Aspects of Molecular Imaging with a Scanning Tunneling Microscope

Aspects of Molecular Imaging with a Scanning Tunneling Microscope ... Scanning Tunneling Microscopy Imaging of Small Adsorbed Molecules on Metal ...
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J. Phys. Chem. 1994,98, 2941-2949

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Aspects of Molecular Imaging with a Scanning Tunneling Microscope Shachar Richter and Yishay Manassen' Department of Chemical Physics, The Weizmann Institute of Science, Rehovot 76100,Israel Received: July 16, 1993; I n Final Form: December 20, 1993'

There is considerable interest in imaging molecules adsorbed on surfaces with a scanning tunneling microscope (STM), due to the potentially high resolution available and the enormous potential of such studies in chemical directions. The results observed on adsorbed molecules on conducting surfaces with the STM are reviewed. From these results, it is clear that the mechanism of tunneling through the molecule as well as the parameters which provide the contrast in the STM images are not fully understood. We then show that the molecules under the tip of the STM are subject to significant forces: both van der Waals forces and electrostatic ones. These forces are different from one molecule to another and from one functional group (in the same molecule) to another. Most measurements were done in room temperature. Therefore both molecules must move with respect to the others and also motions of functional groups within the molecule must occur: The activation energies of these processes are of the same order of magnitude as the tipmolecule interactions. We try to evaluate quantitatively the extent to which these motions occur and their effect on the observed contrast. Most studies so far claimed that the observed STM image is related to the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of the molecules. The HOMO and the LUMO were calculated for some molecules which were studied experimentally, and simulated images in a gray scale representation were calculated and were compared with the STM images. The lateral solution of the simulated image was artificially reduced to 2.5 A (which is the resolution of the STM). The simulated images were calculated for the different possible conformations of the molecule on the surface. The STM image is probably an average of the different states at the different conformations.

Introduction The invention of the STM'J (scanning tunneling microscope) was a real revolution in the fields of surface science and microscopy. After extremely high atomicresolution was achieved on clean semiconductor and metal surfaces, many attempts were made to perform STM studies on surfaces which are covered with molecules,in order to observe structural information, together with measuring the molecular energy levels, in a way similar to the studies on clean conductingsurfaces. These experimentscan be divided to several classes. The first one is on surfaces where as a result of a chemical reaction between an adsorbed small molecule and the surface, the surface densityof states is modified and this provides the contrast for the STM image, without distinguishing between the molecule and the surface. Such systems are NH3 adsorbed on a Si( 111) 7x7 surface3and 02 on Si(ll1) 7X7.45 The study on the second system showed that a single molecule might create a different contrast due to the different reaction products on the surface. The same type of study was made on 0 2 adsorbed on GaAs.6 In addition to adsorbate-inducedmodifications of the local density of states, it has been shown that since the adsorbed oxygen is charged, it interacts with the mobile carrier of the semiconductor. This interaction affectedthe surface density of states over ranges much larger than the molecular size. A similar study was made also on metallic ~urfaces:~*8 The adsorption of 0 2 on Cu( 110) gave a similar modification of the metallic surface density of states. A second class of experimentswas done on conductingorganic molecular crystals. The first study was done on a crystal of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCQN)9 (All the molecliles mentioned in the text are shown in Chart 1). In this work, it was shown that since the largest contribution to the delocalized state at the Fermi level is coming from the highest occupied molecular orbital (HOMO)of the TTF molecule and the lowest unoccupied molecular orbital (LUMO) of the TNCQ molecule, the simulation of these molecular orbitals are in Abstract published in Advance ACS Absrrocts, February 1, 1994.

0022-365419412098-294 1$04.50/0

agreement with the STM image. In these images, thedifferences between the molecules as well as the central ring and the cyano groups in the TNCQ molecule are distinguishable. Several successful experiments were done on chemisorbed molecules on metallic surfaces. Such studies were performed by coadsorption of benzene (C6H6) and CO on Rh( 111).l0J1 The benzene appearsas a 3-fold ring. The CO molecules were required in order to restrict the diffusion of benzene. Two structures were formed: a 3x3 structurelo and a 4 2 4 X 4)rect structure.11 The CO molecules were visible only in the second structure. A smaller molecule was seen only after cooling the surface. An example to a study of this type is the adsorption of ethylene (C2H2) on Pt(l1 l).lZ This study demonstrated the capability of the STM to follow a chemical reaction of the adsorbed molecule: Annealing the Pt surface which was covered with ethylene converted the ethylene first to ethylidene (C-CH3) and then to graphite. This work demonstrated the enormous potential of STM studies in the chemical direction. In addition to benzene, several successful attempts were made in imaging larger molecules such as naphthalene,13.60.61 azulene derivatives,60.61and copper phthalocya11ine.1~J5The naphthalene molecules were adsorbed on a Pt( 111) surface.13 They appear as a bilobed structure, which looks similar to the LUMO orbital (Figure 5). The copper phthalocyanine is an extremely rigid molecule, which' was imaged relatively successfully even with the field ion microscope. It was successfully imaged on Cu(lOO)l4 and on GaA~(100).'~ Theimageon Cu( 100) showsa remarkable similarity to the HOMO. It demonstrates that in principle the STM is capable of providing information on the molecular structure. The successful observation is probably due to the fact that the surfacehas a wave function which is approximately a superposition of the molecule and the metallic states. This is possible, since there is a strong chemical bond between the molecule and the surface, making the molecular orbital part of the delocalized state, and the molecule becomes conducting. (8

1994 American Chemical Society

Richter and Manassen

2942 The Journal of Physical Chemistry, Vol. 98, No. 11, ISp94

CHART 1: Molecules Imaged in the STM Experiments' a.

b.

