Desorption Of Adatoms Research Articles

Adsorption and desorption of hydrogen on bare and Xe-covered Cu(111)

Using polarization-modulated ellipsometry to monitor adsorbate coverage in-situ, we studied the activated adsorption of filament-heated molecular hydrogen on Cu(111) and subsequent isothermal desorption of hydrogen adatoms. The adsorption is characterized by a zeroth-order kinetic with a constant sticking probability of S0=0.0062 up to θ=0.25, followed by a Langmuir kinetic until the saturation coverage θs=0.5 is reached. The desorption follows a second-order kinetic with an activation energy of 0.63 eV and a pre-exponential factor of 1×109 /s. A pre-adsorbed monolayer of Xe atoms on Cu(111), with a desorption activation energy of 0.25 eV and a pre-exponential factor of 8×1014 /s, efficiently blocks the subsequent adsorption of hot molecular hydrogen, making physisorbedXe useful as templates for spatial patterning of hydrogen adatom density on Cu(111).

Kinetics of underpotential deposition and nucleation of copper on the Pt(111) face in the presence of acetonitrile

The kinetics of underpotential deposition, three-dimensional nucleation, and growth of copper deposits at cathodic overpotentials on a Pt(111) electrode in solutions containing 0.5 M H2SO4, 10 mM CuSO4, and 0–200 mM acetonitrile (AcN) is studied by the cyclic voltammetry, potentiostatic current transients, and scanning probe microscopy methods. At low volume concentrations of acetonitrile ([AcN] ≤ 4 mM), adsorbed acetonitrile molecules accelerate the formation of a co-adsorption lattice of copper adatoms with anions due to local electrostatic effects at the charged interface. At higher concentrations, the underpotential deposition process is hampered, but the desorption of copper adatoms occurs at potentials more positive than those at low acetonitrile concentrations. This effect is attributed to a stabilizing action of acetonitrile molecules situated on the layer of copper adatoms and, in part, on platinum. At [AcN] = 0.4–40 mM, adsorbed acetonitrile molecules accelerate the growth of the bulk copper deposit, but the nucleation stage is hindered. The dependence of the copper amount on the deposition potential at [AcN] = 40 mM exhibits a maximum at 0.15–0.17 V. This effect was previously observed in weakly acid solutions (pH 1.7–3.0) containing no acetonitrile. The maximum rate of the deposit growth corresponds to an optimum number of crystallites (which is not too great) and an optimum distance between the growing centers in conditions of mixed kinetics “diffusion + electron transfer.” A substantial number of complexes Cu(I)-AcN forms at high acetonitrile concentrations.

Behaviour of discontinuous gold films on SrTiO 3 substrates under annealing

Morphological changes of thin, discontinuous gold films on SrTiO 3 substrates, resulting from evaporation in the temperature range of 1143–1278 K, have been investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). If the gold covered fraction of the surface is small, the evaporation kinetics can be related to the desorption of adatoms. Measuring the density of the gold beads and the time dependence of the effective thickness of the film as calculated from the diameter of the beads, the following parameters have been determined: the surface diffusion length of the gold adatoms, λ s ( m ) = ( 2.2 − 2.0 + 26.7 ) × 10 − 3 × exp [ − ( 77 ± 26 ) kJ / mol / R T ] , the mass transfer surface diffusion coefficient, D ′ s ( m 2 / s ) = ( 3.25 − 3.23 + 433 ) × 10 10 × exp [ − ( 560 ± 49 ) kJ / mol / R T ] and the evaporation flux, J ( m − 2 s − 1 ) ( 2.79 − 2.76 + 267 ) × 10 35 × exp [ − ( 426 ± 46 ) kJ / mol / R T ] .

