working electrode

and a Cu/CuSO4 reference electrode [15]. At the start of the CV, the substrate is clean of copper, as shown

in the AFM image in figure 2.3a, that is acquired simultaneously. Going to more negative potentials gives rise to a reduction current, where electrons transfer from the electrode to the ions electrolyte. In this case, the following reduction reaction takes place, depositing solid copper on the electrode as can be seen in figure 2.3b:

Cu2++ 2e −→ Cu(s) (2.1)

The decrease in current right after point b is explained by the fact that the current becomes limited by diffusion of ions towards the electrode. In figure 2.3c, at the stage right before the oxidation potential is

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Figure 2.2: A simplified picture of what is happening at the electrode during different stages of growth in a CA experiment (left) and the corresponding part in a typical current transient (right) [15].

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Figure 2.3: A cyclic voltammetry graph of an electrochemical system consisting of a CuSO4 and diluted

sulfuric acid solution, a glassy carbon working electrode and a Cu/CuSO4 reference electrode. AFM images

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reached, more nuclei have been formed. After that point, going to more positive potentials will cause the opposite reaction of 2.1 to happen: the oxidation of the deposited copper. Right after the oxidation peak, almost all copper has been oxidised as can be seen in figure 2.3d. The current will then drop to zero and the electrode is completely stripped from copper as in the beginning.

2.1.3 Nucleation

A typical CA curve with nucleation and mass transport in the form of diffusion as mechanisms that control the shape of such a curve, was already discussed in the previous section. Scharifker and Hills [17] studied both the increasing and decreasing current in the current transient of different combinations of electrode and electrolyte and came up with models of different nucleation mechanisms. In their paper they discuss the two limiting nucleation mechanisms: instantaneous and progressive nucleation. Instantaneous nucleation corresponds to fast nucleation of a relatively small amount of active sites, after which barely no new nuclei are formed. Progressive nucleation corresponds to slow nucleation, where the new nuclei are formed over the course of electrochemical deposition. A nucleation process does not necessarily only include one of the two mechanisms. A general feature of nucleation in multiple systems where nucleus growth is mass-transfer controlled is that after a period of progressive nucleation, further nucleation stops and instantaneous behavior follows [17].

In order to compare current transients with the Scharifker-Hills model, it is easiest to normalise both the current and time to the value at the current peak, which leaves us with the dimensionless parameters I/Im

and t/tm. The dimensionless form of the transients for instantaneous and progressive nucleation are given

by equation 2.2 and 2.3 respectively. I2 I2 m =1.9542 t/tm {1 − exp[−1.2564(t/tm)]}2 (2.2) I2 I2 m =1.2254 t/tm {1 − exp[−2.3367(t/tm)2]}2 (2.3)

2.2 Meniscus confined electrochemical deposition

As already explained in the introduction, the electrochemistry in MCED is confined to a micro pipette filled with electrolyte and the meniscus formed between the pipette and a substrate. In this system, the substrate is used as one electrode and a metal wire inserted in the pipette is used as the counter electrode. Where the previous section discussed some general features of electrochemical deposition, this section focuses on the features specific for MCED.

2.2.1 Dimensions of the system

In order to grow continuously, a stable meniscus is needed. Both the maximum height of the meniscus and the diameter of the deposited wire are related to the the nozzle diameter. In this section the size limits of a stable meniscus as a function of nozzle size are discussed, as it follows from calculations and simulations in literature. The stability of the meniscus mainly depends on the wetting conditions at the pipette nozzle and the substrate. The growth angle φ0as depicted in figure 2.4, can be calculated as function of the surface

energies of the electrolyte and the metal wire and the interfacial energy of metal-liquid interface. For a copper-water-air system φ0 is 12o[10].

The following relations for wire diameter [10] and meniscus height [12] are given as function of diameter of the pipette nozzle dn:

dw: 0.5dn - dn

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Figure 2.4: A schematic of the MCED system during growth, showing the growth angle φ0.

2.2.2 Growth rate

We define a steady state growth as growth where the retraction speed equals the average growth rate of the wire. The growth rate of the wire can be expressed using Faraday’s law, by assuming that all current is due to the electrochemical reduction of copper ions and that solid copper is formed at the electrode-electrolyte interface [10]: vgrowth= iM nF ρπr2 i , (2.4) with:

• vgrowth: the growth rate of the wire in z-direction

• i: the Faradaic current

• M : the molar mass of the deposited material • n: the number of electrons used per reduced ion

• F : Faraday constant, i.e. the number of Coulomb per mol of electrons • ρ: the density of the deposited material

• r: the radius of the wire

If both the growth rate and the current are integrated over time, one finds a formula that expresses the height of the wire h as a function of the total charge Q:

h = QM nF ρπr2

i

(2.5) The growth rate of the electrochemically deposited material is partly determined by the potential that is applied on the electrodes. However, the availability of ions at the electrode-electrolyte interface also determines the growth rate. As an example mentioned in the previous section, diffusion could limit the reduction current. Other ion transfer mechanisms that affect the growth rate are migration and convection. Although the types of ion transport mechanisms are the same in regular electrochemical cells and our system, the contribution of the three types differs. The three different modes of ion transport and their contribution in the MCED system are simulated by multiphysics finite element modelling by Morsali and coworkers and are shortly discussed here [12]. The first ion transfer mechanism is diffusion. This mechanism is driven by a gradient in concentration, and transfers ions from high a concentration to a lower concentration. The second mechanism is called migration and is driven by a gradient in electrical potential. In case of positive ions, this mechanism drives ions from the more positive electrode to the more negative electrode. The last ion transfer mechanism is convection. This is transfer of ions as a result of flow of the solution. In a meniscus confined electrochemical system, it is evaporation at the meniscus-air interface that drives the convection flow from bulk solution to the meniscus.

