Single Stacking Research Articles

Raman spectroscopy as a probe of molecular order, orientation, and stacking of fluorinated copper‐phthalocyanine (F16CuPc) thin films

We report Raman scattering measurements on azimuthally ordered thin films of F16CuPc, prepared by organic molecular beam deposition on A‐plane sapphire substrates. The observed peak frequencies have been compared both to the results of a model calculation for the vibrational modes of the free molecule and to those reported by other authors in related materials. This analysis provides a plausible identification of the modes responsible for the strongest spectral features. Detailed evaluation of the spectra reveals that some observed modes, which correspond to vibrations of the macrocycle inner ring, largely retain the intramolecular character and their polarisation properties can be used to study the orientation and stacking configuration of the molecules. We provide structural parameters deduced either in molecular or crystal symmetry considering the simpler possibilities, i.e. a single column molecular stacking and a herringbone‐like structure. The results suggest that the thicker and most ordered film is structurally close to the recently reported crystal organisation of bulk ribbon samples of this compound. The crystalline quality of the ordered films is mainly reflected in some other Raman peaks which are related to the motion of peripheral atoms and dominate the high wavenumber part of the spectra. These modes are affected by intermolecular interactions inducing Davydov splittings that are unequivocally identified by the observed Raman selection rules. The performed analysis also provides quantitative estimates of the degree of in‐plane ordering. Copyright © 2013 John Wiley & Sons, Ltd.

Electron backscattering from stacking faults in SiC by means ofab initioquantum transport calculations

We study coherent backscattering phenomena from single and multiple stacking faults (SFs) in 3C- and 4H-SiC within density functional theory quantum transport calculations. We show that SFs give rise to highly dispersive bands within both the valance and the conduction bands that can be distinguished for their enhanced density of states at particular wave-number subspaces. The consequent localized perturbation potential significantly scatters the propagating electron waves and strongly increases the resistance for $n$-doped systems. We argue that resonant scattering from SFs should be one of the principal degrading mechanisms for device operation in silicon carbide.

Numerical Exploration of the Defect’s Effect on Mechanical Properties of Nanowires under Torsion

Molecular dynamics (MD) simulations have been carried out to investigate the defect’s effect on the mechanical properties of single-crystal copper nanowire with different surface defects, under torsion deformation. The torsional rigidity is found insensitive to the surface defects and the critical angle appears an obvious decrease due to the surface defects, the largest decrease is found for the nanowire with surface horizon defect. The deformation mechanism appears different degrees of influence due to surface defects. The surface defects play a role of dislocation sources. Comparing with single intrinsic stacking faults formation for the perfect nanowire, much affluent deformation processes have been activated because of surface defects, for instance, we find the twins formation for the nanowire with a surface 45odefect.

Drift Phenomena of Forward and Reverse Recovery Characteristics in 0001 4H-SiC p–i–n Diode

4H-SiC p–i–n diodes fabricated on the (0001) C-face showed smaller forward voltage drift and minimal changes in reverse recovery characteristics after a forward bias stress test compared to those observed on the (0001) Si-face. These drift phenomena in 4H-SiC p–i–n diodes could be explained by increased recombination along the perimeter of single Shockley-type stacking faults. It is suggested that the number of single Shockley-type stacking faults significantly decreased in the drift layer fabricated on (0001) C-face in comparison with that on (0001) Si-face.

Mold Growth on Sapwood Boards Exposed Outdoors: The Impact of Wood Drying

Mold growth on dried Norway spruce and Scots pine sapwood boards was investigated in an accelerated outdoor field test for 96 days. The boards were dried using three different methods of stacking: single stacking, double stacking with the sapwood sides in each pair facing toward each other, and double stacking with sapwood sides facing outward. Drying was performed at three temperatures: 25°C, corresponding to air drying, and kiln drying at 70°C and 110°C. The degree of mold growth was visually assessed on both sides of each board. On average, pine boards showed a higher level of mold growth than the spruce boards. The highest average level of mold growth was found on the boards kiln dried at 70°C, whereas the air-dried boards and the boards kiln dried at 110°C showed considerably less mold growth. Stacking the boards during drying had a large impact on mold susceptibility of the sapwood. This study confirmed that, during the drying process, it is possible to direct the migration of nutrients in sapwood toward one chosen side of each board by double stacking; the opposite side leaches out, which has a great impact on surface mold growth. Chemical analyses of monosaccharide sugar gradients beneath the boards' surfaces confirmed the results.

Development of Fine Filament NbTi Superconducting Strands for ITER

The NbTi superconducting wires are used for the Poloidal Field (PF) coil of International Thermonuclear Experimental Reactor (ITER). Western Superconducting Technologies Co., Ltd. (WST) has produced conventional NbTi superconducting strand by single stacking for ITER. The final strands has been heat-treated with different duration. The properties of the NbTi strands meet the requirements to ITER PF strand specification. The diameter of the strand is 0.73 mm and filament diameter is 7.8 . The multifilament composite strand is made up of Nb-Ti filaments embedded in a high purity Cu matrix. This paper presents the results of comparative investigations on critical currents(Ic), critical current density(Jc), AC losses of the strands.