Surface Ostwald-ripening and evaporation of gold beaded films on sapphire

The kinetics of morphological evolutions of gold nanoparticles on alumina, resulting from evaporation and surface Ostwald-ripening coarsening, have been investigated by means of Auger electron spectroscopy. When the fraction of the covered area is small, the kinetics of evaporation can be related to the desorption of adatoms. In the temperature range 943–1043 K we obtained the evaporation flux J(m-2s-1)=4.8×1027exp[-196±9 (kJ/mol)/RT]. The experimental activation energy of evaporation of gold from a sapphire surface, Qevap=196±9 kJ/mol, is lower than the tabulated value of enthalpy of sublimation of gold, ΔHsubl=368 kJ/mol. At lower temperatures, in the range 623–778 K, Ostwald-ripening experiments, carried out on nanosized clusters, yield the mass transfer surface diffusion coefficients of gold on alumina, Ds(m2/s)=2.6×10-14exp[-58±9 (kJ/mol)/RT]. These results, providing information on the evolution of granular gold films such as those used in catalysts or sensors, are compared to previous data on similar systems.

Electron-stimulated desorption from an unexpected source: Internal hot electrons for Br–Si(1 0 0)-(2 × 1)

The desorption of Br adatoms from Br-saturated Si(1 0 0)-(2 × 1) was studied with scanning tunneling microscopy as a function of dopant type, dopant concentration, and temperature for 620–775 K. Analysis yields the activation energies and prefactors for desorption, and the former correspond to the energy separation between the Fermi level and Si–Br antibonding states. Thus, electron capture in long-lived states results in Br expulsion via a Franck–Condon transition. Analysis of the prefactors reveals that optical phonons provide the energy needed for the electronic excitation. These results show that desorption induced by an electronic transition can occur in closed system without external stimulus, and they indicate that thermally-excited charge carriers may play a general role in surface reactions.

Nonlinear dynamics of vicinal surfaces

We report on step bunching and meandering dynamics on vicinal surfaces during MBE growth. We list four generic meandering scenarios during MBE growth: (i) when adatom desorption is significant, spatio-temporal chaos is found. When desorption is negligible three regimes are found; (ii) meandering with a wavelength which is frozen from the first stages of the instability, but whose amplitude diverges as a power law ( t 1 / 2 ) ; (iii) endless coarsening in the presence of elastic interactions between the steps; (iv) interrupted coarsening in presence of anisotropy. In the case of step bunching, we analyze two scenarios: (i) when desorption is important, ordered bunches appear. (ii) In the opposite limit, bunches separate, and wide terraces appear.

Monte Carlo simulation of CdTe layers growth on CdTe(0 0 1) and Si(0 0 1) substrates

A very promising method for thin (of nanoscale thickness) AIIBVI layer fabrication is the pulsed laser deposition (PLD) method. The initial stage of the CdTe layer growth on Si(001) and CdTe(001) substrates was simulated using the Monte Carlo (MC) procedure. A model for the layer growth included: surface diffusion, absorption and desorption of adatoms, kinetic energy of atoms or ions in the plasma plume and the time distance between consecutive pulses. The shape of the clusters formed clearly depends on these conditions. Layers were obtained for various substrate temperatures. We analysed the morphology and the cross-sections of the layers obtained. The quality of the layers obtained was studied by a fractal description. The results of the simulations allow to determine the correct choice of the layer growth parameters in the real process.

Site-selective silicon adatom desorption using femtosecond laser pulse pairs and scanning tunneling microscopy

We performed an experimental study of silicon adatom desorption from the Si(111)-7×7 surface using femtosecond laser pulse pair excitation with 80 fs pulse duration, 800 nm center wavelength, 300 mW average power, and a 100 MHz repetition rate. Using scanning tunneling microscopy, we directly recorded the desorption characteristics at each delay setting for each of the four adatom binding sites. The study revealed a preferential dependence between the delay time and the adatom sites within a 66.6–1000 fs delay range.

