Nanoelectrochemistry and nanophysics at electrochemical interfaces

Surface Science 597 (2005) 156–172 www.elsevier.com/locate/susc

Nanoelectrochemistry and nanophysics at electrochemical interfaces M. Hugelmann a, P. Hugelmann a, W.J. Lorenz b, W. Schindler a

a,*

Institut fu¨r Hochfrequenztechnik und Quantenelektronik, Universita¨t Karlsruhe (TH), 76128 Karlsruhe, Germany b Institut fu¨r Ho¨chstfrequenztechnik und Elektronik, Universita¨t Karlsruhe (TH), 76128 Karlsruhe, Germany Available online 28 July 2005

Abstract Electrochemical interfaces provide fundamental advantages for the preparation of low-dimensional structures on metal and semiconducting substrates without irreversible modifications induced by the preparation process. Delocalized, and in particular localized electrodeposition using a scanning probe microscope tip as a nanoelectrode, allow a solely electrochemical bottom-up growth of nanostructures under defined nucleation and growth conditions. Localized electrodeposition can be utilized to grow nanoscale structures with lateral sizes of a few nanometers at defined sites both on single-crystal metal surfaces as well as on well defined hydrogen terminated n-Si(1 1 1):H surfaces. In addition, scanning probe microscopy at electrochemical interfaces allows the application of locally resolved in situ investigation techniques like tunneling and contact spectroscopy. Sophisticated in situ tunneling spectroscopy reveals important details of the molecular structure of the solid/liquid interface, and allows for probing electronic states in situ in a bias voltage interval as large as the stability range of the electrolyte, which is approximately 1 V in the case of aqueous solutions. In situ contact spectroscopy, based on the defined formation of quantized contacts between a scanning probe tip and a nanostructure underneath the tip, can be utilized for the investigation of the electronic structure of, e.g., metal/silicon interfaces, which is shown at the example of nanoscale Au diodes on n-Si(1 1 1). Thus, the combination of both, growth and in situ investigation of nanostructures at electrochemical interfaces under defined conditions opens up a fascinating perspective in view of a future nanotechnology utilizing solid/liquid interfaces.
2005 Elsevier B.V. All rights reserved. Keywords: Electrochemical nanotechnology; Nanostructuring; Solid/liquid interface; In-situ scanning probe microscopy; In-situ scanning probe spectroscopy; Localized electrochemical processes

*

Corresponding author. Present address: Physik-Department E19, Technische Universita¨t Mu¨nchen, James-Franck-Strasse 1, 85748 Garching, Germany. Tel.: +49 89 289 12538; fax: +49 89 289 12530. E-mail address: [email protected] (W. Schindler).

1. Motivation Nanotechnology has been often defined by the semiconductor industry to be concerned with the down-scaling of structures according to MooreÕs

0039-6028/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.08.045

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Fig. 1. MooreÕs law describing the decrease of the smallest feature size in ultra large scale integrated (ULSI) memory devices with the year of introduction into production. Room temperature quantum effects are observed in the range of feature sizes between the focus of extreme UV lithography and single atoms.

law [1] (Fig. 1). The so-called extreme UV lithography using a wavelength of 13 nm is currently developed to access the range of lateral structure sizes down to approximately 15 nm [2]. However, besides the problem of wavelength in lithographic processes, there are a variety of open fundamental questions about the lithographic top-down route, which are not yet answered [3]. Furthermore, from Fig. 1 can be seen that there is plenty of space left between the focus of extreme UV lithography around 15 nm and the size of single molecules or atoms. In particular this range of structure sizes is expected to provide novel properties of structures like quantum (size) effects at room temperature [4,5] or unprecedented sensitivity in molecular detection [6,7], which could be exploited in novel applications. Solid/liquid interfaces may play a major role in nanotechnology with such small structure sizes,

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since they provide fundamental advantages for the preparation of well-defined nanostructures without irreversible modifications due to the preparation process [8]. Both, the flux and the supersaturation can be precisely adjusted during electrochemical nucleation and growth, whereas the supersaturation is usually a fluctuating parameter in ultrahigh vacuum deposition processes. This feature results in well-defined nucleation processes at solid/liquid interfaces [9], which are important for the defined growth of nanostructures. In addition, electrochemical nucleation and growth can be performed near thermodynamical equilibrium, whereas laser ablation or sputtering processes involve much higher energies of the involved particles. Nucleation processes at electrochemical interfaces can be directed via nanoelectrodes, which can be realized by scanning probe microscope (SPM) or in particular scanning tunneling microscope (STM) tips, as will be shown in this paper. Thus, electrochemistry allows the bottom-up growth of nanostructures avoiding irreversible modifications during the preparation process. This is of particular importance in nanotechnology since the properties of nanostructures are determined by their surface and interface atoms due to their small volume, and a degradation of the surface by, e.g., defects or passivation may result in completely different physical or chemical properties. We show in this paper, that these unique advantages of solely electrochemical nucleation and growth can be utilized to prepare Ôas grownÕ nanostructures both delocalized on large surface areas, as well as localized at predefined surface sites with adjustable aspect ratios. A more detailed understanding of the intrinsic properties of nanostructures requires in situ investigation techniques which are at present still poorly developed. Locally resolved investigation techniques like tunneling spectroscopy or contact spectroscopy at single nanostructures are desirable, since they would allow for a precise correlation of properties and structure. We have improved in the recent years locally resolved in situ spectroscopy at electrochemical interfaces. It is shown in this paper, that the developed techniques for tunneling or contact spectroscopy allow to investigate the properties of nanostructures on a

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nanometer scale in very detail. In situ spectroscopy can be also advantageously applied to probe for example the electronic structure of solid/liquid interfaces, which is shown at the example of Au(1 1 1) in perchloric acid. The possibility to combine solely electrochemical preparation with these in situ spectroscopies at single nanostructures under electrochemical conditions opens up a fascinating perspective for a serious nanophysics at present and nanotechnology at solid/liquid interfaces in the future.