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2.2.3 The effects of pipette’s retraction speed

The (relative) contributions of the different ion transport mechanisms to the total ion flux is affected by the retraction speed. A faster retraction speed results in an elongated meniscus with a smaller base. This impacts the growth rate in two ways. Firstly, the elongation of the meniscus increases the ion flux through the pipette nozzle. This is partly due to an increased migration flux, but mostly due to an increased convection flow as a result of an increase in volumetric evaporation. As can be seen from figure 2.5, these effects are partly counterbalanced by a decreased diffusion flux, but the total ion flux through the pipette nozzle is increasing for higher retraction speeds [12]. At the same time, because of the smaller meniscus base, the effective working electrode is smaller. This results in less ions being consumed at the electrode-meniscus interface. The combination of a slightly higher ion flux towards the meniscus and a decrease in total ion consumption, results in a higher ion concentration in the meniscus for faster retraction speeds. The higher ion concentration enables faster growth rates in the z-direction. Therefore, for a fixed potential, even with a faster retraction speed the meniscus is stable. The window of retraction speeds that enable a stable meniscus will be referred to as the stability window.

Figure 2.5: The contributions of convection, diffusion and migration to the total ion flux through the pipette nozzle towards the meniscus for five different retraction speeds obtained from simulations. The total ion flux increases for higher retraction speeds [12].

The higher volumetric evaporation as a result of a higher retraction speed has another consequence. Figure 2.6 shows the ion flux due to convection flow at two different retraction speeds. It shows a larger difference in ion flux between the centre and the edge of the meniscus for the higher retraction speed than the lower retraction speed. Therefore, the ratio between growth rate at the edge of the meniscus (GRE) and

the centre (GRC) increases with increasing retraction speed. For the specific pipette (730 nm diameter) and

retraction speeds simulated in figure 2.6 these ratio’s are:

R88 nm/s= GRE GRC = 1.19 nm/s R263 nm/s= GRE GRC = 1.94 nm/s

According to the simulations, this means that faster retraction speeds will lead to the growth of hollow structures.

To conclude, there are two important tuning knobs in our system that affect the growth process. As we have seen before, the potential can enable faster growth. On one hand it enables charge transfer from the electrode to the electrolyte as given by the Butler Volmer equation. Secondly, a higher potential means a greater potential gradient in the system, increasing the ion flux towards the more negative electrode by the migration ion transport mechanism. Secondly, retraction speed influences the shape of the meniscus and thereby the ion concentration in the meniscus and the ion flux from the bulk of the solution towards the

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Figure 2.6: The convection flux in a mol/m2s colour scale at the minimum (left) and maximum (right) retraction speed within the stability window of a pipette with a 730 nm diameter nozzle [12].

meniscus. In these two ways, retraction speed influences the growth rate. Simulations also show that the growth rate influences the ratio between the growth rate at the centre of the meniscus and the edge of the meniscus. The rest of this thesis shows the experimental investigation of the effect of these two main tuning knobs on the growth process in MCED.

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3

System set-up and characterisation tools

This chapter consists of two parts. The first part focuses on the materials and tools that are used in the set-up to do the actual meniscus confined electrochemical deposition. The second part will be focused on the tools used for characterisation of the structures that are grown using that set-up. Since the pipette is such an important part of the system and are fabricated in-house, the tools used for fabrication and characterisation of the pipettes are discussed separately in chapter 4.

3.1 System set-up

For the meniscus confined electrochemical deposition, the substrate is the working electrode of the system. Although the substrate itself can be any size, the active area of the electrode is confined to the base of the meniscus. In this project, a metallic substrate is used to grow on, to ensure high conductivity. The fabrication of the substrate process of the substrate is described in appendix A.3. The second electrode is a thin wire that is inserted in the pipette serving as the counter electrode and quasi reference electrode. Because a two electrode system is used, the potential applied in the experiments in this project is the potential difference between the working and counter electrode, and is distributed over the whole system including both electrode-electrolyte interfaces and the electrode-electrolyte. In order to grow 3D structures, we use a system that combines a potentiostat with a stage that moves with sub micrometer steps in three dimensions.

The system used for this project is the Bio-Logic Model 470 Scanning Electrochemical Workstation in combination with the Bio-Logic 3300 bipotentiostat. As a third part of the system, a video microscope (model VCAM3) is used, to provide continuous visual access to the position of the pipette relative to the substrate. This microscope has a field of view of 1.4 mm at maximum magnification. In practice this means that the pipette can be brought 5-10 µm close to the surface on optical feedback. The complete system, also including the video microscope, is shown in figure 3.1. It allows to perform electrochemical techniques while moving the pipette at the same time. It has step motors in 3 dimensions with a step resolution of 20 nm. Additionally, it has a piezo element in the z-direction. The different experiments and the settings that can be adjusted are discussed in the coming section, first focusing on techniques for approaching the surface and secondly on techniques used for growing.

3.1.1 SECM modes for engaging

A key requirement in MCED is to find the substrate with the pipette, i.e. to bring the pipette close enough to establish a meniscus, but without damaging the pipette by crashin into the substrate. This cannot be done on optical feedback entirely and therefore, another type of feedback is required. In this project, two types of feedback are explored: mechanical and electrochemical feedback. The technical details and a summary of the findings are discussed in the following section.

Intermittent contact surface approach

In an intermittent contact (IC) surface approach, mechanical feedback is used to find the surface. This method is fast compared to using electrochemical feedback and works as follows. The probe, in this case

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Figure 3.1: The set-up used for meniscus confined electrochemical deposition, showing the three main parts: The potentiostat, the scanning electrochemical workstation and the video microscope system. For growing, the conductive substrate is used as working electrode, the connection of which is shown in red. A metal wire inserted in the pipette serves as counter electrode, indicated in green. Figure adapted from Bio-Logic Science Instruments.

the pipette, is moving towards the substrate. Meanwhile, it is vibrated in the z-direction in a sinusoidal manner with a set vibration frequency using the z-piezo. While slowly moving downwards, the vibration is probed with a strain gauge. This signal is compared to an expected value, and the position of the probe is adjusted using a PID control loop if a deviation from the expected signal is measured. Such a deviation can be detected when the probe is so close to the substrate that the mechanical interaction between probe and substrate changes the amplitude and frequency of vibration of the probe.

