Mapping of local band gap opening in hydrogen functionalized graphene

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Mapping of local band gap opening in hydrogen functionalized graphene

Bachelor thesis by Jakob Holm Jørgensen

20094849

Interdisciplinary Nanoscience Center, Aarhus University

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English abstract

In this bachelor project the primary task has been to implement a new Low Temperature experimental Scanning Tunneling microscopy (STM) setup. Atomic resolution on graphite was achieved at both room- and 𝐿𝑁 temperature. Noise measurements were performed to determine which sources contribute. This showed that the main contributors are turbo- and ionpumps which was expected. Surprisingly it was observed that the ionpump controller contributed significantly to the noise level.

During the project, we installed a hydrogen source to enable hydrogen dosing of graphene. Initially we observed some hydrogenation on graphite but line structures were seen as well. This indicates that the source is polluted and degassing of the tungsten capillary was performed. After degassing, no adsorption was achieved. What causes this lack of hydrogen adsorption is not clear and will be investigated in further detail.

STM was used to study hydrogen chemisorption on graphene on Ir(111) substrate. Graphene was grown using the temperature programmed growth technique after cleaning the Ir crystal for several days. After confirming the presence of graphene, identified by the characteristic Moiré structure, atomic hydrogen was dosed for different times to obtain different levels of coverage. These experiments showed that hydrogen does not adsorb randomly but at certain crystal sites called hexagonal close packed, hcp, and cubic close packed, ccp. Atop sites seem not to be favorable since these are always hydrogen-free. At medium coverage circular hydrogen clusters start to form and at high coverage elongated hydrogen structures are seen. To study bias dependent depiction of the hydrogen clusters, images were recorded at the same position with different bias voltage. The images showed a depletion of density of states at a low bias, but when this was increased no depletion was observed. We would like to perform scanning tunneling

spectroscopy to determine whether this is due to a local bandgap at the hydrogen cluster or just a strange behavior of the electronic bands. Finally desorption experiments were performed, confirming that

hydrogen can indeed be desorbed from the surface by scanning the surface at high current at low voltage, meaning that the tip is very close to the surface and thus can pick up hydrogen atoms.

Because of experimental difficulties we did not manage to produce our own graphene data since we did not have graphene available until the very last week of my lab work. The presented data has been

produced by others from the group. The aim in the future is to get a high quality graphene into the LT STM to obtain tunneling spectra at cryogenic temperatures. This should shed light on the physics behind bandgap openings on hydrogen functionalized graphene.

The fact that atomic resolution was achieved in LT STM means that mapping of the electronic structure by STS in principle can be performed with this high resolution.

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1. Introduction

The aim of this bachelor project is to map local bandgap opening in hydrogenated graphene on an Ir(111) substrate. Earlier studies by this group have shown that a bandgap can be induced in graphene by

chemisorption of hydrogen in a periodic pattern [6]. To investigate the mechanism behind the gap opening, we would like to map the local density of states and see how it varies when going from hydrogenated to hydrogen-free areas.

Graphene is a promising candidate for replacing silicon in electronic devices because of its superior electronic properties as e.g. ballistic electron transport. Finite bandgap is essential because the carrier density needs to be controlled. This is why a lot of research is aiming at opening and controlling the size of the energy gap in graphene, and several promising ways have already been developed, though commercial electronic devices are far from being available. The two most promising ways to control the size of the gap in graphene are by chemical functionalization and by utilizing bilayer graphene.

1.1 About this project

This bachelor project has been conducted in the group of Associate Professor Liv Hornekær at Aarhus University. The laboratory work was initiated in September 2011, where the aim was to grow epitaxial graphene on SiC in collaboration with Mikkel Kongsfelt from Kim Dåsbjerg’s group, Aarhus University. This project was abandoned in November because the induction furnace used to grow graphene did not allow a continuous gas flow, which is essential when growing graphene by thermal decomposition at high pressure.

The project description was changed to measure local bandgap opening in hydrogenated graphene on Ir(111) substrate, which was initiated in January 2012. In this project I have been working closely with Richard Balog, where the primary work has been of more practical character, since we first needed to implement the low temperature experimental system. We succeeded in achieving atomic resolution on graphite at both room temperature and liquid nitrogen temperature and were able to calibrate

approximate piezo constants at both temperatures. Because low temperature experimental setup does not allow in situ growth of graphene on iridium substrate, the graphene sample was not available for

measurements before the very last week of my experimental work, and therefore no data on the graphene- Ir(111) system was produced during my bachelors project.

During my project I have taken part in one opening of the UHV chamber, where we improved a lot of things on the setup and installed a hydrogen source and molecular doser.

I would like to thank Richard Balog for taking the time to acquainting me with the experimental setup and for great patience with all my questions.

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2. Scanning tunneling microscopy

The scanning tunneling microscope (STM) was invented by Binnig and Rohrer in 1981 and described in Physical review letters in 1982 [7]. Their achievement was awarded with the Nobelprice five years later.

Since then the STM has become one of the most powerful techniques for high resolution imaging of conducting and semiconducting surfaces. In the following section the theory behind the STM imaging and scanning tunneling spectroscopy (STS) technique, will be described.

2.1. Theory of Scanning Tunneling Microscopy [1]

The STM builds entirely on the quantum mechanical effect of tunneling. Tunneling of e.g. electrons through a vacuum barrier is possible because in quantum mechanics, the electronic wave function describing the electron may protrude into a barrier and beyond. Thus, when two materials, in this case the sample and the STM tip, come close enough so the width of the barrier is decreased, electrons ‘make it’ through the barrier and an electron is transferred from one material to another without contacting them physically.

The amount of electrons per unit time tunneling through the barrier can be calculated using the Wentzel- Kramers-Brillouin approximation, expressed as [1],

𝐼 = ∫ 𝜌 (𝐸)𝜌 (𝐸 − 𝑒𝑉 )𝑇(𝐸, 𝑒𝑉 )𝑑𝐸 (2.1)

where 𝜌 , 𝜌 is the local density of states (LDOS) at energy E at the surface of A and B, respectively, and 𝑇(𝐸, 𝑒𝑉 ) is the tunneling transmission probability of an electron with the energy E at an applied bias 𝑉 . The latter is given by [1],

𝑇(𝐸, 𝑒𝑉 ) = exp − ^√

ℏ + − 𝐸 (2.2)

where 𝜙 , 𝜙 are the workfunctions of material A and B, z is their mutual separation and m is the mass of the electron. An evaluation on this expression shows that the transmission probability is highest for electrons around the Fermi energy, 𝐸 , of a material that is negatively biased (𝑉 < 0) and falls off exponentially for lower energies up to 𝐸 of a second material .

Equation 2.1 can be simplified by assuming constant density of states (independent of energy). That is true in the limit of low bias voltage

, ≪ 1 [1]:

𝐼 = 𝜌 𝜌 𝑉 exp −2 ⁽ ⁾

ℏ (2.3)

As can be seen from equation 2.3, the tunneling current depends exponentially on the separation z. That exponential dependence makes STM a superior height resolution technique, with height sensitivity as high as 0,1𝑝𝑚. Also it is clear from expression 2.3 how the outstanding lateral resolution can be achieved.

Because of the strong dependence of tunnelling current on material separation, the majority of electrons will tunnel through the outermost atoms of the tip, so if this can be prepared such that the tip is only one

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3 atom thick at the end, one can achieve lateral resolution at atomic level. Such tips are routinely prepared at present using standard tip etching techniques.