2. Low-dimensional systems (LDSs) Structures become low-dimensional with a reduced dimensionality i (iD, 0 6 i < 3) when the electronic system of the considered structure is confined in at least one spacial direction. Examples for low-dimensional systems prepared at electrochemical interfaces would be metal clusters on graphite or silicon surfaces with diameters in the lower nanometer range, and with volumes of several hundreds of atoms [10,11] (zero-dimensional, 0D systems), or nanowires a few atomic distances in diameter, and up to micrometer lengths [12] (one-dimensional, 1D systems). Two-dimensional (2D) systems would be for example 2D expanded or condensed surface layers, or adsorbate layers like hydrogen or self-assembled monolayers on metal surfaces [13]. Metal nanostructures on metal substrates may not show a low-dimensional behaviour since their electron system is expected to be strongly coupled to the electron system of the metal substrate. Therefore, from the electronic point of view, underpotential deposition (UPD) layers may not show a low-dimensional behaviour with respect to their electron system. Despite, Pd nanoparticles on Au(1 1 1) substrates have been found to show catalytic properties which are quite different from the catalytic properties of bulk Pd [14]. A solution to this discrepancy may be provided by recent findings that the unusual catalytic activity is not primarily caused by the low-dimensional electron system, but by an expanded lattice constant of the top surface layer of the Pd cluster [15]. In contrast, metal nanostructures on isolating or semiconducting surfaces show a low-dimen-

sional (2D) electronic behaviour [16], which demonstrates the importance of semiconductor substrates for the preparation of low-dimensional systems. The low dimensionality results in a quantization of electronic states, which in turn results in a variety of exciting, and at present not yet foreseeable, novel physical or chemical properties, single electron effects, coulomb blockade behaviour, possibly unusual catalytic features, or unknown stability effects, compared to the corresponding bulk system. It has been found during studies of the nucleation and the initial stages of growth of Ag on Ag(1 1 1) several years ago, that the activity of low-dimensional systems seems to be smaller than the activity of the corresponding bulk systems [17]. Thus, low-dimensional iD systems like clusters should be thermodynamically more stable than corresponding bulk systems [18–21]. Since lowdimensional systems are grown on substrate surfaces, stress or strain and alloying or intermixing processes on the atomic scale determine the electronic structure as well as the geometrical confinement. It is one issue of nanophysics to clarify the impact of these different processes on the stability and other properties of low-dimensional systems, which is at present mostly discussed by calculations rather than by experiments [15,22,23].

3. Experimental techniques The experimental techniques used for performing investigations on (single) nanostructures are crucial for the reliability and reproducibility of the measurements. Ex situ investigations at nanostructures are in general questionable since modifications of the as grown structures by, e.g., passivation processes can hardly be ruled out. Inhibition of nanostructures may solve this problem but may result in a change of the nanostructure properties. Therefore, reliable in situ experiments at electrochemical interfaces are essential for a more rapid development of nanophysics at electrochemical interfaces. Experiments at nanostructures at electrochemical interfaces require the lowest achievable contamination level in the electrolyte, which is al-

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Fig. 2. Schematic drawing of the instrumental setup used for advanced SPM investigations at solid/liquid interfaces under potential control. The bipotentiostat BP-600 operates our homebuilt electrochemical quartz glass cell independently of the SPM control electronics. The STM images in this paper have been recorded using a digital instruments nanoscope control electronics.

ready standard in electrochemistry and corresponds to ultrahigh vacuum conditions [24]. The materials in contact with the electrolyte are chosen to be as inert as possible, and to withstand aggressive cleaning solutions as for example 50-vol.% 98% H2SO4/50-vol.% 35% H2O2. Thus, the utmost purity is achieved using solely glass, for more critical parts like the electrochemical cell quartz glass, or Teflon (PTFE or PCTFE). The aqueous electrolytes are solutions of ultrapure/suprapure chemicals (99.999% purity) in ultrapure water. Utmost care must be drawn to the water purification system to avoid that for example organic molecules from fittings, tubings, or filter modules contaminate the water. The removal of oxygen from the electrolytes is essential for several reasons: Passivation of nanostructures is particularly critical in the presence of oxygen since these structures consist mainly of surface atoms. n-Si(1 1 1):H surfaces oxidize in oxygen containing electrolytes already at negative potentials with respect to the standard hydrogen electrode [25]. In situ VTS can be carried out with the required resolution only in well deaerated electrolytes where oxygen does not cause Faraday current changes upon changes in the electrode potential. Although in the literature mostly neglected, this effect is rather substantial [26]. The solely electrochemical preparation of nanostructures using a SPM tip as a nanoelectrode, and