Time Sequential Evolutions of Optically-Induced Single Shockley Stacking Faults Formed in 4H-SiC Epitaxial Layers

Time-dependent evolutions of single and quadruple Shockley stacking faults (sSSF and 4SSF) in 4° off 4H-SiC epitaxial layers have been investigated. UV illuminations using an Hg-Xe lamp light source generate dissociations of basal plane dislocations (BPDs) into sSSFs whereas for 4SSFs no significant changes in shape occur. Detailed analyses of Photo-luminescence (PL) signals suggest that Si- and C-core partials have different PL spectrum distributions in the wavelength range larger than 750 nm, giving rise to images with different contrasts in PL mappings.

Condition Dependences of Extended Defect Formation in 4H-SiC by Ion-Implantation/Activation-Anneal Process

Condition dependences of defect formation in 4H-SiC epilayer induced by the implantation/annealing process were investigated using synchrotron reflection X-ray topography and transmission electron microscopy. Nitrogen, phosphorus or aluminum ions were implanted in the 4H-SiC epilayers and then activation annealing was performed. To compare the implantation/annealing process, a sample receiving only the annealing treatment without the implantation was also performed. Two different crucibles (conventional and improved) were used in the annealing process. The formation of single layer Shockley-type stacking faults near the surface was found to have no ion-implantation condition or crucible dependence. The formation of BPD half-loops and the glide of pre-existing BPDs showed clear dependence on the crucibles.

2-Cyano-N-(2,6-dimethoxypyrimidin-4-yl)-3-[4-(dimethylamino)phenyl]acrylamide and dimethylammonium 6-cyano-1,3-dimethyl-2,4,5-trioxo-1,2,3,4,5,8-hexahydropyrido[2,3-d]pyrimidin-8-ide: a π-stacked dimer and a hydrogen-bonded ribbon containing three types of ring

The bond distances in 2-cyano-N-(2,6-dimethoxypyrimidin-4-yl)-3-[4-(dimethylamino)phenyl]acrylamide, C(18)H(19)N(5)O(3), (I), and in the anionic component of the salt dimethylammonium 6-cyano-1,3-dimethyl-2,4,5-trioxo-1,2,3,4,5,8-hexahydropyrido[2,3-d]pyrimidin-8-ide, C(2)H(8)N(2)(+).C(10)H(7)N(4)O(3)(-), (II), provide evidence for the occurrence of electronic polarization. There are no hydrogen bonds in the structure of (I), instead pairs of molecules are linked into centrosymmetric dimers by a single pi-pi stacking interaction. In (II), a combination of a two-centre N-H...O hydrogen bond, a three-centre N-H...(O)(2) hydrogen bond, together utilizing all three O atoms, and a two-centre C-H...N hydrogen bond, link the components into a ribbon containing R(1)(2)(6), R(2)(2)(10) and R(6)(6)(30) rings.

Improvement of green cathodoluminescence of zinc oxide stacked films prepared by high-power excimer laser irradiation of sol–gel-derived precursors

Zinc oxide (ZnO) stacked films were prepared by repeating a procedure involving KrF excimer laser irradiation at various energy fluences ( E f) of sol–gel-derived precursors. All the stacked films are wurtzite-type and highly c-axis oriented on glass substrates. The green cathodoluminescence (CL) efficiency of the films is strongly influenced by the preparation conditions. Especially, laser E f and stacking number are important control factors. The stacked film prepared under optimized preparation conditions consists of large grains and indicate low electric resistivity. The green CL efficiency of the stacked film is 0.0139 lm/W, which is higher than those of ZnO thin films prepared by a single stacking.

In-line coupling of two-phase single drop microextraction and large volume stacking using an electroosmotic flow pump in nonaqueous capillary electrophoresis

In order to improve the concentration sensitivity of capillary electrophoresis (CE), two sample preconcentration techniques, single drop microextraction (SDME) and large volume stacking using an electroosmotic flow pump (LVSEP), were coupled in-line in a commercial CE instrument. By simple programming of liquid handling sequences, a pentanol drop was prepared at the tip of a fused silica capillary over which a Teflon tube had been sleeved to serve as a hydrophobic support. After extraction of the analytes from an aqueous donor solution into the drop, the entire capillary column was filled with enriched pentanol extract. LVSEP, in which the sample matrix is automatically removed by the EOF, was then carried out using a methanolic run buffer. The overall enrichment factors for the analytes pentachlorophenol (PCP), 3-bromobenzoic acid (3-BBA), and 4-iodobenzoic acid (4-IBA), from a combination of 30 min SDME and LVSEP on a 27 cm capillary, were about 7000, even without agitation of the donor solution. The resulting limits of detection for PCP, 3-BBA, and 4-IBA were 0.7, 0.3 and 0.7 nM, respectively. Since no modification of the existing CE instrument is necessary and a bare capillary is used for LVSEP, this scheme can be adapted quite easily for many CE applications that require high concentration sensitivity.

