faculty of natural science department of physics

VYTAUTAS MAGNUS UNIVERSITY

FACULTY OF NATURAL SCIENCE

DEPARTMENT OF PHYSICS

MICHAEL OLUWASEUN IDOWU

“INVESTIGATION OF THE CATALYST MATERIALS DIFFUSION DURINGHYDROGENATION OF Pt-Mg-Ni THIN FILMS”

Master thesis

Energy and Environment Study program, Study code 621F35001Physical Sciences (Physics)

Supervisor: Prof. Dr Darius Milcius _____________ ____________

Defence: Prof. habil. Dr. Algimantas Paulauskas __________ ____________

KAUNAS, 2015

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CONTENTS

SUMMARY................................................................................................................................... 3

SANTRAUKA............................................................................................................................... 4

LIST OF TABLES.......................................................................................................................... 5

LIST OF FIGURES........................................................................................................................ 6

INTRODUCTION.......................................................................................................................... 8

1 MAGNESIUM NICKEL FILMS...........................................................................................10

2 MATERIAL SYNTHESIS BY MAGNETRON SPUTTERING..........................................11

2.1 PRINCIPLES OF MAGNERTON SPUTTERING.........................................................11

3 MATERIAL ANALYSIS .....................................................................................................13

3.1 Scanning Electron Microscopy...................................................................................13

3.2 Energy Dispersive Spectroscopy................................................................................17

3.3 X-ray Photoelectron Spectroscopy.............................................................................20

3.4 X-ray Diffraction........................................................................................................23

4 MATERIAL ANALYSIS METHODS..................................................................................27

4.1 Material Synthesis by Magnetron Sputtering.............................................................27

4.2 Material Analysis by Scanning Electron Microscope................................................28

4.3 Material Analysis using Energy dispersive Spectroscope..........................................29

4.4 Material Analysis using X-ray Photoelectron Spectroscope......................................30

4.5 Bulk Analysis using X-ray Diffraction.......................................................................31

5 MATERIAL ANALYSIS RESULTS....................................................................................34

5.1 Results from Scanning Electron Microscope..............................................................34

5.2 Results from Energy dispersive Spectroscope............................................................35

5.3 Results from X-ray Photoelectron Spectroscope........................................................36

5.4 Results from X-ray Diffraction...................................................................................40

CONCLUSIONS...........................................................................................................................43

REFERENCES...............................................................................................................................44

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SUMMARY

Author of diploma paper: Michael Oluwaseun Idowu

Full title of diploma paper: Investigation of the catalytic material diffusion during

hydrogenation of Pt-Mg-Ni Thin films

Diploma paper advisor: Prof. Dr Darius Milcius

Presented at: Vytautas Magnus University, Faculty of Natural Science, Kaunas,

2015

Number of pages: 46

Number of tables: 3

Number of pictures: 28

Mg-Ni alloy thin films have attracted much attention as hydrogen storage materials . The

achievable high H-storage capacity and the low absorption and desorption pressures are the advantages

of this material. However, because of the high sorption temperature and the relatively slow kinetics the

direct use of magnesium is not common. Using catalysts is one direction to optimize the properties. It is

important to discover the best catalyst for magnesium-based systems to enhance the kinetics of

absorption and desorption. In this thesis work, The aim of the work is to investigate what happens to

the Platinum coating which is the catalyst during hydrogenation process of the Magnesium-Nickel

samples/films.

Three samples of Magnesium and Nickel coatings on Silicon substrate were obtained by

magnetron sputtering. Each sample was divided into two equal parts and one part was covered with a

catalytically active platinum layer. The coating was hydrogenated using the sky hydrogenation under

high hydrogen pressure and temperature technologies. The amount of hydrogen absorption was large

upon exposure to hydrogen gas; the films take up hydrogen and become transparent. The platinum

layer enhanced the kinetics of hydrogen adsorption and desorption, while also protecting the coatings

from further oxidation. It is shown that in spite of all the advantages of platinum after hydrogenation

part of the platinum diffused into the Mg-Ni coating structure and in the future it may stop

hydrogenation process while trying to realize multiple coatings hydrogenating - dehydrogenating. It

was also noted that this diffusion from the Mg-Ni coating initiated the formation of undesirable Mg-

Silicates formation.

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SANTRAUKA

Šiame darbe, panaudojant magnetroninio garinimo metodą buvo gautos Mg-Ni dangos ant

silicio padėklo. Kiekvienas bandinys buvo padalintas į dvi lygias dalis ir viena iš dalių buvo padengtas

katalitiškai aktyviu platinos sluoksniu. Dangos buvo hidrintos panaudojant dangų hidrinimo aukštame

vandenilio slėgyje ir temperatūroje technologijas. Gauti metalų hidridai buvo skaidrūs ir sluoksnis,

kuris buvo padengtas platinos katalizatoriais pademonstravo geresnę hidrinimosi kinetiką. Taigi

platinos sluoksnis pasitarnavo kaip hidrinimo proceso aktyvatorius ir kartu apsaugojo Mg-Ni dangas

nuo papildomos oksidacijos.

Pagrindinis šio darbo tikslas buvo ištirti kas atsitinka su platina Mg-Ni dangų hidrinimo metu.

Darbe parodyta, kad nepaisant visų platinos privalumų po hidrinimosi dalis platinos difundavo

į Mg-Ni dangos struktūrą ir ateityje tai gali sustabdyti hidrinimosi procesą, siekiant realizuoti

daugkartinį dangų hidrinimą – dehidrinimą. Taip pat buvo pastebėta, kad Si difundavo iš padėkliuko į

Mg-Ni dangas taip iniciuodamas nepageidautinų Mg – silicidų formavimąsi.

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LIST OF TABLES

Table 1 showing the variations in quantities of different elements in the sample with and withoutplatinum after hydrogenation for first sample.

Table 2 showing the variations in quantities of different constituent elements in the sample with andwithout platinum after hydrogenation for the second sample.

Table 3 showing the variations in quantities of different constituent elements in the sample with andwithout platinum after hydrogenation for the third sample.

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LIST OF FIGURES

Figure 1 Schematic representation of a magnetron sputtering process

Figure 2 Schematic representation of the scanning electron microscope

Figure 3 Electron beam-specimen interaction diagram and the different signals generated

Figure 4 Interaction volume increases with increasing acceleration voltage and decreases withincreasing atomic number

Figure 5 The Peltier Cooler

Figure 6 Elemental Energy dispersive X-Ray microanalyses, the peaks are labeled with the line of thecorresponding element

Figure 7 Photoelectron by X-ray Photoelectron spectroscopic technique

Figure 8 Kinetic energy of ejected photoelectrons

Figure 9 Schematic representation of the X-ray Photoelectron Spectroscopy

Figure 10 Schematics of the Bragg spectrometer

Figure 11 Braggs Equation describing x-ray diffraction

Figure 12 Magnetron Sputtering System at the Lithuania Energy institute

Figure 13 Scanning Electron Microscope: Model – S-3400N at the Lithuanian Energy Institute.

