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Using lattice Boltzmann simulations we investigate the impact of the amplitude of concentration fluctuations in binary fluid mixtures prior to demixing when in contact with a surface that is preferentially wet by one of the components. We find a bicontinuous structure near the surface for an initial, prequench state of the mixture close to the critical point where the amplitude of concentration fluctuations is large. In contrast, if the initial state of the mixture is not near the critical point and concentration fluctuations are relatively weak, then the morphology is not bicontinuous but remains layered until the very late stages of coarsening. In both cases, it is the morphology of a depletion layer rich in the nonpreferred component that dictates the growth exponent of the thickness of the fluid layer that is in direct contact with the substrate. In the early stages of demixing, we find a growth exponent consistent with a value of 1/4 for a prequench state away from the critical point, which is different from the usual diffusive scaling exponent of 1/3 that we recover for a prequench state close to the critical point. We attribute this to the structure of a depletion layer that is penetrated by tubes of the preferred fluid, connecting the wetting layer to the bulk fluid even in the early stages if the initial state is characterized by concentration fluctuations that are large in amplitude. Furthermore, we find that in the late stages of demixing the flow through these tubes results in significant in-plane concentration variations near the substrate, leading to dropletlike structures with a concentration lower than the average concentration in the wetting layer. This causes a deceleration in the growth of the wetting layer in the very late stages of the demixing. Irrespective of the prequench state of the mixture, the late stages of the demixing process produce the same growth law for the layer thickness, with a scaling exponent of unity usually associated with the impact of hydrodynamic flow fields.

We have obtained an ultralong lifetime exciton emission in InAs/GaAs single quantum dots (QDs) when the QD films are transferred onto the Si substrate covered by Ag nanoparticles. It is found that when the separation distance from the QD layer (also the wetting layer) to the Ag nanoparticles is around 19 nm, the QD emission lifetime changes from approximately 1 to 2000 ns. A classical dipole oscillator model is used to quantitatively calculate the spontaneous radiation decay rate of the excitons in the wetting layer (WL), and the simulated calculation result is in good agreement with the experimental one, revealing that the long lifetime exciton emission is due to the existence of the dark state in the WL. The self-induced dark state stems from the destructive interference between the exciton emission field and the induced dipole field of the Ag nanoparticles.

We provide an extensive and systematic investigation of exciton dynamics in droplet epitaxial quantum dots comparing the cases of (311)A, (001), and (111)A surfaces. Despite a similar s-shell exciton structure common to the three cases, the absence of a wetting layer for (311)A and (111)A samples leads to a larger carrier confinement compared to (001), where a wetting layer is present. This leads to a more pronounced dependence of the binding energies of s-shell excitons on the quantum dot size and to the strong anti-binding character of the positive-charged exciton for smaller quantum dots. In-plane geometrical anisotropies of (311)A and (001) quantum dots lead to a large electron-hole fine interaction (fine structure splitting (FSS) ∼100 μeV), whereas for the three-fold symmetric (111)A counterpart, this figure of merit is reduced by about one order of magnitude. In all these cases, we do not observe any size dependence of the fine structure splitting. Heavy-hole/light-hole mixing is present in all the studied cases, leading to a broad spread of linear polarization anisotropy (from 0 up to about 50%) irrespective of surface orientation (symmetry of the confinement), fine structure splitting, and nanostructure size. These results are important for the further development of ideal single and entangled photon sources based on semiconductor quantum dots.

The presence of a surface preferably attracting one component of a polymer mixture by the long-range van der Waals surface potential while the mixture undergoes phase separation by spinodal decomposition is called long-range surface-directed spinodal decomposition (SDSD). The morphology achieved under SDSD is an enrichment layer(s) close to the wall surface and a droplet-type structure in the bulk. In the current study of the long-range surface-directed polymerization-induced phase separation, the surface-directed spinodal decomposition of a monomer&ndash, solvent mixture undergoing self-condensation polymerization was theoretically simulated. The nonlinear Cahn&ndash, Hilliard and Flory&ndash, Huggins free energy theories were applied to investigate the phase separation phenomenon. The long-range surface potential led to the formation of a wetting layer on the surface. The thickness of the wetting layer was found proportional to time t*1/5 and surface potential parameter h11/5. A larger diffusion coefficient led to the formation of smaller droplets in the bulk and a thinner depletion layer, while it did not affect the thickness of the enrichment layer close to the wall. A temperature gradient imposed in the same direction of long-range surface potential led to the formation of a stripe morphology near the wall, while imposing it in the opposite direction of surface potential led to the formation of large particles at the high-temperature side, the opposite side of the interacting wall.

