PULSED PLASMA DEPOSITION OF SURFACE FUNCTIONAL THIN FILMS

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Nickolas R. Kaiser

May 2017 PULSED PLASMA DEPOSITION OF SURFACE FUNCTIONAL THIN FILMS

Nickolas R. Kaiser

Thesis

Approved: Accepted:

______Advisor Dean of the College Dr. Ali Dhinojwala Dr. Eric J. Amis

______Faculty Reader Interim Dean of the Graduate School Dr. Coleen Pugh Dr. Chand Midha

______Department Chair or School Director Date Dr. Coleen Pugh

ii ABSTRACT

Radio Frequency (RF) Plasma deposition has proven to be an unusually convenient and universal surface-modification and coating technology for grafting thin film for applications in which solution chemistry is difficult or entirely impossible, or adhesion to a low energy substrate surface is desired1. The one-step gas to solid-phase nature of the process eliminates liquid solvents, which are otherwise required for spin coating, electro- deposition, and other traditional coating processes. The technique uses excited plasma in a volume of monomer vapor, forming reactive energetic species (radicals and ions). The recombination of surface-bound free radicals and ions with airborne radicals and oppositely charged ions creates strong substrate-independent covalent attachment at the interface2. Pulsing of the incident electrical energy significantly reduces the total energy absorbed by the targeted vapor, subsequently minimizing bond scission and energetic structure rearrangement to retain useful functional groups.

In this work, Terpyridine was tethered to various substrates and complexed with iron, forming a film that may readily complex with other Terpyridine-coated substrates to form an adhesive bond. Thin films of reactive anhydride were first deposited by maleic anhydride vapor in a pulsed plasma process. The highly reactive anhydride group was retained in the plasma with low input power, short duty cycle on-time, and long duty cycle off-times3. A primary amine-functional Terpyridine was tethered to the anhydride film via aminolysis and heated to form a stable maleimide linkage.

iii TABLE OF CONTENTS

List of Figures ...... vii

List of Tables ...... ix

CHAPTER

I. FUNDAMENTALS OF PLASMA

Introduction ...... 1

Plasma state ...... 1

Fundamentals of Gas and Molecular collisions ...... 3

Classification of plasmas ...... 6

Hot vs. Cold plasma ...... 7

Hot plasmas (near-equilibrium plasmas) ...... 8

Non-equilibrium (cold plasmas) ...... 9

Low-pressure, non-equilibrium plasmas ...... 12

Atmospheric pressure, non-equilibrium plasmas ...... 13

Modulated plasmas (pulsed plasmas) ...... 14

II. APPLICATIONS OF PLASMA TECHNOLOGY

Applications of non-equilibrium plasmas ...... 16

Plasma Sputtering ...... 18

Plasma Surface Modification ...... 19

Plasma Etching...... 22

Plasma Sterilization ...... 23

Plasma Polymer Deposition ...... 24

iv III. DEPOSITION AND DERIVITATION OF FUNCTIONAL FILMS

Maleic Anhydride Chemistry ...... 28

Maleic Anhydride Deposition ...... 31

Maleic Anhydride Derivatization ...... 33

IV. DERIVITIZATION AND ADHESION

Introduction ...... 34

Reversible Adhesion ...... 34

Gecko Tape ...... 35

Click Chemistry ...... 36

Terpyridine ...... 37

V. EXPERIMENTAL METHODS

Introduction ...... 39

Equipment and Apparatus ...... 40

Materials ...... 43

Methods...... 45

VI. RESULTS AND DISCUSSION

Maleic Anhydride Plasma Polymerization ...... 48

Maleic Anhydride Plasma Polymer Derivitization Reactions ...... 50

Diethylene Triamine ...... 50

Aminopropyltriethoxysilane ...... 52

Rubidium-complexed Aminophenyl terpyridine ...... 54

Pentylamine Terpyridine ...... 58

v VII. CONCLUSION AND FUTURE WORK ...... 63

REFERENCES ...... 65

vi LIST OF FIGURES

Figure Page 1.1 Range of plasmas ...... 2

1.2 Common States of Matter ...... 3

1.3 Debye Sphere Illustration ...... 4

1.4 Schematic of Plasma Stars ...... 7

1.5 Plasma Reactor Schematic ...... 10

1.6 Atmospheric Plasma Reactor Schematic ...... 14

2.1 Schematic of plasma chamber ...... 19

2.2 Pulsed Plasma Reactor Electrical System Schematic ...... 27

3.1 Molecular Structure of Maleic Anhydride ...... 28

3.2 Examples of Cycloaddition with Maleic Anhydride ...... 30

3.3 Molecular Structure of poly(Maleic Anhydride) ...... 31

3.4 Schematic for the Aminolysis of Maleic Anhydride ...... 33

4.1Schematic of Gecko Foot Pad Adhesion ...... 36

4.2 Mechanism of A Click Reaction ...... 37

4.3 Aminolysis and Amidization of Maleic Anhydride with Aminoterpyridine ...38

5.1 Photograph of Pulsed Plasma Reactor Used in Studies ...... 41

5.2 Molecular Structure of Aminophenyl Terpyridine ...... 43

5.3 Synthesis Schematic of Aminopentyl Terpyridine ...... 44

5.4 H1-NMR Spectrum of the Aminopentyl Terpyridine Product ...... 45

6.1 Grazing angle FT-IR spectrum of Maleic Anhydride film ...... 49

6.2 XPS Elemental analysis of MA film ...... 50

vii 6.3 Grazing angle FTIR Spec of Maleic Anhydride/Diethylene Triamine Film ...51

6.4 Magnified FTIR Spectrum of Maleic Anhydride/Diethylene Triamine Film .51

6.5 frazing angle FTIR Spectrum of Maleic Anhydride/Aminosilane Film ...... 52

6.6 Magnified FTIR Spectrum of Maleic Anhydride/ Aminosilane Film ...... 53

6.7 XPS Elemental analysis of Maleic Anhydride/ Aminosilane film ...... 54

6.8 FTIR Spectrum of Maleic Anhydride/Aminophenyl Terpyridine ...... 55

6.9 XPS analysis of Maleic Anhydride/ Aminophenyl Terpyridine film ...... 56

6.10 Magnified XPS Peak of Nitrogen ...... 57

6.11 Magnified XPS Peak of Rubidium ...... 57

6.12 Grazing angle FTIR of Maleic Anhydride/Aminopentyl Terpyridine ...... 59

6.13 Magnified FTIR of Maleic Anhydride/Aminopentyl Terpyridine...... 59

6.14 XPS spectrum of MA film ...... 61

6.15 XPS spectrum of MA film with Amide ...... 61

6.16 XPS spectrum of MA film with Imide ...... 62

6.17 XPS spectrum of MA: Terpyridine: Iron Complex ...... 62

viii LIST OF TABLES

Table Page

2.1 Bond Dissociation Energies ...... 17

6.1 FTIR Absorption Bands Important to this Research...... 48

ix CHAPTER I

FUNDAMENTALS OF PLASMA

The chemical modification of surfaces using a glow plasma discharge has been performed for a large part of the 20th Century, beginning with Linder and Davis4, who measured the gas and solid products of hydrocarbons exposed to a glow discharge.

Investigations probing the change in dielectric properties of inherently insulating olefin materials in the presence of strong electric fields2 precluded irradiation of starting materials with charged particles to induce chemical changes. An undesirable deposit of carbon formed in certain electric discharge processes was found to be an excellent coating under controlled conditions. Further research involved variation of process parameters to control chemical and physical properties of exposed materials5. Later, utilization of a pulsed discharge to retain starting material characteristics by limiting overall power input6 would enable solvent-less film deposition by mechanisms which closely resemble those of more traditional polymerization methods.

Plasma State

Plasmas are by far the most abundant phase of matter in the known universe, both by mass and by volume. Due to the tremendous heat that is found at the surface of stars, all the material that they are comprised of is found in the plasma state. Even the space between stars is populated by very sparse plasma. This interstellar medium can have a density as low as one particle per cubic meter, or as high as 1030 per cubic meter.

Ionization, or the separation of polar components of atoms and molecules into oppositely

1 charged states, is the only thing that is necessary for a gaseous substance to be considered in the plasma state.

Figure 1.1 Illustration of plasmas by temperature and electron density.

