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Chapter 2 Surface Science Methods
Chapter 2
SURFACE SCIENCE METHODS
Since a great number of books has already appeared, giving detailed descriptions of the surface science methods (e.g. refs. [I, 2]), a brief classification and a description of the most common techniques will suffice. 2.1
DYNAMIC METHODS
The techniques used, when applying these methods, cause the adsorbed molecules, atoms or radicals to be desorbed from the surface and analyzed in the gas phase. A mass-spectrometer is normally used as the gas analyzer. The desorption of the surface species can be achieved in various ways, e.g. by means of a programmed increase of the substrate surface temperature or by irradiation of the surface layer by electrons, photons, atoms or ions. 2.1.1
Thermal Desorption
This widely used method is based on measuring the flux of the desorbing species for a given adsorption system [3-81. The desorption is induced by heating the sample using an adequate temperature ramp (temperatureprogrammed desorption, TPD) [3-71, or pulse laser beam (laser-induced desorption, LID) [8]. The desorption flux is usually recorded and analyzed by a mass spectrometer. For a linear temperature ramp the desorption rate can be described by the relationship:
dN - V dP S _ dt - w[dt+l/p]l where N is the adsorbate surface concentration in molecules per cm ', P is the pressure change induced by the desorbing particles, S is the pumping rate of the system in 1 per sec, A is the adsorbent surface in cm 2 , V is the volume of the system and Tg is the temperature in the gas phase. With S >> V as the limit and a slow heating rate ( d T / d t ) the desorption rate ( d N / d t ) becomes proportional to the pressure changes induced by the desorbing particles. Taking into account that the modern experimental chambers are characterized by V 5 20 1, and S > 400 I.sec-', aiid d T / d t is usually less than 20' sec-l, 5
Chapter 2.
6
the mathematical description of the T P D spectra obtained is simplified considerably. Thus the traces of the T P D spectra usually reflect the desorption rate directly from the i-th adsorption state, as described by the Polanyi Wigner equation [9, 101:
where ni is the order of desorption, E, is the activation energy of desorption, v, is the frequency factor and N , is the surface concentration for the i-th adsorption state. Thus, in the case of E,, n, and v, independent of the adsorbate surface concentration the relation between the desorption parameters and the experimental thermal desorption curves for n, = 1 can be described by [4]:
Ei RT
- = In
(y) vi.T .
3.46,
(3)
where p = dT/dt, and Tpd is the temperature at the maximum of the corresponding T P D peak. For ni = 2, the relation can be given by [4]:
where Nio is the initial coverage. In actual fact, the desorption parameters can be independent of coverage only with the limits of very low coverages. This calls for full analyses of the complex T P D spectra obtained at moderate and high coverages [5, 9-11]. The T P D method is rather popular because it is relatively cheap and can be applied to many adsorption systems. The T P D data provide information about the adsorbate surface coverage (the area under the T P D spectra is directly proportional to the adsorbate surface concentration), the existence of different adsorption states (an appearance of several TPD peaks), the adsorption and desorption kinetic parameters, the existence of different adsorption states at different adsorbate coverages (from the changes in the shape of the T P D spectra), the interactions between the adsorbed species, the phase transitions in the adsorbate layer, etc. In order to obtain reliable information, it is necessary to avoid side effects, such as recording desorption from the sample supports, a temperature gradient across the crystal, possible laser-induced damage t o the surface, etc. [6,7]. When interpreting the data, especially the multi - peak T D spectra, care must be taken, because in some cases the temperature rise might cause interconversion between the adsorption states. Consequently, the different peaks in the T P D spectra do not necessarily reflect a coexistence of different adsorption st,at,es at the given adsorption temperature. 2.1.2
Molecular Beam Technique
Rapid progress has been made recently with molecular beam experiments because they represent a good approach to the heterogeneously catalyzed surface
2.1. Dynamic Methods
7
reactions [12-141. In these experiments, the sample or t h e detectors (or both) can be moved so that the molecular beams of the molecules, atoms or fragments under study can be directed and reflected elastically or inelastically by the substrate surface a t different angles. This ensures variation of the angle of incidence of the molecular beam, an independent change of the gas and surface temperature, and allows direct analysis of the reaction products by means of a mass spectrometer. The use of a mass spectrometer as an analyzer and lock-in phase sensitive detection restricts the measurements only to the primary reaction products of interest. Furthermore, by using a modulated molecular beam and by first analyzing the phase shift between the input and output signal, it is possible to obtain information about the residence time of the adsorbing species at different substrate temperatures and to deduce the adsorption/desorption parameters and the mechanism of the surface processes. 2.1.3
Electron and Photon Stimulated Desorption (ESD and PSD) and Electron Stimulated Desorption - Angular Distribution (ESDAD)
This method is based on analyses of the particles (ions and neutrals) desorbed as a result of electronic excitations of the adsorbed surface species, induced by electron or photon irradiation 11, 15-17]. A variety of molecular and fragment ions and neutrals ( either in ground or in electronically excited states) have been detected from the adsorbed overlayers as electron (photon)-stimulated products. Two mechanisms are proposed for explaining the electron- stimulated desorption. According to the first mechanism (known as Menzel, Gonier and Redhead model) [15, IS], the first step of electronic excitation is a FrankCondon transition from a ground to nonbonding (repulsive) state. This primary excitation can be followed by different events leading either to quenching of the excitation (by delocalization and trapping of the particles in an attractive potential well) or t o the removal of the excited particle from the surface (by transfer of the electronic energy to a nuclear motion as a result of the electronic rearrangements). On it,s way away, the excited particle may be transferred to another repulsive potential curve which determines the nature of the detected products (positive or negat>iveions or neutrals). In this model, the total desorption cross section is usually given by the relationship: u = u,.P,
where u, is the excitation cross section for the gas phase and P is the escape probability. P depends on the shape of the repulsive potential curve which determines the kinetic energy of the desorbing particle and the lifetime of the excited state a t the surface. According t o the second model suggested by Knotek and Feibelman [17], the primary excitation is a core excitation that decays by an interatomic Auger transition. As a result of this Auger decay, the ion which was negatively charged originally, loses electrons and becomes positively charged. This leads to the desorption of positive ions due to the inversion of the Madelung
8
Chapter 2.