Equation 2.3 states that the tunneling current also depends strongly on LDOS of the tip and sample. It is therefore often difficult to separate these two contributions in the image.

An STM image is usually obtained by rastering the tip over the surface and measuring the tunneling current point by point. Two different scanning modes are widely used: the constant-current and constant-height mode.

In constant-current mode, the aim is to keep the tunneling current constant by varying the distance of the tip to the substrate using a fast feedback loop. The height is changed by piezoelectric electrodes, which are special materials that contract or expand when a voltage is applied. The voltage changes applied to the tip’s z-position (height) are then monitored by the computer.

Constant-height mode means that the z-position of the tip is fixed and the STM image is created by measuring changes in the tunneling current. While this mode is faster than constant- current mode, it requires an extremely stable system without any thermal drifts, thus avoiding tip crashing against the surface. Because of these limitations, the constant-current mode is by far the most used scanning mode.

The material from which the tip is made is very important for the performance of the STM. The preferred materials are d- band metals, due to a directional nature of the d-wave function. These tips give the highest spatial resolution. In vacuum tungsten is often used but when scanning at atmospheric pressure, W oxidizes and an alloy like PtIr is preferred because of its stability.

There are many advantages of using STM in surface science. The atomic resolution images in real space are obviously a major strength of the technique. The STM probes the surface very locally, this means that local surface defects can be investigated, which would have been averaged out in e.g. diffraction data. On the other hand, it is not possible to say anything statistically reliable at macroscopic scales just by using the STM. One can, of course, move the tip to different areas of the sample, but it is impossible to systematically cover large parts of the sample. Another drawback of STM is that images do not possess any information about chemical specificity. This means that the image will only give information about the size and position of an atom or molecule at the surface but not the chemical composition. Comparison between simulated and experimental images needs to be performed in order to obtain information regarding chemistry, so an initial knowledge about surface chemical composition is needed. Thus depositing known atoms or

molecules onto the clean surface can often circumvent this problem. Another possibility is to utilise the scanning tunneling spectroscopy (STS) technique, which may partly solve the problem with identifying the chemical composition of the sample.

Figure 2.1: Principle of the STM technique. The tip is scanning the surface, using one of two possible modes. The feedback loop is used to adjust piezo motors when scanning in constant-current mode, ref. [4]

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2.2. Scanning tunneling spectroscopy

Scanning tunneling spectroscopy (STS) is mainly used to characterize the electronic structure of the surface.

This is done by holding the tip at the point where the spectrum is to be measured while turning the

feedback loop off. The bias is varied over an interval of interest while measuring the tunneling current. This gives a local 𝐼(𝑉) spectrum with characteristic features related to positions of energy states. The current is often differentiated with respect to the bias voltage to obtain ^{( )} which is then plotted against V. This representation is very convenient since ^{( )} reflects the density of states of the surface at that energy, 𝜌 (𝑒𝑉). The expression is often normalised to avoid divergence in

both biasvoltage and tip-sample separation: ^{( )}/(𝐼/𝑉).

A standard convention is that the bias voltage is applied to the sample and therefore scanning with a positive bias is often called empty-state STM, while a negative bias is called filled-state STM.

This is because, at positive bias, electrons are tunneling from the valence band of the negatively charged tip to the conduction band of the positively charged sample and vice versa for negative bias voltage, as shown in figure 2.2a,b.

STS is a very convenient technique for determining bandgaps of semiconductors. This is illustrated in figure 2.3a. It is seen that the tunneling current is vanishing around the Fermi energy (𝑉 = 0), but suddenly rises at higher voltages. The bandgap can be read directly of the spectrum as the distance between the non-zero 𝑑𝐼/𝑑𝑉 at positive and negative bias as showed in figure 2.3a.

Figure 2.3: a) STS spectrum of an intrinsic semiconductor, ref [8], b) GaAs semiconductor, upper figure is p doped GaAs, lower is n doped GaAs, ref [1]

STS can also be used to determine the doping characteristics of a semiconductor. This is illustrated in figure 2.3b for GaAs. The C peak is the conduction band localized on Ga atoms while V is the filled valence band located on the As atoms. When the semiconductor is p doped, this will show as a “bump” at positive bias voltage from the dopant states close to the conduction band. On the other hand when the material is n- doped, the dopant states are lying close to the valence band and the bump from the dopants can be found at negative bias voltage.

Figure 2.2: Tunneling with filled- and empty -state STM [1]

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3. Experimental setup

The setup used in this bachelor project is a Createc Low Temperature Scanning Tunneling Microscope developed in 1996 [9]. The STM has very recently been acquired by the group and therefore much effort has gone into implementing the system, which was the primary task of this bachelor project. The STM is capable of working at liquid nitrogen and liquid He temperatures at ultra-high vacuum (UHV). This provides very low thermal drifts and reduces thermal fluctuations, which is essential when performing STS

measurements.

3.1 The STM

Figure 3.1 depicts the STM unit which is based on a Besocke “beetle” design [9]. In experimental setup the STM unit is surrounded by two radiation shields in order to keep the STM thermally stable during low temperature measurements. The outer shield is cooled by liquid 𝑁 to 77K, while the inner shield can be cooled down to 4K by liquid He. As can be seen on the image, the wires are very thin to avoid heat conductance to the STM and thereby destabilize it thermally. The basic STM parts are the following:

1) Scanner tube

2) Piezoelectric moters for approach

3) Tip

4) Suspension connecters 5) Sample position 6) Sample connections

1. Scanner tube: The scanner tube is responsible for movement of the tip during the scan. It is made from a piezoelectric material, which allows very accurate movement by applying a voltage to it. The tube is coated by a metal both inside and outside and divided into a large central electrode and two upper and lower electrodes. The outer electrodes enable the piezoelectric material to move in the xy-plane which is needed for raster scanning the surface. The inner electrode is for movement in the z direction.

Figure 3.1: STM unit of the Besocke "beetle" design

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6 2. Piezoelectric scanner tubes for approaching: These three scanner tubes are responsible for coarse approach of the tip to the sample. The scanner tubes carry sapphire balls on which the central scanner tube rests. The approach is done by applying a saw tooth wave to the piezos and thereby performing a “stick and slip” movement, i.e. the ring carrying the tip is rotated during the slow rise of the voltage after which the sapphire balls slip back to the original position during the fast drop of the voltage. This cycle is repeated many times until the tip approaches the sample. Tip movement in the xy-plane is done by applying an appropriate voltage to the scanner tubes such that all of them are shifted in the same direction.

3. Tip: The tip is in our case made of either tungsten or a PtIr alloy. It is attached to the piezo material by a magnet and can easily be exchanged without opening the chamber. The tungsten tip is made by etching.

To clean the tip from impurities after the etching it is annealed in vacuum, using electron bombardment from a filament ring mounted on the sample garage.

4. Suspension springs: To reduce mechanical vibrations, the STM is suspended by three helical stainless- steel springs which are mounted on the He cryostat. The springs are often very soft to make the largest mismatch between the mechanical modes of the springs and the high resonance frequency of the stiff STM unit.

5. Sample position: The sample is loaded into the STM by pulling the connections (6) down and transferring the sample holder (Fig. 3.2) into the available space. When the sample is in position, (6) is pulled up to release the STM and lock the sample holder into position.