C.

5 -nonylZ-(n-hexoxyphenyl)pyrimidine

i.

didodecybenzene

i. I

C

O

O

H

behenic add k. H

x)=(xH

H

H

..

k

H

tetrathiafulvalene (TTF)

tetracyanoquinodimelhane(TCNQ)

m.

n.

/

4-(4'-n-prcpylcyclohexyl)

cyanocyckhexane (CCHS)

4-n-hexyI4-cyanobiphenyl (WE)

This is no longer true, however, when physisorbed molecules are imaged. The fact that STM measurements of such systems gave many rather sharp and high-resolution images is one of the most important unresolved problems in the field of tunneling microscopy. The STM images of the cadmium arachidate bilayersI6 demonstrate the problem: In this system, the molecules are oriented perpendicular to the surface, creating an insulating film (at least in principle) of a thickness of 54 A. Despite this huge tunneling distance, an array of bright spots was observed, where each spot was assigned as a single molecule. The array of spots was clearly distinguishable from the graphite substrate. Most of the successful molecular imaging experiments on surfaces with physisorbed molecules were done on graphite. Most of these works used the strong affinity of the alkyl chains to the graphite surface in order to immobilize the molecules. This is due to the fact that the alkyl chain is commensurate with the graphite surface. As is expected, the images of alkanes are with very good molecular resolution. Such images were taken from C32& and from Cl7H%.l7J* A careful examination of the results showed that the observed atomic features are due to the graphite atoms and not due to the atoms of the observed molecule. The tunneling is e n h a n d , however, by the presence of the molecule, and the graphite sites under the molecule appear brighter. Severalworks used thisaffintyof the alkyl chain to thegraphite substrateto look at several different organicmoleculeson graphite. Such studies were performed on alkylcyanobiphenyls or more accurately, 4-n-alkyl-4'-cyanobiphenyls(mCB wheren m = 6,8, 10,12).ls22 In all these images, the bright regions are assigned as the biphenyl rings, while the alkyl chains appear darker (Figure 5). Other molecules that were images with the STM on graphite are 5-nonyl-2-(n-nonoxylphenyl)pyrimidine(PYP 909) molecules.23126 Thew molecules are made of two aromatic rings and two alkylchains. In the image it is possible todistinguishbetween the alkyl chains and the aromatic parts of the molecule, which again appear brighter in the image. It is not possible (unlike the case of the cyanobiphenyls) to distinguish between the two aromatic rings. The boundary formed between the rows of the molecules is distinguishable. A different molecule which was also imaged is e(4'-~p~l~c~h~yl-propylcyclohexyl)cyanocyclohwtane (CCH3). The cyclohexane ring is not aromatic, unlike the rings in the molecules imaged so far. Nevertheless, it is not possible to say, accordingto these results, that the STM can distinguishbetween the cyclohexane and the benzene rings which are approximately the same size but have a completely different electronic structure (Figure 5). Another molecule which was imaged on graphite is d i d o ~ y l b e n z e n e . Here, ~ ~ ~ ~again, ~ the benzene appears brighter than the alkyl chain (Figure 5). The boundary between the rows of molecules as well as the molecular domain boundaries are beautifully distinguishable. As was mentioned previously, the mechanism by which the electrons are transferred from the tip to the molecule and from the molecule into the surface and the mechanism of contrast are not completely understood. An interesting proposal for the contrast mechanism was that the variations in the polarizability of the different parts of the molecule causes changes in the work functionof the surface under the molecular layer and the contrast mechanism is due to a spatially dependent work function. The possibility that a change in the work function is responsible for the molecular contrast was mentioned also for the system benzene CO on Rh( 11l).lO The fact that in the topographic imaging of alkanes on graphite17J' the atomic corrugations that are seen are of the graphite and not of the molecular orbitals is a point that supports the barrier height mechanism, since it indicates that the charge density of the substrate is measured and the molecule only applies a perturbation to the tunneling process. However it was show# that even for alkylcyanobiphenyl molecules on graphite (which is a clear exampleof physisorption)

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0.

N adenine a Theoretical simulations were done for several of them. This shows theorientationsof the moleculcain the simulationsand in the comepnding experimental imagcs.

