in situ scanning tunneling microscopy

J. Electroanol. Chem., 290 (1990) l-20 Elsevier Sequoia S.A., Lausanne

Review

In situ scanning tunneling microscopy

New insight for electrochemical electrode-surface investigations

Tommaso R.I. Cataldi l , Ian G. Blackham, G. Andrew D. Briggs and John B. Pethica

Department of Materials, Oxford University, Parks Road, Oxford OXI 3PH (Great Britain)

H. Allen 0. Hill **

Inorganic Chemistry Laboratory and The Oxford Centre for Molecular Science, South Parks Road,

Oxford OX1 3QR (Great Britain)

(Received 23 March 1990)

ABSTRACT

The successful expansion which the scanning tunneling microscopy (STM) has had is dependent on its ability to examine surfaces on a sub-nanometric scale and on providing in situ (i.e. in the presence of bulk electrolyte) sample examination. In addition to the ability to study metals and semiconductors in vacua, the application of the technique to surfaces in contact with an electrolytic solution has prompted increased interest amongst electrochemists. We discuss herein the technique, with particular reference to advances in electrochemical applications. A new scanning tunneliig microscope for operation in electrolytic environments is described. Atomic force microscopy, scanning electrochemical microscopy and scanning ion-conducting microscopy are compared with the STM.

INTRODUCTION

The invention of STM by Gerd Binnig and Heinrich Rohrer and their associates [l-4] at the IBM Research Laboratory in Zurich in the early 198Os, has heralded a new area for research into surface topography. That work resulted in their sharing the 1986 Nobel Prize in Physics with Ernst Ruska, who constructed the first electron microscope.

l Permanent address: Universita’ degli Studi della Basilicata, Istituto di Chimica, Facolta’ di Scienze, Via N. Sauro, 85, 85100 Potenxa, Italy. l * To whom correspondence should be addressed.

OO22-0728/!90/%03.50 6 1990 - Elsevier Sequoia S.A.

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Unlike electron microscopes(SEM, TEM, STEM), electron diffraction techniques (LEED, RHEED, XRD) or electron spectroscopies (XPS, UPS, AES), which can operate only in ultra-high vacuum (UHV) due to the short scattering length of electrons in the presence of air or fluids, STM (based on quantum mechanical tunneling of electrons over short distances) can be used in various media. Further- more, although each analytical technique is capable of probing a small area of the sample and is valuable in providing complementary information for the system under study, the topographic resolution is not comparable with the STM. Hansma and co-workers have demonstrated that the tunnel effect and consequently the functioning of the STM is not limited to experiments in vacua; but is also successful in air, paraffin oil and solutions of electrolytes [5-71. These unique features make STM a suitable method for the investigation of chemical and electrochemical processes and their dependence on surface structure. The aim of this paper is to describe the relevant characteristics, and electrochemical applications of STM. A number of valuable reviews dealing with the general theory and practical aspects (instrumental characteristics, isolation of the vibrations, electronics, and tip preparation) of STM have already been reported [8-131

PRINCIPLES OF OPERATION

Tunneling current vs. tip-sample separation STM, with its ability to position a metal tip at molecular distances from a solid

conducting surface and obtain significant topographical information with atomic resolution, has stimulated a wide range of new applications and investigations [14-171. The basic principles of STM involve the positioning of a chemically inert, fine metal tip (W, Ta, Au or Pt(Ir) within a few angstroms (5-10 A; 1 A = lo-” m) of the sample surface so that the electron wave functions of the tip and the sample overlap. Application of a low bias voltage (2 mV-2 V) between the tip and the sample causes electrons to tunnel across the gap, typically yielding a current of nano-amperes or less. From a theoretical point of view, Simmons [18] has derived an explicit formula for the density of the tunnel current between two planar electrodes, by the solution of the one-dimensional barrier potential:

Z=Z,@‘/*Vexp -4$EGX;s] (

with

I, = (e/h)*(2m)“*/s (lb)

where ip = (+i + (p2)/2, is the mean barrier height above the Fermi level, with +i and & the respective work functions of the two electrodes, and s is their tunnel gap width; m and e are respectively the mass and charge of electron, h is Plan&s constant, and Y is the applied voltage. When adsorbed molecules are involved, the barrier height (0) corresponds to the vertical ionization energy to the lowest unoccupied molecular orbital (LUMO).

