Half Unit Cell Research Articles

Existence probabilities of single Si atoms diffusing in Si(111)-(7 × 7) half-unit cells at room temperature

Existence probabilities of single Si atoms diffusing in Si(111)-(7 × 7) half-unit cells at room temperature

Interface atomic structures in a cadmium arsenide/III–V semiconductor heterostructure

The interface atomic structure between an epitaxial thin film of the prototype topological semimetal cadmium arsenide (Cd3As2) and a III–V semiconductor layer is investigated using high-angle annular dark-field imaging in an aberration-corrected scanning transmission electron microscope. We find that the interface unit cell adopts a defined stoichiometry that is CdSb-like, which is achieved through the insertion of periodically arranged Cd vacancies in the terminating Cd-plane of Cd3As2. This interface stoichiometry is consistent with the Sb-termination of the III–V layer and the fact that CdSb is the thermodynamically stable phase in the Cd–Sb binary system. We find at least two distinct alignments of the film with respect to the buffer layer, which are characterized by a 14100Cd3As2 shift parallel to the interface. We show that steps of half unit cell height in the III–V layer can produce these distinct interface structures.

Influence of fluorine content of potassium trioctahedral micas on plant availability of interlayer potassium and effectiveness to reduce radiocesium uptake by paddy rice

Characterization of 10 potassium trioctahedral micas alongside pot cultivation of paddy rice was conducted here to elucidate the influence of F content on the plant availability of non-exchangeable K in the interlayers of these micas. Additionally, their effectiveness as soil amendments in reducing radiocesium uptake by paddy rice was investigated. Chemical analysis confirmed that all micas were potassium trioctahedral type, whilst F content varied between 0.09 and 1.24 per half-unit cell. The application of potassium trioctahedral micas did not increase soil exchangeable K before planting; however, exchangeable K after harvest was higher in mica-amended pots than in control pots, except for two micas. This indicated the release of non-exchangeable K during the growing season of paddy rice. Furthermore, the soil K status during the later growth stage was reflected by higher K uptake by rice shoots and lower radiocesium transfer to brown rice compared with the control, thus suggesting the effectiveness of phlogopite and biotite as soil amendments for increasing K uptake by paddy rice, whilst reducing radiocesium uptake. The strong positive correlation of tetraphenylborate-extractable K with soil K status during the late growth stage with K uptake by rice shoots suggested that the plant availability of non-exchangeable K in potassium trioctahedral mica amendments could be evaluated through tetraphenylborate extraction. An increase in the F content of Fe-poor potassium trioctahedral micas resulted in decreased soil K status during the late growing season, alongside decreased K uptake by shoots and increased radiocesium transfer to brown rice. However, the negative influence of F on the plant availability of non-exchangeable K appeared to be less significant in Fe-rich potassium trioctahedral micas than in Fe-poor ones.

Automatic drift compensation for nanoscale imaging using feature point matching

An implementation of drift compensation for imaging at the nanoscale is presented. The method is based on computer vision techniques and hence applicable to any microscope that generates images through a computer interface. The algorithm extracts and matches pairs of feature points from consecutive images to compute and compensate for probe–sample misalignments over time. The protocol also applies selection rules that enable it to withstand significant changes in image contrast. We demonstrate our fully automatic implementation by continuously imaging the same area of a Si(100) surface at the atomic scale with scanning probe microscopy over a period of 25 h at room temperature, showing that the method is robust even under the presence of non-linear drift or spontaneous changes of the probe apex. We apply our method to study the movement of pairs of tin atoms confined within a half-unit cell of the Si(111)-(7 × 7) surface and estimate the energy barrier for their diffusion at room temperature.

Molecular Modeling to Predict the Optimal Mineralogy of Smectites as Binders of Aflatoxin