Nucleation and initial growth kinetics of GaN on sapphire substrate by hydride vapor phase epitaxy

The nucleation and initial growth kinetics of GaN on sapphire grown using hydride vapor phase epitaxy technique were measured over a broad range of typically employed growth conditions. GaN nucleation and growth were investigated by a series of short time growth and quench experiments. Atomic force microscopy was used to examine the island density, size, and shape as a function of growth time and substrate temperature. The nucleation temperatures were studied over the range of 985–1100°C. As the temperature increased, the island density decreased due to adatom desorption and higher surface diffusion rates. The initial growth islands transformed from an irregular shape to a hexagonal shape and secondary nucleation was observed as the temperature increased. The activation energy for adatom surface diffusion was estimated to be ∼2.3 eV on the nitridated (0 0 0 1) sapphire. At high temperatures (⩾1050°C), different surface processes, including adatom desorption and island mobility were taking place.

The effects of monovacancies on the terrace width during sublimation from the (111) surface of a diamond-like crystal

Using the three-dimensional Monte Carlo model, the effect of monovacancies on the diffusion exchange between the steps on the (111) surface of a diamond-like crystal during sublimation was investigated. The critical terrace width L cr (the distance from the step edge to the nearest stable vacancy islands) was determined as a function of the relationship between the energy of adatom formation at the smooth terrace E n and the energy of adatom desorption from the terrace E d with retention of the sum of these energies E sub=E n+E d, which characterizes the sublimation of a substance. A well-pronounced maximum exists in the dependences L cr(E n) obtained. The formula is suggested which well describes the dependences L cr(E n) for various values of E sub. The extrapolation of the dependence L cr(E n) to the value E sub=4.2 eV, which is characteristic of Si, permits us to compare the model results with the experimental data of other authors. The explanation of a threefold increase in the terrace width, which is observed experimentally at about 1500 K, is suggested.

Reconstruction of Br-chemisorbed Si(111) surfaces under electron-stimulated desorption

We present atomic structural changes of the rest-atom layer under the electron-stimulated desorption of bromine-chemisorbed Si(111) surfaces. We used a scanning tunneling microscope as an electron-beam source, as well as for structural analysis. By the desorption of bromine atoms and adatoms, underlying rest atoms were imaged in a $7\ifmmode\times\else\texttimes\fi{}7$ structure along with corner holes and dimer walls. These rest atoms are adsorbate-free. The structure was uniquely observed by electron-stimulated desorption. Furthermore, we removed many adatoms by thermal etching in ${\mathrm{Br}}_{2}$ gas in order to observe the rest-atom layer more clearly. By etching at 400 \ifmmode^\circ\else\textdegree\fi{}C, the rest atoms were imaged in the $7\ifmmode\times\else\texttimes\fi{}7$ structure and are terminated with Br atoms. After the desorption of the Br atoms by electron irradiation, the rest atoms maintained the $7\ifmmode\times\else\texttimes\fi{}7$ structure, similar to the case in which adatoms are removed by electron irradiation. In contrast, etching at 450 \ifmmode^\circ\else\textdegree\fi{}C produced a wide area of Br-terminated $1\ifmmode\times\else\texttimes\fi{}1$ structure such as that of a bulk, quenching the corner holes and dimer walls. Electron irradiation changed this surface into a $2\ifmmode\times\else\texttimes\fi{}1$ structure through Br desorption. We discussed the above results from the viewpoint of reconstruction behavior of the local structure produced by the electron-stimulated desorption.

Kinetic faceting of multiply twinned diamond crystals during vapor phase synthesis

Multiply twinned particles in diamond cubic crystals such as decahedron and icosahedron were explained using Kinetic Monte Carlo (KMC) simulations by the sequential occurrence of two and three stacking faults on different {1 1 1} surfaces during vapor-phase growth. However, the same simulations, using single adatom adsorption and desorption of singly bonded sites, failed to predict the kinetic faceting of another commonly observed ‘star-shaped’ decahedron. In this work, a set of KMC simulations were performed using an additional reaction step for etching of doubly bonded surface sites during growth starting with a seed crystal containing two stacking errors. These simulations produced star-shaped decahedral crystals and thus pointed to a possible explanation of the observed ‘star-shaped’ decahedral morphology for diamond crystals. The experimental results on the morphological evolution of star-decahedral crystals exhibited a sequence in which an intermediate shape, between decahedron and icosahedron, developed into a more complete icosahedral shape upon further growth.

