However, there have been virtually no reports on experiments that examine the interaction between light and substances while observing individual molecules. Photocatalysts are a firm research target. This time, I intend to conduct research into the essence of photocatalysts in my own right based on the technology and experience I gained over the years at RIKEN. On a single-molecular scale, nobody knew the position on titanium oxide at which a photocatalytic reaction occurs. Our experiments with an STM probe tip clarified that photocatalytic reactions actually occur across wide electronically active areas around the positions where oxygen atoms are missing.
The Surface and Interface Science Laboratory is also conducting research into organic solar cells. Many researchers from around the world have wanted to perform single-molecule experiments while observing individual molecules, but such experiments have been too difficult to handle. We have accumulated STM technology that I am confident will enable such experiments.
In the future I also plan to start research that helps us link that knowledge to practical applications. He specializes in organic synthesis and can synthesize any organic molecule. I always have a good time with him, talking about our dreams. Molecules and matter exhibit different characteristics on the nanometer or molecular scale compared with the macroscale behavior scientists are most familiar with.
This is the reason for the widespread scientific interest in nanotechnology over the past ten years, and the origin of the expectations for a nanotechnology revolution. Although many theoretical papers have been published on what is actually going on in the nanometer world, only a few study have been reported because of the technical difficulty in directly observing the nature and functions of individual molecules.
Many conventional application studies have been conducted without fully understanding the basic mechanisms of nanotechnology. I plan to make use of the STM to study the nature of individual molecules and open a new frontier in nanoscience that will allow us to explore the essence of the nanoworld. I would like to follow the research concept of sci-engineering in the Surface and Interface Science Laboratory.
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This technique can also be used to induce a chemical reaction. Letters drawn using an STM tip to move molecules. Information about reproducing material from RSC articles with different licences is available on our Permission Requests page. Fetching data from CrossRef. This may take some time to load.
Jump to main content. Jump to site search. Journals Books Databases. Search Advanced. Current Journals. Archive Journals. All Journals. New Titles. Pick and Choose. Literature Updates. For Members. For Librarians. RSS Feeds. Chemistry World. Education in Chemistry. Open Access. The evidence of interaction between electrons of molecular adsorbates with the substrate was subsequently found in the adsorption of single Lander molecules Cg0Hg8 on C u l l 0 surfaces by Rosei et al.
The molecules were deposited on the surface at room temperature and were removed from their original adsorption sites at lower temperature K. The authors observed that the adsorption sites previously occupied by the molecules were transformed into a characteristic nanostructure two Cu atoms wide and eight Cu atoms long. This observation pointed t o the limited diffusion of Cu atoms at kink sites under room temperature that enables the restructuring of surface atoms and retained at lower temperature. The individual molecules were adsorbed a t the lowest energy conformation at the step edges, and subsequently transformed the underlying atom arrangements at the adsorption sites.
In a similar study, molecular-induced imprints were identified on C u l l 0 at thc adsorption sitcs prcviously occupicd by hcxa-tcrt-butyl-dccacyclcnc HtBDC, CsoHs6 molecules Figs. The cause for the reconstructed local lattice was ascribed t o a possible steric effect of the t-butyl groups that contacted the surface, or the enhanced reactivitv due to the molecule-substrate interaction based on the relationship of bonding strength and the metal coordination number a t the adsorption site [3. The conformations of L-Lander molecules adsorbed on Cu surfaces were characterized by the distance between the observed lobes using SThI [3.
The interaction between the central molecular board and the substrate causes the rotation of the attached legs. The adsorbate-induced removal of substrate atoms was also observed by chiral molecules of cystein on A u l l 0 surfaces [3. The preferential adsorption behavior of Lander molecules was observed on pre-oxidized C u l l 0 surfaces [3. Structure of HtBDC molecules extracted from [3. In addition, the orientation of the Lander molecules could also be adjusted in order to fit in the metallic part of the surface.
