Characterization of Morphological and Chemical Properties of Scandium Containing

Cathode Materials

A dissertation presented to

the faculty of

the College of Arts and Sciences of Ohio University

In partial fulfillment

of the requirements for the degree

Doctor of Philosophy

Michael V. Mroz

May 2020

This dissertation titled

Characterization of Morphological and Chemical Properties of Scandium Containing

Cathode Materials

MICHAEL V. MROZ

has been approved for

the Department of Physics and Astronomy

and the College of Arts and Sciences by

Martin E. Kordesch

Professor of Physics and Astronomy

Florenz Plassmann

Dean, College of Arts and Sciences

ABSTRACT

MROZ, MICHAEL V., Ph.D., May 2020, Physics and Astronomy

Characterization of Morphological and Chemical Properties of Scandium Containing

Cathode Materials

Director of Dissertation: Martin E. Kordesch

Understanding thermionic cathodes is crucial for the future development of communication technologies operating at the terahertz frequency. Model cathode systems were characterized using multiple experimental techniques. These included Low

Energy Electron Microscopy, X-Ray Photoemission Spectroscopy, and Auger Electron

Spectroscopy. This was done to determine the mechanisms by which tungsten, barium, scandium, and oxygen may combine in order to achieve high current densities via thermionic emission. Barium and scandium films are found to dewet from the tungsten surfaces studied, and not diffuse out from bulk sources. The dewetted droplets were found to contribute the most to thermal emission. Barium oxide and scandium oxide are also found to react desorb from the emitting surface at lower temperatures then the metals themselves. The function of scandium in scandate cathodes was determined to act as an inhibitor to oxide formation. These observations are not compatible with certain models of cathode operation, mainly the dipole and semi-conductor models.

DEDICATION

To Eli, my parents Lisa and Michael, and my sister Kalie,

And to everyone who helped light my path on this adventure.

ACKNOWLEDGMENTS

This work was supported by the DARPA INVEST (Grant No. N66001-16-1-

4040). This research used resources of the Center for Functional Nanomaterials and

National Synchrotron Light Source II, which are U.S. Department of Energy (DOE)

Office of Science Facilities, at Brookhaven National Laboratory under Contract No. DE-

SC0012704.

TABLE OF CONTENTS

Page

Abstract ...... 3 Dedication ...... 4 Acknowledgments...... 5 List of Tables ...... 8 List of Figures ...... 9 Chapter 1 : Introduction ...... 12 History...... 12 Thermionic Emission ...... 14 Dispenser Cathodes ...... 16 DARPA INVEST ...... 20 Chapter 2 : Experimental methods ...... 24 Low Energy Electron Microscopy (LEEM) ...... 24 Photoemission Electron Microscopy (PEEM) ...... 27 Thermionic Emission Electron Microscopy (ThEEM) ...... 28 Low Energy Electron Diffraction (LEED) ...... 29 Auger Electron Spectroscopy (AES) ...... 30 X-Ray Photoelectron Spectroscopy (XPS) ...... 32 Samples and Preparation ...... 35 Chapter 3 : Data analysis ...... 46 Particle Analysis ...... 46 Auger Data Smoothing ...... 48 Gnuplot ...... 50 Chapter 4 : Results ...... 51 Overlapped Ba/Sc Depositions ...... 51 Dewetting ...... 67 Adsorbate Surface Structure ...... 86 X-Ray Spectroscopy ...... 97 Auger Spectroscopy ...... 107 Chapter 5 : Discussion ...... 115 Dipole Model ...... 115 7

Semi-Conductor Model ...... 118 Function of Scandium ...... 119 Emission from Barium ...... 120 Role of Field Emission...... 121 Future Direction ...... 122 Chapter 6 : Conclusions ...... 123 References ...... 127

LIST OF TABLES

Page

Table 1.1 Work functions of relevant metals...... 21 Table 4.1 Temperatures at which phenomena occur in the overlapped deposition...... 67 Table 4.2 Surface coverage of dewetted scandium films...... 75

LIST OF FIGURES

Page

Figure 1.1. Filament of an Edison bulb...... 13 Figure 1.2. Primary principle of thermionic emission...... 15 Figure 1.3. Schematic of a generic dispenser cathode...... 17 Figure 1.4. Cathode current density increase over time...... 19 Figure 1.5. Scanning Electron Microscope (SEM) image of scandate cathode...... 22 Figure 2.1. LEEM III at NSLS-II...... 24 Figure 2.2. LEEM V at CFN...... 25 Figure 2.3. Simple LEEM Schematic...... 27 Figure 2.4. Schematic of the photoemission process...... 28 Figure 2.5. MicroCMA on the LEEM V at CFN...... 30 Figure 2.6. Auger electron emission process...... 31 Figure 2.7. microCMA diagram...... 32 Figure 2.8. NSLS/XPS setup...... 33 Figure 2.9. Resulting spectrum from hemispherical analyzer...... 34 Figure 2.10. W(100) single crystal...... 36 Figure 2.11. Sample cartridge with loaded W(100) crystal...... 37 Figure 2.12. Heated W(100) crystal in LEEM V...... 39 Figure 2.13. Clean 1X1 pattern of a W(100) surface...... 40 Figure 2.14. Clean tungsten Auger spectrum...... 41 Figure 2.15. Clean W(100) surface...... 42 Figure 2.16. Shielded barium and scandium evaporators...... 44 Figure 3.1. Particle counting process...... 48 Figure 3.2 Differentiated Auger data treated via exponential smoothing...... 50 Figure 4.1. Cleaned W(100) surface...... 52 Figure 4.2. Overlapping deposition pattern ...... 53 Figure 4.3. Survey of overlapping deposition areas...... 54 Figure 4.4. Barium region during annealing...... 56 Figure 4.5. Edge of the barium deposition area...... 57 Figure 4.6. Scandium region during annealing...... 58 Figure 4.7. Edge of scandium deposition area...... 59 10

Figure 4.8. Desorption of barium near the overlap region boundary...... 60 Figure 4.9. Desorption of the overlap region near scandium only region...... 61 Figure 4.10. Sample of each deposition region after oxygenation...... 62 Figure 4.11. Desorption in the oxide overlap region...... 63 Figure 4.12. Overlapping deposition area using 0.5 mm aperture for Ba...... 64 Figure 4.13. Barium microdot deposited on top of scandium...... 66 Figure 4.14. Schematic of dewetting process...... 69 Figure 4.15. Dewetting parameter diagram...... 71 Figure 4.16. LEEM images of before and after scandium deposition...... 72 Figure 4.17. 30 nm scandium film dewetting in ThEEM at 770 ℃...... 74 Figure 4.18. ThEEM images of dewetted scandium films of varying thicknesses...... 75 Figure 4.19. Distribution of scandium particle sizes as a function of film thickness...... 77 Figure 4.20. Scandium drop diameter vs. film thickness on W(100)...... 78 Figure 4.21. Dewetting temperature dependence on scandium film thickness...... 79 Figure 4.22. Clean W(112) surface...... 80 Figure 4.23. Dewetting of barium on W(112)...... 81 Figure 4.24. Barium drop diameter vs. film thickness on W(112)...... 82 Figure 4.25. Dewetting temperature dependence on barium film thickness...... 83 Figure 4.26. BaO on W(100) dewetting...... 84

Figure 4.27. Sc2O3 on W(111) dewetting...... 86 Figure 4.28. LEED image of barium dewetting on W(100) ...... 87 Figure 4.29. Surface unit cell of barium on W(100) after dewetting...... 88 Figure 4.30. LEED pattern of scandium dewetting on W(100) ...... 90 Figure 4.31. Schematic of dewetting scandium strand breaking...... 91 Figure 4.32. LEED images of scandium dewetting on oxygenated W(100)...... 92 Figure 4.33. Overlapped scandium/barium LEED pattern...... 93

Figure 4.34. LEED patterns of annealed BaO and Sc2O3 films...... 94 Figure 4.35. Proposed surface structure of annealed BaO films...... 95 Figure 4.36. LEED resulting from annealed barium and scandium films on W(112). .... 96 Figure 4.37. Proposed surface features of dewetted films on W(112)...... 97 Figure 4.38. Spectra of annealed barium film on W(100)...... 99 Figure 4.39. Spectra of annealed oxygenated scandium film on W(100)...... 101 Figure 4.40. Valence band spectra of annealed scandium film on oxygenated W(100). 102 11

Figure 4.41. Spectra of annealing Sc-Ba-O film on W(100)...... 104 Figure 4.42. Spectra of annealed Ba-O film deposited on system from Figure 4.41. ... 105 Figure 4.43. Spectra of valence bands of Ba/Sc co-deposited films on W(100)...... 107 Figure 4.44. Auger spectra of annealed Sc/Ba films on W(100)...... 109 Figure 4.45. Normalized Ba/Sc peak height ratio vs. temperature...... 110 Figure 4.46. Auger spectra of thick Ba film dewetting on W(112)...... 111 Figure 4.47. Auger spectra of annealed oxygenated Sc/Ba films on W(100)...... 113 Figure 4.48. Normalized Ba/Sc peak height ratio vs. temp. from Sc-Ba-O systems...... 114 Figure 6.1. Phase diagram for the metallic systems studied...... 124 Figure 6.2. Phase diagram for the oxygenated systems studied...... 125

CHAPTER 1 : INTRODUCTION

History

The early history of thermionic emission, like that of many other topics in the sciences, is littered with discovery, rediscovery, accidental advancements, and the slow accumulation of an understanding into a mechanism of nature that would lead to innovations all the way up until the modern era. The recorded birthplace of this field of inquiry is France, 1853. Edmond Becquerel noticed that electricity could be transported through air between pieces of heated platinum with a voltage applied across them, although it would not be until the 20th century where the physical mechanism would begin to be more fully understood [1,2]. Thirty years after Becquerel, across the Atlantic

Ocean, the most historically noted rediscovery would take place. In 1880, Thomas

Edison was studying the lifetime behavior of his invention, the incandescent light bulb, shown in Figure 1.1. Inside the bulb, he observed that parts of the bulb were being damaged over time. In particular, the area around the positively charged side of the filament power supply was consistently blackened [3]. In addition, this damaged area also seemed to act as a shadow to the heated filament in the bulb, suggesting that the metal filament was the source of whatever it was that was causing the issue. Three years later, Edison discovered that by adding an electrode to the area that was blackened, an electrical current could be detected. For that to occur, the electrical charge had to be transported through a vacuum system. 13

Figure 1.1. Filament of an Edison bulb.

Up until that point, it was thought that electricity needed a medium to be through which to be transmitted. In 1897, Joseph John Thomson published a paper in which he had hoped to settle a debate between two current schools of thought, that in which electricity travels though the aether medium, and that where electricity is composed of negatively charged particles. Using a heated filament in a cathode ray tube to produce 14 electricity, and a magnetic field to steer its path, Thomson was able to measure the mass/charge ratio of whatever was “carrying” the electricity.

“The smallness of m/e may be due to the smallness of m or the largeness of e, or to a combination of these two.” [4].

This charge carrier was an electron, which has an incredibly small mass, and the smallest unit of charge, and with this knowledge, the area of thermionic cathodes began to grow in prominence.

Thermionic Emission

By the turn of the 20th century, the process of thermionic emission was understood in its most basic form. When a metal is heated, negatively charged electrons are freed from the metal. Applying a voltage with the negative side on the heated metal, the cathode, and a positive side to another nearby piece of metal, the anode, it is possible to draw electrons away from the cathode across a vacuum to the anode. This principal is primary driving force behind thermionic vacuum electronics.

The prevailing theoretical framework for this mechanism continues to be based on the work of Owen Richardson and Saul Dushman [5]. This model is summarized as an expression for current density J in Equation 1.

