Heinrich Rohrer Medal Lecture Atomic Force Microscopy for Imaging , Identification and Manipulation of Single Atoms

Measuring tiny interatomic forces between tip and sample has been an important challenge in the development of atomic force microscopy (AFM). Present force sensitivity achieved by a frequency modulation (FM) technique allows us to obtain atomic resolution routinely using FM-AFM. We applied the capability to measure the chemical bond between two atoms to identify the chemical species on surfaces. The chemical bonding force can also be used for single atom manipulation at room temperature. Recently, these AFM capabilities have led to the creation of various artificial nanostructures atom-by-atom, such as atomic clusters. By AFM characterization combined with scanning tunneling microscopy (STM), we found that some clusters work as atomic switches, which can be controlled by atomic force as well as carrier injection. [DOI: 10.1380/ejssnt.2016.28]


I. INTRODUCTION
I start from scanning tunneling microscopy (STM) invented by Binnig and Rohrer [1].Using STM, we can see individual atoms by measuring tunneling current between scanning tip and sample surface.In the famous first atomic-resolution STM image on Si(111)-(7×7) surface which they reported [2], individual Si adatoms were clearly imaged, giving big impact in the world.Note that in this article, I will spend much space to present our works on this 7×7 surface.
In the very early stage of STM development, in 1986, Rohrer recognized the interatomic forces in STM when he measured the STM corrugations on graphite surface [3].Tip to sample distances can be reduced by increasing the setpoint of the tunneling current or decreasing the gap voltage.They observed enhancement of the atomic corrugation with decreasing the tip-sample distance.At the closest distance, the corrugation reaches almost 1 nm: It never be real topography on Graphite surface.It is apparent corrugation amplified by elastic deformation of the surface.When tip is located on top of an atom, Graphite layers are pulled up by the attractive interaction force with the tip.When tip is located off positions, then, graphite layers are pushed down by the repulsive interaction.This is the mechanism for the giant corrugation.This pioneering work tells us two things.The surface atoms exert force on the tip.On the other hand, the tip exert force on the surface atoms; this is driving force for atom manipulation.
In the same year, atomic force microscope (AFM) was born [4].The AFM was invented by Binnig and the coworkers to expand STM capability to insulator surfaces.In the AFM, a cantilever with a sharp tip is used.When the tip apex atoms interact with the sample atoms, the cantilever bends.This deflection is detected by the sensor.This is the basic principle of AFM.
To get true atomic resolution by AFM, frequency modulation technique is usually used [5].A cantilever is oscillated at its resonance frequency f 0 .When the tip approaches the sample surface, the resonance frequency is changed by the interaction force between tip and sample, F .This frequency shift ∆f is observable and we obtain AFM topographic images by keeping the frequency shift constant.The relation between ∆f and F is slightly complicated.∆f is written by the weighted average of F over one oscillation cycle T : We can also invert the observed ∆f into F , the physical quantity we are interested in [6,7].
In this article, I present our atomic technology using AFM/STM at room temperature.Firstly, I will show atom imaging using AFM.Then, I will explain the method for atom identification based on the force measurements.Next, I will show atom manipulation at room temperature.Finally, I will show switching operation of assembled structures.

II. ATOM IMAGING
Figure 1 shows the AFM images we obtained.[8][9][10].In the image of a single PTCDA molecule, chemical structure inside the molecule can be resolved by Pauli repulsive force even at room temperature [11].
Note that AFM image well reflects real topographic height.Figure 2(a) shows AFM image of very initial stage of oxygen adsorption on the Si(111)-(7×7) surface [12].As shown in Fig. 2 the structure formed after single oxygen molecule is dissociated: Two oxygen atoms are inserted into the backbond sites.During in situ oxygen adsorption, we also observed that such bright spot turns into much brighter spot.This is formed after the second oxidization: In this case, one oxygen atom is located on top of Si adatom, so it is imaged very bright in AFM.The AFM topographic heights well coincide with the atomic positions obtained by density functional theory (DFT) calculations [12].When we scanned the exactly same area by STM (Fig. 2(b)), this site, the brightest spot in AFM, looks dark due to the disappearance of density of states (DOS) near Fermi level.As can be seen in this example, AFM combined with STM can provide complementary information, the atomic force and DOS.

