Thermal Desorption Studies of the Hydrogenation of ... · Anders Lind Skov Supervisor: Liv Hornekær Institute for Physics and Astronomy, Aarhus University Bachelor Thesis in Physics

Thermal Desorption Studies of the Hydrogenation of Coronene on HOPG

Anders Lind Skov Supervisor: Liv Hornekær

Institute for Physics and Astronomy, Aarhus University

Bachelor Thesis in Physics September 2010

i

Dansk resume

I dette projekt er desorption af coronen fra højt orienteret pyrolytisk grafit (HOPG),desorption af hydrogeneret coronene fra HOPG og desorption af hydrogen fra coronenpa HOPG blevet undersøgt. Dette er gjort med temperaturprogrammeret desorption(TPD).

Coronen er et molekyle fra gruppen polycykliske aromatiske hydrokarboner (PAH’er).Disse molekyler bestar alle af sammensatte benzen ringe, altsa karbon molekyler bun-det sammen af sp2-hybridiserede bindinger. PAH’er er tilstede i det interstellaremedium (ISM), og det er blevet foreslaet, at hydrogenerede PAH’er i nogen omraderaf ISM’et virker som katalysator for dannelsen af molekylært brint, H2. Desuden erhydrogenering af PAH’er blevet foreslaet som en mulig metode til brintlagring.

TPD eksperimenterne blev udført under ultrahøjt vakuum (UHV) i et vakuumkam-mer. HOPG prøven blev beskudt med coronen molekyler fra en Knudsen-celle, hvorcoronen blev varmet op til 180 ◦C. I forsøgene med hydrogeneret coronen blev prøvenderefter beskudt med atomart deuterium fra en atomkilde, hvor molekylært deuterium,D2, blev knækket ved at sende det gennem et varmt rør. Derefter blev prøven op-varmet, sa molekylerne kunne desorbere og detekteres med et quadropol massespek-trometer.

Vi erfarede at desorption af coronen fra HOPG følger en desorptionskinetik derikke ændre sig ved lave dækningsgrader og har en fraktionel reaktionsorden. Nardækningsgraden stiger og kommer tæt pa et monolag, ændrer desorptionskinetikkensig dog, da reaktionsordnen, n, falder. Dette tyder pa, at coronenemolekylerne tilternar dækningsgrader tæt pa et monolag opnas.

Vi erfarede ogsa at hydrogenering af coronen er muligt, ved at beskyde coronenenmed atomart deuterium. Dette er den vigtigste observation i dette projekt, da hy-drogenering PAH’er ved addition indtil nu kun er forudset teoretisk. Vi observerededesuden, at en abstraktions reaktion muligvis kan forekomme, hvilket føre til at et deu-terium atom kan bytte plads med et af de normale brint atomer, der er pa coronenenfra starten.

Contents

Dansk resume i

Contents ii

Abbreviations iv

1 Introduction 1

2 Motivation 1

3 Polycyclic aromatic hydrocarbons 23.1 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23.2 Astrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.3 Interaction with hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . 43.4 Hydrogen storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Adsorption 6

5 Temperature Programmed Desorption 65.1 Polanyi-Wigner equation . . . . . . . . . . . . . . . . . . . . . . . . . . 75.2 Kinetic parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.3 Experimental parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 115.4 Numerical TPD simulations . . . . . . . . . . . . . . . . . . . . . . . . 12

6 Experimental Setup 146.1 Ultrahigh vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146.2 The Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6.2.1 Roughing pump . . . . . . . . . . . . . . . . . . . . . . . . . . . 166.2.2 Turbomolecular pump . . . . . . . . . . . . . . . . . . . . . . . 176.2.3 Diffusion pump . . . . . . . . . . . . . . . . . . . . . . . . . . . 176.2.4 Ion pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186.2.5 Titanium sublimation pump . . . . . . . . . . . . . . . . . . . . 196.2.6 Pressure measurement . . . . . . . . . . . . . . . . . . . . . . . 196.2.7 Manipulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206.2.8 Hydrogen source . . . . . . . . . . . . . . . . . . . . . . . . . . 216.2.9 Coronene source . . . . . . . . . . . . . . . . . . . . . . . . . . . 226.2.10 Quadrupole mass spectrometer . . . . . . . . . . . . . . . . . . 22

7 Results 247.1 Temperature programmed desorption of coronene from HOPG . . . . . 247.2 Temperature programmed desorption of hydrogenated coronene from

HOPG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267.3 Temperature programmed desorption of hydrogen from hydrogenated

coronene on HOPG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

8 Conclusion 30

ii

CONTENTS iii

9 Acknowledgements 30

Bibliography 31

iv ABBREVIATIONS

Abbreviations

DFT - Density functional theoryDOE - United States Department of EnergyHABS - Hydrogen atom beam sourceHOPG - Highly oriented pyrolytic graphiteIR - InfraredISM - Interstellar mediumLEED - Low energy electron diffractionOFHC - Oxygen-free high thermal conductivityPAH - Polycyclic aromatic hydrocarbonPID - Proportional-integral-derivativeSTM - scanning tunneling microscopyTPD - Temperature programmed desorptionTSP - Titanium sublimation pumpUHV - Ultrahigh vacuumUIR- Unidentified IR bandsUV - UltravioletQMS - Quadrupole mass spectrometer

1

1 Introduction

During my bachelor project I have been a part of the Surface Dynamics group atthe Institute for Physics and Astronomy at Aarhus University. I have mainly workedtogether with PhD student Bjarke Jørgensen and Post doc. John Thrower and alldata presented in this project has been acquired with them.

The aim of my project was to investigate the reaction between coronene and atomichydrogen as suggested by Rauls and Hornekær [14]. This was achieved through tem-perature programmed desorption (TPD) experiments using a highly oriented pyrolyticgraphite (HOPG) substrate in ultrahigh vacuum (UHV).

Three types of TPD experiments were performed during my project, TPD ofcoronene on HOPG, TPD of hydrogenated coronene on HOPG and TPD of hydrogenfrom hydrogenated coronene on HOPG. The TPD experiments of coronene on HOPGwere made to investigate the general desorption kinetics of coronene on HOPG, beforehydrogenation of coronene was attempted. This was done as a base for further in-vestigations. The TPD experiments of hydrogenated coronene on HOPG were mostlyperformed to test, if hydrogenation of coronene is even possible, as suggested by Raulsand Hornekær [14]. Furthermore the experiments were made to observe the generaleffect of the hydrogenation. The TPD experiments of hydrogen from hydrogenatedcoronene on HOPG were performed to investigate the interaction between hydrogenand coronene.

2 Motivation

Polycyclic aromatic hydrocarbons (PAHs) are a group of hydrocarbon molecules, whichinclude coronene. PAHs are described in detail in the next chapter. PAHs has beenstudied for many years, both physically, chemically and biologically, as they can beformed by incomplete combustion of carbonaceous materials. Therefore they are verycheap to produce. In 1985 Allamandola et al. suggested that PAHs are present in theInterstellar medium (ISM) which also makes them interesting for astrochemistry [2].This opened up for a whole new range of research possibilities on PAHs.

There are two main motivations for the TPD investigation performed in thisproject. PAHs might take part in the formation of molecular hydrogen in the ISM.PAHs might also have an application as a hydrogen storage material.

Molecular hydrogen, H2, is the most abundant molecule in the ISM. However insome interstellar environments, no efficient gas-phase routes exist for the formationof H2. Density functional theory (DFT) calculations made by Rauls and Hornekærhowever suggest that an efficient route to formation of H2 can be found through hy-drogenation of PAHs in some of these environments [14]. The TPD investigations ofcoronene can substantiate the possibility of this catalytic effect that the PAHs mayhave. This hopefully will lead to a better understanding of the astrochemistry in someinterstellar environments.

The amount of available fossil fuel in the wolrd is getting smaller and an increasingamount of research has suggested that CO2 emission from the combustion of fossil fuelsmight affect the climate. Several alternative energy sources exist, such as wind-, water-, sun- and nuclear-energy. These energi sources are, however, difficult to transport and

2 CHAPTER 3. POLYCYCLIC AROMATIC HYDROCARBONS

use, for instance in cars. Furthermore all except nuclear-energy are weather dependent,which makes them somewhat unreliable.

This means that an effective and mobile energy storage solution is needed. It hasbeen suggested that the energy obtained from alternative energy sources can be usedto generate hydrogen from water through electrolysis. However a good storage solutionis needed, as it is too expensive and possibly dangerous to store hydrogen in pressurebottles. A possible solution to this is to store the hydrogen in solid materials byreacting with them. Many possibilities for this kind of storage have been suggested,including several carbonaceous materials. In this category PAHs might be a goodcandidate, due to the low atomic weight of carbon, and the fact that PAHs are cheapto produce. The TPD investigations of coronene should tell us, if it is possible to storehydrogen in PAHs and give information, about how much hydrogen can be stored inPAHs, if it is possible.

