Surface Growth Kinetics Research Articles

Strain-graded quantum dots with spectrally pure, stable and polarized emission

Structural deformation modifies the bandgap, exciton fine structure and phonon energy of semiconductors, providing an additional knob to control their optical properties. The impact can be exploited in colloidal semiconductor quantum dots (QDs), wherein structural stresses can be imposed in three dimensions while defect formation is suppressed by controlling surface growth kinetics. Yet, the control over the structural deformation of QDs free from optically active defects has not been reached. Here, we demonstrate strain-graded CdSe-ZnSe core-shell QDs with compositionally abrupt interface by the coherent pseudomorphic heteroepitaxy. Resulting QDs tolerate mutual elastic deformation of varying magnitudes at the interface with high structural fidelity, allowing for spectrally stable and pure emission of photons at accelerated rates with near unity luminescence efficiency. We capitalize on the asymmetric strain effect together with the quantum confinement effect to expand emission envelope of QDs spanning the entire visible region and exemplify their use in photonic applications.

Helium bubble size effects on the surface morphological response of plasma-facing tungsten

Abstract We report a simulation study on the effects of helium (He) bubble size on the morphological evolution and pattern formation on the surface of tungsten used as a plasma-facing component (PFC) in nuclear fusion devices. We have carried out a systematic investigation based on self-consistent dynamical simulations of surface morphological evolution according to an atomistically-informed, 3D continuum-scale model that captures well the relevant length and time scales of surface nanostructure formation in PFC tungsten. The model accounts for PFC surface diffusion, driven by the biaxial compressive stress originating from the over-pressurized He bubbles in the near-surface region of PFC tungsten as a result of He plasma exposure, combined with the formation of self-interstitial atoms in tungsten that diffuse toward the PFC surface and the flux of surface adatoms generated as a result of surface vacancy-adatom pair formation upon He implantation; this transport of surface adatoms contributes to the anisotropic growth of surface nanostructural features due to the different rates of adatom diffusion along and across step edges of islands on the tungsten surface. Our detailed analysis reveals that varying the average He bubble size plays an important role in the PFC surface growth kinetics as well as the resulting surface topography. Specifically, we find that an increase in the He bubble size leads to a deceleration in the growth rate of the tungsten nanotendrils that emanate from the PFC surface. We also find that the separation distance between the resulting surface features increases with increasing He bubble size, as well as over time. This coarsening effect is a thermally activated process resulting in an accurate description of the temperature dependence of the average surface feature separation by an Arrhenius relation.

Effects of surface vacancy-adatom pair formation on PFC tungsten surface morphological response

We report a simulation study of the effect of He-irradiation-induced surface vacancy-adatom pair formation on the surface morphological evolution of plasma-facing component (PFC) tungsten and examine a number of factors that impact such evolution. Our analysis is based on self-consistent dynamical simulations according to an atomistically-informed, continuum-scale surface evolution model that has been developed following a hierarchical multiscale modeling strategy and can access the spatiotemporal scales of relevance to fuzz formation. The model accounts for the flux of surface adatoms generated as a result of the surface vacancy-adatom pair formation effect upon He implantation, which contributes to the anisotropic growth of surface nanostructural features due to the different rates of adatom diffusion along and across step edges of islands on the tungsten surface. We have carried out atomic-scale computations of optimal diffusion pathways along and across island step edges on the W(110) surface and calculated Ehrlich–Schwoebel (ES) barriers in adatom diffusion along and across such step edges. This aspect of surface adatom diffusion contributes to anisotropic surface atomic fluxes, terrace and step diffusive currents, and has been incorporated into our PFC surface evolution model, which predicts the formation of preferentially aligned nanoridge stripe patterns on the PFC surface. We establish that these anisotropic diffusive currents accelerate nanotendril growth on the PFC surface and the onset of surface nanostructure pattern formation. We also explore systematically the dependence of the PFC surface morphological response on the surface temperature and He ion incident flux, characterize in detail the resulting surface topographies and growth kinetics, and compare the predicted surface morphologies with experimental observations. Our simulation predictions for the emerging surface nanostructure patterns under certain plasma exposure conditions are consistent with experimental findings in the literature.