Figure 14 Energy Dispersive Spectroscope Model: Quad Detector 5040

Figure 15 XPS Model: PHI 5000 Versaprobe Manufactured by Physical Electronics, Inc. at theLithuania Energy institute

Figure 16 X-ray Diffractometer at the Lithuania Energy institute

Figure 17 Scanning Electron Microscope images of the two sample after hydrogenation with andwithout platinum



Figure 20 XPS graph showing the concentration of the different elements against depth in the thin filmafter Platinum was cleaned off for first sample

Figure 21 XPS graph showing the concentration of the different elements relative to the depth in thethin film for first sample

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Figure 22 XPS graph showing the concentration of the different elements against depth in the thin filmafter Platinum was cleaned off for second sample

Figure 23 XPS graph showing the concentration of the different elements relative to the depth in thethin film for second sample

Figure 24 XPS graph showing the concentration of the different elements against depth in the thin filmafter Platinum was cleaned off for third sample

Figure 25 XPS graph showing the concentration of the different elements relative to the depth in thethin film for third sample

Figure 26 X-ray diffractogram of the first hydrogenated Mg-Ni thin film before and after being coatedwith platinum

Figure 27 X-ray diffractogram of the second hydrogenated Mg-Ni thin film before and after beingcoated with platinum

Figure 28 X-ray diffractogram of the third hydrogenated Mg-Ni thin film before and after being coatedwith platinum

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INTRODUCTION

Hydrogen is very attractive as an energy carrier. To store and transport hydrogen there are

several options, each with its own advantages and problems. Hydrogen metal systems receive a lot of

attention due to the fact that the hydrogen density in a metal-hydride is often higher than in a liquid or

even in a solid hydrogen phase since it has a high diffusion co-efficient in metals [1]. To store

hydrogen, the metallic host needs to have certain properties like high hydrogen volume and weight

density, a good cycling stability, appropriate thermodynamics and low production costs. These

requirements limit the number of possible candidates strongly [2]. For its industrial use many criteria

have to be met. The achievable maximum capacity is determined by the selected storage solution.

However, there are a number of important secondary properties, which should also be considered:

hydrogen absorption-desorption kinetics and temperature, impurity effects, cycling stability, safety, raw

material cost and ease of manufacture [3].

Beside these basic hydriding properties there are other ones that should be put into

consideration like activation, a procedure that is important for hydriding a metal for the first time and

bringing it up to maximum H-capacity and hydriding-dehydriding kinetics. The ease of initial H2

penetration depends on surface structures and barriers, such as dissociation catalytic species and

passivation oxide films [4]. The time required for absorption-desorption is also an important property

[5]. Applications require as fast hydrogenation as possible, and desorption that is also fast enough to

meet the continuous production of energy in demand. Kinetics, the time required for a full absorption

or desorption, can vary on a large scale from alloy to alloy. However, there are some materials with

high hydrogen capacity, which are kinetics limited, especially at low temperature [6]. In cases like

these, enhancement of the hydriding processes is the main objectives.

The first step of all metal-hydrogen reaction is the mass transport of hydrogen molecules onto

the solid-gas interface, the dissociation of the molecules on the surface also called chemisorption at

special dissociation sites, possible migration to such sites, and penetration of hydrogen atoms through

the surface into the bulk metal [7]. At the end of this process, the hydrogen atoms are dissolved in the

bulk metal, near to the solid-gas interface [8].

Hydrogenation of a metal host material is done by hydrogen gas loading or electrolytic

charging. However, hydrogenation and the repeated cycling have a negative effect on the

microstructure of the bulk metal thereby making the material brittle due to expansion upon hydrogen

absorption. Eventually this will lead to a pulverization of the host material [9]. As a result

determination of the physical properties of the metal hydrides is not always easy. The problem is

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prevented by depositing a thin film of the material. Due to the interaction with the substrate, the film

remains intact during hydrogenation. Thin metal hydride films can thus be used as one of the model

systems to study the hydrogen storage properties.

Among the numerous hydrogen storage materials, magnesium and magnesium based alloys

are attractive because of their high theoretical hydrogen storage capacities (7.6 wt.% for MgH2, 3.6

wt.% for Mg2NiH4), lightweight and low cost. Mg-Ni alloy thin films have attracted much attention as

hydrogen storage materials [10], hydrogen sensors [11] and switchable mirrors [12]. But their setback

is the slow kinetics and high hydriding/dehydriding temperature of the Magnesium-based materials

seriously hamper their practical application. Many methods have been used to overcome these

limitations, like the addition of alloying elements and catalysts, surface modifications etc.

Subsequently, scientists became increasingly interested in investigating the relationship between the

microstructure and the hydrogenation properties of Mg based materials.

The achievable high hydrogen-storage capacity and the low absorption and desorption

pressures are the advantages of this material. However, because of the high sorption temperature and

the relatively slow kinetics, the direct use of magnesium is not very common. Making use of catalysts

is one pathway to optimize the properties. It is equally pertinent to find the best catalyst for

magnesium-based systems to enhance the kinetics of absorption and desorption, while the real, physical

role of catalysts in these processes is still not clear [13]. Platinum catalyst helps to split H2 molecules

to H atoms and increases hydrogenation kinetics which is why it is considered here.

MAIN GOAL OF THE WORK:

Investigation of the catalytic material diffusion during hydrogenation of Pt-Mg-Ni thin films.

TASKS OF THE WORK:

(i) Deposition of Mg-Ni thin films on silicon substrate by magnetron sputtering.

(ii) Carrying out the sample analysis with and without platinum using Scanning electron

microscope, Energy dispersive spectroscope and X-ray photoelectron spectroscope.

(iii) Carrying out bulk analysis using the X-ray diffraction.

(iv) Make conclusions on the fate of platinum after analysis using the instruments above.

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1. MAGNESIUM NICKEL FILMS (Mg2NiH4)

This is a semi-conductor and non-metallic hydride derived from metallic Mg2Ni after its

absorption of hydrogen by a relatively important rearrangement of its hexagonal metal substructure.

Four hydrogen atoms bond with single a Ni atom and the two Mg atoms donate two electrons each to

stabilize the high-H transition metal complex [14]. In this hydride two Mg² ions donate two electrons

each to stabilize the NiH4 complex. It is known as the only stable compound known for the Mg-Ni-H

system with a theoretical hydrogen capacity of 3.6 wt%. The structure of consists of tetrahedral NiH4-

complexes in a framework of magnesium ions.

The hydride exhibits a transition at 235 °C. In the low-temperature (LT) phase, magnesium

ions and NiH4-complexes assume an ordered arrangement to form a monoclinic structure, whereas in

the high-temperature (HT) phase, the arrangement becomes disordered to form a cubic anti-fluorite

structure. This phase transformation is, however, known to be severely disturbed by the internal stress.

The HT phase was produced by ball-milling at room temperature and also by static compression to

50MPa during heat treatments. The reverse transition from HT to LT phase was also affected by

introduction of micro twinning, which is characterized by the appearance of a diffraction peak of a

lattice spacing of 3.24 Å [15]. To make Mg2NiH4 less stable and more suitable for hydrogen storage it

is essential to avoid the presence of Mg/MgH2 [16].

Magnesium alloys are promising storage materials as a result of their lightweight, high

specific storage capacity (7.6 wt% for MgH2 and 3.6 wt% for Mg2NiH4) [17], abundant raw materials

and low environmental impact. Due to its low weight and high hydrogen content, it has been

intensively investigated both theoretically and experimentally as a hydrogen storage material and also

for increasing the negative electrode capacity in nickel metal hydride (NiMH) batteries. Nevertheless,

Magnesium is inadequate to be used in hydrogen storage applications due to its high hydrogen

desorption temperature and relatively slow hydrogen absorption/desorption kinetics [18]. Storage

capacity of Mg2NiH4 is smaller; however the kinetic is better and desorption temperature lower

meaning it has more potential for hydrogen storage if more solutions are discovered via effective

experimental investigations.

