Imaging of sodium decatungstocerate (IV) by scanning tunneling and atomic force microscopy

‘surface scienC6

Surface Science 264 (1992) 271-280 Noah-Holland

Imaging of sodium decatungstocerate and atomic force microscopy

(IV) by scanning tunneling

B. Ke’ita, F. Chauveau ‘, F. ThCobald 2, D. BClanger 3 and L. Nadjo * Laboratoire d’Electrochimie et de Photo&fectrochimie, URA 1383 CNRS, Universdte Paris XI, B&iment 420, 91405 Orsay Cedex, France

Received 22 July 1991; accepted for pubIication 1 November 1991

.30H,O has been selected as a representative example of discrete, molecular Sodium decatungstocerate (IV) Na,H,CeWt,Os, mineral material for comparison of the observations made by the AFM and STM techniques. The surface of the single crystal has been imaged with molecular resolution by the AFM, showing a regular array. The AFM images of the single crystal wet with solvent are less ordered but the dimension of individual molecules could also be measured. STM images of sodium decatungstocerate (IV) deposited from methanolic solution on highly oriented pyrolytic graphite (HOPG) give the same molecular dimension as determined from the AFM images. The close similarity of the AFM and STM images demonstrates the reproducibility of the observed structure and the good correlation between the two techniques in the present case.

1. Intrusion The development of an emerging family of scanning probe microscopes [l-3] is setting down new powerful tools for the characterization of surface structure or molecular structure. Images are obtained by moving a sharp tip on or near a surface and sensing and monitoring the changes in tip position or some other tip variable. Among these techniques, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have proved to image with astounding resolution. Molecules down to atoms can be “seen” in favorable cases. In principle, STM cannot be used with insulating substrates. Early results for molecular adsorbates appeared somewhat disappointing [46]. However, it soon came out that conditions can be found to observe by STM various nonconduc-

* To whom correspondence should be addressed. ’ ITODYS, Wniversiti Paris VII, 1 place Jussieu, 75232 Paris Cedex 05, France. ’ Laboratoire de Cristallographie, UniversitC Paris XI, BItiment 490,91405 Orsay Cedex, France. 3 Departement de Chimie, Universitd du Quebec B Montreal, Montreal, Canada, H3C 3P8.

tive materials deposited on a conductive layer and several pioneering works in this domain have been published [7,8]. This communication shows that AFM and STM can achieve high resolution in the imaging of discrete heteropoly and isopoly oxometalate species. Our interest in this class of chemicals has been triggered for several years and we have demonstrated that oxometalates can be used to modify persistently electrode surfaces and bring about strikingly favorable catalytic properties as regards several challenging electrochemical processes as the hydrogen evolution reaction or the reduction of oxygen [9]. However, the chemistry and geometric factors that lead to such remarkable properties are, at best, partly understood. Hence the idea that scanning near-field microscopies coupled with electrochemistry, may also help in the elucidation of the transfo~ations which induce this surface activity. A necessary step in this direction is that high resolution images of the starting oxometalates could be obtained. The present study is conducted on sodium decatungstocerate (IV) which is a representative compound of this class of chemicals. The molecular dimensions, measured from ABM and STM

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E. Ke?ta et al. / Imaging o~.~~d~~~ decut~ffgstocerate #“iY~by STM and AFM

images respectively, are compared. Several reasons make such a comparison most interesting. The STM responds to the local density of states at the Fermi level of the sample. Therefore, the image does not necessarily represent pure topography. Rather the point-to-point variations of the controlled parameter due to spatial variations of the sample can be superimposed, altering the observed pattern. The AFM, on the other hand, samples the total electronic density at the surface which contributes to the atomic force measured by the AFM tip. The principles are thus completely different and may not give the same results.

the surface arrangement of the single crystat. Such crystals proved still suitable also for crystallographic determinations after AFM experiments. Some other crystals were completely impregnated by the solvent of the glue and these “‘wet crystals” gave more “faulted” AFM images as will be described in the following. For the STM images, a crystal of sodium decatungstocerate was dissolved in methanol (typically 10-4M). A drop of that solution was deposited on a freshly cleaved surface of highiy oriented pyrolytic graphite (HOPG) and the solvent was allowed to evaporate at room temperature, then at 60°C in an oven overnight (typically 12 h).