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The two planar electrodes in Simmons’s model do not adequately represent the tip and the sample in STM; nevertheless at small voltage (V < Q/e), the current-exponential dependence on gap separation and barrier height is still valid [4,9,11]:

I = kl/ exp( --A@“2s) (2)

where A is a constant with value 1.025 (eV)-“2 A-‘. With a typical barrier height of 4 eV, the tunnel current decreases roughly by an order of magnitude when the gap s is increased by only 1 A. This great sensitivity to changes in height suggests a marked potential for STM as a tool for investigating atomic and molecular phenomena occurring on a surface. Thus it should be emphasized that STM is a real space technique, providing the observation of individual species (atoms, molecules, clusters) and of surface structures, gaining information on several physicochemical phenomena including ad- and desorption, electrochemical processes and catalysis. As far as the instrument resolution is concerned, in the direction perpendicular to the sample surface (z direction) it can be Obetter than 0.1 A, and the lateral resolution (x, y) has been quoted as 4-7 A [19]. The ability to detect atomic corrugation depends on both the lateral resolution and the noise level. With a noise amplitude of 0.1 A it can be calculated that only atoms separated by at least 4 A can be resolved [ll]. Nevertheless, close-packed (111) Au [20] and Al [21] surfaces have been resolved regularly. Although atomic corrugation has been obtained on graphite [22-251, there is controversy over the imaging process as a result of various anomalous phenomena exhibited by such images [26,27]. Todd and Pethica [27] have postulated that for images which show periodicity over a large scale and a surprising absence of single atom defects, mechanical contact of at least a few atoms between tip and sample occurs.

Further information from the tunneling current While eqn. (2) provides a quite simple description of the current-gap relation-

ship, and implies that images gained are direct topographs of the surface, image analysis requires a little more care as the tunneling problem is three-dimensional involving states at the Fermi level. Tersoff and Hamann [28] based a now widely accepted analysis on the transfer Hamiltonian approach of Bardeen [29], where the tip and substrate are assumed to be a weakly coupled system acted upon by a perturbation, thus yielding a model of the tunneling barrier. The same workers showed that, if the tip wavefunction could, at the Fermi energy, be approximated by an s wavefunction, the STM image may be interpreted as a contour map of the local density of states (LDOS). Thus, although the image is a convolution of the electronic interaction of both the tip and the surface, it may be taken to be independent of the tip at a small gap. A very appealing application of the STM is spatially resolved surface tunneling spectroscopy [30]. In this technique one varies the tunneling voltage in order to obtain current-voltage spectra for specific points on the surface. Peaks in the derivatives of these spectra are interpreted as corre- sponding to energies of high LDOS, for example, surface states or resonances. Alternatively, images (or their voltage derivatives) may be taken, at specific bias

voltages, to highlight the spatial distribution of particular states. It is noteworthy that in the constant current mode, the STM image is not always a simple function of sample topography, but with electronically heterogeneous substrates, the contrast also depends on the local electronic structure. Thus, the image is a combination of topographic, chemical and electronic information [3], which is, in simple terms, respectively contained in the parameters: tunnel-gap (s), barrier height (@) and the pre-exponential factor of eqn. (2).

THE DESIGN OF THE SCANNING TUNNELING MICROSCOPE

The instrumental stage Mechanically, the system involves supporting the tip over the surface by a

piezo-electric system which allows three-dimensional (x, r,z) scanning over the surface while the tunneling current, 1, is monitored. After imposing a bias voltage between tip and sample, the image of the surface can be obtained by operating in two different ways [8,11]. In the constant current mode, an electronic negative feedback loop changes the relative tip-substrate distance (s) in order to keep a constant (preset) tunneling current, i.e. it maintains the product, &s, fixed. When the surface is electronically homogeneous, i.e. the barrier height is approximately constant, the gap separation, s, is practically constant. A raster voltage, applied to the x and y piezo-electric system, results in scanning and the image is obtained as a map of the tip displacement z(x, v) plotted against the lateral coordinates x and y: it consists of multiple line scans displaced laterally from each other in the y or x direction. For atomically flat surfaces, with roughness of a few angstroms, the high-speed or variable current mode [23] can be used. The tip is rastered over the surface at nearly constant height and the current 1(x, v) is recorded as a function of x and y. Even though the frequency must be below the lowest mechanical resonance of the STM, it is possible to achieve faster scan rate, because the piezoelectric z translator is not required to respond to individual topographical features. In this case the feedback loop does not follow the high frequency components of the tunneling current, but serves only to maintain the average current constant.

Though the technique is conceptually quite simple, careful considerations are required in designing the mechanical stage. Foremost among these is to take into account that the tip-to-sample distance has to be stabilised on a sub-angstrom scale, therefore the system stage should be isolated efficiently from e.g., building vibrations. The resonant frequency of the isolation system should be as low as possible while that of the scanner unit should be as high as possible [11,31,32]. Thermal drift [31] (of the sample with respect to the tip), electric interference and acoustic noise need to be minimized. Additionally, the design of an STM for electrochemical studies should also have the possibility of solution degassing and should avoid changes in solute concentration by solvent evaporation (over the duration of the experiment). The best compromise to satisfy these demands, which are to be included in a simple design, is fulfilled by reducing the number of construction

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elements to a minimum and making the instrument as small, compact and rigid as possible. This has led to a trend towards “pocket-sized” STM design [33]. Moreover, the advent of the piezo-electric tube scanner [34,35] for x,y,z translation, replacing the original three-orthogonal piezo block type configuration [36], has reduced the size and complexity of microscopes, and has raised the resonant frequency of the system, enabling higher scan rates to be used.