However, under normal conditions Mg2NiH4 is too stable, i.e. at room temperature the

equilibrium pressure is so low that Mg2NiH4 practically does not desorp hydrogen. Therefore, it is

essential that the stability of Mg2NiH4 be decreased to produce a suitable material for practical

hydrogen storage. To make Mg2NiH4 less stable and more suitable for hydrogen storage it is essential

to avoid the presence of Mg/MgH2 [19].

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2. MATERIAL SYNTHESIS BY MAGNETRON SPUTTERING

2.1. MAGNETRON SPUTTERING PRINCIPLES

In the basic sputtering process, a target or cathode plate is bombarded by energetic ions

generated in glow discharge plasma, situated in front of the target. The bombardment process causes

the removal and sputtering of target atoms, which may then condense on a substrate as a thin film.

Secondary electrons are also emitted from the target surface as a result of the ion bombardment, and

these electrons play an important role in maintaining the plasma [20]. While in Magnetron Sputtering,

The magnets produce the magnetic field that makes the electrons go in a curved path which are longer

bringing about more collisions which in turn produces more ions needed to strike plasma.

Fig.1 Schematic representation of a magnetron sputtering process [21]

Magnetrons make use of the fact that a magnetic field configured parallel to the target surface

maintains secondary electron motion to the surroundings of the target. A magnetic field is applied at

right angle to electric field by placing large magnets behind the target; this traps the electrons near the

target surface, causing them to move in spiral motion until they collide with an Argon atom [22].

Trapping the electrons in this way increases the frequency of an ionizing electron-atom collision

occurrence. A dense plasma is the result of the increased ionization efficiency of a magnetron in the

target region resulting into increased ion bombardment of the target resulting in higher sputtering rates

and consequent higher deposition rates at the substrate. In addition, the increased ionization efficiency

achieved in the magnetron mode allows the discharge to be maintained at lower operating pressures (up

to 10~2mbar) and lower operating voltages (typically, -500V, compared to -2 to -3 kV) than is possible

in the basic sputtering mode since lower voltage is needed to strike plasma and deposition rate

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increases up to 10-100 times faster than without magnetron configuration. The use of the magnets is to

increase the percentage of electrons taking part in the ionization events thereby increasing the

probability of electrons striking Argon, increases electron path length which increases efficiency of

ionization. Another reason why the magnetron sputtering process is more efficient is that it controls

uniformity; electrons paths are more curved near stronger magnetic field. More ions collide with target

in regions of high magnetic field. More ion collisions lead to more target atoms sputtering.

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3. MATERIAL ANALYSIS

3.1. MATERIAL ANALYSIS USING SCANNING ELECTRON MICROSCOPE

It is an electron microscope that creates images of the sample surface by using an electron

beam that scans the surface of a specimen inside a vacuum chamber. The electrons interact with the

atoms that make up the sample producing secondary electrons, backscattered electrons, x-rays that

contain information The scanning electron microscope provides characteristic information like the;

Topography which are the surface features of the material/sample or “how the sample/s look

like”, texture and the direct relation between these features and the properties of the material/sample.

Morphology which is the size and shape of the particles that makes up the material and the

direct relation between these structures and the properties of the material/sample.

Composition which is the elements and compounds that the material is composed of with their

respective relative amounts; direct relation between composition and the properties of the

material/sample. A scanning electron microscope may be equipped with an energy dispersive

spectroscopic analysis system to enable it to perform compositional analysis on specimens.

3.1.1 WORKING PRINCIPLES OF THE SCANNING ELECTRON MICROSCOPE

The principle is based on interaction between a focused beam of electron and the sample

which eventually produces the signal on the screen. The scanning electron microscope uses electrons

instead of light to form an image. A beam of electrons is produced at the top of the microscope by the

electron gun. The electron beam follows a vertical path through the column of the microscope. It makes

its way through electromagnetic lenses which focus and direct the beam down towards the sample [23].

Once it hits the sample, a large number of signals are generated including other electrons like

backscattered or secondary. Detectors collect the secondary or backscattered electrons, and convert

them to a signal that is sent to a viewing screen similar to the one in an ordinary television, producing

an image. The energy of the primary electrons determines the quantity of secondary electrons collected

during interaction.

The emission of secondary electrons from the specimen increases as the energy of the primary

electron beam increases, until a certain limit is reached. Beyond this limit, the collected secondary

electrons diminish as the energy of the primary beam is increased, because the primary beam is already

activating electrons deep below the surface of the specimen [24]. Electrons coming from such depths

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usually recombine before reaching the surface for emission. Apart from secondary electrons, the

primary electron beam results in the emission of backscattered electrons from the specimen.

Backscattered electrons possess more energy than secondary electrons, and have a definite direction.

Due to this, they cannot be collected by a secondary electron detector, unless the detector is directly in

their path of travel. All emissions above 50 eV are known to be backscattered electrons. Secondary

electrons are generated from the collision between the incoming electrons and the loosely bonded outer

electrons low energy electrons of about 10-50 eV and only secondary electrons generated close to

surface escape which are the source of the topographic information.

Fig. 2 Schematic representation of the scanning electron microscope [25]

These interactions can be divided into two major categories: elastic interactions and inelastic

interactions. Elastic scattering results from the deflection of the incident electron by the specimen

atomic nucleus or by outer shell electrons of similar energy [26]. This kind of Interaction is

characterized by negligible energy loss during the collision and by a wide-angle directional change of

the scattered electron. Incident electrons that are elastically scattered through an angle of more than 90˚

are called backscattered electrons (BSE), and yield a useful signal for imaging the sample. Inelastic

scattering occurs through a variety of interactions between the incident electrons and the electrons and

atoms of the sample, and results in the primary beam electron transferring substantial energy to that

atom [27]. The amount of energy loss is dependent on whether the specimen electrons are excited

independently or collectively and the binding energy of the electron to the atom. As a result, the

15

excitation of the specimen electrons during the ionization of specimen atoms leads to the generation of

secondary electrons (SE), which are defined as possessing energies of less than 50eV which can be

used to image or analyze the sample. In addition to those signals that are utilized to form an image, a

number of other signals are produced when an electron beam strikes a sample, including the emission

of characteristic x-rays, Auger electrons, and cathodaluminescence. The tear drop zone below shows

the regions from which different signals are detected. The combined effect of the elastic and inelastic

interactions is to limit the penetration of the beam into the solid. The region of interaction between the

solid and the beam is known as the interaction volume.

Fig. 3 Electron beam-specimen interaction diagram and the different signals generated [28]

Two major factors control which effects can be detected from the interaction volume.

i) A beam of electrons lose energy as they probe the sample due to interactions with it

and if too much energy is required to produce an effect, it will not be possible to

produce it from deeper portions of the volume due to the increasing loss of energy.

ii) The degree to which an effect, once produced, can be observed is controlled by how

strongly it is diminished by absorption and scattering in the sample. E.g. although

secondary and Auger electrons are produced throughout the interaction volume,

they have very low energies and can only escape from a thin layer near the sample's

surface [29].

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Fig. 4 Interaction volume increases with increasing acceleration voltage and decreases with increasing atomicnumber [30]

3.1.2 MAIN COMPONENTS OF THE SCANNING ELECTRON MICROSCOPE

These are subdivided into three parts which are the Electron Optical Column consisting of the

electron gun, objective lenses, condenser lenses, apertures and the scanning coils, Vacuum systems

consisting of the chamber and valves and pumps which produce the vacuum while the signal detection

and display comprise of detectors and electronics which produce the images from the signals.

(i) ELECTRON GUN: It is used for producing an intense beam of electrons using either the

thermionic gun (thermal energy) TEG electron emission from heating of a solid e.g. LaB6

or a field emission gun (electric field) where a strong electric field is used to extract

electrons.

