Dynamics of epitaxial graphene growth and adsorptions of cobalt 3

3 Research Methods and Procedures Advances in physics follow advances in instrumentation, once the instruments are made, many people will make the discoveries – Abraham Pais, 1986. Inward Bound: Of Matter and Force in the Physical World At the beginning of this chapter, the ultra-high vacuum (UHV) system used for this research programme is introduced. This system is used for preparation of clean surface, growth of thin films and also in-situ characterisations. Procedures used to prepare clean graphite and silicon carbide surfaces are followed immediately. The details of the growth conditions and deposition techniques i.e. electron beam evaporation are also provided. This is followed by descriptions on main characterisation techniques used i.e. X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and atomic force microscopy (AFM) where the theory, operation and interpretation of data are included. This chapter ends with explanations on sampling procedures used to collect statistics from STM and AFM results. Research Methods and Procedures 3.1 The UHV system Ideally terminated surface will reconstruct (semiconductor) or relax (metals) to reduce the dangling bonds density on the surface. However, this does not assure the surface is completely free of dangling bonds. In air, it will oxidise and their well-ordered surface structure will be destroyed. Even inert surfaces such as graphite and gold are not spared from weak physical adsorption of particles and other gaseous species. These contaminants prevent any efforts in studying the true dynamic reflected by this surface when exposed to vapour of film materials. Hence, it becomes necessary to work under a strictly clean environment during the duration of experiment. Carrying out the experiments under vacuum conditions is one of the ways to accomplish this. Gas kinetic theory (see Appendix for more details) predicts that pressure better than 1.0 x 10-10 mbar (ultra-high vacuum, UHV regime) is needed to prevent contamination up to hours (typical duration for adsorption studies experiment). UHV also permits techniques such as low energy electron diffraction (LEED) and XPS to function without scattering from stray gaseous molecules. At x 10-10 mbar, the mean free path of stray molecules is x 106 m which far exceeded the diameter of any UHV chambers (30 to 60 cm). 3.1.1 Layout of UHV system used All in-situ STM experiments were done at Surface Science Laboratory at Physics Department, National University of Singapore (NUS) while all the in-situ and ex-situ XPS experiments were done at Institute of Materials Research and Engineering (IMRE). 52 Chapter The STM system is designed by Omicron GmbH while the XPS system is designed by VG Scientific. The STM system will be described in greater details since 70% of the work were carried out here. to N2 cylinder Manipulator on bellow Valve Analysis chamber (ANA) Rotary pump Load-lock door Preparation chamber (PREP) Ion pump Turbo pump STM Wobble stick FEL Turbo pump Transfer arm Rotary pump Ion pump Manipulator on bellow Fig. 3.1 Schematic drawing of UHV system at Surface Science Laboratory made by Omicron GmbH that consists of three chambers i.e. fast-entry lock, preparation chamber and analysis chamber. A scanning tunneling microscope is housed here, attached to analysis chamber. Other ports are not drawn. The STM system has three chambers (likewise for the XPS system) as seen in Fig. 3.1 i.e. a fast-entry lock (FEL), a preparation chamber (PREP) and an analysis chamber (ANA). All three chambers are separated by hand operated gate valve. Only one sample can be loaded at a time. FEL serves as the entry and exit point to the UHV system. To introduce sample into the FEL, the FEL is vented using 99.999% nitrogen. Once sample is loaded onto the transfer arm inside FEL, the load-lock door is closed and FEL is immediately pumped down using turbo pump station. After 30 minutes, the pressure will 53 Research Methods and Procedures normally achieve mid 10-9 mbar. The gate valve between FEL and PREP is opened to transfer sample into PREP and is immediately closed back after sample transfer is completed. This loading procedure prevents the PREP from direct exposure to atmosphere. Due to frequent venting, the base pressure of FEL is often at 10-9 mbar, one order higher than PREP. All the growth techniques such as home-made Si source and electron-beam evaporator are mounted on the PREP. Preparation of clean surface and growth of materials are carried out here. The PREP is pumped by a turbo pump that is supported on a rotary pump and also an ion pump. Only ion pump is used most of the time since its operation is cleaner than turbo pump which may experience contamination from oil-based rotary pump. Turbo pump is used when (i) inert gas is needed to be pumped away (ion pump has low efficiency for inert gas), (ii) as an additional support when pressure rises to high 10-8 mbar during sample preparation or (iii) during system baking where ion pump is not suitable for pressure 10-5 and above (operating at these pressures reduces its life expectancy). Due to repeatable sample cleaning and exposure to FEL, the base pressure of PREP (5.0 x 10-10 mbar) is slightly poorer than ANA (1.1 x 10-10 mbar). The ANA is supported by an ion pump only. All the characterisation work is carried out here i.e. STM (or XPS for system at IMRE). For the XPS system, the ANA has a layer of mu-metal to shield electrons emitted