Kelvin Probe Force Microscopy on MgO(001) - American Chemical

Dec 10, 2008 - Kelvin probe force microscopy measurements of the clean (001) surface of UHV ... Kelvin images of supported Pd nanoclusters exhibit a m...
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J. Phys. Chem. C 2009, 113, 247–253

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Kelvin Probe Force Microscopy on MgO(001) Surfaces and Supported Pd Nanoclusters Clemens Barth* and Claude R. Henry CINAM-CNRS,† Campus de Luminy, Case 913, 13288 Marseille Cedex 09, France ReceiVed: August 16, 2008; ReVised Manuscript ReceiVed: October 3, 2008

Kelvin probe force microscopy measurements of the clean (001) surface of UHV cleaved MgO and of the same surface with supported palladium nanoclusters are presented. Kelvin images of the clean surface show a bright contrast at steps, pits, and fragments of MgO. The bright contrast is due to negatively charged defects, which deliver the surface a net negative charge. Kelvin images of supported Pd nanoclusters exhibit a mean voltage difference of 2.4 V between Pd clusters and the MgO(001) surface. It is shown that the voltage difference equals the work function difference between MgO(001)/Mo and Pd, which was found by recent MIES and UPS measurements. 1. Introduction In recent years magnesium oxide has become very important in research and industry. Thin films of MgO are under consideration for instance, if they can be used as magnetic tunnel junctions for electronic devices,1,2 which is because MgO films exhibit an excellent high-temperature stability as well as a thermal conductance and good growth characteristics on, e.g., silicon.3 The MgO(001) surface serves also as a support for, e.g., palladium,4-7 gold,8-10 or bimetallic11-13 nanoclusters, which form important model catalysts in nanocatalysis.14-16 The nanocatalysis depends on many key factors like the size, morphology, and nucleation of the clusters.14 In the last years, research has been focused especially on the electronic configuration of the cluster-oxide system, which plays a key role in nanocatalysis. For 1 ML ultrathin MgO(001) films grown on metal substrates for instance, the work function of the MgO(001)/ metal system is so low that deposited gold nanoclusters get charged by the metal substrate changing the geometric, electronic, and catalytical properties of the cluster-oxide system, a considerably large difference in comparison to thicker films and bulk MgO(001).17-21 More important for thick MgO films and bulk MgO are defects like F centers on the surface. The F centers change the electronic structure of the nanoclusters by a charge transfer, which has a large influence on, e.g., the CO oxidation at gold nanoclusters on MgO(001).22 The influence of surface defects on metal nanoclusters can be directly observed via work function changes of the whole cluster-oxide system.23 The work function is strongly correlated with the electronic structure of surfaces and can be directly measured. Integral analysis methods such as ultraviolet photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES),23-26 or two-photon photoemission spectroscopy (2PPS)27 are used to perform work function measurements. Another direct method is the classical Kelvin method, where a macroscopic metal plate is vibrating in front of the surface at a very close distance of some microns.28-31 Although these analytic tools give a direct access to the work function and supply information averaged over a large surface area, no local information can be extracted. Furthermore UPS or MIES measurements are difficult to * To whom correspondence should be addressed. E-mail: barth@ cinam.univ-mrs.fr. † The CINAM is associated with the Aix-Marseille University.

perform on bulk insulating oxides, which is due to a surface charging that arises during the measurement.25 In recent years dynamic scanning force microscopy (dynamic SFM)32-34 has shown that it allows bulk oxide surfaces to be imaged with true atomic resolution.35,36,78 There is no restriction for the imaging of supported nanoclusters.37-41 A recent study has even shown that the correct cluster shape and size can be precisely imaged if the constant height imaging mode of the dynamic SFM is used.42 An advantage of the dynamic SFM technique is the implementation of the Kelvin modulation technique.43 This so-called Kelvin probe force microscope (KPFM) provides an image of the electrostatic surface potential exhibiting differences in the work function of different conducting materials on the surface. For semiconductor and insulating materials, the distribution of surface charges is additionally imaged.44,45 The Kelvin microscope can be used also for the imaging of metal nanoclusters, where Kelvin images with a high spacial resolution can be achieved,46,47 even if they are supported on bulk insulator surfaces.48 In this paper, a KPFM study of the clean (001) surface of bulk MgO and of supported palladium nanoclusters is presented. It is discussed to what extent the Kelvin microscope can contribute to work function measurements on these surfaces. After a discussion of the clean (001) surface of UHV cleaved MgO, on which preferentially negatively charged surface defects could be found, Kelvin measurements of supported palladium nanoclusters are shown. Large differences in the minimizing bias voltage between the clean MgO(001) surface and Pd clusters of up to several volts were measured. These findings areexplainedbyacomparisonwithrecenttheoreticalcalculations17,18 and with recent UPS and MIES measurements.26 2. Experimental Methods Dynamic SFM and KPFM experiments were performed in the low 10-10 mbar pressure range and at room temperature with an Omicron STM/AFM (digital demodulator from NanoSurf). A conducting silicon cantilever (Nanosensors, p-Si, 0.015Ω cm, 318.2 kHz resonance frequency, 37 N/m spring constant, 8 nm peak-to-peak amplitude) was used. Because the tip was originally exposed to the atmosphere, the tip carried a native oxide layer before taking measurements.