Semiconductors interfaced with heavy elements possessing a strong atomic spin–orbit coupling are important building blocks for the development of new spintronic devices. Here, we present a microscopic and spin-resolved spectroscopic investigation of ultrathin Bi films grown onto a Ge(111) substrate. At monolayer coverage, a Bi wetting layer is formed, characterized by a semiconducting behavior and a ( 3 × 3 ) R 30 ° superstructure. The wetting layer supports the subsequent growth of Bi islands with a pseudo-cubic structure similar to that of Bi(110), showing a well-defined orientation with respect to the substrate high-symmetry directions. We performed photoemission and spin-resolved inverse photoemission experiments at off-normal electron emission and incidence, respectively, along the substrate Γ ¯ K ¯ direction. Inverse photoemission, in particular, highlights the presence of a spin-polarized empty Bi state, not reported so far, due to the strong spin–orbit effects characteristic of the Bi surface and thin layers. Finally, scanning tunneling spectroscopy is employed to link the observed spectroscopic features to either the wetting layer or the Bi islands.

An overview is given of the many applications that nm-thin pure boron (PureB) layers can have when deposited on semiconductors such as Si, Ge, and GaN. The application that has been researched in most detail is the fabrication of nm-shallow p+n-like Si diode junctions that are both electrically and chemically very robust. They are presently used commercially in photodiode detectors for extremeultraviolet (EUV) lithography and scanning-electron-microscopy (SEM) systems. By using chemicalvapor deposition (CVD) or molecular beam epitaxy (MBE) to deposit the B, PureB diodes have been fabricated at temperatures from an optimal 700 °C to as low as 50 °C, making them both front- and back-end-of-line CMOS compatible. On Ge, near-ideal p+n-like diodes were fabricated by covering a wetting layer of Ga with a PureB capping layer (PureGaB). For GaN high electron mobility transistors (HEMTs), an Al-on-PureB gate stack was developed that promises to be a robust alternative to the conventional Ni-Au gates. In MEMS processing, PureB is a resilient nm-thin masking layer for Si micromachining with tetramethyl ammonium hydroxide (TMAH) or potassium hydroxide (KOH), and low-stress PureB membranes have also been demonstrated.

We study the assembly of magnetite nanoparticles in water-based ferrofluids in wetting layers close to silicon substrates with different functionalization without and with an out-of-plane magnetic field. For particles of nominal sizes 5, 15, and 25 nm, we extract density profiles from neutron reflectivity measurements. We show that self-assembly is only promoted by a magnetic field if a seed layer is formed at the silicon substrate. Such a layer can be formed by chemisorption of activated N-hydroxysuccinimide ester-coated nanoparticles at a (3-aminopropyl)triethoxysilane functionalized surface. Less dense packing is reported for physisorption of the same particles at a piranha-treated (strongly hydrophilic) silicon wafer, and no wetting layer is found for a self-assembled monolayer of octadecyltrichlorosilane (strongly hydrophobic) at the interface. We show that once the seed layer is formed and under an out-of-plane magnetic field further wetting layers assemble. These layers become denser with time, larger magnetic fields, higher particle concentrations, and larger moment of the nanoparticles.

Blended organic thin films have been studied during the last decades due to their applicability in organic solar cells. Although their optical and electronic features have been examined intensively, there is still lack of detailed knowledge about their growth processes and resulting morphologies, which play a key role for the efficiency of optoelectronic devices such as organic solar cells. In this study, pure and blended thin films of copper phthalocyanine (CuPc) and the Buckminster fullerene (C60) were grown by vacuum deposition onto a native silicon oxide substrate at two different substrate temperatures, 310 K and 400 K. The evolution of roughness was followed by in-situ real-time X-ray reflectivity. Crystal orientation, island densities and morphology were examined after the growth by X-ray diffraction experiments and microscopy techniques. The formation of a smooth wetting layer followed by rapid roughening was found in pure CuPc thin films, whereas C60 shows a fast formation of distinct islands at a very early stage of growth. The growth of needle-like CuPc crystals loosing their alignment with the substrate was identified in co-deposited thin films. Furthermore, the data demonstrates that structural features become larger and more pronounced and that the island density decreases by a factor of four when going from 310 K to 400 K. Finally, the key parameters roughness and island density were well reproduced on a smaller scale by kinetic Monte-Carlo simulations of a generic, binary lattice model with simple nearest-neighbor interaction energies., Comment: 30 pages, 11 figures