III. ATOM IDENTIFICATION
Firstly, let me explain the interaction force measured on Si adatom of the Si(111)-(7×7) surface [13].When we get clear atomic resolution like Fig. 3(b), the force curve on top of a Si adatom as a function of the tip-sample distance looks like the red curve without symbols in Fig. 3(a).The negative values mean that the force is attractive.This is attributed to the chemical bonding force between dangling bonds on Si adatom and tip apex atom [14,15].
Sometimes, we also get unclear image like Fig. 3(c).Then, the force curve on top of a Si adatom looks like the blue curve without symbols in Fig. 3(a), which indicates a tiny, attractive force.This kind of tip is inert, no dangling bond on the tip apex to form chemical bond.So, only weak physical force remains.This idea is supported by DFT calculation [13].The tip model of Si cluster with a dangling bond at the apex reproduces the large attractive force in the red curve with filled circles, while the OH terminated inert tip reproduces the tiny attractive force in the blue curve with filled circles.The calculated DOS projected on a Si adatom is not modified under the interaction with the inert tip, while the DOS is changed a lot by the interaction with the reactive tip.We can also see the split of the states to bonding states and antibonding states [13].The feature of the chemical bond also appears in the electron density map.Electron accumulation in the center of the junction was theoretically obtained.
The magnitude of such chemical bonding depends on the elements.We proposed a method for chemical identification of single atoms based on the force measure- ments [16,17].For example, Figure 4(a) shows an AFM image of Si, Sn, Pb mixed surface.In Fig. 4(a), you can see two atomic contrasts, but we cannot discriminate three atomic species.Then we performed force measurements over all atoms in this image.Figure 4(c) shows the measured force curves.We found that the force curves can be classified into three groups.The absolute values of the force depend on the tip.However, it was found that the relative ratio of the maximum attractive chemical interaction forces for the same tip remains nearly constant  Xe atoms on Ni [18] Sn atoms in Ge [20] regardless of the tip.The ratio of the force values between Sn and Si is 0.77 and that between Pb and Si is 0.59.Using these tip independent values, we can clearly identify the three atomic species for all atoms in this image as shown in Fig. 4(b).