−휑 퐽(휑, 푇) = 퐴푇2exp ( ) (1) 푘푏푇

Here, φ is the work function of the metal, T is temperature in Kelvin, kb is the Boltzmann constant, and A is the Richardson constant, which is defined as 15

4휋푚푞푘2 퐴 퐴 = 푏 = 120.17 (2) ℎ3 푐푚2퐾2

Where m is the electron mass, q its charge, and h is Planck’s constant. A schematic for this phenomenon in vacuum devices is shown in Figure 1.2.

Figure 1.2. Primary principle of thermionic emission. When a metal is heated, electrons in the high energy tail of a thermal distribution requires little energy to escape. Colored gradient representative of Fermi function. Freed electrons can be pulled from the surface to a positive anode to be used.

This model gives two parameters to consider when designing thermionic vacuum electronics for use at a desired current density. The first to consider is operating temperature. According to Equation 1, any current density can be achieved if the right temperature is reached. This is unrealistic, as materials can undergo phase changes, or the device itself could not operate at those high temperatures. This leads to the second parameter, the material’s work function. The work function φ is a measure of how much energy it needed to free electrons from a material. In order to increase emission, materials with lower work functions may be incorporated. 16

One such material was found due to a leaky vacuum chamber in 1904. Arthur

Wehnelt noticed that sealant grease containing barium oxide coated the platinum wires from which he was measuring thermionic current. This resulted in a much higher current being measured then simply pure platinum alone [6]. The following year, he confirmed these findings in a more controlled environment for multiple oxides, including barium, calcium, and strontium oxide [7]. These materials would become the basis of high current density vacuum electronics, which would find widespread use in the coming decades.

Dispenser Cathodes

One category of device that utilizes these high current emitting materials is dispenser cathodes. These cathodes were, and in some cases still are, used in radio and microwave technologies, television sets, as well as numerous electron beam applications

[6]. Dispenser cathodes serve to solve two issues simultaneously. The first is to have a low work function present on the emitting surface of the cathode, and the second is to maintain its presence there by replenishing material lost through thermal desorption via a reservoir of said material [8-10]. Figure 1.3 shows a simple schematic of a dispenser cathode. 17

Figure 1.3. Schematic of a generic dispenser cathode. Heater both releases reservoir material to tungsten, and heats cathode for thermionic emission.

Thermionic cathodes in vacuum have operating temperatures upwards of 1000 ℃.

The emitting material, typically barium compounds, tend to desorb from a tungsten surface at such temperatures [11]. The reservoir is in place in order to resupply the surface with the functional barium. The barium is transported up through the porous metal to where it can contribute to the overall cathode current. This process repeats until the reservoir is depleted, where the cathode would then be needed to be replaced. It should be noted that the reservoir is sometimes a mixture of barium, calcium, strontium, 18 and aluminum oxides, and the mixture ratios may also vary. The function of these other oxides is debated, but it is agreed that the barium is the primary source of current, so that will be the material of focus going forward.

In the late 1960’s and 1970’s, large advances in surface analysis were being made, including widespread development of techniques such as Low Energy Electron

Diffraction (LEED) and higher precision thermal desorption studies [12,13]. These advances also translated over to the ongoing studies of dispenser cathodes. It was in this time period that overall performance of these devices was increasing. In particular, the overall current density and lifetime of the devices was on the rise alongside the development of surface analysis techniques. These improvements came to a head when it was discovered that the inclusion of scandium into the cathodes greatly improved these qualities of the dispenser cathodes, as displayed in Figure 1.4 [14-16]. Scandium became the common thread that linked all these new high-performance cathodes together, which lead to the modern-day effort to understand the mechanism in which scandium improved thermal emission of these cathodes. 19

Figure 1.4. Cathode current density increase over time. Figure from Gärtner et al., 1997 [16].

DARPA INVEST

An agency of the United States Department of Defense, the Defense Advanced

Research Projects Agency (DARPA) began the Innovative Vacuum Electronic Science and Technology (INVEST) program with the intent to develop next generation vacuum electronics, of which thermionic cathodes are key components [17]. The idea is that to achieve high power, high current density cathodes that preferably operate at lower temperatures, a solid understanding must be developed for the physical mechanisms underlying the current leading candidate, the scandate cathode [18].

There have been two prevailing theories as to how these current densities are achieved. One proposes that it is due to the thick semiconductor layer on the tungsten surface, on the order of 400-500 nm, while the other suggests it is due to a partial monolayer dipole effect [18-20]. In the semiconductor model, the electric field between the cathode and anode can penetrate the thick layer due to a sparsely populated conduction band. This results in a lowering of the layers effective work function and thus enhancement in thermionic emission as shown in Equation 1. The dipole layer suggests that since barium and scandium are less electronegative then tungsten, an array of surface dipoles caused by adsorbed material point out of the surface, lowering the work function.

Furthermore, as part of the DARPA collaboration, the dipole model was expanded using current Density Functional Theory techniques [21,22]. These calculations were used to predict what surface concentrations and distributions of barium, scandium, and oxygen would result in low work function environments. Standard work functions for the relevant bulk metals are provided in Table 1.1. 21

Table 1.1 Work functions of relevant metals. Metal Φ (eV)

W 4.61

Ba 2.52

Sc 3.50

Another factor to consider when investigating dispenser cathodes is the tungsten substrate itself. Historically, the porous tungsten is formed via sintering tungsten powder forming a network of faceted grains, as seen in Figure 1.5. A method to determine the three-dimensional shape of a crystal under certain environmental conditions via surface energy minimization is the Wulff construction [23]. Using this method of analysis, the structure of tungsten grains in well performing cathodes were found to primarily have

{100}, {110}, and {112} surfaces [18,22]. 22

Figure 1.5. Scanning Electron Microscope (SEM) image of scandate cathode. Facets of the sintered grains matched to facets found from Wulff analysis. Figure from Kirkwood et al., 2018 [18]. Copyright ©2018, IEEE.

The work performed at Ohio University is an exploration at how the three primary ingredients to dispenser cathodes, barium, scandium, and oxygen, interact with low

Miller index surfaces of tungsten up to the standard operating temperatures of the cathodes. The goal is to characterize these materials to shed light on the mechanisms which provide these devices with the large current densities they are capable of emitting, as well as develop ideas in which the densities can be increased further. This work was done under the INVEST program with DARPA, and in collaboration with the Center of 23

Functional Nanomaterials (CFN) at Brookhaven National Laboratory (BNL) in Upton,

New York.

CHAPTER 2 : EXPERIMENTAL METHODS

Low Energy Electron Microscopy (LEEM)

The primary instruments used in this research was the Elmitec Low Energy

Electron Microscope III and V at Brookhaven National Laboratory (BNL), depicted in

Figure 2.1 and Figure 2.2 respectively. The LEEM III is located at the National

Synchrotron Light Source II (NSLS-II) at the Electron Spectro-Microscopy beamline, and the LEEM V is situated in the Center for Functional Nanomaterials (CFN).

Figure 2.1. LEEM III at NSLS-II. Figure from Brookhaven National Lab, 2020 [24]. 25

Figure 2.2. LEEM V at CFN. Figure from Brookhaven National Lab, 2020 [24].

LEEM utilizes a high intensity electron source, such as a thermionic cathode, and elastically backscattered electrons from the sample to form an image of the sample’s surface [25]. A simplified schematic of a typical LEEM setup is provided in Figure 2.3.

The entire system is held at a pressure of approximately 10-9 to 10-10 Torr. A sample preparation chamber is kept isolated from the main LEEM system to avoid contamination. The electron gun is held at a high voltage of 20 kV in both the LEEM III and V. The electrons are accelerated away and focused into a beam in the first set of electromagnetic quadrupole lenses. The beam passes through a deflector which steers the electrons towards the sample. This deflector separates out the incoming illuminating 26 beam and the outgoing imaging beam. Electron paths are then made parallel by passing through a collimator lens. The objective lens is grounded, which has the effect of slowing the electron beam down, making the electrons surface sensitive.

The sample is held near the gun voltage with an offset called the start or impact voltage, which typically results in incoming electrons with an energy between 1-100 eV.

The precise energy of the incident electrons is related to the difference between the start voltage and the samples work function. This energy dictates the electrons penetration depth. It should be noted that the sample may be biased such that nearly all electrons are reflected, known as Mirror Electron Microscopy (MEM), which is very surface sensitive.

Impacting electrons elastically backscatter towards the objective lens, carrying surface structure information with them.

The image beam then passes back through the beam deflector, towards the detector. The image is refocused and steered with the use of projector lenses. Apertures may be added to the beam path to assist with brightness and contrast adjustments.

Electrons are collected by a Charge-Coupled Device (CCD) camera and an intensity map image is produced via the Elmitec LEEM software, UView2000. Images can have a field of view between 10 and 100 micrometers and has a spatial resolution of 5 nanometers.

This microscope allows for multiple characterization techniques to be run on a sample in real time, as no scanning is involved, which is crucial in determining the properties of the materials as they are being heated. 27

Figure 2.3. Simple LEEM Schematic.

Photoemission Electron Microscopy (PEEM)

Photoemission electron microscopy works synergistically alongside LEEM in that it provides an electron yield map from which surface composition and work function can be inferred, alongside the morphological information from the LEEM. As such, an

Ultraviolet light source is often included on a LEEM system, as shown in Figure 2.3.

Photoemission functions by having a photon of enough energy, on the order of a few electron volts for metals, excite valence electrons to the vacuum, as seen in Figure 2.4

[7]. These electrons can then be accelerated away from the sample the same way the

LEEM electrons are, and thus, an intensity map of the sample can be made. Surfaces with lower work function will appear brighter in the final image. 28

Figure 2.4. Schematic of the photoemission process. Incident UV photon excites an electron from the Fermi level to the vacuum.

Thermionic Emission Electron Microscopy (ThEEM)

A key feature of the LEEM systems as it relates to this study is its sample heating capabilities. Temperatures of up to 1800 ℃ can be reached. In addition, the sample may be viewed in real time as it heats. This is key to determining what on the surfaces thermally emits and at what temperatures. The number of electrons emitted thermally depends on both the work function and temperature of the sample, as given by the

Richardson Dushman equation (Equation 1). For a given material, the current density varies as the square of the temperature. So as thermal emission begins, it increases 29 rapidly with temperature. The 20kV voltage driving the LEEM system is set to shut off when the total current landing on the objective lens exceeds 1 mA. Currents higher than that run the risk of damaging both the objective lens itself as well as the CCD camera due to over saturation. This results in only being able to view the sample up to a given temperature, and not up to the potential 1800 ℃. When this limit is reached, the sample is heated up past that limit with the imaging turned off. The sample is then cooled to below the limit and characterization may continue, with higher temperature behavior being inferred. Electrons are imaged via the same path as the LEEM and PEEM processes.

Low Energy Electron Diffraction (LEED)

The electrons impacting the sample in the LEEM system are surface sensitive, meaning they only penetrate a few atomic layers into the sample before backscattering out. If the sample is an ordered crystal face, or a surface with ordered adsorbates, then an electron diffraction pattern may be observed [26,27]. This pattern will have a characteristic ordering determined by the arrangement of atoms at the sample surface. For example, a simple square lattice will have a simple square pattern. Typically, LEED measurements are performed using a hemispherical phosphorescent screen surrounding the sample in order to collect the backscattered electrons. In the LEEM microscopes however, the lenses along the imaging path of the beam can be configured to image the back focal plane of the objective lens. With the addition of an aperture, a diffraction pattern can be produced in the CCD. It should be noted that patterns formed by adsorbates only provide information as to how they are ordered with respect to 30 themselves, not the layer onto which they are adsorbed. In other words, not whether they are adsorbed on top of the surface atoms, or in hollow sites on the lattice.