potential. This model is primarily applicable to systems with an ionic bond at the highest valence state. As was mentioned above, while the desorbing particles are leaving the surface, they can undergo different transitions leading to the recapture or a change in the charged state. What happens very much depends on the environment of the adsorbed particles. Depending on the type of the excitation process involved, the cross section of the different ESD products can vary substantially with the changes of the adsorption state and the adsorbate coverage. These variations provide inherent information on the nature of surface - adsorbate interactions and bonding and the changes induced by changes in coverage or modifications of the surface. It is assumed that the directions of the desorbing beams are exclusively determined by the bond orientation of the initial state which allows the angular distribution measurements of the species desorbing by electron or photon stimulation, as developed by using the ESDAD method [18-21). A movable mass - spectrometer [20], a phosphorus screen [I91 or a resistive anode [21] can be used as detector for the ESDAD patterns of the excit,ed ESD products. The uniqueness of the ESDAD method is that i t is passing on information about single species on the surface. It does not require long-range ordering but azimuthal ordering for abnormal directions. This method turns out to be a sensitive probe, concerning the chemisorption bond angles, the site location, the amplitudes of the soft bending molecular vibrations parallel to the surface and their dependence on temperature, coverage and surface composition.
2.1.4
Secondary Ion Mass Spectrometry (SIMS)
This method is based on mass analysis of the species sputtered from the surface as a result of bombardment with highly energetic particles, usually Ar ions with energies in the keV range [I, 221. The emission of the surface particles is induced by energy transfer from the impact ions to the substrate lattice atoms or to the adsorbates. The mass analysis of the sputtered particles contains information about the surface composition. It also provides information on the local structure using the kind of the fragments monitored by the mass spectrometer and the angular dependence of the ion emission as a fingerprint. Depending on the density of the incident-beam current (ranging from 1 nA cm-2 t o 1 mA emd2), this technique can be used for accurate surface and bulk analysis, which provides information on a depth scale.
2.2
STATIC METHODS
These methods involve analyses of the stationary adsorbed phase by means of various spectroscopies based on the interaction of electroils and photons with the surface layer.
2.2.1
Low Energy Electron Diffraction (LEED)
This method is based on recording and doing spatial analysis of the elastically back scattered low energy (15-350 eV) primary electrons from the surface [l, 2, 23, 241. According t o the Lue de Broglie equation [l],which describes the
2.2. Static Methods
9
interference phenomena in electrons scattered by a crystal, the pronounced maxima in the angular distribution of the back scattered electrons displayed on the detector (phosphorus screen or resistive anode) reflect the periodicity of the surface and the possible variations as a result of reconstruction or formation of ordered adsorbate superstructures. In the case of a two - dimensional lattice consisting of parallel rows of atoms in the directions [k,h] and interatomic distances dk,h the Lue de Broglie equation becomes:
where A is the wave length, 90 the angle of the primary electron beam, andps the angle of the reflected electrons. Usually in the LEED systems, ‘po is 0’. Thus the diffraction pattern reflects the periodicity on the surface and the changes induced by the increase of coverage, introduction of coadsorbates, reconstruction of the substrate surface etc. The main driving force for obtaining ordered adsorbate structures on a single cryst,al substrate is determined by the type of adsorbate - substrate and adsorbate - adsorbate interactions. Complete structural analysis of the LEED patterns is possible applying the kinematic theory and measuring the intensity of the diffracted beam as a function of the direction and energy of the primary electron beam [23,24]. This ensures determination of the site location of surface species within the unit cell, and the corresponding adsorbate - adsorbate distances and adsorbate - substrate bond lengths. 2.2.2
W o r k Function Measurements
The change of the work function upon structural and/or composition changes on the surface provides valuable information on the electrostatic potential of the surface which affects its reactivity [25]. Work function is usually described by the relationship: 4 = A4 - m / e , (7) where A@ represents the electrostatic potential of the surface double layer and xel the chemical potential of the electron in the bulk. Since the chemical potential is a bulk property and is not much affected by modification of the surface, it is the change of the double layer that generally determines the work function changes that are observed. The surface potential in the adsorption systems arises because of the presence of dipoles. The potential difference on the two sides of the dipole layer is given by the equation:
Ad = 4 ~ , u N ,
(8)
where N and p are the concentration of the dipoles and the dipole moment of each species on the layer. When the concentration of the surface dipoles increases, the depolarization effects should be ascribed to dipole-dipole interactions. In this case, the surface potential can be expressed by the following equation: 4?rp0N A$ = (9) 1 +9a”
10
Chapter 2.