10.1021/jp807340k CCC: $40.75  2009 American Chemical Society Published on Web 12/10/2008

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Figure 1. KPFM measurement of the clean bulk MgO(001) surface. The topography image (a) and the corresponding Kelvin image (b) are shown. Profiles in (c) and (d) were taken at the green lines of image (a) and (b). Graph (e) shows the voltage distribution of the Kelvin image (b). 200 × 200 nm2, ∆f ) -34.0 Hz, VScan ) 0.5 Hz, Uac ) 1.5 V, fac ) 474 Hz.

In frequency modulated KPFM, a dc (Udc) and ac voltage (Uac) with frequency fac are applied at the rear side of the sample (tip at ground).43,49 In such measurement, the electrostatic tip-surface interaction is minimized at each point on the surface by the bias voltage, which yields the work function difference Udc,0e ) φsample - φtip between a metallic tip and sample. If at two different places on the surface (position 1 and 2) the latter contact potential is measured, the difference Udc,0,pos1-pos2e ) (φpos1 - φtip) - (φpos2 - φtip) ) φpos1 - φpos2 yields the work function difference between the materials at the two surface positions.43 Note that in KPFM the tip and sample are connected between the tip and the metallic back-plate supporting the sample via the ac and dc voltage and not by a direct contact between tip and surface. In the case of an insulator, the work function difference is measured between the tip and the metallic back-plate supporting the sample,29,31 whereas charges in the bulk or on the surface of the insulator strongly modify the Kelvin signal (Udc,0). Because surface charges are close to the tip apex, the surface charge distribution determines the Kelvin contrast in most cases.45 Despite this modification of the work function by charges, the term “work function” is kept throughout the paper for simplicity reasons. The Kelvin modulation is applied during the normal topography imaging, so that a topography and a Kelvin image (Udc,0) are gained the same time in one Kelvin measurement. Images were acquired with the Omicron SCALA system and prepared with the WSxM software.50 The sample was taken from commercial MgO single crystals of highest available quality (4N purity, Pi-Kem, England, size: 5 × 3 × 3 mm3) and mounted on a molybdenum sample holder. The surface was prepared by UHV cleavage at room temperature along the (001) cleavage plane.51 The crystal was then annealed in an UHV oven at 350 °C overnight (base pressure 5 × 10-10 mbar) in order to put the crystal into its equilibrium charge state.49,51 Palladium clusters were then epitaxially grown by condensing a calibrated beam of neutral palladium atoms from a Knudsen cell on the surface. The cleavage, the deposition of palladium and scanning force microscopy were done in the same UHV system.51 3. Experimental Results 3.1. Clean MgO(001) Surface. A Kelvin measurement is shown in Figure 1, which was performed on the MgO(001) surface directly after the UHV cleavage of the MgO crystal. The topography image (a) shows details, which are typical for the MgO(001) surface:35 Alongside atomically flat surfaces, which are intersected by monatomic high steps, rectangular pits