AbstractNumerous experiments have verified that smectites can adsorb aflatoxin B1 (AfB1) effectively and the efficiency of this process depends heavily on the chemical, physical, and mineralogical characteristics of the smectite. Several relationships between these characteristics and AfB1 sorption have been determined experimentally, but the molecular mechanisms underlying these were not investigated. In the current study the effects of charge density, type of exchange cation, and charge origin (octahedral vs. tetrahedral) on AfB1 sorption on smectites were analyzed by a series of molecular simulations. The calculations confirmed the formation of water bridges between carbonyl groups of AfB1 molecules and interlayer cations. Flat orientation of AfB1 molecules on smectite surfaces was also confirmed. For larger amounts of AfB1 molecules in the intercalates, self-association of two AfB1 molecules bound by π–π interaction was shown. The thermodynamics of AfB1 sorption depends heavily on the water content in the structure, being optimal for basal distances corresponding to two layers of water. A clear preference for sorption of AfB1 on smectites with bivalent cations (Ba2+, Ca2+) and an octahedral origin of its layer charge was confirmed and this was explained as steric hindrance between hydrated ions and AfB1 molecules, which tend to lie flat on smectite surfaces devoid of ions. Ba-montmorillonite with a charge of 0.4 per half unit cell was shown to have the smallest and thus the best potential energy of adsorption compared to the other layer charges.

Engineering the stoichiometry of a TiO2-rich SrTiO3(001) surface

We investigate the stoichiometry of a TiO2-rich SrTiO3(001) surface by depositing less than one monolayer of SrO under typical epitaxy conditions. We find that this deposition yields atomically flat islands on the step-and-terrace surface. All these islands have a height of exactly one full SrTiO3 unit cell, instead of the expected half unit cell. Reflection high-energy electron diffraction indicates that a reconstruction occurs on the bare surface, which disappears with SrO deposition. To explain these results, we propose a simple model with two key elements: (i) a TiO2 double layer must intrinsically be present on the as-prepared SrTiO3(001) surface, and (ii) with deposited SrO, this top layer rearranges itself to form islands with a height of one unit cell.

A study of nanoconfined water in halloysite

Recent years have witnessed a significant level of interest on the properties and applications of halloysite. Despite the widespread interest, little is known about the nature of nanoconfined H2O and its role in governing the dehydration of halloysite (10 Å) to halloysite (7 Å). This is because most studies have utilized halloysite that has irreversibly transformed from fully hydrated halloysite (10 Å) to halloysite (7 Å). This study examined a naturally occurring fully hydrated halloysite (10 Å) that was collected in a wet state and preserved in a sealed desiccator. Thermogravimetric (TGA/DTG) and low-temperature differential scanning calorimetry (LT-DSC) were used to quantify the amount of H2O present in halloysite (10 Å) as interlayer water (ILW), lumen water (LUW) and unconfined water external to the halloysite tubes (UCW). The LT-DSC results, in particular, identified two exothermic freezing peaks and two endothermic melting peaks that could distinguish between unconfined and lumen H2O. H2O confined in the lumen is characterized not only by a depression in the freezing temperature, but also by a significant reduction in the freezing enthalpy. Consistent with work on other phyllosilicates, interlayer H2O in halloysite (10 Å) remains unfrozen, as detected by DSC scans down to −75 °C. For fully hydrated halloysite (10 Å), the half-unit cell formula is Si2Al2O5(OH)4 (H2O)2(ILW)(H2O)3.4–5.0(UCW+LUW). LT-DSC revealed that about 2.31 ± 0.09 molesH2O/molekaol of the 5.4–7.0 molesH2O/molekaol remain unfrozen down to −75 °C, consistent with 2.0 molesH2O/molekaol assigned to the interlayer and 0.31 assigned to the δ-layer on the hydrophilic surface of the inner lumen. Dehydration experiments using LT-DSC, XRD and ATR-FTIR showed that unconfined + lumen H2O was lost rapidly, with a slower release of ILW + δ-layer. ATR-FTIR spectra of dehydrated halloysite exhibited a significant shift in the δ(H-O-H) bending band of sorbed H2O, also termed hole water, to a higher frequency with a concomitant narrowing. Compared to unconfined water, which was indistinguishable from bulk water, these isolated H2O molecules trapped in siloxane ditrigonal cavities are more hydrogen bonded and more ordered.