This was the first time that STS was used t o study vibrational characteristics of a single molecule. The acetylene molecule is stabilized on Cu surfaces through C-Cu bonds, with the C-H pointing outward from the Cu surface. A deuterium isotope effect was also confirmed in the tunneling spectrum. The results opened possible applications of the STS method for inelastic electron tunneling spectroscopv of single molecules. Theoretical analysis suggested that the coupling between tunneling electrons and the molecule is shortrange and non-resonant, and further confirmed that the contribution from C-H or C-D stretch modes is dominant, compared with other vibrational modes [3.
The adsorbed single acetylene molecules on P d l l 1 were recorded as individual protrusion-depression pairs by STM Fig. The molecules were found t o rotate randomly in three equivalent orientations 60' apart a t temperatures around 44 K. This is considered as evidence of threefold adsorption sites, i. The apparent shape of the molecule was confirmed by simulations t o bind t o both hcp and fcc hollow sites with the molecular axis C-C bond parallel to the surface.
A slight tilt of the molecular plane was conjectured t o achieve optimized overlapping between the i7 orbital of the molecule and the STM tip, thereby causing the asymmetric topographic appearance. It was also found that lateral diffusion of molecules begins at around 70 K [3. In another study, ethylene 3. Such immobilization may not be obvious for assembled structures that could yield very high imaging qualities. However, success in imaging isolated, individual molecules requires more stringent conditions.
The key is t o reduce diffusion t o sustain inevitable disturbances associated with the scanning probe. One prevalent approach suitable for ultra-high vacuum STM studies is cooling t o lower temperatures, which would reduce the diffusion mobility of adsorbed species on surfaces. Such a preparation approach by direct cooling is still widely adopted in current STM studies on single molecules.
Other approaches involve the assisted immobilization effect from different coadsorbate species. One of the first demonstrations of this approach was seen as benzene coadsorbed with CO on R h l l 1 surfaces. CO molecules were introduced to inhibit the lateral diffusion of benzene molecules [3. This immobilization scheme can be extended for studies carried out in ambient conditions and solvents, as shown in the following discussions. Assembled networks with cavities of various sizes and geometries provide another viable approach t o explore guest-host interactions by immobilizing guest species a t single molecule level.
Such efforts could be expanded into the generalized areas of host structure construction and physicochemical properties of the resulting complexes [3. These interaction properties have been extensively studied and used in molecular self-assemblies. In a recent study, the hydrogen bond configuration of 1,3,5benzenetricarboxylic acid trimesic acid, TMA adsorbed on Cu surfaces was observed t o depend on temperature At low temperatures around K , a honeycomb-like network with core diameter of about 20 A is the stable structure, whereas the striped structure prevails a t room temperature.
The distortion of the hexagonal molecular lattice is due t o the preferential adsorption a t the hollow sites of the fourfold lattice of Cu substrates [3. In addition t o the hexagonal honeycomb lattice, the "flower" structure is also stabilized by hydrogen bonding with two different cavity sizes [3.
Reproduced with permission from [3. The entrapped guest TMA molecule was suggested t o be stabilized by two hydrogen bonds to the host network, as shown in Fig. Little effect on the host lattice geometry from the guest molecule can be observed. This supramolecular network adopts a hexogonal pattern with a lattice constant of The open pores can be packed with up to seven Cso molcculcs. This is a reflection of the effect of the host lattice structure. In addition, the Cso can also adsorb directly on top of PTCDI and melamine molecules, leading to a replicated lattice structure.
As another example, a monolayer of 1,3,5-tris carboxymethoxy benzene TCME shows two-dimensional hexagonal networks formed by hydrogen bonds, whereas the monolayer of 1,3,5-tris carboxydecyloxy benzene TCDB shows two-dimensional tetragonal networks on highly oriented pyrolitic graphite HOPG [3. The inclusion effect of hydrogen-bonded twodimensional networks of TCDB was demonstrated on the surface of highly oriented pyrolytic graphite HOPG in ambient conditions [3. An appreciable variation of the lattice dimension was observed as a result of the guest-host interaction.