Vibration frequency and other settings

For a IC surface approach, PID and probe control parameters can be adjusted. However, for a lot of these parameters it is hard to predict what their effect will exactly be, because their effect is dependent on how other parameters are chosen. One parameter that is relatively straight forward is the control point. The control point will set the percentage of the maximum vibration amplitude that the PID loop will seek, with a maximum value of 95%. For example, a value of 60% will aim to reduce the vibration amplitude to 60% of the maximum vibration amplitude. In order to do so, relatively forceful contact with the surface is needed. On the other hand, values close to 95%, will tap the surface softly. Since the pipettes need to be protected from crashing in the substrate, a high percentage control point is preferred. What the maximum vibration amplitude is, and therefore, the maximum absolute control point, depends on what vibration frequency is chosen. Next, the selection of a suitable vibration frequency is explained.

A vibration frequency should be chosen that is close to the resonance frequency of the probe. In this way, the system is very sensitive to changes, because close to the resonant frequency a small change in frequency gives a large change in signal output. It is advised by Bio-Logic to use a vibration frequency that is 10-15 Hz lower than the resonance frequency. The resonance frequency is dependent on the device and probe that are used and should therefore be determined before every IC surface approach. The resonance frequency is determined by scanning over vibration frequencies in the range of 70-600 Hz, where the signal gain is measured using the strain gauge. Here, all spectra are plotted in the 400-600 Hz range, since none of the obtained spectra show features in frequencies outside that range. Not all peaks in the resulting spectrum are related to the pipette. Some features are for example related to the holder that is used and are therefore not sensitive to pipette-surface interaction.

To explore the possibilities of using the IC surface approach in combination with a pipette, spectra are obtained for two different pipette holders, all parts of which are shown in figure 3.2. Both holders are mounted on the SECM by using the parts shown in 3.2.a. The backside can directly be mounted on the SECM, while the front part can be screwed on the backside, clamping one of the holders in between. The first holder,

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Figure 3.2: The two different holders that are used to mount the pipette. In (a) the parts are shown that are used to fasten the holders onto the SECM. (b) shows the pipette holder that consist of a perspex block with a small groove and screw, while (c) shows the metallic tube holder.

shown in figure 3.2.b, consists of a small rod with a perspex block attached to it. The perspex block has a groove on one of its sides, with the same thickness as the outer diameter of the pipette. The pipette is positioned in the groove and tightened with a screw. This holder can be fastened in two configurations, both of which are shown in figure 3.3. In 3.3a, the perspex is not attached to the front and backside of the holder, while in 3.3b it is. The second holder, a metal tube with a slit in it, is shown in figure 3.2.c. Here the

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Figure 3.3: Two different configurations to affix the perspex holder. In (a) configuration 1 is shown, with a ∼ 1.5 cm gap between the perspex block and the black part of the holder. (b) shows configuration 2, where the perspex glass is attached to the black part of the holder

pipette is inserted in the metal tube and a small rubber ring helps to ensure that the pipette will not fall out easily. Because of the slit, the pipette will be held in place if pressure is exerted on the tube when the tube is clamped between the front and backside shown figure 3.2.a.

For all the parts of the different holders, the AC-characterisation is performed, in order to try to un-derstand what resonance frequencies in the spectra are caused by the holder rather than the pipette. The resulting spectra are shown in figure 3.4. It can be seen that the addition of the perspex block holder changes

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the spectrum significantly, with two resonance peaks appearing at 480 and 510 Hz. Furthermore, the spec-trum is virtually the same for the two configurations of that holder, as shown in figure 3.4a. On the contrary, the metal tube holder does not change the spectrum of the bare clamp much, only shifting the resonance peak from 525 to 520 Hz, as can be seen in figure 3.4b. Now that the spectra for all holders are known, the effect of the addition of a pipette can be explored.

4 0 0 4 2 5 4 5 0 4 7 5 5 0 0 5 2 5 5 5 0 5 7 5 6 0 0 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 S ig n a l G a in a t F re q u e n c y ( A .U .) F r e q u e n c y ( H z ) B a r e p i e z o + B a c k s i d e + B a c k s i d e a n d f r o n t H o l d e r c o n f i g . 1 H o l d e r c o n f i g . 2 (a) 4 0 0 4 2 5 4 5 0 4 7 5 5 0 0 5 2 5 5 5 0 5 7 5 6 0 0 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 S ig n a l G a in a t F re q u e n c y ( A .U .) F r e q u e n c y ( H z ) B a r e p i e z o + B a c k s i d e + B a c k s i d e a n d f r o n t C o m p l e t e h o l d e r (b)

Figure 3.4: The AC characterisation spectra of different parts of the two different holders. In both graphs the spectra are shown of the bare piezo element (black), the piezo plus the backside of the holder (blue) and the piezo, backside and front part of the holder together (green). In (a) also the spectra of the two configurations of the complete perspex holder is shown, with the two configurations as shown in figure 3.3a and 3.3b. In (b) the spectrum of the complete holder including the metal tube is shown.