An STM design for electrochemical applications A schematic STM mechanical assembly is shown in Fig. 1. The system, rigidly

constructed, can work in solution under potentiostatic control of the tip and sample (Fig. 2). Two stages of vibration isolation are exploited; a pyramidal stack of mild steel plates with rubber dampers (Viton) in between, and four springs which suspend the whole assembly, situated on a wooden plate. A bell jar, covered with aluminium foil (acting as a Faraday cage), is used to reduce acoustic and electric noise and also to prevent solution contamination by an exposure to the laboratory atmosphere.

Following the design described by Binnig and Smith [34], the x,y,z motion of the tip, relative to the specimen, is achieved by a tube scanner (PCS-Unilator Technical Ceramics Ltd. [U.K.]). Positioning of the tip near to the sample is accomplished with a piezo electric motor (Inchworm controlled approach UHVC-

Bell jar covrrcd with 111 foil

Fig. 1. STM assembly combined with electrochemical cell. The Al alloy frame supports: Inchworm motor system approach, piezotube (P), insulated tip (T) and electrochemical cell. Counter electrode (C), reference electrode (R) and sample (S) are directly connected to the bipotentiostat. CP, RS and W are respectively: screws for cell controlling position, reference electrode support and plates for wire CoMection.

1~CliU0Rfl MOTOR

.,_., . . ._ ., . . ._ ._ . . . . ._ . . . . . . ._;. ‘.. ‘._ . . 1 ELECTROCHEMICRL CELL 1: j;

REFERENCE

DIFFEREWTIfiL AHPLIFIER

IRTEGRI)TOR ‘:’ Fig. 2. Schematic representation of ‘electrochemical cell, piezotube and Inchworm motor controlled approach with electronics configuration. The bipotentiostat allows independent potential control of the tip and the sample. The tungsten tip is insulated by Apiwn wax coating.

100, Burleigh Instruments Ltd. [U.K.]), and it is controlled remotely by an Inchworm Controller (6000-1-C). The Inchworm motor provides: 1 nm step size, 0.5 mm/s as maximum speed and the mechanical resonance is approximately 1 kHz. It acts without slip or backlash.

Ultramicroelectrode tips It has not been possible to apply the STM in electrochemical cells without

overcoming a few hurdles! The first problem is that, except in an ideally pure system, a faradaic current will flow between tip and specimen (i.e. working electrode) on application of a bias potential. In order to make the experiment workable, the faradaic current through the tunneling tip must not exceed the tunneling current. The recognised answer to minimize faradaic processes is to use a tip insulated except for the very end. Earlier experiments used commercial and “home built” glass insulated Pt/Ir tips [6,37-401. These suffered from relatively large exposed areas and were inconvenient to handle. Subsequently, methods of tip preparation using glass, polymer [40-41 or Apiezon wax coating [44,45] have been outlined. Gewirth et al. [46] have measured the exposed area of varnish-coated tips by cyclic voltammetry. They assumed that the equation for an ultramicrodisk electrode was valid for tips and found tip radii ranging from 0.01 to 1 pm. Very recently, Lewis and co-workers [47] described the preparation and electrochemical characterization of conical and hemispherical tips. After electrochemical ac etching

and glass or polymer coating procedures, Pt/Ir tips yielded radii of 0.5-10 pm. The exposed area and the shape (hemispherical/conical) was established with scarming electron microscopy, cyclic voltammetry and chronoamperometry. A unified view of the hemispherical-conical tip geometry is provided, depending on the tip coating process. Only when tips have radii less than ca. 1 pm, does the exposed area shape show a hemispherical geometry.

Potentiostatic control of tip and substrate The first demonstrations of imaging under electrolyte [6,37] involved only the tip

and substrate, so the potential of the surface was not referenced to a standard electrode and was consequently electrochemically undefined. In an electrochemical experiment with potentiostatic control, the potential of a substrate is controlled relative to that of a known reference through which there is no current flow (even a small current flow could polarise the electrode and change its potential). The current flow is between the substrate and a counter (or auxiliary) electrode. Although most of the groups working with electrolytic solutions have different scanning stages, the electrochemical cells and experiments are all based on accepted electrochemical principles [48].

Potentiostatic control of the substrate during imaging is now used in all in situ work, but the methods of achieving it differ slightly from group to group. W&hers et al. [49], Green et al. [50], Otsuka and Iwasaki [51], Hottenhuis et al. [52] and Trevor et al. [43] use a conventional three-electrode configuration with the tip as a fourth electrode held either at a constant potential relative to the reference [43,49], or with a different electronic configuration at a constant bias voltage relative to the working electrode [50,51]. Itaya and Torn&a [48], Uosaki and Kita [54,55], Fan and Bard [56] and Christoph et al. [57] have instead employed designs based upon a bipotentiostat [48], allowing independent control of the tip and sample with respect to the reference electrode.

Since the tip-sample bias voltage is not a rigid constraint (only when large differences are used, are spectroscopic effects mixed with topographic information [SS]), the tip in an electrochemical environment should be held in a “potential window” so as to minimize the faradaic processes. Thus the working electrode potential can be varied, allowing surface structural modification to occur.

