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Atom-by-atom assembly

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Reports on Progress in Physics Rep. Prog. Phys. 77 (2014) 056502 (16pp)

doi:10.1088/0034-4885/77/5/056502

Review Article

Atom-by-atom assembly Saw Wai Hla Center for Nanoscale Materials, Argonne National Laboratory, 9700 S Cass Ave., Lemont, IL 60661, USA Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA Received 13 May 2013, revised 18 February 2014 Accepted for publication 26 February 2014 Published 2 May 2014 Abstract

Atomic manipulation using a scanning tunneling microscope (STM) tip enables the construction of quantum structures on an atom-by-atom basis, as well as the investigation of the electronic and dynamical properties of individual atoms on a one-atom-at-a-time basis. An STM is not only an instrument that is used to ‘see’ individual atoms by means of imaging, but is also a tool that is used to ‘touch’ and ‘take’ the atoms, or to ‘hear’ their movements. Therefore, the STM can be considered as the ‘eyes’, ‘hands’ and ‘ears’ of the scientists, connecting our macroscopic world to the exciting atomic world. In this article, various STM atom manipulation schemes and their example applications are described. The future directions of atomic level assembly on surfaces using scanning probe tips are also discussed. Keywords: STM, atom manipulation, atomic level assembly (Some figures may appear in colour only in the online journal)

a stick to relocate an atom across the surface (figures 1(c) and (d)). Here, the lateral force of the STM tip must overcome the surface retaining forces to move the atom. To date, a number of reviews on atomic and molecular manipulation have been published in the literature [5–8]. In addition to general explanations of atom manipulation schemes described in previous reviews, the atomic assembly processes based on the author’s own experience are included in this article, and some example applications of atomic level assembly are also given.

1. Introduction

One of the dreams of scientists is to build structures atomby-atom; a process that is analogous to many macroscopic constructions in our world where basic building blocks, such as bricks, are used. Because of the minuscule sizes of the atoms, such atomic level assembly requires specialized tools. The invention of scanning tunneling microscopes (STMs) in the early 1980s [1, 2] has enabled the real space imaging of atomic structures on a material’s surface. Later, a variety of scanning probe microscopes have been developed where a sharp needle, commonly known as the ‘tip’, is used to probe atomic structures. In an STM, the tip needs to be positioned at the proximity of the sample surface, typically less than 1 nm distance. When a bias voltage is applied, electrons can tunnel between the tip and the sample, and the tunneling current varies exponentially with the tip-sample distance [3, 4]. The tip of a scanning probe microscope that is used to acquire atomic scale images can also be used to perform atom-by-atom assembly. An analogy of this process is demonstrated in figures 1(a) and (b), where oranges are laterally moved across a table using a wooden stick to form a line. To move an orange, the magnitude of the lateral force applied by the wooden stick needs to encompass the surface friction. An analogous process is used in an atom manipulation process, where an STM tip is used as 0034-4885/14/056502+16$88.00

2. Basic atom manipulation schemes

During STM imaging the location of the STM tip near the surface can cause tip-sample interactions. When acquiring an STM image, the effects of tip-sample interactions are kept as low as possible. But these undesired effects can be systematically used to manipulate atoms and molecules to produce atomic assemblies on surfaces [9–47]. Atom manipulation can be broadly classified into parallel and vertical processes. In the parallel process, the manipulation is focused along the directions parallel to the surface plane, which is commonly known as the lateral manipulation (LM). In the vertical manipulation (VM) process, the manipulation is along the surface normal direction. 1

© 2014 IOP Publishing Ltd Printed in the UK

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Figure 1. Orange and atom assembly. (a) An orange is laterally

moved along the arrow by a wooden stick, and (b) an assembled line of four oranges. (c) Four silver atoms are laterally moved to assembled to form a line on Ag(1 1 1) (d). 2.1. Lateral manipulation

Unintentional dragging of atoms and molecules by the STM tip can occasionally occur during imaging. Eigler and co-workers used this process to demonstrate atomically precise positioning of individual atoms on a surface in the early 1990s [9]. This atom manipulation process involves the relocation of individual atoms across the surface with sub-atomic precision. The atom manipulation here is parallel to the surface and, therefore, it is a LM process. The mechanisms of atom manipulation have been studied and the LM procedure has been used to assemble atomic structures by a number of groups [10, 23, 42–58]. The LM of individual atoms involves three experimental steps (figure 2(a)): (1) approaching the tip toward the atom, (2) lateral movement of the tip parallel to the surface and (3) retraction of the tip back to an image height. For the LM of an atom across a surface, it is necessary to increase the tip-atom force. When the tip approaches vertically toward an atom that is adsorbed on a surface, the force between the tip and the atom (the total force F T ) increases (figures 2(a) and (b)). When the desired force strength is reached, the tip approach is terminated and the tip-atom distance remains fixed throughout the manipulation process in the constant current scanning mode. Next, the tip is laterally moved along a chosen path to a final destination where the atom will be relocated. During this, the atom moves together with the tip. At the final

Figure 2. (a) LM procedure. (b) Tip-atom geometry. (c) Drawings of pulling (blue), pushing (green), and sliding (red) curves (top) and corresponding manipulation signals (bottom). (d) A pulling signal (top) and LM process (bottom) for the dashed box region in the signal. (e) Distance dependent tip-atom potential. Here, the green and blue areas indicate pulling and sliding regions.

