A versatile variable-temperature scanning tunneling

REVIEW OF SCIENTIFIC INSTRUMENTS 79, 083903 共2008兲

A versatile variable-temperature scanning tunneling microscope for molecular growth Stefan Kuck,a兲 Jan Wienhausen, Germar Hoffmann,b兲 and Roland Wiesendanger Institute of Applied Physics, University of Hamburg, Hamburg 20355, Germany

共Received 29 April 2008; accepted 26 July 2008; published online 22 August 2008兲 We describe and discuss the design of a variable-temperature scanning tunneling microscope 共STM兲 system for the study of molecules at temperatures between 18 and 300 K in ultrahigh vacuum. The STM head is a refinement of a very rigid design developed and successfully operated in Hamburg. In the current version, the head is connected to a liquid helium flow cryostat, thereby reaching a base temperature of 18 K. To minimize the heat load on the STM head, a helium back flow cooled radiation shield is installed. The dimensions and the choice of materials are based on simulations of the heat dissipation. The STM is galvanically isolated from the vacuum chamber to minimize electronic noise and mechanically decoupled by means of springs and an eddy current damping stage. Additionally, the design of the STM head allows the deposition of several molecular materials onto the same cold sample surface. The operation of the STM in imaging mode is demonstrated for TPP/Cu共111兲 and FePC / NaCl/ Cu共111兲. Spectroscopic capabilities of the system are shown for electronic states on NaCl/ Cu共111兲 and TPP/Cu共111兲. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2972971兴

I. INTRODUCTION

With the invention of scanning tunneling microscopy 共STM兲 in the early 1980s by Binnig and Rohrer,1 real-space access to structures on the atomic scale became possible. The atomic resolution originates from the highly localized source of tunneling electrons, resulting in a detectable current I from a tip in close proximity to a surface under an applied bias voltage U. STM developed into an invaluable and powerful technique when it was realized that this tunneling current I grants access to many other properties such as electronic2 and magnetic structures3–5 as well as optical6,7 and electronic8,9 excitations. Moreover, the temporal evolution of atomic and molecular structures and properties under external thermodynamic conditions such as temperature10 or pressure11 can be thoroughly investigated, which is relevant to understand surface diffusion,12 catalysis,13 phase transitions,14 or growth of structures.15 However, this requires a sophisticated STM design to enable the precise control of measurement conditions. The instrumentation for high resolution tunneling spectroscopy is governed by the requirements of thermal stability for the precise positioning of the local probe. Several different designs of STM heads are reported,16–18 most often mounted to a 4He bath cryostat.19,20 Under these conditions the temperature and therefore the positioning are highly stable and the energy resolution can reach kT. However, the ability to heat and to stabilize at higher temperatures is limited by the desorption and the consumption of liquid helium.19 The design of a variable-temperature STM 共VT-STM兲 is a兲

Electronic mail: [email protected]. Electronic mail: [email protected].

b兲

0034-6748/2008/79共8兲/083903/7/$23.00

more challenging.21–26 Efficient cooling can be achieved by using a liquid helium flow cryostat but inherent mechanical cross talk introduced by boiling of helium and rush of liquid helium cannot be avoided but only minimized. Moreover, different expansion coefficients of materials used in the STM head lead to additional drift. All these effects have to be considered. The design presented here is based on the original design by Pan.27 For mechanical isolation, the STM rests on an eddy current damped platform only connected by copper braids for cooling of the STM and of the thermal shielding. Atomic resolution and local scanning tunneling spectroscopy down to the lowest temperature of 18 K are demonstrated. With in situ exchangeable molecular material sources, molecular deposition of several different molecules at low temperatures is feasible. The growth of highly ordered molecular structures is a natural effect commonly observed, and the extension towards metallic-molecule heterosystems is appealing.28 However, to understand the underlying mechanisms of molecular growth29 and molecular interactions30 as well as the development of electronic properties31 from the single molecule up to molecular films, it is desirable to start with individual molecules. Most often thermally induced mobility during preparation at room temperature prevents such an approach due to self-assembly into larger molecular clusters.32 The ability to deposit individual molecules at cryogenic temperatures overcomes this limitation. Even the preparation of single molecules on insulating substrate materials, such as NaCl, becomes feasible in this case.33 II. DESIGN AND CONSTRUCTION

