Electronic and geometric properties of silver and gold nanoparticles

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Electronic and Geometric Properties of Silver and Gold Nanoparticles

hv e- Tip

Dissertation

Zur Erlangung des akademischen Grades des Doktors der Naturwissenschaften

An der Universtität Konstanz,

Mathematisch-Naturwissenschaftliche Sektion, Fachbereich Physik

Vorgelegt von Ignacio López Salido

Tag der mündlichen Prüfung: 29. Januar 2007 Referent: Prof. Dr. Gerd Ganteför

Referent: Prof. Dr. Paul Leiderer

Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/2707/

URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-27075

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Cover picture: Nanoparticles on a substrate being investigated by means of Spectroscopic and Microscopic Techniques.

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List of Publications

1 Defect Formation of Au thin Films on SiO2/Si upon annealing:

D. C. Lim, I. Lopez-Salido, R. Dietsche and Y. D. Kim:

Philosophical Magazine, 2005, Vol. 85, N 29, 3477–3486.

2 Ag Nanoparticles on Highly Ordered Pyrolytic Graphite (HOPG) surfaces

studied using STM and XPS:

I. Lopez-Salido, D. C. Lim and Y. D. Kim:

Surface Science, 2005, Vol 588, Issues 1-3, 6-18.

3 Size Selectivity for CO-Oxidation of Ag Nanoparticles on Highly Ordered

Pyrolytic Graphite (HOPG):

D. C. Lim, I. Lopez-Salido and Y. D. Kim:

Surface Science, 2005, Vol 598, Issues 1-3, 96-103.

4 Oxidation of Au nanoparticles on HOPG using atomic Oxygen:

D. C. Lim, I. Lopez-Salido, R. Dietsche, M. Bubek, Y. D. Kim:

Surface Science, 2006, Vol 600, Issues 3, 507-513.

5 Electronic and Geometric Properties of Au Nanostructures on HOPG

studied using XPS and STM:

I. Lopez-Salido, D. C. Lim, R. Dietsche, N. Betram, and Y. D. Kim Journal of Physical Chemistry B, 2006, Vol 110, Issue 3, 1128-1136.

6 Characterization of Ag Nanoparticles on Si wafer prepared using Tollen’s

Reagent and Acid-Etching:

D. C. Lim, I. Lopez-Salido, Y. D. Kim:

Accepted in Applied Surface Science (2006).

7 Size-Selectivity of the Oxidation Behaviors of Au Nanoparticles:

Angewandte Chemie International Edition, 2006, Vol 45, Issue 15, 2413- 2415.

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Highly Ordered Pyrolytic Graphite (HOPG):

I. Lopez-Salido, N. Bertram, D. C. Lim, G. Ganteför and Y. D. Kim:

Bull. Korean Chem. Soc. 2006, Vol 24, No 4, 6-562.

9 Electronic and Chemical Properties of supported Au Nanoparticles:

Submitted in Chemical Physics (2006).

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1 Introduction

The creation and utilization of new materials, devices, and systems through control of matter at nanometer scale (10^-9 m), and the ability to work at these levels with nanostructures is known as Nanotechnology. At least one dimension at nanoscale is necessary to generate building blocks for these nanostructures. Candidates for building blocks can be nanowires, thin films, supramolecular aggregates, and nanoparticles. In this context, noble metal particles consisting of two to about several thousand atoms have attracted considerably the attention over the past few years. The reason is that these particles at nanometer scale reveal novel properties different from individual atoms, molecules and bulk materials. A basic understanding of these properties opens fascinating routes to design devices and potential technological applications. Some of these novel properties have been already explored and exploited in photography [1], biological labelling [2], photonics [3], information storage [4], optoelectronics [5], etc.

Gold as bulk, usually appreciated not only for its beauty but also for the resistivity against corrosion, is a special case at nanoscale. In the form of small nanoparticles, it becomes especially catalytic active at certain sizes. The Industrial Technology Research Institute in Japan, for example, claims that gold nanoparticles are ready to hit commercial markets for gas filters, air conditioners, and air purifiers [6]. In other areas of applications like aerospace technology, gold is used as protective coating of the Hubble telescope to reflect heat flow whilst permitting light very well [7]. In electronics, for example, single-electron switches and transistors of metal quantum dots¹ are developed with gold (or silver) nanoparticles.

In the case of silver nanoparticles, there are already many interesting applications. The activity of silver as catalyst for the selective oxidation of hydrocarbons, such as ethylene or methanol, is well known [8, 9], opening the interest for silver nanoparticles as alternative material to the excellent properties of gold nanoparticles in this field. In other fields, like pharmacology, silver is employed as a bactericide for water purification and to prevent the buildup of bacteria and algae in filters since more than a decade. Studies on the function of

1 Quantum dot: If we cut off a small piece of metal nanowire, we reach a zero dimensional situation, where the remaining electrons are constrained in such a way that they begin to occupy discrete energy levels. This particle can be only described by quantum mechanical rules.

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2 1 Introduction silver nanoparticles as antimicrobial agent are very promising [10]. Due to the sensitivity of noble metals to light and temperatures of wide ranges, recently, in the field of medicine, some attempts to use these properties for applying silver (or gold) nanoparticles to fight against cancer, were pursued [11]. For example, microcapsules filled with a drug, can be delivered directly to its target location within a cancer cell, avoiding undesired side effects of medications. In order to release the drug, these nanoparticles are introduced into the shells of the capsules and the microcapsules can be heated in the presence of a fluorescence dye. These metal nanoparticles act as absorption centers for this light so that a laser pulse can be used to break apart the capsules and release their contents.

Along this line of medical applications, silver and gold nanoparticles in combination with bio molecules are already at a point where even commercial application in medical diagnostic has become known [12].

Many of these special properties are based upon quantum confinement effects at nanometer scale. A basic explanation of this is the following: the properties of a material depend on the type of motion its electrons can execute, which depends on the space available for them. If the physical size of the material is reduced to nanometer size, its properties change unpredictably, becoming sensitive to its size, shape and electronic structure [13].

As already mentioned, one of the properties, which can vary extremely as the size of the particle becomes very small, is its catalytic activity,. At nanometer scale, this activity can depend on the geometric and electronic structure of the particle. For supported nanoparticles, the influence of the substrate should be also taken into account. The catalysis of noble metal nanoparticles became especially important at the end of the 1980´s as M.