International audience; We report experimental evidence for a spontaneous shape transition, from regular islands to elongated nanowires, upon high-temperature annealing of a thin Mn wetting layer evaporated on Ge(111). We demonstrate that 4.5 monolayers is the critical thickness of the Mn layer, governing the shape transition to wires. A small change around this value modulates the geometry of the nanostructures. The Mn–Ge alloy nanowires are single-crystalline structures with homogeneous composition and uniform width along their length. The shape evolution towards nanowires occurs for islands with a mean size of ≃170 nm. The wires, up to ≃1.1 μm long, asymptotically tend to ≃80 nm of width. We found that tuning the annealing process allows one to extend the wire length up to ≃1.5 μm with a minor rise of the lateral size to ≃100 nm. The elongation process of the nanostructures is in agreement with a strain-driven shape transition mechanism proposed in the literature for other heteroepitaxial systems. Our study gives experimental evidence for the spontaneous formation of spatially uniform and compositionally homogeneous Mn-rich GeMn nanowires on Ge(111). The reliable and simple synthesis approach allows one to exploit the room-temperature ferromagnetic properties of the Mn–Ge alloy to design and fabricate novel nanodevices.

We present a methodical study of grazing-incidence small-angle x-ray scattering performed in situ during pulsed-laser deposition of Pt on sapphire substrates. From measured two-dimensional intensity distributions in reciprocal space we calculated horizontal and vertical intensity projections and compare them to numerical simulations. The structure of the Pt layers was described using a simple Monte-Carlo model and the Ornstein-Zernicke theory with the Percus-Yevick approximation, and the scattering process was simulated using distorted-wave Born approximation. The validity of the structure models as well as the effect of the indirect scattering processes are discussed. From the comparison of the measured and simulated data we determined the lateral and vertical sizes of surface islands, the surface coverage, the island coalescence, as well as the thickness of the wetting layer at the substrate surface. We studied the time evolution of these parameters and their dependence on the substrate temperature.

Droplet epitaxy allows the efficient fabrication of a plethora of 3D, III&ndash, V-based nanostructures on different crystalline orientations. Quantum dots grown on a (311)A-oriented surface are obtained with record surface density, with or without a wetting layer. These are appealing features for quantum dot lasing, thanks to the large density of quantum emitters and a truly 3D lateral confinement. However, the intimate photophysics of this class of nanostructures has not yet been investigated. Here, we address the main optical and electronic properties of s-shell excitons in individual quantum dots grown on (311)A substrates with photoluminescence spectroscopy experiments. We show the presence of neutral exciton and biexciton as well as positive and negative charged excitons. We investigate the origins of spectral broadening, identifying them in spectral diffusion at low temperature and phonon interaction at higher temperature, the presence of fine interactions between electron and hole spin, and a relevant heavy-hole/light-hole mixing. We interpret the level filling with a simple Poissonian model reproducing the power excitation dependence of the s-shell excitons. These results are relevant for the further improvement of this class of quantum emitters and their exploitation as single-photon sources for low-density samples as well as for efficient lasers for high-density samples.

In this paper, we comprehensively consider the effect of the dependence of elastic strain on the thickness of deposited material on the formation of two-dimensional layers and quantum dots by the Stranski–Krastanow mechanism. The nucleation and growth of germanium quantum dots on silicon surface (100) are used as a model system for conducting experimental studies and theoretical calculations. A detailed dependence of the value of elastic strains on the effective thickness of deposited germanium is obtained. It is also shown that the magnitude of the 1/N superstructural periodicity in this system reaches 12.5%. Based on the obtained thickness dependence of lattice mismatch, a new theory is constructed for calculating the parameters of the formed islands, generalizing previously used thermodynamic models. The equilibrium and critical thicknesses of the wetting layer are determined for the first time under the assumption that lattice mismatch depends on the thickness of the deposited material. In this approximation, some unexpected results are obtained that refine traditional thermodynamic models and confirmed by experimental data. The results of this work and proposed theoretical model may be applied for strain engineering in other material systems where growth of two-dimensional materials and quantum-sized islands by the Stranski–Krastanow mechanism is realized.