Measurement of local contact potential difference of atomic scale Au/Si(111)-(7×7) delocalized adsorption state in room-temperature and ultra-high vacuum environment

The structural properties and local contact potential difference of Au on Si(111)-(7×7) surface are studied by the homemade ultra-high vacuum non-contact Kelvin probe force microscope. Although scanning tunneling microscopy has been widely used to study the metal- adsorbed semiconductor surfaces on an atomic scale, the tunnel current measured by scanning tunneling microscopy is easy to lead the charge states to accidentally switch in the measurement process, and it is limited only to the observation of metal and semiconductor surfaces. Kelvin probe force microscope allows us to directly measure the charges at different positions of various flat surfaces by local contact potential difference on an atomic scale, which has become a more convenient and accurate means of charge characterization. In this paper, the topography and local contact potential difference of Au adsorbed Si(111)-(7×7) surface are measured on an atomic scale by Kelvin probe force microscope at room temperature, and the corresponding adsorption model and first principle calculation are established. The differential charge density distribution of the stable adsorption position of Au/Si(111)-(7×7) is obtained, and the local contact potential energy difference relationship of the stable adsorption position of Au on Si surface is given, The mechanism of charge transfer between Au atom and Si(111)-(7×7) surface during adsorption is analyzed. The experimental results show that at room temperature, single Au atom will form triangular delocalized adsorption state in the half unit cell of Si(111)-(7×7). The delocalized adsorption state is due to the fact that the moving speed of a single Au atom in the HUC is faster than the scanning speed of Kelvin probe force microscope, and the local contact potential difference measurement of Au/Si(111)-(7×7) adsorbed surface can effectively identify Au and Si atoms. Obviously, this research is of great significance in promoting the development of surface charge precision measurement, and is expected to provide some insights into the charge properties of metal adsorbed semiconductor surfaces.

Organic molecule intercalation sites in Sr2CaCu2Oy superconductor

AbstractThe organic molecule adsorption sites in a Sr2CaCu2Oy (0(Sr)212) half unit cell upon water molecule intercalation were estimated by simulation using Molecular Orbital PACkage (MOPAC) program. For water molecules, the unit cell was elongated with increasing the number of molecules. Upon four water molecules intercalated between the two SrO layers, the upper Sr atom moved along the c‐axis by 1.49 Å. This value was smaller than but comparable to the experimentally obtained elongation of half of the c‐parameter. The primary reason of the elongation is the repulsive force between Sr–H atoms in the two SrO layers. For ethanol and acetone molecules, stable intercalation sites were not found in the 0(Sr)212 crystal. These results coincided to the experimental results, which did not show intercalations. It was concluded that MOPAC can reproduce the organic molecule intercalation phenomena in oxide crystals.

Synthesis of ferrian and ferro-saponites: Implications for the structure of (Fe,Mg)-smectites formed under reduced conditions

Abstract Clay minerals are widely distributed on the surface of Earth, Mars, and Ceres in the solar systems. Among numerous clay minerals, smectites can record the history of the environment through the exchange of interlayer cations with those in water or through redox reactions with the atmosphere. Therefore, characterization of chemical compositions and crystal structures of smectites is crucial for revealing the paleoenvironment. For instance, the crystal structure within octahedral sheets of iron-bearing smectites changes to trioctahedral sheets under reduced or dioctahedral sheets under oxidizing conditions. Orbital infrared and X-ray diffraction (XRD) analyses by Mars orbiters/rovers revealed the presence of (Fe,Mg)-smectites on the surface of Mars; however, it has been difficult to characterize the properties of these (Fe,Mg)-smectites, which are rare on the surface of Earth. In this study, we synthesized ferrian (ferric ion-rich) and ferrous (ferrous ion-rich) (Fe,Mg)-saponite and revealed the effect of valence states and iron contents on the crystal structures. These saponites were synthesized using a hydrothermal method under reduced conditions. The crystal structures and valence states of iron were analyzed by XRD, Fourier-transform infrared spectroscopy, transmission electron microscopy, Mössbauer spectroscopy, and X-ray absorption near edge measurements. The synthesized clays were trioctahedral swelling clays and were identified as saponites. The valence state of iron in these synthesized saponites is altered by oxygen and a reducing agent in water; however, the trioctahedral structures are maintained under both oxidizing and reduced conditions, following a reversible reaction. This mechanism can be interpreted by the desorption and adsorption of hydrogen in the hydroxyls of the octahedral sheets of the smectite layers. The maximum basal spacing of the (02l) lattice plane in the octahedral sheets was defined by compiling various smectite data. When the basal spacing of (02l) is larger than the maximum in dioctahedral smectites, smectite can be identified as trioctahedral smectite. The redox state of iron in the octahedral sheet cannot be determined from the basal spacing of (02l). We revealed that the iron content in the trioctahedral sheet has a linear relationship with lattice parameter b. This provides a method to estimate the iron content in saponite from XRD data. The XRD profiles of smectites found at the Yellowknife Bay on Mars can be explained only by trioctahedral smectites, and the iron content in the octahedral sheet is roughly estimated to be 0.5–1.7 in a half-unit cell. These results indicate that the presence of (Fe,Mg)-saponite implies a reduced environment during the formation and that this iron-bearing saponite has both oxidation and reduction capabilities depending on the environment.