Haruta and co-workers discovered that gold nanoparticles can exhibit a high catalytic activity for reactions such as CO-oxidation, propylene epoxidation and other industrial reactions, specially at low temperature (about 200 K) [14-18].

This discovery caused excitement in the surface science community, opening many questions for a better understanding of catalytic processes. It was suggested that a characterisation of these particles could shed light onto these catalytic behaviours. Since the substrate may reshape the geometry and electronic structure of the particle, investigations of deposited particles on a substrate are essential. In line with this idea, the results presented in this work concentrate on the geometric and electronic characterization of silver and gold nanoparticles supported on two different substrates.

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3 1 Introduction A promising method to investigate these properties is given via controlled deposition of size selected clusters². Clusters can be generated by cluster sources (e.g. laser vaporization source [19], arc discharge source [20], magnetron sputter source [21, 22]) in the gas phase, subsequently mass-selected (via mass-spectrometer) and deposited on the substrate. Additional information can be obtained, preparing relative narrow size distributions by nucleation and growth on the support. Narrow size distributions of supported nanoparticles can be achieved via conventional methods as metal vapour deposition [23], and chemical methods such as synthesis of colloidal particles surrounded by organic stabilizers [24] or by using the reduction of Tollen’s reagent and chemical etching [25]. Thus, information gained from mass-selected clusters and supported nanoparticles (not mass-selected but with narrow size distributions) can be compared and complemented.

Finding out the mechanisms that make noble metal particles (e.g. gold and silver) a very active catalyst, is still an open question. Many factors, which can contribute to this activity, should be further investigated, for example, the geometry of a metal nanoparticle with its atoms at edges or corners. It could also occur that these particles have a special electronic structure or that the charge of these clusters is relevant for the activity. Possibly, these nanoparticles need to sit at special sites (such as a defect) on a specific substrate (e.g. oxide support). In any case, the size of these particles seems to play a key role for the catalytic activity and therefore, particles at nanometer scale should be prepared on a substrate in order to be investigated. In this thesis, additional information about these mechanisms for Ag and Au nanoparticles has been provided. To investigate how these different factors are involved, two stable model systems were prepared: Ag (or Au) nanoparticles on a Van-der-Waals surface and Ag(Au) nanoparticles on an oxide substrate. Both systems were as well defined as possible, resulting to be highly interesting as a potential catalyst for CO- oxidation. The characterization of the geometric and electronic properties of these particles is an essential step to understand why these particles have a size- selective catalytic activity.

Last but not least, it is worth mentioning that cleaner cars, non-polluting industrial-process or eco-friendly power plants could be in a promising future some of the wonderful and idealistic achievements based upon the knowledges acquired by basic scientific works related to this dissertation. For this purpose,

2 Clusters are particles consisting of 1 to about several hundreds of atoms with properties that diverge from single atoms, molecules or bulk features.

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4 1 Introduction it is still necessary to investigate these systems but at ambient conditions³.

In the next section, a short review over the last years is presented, mentioning the most significant results on gold and silver nanoparticles and mass-selected clusters. In doing so, I focus more on those aspects which are of interest to this work and refer to the important results from the literature.

3 In this work, experiments were done under Ultra High Vacuum conditions (< 10^-9 mbar).

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2 State of the Art

After Haruta’s discovery [26], many studies have been focused on explaining why gold is a good catalyst for the low temperature CO-oxidation⁴, while Pt-metal catalysts, for example, need a higher temperature for this oxidation reaction. Gold nanoparticles as model catalyst have been widely investigated over the last years, experimentally as well as theoretically.

Although the highest number of studies is concentrated on gold, in principle, the interest of the community is also recently attracted to Ag nanoparticles.

One of the first experiments demonstrating that the catalytic activity of gold nanoparticles can be related to the electronic structure was performed by Goodman et al.[15]. In this experiment, gold nanoparticles in a size regime between 1 and 6 nanometers were prepared on a TiO2 (110) surface and investigated using STM⁵. Goodman and coworkers found a maximum in the catalytic activity for the CO-oxidation as the particle size becomes about 3-4 nm. Moreover, the catalytic activity was found to coincide with an onset of a band gap (metal-nonmetal transition) in the electronic structure of the particle (see Fig 2.1). Other groups have also predicted this kind of transition for these nanoparticles on other supports. Metal-nonmetal transition was predicted with X-Ray Photoelectron Spectroscopy (XPS, see Chapter 4) for gold nanoparticles about 150 atoms by means of an extrapolation of the XPS data obtained by size selected clusters (Au33, Au27, Au7 and Au5) supported on amorphous carbon [27]. In this system, different shifts in the valence and core levels, depending on the cluster size, were observed. The authors in ref [27] correlated these shifts with the average coordination number of the atoms in the particle. In line with ref [27], Whetten and co-workers found for gold nanoparticles supported on Aluminia that discrete states (interband gap 5d-6sp), in the optical absorption spectra began to emerge at diameter below 2 nm [28]. They noted that this transition takes place as a step-like structure in the particle emerges. Atoms at steps are undercoordinated, differing in the electronic structure from atoms in the bulk. Therefore, it is reasonable that a reduction in the average coordination number of the atoms partly affects the electronic structure of the particle [29].

4

2 2

1

CO+2O →CO

5 STM (Scanning Tunneling Microscope). For further explanations, see Chapter 3.

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6 2 State of the Art Theoretical studies about the dependence between the electronic structure of undercoordinated atoms and geometry of the particle have been done by U.Landman et al. [30]. They calculated that a strong hybridization of s and d orbitals for Au cluster up to approx. 13 atoms is responsible for a planar structure of these clusters.

Fig 2.1 Catalytic activity and electronic structure of gold nanoparticles on TiO2 (100) for the CO-oxidation as function of particle size. By Goodman et. al.,[15]

Shifts of core levels in the particle to higher binding energies with decreasing particle size have been frequently observed [31-37]. In addition, oxidation states of the particles were often found to lead to core level shifts, or new peaks at higher binding energies, related to new oxygen species of the particle [38]. For these reasons, an experimental method, such as XPS, is a useful method to study the electronic structure of the particle (see Chapter 4).

Initial and final states effects, before and after the photoemission of an electron from the particle, have been suggested to explain these changes in the electronic structure observed by XPS [39]. These effects are widely examined for Ag and Au nanoparticles in Chapter 6.