2

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destination, the tip is retracted back to normal image height and the atom now remains bonded on the surface. Among these steps, the second step is the most important and several physical processes occur depending on the tip-atom interactions, the chemical nature of the tip-apex, the surface material, and the manipulated atom. The LM process can also be used to relocate larger molecules, and to operate molecular machines and motors [59–76]. More complex mechanisms can occur in molecular manipulations because molecules are composed of several atoms and molecular conformation can be changed during manipulation. In LM, successful atom relocation can be confirmed by comparing the initial and final STM images acquired before and after the manipulation. During the actual relocation of the atoms, the tip-height signal, which is often called the manipulation signal, can be monitored [23, 45]. The detailed atom movement mechanisms can be extracted from this manipulation signal. Depending on the nature of the atom movement and the forces involved in the manipulation, the LM process can be roughly divided into three regimes, which are: pulling, sliding and pushing [45] (figure 2(c)). In a pulling regime, the tip first traces the contour of the top part of the atom being at rest on the surface in a constant current scanning mode (figure 2(d)). Because the tip-atom distance is constant, the total force FT is also assumed to be constant. But shifting the tip-apex position with respect to the atom causes the lateral force FL to gradually increase (figure 2(d)). FL is the vector component of FT , and it can be described as [54, 58]: FL = FT cos φ =

dU (r) cos φ dr

(figure 2(e)). A pulling manipulation typically involves the attractive part of the tip-atom interaction, which is located on the downward slope of the potential shown in figure 2(e). During pulling, the electronic wave functions between the tip and atom are overlapping due to a close distance between them, and the attractive force here is manifested from a weak temporary bond formation between the tip and the atom [79]. However the atom-surface binding is stronger than that of the tip-atom attraction and, therefore, the manipulated atom remains on the surface throughout the process. Thus, maintaining a delicate balance between the forces involved is a stringent requirement for a successful atom manipulation. In the case of a pushing process [45, 46], the trace of an atom’s contour is produced when the tip climbs the up-ward slope of the manipulated atom being at rest. When the lateral force inserted by the tip to the atom exceeds the hopping barrier, a repulsive tip-atom interaction pushes the atom, and it hops to the next surface site. This suddenly increases the tip-atom distance. In the constant current scanning mode the atom hopping alerts the STM feedback system to abruptly move the tip toward the surface in order to maintain the tip-atom distance constant. As a result, the tip height abruptly decreases (figure 2(c)). By repeating this rest-hop cycle, the atom moves to the desired destination in a discontinuous manner, similar to the pulling. The tip-height signal for pushing also exhibits a saw-tooth like shape, but in a reverse order to the pulling. On a metal surface, it is necessary for the STM tip to be much closer to the atom for pushing than pulling [45, 46, 80]. The repulsive force in pushing can originate from different sources. For example, when the manipulated atom is adsorbed at the lower part of a step-edge, the atom-substrate binding is stronger than the one adsorbed on a bare surface plane. Then, a stronger force is required to move the atom and thus a much closer tip-atom distance is necessary for manipulation. In the case of a molecular manipulation using a molecule tip (i.e. another molecule located at the tip-apex), a steric repulsion between the two molecules can trigger a pushing process. In a sliding manipulation, the tip-atom distance is closer than that of pulling (figure 2(e)) and the atom is locked inbetween the tip and the surface during manipulation. The atom moves smoothly together with the tip [45, 46, 53, 80] and the sliding signal generally shows just the contour of the surface atoms without the abrupt tip retractions typically observed in pushing and pulling processes. In some sliding manipulations, the atom still moves in a discontinuous manner; however, the close tip-atom distance causes the manipulation signal to appear smooth [53].

(1)

where ‘r ’ is the tip-atom distance, U (r ) is the tip-atom interaction potential, and φ is the angle between the total and the lateral force directions (figure 2(b)). When FL overcomes the surface retaining force FS , which originates from the binding of the atom to the surface, the atom hops to the next surface site to follow the tip [54, 55, 58]. The atom is now closer to the tip and it triggers an abrupt retraction of the tip by the STM feedback to maintain a constant tip-atom distance. Accordingly, a sharp rise in the tip-height occurs. At the moment of atom hopping, the magnitude of threshold lateral force FLth exceeds the Fs ; FLth
Fs . By repeating this rest-hop sequence of movement, the atom is laterally relocated across the surface. The tip-height signal for this process exhibits a saw-tooth like shape [45, 58] (figures 2(c) and (d)). Because the atom has to follow the tip, it is called ‘pulling’ [45, 46]. Pulling can also be performed in a constant height scanning mode. In this case, the tunneling current is recorded as the manipulation signal instead of the tip-height. Here, the tip height does not change during atom hopping but a rapid increase in current occurs due to changes in tip-atom distance [66]. In an LM process, typical tip-atom distances are in the order of an atomic size or less. When the tip is located very close to an atom on a surface, the interaction between the tip-apex atom and the manipulated atom become dominant. Then, a two-atom potential model [77, 78] may be used to approximate the interaction where one atom is located at the tip-apex and the other is the manipulated atom [58]

2.2. Vertical manipulation

During STM imaging, occasional picking or dropping of adsorbates by the tip often occurs. Such transfer of adsorbates between the tip and sample can be realized in a controlled manner and this is known as the VM (figure 3) [81–97]. The VM process is somewhat analogous to the loading and unloading of a crane in our macroscopic world (figures 3(a) and (b)). The easiest way to transfer an atom to the tip-apex is by means of a direct tip-atom contact. In this process, the STM 3

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bias, or a mechanical movement of the tip depending on the chemical nature of the atom and the type of bonding to the tip. For the transfer of CO molecules between the surface and the tip, an inelastic electron tunneling (IET) can be used. When a CO molecule adsorbed on a Cu(1 1 1) surface is injected with 2.4 eV tunneling electrons from an STM tip located directly above the molecule, the molecule can desorb from the surface [85, 86]. The CO molecule can be re-adsorbed either on the surface or at the tip-apex. Because CO normally adsorbs on metals via its carbon atom, this process also involves 180◦ flipping of the molecule. For the VM of some atoms, such as xenon (Xe), the atom can be transferred between the tip and the surface just by changing the bias polarity [81, 82]. There are a number of different theoretical explanations for the case of Xe atom transfer process. It is proposed that the polarization of the atom by the electric field leads to an attractive or a repulsive interaction [81, 82]. Here, the barrier at the W shape potential well is reduced due to the close proximity of the tip and the surface, while the applied bias provides additional lowering of the barrier and a directional driving force (figure 3(c)) [100]. However, Gao et al [101]. considered the Xe atom transfer process between the tip and the surface as induced by a vibrational excitation of inelastic tunneling electrons, while Brandbyge and Hedegard [102] modeled the Xe atom as the quantum Brownian particle interacting with the environment of electrons from the tip and the sample. Saenz and Garcia [83] assumed that the Xe atom transfer is caused by a single atom tunneling process. The Xe atom transfer is also proposed to be responsible for a vibrational heating [103, 104]. For the transfer of Na atom and O2 molecule between the tip and the surface, coherent multiple excitations are considered to be dominant [103]. More complete theoretical formalisms for atom transfer process between the tip and surface are later proposed [105, 106]. For a metal atom adsorbed at the metal tip-apex, the bonding is metallic in nature and it may not be able to transfer just by an electric field. In this case, approaching the tip close to a direct mechanical contact with the transfer atom is often required [94, 107]. A useful application of VM is the reshaping of the tip-apex. By transferring an atom from the surface to the tip-apex, an atomically sharp tip can be formed. In addition, the elemental nature of the tip-apex atom is also known [94]. By means of the VM process, atoms and molecules can be deposited on surfaces from the tip and, therefore, it can be used for atomic assembly processes [39, 40].