Contrary to simplified theoretical models, atomistic calculations presented here reveal that sufficiently large in-plane shape elongation of quantum dots can not only decrease, but even reverse the splitting of the two lowest optically active excitonic states. Such a surprising cancellation of bright-exciton splitting occurs for shape-anisotropic nanostructures with realistic elongation ratios, yet without a wetting layer, which plays here a vital role. However, this non-trivial effect due to shape-elongation is strongly diminished by alloy randomness resulting from intermixing of InAs quantum-dot material with the surrounding InP matrix. Alloying randomizes, and to some degree flattens the shape dependence of fine-structure splitting giving a practical justification for the application of simplified theories. Finally, we find that the dark-exciton spectra are rather weakly affected by alloying and are dominated by the effects of lateral elongation.

Oxide/metal/oxide (OMO) layer stacks are used to replace transparent conductive oxides as front contact of thin-film solar cells. These multilayer structures not only reduce the overall thickness of the contact, but can be used for colouring of the cells utilizing interference effects. However, sheet resistance and parasitic absorption, both of which depend heavily on the metal layer, should be further reduced to reach higher efficiencies in the solar cells. In this publication, AgOX wetting layers were applied to OMO electrodes to improve the performance of Cu(In,Ga)Se2 (CIGS) thin-film solar cells. We show that an AgOX wetting layer is an effective measure to increase transmission and conductivity of the multilayer electrode. With the presented approach, we were able to improve the short-circuit current density by 18% from 28.8 to 33.9 mA/cm2 with a metal (Ag) film thickness as low as 6 nm. Our results highlight that OMO electrodes can be an effective replacement for conventional transparent conductive oxides like aluminium-doped zinc oxide on thin-film solar cells.

Two-dimensional (2D) Pb nanoisland has established an ideal platform for studying the quantum size effects on growth mechanism, electronic structures as well as high-temperature superconductivity. Here, we investigate the growth and quantum well electronic states of the 2D Pb nanoislands on Nb-doped SrTiO3(0 0 1) by scanning tunneling microscopy and spectroscopy. In contrast to Pb/Si(1 1 1), Pb/Cu(1 1 1) and Pb/Ag(1 1 1), there is no wetting layer of Pb formed on the Nb-doped SrTiO3(0 0 1) surface, resulting in isolated Pb nanoislands with an apparent height of 4 atomic layers as the building blocks for the island growth. According to the thickness-dependent quantum well states resolved in both occupied and unoccupied energy regions, the constant group velocity vg = 1.804 × 106 m/s and the Fermi wavevector kF = 1.575 A−1, have been extracted from a linear fit of the Pb(1 1 1) band dispersion along the Γ - L direction. In addition, the energy-dependent scattering phase shift φ(E) obtained by means of the phase accumulation model shows a metallic-like scattering interface analogous to Pb/Ag(1 1 1). These spatially decoupled 2D Pb nanoislands thus realize an opportunity to explore the intrinsic quantum confinement phenomena in nanoscale superconductors on the doped titanium-oxide-type substrate.

Self-assembled quantum dots grown by molecular beam epitaxy have been a hotbed for various fundamental research and device applications over the past decades. Among them, InAs/GaAs quantum dots have shown great potential for applications in quantum information, quantum computing, infrared photodetection, etc. Though intensively studied, some of the optical nonlinear properties of InAs/GaAs quantum dots, specifically the associated two-photon absorption of the wetting and barrier layers, have not been investigated yet. Here we report a study of the photoluminescence of these dots by using direct two-photon excitation. The quadratic power law dependence of the photoluminescence intensity, together with the ground-state resonant peak of quantum dots appearing in the photoluminescence excitation spectrum, unambiguously confirms the occurrence of the direct two-photon absorption in the dots. A three-level rate equation model is proposed to describe the photogenerated carrier dynamics in the quantum dot-wetting layer-GaAs system. Moreover, higher-order power law dependence of photoluminescence intensity is observed on both the GaAs substrate and the wetting layer by two-photon excitation, which is accounted for by a model involving the third-harmonic generation at the sample interface. Our results open a door for understanding the optical nonlinear effects associated with this fundamentally and technologically important platform.