3.2 Sample holder

Figure 3.2 shows the sample holder. The sample (1) is mounted on a ceramic heater and tightened by a molybdenum cap. The sample temperature is monitored by a thermocouple (2) mounted on the Mo cap

and shows the temperature difference between the sample holder and the sample. For this reason, the temperature cannot be monitored during cooling, since the sample and sample holder are cooled simultaneously to the same temperature. The connections for the thermocouple and the heater are located on (3). This is also where the bias is applied during the scan. The holder is grabbed by the manipulator in the groove (4).

Figure3.2: Sample holder for LT STM

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3.3 Instruments on the chamber

The Low Temperature setup is imaged in figure 3.3. The chamber consists of two main chambers and a load lock. The preparation chamber is where samples are cleaned by annealing and, for metals, sputtering. This is also where the hydrogen source and molecular doser are mounted. The STM chamber is of course where measurements are performed and is not really used for anything else although it would be possible to mount a doser on this chamber to do in situ dosing.

1)-4) Manipulator used for sample transfer

5) Ion gauge

6) Garage controller 7) Transfer stick 8) Load lock 9) Sputter gun 10) Hydrogen source 11) Quadrupole mass spectrometer 12) Cryostat

(1) Manipulator is used to transfer sample between the preparation and the STM chamber. The sample position can be adjusted in x and y directions by the micrometer screw gauges (2) and (3) respectively, in z- direction on the handle below the manipulator (not shown) and rotated around the z-axis on (4). Sample cleaning and preparation, e.g. sputter/ annealing, dosing molecules or atomic hydrogen is done while the sample is held in the manipulator. Since the bearings in the rotating part of the manipulator give rise to vacuum leaks, this part is at all times pumped to keep the chamber pressure in the UHV region.

(9) Sputter gun is used for cleaning the sample. Neon or Argon atoms are ionized, accelerated through a potential ranging from 1𝑘𝑉 − 3𝑘𝑉 and bombarded onto the sample to remove dirt. After the sputtering, the sample is usually annealed to reconstruct the surface. Sputtering is primarily used on metals since the atoms are quite mobile and therefore the surface is easy to reconstruct.

(5) Ion gauge is used for pressure measurements in the range of 10 𝑚𝑏𝑎𝑟 − 10 𝑚𝑏𝑎𝑟. A hot filament emits electrons which are attracted towards a positively biased metal grid. On the path from the filament to the grid, electrons hit rest gas molecules and ionize them. The ions are collected at a wire in the middle of the grid. The measured ion current is then related to the pressure. The experimental setup is equipped with three of these gauges for separate reading of the pressure of different parts of the setup– the preparation chamber, the STM chamber and the load lock.

Figure 3.3: The Low Temperature STM experimental setup

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8 (10) Hydrogen source is of the Juelich design [10]. It consists of a tungsten capillary which is heated to around 2100K. A flow of hydrogen molecules at a feeding pressure of ~10 𝑚𝑏𝑎𝑟 is passed through the capillary to crack the molecule into hydrogen atoms, which are then dosed to the sample surface. The exposure can be controlled by a shutter.

(12) Cryostat is used for cooling the STM to cryogenic temperatures. The cryostat consists of an inner and outer dewar, which are in mechanical contact to the inner and outer radiation shields of STM, respectively.

The outer shield is always cooled by liquid 𝑁 while the inner can be cooled by either liquid 𝑁 or liquid He.

When using He, the STM can be cooled down to 4,2K at which the thermal fluctuations are minimal and thus the surface is very stable.

(8) Load lock enables loading samples without venting the chamber. In the load lock the sample is loaded onto a transfer stick at ambient conditions. Afterwards the load lock is pumped down for a few hours until high vacuum is reached. The valve separating the load lock and the preparation chamber can then be opened without compromising the vacuum too much.

3.4 Pumping of the chamber

In order to achieve UHV, several different pumps are needed. The chamber is equipped with a roughing pump as well as turbomolecular pumps, ion pumps and sublimation pumps:

Roughing pump: Uses a rotating arm to push the gas out through an oil-filled compartment. Is capable of pumping the chamber to around 10 𝑚𝑏𝑎𝑟, which is necessary since the turbomolecular pump does not work at ambient pressure and after pump down to keep the pressure behind the turbomolecular pump.

Turbomolecular pump: This pump is built of a fast spinning turbine rotor. The principle of operation is that gas molecules can be given a momentum in a desired direction by repeated collisions with the rotor blades.

The molecules at the inlet hit the rotor and are sent towards the exhaust, where the roughing pump is connected, thus lowering the pressure. UHV can be obtained using the turbomolecular pump but ion pumps are used on most chambers due to better efficiency of removing some gasses. The Low

Temperature chamber has two turbomolecular pumps, one on the preparation chamber and one on the load lock. The first is primarily used to keep the pressure during sputtering and dosing where the ion pump is turned off while the last is used for sample loading, differential pumping of the sputter gun, and pumping of the hydrogen source and molecular doser.

Ion pump: Ionizes gas molecules by a plasma discharge using a large potential of 7𝑘𝑉 to accelerate the ions into a solid from which they cannot escape. The ion pump does not function above pressures higher than 10 − 10 mbar and hence the turbo pump is needed at higher pressures. Ion pumps are employed on most chambers because it has high pumping efficiency for different atomic masses than the turbomolecular pump. Also it is often kept running during STM measurements which would be impossible with the

turbomolecular pump due to a very high mechanical noise level.

Titanium sublimation pump: Works by sublimating reactive Ti atoms from a filament which is heated by applying a large current. The sublimated Ti atoms are deposited to the chamber walls where rest gas molecules can stick to the atoms. It is only necessary to run the sublimation pump occasionally, but after

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9 some time the Ti coating is no longer active and a new layer is needed. A major advantage of the

sublimation pump is that hydrogen easily sticks since this is usually hard to pump out with other types of pumps.

3.5 Noise measurements of the STM setup

Since STM is a very delicate experimental technique, high levels of noise are not tolerated. Because of pumps and electronics connected to the chamber, they all can contribute to the noise, which can spoil the measurements. This will be investigates in this section.

Figure 3.4: Spectrum with everything turned on

Figure 3.4 shows a frequency spectrum of the noise in tunneling current for a situation where both the turbomolecular- and ionpumps are turned on. As seen in the figure there are two frequency ranges with large level of noise intensity. The root mean square of the noise level is about 48 meV. To identify partial contribution from different sources, the pumps will be turned off one after each other while measuring the frequency spectrum of the tunneling current.

Figure 1.5: Ion pump turned off

Figure 3.5 depicts the situation where only ion pumps are turned off. All the frequencies have been reduced drastically except for the peak at 1𝑘𝐻z. That frequency corresponds to the spinning frequency of the turbomolecular pumps. The root mean square (RMS) of the noise has been reduced substantially from 48𝑚𝑉 to 16𝑚𝑉. The spectrum, however, still shows two smaller peaks at ca. 0.8𝑘𝐻𝑧 and 5 peaks between

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10 1𝑘𝐻𝑧 and 2.5𝑘𝐻𝑧. These I cannot account for, but it is probably electronic noise, since they are found at relatively high frequencies.