where LY is the polarizability of the adsorbed species and po is the initial dipole moment. The absolute values of the work function can be determined by three methods only: thermo-ionic, field emission methods and a He1 ultraviolet photoemission method [25, 271. Since usually the relative change of the surface potential, Ad, is the only important factor, many other methods can be used, such as a vibrating capacitor, a diode (retarding potential) method, a secondary electron method (cut-off of the low energy secondary electrons created by electron or photon irradiation on the surface), etc. [25]. It should be pointed out that, together with information about the sign and strength of the surface dipoles, some of the informat,ion on the uniformity of the surface layer is obtained by using these methods, e.g. by the diode method [26, 271. In mixed overlayers the coexistence of patches of different work functions, compositions and structures on the surface (which is difficult to detect with other methods) can be established. 2.2.3
Emission Spectroscopies
Depending on the kind of emission which is recorded and used for characterization of the overlayer, the emission spectroscopies can be classified into two general groups: A. Electron emission spectroscopies. These involve spectroscopies based on measurements of the intensities and the kinetic energies of the electrons emitted from various electron levels of the matter under investigation. Depending on the primary irradiation that causes the electron emission, these spectroscopies can be divided as follows: (a) Spectroscopies in which the primary irradiation is performed by electrons. The Auger Electron Spectroscopy (AES) falls in this category [1,2]. This is the most widely used technique for the quantitative and qualitative analysis of surface layers. The principle of the method is based on detection and analysis of the energy distribution of the Auger electrons ejected from the surface as a result of excitation induced by irradiation with electrons of primary energy (typically between 1500-5000 eV). The energies of the secondary Auger electrons is determined by the following relationship: where E,, Ey and E , are the binding energies of the three electron levels involved in the Auger process, E, is the correction for relaxation of the levels as a result of changes in the amounts of the charges, caused by the creation of a core hole, and e4sp is the work function of the spectrometer. It is obvious from eq.(lO) that the kinetic energy of the emitted Auger electron is independent of the primary excitation energy. Information derived from the energy position, intensity and shape of the Auger spectra has been successfully used for the determination of surface concentration and changes in the chemical state of the adsorbate and substrake. The surface sensitivity of AES depends on the primary excitation energy and the energy of the given Auger transition. Usually, Auger electrons with a low kinetic energy are operative at valence electron levels and are much more sensitive to the interactions in the overlayer and changes in the chemical d a t e of the interacting species.
2.2. Static Methods
11
(b) Photoelectron spectroscopies, where the primary irradiation is electromagnetic (photons). In this class are: X-ray excited Auger electron spectroscopy (XAES), X-ray photo-electron spectroscopy (XPS), ultraviolet electron spectroscopy (UPS), extended X-ray absorption fine structure (EXAFS) etc. [1,2]. The XPS and UPS methods are based on detection of the kinetic energies, &n, of the photoelectrons ejected from discrete electron levels of the adsorbate or substrate as a result of irradiation by a monochromatic photon beam with an energy hv. When using the relationship: one can determine the binding energy in electron level 12, E g , with respect to the vacuum level. XPS and UPS differ in the energy of the primary excitation beam. Usually, the irradiation sources for XPS give beams with energies by 150-8000 eV. XPS is most appropriate for the determination of the electron states in the core levels. The information originating from the XPS spectra includes data. on : the m t u r e of adsorbates a.nd suhstmtes, the surface concentration (which is directly proportional t.0 the int,ensity of the photoelectrons emitted from tlie deep core levels), the chmiges i n chemical state of the surface species (from the energy shifts), tlie coordination of the adsorbed molecules etc. The energy of tlie primary photon beam for UPS is less than 100 eV (usually 10-45 eV). The information based on UPS concerns mainly the valence electron states, binding energies of the molecular orbitals, and the changes induced by interactions on the surface. As has been mentioned in subsection 2.2.2., t8he UPS method can also be applied for work function measurements [I]. The EXAFS method is based 011 the fact, that the photoelectrons a,re emitted as a result of X-ray a.bsorption hack scattered off neighboring atoms. This results in interference between the outgoing and back - scattered photoelectron waves. This interference process produces a,n oscillatory modulation in the X-ray absorption spectrum within the energy range beyond the absorption threshold. Analyses of this oscilhtory fraction in the X-ray absorption spectrum provide information on the local structure around the absorbing species. The advantage of this met,Iiod is that, contrary to the diffraction methods (such as LEED and X-ray diffraction), it does not require a longrange periodicity. (c) Spectroscopies where the primary irra.diation is performed by noble gas ions and metastable atoms with low kinetic energies. To this class belong the metastable quenching spectroscopy (MQS) or penning ionization electron spectroscopy (PIES) [l]. This method is b a e d on t,he fact that the interaction of noble gas ions or excited neutrals with a surface layer forces neutralization accompanied by a.n electron emission. Depending on t,he deexitation mechanism, the resulting electron energy spectra. conbain information on t.he energy position of the molecu1a.r orbitals, the electron levels near t,he Fermi level etc. B. Photoemission spectroscopies. These a.re the spect,roscopies based on measurements of tlie intensities and the energies of the electroma.gnetic radiation emitted as a result of the interaction of slow electrons with the surface layer. In this class a.re the appearance potential spectroscopy (APS) [l]and the inverse photoemission spectroscopy (IPS) [1,28].
12
Chapter 2.