2. Experimental 3. Results Measurements were made in air at room temperature with a Nanoscope II (Digital Instruments, Santa Barbara, California). Pt-Ir wires were employed as tunneling tips for the STM image. The same instrument, properly implemented, was used for the AFM, in the constantforce mode. It combines a microfabricated Si,N, cantilever with the optical lever technique. The cantilever has a spring constant of appro~mately 0.58 N/m and during the imaging exerted a force of approximately lo-’ N on the surface of the sample. Some images have been fast-Fouriertransform filtered to remove high-frequency noise. Multifaceted yellow single crystals of sodium decatungstocerate (IV) of formula Na~H*CeW~~~O~~ .3OH,O, were grown, following the literature [ll]. X-ray diffraction analysis gives exactly the same parameters as published previously by Weakley [12] for this structure, thus confirming the identity of the compound. Sample preparation proved important; it depended on the observation technique to be used. For the AFM images, crystals showing visually large flat terraces have been selected and glued onto the steel sample-holder. Two situations were observed. Some crystals were thick enough not to become soaked up to their surface by the solvent contained in the cyanocrylate adhesives (Ioctiten or Super Glue 3TM). The images of these “dry crystals” can be considered as representative of

Fig. 1 represents the AFM image of the surface of a “dry” single crystal. Fig. la is the unfiltered line-plot of this surface. It is remarkabIe that regular contours can already be distinguished on this image. Therefore, the FFT filtering process is expected to remove mainly highfrequency noise without altering the pattern. Figs. lb and lc represent such filtered images. Two different displays of the same surface area underscore the main features of the sample. Fig. lb shows clearly that several domains exist on the surface: a relatively regular pattern seems to dominate over more “faulted” areas. The pseudo-three-dimensional representation of fig. lc best visualizes the subtle detaiIs of the surface structure. Even in that display, areas appear in which reliable crystal parameter determinations are possible. That some of the observed periodic contours may represent individual sodium de~atungstocerate species stems from two facts: (8 the AFM records the surface topography, (ii) most transition metal polyoxoanions are discrete, molecular compounds, with their structures based on close-packed oxygen stackings containing interstitial metal centers. The sodium decatungstocerate (IV) belongs to such a family and crystallizes with 30 water molecules in the monoclinic system [12], space group C2/c, a = 18.14 A, b = 18.62 A, c = 18.51 A, /3 =95.9”, 2 = 4. Fig. 2

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Fig. 1. Line-plot AFM images of the surface of a single crystal of sodium decatungstoceracte (IV) in air at room tempexature. (b) and (c) have been filtered by two-dimensional fastFourier transfo~ation. (a) Raw data; pitch angle 60”. (b) Pitch angle 60“. Cc)Pitch angle 30”. Three dimensional line-plot image.

Fig. 2. Structure of a sodium decatungstocerate (IV) crystal, shown in the coordination polyhedral model and the spacefilling model, respectively, the spheres essentially representing close-packed oxygen atoms.

exhibits this heteropoly~ion as an assembly of WO, octahedra surrounding the cerium (IV) species and also the corresponding space-filling model is represented. Obviously, this model shows the contour which contributes mainly to the atomic force measured by the AFM tips. Taking into consideration the presence of sodium counter cations and of water molecules, such a contour would explain that this molecular species can hardly be expected to be imaged in two symmetrical halves. “Wet crystals” have been imaged also by the AFM. Corresponding images appear on fig. 3. These images are strikingly different from those

B. Ke2a et al. / Imaging of sodium deca~angs~ocera~e (VII by STM and AFM

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Fig. 3. Line-plot

AFM image of the surface of a “wet” single crystal of sodium decatungstocerate pitch angle 60”. (a) Raw data. (b) Image filtered by twt a-dimensional fast-Fourier

obtained from the “dry” crystal. The pattern is much less regular than previously. However, even the unfiltered image (fig. 3a) suggests the presence of well-resolved contours. The fiItered images show more cIearly small areas in which molecular contours can be distinguished. The STM images of sodium decatungstocerate (IV) deposited on HOPG are shown in fig. 4. The observed pattern is much the same as that of the AFM images of “wet crystals”. The reiative dis-

Fig. 4. Line-plot

STM image of sodium decatungstocerate (a) Raw data. (b) Image filtered

(IV) in air, at room temper transformation.

rature

order observed on the STM images is not surprising, as no attempt has been made to obtain a uniform monolayer of the heteropolyanion on the HOPG and it is anticipated that the deposition should be random and give a multilayer coverage. Also, evaporation of the solvent during the preparation of the STM samples could bring about some dehydration of the sodium decatungstocerate (IV) species, inducing that random deposition. Even so, on the unfiltered image

(IV) deposited on HOPG (I = 0.76 nA, U = 57.7 mV). pitch angle: by two-dimensional fast-Fourier transformation.