Figure 3.6a shows the contribution to the noise from the ion pumps only. In that case the sharp peak at 1kHz from the turbopumps has disappeared. The broad range of frequencies seen in the spectra is due to a high voltage (HV) generator for the ion pump. If this is switched off the noise level is reduced from ~39 meV to ~ 9meV, figure 3.6b. However, as figure 3.6b shows it is not only contributions of the HV generator to the noise which is seen in figure 3.6a. Even when HV generator has been turned off, a significant noise level is apparent. To find out the source, the ion pump controller itself was un-plugged from the power socket, which had indeed a quite large effect. The RMS was reduced by almost a factor 3 to 3.6𝑚𝑉 which is an acceptable noise level, figure3. 6c.

Figure 3.6: Ion pump: a) ion pump running, b) ion pump turned off c) Controller turned off

These results show that a lot of electronic noise can be eliminated by turning off the ion pump controller when scanning for atomic resolution. This result is rather unexpected since electronics should not contribute that much to the noise.

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4. Graphene

As already mentioned, graphene has unique properties which makes it an ideal material for many

applications. These will be summed up in the first section. The main interest in this project is the possibility of opening a band gap and for this reason bandgap opening mechanisms are discussed in some detail in section two. In the third section hydrogenation of graphene is discussed, since this is the mechanism we would like to investigate in further detail.

4.1. Properties of graphene

Graphene is a two-dimensional material consisting only of 𝑠𝑝 - hybridized carbon atoms in an “infinite”

hexagonal lattice, the Honeycomb lattice (Figure 4.1). Interestingly, it was predicted by Landau & Peierls [11] more than 70 years ago, that 2D materials are not thermodynamically stable and therefore should not exist. Nonetheless A.K. Geim & K.S. Novoselov succeeded in isolating and characterizing a single layer of graphene [12] in 2004 for which they were awarded the Nobel Prize only 6 years later. The main reason for

the stability of the graphene is that it is not truly a 2-dimensional freestanding material as figure 4.1A suggests; it buckles into the third

dimension, which lowers its energy and makes it thermodynamically stable. Graphene is the fundamental building block for several other carbon structures. As indicated in figure 4.1B, fullerenes and carbon nanotubes can be seen as folded from small graphene parts, while graphite consists of graphene sheets stacked upon each other by AB stacking.

The reason why graphene is a very hot topic is because its, a) superior electronic properties b) remarkable mechanical properties, c) chemical inertness and d) optical properties.

a) Electronic properties: Freestanding graphene is a semimetal [13], which means that there is no bandgap, contrary to semiconductors, but the density of states vanish at the Fermi Energy. The honeycomb structure of graphene can be described by two sublattices as identified in figure 4.2b, each of them with a hexagonal closed packed structure. The dispersion relation is represented in figure 4.2a for freestanding graphene.

The conduction and valence bands meet at the K and K’ points (figure 4.2b), for the two sublattices respectively, which are the high symmetry points of the graphene Brillouin Zone. The hexagonal structure of graphene gives rise to a linear dispersion around the Fermi energy given by [2]:

𝐸(𝑘) = ħ𝑐^∗𝑘 (4.1)

Figure 4.1 A Graphene and B the deriveritives, adapted from ref. [2]

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12 Where k is the wave vector and 𝑐^∗ is the effective speed of light and 𝑝 = ħ𝑘 is the momentum of the electrons, Because of the linear dispersion, electrons mimic massless Dirac fermions, which means that they travel through the material as photons with an effective ‘speed of light’ 𝑐^∗= 10 𝑚/𝑠.

Figure 4.2: a) Dispersion relation of graphene, B) represents the symmetry elemente of the graphene Brillouin Zone, ref [14]

Bolotin et al. achieved electron mobility as high as 200,000𝑐𝑚 𝑉 𝑠 in a suspended single layer sheet with a mean free path for free charge carriers exceeding 1𝜇𝑚 [15], 10 times higher than that of copper.

This makes graphene a very interesting material in high speed electronic devices. Unfortunately graphene is not a semiconductor but it is possible to induce a bandgap, as will be discussed in next section. This would open for the possibility of replacing Si as the base material in electronics.

Another unusual characteristic is the half integer quantum Hall effect (QHE) described by Novoselov [13], contrary to conventional integer QHE. The QHE is a phenomenon only observed in two-dimensional metals and graphene is therefore interesting in this respect. Graphene differ from other materials because of the exceptional electronic structure, and the half integer QHE has been theoretically predicted from the Dirac Equation. QHE is usually only observed at very low temperatures (<30K) but for graphene, this has been seen even at room temperature for high magnetic fields [16].

b) Mechanical properties: Graphene has a Young Modulus of 1 TPa and a breaking strength 200 times higher than that of steel [17]. Because of the high strength to weight ratio, graphene is a promising material for structural uses in e.g. aircrafts.

c) Chemical stability: Graphene is a very inert material and is therefore ideal for coating of surfaces

operating in chemically harsh environments [18]. The graphene layer does not need to be of high quality in order to protect the surface and thus is expected to be used commercially within few years.

d) Optical properties: Since graphene is conducting and transparent, it has already been employed in touch screens because the transparency is higher than other materials used [19]. This property also makes it usable in organic dye-sensitized solar cells as electrode material, though high quality graphene is needed.

4.2 Bandgap opening in graphene

As already stated, graphene has remarkable electronic properties and for that reason the electronics industry is very interested in graphene as a potential successor of silicon in electronic devices. The major

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13 problem in utilising graphene in circuits is the fact, that it does not possess a bandgap around the Fermi energy. A bandgap can be opened in graphene by at least three different mechanisms: quantum confinement of electrons, symmetry breaking of the lattice and Anderson localization.

Quantum confinement means that electrons are forced to move in only certain areas of the graphene. This in turn induces a bandgap opening. The principal of confinement is beautifully seen in graphene

nanoribbons, which are infinitely long graphene sheets of a certain width. By cutting the graphene in different directions one obtains two different types of ribbons recognized by their edges, namely the zigzag and armchair [20]. The size of the bandgap in both types of nanoribbons is inversely proportional to the width 𝑤 of the ribbon 𝐸 ∝ 𝑤 . Edge effects are important, however, when considering the electronic characteristics of the ribbon[20, 21].

Confinement effects play also important role in hydrogen functionalized graphene on Ir(111). Here, the graphene layer is periodically hydrogenated due to influence of the Moiré structure. The hydrogenated areas show a diamond-like 𝑠𝑝 hybridization of carbon atoms, where the carbon atoms within the hexagon bind alternating to Ir and H atoms. These areas are insulating which means that electrons can’t move within these areas. For that reason, electrons are forced to move only in the hydrogen-free areas which

consequently causes bandgap opening.

The similar confinement effect as observed in periodically functionalized graphene with hydrogen can be induced in graphene antidot lattices (GAL). Here the insulating areas are formed by periodic array of holes.

It was found that the energy gap is proportional to the size of the hole [22]:

𝐸 ∝⁽ ⁾^½ (4.2)

Thus, the larger the hole is at a constant unit cell size, the larger bandgap opens. GAL’s have primarily been investigated in theoretical papers because of the production issues involved. The GAL’s requires a nice graphene layer with the holes distributed periodically over the entire sheet which is technically very demanding. One of the reasons why the GAL is attracting attention is the possibility of making spin qubits by making defects in the regular spaced holes, that is omitting some of the antidots in the pattern and thereby making localized spin qubit states [23].