The VT-STM is part of a vacuum system for the investigation of the temperature dependent growth of molecular 79, 083903-1

© 2008 American Institute of Physics

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Rev. Sci. Instrum. 79, 083903 共2008兲

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TABLE I. Materials used for the STM. Part Isolating ceramics Scanner tube Sapphire prism Isolating plates Walker stacks Conductive glue Nonconductive glue Contact wires Precision contacts UHV coaxial cables Low-noise BNC cables Temperature diode Preamplifier STM control unit Pneumatic damping legs

Material

Supplier

Macor EBL 4 PZT ceramic Sapphire Sapphire EBL 4 PZT ceramic H20 E epoxy glue H77 epoxy glue Kapton isolated copper Copper MMK 5001 G 01130 DT-670 DLPCA-200 SPM 1000 PD series

Schröder Glas EBL Products Inc. Gebrüder Wild Helmut Günther EBL Products Inc. Epotek Epotek Detakta Muetron Elspec GmbH Huber and Suhner Lakeshore Femto RHK IDE

systems on various surfaces. Therefore, we will first give a short introduction into the complete setup and discuss the setup of the microscope in more detail afterwards. Table I lists sources of materials where not common. A. UHV system

The experimental setup resides in a three chamber vacuum system 共see Fig. 1兲: a load lock for the exchange of samples, tips, and evaporation materials, a preparation chamber for sample and tip cleaning 共i.e., argon ion etching and heating兲, and a STM chamber for the local characterization of molecular systems. Possible materials for evaporation range from metals 共Fe, Co, Cr兲 and insulators 共NaCl兲 to all kinds of volatile molecules. The materials are evaporated from Knudsen cells with flux control via the ion rate 共for metals兲 and via a quartz microbalance 共for molecules兲. Deposition of adlayers and molecules at room temperature and above takes place in the preparation chamber, whereas low temperature deposition takes place in the STM chamber with the sample stored in the cooled microscope head 共see B5 in Fig. 2 and Sec. II B兲. The vacuum chambers are pumped via an ion getter, titanium sublimation, and turbomolecular pumps with the base pressure maintained in the lower 10−10 mbar range.

FIG. 1. 共Color online兲 Setup of the vacuum system with three separate chambers for loading samples, sample preparation, and sample analysis. The complete setup is mounted on a rigid frame and is mechanically isolated from building vibrations via passive damping legs.

FIG. 2. 共Color online兲 Schematic view into the STM chamber 共A兲. Labeling of the parts is related to their appearance in sections A–D. Relevant parts are the damping and cooling stages 共B1–B5兲, the housing of the thermal isolation 共C1–C3兲, and the microscope body 共D兲.

For mechanical stability and mechanical isolation the vacuum system is mounted on a rigid steel frame with additional, stabilizing frames for the transfer rods. The complete system rests on four pneumatic, passive damping legs. Figure 2 gives a detailed insight into the STM chamber 共A兲 including the components for damping 共B1 + B2兲, cooling 共B3 + B4兲, thermal isolation 共C1–C3兲, and STM 共D兲. Characters A–D refer to the sections where these parts are discussed in more detail.