in particular the in situ investigation of nanostructures by spectroscopy, require a sophisticated and flexible electrochemical instrumentation. Therefore, all measurements presented in this paper have been carried out with a BP-600 bipotentiostat (ECTec) which operates the electrochemical cell independently of the SPM measurement system, which allows to adjust tip and substrate potentials independently of each other, and which provides a high electronic bandwidth of several MHz (Fig. 2).1 This device allows us to carry out spectroscopic investigations in the millisecond time range, and, thus, to minimize any influence of thermal drift on our measurements. As a result, nucleation and growth by defined potential step and double step pulses can be imaged (see Fig. 3), in situ tunneling spectroscopy can be brought up to a new level of performance (see Fig. 9), and in situ contact spectroscopy using the STM tip as a contact electrode can be performed with unprecedented high accuracy (see Fig. 13). The influence of the bipotentiostatic instrumentation on the measured results is widely underestimated, although a state of the art bipotentiostat is crucial for the reliability of particularly spectroscopic measurements at solid/ liquid interfaces. Without a detailed knowledge of

1 See the EC-Tec homepage for details: http://www. ec-spm.com/.

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whereas the Au(1 1 1) surface reconstruction can be nicely resolved, the structure of the Co clusters remains unresolved by the same STM tip. A more advanced field emission/sputtering technique is able to provide STM tips with promising features: They can be reproducibly prepared with welldefined radii of less than 10 nm, and they show a decent electrochemical behaviour as nanoelectrodes [29].

4. Delocalized electrodeposition of nanostructures (iD LDS’s)

Fig. 3. In situ STM image of delocalized deposited Co clusters on Au(1 1 1). The nucleation has been carried out at a supersaturation of g = 200 mV for 2 ms, and subsequent growth of the clusters has been achieved at g = 20 mV for 20 ms. pffiffiffi The top view image shows an enlarged surface area with the 3  22 reconstruction of the Au(1 1 1) surface. Electrolyte: 0.25 M Na2SO4 + 5 mM CoSO4. Imaging conditions: EWE = 460 mV, Etip = 410 mV, both potentials quoted with respect to the standard hydrogen electrode, Itip = 1.2 nA.

the performance and specification of the used bipotentiostat any reported results, at least in the field of spectroscopy at electrochemical interfaces, must be taken with precautions. A serious restriction for nanophysics at electrochemical interfaces with in situ SPM techniques is at present the low resolution provided by the conventional STM tips used. Despite atomic resolution can be achieved in situ with conventional, electrochemically etched STM tips, there are only very few STM images published which show at least the step edge surface structure of nanometer scale clusters (Fig. 6, or [27]). The reason is probably the fact that atomic resolution on atomically flat surfaces can be achieved with relatively blunt tips. Such tips result in a low resolution when imaging vertically extended objects due to the convolution of the real cluster shape with the tip shape. This problem is not new [28]. An example proving this assumption is shown in Fig. 3;

The so-called delocalized nucleation and growth on native or foreign substrate surfaces has been studied intensively since several decades [9]. 3D Metal (Me) nucleation and growth on a substrate surface is initiated at supersaturation conditions which can be adjusted either by lowering the substrate potential below the equilibrium potential of the Mez+/Me system at a fixed Mez+ ion concentration in the electrolyte, or by increasing the Mez+ ion concentration at a fixed substrate potential. Delocalized nucleation and growth studies utilize usually the first method [9]. Fig. 3 shows an example of delocalized deposited Co clusters on a Au(1 1 1) surface, as imaged with in situ STM. The advantage of delocalized electrodeposition is its capability to decorate large areas of metal (Fig. 3) or semiconductor surfaces [30] with nanostructures of a narrow size distribution. Stable nuclei can be formed consisting of only one to a few atoms [9,31]. The nucleation density and the size of the clusters can be precisely adjusted by choosing appropriate supersaturation conditions, which are controlled by an appropriate nucleation and (subsequent) growth pulse applied at the substrate electrode at a constant Mez+ ion concentration in the electrolyte [9]. The formation of stable nuclei is thermodynamically more favorable at surface inhomogeneities like step edges than on an atomically flat ideal surface. The different sites where nuclei are formed are thermodynamically not equivalent, and can thus be controlled by the adjusted supersaturation, i.e. the electrode potential. This feature is exploited for example in the electrochemical growth of nanoclusters on graph-

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ite surfaces [10,11], or nanowires with lengths of several micrometers at the step edges of graphite substrates [12], which are decorated at the lowest supersaturation before nuclei are formed at other surface inhomogeneities. As can be seen from Fig. 3, a disadvantage of delocalized electrodeposition is the fact, that the nucleation centers are randomly distributed across a surface. The nucleation events follow a Poissopffiffiffi nian distribution [9,32]. In the case of the 3x22 reconstructed Au(1 1 1) surface, the preferential nucleation sites are the elbows of the herringbone reconstruction (inset of Fig. 3). The fabrication of regular arrays of nanoclusters requires usually a template on the substrate surface, which defines the areas where nuclei can be formed on the substrate surface, and which inhibits the rest of the substrate surface.