Auger Electron Spectroscopy (AES)

A method used to determine the surface elemental composition was using Auger electron spectroscopy. A micro Cylindrical Mirror Analyzer (microCMA) from RBD

Instruments was integrated into the LEEM V system, pictured in Figure 2.5. The system is mounted on the sample preparation chamber, so the surface composition may be monitored after each material deposition.

Figure 2.5. MicroCMA on the LEEM V at CFN.

The microCMA has a working distance of 3.8 millimeters and uses 2 keV electrons accelerated from a thermionic emission source to perform the Auger process, 31 seen in Figure 2.6. Electrons from the gun interact with the core shell electrons of the atoms near the surface of the sample, knocking the electron out of the atom leaving it in an excited state. An outer shell electron drops into the vacancy left, and an additional electron will be ejected from the atom with some kinetic energy due to energy conservation. This is the Auger electron, and the kinetic energy with which it leaves is characteristic to the element from which in originated [27,28]. The de-excitation of the atom may also occur via the emission of an x-ray when an electron fills the inner shell vacancy, which is known as X-Ray fluorescence.

Figure 2.6. Auger electron emission process. The electron collected is the ejected electron with a characteristic kinetic energy.

The Auger electrons then travel back towards the gun and are selected by the entrance of the cylindrical analyzer at some angle, which is 42.3° for this instrument. 32

The cylinder sweeps through voltages which serves to sweep through the incoming electron energies. The step size in kinetic energies checked is 0.2 eV/step. The electrons that make it to the electron multiplier at the rear are then counted, and a spectrum is produced of the sample. The energy resolution of this analyzer is (∆E/E) of 0.7%. The schematic of the analyzer is provided in Figure 2.7.

Figure 2.7. microCMA diagram. The cylinder serves as an energy filter for the Auger electrons.

X-Ray Photoelectron Spectroscopy (XPS)

Closely related to AES is XPS, where X-rays are used to excite core electrons out of atoms. The kinetic energy of these electrons is characteristic of their source element, and the shell from which they originated [25,29,30]. X-Rays are produced from NSLS-II via synchrotron radiation, or accelerating radiation emitted from electrons traveling around the synchrotron’s storage ring. Photons with energies of 80 and 140 eV were used to characterize the samples. The x-rays travel down the beam line offshoot from the 33 ring, to where the LEEM III system is placed. This allows for the use of the x-rays to perform XPS on the sample in the LEEM. A schematic for this process is provided in

Figure 2.8. The electrons produced are then collected in a similar fashion as the LEEM and PEEM electrons, as discussed prior.

Figure 2.8. NSLS/XPS setup. X-rays excite electrons from the core to the vacuum.

However, the LEEM III has an addition to the imaging beam path as depicted in

Figure 2.3. A hemispherical energy analyzer sorts electron by their kinetic energy. This is accomplished by applying a voltage across the inner and outer spheres of the analyzer and utilizing the resulting electric field to bend the path of the incident electron beam. 34

Electrons of varying kinetic energy get deflected differently, and this dispersive plane may be imaged, allowing for an energy spectrum to be produced [25]. A demonstration of this method is presented in Figure 2.9.

Figure 2.9. Resulting spectrum from hemispherical analyzer. The analyzer filters the electrons by energy and focuses the beam into a line. The intensity of each pixel along the line is then plotted.

While XPS and AES are similar, there is a fundamental distinction. XPS is a measurement of the atoms current state. AES is a measurement of an atom after an excitation event, as it is a two-step process. This difference between initial and final state 35 spectroscopic techniques is why determining chemical information is simpler using XPS rather than AES [31].

Samples and Preparation

The substrate for each of these studies was a high purity (≥ 99.999%) tungsten single crystal. The faces studied are the (100), (110), (111), and (112) crystal faces. The

(100) and (110) crystals were purchased from Accumet Materials, Inc., Ossining, NY.

They are 9 millimeters in diameter, 0.25 millimeters thick and polished to ≤ 0.5 degrees of the crystal plane. The (112) crystal is 10 millimeters in diameter and is 2 millimeters thick. It was polished to within 0.1 degrees of the (112) plane and had a surface roughness under 0.01 microns. 36

Figure 2.10. W(100) single crystal. 37

The samples were mounted to an Elmitec sample cartridge, pictured in Figure

2.11. The entire holder is made of non-magnetic materials such as tantalum and molybdenum, to not disrupt the electron beam path in the LEEM. Behind the sample is a built-in tungsten-rhenium thermocouple and a heating filament which can heat the sample via electron bombardment. Temperatures upwards of 1800 ℃ can be reached and the thermocouple can accurately determine the temperature to within 100 ℃. The accuracy of the temperature measurement is sample holder dependent as well. The LEEM manipulator locks into the holder via a spring-loaded twist lock, ensuring sample security in the microscope.

Figure 2.11. Sample cartridge with loaded W(100) crystal.

The tungsten crystals were prepared for film deposition by first cleaning the surface. This was primarily accomplished via heating the sample in the main LEEM chamber up to approximately 1600 ℃, pictured in Figure 2.12. The sample is also flashed to that temperature in oxygen at 5*10-7 Torr in order to react away additional residuals, particularly carbon. Performing LEED on the crystal would then show clean

1X1 patterns, seen in Figure 2.13. Auger electron spectroscopy was also regularly performed to verify the surface composition in the LEEM V, and XPS in the LEEM III.

A standard Auger spectrum of clean tungsten is shown in Figure 2.14. LEEM images were also taken of the clean surface, seen in Figure 2.15, which served as a base line comparison for all experiments performed. 39

Figure 2.12. Heated W(100) crystal in LEEM V. 40

Figure 2.13. Clean 1X1 pattern of a W(100) surface. FCC pattern shown in reciprocal space as tungsten is a BCC crystal in real space. 41

Figure 2.14. Clean tungsten Auger spectrum. This is raw data that has been differentiated, dN(E)/dE vs. KE, and not treated via smoothing. 42

Figure 2.15. Clean W(100) surface. Imperfections in the crystal surface are visible, including pinned defects (circular features) and step edges (long line features)[32].

Barium and scandium are both delivered to the surface via thermal evaporation.

Tungsten wire 0.375 millimeters in diameter was used to form coils 4 millimeters wide with 5 turns of wire. The coils were then attached to vacuum system feedthroughs, and pieces of barium and scandium metal were placed inside each of the coils. Shields are 43 added to direct the depositions as thermal deposition is a line of sight process, as well as prevent cross contamination of evaporation sources. An example of a complete evaporator setup is shown in Figure 2.16. Scandium is relatively inert in atmosphere compared to the alkali barium; hence no special preparation is necessary. Therefore, the full process of removing the barium metal from the immersion oil, placing it in the coil, and attaching the feedthrough to the LEEM’s preparation chamber, and pumping down the system is done in as little time as possible. 44

Figure 2.16. Shielded barium and scandium evaporators. 45

Residual gases from the atmosphere are removed during preheating of the sources. Preheating only heats the sources to under their evaporation temperatures in the ultra-high vacuum environment, i.e. approximately 10-9 Torr. At this pressure, barium evaporates at around 830 ℃ and scandium at around 970 ℃, or to where the filaments are red and yellow respectively [33,34]. The current in the barium and scandium containing coils are set to approximately 4 and 7 amperes respectively in order to preheat.

Deposition occurs at 5 amperes for the barium and 10 amperes for the scandium. The amount of time the deposition occurs determine the resulting film thickness. Thicknesses were determined via a quartz crystal monitor using the same setup, in a separate vacuum system. Again, Auger and XPS were regularly used to verify successful metal depositions.

CHAPTER 3 : DATA ANALYSIS

Particle Analysis

A large portion of the work described here involves the formation of droplets on the tungsten surface. The software used to perform the analysis on these particles was

ImageJ, an open source software package developed by the National Institute of Health

[35]. The main goal was the development of a method in order to accurately determine the sizes of the particles formed [36,37].

Figure 3.1 shows the step by step process in determining particle count and size.

Figure 3.1 a. shows the raw ThEEM image of dewetted particles of barium. First, the image is duplicated, and a Gaussian Blur filter is applied. The filter is set at 40 pixels, which means a given pixel is assigned a new color value based on a weighted average of the pixel values within a 40-pixel radius, effectively smearing the image. The resulting smeared image is then subtracted from the original. A Median filter is then applied to further reduce noise. Each pixels color value is then set to the average value of all the pixels in a 5 pixel radius [38-40]. After the contrast is re-adjusted, the resulting image has a reduced background signal, as seen in Figure 3.1 b.

The background must be subtracted in order to assist the edge detection of particles in an image. The algorithm used to separate out the background from the particles is the Max Entropy algorithm [41]. This determines a particles edge based on the brightness of the surrounding pixels using a probabilistic approach. A binary watershed was then applied in order to separate clusters of particles into individual particles [35]. This is done by checking for concavity in the particle boundary. The 47 result of the Max Entropy thresholding and the watershed application is given in Figure

3.1 c.

Lastly, the particles are counted and their corresponding area in square pixels is placed in a data file, see Figure 3.1 d. Particles whose boundaries extend outside the viewing area are not counted. Knowing the current field of view from the LEEM, pixels can be converted in microns. Particle sizes can then be put into a histogram for statistical analysis. 48

Figure 3.1. Particle counting process. a. Raw image taken from LEEM. b. Background subtracted image. c. Threshold and watershed algorithms applied. Note separation of clusters in zoomed inset. d. Particles counted and area determined.

Auger Data Smoothing

Auger data is first acquired by counting the number of electrons detected at a given kinetic energy. However, this data is traditionally differentiated with respect to 49 kinetic energy in order to make the spectroscopic peaks more pronounced [42]. This also has the effect of enhancing any initial noise in the original data. In order to measure signal peak heights, the differentiated data must therefore be smoothed.

Microsoft Excel’s exponential smoothing feature was utilized to accomplish this.

Exponential smoothing averages the data in a range by assigning exponentially decreasing weights to values around a data point and taking a weighted average [43].

The degree to which the data is smoothed is given as a damping factor with ranges from zero to one: or no smoothing to complete smoothing. A value of 0.9 was chosen in order to minimize the error [44]. The result of smoothing is displayed in Figure 3.2. A clean tungsten spectrum is shown in Figure 3.2 a. After smoothing, the new spectrum is seen as the black line in Figure 3.2 b. The error bars, seen in green, are the result from the noise in the data. Note the noise is strongest at lower energies. Peak heights are measured from the maximum and minimum values in the smoothed data, and the corresponding error bars are propagated into the calculated values of peak height ratios. 50

Figure 3.2 Differentiated Auger data treated via exponential smoothing. a. Raw data from a clean tungsten surface. b. Smoothed spectra data (black) with error bars from the noise in a. (green).

Gnuplot

Gnuplot was used to create all plots seen in this work [45]. Line fits to data are accomplished via the nonlinear least-squares Marquardt-Levenberg algorithm. This iterates on fits in order to minimize the sum of squared errors [46]. These fits were primarily performed for initial film thickness vs. annealing temperature data.

CHAPTER 4 : RESULTS

Overlapped Ba/Sc Depositions

This experiment was performed in the LEEM V system. A W(100) crystal was cleaned via heating to 1800 ± 100 ℃ and was flashed in oxygen to assist in the removal of residuals. Before the scandium and barium were added, the sample was imaged to confirm cleanliness, see Figure 4.1. The depositions were performed in the preparation chamber at a pressure of 10-9 Torr. Barium and scandium depositions were performed via thermal evaporation from tungsten wire coils. For this deposition, apertures were added in the evaporation path to limit the coverage of the tungsten crystal to small spots.