Plasma has come to be known as the 4th state of matter, due to its unique characteristics in comparison to solids, liquids, and gases. However, plasma shares a few of the characteristics of each of the other states of matter. Like many solids and liquids, plasmas are able to transfer charge by flow of electrons from negative to positive electrodes or charge centers. Like gas, plasma has an undefined shape and volume. The defining characteristic of plasma that separates it from the similar gaseous state is plasma’s inherent ability to conduct electricity. The mobility of discrete positive and negative charge carriers in the material allows it to be manipulated in the presence of an electric field7.

2 Figure 1.2 The most common states of matter in everyday life.

Fundamentals of Gas and Molecular Collisions

Plasma is defined as a conductive, yet electrically quasi-neutral state of ionized gas which consists of mainly positively charged molecules or atoms (ions) and negatively charged electrons. The densities of positive and negative species contained in a plasma realm tend to be equal, thus the system remains electrically neutral overall. The distinguishing feature of a true plasma is that the charged particles are close enough together that each particle can magnetically influence multiple nearby particles, rather than just their closest neighbors. This definition is known as the plasma approximation.

It is valid only when there are at least two charge carriers within the Debye sphere, a sphere whose radius is the distance, or Debye screening length, by which a particle can influence other particles.

3 Figure 1.3 Charged particle alignment within an electron’s Debye sphere of influence.

When two particles collide with one another, many different physical changes may occur depending on the mass, charge, energy, temperature, and shape of the particles.

For example, the particles may exchange momentum or energy, neutral atoms ionize, or ionized particles neutralize. When momentum is the only energy that is transferred from one particle to another, the collision may be called an elastic collision. If excitation and/or ionization occur, the collision is referred to as inelastic. In a classical and limited- scope introduction to particle collisions, it is sufficient to discuss the interactions between electrons, positive and negatively charged ions, and neutral gas molecules. In the event that electrons elastically hit much more massive atoms, electron momentum is most

4 severely affected while the atom’s momentum may be only slightly changed. Inelastic processes observed when electrons collide with larger particles can involve excitation or ionization of the atom. However, when ions collide with atoms, resonant charge transfer is found in addition to the elastic scattering mechanisms where momentum and energy are exchanged. The processes that can manifest in molecular gases are much more diverse. These may include dissociation, dissociative recombination, attachment and detachment, positive-negative ion charge transfer, and processes involving excitation of molecular vibrations and rotations.

According to the first law of Classical Thermodynamics, energy may not be created nor destroyed. The total momentum and energy of particles at the end of a collision are equal to the momentum and energy before the collision. In an elastic collision, if internal energy does not change, the sum of kinetic energies of the two particles doesn’t change.

In most inelastic collisions, which involve energy increases by excitation and ionization in addition to momentum changes, the total kinetic energy is decreased. Conversely, an excited atom may be de-excited by a collision, increasing the total kinetic energy of the system. Essentially, excitation energy is converted to kinetic energy, or motion. This type of collision is called Super-Elastic. For electrons and fully stripped ions, only kinetic energy is available for transfer. This is due to the single body nature of these species. A more complex atom and partially stripped ions are in possession of internal energy level structures, and can be excited, de-excited, or ionized, corresponding to potential energy changes in the spatially diverse environment. This transfer of energy levels from one molecule to another is the basis of the useful chemistry associated with plasma reactions. 5 Under the influence of a glow discharge, many different chemical species can be

formed. Chemistry changes within the plasma sheath depend on the qualities and

quantities of particles in exposure to the plasma glow, power and frequency of the energy

introduced to maintain the plasma state, and other process parameters.

Classification of Plasmas

Plasmas can be broken down into many classifications based on the mechanism by which the material comprising the plasma is broken down into its charged substituents, and the amount of energy that the particles possess. Thermal energy produced by thermonuclear reaction is the most abundant source of plasma in the universe. This type of plasma is what gives stars their glow, as illustrated by the schematic of a star, Figure

1.4. Electrical energy is the most common form of energy to be used for artificial, or man-made plasma generation. The first classification of plasma is based on the thermal energy, or temperature, a particle possesses.

6 Figure 1.4 Components of a Star. The star’s material is in a plasma state.

Hot vs. Cold Plasma Characteristics

It is common to characterize plasmas by the average temperature of the particles which they are comprised of. As the term average implies, the particles in a plasma discharge exist in a range of widely varying temperatures. Because of the large difference in mass between dissociated particles of a molecule, the lighter particles, namely electrons, come to thermal equilibrium with other electrons faster than they come to equilibrium with ions and neutral atoms. For this reason, the temperature of large ions and particles may be much lower than the temperature of neighboring electrons. This condition gives the name of “non-equilibrium” to these plasmas. They are also commonly called cold plasmas because of the relatively low temperature of the heavy

7 particles. Cold plasmas are often man-made, and are intentionally sustained by externally-applied energy sources.

Despite heavy particles’ higher capacity to absorb heat, there still exists plasmas in which the average thermal energy of these ions and neutral atoms is equal to the energy of the electrons. These plasmas are often referred to as hot plasmas because of the high average temperature of both the light and heavy particles. Since the temperatures of all of the species (electrons, ions, and neutrals) are equal in this situation, this type of plasma may be called equilibrium plasma. There are still many subcategories of equilibrium and non-equilibrium plasmas to discuss.

Hot Plasmas (Equilibrium)

As energy is added to a gas or plasma, the degree of ionization rises, and the ion/particle temperature reaches thermal equilibrium with the high-energy electrons.

When the ion temperature has increased and the degree of ionization approaches 100 percent, the plasma may be referred to as an equilibrium, or hot plasma. Terrestrial examples of hot plasmas include electrical arcs, plasma jets of rocket engines, and plasmas generated by a nuclear reaction. Natural plasma accounts for more than 99% of the matter in the universe, although much of this is extremely low particle density.

Extremely dense plasmas are also found in outer space, at the center of stars. The extreme heat given off from the nuclear fusion reaction sustains equilibrium plasma in and around a live star.

8 Cold Plasmas (Non-Equilibrium)

Non-equilibrium plasmas are so named because the temperatures of their particles are not at thermal equilibrium with one another. In general, the charged species in the plasma possess a much higher kinetic energy than the neutral species. This form of plasma requires energy to be continuously added to the system to maintain thermal ionization of the atoms. Energy is mainly distributed among particles by collisional and radiative processes. It is introduced to a system by thermal, mechanical, chemical, radiant, nuclear, and electrical processes. This energy should be equal to or greater than the amount of energy that is lost to the walls of the container and to recombination of oppositely charged species. All forms of artificial plasmas could be characterized as non- equilibrium plasmas, because it is not possible to maintain a controlled system that completely conserves or produces its energy. It is possible, however, to control the density of particles in a system by controlling the air pressure within the plasma cloud.

Man-made plasmas can be created by raising the energy content of matter by mechanical, thermal, chemical, nuclear, or electrical means. Unlike naturally occurring equilibrium plasmas, such as those found around stars, man-made plasmas may only be sustained by continuously supplying energy, which is lost to the environment.

Electrical Energy may be introduced into matter either by capacitive coupling, in which electrons are accelerated from an anode toward a cathode, or by inductive coupling, in which a spiral-shaped “antenna” wrapped around a quartz vessel induces a current into matter inside the vessel. The electrical supply for these systems may be a

9 direct current (DC), or an alternating current (AC), although AC has become much more

common as plasma equipment evolves.

Figure 1.5 Schematic of inductively- and capacitively coupled plasma reactors.

DC plasma is the inaugural form of low-temperature non-equilibrium plasma whose

properties are highly system dependent. System pressure, applied voltage between

electrodes, and the distance between electrodes all play important roles in the

characteristics of a DC plasma8.

Some of the highly system-dependent characteristics of direct-current (DC) plasma

are its visual glow, distinguishable zones of varying particle identity, and a constant

potential difference between electrodes9. Process parameters, including pressure, electrode separation, and applied voltage all have a role in determining the identity and characteristics of the DC plasma glow7. DC plasma consists of two metal conductive electrodes configured parallel to each other inside a reactor vessel. Unfortunately, electrodes which are encased inside the reacting vessel of a DC system will eventually be

10 covered with insulating deposits of organic monomers, extinguishing the glow10.

Therefore, electrodes are preferentially configured outside the reactor to retain conductivity of the electrode material, and avoid contamination of the end product. The inherent problems of DC configurations can be avoided with the use of alternating current (AC) to cyclically switch the polarity of the electrode plates.