The APS method is based on measuring the X-ray intensity emitted from the surface irradiated with electrons, on which , in the background, which is steadily gaining in strength, a characteristic emission is superimposed. The latter appears when the primary beam energy equals the threshhold energy of excitation of an electron from a core level to an unfilled state above the Fermi level. APS spectra contain information about the core level binding energies and the density of unfilled states above the Fermi level. IPS is based on a process which is to photoemission, i.e. a radiative deexcitation of electrons. Thus, the energy distribution of the emitted photons reflects the electron density of the unoccupied stat,es of the adsorbate and substrate. It should be pointed out that the photoemission spectroscopies provide information about the empty electron density of states above the Fermi level and the unoccupied adsorbate electron orbitals, whereas the electron emission spectroscopies carry information exclusively on the occupied electronic states of the substrate and the occupied molecular or atomic orbitals of the adsorbates. 2.2.4
Absorption Spectroscopies
The absorption spectroscopies are based on monitoring of the energy spectra of the inelastically back-scattered primary electrons (electron loss spectroscopies) or reflected electromagnetic radiation (infrared spectroscopy - IR). A. Electron loss spectroscopy. Depending on the characteristic energy losses two categories of electron loss spectroscopies are distinguished on the basis of the energy of the primary electron beam: (a) Electron Energy Loss Spectroscopy (EELS), where the primary electron energies are of the order of 100 eV. The characteristic energy spectra are a result of energy losses caused by the induction of interband and intraband transitions, plasmon and core level electron excitations in the surface layers and one electron transitions in molecules, atoms or molecular fragments present on the surface. These losses are usually observed in the 1-50 eV range and the energies of the loss peaks are determined by the separation of the two electron levels involved in the induced electron transition or the excitation energy for the plasma oscillations. (b) High Resolution Electron Energy Loss spectroscopy (HREELS), where the primary energy is of the order of a few eV and the electron losses are caused by excitations of the vibrational modes (phonons) at the surface or in the adsorbed species. These losses are nearly of the order of 100 meV. HREELS has been successfully applied in studies of interfacial properties of thin films, and bonding configurations of adsorbed species. Information obtained by HREELS regarding the vibrations excited in the adsorbed molecular species is very similar to that offered by infrared spectroscopy, where the same excitations are induced by electromagnetic irradiation.
References
13
B. Infrared reflection absorption spectroscopy [29]. The advantage of this vibrational spectroscopy is the higher resolution than that of HREELS and the absence of possible electron beam effects. Recently, it is widely applied for the determination of the bonding mode orientation of adsorbed molecules and interactional effects between adsorbed species. It is worth mentioning here that the vibrational spectroscopies are directly related t o the fact that any adsorbate on the surface is vibrating. The possible vibrational modes are determined by the symmetry of the adsorbed species. The symmetry depends on the number of the substrate atoms participating in the formation of the adsorption bond. The possible adsorption sites on single crystal surfaces are determined by the crystallographic orientation of the surface plane. For example, for a fcc (111) surface there are one-, two- and three-fold adsorption sites, depending on the number of the nearest substrate surface atoms.
REFERENCES G . Ertl and J. Kiippers, Low Energy Electrons and Surface Chemistry, 2nd ed. (Verlag Cheniie, Weinheim, 1986) D. P. Woodruff and T. C. Delchar, Cambridge Solid State Sciences Series, eds. R. Cahn, E. Davies and I. Ward (Cambridge, 1986) G. Erlich, J. AppJ. Phys. 32 (1961) 4; Adv. Catalysis 14 (1963) 255 P. A. Redhead, Vacuum 12 (1962) 203 L. D. Schmidt, Catalysis Rev.-Sci. Eng. 9 (1974) 115 L. P. Levine, J. F. Ready and E. Bernalg, J . appl. Phys. 38 (1967) 531; 1EE J . Quantum Electron. QE-4 (1968) 18 [71 D. Menzel, in: Chemistry and Physics of Solid Surfaces, eds. R. Vanselow and R. Rowe (Springer Series in Chemical Physics, 1981) p.389 J. T. Yates, in: Experimental Methods of Experimental Physics vol.22, ed. R. L. Park (Academic Press, 1985) p.425 D. A . King, Surface Sci. 47 (1975) 384 E. G. Seebauer, A. C. F. Kong and L. D. Schmidt, Surface Sci. 193 (1988) 417
J. B. Miller, H. R. Siddiqui, S. M. Gates, J. N. Russel Jr., J . T. Yates Jr., J. C. Tully and M. J. Cardillo, J . Chem. Phys. 87 (1987) 6725 P. M. Merrill, Cat. Rev. 4 (1970) 115 M. P. D’Evelyn and R. J. Madix, Surface Sci.Reports 3 (1983) 413 J. A. Barker and D. J. Auerbach, Surface Sci. Reports 4 (1984) 1 D. Menzel and R. Gomer, J . Chem. Phys. 41 (1964) 3311 P. A. Redhead, Can. J . Phys. 42 (1964) 886 M. L. Knotec and P. J. Feibelman, Phys. Rev. Lett. 40 (1978) 904 J. J. Czyzewsky, T. E. Madey and 3. T. Yates Jr., Phys. rev. Lett. 32 (1974) 777
T. E. Madey, D. L. Doering, E. Bertel and R. Stockbauer, Ultramicroscopy I1 (1983) 187 and references therein D. Menzel, Nucl. Instr. and Methods in: Physics Research, Vol. B13 (1986) 50
M. Alvey, M. J. Dresser and J. T. Yates Jr., Phys. Rev. Lett. 56 (1986) 367 A. Benninghoven, J . Phys. 230 (1970) 403; Surface Sci. 57 (1975) 596
14
Chapter 2
[23]