In the cluster community, the possibility of creating mass-selected cluster by means of diverse cluster sources [40, 41] or chemical methods has permitted

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2. State of the Art 7 a systematic study on gradual changes in the electronic structure with increasing cluster size on the atom-by-atom basis. It is well known, that a large energy gap and a closed electron shell configuration is a prerequisite for the chemical stability of a cluster. Thus, for example, the large HOMO-LUMO⁶ gap of C60 is responsible for its chemical inertness [42]. Au20 has also a closed shell electronic structure [43, 44]. In case of Au55 (~1.4 nm) prepared on top of a silicon wafer, a maxima resistance to be oxidized was found in oxidation experiments performed by Boyen and coworkers [45]. In this work, this resistance was not observed for gold nanoparticles with other sizes (between 1 to 8 nanometers), prepared in a similar way. In contrast to Au20, Au55 was suggested to be magic due to geometric reason even though it is metallic in electronic structure. For this reason, Au55 was proposed to be special candidates as catalysts for the CO-oxidation.

Evidences in the gas phase, that the electronic structure can determine the reactivity of Agn and Aun metal clusters were also reported by Ganteför et. al., [46-47]. In this work mass spectra taken for silver and gold cluster anions; i.e., Ag^-n and Au^-n (n=2, 3, 4… 19, 21) show a pronounced even-odd alternation of O2 uptake, which is directly related to their electron affinity. This alternation is shown in the case of free Au^-n clusters in Fig 2.2.

Fig 2.2 Mass spectra of free Au^-n cluster anions for the mass regime (3 < n < 9) after reaction with O2. Only cluster with even numbers of atoms reacted with O2. By D. Stolcic et. al., [46].

6 Energetic gap in a cluster between the Lowest Unoccupied Molecular Orbital (LUMO) and the Highest Occupied Molecular Orbital (HOMO).

3 4 5 6 7 8 9

Au₃O₂^-

Au₉^- Au₃^-

Au₇^- Au₅^-

Au₈O₂^- Au₆O₂^-

Au₄O₂^-

Au

_n^-

+ O

₂

Intensity (arb.units)

Number of Au atoms

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8 2 State of the Art It is worth mentioning that time-resolved experiments on metal clusters in the gas phase are becoming very important. Exciting studies about the dynamic of the photodissociation of O2 molecules from Ag^-n, and Au^-n metal clusters using spectroscopy methods such as Photoelectron Spectroscopy (PES), can provide information in real time (femtosecond scale) about the underlying mechanisms of O2 dissociation and CO-oxidation ongoing on these particles [48-50].

Fig 2.3 XPS spectra of Aun cluster (n=2 - 10) deposited on SiO2. An even-odd alternation in the reactivity of this clusters for the absortion of atomic oxygen is observed . By D. C. Lim et.al.,[51, 52].

The strong alternation of the chemical properties of the coinage metals observed in the gas phase can become less pronounced, when clusters interact with a substrate. Fig 2.3 shows XPS spectra measured by Y. D. Kim et al. [52], in which this even-odd alternation is again observed. Low reactivity for Au3, Au5 and Au7, and high reactivity for Au2, Au4, Au6 and Au8 is demonstrated.

Clusters with even number of atoms present additional peaks, at higher binding energies respect to the Au 4f states, which are related to the new oxidation state of the particle [51, 52]. Although this alternation still exists for all even- numbered clusters in the figure, attenuation in the alternation-pattern can also

Au5

Au3

Au6 Au2

Au7

92 88 84 80

Au10

92 88 84 80

Au9

92 88 84 80

Au8

Au4

B ind ing Ene rgy / eV

Intensity /arb. units

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2. State of the Art 9 be observed for bigger clusters, i.e. for n>8, the even-odd pattern is much less pronounced than in the case of the respective gas-phase clusters. Scott L.

Anderson and coworkers discovered a strong size dependence on the CO- binding and oxidation of Aun (n=1, 2, 3, 4, 7) clusters deposited on TiO2, which is much different from the size dependence of the chemistry of gas phase clusters [53]. In order to investigate the oxidation-pattern of larger particle size regimes, Y.D. Kim and coworkers investigated supported Au nanoparticles (between approx. 1 and 10 nm) on SiO2/Si, grown via evaporation [54]. A size selectivity of the catalytic activity of these particles for the CO-oxidation was found⁷. Only very small particles with the particle heights of 2-3 atomic layers or less were found to be inert towards oxidation.

The fact that the chemistry of deposited clusters is much different from the respective free clusters points towards importance of metal-support interaction as well as electronic properties of clusters themselves. Campbell and coworkers, for example, demonstrated the importance of the role of the substrate by preparing Au particles with controlled thicknesses from one to several monolayers on TiO2(110) [55]. In this work, the authors studied oxygen-adsorption on Au/TiO2(110) as a function of Au particle thickness.

Experiments carried out with Thermo-Desorption Spectroscopy (TDS⁸) indicate higher O2 desorption temperatures (741 K) for ultra thin gold particles than for thicker particles (545 K). This implies that oxygen molecules bind much more strongly to the thin gold particles and consequently oxygen molecules dissociates more easily from these particles. Furthermore, it is discussed whether the catalytic activity can be explained by the presence of active sites (defects) on the support or by highly reactive sites on the particle (low coordinated atoms). The catalyzed combustion of CO for Au^-n (n≤20) clusters deposited on MgO, using soft landing deposition, was reported by U. Heiz et al.,[18, 56]. The chemical reactivity of these clusters was investigated using TDS. In this work, a significant size-dependence of the CO-oxidation reactivity of the Au^-n cluster was found. Especially, Au8 and Au18 were found to be the smallest and the highest catalytically active sizes, respectively. Size-dependent CO-oxidation for Ag clusters has been also reported, elsewhere [57]. U.Heiz and coworker concluded that electron transfer from the surface to the gold cluster due to the F-center defects created by the vacancy of oxygen atoms can

7 For additional information , see Chapter 7

8 Thermo-Desorption Spectroscopy (TDS), also known as temperature programmed desorption (TPD), is a UHV technique, which consists of observing desorbed molecules form a surface by means of a mass spectrometer when the surface temperature is increased.