When a voltage is introduced between two parallel plates under the influence of an alternating current (AC), the plates switch between cathode and anode with a specific frequency of alternation. When the frequency of alternation between polarities is slow, there is a comparatively long pause in radiation during which the voltage between electrodes drops below the breakdown voltage, which must be exceeded to retain a plasma glow. At elevated frequencies, the polarity alternates quickly enough that the energy required to retain the discharge during the cycle is lowered by the continued existence of residual ionized species from the previous cycle10. Therefore, with higher frequency alternation, plasma glow can be retained, despite a short zero-voltage drop during each polarity switch of the applied field, with a lower applied voltage than those required with low frequency alternation.

The majority of AC-powered high-frequency plasmas are regulated for Industrial,

Scientific, and Medical (ISM) uses at 13.56 MHz to avoid interference with amateur radio operator bands operating in the 10 and 24 MHz HF bands11. For this reason, they are referred to as radio frequency (RF) plasmas. At this relatively high frequency of oscillation, a majority of the electrons gain energy through oscillation in the electric field without extinguishing by contact with the electrodes. The plasma can be coupled to the

11 energy supply’s electromagnetic energy with either capacitive electrodes or an inductive antenna wrapped around the reaction vessel. Two advantages of the RF type plasma are that an RF plasma can be sustained with an external electrode as well as an internal electrode, and it can be ignited and maintained by either conductive or nonconductive electrodes.

High-frequency plasmas which utilize frequencies of 2.45 GHz are commonly referred to as Microwave (MW) plasma. The mechanism which produces a plasma glow with microwave-frequency oscillation is similar to the excitation with RF plasma, except that 2.45 GHz nearly matches the collision frequency of electrons. Because of this matching frequency level, power absorption by electrons is more effective in MW plasma than in RF plasma. This allows microwave plasmas to be used more effectively with high-density plasmas, as it is easier to drive more particles with efficient frequency matching.

Low pressure, non-equilibrium plasma

Mean free path of a particle is a measure of the average, or mean distance a particle travels in a system before making contact with another particle, and therefore changing its energy, velocity, and occasionally chemical makeup. The particle density, or pressure, of a system has a strong influence on the interaction between the particles contained within the system. Impedance, or resistance, to particle movement of a discharge decreases with increasing frequency; but the frequency of the alternating current must be extremely high at pressures near atmospheric (1 Bar). When the pressure of a system is

12 large, the mean free path is very short, which does not allow particles to gain a large amount of energy in a low-power system.

There are many advantages to using Radio-Frequency (RF) AC systems over DC systems. RF systems may operate at low pressures, which is important because the impedance of a discharge increases with the decreasing collision frequency at low pressure. The ionization mechanism of AC systems is more efficient than a DC system because electrons gain energy throughout the whole cycle. Since the transfer of energy to electrons is more efficient, the plasma sheath of an RF-AC system is more spatially uniform than a capacitive DC system.

Atmospheric pressure, non-equilibrium plasma

Plasma that is produced in atmospheric pressure conditions is very useful for functional environments where it is not feasible to produce the discharge in the closed batch process that is required in high-vacuum setups. Although the atmosphere surrounding the discharge must be carefully regulated to avoid contamination of the desired product, the absence of complex vacuum systems and closed reactors makes this a cheap and convenient processing method.

13 Figure 1.6 Schematic of atmospheric plasma torch reactor.

Modulated (Pulsed) Plasmas

In order to control the energy that is introduced in a plasma processing system,

technology has been created to pulse the plasma glow. This technique significantly

reduces the overall energy introduced to a vaporous material, limiting the occurrence of

bond scission and reducing concentration of reactive centers to retain chemical structure

and avoid crosslinking. The parameters of plasma processing that are varied for this

purpose are: carrier frequency, pulse period (time on + time off), and duty cycle (time

on/pulse period). The duty cycle dictates the average power Pintroduced to a system

by the equation12:

P= Ppton/(ton + toff)

where ton/(ton + toff) is defined as the duty cycle and Pp is the peak power, or the power of the unit under continuous equilibrium.

14 The carrier frequency is chosen to match the mean free path, λ, of a particular system, which is controlled by particle density, N, and therefore pressure, by the following equation1:

2 λ = 1/π(r1+r2) N

with r1 and r2 being the radius of the colliding particles,

and N = number of particles per unit volume.

The average power, P absorbed by an electron in this radio frequency (RF) AC system is characterized by the equation8:

e2E 2  P  0 e , 2 2 2me  e  

where νe = electron collision frequency, ω = frequency of field, and E0 = amplitude of the E-field. The mean power absorbed by electrons and ions in a plasma discharge may dictate the type and degree of physical and chemical change taking place. Pulsing a low power plasma discharge significantly lowers the overall power introduced to a system, allowing one to use plasma for chemical reactions requiring much lower activation energies.

15 CHAPTER II

APPLICATIONS OF PLASMA TECHNOLGY

Applications of non-equilibrium plasmas

The appeal of plasma polymerization could be attributable to both its solvent-less nature and substrate independence. The one step involved in the vapor to solid deposition process of plasma is far more economical in contrast to the several steps involved in a conventional solution polymerization. Free electrons within the plasma sheath collide with the substrate, forming radicals on its surface. These charged surface species react with floating monomer molecules, forming strong covalent bonds and a strong bulk adhesion between the monomer and substrate. It has been found that the nature of the substrate is not a significant factor in the strength of this adhesion.

Controlled formation of ions, electrons, and free radicals in a low temperature glow discharge has many uses in chemical and physical modification of material surfaces.

Man-made plasma conditions produce an electron energy range of roughly190 to 480 kJ/mol (2 to 5 eV), which is intense enough to dissociate a majority of chemical bonds in an organic structure, and to create free radicals capable of reorganizing into macromolecular structures. A sampling of typical bond dissociation energies is listed in

Table 2.1.

16 Table 2.1 Typical Bond Dissociation Energies in kJ/mol.

A few current applications of low-temperature plasma processes include modification

of material surfaces for specialized applications, sterilization of surfaces for clinical use,

and anti-corrosion deposition on surfaces of industrial metals. Plasma facilitates precise

modification of surface properties, either by deposition of material on the surface, or

sputtering material from it. Based on the kinds of monomer gases, low-temperature

plasma can be sorted into three groups1: chemically non-reactive plasma, chemically reactive but non-polymer-forming plasma, and chemically reactive and polymer-forming plasma.

The first group, chemically non-reactive plasmas, are primarily comprised of monatomic inert gases such as argon and helium. These plasmas can help ionize other molecules, modify the solid surface, or sputter materials but are not consumed by chemical reactions.

17 The second group of plasma species are chemically reactive but non-polymer-forming

plasma, are organic or inorganic molecular gases such as N2, CF4 and O2. Although these compounds can form reactive species such as ions and radicals in the plasma sheath, they will not polymerize in pure gas plasmas. However, it is possible to form a deposit when the gas mixture contains specific impurities. Volatile gas compounds in these plasmas can chemically etch the substrate surface.

The third group concerns chemically reactive and polymer-forming plasmas, which are organic gases or vapors such as methane and maleic anhydride. Thin films are deposited on the surface at the interface with active vapor species. The process of deposition on the solid surface that is formed with the polymer-forming plasma is called plasma polymerization or plasma chemical vapor deposition (CVD).

Plasma Sputtering

Plasma sputtering is the reaction of atoms that are ejected from the cathode surface when ions strike the cathode in the plasma system13. The sputtered atoms can deposit on the surface of a target or substrate material. The deposition rate of plasma sputtering on the substrate would rely on the amount of the ion energy and the ion generation on the target14–16. For most sputtering applications, the sputtering gas must not chemically react with the target or substrate, thus inert gases are used for sputtering processes. Plasma sputter etching is the process that occurs when material is removed from a surface by sputter ejection17.

18 Figure 2.1 Schematic of Plasma Sputtering of Titanium substrate.

Reactive plasma sputtering is widely used in thin film deposition where it is beneficial to add chemically reactive gases to the inert gas in the sputter deposition of compound films18,19. This technique offers almost unlimited opportunities to produce thin films of unique compositions including thin films of oxides, nitrides, hydrides, carbides, silicides, sulfides, selenides, etc20–22. However, this great adaptability is accompanied by reproducibility problems when metastable thin films are being deposited. In these cases, the film stoichiometry will depend on the relative anion and cation fluxes, the former coming from the injected gas and the latter from the sputtering target23. Precise control of the relative anion and cation fluxes is hard to achieve.

