Andrzej Daniluk

Solvent effects and vibrational dependence in electrochromic spectra of carotenoids

Abstract Electrochromic (Stark effect) spectra of three carotenoids, β-carotene, lutein and violaxanthin, were obtained in glassy matrices at low temperature. When analyzed in the framework of the theory of electrochromism they were found to contain a remarkable contribution from the second derivative of the absorption spectrum, equivalent to a substantial change in dipole moment (3–5 D) on electronic excitation, in addition to the usual polarizability term. These dipole moments only weakly depend on solvent polarity; this puts in doubt the induced dipole model. In the case of violaxanthin, a variability of the electro-optical parameters along the electrochromic spectrum was found, which is related to the type of vibration involved in the electronic transition. An analogous effect was also noted for tetradecaheptaene chromophore in amphotericin B. These observations strongly indicate an essential role of vibronic coupling in determining the electro-optical parameters of carotenoids.

Studies on RHEED oscillations at low glancing angles

Abstract A new approach for interpreting RHEED intensity oscillations is presented. Only low values of the glancing angle of the incident electron beam are considered. The intensity of the specularly reflected beam is calculated by solving the Schrodinger equation with a one-dimensional model of a scattering potential of a growing film. For low values of the glancing angle (less than 0.5° for an electron energy of 20 keV) such calculations may be expected to lead to similar results to those obtained assuming a three-dimensional model of the potential. In the present paper numerically determined shapes of the intensity oscillations are shown for different cases of deposition of atoms at surfaces of growing films.

RHEED intensity oscillations observed during growth of Ge on Si(111) substrates

Reflection high-energy electron diffraction (RHEED) intensity oscillations were observed during molecular beam epitaxy growth of Ge on a Si(111) surface. At 250°C the oscillations continue up to 6 ML. When Ge is grown at room temperature on the GeSi(111) surface the oscillations continue up to 14 ML. During the growth of Ge thin films on a clean Si(111)-7 × 7 surface at room temperature the oscillations continue up to 10 ML. The intensity of the reflected beam is calculated by solving the one-dimensional Schrodinger equation. A simple birth-death model was used for the growth simulation in order to investigate fundamental behaviours of reflectivity change during the Stranski-Krastanov growth of Ge on the Si(111) surface at 250°C.

Dynamical calculations for RHEED intensity oscillations

Abstract A practical computing algorithm working in real time has been developed for calculating the reflection high-energy electron diffraction from the molecular beam epitaxy growing surface. The calculations are based on the use of a dynamical diffraction theory in which the electrons are taken to be diffracted by a potential, which is periodic in the dimension perpendicular to the surface. The results of the calculations are presented in the form of rocking curves to illustrate how the diffracted beam intensities depend on the glancing angle of the incident beam. Program summary Title of program: RHEED Catalogue identifier: ADUY Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADUY Program obtainable from: CPC Program Library, Queens University of Belfast, N. Ireland Computer for which the program is designed and others on which it has been tested: Pentium-based PC Operating systems or monitors under which the program has been tested: Windows 9x, XP, NT, Linux Programming language used: Borland C++ Memory required to execute with typical data: more than 1 MB Number of bits in a word: 64 bits Number of processors used: 1 Distribution format: tar.gz Number of lines in distributed program, including test data, etc.: 982 Number of bytes in distributed program, including test data, etc.: 126 051 Nature of physical problem: Reflection high-energy electron diffraction (RHEED) is a very useful technique for studying growth and surface analysis of thin epitaxial structures prepared by the molecular beam epitaxy (MBE). Nowadays, RHEED is used in many laboratories all over the world where researchers deal with the growth of materials by MBE. The RHEED technique can reveal, almost instantaneously, changes either in the coverage of the sample surface by adsorbates or in the surface structure of a thin film. In most cases the interpretation of experimental results is based on the use of dynamical diffraction approaches. Such approaches are said to be quite useful in qualitative and quantitative analysis of RHEED experimental data. Method of solution: RHEED intensities are calculated within the framework of the general matrix formulation of Peng and Whelan [Surf. Sci. Lett. 238 (1990) L446] under the one-beam condition. The dynamical diffraction calculations presented in this paper utilize the systematic reflection case in RHEED, in which the atomic potential in the planes parallel to the surface are projected on the surface normal, so that the results are insensitive to the atomic arrangement in the layers parallel to the surface. This model shows a systematic approximation in calculating dynamical RHEED intensities, and only a layer coverage factor for the n th layer was taken into account in calculating the interaction potential between the fast electron and that layer. Typical running time: The typical running time is machine and user-parameters dependent. Unusual features of the program: The program is presented in the form of a basic unit RHEED.cpp and should be compiled using C++ compilers, including C++ Builder and g++.