Photoluminescence linewidth narrowing of InAs/GaAs self-assembled quantum dots

The effects of desorption and diffusion of indium adatoms on the photoluminescence (PL) from InAs self-assembled quantum dots (QDs) are investigated by introducing growth interruptions after QD formation. Large, low-density and small, high-density QDs were grown by molecular beam epitaxy using low (0.01 ML/ s) and high (0.2 ML/ s) growth rates, respectively. The PL from the QDs grown at 0.01 ML/ s and with various growth interruption times exhibit decreasing linewidths from 40 to 32 meV with increasing growth interruption time up to 30 s . The narrowing of the PL linewidth results from improved size homogeneity due to desorption and diffusion of adatoms from small (<30 nm) InAs clusters. The narrowing of the PL linewidth from the InAs dots is combined with low-temperature GaAs capping to obtain a linewidth of 26 meV .

欠陥生成を取り入れた薄膜成長のモンテカルロシミュレーション 孔埋め込みへの応用

Monte Carlo simulations of via filling have been performed using the lattice model of crystal growth, which allows vacancy formation in the film during the growth. Adsorption, desorption and surface diffusion of adatoms are taken into account. Using a two-dimensional model, we examined the film growth from the initial surface with hollow parts (wedge-shaped and flat-bottomed holes), the aspect ratio of which ranged from 0.5 to 4. When a wedge-shaped hole is filled with deposited atoms, small voids appear in the film, which are aligned in the growth direction. In the case of the flat-bottomed hole, on the other hand, large voids with several hundred vacant sites appear in the middle of the hole. The voids are elongated in the growth direction as the aspect ratio becomes large. These voids have a similar structure to those of the experiment of filling vias and trenches in Cu interconnections from the solution without additives. The mechanism of the void formation is discussed in relation to the aspect ratio and the overpotential.

Modelling of growth of thin solid films obtained by pulsed laser deposition

Pulsed laser deposition (PLD) method of thin films fabrication requires appropriate choice of technological parameters. These are: substrate temperature, the energy density of the laser radiation, its wavelength, the pulse duration, the distance target–substrate, etc. Exact analysis of the influence of these parameters on the layer growth is too difficult to be performed. Therefore, the computer simulation of the layer growth is very promising method for determining the growth parameters. In the paper, the growth of Si and CdTe films on the (001) substrate surface was simulated by the Monte Carlo method. The model for layer growth included: surface diffusion, adsorption and desorption of adatoms, kinetic energy distribution of atoms or ions in the plasma plume and its space and time structure. Results obtained have been compared with some experimental data.

Sequential Selective Formation of Adsorbedtert-Butoxy andtert-Butyl Groups fromtert-Butyl Formate on Ni(111)

The sequential preparation of tert-butoxy and tert-butyl groups on Ni(111) was achieved through the thermal decomposition of adsorbed tert-butyl formate. The two intermediates were characterized using infrared reflection−absorption spectroscopy, XPS, and TPD measurements. The initial step, below 250 K, yielded coadsorbed tert-butoxy, carbon monoxide, and atomic hydrogen. The second step, in the temperature range 310−370 K, produced adsorbed oxygen atoms and surface tert-butyl groups through scission of the tert-butoxy C−O bond. The complete removal of the tert-butoxy groups coincided with the recombinative desorption of hydrogen adatoms derived from the formyl group of the parent molecule. The decomposition of the tert-butyl groups at 410 K coincided with the desorption limited removal of CO from the surface. Quantitative XPS analysis confirmed that the decomposition of tert-butyl formate on Ni(111) proceeds exclusively through the sequential formation of tert-butoxy and tert-butyl groups.