with a depth of one monolayer and with base lengths of 25 nm and more can be seen in the image. The steps and rectangular pits are randomly decorated by some bright spots or patches, which can be also sometimes found on the flat terraces. The patches have a lateral size of a few nanometers and exhibit apparent heights of up to a few nanometers. In the normal topography imaging mode of the dynamic SFM, nano-objects, which are much smaller than the tip apex, are imaged with a larger size - a result of the tip-surface convolution effect.42 If the objects are charged, the height in the topography image can even be falsely reproduced. However, small nano-objects can be imaged at least in their correct lateral shape and size if the constant height imaging mode is used.42 The topography image (a) in Figure 2 shows some patches at the top of the image, which seem to be attached at a dislocation line. Three of the patches were imaged in the constant height mode with a larger magnification and without the Kelvin modulation technique (b). The patches appear in a clear contrast with sharp edges along surface directions and exhibit a mean lateral size of 1-2 nm2. Around the patches the atomic resolution could be gained on MgO(001), which appears very faint in the image. A stronger atomic contrast can be seen inside the patches, which shows an atomic lattice that is ordered and oriented along the direction. Because of the large volume fraction of these features, an aggregation of impurities in the bulk onto the surface can be excluded as an explanation. Note that no impurities or metallic magnesium were ever detected after UHV cleavage of MgO crystals.25,52 It is anticipated, that the patches are rather clusters or fragments from MgO, which would explain the specific epitaxy of the fragments along the surface direction. Such MgO fragments are indeed believed to be stable on the MgO(001) surface,53 even if they are not stoichiometric.54 The fragments were probably created during the UHV cleavage and adsorbed at defects on MgO(001). The fragments show interesting contrast features in the Kelvin image (b) of Figure 1: Alongside an almost uniform dark contrast on the flat terraces, a bright contrast can be observed at steps of terraces and pits but also at the fragments. The bright contrast corresponds to more positive voltage values (Udc,0) exhibiting differences of up to 1.5 V with respect to the mean voltage of terraces (d). The histogram (e) of the Kelvin image permits a more quantitative analysis of the Kelvin contrast. A high peak on the left side, which belongs to the average voltage T ), can be found but also a smaller one above terrace sites (Udc,0 on the right side, which belongs to the average voltage above F ). A fit with two Gaussian curves shows that fragments (Udc,0

Kelvin Probe Force Microscopy on MgO(001) Surfaces

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Figure 2. (a) Topography image of the MgO(001) surface with some bright patches. (b) Detuning image, which was gained in the surface region marked by the dotted rectangle in image (a). A bright color belongs to more negative detuning values and vice-versa. Image (b) was recorded after a couple of other constant height images, where a tip change lead to a high-symmetric tip and therefore to a much higher resolution. Because the tip was a multiple tip before, the three patches in image (a) exhibit an irregular shape due to a complex tip-surface convolution.42 The dark shadows at the sides of the patches are a result of the imaging in the constant height mode.42 32 × 17 nm2, ∆f ) -20.2 Hz, VScan ) 0.5 Hz. (b) 12 × 9 nm2, ∆f ) -27.0 Hz, VScan ) 10 Hz, Udc ) -7.33 V.

Figure 3. Kelvin measurement of the MgO(001) surface with a rare dark Kelvin contrast at a few fragments. Shown are the topography image (a) and corresponding Kelvin image (b). A dark contrast in image (b) is in average 300 mV below and a bright contrast 300 mV above the mean value of -5.1 V. 200 × 150 nm2, ∆f ) -16.1 Hz, VScan ) 0.5 Hz, Uac ) 1.5 V, fac ) 474 Hz.

T the left peak is located at Udc,0 ) -0.20 V and the right one at F F T ) 0.13 V, yielding a mean difference of Udc,0 - Udc,0 ∼ Udc,0 350 mV. Note that this mean value was found for image (b) in Figure 1. However, the value can slightly vary from place to place on the surface but also from cleavage to cleavage. As mentioned before,49 additional soft variations in the Kelvin contrast can be sometimes observed apart from the bright Kelvin contrast at steps and fragments (compare lower left quarter with upper right quarter of image (b) in Figure 3). It is believed that this type of contrast is due to charges below the surface, which are far away from the tip and which cannot be well resolved.49 In very rare cases, also a sharp dark contrast with respect to terrace sites could be found. The Kelvin measurement shown in Figure 3 exemplifies such a rare dark contrast, which can be seen at a few fragments on the surface (e.g., at positions (1) and (2)). Because no other material than MgO is present on the surface, which could produce a bright Kelvin contrast, it can be anticipated that the bright Kelvin contrast is due to “charged species”, which are located at the steps of terraces, pits and at the fragments. As explained in ref 45, a bright Kelvin contrast belongs to something more negative on the surface, so that the species are more negatively charged with respect to the terraces. The Kelvin images suggest that the charges seem to have a lateral size of some nanometers. However, an agglomeration