Plasma Surface Modification

In recent years, there has been an appreciable growth in the use of low-temperature plasmas in surface modification processes to avoid the chemical waste problems related with wet processing. The effluent from plasma reactors, which use chemically reactive

19 gases, might be toxic or corrosive, and must be carefully processed. Still, the much

smaller quantities of condensed plasma products compared to wet processing products

make this problem much less severe in plasma surface processing. Low-temperature

plasma surface modification is the process that alters the physical and chemical

characteristics of a surface of material without changing its bulk material properties24. It is becoming attractive and can be done on metals and alloys as well as ceramics and polymers. Some of the surface properties that can be modified by low-temperature plasma process include surface hardness, adhesion, corrosion resistance, fatigue, coefficient of friction, oxidation, electrical resistivity, toughness, and abrasive wear properties25,26. Examples of low-temperature plasma surface modification processes are the previously-mentioned plasma oxidizing and nitriding, along with oligomer and polymer surface modification.

The use of oxygen plasmas to deposit oxide films on metal or semiconductor surfaces is one of the oldest plasma-based surface processing techniques. As with plasma chemical vapor deposition systems, the major advantage of plasma oxidation is the ability to deposit oxide films at low temperatures or on surfaces that do not oxidize readily upon exposure to molecular oxygen. The applications of plasma oxidation are mostly in the oxidation of silicon, gallium arsenide and indium phosphide, and in the growth of tunneling junctions on niobium, lead and related alloys14. These films are typically deposited under energetic ion bombardment conditions and relatively quickly attain a steady-state thickness, after which the oxide deposition rate and the removal rate by sputter etching are equal. The precise control of oxide deposition film thickness can be achieved by adjusting the plasma operational parameters such as power, pressure, gas 20 composition, in that each set of parameters results in a different steady-state oxide thickness.

Plasma nitriding is very similar to plasma oxidation. The major difference is that oxygen tends to become a negative ion reacting on metal or semiconductor surfaces, while atomic nitrogen does not27. For that reason, film growth by negative ion migration to the film-substrate interface is not a concern in plasma nitriding. Plasma nitriding is typically performed as a relatively energetic positive ion bombardment process and can be expected to behave similarly to cathode oxidation. Therefore, nitrogen atoms as well as energetic and molecular nitrogen ions play an important role in the nitride film- deposition process. At low temperatures where thermal diffusion rates are very low, the nitride film thickness is determined by the enhanced diffusion often associated with energetic ion bombardment of surfaces14. One of the most important commercial applications of plasma nitriding is the case-hardening of machine tools and other mechanical components28. In the microelectronics industry, there is attention in the plasma nitriding of semiconductor surfaces such as Si and GaAs.

An important function of low-temperature plasma polymer surface modification concerns polymer surface energy change24, and specifically the improvement of either wetting or hydrophobic properties of a polymer. Generally speaking, low-temperature plasma modification processing leads to impressive modifications of the wetting properties of polymers, but most studies have demonstrated that ion bombardment energy is the key factor for stability after treatment or for the treatment durability. The generally accepted plasma modification mechanism is that improved hydrophilicity is due to the

21 formation of oxygen-containing hydrophilic functional groups, and that the high-energy

ion bombardment produces deeper cross-linking of polymer chains, with the intention

that the modified layers are more resistant to hydrophobic recovery with time29. Much

of low-temperature plasma surface modification research is devoted to the polymer

surface property, such as cross-linking, chain scission, creation of functional groups, and

their relationship with the surface properties obtained after treatment.

Plasma Etching

Plasma etching involves the layer-by-layer removal of atoms from the surface of a

substrate by ions excited by plasma radiation. It results from ion bombardments that

move along electric field lines, since electric field lines are always vertical to an

equipotential surface, and resulting etch profiles are inherently perpendicular

(anisotropic)1,30. It contrasts with the isotropic profiles observed with wet chemical etching. Plasma sputter etching can be carried out in a conventional sputtering system with a plasma glow discharge or with an externally generated ion beam. When using either a plasma or an ion beam source, there is the possibility of charge-exchange collisions occurring with neutral atoms in the gas producing energetic neutrals which are not affected by the electric field lines and do not directly follow them to the target. In addition, the sputtered material from the target support could suffer collisions with the gas and be scattered back onto the substrate. This could lead to inaccurate results.

Precise pattern transfer to silicon wafers by physical etching has become a useful technique for semiconductor fabricating, where critical feature dimensions are less than micro-scale. Using plasma etching, it is possible to selectively remove material 22 efficiently normal to the surface, resulting in anisotropic etching13. This permits smaller

features to be etched onto a smaller surface area, a requirement for improvements in the

microelectronics industry. This has allowed plasma etching to become the primary

method in semiconductor fabricating processes, and it is the key to produce smaller,

faster, and cheaper microchips.

Reactive-ion etching (RIE) is defined as plasma etching with required simultaneous

energetic ion bombardment of the processed surface. In fact, the most important reason

that RIE has replaced wet chemical etching in semiconductor fabricating technology is

the ability of RIE to achieve anisotropic etching. Materials such as silicon, silicon

dioxide, silicon nitride, tungsten, and titanium used in micro fabrication readily lend

themselves to plasma etching in that they form volatile compounds under proper

conditions14. In addition to the applications in semiconductor fabricating technology, the ability of RIE processing to reduce the large quantities of liquid chemicals needed for wet processing is a decisive improvement.

Plasma Sterilization

A useful sterilization method capable of rapidly killing microorganisms and cause

less substrate damage is low-temperature plasma sterilization. Plasma sterilization takes

place at low temperature without damage to polymeric materials and is safer than gaseous

ethoxy- chemical sterilization31,32. To sterilize a surface, plasma is generated by radio

frequency and microwave discharges in a non-equilibrium state. In a low-pressure

plasma environment, the plasma energy and UV radiation are set to be weak for

23 sterilization, and as a result, the duration of a vacuum process treatment must be several minutes long.

Plasma Polymerization

Plasma polymerization or polymer deposition can be characterized by two mechanisms, one involving chain propagation and formation within the plasma discharge, the other taking place in the absence of plasma. Plasma-state polymerization involves polymer growth while in direct contact with plasma radiation. This mechanism represents the dominant condition in continuous wave (non-pulsed) reactions.

Conversely, plasma-induced polymerizations involve polymer growth in absence of a plasma discharge involving active species created while in contact with the active discharge. Plasma-induced polymerization is widely accepted to involve traditional polymerization pathways, with radical species dominating the propagation1,33. Additional active species are also present in the reaction vessel, but the very fast recombination of charged species leaves radical species as the main intermediate for plasma polymerization. Plasma-induced polymerization may take place during a plasma off-time in pulsed plasma polymerization, or spatially in the region outside of the glow discharge.

Low temperature plasma processes are usually used to deposit materials on surfaces.

Plasma chemical vapor deposition (PCVD) is the technique of forming solid deposition by initiating chemical reactions in plasma state24. Its major advantage over thermal chemical vapor deposition is the ability to deposit films at relatively low substrate temperatures34. PCVD is especially useful for materials that might vaporize, flow, diffuse, or undergo a chemical reaction at the higher temperatures. In this method, 24 vapor-state materials are introduced into a relatively low power density plasma system.

The gas molecules are dissociated by plasma, creating reactive radicals or atoms that

condense on the substrate24. Sometimes, the substrates are heated to improve the film

quality, but the substrate temperatures which are required are usually significantly lower

than those required for thermal chemical vapor deposition35. As with higher energy

methods, energetic ion bombardment of the growing film has a large influence on films

deposited by plasma chemical vapor deposition, but research into the mechanism is

limited.

Plasma polymerization is very similar to plasma chemical vapor deposition, the major

difference being that plasma chemical vapor deposition is concerned with depositing

inorganic films, whereas from plasma polymerization the resulting film is an organic

polymer27. Plasma polymerization is the formation of polymeric materials under the

influence of plasma1. It is an atomic polymerization different from conventional

polymerization24. Most of organic gases and vapors, even gases that cannot produce polymer by conventional polymerization methods such as methane, would produce polymeric material in the glow discharge. In the system, polymeric materials are deposited on the solid surfaces forming ultra-thin solid films called plasma polymers.

Plasma polymers have several advantageous properties such as being pinhole free, highly branched, and highly crossed linked37. Although they can form powdery and oily

products in certain cases, the formation of such products can be prevented by controlling

the system conditions and the reactor design. Due to the reaction, in which high energetic

electrons break the chemical bond in plasma polymerization system, plasma polymers

25 have no manifestly repeating units as conventional polymers. Therefore, it is hard to determine the properties of plasma polymers from used monomers in contrast to conventional polymerization which are linked together with a mere alternation of the chemical structure of monomer.