B. Damping, cooling, and sample preparation

This section covers the mechanical damping 共B1 and B2 in Fig. 2兲, the cryostat 共B3 + B4兲, and the preparation of molecules on cold surfaces 共B5兲. The concept of the mechanical mounting is based on a commercial setup.34 There, the STM head is fixed on a goldplated platform. This platform has cut-outs for optical access and for the liquid helium flow cryostat and is connected via springs 共B1兲 and an eddy current damping stage 共B2兲 to the base flange. During sample and tip exchange the platform is fixed to a predefined position via a z manipulator. During STM operation the platform is released and the sample is cooled from the side 共above the platform兲. In our system, some modifications were introduced to host a custom-made STM head and to permit deposition of molecules onto the cold sample. The base flange is a CF 250 flange with ports for electrical feedthroughs 共three floating BNC and a 20 pin feedthrough兲, for the liquid helium flow cryostat, and for a z manipulator to lock the platform during tip and sample exchange. The length of the springs to hold the platform is adjusted to the weight of our STM design. The liquid helium flow cryostat ends below the platform to enable effective cooling of the whole microscope and not only of the sample. The STM head is connected to the liquid helium cryostat via a short and flexible copper braid 共B3兲. Additionally, to reduce the thermal load on the STM, a radiation shield is installed 共see Sec. II C兲. The radiation shield is connected to a second flexible copper braid and cooled by the backflow line of the liquid helium flow cryostat 共B4兲. A special cut-out is introduced into the platform opposite the

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Rev. Sci. Instrum. 79, 083903 共2008兲

A VT-STM for molecular growth

Wx Lx 1

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FIG. 4. Each component of the housing is approximated by a part of length L and of surface area A. The heat conductance ␭ and capacity C depend on the properties of the material and vary with the temperature T of the part. FIG. 3. 共Color online兲 Left: The inner housing is a series of stainless steel tubes connecting the STM body with the damping platform and holds the radiation shield. Right: Photograph of the inner and outer housings. For protection against heat radiation, the outer housing consists of a cooled copper shield and a rotatable shutter.

STM for accommodating a home-made molecule evaporation stage 共B5兲. With small mobile evaporators it is possible to exchange the molecular material source in situ. This way, more than one kind of molecule can be evaporated on the same cold substrate. During evaporation with the shield of the STM left open, the temperature of the sample in the microscope head rises to 20 K when operated at the highest helium flux. Molecules from the evaporation stage impinge on the sample surface under an angle of 25°. C. Thermal isolation

To maintain a low and stable temperature in the STM head, it is thermally decoupled from the room temperature environment. A helium backflow cooled housing 共see C1 in Fig. 2兲 and a rotatable shutter 共C2兲 for tip and sample exchange shield the STM head from thermal radiation. The housing is rigidly mounted to the inner STM head and the outer platform via a cascade of tubes with low thermal conductivity 共C3兲. The final concept was selected after numerically modeling different designs. 1. Realization

Figure 3 共left兲 shows the supporting skeleton of the radiation shield. It consists of four layers of tubes and five stainless steel rings 共I–V兲 interconnecting the tubes. The outer ring 共I兲 fixes the skeleton to the eddy current damped platform at room temperature. The radiation shield 共see Fig. 3, right兲 is mounted on ring III. The STM head itself is fixed to the innermost ring 共V兲, whereas the other two rings 共II and IV兲 give an additional extension of the tubes to increase the length of the heat dissipation paths. Rings I and II are interconnected by four quartz rods 共␾ = 3 mm兲. Quartz is an ideal material for thermal and electrical isolation but is also rather fragile. During tip and sample exchange, additional side forces appear, which limit the usage of quartz to short lengths. Therefore, for further thermal isolation the remaining skeleton is made of stainless steel tubes. The tubes are circularly arranged around the center for a high stiffness. To compensate for the increased heat conductivity, the wall thickness is reduced 共␾ = 3 mm, dwall = 0.1 mm兲. As radiation shields, several stages are used. The inner shield 关oxygen-free hugh conductivity 共OFHC兲 copper兴 is rigidly fixed at one end to ring III, whereas the opposite end is directly cooled through the helium backflow line. The in-

ner shield has an opening to the front to grant direct access to the STM head. During STM operation, this hole is completely covered by the outer shutter. The outer stainless steel shield is sustained by a ball bearing for rotation and is in situ handled by a wobble stick. 2. Simulation