Early stage growth of amorphous thin film: Average kinetics, nanoscale dynamics, and pressure dependence

We used the coherent grazing incidence small angle x-ray scattering technique to study the average kinetics and nanoscale dynamics during early-stage a-${\mathrm{WSi}}_{2}$ sputter deposition. The kinetic and dynamic properties are examined as a function of pressure, which is known to be a critical factor in determining final surface roughness. Surface growth kinetics and dynamics are characterized by time parameters extracted from the height-height structure factor and correlation functions. The roughness at a given length scale reaches a maximum before relaxing down to a steady state. The lateral length scale dependence and pressure dependence are then compared among the measured kinetics and dynamics time parameters. Surfaces grown at lower pressures are smoother, leading to longer correlation times. The time parameters to reach a dynamic steady state and a kinetic steady state show contrasting pressure dependence. A dynamic steady state is reached earlier than the kinetic steady state at high pressure. A more random deposition direction and lower kinetic energy at higher pressures can explain these phenomena, along with the hypothesis that larger nanoclusters form in vapor before arriving at the surface. A continuum model is applied to simulate the overall behavior with mixed success.

Identification of regularities of formation of the phase-structural state and properties of coatings obtained by micro-arc oxidation of high-strength V95 alloy

The influence of electrolyte composition and electrolysis modes on the surface morphology, growth kinetics, phase-structural state, and mechanical properties of MAO coatings on the V95 aluminum alloy (with Zn as the main alloying element) was studied. The possibility of forming a continuous MAO coating on the V95 alloy with a base layer thickness of more than 100 μm and a surface roughness of less than 5 μm was found. The study of the growth kinetics of MAO coatings on the V95 alloy showed that the highest growth rate of the base coating layer (about 0.83 μm/min) occurs in the 1 g/L KOH+6 g/L Na2SiO3 electrolyte. Although an increase in the relative content of the silicate component (Na2SiO3) provides a fairly high growth rate of the coating, the growth rate of the base layer remains rather low (0.38–0.40 µm/min). This makes the oxidation process in these electrolytes less technological.The study of the phase-structural state of the base coating layer showed that it has a crystalline structure, in which the main phase is aluminum oxide γ–Al2O3 (90–97 %). Crystallites of α-Al2O3 and mullite (3Al2O3∙2SiO2) are formed as the second phase. With an increase in process time in electrolytes with the highest relative content of the alkaline component (1 g/L KOH+6 g/L Na2SiO3), the relative content of the hardest α-Al2O3 phase increases (up to 5 %). The hardness of such coatings with an oxidation time of 180 minutes reaches 14,000 MPa. Thus, the study has shown the advisability of using MAO treatment for the V95 alloy, since it can significantly increase the surface hardness and thereby guarantee its high wear resistance. The combination of high hardness with a relatively high coating growth rate makes it possible to recommend oxidation in the 1 g/L KOH and 6 g/L Na2SiO3 electrolyte as the most technologically advanced for improving the mechanical characteristics of the surface of products made of V95 alloy

Why Do Colloidal Wurtzite Semiconductor Nanoplatelets Have an Atomically Uniform Thickness of Eight Monolayers?

Herein we employed a first-principles method based on density functional theory to investigate the surface energy and growth kinetics of wurtzite nanoplatelets to elucidate why nanoplatelets exhibit a uniform thickness of eight monolayers. We synthesized a series of wurtzite nanoplatelets (ZnSe, ZnS, ZnTe, and CdSe) with an atomically uniform thickness of eight monolayers. As a representative example, the growth mechanism of 1.39 nm thick (eight monolayers) wurtzite ZnSe nanoplatelets was studied to substantiate the proposed growth kinetics. The results show that the growth of the seventh and eighth layers along the [112̅0] direction of 0.99 nm (six monolayers) ZnSe magic-size nanoclusters is accessible, whereas the growth of the ninth layer is unlikely to occur because the formation energy is large. This work not only gives insights into the synthesis of atomically uniform thick wurtzite semiconductor nanoplatelets but also opens up new avenues to their applications in light-emitting diodes, catalysis, detectors, and lasers.