J. B. Pendry, Low Energy Electron Diffraction eds. G. M. Conn and I<. R.
[24]
M. A. Van Hove and S. Y. Tong, Surface Crystallography by LEED,(Springer
[25]
J. Holzl, F. Schulte in: Solid Surface Physics , vo1.85 of Springer Tracts in
[26] [27] [28] [29]
Coleman (Academic Press, London, 1974) Berlin, 1979)
Modern Physics (Springer, Berlin, 1979) p.85 A. G. Knapp, Surface Sci. 34 (1973) 289 I. I. Ionov, Soviet Phys. Tech. Phys. 43 (1973) 159 V. Dose, Surface Sci. Reports, 3 (1985) 337 and the references therein F. M. Hoffmann, Surface Sci. Reports 3 (1983) 107
SURFACE SCIENCE METHODS
Since a great number of books has already appeared, giving detailed descriptions of the surface science methods (e.g. refs. [I, 2]), a brief classification and a description of the most common techniques will suffice. 2.1
DYNAMIC METHODS
The techniques used, when applying these methods, cause the adsorbed molecules, atoms or radicals to be desorbed from the surface and analyzed in the gas phase. A mass-spectrometer is normally used as the gas analyzer. The desorption of the surface species can be achieved in various ways, e.g. by means of a programmed increase of the substrate surface temperature or by irradiation of the surface layer by electrons, photons, atoms or ions. 2.1.1
Thermal Desorption
This widely used method is based on measuring the flux of the desorbing species for a given adsorption system [3-81. The desorption is induced by heating the sample using an adequate temperature ramp (temperatureprogrammed desorption, TPD) [3-71, or pulse laser beam (laser-induced desorption, LID) [8]. The desorption flux is usually recorded and analyzed by a mass spectrometer. For a linear temperature ramp the desorption rate can be described by the relationship:
dN - V dP S _ dt - w[dt+l/p]l where N is the adsorbate surface concentration in molecules per cm ', P is the pressure change induced by the desorbing particles, S is the pumping rate of the system in 1 per sec, A is the adsorbent surface in cm 2 , V is the volume of the system and Tg is the temperature in the gas phase. With S >> V as the limit and a slow heating rate ( d T / d t ) the desorption rate ( d N / d t ) becomes proportional to the pressure changes induced by the desorbing particles. Taking into account that the modern experimental chambers are characterized by V 5 20 1, and S > 400 I.sec-', aiid d T / d t is usually less than 20' sec-l, 5
Chapter 2.
6
the mathematical description of the T P D spectra obtained is simplified considerably. Thus the traces of the T P D spectra usually reflect the desorption rate directly from the i-th adsorption state, as described by the Polanyi Wigner equation [9, 101:
where ni is the order of desorption, E, is the activation energy of desorption, v, is the frequency factor and N , is the surface concentration for the i-th adsorption state. Thus, in the case of E,, n, and v, independent of the adsorbate surface concentration the relation between the desorption parameters and the experimental thermal desorption curves for n, = 1 can be described by [4]:
Ei RT
- = In
(y) vi.T .
3.46,
(3)
where p = dT/dt, and Tpd is the temperature at the maximum of the corresponding T P D peak. For ni = 2, the relation can be given by [4]:
where Nio is the initial coverage. In actual fact, the desorption parameters can be independent of coverage only with the limits of very low coverages. This calls for full analyses of the complex T P D spectra obtained at moderate and high coverages [5, 9-11]. The T P D method is rather popular because it is relatively cheap and can be applied to many adsorption systems. The T P D data provide information about the adsorbate surface coverage (the area under the T P D spectra is directly proportional to the adsorbate surface concentration), the existence of different adsorption states (an appearance of several TPD peaks), the adsorption and desorption kinetic parameters, the existence of different adsorption states at different adsorbate coverages (from the changes in the shape of the T P D spectra), the interactions between the adsorbed species, the phase transitions in the adsorbate layer, etc. In order to obtain reliable information, it is necessary to avoid side effects, such as recording desorption from the sample supports, a temperature gradient across the crystal, possible laser-induced damage t o the surface, etc. [6,7]. When interpreting the data, especially the multi - peak T D spectra, care must be taken, because in some cases the temperature rise might cause interconversion between the adsorption states. Consequently, the different peaks in the T P D spectra do not necessarily reflect a coexistence of different adsorption st,at,es at the given adsorption temperature. 2.1.2
Molecular Beam Technique
Rapid progress has been made recently with molecular beam experiments because they represent a good approach to the heterogeneously catalyzed surface
2.1. Dynamic Methods
7
reactions [12-141. In these experiments, the sample or t h e detectors (or both) can be moved so that the molecular beams of the molecules, atoms or fragments under study can be directed and reflected elastically or inelastically by the substrate surface a t different angles. This ensures variation of the angle of incidence of the molecular beam, an independent change of the gas and surface temperature, and allows direct analysis of the reaction products by means of a mass spectrometer. The use of a mass spectrometer as an analyzer and lock-in phase sensitive detection restricts the measurements only to the primary reaction products of interest. Furthermore, by using a modulated molecular beam and by first analyzing the phase shift between the input and output signal, it is possible to obtain information about the residence time of the adsorbing species at different substrate temperatures and to deduce the adsorption/desorption parameters and the mechanism of the surface processes. 2.1.3
Electron and Photon Stimulated Desorption (ESD and PSD) and Electron Stimulated Desorption - Angular Distribution (ESDAD)
This method is based on analyses of the particles (ions and neutrals) desorbed as a result of electronic excitations of the adsorbed surface species, induced by electron or photon irradiation 11, 15-17]. A variety of molecular and fragment ions and neutrals ( either in ground or in electronically excited states) have been detected from the adsorbed overlayers as electron (photon)-stimulated products. Two mechanisms are proposed for explaining the electron- stimulated desorption. According to the first mechanism (known as Menzel, Gonier and Redhead model) [15, IS], the first step of electronic excitation is a FrankCondon transition from a ground to nonbonding (repulsive) state. This primary excitation can be followed by different events leading either to quenching of the excitation (by delocalization and trapping of the particles in an attractive potential well) or t o the removal of the excited particle from the surface (by transfer of the electronic energy to a nuclear motion as a result of the electronic rearrangements). On it,s way away, the excited particle may be transferred to another repulsive potential curve which determines the nature of the detected products (positive or negat>iveions or neutrals). In this model, the total desorption cross section is usually given by the relationship: u = u,.P,
where u, is the excitation cross section for the gas phase and P is the escape probability. P depends on the shape of the repulsive potential curve which determines the kinetic energy of the desorbing particle and the lifetime of the excited state a t the surface. According t o the second model suggested by Knotek and Feibelman [17], the primary excitation is a core excitation that decays by an interatomic Auger transition. As a result of this Auger decay, the ion which was negatively charged originally, loses electrons and becomes positively charged. This leads to the desorption of positive ions due to the inversion of the Madelung
8
Chapter 2.