IN SITU ELECTRODE SURFACE CHARACTERIZATION

Spectroscopic techniques Knowledge of the structure and properties of the electrode/electrolyte interface

is decidedly important for a better understanding of electrode reactions. As with the study of most reactions it is advantageous to know the structure and composition of the reactants, so in electrochemical processes [59], which are inherently interfacial ones, the electrode/solution interface plays an important role, being one of the reactants.

Currently the techniques used most for in situ electrode characterization are based upon visible and infrared optical spectroscopies (i.e. surface-enhanced Raman spectroscopy SERS [60], ellipsometry [61] and infrared (IR) spectroscopy [62]). A major problem, for instance, in IR studies is the large absorbance of electrolytes. This problem has been solved either ,by using thin-layer ( - 1 pm) spectroelectro- chemical cells, or, for semiconducting specimens, by using a total-internal reflection geometry. Moreover, in order to select the weak interface absorption, the electrode potential is modulated, thereby measuring the associated change in absorption. Even though the electrode/electrolyte interface can be investigated with electrochemically modulated infrared spectroscopy [63] (EMIRS) or subtractively normal- ized interfacial Fourier transform infrared spectroscopy [64] (SNIFTIRS), none of these techniques can provide direct real-space information on electrode surface structure. Up to now, only electrochemical techniques have been used to study the structure and properties of the solid/electrolyte interface. In this context, STM has the potential to become a valuable source of topographic information which will be complementary to that obtained from the established spectroscopic techniques or from analysis involving the simulation of molecular bonding. STM images, in vacua, of adsorbed isolated copper-phthalocyanine molecules have been reported [65,66] exhibiting four-fold symmetry, which is in agreement with the HOMO and LUMO charge-density distribution of Htickel molecular orbital calculations.

STM applications in electrochemical environments It has recently been the goal of several research projects to try to take advantage

of STM in studying processes occurring in electrochemical environments. The first example of an STM instrument, designed specifically for electrochemical investigations, was reported by Dovek et al. [67]. The system offers automatic tip approach and positioning of the tip at different distances from the sample. The STM base stage is made of Pyrex whilst brass plates, separated by silicon rubber spacers, were used for vibration isolation. The same authors have also compared [68] STM with in situ and ex situ electrode surface characterization techniques. Elsewhere, Arvia [69] has offered a stimulating overview of the STM potentiality in the electrochemical field. He dealt with problems of electrode surface topography under equilibrium and non-equilibrium conditions, postulating possible areas of research.

Sonnenfeld and Hansma [6] have given the first example of STM imaging in water. Using a glass insulated Pt/Ir tip (except for 50 pm at the end), they obtained atomic resolution of high ordered pyrolytic graphite (HOPG) under deionized water. A gold film, immersed in a NaCl(2 mM) solution, was also imaged, but with lower resolution. At about the same time, Bard and coworkers [37] described an apparatus for scanning electrochemical and tunneling microscopy with a resolution in solution of ca. 30 run. Neither of these reports operated with the sample under potentiostatic control.

Sonnenfeld and Schardt [40] have imaged silver, electrodeposited onto graphite, using a three-electrode system (HOPG as substrate, Ag/AgCl reference electrode

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and stainless-steel counter electrode). In this case, before performing Ag electroplat- ing, the tip was retracted and disconnected from the STM electronics. The surface was imaged before and after, firstly plating and then stripping, potential sweeps. The pictures were taken at the rest potential of the system. The repeated failure to observe monolayer coverage of the surface supported the pure island growth mechanism as opposed to the “film-and-island” mixed mechanism of surface coverage. This separate electrochemistry and STM experimentation technique was also used by Itaya and Sugawara [70] when they imaged platinum particles on graphite. In situ monitoring, of the anodic oxidation of HOPG in sulfuric acid under potentiostatic control, has been reported by Gewirth and Bard [71]. Prior to electrochemical oxidation, constant height STM images of the electrode exhibited the usual atomic lattice. After 20 potential cycles between 0 and 1.8 V with respect to a silver wire quasi-reference electrode (AgQRE), a partially insulating oxide was formed by irreversible cleavage of C-C bonds. STM topographic images showed large areas of corrugation adjacent to flat regions where atomic resolution could be obtained. Further oxidation gave rise to even rougher surfaces with higher corrugation. Local barrier-height images [3] were provided by gap-width modulation of 0.5 A at 15 kHz, and recording of (log I)/ds, where I is the tunneling current and s is the gap separation. After oxidation, the sample of graphite exhibited a lower value of barrier height: 0.9 and 0.25 eV, respectively. For gold substrates in water and in air, Lindsay and Barris [72] have also reported very low values, widely scattered between 1 eV and 1 meV, which were rationalized by the absence or presence of adsorbates on the specimen. Despite these very low values [see ref. 731 we note that in situ measurements still show the expected exponential variation of current with separation [49,57,74].

Lev et al. [38] have monitored the anodic dissolution of nickel substrates in sulfuric acid. During STM operation, the sample was biased positively with respect to the counter electrode by using a battery, independent from the STM control electronics. This allowed the sample to be biased away from the rest potential of the solution, and so avoided its etching.