Figure 3. VM. Loading/unloading of a crane in (a) is analogous to the transfer of an atom between the tip and sample (b). (c) The double well potential (red) for two possible adsorption sites of the atom: on the surface and at the tip. A single well (dashed curve) is formed when the tip is in contact with the atom. When an electric field is applied, the barrier between the two wells is reduced (green).

tip is approached toward the atom until a mechanical contact is achieved. Here, the initial tip and surface binding sites can be represented by a ‘W’ shape potential well (figure 3(c)), where one well is located on the surface while the other is at the tip-apex [98, 99]. Upon the tip-atom contact, the W shape potential is changed to a single potential well. Because the tip-apex generally has fewer neighboring atoms than the surface, the tip-atom binding can be stronger than the atomsurface binding. When the STM tip is retracted back to the normal image height, the atom is now located at the tip-apex. However, the process to transfer the atom from the tip to the surface is not so straightforward. The atom may preferentially remain at the tip-apex due to a stronger binding there than on the surface. The atom transfer from the tip to the surface can be realized either by means of tunneling current or applied

3. STM tip for atomic assembly

While an average tip can be used for STM imaging, the well defined shape, structure, and chemical identity of the tip-apex are vital for atom manipulation. Although theoretical calculations indicate that a single atom, as well as dimer and trimer tip-apexes, can be used for atom manipulation [108], a precise positioning of the atoms at specific locations on a surface requires a symmetric tip-apex. Thus, it is often required to reshape the tip for atom manipulation. A useful tip preparation technique is to make a direct contact between the tip and the sample [50]. If a tungsten (W) tip is used on softer 4

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(3) construction of atomistic structures and (4) cleaning up the debris and defects. 4.1. Atom extraction with an STM tip

There are several ways to get atoms on a surface for atomic level assembly. A standard procedure is to deposit atoms by thermal evaporation [14]. But diffusion of atoms occurs at very low temperatures on metal surfaces, which often results in undesirable products, such as islands. To circumvent the thermal diffusion of atoms, low substrate temperatures are often required during the atom deposition process. In most cases, the atoms are directly deposited on a substrate maintained at a low temperature inside the STM scanner. However, there are other ways of getting atoms on a surface. For example, the surface itself is composed of atoms, and it is possible to locally extract the atoms from the native surface with an STM tip [43, 50]. Atoms can be also extracted from nanoscale metallic clusters [79], individual molecules [58], or from the tip itself. Such local atom extractions are also interesting for the understanding of the tip-atom-surface interactions. Depending on the nature of the experiment, several atom extraction procedures may be combined to locally extract chemically distinct atoms on a surface [58]. A number of local atom extraction procedures using an STM tip are discussed in this section.

Figure 4. (a) Distorted shapes of the atoms are produced by an asymmetric tip. (b) After reshaping the tip-apex, the atoms appear as more symmetric shapes.

metallic surfaces, such as gold, silver or copper, the tip-apex can be reshaped by dipping it into the substrate [50]. Local heat produced in this process can reshape the structure of the tip-apex. An electric field can also aid the reshaping of the tip. During this process, the tip-apex is also coated with the substrate material and, hence, its chemical identity can be known. Although the tip-apex can be reshaped, it is not easy to image the structure of the tip-apex. An indirect way to identify a good tip-apex is by taking STM images of individual atoms. figures 4(a) and (b) show sample STM images of two Ag atoms on a Ag(1 1 1) surface. In figure 4(a), the shapes of the atoms are distorted. Since the STM image is formed by a convolution of the tip and the surface electronic structures, such a distorted atom shape is due to an asymmetric tip-apex. After reshaping the tip-apex in figure 4(b) by means of tip-sample contact, the atoms appear as circular shapes indicating that the tip-apex now has a symmetric shape. Such a symmetric tip-apex is the first step for a successful atom manipulation for the atomic assembly processes discussed in this article.

When the STM tip crashes on a surface, it destroys the surface area. Although such an event is not desirable for imaging, a controlled tip crash process is useful to extract individual atoms from the surface [50]. For example, the extraction of silver (Ag) atoms on a Ag(1 1 1) surface (figures 5(a)–(d)) has been demonstrated by means of tip-crashes using tunneling biases between +1.5 and +4 V [50]. Here, after the STM tip dips into the substrate, a nano-size crater and displaced clusters are formed (figure 5(b)). While the larger clusters are located close to the crater, smaller clusters and scattered individual atoms can be found at further distances from the crater (figures 5(a) and (b)). These extracted atoms have the same chemical element as the substrate, which can be confirmed by filling the atoms back into small holes that are also created by tip-sample contacts [50]. The atom extraction here depends both on the tip-sample contact area and the applied bias at the tip-sample junction. An increase in the tip-sample contact area results in a larger displacement of extracted atoms from the crash site (figure 5(d)). A similar effect is found when the tunneling bias is increased: the larger the bias, the farther away the atoms are displaced from the crash site. Both the tip-sample contact and applied bias generate heat at the tip-sample junction. Therefore, increasing the contact area or applied bias enhances thermal diffusion of the extracted atoms [50]. In this type of atom extraction, the mechanical impact of the tip and the local electric field at the tip-sample junction are the key parameters that control the process. Further theoretical simulations are needed to understand the detailed mechanism of such atom extraction.