First, apertures of diameter 1 millimeter were used, resulting in a deposition pattern shown in Figure 4.2. Both evaporations took 1 minute at 10 amperes for the coil containing scandium and 8 amperes for the barium. The purpose is to study how the individual depositions of barium and scandium behave compared to the co-deposited region [47]. 52

Figure 4.1. Cleaned W(100) surface. a. PEEM image taken at 220 ℃. Surface features on this crystal are a result of damage from the cleaning process. b. 1X1 LEED pattern at 300 ℃. Start voltage: 30V.

Figure 4.2. Overlapping deposition pattern via evaporation through a 1 mm aperture (not to scale).

Once the deposition process was complete, the crystal was moved back into the main chamber for characterization. The first step was to confirm the desired deposition pattern was achieved. Since both scandium and barium have lower work functions then tungsten by an electron volt or more, they appear brighter while imaging in PEEM. Once the deposition spots were located, a spatial survey was taken, as seen in Figure 4.3. The composition of the spots is inferred by the contrast with the underlying tungsten surface.

Comparing Figure 4.3 a. to d., the deposition area is dimmer in a. then in d. Since barium as a work function lower than scandium, it is determined that a. is the edge of the 54 scandium deposition and d. is the barium edge. Figure 4.3 b. and c. are therefore the boundaries of the overlap region.

Figure 4.3. Survey of overlapping deposition areas. PEEM images taken at 130 ℃. b.c. Lines demarking overlap boundaries added for clarity. Insets of deposition schematic included for reference (areas are not perfectly circular). 55

The sample was then annealed slowly, and repeated surveys were taken to observe the behavior of each region as a function of temperature. The barium only region is summarized in Figure 4.4. As the barium film heats, it undergoes a morphological change. Here, at around 400 ℃, the barium undergoes dewetting. This process is primarily driven by the surface energy of the tungsten in combination with the kinetic energy associated with the heating of the sample. Tungsten is a metal with one of the highest surface energies [48]. This means that thin films of material adsorbed to a tungsten surface are unstable when heated, as the material is more stable adhering to itself rather than the surface. The thermal energy provides the film energy to reach a more stable state by rupturing and contracting into droplets on the surface, even at temperatures below the melting point of the film material [49]. The melting point of barium is 725 ℃, so this process occurred at roughly half of the melting temperature.

The formation of these droplets can be seen in Figure 4.4 b. as small dark circles.

They are dark because of the increasing height of the droplets, which takes them out of focus with respect to the initial image. Here, the average droplet diameter was found to be 0.27 ± 0.09 µm. Note that due to the intensity of thermionic emission, the size is over estimated, as the brightness increases the apparent size. As the sample continues to heat, thermionic emission occurs. It is seen in Figure 4.4 c. that the droplets are the primary thermionic emitters in the image. The background that was bright in PEEM is now dark by comparison. This is because the image is normalized to the highest electron emission area such that it is shown as white. This demonstrates that thermal emission dominates photoemission in terms of electron yield. 56

Figure 4.4. Barium region during annealing. a.b. PEEM images of Ba on W(100). b. Droplet formation occurs. c. ThEEM image of emitting droplets. Note the background is largely dark.

Since these depositions are limited in spread, the spots on the tungsten surface also have a thickness gradient, where it is thickest near the center and thinnest near the edge. Therefore, using a quartz crystal monitor to calibrate a thickness to the deposition time is difficult. The behavior of the thinner film around the edge is displayed in Figure

4.5. The density of droplets sharply decreases near the edge of the deposition. In addition, the very edge appears brighter. This has two possible explanations, including a lower work function than bulk barium caused by a partial monolayer reducing the emission barrier via a dipole interaction [21]. The other explanation is a dispersion of smaller, more tightly packed droplets like those in the center of the deposition area. Final droplet size is dependent on initial film thickness and is a topic of another experiment to be discussed in a later section [49]. 57

The scandium only region follows similarly to the barium. The effects of annealing process and the characteristics of the edge of the deposition are provided in

Figure 4.6 and Figure 4.7 respectively. It can be seen in Figure 4.6 b. that after heating to

750 ℃, thermally emitting droplets on the order of nanometers begin to admit. These are difficult to resolve due to their small size and the competing intensity from PEEM.

Increasing the magnification in the LEEM system further resulted in caustics in the image. Scandium also undergoes the dewetting process on tungsten. Its melting point is

1541 ℃, so this too is occurring at half the melting temperature. The edge of the deposition is also bright in Figure 4.7 because it is the left-over scandium that was not 58 pulled into the center of the deposition area via dewetting. The size discrepancy between droplets in the barium and scandium is due to the initial film thickness as stated prior.

Figure 4.6. Scandium region during annealing. a. PEEM image of scandium as deposited. b. PEEM/ThEEM image. 5 µm x 5 µm area shown in inset to make thermally emitting scandium particles more apparent. 59

Figure 4.7. Edge of scandium deposition area. PEEM images taken at 220 ℃ after annealing to 1100 ℃. This is due from the system shutting down due to reaching the 1 mA limit at approximately 800 ℃.

Another process that takes place in the annealing process is desorption. The overlapped deposition setup provided a straightforward method to determine the temperature range each section evaporates from the tungsten surface. The sample was heated to a given temperature, and then cooled in order to view it without saturating the camera. A survey was conducted to see how the signal in PEEM changed in each section. The boundaries between the barium and scandium only region and the overlap region were the focus. The result from the barium/overlap boundary is given in Figure

4.8. By 1100 ℃, the PEEM intensity of the barium is already low. This is in part due to the dewetting process, as well as the beginning of complete desorption. A monolayer of barium stays adsorbed to the surface at higher temperatures then multilayers. The multilayers desorb by 800 ℃, while the monolayer completely desorbs by 1250 ℃. 60

Comparing Figure 4.8 b. and c., it was observed that the barium signal completely disappears between 1300 ℃ and 1400 ℃, which is within the ±100 ℃ error for the desorption of a monolayer [11].

Figure 4.8. Desorption of barium near the overlap region boundary. PEEM images taken at approximately 230 ℃. Colored circles demark surface features between pictures for reference.

The boundary between the scandium deposition and the overlap region was also observed after successive heating to progressively higher temperatures. The results of this are displayed in Figure 4.9. By 1300 ℃, the overlap region is already distinct due to the barium desorbing, as seen in Figure 4.9 a. and b. After heating to 1600 ℃, the overlap region completely vanishes in PEEM, whereas the scandium only region remained. This implies two properties. First, since the barium region desorbed completely by 1300 ℃ and only the scandium area remained up to 1600 ℃, the overlap region may still contain barium in some capacity. Barium may bond to scandium 61 preferentially over tungsten, allowing it to persist to higher temperatures. Second, scandium itself desorbs between 1600 ℃ and 1800 ℃, the temperature at which the crystal is cleaned.

Figure 4.9. Desorption of the overlap region near scandium only region. PEEM images taken at 220 ℃, 330 ℃, and 390 ℃ for a., b., and c. respectively. Circles highlight same feature across all images.

The overlapped deposition experiment was also conducted in the same manner as described above, with the addition of oxygen. After the deposition, oxygen was allowed into the preparation chamber for one minute at a pressure of 5x10-7 Torr, or 30 Langmuir, in order to oxidize the deposition areas. The surface was surveyed, and a characteristic image of each area is provided in Figure 4.10. A mixture of materials is confirmed in

Figure 4.10 b. due to a patchwork of brightness that is observed. Comparing the Sc2O3 in

Figure 4.10 a. to the BaO in c., the scandia has a weaker signal. It can thus be inferred that the bright patches in Figure 4.10 b. is BaO at the surface while the rest is Sc2O3. 62

Figure 4.10. Sample of each deposition region after oxygenation. PEEM images taken at approximately 300 ℃ after annealing to 800 ℃. a. Sc2O3, PEEM signal is weak. b. Sc2O3/BaO, small bright areas can be seen show different materials present. c. BaO, uniform brightness observed.

While the barium and scandium oxide only regions remained largely unchanged as the sample was heated to higher temperatures, as seen using the available magnification, the overlap region had a varying behavior than that of the pure metals.

This is demonstrated in Figure 4.11. Desorption for the mixed oxide region begins in the temperature range of 800 ℃ to 1100 ℃. Comparing to the pure metal case shown in

Figure 4.9, the oxides begin to leave the surface 500 ℃ to 700 ℃ lower. This agrees with previous reports of this process occurring near 1000 ℃ [50,51]. 63

Figure 4.11. Desorption in the oxide overlap region. a. PEEM image taken at 310 ℃ . b. PEEM image taken at 240 ℃, much of the photoemission intensity is now gone. Circles mark same location.

In addition to the first deposition pattern shown in Figure 4.2, another pattern was created by changing the size of the aperture through which the barium was evaporated from 1 millimeter to 0.5 millimeter. First, scandium was evaporated to the surface for 3 minutes through the 1 mm aperture with a coil current of 10 amperes. The barium coil was run at 6 amperes for 1.5 minutes. Figure 4.12 shows the resulting deposition pattern for this scenario. 64

Figure 4.12. Overlapping deposition area using 0.5 mm aperture for Ba.

The characterization of this deposition pattern is summarized in Figure 4.13. The images were taken after annealing to 900 ℃. As with the previous deposition pattern, dewetting still occurs below the melting point of both scandium and barium. In Figure

4.13 b., a clear separation between barium concentration regimes can be seen, where the 65 very edge shows dendritic dewetting patterns. Compared to the overlap region, the scandium only area has less dense droplet formation. Thermionically emitting droplets can be seen in Figure 4.13 c. Much of the barium deposition area has thermionic emission areas with a diameter of 9.8 ± 4.5 micrometers. At this magnification in ThEEM, it is difficult to resolve individual droplets from clusters due to the electron intensity. Near the edge where the barium concentration decreases in Figure 4.13 d., the droplets become too small to individually resolve at this magnification. This area is also the most intense thermionic emitter. Since this is where the barium is thinnest, the resulting dewetted drops are smaller, and more densely packed on the surface. 66

Figure 4.13. Barium microdot deposited on top of scandium. Sample annealed to 900 ℃. a.b. PEEM images taken at 150 ℃. Black spots are overfocused dewetted droplets. Note differing dewetting pattern in each region in b. c.d. ThEEM images taken at 320 ℃. Highest intensity near the edge of the Ba deposition.

A summary of the relevant phenomena and the temperatures at which they occur is provided in Table 4.1. It was observed that the co-deposited metals gave barium a 67 higher desorption temperature then barium alone. Conversely, the overlapping oxides lowered the temperature at which barium left the surface, implying a high temperature

reaction between BaO and Sc2O3. Dewetting temperatures are also reported for the individual metals. Dewetting was not observed in the oxide cases, and in the case of the adsorbed metals, a definitive temperature is difficult to obtain. Dewetting temperatures are typically framed in two material systems as opposed to three.

Table 4.1 Temperatures at which phenomena occur in the overlapped deposition. Temperatures reported have an error ± 100 ℃.

Compound Full Desorption Dewetting Temp. (℃)

Temp. (℃)

Ba 1300 – 1400 570

Sc > 1600 750

Ba/Sc 1500 – 1600 < 900

BaO > 1400 -

Sc2O3 > 1600 -

BaO/ Sc2O3 800 – 1100 -

Dewetting

In the previous section, the process of dewetting was identified as a central behavior of materials present in scandate cathodes. The following section will serve to expand upon the details of that process and how it impacts potential cathode 68 performance. Dewetting of heated films ultimately stems from the inherit instability of deposited thin films. When provided enough thermal energy, the atoms in the film can become mobile, which provides a means to lower the overall free energy of the surface by contracting into droplets.