3 Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbon (PAH) is a collective term for a class of hydrocarbonmolecules which consist of fused benzene rings. The molecules are generally planar andeach carbon atom is sp2-hybridized and has a p orbital perpendicular to the plane.Because all six p orbitals of the carbon molecule are equivalent, it is impossible todefine three localized π-bonds. This leads to a delocalized electron structure in theplanes above and below the molecule. Different PAH molecules are seen in figure 1,the smallest one, benzene (C6H6) (fig. 1.a), only consists of a single aromatic ring.C6H6 is really not a PAH, but it is often mentioned as one, because it has relatedproperties. The PAH used in this project is coronene (fig. 1.e), which consists of sevenaromatic rings. This is the smallest ”large” symmetric PAH, because it exhibits threedifferent sites for H addition as seen in figure 2. Because of the increasing size of thedelocalized system, PAHs becomes more stable with increasing size.

3.1 Properties

Much can be said about the properties of PAHs, physically as well as chemically andbiologically. In this section properties, which are interesting for this project and thegeneral work with PAHs in the Surface Dynamic group, will be described.

The photophysical properties is one of the more interesting characteristics of PAHs.PAH molecules can be detected both through absorption and emission. Both situationshave been investigated through theoretical and experimental research by Allamandolaet al. [2]. A PAH molecule in its ground singlet state, S0, can absorb a UV photonand get excited to higher electronic singlet states. For chrysine (C18H12) Allamandolaet al. found a strong absorption peak at 267.5 nm connecting the S0 and S3 electronicstates [2]. After this excitation there is a very big probability that the molecule willundergo an internal conversion to a high vibrational level in the S1 state, hence UV-light can be used to pump vibrational modes in PAHs. Further system inversions cantake place and only 12% of the S1 state decay through optical fluorescence, the restwill decay from other states through infrared (IR) emission.

3.2. ASTROCHEMISTRY 3

Figure 1: Molecular structures of PAHs. a) Benzene C6H6. b) Naphthalene C10H8. c)Anthracene C14H10. d) Pyrene C16H10. e) Coronene C24H12. [16]

IR emission can be achieved by UV excitation, as well as other kinds of excitation,for instance thermal excitation. The IR emission spectra are very distinctive for PAH,and are often used for PAH detection, for instance in astrochemistry (See Astrochem-istry section for further information). The features seen in a PAH emission spectrumconsist of several C-H bending and stretching modes as well as a lot of C-C stretchingmodes. An IR spectrum of coronene can be seen in figure 3

Studies of interactions between PAHs and graphite, as in this project, have beenperformed by Zacharia et al. [21]. They did TPD experiments of benzene (C6H6),naphthalene (C10H8), coronene (C24H12) and ovalene (C28H14) on graphite. Experi-ments suggested that the PAH molecule at low coverages adsorb with their aromaticrings parallel to the surface. However at higher coverages the molecules tilt and areoriented with the plane of their aromatic rings at some angle to the surface. Further-more their experiments suggested that coronene and ovalene follow fractional-orderdesorption kinetics (see TPD section for further information). This was suggested tobe because of the formation of two-dimensional islands, which were stable up to thedesorption temperatures. That island formation happens has also been suggested byLackinger et al. based on STM studies. They however believe that the island forma-tion only happens for multilayers [8]. The reason that Zacharia et al. suggest thatisland formation also happens for monolayers, can be explained by their use of an iongauge for coronene detection [21]. An ion gauge is not that precise and therefore it isdebatable that monolayers were even detected.

3.2 Astrochemistry

PAHs are commonly thought to be present in the ISM and it is believed that theyaccount for a significant proportion of the carbon in the galaxy. Extensive researchsuggests that PAHs are the source of a part of the IR emission bands known as theUnidentified IR bands (UIR). Some of these features are found at 3.3, 3.4, 6.2, 7.7,8.6 and 11.3 µm. Allamandola et al. suggested as early as 1985 that these featurescould be explained by PAHs [2]. The 11.3 µm can, for instance, be explained by C-Hbending and the 3.3 µm can be explained by C-H stretching and so on. Theoretical

4 CHAPTER 3. POLYCYCLIC AROMATIC HYDROCARBONS

Figure 2: A figure showing the different kind of sites in the coronene molecule. [14]

calculations also suggested that most interstellar PAHs contain about 20 - 40 carbonatoms (coronene has 24 carbon atoms) and that the abundance of PAHs is about2 · 10−7 relative to hydrogen [2] [1].

PAHs are also suspected to play a role in the formation of H2 in the ISM. This isbecause there exist no efficient gas-phase routes for formation of H2 at low temper-atures and densities. Several routes for formation have been suggested, for instancethrough physisorption on interstellar dust grains. Experimental research by Hornekæret al. however shows that this will only be efficient below 20 K [7]. At intermediatetemperatures no efficient routes have yet been found. PAHs has been suggested tocatalyze the formation at these temperatures. Rauls and Hornekær suggest that su-perhydrogenated PAHs might be the catalyst in areas with low UV flux, based on theDFT calculation explained in the next section [14]. In areas with intermediate UVflux Le Page et al. suggests that PAH cations might be the catalyst [9].

3.3 Interaction with hydrogen

When looking at the interaction with hydrogen, it is in connection with this projectinteresting to look at the possibility of hydrogenation of PAHs. On this subject DFTcalculations of the hydrogenation of coronene have been made by Rauls and Hornekær[14]. When considering the most energetically favorable addition routes they foundthat the first hydrogen atom had to overcome a barrier of 60 meV to attach itself to thecoronene. After the attachment of the first hydrogen atom the barrier for attachmentof the following hydrogen atoms are zero, except for the third atom which has a barrierof 30 meV. So if the first hydrogen atom can be attached, multi-hydrogenated PAHsshould exist. The DFT calculations also showed that the hydrogenated PAHs in severalcases could form molecular hydrogen, H2, by Eley-Rideal abstraction with no energybarrier. Because the energy barriers for addition of an extra hydrogen atom, in someof these cases were zero as well, the hydrogenated PAH would here act as a catalystfor H2 formation. The binding energy of the added H atoms depends on the site atwhich they are added, but they are all between 1.4 and 3.2 eV. This is a fairly strongbond.

Much research has been made on PAH cations as catalysts for H2 formation. AsPAH ions are not in the scope of this project, I will only mention it briefly. Theoretical

3.4. HYDROGEN STORAGE 5

Figure 3: IR spectrum of coronene from NASA ames database. [3]

research by Le Page et al. suggest that PAH cations as well can work as catalysts forformation of H2 [9]. It has also been suggested that ionizing the PAHs can lower thebarrier for addition of the first hydrogen atom to coronene and other PAHs [4].

3.4 Hydrogen storage

Hydrogen is predicted to be one of the future replacements for fossil fuels in, forinstance, the car industry. However a good storage solution is needed. Here PAHshave been suggested as a possible storage medium, for instace by Patchkovskii et al.[13]. As mentioned in section 3.3 the barriers for hydrogenation of PAHs are relativelylow, so it should easily be possible to hydrogenate the PAHs. Fully hydrogenatedPAHs would have a gravimetric capacity of 7.4wt% stored H, which is close to thegoals of 9wt% set by the United States Department of Energy (DOE) for 2015 [12].Further more fully hydrogenated PAHs have a high volumetric capacity, coronene forinstance has a volumetric capacity of 0.11 kg/L. This is above the goals set by DOEon 0.081 kg/L for 2015 [12].

To get the hydrogen off the hydrogenated PAHs UV excitation has been suggested.As the UV can be used to pump the vibrational states in PAHs, it is suspected that itcan be used to separate the added hydrogen from the PAH. PAHs also have advantagescompared to some other carbonaceous materials suggested for hydrogen storage such asgraphene. When the graphene sheets are stacked in the form of graphite, the materialis relatively cheap. But it is impossible to get a high degree of hydrogenation, becausethe hydrogen only can reach the top sheets. This of course can be dealt with if singlegraphene sheets are used. They however are very expensive to make and would at themoment not be an ideal choice for hydrogen storage. PAHs however are both cheapto produce, and do not stick together in layers.

6 CHAPTER 5. TEMPERATURE PROGRAMMED DESORPTION

4 Adsorption

Adsorption describes a process where a molecule or an atom, the adsorbate, forms abond with the surface of another material, the adsorbent. This term should not beconfused with the term absorption, which refers to molecules or atoms entering thebulk of a material. Generally, two different kinds of adsorption can be distinguished.If the molecule adsorbs as a single unit without fragmentation, it is called associativeadsorption. If the molecule is dissociated upon adsorption, it is called dissociativeadsorption.

The bond between the adsorbate and the adsorbent is similar to the bond be-tween atoms in a molecule. The major difference is that the adsorbent contains verymany electrons and is much larger than the adsorbate. Generally there also can bedistinguished between two different types of adsorption, this is as physisorption andchemisorption. In physisorption the interactions which lead to the bond between theadsorbate and the adsorbent is a van der Waals-type interaction. This is caused by aredistribution of the electron density in both the adsorbate and the adsorbent. Thisleads to a polarization of both the adsorbate and the adsorbent, and thereby an elec-trical interaction. The bond is usually long ranged, but weak. The binding energy istypically in the order of a few meV (kBT ≈ 25 meV) for small molecules. Coroneneadsorbs to graphite via physisorption, here the binding energy however is expected tobe in the order of eV, due to the delocalized electron structure of both the coroneneand the graphite and the number of carbon atoms in the coronene.