To see if the pipette adds a clear resonance feature to the spectra, also an AC characterisation is done with a pipette mounted. This is done for the two different holders, both with an empty and filled pipette and the results are compared to the spectrum of the complete holders without pipette. For the perspex holder, only configuration 2 is shown (figure 3.3b), since in the other configuration (figure 3.3a) the spectra without and with (filled) pipette were identical. The results are shown in figure 3.5. In figure 3.5a, it can

4 0 0 4 2 5 4 5 0 4 7 5 5 0 0 5 2 5 5 5 0 5 7 5 6 0 0 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 S ig n a l G a in a t F re q u e n c y F r e q u e n c y ( H z ) N o p i p e t t e E m p t y p i p e t t e F i l l e d p i p e t t e (a) 4 0 0 4 2 5 4 5 0 4 7 5 5 0 0 5 2 5 5 5 0 5 7 5 6 0 0 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 S ig n a l G a in a t F re q u e n c y ( A .U .) F r e q u e n c y ( H z ) N o p i p e t t e E m p t y p i p e t t e F i l l e d p i p e t t e (b)

Figure 3.5: AC-characterisation spectra for the perspex block holder in configuration 2 (a) and metal tube holder (b), without and with and empty and filled pipette.

be seen that with a pipette, the signal gain at 510 Hz in the AC-characterisation spectrum of the perspex holder increases compared to without a pipette. This suggests that the frequencies close to 510 Hz are the interesting frequencies to use for the vibration frequency in an IC approach experiment with this holder. For the metal tube holder, the peaks at 490 Hz and 525 Hz increase in signal gain with the addition of the pipette and are therefore the interesting frequencies to use when the pipette is mounted in this holder.

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Results and findings

The explored configurations are summarised in table 3.1. Approaching with the perspex holder in configura-tion 2 and using a 505 Hz vibraconfigura-tion frequency, resulted in finding the surface without the pipette visibly being broken. However, only a small change in frequency of 2 Hz, did result in the same pipette being broken. In combination with the metal tube holder, two vibration frequencies based on the resonance frequencies found in figure 3.5b have been tested with different pipettes. Using the 490 Hz vibration frequency, it is able to find the surface without breaking a pipette only for pipette with a nozzle diameter larger than 50 µm. The second vibration frequency that was used was 510 Hz, which is 15 Hz lower than the resonance frequency as shown in figure 3.5b. Also for this frequency, using a control point value of 95%, pipettes in the order of magnitude of 1 µm would break.

Table 3.1: Results of the explored configurations and settings for IC surface approach. Holder Vibr. freq. (Hz) Pipette size (µm) Control point (%) Result

Perspex config. 2 505 6 ± 4 80 Not visibly broken

Perspex config. 2 503 6 ± 4 80 Visibly broken

Metal tube 490 >50 80 Visibly broken

Metal tube 490 125 ± 75 95 Not visibly broken

Metal tube 490 27 ± 13 95 Visibly broken

Metal tube 490 0.9 ± 0.5 95 Visibly broken

Metal tube 510 ∼ 1 95 Visibly broken

The order of magnitude of the nozzle diameter that is used in this project for meniscus confined electro-chemical deposition is 1 µm. The combination of holders and vibration frequencies as discussed above, are all not suitable for using IC surface approach with pipettes of this size.

Conclusion and recommendations

For the settings explored so far, it can be concluded that the intermittent contact surface approach only works for pipettes with a nozzle diameter bigger than 50µm. Since finding the surface with this technique is a fast process, further exploration of different settings is recommended. Another set of AC characterisation spectra, shown in figure 3.6, suggest that there is another vibration frequency that does seem promising for approaching the surface with small pipettes. In this set, the AC characterisation spectra of a filled pipette in air is compared to the spectra of that same pipette when it is engaged, i.e. when a meniscus has already been formed. After engaging, the pipette was retracted for different distances between 1-10 µm. The pipette used here was relatively big (nozzle diameter 20-100 µm) and mounted using the metal tube holder. The pipette is engaged using electrochemical feedback, the details of which are discussed after this section. The biggest change in signal gain between the pipette in air and the pipette when engaged happens at a frequency of 555 Hz. Therefore, it seems that the system with the old holder is the most sensitive to the interaction between the pipette and substrate at this frequency, which was not captured by table 3.1.

Because of limited time, this and some other options for combinations of holder and vibration frequencies have not been fully explored. If more time is available, the following options are recommended for further exploration:

• Perspex holder (config. 2), using a vibration frequency even closer to or at resonance frequency of 510 Hz, using control point of 95%. The reason behind this recommendation is that using a vibration frequency of 505 Hz seemed to work, but a slightly lower vibration frequency would cause the pipette to crash.

• Metal tube holder closer to or at resonance frequency of 525 Hz, still using control point of 95%. Closer to the actual resonance, the sensitivity to surface interaction can be higher than 15 Hz away from resonance.

• Metal tube holder, using vibration frequency of 555 Hz and a control point of 95%. As appears from figure 3.6 a peak in the characterisation spectrum is sensitive to contact between the pipette and the substrate through the meniscus.

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4 0 0 4 2 5 4 5 0 4 7 5 5 0 0 5 2 5 5 5 0 5 7 5 6 0 0 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 S ig n a l G a in a t F re q u e n c y ( A .U .) F r e q u e n c y ( H z ) N o t e n g a g e d E n g a g e d , 1 0 µm r e t r a c t e d E n g a g e d , 5 . 8 µm r e t r a c t e d E n g a g e d , 3 . 4 µm r e t r a c t e d E n g a g e d , 1 . 4 µm r e t r a c t e d

Figure 3.6: The AC characterisation spectra of a pipette in air and the same pipette when engaged and retracted for different distances.

Approach curve experiment

Although a few more options are to be explored for using IC surface approach, the results so far have shown that this method for approaching only works for pipettes with a nozzle diameter bigger than 50 µm. Since that is too big for the purpose of this project, another surface approaching method needs to be used that allows smaller pipette nozzles. Another method available for the used set-up is an approach curve experiment, which makes use of electrochemical feedback. In such an experiment, the pipette is moved towards the sub-strate while either applying a bias potential or forcing a current. In total there are three main modes to use within an approach curve experiment, dependent on the parameter that is measured: Open circuit potential, potential or current. It is possible to set a trigger for the measured parameters at which the movement of the pipette is terminated. If these triggers are chosen the right way the approach curve can be terminated at the moment contact is made between the pipette and the substrate in the form of a meniscus. A setting that is specifically important for approaching the substrate is the step size. The smallest step size that can be set in an approach curve experiment is 20 nm. However, this does not guarantee that only steps of 20 nm are taken. It happens often in approach curves that by an error in the step motor, the distance is overshot and then corrected, for example first taking a step 100 nm and then jumping back 80 nm. The following sub sections discusses the three different modes that can be used in an approach curve experiment.