Commensurate-incommensurate transition and strain relief patterns in monolayer C60 on Cd(0001)

We have studied the commensurate-incommensurate phase transition, rotation epitaxy, and strain relief patterns in ${\mathrm{C}}_{60}$ overlayers grown on Cd(0001) with a low-temperature scanning tunneling microscopy. When deposited at low temperature (\ensuremath{\sim}200 K), ${\mathrm{C}}_{60}$ molecules form the 10\ifmmode\times\else\texttimes\fi{}10 high-order commensurate (HOC) phase with two different half-unit cells, resembling the $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface. Postannealing at room temperature (RT) results in the transition from HOC phase to incommensurate phase, in which strain relaxation takes place in the form of periodic vacancies and ${\mathrm{C}}_{60}$ heptamer arrays. In the case of RT deposition, parallel stripe domain walls appeared in the commensurate $2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3}$ R30\ifmmode^\circ\else\textdegree\fi{} phase. These results provide essential information for understanding the strain relaxation mechanism, and the role of substrate temperatures in the process of ${\mathrm{C}}_{60}$ thin films growth.

Effect of Montmorillonite Layer Charge on the Thermal Stability of Bentonite

AbstractThe thermal stability of bentonite is vitally important for its application in the casting field and the layer charge of montmorillonite (Qm) is one of its central crystal-chemical parameters. As the main component of bentonite, the influence of Qm on montmorillonite properties and behavior needs to be considered if bentonite is to be used in high-temperature environments. The objective of the current study was to investigate the relationship between Qm and the thermal stability of Chinese bentonite samples collected from Wuhu, Anhui Province (marked as WH); Xinyang, Henan Province (marked as XY); and Santai, Sichuan Province (marked as ST) below. The values of Qm were obtained using the O (11) method, and the structural properties of the bentonite samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry-differential scanning calorimetry (TG-DSC), and field emission scanning electron microscopy (FESEM). The results showed that, in the samples investigated, Qm was inversely related to the thermal stability of bentonite. The Qm value (electrons per half unit cell, e/huc) was greatest for sample ST (0.725 e/huc), followed by sample XY (0.470 e/huc), and by sample WH (0.354 e/huc). The dehydroxylation temperature changed related to Qm; the sample with the largest Qm value was WH (701°C), followed by sample XY (684°C), and sample ST (630°C). After the samples were calcined at 600°C, sample WH had the best montmorillonite structural integrity with the greatest degree of reusability (78.21%); while the montmorillonite structures of samples XY and ST were destroyed, and their reusabilities were only 9.48 and 6.01%, respectively.

Structure of glycerol-Mg2+-smectites/vermiculites complex based on molecular dynamics and implementation of the model for X-ray diffraction modeling

Smectites and vermiculites can be distinguished from each other after Mg2+-saturation and solvation with glycerol (GLY). Smectites generally expand to c.a. 18 Å, forming a two-layer complex, while vermiculites to c.a. 14.5 Å, forming a one-layer complex. Additionally, the distribution of layer charges within one population of crystallites can lead to mixed-layered one−/two-GLY layer complexes. Thus far, the model of the GLY intercalate has not been implemented in programs used for XRD modeling of clay minerals. The aim of this study was to obtain a model of the structure of Mg2+-GLY-smectite/vermiculite with the help of molecular dynamic (MD) simulations.A wide range of GLY and H2O contents, in the simulated structures of montmorillonites with charges: 0.3, 0.5, and 0.75 per half unit cell (phuc), and beidellite with a charge of 0.75 phuc, were considered.The average atomic density profiles along the z-direction were calculated for all of the simulations. Subsets corresponding to certain basal spacings (plateaus) were chosen as representative for one- and two-layer intercalates. The electronic density profiles of GLY, H2O, and Mg2+ ions were fitted with Gaussian functions. The purpose of the work was to minimize the number of functions in order to achieve a maximally simple, yet flexible model of the interlayer structures.It was possible to determine relationships between positions of the atomic distributions and the basal spacings. All the relationships were implemented in Sybilla and BGMN computer programs. The new model of the complex was used for fitting of diffractograms of Mg2+-montmorillonites and Mg2+-vermiculite with different layer charges.