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The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2943

Aspects of Molecular Imaging with an STM

the observed image is similar to both the HOMO and the LUMO. (As can be seen in Figure 5 , the similarity is particularly remarkable for 6CB). It is important to emphasize at this point that this similarity is not necessarily in a concentration with the proposal that the mechanism is due to changesof the workfunction as a result of a different polarizability. The polarizability is directlyrelated to the molecular orbitals:Z7(Yk = 2e?&/(ilX&)lz/ (E,-EJ where i representsthe occupied orbitals, u theunoccupied ones, and k = x, y , z. Looking at the spatial variation of this value, it might have a similar spatial dependence to the HOMO and the LUMO. However, measurements on behenic acid Langmuir-Blodgett filmsz8 showed that the barrier height mechanism might be incorrect in explaining the contrast. In these experiments, the d(ln J ) / d z image is measured. This imaging mode mainly reflects the local barrier height variations. It was shown that in the regular topographic image, the carboxylic group appears brighter than the alkyl chain. However, this contrast did not appear in the image. This shows that at least in this particular case, the work function mechanism for the molecular contrast is not correct. Of course it must be tested experimentally whether this conclusion can be generalized. In STM measurements of clean surfaces, it is possible, by changing the bias voltage, to study the local surface density of states at different energies. Changingthe bias voltage on a surface covered with molecules so far had not given a similar result. The response of the system to changes in the bias voltage is almost opposite in chemisorbed and in physisorbed molecules. On chemisorbed molecules, for example, copper phthal~yaninel~ adsorbed on Cu( loo), decreasing the current and voltages improves the molecular resolution. A similar behavior was observed in the system Rh( 111)(3X3)(C& 2C0),1° where decreasing the bias voltage from -1.4 to -0.01 V prevented instabilities and enabled better resolution. As will be suggested later, this phenomena is due to the influence of the electric forces applied on the molecule by the tip: Working with low-bias voltage and tunneling current decreases electric forces applied on the molecule by the tip and minimizes the molecular motion. To a certain extent, the behavior in physisorbed molecules is opposite: For example in the imaging of alkanes on graphite,” working at 2 V gives a good image of the adsorbed molecules. Decreasing the voltage below 0.3 V resulted in very noisy images. Better contrast was achieved at conditions where strong electric fields were applied. This might indicate that molecular motions due to these strong fields are partly responsible for the contrast which is seen. This point will be discussed below in detail. In other works, decreasing the bias voltage leads to observation of the substrate and to disappearanceof themolecules. An example to this behavior was observed on 7CB and 8CB molecules on graphite.22 In 8CB this transformation occurs when the bias voltage was reduced from 1 to 0.1 V. On 7C3 it occurs in a reduction from 1.9 to 0.05 V. The trivial explanation that the molecules are pushed away by the tip was ruled out, since the transformation occurs immediately, much faster than the time which is required for molecular rearrangements. No damage to the molecular layer was observed after returning to 1V. A similar behavior was observed in the topographicimage of behenic acidZ8 and in other systems. An attempt to explain this behavior by a resonance tunneling mechanism was made? When a large bias voltage is applied, resonance tunneling throughout the localized molecular state is dominant, giving the molecular structure. At lower voltages, normal tunneling is dominant, and corrugations of the substrate are detected. Although this model can explain the observed voltage dependence, it cannot provide any explanation for the contrast observed. Other possibilities for contrast might be a reduction of the barrier height due to changesin the dielectric constant of the mediumin which the tunnelingelectron propagates. In general, however, the conduction mechanism and the observed contrast are not fully understood.

+

-qn, -dn

If

Figure 1. Scheme of the configuration of the conducting sphere and the conducting plane. The problem of calculatingthe potential between the sphere and the plane is reduced in the method of images to the problem of calculatingthe potential from the point charges qnand from the image charges -qn.

Electric Forces on Molecules during STM Measurements Tip sample interactions during STM operation have attracted significant attemption during recent years. Severalexperimental studies (for example, see ref 29) showed that the forces applied by the tip on the sample are rather strong and are of the order of 10-9 N. In a theoretical work30 the bonding energy between a graphite surface and an aluminum tip was calculated. A significant difference in energy was found when the apex of the tip was above a different location on the surface. When the tip-sample distance is such that the total energy is minimized above a carbon atom, the bonding energy was -0.33 eV and above a hollow site it was -0.61 eV. These are attraction energies. When the tip is pushed further toward the surface, to a distance which is closer than themost stableconfiguration,strong repulsive forces are observed. It is clear that when molecules are physisorbed on the surface, the forces applied on the molecules are even stronger. These forces can be significantly different in different functional groups of the molecules. Larger atoms and more polarizable groups will feel larger forces. A similar and even simpler result is observed when the electric forces applied on physisorbed molecules as a result of a biased tip are taken into account. The difference between different moleculesand different functional groups is even larger: The electric forces applied by a biased tip leads to the ability of manipulating atoms and molecules on the atomic scale, as was demonstrated in many st~dies.~l-3~ The questions raised here are the impact of these electric forces and of the induced molecular motions on the observed image. To calculate the electric field due to the bias voltage, one must use the method of images.37 The field between a spherical tip and a plane (a conducting surface) is expressed as a sum of fields from a series of point charges on the line which is perpendicular to the plane and is connecting the center of the sphere and the plane (Figure 1). The point charges q,, are located at a distance d,, from the plane (inside the sphere; see Figure 1). The image charges -Qn (behind the plane) have a distance +in.The charge of the nth point charge is given by q n = qn-lR/[d,+l + S R] where qo = V& and d, = +{S R -RZ/[d,, +S+R]Jwheredo=S+ R. SandRarethedistance between the sphere and the plane and the radius of the sphere, respectively. uo is the bias voltage in volts. The charges are calculated recursively and the potential at a certain location P on the surface (Figure 1) is given by U p )= E,,qn/rPm where the summation is taken over both the point and image charges. Since the surface is conducting,the electric field parallel to the surface is zero, and only components perpendicularto the surface (in the I direction) are left: E g = aW(p)/aZ. The electric

