Adatom-vacancy Pair Research Articles

(Invited) Nucleation and Growth on Pt(111) during Oxidation-Reduction-Cycles

In order to realize the energy transition, and thus not to leave our children with a future problem, we need to switch to renewables and the introduction of H2-economy. Platinum is of crucial importance for this, as it is widely used as an electrocatalyst in several applications like fuel cells and electrolyzers, due to its high catalytic activity. However, platinum is scarce, expensive, and its economical feasibility is limited by its degradation under oxidizing conditions. Repeated oxidation and reduction of a platinum electrode, which can be compared to the switching ON and OFF of a device, leads to the roughening of the surface, caused by the nucleation and growth of nano-islands, which first grow only laterally (2D) and then exclusively in height (3D) [1,2]. Surprisingly, this particular nucleation and growth combines several remarkable aspects, discussed in this talk, that all deviate from standard text book- and literature-knowledge. It starts with the fact that the growth of these islands is promoted by the creation of adatom-vacancy pairs [3] such that not only growth processes but also etching processes have to be considered. As a precursor of these adatom-vacancy pairs, an adatom gas of so-called “place exchanged” (PE) atoms is created that is fully reversible and does not lead to any roughening, as long as one stays below a critical coverage and thus oxidizing potential [4-7]. With each added PE-atoms the surface stress is increased, until the critical value is reached, atoms are kicked out of the surface, and adatom-vacancy pairs are created [8]. A long ongoing debate about a possible ordering of these PE atoms into spokes (lines) [5-8] or even the famous “spoke wheels” [9] has just been settled with the conclusion that they do not exist [10]. Surprisingly, our most recent analysis of the island-island distance distribution function clearly shows a strong repulsive interaction during the nucleation stage, which can only be explained with the existence of the spokes [11]. Next to this mysterious nucleation, it is striking that the islands first grow only in 2D and upon touching at the base only grow further in height (3D) without any coalescence or Ostwald ripening that is expected to take place, as it would lower the free energy of the system. We unraveled the underlying atomic processes and pinpoint the reason to a growth instability for vacancies that stands in full analogy to the famous mound formation instability for adatoms based on the Ehrlich-Schwoebel barrier [3]. An Ehrlich-Schwoebel barrier for vacancies must exist, and we formulated a complete set of equivalent growth modes for vacancies (etching) next to the well-known homoepitaxial adatom growth modes [3]. Finally, the analytical description of the combined adatom and vacancy growth modes nicely fits our experimental observations, thereby verifying both their existence and our model. [1] Jacobse, L., Huang, Y., Koper, M.T.M., and Rost, M.J., Nat. Mater. 17, 277 (2018)[2] Jacobse, L., Rost, M.J., and Koper, M.T.M., ACS Cent. Sci. 5 (12), 1920 (2019) [3] Rost, M.J., Jacobse, L. and Koper, M.T.M., Nat. Commun. 10, 5233 (2019)[4] H. You, et al. , R. P., Proc. 6th Int. Symp. Electrode Processes (1996)[5] J. Drnec, et al., Electrochimica Acta, 224 220 (2017).[6] M. Ruge, et al., J. of The Electrochem. Soc., 164,9 H608 (2017).[7] M. Ruge, et al., J. of the Amer. Chemi. Soc. 139,12 4532 (2017).[8] Hanselman, S., McCrum, I., Rost, M.J. and Koper, M.T.M., Phys. Chem. Chem. Phys. 22, 10634 (2019).[9] M.A. van Spronsen, et al., Nature Commun. 8 429 (2017), https://doi.org/10.1038/s41467-017-00643-z [10] L. Jacobse, V. Vonk, I.T McCrum, C. Seitz, M.T.M. Koper, M.J. Rost, and A Stierle, Electrochimica Acta, submitted [11] M.J. Rost, L. Jacobse, M.T.M. Koper, Angew. Chem to be submitted

Defect engineering of carbon nanotubes and its effect on mechanical properties of carbon nanotubes/polymer nanocomposites: A molecular dynamics study