5. Localized electrodeposition of nanostructures (iD LDS’s) There is a variety of reports in the literature where nanostructures have been deposited at electrochemical interfaces, using scanning probe microscope (SPM) tips as preparation tools [33– 46]. The advantage of the SPM technique is its precise control where nanostructures are formed on the underlying substrate surface. However, a closer view at the reported techniques reveals that nearly all are based on a mixture of several physical effects as for example electrochemistry combined with a jump to contact mechanism [38–41,46], tip and electric field induced generation of clusters [35,42], or electric field induced surface modifications at solid/liquid interfaces [43–45]. Secondly, all reported techniques tried to deposit nanoclusters with the STM tip in tunneling contact to the substrate surface [33–46], which is not necessary in a solely electrochemical deposition process. Nanostructuring of particularly silicon surfaces with STM tip induced techniques has been reported to be a difficult task [47,48]. There has been only one report of a successful STM tip induced deposition of Pb clusters on n-Si(1 1 1):H [42]. The reason for these difficulties is not clear, but

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it is certainly not correlated with electrochemistry, since electrodeposition of, e.g., metals onto silicon surfaces seems to work quite well [49]. We will show below that solely electrochemical routes can be successfully used to grow nanostructures on silicon surfaces. A solely electrochemical route to nucleate and grow nanostructures on surfaces must be based on the generation of laterally changing superand undersaturation conditions at a substrate surface. Such a variation can be achieved by a local increase of the Mez+ ion concentration at a substrate surface kept at a fixed potential. Then, localized electrodeposition is achieved in surface areas where the locally varying Mez+ ion concentration results in supersaturation conditions, whereas no deposition is observed in surface areas where undersaturation conditions exist. This process is utilized by the Scanning Electrochemical Microscope (SECM) which uses a capillary to provide locally a higher Mez+ ion concentration [50]. It has been shown that this technique is capable to grow metal structures on substrate surfaces, however, with diameters in the range of micrometers [51–53]. Utilizing the tip of a Scanning Tunneling Microscope (STM) as a ‘‘nanoelectrode’’ provides a much higher resolution than SECM. This technique can be used to grow nanostructures with lateral diameters below 10 nm. It consists of a two-step procedure which generates in the first step a nanoelectrode by deposition of Mez+ onto the STM tip, and in a second step the required local supersaturation conditions by an abrupt dissolution of Mez+ from the STM tip (Fig. 4) [27,54– 56]. We use a STM tip of electrochemically inert material like Au, and generate the nanoelectrode by deposition of Mez+ from the electrolyte onto the STM tip to be able to use the STM tip before and after the deposition procedure for in situ STM imaging or tunneling investigations. The geometry of the electrodeposited structures depends on different process parameters as shown in Fig. 5. It is an important feature that the various parameters can be adjusted independently which allows to control height and diameter of the deposited structures independently [55]. Thus, nanostructures can be grown with adjustable aspect ratios.

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Fig. 4. Schematic of the two step process of localized electrodeposition utilizing a STM tip as a nanoelectrode. The distance between tip apex and substrate surface during the deposition process is of the order of 20 nm, i.e. much larger than a tunneling gap. Step 1: Electrodeposition of Mez+ from the electrolyte onto the STM tip and generation of the nanoelectrode. Step 2: Dissolution of Mez+ from the STM tip, generation of local supersaturation conditions, and nucleation of Mez+ on the substrate surface underneath.

Fig. 5. Parametric dependencies of the height and diameter of localized electrodeposited structures, using a STM tip as a generator electrode to achieve the localized increase of the Mez+ ion concentration. The independently adjustable parameters allow to control precisely the aspect ratio during growth.

A solely electrochemical process does not require the tip to be in tunneling distance from the substrate surface. Rather, typical distances between tip and substrate surface are 10–20 nm [27,54–56]. Such large distances avoid any influences of parasitic effects like mechanical or electric field interaction between tip and substrate surface on the deposition process. The electrochemical double layers around tip and substrate surfaces are well-separated at such large distances, and at the electrolyte concentrations used in the experi-

ments [27,54–56]. In this case, the kinetics of the deposition process is determined by the diffusion of Mez+ from the STM tip to the substrate surface [55,56]. Nanostructuring techniques with the STM tip in tunneling mode to the substrate surface are probably activation controlled [57] in absence of additional, non electrochemical processes. Since localized electrodeposition is a solely electrochemical process, nucleation and growth of metal nanostructures can be achieved in deposit/ substrate systems with either a weak or a strong deposit/substrate interaction, which is expressed in an overpotential deposition (OPD) or underpotential deposition (UPD) behavior, respectively. Therefore, localized electrodeposition allows in particular to grow nanostructures on semiconductor surfaces [27] which show usually a weak interaction with metal adatoms (no UPD phenomena). This feature will be of importance for a future technology which will probably require technologically relevant substrates. Furthermore, semiconductor substrates will allow the fabrication of nanostructures for nanoelectronic applications which cannot be realized on metal substrates.

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Fig. 6. Localized electrodeposited Co cluster on Au(1 1 1), which consists of five atomic layers. A detailed look at the height of the step edges points to a more complicated structure. Electrolyte: 0.25 M Na2SO4 + 5 mM CoSO4. Imaging conditions: EWE = 340 mV, Etip = 210 mV, both quoted with respect to the standard hydrogen electrode, Itip = 1 nA. Cluster deposition parameters: EWE = 460 mV with respect to the standard hydrogen electrode, cathodic predeposited tip charge Qcat = 2500 pC, Co2+ ion current during dissolution from the tip I 2þ Co ¼ 120 nA, tip substrate distance Dz = 20 nm.