Atomic structural changes of a Br-chemisorbedSi(111)−7×7surface under 10–150 eV electron impact

Electron-stimulated desorption from Br-chemisorbed $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7$ surfaces was investigated by scanning tunneling microscopy. Irradiating these surfaces with electrons field emitted from the tip of the scanning tunneling microscope induced various desorption behaviors depending on the initial Br coverage and electron energy. At low Br coverage, Br atoms desorb but no atomic changes occur on the Si surface. At saturation coverage, Si adatoms as well as Br atoms desorb to a large extent. The cross section of Br atom desorption first increases near 15 eV and then increases by orders of magnitude with electron energy up to 150 eV. The cross section of adatom desorption, in contrast, is much less dependent on the electron energy in this energy region. These desorption behaviors are discussed from the viewpoint of electronic excitation at the Br-chemisorbed Si(111) surface.

Platinum electrodeposits on glassy carbon: The formation mechanism, morphology, and adsorption properties

The deposition of platinum on glassy carbon (GC) is studied by chronoamperometry. Basic tendencies of the formation of aggregate platinum particles on the oxidized carbon surface are established. These include a primary instantaneous nucleation of platinum under diffusion control and the beginning of a secondary nucleation prior to filling primary active centers. The deposit morphology is examined byex situ methods of scannng electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM). A globular structure of platinum, formed by crystallites 3–5 nm in size, is revealed. A comparison of the STM, SEM, and TEM data demonstrates a high information value and accuracy of STM in studies of disperse materials in both nanometer and submicron ranges. Various coulometry techniques intended for the determination of the true surface area of deposited platinum are compared. The most informative techniques are the voltammetry of desorption of copper adatoms and chemisorbed carbon monoxide at, respectively, low and high platinum contents. Differences in the formation kinetics and properties of aggregate particles in Pt/GC and Pt/Pt are found, specifically, smaller Pt/GC crystallites and higher degrees of their concrescence (screening)

The deduction of desorption parameters using an exponential tempering function

It is shown how, using the exponential tempering function T=T 0 exp r 1t , the parameters of initial surface adatom coverage n 0, attempt frequency factor ν 0, and activation energy for desorption E, when desorption is described by first-order reaction kinetics and the activation energy is single valued, can all be measured to an accuracy of about 3% in a first approximation by evaluating the adatom desorption rate as a function of increasing temperature only up to the maximum in the desorption rate.

Kinetics study on the hydrogen atom-induced abstraction and associative desorption of deuterium adatoms from the Si(100) surface at 573 K

Hydrogen atom-induced deuterium adatom abstraction has been studied on the Si(100) surface from a desorption point of view. Abstraction of HD as well as collision-induced D2 desorption are observed on the D/Si(100) surface during H beam exposure at 573 K. For 1 ML Dad precoverage, about 30% of the desorbed D atoms are ejected as collision-induced D2 desorption at the very beginning of H(g) exposure. It is found that the decay of the HD and D2 rates are affected by the total coverage, θH+D. The decay rate of the HD rate is much lower in the low coverage regime than in the high coverage regime. The nominal cross section of the former is 2.3±1.0 Å2, while for the latter 7.2±0.5 Å2. The HD rate curves show that the abstraction obeys nearly first-order kinetics towards D coverage, θD. On the other hand, the D2 rate curves show that the collision-induced desorption obeys nearly fourth-order kinetics towards θD. Such HD abstraction is also observed on an oxygen-deposited D/Si(100) surface where the oxygen atoms are known to be incorporated between Si atoms. The observed decay rate of the HD rate is extremely low, with a cross section of 1.5±1.0 Å2. The reaction kinetics are well understood by adopting the kinetics model recently proposed by Flowers et al. [Surf. Sci. 396, 227 (1998)]. The abstraction of HD may be due to an Eley–Rideal mechanism rather than to a hot atom mechanism. Collision-induced D2 desorption may take place upon encounter of two dideuteride species via an isomerization reaction at quasiequilibrium.