(ii) LENSES: Here, we have the condenser lens that reduces the diameter of the electron beam

which is important for good resolution; which is the ability to resolve two closely spaced

points which can be improved by reducing the size of the electron beam that strikes the

sample and also by decreasing the working distance. The condenser lens is used to form the

beam and limit the amount of current in the beam while also helping to eliminate the high-

angle electrons from the beam with the help of the condenser aperture which also prevent

electron spray since the electrons coming from the electron gun have different kinetic

energies and directions of movement, they may not be focused to the same plan to form a

sharp spot, by inserting the aperture, the stray electrons are blocked and the remaining

narrow beam will come to a narrow point of least confusion. The objective lens plays the

role of strictly concentrating the electron beam on the sample by changing the magnetic

field strength.

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(iii) SCANNING COILS: This raster or sweep the beam across the sample surface by also

moving the beam too.

(iv) SAMPLE CHAMBER: This is where the sample is placed and can also manipulate and

move the sample.

(v) DETECTOR: This is used to detect the secondary and backscattered electrons. Detectors

can be the Backscattered electron detector (Solid- State detector), the secondary electron

detector and the energy dispersive spectroscopy which detects x-rays.

(vi) VACUUM CHAMBER: This holds the vacuum and is very essential because without it the

electron filament will be damaged via burning and oxidation and electrons and sample/s will

collide with other gas molecules.

3.2 MATERIAL ANALYSIS USING THE ENERGY DISPERSIVESPECTROSCOPE

The energy dispersive spectroscopy system determines atomic qualitative and quantitative

information from the specimen. The analysis is useful in identifying materials and contaminants, as

well as estimating their relative concentrations on the surface of the specimen. It relies on the

investigation of sample through interactions between electromagnetic radiation and matter analyzing x-

rays emitted by the matter in response to being hit with charge particles. Its characterization capabilities

are due in large part to the fundamental principle that each element has a unique atomic structure

allowing x-rays that is a characteristic of an element's atomic structure to be identified uniquely from

each other. Using X-rays to produce electron-hole pairs (total summed charge which is proportional to

incident x-ray energy), which are amplified and then “digitized” by voltage, displayed as a histogram

of the number of x-rays pulses (y axis) versus x-ray energy (x axis) [31].

3.2.1 OPERATING PRINCIPLES OF THE ENERGY DISPERSIVE SPECTROSCOPE

Due to interaction between high energy electrons and the sample that is being investigated in

the electron microscope, the atoms of this sample are caused to emit X-rays. Putting the energy

dispersive spectroscope into use confirms that atoms of different chemical elements irradiate X-rays of

different characteristic energy. The evaluation of the energy spectrum collected by energy dispersive Si

(Li) or SDD X-ray detector allows us to determine the qualitative and quantitative chemical

composition of the sample at the current beam position. This technique provides a very high spatial

resolution since the information is obtained from a very small sample volume in the order of only a few

microns. It is therefore also referred to as X-ray microanalysis. When used with scanning electron

18

microscopes, the EDS analyses element distributions along a line (line scan) or within an area of

interest (mapping). Another advantage of the energy dispersive spectroscopy is that all elements from

atomic number 5 (Boron) up to 92 (Uranium) contained in the sample can be detected and analyzed

simultaneously [32].

A Bruker detector which uses Peltier cooler, a solid state cooler is used that has no moving

parts and doesn’t encourage any form of vibration that could degrade the operation of the microscope

and is based on the Peltier effect that occurs whenever electrical current flows through two dissimilar

conductors that is dependent on the direction of current flow, the junction of the two conductors will

either absorb or release heat.

Simply, the cold side of the Peltier will just absorb the heat and the flow of electrons will then

transfer the heat to the hot side to be dissipated. All this is achieved with only electron flow from a DC

source [33].

Fig. 5 The Peltier Cooler [34]

The X Flash detector is an energy dispersive X-ray detector based on the silicon drift chamber

principle (SDD). A monolithically integrated on-chip FET (Flash Emulation Tool) acts a signal

amplifier and supports energy resolution. The X-flash Detectors cope with extremely high count rates

while also maintaining an energy resolution unrivaled by any other energy-dispersive detection system

[35].

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3.2.2 THE SDD WORKING PRINCIPLE

The detector crystal is moderately cooled by vibration free thermo-electric coolers; any

generated heat is dissipated by unforced convection without the need for external cooling means. A

super thin radiation entrance window separates the sensitive detector area from the ambient atmosphere

and guarantees a good transmission for the X-rays of interest [36]. An SDD is an actual semiconductor

device with a greatly reduced anode spot and drift rings (from which the technology gets its name) that

apply a drift field to guide the charge clouds from anywhere within the active area to the designated

anode spot. This eliminates the related anode capacitance while maintaining the original active area of

the detector [37]. With reduced noise levels, the resolution of the detector is better and the required

shaping time is small.

Bruker X-Flash SD detectors go one step further than other SDD devices by integrating the

FET monolithically into the detector chip. This eliminates noise introduced by an external FET with

wiring connection to the crystal. This gives unprecedented energy resolution enables fast signal

processing, resulting to a significant increase in throughput count rate at a given resolution. Optimum

resolution can be achieved with count rates exceeding 100,000 cps [38].

A side effect of the fast filtering is a strongly reduced sensitivity of SDDs to thermally

generated noise, relaxing the requirement on cooling. The moderate SDD operating temperatures

(around -20°C) can easily be achieved by thermoelectric coolers (Petier). This vibration free cooling

method excludes any need for cooling agents like liquid nitrogen or other external cooling means [39].

Fig.6 Elemental Energy dispersive X-Ray microanalyses, the peaks are labeled with the line of thecorresponding element [40]

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Peak intensity gives information about the element composition to find the changes in

concentration of the elements; a quantitative information while Peak energy gives qualitative

information about the constituent elements.

3.3 MATERIAL ANALYSIS USING X-RAY PHOTOELECTRONSPECTROSCOPE

It was developed in Sweden by Kai Siegbahn and his research group in the mid-1960s. The

technique was initially called ESCA (Electron Spectroscopy for Chemical Analysis). Examination of

elemental and chemical state information of a wide range of inorganic and organic solid materials,

films, powders is possible using this technique. Sample is irradiated with mono-energetic x-rays

resulting in emission of photoelectrons from the sample surface. An electron energy analyzer

determines the binding energy of the photoelectrons. From the binding energy and intensity of a

photoelectron peak, the elemental identity, chemical state, and quantity of an element are determined

which is an important advantage of the technique since the number of photoelectron of an element is

dependent upon the atomic concentration of that element in the sample. Identification of the elements

in the sample can be determined directly from the kinetic energies of the ejected photoelectrons which

are measured by the electron energy analyzer and a photoelectric spectrum is then recorded [41].

3.3.1 WORKING PRINCIPLE OF XPS

It is based on the photoelectric effect. Each atom in the surface has core electron with its

characteristic binding energy. When an x-ray beam is irradiated on the sample surface, the photon is

absorbed by an atom in the sample, resulting in ionization and the emission of an inner shell electron

from the surface of the sample after the photon energy is adsorped by the electron only if the photon

energy is large enough to cause the emission otherwise photoelectrons from deep within the material

will be reabsorbed. While the x-rays may penetrate deep into the sample, there is a limit to the depth at

which the ejected electron can escape e.g. for energies around 1400eV, ejected electrons from depths

greater than 10nm have a low possibility of leaving the surface without undergoing an energy loss

occurrence, and therefore contribute to the background signal rather than well defined primary

photoelectric peaks [42]. The emitted electron with the kinetic energy Ek is the photoelectron.