Molecular dynamics simulations have been performed to explore the effect of intrinsic defects of carbon nanotubes (CNTs) on the mechanical properties of CNTs/polyethylene (PE) composites. Three kinds of defects including single adatom defect, single vacancy defect and an adatom-vacancy pair defect have been considered here. The longitudinal Young's moduli of individual CNTs and CNTs/PE composites with different defects were calculated. On this basis, the interfacial interaction energy between CNTs and PE matrix and the adsorption behavior of PE molecular chains on the surface of CNTs were characterized. The results show that the elastic properties of individual CNTs and the interfacial characteristics between CNTs and matrix both contribute to the elastic properties of CNTs/PE composites. Meanwhile, due to different interaction energies, there exists three different adsorption modes of PE molecular chains on the surface of CNTs. Compared with the MD simulation results, the rule of mixture does not solidly hold in predicting the Young's modulus of composites for large defect degree.

Vacancy formation on C60/Pt (111): unraveling the complex atomistic mechanism

The interaction of fullerenes with transition metal surfaces leads to the development of an atomic network of ordered vacancies on the metal. However, the structure and formation mechanism of this intricate surface reconstruction is not yet understood at an atomic level. We combine scanning tunneling microscopy, high resolution and temperature programmed-x-ray photoelectrons spectroscopy, and density functional theory calculations to show that the vacancy formation in C60/Pt(111) is a complex process in which fullerenes undergo two significant structural rearrangements upon thermal annealing. At first, the molecules are physisorbed on the surface; next, they chemisorb inducing the formation of an adatom–vacancy pair on the side of the fullerene. Finally, this metastable state relaxes when the adatom migrates away and the vacancy moves under the molecule. The evolution from a weakly-bound fullerene to a chemisorbed state with a vacancy underneath could be triggered by residual H atoms on the surface which prevent a strong surface-adsorbate bonding right after deposition. Upon annealing at about 440 K, when all H has desorbed, the C60 interacts with the Pt surface atoms forming the vacancy-adatom pair. This metastable state induces a small charge transfer and precedes the final adsorption structure.

First-Principles Calculations of Adatom–Vacancy Pairs on the Graphene

First-Principles Calculations of Adatom–Vacancy Pairs on the Graphene

First-Principles Calculations of Adatom–Vacancy Pairs on the Graphene

By using first-principles calculation, we study adatom–vacancy pairs on graphenes. We found that the healing barrier of the adatom–vacancy pair is very small (0.06 eV) when the adatom is bonded to a nearest carbon atom of the mono-vacancy. Therefore, this pair is easily healed. On the contrary, when the adatom is located 4.26–5.54 Å far from the vacant site, the healing barrier becomes high (0.24–0.32 eV), but these barriers are lower than that of the adatom diffusion. It is therefore expected that these adatom–vacancy pairs are healed in low temperature range where the adatom does not diffuse.

First-principles study on the reconstruction induced by the adsorption of C60 on Pt(111)

The adsorption of C60 on a Pt(111) surface and the origins of the √13 × √13R13.9° or 2√3 × 2√3R30° reconstruction of the C60/Pt(111) system have been investigated by means of first-principles calculations. In agreement with the experimental observations, our calculations reveal that the C60 molecule binds covalently on the Pt(111) surface. The C60 molecule adsorbs on the Pt(111) surface with the center of a hexagonal ring located on top of a surface Pt atom. The surface Pt atom can be removed easily, forming a Pt vacancy upon the adsorption of C60 molecule. Our calculation results show that the strong covalent bonds between C60 and the Pt(111) surface and the formation of adatom-vacancy pairs in the C60/Pt(111) system may be the main driving forces promoting the substrate reconstructing pattern observed in experiments.

Spontaneous edge-defect formation and defect-induced conductance suppression in graphene nanoribbons

We present a first-principles study of the migration and recombination of edge defects (carbon adatom and/or vacancy) and their influence on electrical conductance in zigzag graphene nanoribbons (ZGNRs). It is found that at room temperature, the adatom is quite mobile while the vacancy is almost immobile along the edge of ZGNRs. The recombination of an adatom-vacancy pair leads to a pentagon-heptagon ring defect structure having a lower energy than the perfect edge, implying that such an edge defect can be formed spontaneously. This edge defect can suppresses the conductance of ZGNRs drastically, which provides some useful hints for understanding the observed semiconducting behavior of the fabricated narrow GNRs.