from sample from stray magnetic fields other than the electric fields created by the lens system of the XPS. The pressure of PREP and ANA are separated from one another using gate valves installed between them. The gate valve also isolates the ANA from contamination during in-situ cleaning of substrate in PREP. 54 Chapter 3.1.2 Baking procedures To attain 10-10 mbar or better, the chamber must first be baked to increase the outgassing rate (removal of gas molecules) of the chamber wall. Baking is also needed for effective removal of moisture trapped in the system. Baking is usually done every time after the system is vented for installation or repair work. Typical baking conditions are wrapping the whole UHV system and kept at 150oC for more than 24 hours while continually pumped by turbo or ion pumps. When the system is cooled after baking completed, high vacuum is generated due to decrease of outgassing rate. The base pressure of the system depends on the outgassing rate of the system and pumping efficiency of the pumps. Since most of the UHV components fitted with filament operate at temperature higher than 150oC, a separate degassing procedure is needed. This is normally carried out when the chamber is still hot from baking to minimise recontamination from gaseous molecules onto the chamber wall. Degas often starts with components with bigger filament such as titanium sublimation pump (TSP) and ion gauges since they release more residual gaseous and followed by smaller filament components such as X-ray gun and manipulators. Degas of growth sources such as silicon (Si) and e-beam evaporator are normally done as the last step. 3.1.3 Sample transfer, heating and current feedthroughs As seen from Fig. 3.1, the sample is maneuvered between the chambers using transfer arms and manipulators. The transfer arm provides linear movement in one direction parallel to it and also rotation, while the manipulator provides linear translation 55 Research Methods and Procedures in all three x, y, z directions and also rotation. A sample stage is mounted on this manipulator for preparation of clean surface and growth studies. Current and voltage are applied to the stage via feedthrough mounted on the atmospheric side. There are two ways that a sample can be heated as shown in Fig. 3.2a. The first method is known as direct heating (DH) where current flows through the sample and can be varied to control temperature. This heating is suitable for doped semiconducting materials. The second method is known as resistive heating (RH) where current flows through a coiled filament sit directly underneath the sample. This filament, wrapped by ceramic beads, forms the base of the sample stage. The heat from the filament is used to heat up the sample. This method is suitable for metals or insulators. The temperature is read using infrared pyrometer with emissivity set to 0.68. Figure 3.2b shows the power-temperature calibration for silicon carbide, 6H-SiC(0001) heated using DH. (b) 1400 o Temperature ( C) (a) Sample holder Contact brush Contact bar DH RH 1200 1000 800 600 400 200 0 10 15 20 25 30 35 Power (W) Fig. 3.2 (a) Schematic of direct (DH) and resistive heating (RH) facilities for sample preparation. The sample slot and actual position of sample holder (dotted line) are also shown. For DH, the current flows via terminal and 3. This circuit is completed if the sample is present where the sample holder’s contact bar will be in physical contact with manipulator’s contact brush. Without the sample, the circuit becomes open. For RH, current flows via terminal and 3. Filament coil below the sample is used to heat up the sample; and (b) shows the power-temperature calibration of silicon carbide (SiC) heated using DH. The emissivity of pyrometer is set to 0.68. 56 Chapter 3.2 Preparation of clean surfaces 3.2.1 Preparation of highly oriented pyrolytic graphite Graphite substrates in the form of highly oriented pyrolytic graphite (HOPG) were purchased from Goodfellow Cambridge Limited. Similar to graphite with AB (Bernel) stacking, these substrates are polycrystalline with average domain size of 1m diameter. Each substrate was cut into 10mm x 5mm x 1mm to fit onto a sample holder. From literature, there are two surface cleaning methods which are also compared in this work. The first method involves cleaning the HOPG ex-situ [1,2]. Since the graphite sheets along the z-direction are held together by weak physical forces, a fresh surface can be easily generated by peeling off first few top layers using adhesive tape. This substrate was then immediately mounted onto a sample holder and loaded into the FEL and PREP. Without any further treatment, this substrate was used directly for experiment. The second method repeats the ex-situ preparation method. However, after transferred into the PREP, the substrate was also annealed [3,4]. Due to low resistivity nature of HOPG (graphite is semimetal), direct heating is not suitable to achieve high temperature. Hence, the substrate was prepared using resistive heating as described earlier. The substrate was mounted onto a sample holder with a window that helps minimised heat blocking (see Fig. 3.3). As the filament is heated up, the temperature of substrate that sits directly above it also increases (Fig. 3.3d). The increment of temperature was carefully controlled to maintain pressure below low 10-8 mbar. To clean HOPG, the substrate was held at 600oC until pressure recovers near the base pressure, a sign that outgassing from substrate is completed. This takes approximately 60 to 90 minutes. 