Manejo de marranas y gorrinas

X-ray photoelectron spectroscopy (XPS) is a standard spectroscopic technique widely applied for the analysis of solid surfaces, for example, for establishing the concentration of atoms on the surface and their chemical (valence) state [10]. The operation principle of XPS is based on the photoelectric effect. If photons impinging on a surface possess sufficient energy, electrons can leave the solid. On its way out, an electron can lose energy in a number of ways, which makes it less probable that electrons emitted deeper in the solid will escape and be detected. Typically, photoelectrons with kinetic energy of 5-2000 eV have an escape depth in the range of several tens of angstroms and less. This means that the electrons emitted into the vacuum originate from atoms that reside in the outermost atomic layers, and this is what makes XPS a surface specific technique. The existence of different local chemical and/or electronic environments gives rise to the appearance in the photoemission spectrum of different components which are shifted in energy. This explains why XPS is such a powerful technique for chemical characterization of surfaces.

When one uses for excitation X-ray photons with energy of > 100 eV, then electrons that are in the core levels of an atom can be emitted. This is employed in the present work and this regime is called core level photoemission spectroscopy.

The kinetic energy of photoemitted electrons from a core level in the first approximation follows the relation: , where E′kin is the kinetic energy of the emitted electrons in the vacuum,

S b kin h E e

E'     



h is the photon energy, Eb is the binding energy of the specific level relative to the Fermi level and eS is the work function needed to extract an electron from the Fermi level into the vacuum. Technically, an electron energy analyzer is used to measure the kinetic energy of the outgoing electrons. If the sample is grounded then it has to overcome the potential difference between the sample and analyzer e(_S_A). Therefore, the kinetic energy detected has to include this work function relation:

. This equation is valid in the so-called adiabatic approximation. It should be realized that this is a greatly simplified view of the photoemission process. The creation of the core hole causes a relaxation of the other electron orbitals, which contract towards the nucleus in order to screen the hole, so that more energy can be available for the outgoing photoelectrons. This leads to a lowering of the photoelectron binding energy (called intra-atomic relaxation shift). Thus one needs to add this energyto the right hand side of the above equation. A schematic diagram of the photoemission process is shown in Fig. 2.6. ) ( S A b kin h E e E     

Figure 2.6:Schematic diagram of the photoemission process.

As the photoemission process is usually faster than the system relaxation (rearrangement of its charge distribution) this results in final states with multiple excitations. This processes lead to the occurrence of so-called satellites in the core level photoemission spectrum. The satellites appear at lower kinetic energies than the main peak, also called the adiabatic peak. They are usually referred to as shake-up and shake-off features, depending on whether excitation occurs into a bound state or into the continuum. The photoemission spectrum is represented not by single lines at certain energy but lines having some width. The reason of this is related to the photoemission process and to the way the photoemission spectra are measured. The first factor that contributes to the natural line broadening is a direct consequence of the uncertainty in the lifetime of the ion state remaining after photoemission. The energy of such a level cannot be precisely determined and will have an uncertainty of the order ћ/τ, where τ - is the lifetime of the excited state of an ion. The process brings Lorentzian broadening to the line, which for the broadest core levels is of the order of about 0.1 eV. There are several other processes that can contribute to the line broadening. Here one could mention energy losses caused by multiple ionization processes, so-called intrinsic losses. On their way to the surface photoelectrons can also lose some energy by electron-electron and electron-plasmon interaction (extrinsic losses). All these processes can contribute to the line broadening when they are situated close to the natural line energy level.

The resolution of XPS is determined by the natural line width of the level under study (ΔEnat), the line width of the X-ray source (ΔEx), and the broadening due to the analyzer (ΔEan). Finally, the width of the photoemission peak at half maximum, taking into account all three terms is expressed as follows: E EnatExEan, where ΔE – is the width of a photoemission

2 ) (

peak at half maximum, ΔEnat – is the natural line width, ΔEx – is the line width of the X-ray source, ΔEan – is the broadening due to the analyzer.