Vacancy Migrations in Carbon Nanotubes

Activities of vacancy defects in carbon nanotubes have been directly monitored by in situ high-resolution transmission electron microscopy at elevated temperatures. Adatom-vacancy pair defects are first prolific due to the knock-on damage, and then the induced vacancies indeed grow up to 1-2 nm in the size by the following Joule heating. Surprisingly, these large vacancies, or "holes", tend to migrate and coalesce with each other to form even larger ones. It suggests that the activation barrier has been substantially lowered due to the contributions of an electromigration and/or irradiation effect.

Kinetic energy effects on morphology evolution during pulsed laser deposition of metal-on-insulator films

We report an experimental comparison of Volmer-Weber metal-on-insulator growth morphology in pulsed laser deposition (PLD) and thermal deposition under identical thermal, background, and surface preparation conditions for Ag on $\mathrm{Si}{\mathrm{O}}_{2}$ and mica. Films exhibit a characteristic morphological progression from isolated three-dimensional islands to elongated clusters to a percolating, electrically conducting film to a pinhole-free film. We observed this same progression for films deposited by PLD. Kinetic Monte Carlo (KMC) simulations that take into account only the pulsed nature of the flux predict that PLD films should advance to percolation with less deposition than thermally deposited films under otherwise identical conditions. At low substrate temperatures, this prediction is confirmed. However, in situ resistance measurements and ex situ atomic force microscopy measurements demonstrate that at high substrate temperatures, PLD films require more deposition to reach percolation. PLD experiments performed at varying kinetic energy of the depositing Ag species suggest a regime in which increasing kinetic energy can delay the percolation transition. Comparison was made with KMC simulations of two-island coalescence in the presence of adatom-vacancy pair creation, which occurs with a greater-than-unity yield per incident ion at kinetic energy $>50\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. A mechanism controlling the delayed percolation of PLD films in the high-temperature regime is proposed: (1) the energetic deposition results in a net uphill flux from adatom-vacancy pair creation, inducing a vertical shape change; (2) taller-than-equilibrium islands coalesce more rapidly; (3) the result is an extended time period over which coalescence is efficient compared to island-island impingement; (4) the percolation transition is delayed.

Atomic simulation of the point defects in three low-index surfaces of BCC transition metals with the MAEAM

The favorable position of an adatom and the formation energies of a single vacancy and an adatom-vacancy pair in three low-index surfaces of body-centered cubic (BCC) transition metals have been calculated by using the modified analytical embedded atom method (MAEAM). The favorable position of an adatom is at the fourfold and twofold positions above the (100) and (110) surfaces respectively, but it is deviated from the threefold position of the (111) surface. Either the heights of the adatom from the top atomic layer, or the formation energies of a single vacancy, or an adatom-vacancy pair decrease in sequence of the (110), (100) and (111) surfaces for each metal. Furthermore, the formation energy of an adatom-vacancy pair is always lower than that of a single vacancy for each low-index surface of each metal, which shown the formation of adatom-vacancy pair is more energetically favorable than the vacancy for the BCC transition metals. Copyright © 2007 John Wiley & Sons, Ltd.

Calculation of the formation energies of isolated vacancy and adatom–vacancy pair at low-index surfaces of fcc metals with MAEAM

The formation energies of isolated vacancy and adatom–vacancy pair (where the two are separated by a large distance) at low-index surfaces of fcc metals calculated by using the modifies analytical embedded atom method (MAEAM). The results predict the prevailing formation of vacancies on the surfaces (1 1 1), (1 0 0) (but Pd), Cu and Ni on the (1 1 0) surfaces at low temperature, and the defect formation energies consistently create in the sequence (1 1 0) → (1 0 0) → (1 1 1). With good accuracy, the calculated energy values coincide with those obtained by the embedded atom methods (EAM) and from experiments. The correctness of the method by which calculated the formation energies of point defects on the surface was proved.