Successful in situ experiments have been also carried out by Trevor et al. [43]. They observed the roughening of a Au(ll1) surface on cycling in perchloric acid when more than a monolayer of oxide was formed and reduced. Pits and terraces were formed on the surface, probably by a place exchange mechanism [75]. Ad- sorbed oxygen atoms and gold surface atoms exchange places and on reduction, gold atoms are displaced from their original positions. The surface was annealed within a couple of minutes of the reduction of the oxide layer i.e. the pits coalesced with each other and the terraces, to give a surface of similar morphology to that observed before the experiment. When chloride ions were added to the electrolyte above the gold surface, no pits could be imaged and step edges appeared to move quickly across the surface. The enhanced mobility of gold atoms in the presence of chloride was also noted by Wiechers et al. [49] and contrasts with the high stability of pits and indentations formed (by tip/surface contact), and imaged in vacua, by Jaklevic and Elie [58]. Similar surface morphology studies on polycrystalline silver

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and gold electrodes in potassium chloride solution have been carried out by Morita et al. [76].

The corrosion process of 304L stainless steel in aqueous chloride solution has been monitored by Fan and Bard [74]. The corrosion rate, as measured by the presence of pits or steps on a previously smooth surface, increases with decreasing values of pH. The study took place over about 18 h and one of the difficulties in this type of experiment is, even allowing for thermal equilibration of the system, that small drifts will mean the area scanned may not be the same in each image! The introduction of an organic corrosion*inhibitor yielded a disordered surface which exhibited non-ideal tunneling characteristics. This was ascribed to strong adsorption of the inhibitor by the surface and subsequent formation of an insulator layer.

In an advance on a non-potentiostatically controlled study of electrocrystallisa- tion on gold(ll1) by Schneir et al. [77], Hottenhuis et al. [52,78] correlated in situ and ex situ STM images of the electrolytic growth of silver with optical data, Their observation of rough and smooth areas on the crystal surface corresponded with what were previously thought to be inactive and active surfaces for growth. STM imaging revealed some faradaic activity on the rough surfaces showing the inactive regions are not completely static, but are subject to slow modification only.

Under-potential deposition (UPD) is probably one of the most appealing electrochemical process to investigate by STM. The UPD consists in the deposition of a metal monolayer on a foreign metal substrate. The phenomenon occurs at a more positive potential than the reversible Nernst potential value for the bulk metal deposition [79]. A detailed understanding of the role of metal-ion to be deposited, the electrolyte, the ions’ adsorption, the potential used for deposition on the substrate and properties of the new phase, are topics of major research interest and activity [80]. Green et al. [50] and Christoph et al. [57] have studied the UPD of lead on Au(ll1) and Ag(100) surfaces, respectively. For the gold surface, the UPD film is found to grow faster at steps and defects. Single plating and stripping cycles caused little alteration to the substrate surface, and lead adsorption was shown by the smoothing of sub-nanometer corrugations. Repetition of the potential cycle gave nanometer steps on the substrate.

Atomic resolution of the copper UPD on Au(ll1) and Au(lOO), in sulphuric and perchloric acids, were observed by Behm and co-workers [X1,82]. In sulphuric acid and Au(lll), two different hexagonal ordered patterns have been obtained [81]: (0 X 6) R30 and (5 X 5) with the transition between these superstructures in a narrow potential range. Even though no atomic resolution was observed for the Au(ll1) substrate, this was the first reported observation in solution of atomic corrugation in a copper UPD layer. Other in situ images of deposition reactions have been provided by Itaya and Tomita [53] (silver on HOPG) and Armstrong and Muller [83] (copper on platinum). Uosaki and Kita [55] attempted to image copper deposition on palladium. This study achieved real-time data storage by using a video cassette recorder and found copper to deposit at step edges, giving particles which grew laterally to coalesce eventually and form a smooth surface. Continuous recording of STM images has also been reported by Robinson [84]. A sequence of

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constant current topographs, taken in real time (each image was acquired in 4-6 s), of the electrochemical dissolution of a thin silver film showed stripping, further nucleation and re-deposition of the silver. These images were gained on silver plated HOPG, which was previously deposited galvanostatically from a commercial plating solution.