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10 2 State of the Art play an important role for the activation of the cluster as a catalyst. Au clusters seem to be active for the CO-oxidation only if the clusters nucleate at oxygen vacancies. The nucleation of Au clusters at oxygen vacancies suggests that surface defects may alter the electronic configuration of the metal particle. This effect enables the activation (or dissociation) of oxygen on the Au particles, which increases the rate of the CO-oxidation reaction. Therefore, the role that defects (i.e. oxygen vacancies) play for the activation of the reaction is very important. Besenbacher and coworkers have demonstrated the correlation between a decrease in the density of oxygen vacancies and the amount of supported Au nanoparticles onto TiO2 (110) [58]. In this study, gold was evaporated on a highly defective TiO2 (110) surface. STM images of the sample were taken and DFT⁹ calculations were carried out in addition. These calculations confirmed that the charging of Au cluster is very important for the catalytic activity. This charging, in case of supported nanoparticles, can be understood as a result of support-to-metal charge transfer. Besides U.Heiz results, many evidences of the importance of the substrate for the activation of CO-oxidation were reported in the literature. For example, large differences in the activity between supported gold nanoparticles on Titania and Zirconia (Au/TiO2 » Au/ZrO2) for identical particles sizes have been attributed to different natures of the substrates, which can be explained due to possible mechanisms, involving the adsorption of oxygen [59]. Moreover, it is important to mention that the electronic structure of supported metal clusters can change by introducing impurity dopant atoms into a cluster. U. Heiz and coworkers used the system Aun/MgO, replacing one gold atom of an Au4 cluster by one Sr atom. This exchange led to an enhancement in the catalyzed oxidation of CO for the system Au3Sr/MgO [60].

As mentioned before, geometry effects can influence the electronic structure of the particle. Shape effects were observed by Haruta´s investigations showing a better catalytic activity of hemispherical gold particles as opposed to more spherical particles using TEM¹⁰ investigation [61]. Moreover, the surrounding area of the nanoclusters can be especially active for the activation of oxygen. In the past, DFT calculations of O and O2 adsorption and CO oxidation on gold surfaces have bee carried out by Mavrikakiss et al.,

9 Density Functional Theory (DFT) is a quantum mechanical method used to investigate the electronic structure of many-body systems, in particular molecules.

10 Transmission Electron Microscopy (TEM) is an imaging technique whereby a beam of electrons is focused onto a sample and an enlarged version of the sample appear on a fluorescent screen.

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2. State of the Art 11 demonstrating that these molecules prefer to chemisorb on stepped surface Au(211) [62]. Fig 2.4 shows the calculated step density as a function of particle size. A maximum in the step density is found as particle size is about 3 nm.

This may be correlated with the onset in the reactivity of small gold particles predicted by Haruta and others [62. 63].

Fig 2.4 Calculated percent of edge Au atoms by particle as a function of particle size. By Norskov et al., [62].

Fig 2.5 Interaction energy in eV per molecule for CO and O adsorption versus coordination number for gold atoms in various geometries. By Jacobsen et. al., [64].

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12 2 State of the Art Another theoretical study carried out by Norskov and coworkers have reported about calculations focused on the catalytic CO-oxidation of Au10

clusters depending on the geometry [17]. Au10 possesses low coordinated atoms which are able to interact more strongly with adsorbates as shown in Fig 2.5.

Eventually, shape of the cluster can alter during a chemical reaction inducing different isomers, which can have different energy barriers of activation. It is possible that one of them can activate better the reaction [60].

Topographic investigation with a STM can be very useful for the characterization of supported metal nanoparticles. Some studies have shown that evaporated Au and Ag metal atoms, assuming to be monomer in the gas phase, can suffer agglomeration upon reaching the surface, forming larger clusters (»10 atoms). For this reason, a characterization of the size and morphology of the particles after deposition is necessary. Burato’s group, for example, investigated with STM cluster size distributions that result from the deposition of Ag⁺n (n= 1, 2, 3) mass-selected clusters on TiO2 at room temperature [65]. It turned out that Ag⁺n clusters sintered to form three- dimensional islands of approximately 30 atoms in size for dimers and 50 atoms for monomers.

Fig 2.6 Two STM images of Au⁺3 clusters on TiO2 (100)-(1x1) surface at room temperature:

(a) 140 Å^2,; (b) 50 Å²; (c) Cluster size distribution. The bright spots are the clusters. The bright stripes are the coordinated Ti atom rows separated by the bridging oxygen rows, which are dark. The dim spots that appear on the bridging oxygen rows are bridging oxygen vacancies. In this case, cluster agglomeration was not found. By Burato et. al., [66].

A similar experimental work was performed by the same group with Au⁺n

(n=2-8) mass-selected gold clusters on TiO2 [66]. STM pictures of soft landed

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2. State of the Art 13 Au single atoms suggest aggregation of monomers into larger clusters. A nearly uniform height was found for Au⁺n clusters with n bigger than one. The preference of particles for certain adsorption sites was also observed. The images suggest that these clusters remain intact after landing. Particle size distribution and two STM pictures of deposited Au⁺3 clusters on TiO2 from Burato’s work of ref [66] are shown in Fig 2.6.

In another work by Besenbecher and coworkers, Au atoms were evaporated onto TiO2 (110) in different doses and temperatures, and the particle size was determined using STM [58]. In absence of oxygen vacancies, aggregation and sintering of gold metal particles to larger particles formed after evaporation were found. There may be some differences between the processes of depositing clusters and of evaporating atoms, which could justify different behaviors among various studies. First, mass-selected clusters such as clusters prepared by Burato and coworkers in ref [66] are charged. In addition, the impact energy by the deposition of these clusters is about 1eV/atom, which is above the thermal energy in the experiment of Besenbecher and coworkers of ref [58]. The group of K.Kern studied the kinetic impact energy and substrate temperature as a function of the cluster size [67]. For this, mass selected Agn

(n=1, 7, 19) were deposited onto a Pt(111) at low temperature. In this work the kinetic energy per cluster atom was found to be the most important parameter for a controlled deposition. The authors concluded that via energy dissipation into a rare gas buffer layer, non-destructive deposition of soft landed clusters can be achieved easier. This technique is widely used nowadays for the deposition of clusters. Information about the experimental setup for a nondestructive deposition of mass-selected clusters has been provided by W.Eberhardt et.al., [68] and U.Heiz et. al., [69]. It is worth mentioning that the stabilization of nanoparticles can be generated artificially through defects created either by the impact process or by sputtering. This was reported by R.