60”.

B. Kei:a et al. / Imaging of sodium decatungstocerate (VI) by STM and AFM

and more obviously on the filtered ones, small regular geometric patterns appear, which should be suitable for molecular size determinations from the images. Clearly, the images cannot be misinterpreted as being those of the underlying HOPG, because the apparent period is much larger than expected for graphite. For quantitative determinations, areas as regular as possible have been selected on each kind of

27.5

image, and are then magnified. Figs. 5 though 7 show such zoomed images, with the sections which have been retained for the present measurements. The lines along which the cross sections are taken are indicated on the corresponding figures. Obviously, the more reliable results must be obtained from fig. 5 which is the AFM image of the single-crystal surface. The main dimension obtained for each entity of the regular array is

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Fig. 5. (a) Zoom on a part of fig. la (AFM image of the single crystal). Pitch angle: 60”. The line along which the cross section is taken is indicated. (b) Section along the line indicated in (a).

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17.0 k 0.5 A, which is very close to the corresponding crystallographic value 17.1 A. This result reinforces the idea that the AFM image represents individual molecular species. The very regular arrangement of molecules on the singlecrystal surface may be tentatively compared to

(VI) by STM and AFM

those expected for various faces of the crystal. Such an attempt has been described in more detail elswhere [13]. For the area examined, it does appear that the surface is a simple termination of the bulk. The main point here is to show that the AFM image represents the topography

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(b) Fig. 6. (a) Zoom on a part of fig. 3 (AFM image of the “wet” single crystal). Pitch angle: 60”. The line along which the cross section is taken is indicated. (b) Section along the line indicated in (a).

B. Kei’ta et al. / Imaging ofsodium decatungstocerate (VI) by STM and AFM

of the surface with the possibility that moiecular entities be identified and quantitatively measured with a good approximation. Figs. 6a and 7a are, respectively, a zoomed AFM image of the “wet crystal” or a zoomed STM image. With the corresponding sections on

figs. 6b and 7b, they show that individual entities can be distin~ished randomIy on the surfaces and their dimension measured. The molecular dimension obtained from the measurements remain in the range of 16.7 k 0.5 A. It must be stressed that the measured values are close to

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Fig. 7. (a) ‘Zoom on a part of fig. 4 (STM image of the sodium decatungstocerate deposited on HOPG). Pitch angte: 60”. The line along which the cross section is taken is indicated. (b) Section along the line indicated in (a).

that measured from the AFM image of the “dry” single crystal. Such an agreement proves that individual molecular species are indeed observed. It appears that the repeat distances obtained from the AFM images of the ‘“dry” single crystal [13] cannot be measured here because the system is disordered and several layers may overlap partly.

4. Discussion Advantage has been taken of the versatility of the AFM concerning the conductivity of the sample, to image directly the surface of a single crystal of sodium decatungstocerate. Furthermore, it was expected that this AFM image may not be distorted by the electronic properties of the sample holder. Even with the crystal soaked by the solvent from the glue, the AFM technique provides us with completely independent measurements to be compared with those obtained from the STM images on HOPG. A very good agreement is obtained between measurements culled from single crystal AFM images, “wet crystal” AFM images and STM images of the sodium decatungstocerate (IV) deposited on HOPG. Apparently, these techniques thus allow the imaging of this molecular species with a good accuracy. The structure observed by AFM for the “wet crystal” can be explained in the following way, starting from that of the single crystal. Contrary to some other salts of heteropolyanions in which the cations play a prominent role in Iinking together the anions [14-161, it has been found that .30H,O, two of the four in Na,H$ZeW,aO,, structurally distinct sodium atoms are present as Na(H20)6+, the other two are bonded to just one terminal oxygen atom of an anion and five water molecules each [12]. Therefore, the participation in hydrogen bonds to water of the remaining terminal oxygen atoms and most of the other oxygen atoms on the outside of the anion is essential to the crystal cohesion. Then, wetting of the crystal by a tiny amount of solvent will destroy hydrogen bonds, at least partly, which results in the disordered structure observed on the