of charges at this scale is highly unlikely, so that it can be assumed that the charges are due to charged defects of atomic size. In this case the point charges image effectively the nanometer sized tip-apex due to the long-range electrostatic tipcharge interaction, which produces nanometer large bright spots in the Kelvin image.45 In the literature many types of negatively charged defects can be found (a good overview of defects on the MgO(001) surface is given by ref 55). Candidates are unscreened, lowcoordinated anions at kinks and corners,56 charged cation vacancies,57 charged divacancies,58 or shallow traps at kinks.59 Because defects of the MgO(001) surface are also quite reactive toward gaseous species like oxygen, water or CO, which are the main constituents of the residual gas of the UHV chamber, negatively charged radicals like O2- or CO- can be produced especially at F centers (see ref 60 and the references therein). Although all of these defects can be created in principle during the cleavage and by a chemical reaction with the residual gas of the UHV, it remains unclear why negatively charged defects preferentially appear on the surface, a disagreement especially in view of the large facility to create positively charged F centers.61,62 However, a specific surface mechanism based on the thermodynamic equilibrium between positively and negatively charged defects on and below the surface could explain this observation: The surface carries a net negative

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Figure 4. KPFM measurement of Pd nanoclusters grown on the same MgO(001) surface shown in Figure 1. Image (a) corresponds to the topography and image (b) to the corresponding Kelvin signal. Profiles are shown in (c) and (d), which were taken at the green lines in the images (a) and (b). Graph (e) shows the voltage distribution of the Kelvin image (b). 400 × 400 nm2, ∆f ) -16.2 Hz, VScan ) 0.5 Hz, Uac ) 1.0 V, fac ) 477 Hz.

surface charge due to the presence of a surface double layer (Debye-Frenkel layer) as in the case of alkali halide (001) surfaces:45 If there is a difference in the free formation energy between negatively and positively charged defects, one type of charged defect is found on the surface and the other type below the surface. This double layer guarantees that the crystal stays globally neutral.63 In the case of MgO, a double layer would be composed of a space charge layer of, e.g., trivalent impurity ions below the surface and a layer of cation vacancies or other negative species directly on the surface delivering the surface a net negative surface charge.45 Indeed, trivalent impurities can be found in any MgO crystal of even highest purity. It has been shown for instance that impurity ions such as Fe3+, Cr3+, or Sc3+are located in the subsurface region of MgO crystals of highest purity.64 Furthermore, a net negative surface charge could be indeed measured.65 The crystals used here contained 40 ppm Fe as a main impurity source so that a double layer is likely. Although the surface carries a net negative surface charge, it seems that positively charged defects are also sometimes present on the surface (Figure 3). Potential candidates are rather F+ centers than F2+ centers because the formation energy is much larger for a F2+ center than for a F+ center.62 The F+ centers find their energetic minimum at low coordinated surface sites and strongly change the electrostatic potential locally on the surface.62,66 Because these defects are quite reactive toward gaseous species,60 it can be speculated that they exhibit a strong electrostatic interaction with the tip. This probably explains the sharp dark contrast in the Kelvin images but also the elongated shape of the dark patches along the fast scanning direction, where the Kelvin regulation was too slow to follow the strong change in the electrostatic tip-surface interaction during passing the defect. Model calculations in combination with Kelvin measurements on F+ center enriched MgO(001) surfaces must be done in future in order to explain the specific dark contrast found on the surfaces here. 3.2. Supported Pd Nanoclusters on MgO(001). If metal is evaporated on a MgO(001) surface, the metal forms 3D clusters, which are mostly attached at steps and point defects on terraces.14 The topography image (a) in Figure 4 shows the MgO(001) surface at room temperature, after an evaporation of palladium at 460 °C sample temperature (2.3 ML, flux ∼1 × 1013 atoms/cm2 s, 300 s deposition time). The surface resembles the Pd/MgO(001) surfaces investigated before.52,37 In the surface region of image (a), a cluster density of 4.2 × 1011 clusters/cm2 was found. The cluster exhibit heights of up to 7 nm (c) counting each about 300 × 1013/4.2 × 1011 ≈ 7000

Figure 5. (a) Pd nanoclusters on MgO(001) after three weeks in UHV (topography image). Image (b) and (c) were recorded in the region labeled by the dotted white square. Image (b) shows the topography signal and image (c) the corresponding damping signal. The corrugation in (a) measures 80 pm and is larger than what was measured and calculated before.35,67 The tip-surface distance was probably very small, which explains also the atomic 10% change in the damping signal.68 (a) 62 × 68 nm2, ∆f ) -15.6 Hz, VScan ) 0.5 Hz (b) 3.8 × 3.8 nm2, ∆f ) -106 Hz, VScan ) 5 Hz.

