Atomic force microscopy (AFM)

Atomic force microscopy (AFM) is one of the most powerful tools for analysing and imaging the surface topography and roughness of the material surface. It differs from optical and electron microscopes which 'look' at the sample surface; the AFM works by scanning probe microscope by 'feeling' the sample surface [138]. AFM operates based on the principle of measuring the deflection of a sharp force-sensing tip which is attached to a flexible cantilever with a specific spring constant as it probes the material’s surface. The sharp tip is commonly made from silicon or silicon nitride [139]. The changes of cantilever deflection are monitored by a four segment-photodiode detector. The computer processes the electrical differential signal from the photodiodes and generates a feedback signal for a piezo-scanner to maintain a constant force between the tip and the sample surface. The data obtained from the cantilever moving vertically (z-direction) at each (x,y) point in the surface which is caused

by the changes in the surface contours are then processed by computer to form the topographic image of surface [138, 139].

Surface topographic images of the thin films in this study were obtained using a commercial atomic force microscope (AFM) (Ntegra Prima, NT-MDT Co., Moscow, Russia) in semi-contact mode. The thin film samples were fixed on adhesive tape before AFM scans were conducted. The probe used for the imaging contained a tetrahedral tip with height 14–16 μm and a typical curvature radius of 6 nm. The tip was mounted on a rectangular single crystal silicon (N-type, antimony doped) cantilever with a thickness of 2 μm, a resonant frequency of 140–390 kHz and a force constant of 3.1–37.6 N/m.

62 4.2.4. X-ray photoelectron spectroscopy (XPS)

XPS is a highly-sensitive surface technique based on the photoelectronic effect to detect the chemistry composition and the electronic structure of the material surface. When an atom in the surface is illuminated by a monoenergetic soft X-ray photon in an ultrahigh vacuum chamber, an electron is ejected from an inner shell and this photoelectron has a kinetic energy (Ek) equal to the following equation;

Ek = λυ – Eb – φ (4.2)

where λυ is the energy of the X-ray photon, Eb is the binding energy of the atomic orbital

from which the electron originates and φ is the work function, a value dependant on both

sample and spectrometer [140]. The ejected electrons are passed through the hemispherical photoelectron energy analyser and selected at a given energy by electrostatic fields prior to arriving at the detector.

An XPS spectrum is obtained by a plot of the electron counting rate versus their binding energy. A peak at a particular energy would indicate the presence on a certain element while the intensity of the peak corresponds to the concentration of the element in the sample. Each element has a characteristic binding energy associated with each core atomic orbital, yielding a set of discrete peaks in the photoelectron spectrum. The spectrum of a mixture of elements may be considered as the sum of the peaks of the individual constituents. Identification of chemical states can be obtained from an accurate estimation of the separations and peak positions, as well as from certain spectral features. In the XPS, the composition of the film can be determined by utilising the peak area and peak height sensitivity factors. Fluctuations in the structural configuration or oxidation states of a chemical element at the surface layers can be identified by examining the shifts in the binding energies of a particular core level.

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Figure 4.5. Schematic diagram of hemispherical photoelectron energy analyser in XPS instrument [140].

XPS system performance is influenced by the power of source and its focusing ability. Most common sources of photons are the MgKα and AlKα lines. The higher resolution of XPS can be obtained from the synchrotron radiation source. The synchrotron radiation XPS (SR-XPS) provides a continuous energy distribution over a large energy region with high intensity and tuneability giving an optimal excitation energy instead of a fixed excitation energy source (either AlKα or MgKα radiation from a sealed-off X-ray tube) used in conventional XPS [141, 142]. The photon energies of the synchrotron source can be varied to various escape depths of out-coming photon electrons and having a better photon ionization cross-section [143]. Furthermore, the beam size of the synchrotron light source is much smaller than the conventional photon sources, which assists to reduce the effects of the non- uniformity of the sample surfaces [143].

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The atomic percentages and surface bonding structures of samples in this study were probed by Kratos Axis Ultra XPS spectrometer (Manchester, UK) with Mg Kα radiation (hν=

1253.6 eV). The samples were mounted, using double-sided Cu sticky tape, horizontally on the holder and normal to the electrostatic lens. The vacuum pressure of the analyser chamber was less than 10-9 Torr. The voltage and emission current of the X-ray source were held at 12

kV and 12 mA, respectively. Initial survey scans used pass energy of 80 eV. To ensure high resolution and good sensitivity for the features of interests, pass energy of 10 eV was used. The charging effects were corrected by using the C 1s of saturated carbon (C-C/C–H) peak as

reference for all samples at a binding energy (BE) of 284.8 eV. The electrostatic lens mode and analyser entrance of the XPS instrument were selected using the Hybrid and Slot mode (iris=0.6 and aperture=49), respectively. Charge neutralisation was employed during the XPS measurements. The CASA XPS (V.2.3.15) software was utilised for quantification analysis with Shirley background subtraction.

The analyses using the synchrotron source of XPS were conducted on the soft X-ray beamline of the Australian Synchrotron under ring operation of 200 mA and 3 GeV. The beamline was equipped with a collimated light plane grating monochromator SX700. The 1200 lines/mm grating and 15 μm entrance/exit slits were used. The samples were mounted on a stainless steel sample holder and characterised under a background pressure 10-10 Torr in

the X-ray spectroscopy end-station. The Co 2p, Cu 2p and O 1s photoelectron lines were

measured in XPS mode using photon energy of 1253.6 eV. The XPS spectra energy scale was calibrated using C 1s (284.8 eV; saturated carbon: C-C/C–H). The data were processed and evaluated using SPECS (V2.75-R25274) and CasaXPS (V.2.3.15) softwares.

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Figure 4.6. Kratos Axis Ultra XPS spectrometer (Manchester, UK) with Mg Kα radiation source, Murdoch University.