The choice of materials and dimensions of the different parts resulted from a numerical simulation of the heat dissipation within the system. The goal was to achieve a base temperature in the range of 15– 20 K. As a boundary condition, the massive platform is assumed to constantly stay at room temperature. The cooling power of the liquid helium flow cryostat is given by the manufacturer 共5 W兲,35 the microscope body 共see next section兲 is thermally highly conductive and the assumption of a constant, i.e., homogeneous temperature, is reasonable. The response of the housing on the cooling process is then described by an iterative approach. This additionally yields information on the time dependence of the cooling process. Starting with the whole system at room temperature, the cooling by the cryostat is switched on at time i = 0. Then, the development of the temperature for each individual component of the setup is calculated up to an equilibrium state. This is illustrated in Fig. 4 for one idealized component x of length Lx and cross sectional area Ax. With a temperature gradient ⌬Tx given between both ends 共Tx , T¯x兲 this results in a net heat dissipation Wx. This heat dissipation reflects the temperature dependent material properties 共heat conductance ␭x兲 whereas the resulting change in temperature reflects the ratio of heat dissipation and heat capacitance Cx. With the material properties known from standard literature,36,37 we calculate the heat dissipation and the new temperature Tⴱx after a time step ⌬i to be T ,T¯x

Wx x

=

Ax ␭x共Tx,T¯x兲⌬Tx Lx

Tⴱx = Tx + Wx x

T ,T¯x

⌬i x兲 C共T x ⌬Tx

共heat dissipation兲,

共1兲

共new temperature兲.

共2兲

Including contributions due to heat radiation 共⬃T4兲, this approach was applied to the final design with the most relevant components schematically shown in Fig. 5. For simplification we introduced some approximations in the simulation: each component is described by its length and an average cross sectional area instead of its real geometry, i.e., with the exact positions of cut-outs and bores. Heat conductivity and heat capacity enter the calculation as homogeneous material properties, i.e., temperature gradients are neglected.

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Rev. Sci. Instrum. 79, 083903 共2008兲

Kuck et al.

1 1 1 1 1

contact points for cooling 1 1 1 1 1 1 1 1

FIG. 5. 共Color online兲 Schematic of the different stages connecting the STM, shield, platform, and cooling braids. The two sources of heat load are heat flow 共→兲 from the platform and heat radiation 共⇒兲.

Instead, for each step in the calculation, values for heat conductivity and heat capacity are taken for a mean temperature 关共Tx + T¯x兲 / 2兴. Interface heat resistances between two linked components were treated in the limit of idealized contacts. The cooling power of the helium backflow line was unknown. Therefore, the same calculation was applied to two other similar VT-STM systems with known temperature behavior. The results of those calculations for the helium backflow line were handled as input parameters for the present calculation.38,39 Figure 6 shows the results of this calculation 共full line兲 along with the measured experimental data of the final setup 共dashed line兲. Experimentally, the temperatures of the cryostat at the sample stage within the microscope body and at the inner shielding were measured in intervals of 5 min by silicon temperature diodes. Overall a convincing agreement between experimental and calculated temperatures is found. 20 min after the start of the cooling procedure the liquid helium cryostat is efficiently cooled down to approximately 30 K. The temperature load now only comes from parts of the microscope behind the cooling point of the cryostat. The slope of the time dependent temperature changes significantly in experiment as well as in the calculations 共dashed and full bottom curves兲. The same effect can be observed after approximately 50 min in the curves related to the cryostat and to the sample stage 共middle curves兲. The microscope body reaches a temperature value of 25 K and the

FIG. 6. 共Color online兲 The cooling process as simulated and experimentally observed for different positions within the setup.