Platinum surfaces have been studied extensively as electrode materials because their catalytic properties are well recognized. Recently, STM studies of different Pt surfaces (single crystal, Pt deposited on automatically smooth mica, polycrystalline) in various protonic media have been performed [56,85-891. Itaya and co-workers [87] observed a polished Pt electrode, before and after repetitive voltammetric cycles in sulfuric acid. STM images of the surface showed a step height of ca. 0.23 nm, as expected for a monoatomic platinum step, with directions which reflected the three-fold symmetry of (111) planes. When a limited number of repetitive electrochemical cycles was performed (scan rate 50 mV/s), a few islands of monoatomic or diatomic height appeared on Pt(ll1) terraces. The images showed increased noise, but the location of monoatomic steps appeared unchanged, while, after extended electrochemical activation of 50 potential cycles, more disordered terraces with hemispherical domains were observed [86]. In the former case of slight electrochemical activation, the results can be explained by place exchange of Pt adatoms accompanying anodic oxidation, so that the atoms regenerated after reduction do not return to their original position [75]. In contrast with Au(ll1) [43], where the pits disappeared with time, changes induced by Pt oxide formation and subsequent reduction appear irreversible. After performing several physicochemically different pretreatments, Fan and Bard [88] used the STM in air, n-heptane and pure water to image platinum surfaces. For the case of Pt(lOO) single crystals subjected to long-term voltammetric cycling between 1.3 and -0.2 V vs. SCE in 1 M HClO,, the STM images exhibited regions of parallel ridges and dome-like structures. Further- more, as the period of cycling increases, the voltammogram shape approaches that typical of polycrystalline Pt. The investigation supports previous work carried out in situ in sulfuric acid by Itaya et al. [85] and ex situ by Vazquez et al. [89], providing further evidence that extended electrochemical activation produces considerable surface roughening. In a related study [56], an annealed polycrystalline platinum electrode has been imaged under potentiostatic control. The tip was withdrawn from the substrate, while the electrode potential was altered. After 14 min equilibration, the surface was imaged. Repeating these procedures for the sequence of potentials 0.15, 0.8 and 0.15 V (vs. SCE) resulted in the formation of irregularly distributed Pt clusters. The same paper reported images of polypyrrole taken in acetonitrile solution. The pyrrole electropolymerization seems to involve formation of islands on the bare substrate, and is consistent with a previous STM [90] investigation.

Surface structure of semiconductors in solution As part of the continuing interest in solar energy photoconversion [91], some

semiconductor electrodes have been studied in aqueous solution under potentiostatic control while being characterized by STM. Since the photoelectrolysis process

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occurring on the photoanode is related directly to the electronic and topographic properties of the surface, it is very important to understand how the stability and the performance of these electrodes can be improved. In situ investigation by STM on various semiconductors as n-TiO, [92], GaAs [93-941, n-ZnO [96], and n-Si [97] have been carried out. These semiconductor materials are quite stable in the dark, even in aqueous solution.

n-TiO, and n-ZnO give rise to an anodic current under illumination due to the photoanodic dissolution of the oxide. Their electron tunneling from the conduction band to the vacant levels in the metal tip seems to be possible when the electrode potential of the semiconductor is negatively biased with respect to the flat band potential (Es,). The absence of energy levels near the surface of the semiconductor at positive potential with respect to +I?,,, results in a sharp decrease in the probability of electron tunneling from the conduction band in the semiconductor. From a study carried out in air, the energy gap for a TiO,(llO) single crystal evaluated by tunneling spectroscopy is between + 1.25 and -0.25 eV with respect to the Fermi level [98]. The topography of a n-ZnO(OO1) single crystal with the potential of the electrode and the tip at - 0.6 and 0.2 V vs. Ag/AgCl respectively, showed randomly distributed “rolling hills” together with flat regions. The semiconductor electrode potential was chosen to be more negative with respect to the E,, in order to obtain the STM images. In the case of n-GaAs [93] no STM imaging was possible when its potential was held between -800 and +250 mV vs. a Pt reference electrode, with the tip at -400 mV with respect to the same reference. During photoanodic dissolution under electrode illumination and with the potential at +600 mV in 2 mM HClO,, no large structural modification of the surface was recorded.

A&orbed molecules The investigation of adsorbed layers and monolayers is of utmost importance in

many areas, from corrosion inhibition, crystal growth and catalysis, to sensors. Particularly relevant examples investigated by STM include: Langmuir-Blodgett films [99-1031; adsorbed ordered sub-monolayers (copper-phthalocyanine on Cu(100) [66], co-adsorbed benzene and carbon monoxide on Rh(ll1) [104] and iodine adlattices on Pt(ll1) [105]); crystals of the organic conductor tetrathiafulva- lene tetracyanoquinodimethane (TTF-TCNQ) [106]; carbon fibers [107]; two-dimensional liquid crystals [log]; several organic molecules, including biological samples of bacteriophage +29 [109], DNA molecules [72,110-1151, sorbic acid [116], n-alkanes of long carbon atoms chain [117], and ordered monolayers adsorbed and imaged in solution [118]. These results seem to suggest that effective tunneling can occur over distances ranging between 20 and 50 A. Monolayers of the detergent 3-[(3-chol~dopropyl)dimethylammonio]-l-propanesulphonate (estimated thick- ness 22-26 A) in aqueous solution and adsorbed on graphite have been imaged only when the tunneling current was less than 2 nA [118]. When higher values (5-8 nA) were used, just the graphite structure was imaged, suggesting demolition of the adsorbed monolayer. Similarly, Horber et al. [102] obtained stable images of Cd-arachidate bilayer only when low values of tunneling currents (0.25-0.5 nA)

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were used. At this point some questions should be addressed: (1) what is the role of the adsorbed molecules in the tunneling process, and (2) how can the experimental results be interpreted?

