Defectless Crystals Research Articles

Synthesis and Evaluation of Charge Transport Property of Ethynylene‐Bridged Anthracene Oligomers

AbstractA set of ethynylene‐bridged anthracene oligomers from monomer to tetramer is synthesized and characterized. The red‐shifts in the UV–vis absorption and fluorescence spectra indicate the π‐extension with elongation of the linear chain. Cyclic voltammetric measurements suggest that their highest occupied molecular orbitals are located lower than –5.0 eV, indicating high air stability. Single‐crystal X‐ray analysis of a trimer needle crystal indicates that anthracene units display a twisted arrangement, adopting a face‐to‐face packing structure. The dip‐coating method with a very slow pulling rate provides crystalline films of dimer and trimer. The top‐contact bottom‐gate organic field‐effect transistor devices reveal that trimer shows better charge transport property (8.0 × 10–3 cm2 V–1 s–1) than dimer (8.9 × 10–4 cm2 V–1 s–1). Furthermore, the single‐crystal field‐effect transistor of trimer shows mobility up to 0.14 cm2 V–1 s–1, which is higher than that of tetramer (3.3 × 10–2 cm2 V–1 s–1). This is likely due to the larger amount of defects in tetramer crystals that are caused by the free rotation of ethynylene units, while the trimer achieves a good balance between the effective π‐conjugation and defectless crystal formation.

Ultrafast synthesis of high-silica Beta zeolite from dealuminated MOR by interzeolite transformation for methanol to propylene reactions

A novel interzeolite transformation method was developed to synthesize the large pore Beta zeolite with high Si/Al ratio. and less defects within 3 h. Such high-silica Beta showed a long lifetime in comparison to the high-silica Beta prepared via the post-synthesis method (Beta-DA) and hydrothermal method in F medium (Beta-F), owing to its nano-size and defect-less crystal.

Solid-State Photon Upconversion Materials: Structural Integrity and Triplet-Singlet Dual Energy Migration.

Triplet-triplet annihilation-based photon upconversion (TTA-UC) is a process wherein longer-wavelength light (lower-energy photons) is converted into shorter-wavelength light (higher-energy photons) under low excitation intensity in multichromophore systems. There have been many reports on highly efficient TTA-UC in solution; however, significant challenges remain in the development of solid-state upconverters in order to explore real-world applications. In this Perspective, we discuss the advantages and challenges of different approaches for TTA-UC in solvent-free solid systems. We consider that the energy migration-based TTA-UC has the potential to achieve ideal materials with high UC efficiency at weak solar irradiance. While the UC performance of such systems is still limited at this moment, we introduce recently developed important concepts to improve it, including kinetic/thermodynamic donor dispersion in acceptor assemblies, defectless crystals, and triplet-singlet dual energy migration. Future integration of these concepts into a single material would realize the ideal TTA-UC system.

Field localization on the defect of inversion type in a one-dimensional fotonic crystal structure

Modification of the transmission spectrum and wave field distribution in a one-dimensional photonic crystal with single or double defect of inversion type is investigated. It is demonstrated that in the presence of the defect, a narrow transmission miniband is formed in the non-transmission band gap of the defectless crystal. In this case, the field is strongly localized in the defect region depending on the defect type, its position, and the number of complete periods in the structure. The formation of the double defect of inversion type leads to the formation of two defect minibands in the spectrum and the redistribution of the wave field in the structure.

Dielectric model of point charge defects in insulating paraelectric perovskites

Some point defects (i.e., oxygen vacancies) create deep trapping levels in the bandgap of the paraelectric phase ferroelectric crystals. Under applied DC field the traps release electrons via the Poole-Frenkel mechanism and become charged. The electric field of a point charge polarizes the crystal locally reducing its permittivity. In this paper a simple theory is proposed for calculating the DC field dependent apparent (measureable) permittivity of a paraelectric crystal with point charge defects. It is shown that the apparent permittivity of a paraelectric crystal may be sufficiently lower as compared with the defectless crystal. This reduction is in addition to the possible reduction of the apparent permittivity associated with the interfacial “dead” layers and strain.