Visual modeling for scientific software architecture design. A practical approach

Abstract This paper presents the problem of Molecular Beam Epitaxy and Reflection High-Energy Electron Diffraction with the help of a unified, modern MDA approach. Model-Driven Architecture (MDA) constitutes a modern and unusually efficient method of improving the process of generating software. It was created at the beginning of the twenty-first century by the Object Management Group as an element of Model-Driven Development, a highly promoted trend in software engineering. In MDA a viewpoint on a system is a technique for abstraction using a selected set of architectural concepts and structuring rules, in order to focus on particular concerns within a system. In MDA, system design begins with defining the problem domain. Next, at a highly abstract level—independent of the system and programming platform—a Platform-Independent Model (PIM) is constructed as well as a general system specification. This specification is created with the help of Unified Modeling Language. The real implementation of the system is performed through the transformation of PIM to Platform-Specific Model (PSM). The essence of Model-Driven Architecture is the replacement of the twentieth century approach to programming, calling that “everything is an object”, to the modern—“everything is a model”.

Kinematical calculations of RHEED intensity oscillations during the growth of thin epitaxial films

A practical computing algorithm working in real time has been developed for calculating the reflection high-energy electron diffraction (RHEED) from the molecular beam epitaxy (MBE) growing surface. The calculations are based on the use of kinematical diffraction theory. Simple mathematical models are used for the growth simulation in order to investigate the fundamental behaviors of reflectivity change during the growth of thin epitaxial films prepared using MBE.

RHEED intensity oscillations observed during the growth of YSi2 − x on Si(111) substrates

Abstract For the first time reflection high-energy electron diffraction intensity oscillations were observed during reactive deposition epitaxy (RDE) growth of YSi2 − x (x ≅0.3) on the Si(111) surface. The YSi2 − x crystallographic structure consists of Y planes alternating with Si planes parallel to the Si(111) substrate planes. The intensity of the reflected beam is calculated by solving the one-dimensional Schrodinger equation. A birth-death model was used for the growth simulation in order to investigate fundamental behaviours of reflectivity change during the growth of YSi2 − x on the Si(111) surface.

Monte-Carlo simulation of Ge on Si(111) MBE growth: analysis of percolative structure

Abstract Reflection high energy electron diffraction (RHEED) intensity oscillations were observed during molecular beam epitaxy (MBE) growth of Ge on Si(111) surface. At 250 °C the oscillations continue up to 6 monolayers. The intensity of the reflected beam is calculated by solving the one-dimensional Schrodinger equation. Monte-Carlo simulation was used for the growth simulation in order to investigate fundamental behaviours of reflectivity change during the Stranski-Krastanov growth of Ge on the Si(111) surface at 250 °C.

Distributed growth model used for the interpretation of RHEED intensity oscillations observed during the growth of Pb on Si(111) substrates

Abstract Analyses of reflection high-energy electron diffraction (RHEED) intensity changes observed during initial stages of heteroepitaxial growth of Pb on Si(111) substrates were presented. The intensity of the reflected beam is calculated by solving the one-dimensional Schrodinger equation. A simple birth-death model was used for the growth simulation in order to investigate the fundamental behaviours of reflectivity change during the growth of Pb on the Si(111) surface.

RHEED intensities from two-dimensional heteroepitaxial nanoscale systems of GaN on a 3C-SiC(111) surface

This paper presents a computer program, which facilitates the calculation of changes to the intensity of RHEED oscillations from the heteroepitaxial structures of (0001)GaN films nucleated on a Si surface. The calculations are based on the use of a dynamical diffraction theory and different models of scattering crystal potential. New version program summary Title of program: RHEED_DIFF_W Program Files doi: 10.17632/hn3pt6ytky.1 Licensing provisions: GNU General Public License 3 Programming language used: C++ Journal reference of previous version: Computer Physics Communications 185 (2014) 3001–3009 Does the new version supersede the previous version?: No. It does supersede version AETW_v1_0. Reasons for the new version: Responding to the users’ feedback we presented a practical procedure of construction of simulation program, which facilitates the calculation of changes to the intensity of RHEED oscillations in the function of the glancing angle of incidence of the electron beam, employing variousmodels of scattering crystal potential for heteroepitaxial structures of hexagonal (0001)GaN films nucleated on a Si surface. GaN has a wurtzite crystal structure with alternating planes of Ga and N atoms and lattice constants a = 3.189 Å and c = 5.185 Å [1]. Choosing the top surface to be z = 0, the N atoms are located at z = nd where d = 5.185/2 Å and n is an integer, and Ga atoms are at z = nd − u, where u = 3c/8 (as shown in Fig. 1). The nature of physical problem: Since the discovery of molecular beam epitaxy, which allows formation of layers (crystals) with a controlled composition of subsequent monolayers, the research attempts of many laboratories have been focused on studying the dynamics of formation of both monolayers and specific heterojunction structures, that is the structures with monocrystalline interfaces created by materials of various chemical compositions (different physical and chemical properties). The fundamental scientific problem in such research is to specify both interface type and growth mechanism for subsequent layers. Researchers and technologists manufacturing two-dimensional nanoscale systems frequently use RHEED rocking curves (the specular beam intensities versus the glancing angle) to control growth of films at the atomistic level of accuracy. The structural properties of the GaN/Si interface have been meticulously studied by many [3–5], and crucial problem of the direct growth of GaN on Si is the formation of amorphous silicon nitride (SiN) at the GaN/Si interface. Typical running time: The typical running time ismachine and user-parameters dependent, i.e. the number of layers for both the substrate and the growing layers included in the calculations. The method of solution: RHEED intensities are calculated within the general framework described in Ref. [2] with the model of the scattering potential for heterostructures: