Triple Acceptor Research Articles

In Ukrainian

The possibility of the existence of clusters of inorganic substances in the state of a molecular associate and a combination of ion pairs is analyzed. The main causes of the appearance of ionic pairs are considered: partial or complete transfer of an electron or electron pair from one molecule to another, transfer of a proton (prototropy) or halide ions (halotropy) between molecules, conversion of contact ionic pairs in microcrystals of salts into salts separated by solvation. The conditions of structural transitions between them are determined. The influence of nanoparticle sizes and temperature on the relative thermodynamic stability of these two forms of clusters is described.               The review covers a wide range of subjects: clusters formed by water molecules, hydrated molecules of oxygen and oxygen-free acids, inorganic bases (alkali metal and ammonia hydroxides) and salts; anhydrous and hydrated clusters containing simultaneously acid and base molecules; clusters of coordination compounds, and some other substances. The results are presented of quantum chemical calculations by Hartree-Fock (HF), electron density functional theory (DFT), coupled single, double and triple excitation (CCSD (T)) and second-order Möller-Plesset perturbation theory (MP2) methods of the spatial structure and energy characteristics of the models under consideration. The theoretical results are compared with the available experimental data.               It is revealed that dissociation of O–H bonds is possible in clusters formed by at least five water molecules and is a complex reaction controlled by the formation of hydrogen bonds, with two water molecules acting as promoters of the hydrolysis reaction where a synchronous displacement occurs of two protons, and the H3O+ and OH– ions act simultaneously as hydrogen bond triple acceptors and triple donors. In clusters of hydrated inorganic acid molecules, both anoxic and oxygen-containing, including 3 to 5 water molecules, collective (i.e. concerted) proton transitions and ionic dissociation are possible. The ionic dissociation of alkali metal hydroxides and halides requires 3 to 7 water molecules in the cluster.               It has been shown that in clusters consisting of the same number of molecules of ammonia and hydrochloric acid (HCl)n•(NH3)n, the transition from the hydrogen bond to the ion pair occurs already at n = 2. Ionic coordination compounds can form oligomeric associates, where the distance between the cation and the anion depends on the coordination number of the cation and is intermediate between the values characteristic of the contact and separated ion pairs.

Triple Acceptors in a Polymeric Architecture for Balanced Ambipolar Transistors and High‐Gain Inverters

The exploration of novel molecular architectures is crucial for the design of high-performance ambipolar polymer semiconductors. Here, a "triple-acceptors architecture" strategy to design the ambipolar polymer DPP-2T-DPP-TBT is introduced. The utilization of this architecture enables DPP-2T-DPP-TBT to achieve deep-lying highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of -5.38/-4.19 eV, and strong intermolecular interactions, which are favorable for hole/electron injection and intermolecular hopping through π-stacking. All these factors result in excellent ambipolar transport characteristics and promising applications in complementary-like circuits for DPP-2T-DPP-TBT under ambient conditions with high hole/electron mobilities and a gain value of up to 3.01/3.84 cm2 V-1 s-1 and 171, respectively, which are among the best performances in ambipolar polymer organic thin-film transistors and associated complementary-like circuits, especially in top-gate device configuration with low-cost glass as substrates. These results demonstrate that the "triple-acceptors architecture" strategy is an effective way for designing high-performance ambipolar polymer semiconductors.

First-principles study of native point defects in Bi2Se3

Using first-principles method within the framework of the density functional theory, we study the influence of native point defect on the structural and electronic properties of Bi2Se3. Se vacancy in Bi2Se3 is a double donor, and Bi vacancy is a triple acceptor. Se antisite (SeBi) is always an active donor in the system because its donor level (ɛ(+1/0)) enters into the conduction band. Interestingly, Bi antisite (BiSe1) in Bi2Se3 is an amphoteric dopant, acting as a donor when μe < 0.119 eV (the material is typical p-type) and as an acceptor when μe > 0.251 eV (the material is typical n-type). The formation energies under different growth environments (such as Bi-rich or Se-rich) indicate that under Se-rich condition, SeBi is the most stable native defect independent of electron chemical potential μe. Under Bi-rich condition, Se vacancy is the most stable native defect except for under the growth window as μe > 0.262 eV (the material is typical n-type) and ΔμSe < −0.459 eV (Bi-rich), under such growth window BiSe1 carrying one negative charge is the most stable one.