The relationship governing the interaction of the photon with the core level is; KE = hν - BE –

Eφ where KE is the kinetic energy of the ejected photoelectron, hv is the X-ray photon characteristic

energy (for monochromatic Al Ka, h = 1486.6 eV since the photon is energy specified), BE is Binding

21

Energy of the atomic orbital from which the electron originates while eΦ is the spectrometer work

function also work function induced by the analyzer about 4-5 eV. XPS spectral lines are identified by

the shell from which the electron was ejected (1s, 2s, 2p, etc.). Binding energy (BE) denotes the

strength of interaction between electron (n, l,m, s) and nuclear charge [43]. In principle, the energies of

the photoelectric lines are defined in terms of the binding energy of the electronic states of atoms. Well

defined energy shifts to the peak energies as a result of the chemical environment of the atoms at the

surface. In the case of conducting samples, for which the detected electron energies can be referenced

to the Fermi energy of the spectrometer, an absolute energy scale can be established, thereby making

identification of species possible. However, for non-conducting samples the problem of energy

calibration is important due to the fact that electrons leaving the sample surface cause a potential

difference to exist between the sample and the spectrometer resulting in a retarding field acting on the

electrons escaping the surface because a positive charge zone will form on the surface of the sample

which brings about shift in the XPS peaks to higher binding energy [44]. To neutralize the surface

charge for data acquisition, we use the electron flood gun to re inject electrons to the sample surface

which then provide the right current to push the XPS to its original position.

Fig.7 Photoelectron by X-ray Photoelectronspectroscopic technique [45]

Fig. 8 Kinetic energy of ejected photoelectrons [46]

The core electron of an element has a distinct binding energy, something like a "fingerprint"

[47]. Thus almost all elements except for hydrogen and helium can be identified by the measurement of

the binding energy of its core electron which is also sensitive to the chemical set-up of the element. The

same atom is bonded to the different chemical species meaning variations in the binding energies of its

core electron resulting in shift of its corresponding XPS peaks ranging from 0.1 eV to 10 eV, an effect

called the chemical shift which can be applied to studying the chemical status of element in the surface.

22

Fig. 9 Schematic representation of the X-ray Photoelectron Spectroscopy [48]

Photoemission process can be summarized into three steps:

(i) Absorption and ionization, an initial effect stage.

(ii) Response of atom and creation of photoelectron, the final effect stage

(iii) Transport of electron to surface and escape (extrinsic losses) with both the first

and second having a significant effect on the binding energy.

XPS spectra show a distinct stepped background as intensity of background to high BE of

photoemission peak is always greater than low BE. Due to inelastic processes (extrinsic losses) from

deep in bulk, only electrons close to surface can, on average, escape without energy loss electrons

deeper in surface loose energy and emerge with reduced KE, increased BE Electrons very deep in

surface loose all energy and cannot escape. XPS is a quantitative technique in the sense that the number

of electrons recorded for a given transition is proportional to the number of atoms at the surface. The

chemical shifts reflect the influence of chemical bond with neighbouring atoms and it’s as a result of

varying binding energies of electrons. It is seen in XPS data as a valuable source of information about

the sample. “Tilting the sample with respect to the axis of the analyzer results in changing the sampling

depth for a given transition and therefore data collected at different angles vary due to the differing

composition with depth” [49].

3.3.2 COMPONENTS OF THE XPS

ELECTRON ENERGY ANALYZER: It is of two different types which are hemispherical and

cylindrical. It determines the binding energy of the photoelectrons.

X-ray source which is the x-ray monochromatic source e.g. twin anode (Mg/Al) source.

23

ELECTRON GUN: This is used to produce the low energy x-ray that is aimed at the target.

ARGON ION GUN: This allows the depth profiling of the surface of the sample which helps

to determine if contaminants are of the surface of the sample or inside the bulk.

VACUUM SYSTEM: Since XPS is a surface sensitive technique; there is tendency of

incorrect analysis of the surface composition if it encounters contaminants. The vacuum is helps in

removing contaminations and sample surface bombarded with argon ions.

ELECTRONIC CONTROLS.

3.3 BULK ANALYSIS USING X-RAY DIFFRACTION

X-ray diffraction is a tool for the investigation of the structure of matter. X -rays are scattered

by interaction with the electrons of the atoms in the material being investigated. Analysis of crystalline

structures and In-situ heating/cooling and phase change are done using the instrument. For

electromagnetic radiation to be diffracted the spacing in the grating should be of the same order as the

wavelength. In crystals the typical inter-atomic spacing ~ 2-3 Å so the suitable radiation is X-rays

therefore Hence, X-rays can be used for the study of crystal structures [50]. A number of concepts

should be understood about X-Rays and diffraction to get the most out of an X-Ray diffraction

machine. The main concepts include: X-ray, Crystals and diffraction.

(i) X-RAY: The typical wavelength of x-rays is 10-10m, whereas the wavelength of visible

light is typically 10-6m. X-rays are produced when any electrically charged particle of

sufficient kinetic energy is rapidly decelerated. Electrons are used as the particles in x-ray

tubes along with two metal electrodes. A high voltage (typically tens of thousands of volts)

is maintained across these electrodes, which draws electrons to the anode or target. X-rays

are produced at the point of impact and radiate in all directions [51]. A continuous spectrum

is produced when electrons hit the target, which is referred to as ‘bremsstrahlung’ or

German for ‘braking radiation’ as it is produced by stopping electrons. However, when the

applied voltage is raised above a critical value, characteristic of the target metal, sharp

intensity maxima appear in the spectrum [52]. These are the ‘characteristic lines’ and they

are produced by exciting an electron out of its shell (only K lines are used in x-ray

diffraction due to absorption).

The Kα lines are produced as an electron from one of the outer shells falling into the vacancy

created in the K shell, thereby emitting energy in the process as x-rays [53]. Many techniques,

including powder x-ray diffraction require the use of monochromatic radiation (single wavelength x-

24

rays). Hence the high intensity and monochromatic nature of characteristic lines make them ideal for

XRD analysis. For Cu radiation, the Kα x-ray is 90 times the intensity of the bremsstrahlung x-rays

[54].

(ii) CRYSTAL: There are 7 crystal systems: cubic, tetragonal, hexagonal (hexagonal and

trigonal), orthorhombic, monoclinic and triclinic. These are defined according to the axial

system used to describe their lattice.

(iii) DIFFRACTION: When a monochromatic X-ray comes into contact with a crystal lattice,

interference patterns are produced by each atom hit by the beam. Many of these patterns

will interfere with each other and cancel each other out. However, at the right distance and

angle these patterns can be in-phase with one another and cause constructive interference.

This is known as diffraction. This type of waveform interaction can be seen in all wave

systems. The image to the right shows a simulated wave pattern where the lattice is placed

on its side and two slits are present. The interfering wave propagation can be seen

amplifying three waves. A crystal lattice can be thought of as a source of multiple tiny slits,

where the X-ray beam is acting like the waveform in the image [55]. A beam of X-rays

directed at a crystal interacts with the electrons of the atoms in the crystal.

The electrons oscillate under the influence of the incoming x-rays and become secondary

sources of EM radiation. The secondary radiation is in all directions. The waves emitted by the

electrons have the same frequency as the incoming X-rays coherent. The emission will undergo

constructive or destructive interference with waves scattered from other atoms.