When adsorbed molecules are involved, electron tunneling can be considered in three parts. Firstly there may be a relatively facile electron transfer process between an edge of the physi- or chemisorbed molecule and the metal substrate. It is well known that work function changes (usually toward lower values [119,120]) are indicative of molecule-substrate interaction which is due to the appearance of a permanent electric polarization. For example, Ernst and Christmann have recently reported ca. 2 eV lowering of the work function, as measured by UV photoelectron spectroscopy, for the Pt(ll1) surface due to the adsorption of glycine [121]. Secondly, the molecule itself will be involved in the electron transfer process, which is more or less efficient, depending on whether the compound has a conjugated a-system or if the backbone is completely u-bonded. Thirdly, the electron tunneling can take place through space from the molecular edge which is nearer to the tip. These latter two processes decay rapidly with the distance as a consequence of a common exponential relationship e (-@) [122], but they are not equally weighted because the attenuation factor /3 is different for these two circumstances. Beratan et al. [122], in their model of electron tunneling in proteins, assume for covalent bonds a /3 value of 0.6 A-’ and about 1.7 A-’ for through-space interactions. In relation to the simple model of electron tunneling (see eqn. 2), the /3 factor can be interpreted in terms of electron barrier height through a square potential gap [123].

Keeping in mind these considerations, the through-space mechanism can be efficient at short distances and, at the limit, there may be significant orbital interactions between substrate, adsorbed molecule and tip [1X]. This means a long range electron transfer process through the bonds in close similarity to the molecular donor/acceptor systems [124,125]. However, at the present stage of knowledge, it is difficult to rationalize how molecules without any conjugated a-system are able to transport electrons so efficiently; this matter requires clarification to allow understanding of the STM images.

Analogous characteristics are exhibited by organic molecules adsorbed on electrode surfaces and involved in electron transfer processes of bio-macromolecules. For instance, the electrochemistry of certain redox proteins can be enhanced dramatically by the pre-adsorption of a monolayer of a so-called promoter on the electrode surface [126-1281. Their function is to provide a suitable interface between the protein or enzyme (molecular masses in the range 5-200 kg), and the electrode surface. These substances, in the potential range of the observed electron transfer, are electroinactive and, in addition, they act to limit or prevent irreversible and degradative adsorption of proteins at the electrode.

Despite much effort to understand better the role of these promoters [129-1321, the present knowledge of their interaction mechanism with the electrode, and of the electron transfer process itself, is inadequate. Relevant problems include the investigation of orientation of the adsorbed molecules at electrodes with reference to changes with coverage, applied potential, the absence/presence of redox proteins

14

and cations, etc. The interest in this field is always increasing because of the gap in our knowledge of the behaviour of in vivo redox systems and their possible applications as biosensors. STM is a new tool which should provide not only relevant topographic information, but also yield valuable explanations and new insight on the appealing subject of intra/intermolecular electron transfer.

ULTRAMICROELECTRODE PROBES: FARADAIC OR TUNNELING CURRENT?

In some recent reports [37.133-1361, Bard and co-workers have outlined a completely new approach in order to characterize chemical and electrochemical processes in the range of 1 nm to 10 pm, near the sample substrates. In this technique, called scanning electrochemical microscopy (SECM), the faradaic current flows at an ultramicroelectrode tip immersed in solution above a conductive, semiconductive or insulating specimen. Although SECM has some affinity to the STM in the scanning of the tip over a substrate and in the method of moving the tip itself, the principle of operation is quite different. Basically, with SECM, an electrochemical process is involved at the ultramicroelectrode tip and eventually at the substrate, and this process is controlled by electron transfer kinetics at the interfaces and mass diffusion processes in solution. The original idea, i.e., to probe the dynamics of processes occurring in the diffusion layer, is due to Engstrom et al. [137,138]. Using microelectrodes with diameters of 10 pm and a bipotentiostat (for independent control of the specimen electrode), and an ultramicroelectrode probe in an electrode-generator and probe-collector mode, they were able to obtain a concentration profile of species electrogenerated at the specimen electrode. An appealing improvement, where the substrate need not necessarily be a conductor, is to employ the SECM in “feedback mode”. Using potassium ferrocyanide at millimolar concentration, and setting the tip-specimen separation at the level of the diffusion layer, one can expect different responses when the potential at the microelectrode is able to oxidize/reduce the species present in bulk solution. With a conductive specimen and a microelectrode in the double-layer region, the current value is enhanced because of recycling of electroactive materials between the two electrodes. If the specimen is non-conductive, the species are probably “diffusion blocked” and there will not be feedback between the electrodes, so diminishing the current value. A theoretical treatment for the steady-state current that flows between the tip and a planar substrate with the system operating in feedback mode is given in ref. 134.

Initial applications of the SECM [135] have included imaging of platinum wire (50 pm) and a glass fiber (50 pm) on a glass slide, and of a gold minigrid ca. 90 x 90 pm. These attempts, although at only micrometer scale resolution, demonstrate the ability to get topographic information from both conductive and non-conductive substrates. The last aspect is particularly attractive if one considers that, for example, a biological material, usually insulating, could be imaged in solution at a resolution approaching that of the scanning electron microscopy. Developments in the application of this instrument will be of great interest.