The line width of the X-ray source is in the order 1 eV, but with the help of monochromatization can be reduced to about 0.3 eV. The broadening due to an analyz

S spectra were measured in a different UHV system. This s

an analytic technique that is commonly used for the etermination of the crystallographic structure, chemical composition and

er depends on the energy at which the electrons travel trough the analyzer and the width of the slits between the energy filter and the actual detector. At low pass energy the analyzer contribution to the line width is negligible, but the intensity of a line decreases.

In the experimental UHV set-up used for STM/AFM measurements the XPS was not available and the XP

ystem was equipped with a VG ESCALAB Mk II electron spectrometer. Filtered Al Kα radiation (1486.6 eV) from an X-ray source operating at 15 kV and 32 mA was used to excite photoelectrons which were analyzed with a hemispherical analyzer operated at 25 eV pass energy. The energy scale was calibrated versus carbon at 284.4 eV. The working pressure was below 2×10-9 mbar. Note that the spatial resolution of the spectrometer used in the present study is ca. 100 μm, thus the net Mn oxidation state within this area is sampled. All spectra were recorded at a photoelectron take-off angle of 52°. Data processing was carried out using “CASA” XPS software [11].

.6 X –ray diffraction 2

X-ray diffraction (XRD) is d

physical properties of materials and thin films. There are several kinds of the technique which permit to get specific information about a material. In this study the analysis of MnO single crystals was carried out in the geometry used for the X- ray powder diffraction technique. The diffraction patterns of the samples were collected using a diffractometer (PanAlytical X’Pert) with Cu Kα radiation, (λ=1.54056 Å, E=8 keV) in the range from 15° to 100° with 0.02° (2θ) step.

The technique is based on the Bragg equation:ndsin, where n - is the reflection order, d - interlayer distance of a crystal, λ - is the wave length of the used X-rays, θ - is the angle between the incoming as well as reflected X-rays and the normal to the reflecting local plane. The equation is based on the fact that if the difference between the X-rays reflected from different planes is a multiple integer of the wave length of incident rays then constructive interference takes place. When one measures the angles 2θ under which constructively interfering X-rays leave the crystal, the Bragg equation gives the corresponding lattice interlayer distance, which is characteristic for a particular compound. Clear X-ray diffraction peaks are only observed when the sample possesses sufficient long- range order. In general, however, the Bragg equation is the necessary but

insufficient condition of effective mirror reflection from the crystal since it does not take into account the location of atoms in an individual reflecting plane. The quality of the diffraction pattern, which appears as a result of the reflection from particular planes, also depends on other two parameters: (i) the so-called scattering factor (or form-factor) for the atoms, from which these planes consist; (ii) the structural factor of these planes. The latter factor depends on the form factors of toms of different kind in a crystal and their locations in the unit cell. In general, t can be large, small or equal to zero [12]. Consequently, XRD spectra of crystals only exhibit the peaks at certain crystallographic directions, permitted in terms of the structural-factor.

a i

.7 Low energy electron diffraction

tron diffraction (LEED) was used as a ualitative tool to check the structure of the MnO(100) surface. Therefore, only

ch rear view LEED/Auger electron spectroscopy stem

2

In the present study low-energy elec q

the basic principles of the technique are described. The fundamentals of the technique can be found elsewhere [13]. The technique is based on elastic scattering of low energy electrons (50-500 eV) impinging perpendicularly on a surface of a single crystal. In this energy region electrons have according to the de Broglie law a wavelength in the order of atomic distances in the solids. The application of low energy electrons, due to their small escape depth from the solid, permits to obtain an image of the topmost layers of a crystal. The maxima of scattered and constructively interfered electron waves are visualized on a fluorescent screen which has the shape of a hemisphere centered on the crystal surface and is situated around the electron gun. The spots on the screen, projected onto the viewing plane correspond to the reciprocal lattice of the surface. LEED provides a snapshot of the 2D reciprocal lattice of the near surface layers. The intensity of the main spots on the screen, their sharpness as well as any changes in their number at particular electron energy contains information about the two- dimensional surface structure.

In the present study, LEED patterns were obtained at ambient temperature with a four-grid VG Microte

sy controlled by VG Microtech Model 8011 electronics. The primary beam energy was in the range 200-280 eV. Diffraction patterns were displayed and recorded using a CCD video camera interfaced to a video monitor and stored on a computer. The LEED patterns were analyzed with the “SMARTLEED V1.51” computer program.