IV. ATOM MANIPULATION
One of our research subjects is the bottom-up nanotechnology, the technology for atom-by-atom construction of nano-devices as predicted by Feynman.After invention of STM, Eigler opened the way to single atom manipulation [18].Since then, the atom manipulation and assembly using STM have attracted much attention in the field of nanotechnology.Eigler and coworkers created letters 'I B M' of individual atoms by manipulating Xe atoms laterally on the Ni(100) surface.This method is called lateral manipulation.This technique reached the ultimate level of creating the logic gate composed of Carbon monoxide molecules on Cu(111) surface [19].
Nevertheless, almost all lateral manipulation has been performed using atoms or molecules weakly bound on the surface.So, the atom manipulation and assembly has been performed only at cryogenic temperature to avoid desorption or diffusion of manipulated atoms.
We found a method that enables us to manipulate single atoms even at room temperature [20].We call it interchange lateral manipulation.Table I shows the comparison between conventional lateral manipulation and interchange lateral manipulation.The conventional lateral manipulation is performed using STM at cryogenic temperature.In the interchange lateral manipulation, different atom species embedded in the surface are interchanged by the tip.The atoms embedded in the surface are strongly bound on the surface.We succeeded in creating atom letters 'S n' using Sn atoms embedded in the Ge surface at room temperature as shown in Fig. 5(a).Sn atoms and Ge atoms were interchanged more than 100 times.This is the first demonstration of atom manipulation and assembly using AFM.
After this work, we found another method, vertical interchange manipulation [21].In this method, two different atomic species are vertically interchanged, that is, a surface atom is replaced by a tip apex atom.So, surface atom goes to the tip and tip atom goes to the surface.Atom letters 'S i' are shown in Fig. 5((b) where single Si atoms on the tip are deposited into Sn surface atom by atom.
We confirmed that interchange phenomena are universal for semiconducting elements.for the interchange lateral manipulation.The upper images are before manipulation and the lower images are after manipulation.Two different atom species marked by rectangles are interchanged.They are Sn and Si, In and Si, and Sb and Si in Fig. 6(a), (b) and (c), respectively.These experimental results show that interchange type of manipulation can be applied to large fields of science and engineering.For example, we can arrange the dopant atoms in Si surface with atomic precision.
Atom manipulation on semiconducting surface at room temperature is still quite rare.So, we investigated simple lateral manipulation of Si adatom on the Si(111)-(7×7) surface.First, we created two single vacancies on the 7×7 surface.One of them was used as a marker, and the other was used as an open space for manipulation.Then, by vector scanning, we can laterally manipulate intrinsic Si adatoms toward the vacancy.Figure 7 is the calculated energy landscape for the manipulated atom [22].In the case without tip, the energy barrier for the migration from the initial position to the final one is 1.1 eV, which means that the thermal diffusion even at room temperature hardly occurs.When the tip interacts with the Si atom to be manipulated, the barrier is reduced by the lateral force as well as the vertical force.The vertical force component pulls the Si atom up and weakens the backbonds of the Si atom.Even under the strong interaction, the energy barrier remains as high as about 0.65 eV.However, this barrier can be overcome by the thermal energy at room temperature.We also proved that atom manipulation process is indeed stochastic as expected for the thermal assisted manipulation [23].
Recently, we also found the correlation between the capability of this manipulation and magnitude of the interaction force measured by AFM [24].All tips used in this study produce clear atomic resolution.However, only the tips of the more reactive group characterized by the maximum attractive force larger than 1.5 nN have a capability of atom manipulation.This suggests that the chemical reactivity on the tip is important for manipulation.
Next, I introduce a different manipulation method to assemble nano-clusters atom by atom.In this study, we used a half unit cell of the Si(111)-(7×7) surface as a nano-space (NS) to confine single atom thermally diffusing at room temperature.Note that single atom that adsorbs on the 7×7 surface thermally diffuses but is confined within the NS at room temperature.Previous STM images of Pb, Au, and Ag adsorbates in the NS show that their diffusion rate within the NS is quite large [25,26].So STM/AFM cannot observe specific adsorption sites at room temperature.In the new manipulation technique to assemble various atom-clusters with atomically defined compositions at room temperature, transfer of single atom diffusing within a NS to an adjacent one can be induced by the reduction of the energy barrier that hinders the diffusion of inter NS.We found that this inter NS transfer can be controlled by the tip proximity near the border, but slightly shifted away from the NS that is occupied by the single atom as shown in Fig. 8(a) [27].When the tip approaches, the energy barrier is reduced.Then an atom is transferred.Various atom clusters can be assembled by collecting atoms one-by-one into a NS with successive manipulation.
Figure 8(b) and (c) show an example of this manipulation.There are three Au atoms in the STM image shown in Fig. 8(b).Three NS look brighter due to the electronic modification by Au atoms diffusing with much higher speed than the scanning speed.First, the tip was positioned above the cross.Then, the distance feedback was opened and the tip gradually approached the surface by 3.3 Å from STM feedback position.Then, one of Au atoms was transferred to the adjacent NS without any other surface and tip modifications.We could achieve such inter NS manipulation for various elements, such as Ag, Pb, Sn, and Si atoms in the same way.So, this manipulation is a very universal technique.Some insights into the manipulation mechanism can be obtained from the distance dependence of tunneling current I t during the manipulation process [27].Figure 9 shows the semi-logarithmic plot of I t for manipulating an Au atom by STM as a function of tip-sample distance.Black curve is taken during the approach and red curve is during the retraction.In the approach curve, I t increases exponentially (0.3 Å<z), and shows jump at a certain point (z=0.3Å), and then increases with a smaller slope (0<z<0.3Å).In the retraction curve, I t decreases with a smaller slope (0 Å<z<0.75 Å), a plateau is seen (0.75 Å<z<1.0Å), and then I t fluctuates (1.0 Å<z<1.4Å).Finally, I t drops by one order (z=1.4Å) and decrease exponentially (1.4 Å<z).
The current jump in the approach curve can be explained by Au atom hopping from the original NS to the adjacent NS below the tip.After hopping, Au atom is trapped below the tip and an atomic junction of tip-Au-Si surface is formed, which causes the sudden increase of I t .With decreasing the tip-surface separation I t increases with a smaller slope, which indicates that the tunneling barrier collapses and ballistic current flows in the junction.
In the retraction curve, the plateau is seen, where the Au atom is still trapped and the atomic junction is maintained.The stabilization of the junction is evident in the relatively small noise in the plateau region.We obtained 10 −4 G 0 as the quantized conductance through single Au atom junction using several tips.These are much smaller than that obtained in pure gold junctions where the conductance is close to 1G 0 for single Au wire [28].Our small conductance probably originates in the mismatch of the energy levels between Au and surface causing the transmission coefficient much smaller than 1.
When the tip is retracted further from the plateau region, the fluctuation is observed due to the stochastic trap and release of single Au atom by the tip.Trapping state give large I t through the junction.When Au atom is released and starts to thermally diffuse, I t decreases.These two states are stochastically switched here.Finally, I t drops by one order where Au atom is released from the bond with the tip and thermally diffuses within the NS all the time.Hereafter, I t decreases with increasing the tip-surface distance following the tunneling law.
In summary, when the tip approaches, barrier is reduced and atom hops.Then it is trapped below the tip and junction is formed.Trap and release occurs stochastically and the atom is released all the time.We can transfer an adsorbate at zero current and zero voltage by AFM.So, we can say this barrier reduction is caused by the chemical bonding force between tip and the adsorbate.
We   three fold symmetry (Fig. 11(a)) and clearly different from Au 12 (Fig. 11(c)) although both Ag and Au belong to the same chemical group.More remarkably, Ag 13 never be formed while Au 13 can be formed (Fig. 11(d)).When single Ag atom is put into a NS adjacent to Ag 12 , the Ag atom thermally diffuses among three NSs surrounding the Ag 12 (Fig. 11(b)).Although the Ag atom has to pass through the Ag 12 for such diffusion, Ag 13 never be formed within observable time scale.

