Vacancy Islands Research Articles

Formation Two Dimensional Negative Islands At High Rate Cooling Of Ultra-Flat Surface Si(111)

In situ ultrahigh vacuum reflection electron and ex situ atomic force microscopy have been applied to investigate morphology transformations of the ultra-flat stepped Si(111) surface with wide (20–50 µm in diameter) singular terraces during sublimation and quenching from elevated temperatures. The formation of two dimensional negative (vacancy) islands has been observed on the wide terraces after the quenching from temperatures above 1 200°C. The increasing of the critical terrace size for the two-dimensional negative island nucleation has been explained by the changing of the atomic mechanism of mass transport on silicon surface.

Atomic structure and surface defects at mineral-water interfaces probed by in situ atomic force microscopy.

Atomic scale details of surface structure play a crucial role for solid-liquid interfaces. While macroscopic characterization techniques provide averaged information about bulk and interfaces, high resolution real space imaging reveals unique insights into the role of defects that are believed to dominate many aspects of surface chemistry and physics. Here, we use high resolution dynamic Atomic Force Microscopy (AFM) to visualize and characterize in ambient water the morphology and atomic scale structure of a variety of nanoparticles of common clay minerals adsorbed to flat solid surfaces. Atomically resolved images of the (001) basal planes are obtained on all materials investigated, namely gibbsite, kaolinite, illite, and Na-montmorillonite of both natural and synthetic origin. Next to regions of perfect crystallinity, we routinely observe extended regions of various types of defects on the surfaces, including vacancies of one or few atoms, vacancy islands, atomic steps, apparently disordered regions, as well as strongly adsorbed seemingly organic and inorganic species. While their exact nature is frequently difficult to identify, our observations clearly highlight the ubiquity of such defects and their relevance for the overall physical and chemical properties of clay nanoparticle-water interfaces.

Argon ion irradiation induced morphological instability of bare and thiol-functionalized Au(111) surfaces.

Ar ion irradiation-induced changes in the morphology of bare and 1-dodecanethiol self-assembled monolayer (SAM) covered Au(111) surfaces have been investigated systematically. The changes were followed by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) measurements while varying the ion charge (Ar(+),Ar(4+)), energy (10-40 keV) and fluency (10(12)-10(13) ions per cm(2)). The impact of flame-annealing of the Au(111) surface on subsequent ion bombardment was considered and more prominent related surface morphology changes were noted. The irradiation of Au(111) surfaces generated Au vacancy and adatom islands and caused roughening of step edges. The size and abundance of these islands and the level of deformation on the step edges depended strongly on the ion energy and fluency. In case of the SAM functionalized surface, the gold vacancy islands present on the surface already from the SAM formation were modified, step edges roughened and gold adatom islands formed. Similarly to the bare surface, the level of surface deformation increased as a function of ion energy and fluency. The Ar(4+) irradiation caused on the average slightly larger vacancy islands on the SAM modified surfaces than the Ar(+) irradiation. Irradiation to fluency of 10(12) ions per cm(2) mostly maintained standing-up orientation of the thiolates whereas irradiation to higher fluency resulted in reduced surface coverage and flat-lying molecules. As a general trend the DDT covered surfaces were more susceptible for irradiation-induced surface morphology changes than the unmodified Au surfaces.

Structure and Electronic and Charge-Transfer Properties of Mercaptobenzoic Acid and Mercaptobenzoic Acid–Undecanethiol Mixed Monolayers on Au(111)

The surface structure, molecular conductance, and charge-transfer properties of pure mercaptobenzoic acid (MBA) and mixed MBA–undecanethiol (UDT) self-assembled monolayers (SAMs) on Au(111) are studied by scanning tunneling microscopy, scanning tunneling spectroscopy, electrochemical techniques, and density functional theory calculations. MBA forms ordered diluted lattices without evidence of Au adatom complexes at terraces (absence of vacancy islands) at low MBA concentrations, whereas disordered lattices with possible formation of these complexes are found at high concentrations (presence of vacancy islands). In addition, some other interesting facts have been established. The stability of the MBA SAMs is improved in mixed MBA–UDT SAMs, revealing aromatic–aliphatic attractive interactions. The mixed SAMs at equivalent amounts of thiols hinder the vacancy island formation typical of the pure UDT SAMs. The MBA SAMs exhibit good molecular conductance with filled states from S and aromatic ring contribution...