Control of adsorption site and geometry of organic molecules in self-assembled monolayers was accomplished by this method. Self-assembled, second-generation Frkhet-type dendrons have been observed to assemble into nearly uniform, disk-shaped structures connected by hydrogen bonds a t the focal points on a graphite surface [3. Two kinds 3. The measured values agree with those of a simplified structural model. The above examples suggest there are ample molecular species that can be used for building molecular lattices via hydrogen bond interaction.
The cavity svmmetry and size can be designed bv using different molecules. Such capability is certainly an expansion of using bare substrates as adsorption support, and will enable studies of novel composite molecular structures. Characteristic hexagonal and quasi-quadratic structures derived from the first generation of n-alkoxy-substituted stilbenoid dendrimers have been observed. The assembly structures are stabilized by the interdigitated alkoxy chains, and Fig. No distortion of the host lattice geometry is observed. Effect of guest molecule insertion on the lattice spacing extracted from [3.
The interaction between the constituent molcculcs is solcly van dcr Waals forcc. Quasi-quadratic molccular latticcs can also bc constructcd by intcrdigitated alkylated copper phthalocvanines [3. The trapping effect of twodimensional assembly of octa-alkoxyl-substituted phthalocyanine PcOC8 for individual molecules of phthalocvanine, prophvrins, and calixarene has been observed Fig.
It was shown that single molecules are trapped in quadratic, rather than hexagonal lattices, and domain boundaries are the preferential trapping sites, compared with sites within the domains [3.
The observed trapping behavior is, in some respects, analogous to the impurity segregation phenomena of point defects, dislocations, and grain boundaries in solid-state materials. The molecular inclusion-induced spacing increase between host molecular rows is presented in Fig. I t is evident that, with an increase in intercalating molecular size, the distances of neighboring PcOC8 rows increase correspondingly. This could be an indication of the flexibility of the PcOC8 lattice.
The insertion of molecules in the initially packed lattices leads t o enhanced repulsive interaction between molecules. On the other hand, the overlapping alkyl parts could readjust t o accommodate additional molecules and compensate the associated increment of repulsions. Alolecular networks thus show a certain degree of flexibility in trapping differently sized and shaped single molecules.
Seeing the world of nanotechnology from a single-molecule perspective
It has been known from a number of reports that double molecular layers can 3. This fact has opened the possibility of studing single molecule adsorption on top of monolayers of organic molecules. This is an additional advantage of the STM method that is readily applicable to studies on bare surfaces, as presented in the preceding sections. The theoretical analysis of single molecule adsorption, such as adsorption configurations and kinetics, is analogous t o that of bare metal and semiconductor surfaces.
The results can also be directly related t o investigations of the interface of heterogeneous organic structures with various functionalities. The examples provided in this section illustrate some progress in this area. Molecular adsorption-induced effects, such as molecular diffusion, and surface restructuring, are well documented in the case of molecular adsorption on metal and semiconductor substrates [3.
By contrast, only few experimental studies have been dedicated t o the microscopic effects of molecular adsorption on an organic molecular supports [3. The pancity of information on microscopic adsorption on organic substrates could be due mainly t o the lack of experimental capabilities suitable for such systems.
With the wealth of functionalities associated with molecular assemblies, it is conceivable that such studies could be highly rewarding. The information obtaincd should bc of high importance t o thc construction of molccular nanostructures in both two and three dimensions. Considering the different polaritv and electronic properties of functional groups from the family of alkane derivatives, monolayers of alkane derivatives could provide ideal templates for the investigation of adsorption behavior of organic molecules. Compared with the nanofabricated surface, this alkane-derivative-modified surface is purely organic, rather than metallic or semiconducting.
The interlayer interaction is an important factor for determining the overlayer structures. The heterogeneous organic-organic interface is generally associated with weak interactions, tvpicallv van der Waals force. The introduction of various functional groups to the buffer layer offers possibilities t o further explore the effect of molecular interfacial interact ions on the assembly structures. The presence of heterogeneous adsorption sites can result in the selective adsorption of single molecules.