Pd atoms in average. The cluster seem to cover 60% of the MgO(001) surface, which is, however, a too large value. The value is a result of the tip-surface convolution effect where the clusters appear larger and more distorted in the images than in reality.42 The true coverage measures only 20%, which was calculated from the mean number of Pd atoms in a cluster and from the cluster shape of a truncated octahedron.4,5 This result agrees well with the value of similar Pd/MgO(001) surfaces determined by TEM measurements.42 Between the clusters, the MgO(001) surface is clean apart from some atomic point defects, and the atomic resolution can be routinely gained, even after some weeks during which the surface is exposed to the residual gas of the UHV (see Figure 5). The Kelvin image (b) in Figure 4 shows a strong bright contrast at all clusters exhibiting voltage differences of up to 3.5 V with respect to flat terraces (Figure 4d). The minimizing C voltage at the clusters (Udc,0 ) is more positive with respect to C T the one above terrace sites (Udc,0 > Udc,0 ). The histogram of the voltage distribution (Figure 4e) shows two clear peaks, which were fitted with two Gaussian curves. The peaks are located at T C Udc,0 ) 5.27 V (left peak) and Udc,0 ) 7.69 V (right peak) C T yielding a mean voltage difference of Udc,0 - Udc,0 ) 2.42 V.

Kelvin Probe Force Microscopy on MgO(001) Surfaces

Figure 6. Kelvin measurement of palladium nanoclusters on MgO(001). (a) Topography image and corresponding Kelvin image (b). 70 × 70 nm2, ∆f ) -16.1 Hz, VScan ) 0.5 Hz, Uac ) 1.0 V, fac ) 477 Hz.

The latter mean voltage difference and deviating voltage differences of up to 3.5 V are very large values in KPFM, especially in view of the nanometer sized Pd cluster. In order to interpret the voltage difference of 2.42 V, it has to be clarified how local the tip probes the clusters or the clean MgO(001) surface. When the tip is above a Pd cluster during scanning, the tip probes the cluster in a close distance of 1-2 nanometers. Frequency modulated KPFM is sensitive on the electrostatic force gradient, which decays much faster than the electrostatic tip-surface interaction, the last nanometers of the tip apex are then the most important part in the tip-cluster interaction.69 Before KPFM images can even be recorded with a high resolution, the tip is first accidentally in contact with the surface in most cases. During such an uncontrolled contact, the tip can pick-up or drop material from the surface, which decreases or increases image resolution (see Figure 8 in ref 70). In the latter case a sharp “nanotip” at the tip apex can be formed by, e.g., picking up palladium from the clusters, what had probably been the case during a tip change before the Kelvin measurement in Figure 4. A nanotip increases the resolution in the topography image but also the lateral resolution in the Kelvin image, as explained by recent calculations.71 The Kelvin measurement in Figure 6 shows a larger magnification of the surface region that is marked by the bright dotted square in image (a) of Figure 4. The profile of the topography (a) and Kelvin image (b) demonstrate that the lateral resolution of both images is almost the same. A value of at least 5 nm can be found for the lateral resolution, which is roughly comparable with the mean lateral size of the palladium clusters (cluster shape of a truncated octahedron: top facet ) 2.5 nm, base length ) 7.0 nm). With good confidence it can be therefore assumed that the electrostatic tip-surface interaction is mainly characterized by the electrostatic interaction between the cluster and the nanotip, which both are then the main elements determining the minimizing voltage Udc,0 between tip and cluster. In KPFM, the dependence of the electrostatic interaction between a tip and the surface of a bulk insulator is of capacitive nature, a quadratic dependence of the detuning ∆f on the bias voltage UDC can be observed:38,49 The tip gets polarized by the crystal surface and in turn couples to the metal back-electrode.29,31,71 Because the clusters are not electrically connected to the metal back-electrode but instead they are supported on the thick dielectric MgO crystal, the compensating bias voltage reflects on the work function of the Pd clusters, which is, however,