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The Journal of Physical Chemistry, Vol. 98, No. 11, I994 0

b

Richter and Manassen of the maximum interaction energy (under the apex of the tip) as a function of different tip molecule distances and for differen bias voltages is seen in Figure 2c. These are significantinteraction energies. What is clear from this result is that in addition to the large lateral forces applied on the molecule in the direction of the point under the apex of the tip, thereare additional strong vertical forces that pulls the molecule toward the tip. Both of these forces are applied in a different intensity on different functional groups of the molecule, and are expected to distort the shape of the molecule. These forces are added to other attraction energies between the tip and the molecule which were mentioned in the beginning of this section. The large and variable forces applied and the response of the molecule must have a significant affect on the contrast achieved in molecular imaging.

Induced Molecular Motions during STM Measurements

Figure 2. (a) Electric field on the surface as a function of the lateral distance from the point on the surface directly under the tip (x [A]). The field was calculated for different tip radii. In this calculation V = 2 V, and s = 10 A. (b) The interaction energy between several selected molecules and the tip. The mokules are C6H.5 (no. l), cc4 (no. 2), CH4 (no. 3), N2 (no. 4) and H2 (no. 5). In this calculation V = 2 V, R = 100 A, and s = 10 A. (c) Contour map of the maximum interaction energy (x [A] = 0) for different tip sample bias voltage and for t i p molecule distance (s [A]) (moleculesdata of polarizability is taken from

ref 62).

field is of course not uniform. Electric fields on the surface as a function of the lateral distance from the apex of the tip, is seen in Figure 2a for different tip radii. The energy of a molecule in an electric field E(r) is given by V(r) = -pE(r) - 0.5~tE(r)~, where p is the permanent dipole moment of the molecule and a is the effective polarizability. The fact that a molecule with a permanent dipole moment which is not perpendicular to the surface will be affected by the electric field is obvious (and has been measured e~perimentally).~8 In most of the casesdiscussed here the molecules are adsorbed parallel to the surface, and the dipole term in the interaction energy V(r) is very small and can be neglected. The second term, however, can be and is significant. Figure 2.b shows the interaction energy between the different molecules and the tip as a function of the lateral distance on the surface. In this calculation,several assumptions were taken: First, the polarizability of the free molecule was used in calculating V(r).Namely, weneglect theaffect ofthesurfaceon theelectronic structure of the molecule. This assumption is certainly good for large physisorbed molecules which are adsorbed on a relatively inert surface, which is the case where this interaction is expected to give a significant affect. In the case of chemisorbed molecules, this assumption is less true, but the calculated energy is probably at the same order of magnitude of the real one. In the calculation, the mean value of the polarizability was taken into account. The results in Figure 2b describe an attraction between the biased tip and the molecule. Since the interaction energy has a minimum at x = 0 A, it means that when the molecule is not there, it feels a lateral force in the direction of the point which is under the apex of the tip. This is true regardless of the sign of the bias voltage. The interaction energy varies significantly in different molecules. Larger molecules,with more unsaturated bonds and with heavier atoms will have a larger interaction. Similarly, a large molecule may have several different functional groups, each one of them might have a different interaction energy with the electric field. The interaction energy was calculated for a tip of radius of curvature of 100 A. (The real radius of curvature can be significantly smaller), and for tip-molecule distance of 10 A (normally this distance is believed to be smaller). A calculation