Scale-free ferroelectricity induced by flat phonon bands in HfO2.

Discovery of robust yet reversibly switchable electric dipoles at reduced dimensions is critical to the advancement of nanoelectronics devices. Energy bands flat in momentum space generate robust localized states that are activated independently of each other. We determined that flat bands exist and induce robust yet independently switchable dipoles that exhibit a distinct ferroelectricity in hafnium dioxide (HfO2). Flat polar phonon bands in HfO2 cause extreme localization of electric dipoles within its irreducible half-unit cell widths (~3 angstroms). Contrary to conventional ferroelectrics with spread dipoles, those intrinsically localized dipoles are stable against extrinsic effects such as domain walls, surface exposure, and even miniaturization down to the angstrom scale. Moreover, the subnanometer-scale dipoles are individually switchable without creating any domain-wall energy cost. This offers unexpected opportunities for ultimately dense unit cell-by-unit cell ferroelectric switching devices that are directly integrable into silicon technology.

Contribution to understand the biomineralization of bones.

The goal is to propose a material scientific hypothesis for the atomic arrangement of calcium phosphates during the mineralization of bones. It was reached by the analysis of bones of healthy and osteoporotic rats using analytical transmission electron microscopic methods. Electron diffraction patterns show hydroxyapatite (HAP) as dominant phase within the mineralized areas. In the electron energy loss spectrum, a double peak of the phosphorous L-edge seems to be a characteristic feature of the phosphorous binding in biological HAP. The hypothesis bases on periodic features on the collagen surface which agree with distances between oxygen atoms in the (200) plane of octacalcium phosphate (OCP). Bridge pillars for the HAP network consist of OCP coupled with a half unit cell on collagen by oxygen-hydrogen bridges. Possibly, the metastable OCP bridges are only a transient step, while the mineralization is starting. OCP and HAP couple by similar distances of calcium atoms in an interface close to the (100) planes of the OCP and the HAP network. To reach the perfect overlap of the equidistant Ca atoms, the HAP network has to be rotated by 22.5° around the a-axis, 11.5° around the c-axis of HAP, and 10.1° around an axis perpendicular to a and c. A supercell based on this idea is able to explain the dominance of HAP in the electron diffraction patterns, the arrangement of the (002) lattice planes perpendicular to the collagen fiber axis, and sections of high-resolution TEM images.

High-temperature superconductivity in monolayer Bi2Sr2CaCu2O8+δ.

Although copper oxide high-temperature superconductors constitute a complex and diverse material family, they all share a layered lattice structure. This curious fact prompts the question of whether high-temperature superconductivity can exist in an isolated monolayer of copper oxide, and if so, whether the two-dimensional superconductivity and various related phenomena differ from those of their three-dimensional counterparts. The answers may provide insights into the role of dimensionality in high-temperature superconductivity. Here we develop a fabrication process that obtains intrinsic monolayer crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212; here, a monolayer refers to a half unit cell that contains two CuO2 planes). The highest superconducting transition temperature of the monolayer is as high as that of optimally doped bulk. The lack of dimensionality effect on the transition temperature defies expectations from the Mermin-Wagner theorem, in contrast to the much-reduced transition temperature in conventional two-dimensional superconductorssuch as NbSe2. The properties of monolayer Bi-2212 become extremely tunable; our survey of superconductivity, the pseudogap, charge order and the Mott state at various doping concentrations reveals that the phases are indistinguishable from those in the bulk. Monolayer Bi-2212 therefore displays all the fundamental physics of high-temperature superconductivity. Our results establish monolayer copper oxides as a platform for studying high-temperature superconductivity and other strongly correlated phenomena in two dimensions.

Fabrication of one-dimensional magic cluster arrays using a vicinal surface as a template

One-dimensional arrays of magic In clusters were grown using the Si(557) surface as a template. We find that magic clusters with an In6Si3 stoichiometry grow on the (111) terraces of the Si(557) surface, which are identical to those formed on the planar Si(111)7 × 7 surface. A preferential formation of the clusters on faulted half unit cells of the 7 × 7 reconstruction enables the growth of two different types of one-dimensional arrays with different cluster-cluster distances. A delayed onset of the cluster growth with an initial formation of atomic wires is found, in contrast to magic In clusters on the planar Si(111) surface.