Plasma deposition is useful for applications where solution chemistry is difficult or entirely impossible, and where adhesion to a low energy substrate surface is required.

Plasma deposition is a vapor to solid phase process, and solvents are not required for deposition as is required for spin coating, electro-deposition, and other traditional processes. In these wet chemistry coating techniques, as many as seven steps are required: (1) synthesis of monomer, (2) polymerization of the monomer into a polymer or intermediate polymer to be further processed in a succeeding step, (3) preparation of coating solution, (4) cleaning and/or conditioning of the substrate surface by application of a primer or coupling agent, (5) application of the coating, (6) drying of the coating, (7) curing of the coating.38 The plasma deposition of a monomer, on the other hand, is essentially a one-step process. Contact of the substrate with plasma radiation produces radicals on the substrate surface which form covalent bonds with monomer molecules.

Generally, adhesion to metals in plasma deposition is stronger than to substrates such as glass and fluorinated materials, although adhesion to the latter is significant.

Pulsed plasma polymerization mechanistically involves both plasma-state and plasma-induced polymerization conditions. Pulsed plasma depositions are customized by operating parameters such as deposition time, input power, and duty cycle, the ratio of plasma on-time to total period (on-time + off-time). The implementation of a long plasma off –time in the pulse cycle involves a mechanism very similar to traditional 26 polymerization, and has been widely utilized to retain functional characteristics of the monomer. In the research which will be described here, pulsed plasma conditions are applied to make use of monomer functional groups for surface derivatization reactions.

Figure 2.2 Schematic of a pulsed plasma reactor system.

27 CHAPTER III

DEPOSITION AND DERIVITIZATION OF FUNCTIONAL FILMS

Maleic Anhydride Chemistry

The strong interest in Maleic Anhydride (MA) as a reactant is based on its dual functional groups: an ideal dienophile double bond and an anhydride moiety that can be hydrolyzed to twin reactive acid carbonyls, Figure 3.1.

Figure 3.1 Molecular Structure of Maleic Anhydride.

Nucleophilic radicals donate hydrogen or halogen to Maleic Anhydride, saturating the double bond. In the presence of a Diels-Alder catalyst, cycloaddition occurs with monomers such as Styrene39, Furan40, Ethylene and Ethyl Acrylate41, Acrylamide42 and many more, Figure 3.2. Electrophilic Addition at the anhydride is perpetrated by reagents including halogens, hydrohalic acid, water, and alcohols. Oftentimes, the double bond is reacted homogenously or in block copolymer fashion to incorporate the anhydride functionality to polymeric materials. Until recently, incorporation of MA into a polymer was accomplished with traditional polymerization methods in the presence of

UV radiation, free radicals, ionic catalyst, or the application of high pressure12. The intact anhydride group can be converted to the diacid for bioconjugation reactions43,44, or

28 react spontaneously with primary amine-terminated structures to easily graft polydimethylsiloxane41, polymer brush structures45, and myriad other structures. Plasma- enhanced chemical vapor deposition has opened new possibilities for this useful material.

29 O O

O O

O O

O O O

O O O

O O

O O

O O

O O O O O

O O O O

O O O

Figure 3.2 Examples of cycloaddition with maleic anhydride monomer.

30 Maleic Anhydride Deposition

The early attempts at plasma enhanced chemical vapor deposition of MA resulted in very low ratios of intact anhydride functionality6. This method utilized continuous plasma irradiation, which endlessly bombarded molecules with high powered radiation.

The materials that resulted from chemical vapor deposition with this high power closely resembled highly crosslinked polyethylene, with limited retention of oxygen-containing and other moieties. In 1971, Westwood observed that the chemical structure of various vinyl monomers polymerized by plasma radiation more closely resembled traditionally polymerized materials as the power input was lowered46. In 1996, Badyal and Ryan pioneered the use of modulated or pulsed plasma to significantly decrease the overall power input applied to plasma-polymerized Maleic Anhydride12, as in Figure 3.2.

Figure 3.3 Molecular Structure of Poly (Maleic Anhydride).

The physical and chemical properties of pulsed-plasma deposited maleic anhydride, and other plasma-deposited monomers, depends primarily on the electrical input power,

31 feed gas composition, monomer pressure, substrate temperature, and substrate position.

As mentioned, lower input power serves to preserve the structure of reactive monomer groups for subsequent functionalization reactions. Good retention of anhydride functionality is favored by short plasma on times and long off times in the pulsed regime.

Using Argon as a carrier gas, for one example, has been shown to enhance excitation of monomers at low power more efficiently. MA is a solid material at room temperature, but possesses a vapor pressure of 2.6 x 10-1 mbar due to above-average sublimation activity. It is necessary to discuss optimum processing conditions for plasma deposited

MA homopolymer with the most ideal chemical structure and functionality.

Maleic Anhydride homopolymer with maximum anhydride functionality intact is useful for many derivatization reactions. The landmark work by Ryan and Badyal increased the anhydride percent from a low of 8% to a high of 28% by raising the duty cycle (ratio of plasma on to plasma off times) while lowering the peak power to 5 Watts.

This study found that the optimum parameters are at 5W peak power with a 20µs on time and 1200µs off time12. Siffer et al. used statistical analysis to further perfect the process of pulsed plasma MA deposition using anhydride group quantity, along with surface roughness and deposition rates3. In this work, parameters of 5W peak power for a 25µs on time and 1200µs off time produced a 15.9nm film with 0.16nm surface roughness, and

32% anhydride functionality. As mentioned, these research teams, along with others working with pulsed plasma MA, have performed a great deal of research functionalizing the film using the aminolysis reaction with high success.

32 Maleic Anhydride Derivitization

Plasma-deposited MA homopolymer film has been used to impart surface groups of carboxylic acid44,47, alkene6,40, epoxide48,49, amine38, and grafted polymers45,50 by reaction of a primary amine with the acid anhydride. The addition of a primary amine to an acid anhydride is energetically favored, and will react at room temperature in minutes, Figure

3.3. This reaction is slower in dilute solutions of the reactants. In a vacuum system, the

MA surface is kept isolated from atmospheric moisture to avoid the competing hydrolysis reaction. The diacid product of anhydride ring hydrolysis is unfavorable, as the reaction kinetics of amine with acid is severely retarded.

There are a great many valuable uses for surfaces functionalized with terpyridine groups. Grafting of any terpyridine-terminated chain or moiety to the surface would be simple, and under FeCl2 complexation conditions, reversible.

Figure 3.4 Schematic for the Aminolysis of Maleic Anhydride

33 CHAPTER IV

DERIVITIZATION AND ADHESION

The surface chemistry of a material dictates the interaction it has with the surrounding environment. Examples of interaction on a material’s surface include wettability and adsorption, catalysis, repulsion and adhesion. Bulk materials often exhibit undesirable surface characteristics for specific applications. Tailoring the surfaces of these materials for specialized applications broadens the possible selection of bulk materials to optimize cost, mechanical and thermal properties, and surface energy. This can be accomplished with various complexations of plasma-deposited polymers with desirable compounds.

Reversible Adhesion

Wet and dry reversible adhesives are a highly sought after materials for manufacturing, construction, underwater exploration, research and everyday do-it- yourself consumers. Most pressure sensitive adhesive tapes are unable to form a bond on a wet surface. A majority of them leave a residue that may ruin the appearance of a surface or even affect the performance of a component. Adhesives that are able to hold a strong bond when required, then easily break the bond with minimal difficulty, are highly sought after and will find universal utility. Surface energy of “sticky” elastomers, such as may be tuned from soft in the direction normal to the surface, to hard in the loading directions in order to adhere and detach from a surface51. Biological researchers have

used photocleavable l-nitrobenzyl groups to reversibly attach polyethylene glycol to glass

slides for selective cell attachment52 Materials researchers have utilized the reversible

34 nature of some organometallic complexes to build metalloblock copolymer material, only to decomplex parts of the matrix to form a three-dimensional scaffolding with nanoporous structure for membrane separation, surface confinement, and other uses53.

Host-guest interactions of chemical groups have enabled systems such as adamantine/β- cyclodextrin54 and aminomethylferrocene/cucurbit[7]uril55 to form a Velcro®-like reversible attachment of two pre-patterned surfaces. Lu found that mussel foot proteins exhibit multiple adhesive mechanisms, including hydrophobic, π-π, cation-π and metal ion coordination to latch onto diverse substrates56. Researchers have also used nanometer-scale hierarchical structures engineered on surfaces to mimic the wall climbing abilities of geckos57.