Surface segregation of the low molecular weight component of a polymeric mixture is a ubiquitous phenomenon that leads to degradation of industrial formulations. We report a simultaneous phase separation and surface migration phenomena in oligomer&ndash, polymer ( O P ) and oligomer&ndash, gel ( O G ) systems following a temperature quench that induces demixing of components. We compute equilibrium and time varying migrant (oligomer) density profiles and wetting layer thickness in these systems using coarse grained molecular dynamics (CGMD) and mesoscale hydrodynamics (MH) simulations. Such multiscale methods quantitatively describe the phenomena over a wide range of length and time scales. We show that surface migration in gel&ndash, oligomer systems is significantly reduced on account of network elasticity. Furthermore, the phase separation processes are significantly slowed in gels leading to the modification of the well known Lifshitz&ndash, Slyozov&ndash, Wagner (LSW) law ℓ ( &tau, ) &sim, &tau, 1 / 3 . Our work allows for rational design of polymer/gel&ndash, oligomer mixtures with predictable surface segregation characteristics that can be compared against experiments.

Heteroepitaxial growth of crystalline ice thin films of water on metal substrates under ultrahigh vacuum provides an excellent opportunity to investigate the interior and surface structures of crystalline ice that are closely related to their physicochemical properties. Here we present the spectroscopic studies of the orientational ordering and the surface relaxation of crystalline ice films grown on two representative metal surfaces: Pt(111) and Rh(111). A versatile tool for exploring these structures is sum frequency generation (SFG) vibrational spectroscopy; homodyne detection of SFG signals serves as a good measure of orientational ordering in the interior of crystalline ice films while heterodyne detection enables us to determine the direction of water molecules at the interface with metal substrates, in the interior of crystalline ice films, and at their surfaces. Water molecules on the wetting layer of Pt(111) are preferentially oriented in H-down configuration, and the configuration is passed along into the interior of crystalline ice films. In contrast, water molecules on Rh(111) are adsorbed in a mixture of H-down and H-up configurations, leading to orientationally disordered crystalline ice films. The inter-layer distance at the top of the surface is modulated alternately in accordance with the orientation of molecules hydrogen bonded to the bilayer underneath. Therefore, the molecular orientation also plays an important role in their surface relaxation.

In this paper, we have investigated the thermally activated carriers transfer mechanism in closely stacked InAs/GaAs quantum dots (QDs) by means of steady-state photoluminescence (PL) and time-resolved photoluminescence measurements. The 10 K PL spectrum exhibits double-emission peaks where the excitation power dependence reveals that these emission peaks are attributed to large and small QD groups. With increasing the sample temperature, an abnormal line-width shrinkage of large QDs (LQDs) is observed. The increase in PL decay lifetime of LQDs versus temperature is nicely explained as the electron and hole wave function overlap between dot layers induced by vertical electronic coupling effect. Using a thermal escape model, the activation energies for PL thermal quenching at high temperatures (above 80K) were derived from fitting the temperature-dependent PL decay lifetime data of LQDs and SQDs. The determined activation energies show that the escape of electron-hole pairs from QDs occurs via transfer channel located below the wetting layer. These results are well reproduced by a rate equation-based model treating the QDs as a localized-state ensemble. Our results emphasize the important role of the vertically stacked InAs/GaAs QDs structures with thin GaAs spacer layer to slow down the carrier PL decay lifetime of the thermal transfer process between QDs. This finding is important for the use of such structures as intermediate band in solar cells.

In this study, Cu-Cu direct bonding at the ultra-low bonding temperature has been successfully demonstrated by using the Cr/Au layer, which can protect Cu from oxidation before the thermocompression bonding (TCB) process and greatly improve the diffusion of Cu atoms into the surface during the thermal process. The bonding quality has been carefully investigated by analyzing Auger depth, observing the SAT, AFM, SEM images, and measuring electrical performance of the samples. In addition, the mechanical test and reliability test have been performed to verify the reliability of the novel structure with the Cr/Au layer in this study.