4.1.1. Atom extraction by tip-sample contact.

4. Atomistic construction processes

Consider a civil construction project in our world. Materials are extracted from the Earth at one place and then relocated to be used in a construction site to build desired structures. An atomistic construction is somewhat analogous to a macroscopic construction but it is realized locally on a surface. An atomistic construction project may involve the following steps: (1) extraction of individual atoms; (2) relocation of atoms on the surface; 5

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Figure 5. (a–d): (a) Atom extraction by tip-crash; (b) STM image showing individual atoms on a Ag(1 1 1) surface after a tip-crash. (c) A zoom in image shows the extracted atoms. (d) Atom displacement as a function of tip-crash area. (e–h): (e) Atom extraction from a cluster. (f ) A silver cluster on Ag(1 1 1) surface. (g) Two Ag atoms are extracted from the cluster. (h) Probability of atom extraction (LAtom /LTip ) vs. tunneling current plot. (i–m): (i) Atom extraction from a molecule. (j ) A Co-TBrPP molecule on Ag(1 1 1). After dissociation (k), a Br atom is moved away from the molecule with the STM tip (l). The tunneling current abruptly drops upon the dissociation of a Br atom (m).

top cluster atoms with the STM tip toward the flat terrace area, the atoms can be extracted one-at-a-time. This process can be reliably and reproducibly used to extract atoms from clusters [24]. A simulation using an embedded atomic potential method [109] reveals that the binding energy of the top cluster atom is greatly reduced when the tip approaches toward the cluster. Experimentally, the probability of atom extraction can be determined by measuring the distance travelled by the STM tip (LTip ) and that of the atom displacement (LAtom ). If the ratio of LAtom and LTip is ‘1’, then the atom is fully extracted; while if it is ‘0’, then the atom is not extracted [79] (figure 5(h)). Using a fixed small bias, the atom extraction probability can be measured as a function of tunneling current. Because the tunneling voltage is fixed, an increase in tunneling current during the tip approach reduces the tunneling resistance. For the atom extraction from silver clusters, the probability plot

When the STM tip is located very close to the surface it perturbs the surface and induces it to alter the surface potential landscape. When the tip is approached toward a nano-size metallic cluster, a similar phenomenon occurs. Here, the closer the tip is to the cluster, the stronger the perturbation becomes and it greatly reduces the binding energy of the cluster atoms [79, 109, 110]. Thus, just by approaching the tip toward the cluster without any applied bias, the cluster atoms located closest to the tip can be extracted (figure 5(e)–(h)) [79, 109]. An example is demonstrated in figures 5(f ) and (g). Here, individual silver atoms are extracted from a silver cluster on a Ag(1 1 1) surface using the STM tip [58, 79]. The silver cluster in this case is also formed by making a tip sample contact with a silver coated W tip. To extract the atoms, the STM tip is approached very close to the top part of the cluster. Then, by laterally pushing

4.1.2. Atom extraction from metallic clusters.

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shows a step function (figure 5(h)) indicating a certain tipcluster distance requirement for the atom extraction. Studies of tip-atom contact mechanisms indicate that atomic relaxation can occur near the tip-contact distance [111], which may also contribute to the atom extraction process. Atoms for an atomic assembly process can also be extracted from organic molecules adsorbed on a surface (figures 5(i)–(m)). By means of IET, selective bond dissociations of a molecule can be performed [112, 113], and individual atoms from a molecule can be extracted [58]. Such atom extraction can be realized by positioning the STM tip above the location of the bond, and then injecting tunneling electrons into the molecule. In an IET process, the tunneling electron energy is transferred to the molecule by means of a temporary electron capture in a resonance state, followed by a vibrational relaxation of the molecule [112, 114]. If the transferred energy exceeds the bond dissociation energy, the bond can be broken and the desired atom can be freed from the molecule. On metal surfaces, the molecules containing halogen atoms are easier to dissociate because the carbon–halogen bond is generally the weakest when compared to C–C and C–H bonds [113]. Therefore, halogen atoms can be selectively extracted by using this technique while the rest of the molecule remains intact [58]. Figures 5(j –l) illustrate a bromine (Br) atom extraction from a cobalt porphyrin molecule (Co–TBrPP) adsorbed on a Ag(1 1 1) surface. A Co–TBrPP molecule contains four Br atoms at its four corners, and the C–Br bonds can be broken by using electron energy of 2.2 eV. Upon bond breaking, an abrupt change in tunneling current occurs (figure 5(m)) due to changes in the electronic structure of the molecule under the tip as well as the displacement of the resulting molecular fragments. Thus, successful atom extraction from a molecule can be known by monitoring the tunneling current changes during the process.

4.1.3. Atom extraction by molecular bond breaking.

Figure 6. (a) An STM image showing a mass relocation of atoms (indicated with an arrow for general directions) to construct a circular corral in b. [70 nm × 70 nm, 50 mV, 1 nA]. (c) Atom manipulation along different surface directions. The background is a 3-D atomic resolution STM image of a Ag(1 1 1) surface.

dynamic of atom movements along different surface directions are interesting from the point of view of fundamental understanding, as well as to monitor the success of the atom manipulation if one wants to automate the process. In STM images, an atom adsorbed on a surface is often imaged as a protrusion or a depression on a flat 2D surface. In reality, the surface itself is made of atoms and, therefore, the manipulated atom encounters different potential barriers when moving in various surface directions. Figure 6(c) illustrates a Ag atom’s movement along different directions on a Ag(1 1 1) surface [23]. Here, the easiest atom movement is along [1 1 0] surface direction, and the atom moves in a typical pulling mode by following rest-hop cycles trailing the tip. But when the atom is moved along other surface directions, then more complicated atom movements are observed. For example, when it is moved along a direction 20◦ deviated from the [1 1 0] surface closedpacked (CP) row, jumping of the atom to the next surface CP row occurs in addition to the usual pulling process. The atom in this case follows the tip in a zigzag path instead of a straight movement path [23]. When the tip path is along [2 1 1] surface direction, which is along 30◦ from the [1 1 0] direction, a different pattern of atom movement occurs. The surface geometry along this path includes repeating units of three hollow sites, where the first two sites are closely spaced then the third site. In this case, the atom moves in a smooth sliding mode between the first two hollow sites. Then, to move the third

Atoms can also be directly extracted from the tip. Using VM, atoms can be picked up from one part of a surface and then dropped back down at another part of the surface [39]. Alternatively, the tip can also be coated with the desired materials for atom deposition. This technique is useful for the atomic assembly processes on semiconductor surfaces [39, 40], where atom extraction using the tip-sample contact can be difficult because of the hardness of the sample surface.