contact pad for piezo walker stacks electrical contacts

FIG. 7. 共Color online兲 The body of the STM shown from the back 共a兲 and the front 关共b兲 and 共c兲兴. It is made out of phosphor bronze with copper parts for homogeneous cooling power. The front view shows the STM after assembly with 共b兲 and without the scan unit 共c兲.

slope of both curves changes. The temperature of the inner shield still decreases for another 12 h 共top lines兲. In the calculations the time dependent temperature curve of the sample stage shows a pronounced kink at 100 K. This results from a drastic change in heat conductivity for copper with its approximation by a mean value. In reality the existing temperature gradient washes out this effect. Additional deviations between simulation and experiment 共see Fig. 6兲 arise from the unknown interface heat resistance and the overestimation of the cooling power of the helium backflow line. After approximately 1 h, the base temperature of the sample stage of about 18 K is reached. D. STM

The design of the STM head derives from the original design by Pan27 and its further development at the University of Hamburg.40,41 The current version is optimized for use in a VT-STM system. To reduce the heat capacity and to minimize the surface area for thermal radiation, its size was reduced compared to the original version. Figure 7 shows three photographs of the homebuilt STM head: a back view 共a兲 and views into the assembled microscope body with 共b兲 and without 共c兲 the scan unit. The body has a tube shape with a diameter of 24 mm. The STM body consists of phosphor bronze, a material with low thermal conductivity, therefore acting as a damper against thermal fluctuations. Moreover, phosphor bronze is advantageous in terms of mechanical properties and its machinability compared to OFHC copper. For an effective and homogeneous cooling, the body is directly connected to a rigid thread made out of OFHC copper, which hosts the contact points for the cooling braids. A temperature diode is installed close to the sample stage with the wiring guided along the back side. At the bottom end a contact pad for electrical connections is mounted. Twenty-three commercial pin sockets are glued into a solid macor block. For maintenance, the electrical connections can easily be plugged and unplugged. The front view 关Figs. 7共b兲 and 7共c兲兴 opens an insight into the assembled microscope. The sample stage is glued into the upper part of the body with nonconductive glue and is separately contacted with a miniature UHV compatible low-noise

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Rev. Sci. Instrum. 79, 083903 共2008兲

A VT-STM for molecular growth

coaxial cable. The potential of the sample is defined by the voltage applied between the sample stage and the grounded body. Below the sample stage four piezowalker stacks for the coarse approach are visible. These stacks are completed by Al2O3 plates on top of each stack. Al2O3 is a hard material keeping the abrasion during operation of the coarse approach minimal. Polishing of the Al2O3 plates results in a very smooth and reproducible coarse movement. The view in Fig. 7共b兲 shows the microscope body after adding the scan unit. The scan unit consists of a sapphire prism with the scanner tube mounted inside. It is a conventional five segment piezotube with four outer segments for the x-y positioning and an inner segment for the z positioning. For easy access, we use a wraparound electrode for the inner segment. Therefore, the contact point is below the sapphire prism and on the outer side. At the other end, the scanner tube extends beyond the sapphire prism and in case of failure, the x-y-z cabling can be optically controlled and easily repaired. The sapphire prism rests on the four piezowalker stacks and is fixed in its position by a counterplate which hosts two more piezowalker stacks 共hidden below the spring兲. The contact pressure between the sapphire prism and the six piezowalker stacks is controlled through the spring force. The spring force is adjusted by three adjustment screws. With optimized spring force, a reliable and reproducible approach speed within the full temperature range is achieved. The approach is operated at 1.5 kHz with a typical step size of ⬃120 nm at room temperature and of ⬃60 nm at 18 K. The travel distance accounts for ⬃10 mm, which is sufficient for in situ tip exchange. The STM is controlled via external electronics for temperature monitoring, the coarse approach, and the STM operation. Sensitive signals are the current I, the bias voltage V, and the high voltage for z positioning 共z兲. To minimize the electronic noise on these signals, the STM and the control units are electrically isolated from the electronics of the vacuum system. We defined one common ground node at the back side of the STM control unit which is rigidly connected to the ground of the building by a thick, low Ohmic copper braid. The potential of the ground node serves as reference for all signals. To avoid the creation of a ground loop via the approach control unit, the approach piezos are electrically isolated from the control unit and grounded at the common ground node through a mechanical switch during STM operation. Outside of the vacuum, conventional low-noise coaxial cables are used for z, V, and I. On the vacuum side, UHV compatible low-noise coaxial cables are used. These cables have a small diameter, minimizing thermal conductivity. The shieldings of the sensitive signals are guided into the vacuum system via floating BNC feedthroughs with the ends of the shieldings left open at the microscope body. All other signals are transmitted in situ via unshielded and Kapton insulated copper wires and in air via a shielded BNC cable bundle. The electric potential of the STM body is defined by the common ground node with a direct connection to it. Further, ground connection through the cooling is prevented by thermally highly conductive but electrically insulating sapphire plates