potential. This model is primarily applicable to systems with an ionic bond at the highest valence state. As was mentioned above, while the desorbing particles are leaving the surface, they can undergo different transitions leading to the recapture or a change in the charged state. What happens very much depends on the environment of the adsorbed particles. Depending on the type of the excitation process involved, the cross section of the different ESD products can vary substantially with the changes of the adsorption state and the adsorbate coverage. These variations provide inherent information on the nature of surface - adsorbate interactions and bonding and the changes induced by changes in coverage or modifications of the surface. It is assumed that the directions of the desorbing beams are exclusively determined by the bond orientation of the initial state which allows the angular distribution measurements of the species desorbing by electron or photon stimulation, as developed by using the ESDAD method [18-21). A movable mass - spectrometer [20], a phosphorus screen [I91 or a resistive anode [21] can be used as detector for the ESDAD patterns of the excit,ed ESD products. The uniqueness of the ESDAD method is that i t is passing on information about single species on the surface. It does not require long-range ordering but azimuthal ordering for abnormal directions. This method turns out to be a sensitive probe, concerning the chemisorption bond angles, the site location, the amplitudes of the soft bending molecular vibrations parallel to the surface and their dependence on temperature, coverage and surface composition.
2.1.4
Secondary Ion Mass Spectrometry (SIMS)
This method is based on mass analysis of the species sputtered from the surface as a result of bombardment with highly energetic particles, usually Ar ions with energies in the keV range [I, 221. The emission of the surface particles is induced by energy transfer from the impact ions to the substrate lattice atoms or to the adsorbates. The mass analysis of the sputtered particles contains information about the surface composition. It also provides information on the local structure using the kind of the fragments monitored by the mass spectrometer and the angular dependence of the ion emission as a fingerprint. Depending on the density of the incident-beam current (ranging from 1 nA cm-2 t o 1 mA emd2), this technique can be used for accurate surface and bulk analysis, which provides information on a depth scale.
2.2
STATIC METHODS
These methods involve analyses of the stationary adsorbed phase by means of various spectroscopies based on the interaction of electroils and photons with the surface layer.
2.2.1
Low Energy Electron Diffraction (LEED)
This method is based on recording and doing spatial analysis of the elastically back scattered low energy (15-350 eV) primary electrons from the surface [l, 2, 23, 241. According t o the Lue de Broglie equation [l],which describes the
2.2. Static Methods
9
interference phenomena in electrons scattered by a crystal, the pronounced maxima in the angular distribution of the back scattered electrons displayed on the detector (phosphorus screen or resistive anode) reflect the periodicity of the surface and the possible variations as a result of reconstruction or formation of ordered adsorbate superstructures. In the case of a two - dimensional lattice consisting of parallel rows of atoms in the directions [k,h] and interatomic distances dk,h the Lue de Broglie equation becomes:
where A is the wave length, 90 the angle of the primary electron beam, andps the angle of the reflected electrons. Usually in the LEED systems, ‘po is 0’. Thus the diffraction pattern reflects the periodicity on the surface and the changes induced by the increase of coverage, introduction of coadsorbates, reconstruction of the substrate surface etc. The main driving force for obtaining ordered adsorbate structures on a single cryst,al substrate is determined by the type of adsorbate - substrate and adsorbate - adsorbate interactions. Complete structural analysis of the LEED patterns is possible applying the kinematic theory and measuring the intensity of the diffracted beam as a function of the direction and energy of the primary electron beam [23,24]. This ensures determination of the site location of surface species within the unit cell, and the corresponding adsorbate - adsorbate distances and adsorbate - substrate bond lengths. 2.2.2
W o r k Function Measurements
The change of the work function upon structural and/or composition changes on the surface provides valuable information on the electrostatic potential of the surface which affects its reactivity [25]. Work function is usually described by the relationship: 4 = A4 - m / e , (7) where A@ represents the electrostatic potential of the surface double layer and xel the chemical potential of the electron in the bulk. Since the chemical potential is a bulk property and is not much affected by modification of the surface, it is the change of the double layer that generally determines the work function changes that are observed. The surface potential in the adsorption systems arises because of the presence of dipoles. The potential difference on the two sides of the dipole layer is given by the equation:
Ad = 4 ~ , u N ,
(8)
where N and p are the concentration of the dipoles and the dipole moment of each species on the layer. When the concentration of the surface dipoles increases, the depolarization effects should be ascribed to dipole-dipole interactions. In this case, the surface potential can be expressed by the following equation: 4?rp0N A$ = (9) 1 +9a”
10
Chapter 2.