57 Research Methods and Procedures HOPG x4 (a) (d) (b) (c) HOPG ~2mm Glowing filament Fig. 3.3 Resistive heating (RH) sample holder. (a) Parts of resistive heating sample holder consist of one sample plate with window, four sets of stud and nut and two pieces of tantalum (Ta) metal strip. The window at the center allows direct heat transfer from underneath to the sample, (b) a sample is placed on top of sample plate, (c) a sample is placed at the center of the window and secured by a pair of screw-tighten Ta metal strips and (d) cross-section of the heating stage with a RH sample holder mounted with a HOPG substrate. 3.2.2 Preparation of (63x63)R30o/6H-SiC(0001) 6H-SiC(0001) substrate (on-axis, produced by CREE Inc.) was used to prepare Crich (63x63)R30o phase (hereafter 63). The wafer was diced into 10 mm x mm dice size to fit the sample holder. Prior loading to UHV chamber, the wafer was submerged inside isopropanol (IPA) solution and sonicated for 15 minutes and followed by acetone by another 15 minutes. Due to the wide band gap nature of SiC, very high voltage was needed to drive current through the substrate and the voltage is above the limit of the power supplies in our laboratory. Instead, a doped Si substrate was stacked underneath the SiC substrate and current was passed through it to heat up the SiC. Prior to that, the Si 58 Chapter substrate was cleaned in similar manner as SiC and together with SiC, they were mounted onto a DH sample holder (see Fig. 3.4). Sample mounting procedures and descriptions of current flow are provided in Fig. 3.4. (a) Ta strip (b) (c) Connector between contact bar and strip Mo base plate I Mo washer Mo threaded stud Ceramic washer Contact bar SiC/Si Ceramic top plate Fig. 3.4 (a) A blank direct heating sample holder. The tantalum (Ta) strip on the left is in contact with the molybdenum (Mo) base plate via Mo studs and washers underneath. The Ta strip on the right and the contact bar are insulated from the Mo base plate using ceramic washers. The two Ta strips will only be in electrically contact with each other if a sample is put across as shown in (b). Current, I flows to the metal contact bar, through the sample, to the Ta strip on the left and then the Mo base plate; and (c) ceramic top plate is fixed for further insulation and to secure the sample onto the holder before loading into FEL. The substrates were degassed for approximately hours at 500oC as shown in Fig. 3.5, after which slow heating process took place by increasing the current slowly while maintaining pressure below mid 10-9 mbar. To remove surface native oxides, Si deposition was carried out at 850oC for 10 minutes where the oxides will desorb as volatile SiOx species. This was followed by annealing without Si flux for another 15 minutes to allow surface gains stability and flashes off excess Si. Next, the temperature was gradually increased systematically to 950, 1050 and 1150oC (20 minutes at each temperature). At these temperatures Si starts to desorb from the surface and generates other phases i.e. Si-rich (3x3)R30o (between 950oC and 1050oC) and C-rich (63x63)R30o (1150oC) [5,6,7,8]. For preparation of C-rich 63 surface, the annealing 59 Research Methods and Procedures process completes at temperatures between 1100oC and 1150oC. The surface was brought to cool gradually by decreasing the current slowly. 1400 1100oC- 1150oC o Temperature ( C) 1200 Si deposition 1000 800 Heating Cooling 600 400 Degas 200 0 10 12 14 12 14 Time (hour) Fig. 3.5 Procedures for preparation of clean 63 on 6H-SiC(0001). 3.2.3 Preparation of graphene on 6H-SiC(0001) 1400 1300oC o Temperature ( C) 1200 Si deposition 1000 800 Heating 600 400 Degas 200 0 10 Time (hour) Fig. 3.6 Procedures for preparation of graphene on 6H-SiC(0001). 60 Chapter (vii) Quantification and Growth Modes Quantitative XPS has been vastly used to probe the growth mode of thin film [22,23,24]. A bare substrate’s signal intensity, Is as shown in Fig. 3.12a is given in Eq. (3.5b) at  = 0o. With a film, f of thickness t deposited above it as shown in Fig. 3.12b, the intensity, Is is attenuated as follows: I s  I s exp   t  f ( KEs )  (3.6) where f(KEs) is the modified IMFP of substrate photoelectrons with kinetic energy, KEs moving in the substrate and through the film. Similarly, the film signal intensity, If can be derived from common XPS formalism as:   t I f  KT ( KE f ) Lij ( ) ij n f  f ( KE f ) 1  exp      f ( KE f ) cos        (3.7a) I f  I f 1  exp   t  ( KE f )   , for  = 0o (3.7b) where f(KEf) is the IMFP of film photoelectron with kinetic energy, KEf moving in the film, nf is the density of atoms packed in the film and I f is the intensity for an infinitely thick film. (a) bare substrate (b) substrate with a thin film hv  Is= KT(KE)Lij(γ)σijniλ(KE) Is= Is exp[-t/λf(KEs)] If = I f  I f [1  exp( t  ( KE f )] Is()= KT(KE)Lij(γ)σijniλ(KE)cos t Fig. 3.12 Intensity of substrate, s and film, f (a) before and (b) after growth of film with thickness t. 77 Research Methods and Procedures Based on the attenuation of Is and increment of If, each of the three growth modes (FM, VW and SK as discussed in Chapter 2) has a distinct I versus coverage, plot. With FM growth (layer-by-layer), the I- plot has unique segments of linear plot with decreasing gradient as seen in Fig. 3.13a. Each segment represents completion of one monolayer. Once a monolayer is completed and the next monolayer begins, the gradient of Is decreases. The