Examples for localized electrodeposited clusters are shown in Fig. 6 for the case of Co on Au(1 1 1), and in Fig. 7 for the case of Pb on n-Si(1 1 1):H. Both systems are characterized by a weak metal adatom/substrate interaction. The reverse process of localized electrodeposition, localized electrochemical dissolution, can be achieved via localized generation of undersaturation conditions. Such conditions can be realized for example by depletion of Mez+ ions in a tunneling gap at positive tip potentials with respect to the Me/Mez+ equilibrium potential while the substrate potential is kept at a small overpotential with respect to the Me/Mez+ equilibrium potential. Examples for this process are observed during STM studies of metal growth where the tip is in tunneling distance to the substrate surface, and have been reported for the systems Cu/Cu2+ [58,59] and Ag/Ag+ [60].

6. In situ investigations Nanotechnology requires a detailed knowledge of the properties of nanostructures. As mentioned

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above, nanostructures consist mainly of surface or interface atoms. Therefore, their properties depend strongly on changes in the surface structure, for example due to passivation processes. Consequently, reliable investigations have to be performed in situ rather than ex situ, and preferably at single nanostructures with a well-defined structure. Integral analytical methods as for example in situ surface X-ray diffraction [61] or in situ magneto-optical Kerr effect which we have established several years ago for measurements of the magnetization of ultrathin magnetic films [24,62–64] do not provide the required sensitivity to investigate single nanostructures, although the techniques provide sub-monolayer resolution. SPM techniques allow to perform spectroscopic investigations in situ at single nanostructures. Thus, SPM techniques provide the ideal situation that nanostructures can be prepared and investigated at the electrochemical interface in the same experiment. In the following, we address three different types of spectroscopy at the solid/liquid interface, which aim at the investigation of different physical quantities: Distance Tunneling Spectroscopy (DTS) [9,65–68] probes the distance dependence of the tunneling current at a fixed bias voltage, which results in information about the tunneling barrier in the tunneling gap, whereas Voltage Tunneling Spectroscopy (VTS) [9,65] at a fixed gap width is sensitive to different electronic states in the tunneling gap. The application of tunneling spectroscopy at a nanostructure whose electron system is isolated from the electron system of the substrate gives also information about the electronic structure of this nanostructure. This technique has been recently used in ultrahigh vacuum environment to investigate the electronic states in low-dimensional (0D) clusters [69,70]. We show here that tunneling spectroscopy at electrochemical interfaces may be very useful for locally resolved in situ investigations. Contact spectroscopy utilizing atomic contacts between STM tip and a nanostructured object can be used to record current–voltage characteristics of nanostructure/ substrate interfaces. This will be shown below at the example of Au/n-Si(1 1 1) nanocontacts. Spectroscopic measurements at electrochemical interfaces require a high bandwidth of the

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Fig. 7. Localized electrodeposited Pb cluster on atomically flat n-Si(1 1 1):H. (a) Bare n-Si(1 1 1):H surface before cluster deposition; (b) Same n-Si(1 1 1):H surface area upon deposition of the Pb cluster; (c) Same n-Si(1 1 1):H surface area upon a change of the substrate potential resulting in a dissolution of the Pb cluster. Electrolyte: 0.1 M HClO4 + 1 mM Pb(ClO4)2. Imaging conditions: EWE = 240 mV, Etip = +640 mV, Itip = 200 pA. Cluster deposition parameters: EWE = 240 mV, cathodic predeposited tip charge Qcat = 2000 pC, Pb2+ ion current during dissolution from the tip I 2þ Pb ¼ 120 nA, tip substrate distance Dz = 20 nm. Potentials quoted with respect to the standard hydrogen electrode.

electrochemical instrumentation to allow a fast data acquisition which minimizes thermal drift during the measurements. Additionally, spectroscopy at electrochemical interfaces must consider not only the tunneling current, or electrical current through the interface in contact spectroscopy, but also the electrochemical currents of both STM tip and substrate electrode. Fig. 8 shows the different contributions to the total STM tip current and total substrate current. Reliable spectroscopic investigations have to minimize the electrochemical currents, or to precisely separate electrochemical currents from tunneling or contact interface cur-

rents. We will also show below that spectroscopy requires in particular the removal of oxygen from the electrolyte to achieve reliable data. Unfortunately, this aspect is so far widely neglected in the literature. Besides spectroscopy, the STM tip can be also utilized to measure laterally resolved reaction currents of redox reactions at the lateral position of the STM tip. An example for such a technique is the determination of the catalytic activity of Pd nanoclusters with respect to the hydrogen evolution [14]. The tip is used as a nanoelectrode to reoxidize H2 which has been previously

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Fig. 8. Schematic drawings of in situ spectroscopy using a STM tip as probe. (a) Tunneling spectroscopy and (b) contact spectroscopy. The currents through STM tip (Itip,EC and Itunnel or Icontact) and substrate surface (IWE,EC and Itunnel or Icontact) are determined by the electrochemical interface of the electrode area exposed to the electrolyte, and by the (tunneling) contact, as shown in the figure.

formed at the Pd nanocluster underneath the STM tip.