E. Palmer et. al., [70]. In this work, Ag⁺n clusters (n=2700) were deposited on a previously sputtered Highly Ordered Pyrolitic Graphite (HOPG) surface. A stable and randomly dispersed array of Ag2700 cluster on the surface was observed.

There is a high interest to study individually, that is, locally, nanoparticles grown on surfaces. The morphology and electronic structure of individual noble metal particles supported on surfaces can be well characterized with Scanning Tunneling microscope (STM) and Scanning Tunneling Spectroscopy (STS) (see Chapter 3). For example, STM images in situ of the nucleation and growth of

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14 2 State of the Art single Au nanoparticles supported on a reduced TiO2 substrate have been provided by H. J. Freund et al., in the past [71]. In this work, a comparison, cluster-by-cluster, was made of the morphological evolution and stability of these particles during thermal annealing. Other experiments, such as studies on the luminescent process at single clusters generated by tunneling electrons with sufficient energy have been also carried out. For example, H. J. Freund et. al performed STS experiments on individual Ag metal particles grown on Al2O3/NiAl (110) for this aim [72]. In this work, light emitted from the Ag particles (Mie plasmon resonance¹¹) was stimulated by electron injection from the tip of the STM. Moreover, STS can be employed to study the electronic structure of single particles, such as its band structure. Thus, information about quantized electronic states on the facets of large particles grown on a substrate can be obtained by using a combination of UPS¹²/STS measurements. Using this methods, Hövel et. al., investigated the electronic structure of the Fermi level for Au nanoparticles deposited on a defective HOPG surface at low temperatures [73]. UPS spectra were compared with STS data, providing additional information about the Local Density of State of the electrons (LDOS) in the Fermi level of the particle This comparison is very useful, because the electronic structure of the tip usually perturb and complicate the interpretation of STS spectra taken at single particles [74, 75]. Similar information about the electronic structure of the surface of Ag nanoparticles grown on Ag(111) has been reported by W. Schneider and coworkers by means of a low-temperature STM [76]. All of these examples illustrate the capability of the STM to acquire detailed geometric and electronic information from nanostructures (islands, nanoparticles, mass-selected clusters) on surfaces, highlighting the potential of this tool at local scale.

In this chapter, some of the accomplishments of several groups worldwide to understand the size-dependent electronic, structural and chemical properties of noble metal clusters on different substrates were summarized. It was shown that information about the electronic structure of Ag and Au clusters in the gas phase can be very valuable to understand further relevant results about some aspects of the catalysis of these particles deposited on diverse substrates. A considerable number of measurements on Ag and Au metal clusters have revealed that among the effects responsible for these properties are metal to nonmetal transitions as well as changes in geometry, core level binding energy

11 See Chapter 4

12 Ultraviolet Photoelectron Spectroscopy (UPS). See Chapter 4.

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2. State of the Art 15 shifts, etc., each depending on particle size. Many models of supported noble metal catalysts have been widely investigated using spectroscopic techniques (eg. XPS, UPS) and microscopy studies (STM) over the last years. In addition, many investigations on reactivity and spectroscopic/microscopic studies on ultra thin oxide films result to be of special interest, yet the conductivity of thin oxide films on metal substrate is sufficient to use STM. Size-dependent catalytic behaviours of Au (or Ag) metal clusters have been frequently found, giving rise to the interest of the cluster community on nanocatalysis. STM experiments have achieved to follow the growth and sintering kinetics on a cluster by cluster basis, providing already significant results. However, some challenges lie ahead. It has been a purpose of this dissertation to better understand the different role of oxide- and Van-der-Wals-supports, which can significantly influence the electronic and geometric properties of Ag and Au nanoparticles. A deeper insight into both properties is the key for unveiling the size-selectivity in the reactivity of these particles toward atomic oxygen and subsequently CO-reduction of metal nanoparticles.

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3 Scanning Tunneling Microscopy

In 1986 Binnig and Rohrer were awarded with the Nobel Prize for the invention of Scanning Tunnelling Microscopy (STM) [77, 78]. Two decades later, STM has become one of the most important techniques in surface science.

Information about the topography and the electronic structure of a surface at atomic scale can be achieved with this technique [79-84].

Control voltages for piezotube Tunneling

current amplifier

Data processing and display Feedback system

Tunneling voltage

Tip-surface interaction Sample

X Z Y

Fig 3.1 Main principle of STM. A feedback mechanism for the piezo controls the distance between tip and sample. Tunneling current and voltage can be monitored as the tip scans over the surface.

The basic principle of a STM is presented in Fig 3.1. A sharp conductive tip is brought very close to the surface (0.5-2 nm). At this distance, a voltage difference V (bias voltage <<4 V) is applied between the tip and the surface (the sample). As a consequence of the tunneling effect (explained later), a current of electrons with intensity I (0.01 nA-50 nA) flows through the vacuum gap (potential barrier). A measurement of this current at constant height (Constant Height Mode), or, of the voltage at constant current (Constant Current Mode) can be done while the tip scans over the surface. To be able to adjust the distance between tip and surface, the tip is attached to a piezo-electric

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18 3 Scanning Tunneling Microscopy element, which has the property of changing its length when an electric field is applied. By adjusting the voltage on the piezo element, the distance between the tip and the surface can be regulated. The combination of three piezo elements enables a fine adjustment of the tip’s height over the sample in the X-, Y-, and Z-directions.

The Tunneling Effect

The physical idea behind STM is based upon a quantum-mechanical effect called tunneling effect. In classical physics, an electron cannot penetrate through a potential barrier if its kinetic energy E is smaller than the potential height Φ of the barrier. A quantum-mechanical treatment of this problem predicts that the electron has a certain probability to traverse the barrier and reappear on the other side. This effect is based on the wavelike nature of the electrons. When an overlap between the electron wave functions of the tip and substrate take places, there is a certain probability of finding an electron of the sample in the tip, or vice versa.

In order to introduce this quantum-mechanical treatment, the Schrödinger equation can be applied, in a simple case, to calculate the spatial wave function of the electron with an energy E that collide with a rectangular barrier, with height Φ, for the spatial interval (-∞<x<+∞) as shown in Fig 3.2. After obtaining the wave functions, the probability that the electron will be found in any of the regions, A, B, C, can be calculated by squaring the absolute value of the wave function.

Fig 3.2 Rectangular barrier with a potential height Φ. From left to right, an electron collides with the barrier with an energy E lower than Φ.