FIG. 8. 共Color online兲 Atomic resolution of a Cu共111兲 surface with a line profile. The corrugation is 20 pm at a noise level of 5 pm peak to peak 共5 ⫻ 5 nm2, −100 mV, 2 nA兲.

and the STM is electrically insulated from the chamber by quartz rods 共see also Sec. II C兲. E. Operation

We will now discuss the abilities of the STM setup as described above. All results 共including noise measurements兲 presented in the following were acquired with all pumps running. Noise coupling of turbopumps is negligible 共although present兲 whereas the external rotary pump causes significant acoustic disturbances. Therefore, the rotary pump is located in a separate room connected only via a flexible highvacuum tube. The STM is fully operational between 18 and 300 K. The lowest temperature is achieved when cooling with the highest liquid helium flux of approximately 2.5 l / h. Higher temperatures can be stabilized by first reducing the helium flux, then by actively heating against the cooling power of the cryostat. Therefore, to reduce helium consumption, most measurements such as those presented are performed at slightly higher temperatures of approximately 25 K, resulting in a liquid helium consumption of 0.8 l / h. At 25 K, the x-y drift after 24 h of cooling accounts for 0.1 Å / min. Figure 8 shows a representative constant current image of a Cu共111兲 surface at 25 K with atomic resolution. The line section next to it shows a corrugation of 20 pm and reveals a noise level of 5 pm peak to peak. The most important issue is the capability of the system to deposit molecules on the cold substrate. Even consecutive deposition of several different types of molecule onto the same cold substrate is possible. Figure 9 shows such an example. Tetraphenylporphyrin with cobalt 共Co-TPP兲 and copper 共Cu-TPP兲 were evaporated one after the other at 25 K onto a Cu共111兲 surface 关see Fig. 9共a兲兴. The formation of molecular clusters due to surface diffusion is hindered by the

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Rev. Sci. Instrum. 79, 083903 共2008兲

Kuck et al.

b)

a) Co−TPP

Cu−TPP

FIG. 9. 共Color online兲 Porphyrin molecules on a Cu共111兲 surface. 共a兲 Topograph. 共b兲 Map of differential tunneling conductance recorded with lock-in technique 共f = 2.777 kHz, Vmod = 15 mV兲 共6 ⫻ 6 nm2, −750 mV, 300 pA兲.