AFM image. It is then conceivable that the surface of the “wet crystal” closely resembles the STM image of sodium decatungstocerate, deposited on HOPG by evaporation of a methanolic solution. Such a sample preparation for STM cannot be expected to give a perfect singIe crystal. Preparation of a monolayer should need particular care which has not been taken here. Therefore, only short range order of individual molecules is possible, as also observed previously for ~~PW~~O~~ -nH*O, deposited on HOPG from a methanolic solution 1171. Concerning the STM images, the problem of electrical conductivity arises. First, several studies are published in which nonconductive materials. deposited on HOPG have been imaged by STM [4-81. It has usually been admitted that the monolayer in contact with the HOPG is effectively imaged. Second, the situation is somewhat more favorable with heteropolyanions, several of which are known to be largely hydrated 1181with high protonic conductivities [19,20] superimposed on semiconducting or even semimetallic electronic properties. Even though the use of methanolic solution to deposit the sodium decatungstocerate (IV), may favor dehydration, it cannot be excluded that some conductivity remains. It is rewarding that the molecular species of the oxometalates series could be imaged efficiently by AFM and especially by STM. Several authors have previously pointed out and discussed the difficulties attached to the imaging of molecular adsorbates [7,8,21-231 or compare and discuss AFM and STM imaging principles [21,241. The failures in STM have generally been attributed to rapid surface diffusion, possibly augmented by electric fields or to the absence of molecular orbitals near the Fermi level (E,) which are accessible by STM f7]. AFM images of molecular adsorbates can also be altered by several artifacts among which the elastic deformation of soft layered surfaces and the simultaneous imaging by multiple tips [211. They can also be unstable if good adhesion of the adsorbate on the sample holder is not achieved. The system of the present study hardly suffers any of these shortcomings: The molecules appear as large rigid cage-like complexes. They strongly adhere to the

3. Keiia et al. / Imaging of sodium decatungstocerate (VT) by STM and AFM

substrates, in particular to carbon. This observation is in line with electrochemical experiments [9] and heterogeneous phase catalyst preparation [25f in which oxometalates have been demonstrated to adsorb on various solids. It is worth noticing that the sample preparation technique used for STM has previously been applied to the preparation of electrode surfaces modified with oxometalates. Such electrodes have been studied then in plain supporting electrolyte without observing the dissolution of the adsorbed oxometalate into the solution [9]. We believe that such properties are particularly favorable for imaging by STM and AFM. First, the good adhesion ensures that the image be stable for several hours, which is actually observed. Also, the molecular resolution appears even on the unfiltered images. Second, a large molecule with semiconducti~g properties should have severai electronic states which can be split by strong molecule-substrate interactions to yield molecular orbitals near the Fermi level. The reasons probably account for the small voltage bias and tunneling current sufficient to obtain high-resolution images of the present molecular species. Correlatively the experimentsll observations confirm the semi~nductor-like or even metal-like behaviour of several representatives of this class of chemicals. The shape of the molecule of sodium decatungstocerate (IV) as well as its dimensions measured from AFM images of the single crystal or the “wet” crystal or from STM images agree with each other and coincide with the expectations from crystallographic determinations. Such a result is most striking and interesting as it pertains to the same molecule studied under very different conditions and with two techniques based on different principles. Indeed, the AFM image samples the entire electron density at the surface of the sample and may be complicated by the influence of the bulk of the crystal and by friction effects between the cantilever and the sample. The STM image samples the electron density of states near the Fermi level and may be severely affected in particular by tip-sample and/or sample-substrate interactions. Here, contrary to other examples [lo], the STM seems to represent pureIy the topography of the surface.

279

At least two contrasting mechanisms res~nsible for the well-resolved images in STM have been proposed in the literature [22,23]. The experiments of Mizutani et al. support a resonant tunneling model [23]. The present results would tend to favour that of Spong et al. [22] which is based on a modulation of the local work function of the substrate by the polarizable molecular adsorbate even though such ~larizabili~ is not straightforward in the molecule under study. Further work would be necessary to get more insight into the imaging mechanisms in AFM and STM of oxometalates. In any case the good agreement between the images obtained by AFM and STM would suggest that all possible distorting influences are minimal or fortuitously parallel each other.

5. Conclusion Sodium decatungstocerate (IV) has been found to be suitable for comparison of two new powerful tools: STM and AFM. The results prove that these two instruments can achieve molecular resolution in the obse~ation of sodium decatungstocerate (IV) which is reproducibly imaged. Furthermore, it can be concluded from this comparison, that the role of the underlying HOPG on the observed STM images is minimal and does not distort them to a perceptible extent in the present case.

This work was supported by the CNRS (URA 1383) and by the Universitk Paris XI. Drs. E. Paris (Instrumat, I-es Ulis, France) and K. Kjoller (Digital, Santa Barbara, California) are thankfully acknowledged for their help in running the AFM experiments.

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(VI) by STM and AFM

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