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3 Scanning Tunneling Microscopy 19 It turns out that the solution of Schrödinger's equation in the barrier region is of the form:

( )

^{2 (} ⁾

( ) 0 ^kx 0

B

m E

x e⁻ where k Φ − x a

Ψ ∝ Ψ = < <

h (1)

The current is proportional to the probability of electron tunneling through the barrier;

2 2 ( )

0

kx m E

I e⁻ where k Φ − x a

∝ = < <

h (2)

Thus, equation (2) predicts that the tunneling current decays exponentially within the barrier and a finite probability of transmission can be expected [85]. This result is surprising, considering that a flux of electrons through the barrier is not allowed with classical arguments.

In a similar way, this tunneling effect is present at STM. An electron located at the Fermi level¹³ needs an additional amount of energy, known as work function Φ, for leaving the metal. When the tip is electrically connected to the sample, the Fermi Levels of sample and tip are aligned, and the energy band diagram of the STM tunnel-junction can be represented as shown in Fig 3.3 (a).

Fig 3.3 Energy band diagram of a STM tunnel junction (a) before applying a bias voltage at equilibrium and (b) after applying a positive voltage relative to the sample. In this case, an electron current flows from tip to sample.

13 The Fermi Level is defined as the highest occupied molecular orbital in the valence band at 0K.

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20 3 Scanning Tunneling Microscopy The slanted top of the potential barrier is, in this case, a consequence of the different work functions of the two metals. If we apply a bias voltage to this tunneling junction by applying a positive voltage, Vbias, onto the sample, a tunneling current is established. This current occurs because electrons of the tip are free to tunnel into the unoccupied states of the sample's conduction band. As shown in Fig 3.3 (b), the electrons in the tip, which are responsible for the current, have energies between the Fermi energies of sample and tip.

For the barrier in Fig 3.3 (b), an average work function can be defined:

tip

bias

+ ; where E=eV 2

sample

Φ Φ

Φ = (3)

In the case of a variable potential barrier, this approximation can be done taking a succession of square barriers, and solving for each square in the same way. This method is known as WKB approximation [86, 87].

Applying this average work function given by equation (3) in the eq. (2), results in

2 2 ( eV ) bias

0

kx m

I e⁻ where k Φ − x a

∝ = < <

h (4)

One can estimate from equations (3) and (4), that changes in tip-sample distance of about 0.1 nm induce current changes by one order of magnitude.

The vertical resolution depends solely upon the z dependence of the interaction between tip and sample. Because of its exponential form, STM achieve a vertical resolution, neglecting other factors, of about 1.0 x 10^-2 nm [88, 89].

Equations (3) and (4) are based on the assumption that a single atom is responsible for the STM junction contact.

Electron tunnelling can be also understood as an interaction between occupied and unoccupied states. This idea was developed by Tersoff and Hamann’s model (T-H model) [90, 91]. They show that the probability of an electron in state ψ1 at energy E1 to tunnel into state ψ2 at energy E2 partially depends on whether there is an unoccupied state with the same energy in the other electrode. Thus, the tunneling current represents electron transfer from the filled state of the tip to an empty state of the sample, or vice versa. Fig 3.4 shows the generation of tunneling current by applying a negative bias voltage on to the sample. In this case, the occupied states of the sample generate the current.

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3 Scanning Tunneling Microscopy 21

Fig 3.4 Schematic energy diagram of electron tunneling with respect to the density of states of the sample. The occupied states of the sample (indicated with dark color) generate the current [92].

In the T-H model, the tunnelling current I(V) can be described as a function of bias voltage V by

( )

( ) _s( ) (_t ) ( , , ) ( , ) ( , )

I V ^∞ ρ E ρ E eV E V z f E eV T f E T dE

∝

∫

−∞ − Τ − − ⁽⁵⁾

where ρs(E) and ρt(E-eV) are the LDOS¹⁴ for the sample and the tip, respectively. The factor T(E,V,z) is the transmission probability of the tunnelling gap. f(E-V,T) and f(E,T) are the Fermi functions¹⁵ before and after applying a bias voltage V.

Neglecting the influence of temperature for small variations and approximating the Fermi functions by step functions, equation (5) simplifies to:

( ) _s( ) (_t ) ( , , ) I V ^∞ ρ E ρ E eV E V z dE

∝

∫

−∞ − Τ ⁽⁶⁾

If the bias voltage V is close to the Fermi level (eV<<Φ), T can be

14 Local Density Of States (LDOS) is the distribution of number of electrons allowed per energy level as function of which energy is considered.

15 The Fermi function, f(E, T), is the probability that a given available electron energy state will be occupied at a given temperature.

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22 3 Scanning Tunneling Microscopy approximated by:

( ) exp 2 m2 ( _s _t)

T z ⎛ z ⎞

= ⎜⎜− Φ + Φ ⎟⎟

⎝ h ⎠ (7)

and the Intensity is given by ( ) ( ) _s( ) (_t ) I V z ^∞ ρ E ρ E eV dE

∝ Τ

∫

−∞ − ⁽⁸⁾

It is evident from equation (8) and Fig 3.4 that information about the local electronic structure of the sample can be obtained by measuring changes in the intensity as function of the position on the sample. The changes in tip height with position under feedback control reflect both the tip-sample separation and the spatial variation of the local density of surface states (LDOS) of the sample.

Thus, the constant current image only reflects true height changes if the LDOS of the surface (the local work function) is constant across the surface. This would be the case for atomic steps on clean metal surfaces, but would NOT be the case for adsorbates on surfaces. By altering the negative voltage applied to the surface, other states contribute to the current. This provides a way to calculate the LDOS of the sample. The first derivative dI/dV, which is proportional to the LDOS, can be obtained from the measured I-V curves taken at any spatial position. Thus, spectroscopic data corresponding to the differentiation dI/dV normalized by I/V (i.e., (dI/dV)/(I/V)) contains information about the electronic structure of the surface. This technique based on STM measurements that gives information about the LDOS at atomic or molecular scale is known as Scanning Tunneling Spectroscopy (STS). STS measurements are usually carried out at lower temperature, because the spectral resolution improves with lower temperature (∆E=3kBT) [93]. For example, several phenomena related with magnetism [94, 95] and superconductivity [96, 97] could be traced by STS at temperatures around 4 K. A recent general review on STS is given by Schneider et. al.,[93].

