Theoretical study of molecular rectification in porphyrin dimer

Carrying out theoretical calculations using the density functional method for nonequilibrium electron transport, we investigated the electric conductibility of a porphyrin dimer coupled to gold electrodes by thiolate bonds. A porphyrin with four electron-donating amino groups in the dimer is connected to a porphyrin with four electron-withdrawing cyano groups by a dimethylene bridge. The calculations demonstrated that this dimer allows more flow of electrons from the first porphyrin (donor) to the second porphyrin (acceptor) than in the opposite direction. This means that the porphyrin dimer has favorable rectifier characteristics that are accounted for by the mechanism of not Aviram and Ratner [Chem. Phys. Lett. 29 (1974) 277] but of Stokbro, et al. [J. Am. Chem. Soc. 125 (2003) 3674].     

Theoretical issues in the design of visual inspection systems

The objective of this review essay is to both chronicle and analyse literature in the area of visual inspection. Classical as well as contemporary papers are included to describe both the historical development and the state of the art of visual inspection theories and technologies. Human operators, despite well-documented problems, often perform visual inspection. While supervized machine systems obviate some of the problems associated with human inspectors, other problems still exist. In particular, accounting for a supervisor's perception of a machine's performance (as gauged, e.g. by trust) and consequent actions. The difficulties associated with these two alternatives have led to the emergence of a third alternative: collaborative human/machine or hybrid systems, which combine the advantages of both alternatives—in theory. However, in practice, how to best distribute the functions between a human and machine, in a dynamic environment in real time, is problematic. Moreover, a human's perceptions of its counterpart's performance remain an issue. These unresolved problems are subjects for future research. In the interim, the alternatives are critiqued to create a basis for establishing guidelines to select the alternative that is best suited for a given situation.     

A modelling framework to support design of complex engineering systems in early design stages

Production, assembly or logistic systems exist in widespread domains. It is agreed that more than 50% of life-cycle performance, costs and environmental impacts of such systems are due to those decisions that are made in their early design stages (Reich, Res Eng Design 28(4):411–419, https://doi.org/10.1007/s00163-017-0270-7, 2017). However, the large scale and multi-disciplinary essence of such systems make their design considerably challenging. Most of the design approaches follow a sequential approach such that the design in each lower level is finalized/frozen before proceeding to the next level. However, such approaches do not properly address the interaction between different design disciplines which may later lead to design inconsistencies. Therefore, this paper aimed to propose a modelling framework that allows having an integrated approach in the early design stages of such systems. To this end, first the framework prescribed developing an executable meta-architecture that can embody all the design requirements. Second, the framework describes the interconnections between the meta-architecture with certain supporting algorithms and optimization models. This allows generating and simulating different design alternatives and observing the impact of different design decisions on system integrated performance. Therefore, the proposed framework with its providing outcomes can be used to support the decision making in early design stages of such systems. The framework is applied in a real case study from the warehousing domain, which serves to show the practical application of the proposed framework.     

Theoretical aspects of the design of smart measurement systems

Translated from Izmeritel'naya Tekhnika, No. 10, pp. 8–10, October, 1991.     

Computer-assisted study of carrier thermalization by hopping in disordered semiconductors

Techniques for the analysis of hopping transport in disordered semiconductors are outlined and applied to the thermalization of charge carriers within a particular distribution of localized states (N(E)). The transient current is superficially similar to that for the case of trap-limited band transport. However, techniques developed assuming the dominance of this process in the calculation of N(E) yield totally erroneous results. Also, the derived values of the attempt-to-escape frequency, ν_ph, and capture cross section, σ, of the trapping centers are unrealistically small. This may well explain the anomalous results obtained in various experimental studies. The use of data obtained at different temperatures to calculate ν_ph and σ is therefore proposed as a valuable new procedure for differentiating between hopping and trap-limited band transport.     

Rule-based modelling and simulation of biochemical systems with molecular finite automata

The authors propose a theoretical formalism, molecular finite automata (MFA), to describe individual proteins as rule-based computing machines. The MFA formalism provides a framework for modelling individual protein behaviours and systems-level dynamics via construction of programmable and executable machines. Models specified within this formalism explicitly represent the context-sensitive dynamics of individual proteins driven by external inputs and represent protein-protein interactions as synchronised machine reconfigurations. Both deterministic and stochastic simulations can be applied to quantitatively compute the dynamics of MFA models. They apply the MFA formalism to model and simulate a simple example of a signal-transduction system that involves an MAP kinase cascade and a scaffold protein.     