Atomic and molecular motions induced by electric fields"-36 were heavily investigated long before the invention of the STM, with the field ion microscope (FIM). Random and induced (by an electric field) motions of single atoms on metallic surfaces were measured for many year~.~9-4~ Molecularmotions as a result of tip-induced forces were observed in several cases: For example, measurements of the phthalocyanine molecule on a copper surfaceI4 as well as measurements of similar systems showed an improvement of the resolution at low bias voltages and tunneling currents. While this behavior might be explained by thevariations of the adsorbate electronic structure with the tip sample distance, the possibility that it is a result of the electric fields induced by the tip cannot be ruled out. These fields were found38 to be capable of changing the orientation of molecules in a liquid crystal. This change is reflected in a change in the molecular contrast. According to our calculations the interaction of the tip with the molecule is different for different molecules. It is believed that this can explainseveral experimental observations: For example,18 the STM images of alkanes on graphite have quite a good quality. As was mentioned already, this quality is a result of the strong affinity of these moleculesto the surface. However, an additional reason for this quality can be the relatively weak interaction of the alkyl chains with the electric field of the tip. An STM image of similar adsorbed alcohols on the graphite surface under the sameconditionswas with a much worse resolution. Thisdifference might be due to the accessible electronic states of the alkanes. Nevertheless, an alternative explanation might be due to the fact that the alcohol molecules have a stronger interaction with the electric field of the tip. A molecule which is adsorbed on the surface feels a potential which prevents free lateral motion of the molecule on the surface. Therefore, the molecule which moves from one site (with the most stable mo1eculs;surface configuration) to a neighboring one must overcome a certain activation energy Ed known as the barrier to lateral diffusion. Ed is normally much smaller than the desorption energy. The motion of the molecule can be described by a random walk and will obey the diffusion equation. Since the diffusion process is dependent on thermal activation the diffusion constant is normally temperature dependent: D = Do eXp(-Ed/kt). A solution to the diffusion equation and the evaluation of the standard deviation of the distribution will give ( x 2 ) I / 2= (2Dt)1l2,where (x2)V2is an estimation of the average distance made by the diffusingparticle (the molecule) as a function of time (t). At a first step the interaction between the molecule and the tip (which can make the situation only worse) is ignored. The question whether a moleculecanbesufficientlyimmobilized under the tip can be answered by the size of (x2)V2if, within the imaging time of the molecule, it moves to a distance smaller than the lateral resolution (2.5 A), themoleculecan be, in principle, imaged. Otherwise, the molecular resolution will deteriorate rapidly. Therefore ( 9 )must be smaller than cm2. Translating this

Aspects of Molecular Imaging with an STM requirement to units of diffusion coefficients means that if the imaging time of a molecule is 1 s, this means a requirement of D < 10-15 cm2/s. In the fast constant height imaging mode, which is normally used in graphite surfaces, the scanning speed can be 2 or 3 order of magnitudes larger than in the constant current mode. This leads to an upper limit to the diffusionconstant of -10-12 cm2/s. A molecule with a larger surface diffusion constant cannot be imaged with the STM. The diffusion of different molecules adsorbed on a surface was thesubject of many studies. It is clear that the diffusionconstants of molecules which are physisorbed on a surface are large enough to prevent STM imaging of these molecules. For example, the lateral diffusion constant of cyclopentane and cyclohexane on Ru( 100)was measured with the laser-inducedthermal desorption technique (LITD) to be -2 X 10-8 cm2/s at a temperature of 150 K.42 Even in this low temperature, diffusion is too rapid. To image molecules in these systems, it is necessary to reduce the temperature to -80 K (Ed = 3.3 kcal/mol for cyclopentane and 4.5 kcal/mol for cyclohexane).42 Even this might be a problem due to tipmolecule interactions. The coverage of the molecules might play a significant role in decreasing the diffusion constant. For example, the diffusion of tetramethylsilane displayed a strong dependence on the surface coverage at 125 K: D = 5.5 X 10-7 cm2/s at 8 = 0.18, (where Os is the coverage of one molecular monolayer). D = 8 X le9 cm2/s at 8 = 8,. On the other hand, on trimethylsilane, D = 6.5 X 10-8 cm*/s in all e o ~ e r a g e s .since ~ ~ Ed in this case is similar to that in the previous case, even on fully covered surfaces (in cases where it helps), very low temperatures are still required. Studies of other isomers of pentane on Ru(100) showed that these other isomers (isopentane, n-pentane) diffuse even faster than cyclopentane.44 Other alkanes (from propane to n-hexane) on the same surface were examined.45 These results showed a decrease of 4 order of magnitudes in the lateral diffusion constant from propane to n-hexane. As a result, for sufficiently large molecules, the diffusion constant might be small enough for successful molecular imaging with the STM, if the internal structural motions are minimized. The situation on a graphite surface is not different. The diffusion of benzene on the basal planes of graphite was measured with neutron scattering.& Diffusion constants which are smaller than 10-6 cm2/s were measured at room temperature. The conclusion from the measurements done on physisorbed molecules is that unless large molecules are imaged, working on cold samples is necessary as done with ethylene on platinum.I2 Recent STM measurementsmof naphthalene and naphthalene derivatives on Pt( 111) showed that the diffusion of naphthalene molecules is on the order of minutes. Considering the lateral resolution of a few angstroms in their experiments implies a diffusion constant of the order of lO-ls/ lo2cm2/s = l@17cm2/ s. The measurements of naphthalene isomers give a diffusion which is 2 order of magnitude faster (and is strongly coverage dependent). This implies a diffusion coefficient of l t 1 5 cm2/s. The situation is of course, entirely different when a certain condensed molecular phase is formed by the adsorbed molecules. A first example is the measurement done with scanning microelipsometrydone on Langmuir-Blodgett multilayers of cadmium arachidate. In these measurements, which were done at an elevated temperature (100 "C),the upper limit to the lateral diffusion was D I10-IOcm2/s.47 This means that this system can be imaged with the STM in room temperature, as was found indeed experimentally.16 As was mentioned previously, many of the molecules which were successfully measured with the STM form a liquid-crystalline phase. The fact that the molecules are in a liquid-crystalline phase is not enough for sufficient immobilizationof the molecules for successful molecular imaging: For example, the order of magnitude of diffusion coefficients on severalliquid crystals using