Chemisorption typically involves much higher binding energies then physisorption.The binding energy is typically in the order of several eV. This is because chemisorptionis characterized by an exchange of electrons between the adsorbent and the adsorbate.Hence chemisorption is characterized by chemical bonds, and can be discussed in termsof traditional notions of covalent, ionic and metallic bonding. The bonding energy forindividual bonds can sometimes be so high that it is favorable to crack the intermolec-ular bond and adsorb the fragments, hence leading to dissociative adsorption.

The absolute coverage of a surface is the number of adsorbate molecules/atomson the surface. The absolute coverage is often very hard to determine, so instead thecoverage of the surface is usually defined by a relative surface coverage, θ, defined as

θ =Number of surface sites occupied by adsorbate

Total number of substrate adsorbtion sites=Ns

N(1)

where Ns is the absolute surface coverage and N often is equal to the total number ofsurface atoms for small adsorbates, for larger adsorbate molecules, like coronene, thisis of course not true. When θ = 1 it is said that there is a monolayer of the adsorbateon the adsorbent.

5 Temperature Programmed Desorption

Temperature programmed desorption (TPD) describes a process where the sample isheated, to study the desorption of adsorbed atoms and molecules from the sample.The desorption of adsorbed atoms and molecules is an elementary surface kineticprocesses and can provide information about the strength of the interaction between

5.1. POLANYI-WIGNER EQUATION 7

the adsorbate and the adsorbent. TPD can also be used to obtain information aboutdifferent binding sites and orientation of adsorbate molecules.

In a typical TPD experiment an adsorbent substrate with a clean well definedsurface is placed in a UHV chamber. At a temperature T0 the sample is exposed tothe atom or molecule of interest, this could be from an atomic or molecular beam.The sample is then heated in order to desorb the adsorbate. Here a temperatureramp, β(t) = dT/dt, is applied to the sample. Typically the ramp is set so thatthe temperature, T , is a linear function of t, hence T (t) = T0 + βt. The rate ofdesorption is now determined by measuring the amount of adsorbate desorbed in tothe gas phase as a function of temperature. This is in our case done with a quadrupolemass spectrometer, which is placed directly in front of the sample.

Based on these measurements the rate of desorption can be plotted as a functionof temperature as a TPD spectrum. Usually TPD spectra are collected as a family ofcurves with different initial coverages.

5.1 Polanyi-Wigner equation

When doing TPD experiments the sample with adsorbate is heated. At some pointsufficient thermal energy to break the bond becomes available and desorption willoccur. For simple cases of an adsorbate, the activation energy of desorption, Ed, isconstant and a single peak is observed. This kind of desorption can be written interms of an Arrhenius dependency as an activated process with the rate constant kd

kd = ν exp

(−Ed

kbT

)(2)

where ν is a pre-exponential factor. If the experiment is performed in a continuallypumped vacuum chamber, the temperature at which the maximum desorption occurs,Tp, corresponds to the maximum desorption rate. The reason that a maximum isobserved, although kd increases exponentially with temperature, is that the surfacecoverage decreases simultaneously. Therefore the observed adsorption kinetics is aconvolution of these two factors and the rate of desorption, rdes, is

rdes = −dθdt

= kdθn (3)

where n is the reaction order. If the expression for kd found in equation 2 it nowinserted, the equation can be rewritten as

−dθdt

= νθn exp

(Ed

kb · T

)(4)

This equation is usually referred to as the Polanyi-Wigner equation and it is one ofthe most important equations when analyzing TPD data. If a linear ramp is used, thefollowing substitution can be made

dθ

dt=dθ

dT· dTdt

=dθ

dTβ (5)

and inserted into the Polanyi-Wigner equation, which yields

− dθdT

=ν

βθn exp

(Ed

kb · T

)(6)


Figure 4: A simple energy diagram with the activation energy of desorption, theactivation energy of adsorption and the heat of adsorption. [6]

It is this modified version of the Polanyi-Wigner equation, which will be used in therest of the project, as a linear ramp has been used in all experiments.

An important condition for using the Polanyi-Wigner equation for data analysis isthat the monitored desorption signal is proportional to the rate of desorption. Hencethe pumping speed of the UHV chamber has to be constant and sufficiently high toprevent significant re-adsorption of desorbed atoms or molecules on to the sample.

5.2 Kinetic parameters

In this section the effects of the kinetic parameters on the Polanyi-Wigner equationwill be investigated.

The desorption order has a great effect on the form of the TPD curve. In orderto investigate the effect I will look at the modified Polanyi-Wigner equation at thedesorption peak maximum, T = Tp, because this is a maximum

d2θ

dT 2= 0 (7)

So by differentiating equation 6 and setting the resulting expression equal to zero, oneobtains:

Ed

kbT 2p

=ν

βnθn−1 exp

(−Ed

kbTp

)(8)

In this way it is possible to get a general expression relating Tp, Ed and θ. Whenlooking at first order the following expression is obtained

Ed

kbT 2p

=ν

βexp

(−Ed

kbTp

)(9)

Both β and Tp are experimentally measurable parameters, and Ed may be evaluatedas long as ν is known. It is usually possible to estimate ν, which will be discussedlater.

5.2. KINETIC PARAMETERS 9

From equation 9 it can be seen that Tp is independent of the adsorbate coveragefor first-order desorption. So with increasing coverage Tp remains constant and onlythe intensity of the peak increases. It should however be mentioned that this analysisassumes that ν and Ed are independent of θ. Furthermore it is also assumed thatthe desorption occurs in a single step, whereas desorption for systems that exhibitprecursor state adsorption kinetics have a more complex desorption mechanism, sincethe precursor state will play a role in the desorption process. Application of simplesingle step desorption kinetics can lead to significant error when calculating Ed, if usedon multiple step adsorption.

For second-order desorption, evaluation of the differential equation yields.

Ed

kbT 2p

=2νθ

βexp

(−Ed

kbTp

)(10)

This means that Tp now is dependent on θ. If θ increases, Tp will decrease for fixedvalues of Ed and ν. Chemisorbed monolayers generally exhibit first- or second-orderdesorption kinetics, whereas multilayer systems generally exhibit zero-order desorptionkinetics. In a multilayer system the first monolayer is typically bonded to the substratevia strong chemisorption. In the following layers the bonding resembles that in acondensed solid of the pure adsorbate. This is due to the fact that the influencefrom the surface is completely screened out. The weaker multilayer bonding resultsin a desorption peak at lower temperature, hence a lower bond strength compared todesorption from the monolayer. Multilayer desorption peaks do not saturate and willincrease in intensity with an increasing number of layers. For zero-order desorption,Tp shifts to higher temperature with increasing coverage and all desorption curveshave a common leading low temperature edge. The shift to higher temperature withincreasing coverage, occurs because the desorption rate increases exponentially withtemperature. This means that the desorption rate will increase, until all multilayershave been stripped away.

As zero-order desorption typically is associated with multilayers, first-order des-orption is normally associated with systems, where the adsorbate molecule/atom canleave the surface individually i.e following associative adsorption. Second-order des-orption normally is associated with systems, where the adsorbate molecule/atom hasto combine two-and-two to desorb. For systems with precursor states, entropic affects,change in surface energy and other things that the Polanyi-Wigner equation does notaccount for, the equation may in some cases still be used. In these cases the solutioncan sometimes be of fractional order.

Another kinetic parameter from the Polanyi-Wigner equation is the pre-exponentialfactor ν. It is commonly assumed that for a first-order desorption process ν is of thesame order of magnitude as the molecular vibrational frequency and is usually assumedto be 1013 s−1. In case of a first-order desorption, where ν/β is between 108 K−1 and1013 K−1, it has further more been shown that equation 9 can be rewritten as

Ed = kbTp ln

(νTpβ− 3.46

)(11)

This is known as the Redhead equation [15]. The assumption, that ν is the sameorder of magnitude as the molecular vibrational frequency, can only be made forsmall molecules. For bigger molecules ν is typically larger then the before mentioned


Figure 5: TPD simulations. a) First-order desorption. b) Second-order desorption.

1013 s−1. For coronene for instance ν was found by Zacharia et al. as 1.7 · 1018 s−1

[21]. For pre-exponential factors this high the Redhead equation should not be used.Typically more analytical methods are used to estimate Ed, such as the Falconer-Madixanalysis [5]. This method will estimate both Ed and ν for the adsorption system.