4.1.4. Atom extraction from the tip.

4.2. Atom relocations

For an atomic assembly the relocation of sufficient number of atoms to the construction site on a surface is often required. The LM procedure that was previously discussed only describes atom manipulation along the close-packed surface directions on fcc surfaces. The actual relocation and construction of atomic structures can involve lateral positioning of atoms in any direction on a two-dimensional (2D) surface (figures 6(a) and (b)). While this can simply be done by dragging the atoms with the STM tip, the 7

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site, it travels along a semi-circular path [23, 51]. A smart way of investigating the atom movement mechanism is by taking an atom manipulation image by continuously manipulating an atom across a 2D surface area [52, 53]. This process, known as manipulation imaging, can also detail how the atom moves in different atomic landscapes on a surface. So far, the studies of atom movements during manipulation have been mostly concentrated on fcc(1 1 1) surfaces [23]. Such studies on different surface orientations at various material surfaces will be valuable for atomic scale tribology information [54]. 4.3. Atomistic constructions

Construction of atomistic structures requires the precise positioning of atoms on a surface, often on a sub-atomic scale. The simplest atomic level assembly on surfaces is onedimensional (1D) atomic rows or chains. Even in this case, it is necessary to position the atoms with sub-atomic precision if an atomically straight chain is desired. For example, figure 7(a) shows two atomic chains where the left chain appears as a straight line but the right chain is clearly distorted. Here, just by shifting between fcc and hcp adsorption sites of the atoms, this atomic chain can be corrected to appear straight. The repositioning of an atom between fcc and hcp sites requires a controlled manipulation at the sub-atomic level. The atom manipulation procedure in this case is similar to the LM process but the tip movement is well within sub-atomic distances. Thus, relocation of an atom between fcc and hcp sites is more of a lateral perturbation of the atom than a normal LM process and it is somewhat analogous to kicking the atom to the side. Atoms on a surface are adsorbed on specific adsorption sites. For instance, on a fcc(1 1 1) metal surface, metal atoms are mostly adsorbed at the surface hollow sites while CO molecules on the same surface preferentially adsorb on top surface atom sites. Such preferential adsorption imposes limitations in design of atomistic structures. For example, fcc(1 1 1) surfaces such as Ag(1 1 1), and Cu(1 1 1) have a hexagonal lattice and building a square box structure on these surfaces is not so straight-forward. Although such a square box can be built, the distance between the atoms cannot be the same between the adjacent edges of the box. An example is shown in figure 7(d), where Ag atoms are positioned along [1 1 0] surface with six atomic distances at the left chain but the same distance cannot be arranged along a perpendicular direction (a [2 1 1] surface direction) because of the differences in the lattice distances (figure 7(b)). For other structures, like the circular corral shown in figure 6(b), the surface symmetry effect is more pronounced. In order to achieve a more circular shape, it is necessary to increase the diameter of the corral so that the atoms can be closely positioned along the circumference of a circle. 4.4. Tidying up schemes

Figure 7. Atomistic Construction. (a) Straight and disordered atomic lines. (b) A structural drawing showing positions of atoms. (c) Atoms positioned with a sub-atomic precision with six-atomic distance. A grid is included for eye guidance. (d) Formation of a 90◦ corner between two silver atomic lines on Ag(1 1 1). Distance between two atoms along a white arrow is 17.9 Å while that of the yellow arrow is 21.5 Å.

After atomistic constructions, the debris and holes can remain near the construction site. These undesired materials not only obstruct the beauty of the structure but also influence the local electronic environment. Therefore, it is desirable to tidy up

the area after completing an atomistic construction. This can also be accomplished by using the STM manipulation schemes. For example, the debris can be moved to distant locations away 8

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possible man-made 1D structures and can exhibit a rich variety of physics [30–37]. The atoms used in 1D atomic chains here are either deposited onto the surface [30, 31] or locally extracted from the native substrate [35]. The formations of the 1D atomic chains in these experiments are realized by using LM or VM procedures. By measuring the dI /dV spectra of the chain as a function of the number of atoms, the electronic structural evolution of a 1D material can be determined [30]. In the case of an Au20 chain on a NiAl(1 1 0) surface (figures 9(a)–(g)), the dI /dV map reveals superposition of a few eigenstates. When the number of atoms is increased, the energetic positions and separations between the states decrease with the increasing number of atoms in the chain [30, 31, 35, 36]. Here, the electronic structure of 1D atomic chains can be mapped with a 1D quantum well model. Using atomic assembly, the role of impurities can also be investigated by mixing Au and Pd atoms in the chain on a NiAl(1 1 0) surface [35, 37, 38]. Moreover, by varying the number of atoms in a 1D silver atom chain on NiAl(1 1 0) surface, the photon energy of STM tip induced light emission can be tuned [34]. In the case of 1D atom chain formed on InAs(1 1 1) surface [39], a VM procedure is used to extract the native surface In atoms. The In atoms are then dropped onto a surface construction site by a tipsample contact. The In atom chain in this case also exhibits chain-localized quantum states. These experiments clearly demonstrate that local electron structures can be studied and manipulated in artificially engineered structures in an isolated environment. Atomic assembly can also be used to build magnetic nanostructures with a well defined number of atoms and shapes. For example, in the case of a 1D manganese (Mn) atom chain on a thin CuN insulating layer on Cu(1 0 0) surface (figure 9(h)) [165], the dI /dV spectra reveal differences between even and odd number atomic chains (figure 9(i)). The odd number of chains show a dip around the surface Fermi level due to a spin-flip excitation, which is absent in the even number chains. The difference between the even and odd number atomic chains also indicates an anti-ferromagnetic coupling between the atoms. This experiment demonstrates that spin interactions can be investigated at the atomic level inside artificially engineered atomic structures. In another experiment, the spin directions of individual magnetic atoms in an artificial atomic chain are investigated [41]. Using a magnetized tip, magnetic nanostructures can be imaged at an atomic scale using spin polarized scanning tunneling microscopy (SP-STM) technique [116]. In SP-STM, changes in tunneling current intensity occur depending on the parallel or antiparallel magnetization directions between the magnetic tip and the sample. This effect can be used to image magnetic structures down to single atoms [41, 53, 115–118]. In order to image the various spin directions of atoms, individual cobalt (Co) atoms are assembled to form a chain on a Mn mono layer grown on a W(1 1 0) substrate (figures 9(j ) and (k)). The substrate Mn monolayer here exhibits an antiferromagnetic spin spiral [115], and the Co atoms are ferromagnetically coupled to the spin of the underlying surface sites. Because the spin directions of the Mn substrate layer are dependent on the