decline of gradient is due to increase of attenuation to the escaping electrons since the thickness, t increases from nd to (n+1)d for each new monolayer as given by Eq. (3.6). d is the interlayer spacing. At the same time, increment of If also decreases due to attenuation of first layer electrons by the newly formed second layer film (Eq. (3.7b)). Hence, the I from substrate and film always complimentary to each other (see Fig. 3.13a). For VW growth (3D islands), a plot with single gradient across all the coverage is observed (see Fig. 3.13b) since the thickness of 3D growth is random and t no longer increases in an orderly manner. Finally for SK growth (layer followed by 3dimensional), the system initially grows layer-by-layer and each time when new monolayer starts, the gradient change, behaving like FM mode. Once the system switches to 3D growth, the gradient will no longer change regardless of the film thickness, behaving like VW mode (see Fig. 3.13c). XPS intensity (a) FM (layer) (b) VW (3D) Substrate (c) SK (layer + 3D) Substrate Substrate Film Film Film Coverage ML Fig. 3.13 The shape of XPS intensity, I vs. coverage,  according to the growth mode. The substrate and film signals are complimentary to each other. The “knees” formed in the curve of FM and SK modes signify completion of one monolayer (ML). 78 Chapter 3.4.2 Photoemission Spectroscopy (PES) from Synchrotron Radiation The photoemission spectroscopy (PES) experiments from synchrotron radiation were also used to study the chemical states of clean surfaces of (63x63)R30o and graphene/6H-SiC(0001) and also adsorption of Co on these surfaces at room temperature. All PES experiments were conducted at SINS (Surface, Interface and Nanostructure Science) beamline located at Singapore Synchrotron Light Source (SSLS), Singapore. All samples were prepared in-situ. Two photon energy were used i.e. 60 eV for valence band measurements and 350 eV for high-resolution core-level measurements. The measurements were also recorded at two different take-off angles i.e. 0o and 50o to elucidate surface- and bulk-related structures. The concept of binding energy measurements in PES is similar to XPS where the core-level or valence electrons are excited by radiation of monochromatic light source and the kinetic energy of ejected photoelectron is measured by energy analyser. However, there are several advantages using PES from synchrotron radiation i.e. (i) tunable photon energy. This allows better surface sensitive than XPS where minimal mean free path of photoelectrons can be selected, and (ii) high energy resolution and the high photon flux of the synchrotron radiation increase the sensitivity and detection limit. Synchrotron radiation (electromagnetic waves) is emitted when particles (such as electrons) travelling near the speed of light are forced by a series of bending magnets to accelerate in a circular path. At each of these magnets, the electric field of electrons is changed and travelling away from the electrons in a transverse motion at velocity of light i.e. synchrotron radiation is produced. At the SSLS, electrons inside a 10.8-meterscircumference storage ring are accelerated to 700 MeV through a magnetic field of 4.5 79 Research Methods and Procedures Tesla. Synchrotron radiation with characteristic photon energy of 1.47 keV and characteristic wavelength of 0.845 nm are generated. The light is most brilliant in the soft X-ray and adjacent harder X-ray range. (a) Top view of SINS beamline Horizontal focusing mirror (plane-elliptical) Vertical focusing mirror (spherical) Gratings (spherical) Entrance slit Storage ring (700 MeV) Exit slit Focusing mirror (toroidal) Experimental station (b) Gratings and energy range 130 l/mm (50 – 110 eV) 300 l/mm (110 – 220 eV) 600 l/mm (220 – 440 eV) 1200 l/mm (440 – 1200 eV) Fig. 3.14 (a) Schematic of SINS beamline at Singapore Synchrotron Light Source (SSLS). Figure adapted from http://ssls.nus.edu.sg/facility/sins.html, and (b) four gratings are used to cover energy range of 50 to 1200eV. After leaving the storage ring, the beam of light travels through a system of steel tubes for “conditioning” before it reaches the experimental station. The schematic layout of SINS beamline is given in Fig. 3.14a. Mirrors are used to collimate the beam while slits are used to control the physical width and angular spread of the beam. At SINS beamline, photon energy ranges from 50 eV to 1200 eV are available. The energy range is obtained by four spherical gratings with different line density as given in Fig. 3.14b. At a resolving power of 2000 a photon flux of about 1010 photons/s/100mA is delivered into a spot size of (1.5 x 0.2) mm2 (FWHM). The experimental station consists of one multichamber Omicron Multiprobe vacuum system with base pressure better than x 10-10 80 Chapter mbar. Omicron EA 125 hemisphere energy analyzer is used. This analyser consists of channeltron electron multipliers. The energy resolution for XPS (at 50 eV pass energy) is better than 0.6 eV (Ag 3d5/2) and for UPS (at eV pass energy), the resolution is better than 30 meV. 3.4.3 Scanning Tunneling Microscopy (STM) A variable-temperature scanning tunneling microscope (VT-STM) manufactured by Omicron NanoTechnology GmbH was used to acquire images up to atomic resolution. This STM is equipped with cooling and heating facilities to cover temperature range from 25K to 1400K. The imaging can be done in two modes i.e. constant height and constant current mode (which will be explained in the following paragraph). In this work, only constant current mode was used. All STM images were acquired at room temperature using self-made tungsten (W) tip. (i) Basic Principles Scanning tunneling microscopy (STM) was invented by Gert Binnig and Heinrich Rohrer in 1981 based on the quantum tunneling concept [ 25 ] and this technique successfully obtained the first atomic resolution [26]. Today this imaging technique is proven pivotal for enormous development within physics which includes studies of surface structure [27,28], surface dynamics [29,30] and dynamics of clusters on surfaces [31,32]. 