Open Circuit Potential

The open circuit potential (OCP) is sometimes also referred to as the zero-current potential or rest potential. The latter term is most appropriate for the definition of open circuit potential in the Bio-Logic 3300 bipo-tentiostat, as in OCP mode the potential is measured when no current is forced or bias is applied. When the pipette is still approaching and there is no closed circuit yet, the OCP generally goes to the clip value of the potentiostat, which is around ±8 V . Once a meniscus is formed, the OCP can actually be measured, and its value is dependent on the materials used as electrodes. If the same material is used for both the wire in the pipette as the substrate, the OCP is expected to be 0 V. A maximum and minimum trigger OCP can be set for the experiment to terminate. The advantage of operating in OCP mode is that the system is at rest during the approach, so no drastic changes in charge distribution in the electrolyte takes place and no current run once contact is established. However, before starting an approach experiment, a stable OCP value is required to make sure that the trigger is not reached before the surface is found. The disadvantage of using the OCP feedback, is that acquiring a stable OCP value before starting the approach takes some time. Galvanostat mode: measuring potential while forcing a current

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One can also choose to force a current, and measure the potential that is needed to let this current run. Whenever the pipette is approaching and no meniscus has been formed, there is no closed circuit and the set current cannot run. The potentiostat, or in this case galvanostat, will still try and therefore ramp up the potential over the two electrodes to clip value. Once a meniscus is formed, a current can run that is dependent on the electrolyte, the potential that is applied at that point (most probably the clip value) and the size of the meniscus that is formed. Because of the plurality of parameters that affect the current, it is difficult to predict what the required setpoint value for the current needs to be. However, if the trigger value is set on a very small current (∼ 10 times noise value), the clip point potential will be big enough to make that current run once the meniscus is established. Although the feedback will work, galvanostat mode is not the favourable mode for engaging. This is mainly because high potentials are applied over the course of the approach, with a possible large current as result once contact is established and an unknown effect on the distribution of ions in the electrolyte.

Potentiostat mode: measuring current while applying a potential

The last of the three main modes for approaching the surface is the potentiostat mode. In potentiostat mode, a potential is applied over the two electrodes and the current is measured. As long as the pipette is approaching and no closed circuit is formed, no current can run. When an appropriate bias potential is cho-sen, a faradaic current can run once the meniscus is formed. The advantage of this mode over the galvanostat mode is that now it is known exactly what potential is applied, while when operating in galvanostat mode the potential can be ramped up to clip value. As mentioned before, the bias potential needs to be chosen such that a current will run once the meniscus is formed. Some preceding experiment needs to be done in order to know what potentials can be used, either to let an oxidation or reduction current run. It should be realised that the reactions that happen for this current to run, can already cause material deposition at the substrate.

3.1.2 SECM modes for growing

After the meniscus is established between the pipette and the substrate, the electrochemical deposition can be initiated. Since both an approach curve and a reversed approach curve will always start at z = 0, it is important to reset z to zero before starting the growth process. In this project two distinct processes are used for growing. The first one is what will be referred to as continuous growth, where the electrochemical deposition is done at the same time as the pipette is being retracted. In order to be able to grow structures in this way, the retraction speed and growth rate should be matched. The other method is step wise growth, where electrochemical deposition is performed for a few seconds while the pipette is positioned at a certain height. After that the pipette is retracted an amount that should be matched with the height of the previous deposited material. During retraction in this method, no bias is applied or current is forced.

Continuous growth

A continuous growth experiment is done by performing an approach curve experiment in reversed direction, i.e. setting a negative value for the scan distance parameter. The resulting graph is a current versus retracted distance curve. Also here, it is both possible to apply a bias potential during retraction or to force a current. In order to force a constant current through the system, the system constantly adjusts the potential in order to keep that current. However, due to the speed of the feedback loop and the low currents, the potential that is needed for this current is overshot, resulting in a potential that is heavily jumping up and down. In figure 3.7a, the potential is shown that is measured when forcing a cathodic (negative) current of 30 nA current through the system while retracting the pipette. The potential widely varies, with values between -1.45V to +0.20V. The resulting wire greatly fluctuates in diameter and morphology across its length, as is shown in figure 3.7b. When instead of forcing a current, a bias potential is applied during retraction, no feedback loop is used. This results in wires that are have a more constant diameter across the length of the grown wire. Therefore, applying a bias potential is used for the electrochemical deposition in the continuation of this project, instead of forcing a current.

As mentioned in the introduction of this chapter, the advantage of the used system is that it can apply a potential, move the pipette and measure the current all at the same time. By integrating the current over time, the total charge can be obtained that was needed to grow a certain structure. This gives an indication of the density of the grown structure. All settings are as straightforward as with a regular approach curve

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0 5 1 0 1 5 2 0 2 5 3 0 - 2 . 0 0 - 1 . 7 5 - 1 . 5 0 - 1 . 2 5 - 1 . 0 0 - 0 . 7 5 - 0 . 5 0 - 0 . 2 5 0 . 0 0 0 . 2 5 0 . 5 0 P o te n ti a l (V ) D i s t a n c e (µm ) (a) (b)

Figure 3.7: The results of a constant growth experiment where a current of -30 nA is used as a setpoint. The retraction speed was 183 nm s−1. In (a) the measured potential (V) is plotted versus the distance retracted (µm). The potential is highly fluctuating, with a minimum and maximum value of -1.45V and +0.20V respectively. In (b) the resulting wire is shown.

experiment, but special attention is needed when setting up the experiment to prevent the pipette from crashing back into the grown wire, damaging both the grown structure and the pipette. These points of special attention are discussed in appendix B.1.