Formation of clay minerals on Mars: Insights from long-term experimental weathering of olivine

Laboratory experiments are useful to constrain the environmental parameters that have allowed the formation of the ancient hydrous mineralogical assemblages observed at the surface of Mars, which are dominated by ferric smectites. Weathering under a dense CO2 atmosphere on early Mars is a process frequently invoked to explain their formation, but has proven difficult to test in the laboratory due to low reaction rates. Here, we present a long-term weathering experiment (470 days, at 45 °C) of forsteritic olivine specially designed to increase as much as possible the amount of reaction products and thus allow their detailed mineralogical, petrological and chemical characterization by FTIR, SEM and TEM. Our results show the formation of crystalline smectites both under 1 bar of CO2 and under ambient air. However, important differences are observed between the two types of conditions. The smectite formed under CO2 has an average chemical formula per half unit-cell of Si3.92Al0.16Fe3+0.78Mg1.66 Cr0.01Ni0.06K0.04Ca0.04.O10(OH)2. It is thus intermediate between a trioctahedral Mg-rich saponite and a dioctahedral ferric smectite. It is also clearly enriched in Fe compared its counterpart formed under ambient air, which has an average chemical formula per half unit-cell of Si3.68Al0.12Fe3+0.37Mg2.61Cr0.01Ni0.02K0.04Ca0.25.O10(OH)2. This result demonstrates that the enrichment in Fe observed for Martian smectites is to be expected if they were formed by low-temperature weathering under a dense CO2 atmosphere. Another difference is the nature of the accompanying phases, which includes amorphous silica (in the form of opal spheres 10 to 100 nm in diameter) and Mg-carbonates under CO2, but is limited to rare kaolinite under ambient air. The observation of kaolinite particles under air and the significant amount of Al measured in smectites under both atmospheres, despite the Al-poor nature of the initial material, shows that this element is easily concentrated by low-temperature weathering processes. At a larger scale, this concentration mechanism could be responsible for the formation of Al-rich upper horizons, as frequently observed on Mars.

Initial Adsorption of Fe on an Ethanol-Saturated Si(111)7 × 7 Surface: Statistical Analysis in Scanning Tunneling Microscopy

We studied the initial stage of iron deposition on an ethanol-saturated Si(111)7 × 7 surface at room temperature using scanning tunneling microscopy (STM). The statistical analysis of the Si adatom height at empty states for Si(111)-C2H5OH before and after the Fe deposition showed different types of adatoms: type B (before the deposition) and type B′ (after the deposition) assigned to bare adatoms, type D and type D′ to C2H5O-terminated adatoms, and type E′ to adatoms with Fe. The analysis of the height distribution revealed the protection of the molecule termination for the Fe capture at the initial stage. The analysis also indicated the preferential capture of a single Fe atom to a bare center-adatom rather than a bare corner-adatom which remain after the C2H5OH saturation, but no selectivity was observed in faulted and unfaulted half unit-cells. This is the first STM-based report proving that a remaining bare adatom, but not a molecule-terminated adatom, captures a metal.

Atom manipulation method to substitute individual adsorbate atoms into a Si(111)-(7 × 7) substrate at room temperature

We demonstrate a method to substitute individual adsorbate atoms into a Si(111)-(7 × 7) substrate using the tip of an atomic force microscope (AFM) at room temperature. We show that single Sn atoms diffusing within the half-unit-cells (HUCs) of the Si(111)-(7 × 7) substrate can be substituted into Si adatom sites via a close approach of the tip, whereby the intrinsic Si adatoms are ejected onto the surface of the adjacent HUCs. The Sn atom substitution sites can be precisely controlled by the approach of the AFM tip toward the surface at certain positions near the boundary of the HUCs but slightly shifted away from the HUC with the diffusing Sn atom. This manipulation method is also demonstrated to replace Si adatoms in the Si(111)-(7 × 7) surface with Pb using scanning tunneling microscopy. This method can provide a way to induce single-atom substitutional doping at certain positions from an adsorbate atom diffusing within a confined space provided by a substrate, which would allow for control of the doping sites in nanostructural materials.