Vacancy defects in indium oxide: An ab-initio study

First-principles density functional theory is employed to study the electrical behavior of oxygen and indium vacancies in indium oxide (In 2O 3). The oxygen vacancy is found to be a double donor. The indium vacancy is a triple acceptor, which >can be a compensation center in n-type In 2O 3, leading to n-type carrier reduction. However, its high formation energy under p-type conditions makes it unlikely to be a source of p-type carriers by itself.

Topological Hydrogen-Bond Definition to Characterize the Structure and Dynamics of Liquid Water

A definition that equates a hydrogen bond topologically with a local energy well in the potential energy surface is used to study the structure and dynamics of liquid water. We demonstrate the robustness of this hydrogen-bond definition versus the many other definitions which use fixed, arbitrary parameters, do not account for variable molecular environments, and cannot effectively resolve transition states. Our topology definition unambiguously shows that most water molecules are double acceptors but sizable proportions are single or triple acceptors. Almost all hydrogens are found to take part in hydrogen bonds. Broken hydrogen bonds only form when two molecules try to form two hydrogen bonds between them. The double acceptors have tetrahedral geometry, lower potential energy, entropy, and density, and slower dynamics. The single and triple acceptors have trigonal and trigonal bipyramidal geometry and when considered together have higher density, potential energy, and entropy, faster dynamics, and a tendency to cluster. These calculations use an extended theory for the entropy of liquid water that takes into account the variable number of hydrogen bonds. Hydrogen-bond switching is shown to depend explicitly on the variable number of hydrogen bonds accepted and the presence of interstitial water molecules. Transition state theory indicates that the switching of hydrogen bonds is a mildly activated process, requiring only a moderate distortion of hydrogen bonds. Three main types of switching events are observed depending on whether the donor and acceptor are already sharing a hydrogen bond. The switch may proceed with no intermediate or via a bifurcated-oxygen or cyclic dimer, both of which have a broken hydrogen bond and symmetric and asymmetric forms. Switching is found to be strongly coupled to whole-molecule vibration, particularly for the more mobile single and triple acceptors. Our analysis suggests that even though water is heterogeneous in terms of the number of hydrogen bonds, the coupling between neighbors on various length and time scales brings about greater continuity in its properties.

Atomic and electronic structure of single and multiple vacancies in GaN nanowires from first-principles