In this Article, we consider quantum dots with different structures and solve the Schrodinger equations for them by finite difference time-domain method and obtain the Eigenfunction’s and the Eigenvalue’s of these points.The FDTD method have an analysis that demonstrates the high accuracy of this method for solving quantum equations.The FDTD method is a suitable method for simulating electromagnetic phenomena, in this Article we present a simple formulation for the Schrodinger equation in FDTD. In fact, we get help from the idea of FDTD and solve quantum equations. The most important structures we have simulated with this method are cubic quantum dot, spherical quantum dot, elliptical quantum dot and partial truncate-shaped quantum dots on the wetting layer. It is very difficult to solve the Schrodinger equation analytically for these structures, but by using the numerical method we obtain the Eigenfunction’s and Eigenvalue’s in a simpler way.

Electromagnetically induced grating (EIG) in the ladder-plus-Y double quantum dot system is modeled and the diffraction grating properties are explored in this structure. A high transmission function is obtained under the high pump field. This function is reduced under increasing the probe field due to the Kerr effect. The phase of this function depends on pumping. It is shown that the application of another two fields from the wetting layer (WL)-quantum dot (QD) type is more efficient in obtaining very high diffraction intensity. So, EIG with high diffraction intensity is obtained under a four-field application. Note that, WL-QD field effect is not studied earlier. Neglecting the WL effect reduces the transmission by five times. The diffraction intensity of this system is six times higher than that obtained from a single QD structure.

In this paper, a theoretical study of the second harmonic generation (SHG) in a coupled lens shaped InAs/GaAs quantum dots is presented within the framework of the compact density-matrix approach and the finite difference method. In literature, many theoretical studies have neglected the wetting layer (WL) effect. As well, other effects such as the pressure, the external electric field, the temperature and the structural dimensions are not introduced together. Furthermore, in many researches only a single quantum dot (SQD) with and without wetting layer is investigated. What is done in this work, a detailed and comprehensive study of a vertically coupled lens-shaped InAs/GaAs coupled to a WL is presented under the external proofs effects mentioned above. Also, the structural dimensions, the electric field, the relaxation rate, the hydrostatic pressure, and the temperature effects are explored in details and the SHG is optimized. We have found out that the wetting layer has been shown to be so influential on the SHG magnitude and on the corresponding resonant energy. So, calculated results show that a strong SHG coefficient can be obtained by choosing a group of the optimized structural dimensions and the external proofs previously mentioned above. With a good choice of these parameters, it’s found that the SHG coefficient reaches the order of magnitude of 10−7 m/V, which is larger than the corresponding values obtained with a SQD. Besides, obtained results revealed that a red or a blue-shifted energy can be obtained. This behavior in the SHG gives a new degree of freedom in regions of interest for optoelectronic device applications.

We report a new way to slow down the spontaneous emission rate of excitons in the wetting layer (WL) through radiative field coupling between the exciton emissions and the dipole field of metal islands. As a result, a long-lifetime decay process is detected in the emission of InAs/GaAs single quantum dots (QDs). It is found that when the separation distance from WL layer (QD layer) to the metal islands is around 20 nm and the islands have an average size of approximately 50 nm, QD lifetime may change from approximately 1 to 160 ns. The corresponding second-order autocorrelation function g(2) ({\tau}) changes from antibunching into a bunching and antibunching characteristics due to the existence of long-lived metastable states in the WL. This phenomenon can be understood by treating the metal islands as many dipole oscillators in the dipole approximation, which may cause destructive interference between the exciton dipole field and the induced dipole field of metal islands.

To estimate the field-emission current density of a Ge/Si heterosystem, 20-nm germanium/silicon (100) samples were grown by molecular beam epitaxy. The surface of one sample was covered with a layer of antimony, which was removed in vacuum prior to the samples being measured. A second sample of Ge/Si was exposed to room air in the absence of antimony. The current–voltage characteristics of both samples obtained by scanning tunneling microscopy (STM) were discovered to be in agreement with classical Fowler–Nordheim theory. The density of emission current from Ge nanocrystal exceeds the density of emission current from the wetting layer of Ge/Si. The density of emission current of pure Ge nanocrystal is less than the density of emission current of Ge nanocrystal with adsorption layers.