Gecko Tape

A hierarchical pattern of hairs on the foot pad of geckos and many insects gives these creatures an intrinsic ability to stick to and scale nearly any surface, with the important requirement that the adhesion be easily reversible to allow free and quick movement.

The universal mechanism of detachment for these adhesive appendages is an orientation- dependent adhesion strength, or elastic anisotropy, that is a result of directional hair orientation58. The function of the hierarchical structure is to maximize real contact area with the substrate, which facilitates maximum adhesion via Van der Waals attraction59.

35 Figure 4.1 Schematic of hierarchical structure of a gecko’s foot pad60.

Click Chemistry

The click reaction involves the spontaneous complexation of an azide with an alkyne under convenient ambient reaction conditions. This method also serves to attach a reactive moiety to a surface in an ambient environment to preserve the delicate features or avoid damaging side reactions. The attachment of metal-terpyridine has been successfully applied to a functional graphene surface to incorporate a versatile reversible complex61.

36 Figure 4.2 Mechanism of the click reaction between an azide and an alkyne62.

Terpyridine

Organometallic complexes are versatile building blocks for applications that require ambient temperature self-assembly, and tunable adhesion. Terpyridine-metal complexes have shown great success in this area because terpyridine has a high binding affinity toward transition metal ions due to dπ-pπ* back-bonding of metal to the terpyridine rings63, and the adhesion is tunable and reversible. The Schubert research group has been the primary researchers in terpyridine chemistry, forming star polymers with a tetrakis terpyridinyl core63, constructing self healing materials based on the reversible nature of the metal:terpyridine complex64,65, and utilizing the optical and catalytic properties of the various metal:terpyridine complexes.61 Other researchers have utilized the tunable modification of carbon nanotubes and graphene66,67 to introduce order and compatibility.

Terpyridinyl groups have been introduced to various systems with click chemistry67, methacrylate polymerization64,65, and more traditional coordination reaction. Reaction of an amine-terminated terpyridine with a carboxylic acid has been used66. This research

37 will exploit the reaction of surface-confined maleic anhydride with amine-terminated terpyridine to introduce terpyridine functionality to any surface.

N

N N N N

O N N N N

H2N O + O

O O OH HN N O O O O O

(a) (b) (c)

Figure 4.3 Aminolysis and amidization of maleic anhydride with amino terpyridine.

38 CHAPTER V

EXPERIMENTAL METHODS

Some reactive molecules can be added to a pulsed plasma maleic anhydride surface by an aminolysis reaction between the dicarboxylic acid ester of the anhydride, and a primary amine. Amine reactants which can vaporize under vacuum could be introduced to the reaction vessel still under dynamic vacuum. This procedure has the advantage of limiting exposure of the anhydride-functional film to water vapor to avoid hydrolysis of the anhydride (dicarboxylic ester) to a dicarboxylic acid.

The incorporation of a terpyridine-functional molecule onto a surface is useful in surface adhesion applications by utilizing the bis-terpyridine/metal ion complex. To confirm the utility of the aminolysis reaction between a pulsed plasma-deposited Maleic

Anhydride film prior to attachment of adhesive terpyridine moieties, available primary amine-terminated molecules were first investigated.

In order to mobilize a terpyridine functionality on a surface, a hydrocarbon linker is necessary so that the reactive groups have adequate mobility to “find” a free metal ion and then bond that metal ion complex, in bis-configuration, to another free terpyridine. A

5-carbon linker was chosen based on the availability of reactants, and a 5-(2,2’:6’,2”- terpyridin-4’-yloxy)pentylamine was synthesized according to the previous procedure of

Schubert et al. Unfortunately, the desired amine-terminated molecule in this work exist as a solid at ambient temperature with no sublimation properties. In this case, the

39 substrates were removed immediately from the reaction chamber and submersed in a solvent containing the desired amine, and allowed to react overnight.

Equipment and Analysis

A Pulsed Plasma Reactor was designed and assembled in our laboratory for deposition of thin polymer films and modification of various surfaces. Similar reactors are often set up for small substrate surface area, to modify one substrate at a time, usually with a reactor vessel volume of one liter or less. An exceptionally large 10-liter reaction chamber (40cm. long and 20cm. diameter) was incorporated in our device to allow deposition on large substrate surfaces, or many samples at one time. The total surface available for deposition in the reactor is 40cm by 15cm for a total surface area of 600 cm.2 The electrical components of the reactor include a 600 Watt Max Power Dressler

Cesar 136 RF Power Generator capable of pulse modulation between 1Hz and 2000 Hz.

This input power is impedance-matched to the gas volume inside the reactor with a

Dressler 1500 Watt Inductively Coupled Plasma Matching Network with L-series capacitors. The power is inductively coupled to the gas with two 2.5 turn inductive antennae wrapped around the reaction vessel. The reaction gases are introduced with single- or double-chamber custom monomer tubes with Teflon valves from Ace Glass.

The vacuum system consists of ½-inch glass tubing and contains an inline liquid nitrogen cold trap and vacuum is provided by a 3.8 cubic ft./min. Varian SD-91 mechanical vacuum pump. Interior gas pressure is monitored with a tungsten filament Pyrani gauge.

Substrates that have been modified in this reactor include polished aluminum, steel, and

40 Ethylene-Propylene Rubber (EPR), Potassium Bromide (KBr) pellets for infrared analysis of the deposited film, and carbon fiber filaments.

Figure 5.1 Photo of the pulsed plasma chamber designed in our laboratory.

Quick and simple qualitative chemical analysis is most often performed with Infrared

Spectroscopy. A sample may be analyzed in a matter of minutes with minimal sample preparation and equipment operation training. Evaluation of thin surface layers, on the other hand, can require a more involved technique. The reason for complications in surface chemical structure analysis is the lack of specular path length available for radiation to travel through the layers to be evaluated. This can result in low signal resolution, or signals that cannot be separated from undesirable molecules in the bulk. X- ray photoelectron spectroscopy (XPS) may be used for surface analysis as long as an

41 electrically conductive substrate is possible. The collisional nature of x-ray irradiation and its analysis of ejected elections requires the use of XPS in a vacuum atmosphere.

High costs of high-vacuum equipment and long signal acquisition times occasional make analysis by XPS either impossible or impractical. Additionally, this technique is only able to represent individual molecules found in the sample irradiation area, and not of the chemical structure of the surface moieties. The probe depth of IR spectroscopy using attenuated total reflectance geometry is also much too deep for this purpose (nearly 200 nm of penetration). For these reasons, another form of Infrared analysis must be made available to evaluate chemical structure of molecules at the very surface of a substrate.

A Vee-Max Specular Reflectance FTIR accessory was purchased from Pike

Technologies for the purpose of chemical analysis of molecules on the very surface of a substrate. This technique of Infrared Spectroscopy involves introduction of infrared radiation to the substrate at variable angle of incidence from 30 to 80 degrees, depending on the thickness and depth of the layer to be qualitatively analyzed. A high angle of incidence is desirable for the analysis of very thin films because this geometry provides a long pathlength for the incident radiation to pass through a thin layer sample. The result is an enhanced signal for thin samples that would otherwise be riddled with background noise and unwanted signals. The technique was used to observe the attachment of molecules to a thin film of plasma deposited Maleic Anhydride on reflective polished aluminum substrates.

42 Materials

The monomer used in this study was solid maleic anhydride, purchased from Fisher

Chemical in briquette form. One or two briquettes were ground to a fine powder using a ceramic pestle and mortar and scooped into a clean monomer tube. Unused monomer was stored in an Argon atmosphere to avoid reaction with water vapor present in air. The special property of solid maleic anhydride to sublimate in a vacuum environment makes it a fine candidate for reaction in a vacuum environment. The vapor pressure of the monomer is approximately 120 mtorr at 20°C.

The diethylene triamine, Figure 5.3, was purchased from Sigma-Aldrich.

The 4'-(4-aminophenyl)-2,2':6',2''-terpyridine was donated by the lab of George

Newkome from unused lab material. Figure 5.2 shows the molecule used.

Figure 5.2 Molecular structure of 4'-(4-AMINOPHENYL)-2,2':6',2''-TERPYRIDINE.