Surface evolution and growth kinetics of Ti6Al4V alloy in pack boriding

The surface of Ti6Al4V alloy was modified using the pack boriding at temperatures of 950–1100 °C with an interval of 50 °C for 5, 10, 20 and 30 h in a powder mixture. The surface and boride layers were examined by scanning electron microscopy, X-ray diffraction, electron probe microanalysis, micro-hardness tester and universal wear and friction tester. Growth kinetics of boride layers on the surface of Ti6Al4V alloy were modelled with two diffusion models of d2 = Dt and d = kt0.5 based on the experimental boride layer thicknesses, boriding times and temperatures. The total thickness of boride layer was determined by TiB layer at T < 1050 °C and by TiB2 layer at T ≥ 1050 °C. The diffusion model of d = kt0.5 turned out more accurate in predicting the thicknesses of boride layers. The diffusion activation energy for B in TiB2 layer was calculated as 231.177 kJ mol−1. The borided Ti6Al4V alloy at 1100 °C for 10 h showed the largest surface hardness and smallest coefficient of friction due to its quite smooth and compact surface.

Selected mode of dendritic growth with n-fold symmetry in the presence of a forced flow

The effect of n-fold crystal symmetry is investigated for a two-dimensional stable dendritic growth in the presence of a forced convective flow. We consider dendritic growth in a one-component undercooled liquid. The theory is developed for the parabolic solid-liquid surface of dendrite growing at arbitrary growth Péclet numbers keeping in mind small anisotropies of surface energy and growth kinetics. The selection criterion determining the stable growth velocity of the dendritic tip and its stable tip diameter is found on the basis of solvability analysis. The obtained criterion includes previously developed theories of thermally and kinetically controlled dendritic growth with convection for the case of four-fold crystal symmetry. The obtained nonlinear system of equations (representing the selection criterion and undercooling balance) for the determination of dendrite tip velocity and dendrite tip diameter is analytically solved in a parametric form. These exact solutions clearly demonstrate a transition between thermally and kinetically controlled growth regimes. In addition, we show that the dendrites with larger crystal symmetry grow faster than those with smaller symmetry.

Edge Dislocations Triggered Surface Instability in Tensile Epitaxial Hexagonal Nitride Semiconductor.

Understanding the semiconductor surface and its properties including surface stability, atomic morphologies, and even electronic states is of great importance not only for understanding surface growth kinetics but also for evaluating the degree to which they affect the devices' performance. Here, we report studies on the nanoscale fissures related surface instability in AlGaN/GaN heterostructures. Experimental results reveal that edge dislocations are actually the root cause of the surface instability. The nanoscale fissures are initially triggered by the edge dislocations, and the subsequent evolution is associated with tensile lattice-mismatch stress and hydrogen etching. Our findings resolve a long-standing problem on the surface instability in AlGaN/GaN heterostructures and will also lead to new understandings of surface growth kinetics in other hexagonal semiconductor systems.

Modeling of lithium-sulfur batteries incorporating the effect of Li2S precipitation

Abstract In this work, we present a one-dimensional model for the discharge behavior of lithium-sulfur (Li-S) batteries. In addition to the consideration of multiple-step polysulfide dissolution and reductions, the surface nucleation and growth kinetics coupled with electrochemical reactions is particularly exploited for describing the Li2S precipitation. Unlike previous models that overlook the rate-dependent precipitation phenomenon, our model reveals that discrete Li2S particle growth becomes suppressed at higher rates, resulting in smaller Li2S precipitates with a more uniform particle size distribution and a limited discharge capacity. Experimental discharge curves and discharge product observation adequately confirm our numerical results. It is further predicted that promoting the growth of Li2S particles, including lowering the initial nucleation rate and providing a suitable amount of initial nucleation sites, can efficiently prolong the Li-S battery's discharge capacity.

Dislocation-Actuated Growth and Inhibition of Hexagonal l-Cystine Crystallization at the Molecular Level

Crystallization of l-cystine is a critical process in the pathogenesis of kidney stone formation in cystinuria, a disorder affecting more than 20 000 individuals in the United States alone. In an effort to elucidate the crystallization of l-cystine and the mode of action of tailored growth inhibitors that may constitute effective therapies, real-time in situ atomic force microscopy has been used to investigate the surface micromorphology and growth kinetics of the {0001} faces of l-cystine at various supersaturations and concentrations of the growth inhibitor l-cystine dimethylester (CDME). Crystal growth is actuated by screw dislocations on the {0001} l-cystine surface, producing hexagonal spiral hillocks that are a consequence of six interlacing spirals of anisotropic molecular layers. The high level of elastic stress in the immediate vicinity around the dislocation line results in a decrease in the step velocities and a corresponding increase in the spacing of steps. The kinetic curves acquired in the ...