Statistical properties of nanosized clusters on a surface in overdamped stochastic reaction-Cattaneo systems

In this work we study an overdamped stochastic reaction-Cattaneo model describing nanosized pattern formation on a surface at monolayer deposition. We study and compare an influence of both primary and secondary mechanisms onto pattern formation processes. The primary mechanisms relate to the rates of chemical reactions and interaction strength of adsorbate; the secondary mechanisms are related to finite atomic disturbance propagation speed and stochastic contribution satisfying fluctuation–dissipation relation. Considering statistical properties of surface structures we discuss transitions between homogeneous phases related to low and high density states. We illustrate that these transitions are accompanied by a formation of adsorbate or vacancy islands. It was found that spherical adsorbate and vacancy islands are characterized by different distribution functions over their sizes for different symmetry of substrate lattice. We have shown that depending on system control parameters island size distributions can change their modality. The size of localized nano-clusters can be controlled by both primary and secondary mechanisms of pattern formation.

Engineering the Band Structure of Nanoparticles by an Incommensurate Cover Layer

The electronic properties of nanometer size Au(111) vacancy islands are modified by covering the islands with an atomically thin NaCl(001) layer. The large-scale commensurability between the atomic lattices of the cover layer and the islands gives rise to various Moire-effect-induced standing wave patterns of which the periodicity and orientation depend on the relative angle between the lattices. The observed standing wave patterns are accompanied by the formation of multiple band gaps resulting from the Moire-effect-related periodic potential. Moreover, the energy level quantization and the coverlayer-induced energy shift give rise to partial depopulation of the NaCl/Au interface state. Targeted use of a thin covering layer and the resulting Moire-effect-related potential offers yet unexploited possibilities to conveniently modify the electronic structure of nanoparticles.

Sublimation of a crystal slowed by sorption of foreign molecules from a gaseous medium

Numerical simulation of the sublimation of the surface of an ice crystal has shown that the presence of a non-condensable gas substantially reduces the sublimation rate. It has been found that the slowing of the sublimation due to the adsorption of gas molecules on the crystal surface that fill the vacancies during sublimation prevents the formation of vacancy islands on the surface of the crystal required for evaporation of the surface molecular layer. The available results of studies that explain the slowing of the sublimation by the presence of a diffusion layer formed in the vapor medium near the sublimating surface are supplemented with new data showing that molecular processes on the crystal surface play an equally important role as the diffusion layer. Cases have been found where crystal sublimation is accelerated by the sorption of gas molecules. The obtained results refining the theory of sublimation can be used to develop methods for controlling sublimation.

Quasiparticle interference scattering of spin-polarized Shockley-like surface state electrons: Ni(111)

We report on a detailed quasiparticle interference (QPI) scattering study of the Ni(111) surface by low-temperature scanning tunneling spectroscopy (LT-STS). While conventional constant-separation STS shows two broad features, which are interpreted as the ${\ensuremath{\Lambda}}_{3}$ bulk band and $sp$-like Shockley-type surface state ($sp$-SS), energy-dependent Fourier-transformed QPI maps reveal the band dispersion of the underlying surface electronic features. We find two electronlike branches in the $sp$-SS dispersion, which are interpreted as the exchange-split minority and majority spin part. The exchange splitting is determined to $\ensuremath{\Delta}{E}_{\mathrm{ex}}=100\ifmmode\pm\else\textpm\fi{}8$ meV. In addition, a holelike $d$-derived surface resonance is found. Band onsets and effective electron masses are determined by fitting the band dispersion with a parabola at small $k$ values. Hybridization effects with bulk electronic states are observed towards larger $k$ values. Prominent quantum confinement phenomena of the $sp$-SS are observed in STS data obtained within vacancy islands. The results can be interpreted within a one-dimensional quantum-well model.