81 Research Methods and Procedures STM imaging is based on electrons tunneling between a surface and a tip that rastering this surface. Although they are not in touch, electrons are allowed to cross the gap between tip and surface according to a quantum mechanical process known as tunneling [33]. In this process electrons with insufficient energy E are allow to surmount a barrier with potential >E despite forbidden classically. This occurs as the electron wavefunction does not terminate abruptly at the wall of the barrier but decrease exponentially outside the wall. Hence there is some probability of finding electron there. The probability for electron tunneling, P(d) is defined as*:  4 d  P (d )  exp   2me  h   (3.8) As shown, P(d) decreases exponentially with the width of the barrier, which in this case equivalent to the gap between surface and tip, d and square root of the mass of the electron, me. h is the Planck’s constant and  is the barrier’s height or equivalent to the work function of surface. Equation (3.8) is not exclusively for electrons but true to any form of particles. However, P(d) is negligible for heavier particles such as diatomic molecules but significant for very light particles such as electrons. Due to exponential decrease of P(d) with d, the tip and sample surface need to be in very close proximity (2 to atomic diameters) so that the wavefunction from surface (tip) cross the gap and extended to the region of the tip (surface). Once this condition is fulfilled, under an applied bias V, the net flow of tunneling current, IT is given as:  4 d  2me  IT (d )  V exp   h   (3.9) As shown IT is analogous to P(d) where it is also an exponential function of d. Since most  are around to eV, under a constant bias IT changes by nearly an order for every Å. * The solutions of Schrödinger’s equation inside a 1D rectangular barrier have the form ψ=exp(κd) where   2m /  . The tunneling current decays exponentially with barrier width, d as IT  e 2 d . 82 Chapter Hence, STM has excellent height resolution. Owing to this, atomically sharp tip is used and only the most protruding part of the tip that locates nearest to the surface acts as a probe. Depending on the bias applied between tip and surface, the tip can probe either the filled or empty states of the surface [34]. Figure 3.15 shows before and after a bias is applied between tip and surface that acts as two electrodes. If the tip is biased negatively, electrons tunnel from the tip to the empty electronic states of the sample surface. If the tip is biased positively, electrons tunnel from the filled states of the surface to the tip. The density of current transmitted via the tunneling not only depends on the gap but also on the density of states of the sample surface. Hence, corrugation from STM image is not just the physical feature of the surface but mixed with mapping of the surface density of states. (b) filled states (a) ground states Evac t s Evac EF,t EF,s Evac EF,s z surface tip (c) empty states surface Evac Evac Evac V IT tip EF,t V EF,s surface EF,t IT tip Fig. 3.15 (a) Ground states of two electrodes i.e. sample surface, s and probing tip, t with their vaccum level, Evac aligned. The Fermi level of surface, EF,s and tip, EF,t are also shown and they define their work function, s and t respectively. When these two electrodes are brought to very close proximity, the wavefunction (red line) of their Fermi electrons extended to the region of the other electrode; (b) when positive bias is applied to the tip, electrons tunnel from the filled states of the surface to the tip and (c) when negative bias is applied to the tip, electrons tunnel from the tip to the empty states of the surface. 83 Research Methods and Procedures (ii) Instrumentation and acquisition procedures Surface Tunneling electrons Tip Sample z Tip holder Scanner (piezo): maximum Z: 2μm x y Iset STM controller Tunneling current amplifier XY Piezo control maximum X,Y: 15μm Coarse control (X:± 5mm, Y:± 5mm, Z: 10mm) Coarse control box Fig. 3.16 Schematic of set-up for a scanning tunneling microscope. An atomically sharp tip (inset) is used to probe the surface. A software-controlled piezoelectric is used control the tip to raster the surface laterally. The z information is adjusted via feedback control. The design of Omicron GmbH’s VT-STM is illustrated in Fig. 3.16 and based on invention of Binnig et al. [25,35]. A sharp W tip is used as a probe and its position (lateral x and y, and vertical, z) is adjustable via a coarse control motion that enable user to select a sampling area on the surface. Once an area is selected, the tip is moved as near as possible to the surface, also using the coarse control. A set of values i.e. tunneling voltage, tunneling current, feedback loop gain, scan angle and scan size is pre-set by user to stimulate tunneling for tip approach process. In this tip approach process, auto-approach