Crystal growth and replacement processes in the Ba-Sr-SO4-CO3-H2O system

It is determined that at atomic-force microscopy (AFM) investigations of water-soluble salt crystal growth the scanning unit does not considerably affect boundary layer and gives a reliable and objective data for statistical set and analysis of kinetic and morphological data [1]. The elementary processes, accompanying morphological surface evolution, are described phenomenologically and interpreted theoretically. First at nanolevel the processes of formation of inclusion and competition of growth centers in the directed solution flow are determined. Thus, e.g., at one of the experiments we observe a face of NaCl crystal with diffusive starvation in the form of small cavities. The cavity under observation was circular and had diameter up to about 1450 nm. Its obliteration occurred due to themotion of thin steps with a height 0.73-2.12 nm. At that fast growing upper layers with a rate 1.9-2.4 nm/s overtook lower layers with a smaller rate (0.2-0.9 nm/s), and within 12minutes an inclusion was formed in the crystal. Based on AFM statistics the methods of measuring normal growth rate in any part of a face and its fluctuations are developed. The conducted calculations allow to determine the degree of deviation of the processes from the permanent growth condition which is the most important parameter for defectless crystal synthesis for industry.

Direct observation of elementary processes of crystal growth

It is determined that at atomic-force microscopy (AFM) investigations of water-soluble salt crystal growth the scanning unit does not considerably affect boundary layer and gives a reliable and objective data for statistical set and analysis of kinetic and morphological data [1]. The elementary processes, accompanying morphological surface evolution, are described phenomenologically and interpreted theoretically. First at nanolevel the processes of formation of inclusion and competition of growth centers in the directed solution flow are determined. Thus, e.g., at one of the experiments we observe a face of NaCl crystal with diffusive starvation in the form of small cavities. The cavity under observation was circular and had diameter up to about 1450 nm. Its obliteration occurred due to themotion of thin steps with a height 0.73-2.12 nm. At that fast growing upper layers with a rate 1.9-2.4 nm/s overtook lower layers with a smaller rate (0.2-0.9 nm/s), and within 12minutes an inclusion was formed in the crystal. Based on AFM statistics the methods of measuring normal growth rate in any part of a face and its fluctuations are developed. The conducted calculations allow to determine the degree of deviation of the processes from the permanent growth condition which is the most important parameter for defectless crystal synthesis for industry.

Two-Dimension Three-Atom Removal Model for (110) Plane Nanocutting of Defectless Crystals of Copper, Nickel and Silver in [001] Direction

Based on the cutting direction of single-point cutting tool being [001] and the orientation of workmaterial being (110), this paper establishes a two-dimension (2-D) three-atom removal model for the removal of single atom in the nanocutting of defectless single crystals of Cu, Ni and Ag. In the model, interatomic potentials of workmaterial are constructed by Morse's potential. Furthermore, by applying 2-D three-atom removal model, this paper infers an analytic exact solution that the threshold energy required by the cutting tool is in a closed form, and then employs the solution in the preliminary study of multiple-atom nanocutting removal mechanism. The acquired threshold energy value is slightly smaller than the vacancy formation energy acquired in other references.

Defects as a cause of continuity of normal-incommensurate phase transitions

Almost all normal-incommensurate phase transitions observed experimentally are continuous. We show that there is not any theoretical reason for this general behaviour in perfect crystals. A normal-incommensurate phase transition that is not too far from the mean-field tricritical point should be discontinuous and it is highly improbable that so far reported normal-incommensurate phase transitions lie very far from this point. To understand this behaviour we study influence of defects on a hypothetical first-order normal-incommensurate phase transition in a pure material. We have found that this influence is strikingly different from that on other kinds of first-order phase transitions. The change of the discontinuity of the order parameter at the transition is negative and formally diverges within our approximate theory. At the same time the diminishing of the phase transition temperature remains finite. We interpret these results as an indication that at least some of the observed seemingly second-order normal-incommensurate transitions would be first-order transitions in defectless crystals.

Quadrupole Interactions and Anisotropic Magnetic Quenching of Positronium in Oriented Crystals

The quadrupole interaction of positronium in noncubic defectless crystals has been investigated theoretically. It is shown to lead to the weak anisotropy of positronium magnetic quenching. The temperature dependence of the quadrupole coupling constant and the estimations of the magnitude of the anisotropy are obtained for delocalized positronium in crystalline quartz. The significant magnitude of the anisotropy is predicted for anomalous positronium in oriented crystalline polymeric structures. PACS numbers: 31.30.Gs, 36.10.Dr, 71.60.+z, 78.70.Βj