The final kinetic parameter is the activation energy for desorption Ed. Ed describesthe energy barrier, the adsorbate has to overcome to desorb. Ed is typically mentionedtogether with two other parameters. One of them is the activation energy of adsorp-tion, Ea, which is the energy barrier the adsorbate has to overcome to adsorb from thesubstrate. The other is the heat of adsorption, Ha, which is the heat released whenthe adsorbate adsorb. The connection between Ed, Ea and Ha is Ed = Ea + Ha andit can be illustrated as in figure 4. As mentioned earlier the Redhead equation canbe used to evaluate Ed for first-order desorption systems, while the Falconer-Madixanalysis can be used for systems of other orders. However, when analysing data, thebest results are usually found by comparing the data to TPD simulations (like thoseseen in section 5.4). The Redhead equation and the Falconer-Madix analysis shouldonly be used to estimate the kinetic parameters.

It is also possible that the adsorbent consists of several types of adsorption siteswith different activation energies. In this case the TPD curve will be the sum of all thedifferent types of desorption curves, and can be described as a sum of Polanyi-Wignerequations

− dθdT

= −∑ dθi

dT=∑ ν

βθni exp

(Eidkb · T

)(12)

where Eid is the activation energy of desorption for the individual adsorption site andθi is the partial coverage of the individual adsorption site, hence the total coverageis θ =

∑θi. This of course builds on the assumption that Ed is independent of the

surface coverage.

5.3. EXPERIMENTAL PARAMETERS 11

Figure 6: TPD simulation. Zero-order desorption.

5.3 Experimental parameters

In this section the effects that the experimental parameters have on the Polanyi-Wignerequation will be investigated.

The relative surface coverage, θ, of the surface is defined as in equation 1. But eventhough it is an experimental parameter, it is very hard to control. It is affected byparameters such as dosing speed, sticking coefficient and activation energy of adsorp-tion. Instead θ can be calculated afterwards from the TPD curve. The relative surfacecoverage can be determined by comparing the integrated area of a TPD curve withthe integrated area of a TPD curve, where the relative coverage is known. Providedthat the pumping speed of the vacuum chamber remains constant during desorptionexperiments, the ratio between these areas is directly proportional to the relative sur-face coverage. This again is dependent on all the other variables that effect the massspectrometer being constant when measuring. These includes detector gain, distanceto detector, ramp, etc. The integrated area under such a TPD curve then correspondsto a known relative coverage, and any unknown coverages may be determined by asimple ratio of integrated areas of the desorption spectra

θ =area under TPD curve for unknown coverage

area under TPD curve for known coverage· known coverage (13)

It is often possible to determined when the coverage θ = 1. This can be done bycomparing several TPD curves and determine when the desorption kinetics changefrom the initial reaction order to zero-order. Because multilayers exhibit zero-orderkinetics it is possible to determine when there is exactly one monolayer, if it can bedetermined exactly when zero-order behavior starts. Based on this, the relative surfacecoverage can be determined for all other TPD curves. Multilayers do not always occurat all temperatures, in that case the sample will get saturated at θ = 1 and longerdoses will make no change to the TPD curve. Because of this, the situation whereθ = 1 is easy to determine, when multilayers do not occur. If the absolute coverageis known for a system in combination with the relative coverage, the absolute surfacecoverage can also be determined by comparing relative coverages.

Another experimental parameter is the temperature ramp rate, β. This can easilybe controlled and is normally set between 0.5 K/s to 2 K/s at the experimental setup


Figure 7: TPD simulations. a) First-order desorption, with varying pre-exponentialfactors, ν. b) First-order desorption, with varying temperature ramps, β.

used in this project. The most important thing to say about this parameter is thatfor equation 6 to work, the temperature has to increase linearly with time, hence βhas to be constant. It is important that small changes in ramp do not occur duringexperiments, as an uneven ramp can induce experimetal artifacts.

5.4 Numerical TPD simulations

When looking at TPD curves it is important to be able to recognize different typesof desorption kinetics such as different reaction orders. It is also important to knowthe effect of changing a parameter in the experimental setup. To do this, numericalTPD simulation is a valuable tool. It is possible to use them to get experience ininterpreting TPD spectra and to use them as comparison with the experimental data.In this section I will present different numerical plots of the Polanyi-Wigner equationthat are all made with a self written routine in MatLab.

The characteristics of the zero-, first- and second order desorption, as well as thedifference between them is the first thing investigated. For all three systems ν =1.7 · 1018 s−1, Ed = 1.48 eV and β = 1 K/s (these are the values for coronene given byZacharia et al. [21]). In figure 5.a a set of first-order desorption curves with differentinitial coverages can be seen. It can be seen that Tp does not shift for increasing θ justas predicted. Furthermore it can be seen that the peaks are not symmetric. These arethe two main characteristics for first-order desorption. In figure 5.b a set of second-order desorption curves, done in the same manner, can be seen. Here Tp shifts to lowertemperatures for increasing θ and the peaks are symmetric around the Tp. These arethe two main characteristics for second order desorption.

In figure 6, a set of the zero-order desorption plots, again done for different surfacecoverages, is shown. It can be seen that all the curves have a common leading edge,as predicted in a previous section. As predicted it can also be seen that Tp shifts tohigher temperature with increasing θ. These are the main characteristics for a zero-order desorption system. But because zero-order desorption is typically associatedwith multilayer systems, figure 6 is actually very unrealistic, it only shows that Tpincreases with increasing θ. A more realistic result would be a combination of a

5.4. NUMERICAL TPD SIMULATIONS 13

Figure 8: TPD simulation. a) First-order desorption, with second adsorption sites ofvarying energy. b) First order desorption with two adsorption sites and varying β.

first-order and a zero-order curve. The resulting TPD curves can look very differentdepending on value of Ed for the first-order desorption, as well as for the zero-orderdesorption. For a system following second or fractional-order, multilayer curves wouldbe a combination of their monolayer kinetic and zero-order kinetics.

The influence from the pre-exponential factor, ν, is seen in figure 7.a. Here θ, βand Ed is kept constant, while ν is varied in several first-order TPD simulations. Thefirst thing to notice is that Tp shifts to a lower temperature with increasing ν. It is alsoseen that the peaks get narrower and higher when increasing ν. It should, however,be mentioned that the area under the curves remain the same, because they havethe same initial coverage. If ν is considered to be related to the molecular-surfacevibrational frequency, the decrease in Tp makes sense. When the molecular-surfacevibrational frequency increases, the probability of desorption will also increase, as aresult of the larger attempt frequency. When the probability of desorption increases,the desorption peak will shift to a lower temperature.

In figure 7.b the influence of the temperature ramp, β, is shown. This time θ, νand Ed are kept constant, while β is varied in several first-order TPD simulations.The effect is the opposite of what is seen with ν, it is seen that Tp shifts to highertemperature with increasing β and the peak get broader and lower with increasing β.Here it should be mentioned as well that the area under the curves remain the same,because they have the same initial coverage.

Figure 8 shows TPD simulations, where the sample has two adsorption sites withdifferent activation energies. In figure 8.a several first-order TPD simulations can beseen, where β and ν are constant. The desorption energy for one of the sites, E1d, iskept constant at 1.48 eV, while the desorption energy of the other site, E2d, is varied.The partial coverages are also kept constant at θ1 = 0.4 θ2 = 0.6. In the situationwhere the difference in binding energy is smallest, it appears as a curve for one siteand it is impossible to know that there are two different adsorption sites. When thedifference gets bigger, it is possible to see the appearance of two adsorption sites, inone of the curves as a small shoulder and in the other as two separate peaks. Thus thelarger the difference between activation energies is, the easier it is to see that thereare two sites, due to the larger difference in peak maximum temperatures.

14 CHAPTER 6. EXPERIMENTAL SETUP

To see the appearance of adsorption sites with different activation energies moreclearly, β can be changed. A simulation of this situation can be seen in figure 8.b. Hereν, n, θ1 and θ2 are kept constant at the same levels as before. The desorption energiesare kept constant at E1d = 1.48 eV and E2d = 1.42 eV, while β is varied. It can beseen that the desorption signal changes from a peak with a shoulder to two separatepeaks, the smaller β gets. This means that it is possible to resolve two adsorption siteswith different activation energies by lowering β. However there is a practical lowerlimit for the value of β. First of all the time it takes to measure a TPD increases thelower β gets, but more importantly the signal-to-noise ratio decreases as well.

6 Experimental Setup

One of the key ingredients in most surface science experiments, and a crucial require-ment for TPD measurements, is a low pressure vacuum chamber. First of all theanalysis of the TPD through the Polanyi-Wigner equation depend on a continuallypumped vacuumchamber. Furthermore, a low pressure is required to keep the sam-ple clean. This gives a lot of special requirements for the equipment used in theseexperiments.

6.1 Ultrahigh vacuum

Ultrahigh vacuum (UHV) is generally defined as a pressure below 10−9 mbar. Anestimate of the contaminations of a surface can easily be calculated, by looking at therate of molecules impinging a surface. This rate is given by kinetic gas theory as:

dN

dt=

P√2πMkbT

(14)

where P is the pressure in the chamber, M is the molecular mass and T is the tem-perature. Of course the contaminations also depend on the sticking probability ofthe molecules. In this estimate a worst case scenario is assumed, hence the stickingprobability is assumed to be unity. The density of surface atoms is typically 1019 m−2.On the basis of this, it is possible to estimate the pressure that is needed to keep thetime it takes for the formation of one monolayer of contamination to around an hour.This is the definition of UHV given by Luth [10]

PUHV <

√2πMkbT

3600s1019 m−2 (15)

Inserting room temperature, 300 K, and a molecular mass of 28 amu, correspondingto CO or N2 which are typical surface contaminants, yields PUHV < 10−9 mbar.

