Stacking Systems Research Articles

Cross-Transition-Dipole Stacking of Conjugated Organic Molecules: Structure, Exciton Behavior and Optoelectronic Property.

Organic conjugated molecules have gained widespread application as organic semiconductors due to their unique optoelectronic properties. The rigidity of these large conjugated structures facilitates strong intermolecular interactions, which significantly influence their properties in the solid state through various molecular arrangements. The study of the relationship among molecular arrangement, exciton behavior, and optoelectronic properties is an eternal research topic. Cross-dipole stacking is a specific molecular arrangement that demonstrates unique characteristics and has received continuous attention over the past decades. This mini-review will first discuss the unique exciton behaviors in cross-dipole stacking based on exciton models, including weak exciton coupling and suppression of Förster resonance energy transfer. These exciton behaviors, determined by molecular stacking arrangements, are fundamental to the optoelectronic properties of cross-dipole stacking systems. Next, we will introduce well-defined cross-dipole systems and summarize their design principles from a molecular structure perspective. Finally, we will present the specific optoelectronic properties of cross-dipole stacking systems and their outstanding performance, such as high solid-state luminescence, good charge carrier mobility, and significant CD/CPL. Through this mini-review, we hope to enhance the understanding of cross-dipole stacking, contributing to the construction of such systems, the exploration of excited-state behaviors, and the discovery of high-performance materials.

Low-frequency Raman active modes of twisted bilayer MoS2

We study the low-frequency Raman active modes of twisted bilayer MoS2 for several twist angles using a force-field approach and a parametrized bond polarizability model. We show that twist angles near high-symmetry stacking configurations exhibit stacking frustration that leads to significant buckling of the moiré superlattice. We find that atomic relaxation due to the twist is of prime importance. The periodic displacement of the Mo atoms shows the realization of a soliton network, and in turn, leads to the emergence of a number of frequency modes not seen in the high-symmetry stacking systems. Some of the modes are only seen in the XZ Raman polarization setup while others are seen in the XY setup. The symmetry of the normal modes, and how this affects the Raman tensors is examined in detail.

Hückel Molecular Orbital Analysis for Stability and Instability of Stacked Aromatic and Stacked Antiaromatic Systems.

Face-to-face stacking of aromatic compounds leads to stacked antiaromaticity, while that of antiaromatic compounds leads to stacked aromaticity. This is a prediction with a long history; in the late 2000s, the prediction was confirmed by high-precision quantum chemical calculations, and finally, in 2016, a π-conjugated system with stacked aromaticity was synthesized. Several variations have since been reported, but essentially, they are all the same molecule. To realize stacked aromaticity in a completely new and different molecular system and to trigger an extension of the concept of stacked aromaticity, it is important to understand the origin of stacked aromaticity. The Hückel method, which has been successful in giving qualitatively correct results for π-conjugated systems despite its bold assumptions, is well suited for the analysis of stacked aromaticity. We use this method to model the face-to-face stacking systems of benzene and cyclobutadiene molecules and discuss their stacked antiaromaticity and stacked aromaticity on the basis of their π-electron energies. By further developing the discussion, we search for clues to realize stacked aromaticity in synthesizable molecular systems.

Layer engineering piezotronic effect in two-dimensional homojunction transistors

The semiconductor junctions based on piezoelectric materials are of great importance for the realization of functionality and versatility in sensing devices. Two-dimensional (2D) stacked materials provide a promising route to design novel semiconductor junctions due to their layer-dependent electronic and optical properties. However, the combination of layer-dependent band structure and piezoelectricity in device designs is rarely explored to date. In this work, we propose piezotronic transistors designed by layer engineering 2D homojunction. A 2D Poisson equation including the piezoelectric polarization is derived and then combined self-consistently with the non-equilibrium Green function to explore the carrier transport. We find a peculiar symmetrical modulation mechanism of piezotronic effect on carrier transport, i.e., flipping the sign of externally applied strain can drive symmetrical transport process regarding bias polarity. Additionally, the results show that the transport is a mixing of thermionic and tunneling current, which are comparable at low bias but predominated by the quantum tunneling effect at high bias. Although our 2D homojunction transistor is similar to Schottky diodes, the on/off ratio and gauge factors are higher than most of metal-semiconductor contact piezotronic devices. Moreover, for the device with symmetrical stacking configurations, increasing layer number in 2D homojunction can decrease the output current but enhance the gauge factors. By contrast, the device performance in asymmetrical stacking systems is insensitive to the variation of layer number. Our demonstration of layer engineering 2D homojunction transistor not only enriches the fundamental physics of piezotronics, but also open an avenue for designing highly sensitive strain sensors based on diverse 2D layered junctions.