Surface Roughness and Critical Exponent Analyses of Boron-Doped Diamond Films Using Atomic Force Microscopy Imaging: Application of Autocorrelation and Power Spectral Density Functions

The evolution of the surface roughness of growing metal or semiconductor thin films provides much needed information about their growth kinetics and corresponding mechanism. While some systems show stages of nucleation, coalescence, and growth, others exhibit varying microstructures for different process conditions. In view of these classifications, we report herein detailed analyses based on atomic force microscopy (AFM) characterization to extract the surface roughness and growth kinetics exponents of relatively low boron-doped diamond (BDD) films by utilizing the analytical power spectral density (PSD) and autocorrelation function (ACF) as mathematical tools. The machining industry has applied PSD for a number of years for tool design and analysis of wear and machined surface quality. Herein, we present similar analyses at the mesoscale to study the surface morphology as well as quality of BDD films grown using the microwave plasma-assisted chemical vapor deposition technique. PSD spectra as a function of boron concentration (in gaseous phase) are compared with those for samples grown without boron. We find that relatively higher boron concentration yields higher amplitudes of the longer-wavelength power spectral lines, with amplitudes decreasing in an exponential or power-law fashion towards shorter wavelengths, determining the roughness exponent (α ≈ 0.16 ± 0.03) and growth exponent (β ≈ 0.54), albeit indirectly. A unique application of the ACF, which is widely used in signal processing, was also applied to one-dimensional or line analyses (i.e., along the x- and y-axes) of AFM images, revealing surface topology datasets with varying boron concentration. Here, the ACF was used to cancel random surface “noise” and identify any spatial periodicity via repetitive ACF peaks or spatially correlated noise. Periodicity at shorter spatial wavelengths was observed for no doping and low doping levels, while smaller correlations were observed for relatively higher boron concentration. These semiquantitative spatial analyses may prove useful in comparing synthesis techniques and varying compositional makeups of diamond films and other technologically important electronic materials. These findings in terms of critical exponents are also correlated with traditional Raman spectroscopy and x-ray diffraction structural properties, thus helping to provide insight into the growth kinetics, albeit in reverse manner.

Surface Stability and Growth Kinetics of Compound Semiconductors: An Ab Initio-Based Approach.

We review the surface stability and growth kinetics of III-V and III-nitride semiconductors. The theoretical approach used in these studies is based on ab initio calculations and includes gas-phase free energy. With this method, we can investigate the influence of growth conditions, such as partial pressure and temperature, on the surface stability and growth kinetics. First, we examine the feasibility of this approach by comparing calculated surface phase diagrams of GaAs(001) with experimental results. In addition, the Ga diffusion length on GaAs(001) during molecular beam epitaxy is discussed. Next, this approach is systematically applied to the reconstruction, adsorption and incorporation on various nitride semiconductor surfaces. The calculated results for nitride semiconductor surface reconstructions with polar, nonpolar, and semipolar orientations suggest that adlayer reconstructions generally appear on the polar and the semipolar surfaces. However, the stable ideal surface without adsorption is found on the nonpolar surfaces because the ideal surface satisfies the electron counting rule. Finally, the stability of hydrogen and the incorporation mechanisms of Mg and C during metalorganic vapor phase epitaxy are discussed.

Surface Reactions and Growth Kinetics of Plasma-deposited Amorphous and Microcrystalline Silicon Thin Films

The growth kinetics of amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) thin films fabricated by plasma-enhanced chemical vapor deposition (PECVD) is discussed for developing low-cost and high-efficiency solar cells. Surface reactions are reviewed associated with the a-Si growth and the nucleation of μc-Si. With respect to the film growth of μc-Si after the nucleation, roughness evolutions were evaluated using an atomic force microscope, and scaling analyses were carried out on fractal structures of growing surfaces, because scaling exponents give a great insight into the growth kinetics. On the basis of results of scaling analyses in conjunction with Monte Carlo simulations, the growth kinetics of microcrystalline silicon thin films, actually used for the high-efficiency solar cells, is discussed.