In the chamber used for the experiments in this project, the typical pressure wasabout 10−10 mbar. If it is again assumed that the sticking coefficient is unity and themolecular mass is 28 amu, the time it takes for the formation of one monolayer is 10hours. To achieve UHV a combination of several different pumps is used, because nosingle pump can pump from atmospheric pressure to UHV.

In addition to the pumping, which will be explained in detail later, an importantstep in achieving UHV is the so-called bakeout process. When the inside of a UHV

6.2. THE CHAMBER 15

Figure 9: A sketch and a picture of the big chamber.

chamber is exposed to air, a thin film of water will cover the walls. When pumping thechamber, the water will slowly desorb, and it is not possible to reach a pressure below10−8 mbar. To remove of the water, the chamber is heated to around 120 - 180 ◦C forup to 24 hours. After performing the bakeout, all filaments should also be degassed,to desorb contaminants.

6.2 The Chamber

All the experiments in this project were performed on a chamber called the big cham-ber, which is seen in figure 9. During my bachelor project the chamber was equippedwith the instruments seen in the sketch of the chamber i.e. two quadropole mass spec-trometers, D/H source and a thermal evaporation source for dosing coronene, theseinstruments will all be described later. The chamber is also equipped with other in-struments including low energy electron diffraction (LEED) optics, Auger optics, gasinlet system, etc, which was not used in this project and will not be described further.Furthermore the chamber is equipped with different pumps, to keep it at UHV, differ-ent gauges to measure the pressure and a manipulator in which the HOPG sample ismounted. All these components are described in the following subsections.

When constructing a UHV chamber, it is interesting to look at the mean-free pathof the rest gas molecules at UHV. This can be calculated by:

λ =kT√

2πξ2P(16)

where ξ is the molecular diameter. When inserting the UHV pressure, room temper-ature and a molecular radius of 5.20 · 10−10 m, which is approximately the molecularradius of N2, the obtained mean-free path is 34 km. This means that the rest gasmolecules have a much higher probability of colliding with the chamber walls thencolliding with each other. This sets very specific demands for the construction of thechamber. When connecting a pump to the chamber, the connection has to be largefor the rest gas molecules to find their way to the pump. If a pump is connected bya small connection or one containing many sharp angles, the many collisions with thewalls will reduce the effective pumping speed.

When considering materials for chamber construction, it is important that thevapor pressure is very low. This is the partial pressure at which the gas of the substance


Figure 10: Pressure ranges for different types of pumps. [10]

is at dynamic equilibrium with its liquid or solid form. The vapor pressure has to bebelow 10−10 mbar, at the temperature at which it operates. Chambers are often madefrom steel, mu-metal (a nickel-iron alloy) or in rare cases aluminum, all three have avapor pressure below 10−10 mbar at both room and baking temperature, this is alsotrue for other materials used in the chamber such as tantalum and Oxygen-free highthermal conductivity (OFHC) copper. The big chamber is constructed from stainlesssteel grade 304. For filament construction, the vapor pressure has to stay below 10−10

mbar at even higher temperatures. Here tungsten is used, which does not reach avapor pressure of 10−10 mbar until a temperature of 2160 K [6].

To keep the connections between the chamber and other components sufficientlyairtight, the so-called ConFlat flange system is used for all UHV parts. Here each faceof the two mating ConFlat flanges has a knife edge, which deforms a copper gasket,and makes an airtight seal. Every copper gasket is only used once, to make sure thatthe seal stays airtight. The reason for using copper gaskets is to make sure that theseal can withstand the baking temperature.

In the following subsections the instruments and pumps, which have been used onthe chamber, will be explained in detail.

6.2.1 Roughing pump

To lower the pressure in the chamber from 1 atm to 10−3 mbar, several roughing pumpsare used. The roughing pumps are needed because neither the turbomolecular pumps,the diffusion pump or the ion pump can work at a pressure above 10−2 mbar, as seenin figure 10. The roughing pumps are connected to the backside of the turbomolecularpumps and the diffusion pumps, to keep a low backing pressure when they are running.The roughing pumps used on the big chamber are oil-sealed rotary vane pumps. Therotary vane pump functions on the basis of changing gas volume, by the rotation ofan eccentric rotor, which has two blades in a diametrical slot, as shown in figure 11.

As mentioned earlier the oil-sealed rotary vane pump will only pump down to apressure of around 10−3 mbar. This is due to the fact that the long mean free pathof the rest gas molecules, at lower pressure, makes it very unlikely for the molecule toenter the pump. Therefore other types of pumps are needed to reach UHV.

6.2. THE CHAMBER 17

Figure 11: An illustration of the principle behind an oil-sealed rotary vane pump. [6]

6.2.2 Turbomolecular pump

The big chamber is equipped with several turbomolecular pumps. A large VarianTurbo V-550, with a pumping speed of 550 l/s (for N2), pumping the chamber itselfand several smaller ones pumping the coronene source, the two quadrupole mass spec-trometers and a gas inlet system, which was not used in this project. An illustrationof a turbomolecular pump can be seen in figure 12.

The principle behind a turbomolecular pump is based upon high speed rotors,rotating at speeds of up to 80.000 rpm. The rest gas molecules then collide with therotors, which are inclined with respect to the rotation direction. The rest gas moleculesthereby attain an impulse in the direction of the roughing pump. The turbomolecularpump consists of several sets of rotor blades and between each set of rotor blades,there is a set of stator blades, which are inversely inclined and thereby enhances thepumping effect. The geometric setup alone has a pumping effect, but the high speedrotation boosts the effect a lot.

Because of the high rotor speeds, the turbomolecular pump cannot operate at apressures above 10−2 mbar. At higher pressure the rotor blades could get bent andthereby collide with the stator blades and destroy the pump.

Since the pumping effect depends on the impact between the rotor blades and themolecule, the effficiency also depends on the molecular mass. A disadvantage of theturbomolecular pump is thus that they have low pumping speeds for light gasses, inparticular H2 . This can be a problem when performing experiments with hydrogen asin this project. Therefore other pumping techniques, such as a titanium sublimationpump, can be an advantage when using hydrogen in UHV.

6.2.3 Diffusion pump

The big chamber is equipped with one diffusion pump. As seen in figure 13 the firststage of the diffusion pump consists of a heater, which heats a reservoir of a highmolecular mass oil and turns it into vapor. The oil vapor travels up through a column,reaches an umbrella like deflector and is directed through a nozzle resulting in an oil


Figure 12: An illustration of a turbo-molecular pump. [20] Figure 13: An illustration of a diffusion

pump. [10]

jet. Here the oil molecules collide with the rest gas molecules entering through theinlet part. The outer part of the pump is cooled either by air or water which resultsin condensation of the oil. The oil then returns to the reservoir. This results in apumping effect, which for a mean-free path greater than the width of the pump intake(this will always be true in the UHV region) is dominated by diffusion, hence thename diffusion pump. The oil flow is responsible for dragging the rest gas moleculestowards the backing region of the pump. As with the turbomolecular pump, the vapordiffusion pump requires a backing pump in order to work.

One major disadvantage of the diffusion pump is that the pumping oil can back-stream into the chamber and this can result in contamination of the chamber andthe experiment. Because of this, a lot of diffusion pumps have been replaced byturbomolecular pumps. One of the advantages of a diffusion pump compared to aturbomolecular pump is that the pump has no moving parts and is easier to repair.Furthermore diffusion pumps generally have higher pumping speeds for light gasescompared to turbomolecular pumps.

The vapor diffusion pump can typically pump from around 10−3 - 10−10 mbar.

6.2.4 Ion pump

The big chamber is equipped with two ion pumps. An ion pump consists of multiplecells, each consisting of a pumping element as seen in figure 14. The rest gas in thepump is ionized by a plasma discharge due to the high voltage between the anode andthe cathode in the order of 5 kV. The ionization probability is further increased by amagnetic field of a few thousand Gauss. This causes the electrons to follow a helicalpath, which greatly increases the length of their path. The rest gas ions are nowaccelerated towards the titanium cathode. The ions have such a high energy that theyeither get captured or chemisorbed by the cathode. Furthermore the ions can sputtertitanium from the cathode, which is then deposited on other parts of the pump, whereit can react with the rest gas and increase the pumping effect.

The ion pump does not remove the rest gas from the chamber, but reacts with it, soit can not contribute to the pressure. Therefore ion pumps do not need a direct backing

6.2. THE CHAMBER 19

Figure 14: An ion pump cell. [6] Figure 15: An ion gauge. [6]

pump. The ion pump works in the range from around 10−4 - 10−12 mbar, hence theycannot be used to pump the chamber down from roughing pressure to UHV. Insteadion pumps are used to help maintain UHV when low pressure is achieved.