is used to bring the tip closer the sample where the tip moves in small steps until current is detected. The small steps are controlled by coarse control and each step involves a z- 84 Chapter element piezoelectric† extends and retracts to sense any current flow. This piezo resides inside a scanner tube that holds the tip holder and tip as seen in Fig. 3.16. The small steps movement is continued until current is detected where the auto-approach is immediately stopped. At this position, this tip is close enough to the surface that electrons can tunnel quantum mechanically through the vacuum barrier separating the tip and surface. The scanning takes off when the tip scans the surface in rows (x coordinate) and columns (y coordinate) while the tip-sample separation, d is recorded. To achieve atomic resolution, the tip is affixed to a scanner tube consists of x-, y- and z-element piezoelectric that each moves in extremely small distances when small voltages are applied to it. The amount of voltages for each distance is pre-calibrated and stored inside a software that control the voltage generators. Because the tip movements are so small that precise control of the tip and sample positions is required, the instrument is limited to vibrations from surrounding that much less than an angstrom. The STM imaging can be operated in two modes i.e. constant current mode and constant height mode. In this work, only constant current mode is used. In constant current mode, the IT is kept constant during the lateral scanning. IT change exponentially with d and d in turn varies with the surface contours. A feedback loop is used to detect the current displacement and it immediately adjusts the vertical position of the tip via zelement piezoelectric to keep the current constant. The height adjustment at each x,y coordinate is recorded and this maps the surface contours. In constant height mode, the vertical position, z of tip is held constant during scanning while IT(d) is recorded. In this mode surface contours are reflected by the fluctuations of current rather than in the tip † Piezoelectric is a type of ceramic that expands and shrinks under applied voltage. 85 Research Methods and Procedures height [36]. However, this mode is only practical for extremely flat surface. Since IT is proportional to the surface density of states, the first method maps constant density of states contours and the second method maps the actual density of states. (iii) Scanner calibration The x, y and z scanners are normally re-calibrated after each system baking. Established surface structures such as (7x7) unit cell of reconstructed Si(111) surface is used for x, y-scale calibration where any discrepancies from the size of the unit cell as depicted in Fig. 3.17a is corrected by putting in correction factors into the STM userinterface software. For the z-scale calibration, atomic steps of graphite are used where they are often n*3.34 Å, where n = 1, 2, 3… and 3.34 Å is the interlayer spacing between graphite sheets (Fig. 3.17b). (a) (b) 26.9 Å (7x7) 6.71 Å 46.6 Å 6.7 Å nm 100 nm Fig. 3.17 (a) STM image of (7x7) surface of Si(111). Line profiles across several (7x7) unit cells along the x and y direction are used to obtain the average size of (7x7) and any deviation from the actual size of (7x7) as shown in inset will be corrected and saved inside the software, and (b) similarly, atomic steps of graphite is used to calibrate the z-scanner. 86 Chapter (iv) Sampling procedures All STM (and AFM) images are processed offline using WSxM software [37]. This software was used to extract step heights, dimensions of structure (diameter, height) via cross-section analysis, surface roughness, cluster density via flooding method and self-correlation. For all in-situ STM experiments, the clean surfaces were checked with STM prior to deposition. The surfaces were checked with batches of W tips and only the most stable, consistent and sharpest tips were used. These tips are able to resolve the atomic structure of the clean surfaces, for e.g. the honeycomb lattice of graphite and graphene. In addition, they were tested by scanning various atomic steps found on the silicon carbide (SiC) surface. An example is shown in Fig. 3.18a where the lateral displacements of upper and lower terrace of a 7.6 Å step was found to be in a range between Å and Å, which are very close to the real-space lateral displacements of three BL SiC (4.5 Å to 6.2 Å ). Step height measurements in Chapter for various carbon surfaces were collected from line profile similar to Fig. 3.18a(i). Approximately 300 steps were collected for each surface. Each data point is an average value from a few line profiles drawn across the same step. Error bar of 0.2 Å is obtained from standard deviation. For cluster width and height measurements, STM imaging under different biases were carried out to investigate bias-dependant of width and height, if any. Threedimensional Co clusters are formed on graphite, 63 and graphene at room temperature. Under similar experimental conditions, Co clusters with same geometrical shape and 87 Research Methods and Procedures similar aspect ratio have been consistently reproduced for each surface. Line profiles are used to extract the width and height of Co clusters and they are found to be insensitive with tunneling bias for Co deposited on all three types of surfaces. An example is shown in Fig. 3.18b for Co/graphene. No corrections are done to the cluster height. The widths of the clusters, however, are