The adsorbed species would also experience the anisotropic diffusion barrier, and would organize in a restricted manner.
Isolated CuPc selectively adsorbed on the hydrocarbon-chain portion of stearic acid left , and CuPc dimer adsorb on the monolayer of 1-octadecanol right [3. Copyright American Chemical Society Site-selective adsorption of copper phthalocyanine CuPc has been observed atop organic surfaces of monolayers of various alkane derivatives stearic acid, 1-octadecanol and 1-iodooctaecane supported on the surface of HOPG. STM studies showed that the alkane derivatives form lamella templates, which dircct thc adsorption of thc CuPc. This sclcctivc adsorption behavior is attributed to a preference of the CuPc for adsorption on the hydrocarbon-chain portions of the supporting layers [3.
When phthalocyanine PC molecules coadsorb with stearic acid, isolated and paired P c molecules can be detected on top of the stearic acid monolayer, as shown in Fig. The P c molecules are located on top of the alkane part of stearic acid lamellae, whereas no P c molecules were observed a t the location of carboxyl groups. This immobilization effect of alkanes may originate from the variation of adsorption barrier due t o van der Waals interaction among molecules.
Two representative adsorption sites are considered in molecular mechanics simulations schematically shown in Fig. The calculated results indicate that the system potentials at site I are higher than those a t site 11, by more than 21 kJ molpl for three alkane derivative systems. This may be caused by two factors. For one, the trough linked by head-to-head in width and the concentration of atoms in functional groups is about 3 the trough is less than that for other sites of the organic monolayers. Consequently, the van der Waals interaction between the adsorbed CuPc and underlying organic substrate decreases when CuPc adsorbs on the top of the trough.
Moreover, seeing that the trough is linked by atoms and groups of A 3. Simulation of the adsorption energetics of CuPc molecules at different sites on top of alkane lamellae. These two factors may drive the selective adsorption of CuPc on the alkyl moiety to achieve the most stable adsorption state. The selectivity is dependent on the relevant functional groups and could vary among alkane derivatives. This analysis suggests that molecular surface decoration can be essential in achieving an optimized immobilization effect of guest species.
The unprotected amino acid groups were found t o be the preferential adsorption sites, as illustrated in Fig. Each amino acid group can adsorb either one or two urea molecules, as reflected by the local clustering of the adsorbates at some adsorption sites in Fig. By contrast, the lamella structure of single-alkyl amino acids did 64 3 Single Molecule Structural Characterization Fig.
However, such saturation of the amino acid groups can be avoided by introducing fatty acids C23H47COOH as thc matrix molcculc [3. Thc alkyl-substituted amino acids are randomly distributed within the matrix lamella of C23H47COOH,and the amino acid groups are available for interaction with adsorbates such as palladium I1 acetate and urea.
The conformation of nitrogen atoms in the amine group of TDA is tetrahedrical, the nitrogen atom sitting on one acme of this tetrahedron Fig. Since the C-N bond is dipolar, amine molecules are also dipolar, the nitrogen being partially negatively charged. Thus, when amine molecules adsorb onto an inert surface of graphite, there exists a net dipole moment pointing nearly perpendicular t o the surface.
Copyright American Chemical Society Benzoic acid was found exclusively on the TDA assembly adsorbed through hydrogcn bonding a t thc sitcs of aminc groups [3. This shows thc possibility of using alkane derivative lamellae as templates t o direct adsorption and assemblage of other organic molecules. By co-deposition of TDA with copper phthalocyanine CuPc , isolated CuPc molecules and clusters can be stabilized in the alkane part of the larnellae Fig. From the large-scale view in Fig.
The lateral diffusion of the single CuPc molecules as well as clusters of adsorbed CuPc molecules was found exclusively along the direction of the TDA lamellae. Such highly directional diffusion behavior can be considered direct evidence of the one-dimensional template effect of TDA lamellae.