6.2.5 Titanium sublimation pump

The chamber is equipped with one titanium sublimation pump (TSP). The TSP isbased on the reaction of the rest gas with titanium. The pump consists of a titaniumfilament through which a high current (40 A) can be passed. This causes the titaniumto reach its sublimation temperature and some of the titanium from the filament canbe deposited on the walls of the pump. Clean titanium is very reactive, so when therest gas molecules collide with the titanium, they react. This most likely results in asolid product, hence the gas molecules can no longer contribute to the pressure. Thefilament has to be heated periodically to deposit new clean titanium.

Most often the TSP is first used when UHV has been reached. This is due tothe fact that the higher the rest gas pressure, the faster the deposited titanium isused. The effectiveness of the pump is dependent on many things, primarily the sizeof the deposit area, the temperature of the chamber and the composition of the restgas. At ideal conditions the TSP can pump down to around 10−11 mbar. The biggestadvantage of the pump is that it also pumps very well for some light gasses, like H2 andD2, which makes it a very valuable pump when doing experiments with hydrogen. TheTSP the big chamber is equipped with has a pumping speed for H2 of approximately1570 l/s right after the filament is heated, while the pumping speed for H2 of theVarian Turbo V-550 turbomolecular pump is 510 l/s.

6.2.6 Pressure measurement

When performing any kind of vacuum experiment, especially in UHV, it is importantto be able to measure the pressure inside the chamber. The big chamber is equippedwith both Pirani gauges and ion gauges for this purpose.

A Pirani gauge consists of a heated filament suspended in a tube connected tothe chamber. Molecules from the chamber will collide with the with the filament andthereby cool it down, thus the temperature of the filament depends on the pressure.The electrical resistance of the wire is depended on the temperature and thus the


Figure 16: A sketch of the sample holder.

pressure inside the chamber can be measured by monitoring the resistance. Piranigauges work from ambient pressure down to about 10−3 mbar.

A diagram of a hot cathode ion gauge is shown in figure 15. The filament emitselectrons, which are accelerated towards the cylindrical cage. In the cage the electronscollide with the rest gas molecules which get ionized. The ions are collected by thewire in the center of the cage, and the current is measured. This current is a measureof the rest gas pressure.

The ion gauge can be used for pressure measurements from 10−4 - 10−11 mbar. Thelower pressure limit, is due to the effect known as the x-ray limit, where an electronhits the cage and emits a photon. This photon can hit the center wire which thenemits an electron. This causes a positive current measurement. Although this effectis very unlikely, it is a dominant contribution to the current at pressures below 10−11

mbar.

6.2.7 Manipulator

The substrate is placed in a sample holder which is supported in a manipulator. Thismakes it possible to rotate the sample 360◦ and switch it between a center and anoffset position, so all the experimental equipment can be reached.

The sample holder, as seen in figure 16, consists of a tantalum plate, in whichthe sample is mounted. The tantalum is bolted to a copper block, which sits in themanipulator arm. Between the tantalum plate and the copper block there is a pieceof sapphire that causes the tantalum plate to be electrically isolated from the copperblock. Furthermore, sapphire is a thermal insulator at high temperatures, which allowsthe tantalum plate to be heated without heating the copper block. At low temperaturesapphire is no longer a thermal insulator, so the plate can still be cooled by the copperblock, which itself is cooled by water or liquid nitrogen.

Behind the sample is a filament, which emits electrons that can heat the sample.To increase this effect a positive voltage of 550V is applied to the sample, relative tothe filament. This attracts the electrons to the sample. To increase the effect evenfurther, a negative bias can be applied to the copper block, which then repels theelectrons towards the sample. This method of heating is used both when annealing

6.2. THE CHAMBER 21

Figure 17: A sketch of the hydrogen source. [19]

the sample, and when doing the TPD experiments. To secure a linear heating of thesample, the filament is controlled by a model 340 LakeShore proportional-integral-derivative (PID) temperature controller. To measure the temperature of the sample,a c-type thermocouple is fitted to the side of the sample. The sample itself is a pieceof highly oriented pyrolytic graphite (HOPG). The HOPG is cleaved using adhesivetape, and the basal plane is the (0001)-plane. This is used because it has a highlyordered surface and good quality HOPG is relatively cheap compared to good qualitynatural graphite.

6.2.8 Hydrogen source

The big chamber is equipped with a hydrogen atom beam source (HABS), which isthe same as described by Tschersich et al. [19] [17] [18]. A sketch of the HABS isseen in figure 17. It consists of a tungsten capillary inserted into a copper housing,which is fed with molecular hydrogen gas. The capillary is heated by radiation from atungsten filament surrounding the capillary. The heat cracks the molecular hydrogeninto atomic hydrogen. In order to minimize heat transfer into the chamber, the filamentand capillary is surrounded by thermal shields and the copper housing is cooled withwater. The HABS is equipped with a shutter, which is used to control the dosing timeof the atomic hydrogen.

The temperature effects the degree of dissociation, α, of the molecular hydrogen.The degree of dissociation is described by Tschersich [17] and is found to be given as

α =

√Kp

4pequp0

+Kp

(17)

where pequ is equal to the feeding pressure, pfeed, and p0 = 1 bar. Kp is the equilibriumconstant, which only depends on the temperature and is given by

log(Kp) = 6.304− 23760 K

T(18)

From this it can be seen that the temperature has a great influence on the degree ofdissociation, and it is very important that the temperature is kept constant duringexperiments in order to get an accurate dose. Typically the HABS was operated at2300 K. In these experiments pfeed can not be measured but is assumed to be closeto 0.01 mbar which gives a dissociation degree of 0.84. In some of the experiments,the temperature was lowered to 2150 K giving a dissociation degree of 0.56. Howevernone of these latter measurements are included in this report.


Figure 18: A drawing of the coronene source.

6.2.9 Coronene source

The chamber is equipped with an evaporative coronene doser as shown in figure 18.The doser itself is a so-called Knudsen cell. The Knudsen cell consists of a smallglass crucible, in which a small amount of coronene is placed together with someglass wool, to prevent the coronene from falling out. A Thermocoax standard SEAheating wire is twisted around the crucible, which can be heated by an external powersource. The heat from the heating wire heats the coronene, which evaporates andcan be deposited on the sample. The temperature of the crucible is measured witha k-type thermocouple and is controlled by a Eurotherm PID controller. To keepthe temperature more stable, it is possible to cool the crucible with water-cooling.However, in this project the water-cooling was not used, due to the fact that thecooling effect was too great and it was not possible to heat the crucible high enoughwithout damaging the heating wire.

The Knudsen cell sits in a metal housing with a hole of 4 mm in diameter, fromwhich the evaporated coronene can escape. In front of the housing there is a CF40VAT gate valve, which makes it possible to prevent coronene from entering the chamberwhen not dosing. The CF40 VAT valve also makes it possible to shut the coronenesource off from the rest of the chamber, so it can be taken off without venting thewhole chamber.

The dosing mechanism itself is placed on an extendable arm, which allows thedozer to be extended in to the chamber and thereby closer to the sample. This makesit possible only to dose on the sample. The coronene source is pumped by one of thesmall turbomolecular pumps.

6.2.10 Quadrupole mass spectrometer

The chamber is equipped with two quadrupole mass spectrometers (QMSs) a PfeifferVacuum Prisma and a QMS from Extrel. A sketch of a QMS is shown in figure 19and consists of three sections. First there is an ionizer, where electrons emitted by ahot filament ionize the molecules entering the QMS. The ionized molecules are thenaccelerated and focused into the next section, which is the quadrupole mass filter. This

6.2. THE CHAMBER 23

Figure 19: A quadrupole mass spectrometer. [6]

section consists of four metal rods that set up a quadrupole field. This is done by pairwise biasing the rods with a superposition of a DC voltage U and an AC componentV cos(ωt). The filter works such that only ions with a paticular m/z-ratio are allowedto pass through it. By changing U and V it is possible to select the m/z-ratios ofinterest. The last part of the QMS consists of a detector, which contains a channelelectron multiplier. It is also possible to detect with a faraday cup. However thefaraday cup is not sensitive enough for measurements in UHV, so here the channelelectron multiplier, which gives a gain of 107 - 109, has to be used. Both of the QMSsare differentially pumped by small turbomolecular pumps.

The QMS is used to measure the desorptionrate of mass selected molecules duringthe TPD experiments. Furthermore it is used to measure the chemical compositionof the rest gas in the chamber and can also be used for leak-testing the chamber.The reason for the chamber having two QMSs is that they have different m/z-ratioranges. The Pfeiffer Vacuum Prisma is primarily calibrated for low m/z-ratios andcan only measure up to 200 amu/e. The Extrel QMS is primarily calibrated for higherm/z-ratios and can measure up to 500 amu/e, this is primarily used for coronenemeasurments (m = 300 amu). The Extrel QMS could of course be calibrated for lowmasses as well and used instead of the Pfeiffer. However the Pfeifer would still providegreater sensitivity, firstly because it is movable, so it can be used much closer to thesample than the Extrel. Secondly because it has a Feulner cap which makes it possibleto focus the QMS only on the sample itself. In the experiments performed in thisproject, the Pfeiffers Feulner cap was placed approximately 2 mm from the sample,while the intake of the Extrel was placed approximately 100 mm from the sample. Forthese reasons the Pfeiffer is used for D2 measurements.