Fig. 10 Bragg spectrometer schematics [56]

In 1912 W. L. Bragg noticed there was a relationship between the wavelength of radiation, the

angle of the X-rays and the internal spacing in the crystal, which is expressed in the form: nλ = 2dsinθ

where n is an integer, λ is the wavelength of the x-rays (in our case 1.54Å for a copper tube

25

source), d is the spacing between planes in the atomic lattice of the sample, and θ is the diffraction

angle in degrees [57]. This is known as Bragg’s law. Although Bragg's law was used to explain the

interference pattern of X-rays scattered by crystals, diffraction has been developed to study the

structure of all states of matter with any beam, e.g., ions, electrons, neutrons, and protons, with a

wavelength similar to the distance between the atomic or molecular structures of interest. For

constructive interference: n = 2d Sin, The path difference between the 2 rays is = 2d Sin. There are

two important things to note about Bragg’s Law are the smaller the distance d, the larger the diffraction

angle θ. The bigger the wavelength λ, the larger the diffraction angle θ. i.e. sinθ = nλ/ 2d.

Fig. 11 Braggs Equation describing x-ray diffraction [58]

3.4.1 PARTS OF THE X-RAY DIFFRACTOMETER

THE SOURCE: The source produces the X-rays used for analyzing samples with X-

ray diffraction. Typically the source is an X-Ray tube. It consist of an evacuated ceramic or glass vessel

that contains a tungsten filament as cathode which emits electrons, and an anode onto which these

electrons are accelerated with a potential of several ten thousands of volts.

PRIMARY OPTICS: The primary optics controls the beam produced by the X-ray source, and

manipulates it into forms more useful for diffraction experiments.

SAMPLE HOLDER AND STAGE: Most diffractometers are supplied with several kinds of

sample holders for different types of specimens such as powders, bulk samples and thin films. The best

results are obtained with rotating sample holders which considerably improve the measurement

statistics, but they are not available for all machines. The most severe error during sample preparation

is to fill the sample holder too high or low.

SECONDARY OPTICS: The secondary optics retrieve the diffracted X-rays from the sample.

DETECTOR: The detector allows the diffracted X-rays to be detected. It consists of a

scintillator, a material that emits visible light on exposure to X-rays. This fluorescent radiation causes a

26

photocathode to emit photoelectrons which are amplified by a photoelectron multiplier. This is then

counted by electronic equipment to quantify the amplitude of the signals.

27

4. MATERIAL ANALYSIS METHODS

4.1 MATERIAL SYNTHESIS BY MAGNETRON SPUTTERING

It was implemented using the equipment PVD-75 with specifications below:

(i) 1500 L/s cryopump, base pressure < 8 x 10-8 mbar;

(ii) Three 400 W 3” magnetrons equipped with DC, pulsed-DC and RF power sources;

(iii) 2 MFC gas inlet channels;

(iv) 1-200 a.m.u. residual gas analysis (RGA);

(v) Heating up to 300 °C.

The task ranges from thin and thick film deposition to plasma treatment for surface

modification or activation.

Fig. 12 Magnetron Sputtering System at the Lithuania Energy institute

The two elements used for the thin film formation are Magnesium and Nickel. The used

substrate is Silicon. The magnetron sputtering method was used for the formation of vapor phase. The

formation of Mg-Ni alloys was realized by co-deposition of the elements arriving from two

independent magnetron sources while the control of the microstructure of films has been done by

changing the substrate temperature during deposition and the ratio of the flux of incident ions to the

flux of neutral atoms. Mg–Ni alloy thin films were prepared on Silicon substrates using direct-current

magnetron co-sputtering of Mg and Ni targets. The base pressure of the deposition chamber

28

was 5×10−5 Pa and the working pressure during deposition was kept at ∼1.0 Pa using a mass-flow

controller of Ar.

4.2 MATERIAL ANALYSIS BY SCANNING ELECTRON MICROSCOPE

Fig. 13 Scanning Electron Microscope: Model – S-3400N at the Lithuanian Energy Institute

Manufacturer – Hitachi. The main parameters are:

(i) SE resolution of 3nm at 30 kV, 10nm at 3kV, BSE resolution of 4 nm at 30 kV;

(ii) Low vacuum mode (6-270 Pa) allows to measure dielectrics without any pretreatment;

(iii) Sample diameter up to 200 mm, height - 80 mm;

(iv) 3D imaging and quantitative analysis;

(v) EDS elemental composition analysis.

4.2.1 PROCEDURE

SAMPLES: Mg-Ni thin films on Silicon substrate

PURPOSE: To produce a clear image of the samples and to determine the elements present in

them using the EDS attached to it.

EXPERIMENTAL: The provided sample was anchored on the sample hold and inserted in a

Hitachi S-3400N SEM (Scanning Electron Microscope). Attached to the SEM is the Energy Dispersive

Spectroscope (EDS). This technique can detect all elements heavier than and including boron (atomic

number 5). Detection limits are on the order of 0.5 to 1 % by weight if the peaks are isolated; however,

in practice the minimum detection limit will typically be 1-2 % by weight due to spectral overlaps,

29

which can make some elements present in minor concentrations e.g. Sulphur and Molybdenum

undetectable. Working voltage of 3 to 15 keV was used to analyze the residue. The analytical volume

is estimated to be approximately 1-micron deep and 1-micron across [18]. Different results for different

magnification ranging from better clarity to size of grain from finer to coarser with decreased and

increased magnification respectively from 100 keV to 10.0 keV with adjustment in Sigma X and Y axis

relative to focus. Three different Mg-Ni samples images from the scanning electron microscope was

viewed and analyzed as the first set of samples by the left are hydrogenated Mg-Ni samples without

Platinum while the ones at the right are hydrogenated with Platinum.

4.3 MATERIAL ANALYSIS BY ENERGY DISPERSIVE SPECTROSCOPE

The EDS data output of EDS system is just atomic ratios of the elements. For a specific thin

film on a substrate, the system gives only the elemental ratio of coating element and the substrate. The

interaction volume of the electrons increases with the energy of the electron so that, by increasing the

energy of the electrons, it is possible to increase the interaction volume resulting, deeper data

collection. Deeper data collection means that electrons can reach substrate and x-rays from the

substrate can be observed. For constant electron energy, for different thicknesses of thin films, EDS

gives different ratios. From this point of view, for different thicknesses of films, by using different

energies of electrons, EDS Atomic Ratio & Electron Energy graphs of thin films can be constructed.

Fig.14 Energy Dispersive Spectroscope Model: Quad Detector 5040

The manufacturer is Bruker AXS. Elemental mapping and compositional analysis is

determined with the aid of the EDS with key features like elemental composition analysis. EDS

30

analysis is useful in identifying materials and contaminants, as well as estimating their relative

concentrations on the surface of the specimen.

4.3.1 PROCEDURE

SAMPLE: Mg-Ni Samples on Silicon substrate

PURPOSE: Determination of the elemental composition of the samples using the Energy

Dispersive Spectroscope attached to the Scanning Electron Microscope.

EXPERIMENTAL: Attached to the SEM is the Energy Dispersive Spectroscope (EDS). This

technique can detect all elements heavier than and including boron (atomic number 5). Detection limits

are on the order of 0.5 to 1% by weight if the peaks are isolated; however, in practice the minimum

detection limit will typically be 1-2% by weight due to spectral overlaps, which can make some

elements present in minor concentrations e.g. Sulphur and Molybdenum undetectable. Working voltage

of 3 to 15KeV was used to analyze the residue. The analytical volume is estimated to be approximately

1-micron deep and 1-micron across [18]. Different results for different magnification ranging from

better clarity to size of grain from finer to coarser with decreased and increased magnification

respectively from 100k SE to 10.0kSE with adjustment in Sigma X and Y axis relative to focus. Three

different Mg-Ni samples images from the scanning electron microscope was viewed and analyzed as

the first set of samples by the left are hydrogenated Mg-Ni samples without Platinum while the ones at

the right are hydrogenated with Platinum.