We present a comprehensive first-principles investigation of single and multiple gallium and nitrogen vacancies in gallium nitride (GaN) nanowires. We consider nanowires in the [0001] growth direction, with diameters of 9.5 and $15.9\text{ }\text{\AA{}}$ and investigate the stability of multiple vacancies for a wide range of configurations to determine the preferred spatial distribution. The influence of saturating the dangling bonds at the edge of the nanowires is also investigated. For one, two, and three nitrogen vacancies, we find that the most favorable configuration is with the vacancies at the edge of the nanowires. We also find that for multiple (two and three) nitrogen vacancies, the vacancies prefer to cluster together rather than remain well separated. For one and two gallium vacancies, the preferred vacancy location is also at the edge of the nanowires, with clustering favored for the two vacancies. Examining the band structure of unsaturated nanowires (with and without vacancies), we observe states in the band gap that can be attributed to edge states. These edge states are removed when the dangling bonds at the edge of the nanowires are saturated with hydrogen. For unsaturated wires, the nature of the single Ga and N vacancy states is similar to in the bulk, acting as a triple acceptor and single donor, respectively. The position of the vacancy states relative to the two regions of edge-induced states is similar to their location relative to the conduction band minimum and valence band maximum in the bulk, suggesting that any conductivity arising from the vacancies will be confined to the outer region of the nanowires. The two- and three-nitrogen vacancy complexes induce additional states in the band gap, acting as double and triple donors respectively, while the two-gallium vacancy complex reconstructs to an ${\text{N}}_{3}$-like structure and induces several fully occupied and unoccupied singlet state in the band gap. Examining the defect-induced states for the gallium vacancy in the saturated wire versus in bulk GaN, we find a similar result in terms of the number, location, and occupation of the defect states, except the states in the wire are slightly deeper in the band gap. For the nitrogen vacancy in the saturated wire versus in bulk GaN, we also find similar behavior in terms of the number and occupation of the defect states. However, the higher lying singly occupied state is closer to the conduction band in the wire, and the location of the fully occupied singlet state is below the valence band maximum in bulk GaN, and above it in the saturated nanowire. Considering the formation energy of gallium and nitrogen vacancies, we find nitrogen vacancies are significantly more stable than gallium vacancies, and thus we expect them to be the major defect in GaN nanowires.

Electronic properties of titanium and chromium impurity centers in germanium

The electronic properties of the 3d transition metal impurities titanium and chromium in crystalline germanium have been investigated by means of deep level transient spectroscopy. The metals were implanted at 90keV and diffused deeper into the bulk during a thermal anneal at 500°C, which yielded spectra specific for the implanted metal. It was found that Ti introduces a deep electron trap and a semishallow hole trap. For Cr one deep electron trap and three hole traps were observed. The capture cross section of the electron traps is thermally activated while three of the hole traps display electric field enhanced emission, which is in agreement with multiple-acceptor states of metal impurities on substitutional sites. It is concluded that Ti in germanium is a double acceptor, while Cr is a triple acceptor with, in addition, a donor level close to the valence band.

1-Methyl-2-(4-methylphenyl)-4-morpholinopyridazine-3,6(1H,2H)-dione

The structure analysis of the title compound, C16H19N3O3, has been undertaken in order to facilitate the inter­pretation of 1H and 13C NMR data and to determine the position of the morpholine residue in this nucleophilic substitution product. The main result is that the morpholine group, with a chair conformation, is connected at the 4-position of the pyridazine ring. The benzene and pyridazine rings make a dihedral angle of 62.17 (5)°. Mol­ecules are linked into a two-dimensional network by non-classical C—H⋯O hydrogen bonds, in which O atoms serve as double or triple acceptors.

Substitutional and interstitial oxygen in wurtzite GaN

Density-functional theory was used to compute energy-minimum configurations and formation energies of substitutional and interstitial oxygen (O) in wurtzite GaN. The results indicate that O substituted at a N site (ON) acts as a single donor with the ionized state (ON+1) being the most stable O state in p-type GaN. In n-type GaN, interstitial O (OI) is predicted to be a double acceptor and O substituted at a Ga site (OGa) is predicted to be a triple acceptor. The formation energies of these two species are comparable to that of ON in n-type GaN and, as such, they should form and compensate the ON donors. The extent of compensation was estimated for both Ga-rich and N-rich conditions with a total O concentration of 1017cm−3. Ga-rich conditions yielded negligible compensation and an ON concentration in excess of 9.9×1016cm−3. N-rich conditions yielded a 25% lower ON concentration, due to the increased stability of OI and OGa relative to ON, and moderate compensation. These findings are consistent with experimental results indicating that O acts as a donor in GaN(O). Complexes of ON with the Mg acceptor and OI with the Si donor were examined. Binding energies for charge-conserving reactions were ⩾0.5eV, indicating that these complexes can exist in equilibrium at room temperature. Complexes of ON with the Ga vacancy in n-type GaN were also examined and their binding energies were 1.2 and 1.4eV, indicating that appreciable concentrations can exist in equilibrium even at elevated temperatures.