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The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2945 magnetic resonance and optical techniques was le7 cm2/s.*50 This together with other works clearly demonstrates (as was mentioned in several publications) that at the interface between the liquid crystal and the solid substrate, the mobility of the molecule is much less than what is observed either in a bulk liquid crystal or in a monolayer of molecules physisorbed on a surface. Although no independent experimental evidence was found for this, the diffusion coefficient of moleculesin the interface between a liquid crystal and a solid substrate is closer to that of a molecular crystal. The diffusion coefficient is probably similar to the selfdiffusion coefficient in molecular crystals at the melting point. These diffusion coefficients are of the order of 10-13-l@17cm2/ $1 and are, therefore, within the size which enables molecular imaging. STM imaging was successfully performed with extremely high resolution on adatoms. Metal adatoms (for example Ta, W, Re, Ir, and Pt on W( 11O)m) showed a random walk with a diffusion constant of 10-17-10-19cm2/s as was measured with the field ion microscope. This is enough of course to enable STM imaging. Diffusion of silicon adatoms for example on Si( 100) surface was found to be at the sameorder of magnitude (10-19cm2/s) at room temperature.32 It is seen, thus, that the immobilization of the imaged molecules is a difficult problem and cannot be taken for granted. So far, however, the behavior of the molecule which is not under the tipwasdiscussed. In the presenceofthe tip thisbehavior is expected to change. The strong interaction between the molecules and the tip can amount to 0.5 eV or more. This interaction can significantly distort the potential which immobilizes the molecule: The impact of this interaction will be dependent on the ratio between the scanning speed and the diffusion time scale. The scanning velocity is such that in the average time between two diffusion events the forces that the molecule feels will change many times. The lateral forces will probably average out, but the molecule will probably feel some average attraction force in a direction perpendicular to the surface. In the case when the average diffusion time is at the same order of magnitude as the time in which the tip-dependent forces are changing, then the effect of the tip on the molecular diffusion begins to be significant. The larger is the gradient of the electric field (for example in the case of a smaller radius of curvature of the tip) the larger is the distortion of the surface-molecule potential. When thedifference in energy between two neighboring binding sites is getting close to the size of the barrier to diffusion, the diffusion is no longer random, and directional diffusion is observed. Ifthemoleculeisnot directlyunder the tip, thediffusion coefficient will be anisotropic and site dependent and the anisotropic motion will be expressed as different diffusion coefficients: D+ and Dr,where D+ is the diffusion coefficient at a certain site in the direction located under the apex of the tip and D,, is the diffusionin the opposite direction. This directional diffusion is the reason why manipulating and moving atoms on the surface with the electric field of the tip became possiblea52 To get an accurate value of the diffusion coefficients in these conditions, a detailed calculation which takes into account the distortion of the surface potential in necessary. An order of magnitude estimation of the effect of the tip can be made by comparing the interaction of the molecule with the tip to the size of the barrier to diffusion. In cases where as a result of the electric field of the tip and other forces, D+ becomes larger than lo-12-10-13 cmz/s. Then, even if the diffusion in the absence of the tip is slow enough, still the electric field of the tip is sufficient to cause directional molecular diffusion and deterioration of the molecular resolution. For example, the barrier to diffusion for alkanes on a Ru(001) surface"*45 are on the order of magnitude of 0.125-0.2 eV. This is certainly on the order of magnitude of the electric energy interaction between the molecule and the tip which can be estimated from the figure. Even if the temperature

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2946 The Journal of Physical Chemistry. Vol. 98, No. 11, 1994