Theoretical modelling of dopants in diamond

One of the hurdles to the use of diamond as an electronic material is the lack of shallow dopants. In this paper, the theoretical modelling of dopants in diamond is reviewed, including some of the more promising suggestions for shallow donors. Nitrogen, phosphorus, sulphur and boron are now quite reliably modelled using density functional theory. More exotic species, such as arsenic, antimony and sodium (in an interstitial site) are promising candidates for shallow donors, although their solubility is likely to be poor. Some of the complexes (N-H-N, NSi₄) may also be shallow donors, but it will be difficult to produce them in enough quantity and purity to be electronically useful.     

Charge carrier recombination via isoelectronic traps involving excitons in compensated semiconductors

The lifetimes of electrons and holes in semiconductors were studied with respect to their recombination via excitons formed on isoelectronic traps. The curve of carrier lifetime versus trap concentration is nonmonotonic and exhibits a maximum. The effect is related to a sharp decrease in the concentration of major carriers under full compensation conditions and is accompanied by sharp increase in the semiconductor resistivity.     

Theoretical considerations of thermally stimulated discharge techniques in amorphous semiconductors

Thermally stimulated discharge of a previously polarized and electroded dielectric, can generate a current with several peaks. The locations of the peaks along the thermally stimulated discharge current spectrum are characteristics of the particular mechanisms for the decay. Systematic analysis of the current peaks will yield information such as dipole relaxation characteristics and activation energies for intrinsic conduction or trapping parameters of electronic charges in the dielectric. When multilayer dielectrics such as amorphous semiconductor photoreceptors are subjected to an electret formation cycle, the heterogeneity in their structures may cause several polarization effects. For example, discontinuities in the intrinsic conductivities and dielectric constant in amorphous selenium (a-Se)-based multilayer photoreceptors can lead to the accumulation of space charges at the interfaces of the individual layers whenever the device experiences an electric stress for a period of time which is of the order of its effective dielectric relaxation time. Charge trapping by states associated with the heterogeneities of the structure cause an electrical polarization which can have a significant impact on the xerographic performance of the photoreceptor. The purpose of the present series of papers is firstly to describe the principles of thermally stimulated discharge techniques, the associated theories and interpretation of the current spectrum and secondly, to discuss applications of these techniques to a-Se:Te/Se double layer photoreceptors. The principles of thermally stimulated discharge and relevant theories are discussed.     

Theoretical modelling of laminated composite plates

Formulation of appropriate governing equations, simpler than the three-dimensional equations of elasticity yet capable of predicting, fairly accurately, all important response parameters such as stress and strain, is attempted in modelling a structural component. Several theoretical models are available in the literature for the analyses of plates. The emergence of fibre-reinforced plastics as an attractive form of structural construction, added a new complexity to the modelling considerations of laminates by requiring the estimation of the interlaminar stresses and strains. In this paper, modelling considerations of laminated composite plates are discussed. The classical laminated plate theory and higher-order shear deformation models are reviewed to bring out their interlaminar stress predictive capabilities, and some new modelling possibilities are indicated. This work has been supported by the Aeronautics Research and Development Board, Ministry of Defence, Government of India.     

Interpretation in design: modelling how the situation changes during design activity

This paper presents a model of the way that designers move between situations when interpreting during design activity. Three hypotheses are presented that arise from this model: that designers change their situation during interpretation, that small changes in a source can lead to large changes in the representation and that changes to the situation have their origins in the experience of the designer. The paper demonstrates how this internal movement between situations can be computationally implemented using three examples. The systems implemented demonstrate the way that interpretation can lead to changes in the situation and present an example of how the changes to a designer’s situation can be guided by past experiences.     

Controlled nanoscale doping of semiconductors via molecular monolayers

One of the major challenges towards scaling electronic devices to the nanometre-size regime is attaining controlled doping of semiconductor materials with atomic accuracy, as at such small scales, the various existing technologies suffer from a number of setbacks. Here, we present a novel strategy for controlled, nanoscale doping of semiconductor materials by taking advantage of the crystalline nature of silicon and its rich, self-limiting surface reaction properties. Our method relies on the formation of a highly uniform and covalently bonded monolayer of dopant-containing molecules, which enables deterministic positioning of dopant atoms on the Si surfaces. In a subsequent annealing step, the dopant atoms are diffused into the Si lattice to attain the desired doping profile. We show the versatility of our approach through controlled p- and n-doping of a wide range of semiconductor materials, including ultrathin silicon-on-insulator substrates and nanowires, which are then configured into novel transistor structures.     