High carrier mobilities and broad absorption spectra in twisted and non-twisted bilayer GaN, BN, and ZnO structures

Stacking is effective in modulating the properties of two-dimensional (2D) materials, and more interesting properties exist in twisted stacking systems. The electronic and optical properties of twisted-13.17°, twisted-21.78°, and non-twisted-AA’ structures of GaN, BN, and ZnO have been studied by using first-principles. The calculated results indicate that all structures are stable, and the band gaps of the three structures are modulated effectively. Compared with the monolayer, the twisted structures of GaN, BN, and ZnO can enhance absorption peaks and broaden the absorption spectra. The carrier mobilities of the twisted and non-twisted structures have been investigated by using DP theory. The twisted-13.17° and twisted-21.78° structures of the BN exhibit much high hole mobilities along the y direction of 55.92 and 44.30 @103 cm2 V−1 s−1 respectively. The twisted-13.17° structure of ZnO exhibits much high electron mobilities of 56.81 (x direction) and 16.57 (y direction) @103 cm2 V−1 s−1. The band gap, carrier mobility, and absorption spectrum of the twisted and non-twisted GaN, BN, and ZnO stacking structures have been modulated effectively, and these stacking structures will promise important application prospects in the field of ultraviolet broadband electronic devices and photoelectric devices.

Design of Integration Framework Towards Industry 4.0: From Shop Floor to Cloud

The capability of the fourth industrial revolution in fulfilling the ever-changing demand of customers has attracted great attention from the small medium enterprises. Industry 4.0 is one of the ideal solutions to advance the manufacturing to a new level to tackle the high variation in productions of roofing industry. A three-layer Industry 4.0 integration framework, adoptable by a considered roofing company was proposed in this study. A conceptual mechanical designs of automated stacking systems also have been proposed to automate the considered stacking process in the roofing company to minimize the manual labour workforce. The proposed framework realized the complete integration from field layer to the Cloud system using Automation Markup Language and OPC Unified Architecture technologies as the communication backbone. The feasibility of the proposed framework was demonstrated through experiment conducted on an experimental system model where the Google Cloud, the OPC UA-AutomationML server, and a microcontroller-controlled system was used as the enterprise layer, communication layer, and the field layer, respectively. The success of integration was proven by controlling the light intensity and ON/OFF status of a physical LED through the Cloud. For future works, the proposed framework could be implemented on a simulated model of real-time prototype or a real-time manufacturing line to test its robustness.

Effects of conjugated structure on electronic and transport properties in organic–inorganic hybrid superlattices Cd2Se2(C2H4N2)1/2

By inducing π-conjugated organic molecule C2H4N2 in group II–VI based CdSe network structure materials, the band structures and carrier transport of organic–inorganic hybrid superlattices Cd2Se2(C2H4N2)1/2 were investigated via first-principles calculations based on the density functional theory. With different stacking patterns, it is found that the carrier mobility can be modulated by 5–6 orders of magnitude. The physical mechanism of the high carrier mobility in the hybrid structures has been revealed, which means dipole organic layers realize electron delocalization via electrostatic potential difference and build-in electric field. Our calculations shown that the dipole organic layers originate from asymmetric π-conjugated organic molecules and the charges movement between molecules, while symmetric organic molecules tend to electrostatic balance. And although the electronic transport properties were highly restrained by the flat bands of organic layers around Fermi energy in most structures, we found that the collective electrostatic effect can lead to very high electron mobility in AA1 and AA2 stacking systems, which might be attributed to the superposition of molecule electrostatic potential along with electrons transfer between molecules. Furthermore, it is also found that the anisotropy of electron mobility can be modulated via the difference directions of dipole layers.

Improvement of Container Terminal Productivity with Knowledge about Future Transport Modes: A Theoretical Agent-Based Modelling Approach

Despite all the achievements in improving container terminal performance in terms of equipment and container stacking systems (CSS), terminal operators are still facing several challenges. One of these challenges is the lack of information about further transportation modes of the container, which leads to extra movements of the container inside the stacking area. Hence, we aimed to examine factors that affect container handling processes and to evaluate a container terminal’s overall equipment effectiveness. This study used data from a container terminal at the Port of Antwerp, Belgium. An agent-based model was developed based on a block-stacking strategy to investigate two scenarios: (1) having information about further transportation modes and (2) a base scenario. The Overall Equipment Effectiveness Index (OEE) was also adopted to evaluate the container terminal’s effectiveness in both scenarios. Results showed that having information on further transportation mode significantly increased the container outflow, and the OEE index improved compared to the base scenario’s results. Therefore, we recommend an integrated data-sharing system where all the stakeholders can share their information with no fear of losing their market share.