Figure 4.14 summarizes the dewetting of a barium thin film on a tungsten surface.

Figure 4.14 a. shows the initial film as deposited. Figure 4.14 b. shows that variations in the film thickness and defects in the underlying surface function as hole nucleation sites once the system is heated. As sections of the film begins to retract, material on the rim of the films can thicken, introducing further defects. Figure 4.14 c. portrays the final dewetted state of the film. Thickened rims can pinch off into smaller droplets. This demonstrates how a film of an average thickness may end in a wide distribution of dewetted droplet sizes [49]. 69

Figure 4.14. Schematic of dewetting process. (Not to scale) a. Film as deposited. Film thickness and crystal defects exaggerated for clarity. b. Heated to dewetting temperature. Initial holes in film due to defects. c. Final state of dewetting. Further droplets may form due to defects introduced during the dewetting process.

For liquids, the benchmark as to whether a given film will dewet on a given surface is summarized by the ideal Young Equation [52].

훾푆푉 = 훾퐹푆 + 훾퐹푉 푐표푠휃 (3)

Here, γ is the surface tension, and S, F, and V stand for surface, film, and vacuum respectively, so each term is the surface tension at an interface. The angle θ is known as the wetting angle, or the angle made between the substrate and the final dewetted droplet surface. A simple diagram is provided in Figure 4.15. For a perfectly stable film, the angle will be zero. A film will thus dewet if γSV < γFV + γFV. However, as will be shown, the thin films studied do not melt, as the dewetting takes place much below the materials melting point [49]. 71

Figure 4.15. Dewetting parameter diagram.

As shown in Figure 4.13, the primary source of thermionic emission after heating to 900 ℃, which is standard cathode operating temperatures, are the dewetted droplets.

The intensity increases as the deposited film gets thinner near the edge. In order to explore this further, scandium films of varying thicknesses were deposited onto a W(100) single crystal and heated to dewet the films [53]. First, the crystal was cleaned by flashing it in oxygen at 5x10-7 Torr to 1800 ℃. The surface was checked in LEED for cleanliness and a 1X1 LEED pattern was observed (see Figure 2.13). Depositions were 72 performed by running 10 amperes through the scandium evaporator coil for 1, 3, and 6 minutes. Using a quartz crystal monitor to calibrate the deposition time to film thickness, it was found that these times corresponded to approximately 5, 15, and 30 nanometers in thickness respectively. Three trials were performed for the 5 and 30 nanometer films, and four for the 15-nanometer film.

The W(100) surface before and after a 3-minute deposition is seen in Figure 4.16.

The fine details of the clean surface become covered by the film. The start voltage needed to image the surface also shifts by 1.5V, which is representative of the difference between the work functions of tungsten and scandium. After the depositions were confirmed, the sample was heated until the film dewetted.

Figure 4.16. LEEM images of before and after scandium deposition. a. Start voltage: 0.1V. b. 15 nm scandium film. Start voltage: 1.48V. Both taken at 150 ℃. Figure adapted from Mroz et al., (2018) [53].

Figure 4.17 is a 30-nanometer film of scandium dewetting over time. Once the temperature reached approximately 770 ℃, the film began to rupture. In Figure 4.17 b., the initial holes in the film are shown to nucleate. Hole nucleation begins where the film is thinnest, typically areas with crystal surface defects sufficiently large to disrupt uniform film growth. It is also observed that the film retracts along crystallographic directions. As the process continues, the film retracts into droplets and strands of material that connect them, as seen in Figure 4.17 c. The strands then break and are pulled into the resulting droplets, completing the dewetting process. 74

Figure 4.17. 30 nm scandium film dewetting in ThEEM at 770 ℃. Note the dewetting film largely aligns with crystallographic directions. Resulting surface is dark compared to the thermally emitting droplets. Figure adapted from Mroz et al., (2018) [53].

A similar dewetting process was observed for all thicknesses tested, and an example final state of each thickness is given in Figure 4.18. As the film thickness is increased, the final droplet size also increases. However, the percentage of the surface 75 covered with thermally emitting droplets decreases with initial film thickness, seen in

Table 4.2. Note that in all cases, the background is dark compared to the drops, showing that the droplets are the primary emitters.

Figure 4.18. ThEEM images of dewetted scandium films of varying thicknesses. a. Image taken at 580 ℃. b. Image taken at 760 ℃. c. Image taken at 770 ℃. Figure adapted from Mroz et al., (2018) [53].

Table 4.2 Surface coverage of dewetted scandium films. Film Thickness (nm) % Area Coverage

5 40

15 29

30 20

The distribution of observed particle sizes with each film thickness is provided in

Figure 4.19. Particles of size less than 0.1 microns in diameter were not included as that is smaller than 3x3 pixels in the original image and are cut off in the detection threshold process. A deviation in observed average particle size is present in the 5-nanometer case.

Larger particles may be allowed to form due to defect effects, such as step bunches on the surface causing initially smaller droplets to aggregate. This process may be the central feature of Figure 4.18 a. This also sets a characteristic size scale of the surface defects, i.e. approximately 10 nanometers. Note that these particle sizes are of the scale seen in the example cathode in Figure 1.5. 77

Figure 4.19. Distribution of scandium particle sizes as a function of film thickness. Bin size for all is 0.05 µm. b. Included is data from scandium dewetting on a W(111) surface as well, with the average drop size agreeing with the W(100) data to within 5%. 78

Figure 4.20. Scandium drop diameter vs. film thickness on W(100). W(111) point marked for comparison. Error bars are ± 1 standard deviation.

The temperature at which a film dewets is also related to the thickness, as well as the melting temperature of the film material [49,54]. Note that this process occurs below a materials melting point; thus, the film itself remains solid. The general relationship between these quantities is given in Equation 4.

퐴푇푚푒푙푡 푇 ∝ (4) 푑푒푤푒푡 ln(푐ℎ−3)

Here, h is the initial film thickness in nanometers, A is the proportionality constant, and c is also a constant. Fitting Equation 4 to the dewetted scandium data gives Figure 4.21.

The consistently lower dewetting temperature for the 5 nanometer films supports the idea that defects on the surface allow for the droplet size to deviate from the expected linear scaling shown in Figure 4.20.

Figure 4.21. Dewetting temperature dependence on scandium film thickness. 10 3 Tmelt = 1541 ℃, A = 6.89, c = 6.30x10 nm . Melting temperature of scandium reached when film thickness is roughly 400 nm. Error is primarily systematic, ±100 ℃.

Similarly, the dewetting phenomena was observed for the barium on tungsten system. While no direct film thickness dependence studies were performed for barium, data from various observations where film thicknesses and dewetting temperatures are 80 known were compiled. The system where this data is available is from barium depositions on the highly oriented W(112) crystal [55]. Before the depositions, the surface was observed in LEEM and LEED to determine a baseline comparison. Figure

4.22 a. lacks the same surface features of the W(100) seen in Figure 2.15 due to a finer degree of polish.

Figure 4.22. Clean W(112) surface. a. LEEM image at start voltage -0.19 V. b. Clean 1X1 pattern at start voltage 36.3V. Images taken at 70 ℃.

Barium was deposited onto the surface using a current of 5 amperes. A 1-minute deposition time was determined to result in an approximately 20 nanometer film. An additional film was grown with a 2-minute deposition, which resulted in a 40-nanometer thick film. Three 20 and one 40 nanometer films were used for this analysis. The initial film and subsequent result of dewetting is seen in Figure 4.23. The difference in start 81 voltage from before and after the deposition, 2.3 volts, is representative of the difference of work function between tungsten and barium, which is 2.1 eV. As with the scandium, it should be noted that the droplets themselves are the source of thermal emission, as compared to the dark background of the tungsten surface.

Figure 4.23. Dewetting of barium on W(112). a. LEEM image at start voltage -2.49 V. b. ThEEM image of dewetted droplets and remaining intact strands of material.

The average barium droplet diameter plotted as a function of initial film thickness is shown in Figure 4.24. The size is relatively constant between the two thicknesses observed. One possible explanation is the sample size, as only one 40 nanometer film was considered. Another source of error could result from the deposition itself. The 2- minute deposition may have depleted the source itself, meaning the barium supply to the 82 surface was not constant over the deposition time. Lastly, Equation 4 was fit to the barium droplet data, as seen in Figure 4.25.

Figure 4.24. Barium drop diameter vs. film thickness on W(112). Error bars are ± 1 standard deviation. 83

Figure 4.25. Dewetting temperature dependence on barium film thickness. 7 3 Tmelt = 727 ℃, A = 3.66, c = 7.71x10 nm . Melting temperature reached when film thickness is roughly 125 nm. Error bars are ±100 ℃.

Dewetting was also observed in an oxygenated barium film. A 20-nanometer film was deposited onto a W(100) surface and exposed to oxygen. The system was then annealed to 700 ℃. While the precise temperature at which dewetting occurred was not observed, it must be between when the temperature at which the 1 mA limit on the

LEEM’s lens was reached, 450 ℃, and 700 ℃. For a similarly thick barium metal film, the dewetting occurred between 150 ℃ and 400 ℃ lower than the oxygenated film. This is in line with metallic barium, as the melting point for BaO is 1923 ℃ [56]. The resulting image is shown in Figure 4.26. The average diameter of the thermally emitting areas is 0.20±0.09 micrometers, which agrees with the results for metallic barium in

Figure 4.24. 84

Figure 4.26. BaO on W(100) dewetting. LEEM/ThEEM image taken at 390 ℃. Start voltage at -1.5 V.

Similarly, oxygenated scandium films on W(100) and W(111) crystals were also observed to dewet when annealed. On each of these crystals, a 15-nanometer scandium film was deposited and exposed to oxygen. For these two depositions, the average 85 dewetting temperature was found to be 695 ℃ ± 125 ℃, which agrees well with the temperatures found for metallic scandium films of similar thicknesses, as seen in Figure

4.21. The resulting surface is shown in Figure 4.27. The average diameter of the thermionic emitting areas is 0.27±0.11 micrometers, which also agrees with the metallic scandium case marked in Figure 4.20. Since the melting point of Sc2O3 is approximately

2400 ℃, the expectation is that the dewetting should occur at a higher temperature then the metallic case, if at all [57]. However, there may be an approximately 5 nanometer oxidation passivation layer that forms on scandium, meaning any deposited film thicker then this may still have metallic scandium at the tungsten surface [58]. 86

Figure 4.27. Sc2O3 on W(111) dewetting. ThEEM image taken at 830 ℃. 1 mA limit on LEEM lens not yet reached.

Adsorbate Surface Structure

In the previous section, it was shown that the primary sources of thermal emission were droplets formed from barium and scandium dewetted films. Dewetting is a dynamic 87 process that involves material transport across the surface. While the result of the emitting droplets is a more apparent feature, the revealed underlying surface must also be characterized. LEED is a tool that can be utilized to gain information regarding the surface structure. Diffraction patterns were acquired before material deposition and after annealing. Patterns observed were independent of initial film thickness.

LEED patterns from before and after a barium film dewetted on a W(100) crystal is seen in Figure 4.28. Once the film dewets, a clear 2x2 pattern is observed on the surface. While much of the barium is transported into the droplets, a partial monolayer does remain adsorbed to the tungsten surface, as diagramed in Figure 4.29. This satisfies the structural conditions for the dipole model for enhanced thermal emission [20-22].

Recall that the background in the ThEEM images post dewetting were dark when compared to the droplets, as seen in Figure 4.23 for example.