Symmetry breaking means that symmetry elements a crystal unit cell possesses are removed. Graphene consists, as discussed of two sub-lattices. Breaking of these can e.g. be achieved when the number of electrons of the K and K’ atoms is different. Theoretical calculations have shown that this symmetry breaking results in a bandgap opening. Substrate induced symmetry breaking is seen for e.g. freestanding bilayer graphene where the second layer acts as the substrate. Bilayer graphene is AB stacked and this causes symmetry breaking with the result that conduction- and valence bands move apart. This would in principle be a semiconductor with a finite bandgap but because bilayer graphene contains four atoms per unit cell, the band structure is changed and does now contain four bands of which two meet in the Dirac point. The interesting property of bilayer graphene is that a bandgap can be introduced by applying a voltage perpendicular to the graphene plane since this alter the two layers differently and the bilayer symmetry is broken.

Anderson localization is a phenomenon where electrons are scattered on e.g. defects. If an electron is scattered many times, it is essentially localized within a region. This leads to a disorder induced metal-

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14 insulator transition, i.e. a bandgap is opened. This has been observed at low hydrogen coverage on

graphene on silicon carbide, but the same effect may probably also be induced on other substrates [24].

For Anderson localization even randomly distributed hydrogen clusters cause this localization.

Anderson localization also plays a major role in the electronic behaviour of nanoribbons [25]. By

introducing only a small degree of edge disorder, calculations show a strong Anderson localization at the edges giving rise to an energy gap.

All the above mentioned mechanisms for achieving a bandgap, can be done by hydrogenation. For example calculations have shown that by chemisorbing parallel lines of hydrogen, the graphene sheet is divided into electronically independent stripes [26], which is essentially a GNR. Periodic hydrogenation of the graphene will also lead to a confinement induced bandgap, as will be discussed in the next chapter.

Symmetry breaking of the two sublattices in graphene can also be achieved by hydrogenation. Breaking the symmetry of the sublattices is bond to induce a bandgap opening, as is supported by theoretical

calculations [16]. Third, recent experiments have shown that Anderson Localization also plays a role since a minor bandgap is already seen at low hydrogen coverage [6]. When hydrogenating graphene on any substrate, all these mechanisms probably contribute to the energy gap but it is presently not possible to say anything about the relative contribution from each of the mechanisms.

4.3 Hydrogenated graphite and graphene on SiC

Since the (0001) surface of graphite is very similar to that of graphene, it is interesting to compare the bonding scheme of these two substrates. Hydrogenation of graphite has previously been investigated by this group, both experimentally and theoretically [27]. These studies showed that atomic hydrogen chemisorbs to the surface primarily forming two distinct dimer structures depicted in figure 4.3. The chemisorption caused a puckering of the C-atom out of the plane by 0.35Å. This due to the rehybridization of the carbon from 𝑠𝑝 , which is a flat bonding scheme, to 𝑠𝑝 which has the well-known tetrahedral structure. Monomer structures were not observed at room temperature, suggesting that the binding energy is on the order of thermal energy. DFT calculations confirmed that the monomer is indeed stabilized by the formation of a dimer. Calculations also suggest that the ortho- and para configurations are the most stable dimer structures with a binding energy of 2.51𝑒𝑉 with respect to the undisturbed surface and two infinitely separated hydrogen atoms. Temperature Programmed Desorption (TPD) experiments (data not shown) show that the two dimer structures do not desorb at the same temperature which one might have expected from the calculations – it turns out that the ortho structure is the most stable. This is because of different desorption processes and cannot be attributed to differences in binding energy.

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15

Figure 4.3: a. and b. Ortho- and para dimer respectively. c, d show calculated STM images while e, f are experimental results [27]

Hydrogenation of graphene on SiC substrate was subsequently investigated in this group [5]. The experiments revealed that the dimer structures similar to those on graphite are also formed, but

monomers were observed as well, figure 4.4. The reason for the increased stability of a monomer is that the graphene sheet is geometrically corrugated by the underlying substrate. This means that there are concave and convex regions contrary to graphite where the surface layer is completely flat. In the convex region, the carbon bonding is slightly 𝑠𝑝 -hybridized, which makes it more favourable for hydrogen to adsorb to the surface. Therefore the hydrogen is more readily adsorbed to the surface and thus monomers can be found even at room temperature.

Figure 4.4: Hydrogenation of graphene on SiC(0001). A and B is ortho and para respectively, C is elongated dimers and D is monomers [5]

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5. Graphene on Ir(111) 5.1 Growth and structure

Relatively large areas of high quality epitaxial graphene can be grown on the (111) face of Ir in Ultra High Vacuum (UHV) [3, 6, 28] using the Temperature Programmed Growth (TPG) technique. The TPG process requires a chamber pressure of a hydrocarbon gas (ethylene is primarily used in this group) as carbon material for the growth. Upon annealing the crystal to 250C, ethylene decomposes through pyrolysis [3]

and covers the Ir(111) surface with carbon. The graphitization is initiated upon a flash anneal to around 1200℃. Depending on how high a coverage you are interested in, the procedure can be repeated a number of times. Experience show that around 8 cycles should be sufficient to make a full monolayer of graphene, but the number of cycles can of course be increased since additional cycles would continue to cover bare iridium parts only.

In order to get a high quality graphene layer, the surface needs to be extremely clean, hence why the reaction is carried out at UHV conditions. The clean surface is achieved by sputtering the sample for 10 min with noble gas ions, accelerated through a potential followed by an anneal from 500℃ to 1000℃ in steps of 100℃, thereby reconstructing the surface. This process can be repeated as needed to get a clean crystal.

Graphene on an Ir(111) substrate is very interesting specially because graphene interacts only weakly with the Ir substrate, unlike the case of graphene on Ru(0001) or Ni(111) [29]. This means that graphene still shows the linear dispersion around the Dirac point as in the case of free-standing graphene. Since graphene and Ir(111) do not have the same lattice parameters, a superstructure, also called a Moiré structure, is observed in STM [3, 6]. The reciprocal lattice vector of the resulting Moiré is given by the difference of the vectors forming the superstructure:

𝑘⃗ = 𝑘⃗ − 𝑘⃗ ₍ ₎ (5.1)

Where the vectors are in the reciprocal space: 𝑘 = , where a is the real space lattice constant of a given material and direction. In general, the reciprocal lattice vectors are obtained by Fourier transform of the real space lattice, hence periodic functions can be described by discrete Fourier coefficients. This makes the

reciprocal space a convenient description of periodic lattices.

The principle of super lattice formation is illustrated in figure 5.1.

In this example it can be seen that the length of the resulting reciprocal lattice vector is smaller than original vectors , thus the lattice constant of the superstructure in real space is much larger than that of original lattices

Figure 5.1: Formation of Moiré superstructure from 2 lattices, ref [3]. a) shows the superstructure in the real space, while b) depicts the reciprocal lattice vectors, ref. [3]

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17 Typically it is observed that the [1120] direction of the graphene layer is perfectly aligned with the Moiré superstructure maxima [3]. Because they are parallel, we can describe the structure in scalars rather than vectors, thus simplify the evaluation:

𝑘 _é= 𝑘 − 𝑘 ₍ ₎ (5.2)

And the distance between Moiré maxima can be calculated from knowledge of the constituting lattices:

é= _⋅ = −

( ) (5.3)

The lattice constant is determined to be 𝑑 _é= 25,3Å ± 0,004Å and 𝑑 = 2.452Å ± 0.004Å.