Such effects have never been observed on lamellae of simple alkanes, possibly due to the lack of functional groups that could establish sufficient diffusion barriers for the overlayer adsorbates. This concept may be generalized to the construction of molecular templates for novel molecular nanostructures. There are many aspects of molecular templates yet t o be explored in a wide range of single molecules and molecular ensembles, such as sensor or catalytic behavior of molecular assemblies, molecular devices based on 66 3 Single Molecule Structural Characterization Fig.
Arrows indicate the migration of the molecules in consecutive scans in a and b [3. Copyright American Chemical Society heterogeneous assemblies donor, acceptor, p-, and n-types , and assemblies of molecular magnets. I t is conceivable that with the rich variety of functional groups that can be incorporated into molecular structures, the pursuit of tcmplatcs with novcl physicochcmical propcrtics could bc fruitful.
Efforts in thcorctical analysis of the asscrnbling processes arc important in gaining deeper insight of the driving mechanisms. Incorporating chemical specificity into the structural resolution capability of STM is the leading demand in the development of STM techniques. The tunneling spectroscopy of the electronic structures and vibrational modes of molecules sets examples for such endeavors. The detection of the electron-spin resonance effect using STM discussed in this section represents an effort from a different perspective.
Electron-spin resonance from single molecules is an important signature for identifying the chemical nature of the molecule. Efforts a t single molecule recognition using the electron-spin resonance effect can be seen from electron tunneling, force and optical detection approaches. The latter two experimen- 3.
In the electron tunneling approach, the spin centers can interact with the tunneling electrons through the magnetic dipole field localized at the spin centers, or affect the local density of states due to exchange interactions [3. It should be noted that there are ongoing discussions on the nature of the interaction between the spin centers and the tunneling electrons. In either of the above two interacting mechanisms, the magnitude of the interaction strength is dependent on the orientation of the spin that can be modulated by an external magnetic field either DC or AC.
As a result, the tunneling current may contain an ac component in the radio-frequency rf regime that corresponds t o the spin-resonance effect. The Larmor precession frequency modulation can be measured from the tunneling current using a spectrum analyzer Fig. There are three reported types of spin centers studied by the STM method, namely, Si radicals on partially oxidized silicon surfaces [3.
The molecules displaying the spin-resonance effect are active only for a short time and recovery can occur after certain delays. In addition, the spectra peak position and linewidth showed a certain degree of variation that could be related t o the molecular orientation within the tunneling junction, and the aggregation of molecular clusters. The pursuit of the single molecule sin-resonance effect has advanced the studv of single paramagnetic atoms, molecular radicals and defects using STM. It can be expected that, with more rigorous evidence on the mechanisms of the spin-resonance effect on tunneling electrons, and the improvement of experimental capabilities, the STM-ESR technique will deliver very useful d a t a on single spin centers, especially the interactions between the spin centers and the surrounding environments.
The observations of single molecules presented in this chapter are based on electron tunneling principles. These results have enriched our general knowledge of molecular adsorption, as well as opened up new fronts in investigating the physical and chemical properties of individually adsorbed species with vcry high structural rcsolutions, as will bc dcmonstratcd in thc following chaptcrs. Thc topography rcsolution of STM will undoubtcdly continuc to reveal interesting insights of single molecules as well as molecular assemblies.
Probably a more interesting and challenging improvement of the STM technique is its chemical specificity, possibly through improved spectroscopy functions. The accomplishment of this task may lead t o further breakthroughs in single molecule studies. The focus is shifted from the adsorption configuration to the dynamic properties of adsorbed species, i. The interest in these processes is related t o many fundamental mechanisms in heterogeneous catalytic reactions, material surface properties, etc. The success of observing individual molecules on various surfaces presented in the preceding chapter provides us with a solid basis for exploring the twodimcnsional kinctics of singlc molcculcs on surfaccs.