Revisiting stacking interactions in tetrathiafulvalene and selected derivatives using tight-binding quantum chemical calculations and local coupled-cluster method.

The engineering of supramolecular architectures needs accurate descriptions of the intermolecular interactions in crystal structures. Tetrathiafulvalene (TTF) is an effective building block used in the construction of promising functional materials. The parallel packing of the neutral TTF-TTF system was studied previously using the high-level quantum chemical method, advancing it as a valuable model system. The recently developed tight-binding quantum chemical method GFN2-xTB and local coupled-cluster method DLPNO-CCSD(T) were used to investigate the stacking interactions of TTF and selected derivatives deposited in the Cambridge Structural Database. Using the interaction energy of the TTF-TTF dimer calculated at the CCSD(T)/CBS level as the reference, the accuracies of the two methods are investigated. The energy decomposition analysis within the DLPNO-CCSD(T) framework reveals the importance of dispersion interaction in the TTF-related stacking systems. The dispersion interaction density plot vividly shows the magnitude and distribution of the dispersion interaction, providing a revealing insight into the stacking interactions in crystal structures. The results show that the GFN2-xTB and DLPNO-CCSD(T) methods could achieve accuracy at an affordable computational cost, which would be valuable in understanding the nature of parallel stacking in supramolecular systems.

Circular bead-like shapes of cell adhesion structures in 2D case

ABSTRACT Due to the strong deformation ability of cells, the cell adhesive stacking systems are more intricate than atom stacking systems. A general method to reveal their configurations still remains challenging. In this work a theoretic model is proposed to study the circular bead-like adhesion structures composed by equal cells in the two-dimensional (2D) case. Based on the free energy minimization theory, the equilibrium shape equations are obtained and analytical solutions are investigated for high symmetrical structures without volume constraints. All results are confirmed by the finite element method. These shapes are close to some bio-structures, such as the cross section of raspberry and orange. The symmetrical breaking is studied for high symmetrical structures with volume constraints and it reveals that there are abundant degenerated states in a cell adhesion system. These results are helpful for understanding the formation mechanism of the self-organizing cell adhesion structures.

Measurement and Analysis of Transport Vibration for LTL Freight Utilizing Stowage Optimization Equipment

The random vibration spectra typically used in lab based simulations towards developing and validating optimized packages for distribution have been developed through past research that have studied global transport vibration environments. Approximately 67% of all the freight tonnage moved in the U.S. utilizes heavy duty Class 8 trucks. While the floor (suspension) based vibration simulation profiles for these types of trucks are available towards lab based simulation, there have been no studies investigating the impact of shelving and stacking systems typically used for optimization of freight stowage in truck trailers by many freight companies. It is important to quantify how these storage systems affect the vibration levels experienced by loads within the trailers. This study utilized data recorders to measure and analyze the vibration levels experienced by products placed on aluminum decking beams and steel decking tables commonly used for stacking of loads on truck trailers in less-than-truckload (LTL) freight. Random vibration profiles were measured from the aluminum decking beams, steel decking tables and floor of the truck trailers. The results of this study show that the vibration levels experienced by LTL cargo, when decking tables and beams are used for stowage, are significantly different in comparison to the commonly used International Safe Transit Association steel spring truck profile. Using the power density spectrums developed in this study, packaging engineers should be able to develop and/or validate optimum protective packaging systems for LTL cargo transportation in North America that implement such stowage practices.

General van der Waals potential for common organic molecules

This work presents a systematic development of a new van der Waals potential (vdW2016) for common organic molecules based on symmetry-adapted perturbation theory (SAPT) energy decomposition. The Buf-14-7 function, as well as Cubic-mean and Waldman–Hagler mixing rules were chosen given their best performance among other popular potentials. A database containing 39 organic molecules and 108 dimers was utilized to derive a general set of vdW parameters, which were further validated on nucleobase stacking systems and testing organic dimers. The vdW2016 potential is anticipated to significantly improve the accuracy and transferability of new generations of force fields for organic molecules.