Richter and Manassen

-A

0.04

II

I

1.1

3n ’ -A

0.03

2

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0.06

1

2I

is lowered sufficiently (according to the lateral diffusion coefficient) to enable molecular imaging, the directional diffusion caused by the tip might prevent successful molecular imaging. In molecular crystals, the barrier to diffusion is 1-5 eV. It is reasonable to expect that in this and similar systems the tip will not have any affect. In the case of metal adatoms on metallic surfaces”.” the order of magnitude of the barrier to diffusion is 0.5-1 eV. In several cases (for example, on Sn adatoms on Si(l I I ) with a barrier to diffusion of 0.32 eV) the effect of the electric field becomes significant and atomic manipulation experiments become possible.sz Hallmark et al. suggested that the scanning tip plays a role in the diffusion direction of the azulene,but they did not findany connection between thescanning rate or direction to the image obtained. However, even if the molecule is successfully immobilized, they might still undergo rapid conformational changes. Any attempt to interpret the images observed must take into account this rapiddynamics. Itisvery importanttoemphasize that several molecules which were successfully imaged with the STM are definitely not rigid on the time scale of the STM measurement, As a first example, we tried to estimate the size of the energy barrier which the molecule has to overcome in moving from one stahleconfiguration toanother. Tocalculate this barrier, together with the internal coordinate ofthe molecule in each conformation, energy minimization programs‘ was used. The first calculation wasdoneon themolecule 8CB. Again, in a moreaccurate model the interaction of the molecule with the tip and the surface must be taken into account. This will definitely impose a significant constraint on the molecular motion and will change the energies and the stable conformation. However, to get an order of magnitude of this problem, five possible different conformations and their energieswerecalculated. Ofcourse, wetookintoaccount the fact that the molecule must he adsorbed in a parallel configurationon thesurface. This will,ofcourse,limitthepossible

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r-

dihedral angles a and @ (Figure 3). The calculation was done on the free molecule. The energiesof thedifferent conformations are seen in Figure 3 at the right. Few things can be seen clearly from this result. First of all, the size of the energy difference is certainly the size that will enable fast molecular conformational changes. This process is thermal. Taking into account a preexponential factor of kTlh will give us a rate of 104 Hz in the case of an energy barrier of 0.5 eV. It is, thus, reasonable to expect that at least conformations 1, 2, and 3 in Figure 3 will exchange rapidly on the time scale of the STM measurement. The STM will be able to observe only some average of them. The dynamicsofmolecularconformationsisasubjectthatwasheavily investigated, in particular with NMR (nuclear magnetic resonance), and although each case must be treated specifically, the order of magnitude of the molecular conformational changes is as described above and faster. A second important conclusion is that these energies are at the same order of magnitude as the electric energies of interaction between the molecule and the tip. In the specific case of 8CB, benzene and cyano groups have a large interaction. This means that the tip not only affects the dynamics of the molecular reorientation (as was mentioned already) but that the different contrasts achieved in different biasvoltages which was mentioned previouslymight be,at leasttosomeextent,becauseoftheelectric field affects of the average molecular conformation. The same conclusions are truealso forthedifferentconformationsof CCH, (Figure 4). Here, the energy difference between the different conformations is smaller, but the electrical interaction of the molecule with the tip is expected to be smaller too (the molecule hasonlyaliphaticchains). Theoverall picture and theconclusions are similar.

Aspects of Molecular Imaging with an STM

i

Figure 4. Two different conformations of the molecule CCH,. Again, t i p - i n d u d wnformational changes and rapid molecular motion must have a significant effect on the observed image. Simulation of STM Images of Molecules

There are several theoretical calculations dealing with the question of how a molecular adsorbate will look when imaged with the STM. In principle, when a molecular is hound to the surface, the surface state near the Fermi level-close to the molecule-hasto includea largecontribution fromthe molecular orbitals which are close to the Fermi level. Therefore, the image must he similar to these molecular orbitals. This approach was used in several studies"."z0 and is performed by comparing the STM image with the relevant orbitals. A more complicated approach was using the Bardeen approximation to estimate the tunneling current in the presence of the adsorbate?' Another method,56 probably the most precise one, is using an ESQC (electron-scattering quantum chemistry) technique to calculate the tunneling current between benzene molecules on a rhodium surface and the tip, giving an image whichisinanexcellentagreement with theexperimentalresults.'o However, until the role of the different physical processes, some of which were described previously, is precisely determined, the advantages of using such accurate methods cannot he exploited. Therefore,keeping in mind the problems which might deteriorate the STM molecular resolution, namely, to what extent the STM image can provide electronic and structural information about the adsorbed molecule, the problem is approximated by considering only the isolated molecule, we adopted the method of calculating the molecular orbitals, by comparing these results with the observed image. An attempt was made to provide qualitative conclusions, regarding the extent to which the STM can provide structural information and information on the role of conformational changes in the STM image. To simulate the image of the adsorbed molecule, extended Hiickelcalculationsweremade,usingamodifiedversionofQCPE (Quantum Chemistry program Exchange)." The chargedensity matrixpi(x,y,ro)wascalculated. The molecular orbital at energy Ei is a superposition of the atomic orbitals: pi(xy,zo) = ~ ~ C ~ X ~ ( X ,The ~,Z electron O ) ~ ~charge . density was calculated at a plane which is LO angstroms above the molecular plane. c.' are the coefficients of the nth atomic orbital in the ith molecular orbital. If the bias voltage is small and the molecule is relatively small, than the most significant contribution to the image will come from the frontier orbitals, namely, the HOMO and the