Thin film growth using hyperthermal ions: a surface morphology study

Abstract Low-energy ion deposition and molecular beam epitaxy were used to systematically study the influence of the deposition energy on the growth of Co on Si(1 1 1). Co ions with a well-defined energy between 0.1 and 155 eV were deposited onto a 7 × 7 reconstructed Si surface, using sub- as well as supramonolayer coverage. For submonolayer coverage (0.3 ML), the initial periodicity degrades gradually with increasing ion energy: while for low energies (∼10 eV), 7 × 7 features remain clearly visible, these features disappear completely for higher deposition energies. In the supramonolayer (2 ML) region, we observe an increase in island density with increasing ion energy, which can be explained by the increasing amount of adatom–vacancy pairs and island breakup. Additionally, hyperthermal deposition yields a lower surface roughness compared to thermal deposition, with an optimal energy window for growing smooth films at ∼25 eV. This optimum can be attributed to enhanced layer-by-layer growth due to the extra energy provided by the incoming ion.

Kinetic Monte Carlo–molecular dynamics investigations of hyperthermal copper deposition on Cu(111)

Detailed kinetic Monte Carlo--molecular dynamics (KMC-MD) simulations of hyperthermal energy (10--100 eV) copper homoepitaxy reveal a reentrant layer-by-layer growth mode at low temperatures (50 K) and reasonable fluxes (1 ML/s. where ML stands for monolayer). This growth mode is the result of atoms with hyperthermal kinetic energies becoming inserted into islands when the impact site is near a step edge. The yield for atomic insertion as calculated with molecular dynamics near (111) step edges reaches a maximum near 18 eV. KMC-MD simulations of growing films find a minimum in the rms roughness as a function of energy near 25 eV. We find that the rms roughness saturates just beyond 0.5 ML of coverage in films grown with energies greater than 25 eV due to the onset of adatom-vacancy formation near 20 eV. Adatom-vacancy pairs increase the island nuclei density and the step-edge density, which increase the number of sites available to insert atoms. Smoothest growth in this regime is achieved by maximizing island and step-edge densities, which consequently reverses the traditional roles of temperature and flux: low temperatures and high fluxes produce the smoothest surfaces in these films. Dramatic increases in island densities are found to persist at room temperature, where island densities increase an order of magnitude from 20 to 150 eV.

Molecular dynamics study of the transport and structural properties of the Cu 3Au and Ni 3Al [formula omitted] surface

We present molecular dynamics results concerning the transport and the structural properties of the Cu 3Au and Ni 3Al(1 1 0) surfaces in presence of adatoms and vacancies. In the Cu 3Au case we found that below 500 K the Cu adatom occupies preferably the so-called “dumb-bell” position, while the Au adatom penetrates into the second pure Cu layer via an exchange type mechanism inducing thereby local disorder. At higher temperatures the Cu adatom resides almost exclusively at fourfold positions, while spontaneous creation of adatom–vacancy pairs is also present. Vacancy diffusion processes induce surface disorder that starts by as much as 150 K below the bulk order–disorder temperature. In the case of Ni 3Al(1 1 0) surface, we found a new Al adatom position situated between two Ni surface atoms. It came out that this position plays an important role in the adatom’s diffusive behavior. Above 800 K adatom–vacancy pairs are spontaneously created and in conjunction with exchange type diffusion mechanisms they affect seriously the surface order. In addition, we found that the Ni adatoms are more active than the Al adatoms, this difference in the adatom diffusivity being accentuated by exchange events, leading very quickly the surface in a disordered state. In conclusion, it comes out that the Cu 3Au and Ni 3Al(1 1 0) surfaces disorder well below the bulk transition temperature (663 K, Cu 3Au) and the melting point (1663 K, Ni 3Al) via diffusion processes of the adatoms and/or vacancies.

STM studies of island nucleation during hyperthermal atom deposition

ABSTRACTWe report fundamental changes in island nucleation dynamics as the kinetic energy of the constituent particles used for film grown is increased. A hyperthermal energy ion beam-line with precise control over ion kinetic energy was used to grow copper islands on a Cu(100) substrate. Dramatic increases in island densities were observed with increasing kinetic energy from thermal energies to 150 eV. We find that sputter erosion and the formation of adatom-vacancy pairs contribute to this increase. In addition, variations in flux and temperature suggest that the mean-field scaling exponent is sensitive to atomistic mechanisms activated by the ion beam.