The main tuning knobs in the process of meniscus confined electrochemical deposition that are to be investigated are applied potential V and retraction speed vretract. The bias potential and retraction speed

vary among different experiments. The applied bias potential is straightforward to set. The retraction speed is a little less straightforward, as is discussed next. In an approach curve experiment there are two ways to set the retraction velocity, dependent on the scan mode: sweep scan mode or step scan mode. In sweep scan mode, the pipette is retracted with a constant velocity as set in the scan velocity. However, in sweep scan mode the measured parameter, i.e. the current, is not shown real-time during the scan, only after the experiment has finished. This means that the experiment cannot be terminated manually based on an observation made during the experiment. As a consequence, for this project the step scan mode is used. In this mode, the probe is moved in small steps, while continuously applying a bias. Data is collected at every step. Both the step size and the step velocity can be set, resulting in different average retraction speeds. How these settings effect the final average retraction speed is discussed in appendix B.2.

Step wise growth

The resulting curve in a continuous growth experiment is a current versus distance curve. In order to obtain more information on the nucleation and growth processes in meniscus confined electrochemical deposition, also step wise growth experiments are performed. This growth process consists of a two step process that is repeated multiple times. The first step is chronoamperometry (CA) experiment, i.e. applying a certain potential for a certain time interval and measuring the current. During this step, material is electrochemically deposited. The second step is retracting the pipette by a set distance without applying a bias potential. The retraction distance has to be small enough not to break the meniscus, in order that the two-step process can be repeated. As explained in the previous chapter (section 2.1.3), from a CA experiment information can be obtained on the nucleation process.

Additionally to the different experiments as described above, the M470 software for the SECM has another useful feature: the sequencing option. This enables some automation of the process and is discussed in appendix B.3.

3.2 Characterisation tools

While the previous section focuses on the tools used for meniscus confined electrochemical deposition, this section discusses characterisation tools that are used. A scanning electron microscope (SEM) is used to

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deter-mine size and morphology of the grown wires, as well as the size of the pipette nozzle diameter. Additionally, electron dispersive X-ray spectroscopy (EDX) and Raman spectroscopy are used to characterise the material of the grown wires. The following three sections shortly discuss the working principles of these techniques.

3.2.1 Scanning Electron Microscopy

In a scanning electron microscope (SEM), images of samples can be made by scanning the sample with a focused electron beam in a vacuum chamber. In an electron gun, electrons are generated and subsequently accelerated by an acceleration voltage between the cathode and anode in the gun. A few electron lenses are used to influence the path of the electrons and to focus the beam. The resulting electron beam is scanned across the sample. The electrons penetrate the sample with a depth of the order of a micron dependent on the electron energy, interact within the sample and generate different signals that can be used for imaging and characterisation, including secondary electrons and x-rays. The different signals resulting from the electron beam-sample interaction and their associated emission volumes are shown in figure 3.8. For a specific type of signal a specific detector is used. As can be seen in figure 3.8, the secondary electron emission is confined

Figure 3.8: The different signals generated by the electron beam-sample interaction and their associated emission volumes. Amongst the different signals are secondary electrons and characteristic X-rays [18]. to a small area closest to the spot of incidence of the electron beam. Therefore, these electrons are sensitive for differences in surface topography [19]. In this project two SEM systems are used: The FEI Verios 460 and the FEI Helios Nanolab 600. The latter combines an electron microscope with a focused ion beam, as will be discussed in the next chapter. Both SEM systems have a Everhardt Thornley Detector (ETD) for the detection of secondary the electrons. Samples that are to be examined are often coated with a conducting layer, to be able to extract the electrons and prevent charge accumulation in the sample, causing a static electric field that can disturb the electron beam [20].

3.2.2 Electron Dispersive X-ray Spectroscopy

As mentioned in the previous section, in the imaging mode of the SEM, secondary electrons are used to image surface topography and create an image with up to nanometer resolution. With the interaction of the

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electron beam and the sample, also radiative transitions can take place in the atoms of the sample. In this process x-rays can be produced from the K, L and M spectral lines, dependent on the energy of the incoming beam. The energy of these x-rays are characteristic for an element of the periodic table. The FEI Verios 460 has an energy dispersive photon detector to detect the characteristic x-rays. From the resulting spectrum of this detector, which shows the counts per x-ray energy, the material of the sample can be characterised [20]. As a rule of thumb, in order to get enough signal, the energy of the electron beam should be about three times the energy of the spectral line that one wants to probe. However, one should take into account that increasing the energy of the electron beam, the interaction volume within the sample increases. When probing microstructures, this can lead to the fact that also x-rays are emitted by the background of the sample.

3.2.3 Raman Spectroscopy

Raman spectroscopy is a spectroscopic method that is able to characterise both organic and inorganic materi-als and measure the crystallinity of solids. It is based on the inelastic scattering of an incident photon from an intense monochromatic light source (e.g. a laser) on the probed material. In the event of inelastic scattering the photon imparts energy to the lattice exiting a phonon. A photon can also absorb energy and momentum (Stokes-shifted scattering) or the inverse happens, where energy and momentum of the phonon is absorbed by the photon (anti-Stokes-shifted scattering). The former one is the stronger mode and is therefore more often used in Raman spectroscopy. The Raman shift (usually given in cm−1) is the difference in wavenumber between the incoming and outcoming photon. In a Raman spectrum, the intensity is plotted versus raman shift [21].