7. In situ distance tunneling spectroscopy (DTS) An example for in situ Distance Tunneling Spectroscopy (DTS) is shown in Fig. 9. DTS measurements require a precise calibration of the point of zero gap width. This can be achieved by defining the point of jump to contact in the current–distance curve as z = 0. Upon jump to contact quantized conductance channels are formed between tip apex and substrate surface (Fig. 9, Dz > 0). The tip current of the first conductance channel corresponds to Ubias · G0, with the conductance quantum G0 = (12 906 X)1 [71]. The mean variation of the gap width with the tunneling current in the tunneling regime (Fig. 9, z < 0) is roughly 0.18 nm per decade variation in the tunneling current. In order to be able to mea-

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pffiffiffi Fig. 9. Current–distance curve at a 3x22 reconstructed Au (1 1 1) surface in 0.02 M HClO4. The z-axis is scaled to zero at the point of jump to contact. The black points are the data measured at different gap width. The grey line is a guide to the eye, and shows the averaged decay of the tunneling current with the gap width. The whole curve is superposed from three individual curves which overlap at least one decade in tunneling current. The total measurement time of the complete current– voltage curve has been less than 200 ms, resulting in a maximum drift of the STM tip with respect to the substrate during the measurement of 0.004 nm. The tip electrometer 3 dB—bandwidth has been higher than 10 kHz, the WE bandwidth selector at BP-600 has been adjusted to 100 kHz. EWE = +140 mV with respect to the standard hydrogen electrode, Ubias = Etip  EWE = 100 mV.

sure significant changes in the tunnel barrier due to changes in the molecular configuration in the tunneling gap being probed by DTS, several decades of tunneling current have to be measured since molecule diameters are usually much larger than 0.18 nm. The influence of the small water molecules at an aqueous electrochemical interface on the current– distance curves can be deduced by a closer look to Fig. 9. The decay of the tunneling current with the distance, which may be interpreted as ‘‘exponential’’ on a first rough view, is in fact modulated with a period corresponding to the thickness of water layers at aqueous electrochemical interfaces. The effect of this small modulation in the current– distance curve is quite large on the corresponding tunnel barrier heights, which vary from values below 1 eV to values around 2.3 eV depending on the actual (time averaged) configuration of interfacial water in the tunneling gap [68]. These results

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indicate that the electronic structure of intragap molecules plays an important role for the tunneling process at electrochemical interfaces, which has been discussed several years ago [65]. Thus, locally resolved and detailed DTS investigations may be very useful to achieve a better understanding of the electrochemical double layer and the formation of adsorbate structures at surfaces, in particular at surfaces of nanostructures. Such investigations may be also useful to clarify the influence of adsorbates on the electronic structure of low-dimensional systems. Relying on these results, the STM imaging process at electrochemical interfaces seems to depend on the adjusted gap width and the actual molecular configuration in the gap at this particular width. Therefore, height variations in STM images may depend not only on local surface work function changes, but also on gap width changes caused by the imaging process for example during constant current imaging. This must be carefully considered when nanostructures are investigated particularly on foreign substrate surfaces. Such effects have been observed at Co clusters on Au(1 1 1) surfaces [27]. Another important feature of tunneling spectroscopy at electrochemical interfaces may be the observation, that the tunneling process seems to be vacuum like at very small gap widths where no water molecules fit into the gap [68]. Such conditions may be utilized to probe electronic properties by tunneling spectroscopy at electrochemical interfaces without the simultaneous influence of interfacial water or adsorbate structures.

8. In situ voltage tunneling spectroscopy (VTS) Voltage Tunneling Spectroscopy (VTS) requires either to change the substrate potential at a fixed tip potential, or to change the tip potential with respect to a fixed substrate potential. As is well known, any electrochemical interface shows a particular capacity which is for surfaces in aqueous electrolytes between 5 and 50 lF cm2. This large capacity causes large time constants for recharging the substrate surface. The substrate potential scan rates in VTS have to be compatible with these

large time constants, which results in long measurement times (practically seconds) causing an unacceptable thermal drift in the STM. A solution to this difficulty may be the variation of the tip potential with respect to the fixed substrate potential. In this case, the small capacitance of a well-isolated STM tip of less than 1 pF allows a fast recharging of the tip electrode, and, thus, allows to apply fast bias voltage ramps which are compatible with the bandwidth of the tip electrometer. Tip electrometer bandwidths can be as high as several hundreds of kHz. Unfortunately, this technique is not applicable with most of the commercial so-called EC-STM instruments for several reasons [72]. This explains probably why there are only very few VTS studies in the literature [65,73]. In contrast, the electrochemical instrumentation used in our experiments has been designed to allow such types of measurements. Fig. 10 shows VTS measurements at initially adjusted tunnel resistances of 45 MX, 2 MX, and 250 kX at Au(1 1 1) in a carefully deoxygenated 0.02 M HClO4 electrolyte. All curves, independent of the initially adjusted tunnel resistance, show a wide range where the tip current varies linearly with the bias voltage, as would be expected for an ideal VTS measurement at a metal surface without contributions of electrochemical currents to the tip current [29]. A contribution of electrochemical currents to the tip current is only observable at very negative potentials due to hydrogen evolution at the STM tip, and at positive potentials due to adsorption/reconstruction phenomena at the Au STM tip [29]. The linearity range extends over a rather large bias voltage interval of approximately 1000 mV, which can be utilized for in situ VTS studies. The hysteresis in the 45 MX tunnel resistance curve corresponds to the charging current of the tip double layer capacitance during the 300 V s1 up and down sweep of the tip potential. This capacitive current is much smaller than the tunneling current in VTS measurements carried out at smaller tunnel resistances. There are basically three requirements to achieve correct VTS measurements at electrochemical interfaces: Better isolation of the STM tip, i.e. a smaller unisolated tip apex area exposed to the electrolyte, decrease of the adjusted tunnel resis-