4.4 MATERIAL ANALYSIS BY X-RAY PHOTOELECTRON SPECTROSCOPE

Identification of the elements in the sample can be determined directly from the kinetic

energies of the ejected photoelectrons which are measured by the electron energy analyzer and a

photoelectric spectrum is then recorded [35].

Key features:

(i) Monochromatic Al radiation;

(ii) Energy resolution on Ag - FWHM < 0.50 eV (Ag 3d5/2), on PET - FWHM < 0.85 (O=C*-

O);

(iii) Large and micro-area XPS, AR-XPS, UPS;

(iv) Dual beam charge neutralization;

(v) Chemical and secondary electron imaging with a raster scanned 10 µm diameter beams.

31

Fig. 15 XPS Model: PHI 5000 Versaprobe Manufactured by Physical Electronics, Inc. at the Lithuania Energyinstitute

4.4.1 PROCEDURE

SAMPLES: Three Mg-Ni thin films on Silicon Substrate with and without Platinum coating.

PURPOSE: Determination of the elemental composition and chemical state of the samples.

EXPERIMENTAL PROCEDURE: Since the technique is a surface sensitive one, we have to

avoid surface reactions and contaminations therefore the surface of the samples are bombarded with

Argon ions of 2KeV and introduced through a chamber that is in contact with the outside environment

where it is closed and pumped to low vacuum. After the first chamber is at low vacuum the sample will

be introduced into the second chamber in which a UHV environment exists. The ultra high vacuum

environment eliminates excessive surface contamination. The Electron Energy Analyzer which is also

located in a high vacuum chamber measures the kinetic energy of the emitted electrons while useful e-

signal is obtained only from a depth of around 10 to 100 Å on the surface. The spectrum plotted by the

computer is from the analyzer signal while computer system controls the x-ray type (AlK) and

prepares the instrument for analysis. The binding energies can be determined from the peak positions

and the elements present in the sample identified.

4.5 BULK ANALYSIS USING X-RAY DIFFRACTION

X-ray diffraction is a tool for the investigation of the structure of matter. X -rays are scattered

by interaction with the electrons of the atoms in the material being investigated. Analysis of crystalline

32

structures and In-situ heating/cooling and phase change are done using the instrument which has key

features like:

Configurations: Theta/Theta;

Smallest step size (theta/2theta): 0.0001°, reproducibility (Theta/2Theta): +-0.0001°;

Motorized divergence and anti-scatter slits, fixed slits, 60 mm Göbel mirror and 0.12° long

soller slit for parallel beam operation;

MRI heating/cooling chamber: temperature range: -190–1400 °C, pressure: 10-11– 2 bar;

3 detectors: Scintillator, Li-drifted Silicon, PSD.

Fig. 16 X-ray Diffractometer at the Lithuania Energy institute

4.5.1 PROCEDURE FOR X-RAY DIFFRACTION

This technique widely applied for the characterization of crystalline materials. XRD can be

used to look at single crystal or polycrystalline materials. A beam of x-rays is sent into the sample and

the way the beam is scattered by the atoms in the path of the x-ray is studied. The scattered x-rays

constructively interfere with each other. This interference can be looked at using Bragg’s Law to

determine various characteristics of the crystal or polycrystalline material. X-rays focused on a sample

fixed on the axis of the spectrometer are diffracted by the sample. The changes in the diffracted X-ray

intensities are measured, recorded and plotted against the rotation angles of the sample. The result is

referred to as the X-ray diffraction pattern of the sample. Computer analysis of the peak positions and

intensities associated with this pattern enables qualitative analysis, lattice constant determination and/or

33

stress determination of the sample. Qualitative analysis may be conducted on the basis of peak height

or peak area. The peak angles and profiles may be used to determine particle diameters and degree of

crystallization, and are useful in conducting precise X-ray structural analysis since every crystalline

substance gives a specific pattern constantly even when in a mixture of other substances giving the

chemical elemental composition. The phase identification of 3 MgNi thin films before and after

hydrogen absorption was carried out by X-ray diffraction (XRD) using an apparatus equipped with Cu-

Ka radiation source. Also we analyzed each relative to influence of Platinum in the aforementioned

process.

34

5. RESULTS

5.1 SCANNING ELECTRON MICROSCOPE ANALYSIS RESULTS

These are the images of each of the samples after hydrogenation with and without Platinum

with the same magnification and depth.

a) b)

Fig. 17 SEM images of the first sample with (a) after hydrogenation without platinum and (b) afterhydrogenation with platinum

a) b)

Fig 18 SEM images of the second sample with (a) after hydrogenation without platinum and (b) afterhydrogenation with platinum

35

a) b)

Fig. 19 SEM images of the third sample with (a) after hydrogenation without platinum and (b) afterhydrogenation with platinum

The pictures above here before introduction of Platinum shows a relatively fine-grained

orientation of the film before only to become coarser after hydrogenation with Platinum where there

seem to be more interaction with the surface bringing about possibility of Platinum diffusing from the

surface during the hydrogenation/dehydrogenation into the bulk of the film.

5.2 EDS RESULTS

The result here shows the differentiation of elements and their relative quantities in the sample

after the analysis with the energy dispersive spectroscope.

Table 1 showing the variations in quantities of different elements in the sample with and without platinum after

hydrogenation.

Elements Hydrogenation without Platinum Hydrogenation with PlatinumMagnesium (%) 80 11Nickel (%) 20 -Platinum (%) - 89

Table 2 showing the variations in quantities of different constituent elements in the sample with and without

platinum after hydrogenation.


36

Table 3 showing the variations in quantities of different constituent elements in the sample with and without

platinum after hydrogenation.


The tables above show similarities in that the weight of Magnesium and Nickel changed

significantly after coating with Platinum. As observed, there was much diffusion of platinum into the

bulk which led to the reduction in weight of the elements especially Nickel which was virtually unseen.

5.3 X-RAY PHOTOELECTRON SPECTROSCOPY ANALYSIS RESULTS

Fig. 20 XPS graph showing the concentration of the different elements against depth in the thin film afterPlatinum was cleaned off

XPS profile of carbon, oxygen, magnesium, nickel and silicon across the thickness (200 nm)

of Mg-Ni film are presented in this figure. Carbon and oxygen concentration was observed on the very

top surface and equals to 80 at.% and 20 at.%, respectively. At the depth of 15 nm concentrations of

magnesium and nickel increase and reach 55 at.% and 40 at.% respectively. Further, distribution of Mg

and Ni slightly varies across the film thickness and sharply decrease at 170 nm approximately. Worth

to mention that oxygen is distributed from 5 at.% to 10 at.% (red curve) across the Mg-Ni film.

Moreover, some amount of silicon diffuse into the Mg-Ni layer from the substrate. This might be the

reason for silicon oxides formation identified in the XRD graph.

37

Fig. 21 XPS graph showing the concentration of the different elements relative to the depth in the thin film

In the XPS graph above for Mg-Ni thin film with thickness of 300nm, concentration of carbon

and oxygen are observed at 65 at.% and 25 at.% respectively on the surface while at depth of 100nm,

concentration of Nickel and magnesium increase to 55 at.% and 40 at.% respectively while a sharp

decrease is observed at depth of 200 nm. Oxygen remains distributed across the thin film at

concentration not higher than10 at.% while significant amount of Silicon diffused into the layer of the

film which could result in the formation of its oxides.