Figure 8. Holes and clusters in the area marked with a rectangle box in (a) are cleaned up in (b).

from the construction site and the holes can be filled back with clusters and atoms using either an LM or VM process. The sequence of STM images in figure 8 demonstrates a tidying work after an atomistic construction process. Here, a hole and materials at the right side of the structure are cleaned up by laterally moving the cluster to fill the hole. After completing this task, the surface area appears smooth. 5. Atomistic structures and their properties

Atomistic structures can be constructed for specific applications. Depending on the experimental task, and the chemical nature of the atoms and surface material, artificial atomic structures can be tailored to fit the experiment. In this session, a selection of atomistic structures and respective experiments in the literature are described as examples. 5.1. Atomic chains

Atomic level assembly can be used to form structures to explore behaviors of materials at reduced dimensions in well-defined shapes. Linear atom chains on surfaces may be the simplest task to build; however, such atomic chains are the smallest 9

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Figure 9. Structural model (a) and construction of Au atom chain on NiAl(1 1 0) (b–f ). (g) dI /dV spectra of Au atom chain with different number of atoms and NiAl(1 1 0) substrate. Figure reproduced with permission from [30]. Copyright 2002 The American Association for the Advancement of Science. (h) Mn10 atomic chain on CuN layer on Cu(1 0 0). (i) dI /dV spectra Mn atom chain with different number of atoms. Figure reproduced with permission from [165]. Copyright 2006 The American Association for the Advancement of Science. Model (j ) and STM image (k) of Co atom chain on Mn monolayer on W(1 1 0).

the STM images are acquired using a magnetic tip over the atomic chain, the shape of each atom appears to be different (figure 9(k)) due to the different spin directions of Co atoms in the chain [41]. Because the height profile of the atomic chain here follows a cosine function, the spin directions of individual Co atoms can be directly deduced. This experiment

surface locations, the directions of the adsorbed Co atom spins are also position dependent. In order to determine the spin directions of the atoms, six Co atoms are relocated to form a linear chain on the Mn layer with three atomic distances apart. This distance avoids magnetic coupling between Co atoms and, therefore, their individual spin directions are preserved. When 10

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represents a novel application of atomic assembly to investigate spin phenomena. 5.2. Quantum corrals

Noble metal surfaces, such as Ag(1 1 1) and Cu(1 1 1), have a Shockley type 2D surface state near the Fermi level. Free electrons from the surface state can be scattered by adsorbates, defects and steps. The interference of the incident and scattered electrons forms standing electron waves [14, 119], which can be directly observed by STM imaging [120]. These surface state electrons can be confined inside atomic walls, which are known as quantum corrals [14, 18–21, 24]. Quantum corrals are fascinating and are also useful as platforms for innovative experiments conducted locally with the STM tip. The confinement of the surface state electrons leads to quantization inside a quantum corral and, thus, it is also useful to explore the ‘particle in a box’ phenomenon where the wavelength of confined electrons changes with tunneling electron energy in dI /dV maps [20]. The first example of a quantum corral, a 7.1 nm diameter wide circle formed with 48 iron (Fe) atoms on a Cu(1 1 1) surface, was demonstrated by Crommie et al [14]. Quantum corrals can be built with different atomic species, shapes and sizes. All of these variations can be used to control the spatial and spectral distributions of confined surface state electrons, as well as their transport properties. For example, figure 10 shows a circular and a linear structure constructed by using Ag atoms extracted locally from the native Ag(1 1 1) surface. In most quantum corrals, the corral wall atoms are positioned with certain distances apart. Thus, the corral walls are rather leaky and they do not act as 100% hard wall barriers. When a surface state electron encounters a corral wall, it has a finite probability of being transmitted outside the corral, reflected inside the corral, or scattered into a bulk state [15]. During the last two decades, many theoretical attempts have been made to understand the properties of quantum corrals [15, 121–139]. In the multiple-scattering theory used by Heller et al [15], the STM tip is considered to be a point-like source and the electron emanates from the tip to the surface state. It is assumed as ‘s’ wave scatterers, and theoretical STM images of the corrals are formed by superposing electron waves circularly scattered from the corral atoms. For the stadium corral constructed with Fe atoms on a Cu(1 1 1) surface, it is estimated that ∼25% of electrons are reflected by Fe corral atoms while another ∼25% are transmitted [15]. The remaining ∼50% of the electrons are adsorbed, some to the bulk states. Here, although the surface state of Cu(1 1 1) is orthogonal to the bulk states, corral atoms can provide coupling to the bulk states. Gauyacq et al [128] calculated the escape of confined surface states electrons from a Cu corral wall on a Cu(1 1 1) surface using density functional theory with a wave packet propagation approach. Their calculations highlight the existence of two electron escape modes: nonresonant electron transmission through the corral wall, and a resonant-induced process on top of the corral wall. For a rectangular shape corral, recent theoretical works reproduce STM images, the local density of state (LDOS), and the

Figure 10. Quantum Corrals. (a) A circular and (b) a linear quantum corral built with native Ag atoms on Ag(1 1 1).

dI /dV maps. Here, an electron confined by double δ-function barriers is considered as a quasi-stationary state, and the eigenfunctions and eigenenergies of the electron are derived. The results agree well with the experiments conducted by Kliewer et al [20]. On the other hand, Barr et al [130] used the Sabine treatment, a macroscopic formula from acoustics of sound spaces, to characterize the lifetimes of generic scattering structures in the corrals. In addition, several groups have also measured the lifetime of surface state electrons in the corrals [20, 21, 126, 131, 140]. These achievements highlight that the experiments conducted on atomic level assemblies are vital for the development or improvement of theoretical frameworks. The spintronic phenomena inside quantum corrals have also been intensely studied. In 2000, Manoharan et al [19]. demonstrated magnetic information transport between two focal points of elliptical quantum corrals constructed by Co 11

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Figure 11. (a) Atom extraction from a Ag cluster on Ag(1 1 1) surface and construction of a linear corral. (b) After completion of the linear electron resonator constructed for molecular shooting. Two sexiphenyl molecules positioned at the lower part of the resonator are diffused along the troughs to hit two target atoms located at the opposite end.