Container relocation problem with time windows for container departure

The blocks relocation problem is a classic combinatorial optimisation problem that occurs in daily operations for facilities that use block stacking systems. In the block stacking method, blocks can be stored on top of each other in order to utilise the limited surface of a storage area. When there is a predetermined pickup order among the blocks, this stacking method inevitably leads to the reshuffling moves for blocks stored above the target block and the minimisation of such unproductive reshuffling moves is of a primary concern to industry practitioners. A container terminal is a typical place where this problem arises, thus the problem being also referred to as the container relocation problem. In this study, we consider departure time windows for containers, which are usually revealed by the truck appointment system in port container terminals. We propose a stochastic dynamic programming model to calculate the minimum expected number of reshuffles for a stack of containers which all have departure time windows. The model is solved with a search-based algorithm in a tree search space, and an abstraction heuristic is proposed to improve the time performance. To overcome the computational limitation of exact methods, we develop a heuristic called the expected reshuffling index (ERI) and evaluate its performance.

Classifier Subset Selection for the Stacked Generalization Method Applied to Emotion Recognition in Speech.

In this paper, a new supervised classification paradigm, called classifier subset selection for stacked generalization (CSS stacking), is presented to deal with speech emotion recognition. The new approach consists of an improvement of a bi-level multi-classifier system known as stacking generalization by means of an integration of an estimation of distribution algorithm (EDA) in the first layer to select the optimal subset from the standard base classifiers. The good performance of the proposed new paradigm was demonstrated over different configurations and datasets. First, several CSS stacking classifiers were constructed on the RekEmozio dataset, using some specific standard base classifiers and a total of 123 spectral, quality and prosodic features computed using in-house feature extraction algorithms. These initial CSS stacking classifiers were compared to other multi-classifier systems and the employed standard classifiers built on the same set of speech features. Then, new CSS stacking classifiers were built on RekEmozio using a different set of both acoustic parameters (extended version of the Geneva Minimalistic Acoustic Parameter Set (eGeMAPS)) and standard classifiers and employing the best meta-classifier of the initial experiments. The performance of these two CSS stacking classifiers was evaluated and compared. Finally, the new paradigm was tested on the well-known Berlin Emotional Speech database. We compared the performance of single, standard stacking and CSS stacking systems using the same parametrization of the second phase. All of the classifications were performed at the categorical level, including the six primary emotions plus the neutral one.

On the abstraction method for the container relocation problem

The container relocation problem or the blocks relocation problem is a classic combinatorial optimisation problem that occurs in day-to-day operations for facilities that use block stacking systems. A typical place where this problem arises is a container terminal where containers can be stacked vertically in order to utilise the scarce resource of yard surface, thus at times resulting in the unproductive reshuffling moves for containers stacked above the target container for retrieval. Due to the problem class being NP-hard, a number of studies on this topic propose heuristic approaches to solve this problem. There are a few exact methods (search-based algorithms or mathematical programming) proposed for this problem but the feasible problem size of such methods is quite restricted, limiting their practical significance. In this paper, we propose a new insight into reducing the search space of this problem by the abstraction method. Our main contribution to the existing literature is two-fold: the reduction in the search space by the abstraction method and the bidirectional search using the pattern database. Our computational results confirm that our approach enables instances of a near-practical size to be solved optimally within a reasonable computation time.

Gene stacking by recombinases.

Efficient methods of stacking genes into plant genomes are needed to expedite transfer of multigenic traits to crop varieties of diverse ecosystems. Over two decades of research has identified several DNA recombinases that carryout efficient cis and trans recombination between the recombination sites artificially introduced into the plant chromosome. The specificity and efficiency of recombinases make them extremely attractive for genome engineering. In plant biotechnology, recombinases have mostly been used for removing selectable marker genes and have rarely been extended to more complex applications. The reversibility of recombination, a property of the tyrosine family of recombinases, does not lend itself to gene stacking approaches that involve rounds of transformation for integrating genes into the engineered sites. However, recent developments in the field of recombinases have overcome these challenges and paved the way for gene stacking. Some of the key advancements include the application of unidirectional recombination systems, modification of recombination sites and transgene site modifications to allow repeated site-specific integrations into the selected site. Gene stacking is relevant to agriculturally important crops, many of which are difficult to transform; therefore, development of high-efficiency gene stacking systems will be important for its application on agronomically important crops, and their elite varieties. Recombinases, by virtue of their specificity and efficiency in plant cells, emerge as powerful tools for a variety of applications including gene stacking.