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Figure 7. Work function in dependence on the nominal coverage of Pd measured by UPS and MIES. The data was taken from the work of Krischok and co-workers (Figure 2 in ref 26). At the lower part, a work function change of 0.5 eV for a coverage at 2.3 ML of Pd can be seen, whereas at 10 ML the work function increased to ∼5.0 eV by 1.5 eV. The increase of the work function until the bulk value for palladium is sketched on the right side.

strongly modified by the tip-crystal capacitor and charges that may exist in the bulk. This probably explains the large absolute C ) values measured between the Pd clusters and the tip (Udc,0 7.69 V). However, the capacitor and bulk charge contribution is only a constant offset, which is not expected to change much locally on the surface. This means that, if the tip is located above the clean MgO(001) surface in the vicinity of a cluster, the same offset value is measured. The work function of the Pd cluster and the one of the MgO(001)/back-electrode system are now regarded. A Pd cluster contains about 7000 atoms in mean. The ionization potential for such large clusters is then close to the work function of the corresponding bulk metal as it was found for many metal clusters.72,73 With good certainty, the work function of bulk Pd(001) (φPd ) 5.7 eV)74-76 can then be taken. In view of the work function of MgO(001), MIES, UPS,24,25 and 2PPS27 measurements can be compared with the findings of this work. MIES and UPS measurements in combination with theory revealed that the electronic configuration of the bulk MgO(001) surface is similar to the one of thin MgO(001) films. A work function of φMgO ) 2.7 eV was found for ∼2 nm thick MgO(001) films (∼10 ML) grown on W(110) and Mo(001).25 A recent investigation conducted by the same authors yielded a value around 3.4 eV for MgO(001) on W(110) (see Figure 2 in ref 26). Theory roughly reproduces these values and has shown that the work function of MgO(001) films does not change if the film has a thickness larger than 2-3 ML.17,18 If it is assumed that KPFM measures the work function difference between MgO(001)/Mo and the Pd clusters and if the value of φMgO ) (2.7 + 3.4)/2 eV ≈ 3.1 eV is the value for the work function of the bulk MgO(001)/Mo system, the difference φPd - φMgO ) (5.7 - 3.1) eV ) 2.6 eV should be almost the same C T - Udc,0 )e, which is indeed as the experimental value of (Udc,0 the case (2.42 eV). For a further interpretation, the MIES measurement in ref 26 can be analyzed, which shows directly the change of the work function difference in dependency on the Pd coverage on MgO(001). The data is represented in Figure 7 for simplicity. For 2.3 ML Pd on MgO(001), a difference of only φPd - φMgO ≈ 0.5 eV can be found, which does not coincide with the result of 2.42 eV of this work. However, both measurements can be compared as follows. The Kelvin microscope images the clusters locally with nanometer resolution yielding always the work function difference between Pd cluster and the MgO(001)/Mo system, whereas MIES measurements yield an average of the

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Figure 8. Same image as the one in Figure 4 in a color scale that increases the contrast at cluster sites. All voltages have the same color below the cutoff voltage at 7.0 V.