The -(2,2’:6’,2”-terpyridin-4’-yloxy) pentylamine was synthesized using the following procedure (Figure 5.4):

43 5-Aminopentanol (192.68 mg, 1.8676 mmol) was added dropwise to a stirred suspension of powdered KOH (122 mg, 2.1748 mmol) in DMSO (5.6 mL) at 40°C. After

10 min, 4’-chloro-2,2’:6’,2”-terpyridine (125 mg, 0.467 mmol) was added. The mixture was stirred at 40 °C for 22 hours and then poured into deionized water (40 mL). The aqueous phase was removed by filtration and the product was washed with deionized water and dried by rotovap, yielding 5-(2,2’:6’,2”-terpyridin-4’-yloxy) pentylamine as a light yellow solid (111 mg, 0.332 mmol, 71% yield). The final terpyridine product was confirmed by NMR, Figure 5.5.

Figure 5.3 Synthesis Schematic for 5-(2,2’:6’,2”-terpyridin-4’-yloxy) pentylamine

44 Figure 5.4 H-NMR of the aminopentyl terpyridine product.

Methods

The pulsed plasma reactor apparatus was pumped down to a base pressure of approximately 1 mtorr. Ambient air was introduced with a vacuum needle valve to a pressure of 150 mtorr, and an oxygen plasma glow was ignited by automatic inductive coupling of the load. The oxygen plasma was maintained at an input power of 60 W for

30 minutes to oxidize any impurities adsorbed on the reactor surface. Following cleaning, the plasma was turned off, and the vessel was evacuated back down to base pressure, then allowed to fill to atmospheric pressure and the reaction vessel was opened to ambient pressure and loaded with substrates to be modified. 45 A monomer tube was filled with powdered maleic anhydride and degassed in liquid nitrogen by the freeze/pump/thaw method five times to remove dissolved gases. This method involved submersing the monomer tube in a dewar of liquid nitrogen until it was at thermal equilibrium, then the valve of the monomer tube was opened to vacuum until the pressure displayed by the Pyrani gauge dropped to an arbitrary low pressure and then began to climb, indicating that the monomer had started to evaporate.

Once the monomer was fully degassed, the system was evacuated to base pressure, and the monomer tube valve was opened until the vapor pressure reached 150 mtorr for deposition. At this point, the plasma was ignited briefly, with the power supply set on automatic. This allowed the matching network to match the impedance to the gaseous monomer load. The power was then turned off, extinguishing the glow, and the power supply was set to a predetermined pulse width for the particular deposition. The ideal settings of plasma on- and off-times and input power for deposition of Maleic Anhydride have been optimized previously by Siffer et al using a full factorial experimental design3.

In this work, it was found that ideal pulsed plasma MA films were deposited at 5 W input power, 25 µsec. on-time, and 1200µsec. off-times in a much smaller reaction vessel with an internal volume of 680 milliliters. Due to the large size of the reaction vessel used in these experiments, it was found that an input power of 35 W was necessary to obtain similar results. These parameters are implemented in this system by pulse settings of 816

Hz. period and 2% on-time ratio. The valve directly preceding the vacuum pump was opened only a small amount to limit flow of monomer through the system to allow reactive species to deposit fully on the substrate surface. Using this monomer system, it is desirable to limit the time that continuous wave (unpulsed) monomer is allowed to 46 deposit, since this high-energy process deposits highly crosslinked material. Once the proper impedance was found in automatic mode, the plasma was turned off and the pulse settings were set. The pulsed plasma was again ignited and allowed to deposit for 15 minutes, while the monomer pressure was closely monitored for fluctuations.

After deposition of the pulsed plasma film, the power was turned off and the monomer was allowed to flow without radiation in order to allow free monomer to react with and quench active radical sites for 5 minutes. After this time, the monomer tube valve was closed, and the system pumped down to base pressure with the vacuum valve again opened fully to allow maximum vacuum flow. Due to high reactivity of maleic anhydride to water vapor in ambient air, samples were left under dynamic vacuum until they were needed, or removed and analyzed immediately following the deposition.

47 CHAPTER VI

RESULTS AND DISCUSSION

Maleic Anhydride Plasma Polymerization

The plasma deposition of maleic anhydride film was confirmed by FTIR at the maximum 80º grazing angle to properly characterize the extremely thin film. It is estimated that the film is approximately 20 nm thick, based on similar studies3,12. The following infrared band assignments could be made for plasma-deposited maleic anhydride (Figure 5.3): saturated C-H stretching (2988 cm-1), C=O stretching (1864 cm-

1), symmetric C=O stretching (1792 cm-1), cyclic anhydride stretching (1239 cm-1 and

1070 cm-1), C-O-C stretching (939 cm-1). The infrared spectrum of the continuous wave plasma polymer, indicated the presence of a few carboxylic acid C=O groups (1728 cm-

1). The infrared spectrum of the pulsed plasma polymer layer was weak, even for a grazing angle accessory. This is consistent with a very thin coating.

Table 6.1 FTIR transmittance bands associated with this research.

peak position/ cm-1 assignment MA Amide 2900-2800 Alkyl C-H Stretching * * 1850 C=O Anhydride Stretching + - 1790 C=O Anhydride Stretching + - 1720 Carboxylic Acid Stretching - + 1650 Amide I (C=O Stretching) * 1560 Amide II (N-H Bending) * 1490 CNH Stretching of Amide * 1240 Cyclic Anhydride Stretching + - 1065 COC Stretch + - 940 Cyclic Unconjugated Anhydride + - * = present; + = more present; - = less present

48 10-01-08-alu-maleic anhydride-80deg-pol.csv: Column 1...

95

92

89 1239 1863 Transmittance 86 1070 1728 940 83 1792 732

80 4000 3000 2000 1000 Figure 6.1 Grazing-angle FTIR spectrum of aWavenumbers pulsed plasma-deposited maleic anhydride film on aluminum.

XPS elemental analysis of the plasma-deposited maleic anhydride film showed

27% oxygen (Figure 5.4). This is close to the 31% reported by Siffer et al in previous optimization studies.

49 Figure 6.2 XPS Elemental analysis of a pulsed plasma-deposited maleic anhydride film, finding 27% oxygen.

Maleic Anhydride Plasma Polymer Derivitization Reactions

Diethylene Triamine

Plasma-deposited maleic anhydride was further reacted with vapor phase

diethylene triamine to form an amide bond with anhydride. The reaction of the thin film

with the amine was confirmed with FTIR by the formation and/or increase of amide

bands (1648 cm-1 and 1560 cm-1), Figures 5.5 and 5.6. The symmetric C=O band near

1781 cm-1 exhibits a decrease as the symmetry is lost to amide bond formation. The

increase in alkyl C-H stretching (2923 cm-1 and 2853 cm-1) is attributable to the increased

ethylene moiety. This experiment was not analyzed with XPS.

50 90

88

86 1065

932 1236 Transmittance 84 1459 1560 2853 1850 1648 82 2923 1782 1719

4000 3000 2000 1000 Wavenumbers Figure 6.3 FTIR spectra of a pulsed plasma-deposited maleic anhydride film (black) and the maleic anhydride film reacted with diethylene triamine (blue), both on aluminum.

88

1459 86 1560 ttance

1850

Transmi 1648

84 1719

1782

82 2000 1800 1600 1400 Wavenumbers Figure 6.4 Magnified FTIR spectra of a pulsed plasma-deposited maleic anhydride film (black) and the maleic anhydride film reacted with diethylene triamine (blue), in the amide region.

51 Aminopropyltriethoxysilane

Plasma-deposited maleic anhydride was further reacted with vapor phase aminopropyltriethoxysilane to form an amide bond with anhydride. The reaction of the thin film with the amine was confirmed with FTIR by the formation and/or increase of amide bands (1648 cm-1 and 1560 cm-1), Figures 5.7 and 5.8. The symmetric C=O band near 1781 cm-1 exhibits a decrease as the symmetry is lost to amide bond formation. The bands representing alkyl C-H stretching (2923 cm-1 and 2853 cm-1) increased, in this case, due to the propyl moiety.

90

88 933

1869 1648 1078 86 Transmittance 1232

2854 1718 2925 1782 84 1735

4000 3000 2000 1000 Wavenumbers Figure 6.5 Grazing-angle FTIR spectrum of a pulsed plasma-deposited maleic anhydride film (black) and the film reacted with aminopropyltriethoxysilane, both on aluminum.