Domain-sensitive in situ observation of layer-by-layer removal at Si(100) in H2 ambient

Double-layer step formation on Si(100) substrates is a crucial prerequisite for antiphase-domain free III–V compound semiconductor heteroepitaxy. Due to its unequaled relevance in microelectronics, the (100) oriented surface of silicon is by far the most studied semiconductor surface. However, Si(100) preparation in hydrogen process gas ambient, which is commonly employed for Si and III–V device preparation, is completely different from preparation in ultra-high vacuum due to strong interaction between H2 and the Si surface, leading to a kinetically driven different step formation. Here, we observe chemical layer-by-layer removal of surface atoms from the terraces at the Si(100) surface during annealing in hydrogen ambient. Mutually perpendicularly oriented dimers on subsequently removed monolayers induce oscillations in the in situ reflection anisotropy spectroscopy (RAS) signal. Scanning tunneling microscopy measurements support a model, where surface atom removal proceeds by formation and anisotropic expansion of vacancy islands on the terraces. We determined an activation energy Ed of 2.75 ± 0.20 eV for Si etching in H2 ambient by transient in situ RAS measurements. In situ control of the highly reactive Si(100) surface preparation is essential for subsequent defect-free III–V heteroepitaxy.

Scanning tunneling microscopy tip-assisted modification of Ti(IV) dithiothreitol self-assembled monolayers on Au(111): restructuring of the gold surface

Au(111) surfaces modified by exposure to a dilute solution of a Ti(IV)/dithiothreitol ligand to metal charge transfer complex (Ti(DTT)2) show a remarkable response to continuous scanning by scanning tunneling microscopy (STM). Vacancy islands on the gold surface, which are formed during the self-assembly of the molecular overlayer, coarsen and even merge during STM scanning at negative sample bias. In certain instances, the vacancy islands even line up to form parallel nanostructures. We believe the effect is due to mechanical interactions between tip and sample, which is enhanced by electrostatic effects. The Ti(DTT)2 complex is anchored to the gold surface via Au−S bonds, but due to the fact that there are multiple thiol groups, there may be “uncoordinated” thiols left “dangling”. The tip-induced modification involves the interaction of the tip with these “dangling” sulfurs, which in turn causes movement of single complex molecules with the attached sulfur-bonded gold. Under negative sample bias, the electric field weakens the binding between the sulfur-bonded gold atoms and the surrounding gold atoms in the surface allowing for the observed tip-induced dynamics. In contrast with the Ti(DTT)2 complex, a similar Ti(IV)/3-mercapto-1,2-propanediol complex (Ti(MPD)2) does not exhibit any tip-induced effects. In this case, there are no dangling sulfurs to interact with the STM tip. In addition, similarly prepared dithiothreitol, dithiothreitol titanium isopropoxide, and 3-mercapto-1,2-propanediol self-assembled monolayers (without dangling sulfurs) do not exhibit the tip-induced effect.

Scanning Tunneling Microscopy of Superfilling in Formula Containing Chloride, Polyethylene Glycol and Bis-3-Sodiumsulfopropyl-Disulfide

In situ scanning tunneling microscopy (STM) was used to study copper deposition at vacancy defects on a copper thin film under potentiostatic conditions at −0.20 V (vs. Ag/AgCl) in a formula containing sulfuric acid, chloride, polyethylene glycol (PEG), and bis-3-sodiumsulfopropyl-disulfide (SPS) – the widely used mixture to facilitate Cu superfilling at recessed features in semiconductor processing. The vacancy island measuring ∼70 nm wide and 12 nm deep sat in the middle of a facetted surface structure at the beginning. Cu deposit nucleated mainly at the rim of the vacancy and grew into stacked Cu(111) facets. These local pyramidal Cu stacks could restructure into wider Cu(111) terraces by transferring Cu atoms rapidly from higher to lower planes. Voltammetric results showed that Cu deposition was suppressed in a plating bath containing 1 mM KCl + 88 μM PEG8000 + 10−7 M SPS. Steps with sharp edges bunched in the course of Cu deposition. The vacancy island was filled with Cu deposit assuming smooth terraces with sharp step edges aligned mainly in the 〈121〉 directions of the Pt(111) electrode, suggesting crystalline packing in the Cu deposit. Atomic-resolution STM imaging revealed a hexagonal array presumed to be the (√3 × √3)R30° – Cl− adlattice.