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Fig. 10. VTS measurements on Au(1 1 1) in a carefully deoxygenated 0.02 M HClO4 electrolyte, at initially adjusted tunnel resistances of 250 kX, 2 MX, and 45 MX. Grey: Up and down scans of the bias voltage Ubias = Etip  EWE; black: linear fit through the data in the interval 0.6 V < Ubias < 0.4 V. The voltage sweep of 300 Vs1 has been applied approximately 20 ms after adjusting the initial tunnel resistance at Ubias = 100 mV, and switching off the feedback loop of the constant current mode of the SPM system. EWE = +240 mV with respect to the standard hydrogen electrode. The deviation from linearity at very negative potentials is due to the hydrogen evolution at the STM tip, and at positive potentials due to adsorption/reconstruction phenomena at the STM tip. The curve for an initially adjusted tunnel resistance of 2 MX has been magnified by a factor 4, the curve for an initially adjusted tunnel resistance of 45 MX has been magnified by a factor 25. The hysteresis in this curve is due to the charging of the double layer at the tip electrode during the voltage sweep.

tance, i.e. gap width, and minimization of the dissolved oxygen in the electrolyte [29]. A better isolation of the STM tip results in a minimization of electrochemical currents at the tip electrode. This is hard to achieve with current tip isolation techniques, and with electrochemically etched tips showing an irregular shape. Our advanced sputter-

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ing/field emission preparation technique of STM tips may help to solve this problem [29]. A minimization of dissolved oxygen in the electrolyte is essential for electrochemistry in general, but requires a sophisticated layout of the STM cell [74]. A decrease of the tunnel resistance increases the tunneling current exponentially according to the current–distance relationship (Fig. 9). A large ratio of tunneling current and capacitive current at the STM tip avoids hysteresis in the tip current during up and down sweeps of the tip potential. However, very small gap widths have to be adjusted, and to be maintained constant throughout the VTS measurements. DTS data (Fig. 9) show that gap widths are approximately 0.2 nm at tunneling currents of 50 nA [68,71]. Such conditions require very fast measurements to minimize thermal drift, and, thus, a high electronic bandwidth of the used instruments. Small gap widths may show the advantage of moreless vacuum like tunneling [68], but may be also correlated with an increased tip–substrate interaction which is at present not well-known. Gap widths in VTS are usually adjusted by a particular tunneling current setpoint which is assumed to correspond to a particular gap width according to the current–distance correlation as shown in Fig. 9. This procedure neglects the dependence of the tunneling process on tunneling states of interfacial molecules, in the simplest case water molecules [68]. VTS at nanostructures must take into account, that the gap width on top of a nanostructure, and on top of an atomically flat terrace may be different at the same adjusted tunneling current. A different double layer or adsorbate structure around a nanostructure as compared to the double layer or adsorbate structure on top of an atomically flat surface may cause such effects. STM tips with apex diameters much larger than the size of investigated nanostructures may also result, at the same adjusted tunneling current, in a smaller gap width on top of the nanostructure as compared to the gap width on top of an atomically flat surface. Therefore, the same tunneling current on top of a nanostructure and on top of an atomically flat surface does not proof that both measurements are carried out with the same gap width. Since different gap widths are correlated

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with different tunnel barrier heights, as seen from our DTS investigations (Fig. 9) [68], they result in a variation of the tunnel resistance, and, thus, in a variation of the slope of current–voltage curves in VTS measurements. These considerations show that investigations at nanostructures must be performed very carefully, taking the whole system nanostructure/electrochemical interface/STM tip into account. Reliable spectroscopic investigations require additional standards of the experimental conditions, which are not yet established. Instruments must be chosen appropriate, and the dissolved oxygen level in the electrolyte must be sufficiently low. Otherwise, properties may be measured which do not correspond to the intrinsic properties of the investigated nanostructures.

9. In situ contact spectroscopy (VCS) Except for tunneling spectroscopy, the STM tip can be also used to directly contact nanostructures. For this purpose, contacts of atomic dimensions can be formed between a STM tip and the opposite surface. As already discussed in the DTS section, these quantized conductance channels are formed at sufficiently small gap widths upon jump to contact. An example for such contacts is shown in Fig. 11. Depending on the adjusted distance between STM tip and adjacent surface conductance channels are formed showing a discrete conductance quantization. The stability of these contacts depends on the stability of the STM tip position with respect to the adjacent surface position. Contacts can be held stable for times of at least 100 ms which is enough time to perform in situ measurements. As is shown in Fig. 12, the quantized conductance channels show an ohmic behaviour, which makes them suitable to contact nanostructures for current–voltage measurements. Such measurements have been performed at a Au/ n-Si(1 1 1) interface (Fig. 13). The corresponding current–voltage curve shows a Schottky diode behaviour which was expected for a Au/nSi(1 1 1) interface. However, the measured Schottky barrier height, as determined from the data, and fitted by the thermionic emission model [75],