Fig. 22 XPS graph showing the concentration of the different elements against depth in the thin film afterPlatinum was cleaned off

This XPS profile of Mg-Ni thin film of 1600nm shows carbon and oxygen concentration at 70

at.% and 25 at.% respectively on the very top surface while they are distributed across the film at a

38

very high depth. Magnesium concentration threshold is 65 at.% at depth of 500 nm alternated by steady

increase and decrease in its distribution across the film. The distribution of Nickel is across high depth

with relatively lower concentration of not more than 20 at.% at depth of 800 nm. There is total

distribution of both elements across the film meaning diffusion with silicon is inevitable.

Fig. 23 XPS graph showing the concentration of the different elements against depth in the thin film

In this XPS profile of an Mg-Ni thin film with depth 1100nm, carbon and oxygen

concentration on the top surface is 60 at.% and 30 at.% respectively as oxygen is distributed across the

film to depth of 900 nm. At depth of 100nm, magnesium and nickel concentrations increase to

approximately 75 at.% and 18 at.% respectively while their distribution varies across the thin film with

sharp decrease in their concentration observed at about 1000 nm. Some amounts of silicon diffused into

the layer from the substrate.

39

Fig. 24 XPS graph showing the concentration of the different elements against depth in the thin film afterplatinum was cleaned off

The XPS profile of the various elements here across an Mg-Ni film thickness of over 1200 nm

shows that carbon and oxygen concentration of 80 at.% and 18 at.% respectively at the surface while

oxygen is distributed across the film at no more than 10 at.%. There are variations in the distribution of

Magnesium and Nickel across the film starting from 60 at.% and about 40 at.% at depth of 100 nm and

a sharp decrease at 1100 nm depth. Silicon diffusion into the film layer is observed too which is the

reason for the formation of the Silicon oxides.

Fig. 25 XPS graph showing the concentration of the different elements against depth in the thin film

In the XPS profile above, oxygen and carbon are observed on the surface with 30 at.% and 60

at.% concentration respectively while their distribution was no more at 100 nm depth of the layer.

40

While distribution of Nickel gradually increased especially at depth of 900 nm with 55 at.%

concentration being the threshold where it decreased sharply. Magnesium increases until depth 200nm

at approximately 80 at.% then decreased steadily in its distribution while diffusion of Silicon into the

layer is observed.

5.4 XRD ANALYSIS RESULTS

Fig. 26 X-ray diffractogram of hydrogenated Mg-Ni thin film before and after being coated with platinum

In the above diagram, Platinum peaks are dominant after coating which acts as the catalyst

layer after with a noticeable peak of Mg2SiO4 observed before coating with platinum there are many

observed peaks of Magnesium compounds like MgH2, Mg2Ni and Mg2NiH4. High intensity peak of

Mg2SiO4 and SiO2 was observed before the addition of Platinum while it became very negligible after

introduction of Platinum and no SiO2 seen meaning problem of oxidation was curbed by the addition of

the catalyst though less peak of the Magnesium Nickel Hydrides were observed which is due to the

thickness of the Platinum layer on the film.

41

Fig. 27 X-ray diffractogram of hydrogenated Mg-Ni thin film before and after coated with platinum

Platinum peaks were observed after the platinum coating onto the sample. Small additional

peaks of Mg2SiO4 and Mg2NiH4 also were identified after platinum layer formation. Peaks of different

Magnesium compounds like MgH2, Mg2Ni, MgH2 and oxides like Mg2SiO4 and SiO2 were observed

before the coating with Platinum. Oxide peaks were not seen which the effect of the Platinum is but for

the hydrides is the effect of the higher thickness of the platinum layer.

Fig. 28 X-ray diffractogram of hydrogenated Mg-Ni thin film before and after coated with platinum

Here similar to the first sample, peaks of Magnesium compounds are observed being coated

with Platinum. Silicon peak with very high intensity and Platinum peaks with different crystallographic

orientation were observed after its introduction. We can identify different phases of Magnesium

42

compounds like MgH2, and Mg2Ni while Mg2SiO4 before Platinum was introduced. A small peak of

Mg2NiH4 is observed after hydrogenation with Platinum which is not the case before it was coated with

Platinum.

43

CONCLUSIONS

(i) The Mg-Ni thin films absorb hydrogen very easily, while in bulk samples, the

hydrogenation requires a temperature of about 500 K – 600 K. For thin films, it occurs at

room temperature with low pressure when capped with Platinum layer while Magnesium

alloy forming elements like Platinum and Nickel increase hydrogen absorption.

(ii) During experimental activities Mg-Ni thin films on silicon substrates were successfully

deposited using magnetron sputtering technique.

(iii) Every sample was divided into two parts: one of it covered with platinum as catalyst and

other left as deposited. According XRD measurements, parts with platinum showed better

hydrogenation kinetics.

(iv) XPS profiling measurements confirmed suspicions that part of platinum diffuses into Mg-Ni

film during high temperature and pressure hydrogenation.

(v) Silicon also diffused from the substrate into Mg-Ni film causing formation of Mg-silicates

which prevent further hydrogenation of Mg-Ni thin films during the diffusion of Platinum.

This makes silicon non-useful substrate for the further investigations.

44

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28 Electron beam-specimen interaction. From P.Ducumb and P.K Shields Brit. J. Appl. Phys. 14(1965) 779, http://www.globalsino.com/micro/1/1micro9992.html


30 http://131.229.88.77/microscopy/semvar.html

31 Goldstein et al,Electro probe microanalysis, 1992

32 Spectral Solutions AB Bredablicksvägen 7 SE-181 42 Lidingö Sweden, Microanalysis, WorkingPrinciple


34 AXS flash detector http://auroscience.hu/edx-mikroanalitika.






40 Elemental Energy dispersive X-Ray microanalyses of the mineral crust of Rimicaris exoculata, froma mineral particle, 28th August 2009.

46

41 http://www.uio.no/studier/emner/matnat/fys/MEF3100/v07/lecture_notes/XPS2.pdf

42 http://www.casaxps.com/help_manual/manual_updates/xps_spectra.pdf

43 http://www.cem.msu.edu/~cem924sg/Topic09.pdf

44 CasaXPS Manual 2.3.15 Rev 1.2

45 http://subato.blogspot.com/2011/05/material-surface-analysis-with-x-ray.html

46 Roger Smart et al. X ray Photoelectron Spectroscopy, Department of physics and materials science,City University of Hong Kong.

47 Xiaotao Guan,Impact of Zinc Orthophosphate Inhibitor on Distribution System Water Quality. 2007

48 https://wiki.utep.edu/display/~dmarrufo/X-ray+Photoelectron+Spectroscopy

49 http://relateproject.eu/participants/wiki/doku.php?id=ankara:kimvandeperreblog

50 B.D. Cullity & S.R. Stock Prentice Hall, Elements of X-Ray Diffraction Upper Saddle River (2001)

51 http://www.ammrf.org.au/myscope/xrd/background/concepts/xrays/

52 A continuous spectrum is produced when electrons hit the target, which is referred to as‘bremsstrahlung’ or German for ‘braking radiation’ as it is produced by stopping electrons

53 Australia Microscopy and Microanalysis research facility, Diffraction, June 18, 2014



56 Bragg's spectrometer. Thornton, Steven T. and Rex, Andrew, Modern Physics for Scientists andEngineers, Saunders College Publishing, 1993, http://hyperphysics.phy-astr.gsu.edu/hbase/quaref.html#c1


58 Bragg's equation, Cambridge Physics.org. http://www.outreach.phy.cam.ac.uk/camphy/xraydiffraction/xraydiffraction7_1.htm

faculty of natural science department of physics

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