Study of underpotential deposited Cu layers on Pt(111) and their stability against CO and CO2 in perchloric acid

The underpotential deposition (UPD) of copper on a Pt(111) electrode and the influence of gas coadsorbates, i.e. CO and CO2, on the thus deposited copper layer were studied in a 0.1 M HClO4 electrolyte by means of EC-STM. By UPD, an atomically flat Cu layer is formed, which exhibits a pseudomorphic (1 × 1) structure. However, it contains several point defects due to which its total coverage is less than a monolayer, in agreement with the measured charge density in the CV curves. Upon exposure to a CO-saturated solution the pseudomorphic structure collapses to a coalescent structure with many vacancy islands. This phase transition is induced by the preferential binding of CO to the Pt(111) surface. In contrast, CO2, which binds stronger to copper, does not affect the pseudomorphic structure of the Cu layer.

Are 4-Mercaptobenzoic Acid Self Assembled Monolayers on Au(111) a Suitable System to Test Adatom Models?

The adsorption of 4-mercaptobenzoic acid (MBA) has been studied on Au(111) by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), electrochemical techniques, and density functional theory (DFT) calculations. Results show that MBA molecules adsorb on the Au surface via a thiolate bond, arranged in a (√3×4) lattice with coverage θRS = 0.25. DFT data show the strong effect of aromatic ring interactions in stabilizing the adsorbed molecules. The total absence of vacancy islands upon thiol adsorption and the surface coverage of gold islands after desorption are difficult to conciliate with the usual gold-adatom models for thiol adsorption. This is an important issue, as the strongest evidence for the existence of gold-adatom complexes arises from MBA-capped Au nanoparticles. Our results raise an interesting point as regards the validity of making a straightforward parallel between the thiol–Au interface in single crystal surfaces and in monolayer-protected clusters.

Relaxation dynamics of labyrinthine submonolayer films

To explore the relaxation process of percolating labyrinthine submonolayer films, we observe in real-time the evolution of a submonolayer film with a coverage of ~0.7 monolayer on SrTiO3(001) at 670°C using scanning tunneling microscopy. We find that the characteristic length begins to increase after an incubation time. During the incubation time, percolating grooves are segmented by local pinch-off events predominantly driven by thermal fluctuations. The occurrence frequency of the pinch-off events markedly decreases with an increase in the characteristic length. In the second stage, reshaping of the disconnected irregular vacancy islands occurs by edge diffusion. Finally, small vacancy islands disappear via Ostwald ripening by terrace diffusion.

A Molecular Dynamics Study of the Role of Adatoms in SAMs of Methylthiolate on Au(111): A New Force Field Parameterized from Ab Initio Calculations

Starting from ab initio calculations and using a force matching procedure, we have developed a new force field for molecular dynamics simulations of self-assembled monolayers of methylthiolate (MT) on Au(111) surfaces. This new force field is able to reproduce several observed features of SAMs of MT on Au(111) surface, such as the formation of gold vacancy islands and the (√3 × √3)R30 lattice. We have studied the dynamics of Au adatoms and monatomic vacancies on the Au(111) surface for the SAM of MT at room temperature. It is observed that monatomic vacancies coarsen to form large vacancy islands while the adatoms group to form clusters. Both results are in agreement with experiments. At elevated temperatures, Au adatoms that are lifted from the surface leave an atomic vacancy on it. The liquid-like diffusion of gold adatoms on the SAM surface occurs by hopping between pairs of methylthiolate to which the adatom is temporarily bound. Our findings indicate that structural models of the c(4 × 2) unit cell i...