A linear electron resonator is also used for the controlled diffusion of a Cu atom on a Cu(1 1 1) surface [26]. When the Cu atom is placed near the center part of the electron resonator, the atom strictly follows the path along the linearly confined standing electron wave. The preferential diffusion direction here is maintained by long-range surface interactions. This behavior is also observed in circular diffusion paths. From theoretical simulations, Stepanyuk et al [153]. have proposed a mechanism of nanostructure growth based on confinement of surface-state electrons inside quantum corrals. Recently, Cao et al [29]. have experimentally demonstrated the diffusion of Gd atoms in a 30 nm wide circular corral formed by Fe atoms on a Ag(1 1 1) surface. The diffusion of Gd here closely follows the LDOS near the Fermi level and form ringlike structures with ∼3.8 nm distance. These experiments demonstrate that precise control of atom or molecule diffusion processes can be achieved using atomically assembled nanostructures.

atoms on a Cu(1 1 1) surface. Here, a magnetic Co atom was placed at one of the focal point. The Kondo signature generated from the many body interactions between the spin of this Co atom and the free electrons host inside the corrals is then captured at the second focal point, which does not have an adsorbed Co atom. This coherent projection phenomenon, which is dubbed ‘quantum mirage’, is widely explored by theory [125, 140–151]. One of the applications of quantum corrals is the controlled diffusions of atoms and molecules. For instance, a controlled diffusion of sexiphenyl molecules has been demonstrated inside a linear electron resonator (an open linear corral) on a Ag(1 1 1) surface [24] (figures 11(a) and (b)). A significant part of this experiment is that the entire resonator structure is constructed by using native substrate atoms extracted locally on a Ag(1 1 1) surface with the STM tip (figure 11(a)) [24, 50, 70, 152]. Ag atoms in this linear corral are positioned along a [1 1 0] direction of the Ag(1 1 1) surface with six atomic distances apart. The two atomic walls here generate straight electron standing wave patterns with ∼3.8 nm distance between the nearest maxima, which is half of the Fermi wavelength. The diffusion process of sexiphenyl molecules is activated by excitation of the molecules with the STM tip. To demonstrate a linear trajectory of the molecular diffusion, two target Ag atoms are positioned at the opposite end of the resonator (figure 11(b)). During this process, the molecules diffuse along atomically straight lines inside the corral guided by the confined standing wave patterns [24].

5.3. Atomically assembled devices

Atomic level assembly processes can also be useful for the design and construction of novel devices. Some of the atomic scale devices built by using STM manipulation schemes include two and three terminal sorter circuits, which are called ‘molecular cascades’, and an atomic scale spintronic logic device (figure 12). 12

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Figure 12. (a) Molecular cascade. Reprinted with permission from [22]. Copyright 2002 The American Association for the Advancement of Science. (b) Atomic scale all-spin base logic device. Reprinted with permission from [117]. Copyright 2011 The American Association for the Advancement of Science.

islands are chosen to have different sizes and they have different coercivities, 1.75 T and 0.4 T, respectively. Therefore, their states can be switched independently by varying an external magnetic field strength and polarity. The input and output states in the device are sensed by a magnetically coated STM tip. This device can be operated as OR, NOR, AND and NAND functions (figure 12(b)), and it does not use electron flow for operation. Therefore, the logic operations here are solely on the information transport via spin interactions between the artificially engineered atomic chains. These experiments demonstrate that atomic level assembly is an extremely robust process to explore novel device concepts.

Two and three input terminal sorter circuits are formed by assembling CO molecules on Cu(1 1 1) surface using STM manipulation [22] (figure 12(a)). CO molecules position on Cu(1 1 1) surface with the carbon atom is located down toward the surface while the oxygen atom is on top of it. When the surface temperature is lower than 6 K, the thermal motion of CO molecules is quenched. Then, the motion of CO molecules can be initiated by quantum tunneling with the STM tip in a controlled manner. In these circuits, the hopping of a CO molecule triggers a cascade motion of other CO molecules. Hence, the assembled structures are named ‘molecular cascades’. In another experiment, an atomic scale spin-based logic device has been demonstrated by Khajetoorians et al [118]. (figure 11(b)). The device here is designed to transmit the information of the spin state of Co input islands to the gate area via Ruderman–Kittel–Kasuya–Yosida (RKKY) coupling in the chains. In this device, two ferromagnetic Co islands on Cu(1 1 1) surface are used as inputs and two individual Fe atom chains (the spin leads), each having five Fe atoms placed at 0.923 nm apart, are connected to Co islands. The Fe atom distances are adjusted to maximize an antiferromgnetic RKKY coupling. The gate region in this device is composed of two end atoms from each spin lead and an output atom, where the logic operation is performed. The output atom here is positioned 1.35 nm distance from the two end Fe atoms of the opposite spin leads, and thus it is in an equilateral triplet arrangement. Depending on the spin direction (i.e. spin up or spin down), the Fe atoms have two different states: ‘0’ and ‘1’. Because of the antiferromagnetic coupling in the Fe atom chains, the end atom transmits a non-inverted or inverted signal depending on the even or odd number of atoms in the chain. The two input Co

6. Future outlook

Atomic level assembly is the most precise technique to date for atom-by-atom constructions of nanostructures on material surfaces. Atomic manipulation combined with tunneling spectroscopic schemes (such as conductance spectroscopy (dI /dV ), inelastic tunneling spectroscopy (d2 I /dV 2 ), as well as spin-polarized scanning tunneling microscopy and spectroscopy) opens an exciting new avenue of research for atomic level assembly processes, and will enable them to explore the uncharted world of quantum phenomena locally. Here, the same STM tip that can be used to image can also be used to construct atomic structures. In addition, the same STM tip is again used to probe the electronic, magnetic and mechanical properties of the engineered atomistic structures. The integration of imaging, manipulation and spectroscopy in an atomic scale setting makes the STM an extremely robust tool for science. In many cases, atomic level assembly can 13