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23

4 Photoelectron Spectroscopy

Photoelectron Spectroscopy (PES), or, Electron Spectroscopy for Chemical Analysis (ESCA), is an experimental surface technique used to determine the composition, the nature of chemical bonds and the electronic structure of a surface region of a sample. It is based upon the photoelectric effect, explained by A. Einstein in 1905 [98], in which monochromatic electromagnetic radiation (photons) is used for the excitation of bound electrons above the vacuum level. The small escape depth of these electrons makes this technique surface-sensitive (see Fig 4.1).

Fig 4.1 "Universal curve" of the electron Inelastic Mean Free Path (IMFP=λ) versus kinetic energies for different materials. By J.T. Yates et al., [90, 100].

Depending on the energy of the photon, usually in the range of X-rays radiation (100 - 1500 eV) and ultraviolet radiation (5 - 40 eV), deep-core electrons from the atoms of the sample or electrons from the valence levels can be photoemitted, respectively. Using a synchrotron source [101], an entire range of photon energies with high resolution enables more extensive energy ranges of radiation (between 5 and 5000 eV). One of the advantages of working with synchrotron radiation is the possibility of using an extremely narrow line- width of the incident radiation. In contrast, the resolution for traditional X-ray sources can be only improved by using X-ray monochromators.

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24 4 Photoelectron Spectroscopy In X-ray Photoelectron Spectroscopy (XPS), an electron in a core level absorbs a photon with energy higher than its binding energy, and is emitted.

The kinetic energy of the photoelectron EK is related to the energy of the photon hν by the following expression:

K B

E =hν −E (1)

where EB is the electron binding energy in eV, h is the Planck constant, and υ is the frequency (Hz) of the radiation.

It is important to mention that the binding energy EB is referenced to the Fermi level. The additional amount of energy that an electron needs to overcome the energy difference between the Fermi energy (EF) and the vacuum energy (Evacuum)is known as work function of the sample (Φsample). Considering this additional energy, the kinetic energy (Ek) that an electron has after leaving the sample, is given by

K B sample

E =hν −E − Φ (2)

Fig 4.2 Schematic energy diagram of X-ray photoelectron spectra. The measured kinetic energy of the ejected electron is given by E^*k. However, the true kinetic energy of the electron leaving the sample is Ek.

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4 Photoelectron Spectroscopy 25 Since the spectrometer possesses its own work function (Φspectrometer), Ek is not the kinetic energy, which is measured by the spectrometer. If the sample is electrically connected to the spectrometer, their Fermi energies are at the same level as shown in Fig 4.2. The kinetic energy Ek* measured by the spectrometer is given by:

*

k B spectrometer

E =hν −E − Φ (3)

It is important to mention that Φspectrometer can be measured with a bulk sample.

One-electron approximation

The first approximation to explain the photoemission effect of a single electron was given by Koopmans Theorem [102] (also called One-electron approximation¹⁶). In this approximation, the binding energy of an electron is considered to be equal to the energy difference between initial state (atom with n electrons) and final state of the atom (atom with n-1 electrons (ion) and free photoelectron). Therefore, the binding energy of this electron is equal to the negative value of the orbital energy ε:

( 1) ( )

B Final Initial

E =E n− −E n = −ε (4)

In this approximation, no relaxation effects of surrounding electrons due to the positive hole created by the ejected photoelectron are considered. This simple picture of the photoemission effect implies, that rearrangement of all the electrons around is neglected, considering as they were frozen. In addition, this approximation neglects relativistic effects and effects of electron correlation.

Initial state effects

When the energies of core levels are investigated in detail; small shifts in the binding energies of the electrons can be found. The initial state structure of a particle¹⁷ can induce core level shifts. For example, chemical bonding

16 It is based on the Hartree-fock approximation. This quantum-mechanical approximation is used to calculate the wave function of an electron orbital, assuming that the rest of the electrons of the system are “frozen”.

17 A particle can be atom, cluster or molecule and can be free or supported on a surface.

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26 4 Photoelectron Spectroscopy influences the electronic configuration in and around the atom. In addition, oxidation state and electro-negativity of neighboring atoms can have also influence the electronic configuration, generally resulting in the appearance of shifts or shoulders at the main peaks [103, 104]. Hence, direct information from these shifts provides a quantitative method of chemical analysis. The ability to resolve atoms exhibiting slightly different chemical shifts is limited by the peak widths. Apart from the instrumental resolution (see Chapter 5), the width of the photoemission peaks is determined by the lifetime of the positive core hole created by the photoemission process. By the Heisenberg uncertainty relation, the intrinsic peak width Г, is inversely related to the core hole lifetime τ by:

h

Γ =τ (5)

where h is the Planck constant.

As consequence of the statistical nature of the screening process of the positive hole, the deeper the core hole, the more de-excitation channels there are which can fill the core hole. This intrinsic lifetime broadening is usually assumed to be of Lorentzian nature.

Final state effects

Final state effects reflect the relaxation energy of the system, which corresponds to the energy difference between the excited electron system of a particle (atom, cluster, molecule, or bulk) after losing a photoelectron and the relaxation of the electron system. There are some differences depending on the material. In a metal solid, i.e. metal bulk, the hole state (positive charge state) created by the photoemitted electron, is completely shielded by the conduction electrons and the cores of neighboring atoms. For isolated particles, in particular, for deposited particles on poorly conducting substrate [34, 35, 105, 106], the number of conduction electrons and neighboring atoms is limited by the particle size and, hence, the positive hole can be screened less efficiently.

Inelastic background

In a typical XPS spectrum, binding energy is plotted versus intensity of photoemitted electrons. Fig 4.3 shows a XPS spectrum taken over a relatively wide range (0 -1400 eV), obtained from an Ag bulk sample using Al-K_α

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4 Photoelectron Spectroscopy 27 radiation of 1486.6 eV. As Fig 4.3 depicts, the peaks ride on a background of secondary electrons arising from higher kinetic-energy electrons. As result, the background increases step-like beyond each major peak. This feature is due to inelastic electron energy loss that happens as electrons from deep core levels with a depth over the IMFP loose their kinetic energy. This background signal can be subtracted by smooth curve-fitting, but may introduce some error in quantitative intensity determinations and peak positions [107]. Moreover, all peaks exhibit inelastic tails toward higher binding energies [108].

Fig 4.3 XPS spectrum obtained from a Ag bulk sample using Al-Kα radiation [108, 109].