surface system. In the case of 2.3 ML Pd on the MgO(001) surface, only 20% of the MgO(001) surface is covered by Pd cluster so that the averaged work function of the whole surface system is 2.42 eV × 0.2 ) 0.48 eV for this work, which is much the same as the MIES value (≈0.5 eV, see Figure 7). Note that for a Pd coverage of 10 ML, 70% of the MgO(001) surface is covered by Pd cluster (see Figure 20 on page 271 in ref 14), which explains that only 3.4 eV × 0.3 + 5.7 eV × 0.7 ) 5.0 eV are measured in a MIES measurement being still far below the Pd work function of 5.7 eV (Figure 7). From the analysis above the following conclusion can be drawn: It appears that also KPFM measures the work function difference between metal nanoclusters and the MgO(001)/Mo system. For this assumption the work function of bulk palladium and of the MgO(001)/Mo system are used as an input. This all then implies that KPFM does not yield the energy of 6.7 eV between the top MgO valance band and the vacuum level,24,66 which can be also seen as the “work function” of MgO(001) corresponding to the energy needed to extract an electron with lowest energy from the MgO(001) surface. The Kelvin microscope is a complementary technique to MIES, UPS, 2PPS and the classical Kelvin method but permits a local analysis of single clusters with a lateral resolution at the nanometer scale. In order to see differences in the Kelvin contrast among groups of clusters or single clusters, image (b) of Figure 4 is displayed in a color scale with an enhanced contrast at cluster sites in Figure 8. In some surface regions of a high cluster density, a higher contrast (green and white) corresponding to absolute voltages (Udc,0) between 9.5 and 8.5 V can be observed (e.g., at (1)) with respect to other regions of a similar cluster density, where a considerably lower contrast (dark green, blue) with values between 7.0 and 8.0 V can be found (e.g., at (2)). Most of the clusters exhibit a blue color, which corresponds to a voltage of 7.5 V and which is a bit smaller than the mean value of 7.7. Some single cluster on terraces are even not visible in this image due to the cutoff color (dark blue) at 7.0 V. There is no unambiguous explanation for these large voltage variations among the clusters so far. An influence of the finite size of the whole macroscopic tip apex can be almost ruled out because the last nanometer of the tip probes each single cluster on the surface as explained above. It is rather likely, that the contrast was due to differences in the electronic properties among the clusters. An explanation could be that the electronic structure of each cluster is different due to differences in size and shape of the clusters. Another, more likely explanation is

Barth and Henry that the electronic properties of some clusters were modified by the MgO(001) substrate, especially at the charged defects of the clean surface. A similar proposition was made for gold clusters grown on (001) surfaces of alkali halide crystals, where groups or rows of clusters exhibited also large differences in the Kelvin contrast.48 If the variations of the Kelvin contrast above the Pd clusters is compared with the defect-induced variations observed on the clean MgO(001) surface (Figure 1) and comparable surfaces like NaCl(001) or KCl(001)48,45 (300-600 mV), it can be roughly estimated that the variations among the Pd clusters are due to charge differences of only a few electrons and below. However, for a unambiguous interpretation of this particular Kelvin contrast, theoretical calculations of charged and uncharged Pd clusters must be accomplished, especially in view of the work function or the ionization potential. A promising start to calculate the ionization potential is the image charge model for instance, which is also known as the “droplet model”.72,73,77 An additional mechanism in KPFM of metal nanoclusters plays probably an important role: The tip polarizes the clusters and induces, e.g., tunneling phenomena of charges among clusters, which keep a very close cluster-cluster gap distance of only a few nanometer in regions with a high cluster density. This “intercluster communication effect” has been observed before on NaCl(001) or KCl(001) surfaces among gold clusters48 and could explain also the uniform contrast, which can be found in regions of a high cluster density like in, e.g., region (1) (Figure 8). In order to investigate this “communication effect” KPFM measurements on surfaces with different Pd coverages in combination with controlled charge injections by the tip into a single cluster could help for a better understanding. For theory it means, that the presence of the tip above the clusters must be included in numerical simulations. 4. Conclusion In conclusion, a KPFM study of the clean (001) surface of UHV cleaved MgO and of the same surface with supported Pd nanocluster is presented. Kelvin images show a preferential bright contrast at steps and especially at fragments of MgO, which were produced by the UHV cleavage and which could be imaged with atomic resolution. The Kelvin contrast is due to negatively charged defects of atomic size, which change locally the electrostatic potential on the surface. It is anticipated that also the MgO(001) surface exhibits a surface double layer (Debye-Frenkel layer) as in the case of alkali halide (001) crystal surfaces. Pd nanoclusters exhibit a bright Kelvin contrast corresponding to a mean voltage difference of 2.4 V with respect to the MgO(001) surface. Because the tip probes locally the clusters and the MgO(001)/Mo system, the work function difference between both of them is measured. It is shown that the results can be well compared with the ones obtained by MIES and UPS measurements. Because of the high lateral resolution of the Kelvin microscope, differences in the Kelvin signal among Pd clusters could be observed, which point to clusters with different electronic properties. The work presented here is a promising step toward further KPFM studies of metal nanoclusters supported on oxide surfaces. In future theory has to be used for a precise understanding of the contrast mechanism in KPFM and for a quantitative interpretation of the Kelvin contrast. An exciting aspect is the study of much smaller nanocluster where size effects are expected to sensitively change the ionization potential of the metal nanoclusters, which can be imaged by KPFM.

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