It is important to point out that it is not the mass, but the mass to charge ratio,that is measured. This means that a signal at half the expected m/z-ratio is observeddue to double ionization, at for instance 150 amu/e for coronene. Higher degreesof ionization are also possible. Other effects can as well result in an unexpectedmass measurement, for instance isotopic effects and fragmentation. This means thatmasses above 300 amu/e are measured for ordinary coronene due to heavier carbon andhydrogen isotopes. Masses below 300 amu/e are also measured, due to fragmentationin the QMS.

24 CHAPTER 7. RESULTS

Figure 20: TPD curves of coronene on HOPG for varying dose lengths.

7 Results

Three types of experiments were performed during my bachelor project. TPD ofcoronene on HOPG, TPD of hydrogenated coronene on HOPG and TPD of hydrogenfrom hydrogenated coronene on HOPG. These three types of experiments will be thesubject of the next three sections. A lot of TPD data was measured during my projectand I have tried to find the data of the highest quality to present in this chapter.

7.1 Temperature programmed desorption of

coronene from HOPG

In figure 20 a set of desorption curves for coronene on HOPG is shown. The coronenesource was operated at 180 ◦C and the dosing time was varied from 2-8 min. Aftereach dose the sample was heated with a linear ramp of β = 1 K/s up to 1300 K anda spectrum was measured with the QMS at 300 amu/e (The mass of coronene is 300amu).

From general observation it can be seen that 7 and 8 min dose exhibit clear zero-order desorption kinetics, as they share the same leading edge. This indicates thata multilayer has been dosed. Shorter doses exhibit kinetics of other orders and aretherefore assumed to be sub-monolayer doses. However as described later on thedefinition of a monolayer of coronene is not clearly defined. The desorption peakmaximum temperature, Tp, has been determined for all desorption curves and canbe seen in table 1 column 3. The uncertainties of the Tp values are uncertaintiesin determination of the peak temperature, hence experimental uncertainties are notincluded. This is the case for all determined parameters. For the 4 min peak the shapelooks different than for the others, which was due to a ramp problem that has yieldedhigher uncertenties for most determined parameters. For the 2 and 3 min dose higheruncertenties are observed as well, which is due to a decrease in signal-to-noise ratiodue to lower signals. The experimental uncertainties of Tp are primarily due to thecalibration of the thermocouple. Tp is not particularly interesting for the multilayers,because Tp is coverage dependent and tells very little about the desorption kinetics.For sub-monolayers Tp is also coverage dependent, as it can be seen that Tp increases

7.1. TEMPERATURE PROGRAMMED DESORPTION OF CORONENE FROMHOPG 25

Dosing time[min] θ Tp[K] ν[s−1] n Ed[eV]

2 0.052 ± 0.005 361 ± 7 1018 ± 1.5 1.3 ± 0.2 1.34 ± 0.123 0.11 ± 0.01 359 ± 7 1018 ± 1.5 1.3 ± 0.2 1.35 ± 0.124 0.39 ± 0.04 366 ± 7 1018 ± 1.5 1.4 ± 0.1 1.35 ± 0.125 0.74 ± 0.07 371 ± 5 1018 ± 1 1.2 ± 0.1 1.40 ± 0.086 1.0 ± 0.1 372 ± 3 1020 ± 1 1.1 ± 0.1 1.54 ± 0.087 1.5 ± 0.1 370 ± 2 1018 ± 1 0.0 ± 0.1 1.38 ± 0.088 1.7 ± 0.2 372 ± 2 1018 ± 1 0.0 ± 0.1 1.38 ± 0.08

Table 1: Experimental and kinetic parameters determined for desorption of coronenefrom HOPG.

with increasing coverage. Because nothing is known about the kinetics, it is a hard totell what this temperature dependence means, but it could indicate that the desorptionkinetics are not of first-order. However a more thorough analysis of the data is neededto determine anything about the kinetics.

This analysis is performed by comparing the data with TPD simulations madewith a selfwritten routine in MatLab. As an initial guess ν was set to 1013 s−1 andthe energy was calculated with the Redhead equation, equation 11. Although it isassumed that the TPD curves do follow first-order kinetics and that ν is much larger,this makes a good initial guess. Subsequently the kinetic parameters were changed toget a better fit. The determined values of ν, n and Ed can be seen in table 1, column4, 5 and 6. For lower coverages all the parameters are pretty much constant within theuncertainties. The desorption order for these coverages is around 1.3, which indicatesthat we are looking at a desorption process, where things that are not described by thePolanyi-Wigner equation occur. The desorption energies are around 1.35 eV for theselow coverages and the pre-exponential factor is in the order of 1018 s−1. For increasingcoverages it is seen that n decreases, while ν and Ed increases. The increase of ν andEd is within the uncertenties, except for the 6 min dose. The change in ν for onlythe 6 min dose makes no sense and if ν = 1018 s−1 was used in the simulation for the6 min dose as well, it would yield Ed = 1.40 eV, hence the change in energy wouldbe within the uncertainties here as well. The change in n can be a result of severaleffects, but it is believed that the main reason for the change in desorption kinetics isdue to the fact that the coronene molecules tilt at higher coverages, as proposed byZacharia et al. [21]. However further experimental research is needed to confirm this.

The tilt of the coronene means that a monolayer is hard to define, as the numberof desorption sites presumably changes with the change in kinetics. Furthermore, thebeginning of multilayer behavior is hard to determine, as the desorption energies arevery similar for multilayer and monolayer, so the multilayer and monolayer desorptionis not seen as separate peaks but as a mixed peak. This is consistent with the similaritybetween coronene and graphite. This mixing makes it hard to determine the beginningof multilayer behavior. In this analysis I define the 6 min dose as a monolayer, becausethe molecular tilt hence the change in desorption kinetics is only expected to be seenvery close to monolayer adsorption. From the definition of a monolayer, the relativecoverage of the rest of the peaks are determined by comparing the areas of the otherpeaks with the area of the 6 min peak. The uncertainties in the definition of the


Figure 21: TPD curves for even massesfor a 3 min coronene dose.

Figure 22: TPD curves for even massesfor a 2 min coronene dose and 1 min Ddose.

monolayer of course yield a rather big uncertainty in the determination of the relativecoverage.

This analysis substantiates the suggestion that PAH molecules tilt at higher cover-ages made by Zacharia et al. [21]. However further experiments are needed to confirmthis. Regarding the kinetic parameters I found ν and Ed to be in the same area asZacharia et al., but a different n was found. This can be due to the fact that in theexperiments done by Zacharia et al. the desorption was measured with an ion-gaugeas the change in pressure. This is normally more uncertain than measuring with aQMS. Furthermore they determined the kinetic parameters with the Falconer-Madixanalysis. In this analysis it is assumed that n is constant and the peak shape is nottaken in to account when determining the parameters.


hydrogenated coronene from HOPG

When hydrogenating coronene, the heavy hydrogen isotope deuterium, D, is usedinstead of ordinary hydrogen, H. This is due to the fact that even though severalpumping techniques are used, a relatively high background of hydrogen is alwayspresent in the chamber. This means that there would be a lot of noise in desorptioncurves of molecular hydrogen measured for ordinary hydrogen at 2 amu. When usingD2 instead of H2, molecular hydrogen TPD curves as seen in the next section can bemeasured at 4 amu, where the noise level is much lower due to low rest gas pressure.D can be used instead of H, because they are chemicaly identical. This means that thesame desorption mechanisms are expected to be involved. No experiments has beenperformed, to detect if there are any changes in desorption mechanisms when using Dinstead of H on PAHs. This has however been done for graphite. Graphite consists ofgraphene, which itself consist of carbon atoms bound by sp2-hybridyzed bonds. Thismeans that graphite is a lot like PAHs. On this subject Zecho et al. found that Hand D atoms adsorbed on graphite desorb in the same way [22]. Only a shift in Tp isdetected due to isotopic effects. When looking at TPD experiments of hydrogenated

7.2. TEMPERATURE PROGRAMMED DESORPTION OF HYDROGENATEDCORONENE FROM HOPG 27

m/z-ratio[amu/e] θm/θTOT Tp[K]

300 0.21 ± 0.01 350 ± 3302 0.10 ± 0.01 346 ± 3304 0.078 ± 0.01 344 ± 5306 0.034 ± 0.01 340 ± 3308 0.029 ± 0.01 338 ± 5310 0.021 ± 0.01 335 ± 3

Table 2: Determined parameters foreven masses of hydrogenated coroneneon HOPG.

m/z-ratio[amu/e] θm/θTOT

300 0.53 ± 0.01302 0.024 ± 0.01304 0.009 ± 0.01306 0.009 ± 0.01

Table 3: Determined parameters foreven masses of coronene on HOPG be-fore hydrogenation.