Analysis of splice donor and acceptor function in a novel Ac-based gene trap construct.

In this study, an Ac-based gene trap construct was engineered to increase gene trapping efficiency by an effective use of triple acceptor sites preceding a reporter gene. The target of the engineering process was a synthetic intron preceding the GUS reporter. Two different gene trap constructs were designed. In one construct, three of the sequence elements serving as signals for recognition of an intron 3' boundary were systematically modified to allow for almost optimal acceptor site recognition, while these sequences remained unchanged in the other construct. To compare recognition of the engineered intron with that of the unmodified intron, tester constructs were transiently transformed into barley (Hordeum vulgare L.) tissue and the accuracy and efficiency of splicing was determined by mRNA mapping and reporter-gene expression frequency analysis. By employing this test system, we could show that systematic engineering of the intron sequence elements results in advanced intron recognition, compared to the unmodified intron, and that all three acceptor sites were activated, but with unequal frequency. The impact of our findings on reporter expression in a gene-trap approach is discussed.

Defect identification for the AsGa family

The AsGa family consists of at least four distinctly different point defects including the technologically important EL2 defect. While the different members are easily distinguished from their MCDA spectra, the differences of the hf and shf interactions as derived from ODEPR and ODENDOR are rather small.We present ab initio calculations using the LMTO–ASA Green's function method for a variety of defect models that might be relevant for the identification of AsGa-related defects. We confirm the identification of the isolated AsGa and show that the {AsGa–X2} defect must be identified with the nearest-neighbor antistructure pair rather than with the {AsGa–VAs} pair. For the {AsGa–X1} defect a distant antistructure pair is a likely candidate. For the EL2, the most important member of the AsGa family, we have not found a conclusive defect model. The recent ODENDOR data are similar to those of the distant orthorhombic {AsGa–VGa} pair, which, however is a triple acceptor and not a donor.

Two polymorphs, with Z' = 1 and 2, of 2-amino-4-chloro-6-morpholinopyrimidine in P21/c, and 2-amino-4-chloro-6-piperidinopyrimidine, which is isomorphous and almost isostructural with the Z' = 2 polymorph.

Crystallization of 2-amino-4-chloro-6-morpholinopyrimidine, C(8)H(11)ClN(4)O, (I), yields two polymorphs, both with space group P2(1)/c, having Z' = 1 (from diethyl ether solution) and Z' = 2 (from dichloromethane solution), denoted (Ia) and (Ib), respectively. In polymorph (Ia), the molecules are linked by an N-H...O and an N-H...N hydrogen bond into sheets built from alternating R(2)(2)(8) and R(6)(6)(40) rings. In polymorph (Ib), one molecule acts as a triple acceptor of hydrogen bonds and the other acts as a single acceptor; one N-H...O and three N-H...N hydrogen bonds link the molecules in a complex chain containing two types of R(2)(2)(8) and one type of R(4)(4)(18) ring. 2-Amino-4-chloro-6-piperidinopyrimidine, C(9)H(13)ClN(4), (II), which is isomorphous with polymorph (Ib), also has Z' = 2 in P2(1)/c, and the molecules are linked by three N-H...N hydrogen bonds into a centrosymmetric four-molecule aggregate containing three R(2)(2)(8) rings.