The lattice constant of graphene appears to be slightly compressed compared to bulk graphite by approximately 0.4%. The reason for this compression is that Ir and graphene have different thermal expansion coefficients. As already mentioned before, graphene is grown at high temperatures and a

deformation is expected at lower temperatures.

There are three distinct sites identified within the

superstructure; the atop, the face center cubic (fcc) and the hexagonal close packed (hcp) sites. These sites are illustrated in figure 5.2. In the atop site, the carbon honeycomb sits directly on top of an Ir atom and the carbon atoms cover the threefold coordinated hollow sites. The characteristic of fcc and hcp sites is that every second carbon atom in the hexagon is directly above an Ir atom while the other carbon atoms are positioned above a bridge site. These two sites are very similar and can only be distinguished by the coordination of the Ir atoms in the second layer.

An STM image of the Moiré structure for the graphene-iridium interface is shown in figure 5.3a. It is seen that the moiré induces a major change in the STM image compared to free-standing graphene images. As mentioned above, this change can be attributed to both electronic and geometric corrugation of the graphene sheet but often it is impossible to establish the extent of their contribution to the image. To get a

definite answer to this question, it is necessary to take other techniques in use in order to determine the

contributions. Sun Z. et al. [29]

showed, using both STM and AFM, that the actual geometric corrugation of the moiré is in the order of

35 ± 10𝑝𝑚. From this evidence, and the fact that the contrast can be inverted by changing the tunnelling conditions [3], suggests that the main

Figure 5.2: Crystal sites, graphene on Ir(111), ref [3]

Figure 5.3: a) STM topograph showing the Moiré structure of graphene on Ir. b) DFT calculations of the corrugation of the Moiré. The corners (dashed circles) represent atop-regions, the full circle is hcp and the short dashed circle is ccp [3]

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18 contributor to the contrast is due to LDOS . This hypothesis was confirmed by N’Diaye A. et al. who

calculated the geometric corrugation using density functional theory (DFT) [30]. The results from these calculations were around 0.27Å which is in fair agreement with the experimental AFM results. Comparison of these values to corrugation observed in STM will be made in chapter 7.

5.2 Bandgap opening in hydrogenated graphene on Ir(111) substrate

Bandgap induced by hydrogen functionalization of graphene on Ir(111) substrate is the system being investigated in this bachelor project and will therefore be discussed in some detail in this section.

Hydrogen adsorption on graphene is one of the most promising ways of inducing a bandgap. Hydrogen functionalized areas resemble the structure of graphane, which is insulating. The structure of graphane can be seen in figure 5.4. The bonding scheme is such that the C atoms bind H alternatingly on both sides of graphene sheet. . For the graphene-iridium interface hydrogenation forms similar structures, but here

hydrogen on one side is substituted by Ir, thus forming graphane-like structures. The binding of hydrogen leads to a local rehybridization of the graphene from 𝑠𝑝 to 𝑠𝑝 [31]. This rehybridization leads to breakage of the double bonds and therefore an elimination of the 𝜋- band in the hydrogen functionalized areas. Because of the insulating behaviour in these areas, the electrons are confined to the

hydrogen-free areas and thereby a bandgap is opened. Certain sites are preferred binding sites for the hydrogen at low coverage, see figure 5.5a. The hcp and ccp parts form a graphane like structure

while atop sites generally does not. This is due to the Moiré described in the previous section. The stability of the graphene- hydrogen hybrid can be investigated in temperature controlled desorption, TPD, experiments [32] where the hydrogen desorption peak was around 700K. This proves that hydrogen chemisorption onto the surface is a very stable

configuration, at least for the Ir(111) substrate.

As the hydrogen dose and hence the coverage is increased, the hydrogen pattern start forming ring like structures along the Moiré, figure 5.5b, and at the highest dose these ring structures melt together to form elongated structures, figure 5.5c,d.

Figure 5.5: Hydrogen chemisorped on graphene-Ir(111). Hydrogen dose is increased from left to right, ref. [6]

Figure 5.4 Structure of graphane. Hydrogen is covalently bond to C atoms where the binding scheme is alternating on top and below the sheet, wikipedia

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19 It has been experimentally shown, using Angle-resolved photoemission spectroscopy, ARPES, that

chemisorption of hydrogen opens a bandgap of at least 450𝑚𝑒𝑉 [6]. Since ARPES only probes occupied states, it is not possible to say anything about the position of the conduction band, but the gap opening could be much larger than this lower limit. ARPES data also shows a general broadening of the 𝜋-band. This can be explained by Heisenberg’s Uncertainty principle and the confinement theory. The confinement of electrons leads to a larger uncertainty on the momentum and hence a broadening is expected.

The observed bandgap of ~0,5𝑒𝑉 is sufficiently big to be used in the semiconductor industry but an issue is that the graphene is covalently attached to a metal substrate. The graphene must be lifted from the surface and transferred to an insulating substrate without damaging the graphene or the hydrogen pattern on top.

This poses a significant challenge and a lot of research is going in to growing the graphene directly on insulating substrates like SiC [33, 34].

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Experimental results

6. Hydrogenation of graphite

Highly oriented pyrolytic graphite (HOPG) is often used in STM experiments since it is very easy to achieve atomic resolution on that surface. This is partly because graphite, like graphene, is very inert and one only needs to anneal the crystal to 800𝐾 − 900𝐾 to get a perfectly clean surface. Graphite consists of AB stacked graphene sheets. This AB stacking gives rise to two distinct atoms named 𝛼 and 𝛽 in the unit cell,

see figure 6.1A. Only one of the atoms is observed in an STM image, independent of the polarity of the bias [35].

Theoretical calculations indicate that only the 𝛽 atom is imaged [35]. This is seen in figure 7.2A where the 𝛽 atoms form a triagonal lattice instead of the well-known honeycomb.

Calibration of the STM is needed regularly. This can easily be done, using an HOPG image of atomic resolution, since the crystal is of very high quality and the lattice is well described. Figure 6.2A shows individual 𝛽 carbon atoms on the graphite surface. The table value for the 𝛽-atom distance is 2.13Å, it is with respect to this value the calibration is performed. Figure 6.2B shows a line scan along the direction of one of the lattice vectors. The distance is averaged over several atoms to ensure an accurate reading. By measuring the distance and angles for two directions, two equations are obtained. From these, the two unknown x and y calibrations can be calculated by solving a least squares problem. This measurement can more conveniently be performed by making a Fast Fourier Transform (FFT) of the image and calibrate the image by using inverse lattice vectors. Since the STM program is quite bad at performing FFT, the simple approach was chosen.

The height (z-direction) can be calibrated as well, using an HOPG step edge (not done in this bachelor project). The step edge has a well-defined size of 334.8pm.

Figure 6.2A: 𝟐𝟑Å𝒙𝟐𝟑Å, 𝑰𝒕= 𝟏. 𝟏𝒏𝑨, 𝑽 = 𝟖𝟕𝒎𝑽. High resolution image of HOPG for calibration of the STM. B. Line scan indicated by the black line in the inset. The interatomic distance is measured over several atoms to minimize the experimental error

Figure 6.1: A Graphite unit cell from the top, , B. unit cell, showing the AB stacking, ref [35]

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21 The STM was calibrated using the indicated line 1 as x-direction and line 2 for calculating the y-direction, figure 6.2A. The line scan in 6.2B is, as indicated in the inset, for the x-direction. The measured distance for seven 𝛽 atoms is 20.61Å, giving a distance 𝑑 = 2.94Å.