A phonon dispersion relation shows all the phonons that can live in a certain material. Figure 3.9a shows an example of a phonon dispersion curve for a diatomic lattice. Because of the conservation of energy and momentum in the scattering events, only the phonons at the center of the Brillouin zone can be probed (q = 0 in figure 3.9a). The phonon dispersion relation, including the energies of the phonons at the centre of the Brillouin zone, is typical for a certain crystal lattice [22]. Therefore the peaks in a Raman spectrum are characteristic for a certain material, and by comparing the measured spectrum with known Raman spectra, the material can be characterised.

Crystallinity, temperature and stress are material properties that can shift and/or broaden the peaks in a Raman spectrum. For example, in amorphous materials the periodicity of the lattice is interrupted. This means that the phonon is confined to a crystal grain, and the wavevector is less well defined. The uncertainty in wavevector result in an uncertainty in phonon frequency and energy, as shown in figure 3.9a. This results in an asymmetric broadening of the corresponding peak in the Raman spectrum [22].

Stress and strain affect the phonons in a crystal lattice, resulting in a shift in measured Raman spectrum [21]. Also temperature affects the Raman spectrum, because it results in expansion of the lattice and a decrease of the force constant. It should be noted that also the Raman laser itself can cause heating effects, especially when the laser is focused on a single spot and heat dissipation is not sufficient. Figure 3.9b show the effect of increasing laser power on the Raman spectrum of a nanostructured CuO film. When the laser power increases, the Raman peaks shift and broaden [23].

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(a) (b)

Figure 3.9: (a) shows a phonon dispersion curve for a diatomic lattice. An uncertainty in wavevector (x-axis) results in an uncertainty in frequency (y-axis) [22]. (b) shows the effect laser power in a Raman spectroscopy experiment for a 633 nm laser on a CuO nanostructured sample. The peaks both shift and broaden as a result of heating by the laser [23].

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4

Pipette fabrication and characterisation

The pipette is a key part of the system used for meniscus confined electrochemical deposition. Therefore, the fabrication and characterisation of the pipettes are important parts of setting up and characterising the system and will be treated in this chapter. The first section discusses how the pipettes are pulled from glass capillaries and further prepared for use and characterisation. The second part of this chapter focuses on the characterisation of the pipettes, by discussing an in-situ method to determine the nozzle size of the pipettes.

4.1 Pipette fabrication

The fabrication of the pipettes consists of three steps. In the first two steps, the pipettes are pulled and coated with a gold layer. The conducting gold is needed to be able to nicely image the pipettes with a scanning electron microscope (SEM) for precise size measurements of the pipette nozzle, as discussed in the previous chapter in section 3.2.2. In the third step, the very tip of the pipette is cut off using a focused ion beam (FIB) in order to create a clean cut nozzle.

4.1.1 Pipette pulling

The pipettes are pulled from 100 mm long borosilicate capillaries of World Precision Instruments, that have an outer diameter of 1 mm and an inner diameter of 0.58 mm. The capillaries have a filament, a glass rod of ∼ 160 µm annealed to the inner wall, creating capillary action and allow more easy back filling of the electrolyte into the pipette taper and tip. The pulling is done using a P-1000 micropipette puller of Sutter Instruments. A schematic of the pulling mechanism of the pipette puller is shown in figure 4.1. The heating filament in the pipette puller heats up the centre of the glass capillary, causing the glass to soften. At the same time, the ends of the capillary are pulled apart until the glass in the middle breaks, resulting in two pipettes of approximately 50 to 60 mm long. Different parameters in the pulling program can be adjusted in order to create pipettes with a desired nozzle size, which is discussed in the following paragraph.

The first parameter to set is the heat parameter, that determines the current through the heating filament and thereby the heat that is used to soften the glass. The required heat is very much dependent on the type of capillary and type of heating filament used, so a test run should always be done in case one of these two is changed or when a new pulling program is written. If the heat setting is set too high or too low, one risks burning out the heating filament or damaging the puller. A gravitational force in the pulling mechanism always exerts a minimal pull, through which the clamps are pulled sideways. The velocity sets the velocity of the clamps at which the heating stops and the cooling starts. The lower this setpoint, the shorter the taper. Apart from the pull of the gravitational force on the pulling mechanism, one can exert an additional hard pull once the cooling has started with the pull parameter, resulting in more tapered pipettes. The delay parameter sets a certain delay before the hard pull is initiated. The longer the pull is delayed, the more the glass is cooled and the more viscous the glass will be at the time of the pull. A longer delay will therefore result in a shorter taper. The last parameter that can be changed is the pressure, that sets the pressure of the air that is used to cool down the glass. A higher pressure results in faster cooling and therefore in a shorter taper.

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Figure 4.1: A schematic of the pipette puller. The capillary is mounted through the heating filament and fixed on both sides with a clamp. While the filament heats up the centre of the capillary, the clamps are pulled to the sides, pulling two pipettes out of one capillary.

Not every combination of settings results in the capillary to split into two pipettes directly after one run of the program (for example when the velocity is set too low). In that case, the pipette puller keeps looping the program until the capillary does split. It is also possible to write a program with multiple lines with different settings. This can help to create pipettes of which the nozzle is gradually tapered close to the base, but is sharply tapered at the very end of the tip, as can be seen in figure 4.2a and 4.2b. A zoom of the very tip is shown in figure 4.2c. This type of pipette is easier to fill than a very long and more gradually tapered tip.

The most used pull program for this project is a two line program that uses the settings as shown in table 4.1. The resulting nozzle diameters of this pulling program are shown in table 4.2. The total amount of pipettes taken into account is 24. The diameter of the pipettes is determined using the SEM and has a mean of ∼700 nm, with some diameters as small as 200 nm. A more elaborate overview of the pull process and the different pull parameters is given in appendix A.1.

Table 4.1: The pull settings of the most used pull program. The program consist of two loops with different settings, executed in order. For the heat parameter, ’Ramp’ is the value obtained from a ramp test and changes for different combinations of type of glass and heating filaments.