Figure 4.28. LEED image of barium dewetting on W(100) Images taken at a start voltage of 30 V. a. W(100) 1X1 pattern before deposition. T = 100 ℃. b. Resulting 2X2 pattern after annealing to 500 ℃. T = 190 ℃. 88

Figure 4.29. Surface unit cell of barium on W(100) after dewetting. Hollow site chosen for location of barium based on DFT results [21,22].

Similarly, the tungsten surface also has a characteristic LEED pattern after the scandium films dewet, seen in Figure 4.30. Streaks are observed to form along the crystallographic directions. As seen in Figure 4.17, the film does dewet preferentially in certain directions. The ordering seen in the LEED may be due to the dewetting strands 89 breaking along these directions, with the smaller strands being limited in size and spacing due to the atomic steps on the tungsten surface, seen in Figure 4.31 [59]. As the strands break, scandium atoms are left on the surface randomly along the crystallographic directions. The bright streaks between the primary (100) spots suggest an average surface scandium atom spacing twice that of the tungsten. A reason as to why this feature is more apparent in the scandium film would be a difference in surface energy between scandium and barium, as well as the bond strength of these two elements to tungsten itself. 90

Figure 4.30. LEED pattern of scandium dewetting on W(100) Image taken after annealing to approximately 550 ℃. Seen are streaks along (100) directions with bright areas suggesting a general 1X2 + 2X1 surface arrangement. 91

Figure 4.31. Schematic of dewetting scandium strand breaking. Scandium atoms placed in hollow sites based on DFT simulations [21].

In addition to scandium dewetting on a clean W(100) surface, the surface was also oxygenated before a scandium film was deposited. The surface was exposed to oxygen for 2 minutes at a pressure of 10-8 Torr, resulting in the characteristic 5X1 for oxygen on

W(100), see Figure 4.32 a. [60]. The system is then annealed up to 530 ℃, Figure 4.32 b., resulting in a pattern similar to Figure 4.30, only diffuse. After annealing to 900 ℃, the clean 1X1 pattern returns, though the bulk scandium drops themselves remain, as noted in Table 4.1. The oxygen adsorbed to the tungsten surface is swept up by the dewetting scandium. The streaks may disappear by this temperature due to solitary Sc2O3 molecules desorbing. This result is further supported by XPS data that will be discussed in a later section. 92

Figure 4.32. LEED images of scandium dewetting on oxygenated W(100). a. 5X1 pattern imaged at start voltage 30 V and T = 100 ℃. b. Diffuse streaks along (100) direction imaged at start voltage 40 V and T = 530 ℃. c. 1X1 pattern after annealing to 900 ℃. Imaged with start voltage of 40 V and T = 200 ℃.

In systems where overlapping barium and scandium depositions were performed, the LEED result after annealing is similarly overlapped, as seen in Figure 4.33. This demonstrates that the dewetting processes of barium and scandium themselves are mostly independent of each other. It should also be noted that barium and scandium do not alloy, as there is no eutectic in their two-component phase diagram [61]. 93

Figure 4.33. Overlapped scandium/barium LEED pattern. Weak 2X2 barium pattern (spot marked for clarity) inside scandium “box” pattern. Annealed to 900 ℃.

The films were also similarly oxygenated for 2 minutes at 10-7 Torr before annealing [53]. The resulting LEED patterns after heating to approximately 950 ℃ are shown in Figure 4.34. Figure 4.34 a. shows a week 4X4 pattern after annealing a BaO film. The annealed Sc2O3 film LEED, seen in Figure 4.34 b., largely shows the 94 underlying W(100) diffraction pattern, but with additional irregular diffraction patterns appearing, suggesting the appearance of crystallized scandate particles on the surface.

Comparing Figure 4.34 a. and c., the annealed overlapping film LEED results in a much sharper 4X4. This agrees with the assertion that Sc2O3 desorbs at lower temperatures when in the presence of BaO, as discussed in the section on overlapping films. Again, the streaks that appear in these LEED patterns demonstrate that the ordering is along the

(100) directions, but the spacing between adsorbed molecules is somewhat disordered.

Also, the lack of bright 2X2 spots in the films that incorporate BaO suggest that the atoms align at 45° angles. This arrangement also provides an explanation to the enhanced brightness of the 2X1 and 1X2 spots seen in Figure 4.34 c. [62]. The 2X1 and

1X2 streaks are also offset to the inside of the primary (100) pattern, suggesting the distance between a barium and oxygen atom is greater than the tungsten lattice constant of 0.316 nanometers [63]. A proposed general surface structure is given in Figure 4.35, which the orientation of the BaO is in agreement with the most stable arrangement as calculated by DFT [21].

Figure 4.34. LEED patterns of annealed BaO and Sc2O3 films. Images taken at start voltage is 40 V and T = 850 ℃. a. BaO film. b. Sc2O3 film. c. BaO/Sc2O3 film. 95

Figure 4.35. Proposed surface structure of annealed BaO films. Vacancies result in the streaked LEED patterns of Figure 4.34.

LEED patterns gathered from the W(112) surface follows similarly to the (100), suggesting that the surface arrangement of atoms does not overly affect the macroscopic thermal emission behavior. A summary of the LEED from the (112) surface is shown in

Figure 4.36. A clean (112) pattern is seen in Figure 4.36 a. After a barium film was 96 annealed to 800 ℃, the resulting hexagonal pattern in LEED is given in Figure 4.36 b.

Lastly, the streaks in the annealed scandium film LEED in Figure 4.36 c. show that the scandium dewets along the direction where the tungsten atoms are more closely packed, leaving scandium atoms on the surface as the film retracts. A proposed schematic of the

W(112) surface after annealing is provided in Figure 4.37.

Figure 4.36. LEED resulting from annealed barium and scandium films on W(112). a. Clean 1X1 imaged at start voltage of 17 V and T = 360 ℃. b. Hexagonal pattern observed from dewetted Ba film. Start voltage of 9 V and T = 110 ℃. c. Streaks in the 1X1 from scandium film annealed to 900 ℃. Start voltage of 19 V and T = 230 ℃.

Figure 4.37. Proposed surface features of dewetted films on W(112). a. Triangular lattice of adsorbed barium atoms. b. Scandium atoms resulting in streaks in LEED.

X-Ray Spectroscopy

X-Ray Photoemission Spectroscopy was utilized to collect composition data at the

LEEM III at NSLS-II at Brookhaven National laboratory [64,65]. A hemispherical energy analyzer allows for the filtering of electrons by their kinetic energies, as depicted in Figure 2.9. The incident photon energy from the light source is selected such that the difference between photon and kinetic energies results in a range of binding energies that contain a signal of the target material [66]. In the case of scandium films on tungsten, W

4f and Sc 3p binding energies are observed between 28 and 35 eV. Barium has no direct spectral overlap with both scandium and tungsten. When applicable, the Ba 4d and 5p peaks were observed at 90 and 15 eV respectively [67,68]. In addition, when the photon and kinetic energies are approximately equal, the valance band of the system may also be measured. In particular, the W 5d, Ba 5d/6s, Sc 3d, and O 2p levels may be resolved 98

[69-71]. This provides a method of direct observation of the chemical reaction between these system components. Spectra are calibrated by adjusting the kinetic energy by 1 eV.

The spectral data is originally collected as pixel number vs. intensity, and this 1 eV change shifts the peaks location by a certain number of pixels. This allows for a conversion from pixel number to electron volts, using a known peak as a point of reference.

A W(100) crystal was used for these experiments. The crystal was cleaned via multiple heating cycles to about 1800 ℃, and lastly flashed to the same temperature in the presence of oxygen. A barium film was evaporated to the surface for 90 seconds at 5 amperes, resulting in an approximately 30 nanometer thick film. X-Ray spectra were gathered before and after annealing to 600 ℃, which as described above, is past the temperature threshold for dewetting. The results are provided in Figure 4.38. The Ba 5p and 4d peaks in Figure 4.38 a. and c. remain largely unchanged after annealing. This demonstrates that even after dewetting, the barium still covers the entire surface as a combination of droplets and a partial monolayer as reinforced by imaging in LEEM and

LEED, as shown in Figure 4.23 and Figure 4.28. Figure 4.38 b. shows that the overall barium signal in the valance band largely gives way to tungsten oxide. The W 4f spectrum in Figure 4.38 d. is shown to develop more pronounced tungsten oxide peaks after annealing. Since no oxygen was purposefully introduced to this system, the oxide is most likely left over from the cleaning process. However, this demonstrates the preference of oxygen to remain bound to the tungsten surface, rather than the barium overlayer. 99

Figure 4.38. Spectra of annealed barium film on W(100). All spectra are normalized, as well as taken at room temperature, and after annealing to 600 ℃. Incident photon energy hν = 140 eV. a. Ba 5p peaks, kinetic energy K.E. = 121 eV. b. Valance band, K.E. = 132 eV. c. Ba 4d peaks, K.E. = 45 eV. d. W 4f peaks with adsorbed oxygen satellite peaks, K.E. = 104 eV.

On a cleaned W(100) surface, 30 L, or 2 minutes at a pressure of 5x10-7 Torr, of oxygen was introduced at 800 ℃. A 15-nanometer scandium film was then deposited onto the oxidized surface. The scandium film itself was then oxidized with 30 L of oxygen. The film was then annealed up to 1000 ℃. It was determined that the following results were independent of scandium/oxygen deposition order. The corresponding XPS series is provided in Figure 4.39. Comparing Figure 4.39 a. and b., a clear tungsten oxide signal appears as higher energy satellite peaks to the primary W 4f peaks. After the scandium deposition, the entirety of the tungsten signal is masked by the scandium overlayer as seen in Figure 4.39 c. The introduction of oxygen onto the scandium film, 100

Figure 4.39 d. and e., nearly converts the entire signal into scandia peak that’s roughly

4.2 eV higher in binding energy, though a small scandium metal signal remains. Once the film is heated to 1000 ℃, the oxygenated scandium film dewets and in the process, sweeps up the oxygen on the tungsten surface as well, removing the WOx signal. This spectrum corresponds to the LEED images in Figure 4.32. 101

Figure 4.39. Spectra of annealed oxygenated scandium film on W(100). Normalized. a.b. Photon energy hν = 80 eV, kinetic energy K.E. = 42 eV. c.-f. hν = 450 eV, K.E. = 402 eV. a. Clean W(100) spectra. b. 30L of O2 on W(100) at 800 ℃. c. 15 nm scandium film. d.e. 30 L of O2 on scandium film. d. Zoomed in 40X in order to show remaining scandium metal peak. f. Spectrum after annealing to 1000 ℃. Figure adapted from Mroz et al., (2019) [64]. 102

In addition to the W 4f and Sc 3p peaks, the valence band was checked in a similar experiment, the results of which are given in Figure 4.40. In Figure 4.40 a., the primary feature of the spectra is a broad tungsten oxide peak, which results from 30 L of oxygen being introduced to the W(100) surface. There may be remnant barium and scandium peaks between 1 and 3 eV here, but this could not be confirmed using more core electron signals. A 15 nm scandium film was then deposited. The W 5d signal is reduced and a Sc 3d peak appears near 1 eV, as seen in Figure 4.40 b. The system was then heated to 900 ℃ in Figure 4.40 c. The Sc 3d peak is reduced as the film dewets, and the WOx peak broadens again as the tungsten surface is revealed. As seen in Figure 4.39, the W 4f peaks lose the oxide satellites, so the O 2p peak in the valence band is correlated to the scandium, rather than the W 5d peak.