where LY is the polarizability of the adsorbed species and po is the initial dipole moment. The absolute values of the work function can be determined by three methods only: thermo-ionic, field emission methods and a He1 ultraviolet photoemission method [25, 271. Since usually the relative change of the surface potential, Ad, is the only important factor, many other methods can be used, such as a vibrating capacitor, a diode (retarding potential) method, a secondary electron method (cut-off of the low energy secondary electrons created by electron or photon irradiation on the surface), etc. [25]. It should be pointed out that, together with information about the sign and strength of the surface dipoles, some of the informat,ion on the uniformity of the surface layer is obtained by using these methods, e.g. by the diode method [26, 271. In mixed overlayers the coexistence of patches of different work functions, compositions and structures on the surface (which is difficult to detect with other methods) can be established. 2.2.3
Emission Spectroscopies
Depending on the kind of emission which is recorded and used for characterization of the overlayer, the emission spectroscopies can be classified into two general groups: A. Electron emission spectroscopies. These involve spectroscopies based on measurements of the intensities and the kinetic energies of the electrons emitted from various electron levels of the matter under investigation. Depending on the primary irradiation that causes the electron emission, these spectroscopies can be divided as follows: (a) Spectroscopies in which the primary irradiation is performed by electrons. The Auger Electron Spectroscopy (AES) falls in this category [1,2]. This is the most widely used technique for the quantitative and qualitative analysis of surface layers. The principle of the method is based on detection and analysis of the energy distribution of the Auger electrons ejected from the surface as a result of excitation induced by irradiation with electrons of primary energy (typically between 1500-5000 eV). The energies of the secondary Auger electrons is determined by the following relationship: where E,, Ey and E , are the binding energies of the three electron levels involved in the Auger process, E, is the correction for relaxation of the levels as a result of changes in the amounts of the charges, caused by the creation of a core hole, and e4sp is the work function of the spectrometer. It is obvious from eq.(lO) that the kinetic energy of the emitted Auger electron is independent of the primary excitation energy. Information derived from the energy position, intensity and shape of the Auger spectra has been successfully used for the determination of surface concentration and changes in the chemical state of the adsorbate and substrake. The surface sensitivity of AES depends on the primary excitation energy and the energy of the given Auger transition. Usually, Auger electrons with a low kinetic energy are operative at valence electron levels and are much more sensitive to the interactions in the overlayer and changes in the chemical d a t e of the interacting species.
2.2. Static Methods
11
(b) Photoelectron spectroscopies, where the primary irradiation is electromagnetic (photons). In this class are: X-ray excited Auger electron spectroscopy (XAES), X-ray photo-electron spectroscopy (XPS), ultraviolet electron spectroscopy (UPS), extended X-ray absorption fine structure (EXAFS) etc. [1,2]. The XPS and UPS methods are based on detection of the kinetic energies, &n, of the photoelectrons ejected from discrete electron levels of the adsorbate or substrate as a result of irradiation by a monochromatic photon beam with an energy hv. When using the relationship: one can determine the binding energy in electron level 12, E g , with respect to the vacuum level. XPS and UPS differ in the energy of the primary excitation beam. Usually, the irradiation sources for XPS give beams with energies by 150-8000 eV. XPS is most appropriate for the determination of the electron states in the core levels. The information originating from the XPS spectra includes data. on : the m t u r e of adsorbates a.nd suhstmtes, the surface concentration (which is directly proportional t.0 the int,ensity of the photoelectrons emitted from tlie deep core levels), the chmiges i n chemical state of the surface species (from the energy shifts), tlie coordination of the adsorbed molecules etc. The energy of tlie primary photon beam for UPS is less than 100 eV (usually 10-45 eV). The information based on UPS concerns mainly the valence electron states, binding energies of the molecular orbitals, and the changes induced by interactions on the surface. As has been mentioned in subsection 2.2.2., t8he UPS method can also be applied for work function measurements [I]. The EXAFS method is based 011 the fact, that the photoelectrons a,re emitted as a result of X-ray a.bsorption hack scattered off neighboring atoms. This results in interference between the outgoing and back - scattered photoelectron waves. This interference process produces a,n oscillatory modulation in the X-ray absorption spectrum within the energy range beyond the absorption threshold. Analyses of this oscilhtory fraction in the X-ray absorption spectrum provide information on the local structure around the absorbing species. The advantage of this met,Iiod is that, contrary to the diffraction methods (such as LEED and X-ray diffraction), it does not require a longrange periodicity. (c) Spectroscopies where the primary irra.diation is performed by noble gas ions and metastable atoms with low kinetic energies. To this class belong the metastable quenching spectroscopy (MQS) or penning ionization electron spectroscopy (PIES) [l]. This method is b a e d on t,he fact that the interaction of noble gas ions or excited neutrals with a surface layer forces neutralization accompanied by a.n electron emission. Depending on t,he deexitation mechanism, the resulting electron energy spectra. conbain information on t.he energy position of the molecu1a.r orbitals, the electron levels near t,he Fermi level etc. B. Photoemission spectroscopies. These a.re the spect,roscopies based on measurements of tlie intensities and the energies of the electroma.gnetic radiation emitted as a result of the interaction of slow electrons with the surface layer. In this class a.re the appearance potential spectroscopy (APS) [l]and the inverse photoemission spectroscopy (IPS) [1,28].
12
Chapter 2.