low temperature conditions and therefore single molecules are observable. The molecules are easily distinguishable by their appearance in topography as well as in spectroscopy 关see Fig. 9共b兲兴. The STM system is also capable of performing spectroscopy on insulating adlayers. As an example, a 2 ML 共monolayer兲 island of NaCl on Cu共111兲 was investigated 共see Fig. 10兲. In the topograph 关Fig. 10共a兲兴 the atomic lattice of the NaCl island is clearly resolved with a corrugation of 15 pm 关see also line section in Fig. 10共c兲兴. Figure 10共b兲 shows the simultaneously recorded dI / dV signal. The dI / dV image shows standing wave patterns of the surface state on the Cu共111兲 surface and of the interface state on NaCl. The wavelengths of both patterns are distinctively different, which reflects different onset energies for the surface state and the interface state.42 On insulating layers, the coupling between the substrate and molecule is reduced in comparison to the metalmolecule system and the mobility is increased. This leads to completely uncovered NaCl islands at low coverages for room temperature deposition 共not shown兲. This is different in the case of low temperature preparation 共see Fig. 11兲. NaCl was first deposited on Cu共111兲 slightly below room temperature, resulting in the formation of first and second layer NaCl islands.43 Afterwards, FePC molecules were deposited on the cold NaCl/ Cu共111兲 substrate. A detailed analysis 共see also Ref. 44兲 reveals that all FePC molecules on NaCl are oriented in parallel, whereas three different molecular orientations can be found for

Cu(111)

FePC on Cu(111)

NaCl FePC on NaCl

FIG. 11. 共Color online兲 FePC molecules on NaCl/ Cu共111兲 deposited at 30 K. Single molecules can be observed on the copper substrate as well as on the NaCl adlayer 共60⫻ 60 nm2, −1 V, 100 pA兲.

FePC / Cu共111兲. This reflects that NaCl grows in 共100兲 facets on Cu共111兲. III. SUMMARY

We presented the design and construction of a versatile STM fully operational in the range between 18 K and room temperature. Several different molecular materials can be evaporated one after the other with the sample directly stored in the cooled microscope head. In addition, the setup gives the experimentalist the flexibility to choose any arbitrary preparation temperature within the full temperature range. Moreover, the realized design benefits from the optimization of heat dissipation paths by a numerical approach. The STM operates at low consumption of liquid helium 共0.8 l / h兲, is compact 共45⫻ 90 mm2 including all shieldings兲, and is very robust against external vibrations. The setup is operational within a short time of 120 min after starting of the cooling procedure and within 20 min after exchange of sample and tip materials. Due to the compact design, the investigation of identical surfaces before and after deposition of molecules is feasible on a regular basis. Performance and stability of the STM were demonstrated for FePC molecules deposited on a Cu共111兲 surface and on a NaCl adlayer. Ackowledgments

We wish to acknowledge funding by the Deutsche Forschungsgemeinschaft through SFB 668 共TP A5兲 and GrK 611. G. Binnig and H. Rohrer, Rev. Mod. Phys. 59, 615 共1987兲, and references therein. 2 M. Crommie, C. Lutz, and D. Eigler, Nature 共London兲 363, 524 共1993兲. 3 R. Wiesendanger, H.-J. Güntherodt, G. Güntherodt, R. J. Gambino, and R. Ruf, Phys. Rev. Lett. 65, 247 共1990兲. 4 M. Bode, M. Getzlaff, and R. Wiesendanger, Phys. Rev. Lett. 81, 4256 共1998兲. 5 W. Wulfhekel and J. Kirschner, Appl. Phys. Lett. 75, 1944 共1999兲. 6 J. K. Gimzewski, B. Reihl, J. H. Coombs, and R. R. Schlittler, Z. Phys. B: 1

FIG. 10. 共Color online兲 共a兲 Topographic image of a Cu共111兲 terrace with a 2 ML island of NaCl. The atomic lattice of the NaCl is clearly resolved. 共b兲 Simultaneously recorded dI / dV map of the same surface area as in 共a兲. Standing waves of the surface state are visible both on copper and on sodium chloride. 共c兲 Line profile of the section in 共a兲. The atomic resolution of the NaCl is imaged at a corrugation of 15 pm. 共d兲 Line section indicated in 共b兲 reflects standing wave patterns with different wavelengths on the different surfaces 共20⫻ 20 nm2, −75 mV, 400 pA兲.

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