In many surface-mediated reactions or heterogeneous catalytic reactions, the promotion and inhibition of the reactions are closely associated with the mobility of the reactants [4. Thc STM method offcrs a rcal-spacc approach t o invcstigatc thc diffusion behavior of single adsorbates on atomicallv resolved substrate structures. This represents a useful complement t o existing in-depth knowledge of diffusion processes from a range of experimental techniques such as field ion microscopy FIM [4.
The diffusion barrier is caused by variations of adsorption energy minimum and maximum, as schematically shown in Fig. The mean square distance of a particle travelling from its original position a t time t can be expressed as: 70 4 Single Molecule Diffusion and Chemical Reactions Fig. Schematic of diffusion energy barrier resulting from the difference of adsorption energy at different adsorption sites. The adsorption potentials for sites denoted I and 2 are schematically illustrated as solid and dotted curves, respectively. The difference of the equilibrium energy is the origin of the diffusion barrier where a is the jump length, v the effective jump frequency, and Ed the activation energy.
The activation energy of diffusion can be provided by a thermal source, light irradiation, or an external field. Thermal-activated diffusion is the most common driving mechanism. Experimental measurements of the diffusion coefficient D can be achieved by observing the variation of macroscopic coverage of adsorbates a t different temperatures. Rigorous derivations of the formalism can be seen in a number Much success has also been shown with STM of reviews and books [4. The molecular motion is exclusively along the direction, i. The results in the temperature range of 42 K K suggest that multimeric CO molecule chains display higher mobility than that of single CO molecules.
The observed difference in mobility has been attributed t o the effect of entropy of the diffusion pre-factor, and the entropy of a molecule in an adsorbed dimer is lower than that of an isolated one. This observation provides an example of adsorbate-substrate interaction in the mobility of adsorbate molecules. As demonstrated in the preceding chapter, a CO-decorated tip can lead to much enhanced resolution of the adsorbate structures.
Another example of vibration-stimulated hopping was shown for CO on P d l l 0 a t 4. A threshold bias voltage of about mV was observed to induce lateral movement of CO on Pd 1l o , and the value is ascribed t o the G O stretching mode. I t was also suggested that discontinuous motion or hopping of CO molecules could be tuned by the internal vibrational mode [4.
The excitation can be initiated by dosing electrons from an ST11 tip. The enhancement of molecular mobility of H 2 0 clusters, compared with single H20 molecules, was observed on Pd [4. In this system, the mobility is increased by several orders of magnitude for molecular clusters. The enhancement is due t o the mismatch of molecular clusters to the substrate lattice, leading t o a reduced diffusion barrier.
The above studies illustrate the difference between single molecules and molecular aggregates from the perspective of diffusion properties. The single molecule behavior is clearly sensitive t o its surrounding environment, in this casc, thc substratc and adjacent molcculcs. The SThl observation was performed at 22K, which is lower than the temperature for typical thermal-originated diffusion [4. In addition t o the dominant diffusion along Cu atom chains of 1 TO , a minor diffusion channel with branching ratio of 0. Considering that the temperature is well below the threshold for thermally driven diffusion, and that such anisotropic diffusion behavior has never been observed under thermally initiated diffusions, it was suggested that the diffusion is driven by an electronic effect.
As a matter of fact, the cstimatcd clcctronic tcmpcraturc incrcasc is about 3, K for a timc pcriod of about 1 ps under these irradiation conditions, in contrast to the lattice healing lemperalure rise ol aboul K under lhe same condilions. Under the experimental conditions of STM, the adsorbed atom or molecule can be polarized under the influence of a strong electric field a t the tip apex. The induced dipole will show a parabolic-shaped potential, the minimum coinciding with the tip position [4.
The gradient of the potential is the force experienced by the adsorbate: Therefore, the inhomogeneous distribution of field strength may result in a directional difhsion behavior. Field-induced difhsion can be observed as directional difhsion of adsorbates either toward or away from the tip position. Therefore, the concentration of the adsorbates should be dependent on scanning conditions such as speed, bias, and polarity. Under positive tip bias, directional diffusion of adatoms toward the tip position has been observed and interpreted in terms of the above-mentioned arguments [4.