52 90

89

1869 Transmittance

88

1718

1735 1648 87 1782

2000 1900 1800 1700 1600 1500 1400 Wavenumbers

Figure 6.6 Magnified FTIR spectra of a pulsed plasma-deposited maleic anhydride film (black) and the maleic anhydride film reacted with aminopropyltriethoxysilane (blue), in the amide region.

XPS elemental analysis of the plasma-deposited maleic anhydride film measured

24% oxygen and 10.7% silicon, Figure 5.1. The origin of the sulfur is not absolutely known. But it may have come from contamination of the reaction vessel from parallel experiments with thiophene.

53 Figure 6.7 Elemental analysis of a pulsed plasma-deposited maleic anhydride film following reaction with vapor-phase aminopropyltriethoxysilane, showing 10.7% silicon.

Rubidium-complexed Aminophenyl Terpyridine

In order the test the efficacy of the aminolysis of pulsed plasma maleic anhydride with aminoterpyridine, a test involving readily-available aminophenyl terpyridine that had already been complexed with rubidium occurred. The material of interest was dissolved in DMSO, a solvent with very obstructive FTIR peaks. Figure 5.10 shows

FTIR spectra of neat DMSO solvent, aminophenyl terpyridine in DMSO, and aminophenyl terpyridine reacted with maleic anhydride in DMSO.

The neat DMSO spectrum is representative of what DMSO should look like. The spectrum of aminophenyl terpyridine showed increased peaks near 3448 and 1650 54 wavenumbers. These peaks can be attributed to either the amine or from the solvent.

After maleic anhydride was added to the aminophenyl terpyridine shows maleic

anhydride peaks near 1848, 1777, and 1644 wavenumbers. This FTIR study is

inconclusive, due to the obstructive peaks from the DMSO solvent.

nk071708terpybenzamine.bsp: DMSO...

60

40

20

nk071708terpybenzamine.bsp: terpy benzylamine in DMSO...

60

40

Transmittance 20

0 nk071708terpybenzamine.bsp: MA in terpy benzylamine2...

0 1848 2914 -50 3448 1778 2998 1310 891 1408 -100 4000 3000 2000 1000 Figure 6.8 FTIR spectra of DMSO solvent neat(black),Wavenumbers terpybenzamine reactant in DMSO (teal), and maleic anhydride reacted with terpybenzylamine in DMSO (green).

XPS analysis of a plasma-deposited maleic anhydride film reacted with the

aminophenyl terpyridine, then washed thoroughly, is presented in Figure 5.11. The

spectrum shows the complexed rubidium and the nitrogen from the amine linker. Figures

5.12 and 5.13 show magnifications of the nitrogen and rubidium peaks, respectively.

55 Figure 6.9 XPS analysis of a pulsed plasma-deposited maleic anhydride film reacted with aminophenyl terpyridine, showing rubidium and nitrogen peaks.

56 Figure 6.10 Magnified XPS analysis of a pulsed plasma-deposited maleic anhydride film reacted with terpybenzylamine showing the nitrogen peak region near 400eV.

Figure 6.11 Magnified XPS analysis of a pulsed plasma-deposited maleic anhydride film reacted with terpybenzylamine showing the rubidium peak region near 465eV.

57 Pentylamine Terpyridine

The grazing-angle FTIR spectra shown in Figure 5.15 show a progression of plasma- deposited maleic anhydride film (blue), the aminopentyl terpyridine reactant evaporated from its methanol solvent, on an aluminum substrate (pink), and the above maleic anhydride polymer reacted with aminopentyl terpyridine solution for 24 hours. Al Figure

5.15 most visibly shows terpyridine peaks, indicating an anchoring reaction has taken place.

Figure 5.16 is a magnified view of the carbonyl region between 1900 and 1600 wavenumbers. This region shows a combination of very little symmetric carbonyls at

~1776 cm-1 more decoupled carbonyls at ~1713 cm-1 (representing the acid side of an opened anhydride ring), and an amide linkage near 1643 cm-1, confirming a great deal of aminolysis between anhydride and the terpyridine’s primary amine.

58 10-01-08-alu-maleic anhydride-80deg-pol.csv: Column 1...

95

92

87

84 10-02-08 aminoterpyridine blank evap from methanol.csv: Column 1...

97

95 1776 93

91 94 10-02-08-ma+aminoterpy reacted-80deg-pol.csv: Column 1... Transmittance 91 88 85 1713 1206 82 1408 1360 795 78 1584 4000 3000 2000 1000 Wavenumbers Figure 6.12 Grazing angle FTIR spectra of a pulsed plasma maleic anhydride film (blue), aminopropylterpyridine evaporated from methanol (pink), and maleic anhydride film reacted with aminopentylterpyridine (black). Transmittance

1776 1713 1643

1900 1800 1700 1600 Wavenumbers Figure 6.13 Magnified view of grazing angle FTIR spectra of a pulsed plasma maleic anhydride film (blue), aminopropylterpyridine evaporated from methanol (pink), and maleic anhydride film reacted with aminopropylterpyridine (black) between 1900 and 1600 cm-1.

59 Following a 24-hour reaction time, attachment of the functional amine-terminated terpyridine molecule was confirmed with X-ray Photoelectron Spectroscopy. The following progression of XPS spectra confirms the success of the steps outlined in this thesis for the addition of a terpyridine complex to a surface. The pulsed plasma- deposition of maleic anhydride on an aluminum substrate, Figure 5.3, contains only carbon and oxygen. The carbon and oxygen 1s and Auger peaks are the only species present in this spectrum. There is no sign of an aluminum peak near 73 eV, which confirms a uniform MA film. The attachment of amine-terminated terpyridine with an amide linker, Figure 5.4, is evidenced by the nitrogen 1s peak at 397 eV. The addition of several oxygen species manifests with multiple oxygen Auger peaks added above 995eV.

The film with amide linking the terpyridine was heated, closing the amide by condensation with surface acid anhydride to form an imide, Figure 5.5. Lastly, the terpyridine-functional surface was reacted with iron chloride to form an Iron: Terpyridine complex, Figure 5.6. The addition of iron is shown in peaks between 710eV amd 920eV.

The dual peaks near 709eV and 715eV represent the iron 2p orbitals, and the other three are Auger peaks.

60 Figure 6.14 XPS Spectrum of pulsed plasma-deposited maleic anhydride film on Aluminum.

Figure 6.15 XPS Spectrum of pulsed plasma-deposited maleic anhydride film functionalized with a terpyridine end group attached by an amide. 61 Figure 6.16 XPS Spectrum of pulsed plasma-deposited maleic anhydride film functionalized with a terpyridine end group attached by an imide.

Figure 6.17 XPS Spectrum of pulsed plasma-deposited maleic anhydride film functionalized with an iron: terpyridine complex attached by an imide. 62 CHAPTER VII

CONCLUSION AND FINAL WORK

This work was performed with the purpose of investigating a new mechanism for surface functionalization and adhesion. The two step process of introducing terpyridine functionality to various surfaces is a simple and convenient process. Terpyridine groups that may be tethered to nearly any common substrate will find utility in catalysis, cell culture, inter-substrate adhesion, sensing, and many other diverse fields requiring ambient surface modification. The research described in this thesis demonstrated the viability of attaching the versatile terpyridine complex to plasma-deposited maleic anhydride films via the aminolysis reaction with amino functional terpyridine. Grazing angle FT-IR and XPS analysis of the reaction surface confirmed the attachment of aminoterpyridine to the anhydride surface, followed by the formation of an iron with terpyridine complex. This complex can form a bis structure and join with another terpyridine group, or the metal ligand substrate can be used on its own. There are many potential applications for surfaces coated with a metal:ligand surface.

There is still a lifetime of work that could be undertaken relating to this line of research in pulsed plasma-deposited functional surfaces. Adequate adhesion testing between terpyridine functional surfaces should be a first priority for the system described in this thesis. Surfaces functionalized with varying metallo-terpyridine complexes could be studied for myriad catalytic needs, photovoltaics, artificial photosynthesis and water splitting, biological intercalation, and countless other surface modification disciplines.

Adhesion performance could be compared between terpyridine surfaces that are pre-

63 complexed with a metallic ligand, and surfaces that are brought together, then introduced to metallic ligands. This complexation system could be optimized with varying the length and flexibility of the amino linkage (here studied pentyl and benzyl linkages).

Varying the linkage could improve adhesion strength and character, aminolysis complexation efficiency and kinetics, and reduce inter-substrate complexation of terpyridine groups. It would be interesting to explore the use of bipyridine in place of terpyridine complexes. There are undoubtedly countless uses for the system discussed in this research and its variations.

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