Service-based manufacturing systems: modelling and control

In service-based manufacturing systems, functionalities are independently developed as services and a central engine orchestrates their integration. As industrial processes tend to be very large, and performance and productivity are expected to be maximised, there is a constant interest in providing (in-advance) quality guarantees for services interactions, which contrasts with the usual non-automated workflow design. This paper provides an alternative to enhance service orchestration capabilities using supervisory control techniques. Initially, each component (atomic and composite activities) belonging to an orchestration language is modelled as a state-machine. Then, activity models are properly combined and composed, reproducing orchestrated workflows. Finally, supervisory control is used to calculate an optimal version of the orchestrator. Practical implications of handling large state-spaces are discussed and examples are provided.     

Atom Probe Tomography and field evaporation of insulators and semiconductors: Theoretical issues

After reviewing the physics and chemistry in high electrostatic fields and summarizing the theoretical results for Atom Probe Tomography of metallic tips, we turn to the new challenges associated with insulators and semiconductors with regard to local fields inside and on the surface of such materials. The recent (theoretical) discovery that in high fields the band gap in these materials is drastically reduced to the point where at the evaporation field strength it vanishes will be crucial in our discussion.     

Related yet diverging sectoral systems: telecommunications equipment and semiconductors in China

This study examines the evolution of the telecommunications equipment and semiconductor industries in China from 1978 to 2012 using the sectoral systems framework. The article advances research on sectoral systems by examining the diverging evolution of two vertically related industries from a common starting point. In one case, system factors worked to foster global competitiveness, while in the other case, system factors slowed the formation of a competitive industry. The findings contribute to research on industrial dynamics and catching up by suggesting that the characteristics of vertically integrated industries may be seen as two sectoral systems that interact and influence the speed and direction of innovation and industrial development of each other’s industries.     

Co-solvent systems in dissolution testing: Theoretical considerations

The situation was considered where a co-solvent, added to enhance drug solubility and thus provide sink conditions for a drug, also resulted in a decrease in solubility and hence dissolution rate of a water soluble excipient present in the dosage form. Theory predicted that, in the absence of disintegration, at sufficiently high excipient content and or co-solvent concentration the drug dissolution rate may decrease, release becoming controlled by the now less soluble excipient. Solubility and dissolution studies using tolbutamide and lactose mixtures in water-ethanol mixtures provided results which were reasonably consistent with theory, indicating that the presence of the excipient may greatly reduce the drug dissolution rate below that expected for the drug alone.     

Computer modelling of molecular ionic materials

This review describes recent advances and results achieved in the computer modelling of molecular ionic materials. These materials offer particular challenges in modelling because both ionic and covalent interactions contribute to their interatomic forces, and both are therefore responsible for their properties. Ionic materials have been extensively studied, as have molecular solids, and in modelling molecular ionic materials some techniques from both of these are borrowed and used.     

Physico-chemical deformations of solidifying cementitious systems: multiscale modelling

At early stages of hydration and in autogenous conditions (no mass transfer with the outside), solidifying cementitious systems exhibit dimensional variations following two main processes: Le Chatelier contraction (also called chemical shrinkage) and self-desiccation shrinkage causing autogenous shrinkage. Chemical shrinkage results from the difference between the specific volumes of reactants (anhydrous cement and water) and hydration products. Early-age autogenous shrinkage is generally attributed to the development of a negative capillary pressure in the porous network related to the water consumption by the hydration reactions. If restrained, deformations associated to these shrinkages can induce the development of internal stresses high enough to generate cracking of the hardening material. The purpose of this study is to propose a multiscale approach to model the rate of self-desiccation shrinkage of cementitious materials at very early-age, between 0 and 48 h. Within the first hours, Le Chatelier contraction is computed from a formulation suggested in a later work which is based on the chemical equations of hydration and the specific volume of each phase. Then, when the setting of the cement paste takes place, the autogenous shrinkage is calculated according to the evolution of the capillary pressure and the stiffness of the cement paste. The stiffness is calculated by applying a classical homogenization method. Computed results are discussed and analyzed. Good agreements between experiments and simulations are achieved and a sensitivity study is performed to assess the influence of the cement fineness and the aggregate volume fraction on early-age autogenous strain.