Nickel Carbide as a Source of Grain Rotation in Epitaxial Graphene

Graphene has a close lattice match to the Ni(111) surface, resulting in a preference for 1 × 1 configurations. We have investigated graphene grown by chemical vapor deposition (CVD) on the nickel carbide (Ni(2)C) reconstruction of Ni(111) with scanning tunneling microscopy (STM). The presence of excess carbon, in the form of Ni(2)C, prevents graphene from adopting the preferred 1 × 1 configuration and leads to grain rotation. STM measurements show that residual Ni(2)C domains are present under rotated graphene. Nickel vacancy islands are observed at the periphery of rotated grains and indicate Ni(2)C dissolution after graphene growth. Density functional theory (DFT) calculations predict a very weak (van der Waals type) interaction of graphene with the underlying Ni(2)C, which should facilitate a phase separation of the carbide into metal-supported graphene. These results demonstrate that surface phases such as Ni(2)C can play a major role in the quality of epitaxial graphene.

Molecular-Scale Surface Chemistry of a Common Metal Nanoparticle Capping Agent: Triphenylphosphine on Au(111)

Phosphine-stabilized Au clusters have been extensively studied and are used in various applications due to their unique structural, catalytic, and electronic properties. Triphenylphosphine (PPh(3)) is a key stabilizing ligand in the synthesis of Au nanoclusters. Despite its intense use in nanoparticle synthesis protocols, little is known regarding its surface chemistry, monolayer structure, density, and packing arrangement, all of which are important descriptors of functionality. Here, in contrast to sparse earlier investigations, we report that PPh(3) forms very ordered structures on Au(111). Atomic-scale imaging reveals that monolayer formation is accompanied by a partial lifting of the Au(111) surface reconstruction and ejection of extra Au atoms in the surface layer. Interestingly, these atoms are trapped and stabilized as two-dimensional Au nanoislands within the molecular layer. This behavior is in contrast to thiols, also common capping agents, which tend to remove Au atoms beyond those extra atoms present in the native reconstruction and form vacancy islands on the surface. Our data illustrate PPh(3)'s milder reactivity and reveal a new picture of its packing structure. These results shed new light on the surface chemistry of this important ligand for organic, organometallic, and nanoparticle synthesis.

Large-Ordered Striped Phase of Didodecyl Sulfide Self-Assembled Monolayers on Au(111)