LOOP # HEAT PULL VELOCITY DELAY PRESSURE

1 Ramp - 5 60 45 150 500

2 Ramp + 5 140 55 140 500

Table 4.2: An overview of the resulting inner diameters of the pipettes pulled with the settings as shown in table 4.1. N is the total amount of pipettes pulled with this settings. The inner diameter of the pipettes is measured using SEM.

N Mean inner diameter (nm) Min. (nm) Max. (nm) St. Dev. (nm)

24 712 213 1160 294

4.1.2 Pipette coating

In the next section on pipette characterisation, an in-situ method for the determination of the nozzle size of the pipette is discussed. However, in order to get more and precise information on the shape and size of the pipette, taking an SEM image of the pipette is the most reliable method to use. To be able to clearly image the pipettes with the SEM, the pipettes need to be coated with a metallic layer.

The pipettes are therefore coated with a 2 nm layer of chromium and an approximately 30 to 40 nm layer of gold, using a Leica 6000 sputter coater. The recipe for this sputtering process can be found in appendix

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(a) (b) (c)

Figure 4.2: SEM images of different magnifications of a typical pipette used in this project, showing the different tapers in the capillary as a result of different lines used in the pull program. It also shows the final nozzle diameter of ∼ 1µm.

A.2. The chromium serves as an adhesion layer, to make sure the gold sticks well to the pipette. Since the chromium and gold need to be applied on all sides of the pipettes, it is easiest to sputter the pipettes from the top. In order to make this possible in the sputter coater, a special holder was fabricated, as shown in figure 4.3.

Figure 4.3: The pipette holder used for sputtering a layer of chromium and gold on the pipettes, ensuring that the pipettes are kept upright so that they are uniformly coated.

4.1.3 Pipette FIB’ing

Since the pipettes are standing up straight when being coated, the opening of the nozzle is also coated. This results in a more roughened tip. In order to get a clean cut edge of the nozzle, the very tip of the pipette is milled away using the FEI Helios Nanolab 600, that combines a SEM and a focussed ion beam (FIB). Also here, a special pipette holder is used to fix the pipettes in the SEM chamber, as shown in figure 4.4. This holder can be tilted in such a way that the ion beam is perpendicular to the pipette’s tip, so that only the very tip can be milled away. The ions used in this process are gallium. The beam current used was 9.7 pA and the ions were accelerated with an acceleration voltage of 30 kV. The step by step process for using the FIB in this configuration is provided in appendix A.3. A representative image of this process is shown in figure 4.5, showing a before and after SEM image of the nozzle of a pipette. In figure 4.5b the clear cut nozzle and the filament of the pipette are nicely visible.

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(a) (b) (c)

Figure 4.4: The special pipette holder used for working with SEM and FIB. In (a) is shown how the pipettes are mounted in the holder. (b) shows how the holder is placed in the SEM/FIB chamber. The stage that the holder is mounted on, is able to tilt (52o_{) such that the ion beam comes in perpendicular with respect}

to the pipettes. This is shown in the schematic in (c). In this configuration the ion beam can mill away the very tip of the pipette, resulting in a clear cut nozzle.

(a) (b)

Figure 4.5: SEM images of the result of the FIB’ing process. In (a) the pipette is shown as it is after coating. (b) shows the pipette after the tip is cut-off using the FIB, where the coated gold layer on the outside of the pipette is clearly visible. For both images the pipette is tilted 52o.

4.2 Pipette characterisation

In order to do predictions on the size of the to be grown structures, and what settings for potential and retraction speed are required for successful growth using a specific pipette, it is important to know the nozzle size of the pipette that is used. In this section, an in-situ method for measuring the pipette size is discussed.

4.2.1 The pipette’s resistance

An electrolyte typically has a certain resistivity for the ions to move through it, just as a certain metal has an resistance for electrons to move through. The total resistance between the two electrodes in the system gets higher as the distance between the electrodes gets longer and as the cross section of the electrolyte volume between the two electrodes gets smaller. In case of a micro pipette, the cross section is only in the order of square microns. As discussed in the following section, the resistance is mainly determined by the size and shape of the pipette, and therefore a resistance measurement can be used to estimate the size of the pipette.

The resistance through an electrolyte volume is given by: R = L

κA (4.1)

where L and A are respectively the length and the cross section of the electrolyte volume, and κ is the conductivity of that specific electrolyte. The conductivity of an electrolyte is, amongst other parameters,

Meniscus confined electrochemical fabrication of copper nanostructures

Master Physics and Astronomy

Science for Energy and Sustainability

Master Thesis 60 EC

Conducted between 05-11-2018 and 03-10-2019

Meniscus Confined Electrochemical

Deposition for 3D microprinting

by

Rosa Rougoor

UvA Student ID: 10610901

Supervisor

Dr. Esther Alarc´

on Llad´

o

Daily supervisor

MSc. Mark Aarts

Second Examiner

Prof. dr. Erik Garnett

Abstract

Contents

1

Introduction to 3D micro printing

2

Electrochemical deposition

2.1

General electrochemical deposition

2.1.1

Electrode processes

2.1.2

Electrochemical techniques

2.1.3

Nucleation

2.2

Meniscus confined electrochemical deposition

2.2.1

Dimensions of the system

2.2.2

Growth rate

2.2.3

The effects of pipette’s retraction speed

3

System set-up and characterisation tools

3.1

System set-up

3.1.1

SECM modes for engaging

3.1.2

SECM modes for growing

3.2

Characterisation tools

3.2.1

Scanning Electron Microscopy

3.2.2

Electron Dispersive X-ray Spectroscopy

3.2.3

Raman Spectroscopy

4

Pipette fabrication and characterisation

4.1

Pipette fabrication

4.1.1

Pipette pulling

4.1.2

Pipette coating

4.1.3

Pipette FIB’ing

4.2

Pipette characterisation

4.2.1

The pipette’s resistance