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produce textbook like results, thereby providing invaluable experimental information to test and improve theoretical frameworks. Although some of the quantum corral structures (like square, circle, ellipse, etc.) and phenomena like mirage and quantum confinement have been studied using atomic level assembly, many more research directions have yet to be pursued. For instance, a number of interesting quantum corrals on different substrate hosts, such as superconductors and topological insulators, have been theoretically proposed [134–136] but have yet to be explored experimentally. New corral structures (like the parabolic corral) [137] and potential applications of corral as devices [138] or quantum projection [135] have been proposed. Moreover, magnetic phenomena including Kondo screenings, and spin interactions inside quantum corrals with various shapes [154–158] have also been explored theoretically but follow up experiments are still needed. Using the atomic level assembly, only one and 2D structures have been demonstrated on surfaces and three dimensional structures have yet to be realized. Thus, new research directions using atomic assembly processes with an STM tip are completely wide open. On the other hand, as the scale of devices continues to shrink, the quest for smaller and faster devices for information storage, computation and other technological applications is increasingly critical. The possible use of single atoms as atomic scale memory, computation, as well as electronic and spintronic device applications have been actively pursued [81, 92, 118, 159–161]. As demonstrated, atomic level assembly using an STM tip on materials surfaces can play a vital role in the development of novel device concepts or in the demonstration of atomic scale devices. However, because of its slow speed and sub-atomic precision requirements, industrial application of STM base atomic level assembly processes is still not a realistic goal. Moreover, the success of atom-byatom assembly using an STM tip requires a stable STM tip apex that is suitable for the atom manipulation process. Although the tip-apex can be locally reshaped in some cases, such as on fcc metal surfaces (see section 3), this is currently realized by a trial and error basis. If atom-by-atom assembly processes were to be used in industry, massively parallel and automated atom manipulation schemes, as well as the technique to fabricate reliable STM tips for atomic manipulation will be required to be developed. In addition, the constructed atomic structures should remain stable at room temperature or higher. Although several room temperature atom manipulation processes have already been demonstrated using STM [17, 162] and atomic force microscopy (AFM) [163, 164], most of the atomically assembled device structures to date have been demonstrated on metallic surfaces using enticing local surface properties, such as a 2D electron gases. Therefore, they may not be suitable for actual useful devices. Thus, in the near future, STM base atomic level assembly will most likely remain as an incredible technique in the exploration of novel quantum phenomena or testing novel device concepts on materials surfaces.

Acknowledgments

We acknowledge financial support by the US Department of Energy, Office of Science, Office of Basic Energy Sciences grant, DE-FG02-02ER46012. Use of the Center for Nanoscale Materials was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract no DE-AC02-06CH11357. References [1] Binning G, Rohrer H, Gerber Ch and Weibel E 1982 Phys. Rev. Lett. 49 57 [2] Binning G, Rohrer H, Gerber Ch and Weibel E 1982 Phys. Rev. Lett. 50 120 [3] Binning G, Rohrer H, Gerber Ch and Weibel E 1982 Appl. Phys. Lett. 40 178 [4] Tersoff J and Hamann D R 1985 Phys. Rev. B 31 805 [5] Gauthier S 2000 Appl. Surf. Sci. 164 84 [6] Hla S W 2005 J. Vac. Sci. Technol. B 23 1351 [7] Tseng A A and Li Z 2007 J. Nanosci. Nanotechnol. 7 2582 [8] Hla S W 2008 Japan. J. Appl. Phys. 47 6063 [9] Eigler D M and Schweizer E K 1990 Nature 344 524 [10] Stroscio J A and Eigler D M 1991 Science 254 1319 [11] Lyo I W and Avouris Ph 1991 Science 253 173 [12] Meyer G, Neu B and Rieder K H 1995 Appl. Phys. A 60 343 [13] Zeppenfeld P, Lutz C P and Eigler D M 1992 Ultramicroscopy 42–44 128 [14] Crommie M F, Lutz C P and Eigler D M 1993 Science 262 218 [15] Heller E J, Crommie M F, Lutz C P and Eigler D M 1994 Nature 369 464 [16] Crommie M F, Lutz C P, Eigler D M and Heller E J 1995 Physica D 83 98 [17] Sloan P A, Hedouin M F G, Palmer R E and Persson M 2003 Phys. Rev. Lett. 91 118301 [18] Crommie M F, Lutz C P, Eigler D M and Heller E J 1996 Surf. Sci. 361–362 864 [19] Manoharan H C, Lutz C P and Eigler D M 2000 Nature 403 512 [20] Kliewer J, Berndt R and Crampin S 2001 New J. Phys. 3 22.1 [21] Braun K-F and Rieder K-H 2002 Phys. Rev. Lett. 88 096801 [22] Heinrich A J, Lutz C P, Gupta J A and Eigler D M 2002 Science 298 1381 [23] Hla S W, Braun K F and Rieder K H 2003 Phys. Rev. B 67 201402R [24] Hla S W, Wassermann B, Braun K-F and Rieder K-H 2004 Phys. Rev. Lett. 93 208302 [25] Moon C R, Lutz C P and Manoharan H C 2008 Nature Phys. 4 454 [26] Negulyaev N N, Stepanyuk S, Niebergall L, Bruno P, Hergert W, Repp J, Rieder K-H and Meyer G 2008 Phys. Rev. Lett. 101 226601 [27] Moon C R, Mattos L S, Foster B K, Zeltzer G and Manoharan H C 2009 Nature Nanotechnol. 4 167 [28] Gomes K K, Mar W, Ko W, Guinea F and Manoharan H C 2012 Nature 483 306 [29] Cao R X, Miao B F, Zhong Z F, Sun L, You B, Zhang W, Wu D, Hu A, Bader S D and Ding H F 2013 Phys. Rev. B 87 085415 [30] Nilius N, Wallis T M and Ho W 2002 Science 297 1853 [31] Wallis T M, Nilius N and Ho W 2002 Phys. Rev. Lett. 89 236802 [32] Nilius N, Wallis T M, Persson M and Ho W 2003 Phys. Rev. Lett. 90 186102 [33] Nilius N, Wallis T M, Persson M and Ho W 2003 Phys. Rev. Lett. 90 196103 [34] Nazin G V, Qiu X H and Ho W 2003 Phys. Rev. Lett. 90 216110 14

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