Auger Peaks

At higher binding energy in Fig 4.3, other peaks known as Auger peaks appear. They are associated to electron transitions between core and valence electrons and consequently to the emission of low energy electrons in an Auger process. This process relies on the coupling between electrons in different energetic levels, in contrast to the one-electron picture described before. An Auger process is initiated by creation of a core hole, for example in the K- shell¹⁸, from where a phoelectron (Auger electron) is emitted above the vacuum

18 K, L, M are the quantum numbers n= 1, 2, 3,… respectively. Inside a shell (n=1, 2, 3,…), levels (p, d, f,…) with a non-zero value of the orbital angular momentum ( l > 0), show spin- orbit splitting and therefore two different energetic states (2p (L2,3), …, 3d (M2,3)…).

Electron counts Ag (Auger) Ag (3p) Ag (3d) Ag (4s) Ag (4p) Ag (4d)

Ag (3s)

Binding Energy (eV)

Electron counts Ag (Auger) Ag (3p) Ag (3d) Ag (4s) Ag (4p) Ag (4d)

Ag (3s)

Ag (Auger) Ag (3p) Ag (3d) Ag (4s) Ag (4p) Ag (4d)

Ag (3s)

Binding Energy (eV)

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28 4 Photoelectron Spectroscopy level. The ionized atom is in a highly excited state and tends to relax back rapidly (ca. 10^-14s) to a lower energy state. Thus, one electron falls from a higher level L1 to fill an initial core hole in the K-shell and the energy liberated in this process is simultaneously transferred to a second electron in the L2,3- shell. A fraction of this energy is required to overcome the binding energy of this second electron; the remainder is retained by this emitted Auger electron as kinetic energy. In this Auger process illustrated in Fig 4.4, the final state is a doubly-ionized atom with core holes in the L1 and L2,3 shells. The notation for the transition illustrated in Fig 4.4 is KL1L2, 3 and can be energetically expressed by:

1 2,3 1 2,3

KL L K L L

E = E - E - E (6)

Fig 4.4 Energetic scheme of the Auger process KL1L2,3 carried out using X-ray radiation.

(Left) The initial state (middle) ionization process occurs by removal of a K-shell electron. One electron falls from a higher level (L1) to the core hole in the K-shell and the energy liberated in this process is simultaneously transferred to a second electron in the level L2,3 (Auger electron).

(Right) Final state is a doubly-ionized atom [110].

Other Auger transitions for the scheme of Fig 4.4, are also possible, like for example, KL1L1, KL2,3L2,3, L1L2,3L2,3. In general the analysis of the Auger peaks can be used to detect elements. It is worth mentioning that Auger peaks always accompany XPS, but they usually rise with broader and more complex

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4 Photoelectron Spectroscopy 29 structure than photoemission peaks. Moreover, the kinetic energy of the Auger electrons is independent of the incident energy of the radiation.

Spin-orbit splitting

The electronic structure of an atom can be described considering the orbital and spin momenta of its electrons. It is well known that, for any electron in an orbital with orbital angular momentum, a coupling between magnetic fields of spin s and angular momentum L exists. For example, an electron is removed from the 4f-level of an Au atom; the remaining 4f electrons may have a spin either parallel or antiparallel to that of the remaining unpaired 4f- electron. Because of different energy configurations in the coupling of two parallel spins (j=L+s=7/2) and two antiparallel (j=L-s=5/2), these configurations give rise to two states, resulting in a peak splitting. This splitting can be observed in the Au 4f XPS spectrum (see Chapter 6). In this case, Au 4f7/2 states are at lower binding energy than the state Au4f5/2. For other orbitals like the s orbitals, no angular momentum exists; therefore s orbitals do not show spin-orbit splitting. These states are called singlet in XPS. In contrast, p, d, f orbitals with angular momentum of 1, 2, 3, show spin-orbit splitting. These states are called doublets.

Satellite peaks

X-Ray emission is generated by electronic transitions inside a metal.

Depending on the material of the X-ray source, different emission lines are generated. These transitions, in which a photon is emitted, provide a characteristic radiation at fixed photon energies. In addition, a continuous background radiation of lower intensity known as “Bremsstrahlung” is observed, too. Monochromatic radiation is very important to generate sharp photoemission lines in XPS. The most popular monochromatic radiations are due to 2p3/2→1s and 2p1/2→1s transitions, which originate from Al and Mg as X-ray source, providing photon energies of Al-Kα1,2 (hν=1486.6 eV) and Mg- Kα1,2 (hν=1253.6 eV). The same transitions in doubly ionized Mg or Al generate Kα3,4 radiation energies of about 9-10 eV higher, inducing “satellite peaks" in XPS spectra at lower binding energies. These satellite peaks are shown in Fig 4.5 for a XPS spectrum of the C1s state of a HOPG surface.

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30 4 Photoelectron Spectroscopy

Shake up, shake-off

In these events, the outgoing photoelectron excites a valence electron to a previously unoccupied state, or, especially in a metal, to an electron-hole formation (shake-up). An excitation of the valence electron above the vacuum level is also possible (shake-off). For these transitions, the photoelectron must give up some of its kinetic energy; hence, new features in the XPS spectrum always lie on the high binding energy side of a direct photoemission transition.

Sometimes, these features don’t have the shape of discrete peaks, because photoelectrons tend to fall into the energy region of inelastic secondary electrons and often show no discrete structure. Fig 4.5 shows this feature in an XPS spectrum of C1s state of a HOPG surface.

Plasmon

Photoelectrons may give up some energy to the electron gas situated in a valence band of a conductor before leaving the material. This energy can be transferred to the electron gas in form of collective oscillations known as Plasmons, with a characteristic frequency. Due to this oscillation, peaks with higher binding energies than the original binding energy can be observed in the photoelectron spectrum (see Fig 4.5).

345 330 315 300 285 270

Satellite (K_α,3, K_α,4) energy loss

plasmon

Shake-up

Intensity (a.u.)

Binding energy (eV) HOPG C1s Al Kα1,2

hv=1486,6 eV

Fig 4.5 XPS spectrum of the C1s state of a HOPG surface. Satellite peaks at lower binding energies are observed. In addition, Shakeup, shakeoff effects and plasmon oscillation can be observed at higher energies [22, 111].

Electronic and geometric properties of silver and gold nanoparticles