Modification of supramolecular motifs: some effects of incorporation of metal complexes into supramolecular arrays

The structures of four supramolecular arrays incorporating concerted triple acceptor–donor–acceptor/donor–acceptor–donor (ADA/DAD) hydrogen bonds are described. The first is comprised of 1 ∶ 1 phthalimide ∶ 2,4-diamino-6-phenyl-1,3,5-triazine units linked into a one-dimensional strand by more loosely connected independent interposed triazine moieties. The second, composed of [bis(dithiobiureto)nickel(II)∶(3-iminoisoindolinone)2] units, incorporates similar complementary triplet hydrogen bonding motifs. The third system involves a 1 ∶ 2 assembly between the dianionic complex, bis(biureto)nickelate(II) and 2,4-diamino-6-phenyl-1,3,5-triazine to yield the bis(tetraethylammonium) salt of the [bis(biureto)nickelate(II)∶(2,4-diamino-6-phenyl-1,3,5-triazine)2] dianion unit, the array again extended by secondary interactions. The final assembly investigated is composed of ‘tri-component’ units of type [bis(biguanidino)nickelate(II)∶(6,7-dimethoxy-2,4(1H, 3H)-quinazolinedione)2] that feature an excess of hydrogen bond donor groups over acceptor groups. As in the above assemblies, it also incorporates hydrogen bonds between complementary triplet motifs as well as bridging hydrogen bonds between individual units. This last assembly is no longer planar; in the lattice being linked into a two dimensional network by dimethylsulfoxide molecules that act as ‘ancillary’ acceptors for the excess donors present and also as bridging molecules between neighbouring tri-component units. The hydrogen bond geometries in these assemblies are discussed comparatively and also compared with related arrays reported in the literature.

JC virus T' proteins encoded by alternatively spliced early mRNAs enhance T antigen-mediated viral DNA replication in human cells.

Alternative splicing of the JC Virus (JCV) precursor early mRNA yields five transcripts that encode proteins that regulate the life cycle of this human polyomavirus. Large T protein (TAg) mediates viral DNA replication and oncogenic activities, and small t protein influences these functions under certain conditions. Recently, three new early proteins, T'(135), T'(136), and T'(165), were discovered that contain sequences overlapping amino-terminal TAg functional domains. Initial studies with the T' proteins suggested they contribute to viral DNA replication and transformation. Mutation of a donor splice site utilized by all three T' mRNAs creates a mutant that exhibits a 10-fold decrease in viral DNA replication compared to wild type JCV. To assess the influence that individual T' proteins have on the replication process, a set of T' acceptor site mutants was created in which the unique second acceptor splice site of each T' mRNA was altered to eliminate production of one, two or all three T' mRNAs. The patterns of early mRNA and protein expression in these seven mutants were examined, and it was found that mutation of the T'(135) acceptor site resulted in the utilization of cryptic splice sites and the generation of new T' species. Additional mutations were made to prevent these aberrant splicing reactions prior to measuring DNA replication potential of the mutants. DpnI assays revealed that each T' protein contributes to TAg-mediated DNA replication activity. The three single mutants that express two T' proteins and the double mutant that only produces T'(136), exhibited levels of replication equivalent to that of wild type virus, whereas the two double mutants that fail to express T'(136) replicated about twofold less efficiently than wild-type JCV. Replication activity of the triple acceptor site mutant, like that of the T' donor site mutant from an earlier study, was impaired significantly.

On the Molecular Basis of Water Hydrolysis. A Detailed ab Initio Study

A refined study of the water hydrolysis is made at the HF-MP2 level, with full counterpoise correction and many-body analysis. The O−H bond dissociation occurring during water hydrolysis was studied in small water aggregates, from the trimer to hexamer, using fully optimized structures at the HF-MP2 level. The breaking of the O−H bond in these clusters is a complex reaction. It is found that the H-bond coordination pattern for each monomer influences its role during the reaction. From a detailed analysis of molecular properties those roles can be clearly identified. The results obtained show that at least two water molecules should act as promoters of the hydrolysis reaction while the molecules intervening in the concerted double proton transfer can be distinguished in the reactants structure by its large three-body nonadditive interaction. Only one final structure, where the ions act as triple acceptors and triple donors of H-bonds, becomes stable. The role of ion hydration in lowering the barrier for the hydrolysis reaction in these clusters is also discussed.