This is used to calculate an x-factor: 𝑥 − 𝑓𝑎𝑐𝑡𝑜𝑟 = ^{. Å}_{. Å}= 0.72, and the x calibration is complete.

A similar calculation is performed for the y-direction: y − factor = 0.74.

After the calibration we aimed to resolve hydrogen structures on graphene. However, to test the newly mounted hydrogen source, we have decided first to dose hydrogen on HOPG, until graphene was available.

The dose time to achieve hydrogen saturated coverage on graphite has been previously found to be longer than that of graphene on Ir(111). This can be caused by two mechanisms: First, the hydrogen

chemisorption barrier could be higher for graphite. The calculated barrier for graphite is ~0.20𝑒𝑉 [27] but has not yet been calculated for graphene-iridium. Second, abstraction of hydrogen might be different for the two materials.

Figure 6.3 shows the first results obtained from hydrogenation. It is seen in 6.3A, that we have a decent coverage of what we assume is hydrogen atoms. However, figure 6.3B shows line structures, which are not expected to be formed from hydrogen. We suspect that the tungsten capillary might be contaminated. This contaminant could be oxygen but further experiments with atomic oxygen is needed for confirmation. The explanation, we believe, is that the capillary was not properly degassed after installation and that some oxygen was deposited onto the surface.

Figure 6.3. A: 𝟕𝟓𝟎Å𝒙𝟕𝟓𝟎Å, 𝑰𝒕= 𝟎. 𝟑𝟑𝒏𝑨, 𝑽 = 𝟏𝟏𝟑𝟕𝒎𝑽. B: 𝟓𝟎𝟏Å𝒙𝟓𝟎𝟏Å, 𝑰𝒕= 𝟎. 𝟏𝟐𝒏𝑨, 𝑽 = −𝟏𝟒𝟑𝟏𝒎𝑽

The hydrogen source was annealed at the current 𝐼 = 15𝐴 for several hours to degas the capillary properly and reduce the tungsten oxide. The resulting images are shown in figure 6.4. The intensive degassing appears not to have worked. A and B shows large overview images where hydrogen is found in small structures, but some lines are still present. Further degassing resulted in the problem that hydrogen could not be adsorbed at all. Several doses were performed without success. We do not know what has caused this malfunction of the hydrogen source, but it seems like hydrogen is not cracked since the sample

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22 position has been laser aligned and should be in line of sight. We suspect the lacking cracking of hydrogen is due to a leak in the source and hydrogen molecules are not passed through the capillary. Figure 6.4C was an attempt to resolve the different dimer structures but we did not succeed in getting a sufficiently good tip which is needed to distinguish these dimers.

Figure 6.4. A: 𝟏𝟓𝟎𝟎Å𝒙𝟏𝟓𝟎𝟎Å, 𝑰𝒕= 𝟎. 𝟏𝟔𝒏𝑨, 𝑽 = 𝟗𝟖𝟒𝒎𝑽. B: 𝟏𝟓𝟎𝟎Å𝒙𝟏𝟓𝟎𝟎Å, 𝑰𝒕= 𝟎. 𝟓𝒏𝑨, 𝑽 = 𝟗𝟏𝟒𝒎𝑽 . C: 249Åx249Å, 𝑰_𝒕= 𝟎. 𝟎𝟐𝟖𝒏𝑨, 𝑽 = 𝟔𝟐𝒎𝑽

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7. Hydrogenation of graphene on Ir(111)

Because of technical difficulties with preparing the graphene sample for LTSTM setup, I did not succeed in producing my own data on hydrogenated graphene. The following results have kindly been provided by Jill Miwa, Louis Nilsson and Line Kyhl from Liv Hornekær’s group, which have been obtained using Aarhus STM setup.

Graphene was grown by the temperature programmed growth method. After cleaning the crystal, it was exposed to an ethylene chamber pressure of 2.0 ⋅ 10 𝑚𝑏𝑎𝑟 for 100s at room temperature followed by flash annealing to 1320𝐾 at 10𝐾/𝑠 where it was held for 60s to decompose the ethylene. Then the temperature was lowered to 522𝐾 at a rate of 10𝐾/𝑠 and held here for 8 min to ensure that the sample reaches this lower limit. This cycle was repeated seven times to ensure high quality graphene growth.

Images of clean graphene are shown in figure 7.1.

Figure 7.1: A: 𝟒𝟏𝟒å𝒙𝟑𝟒𝟑Å, 𝑰𝒕= 𝟎. 𝟐𝟗𝒏𝑨, 𝑽 = 𝟏𝟎𝟔𝟔𝒎𝑽. B :𝟏𝟗𝟕Å𝒙𝟏𝟔𝟔Å, 𝑰𝒕= 𝟎. 𝟐𝟒𝒏𝑨, 𝑽 = 𝟐. 𝟕𝒎𝑽. C: 𝟓𝟎Å𝒙𝟓𝟎Å, 𝑰𝒕= 𝟎. 𝟓𝟕𝒏𝑨, 𝑽 = 𝟏𝟐. 𝟖𝒎𝑽. Figures depict graphene at different resolutions. The Moiré structure is easily seen, and at highest resolution the graphene honeycomb is seen as well as the Moiré pattern.

Figure 7.1 A. shows that a nice, uniform graphene layer was grown. The layer extends all the way to the step edges and continues on the next terrace. A closer inspection reveals the Moiré structure, but this is seen much clearer on figure 7.1 B with magnification on figure 7.1 C. In the latter figure, in addition to the Moiré supercell also the graphene honeycomb is resolved. Because that there is no AB staking in graphene on Ir(111), all atoms in the honeycomb are imaged as seen in 7.1C. The Moiré unit cell has been marked with white in C. The unit cell length was measured from this image and we obtained a length of 21.1 ± 0.7Å. This is 4Å less than expected, which could indicate that the STM needs to be recalibrated.

The corrugation of the graphene sheet was next investigated. These results are presented in figure 7.2 and 7.3

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Figure 7.2: 𝟏𝟎𝟎Å𝒙𝟏𝟎𝟎Å, 𝑰𝒕= 𝟎. 𝟏𝟕𝒏𝑨, 𝑽 = 𝟏𝟎. 𝟒𝒎𝑽

Figure 7.3: 𝟑𝟎𝟎Å𝒙𝟑𝟎𝟎Å, 𝑰𝒕= 𝟎. 𝟒𝟗𝒏𝑨, 𝑽 = −𝟒𝟒𝟗𝒎𝑽

The STM images have been recorded at different bias. The line scans indicated on the images show the corrugation of the graphene layer. In figure 4.2A, this was measured to approximately 0.8Å while a corrugation of 0.5Å was found in B. Comparison of these results to the values stated in section 5.1, shows that a significant higher corrugation is observed in STM compared to AFM and that the corrugation is very bias dependent. These findings support that electronic corrugation does indeed contribute significantly to the contrast, although it is not clear how well the calibration is, and this might influence the results.

After confirming that a high quality graphene monolayer was grown, hydrogenation experiments were performed. The hydrogen dosing was performed in situ through a small hole in the STM. Therefore longer dose times were expected compared to direct dosing outside the STM. The hydrogen was dosed at a capillary current of 𝐼 = 13.72𝐴. According to calibration this corresponds to a W capillary temperature of 2115K. The chamber pressure varied slightly for different doses, but was usually in the 10 𝑚𝑏𝑎𝑟 range.