Point defects at low-index surfaces of fcc metals: Formation energies of vacancies and adatom-vacancy pairs

Point defects at low-index surfaces of fcc metals: Formation energies of vacancies and adatom-vacancy pairs

STM analysis of copper thin films using hyperthermal copper ions

AbstractSTM analysis of thin copper films grown on Cu(111) and Cu(100) at room temperature using hyperthermal ions reveals several morphological features not present in thermally grown films. Hyperthermal deposition of thin films has become a popular industrial technique due to observed decreases in film roughness and stress with increased grain sizes, but the link between incidence energy and these properties is poorly understood. Using a sophisticated molecular dynamics (MD) simulation, morphologies observed in experimentally grown films are correlated with the activation of hyperthermal atomistic mechanisms in the MD. Previous work has provided strong evidence for activation of adatom-vacancy pair production near 20 eV on Cu(111). Evidence will be presented here for the activation of sputter erosion near 40 eV. A proposal for quantifying the effect of energy on nucleation density by modifying the flux term in mean field nucleation theory will be presented. Other processes which directly contribute to smooth growth, such as atomic insertion near 10eV, have been proposed but are more difficult to correlate with the data due to their subtle morphological signatures.

Thermal Contraction and Disordering of the Al(110) Surface

Al(110) has been studied for temperatures up to 900 K via ensemble density-functional molecular dynamics. The strong anharmonicity displayed by this surface results in a negative coefficient of thermal expansion, where the first interlayer distance decreases with increasing temperature. Very shallow channels of oscillation for the second-layer atoms in the direction perpendicular to the surface support this anomalous contraction, and provide a novel mechanism for the formation of adatom-vacancy pairs, preliminary to the disordering and premelting transition. Such characteristic behavior originates in the free-electron-gas bonding at a loosely packed surface.

Mass transfer surface diffusion of noble gases

Molecular dynamics simulations employing the Lennard-Jones potential have been performed for Ne, Ar, and Kr on Ar(111) to investigate the nature of surface diffusion at elevated temperatures. Despite the inapplicability of such a simple potential to materials other than noble gases, these simulations qualitatively reproduce the characteristic experimental features of high-temperature surface diffusion on metals and semiconductors. All species exhibit Arrhenius curves with discontinuities at temperatures ranging from 0.65–0.73 T m. Below this temperature, Ne does not island and diffuses by a standard site-hopping mechanism, whereas mobile adatoms of Kr and Ar are formed from the edges of immobile islands, yielding an activation energy which includes both an enthalpy of formation and an enthalpy of migration. Above the discontinuity in the Arrhenius curve at which the surface disorders, surface diffusion is dominated by mass transfer effects, with all surface species diffusing predominantly by a novel mechanism of adatom-vacancy pair formation, which sharply increases the number of mobile species. Perhaps surprisingly, this mechanism is operative for Ne surface diffusion at elevated temperatures despite surface segregation. In addition, high-temperature heterodiffusion is seen to decrease with increasing surface coverage as the adspecies quench surface disordering in the top substrate layer. These and other results demonstrate the importance of surface preparation during high-temperature surface diffusion measurements.

Surface diffusion of Ge on Si(111): Experiment and simulation

Surface diffusion of Ge on Si(111) at high temperatures has been examined experimentally by second-harmonic microscopy and computationally by molecular-dynamics simulations with a Stillinger-Weber potential. Experimentally, the activation energy and preexponential factor for mass-transfer diffusion equalled 2.48\ifmmode\pm\else\textpm\fi{}0.09 eV and 6\ifmmode\times\else\texttimes\fi{}${10}^{2\ifmmode\pm\else\textpm\fi{}0.5}$ ${\mathrm{cm}}^{2}$/s, respectively. Simulational results yielded essentially the same numbers, confirming the utility of the Stillinger-Weber potential for diffusional studies. A previously developed semiempirical correlation also did fairly well. The simulations also provided estimates for the corresponding parameters for intrinsic diffusion and for the enthalpy and entropy of Ge adatom-vacancy pair formation on Si. The simulations further yielded evidence for minor contributions of atom exchange to intrinsic diffusion, as well as the complex high-temperature islanding phenomena on picosecond time scales.