The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2941 LUMO. For each molecule, the calculation was done for these levels. For large molecules, the spacing between the energies of the different orbitals is becoming smaller. The electric field of the tip and the interaction with the surface is expected to induce significant mixing between the molecular orbitals. In these cases, of course, other orbitals will contribute to the image. However, a comparison with the experimental images was done when the HOMO and the LUMO were used, hearing in mind that other states may contribute, too. The plane in which pi was calculated was chosen in each case tohe theone with thelargestchargedensity. Thisisatadistance of -0.7 A above the molecular plane. This is the point of maximum probability density of the aromatic P, orbitals of the carbon atoms. After the image simulation is done, digital averaging of neighboring pixels is performed in order to reduce the lateral resolution to 2.5 A, which is believed to be the lateral resolution observed in STM measurements. The simulated image is displayed at on gray scale representation, where a darker color represents smaller electron densities. The images forthe HOMO and the LUMO were calculated and compared with the experimental image. The question which is asked here is the extent to which the STM is capable of providing information on the structure of a single molecule (rather that the structure formed in the molecular film by many molecules). Therefore, the comparison is made with an image of a single molecule, which is eliminated from published experimental STM images. A comparison of the simulated images with the experimental images leads to several interesting conclusions: In the molecule 6CB (no. 1 in Figure 5). there isa remarkablesimilarity between the experimental image and the simulated LUMO image. The simulation results in an almost identical bilobed structure that was obtained in the experiment. The image was taken with a positivesample bias, namely, thecorrugationsof unoccupiedstates were measured. This is in agreement with the similarity to the LUMO image. However, in the experiments, an inversion of the polarity did not change the observed image. This fact is in a contradictionwith the fact that tunnelinginto empty sample states is normally quite selective in energy since the tunneling electrons have mainly the Fermi level of the tip, and tunneling is done into the corresponding emtpy sample states. The simulation of the molecule 8CB (no. 2) demonstrates that, as expected, changing the conformation of the molecule results in significant changes in the simulated HOMO and LUMO images. The experimental image of 8CB looks significantly different from that of 6CB (hut is rather similar to the image of IOCB). The simulation (unlike the case with 6CB) is not capable of giving something which is really similar to theexperimentalimage. This failure may indicate that perhaps other conformations or an average of several of them are responsible for this image. The same conclusion holds for the molecule IOCB (no. 3). All these measurements were done with a positive sample hias, hut changing polarity did not change the experimental results. In the molecule CCH, (no. 4) the experiment was done with a positive rip bias (+0.8 eV). Namely, imaging of occupied sample states was done. Indeed the HOMO image shows a larger similarity to the experimental image than the LUMO. The similarity is not great anyhow, and this might be as in the previous case due to the significantly different conformations which are possible for this molecule (Figure 4). In the naphthalene molecule (no. 5 ) , tunneling was done into empty states and the image shows a similarity to the simulated LUMO image. In the didodecylhenzenemolecule (no. 6) there is a fundamental disagreement between the simulated and the experimental images. Both the HOMO and the LUMO images show an electron density only in the region of the benzene ring. This is in a contradiction to the experimental observation which also shows the alkyl chains. It is important to recall that as was mentioned previously, the images of alkanes on graphite

Richter a n d Manassen

2948 The Journal of Physical Chemistry, Vol. 98. No. 11, 1994

II 3

!

C

a

C

b

d

4

e

C

15

I

Ib I

C

Figure 5. Simulated and experimental images of6CB (no. I), 8CBH (no. 2) IOCB (no. 3). the experimenl mages for the CB molecules were taken from ref 58, CCH3; (no. 4). the experimental image was taken from ref 26; naphthalene (no. S), the experimental image was taken from ref 13; didodecylbenrene (no. 6). the experimental image was taken from ref 24; and adenine (no. 7), the experimental image was taken from ref 59. The images in nos. I and in 3-7 are in the order (a) the simulated HOMO image, (b) the simulated LUMO image, and (c) the experimental image. The images in no. 2 are in the order (a, c) simulated HOMO image for two different conformations and (b, d) are the simulated LUMO images for these conformations, (e) is the experimental image. The conformation of 8CB in (a) and (b) is identical to wnformation no. 4 in Figure 3. The conformation in (c) and (d) is identical to conformation no. 2 in Figure 3. The wnformation in the simulations of 6CB and IOCB are with the two aromatic rings parallel to thesurface. The scale and the orientation afthe molecule in the simulated image is the Sameas in the experimental image. These orientations can be seen in Chart 1. show t h e atomic features d u e to the graphite a t o m s a n d not t h e atoms of t h e molecules. Probably a similar phenomenon is responsible for the appearance of the alkyl chains in didodecyl-

benzene. In t h e alanine molecule (no. 7) a significant similarity is observed to both t h e simulated HOMO a n d LUMO images. To summarize, t h e results of this work demonstrate t h a t in

Aspects of Molecular Imaging with an STM order to understand the STM images of molecules, the motion of either the whole molecule or different parts of it as well as the interaction with the tip must be taken into account.

Acknowledgment. This work was done under a partial support of the US-Israel Binational Science Foundation, the Basic Research Foundation administered by the Israeli academy of Science, and a grant from the Glickson Foundation in memory of Mordechai “Moma” Glickson. References and Notes (1) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett.

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