found subjected to tip curvature effect. They are slightly enlarged when the initial edges of the Co clusters did not change smoothly as the tip scans from bare surface to the edge of these Co clusters. All the widths presented in this thesis have been corrected for this tip curvature effect. An example is shown in Fig. 3.18c where the line profile is fitted to a cross-section of a dome and accepted the fitted width as the width of the cluster. The measured height and amended width are used to compute the volume of every Co clusters. Samplings of Co clusters are taken from area away from steps. About 200-300 clusters per coverage are collected. Error bars are derived from standard deviation. For cluster density of a given coverage, an average is obtained from several STM images sampled from different area. All the cluster densities in this thesis are measured manually. Error bar are derived from standard deviation. 88 Chapter (a) (i) Line profile of a 7.6 Å step found on 63 surface (from STM) 7Å z (Å) 5Å 7.6 Å 50 100 150 x (nm) 200 250 300 (ii) Atomic structure of SiC with three bilayers (BL) step height equivalent to 7.56 Å 4.5 Å  0001   6.2 Å 1100    1120    (b) 7.56 Å (b) -0.8V (a) -0.5V 7.9Å (h), 3.6nm (w) (c) -1.3V 7.9Å (h), 3.7nm (w) 7.6Å (h), 3.6nm (w) Co (c) vertical (nm) nm Line profile from STM width 0 10 11 lateral (nm) 12 13 14 15 16 17 18 19 Fig. 3.18 (a) (i) Line profile of a 7.6 Å step obtained from a 63 surface. Lateral displacements between upper and lower terrace are provided, which are very similar to lateral displacements in real–space atomic structure of silicon carbide (SiC) of similar step height as shown in (ii); (b) bias-dependant STM imaging of Co clusters on graphene/6H-SiC(0001). The imaging under other biases was not stable. Both height and width of the Co cluster are insensitive with tunneling bias; and (c) cross-section of a cluster that shows the cluster height and width. The cluster width is corrected for tip-shape effect by fitting the profile to an appropriate geometry (dome). 89 Research Methods and Procedures 3.4.4 Atomic Force Microscopy (AFM) (i) Basic Principles Similar to operation of STM, the atomic force microscopy (AFM) also depends on the interaction between the surface and tip scanning above it. However instead of tunneling current, the physical force (van der Waals force or dipole-dipole interaction, capillary forces and electrostatic forces) between tip and surface is employed. A cantilever that holds a tip is externally oscillated at or close to its fundamental resonance frequency or a harmonic. The oscillation amplitude, phase and resonance frequency are modified by tip-sample interaction forces; these changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics. Improving the sensitivity such as frequency modulation and using stiffer probe allows AFM to provide true atomic resolution in UHV conditions [38]. (ii) Instrumentation and acquisition procedures In this work, all the AFM images collected are obtained using tapping mode, unless stated otherwise. The AFM unit used here is manufactured by Digital Instrument model Nanoscope III Dimension. The physical set-up of an AFM is as illustrated as in Fig. 3.19. The sample is brought to as close as possible to the cantilever using coarse control motor. With the help of a camera, the focus point for the tip is fine tune using the software-operated fine-motion control and the tip position is adjusted to the center of the viewing camera via mechanical knobs. Once the tip is focused and located, the focus 90 Chapter point for the surface is fine tune using another software operated fine-motion control. This followed by selecting the desired sampling area using the coarse-control motor. These steps ensure that the tip is probing the selected sampling area. Next, the laser is aligned onto the cantilever by observing the intensity counter. Maximum intensity will be obtained if the laser beam hits the cantilever and reflected by the mirror onto the center of detector i.e. photodiode. The photodiode is divided into sectors. The photodiode can be re-positioned such that the reflected laser beam will hit the center of the photodiode as shown in Fig. 3.19. Aluminium coated cantilever is used. This cantilever is more reflective than the conventional Si cantilever, hence improves the signal to noise ratio. Once the laser is aligned, the cantilever is auto-tune to find the resonance frequency which often falls between 250 to 290 kHz. The auto-tuning also help find the best amplitude setpoint that set as reference for a feedback looping. mirror photodiode laser beam Cantilever with oscillating tip sample X-Y-Z scanner piezo tube Fig. 3.19 Typical set-up of a atomic force microscope designed by Digital Instrument’s Nanoscope III Dimension. See text for description of working procedures. 