PAHs this means that we expect signals at 302 amu/e, 304 amu/e, 306 amu/e etc. ifhydrogenation occurs.

For the experiments performed on hydrogenation of coronene the coronene sourcewas operated at 180 ◦C, the HABS was run at 2300 K and the QMS was set to scanin the m/z range from 300 amu/e to 350 amu/e. In figure 21 a set of TPD curves fordifferent masses from the same experiment is shown. Here coronene has been dosedfor 3 min, but no D has been dosed, this is shown as a reference. In figure 22 a set ofTPD curves for different masses from the same experiment is shown as well. But herecoronene has been dosed for 2 min and D has been dosed for 1 min.

In figure 21 no real signal is detected for m/z-ratios above 300 amu/e and the signalseen in the graph is mostly due to noise and some signal at 302 amu/e is present asa result of isotopic components in the coronene. In figure 22 TPD signals are clearlypresent for m/z-ratios of 302 - 310 amu/e which are plotted, the data also suggeststhat higher masses are detected. When comparing these two graphs it is clear thathydrogenation has occurred in the second experiment, where D has been dosed. Tocompare the amount of different hydrogenated coronene molecules, the coverage forthe specific m/z-ratio, θm, is compared with the total coverage, θTOT . This can bedone by comparing the area under the TPD curve with the area under a TPD curvefor the total ion count in the region 300 amu/e to 350 amu/e. These fractions can beseen in table 2 column 2. The θm/θTOT values can be compared to the same valuesdetermined for coronene without a D dose seen in table 3 and a clear change can beseen. Tp has been determined for the peaks for the different masses and these can beseen in table 2 column 3. It can be seen that the desorption temperature decreaseswith increasing hydrogenation. As no change in desorption mechanisms is expected,this indicates that the higher the degree of hydrogenation is, the weaker the moleculeis bound. However this is actually also expected to happen, as the extra bound Datom will decrease the delocalization of the system and therefore the binding strengthof the van der Walls interaction. Actually, a larger decrease in Tp was expected, whichindicates that other kinds of bonds may occur. However further investigation is neededto confirm this.

From table 2 it can also be seen that the coverage for the individual masses isfairly low compared to the total coverage. This is of course partly due to the factthat higher degrees of hydrogenation also has happened. But when looking at theodd masses, as in figure 23, it can be seen that these are also present in the TPD


Figure 23: TPD curves for uneven masses for a 2 min coronene dose and 1 min D dose.

curves. As the adding of D will lead to an increase of 2 amu in the molecular mass,this is partly unexpected. The appearance of the odd masses in the TPD spectrumsuggests that the adding of the second D atom in some cases leads to abstractionwhere HD leaves the molecule. As the odd curves are fairly significant it suggests thatthe reaction barrier is fairly low for abstraction. This could of course also explain theappearance of the even masses, as several abstractions could yield the same result.Some of the detected molecules will properly also be a result of this abstraction, butscanning tunneling microscopy (STM) investigations made in the group also suggestthat higher degrees of hydrogenation occur [11]. The appearance of the odd m/z-ratios can also be due to fragmentation of the coronene molecule in the QMS. If, forinstance, the hydrogenated coronene molecule loses an H atom in the QMS, it wouldlead to a decrease in the molecular mass of 1 amu, hence an odd m/z-ratio will bedetected. In this case it would mean that hydrogenation still takes place, because theeffect only can lower the molecular mass. This would mean that the observed degreesof hydrogenation would only be a lower limit for the possible degrees of hydrogenation.Although the apperance of the odd m/z-ratios is expected to be due to the abstractionreaction, further investigations are needed, to observe if they are due to abstractionor fragmentation in the QMS.

The appearance of hydrogenated coronene in the TPD spectrum is a very importantresult. This shows that hydrogenation of coronene is possible as suggested by Raulsand Hornekær [14]. Furthermore the observation of molecules of odd masses suggestthat abstraction of HD is possible, which suggest that abstraction of H2 is possible,hence H2 formation can happen from hydrogenated coronene. As hydrogenation ofPAHs is possible and abstraction of HD is probable, it further more substantiates thetheory that hydrogenated PAHs can play a role in the formation of H2 in the ISM, assuggested by Rauls and Hornekær [14].

7.3. TEMPERATURE PROGRAMMED DESORPTION OF HYDROGEN FROMHYDROGENATED CORONENE ON HOPG 29

Figure 24: TPD curves of hydrogen fromhydrogenated coronene for a 10 min Ddose and a varying coronene dose length.

Figure 25: TPD curves of hydrogenfrom hydrogenated coronene for a 5 mincoronene dose and a varying D doselength.


hydrogen from hydrogenated coronene on

HOPG

For these investigations the same settings were used as in the previous section. Infigure 24 a set of TPD curves is shown, where the coronene dosing time was variedfrom 60 s - 300 s and the D dosing time was kept constant at 10 min. Together withthese TPD curves a TPD curve of a 2 min D dose on pure HOPG is showed, experiencehas showed that the HOPG is saturated with D at this dosing time. The curve seenfor the 2 min D dose is a typical desorption curve for D on graphite as described byZecho et al. [23]. For the short coronene doses (60 s and 90 s) it can be seen that thegraphite peak is still present, which suggests that a sub-monolayer of coronene hasbeen dosed. For longer doses (150 s and 300 s) the graphite peak is missing, whichsuggests that above a monolayer has been dosed. This is however a very uncertainway to determine monolayer, as the D might be able to adsorb and desorb from thegraphite at doses slightly above monolayer.

When coronene is dosed, it can also be seen that a new peak appears at highertemperature than the graphite peak. The peak is very broad, which indicates thatthere are several desorption sites with different binding energy. This desorption peakcan however not come from the hydrogenated coronene, as they desorb at lower tem-perature. This peak could come from stronger bound coronene or larger structuresformed on the surface, however further investigation has to be made to find the ex-act reason for this. Furthermore it can be seen that Tp for the peak decreases withincreasing coronene dose. The reason for this temperature decrease is unknown, sofurther investigation is needed here as well.

Another interesting effect can be seen in figure 25, where the coronene dose hasbeen kept constant at 5 min and the D dose has been varied from 150 s - 10 min. Hereit is seen that Tp increases with increasing D dose, the reason for this might be due tothat more sites become available for higher degrees of hydrogenation, however further

30 CHAPTER 9. ACKNOWLEDGEMENTS

investigations are needed here as well, to determine the exact reason.

8 Conclusion

Through TPD investigations we have shown that the desorption kinetics of coronenedo not change at low coverages and that they follow a fractional reaction order withconstant Ed. When the coverage increases and gets close to monolayer, the reactionorder decreases. This supports the theory that the coronene molecules tilt with respectto the substrate at coverages close to a monolayer, as this would lead to the observedchange in desorption kinetics.

We have also shown that hydrogenation of coronene is possible through dosing ofatomic deuterium on coronene adsorbed on HOPG. This is one of the most importantresults in this project, as this has only been suggested through theoretical calculationsby Rauls and Hornekær [14]. It furthermore opens up for a lot of further investigationpossibilities. The hydrogenation leads to a decrease in binding energy between thecoronene and the HOPG, which was seen as a decrease in Tp. We have also observedevidence that an abstraction reaction probably can occur, which leads to a deuteriumatom taking the place of an ordinary hydrogen in the coronene molecule, which leadsto an increase in the molecular mass of 1 amu.

There are a lot of possibilities for future work on this subject, especially as all ofthe experimental research discussed in this project is still in progress. First of all amore thorough investigation can be performed on the adsorption kinetics of ordinarycoronene on HOPG to substantiate the possibility of the tilting mechanism further.This could also be done with other techniques, such as STM or atomic force mi-croscopy. A more thorough investigation can as well be performed for the desorptionof hydrogenated coronene. It would be interesting to investigate, if there is a limit tothe degree of hydrogenation of coronene, or if fully hydrogenated coronene is possibleto achieve through this method. Furthermore it would be interesting to investigatethe exact reason for the observation of odd m/z-ratios in the hydrogenated coronenespectrum. It would also be interesting to investigate, what could be the reason for thechange in Tp for the desorbed D from hydrogenated coronene. In the more distant fu-ture it would be interesting to investigate, how UV light would effect the hydrogenatedcoronene and how the degree of hydrogenation affects the UV-visible spectra. Thiswould be interesting for astrochemistry as well as for possible applications in hydrogenstorage.

9 Acknowledgements

I would like to acknowledge the help and support of the whole Surface Dynamicsgroup and thank them for being supporting and welcome during the time I spend inthe group. I would furthermore like to give a special thanks to the following people.

Liv Hornekær, for being my supervisor and providing me with the opportunityto make this project. Bjarke Jørgensen and John Thrower for introducing me to thelaboratory and the experimental techniques, for answering all my question no matterhow stupid they were, and for good company during all the time I spend in thelaboratory.

BIBLIOGRAPHY 31

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Thermal Desorption Studies of the Hydrogenation of ... · Anders Lind Skov Supervisor: Liv Hornekær Institute for Physics and Astronomy, Aarhus University Bachelor Thesis in Physics