Another observation of the tip-induced effect is the reversible rotation of Sb dimers on Si [4. By counting the population of Sb dimers on Si surfaces annealed at various temperatures, the diffusion barrier of Sb across the substrate dimer row was determined as 1. Thc dissociative adsorption of Sb4 on Si surfaces was observed as five distinctive types of Sb dimer configurations.
By annealing the sample a t several temperatures and observing the population variation for each type of Sb dimer, the energy barrier can be estimated in each case [4. The diffusion of Si adatorns on Si surfaces was identified as being predominantlv along surface dimer rows with an activation energy of 0.
Details of the diffusion of Si dimers on Si surfaces have been studied by STM with atom-tracking capability [4. The STM tip is locked onto the target atom and the migration of the target is followed bv dithering the tip alound the target. The STM is used mainly for measuring the kinetics of the target rather than scanning the large area. Such improvement enhances the capability of STM t o studv dvnamic events by 3 orders of magnitude. Using this approach, the diffusion barrier for Si dimers on Si surfaces was determined as 0. In a separate study, a tunneling-current-stimulated migration mechanism was proposed for inducing diffusions of adsorbed Br atoms on Cu surfaces [4.
It was observed that, under the same tunneling bias, the current density is the determining factor for the migration of Br atoms. This mechanism could be due to the localized thermal activation of the adsorbates resulting from inelastic scattering of the injected electrons. Since the thermal activation t o the adsorbates can be dissipated through the adsorbate-substrate 4. STM image of Sb dimers a before and b after annealing at K for 2 min extracted from [4.
The Arrahnius plot of temperature dependence of diffusion coefficients for Sb dimers extracted from [4. In practice, there are certain limitations for studying the diffusion process of individually distributed adsorbates using STM. The limited scanning speed of STM is obvious here, and the unavoidable interaction between the tip and adsorbate would introduce a force on the order of 1 0 ' N [4.
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The latter effect could be reduced somewhat by increasing the tip-sample separation, yet it can not be completely eliminated. If the adsorbates diffuse too fast, they will not be observable by STM, this being the case also if the adsorbates interact too weakly with surfaces cf. By optimizing the tip-sample interaction strength, STM can be used to move adsorbates such as metal particles, atom clusters, or single atoms from one place t o another.
So far, precision manipulations by STM have been demonstrated for a variety of atoms and molecules, and this section provides only a short summary of some representative cases. This manoeuvering capability of single molecules could be useful for building devices made out of small particles, perhaps from particles of different materials. I t could also be useful for studying interactions between particles, and between part icles and substrates. Eventually, it may become possible t o build molecules in an atom-by-atom manner.
Even though such a construction approach is inhcrcntly slow, it ncvcrthclcss can providc a great deal of insight into processes of intermolecular interaction, and possibly also novel reaction pathways. The in-depth exploitation of the tip-sample interaction can lead t o permanent restructuring of the surface, such as in the study of direct writing, lithography, electron-beam-induced deposition, and etching.
It is possiblc t o move these atoms according t o predetermined paths using STM [4. Due t o the strong field strength, the tip of an STM always exerts finite force on an adsorbate atom [4. This force has both van der Waals and electrostatic contributions. By adjusting the tip-sample separation and the voltage of the tip, the magnitude and direction of this force can be tuned. Seeing that it generally requires less force t o move an atom along a surface than t o pull it away from the surface, this makes it possible t o optimize these parameters such that the SThI tip can pull an atom across a surface although the atom nevertheless remains bound t o the surface.
The electrical conductance between the tip and the substrate depends on the position of the Xe atom, which results in a switching device with a low-conductance state when the Xe atom is on the substrate, and with a high-conductance state when the Xe atom is on the tip. This atomic switch is a bistable element, and this type of components is important to microcircuit constructions. A threshold tip height was identified in the above study. Only when the tipsample distance was lower than the threshold, i.