The APS method is based on measuring the X-ray intensity emitted from the surface irradiated with electrons, on which , in the background, which is steadily gaining in strength, a characteristic emission is superimposed. The latter appears when the primary beam energy equals the threshhold energy of excitation of an electron from a core level to an unfilled state above the Fermi level. APS spectra contain information about the core level binding energies and the density of unfilled states above the Fermi level. IPS is based on a process which is to photoemission, i.e. a radiative deexcitation of electrons. Thus, the energy distribution of the emitted photons reflects the electron density of the unoccupied stat,es of the adsorbate and substrate. It should be pointed out that the photoemission spectroscopies provide information about the empty electron density of states above the Fermi level and the unoccupied adsorbate electron orbitals, whereas the electron emission spectroscopies carry information exclusively on the occupied electronic states of the substrate and the occupied molecular or atomic orbitals of the adsorbates. 2.2.4
Absorption Spectroscopies
The absorption spectroscopies are based on monitoring of the energy spectra of the inelastically back-scattered primary electrons (electron loss spectroscopies) or reflected electromagnetic radiation (infrared spectroscopy - IR). A. Electron loss spectroscopy. Depending on the characteristic energy losses two categories of electron loss spectroscopies are distinguished on the basis of the energy of the primary electron beam: (a) Electron Energy Loss Spectroscopy (EELS), where the primary electron energies are of the order of 100 eV. The characteristic energy spectra are a result of energy losses caused by the induction of interband and intraband transitions, plasmon and core level electron excitations in the surface layers and one electron transitions in molecules, atoms or molecular fragments present on the surface. These losses are usually observed in the 1-50 eV range and the energies of the loss peaks are determined by the separation of the two electron levels involved in the induced electron transition or the excitation energy for the plasma oscillations. (b) High Resolution Electron Energy Loss spectroscopy (HREELS), where the primary energy is of the order of a few eV and the electron losses are caused by excitations of the vibrational modes (phonons) at the surface or in the adsorbed species. These losses are nearly of the order of 100 meV. HREELS has been successfully applied in studies of interfacial properties of thin films, and bonding configurations of adsorbed species. Information obtained by HREELS regarding the vibrations excited in the adsorbed molecular species is very similar to that offered by infrared spectroscopy, where the same excitations are induced by electromagnetic irradiation.
References
13
B. Infrared reflection absorption spectroscopy [29]. The advantage of this vibrational spectroscopy is the higher resolution than that of HREELS and the absence of possible electron beam effects. Recently, it is widely applied for the determination of the bonding mode orientation of adsorbed molecules and interactional effects between adsorbed species. It is worth mentioning here that the vibrational spectroscopies are directly related t o the fact that any adsorbate on the surface is vibrating. The possible vibrational modes are determined by the symmetry of the adsorbed species. The symmetry depends on the number of the substrate atoms participating in the formation of the adsorption bond. The possible adsorption sites on single crystal surfaces are determined by the crystallographic orientation of the surface plane. For example, for a fcc (111) surface there are one-, two- and three-fold adsorption sites, depending on the number of the nearest substrate surface atoms.
REFERENCES G . Ertl and J. Kiippers, Low Energy Electrons and Surface Chemistry, 2nd ed. (Verlag Cheniie, Weinheim, 1986) D. P. Woodruff and T. C. Delchar, Cambridge Solid State Sciences Series, eds. R. Cahn, E. Davies and I. Ward (Cambridge, 1986) G. Erlich, J. AppJ. Phys. 32 (1961) 4; Adv. Catalysis 14 (1963) 255 P. A. Redhead, Vacuum 12 (1962) 203 L. D. Schmidt, Catalysis Rev.-Sci. Eng. 9 (1974) 115 L. P. Levine, J. F. Ready and E. Bernalg, J . appl. Phys. 38 (1967) 531; 1EE J . Quantum Electron. QE-4 (1968) 18 [71 D. Menzel, in: Chemistry and Physics of Solid Surfaces, eds. R. Vanselow and R. Rowe (Springer Series in Chemical Physics, 1981) p.389 J. T. Yates, in: Experimental Methods of Experimental Physics vol.22, ed. R. L. Park (Academic Press, 1985) p.425 D. A . King, Surface Sci. 47 (1975) 384 E. G. Seebauer, A. C. F. Kong and L. D. Schmidt, Surface Sci. 193 (1988) 417
J. B. Miller, H. R. Siddiqui, S. M. Gates, J. N. Russel Jr., J . T. Yates Jr., J. C. Tully and M. J. Cardillo, J . Chem. Phys. 87 (1987) 6725 P. M. Merrill, Cat. Rev. 4 (1970) 115 M. P. D’Evelyn and R. J. Madix, Surface Sci.Reports 3 (1983) 413 J. A. Barker and D. J. Auerbach, Surface Sci. Reports 4 (1984) 1 D. Menzel and R. Gomer, J . Chem. Phys. 41 (1964) 3311 P. A. Redhead, Can. J . Phys. 42 (1964) 886 M. L. Knotec and P. J. Feibelman, Phys. Rev. Lett. 40 (1978) 904 J. J. Czyzewsky, T. E. Madey and 3. T. Yates Jr., Phys. rev. Lett. 32 (1974) 777
T. E. Madey, D. L. Doering, E. Bertel and R. Stockbauer, Ultramicroscopy I1 (1983) 187 and references therein D. Menzel, Nucl. Instr. and Methods in: Physics Research, Vol. B13 (1986) 50
M. Alvey, M. J. Dresser and J. T. Yates Jr., Phys. Rev. Lett. 56 (1986) 367 A. Benninghoven, J . Phys. 230 (1970) 403; Surface Sci. 57 (1975) 596
14
Chapter 2
[23]
J. B. Pendry, Low Energy Electron Diffraction eds. G. M. Conn and I<. R.
[24]
M. A. Van Hove and S. Y. Tong, Surface Crystallography by LEED,(Springer
[25]
J. Holzl, F. Schulte in: Solid Surface Physics , vo1.85 of Springer Tracts in
[26] [27] [28] [29]
Coleman (Academic Press, London, 1974) Berlin, 1979)
Modern Physics (Springer, Berlin, 1979) p.85 A. G. Knapp, Surface Sci. 34 (1973) 289 I. I. Ionov, Soviet Phys. Tech. Phys. 43 (1973) 159 V. Dose, Surface Sci. Reports, 3 (1985) 337 and the references therein F. M. Hoffmann, Surface Sci. Reports 3 (1983) 107