Control of Ge/Si intermixing during Ge island growth

The surface energy and growth kinetics during Ge deposition on Si(001) were modified by growing the films in a phosphine environment. Islands were formed under a H2 flux as well as in a PH3/H2 atmosphere, but the morphologies were different. The presence of PH3 not only affects the island shape and size but also the composition profile. The dramatical inhibition of Ge/Si intermixing during growth leads to islands richer in Ge compared to undoped islands.

Influence of impurity on surface morphology and growth kinetics of a lysozyme crystal

采用相差显微镜研究了两种杂质蛋白(荧光标记的溶菌酶和商品溶菌酶中的杂质)对四方溶菌酶晶体表面形貌、{110}和{101}表面生长速度以及{110}面台阶生长动力学的影响. 杂质会影响晶体表面二维生长岛的形貌,并显著降低晶体{110}和{101}面法向生长速度. 杂质会显著降低{110}表面台阶的生长速度, 且对〈110〉方向速度的抑制效果显著强于〈001〉方向. 根据切向台阶速度和过饱和度的关系, 计算得到了台阶动力学系数β和台阶的有效表面能及棱边自由能.

Measurement of Soot Surface Growth Kinetics

Surface growth is one of the major steps in the soot formation processes where most of the total mass is formed. Freshly generated soot from an ethylene diffusion flame was sampled and mixed with ethylene in a flow tube at elevated temperatures and using nitrogen as the carrier gas. The changes in the size of the soot particles after hydrocarbon addition were measured using a differential mobility analyzer. These measurements showed a significant mass increase of soot particles as a result of the hydrocarbon addition where the total number density remained the same. Soot surface growth kinetics were obtained from the changes in the size, surface area and volume distributions. The activation energy (Ea) for ethylene was obtained. Pure pyrolysis experiments for ethylene were also performed. The surface growth process yield was found to be higher than the yield of the pyrolysis process.

Morphology and growth of electrodeposited silver powder particles

The effect of overpotential pulsing and periodic current reversal on the morphology of silver powders was investigated using a scanning electron microscope (SEM). It is shown that the profile of pulses or current reversal determines the micromorphology of the electrodeposited silver particles. Generally, three forms of silver were detected: (1) dendrites, (2) equiaxial crystals (independent and/or agglomerates) and (3) rods. The shape of particle seems to be the result of an interplay between the surface energy and growth kinetics.

Observation of competing modes in the growth of diindenoperylene on SiO2

Diindenoperylene (DIP) thin films deposited on SiO2-surfaces by organic molecular beam deposition in ultra-high vacuum at room temperature have been investigated by atomic-force-microscopy and X-ray scattering as a function of film thickness. We observe the evolution of two distinct molecular orientations; the long axis of the DIP molecule is pointing either along the surface normal (σ-orientation) or along the surface (λ-orientation). Initially, the films exhibit predominantly σ-orientated islands. Increasing the film thickness, elongated, fibrous λ-orientated islands nucleate on top of the σ-oriented islands and dominate the growth front due to their fast 3D growth mode. Upon annealing at 150 °C, the fibers disappear. We argue that these observations can be understood as a competition between surface energy (favoring the σ-orientation) and growth kinetics (favoring the λ-orientation).

Kink density and growth velocity in the kinetic BCSOS model

The kink density and its influence on the growth velocity in the kinetic BCSOS model are studied using an exact finite-size analysis based on the transition matrix method and a kinetic Monte Carlo simulation at equilibrium and during growth. The bcc(0 0 1) surface fluctuates in the model through condensation and evaporation of atoms at the surface considered as Markov processes according to a generalized Glauber dynamics. A low-temperature kink energy is estimated from the kink density and its behaviour as a function of the driving force Δ μ is discussed. The numerical results on the ( 0 0 1 ) surface and some vicinal surfaces reveal that the kink site usually considered where an incoming atom can get two perpendicular horizontal bonds is important in deriving the surface growth kinetics and modes.