Simple investigations showed that neither the magnitude nor the sign of the applied voltage had significant effects on the threshold tip-sample distance. This suggests that 4. The notion that atoms of a noble element such as Xe should be observable bv STM is not a foregone conclusion. According t o the theory of Tersoff and Hamann [4. The extent to which an adsorbate will be "visible" by STM depends on how it locallv changes this densitv of states. Lang [4. In the case of noble gas species of Xe, this is not expected t o contribute significantly t o the density of states a t the Fermi level when adsorbed on a metal surface.
The highest filled state of Xe is 5p, which is about eV below the Fermi level, and the lowest unfilled state is 6s, which is about 4. However, the calculated 6s state density displays a broad distribution that spans the Fermi level. This residual 6s density of state a t the Fermi level is considered as the origin for the observed contrast of Xe atoms in STM imagcs [4. STM studies indicated that the Xe atom appears as a nearly cylindrically symmetric protrusion of 1. Similar images for Xe adsorbed on Pt ll1 surfaces a t 4 K have also been obtained [4.
Xe atoms were also shown t o be transferable between a Ni ll0 surface and tungsten tip by pulsing bias voltage [4. The observed sideway motion at smaller tip-sample separation may be due to the increased van der Waals attraction to the tip as the tip is brought closer to the surface. An example of such experiments is for Si ll1 surfaces [4. As the tungsten tip is brought t o the sample surface, the measured apparent barrier collapses a t about 3 A from the surface. At even closer distance t o the sample surface, the chemical interaction between the tip and sample becomes more pronounced.
The Si atoms and clusters can be removed from the basal plane of the S i l l 1 surface by applying a positive voltage pulse of 3 V at the tip-sample separation of approximately A. By this approach, as many as tens of atoms can be successfully removed from the Si ll1 surface and transferred t o the tip. The Si clusters that adsorbed to the tip apex were shown to be transferable back t o the surface with negative voltage pulses. Several mechanisms have been proposed to account for the reported manipulation of single atoms, including possible field evaporation of negative 76 4 Single Molecule Diffusion and Chemical Reactions ions, and electrornigration of atomic adsorbates [4.
Ionization followed by field evaporation has been suggested [4. The required threshold field strength for tip-induced Si atom manipulation is estimated at around 1 VIA, which is much lower than the typical value for evaporating atoms in field-ion microscopy about 3 VIA.
Such reduction of threshold field strength can be attributed t o the strong chemical and mechanical interaction between the tip and sample surface, as well as to high current density during manipulation bias voltage pulsing. Tunneling characteristics revealed a strong change of the electronic states as the gold atoms were added t o the chain one by one.
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The measured conductivity of the Au atom chains can be considered as resulting from the delocalized electron transport within the one-dimensional quantum well structure and the wave function of the Au atoms. The sequential evolvement of the electronic structure, from single atom resonance, coupling between adjacent Au atoms, t o the appearance of a onc-dimcnsional clcctronic band, has bccn idcntificd.
An effective mass of 0. More rigorous theoretical treatment of the electronic properties of the one-dimensional metal atom chains should include possible coupling with the substrate. This experimental demonstration can be further developed t o study the electronic characteristics of complex structures, especially artificially constructed ones. Recent achievements in moving Cu atoms from different sites on Cu surfaces have led to the possibility of constructing metallic clusters from individual atoms.
The shift of Cu atom positions a t the step edge with adsorbed CO molecules was shown to reveal the adsorption registry of CO [4. The lateral manipulation of CO molecules on Cu was found t o be achieved via repulsive interaction between the tip and C O due t o the confinement of C O molecules t o the step edges [4. It should be noted that attractive interaction between the manoeuvering tip and C O prevails for controlled movements of CO on low-index surfaces.
Energy diagram of CO transfer between C u l l 1 and the tip extracted from [4. A threshold voltage of about 2. Individual CO molecules were imaged and manipulated bv using STM on A g l l 0 surfaces a t a temperature of 13 K a t the presence of coadsorbed Fe atoms [4. The tilt angle of the CO ligand was also determined from the topography image.