Figure 4.40. Valence band spectra of annealed scandium film on oxygenated W(100). Photon energy hν = 80 eV, kinetic energy K.E. = 73 eV. Normalized. a. Oxidized W(100), contaminants from previous depositions may be present. b. 15 nm scandium film. c. Annealed to 900 ℃. 103

Lastly, a barium/scandium co-deposited system was studied using XPS. Here, a very small amount of scandium was deposited, only 10 seconds at 10 amperes, which corresponds to 1 nanometer. Barium was then evaporated for 2 minutes at 5 amperes, or approximately 20 nanometers. Oxygen was then introduced to the system at 5X10-8 Torr for 7 minutes, which converts to 20 L. The system was then heated up to 1200 ℃. The resulting spectra series is provided in Figure 4.41. In the case of barium, Figure 4.41 a., aside from a shift in the relative peak heights of Ba 4d3/2 and 4d5/2, the oxygen had little effect. The most substantial change is when the system was heated to 800 ℃. By that temperature, the barium has completely desorbed. This is consistent with the oxygenated films imaged in PEEM in Figure 4.11. Similarly, in Figure 4.41 b., the introduction of oxygen broadens out the W 4f peaks and quenches the Sc 3p signal. As in Figure 4.39, the Sc 3p shifts to a Sc 3+ signal approximately 4 eV higher. This explains the difference in peak heights as the system is heated, as the W 4f5/2 and Sc 3+ signal overlap. By 1200

℃, the Sc2O3 signal is also largely reduced. However, a sparse amount of Sc 3p could still be seen. 104

Figure 4.41. Spectra of annealing Sc-Ba-O film on W(100). Photon energy used was hν = 140 eV. Normalized. a. Ba 4d spectra with O2 added and heated to temperatures shown. Kinetic energy K.E. = 45 eV. b. W 4f/Sc 3p spectra with O2 added and heated to temperatures shown. K.E. = 103 eV. Figure adapted from Mroz et al., (2019) [65].

In order to determine if the scandium oxide is reacting with the barium oxide and leaving the tungsten surface, an additional barium film was deposited onto the annealed system from Figure 4.41. Just as before, a 20-nanometer barium film was deposited and exposed to an additional 20 L of oxygen. The system was then heated to 1200 ℃ once again, and the resulting series of spectra is given in Figure 4.42. The primary observation from Figure 4.42 a. is the persistence of barium up to a temperature of 1200 ℃, at least

400 degrees more than the previous deposition. Figure 4.42 b. has two main points. The 105 first being the complete oxidation of the tungsten surface, made most apparent after heating to 400 ℃. The second being the lack of signal from scandium, suggesting its total removal from the system. This is due to the combination of observations of Sc2O3 and BaO reacting to desorb, as seen in PEEM in Figure 4.11, and the inhibition of tungsten oxide formation due to scandium, as seen in the XPS spectra in Figure 4.39.

Figure 4.42. Spectra of annealed Ba-O film deposited on system from Figure 4.41. Photon energy used was hν = 140 eV. Normalized. a. Ba 4d spectra with O2 added and heated to temperatures shown. Kinetic energy K.E. = 45 eV. Ba signal remains to 1200 ℃ .b. W 4f/Sc 3p spectra with O2 added and heated to temperatures shown. K.E. = 103 eV W completely oxidized, and Sc signal mostly diminished. Figure adapted from Mroz et al., (2019) [65].

106

A comparison between valence bands of a low and high oxygen amounts in co- deposited systems was also made. The low oxygen system had no oxygen purposefully introduced, but a signal in XPS confirmed its presence. The barium and scandium films deposited were approximately 30 and 15 nanometers respectfully. The high oxygen system is the same as in Figure 4.42. In Figure 4.43 a., the Sc 3d/Ba 6s signal is seen to reduce in magnitude with respect to the Ba 5d peak after heating to 800 ℃. Since the relative peak height of the Ba 5d peak as compared to the W 5d peak is 50±2% before and after annealing, it can be concluded that the Sc 3d contribution is reduced after heating. Comparing the low binding energy regions of Figure 4.43 a. to b., it can be concluded that oxidation decreases the electrons available near the vacuum level. These results indicate that scandium inhibits oxide formation and allows for metallic barium to be more available for emission purposes. 107

Figure 4.43. Spectra of valence bands of Ba/Sc co-deposited films on W(100). Photon energy hν = 140 eV, kinetic energy K.E. = 132 eV. Normalized. a. Low O2 system. Sc 3d signal reduced after heating to 800 ℃. b. High O2 system. Spectra dominated by WOx signal at all temperatures.

Auger Spectroscopy

These experiments were performed using a microCMA on the LEEM V, as pictured in Figure 2.5 [72]. Auger spectra were taken in the systems preparation chamber, which is isolated from the main chamber where the sample heating occurred.

Each chamber was held at a pressure of approximately 10-9- Torr. Both the W(100) and

W(112) crystals were used for these spectroscopic studies. All of the relevant elements have a signal below 620 eV, so the microCMA was set to sweep through kinetic energies 108 up to that point [28]. Tungsten has three peaks at 48, 169, and 179 eV. Barium has five peaks within this range, 57, 73, 77, 584, and 600 eV. Scandium’s primary peak is located at 340 eV. Carbon and oxygen signals also appear in the following spectra, at 272 and

503 eV respectively. Analysis of these systems involves measuring elemental peak heights and taking ratios of heights of two elements [73]. Error in the heights is primarily from the noise in the initial data, which is taken as counts vs. kinetic energy, N(K.E.) vs.

K.E. However, this error is enhanced due to spectra typically being differentiated and plotted as dN(K.E.)/dK.E. vs K.E. Temperature measurements are ±100 ℃ from the thermocouple.

A W(100) crystal was heated in cycles up to 1800 ℃ and flashed in oxygen to remove atmospheric contaminants. A 5 and 20 nanometer film of scandium and barium respectively were then deposited successively. The sample was then heated in stages, with spectra taken after each temperature reached. The resulting series of spectra is provided in Figure 4.44. Note only one complete run of this setup was performed. The barium signal largely diminishes by 800 ℃, which corresponds to the dewetting phenomena described previously. The scandium signal only disappears once heated to

1600 ℃, which agrees with the desorption temperature range noted in Table 4.1.

Interestingly, carbon and oxygen were detected after the depositions occurred. After heating to 1200 ℃ however, the oxygen signal vanishes. This may either be related to scandium sweeping up oxygen after dewetting, or the reaction between scandium and barium oxides as observed before. The sample appears completely clean after heating to

1600 ℃. 109

Figure 4.44. Auger spectra of annealed Sc/Ba films on W(100). Elemental peaks labeled, heated to temperatures shown. Normalized to maximum peak heights.

The Ba/Sc peak height ratio as a function of temperature is plotted in Figure 4.45.

Both barium peaks near the low and high kinetic energies in the observed range are used to determine the ratio with scandium, as the results between the two agree to within 5%.

Here it is seen that there is a substantial decrease in barium compared to scandium at 500 110

℃. As noted, prior, this is in the range of temperatures where thin barium films dewet.

So, while the overall surface concentration my decrease, the absolute amount of barium may remain unchanged. The scandium signal remains relatively constant below 1200 ℃.

While scandium also dewets below that temperature, the film was also initially 5 nanometers, which results in a larger surface area covered by droplets after dewetting as noted in Table 4.2. This could result in the overall surface concentration change of scandium being outside the spatial resolution of the microCMA.

Figure 4.45. Normalized Ba/Sc peak height ratio vs. temperature. Normalized to maximum ratio. Error in ratio due to noise in the data. Temperature error is ±100 ℃. Point at 1600 ℃ excluded, as signal/noise ratio decreases as both barium and scandium desorb.

111

To further demonstrate the effect of dewetting on the Auger spectra, a very thick film of barium was deposited on the W(112) crystal. First, a 20-nanometer scandium film was deposited and annealed to 1400 ℃. This dewetted the film but was not hot enough to cause it to completely desorb. The barium evaporator was then run at 7 amperes for 90 seconds, which would result in the film greater than 40 nanometers. The system was then heated to 550 ℃, and the resulting spectra are shown in Figure 4.46. It is observed that the barium film almost completely covers the underlying elemental signals apart from a small amount of oxygen. After heating to 550 ℃, the film dewets and the tungsten and scandium signals return. The Ba/Sc peak height ratio would decrease dramatically in this case; however, little barium has left the surface and has only been largely localized into droplets.

Figure 4.46. Auger spectra of thick Ba film dewetting on W(112). Normalized to maximum peak heights. Ba film completely covers underlying W/Sc signal, which only returns after heating. 112

Oxidized films were also annealed on W(100) and W(112). The following results uses five runs using both crystal faces, as they were found to be independent of these.

Scandium films of thicknesses between 1 and 3 nanometers, and barium films in the range of 5 and 20 nanometers were deposited. These were oxidized with 10 to 75 L of oxygen. The general behavior of these spectra after heating was also independent of film thicknesses. An example series of spectra of a Sc-Ba-O system on W(100) after heating is provided in Figure 4.47. Comparing Figure 4.44 and Figure 4.47, the barium signal once again diminishes with heating, mostly below 800 ℃. However, it persists to higher temperatures as well. Carbon and oxygen signals also decrease with heat, which agrees with previous observations, particularly in the presence of scandium. Also note the 169 and 179 eV tungsten peaks return after heating to 800 ℃. As with Figure 4.46, this could be a sign of dewetting. 113

Figure 4.47. Auger spectra of annealed oxygenated Sc/Ba films on W(100). Elemental peaks labeled, heated to temperatures shown. Normalized to maximum peak heights. Figure adapted from Mroz et al., (2020) [72].

The general trend in these systems regarding barium and scandium content is summarized in Figure 4.48. As the barium signal depletes up to 800 ℃ in Figure 4.47, the Ba/Sc ratio in Figure 4.48 remains relatively constant. This supports the hypothesis that the change in peak heights at temperatures ≤ 800 ℃ is primarily an artifact of the 114 dewetting process. As the temperatures increase to ≥ 800 ℃, the desorption processes that affect these barium and scandium oxide systems seems to dominate. In these experiments both barium and scandium appear to completely leave the tungsten surface by 1400 ℃.

Figure 4.48. Normalized Ba/Sc peak height ratio vs. temp. from Sc-Ba-O systems. Normalized to maximum ratio. Low and high energy Ba peaks used to calculate ratios. Error in ratio due to noise in the data. Temperature error is ±100 ℃. Point at 1400 ℃ excluded. Figure adapted from Mroz et al., (2020) [72].

115

CHAPTER 5 : DISCUSSION

Dipole Model

The experiments described studied materials common in high performing cathodes, barium, scandium, and oxygen, on low index single tungsten crystals. They were designed in order to test the proposed models of the mechanisms in which these cathodes function. One such model is the dipole model, where positive ions on a metal surface lower the barrier for electron emission, the work function, via a dipole interaction

[20-22]. Cathodes that incorporate these materials are typically run at temperatures upward of 1000 ℃, so phenomena that occur at or below this temperature are considered more impactful regarding these systems.

DFT provides four possible arrangements of surface atoms that may result in a low work function surface, or less than 2 eV, on a W(100) surface [21]. This is less than the work function of bulk barium, which is approximately 2.5 eV. Lower work functions result in larger current densities according the Richardson-Dushman equation, Equation

1. These low work function arrangements are Ba0.25, Ba0.25O0.25, Ba0.25O, and

Sc0.25Ba0.25O.