Alkanethiol self-assembled monolayers (SAMs) on metal surfaces are key elements for the fabrication of functional organic layers and devices in the broad fields of nanotechnology and biotechnology. However, it was found that alkanethiol SAMs were usually composed of structural defects such as domain boundaries and vacancy islands (VIs), which make them more amenable to oxidation. Compared to alkanethiols, thioethers (RSR') are more robust to oxidation, and their chemical structures with various alkyl chains can be modified readily by a simple synthetic method. Therefore, SAMs prepared using thioethers provide a very useful means for tuning the characteristics of metal surfaces. Despite these advantages, there have only been a limited number of papers involving the formation and structure of thioether SAMs on gold surfaces. It has been revealed that dioctadecyl sulfides (DODS, CH3(CH2)17S(CH2)17CH3) at the initial growth stage form SAMs with striped phases in which the molecular backbones are oriented parallel to the surface, whereas DODS SAMs formed after long immersion have two mixed phases containing closely packed and loosely packed standing-up phases where the molecular backbones are oriented perpendicular to the surface. High-resolution scanning tunneling microscopy (STM) observation revealed that the SAMs of dimethyl sulfides (DMS, CH3SCH3) with the shortest alkyl chains on Cu(111) have a herringbone-like packing structure, whereas dibutyl sulfides (DBS, CH3(CH2)3S(CH2)3CH3) with slightly larger alkyl chains form striped phases. From these results, we assumed that the formation of dialkyl sulfide SAMs strongly depends on van der Waals interactions between alkyl chains. So far, there no data has been reported on the formation and structure of didodecyl sulfides (DDS, CH3(CH2)11S(CH2)11CH3) with medium alkyl chains from a molecular-scale perspective. In this paper, we report the first STM results showing that the adsorption of DDS molecules at 70 oC generating long-range ordered SAMs with a 7.5 × √3 striped phase with VIs-free surfaces. Dodecanethiol (DDT, CH3(CH2)11SH) and DDS were purchased from Tokyo Chemical Industry (Tokyo, Japan). DDT and DDS SAMs were prepared by dipping the Au(111) substrate in 1 mM ethanol solutions of corresponding compounds at room temperature for 24 h. To understand the effect of solution temperature on the formation of DDS SAMs, the SAMs were prepared at 70 C for 1 h. STM images were obtained using a NanoScope E (Veeco, Santa Barbara, CA, USA) with a commercial Pt/Ir (80:20) tip under ambient conditions. The STM images in Figure 1 show remarkable structural differences in the formation of ordered domains and VIs (dark holes) for DDT and DDS SAMs on Au(111) formed after 24 h immersion at room temperature. Figure 1(a) shows a typical packing structure of DDT SAMs with a c(4 × 2) superlattice formed at saturation coverage. DDT SAMs were mainly composed of ordered phases with three domain orientations (Regions A, B, and C) separated by domain boundaries. The proportion of the VI areas to the total surface area was measured to be approximately 8%-12%, values which are similar to those observed from other alkanethiol SAMs. Unlike DDT SAMs, Figure 1(b) shows that the DDS SAMs have two mixed phases: the ordered phase with three domain orientations (Regions A, B, and C) and the disordered phase (Region D). The existence of three domain orientations for DDS SAMs on Au(111) means that the formation of SAMs was influenced by interactions between the sulfide atoms and gold surfaces. On the other hand, DDS SAMs contained few VIs comprising a small (23%) area fraction of the total surface area. The presence of disordered phases and a smaller VI fraction for DDS SAMs can be attributed to the weaker interactions between the sulfide atoms and gold surfaces compared to those between the sulfur atoms of thiols and gold surfaces for DDT SAMs. This finding is consistent with a previous result for the SAMs of DODS with long alkyl chains of 18 carbon units. The ordered domains have a row structure with an inter-row distance of 1.59 ± 0.03 nm, which is nearly half of the entire

Ehrlich-Schwoebel barrier and interface-limited decay in island kinetics on Ag(100)

We investigate diffusion and decay of adatom and vacancy islands on Ag(100) between $\ensuremath{\approx}10$ and $\ensuremath{\approx}1000$ nm${}^{2}$ in size at room temperature by fast scanning tunneling microscopy. Adatom and vacancy islands decay in the diffusion and in the attachment limit, respectively. This adatom-driven kinetics confirms the existence of an Ehrlich-Schwoebel barrier on Ag(100). The dependence of the diffusivity of vacancy islands on island size is consistent with a kink-dominated periphery diffusion mechanism. Quantitative differences to previously published work are discussed.

Adsorption characteristics of OH-terminated alkanethiol and arenethiol on Au(111) surfaces

Adsorption behaviors of hydroxyl-terminated alkanethiol, 3-mercapto-1-propanol (MPO), and arenethiol, 4-mercaptophenol (MPH), on Au(111) electrodes were studied by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM). The effects of the molecular structure and the chain length on the characteristics of the self-assembled monolayers (SAMs) were investigated by comparison with the results of 11-mercapto-1-undecanol (MUO) and 6-mercapto-1-hexanol (MHO) reported in the literature. All the alkanethiol SAMs have the same coverage ratio (0.33 ML). For MUO which has a longer chain length, a hexagonal lattice, the (√3 ×√3) structure was observed. However, for shorter thiols (MHO and MPO), the adsorbed molecules exhibit different contrasts under the imaging of STM, ascribed to the different conformations of adsorbed molecules. The CV results indicated that a longer chain length triggers a SAM with higher adhesion and higher resistance to the charge transfer across a SAM. For the SAM of arenethiol, MPH, the π-π stacking interaction of the phenyl ring leads to a lower surface mobility of thiol/Au complexes, lower coverage ratio, and less uniform structure of the adlayer. Furthermore, the vacancy islands commonly observed on alkanethiol-modified Au(111) electrodes do not appear on the MPH-modified surface. Instead, 2-dimensional patch islands formed on the terrace due to the aggregation of moveable MPH/Au complexes.