We analyze topographic scanning force microscopy images together with Kelvin probe images obtained on Pb islands and on the wetting layer on Si(111) for variable annealing times. Within the wetting layer we observe negatively charged Si-rich areas. We show evidence that these Si-rich areas result from islands that have disappeared by coarsening. We argue that the islands are located on Si-rich areas inside the wetting layer such that the $\mathrm{Pb}/\mathrm{Si}$ interface of the islands is in line with the top of the wetting layer rather than with its interface to the substrate. We propose that the Pb island heights are one atomic layer smaller than previously believed. For the quantum size effect bilayer oscillations of the work function observed in this system, we conclude that for film thicknesses below 9 atomic layers large values of the work function correspond to even numbers of monolayers instead of odd ones. The atomically precise island height is important to understand ultrafast ``explosive'' island growth in this system.

The growth of mixed cobalt-iron oxides on Ru(0001) by high-temperature oxygen-assisted molecular beam epitaxy has been monitored in real time and real space by x-ray absorption photoemission microscopy. The initial composition is a mixed Fe-Co(II) oxide wetting layer, reflecting the ratio of the deposited materials. However, as subsequent growth of three dimensional spinel islands nucleating on this wetting layer takes place, the composition of the oxide in the wetting layer changes as iron is transferred into the spinel islands. The composition of the islands themselves also changes during growth.

In this work, we have theoretically investigated the indium segregation and In/Ga intermixing effects in lens-shaped InxGa1-xAs/GaAs quantum dots coupled to their wetting layer. In the wetting layer, the segregation phenomenon is described by Muraki's et al. model. However, the radial and vertical indium distributions resulting from In/Ga intermixing effect in the quantum dot were taken into account assuming a three-dimensional Gaussian distribution. The results obtained via the Finite Difference Method clearly showed that the introduction of such effects is of a great importance to match the photoluminescence data. The obtained results also indicated that the indium segregation in the wetting layer and the inter-diffusion inside the quantum dots dramatically affected the linear, nonlinear and total absorption coefficients of the strained nanostructure under investigation. Besides, the obtained results showed that the total absorption coefficient achieved an optimum value with a red shift of the resonant energy upon increasing the pressure and a blue shift when increasing the temperature.

Periodic modulations in chiral nematic liquid crystals (CLCs) are used in several optical and photonic devices. A remarkable way of fabricating and controlling such patterns is in a thin wetting layer that grows as temperature is decreased. Here, we study this stripe formation process experimentally and by means of Monte Carlo simulations. We report on samples of CLCs that present, after the wetting transition, a stripe/fringe formation occurring either through a nucleation process or homogeneously across the cell. These patterns depend on the ratio of the pitch to thickness of the layer, elastic anisotropy, and anchoring strength. Although such process is well documented for CLCs under applied field, we show here that similar textural transitions occur in thin layers mediated by the weak anchoring at the CLC-isotropic interface; it happens for values of pitch to thickness ratio much smaller than the ones reported in field driven situation. The Monte Carlo method takes action in simulating the experimentally observed textures by using a pairwise additive potential model for CLCs. A qualitative comparison with the experimental phenomena is possible to be made in which both the stripes formation and their behavior agree with the experiments.

Based on the data on the atomic density of a film and degree of its homogeneity during the formation of the interface between 3d transition metals (Cr, Co, Fe, or Cu) and silicon, a new concept of forming a contact between a reactive metal and a semiconductor has been justified. According to this concept, the low-temperature vapor-phase deposition of a metal onto a semiconductor is accompanied by the formation of a two-dimensional nanophase wetting layer of a metal or its mixture with silicon with a thickness of several monolayers, which significantly affects the interface formation and structure. This concept changes a perspective of forming a contact between a metal and a semiconductor substrate: it is necessary to take into account not only the formation of surface phases and clusters and/or the mixing process, but also the effect of elastic wetting of a substrate by the forming phases.

Heterostructures with InAs/AlAs quantum dots are grown on GaAs/Si hybrid substrates. The experimentally observed low-temperature (5–80 K) photoluminescence spectra of InAs/AlAs/GaAs/Si heterostructures exhibit bands defined by excitonic recombination in quantum dots and a wetting layer, i.e., a thin quantum well lying at the base of the array of quantum dots. Temperature quenching of the photoluminescence of quantum dots occurs due to the direct trapping of charge carriers at defects localized in the AlAs matrix, in the vicinity of the quantum dots.