In the case of metallic barium on W(100), the resulting LEED pattern after annealing to the point of dewetting, ≤ 500 ℃, does appear to support the Ba0.25 stoichiometry. Figure 4.28 b. shows a strong 2X2 pattern, which means that after the barium film retracts, it leaves behind a 0.25 ordered monolayer. The PEEM studies in

Figure 4.8 also show that a weak signal from barium remains upwards of 1400 ℃ ± 100

℃. However, it should be noted that the barium region is dark when compared to the 116 neighboring Sc/Ba overlap region. This means that the work function is at least higher than scandium’s work function of 3.5 eV. This is most likely due to the amount of bare tungsten visible. Auger data from Figure 4.44 also suggests this partial barium monolayer is stable to upwards of 1200 ℃ ± 100 ℃. These observations suggest that the

Ba0.25 surface is available at temperatures relevant for thermionic emission.

The LEED that results from annealed oxygenated barium films, Figure 4.34 a., does not seem to support the Ba0.25O0.25 or Ba0.25O surface arrangements. The proposed surface structure in Figure 4.35 represents a Ba0.5O0.5 stoichiometry. The 45° orientation with respect to the W(100) surface does match with the proposed Ba0.25O0.25 arrangement from DFT. The XPS data in Figure 4.42 shows that BaO is stable on the tungsten surface to at least 1200 ℃ ± 100 ℃.

The Sc0.25Ba0.25O surface arrangement does not form in the oxygenated Sc/Ba co- deposited systems. The scandium atom is thought to act as a barium dipole stabilizer in some capacity. A full monolayer of oxygen does not form in the presence of scandium, as scandium removes it from the surface during the dewetting process. The LEED in

Figure 4.32 shows that an oxidized tungsten surface with a scandium film deposited onto it will become unoxidized after the film dewets, at temperatures ≤ 750 ℃. The oxygen cleaning properties of scandium is further enforced by the XPS data shown in Figure

4.39, and the Auger spectra provided in Figure 4.44. It is seen that the tungsten is oxidized due to the satellite peaks at higher binding energies in XPS, and that the oxygen signatures are entirely absent at temperatures between 1000 ℃ and 1200 ℃ in both XPS and AES. 117

It has also been observed that BaO and Sc2O3 react in some manner that causes their removal from the surface. In the PEEM images in Figure 4.11, it is seen that the oxides desorb at temperatures between 800 ℃ and 1100 ℃. The high work function tungsten surface becomes visible after the reaction takes place, showing up dark in photoemission. This observation is further supported by the XPS results in Figure 4.41 and Figure 4.42. After the addition of more BaO to an annealed BaO/Sc2O3 system, in which the BaO had been completely reacted away, the new BaO remained to a higher temperature, as the remaining Sc2O3 was depleted in the same reaction. This was also observed to occur near 800 ℃ ± 100 ℃. These factors prohibit the formation of a low work function Sc0.25Ba0.25O surface.

In all cases, when viewed in ThEEM, the primary source of thermal emission was the dewetted droplets, not the monolayer background. The background was always dark when compared to the droplets, suggesting that a monolayer of material is not responsible for the high current densities achieved. Dipole effects on the work function of a surface, as well as temperature effects of atom adsorption and desorption, are well established

[11,74-76]. It may be that these dipole effects become less significant at temperatures required for thermal emission. The surface dipole itself may even depolarize, negating any potential benefit once a certain temperature is reached [77,78]. It should also be noted that temperature directly changes the work function of a material, though the change is limited to < 1% over a range of 500 ℃, meaning it is not significant enough to affect these cathode systems in a practical manner [79]. 118

Semi-Conductor Model

Another model proposed to explain the high current densities observed in scandate cathodes is the semi-conductor model [19]. This functions by having the electric field used to extract electron from the cathode penetrate a semi-conductor layer on the cathode surface. This lowers the effective barrier of emission. The fields used in cathode tests are on the order of kV/cm, which is approximately the fields experienced by samples in LEEM systems [25]. For this effect to be large enough to achieve the current densities desired, the semi-conductor layer must be on the order of 0.4 – 0.5 microns thick in the case of scandate cathodes.

Films of scandium and scandium oxide between 1 and 30 nanometers were studied in this work. However, the similarities between scandium and scandium oxide dewetting behavior and temperatures, noted in Figure 4.27, may hint at the existence of an oxidation passivation layer on scandium [58]. The scandium films were also studied in the presence of barium as well. In all cases, the films dewetted from the tungsten surfaces when annealed to temperatures < 750 ℃. This is primarily a surface energy driven phenomenon, where the addition of heat allows the thin films the energy to reconfigure into lower free energy droplets on the surface. The self-binding energy of the barium and scandium is greater than that of the W-Ba and W-Sc interaction. Similar film instabilities were previously seen up to thicknesses of 200 nanometers [80]. It should be noted that tungsten, scandium, and barium do not alloy at any weight percentage or temperatures [61,81]. 119

Dewetting ruptures any continuous film into droplets, which may explain the previously observed “patch” effects seen in semi-conductor model studies [19,82]. The size of the final droplets observed from the 5-30 nanometer films were between 0.3 and

0.7 microns. Similar sized nanoparticles are observed in well performing cathodes, as pictured in Figure 1.5. This does meet the size requirement for the semi-conductor models for enough field penetration, however.

The other factor needed for the semi-conductor model, is the incorporation of barium into the makeup of the thick layer itself. As described in the previous section,

BaO and Sc2O3 react in a manner that removes them from the surface. The temperature range at which this occurs is in line with standard cathode operating temperatures, between 800 ℃ and 1100 ℃.

Due to the film instabilities, which results in rupturing at dewetting below cathode operating temperatures, and the reactive nature of BaO and Sc2O3 at operating temperatures, the semi-conductor model appears insufficient in explaining the observed current densities.

Function of Scandium

The role of scandium in cathodes has been frequently labeled as not being fully understood, and its function has been debated [18,65]. The experiments described previously demonstrate that the function of scandium is largely a regulatory one. The presence of scandium alters how oxygen interacts in the system.

The XPS spectra of Figure 4.39 and Figure 4.42 demonstrate how scandium inhibits the formation of tungsten oxide on the surface, and also how tungsten oxide 120 forms when scandium is no longer present. This is beneficial for thermal emission, as tungsten oxide has a large work function, approximately 6.7 eV [83]. Having an oxide free surface allows for a more stable adsorption of barium to the surface, allowing it to persist up to 1200 ℃ as a partial monolayer, as confirmed by the LEED in Figure 4.28 and XPS spectra in Figure 4.42.

Scandium has also been observed to remove BaO from the system as well at cathode relevant temperatures. It’s possible that the removal of BaO could free up metallic barium for further thermal emission. BaO has an effective work function, band gap plus electron affinity, of approximately 4.97 eV, as compared to the 2.5 eV work function of metallic barium [84,85].

The inclusion of scandium seems to prepare the tungsten surfaces in its vicinity for more effective barium adsorption. It also removes less thermally emissive BaO. In this regard, scandium serves as a cleaning agent when it comes to oxygen. This may be beneficial in commercial devices, as the manufacturing processes may not be as controlled as in the model systems studied here.

Emission from Barium

The primary source of emission is understood to be barium, or a barium compound of some type. This is the reason as to why barium reservoirs are included in dispenser cathodes, to act as a source of barium to replenish the emitting surface [8-10].

The main form of transport that barium underwent is thought to be surface diffusion. As the cathodes are ran at operating temperature, the barium diffuses from the reservoir to the surface. However, as seen in every case of barium or barium oxide on tungsten, most 121 barium dewets from the surface rather than diffuses across it. Furthermore, barium metal itself is found to be the most efficient emitter in these systems [80]. As displayed in

Figure 4.25, the dewetting temperatures of barium are below cathode operating temperatures by at least 500 ℃, meaning this process is in play during cathode operation.

Once bulk barium dewets on tungsten, it becomes largely immobile.

If barium atoms were to diffuse out of the bulk droplets to replenish the monolayer, the effect of this may be minimal in terms of thermal emission. Figure 4.23 shows that the droplets themselves are the primary thermal emitter, not the partial monolayer background. In some sense, diffusion of barium into the monolayer may be a hindrance to cathode lifetimes.

Replenishing barium to the surface droplets may result in the greater benefit.

However, since dewetting operates in the opposite direction then surface diffusion, another transport mechanism must be utilized to accomplish this. An example would be

Knudsen Diffusion considering the porous nature of cathodes [84].

Role of Field Emission

Cathodes are designed to generate electrons, but electric fields are needed to extract these electrons for use. Thermal emission is therefore tied to the phenomena of field emission [6,87]. The effect of a metal in a high electric field results in the lower barrier for electron emission. Fields experienced in cathodes are of the order of kV/m, and fields upwards of a GV/m are need for full field emission. Due to the relatively low field and high temperatures in these systems, they exist in the regime for Schottky

Emission, also known as Field Enhanced Thermal Emission [88]. Surface geometry also 122 plays a role in field emission, and the degree to which it does is referred to as an enhancement factor [89,90].

In the model cathode systems studied here, the primary thermally emitting feature are the dewetted droplets. Droplets form an approximate hemisphere on the surface.

Hemispheres have a field emission enhancement factor of 3 in ideal conditions [90]. This further reinforces the benefits of optimizing droplet lifetime and distribution. Figure 4.13 d. shows a dewetted scandium film with a barium microdot deposition. The concentration of barium decreases near the edge of the microdot, yet the intensity increases. This is due to smaller droplets covering more of the tungsten surface, as summarized in Table 4.2. The result of which may be a more densely packed area of thermally assisted field emitters, increasing the current density.

Future Direction

In order to pursue higher current density cathodes that potentially operate at lower temperatures, controlling the distribution of surface barium metal seems key. Barium is more stable on a clean tungsten surface, as evidenced by the cleaning function of scandium. The crystal face of tungsten also had no apparent effect on the results of these experiments. Therefore, purposeful texturing of a clean tungsten surface would influence the distribution of barium droplets after dewetting. The geometry of the texturing may also allow for field emission to play a more predominant role. 3-D printing techniques may also be utilized in order to further streamline the manufacturing process [49,91-93]. 123

CHAPTER 6 : CONCLUSIONS

In this work, model cathode systems were studied in order to understand the mechanisms by which the primary materials contribute to high current densities via thermionic emission. Figure 6.1 and Figure 6.2 are the phase diagrams for the metallic and oxygenated systems respectively. At temperatures relevant to cathode operation, 800

℃ to 1000 ℃, barium and scandium are both observed to dewet from the tungsten surfaces. Thermionic electron emission microscopy revealed that the resulting droplets that formed are the primary contributor for thermal emission, not the remaining monolayer of material. This runs contrary to the idea that these materials diffuse across this surface to resupply an emitting dipole layer. Barium oxide and scandium oxide were also observed to react at relevant temperatures in a manner that removes them from the emitting surface. This behavior, and the film instabilities made evident by dewetting, rules out the thick semi-conductor layer model of thermal emission. The function of scandium is one of oxygen regulation, where it inhibits the oxidation of tungsten, allowing for more stable conditions for barium adsorption. Barium still functions as the primary emission source due to its lowest work function, however this emission may be enhanced due to the geometry post dewetting. 124

Figure 6.1. Phase diagram for the metallic systems studied. 125

Figure 6.2. Phase diagram for the oxygenated systems studied.

With the advent of 5G communication technologies, development will move onto the next generation, 6G technologies. This is further motivated by the Federal

Communications Commission (FCC) opening of terahertz frequency atmospheric 126 windows for experimental uses. Thermionic cathodes are a primary component in high powered terahertz communication devices. A more detailed understanding will be necessary in order to improve and optimize them for widespread production and use.

Data transfer rates will need to be pushed to terahertz speeds in order to facilitate technologies such as networked autonomous vehicles and the idea of the Internet of

Things [94-96].

This multi-faceted approach for characterization has shed further light on a technology that has existed for more than a century. Despite the time thermionic cathodes have been in use, improvements to development and efficiency are still possible.

What was considered old, may yet still be made new.

127

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