V. SWITCHING OPERATION
We can fabricate various clusters, Au, Ag, Pb, Si and even bimetal clusters.We found some of assembled clusters work as atom switches [29,30].

Sugimoto
Si 4 and STM images are shown in Fig. 12.It has two equivalent forms, L type and R type.We found that current injection induces the switch between them.When current is injected into the upper atom, the upper atom is moved downward by inelastic process.Interestingly, we can also induce atom switch by force.When tip approach down atom, this down atom can be pulled up.So, Si 4 switch works by both current and force.

VI. CONCLUSION
We have developed atomic technology using AFM/STM at room temperature.The use of AFM is complementary to use of STM.Using AFM, we can image insulator surfaces, measure real topographic height of chemical species on the surfaces, and resolve inner molecular structures of organic molecules.And, the capability of force measurement provide deep insight into chemical nature of the tip apex, which dominates AFM image contrasts and atom manipulation by force.The chemical identification based on the force measurements will be used as a semiconductor technology and nanotechnology as well as a powerful surface science technique.Atom manipulation at room temperature is also interesting research topic.Three dimensional nano-structure such as atom clusters as well as two dimensional pattern can be fabricated by single atom manipulations.We found that some of assembled nano-structures work as atom switches controlled by force as well as carrier injection.Other intriguing properties such as quantum dot behavior, rectification, and magnetic moment will be found in artificial nano-structures in future.
Figure 1 shows the AFM images we obtained.Figure 1(a) shows the image of Si(111)-(7×7) surface, where dangling bonds on individual Si adatoms are imaged.We can image even insulators such as KCl (Fig. 1(b)) using AFM.In the image of TiO 2 surface with adsorbed H atoms (Fig. 1(c)), individual H atoms that are adsorbed on O atoms are imaged[8][9][10].In the image of a single PTCDA molecule, chemical structure inside the molecule can be resolved by Pauli repulsive force even at room temperature[11].Note that AFM image well reflects real topographic height.Figure2(a) shows AFM image of very initial stage of oxygen adsorption on the Si(111)-(7×7) surface[12].As shown in Fig.2(a), slightly brighter spots correspond to Figure 1 shows the AFM images we obtained.Figure 1(a) shows the image of Si(111)-(7×7) surface, where dangling bonds on individual Si adatoms are imaged.We can image even insulators such as KCl (Fig. 1(b)) using AFM.In the image of TiO 2 surface with adsorbed H atoms (Fig. 1(c)), individual H atoms that are adsorbed on O atoms are imaged[8][9][10].In the image of a single PTCDA molecule, chemical structure inside the molecule can be resolved by Pauli repulsive force even at room temperature[11].Note that AFM image well reflects real topographic height.Figure2(a) shows AFM image of very initial stage of oxygen adsorption on the Si(111)-(7×7) surface[12].As shown in Fig.2(a), slightly brighter spots correspond to

FIG. 3 .
FIG. 3. (a) Force vs. tip-sample distance curves for the cases of reactive (red curves) and inert (blue curves) tips [13].Curves without filled circles are measured by AFM, while those with filled circles are calculated using DFT.(b) AFM image obtained with the reactive tip, and (c) the one obtained with the inert tip.
FIG. 4. (a) AFM image of Si, Sn, Pb mixed surface.(c) Force curves measured over all atoms in (a).(b) Identification of atomic species based on the force curves shown in (c) [16].
FIG. 10.Au clusters on the Si(111)-(7×7) surface assembled by inter-nanospace manipulation at room temperature [27]. Au FIG.12.Structure model and STM images of Si4.Both current injection and force can cause switch between L type and R type[29].

TABLE I .
Comparison between conventional lateral manipulation and interchange lateral manipulation.