Fig. 11. Au conductance channels as formed between Au STM tip and Au (1 1 1) substrate surface during variation of the distance z between STM tip and substrate surface. An approach of the STM tip towards the substrate surface results in an increase of the number n of conductance channels, a retraction of the tip results in a decrease in the number n of conductance channels between STM tip and surface. The constant bias voltage Ubias = Etip  EWE = 64.5 mV results in a constant height of the tip or WE current jumps according to Itip = Ubias · nG0 with G0 = (12906 X)1 and n = 1, 2, 3,. . . The bias voltage sweep applied at a contact in the state n = 1 allows to analyze the current–voltage behaviour of quantized contacts as shown in Fig. 12.

is smaller than Schottky barrier heights usually measured in bulk Au/n-Si(1 1 1) contacts, and the current density across this nanoscale contact is orders of magnitude larger than the current density in bulk contacts [76]. The gentle approach of a STM tip to a substrate surface, and the subsequent formation of a conductance channel upon jump to contact allows to contact nanostructures in situ, and to perform in situ contact spectroscopy. This technique seems to be promising to study in situ electronic properties of nanostructures or interfaces. The size of quantized contacts has been shown to be of atomic dimension [77], and, thus, much smaller than the actually investigated nanostructures (see Fig. 13).

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to the size of the contact. This problem is of course independent of electrochemistry, and a general problem in a future nanotechnology.

10. Conclusions

Fig. 12. Current–voltage characteristic of a single quantized conductance channel in the state n = 1, recorded by a bias sweep as shown in Fig. 11 (black curve, left axis). Within the range of the measurement, the current–voltage characteristic shows an ohmic behaviour. Its slope corresponds to G = 1 · G0 (grey curve, and right axis). The arrows indicate the direction of the voltage sweep.

Fig. 13. Schottky diode characteristics of a Au/n-Si(1 1 1) nanocontact with a contact area of approximately 2 · 1012 cm2. The voltage across the Au/n-Si(1 1 1) interface is Ubias = Etip  EWE. EWE = 10 mV with respect to the standard hydrogen electrode. The variation in Ubias has been achieved by ramping Etip with 40 Vs1.

The influence of such types of contacts (leads) on the nanostructure properties may become important if the size of the nanostructure is comparable

The presented results show that the research on nanostructures at electrochemical interfaces is still at its beginning, despite STM in liquids has been established for approximately 20 years. So far, scanning probe techniques have been widely used to image surface structures, but their full potential has not yet been exploited. Well-defined electrochemical conditions are an essential requirement for reliable preparation and in situ investigations of nanostructures since such structures are much more sensitive to passivation or degradation processes than bulk systems. In addition to delocalized electrodeposition, the localized growth of nanostructures, utilizing the tip of a scanning tunneling microscope as a nanoelectrode, allows for a precise nucleation and growth of nanostructures at predefined surface sites. This feature is important from a technological point of view, and may be useful for a detailed correlation of structure and properties of nanostructures. Since localized electrodeposition is a solely electrochemical process, any type of conducting or semiconducting substrate, showing usual electrodeposition of metals, can be nanostructured. This unique feature has been demonstrated at the examples of localized Co deposition on Au(1 1 1) and localized Pb deposition on n-Si(1 1 1):H substrates. Semiconducting substrates are in particular of great importance for technological applications. Although the large scale preparation of nanostructures with scanning probe based techniques is not yet feasible, recent progress in parallelizing of tips [78] may provide a technical solution to the serial operation of a single tip nanoelectrode. The analytical facilities of scanning probe techniques provide exciting possibilities for a detailed and reproducible nanophysics at electrochemical interfaces under precise potential control, if appropriate electrochemical instrumentation is used. In contrast to the poor literature in this field, the novel in situ spectroscopic facilities shown in this paper

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are expected to stimulate further the locally resolved in situ research on nanostructures. The presented results of in situ Distance Tunneling Spectroscopy (DTS) indicate, that the electronic structure of solid/liquid interfaces can be precisely investigated by in situ tunneling spectroscopy. This feature may allow for locally resolved detailed studies of the electronic structure of, e.g., adsorbates at solid/liquid interfaces. The available bias voltage range for Voltage Tunneling Spectroscopy (VTS) has been found to be approximately 1000 mV in aqueous electrolytes, when appropriate experimental conditions are applied. The STM tip may be further utilized as an electrical contact via formation of quantized conductance channels between STM tip and a nanostructure surface. This technique has been demonstrated to be useful in the investigation of metal/n-Si(1 1 1) interfaces showing a Schottky diode behaviour. The analytical possibilities presented in this paper enhance greatly the previous situation where the STM has been mainly used to image surfaces. Much more detailed investigations at defined conditions will stimulate the progress in nanostructure research, and finally help to exploit the fundamental advantages of electrochemical interfaces in a future nanotechnology. Acknowledgements We would like to acknowledge support of the project by J. Kirschner, MPI fu¨r Mikrostrukturphysik, Halle/Saale. Process parameters and the kinetics of localized electrodeposition have been studied in the PhD work of D. Hofmann, MPI fu¨r Mikrostrukturphysik, Halle/Saale. The authors acknowledge financial support by the DFG Schwerpunktprogramm ‘‘Grundlagen der elektrochemischen Nanotechnologie’’ under contracts SCHI492/1&2, which provided the financial basis to achieve the reported, in part fundamental, new results. References [1] G. Moore, IEDM Tech. Dig. (1975) 11. [2] Emerging lithographic technologies II, in: Y. Vladimirsky (Ed.), Proc. SPIE, vol. 3331, 1998.

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