DFT studies on hydrogen-bonding, Stacking, and XH···π-Bonded systems in presence of external electric field

Effect of external electric field on interaction energy as well as stability of the hydrogen-bonding, stacking, and OH π-bonded systems are analyzed in the light of density functional theory (DFT) and conceptual DFT. Interaction energy and stability measured in terms of global hardness and highest occupied molecular orbital energy of the considered systems are observed to be sensitive toward the strength and direction of the applied external electric field. The curvature of the potential energy surfaces gets changed in presence of an external electric field. © 2015 Wiley Periodicals, Inc.

Functional Surfactants for Carbon Nanotubes: Effects of Design

Surfactants are needed to create stable suspensions of carbon nanotubes. Increasingly, these surfactants are given additional functionalities, resulting in bigger and more complex molecules with several subunits. We investigate the effect of assembly of these subunits for a class of perylene-based functional surfactants. The subunits that all surfactants are based on are a perylene core, hydrophilic polyglycerol dendrons, and alkyl chains of different orientations and lengths. The assembly of these subunits affects both the molecules' performance as a surfactant and the efficiency of the energy-transfer complexes formed by the nanotube and surfactant through a π–π stacking mechanism. This results in a best practice guide for designing functional surfactants with π–π stacking cores, and affords more general insights that are applicable to non π–π stacking systems as well.

Aromatic interaction profile to understand the molecular basis of raltegravir resistance

Integrase (IN) is the enzyme of human immunodeficiency virus (HIV) which inserts the viral DNA (vDNA) into the host genome for successful viral replication leading to the infection. However, the chemical basis of HIV IN catalysis is speculative due to lack of complete co-crystal structure. Using the recently published prototype foamy virus IN crystal structure, we developed a model structure of HIV IN showing interaction of vDNA, the metal (Mg2+) cofactor, and raltegravir (RLT) in the active site. Molecular docking and dynamics simulations studies showed that RLT uses it core central ring with diketo motif for Mg2+ chelation and bridge interaction with DDE motif. The triple arene interactions mediated by RLT with neighboring molecular motifs (Y143, cytosine, and adenine) is maintained during long simulation in wild type (WT). The fluorobenzyl and oxadiazole moieties of RLT forms aromatic stacking with cytosine base (head stacking) aromatic side chain of Y143 (tail stacking), respectively, while central ring further establishes aromatic stacking with distorted adenine base of vDNA (central stacking). The novel triple stacking systems were further explored to understand the molecular basis of drug resistance by molecular simulation. The in silico mutation (N155H, Q148H, and Q148H + G140S) and simulation studies elucidated the structural mechanism of resistance to RLT. The simulation studies provided the molecular basis for interdependency observed for the primary and secondary (Q148H and G140S) mutations and also explained the mechanism of viral fitness regain. Our study reveals that triple stacking and its consequence in terms of VdW energetic profile acts as a critical point to understand the drug-resistance. Here, we demonstrate that the root mean square deviation of centroid system (aromatic stacking) can be used as a major determinant of RLT binding toward the fold resistance. This is first kind of report, which discloses a strategy to explore the molecular level of drug resistance profile using aromatic interactions.

Intermolecular interaction and magnetic coupling mechanism on a mononuclear CoII complex

Anti-ferromagnetic interaction was observed in a new crystal that consists of mononuclear CoII complexes, namely [Co(PMP)(N3)] (PMP = 2,9-bis(pyridin-2-methoxyl)-1,10-phenanthroline); in the mononuclear complex CoII has a distorted trigonal-bipyramidal geometry. Analysis for the crystal structure indicates six magnetic coupling pathways among adjacent complexes, in which three involve π–π stacking and the other three deal with intermolecular interactions. The fitting for the variable-temperature magnetic susceptibilities with the Curie–Weiss formula shows an anti-ferromagnetic interaction between adjacent CoII ions with θ = −5.49 K = −3.82 cm−1. Theoretical calculations on the spin section reveal that the three π–π stacking systems result in magnetic coupling constants 2J = −0.10 cm−1, −0.10 cm−1, and 1.24 cm−1, respectively, and the three intermolecular interactions lead to weak anti-ferromagnetic interactions with 2J = −0.36 cm−1, −0.26 cm−1, and −0.32 cm−1, respectively. The theoretical calculations and the experimental magnetic data imply that the anti-ferromagnetic interaction involves the orbital contribution of the relevant CoII ions.