In this paper, we theoretically report competition behaviour of size and strain effects on the InAs/GaAs quantum dots (QDs) eigenenergies, the envelop functions and also transition energies of the z-polarized WL-to-S states with varying the QD height and WL thickness. The obtained result for the bounded ground state of S implies that although the size-dependence behaviour with the inclusive of the strain effects generally resembles that of unstrained case, the energy values have changed appreciably for the short dots with value of 200 meV and for tall dots with nearly 300 meV. The competition behaviour of strain and size effects on the electronic properties of the structure revealed that the WL state is more sensitive to the size and WL thickness in presence of strain. For the weakly bounded WL state, the envelop function was localized in the wetting layer or bulk regions as a continuum state for the thick WL within the strain-modified structure. The calculated optical wavelength from z-polarized transition of WL-to-S in the strain-modified structure agrees well with previous experimental photoluminescence data from other studies.

Carrier dynamics including carrier relaxation and tunneling within a coupled InAs quantum dot (QD) – In0.15Ga0.85As quantum well (QW) hybrid nanostructure are investigated using photoluminescence (PL) spectroscopy. This coupled hybrid nanostructure shows a single PL peak from the QD emission at low excitation intensity and a band filling behavior is observed as the excitation intensity increases, suggesting that there exists a channel to capture carriers from the QW to the QDs. Time resolved PL (TRPL) measurements extract a carrier tunneling time of 103.7 ps, which is only one third of the theoretical prediction. A double-channel resonant carrier tunneling mechanism from the QW to the wetting layer and to the fifth excited state of the QDs and then carrier relaxation into lower discrete QD energy states is proposed to explain this fast carrier tunneling. The double-channel resonant carrier tunneling mechanism is qualitatively supported through the analysis of the excitation-dependent PL spectra as well as the PL excitation spectra. These results enrich our understanding of carrier dynamics in coupled QD and QW hybrid structures.

The properties of organic films are strongly influenced by their growth and structure. In this work, we link the results from three different techniques to elucidate the interplay between the morphology and the electronic structure of α-sexithiophene films grown on Au(111) surfaces. Photoelectron emission microscopy and thermal desorption spectroscopy reveal the growth of a two-layer thick wetting layer at room temperature. On top of the wetting layer, the molecules start to form μm-sized crystallites, which are statistically distributed over the surface. This transition in the growth mode is reflected in the electronic structure of the system, studied by two-photon photoemission spectroscopy. While in the wetting layer, exciton formation is suppressed by the interaction with the metal substrate, Frenkel excitons with picosecond lifetimes are observed in the three-dimensional crystallites.

A wetting layer with a nanophase structure was detected and identified before the first bulk phase during the formation of the metal-silicon interface by vapor-phase deposition at room temperature of the substrate. This became possible due to the developed technique for complex analysis of the structural-chemical state of the surface/ interface with help of Auger electron spectroscopy and electron energy loss spectroscopy, and also due to the method of physical vapor deposition at low temperature of vapor. The discovery this wetting layer and stage of its formation fundamentally changes the approach to the formation of contact between metal and silicon.

In this paper, we theoretically study the effects of the structural dimensions, pressure (P) and temperature (T) on transition lifetime (τ), on the linear (α(1)), nonlinear (α(3) ) and total α absorption coefficients for Tera-Hertz range (1.2–2.88 THz). The structure under investigation consists of two laterally coupled InAs/GaAs lens-shaped quantum dots (LCQDs) connected to a wetting layer. This structure is analyzed by using the Finite Difference Method (FDM) and the effective mass approximation. The analytical expressions of the transition life time, linear, nonlinear and total absorption coefficients are discussed by using the density matrix formalism. Obtained results show that the total absorption coefficient achieves a maximum value in Tera-Hertz range and the resonant peaks shift toward the lower energies (higher) by increasing the pressure (temperature). Also, the transition lifetime decrease (increase) by increasing the temperature (pressure). As well, the spacer width (between quantum dots) effect is well studied and can be controlled and adjusted. This theoretical study gives a new degree of freedom in describing devices based on laterally coupled QDs and on the intra-band transitions of electrons to achieve desired nonlinear optical properties in Tera-Hertz field.