The results of hydrogenation are presented in figure 7.4.

Figure 7.4 A: Low coverage 𝟐𝟎𝟎Å𝐱𝟐𝟎𝟎Å, 𝑰𝒕= 𝟎. 𝟏𝟗𝒏𝑨, 𝑽 = −𝟏𝟎𝟑𝟎𝒎𝑽. B: Medium coverage 𝟏𝟒𝟕𝒙𝟏𝟑𝟗Å^𝟐, 𝑰𝒕= 𝟎. 𝟐𝟓𝒏𝑨, 𝑽 = −𝟏𝟎𝟐𝟓𝒎𝑽. C: High coverage 𝟐𝟎𝟎Å𝒙𝟐𝟎𝟎Å, 𝑰𝒕= 𝟎. 𝟏𝟗𝒏𝑨, 𝑽 = −𝟗𝟑𝟗𝒎𝑽

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25 Figure 7.4A shows the hydrogen adsorption after only a few minutes of exposure. The Moiré is still visible but hydrogen clearly starts to adsorb, appearing as dark spots in some areas. A closer inspection of 7.4A also reveals that the hydrogen binding only takes place in the bright regions of the Moiré, corresponding to hcp and fcc sites of moiré. This is consistent with earlier experiments described in section 4.1. Since every other carbon atom of the graphene honeycomb is placed directly above an Ir atom in these areas, these can form a covalent bond to iridium simultaneously with C atoms binding to hydrogen. This binding scheme is still conserved at medium coverage in 7.4B. The hydrogen clusters have formed ring shaped structures and are now more tightly packed, but atop regions still appear to be hydrogen-free. At this coverage, some of the smaller clusters have merged to form the elongated structures in the lower left corner of 7.4B. After approximately 30 min of dosing, saturation was achieved, figure 7.4C. This image shows that all the small clusters have merged to form elongated structures - no more Moiré is seen because of the high coverage.

However one clearly sees that hydrogen still binds preferentially to some regions over others; in between the structures, there are hydrogen-free areas which must correspond to atop sites. Since the study was performed in situ, tip shadowing was observed during the scan. Tip shadowing means that the tip casts a

“shadow” on the surface with the result that hydrogen is not adsorbed in this region. Image 7.4C was recorded after finishing the dosing and at another site. This probably means that the area was not affected by tip shadowing and could account for the hydrogen saturation observed.

To study the contrast of hydrogen clusters, images were made using different sample biases for the high coverage images. This should give an idea of the electronic states in hydrogen clusters, though STS measurements are needed to say anything conclusive about this. Results are presented in figure 7.5.

One clearly sees a big contrast between the images. 7.5A shows holes in the middle of the clusters, which means that there is lower density of states at this sample bias. If the bias is decreased from −734𝑚𝑉 to

−999𝑚𝑉, the holes appear bright i.e. density of states is higher at that voltage difference. This result could indicate that the corresponding regions possess a bandgap. This is however not a conclusion that can be drawn from these data – it could be, that the band structure does not have any states at −734𝑚𝑉 but does at −999𝑚𝑉. This is not tantamount to a bandgap opening. To conclude anything regarding electronic structure, STS measurements are required. These measurements will be undertaken in the future in the low temperature STM.

Figure 7.5: A: 𝟏𝟓𝟎Å𝒙𝟏𝟓𝟎Å, 𝑰𝒕= 𝟎. 𝟏𝟔𝒏𝑨, 𝑽 = −𝟕𝟑𝟒𝒎𝑽. B: 𝟏𝟓𝟎Å𝒙𝟏𝟓𝟎Å, 𝑰𝒕= 𝟎. 𝟏𝟑𝒏𝑨, 𝑽 = −𝟗𝟗𝟗𝒎𝑽

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26 Finally desorption experiments were performed by scanning the same area with high current and low voltage several times. When scanning with these conditions, the tip is, according to equation (2.3), very close to the surface and hydrogen atoms can be removed. The results are summed up in figure 7.6.

Figure 7.6A: 𝟑𝟎𝟎Å𝒙𝟑𝟎𝟎Å, 𝑰𝒕= 𝟎. 𝟒𝟗𝒏𝑨, 𝑽 = −𝟒𝟓𝟎𝒎𝑽. B. Zoom of figure A.

Figure 7.6A shows the sample after scanning at high current and low voltage. The middle region is clearly hydrogen-free as the Moiré is seen. A few hydrogen defects are still seen. The desorbed area was intended to be a square but this was only partially achieved. This could happen due to surface drifts. 7.6B is a zoom of the boundary between hydrogenated and hydrogen-free areas. The boundary is very well-defined except for a few defects. This indicates that it might be possible to make fine structures in a hydrogenated layer simply by scanning with the tip very close to the surface. This could be used as a nano patterning technique where much smaller structures can be “grafted”.

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8. Conclusion

The main part of this project has been used implementing the new Low Temperature experimental setup.

This has involved trouble shooting of the system, installing new equipment and performing noise

measurements of the system. The latter showed that most noise contributions can be assigned to turbo- and ionpumps. Interestingly, a significant reduction was observed by plugging out the power cord to the ion pump controller. Electronics are not expected to contribute much to noise, but this seems not to be the case in this setup.

The implementation of the STM has reached the final stage. Atomic resolution is achieved routinely on graphite both at room temperature and liquid 𝑁 temperature – cooling to liquid He has not been attempted yet but we do not expect that this should be any different from cooling with L𝑁 . The main concern now is the hydrogen source. Our hydrogenation experiments suggest that no hydrogen is adsorbed to the graphite surface after intensive degassing. We believe that molecular hydrogen is not cracked for some reason, but this need to be investigated in further detail and a chamber opening might be necessary.

When this issue has been solved and a high quality graphene sample is available, we would like to perform STS measurements on hydrogenated graphene to determine the physics behind bandgap opening. The long term objective is to measure STS on the graphene-Ir system as well as graphene-Pt and graphene-SiC, since these systems show different hydrogen adsorption schemes.

Data of the graphene-Ir system was produced by others from the group in Aarhus STM. First it was

conformed that a monolayer of graphene had indeed been grown and was of high quality as desired, with a low degree of defects. The moiré was observed with atop regions appearing dark whereas hcp and ccp regions appeared bright, consistent with theory. The size of the moiré was measured and found to differ from theoretical values. This suggests that a calibration of the STM is needed.

Hydrogen experiments were subsequently performed. These showed that hydrogen primarily binds to hcp and ccp regions and atop sites appear to always be hydrogen-free. At medium coverage ring shaped hydrogen start to form and it is seen that some of the clusters start to merge into larger structures. At high coverage elongated structures are found everywhere except for hydrogen-free channels in between.

To investigate the bias dependence of the depiction of hydrogen, images were made at different bias, this showed that at low voltage (-734mV), the hydrogen clusters appeared dark in the middle but this was not seen when the voltage was increased to −1𝑉 . This could indicate that a bandgap has been opened in the corresponding regions, but further investigations with STS is needed before any conclusions regarding this matter can be drawn.

Finally desorption experiments were performed by scanning the same area several times at high current at low voltage. This showed that it is possible to desorb hydrogen in a controlled fashion. This opens for the possibility of making nanopatterning.

Mapping of local band gap opening in hydrogen functionalized graphene