15

The family of probes which can be applied under electrolytic solution is further increased with the scanning ion-conductance microscopy (SICM) [139]. Although submicrometer resolution has been quoted, the technique may be useful in biological applications, as it images non-conducting surfaces covered by electrolytes by the flow of ions through a micropipette probe. The possibility of having a multiprobe microscope with ion specificity would be of potential in the study of ion flow over biomembranes.

ATOMIC FORCE MICROSCOPY

From the above discussion it is clear that, although STM is a powerful tool, it is limited to specimens of reasonably high conductivity. During the last few years several authors have demonstrated the viability of a new high-resolution surface scarming probe: Atomic Force Microscopy (AFM). Instead of adjusting the tip- substrate distance to keep a tunneling current fixed, one keeps the force acting between the atoms of the surface and a diamond microtip constant. Therefore the AFM does not require that the tip or substrate is conducting and can be used to study insulating materials [140-1421 and in particular biological molecules.

Plotting the z position against lateral x-y position as the tip is scanned, yields an image of the sample. The lateral resolution in aqueous solution is quoted better than 0.5 nm [143]. The small repulsive tracking forces between the tip and the sample, usually in the range of lo-’ to 10e9 N [143,144], are recorded by measuring minute deflections of the cantilever upon which the tip is built. Atomic resolution images in water [143,145] have already been produced by AFM. It has been used successfully to image polyaniline adsorbed on glass and the real time thrombin- catalyzed polymerization process of the protein fibrin [145].

Clearly, AFM used in conjuction with the STM will allow one to study a wide range of molecular specimens at the atomic level.

CONSIDERATIONS, PROBLEMS AND STM DEVELOPMENTS FOR ELECTRODE/ELECTRO- LYTE INVESTIGATIONS

At present, there are no doubts that the STM can be a powerful tool for studying the physical chemistry of surfaces. Since 1982 the ever-increasing number of reports on STM are indicative of the technique’s worth in the field of surface study. Much of the most striking work has involved the UHV investigation of semiconductors, but now the potential applications to the study of the the solid/liquid interface are beginning to be realised. Phenomena such as electron transfer through small and large molecules adsorbed on well-defined surfaces, the nucleation mechanism of conducting polymers by chemical or electrochemical polymerization, sub- and monolayers of foreign metal with catalytic properties, and adsorption of species from solution where the molecular orientation might be probed directly rather than inferred indirectly [146], are just a few examples of the potential applications of in situ STM. Whilst initial studies on electropolymerization of a conducting polymer

16

[86,90] and underpotential deposition (see above) have been carried out, the scope for further research is huge. The study of the actual oxidation processes of relevance to fuel cell technology and the stability of photoanode semiconductors, will require advances in technique to attempt to characterize fully the process at the level of individual molecules. However, within such experimental results, there are some questions still unanswered. For example, the contrast mechanism leading to imaging of adsorbed organic molecules by STM is still unknown and is a challenge to theoreticians and experimentalists alike. This may be regarded both as a problem (how to interpret STM images) and as an opportunity (STM provides a unique probe of focal molecular conductor processes). It is also worth noting the difficulties relating to the interpretation of low barrier height values obtained in some experiments. At present, the most plausible explanation involves possible physical contact between tip and sample [73]. These aspects will be elucidated only by further carefully planned experimental work.

Problems for in situ work include evaluation of the possible reorganization of the double layer caused by the close approach of the ultramicroelectrode probe to the working electrode surface, and consequent repercussions for the apparent ordering of admolecules at the interface. Taking advantage of electrochemical measurements of the heterogeneous rate constant, nanoscopic inspection of the surface may lead to an improved understanding of electrode pretreatment [147]. Moreover, these electrochemical aspects can be investigated in combination with the SECM, as the two techniques can use the same apparatus.

From an instrumental point of view, technological improvements are expected to increase the lowest mechanical resonance frequencies, and provide higher data collection rates [35]. This should provide real-time monitoring of fast surface processes, in a similar way to the AFM [145]. Most successful STM studies have been carried out on flat conductor surfaces of single crystals (in vacua and solution), and this requirement means that flat substrates must be used to study electrodeposited or adsorbed species. Further challenges include minimization of molecular diffusion and thermal drift, and the development of spectroscopy in solution.

Even though the STM has demonstrated its power for in situ applications, its real capability has yet to be discovered. Now that the family of scanning probe microscopes (STM, AFM, SECM and SICM) offers a variety of opportunities for studying the electrode/electrolyte interface, the electrochemical investigation can be analysed in more detail and the present interest in many areas of research will ensure the continued increase in the use of these new tools.

ACKNOWLEDGEMENT

Two of us (T.R.I.C. and I.G.B.) wish to thank the SERC for a postdoctoral fellowship and a research studentship respectively, and we gratefully acknowledge Prof. Behm and his group for allowing us to perform some experiments at Munich University (F.R.G.). Thanks are also extended to Prof. K. Uosaki for a careful reading of the manuscript.

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in situ scanning tunneling microscopy

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