91 Research Methods and Procedures Similar to STM, auto-approach is used to bring tip and surface to working proximity and the movement is facilitated by piezoelectric elements. However, for Nanoscope III scanner “J”, the sample is mounted on the piezo tube and not the cantilever. Voltages are applied to move the sample in x- and y- coordinate during scanning. In amplitude modulation, there are three operational modes i.e. contact mode, non-contact mode and tapping mode (intermittent contact). As mentioned earlier, tapping mode is used in this work where cantilever oscillates up and down near its resonance frequency. When the tip comes close to the surface, the amplitude of this oscillation decreases due to the interaction of forces acting on the cantilever. As the cantilever scanning the surface in lateral (x,y) coordinates, the height information, z comes from height adjustment made by a feedback loop to keep the amplitude set-point constant. This tapping mode is an improvement on conventional contact mode imaging where the cantilever drags across the surface at constant force during scanning and may cause surface damage. Using tapping mode, the conformation of molecules was found to remain unchanged for hours [39]. In non-contact mode, the tip is in proximity with the surface where the short-range forces are just detectable. However most surfaces develop an attractive nature meniscus layer with atmospheric air, hence preventing the tip from sticking to the surface becomes a major hurdle for this mode. Tapping mode was developed to overcome this issue [40]. 92 [...]... volume of 1 ML (i.e area A*0.2nm) and deposition time t The 0.2nm used here is the interlayer spacing of hcp Co Since Co does not form hcp structures but 3D clusters on graphite, this method also overestimate the Co coverage 64 Chapter 3 3 .3. 2 Growth of Co on HOPG, (63x6 3) R30o and graphene Co was evaporated on HOPG, 6 3 and graphene using Omicron’s e-beam evaporator Various fluxes from 10 -3 to 10-2... attenuation of first layer electrons by the newly formed second layer film (Eq (3. 7b)) Hence, the I from substrate and film always complimentary to each other (see Fig 3. 13a) For VW growth (3D islands), a plot with single gradient across all the coverage is observed (see Fig 3. 13b) since the thickness of 3D growth is random and t no longer increases in an orderly manner Finally for SK growth (layer... where they are often n *3. 34 Å, where n = 1, 2, 3 and 3. 34 Å is the interlayer spacing between graphite sheets (Fig 3. 17b) (a) (b) 26.9 Å (7x7) 6.71 Å 46.6 Å 6.7 Å 5 nm 100 nm Fig 3. 17 (a) STM image of (7x7) surface of Si(111) Line profiles across several (7x7) unit cells along the x and y direction are used to obtain the average size of (7x7) and any deviation from the actual size of (7x7) as shown... KT(KE)Lij(γ)σijniλ(KE)cos t Fig 3. 12 Intensity of substrate, s and film, f (a) before and (b) after growth of film with thickness t 77 Research Methods and Procedures Based on the attenuation of Is and increment of If, each of the three growth modes (FM, VW and SK as discussed in Chapter 2) has a distinct I versus coverage, plot With FM growth (layer-by-layer), the I- plot has unique segments of linear plot with... states of clean surfaces of (63x6 3) R30o and graphene/ 6H-SiC(0001) and also adsorption of Co on these surfaces at room temperature All PES experiments were conducted at SINS (Surface, Interface and Nanostructure Science) beamline located at Singapore Synchrotron Light Source (SSLS), Singapore All samples were prepared in-situ Two photon energy were used i.e 60 eV for valence band measurements and 35 0... of this element The values are typically taken from the Scofield library [21]; ni (z)= concentration of element i; λ(KE) = IMFP as defined by Eq (3. 3) and; θ = take-off angle of the photoelectron with respect to surface normal θ Integration of Eq (3. 4) gives: I ij  KT ( KE ) Lij ( ) ij ni  ( KE ) cos  (3. 5a)  I ij  I ij  KT ( KE ) Lij ( ) ij ni  ( KE ) , at  = 0o (3. 5b) When using Eq (3. 5),... (layer + 3D) Substrate Substrate Film Film Film 0 1 3 2 Coverage ML 0 1 2 3 0 1 2 3 Fig 3. 13 The shape of XPS intensity, I vs coverage,  according to the growth mode The substrate and film signals are complimentary to each other The “knees” formed in the curve of FM and SK modes signify completion of one monolayer (ML) 78 Chapter 3 3.4.2 Photoemission Spectroscopy (PES) from Synchrotron Radiation The...Chapter 3 To prepare epitaxial graphene on 6H-SiC(0001), the initial procedures described in Section 3. 2.2 to prepare 6 3 were repeated and followed by annealing to temperatures above 130 0oC (see Fig 3. 6) The coverage of the graphene monolayer can be control using time and temperature In order to follow the graphitisation process systematically,... escaped without inelastic collision and (ii) spectra background contributed from inelastic collision where the kinetic energy of photoelectrons lost at varying amount Only the main peaks reflect the density of states (DOS) of the material (see Fig 3. 9) and can be used to interpret the XPS data 66 Chapter 3 hv M-shell 3d 3p L-shell 3s 2s 2p K-shell 1s Energy (E) n(E) Fig 3. 9 An electron is ejected from... at one time Pass energy 40 eV 20 eV 5 eV 2 eV 38 2 37 7 37 2 36 7 Binding energy (eV) 36 2 Fig 3. 11 The resolution of Ag 3d spectra acquired under a series of pass energies Spectra are normalised for better comparison Figure adapted from Ref [14] Table 3. 1 also shows the importance for the analyser’s work function,  to be accurately determined The calibration of binding energy, BE scale is also needed to . points; and (b) calibration of power with flux extracted in a same manner as in (a) for different power settings. (a) (b) Chapter 3 65 3. 3.2 Growth of Co on HOPG, (63x6 3) R30 o and graphene. from the surface and generates other phases i.e. Si-rich (3x 3) R30 o (between 950 o C and 1050 o C) and C-rich (63x6 3) R30 o (1150 o C) [5,6,7,8]. For preparation of C-rich 6 3 surface, the. Methods and Procedures 58 3. 2.2 Preparation of (63x6 3) R30 o /6H-SiC(0001) 6H-SiC(0001) substrate (on-axis, produced by CREE Inc.) was used to prepare C- rich (63x6 3) R30 o phase