Doping of AlxGa1−xN

N- type AlxGa1−xN exhibits a dramatic decrease in the free-carrier concentration for x⩾0.40. Based on first-principles calculations, we propose that two effects are responsible for this behavior: (i) in the case of doping with oxygen (the most common unintentional donor), a DX transition occurs, which converts the shallow donor into a deep level; and (ii) compensation by the cation vacancy (VGa or VAl), a triple acceptor, increases with alloy composition x. For p-type doping, the calculations indicate that the doping efficiency decreases due to compensation by the nitrogen vacancy. In addition, an increase in the acceptor ionization energy is found with increasing x.

Pairwise-InterwovenR66(48) Hexagonal Nets in the Hydrogen-Bonded Structure of Hexamethylenetetramine–1,1,1-Tris(4-hydroxyphenyl)ethane (1/1)

In the 1:1 adduct formed by hexamethylenetetramine (HMTA) and 1,1,1-tris­(4-hydroxyphenyl)ethane, C6H12N4.C20H18O3, the tris-phenol acts as a triple donor of hydrogen bonds of type O-H⋯N and the HMTA as a triple acceptor of such hydrogen bonds, with O⋯N distances of 2.777 (4), 2.781 (4) and 2.795 (4) A. The components are assembled into infinite nets comprising hexagonal rings and pairs of such nets are interwoven; the pairs of interwoven nets are, in addition, connected by C-H⋯O hydrogen bonds with C⋯O distance 3.339 (5) A.

Carrier compensation and scattering mechanisms in p-GaSb

The hole transport properties of gallium antimonide with various degrees of tellurium compensation have been investigated in the temperature range of 4.2–300 K. For the undoped GaSb, the p-type conductivity arises from a doubly ionizable native defect VGa GaSb. In the Te compensated samples, apart from the Te-donor level and the VGaGaSb center, an acceptor level resulting from complexation of VGa GaSb with TeSb has been found. This acceptor level lies ∼70 meV above the valence band edge. The concentration of this center depends on the melt composition and the level of Te present in the melt during growth of crystals. Most interestingly, at very low level of Te concentration, an additional triple native acceptor (VGa GaSb VGa) has also been observed. With the increase in Te concentration, the mobility decreases and a shift in the mobility peak to higher temperature is observed. The low-temperature mobility is limited by ionized impurity scattering. At higher temperatures, the scattering mechanisms depend on Te concentration in the sample. In this regime, significant contributions from acoustic, nonpolar optical, and polar optical phonon scattering are observed for samples with low levels of Te. In contrast, the impurity scattering dominates even at room temperature for highly compensated crystals.

Copper acceptors in uniaxially stressed germanium: 1s3 to 1s2 2s1 ground-state transformation.

We have studied uniaxially stressed Ge doped with Cu, a substitutional triple acceptor, by photoconductivity spectroscopy. We find that the ground state of this triple acceptor, normally accommodating three 1s holes, reconfigures into an atomic Li-like 1${\mathit{s}}^{2}$ 2${\mathit{s}}^{1}$ arrangement by the application of [100] stress. Above 4 kbar the first ionization potential reaches a constant value of 17.4 meV corresponding to the hole excitation from the now partially occupied 2s energy level. Excellent agreement was obtained between the experimental results and variational calculations.

Tight-binding studies of crystal stability and defects in Bi 2Te 3

The stability of the crystal structure and the electronic state of isolated point defects have been studied in Bi 2Te 3. The structure is related to p-bonding, giving a basic cubic cell further stabilized by the formation of units of five layers of planes along the [111] axis of the cubic cell. The impurities are treated via the Lifshits-Koster-Slater method. Antistructure defects appear as single donors (Te Bi) or single acceptors (Bi Te). Vacancies are strong perturbations: V Te are double donors and V Bi are triple acceptors. The impurities of column IV when substituted for Te give a resonance (or bound state) close to the gap. Their electrical activity may then vary from double acceptor to inactive.