High efficiency diffractive grating coupler based on transferred silicon nanomembrane overlay on photonic waveguide

We report here the design of a new type of high efficiency grating coupler, based on single crystalline Si nanomembrane overlay and stacking. Such high efficiency diffractive grating couplers are designed for the purpose of coupling light between single mode fibres and nanophotonic waveguides, and for the coupling between multiple photonic interconnect layers for compact three-dimensional vertical integration. Two-dimensional model simulation based on eigenmode expansion shows a diffractive power-up efficiency of 81% and a fibre coupling efficiency of 64%. With nanomembrane stacking, it is feasible to integrate the side-distributed Bragg reflector and bottom reflector, which can lead to the diffractive power-up efficiency and the fibre coupling efficiency of 97% and 73.5%, respectively. For a negatively detuned coupler, the bottom reflector is not needed, and the diffractive power-up efficiency can reach 98% over a large spectral range. The device is extremely tolerant to fabrication errors.

Four 7-aryl-substituted pyrido[2,3-d]pyrimidine-2,4(1H,3H)-diones: similar molecular structures but different crystal structures

Molecules of 1,3-dimethyl-7-(4-methylphenyl)pyrido[2,3-d]pyrimidine-2,4(1H,3H)-dione, C(16)H(15)N(3)O(2), (I), are linked by paired C-H...O hydrogen bonds to form centrosymmetric R(2)(2)(10) dimers, which are linked into chains by a single pi-pi stacking interaction. A single C-H...O hydrogen bond links the molecules of 7-(biphenyl-4-yl)-1,3-dimethylpyrido[2,3-d]pyrimidine-2,4(1H,3H)-dione, C(21)H(17)N(3)O(2), (II), into C(10) chains, which are weakly linked into sheets by a pi-pi stacking interaction. In 7-(4-fluorophenyl)-3-methylpyrido[2,3-d]pyrimidine-2,4(1H,3H)-dione, C(14)H(10)FN(3)O(2), (III), an N-H...O hydrogen bond links the molecules into C(6) chains, which are linked into sheets by a pi-pi stacking interaction. The molecules of 7-(4-methoxyphenyl)-3-methylpyrido[2,3-d]pyrimidine-2,4(1H,3H)-dione, C(15)H(13)N(3)O(3), (IV), are also linked into C(6) chains by an N-H...O hydrogen bond, but here the chains are linked into sheets by a combination of two independent C-H...pi(arene) hydrogen bonds.

Expansion of Stacking Faults in 4H-SiC Epitaxial Layer under Laser Light Excitation during Room Temperature Photoluminescence Mapping

We investigated the expansion of single Shockley stacking faults (SSFs) in a 4H-SiC epitaxial layer under high-intensity scanning laser beam during room temperature photoluminescence mapping, which is similar to the degradation of bipolar pin diodes during forward current injection. In an epitaxial layer on an 8 off-axis (0001) substrate, the SSF-related intensity patterns induced by scanning high-intensity laser beam were classified into two types. The first one was a triangular pattern and the second a pattern which expanded in accordance with the motion of the scanning laser beam. The origins of the SSFs responsible for both patterns are presumably due to the preexisting basal plane dislocations and the dislocation-loops on the basal plane in the epitaxial layer, respectively. On the other hand, most of the SSF-expansion in on-axis (11 2 0) epitaxial layers were similar to the second type in the (0001) epitaxial layer. We, therefore, suggest that the dislocation-loops, which were located close to the surface, were dominant nucleation-sites of the SSFs in the (11 2 0) epitaxial layers.

Thermodynamics of single strand DNA base stacking

The thermodynamics of the stacking to unstacking transitions of 24 single-stranded DNA sequences (ssDNA), 10-12 bases in length, in sodium phosphate buffer were determined from 10 to 95 degrees C, using differential scanning calorimetry (DSC). An additional 22 ssDNA sequences did not exhibit an S<=>U transition in this temperature range. The transition properties of the ssDNA sequences with <or=60% self-complementarily in the reverse direction were independent of concentration with transition temperatures ranging from 15 to 70 degrees C, van't Hoff transition enthalpies from 92 to 201 kJ mol(-1) and transition enthalpies from 5 to 75% of the corresponding van't Hoff transition enthalpies. Since all the 16 doublets and 60 of the 64 triplets are present in both the transition and the non-transition ssDNA sequences, it is unlikely that the nucleation subset initiating stacking of the sequence is a specific doublet or triplet subset. Of the 141 quadruplet subsets of the 46 sequences, each transition ssDNA sequence contained at least one or more quadruplets not found in the non-transition ssDNA sequences. It could be concluded that the thermal stability of the stacked conformation was dependent on the presence of a possible nucleation quadruplet and the length of the ssDNA sequence and not on the G or C content of the ssDNA sequence, nor on the number of purine bases in the sequence.

Glide and multiplication of basal plane dislocations during 4H-SiC homoepitaxy

Basal plane dislocations (BPDs) are an important category of extended defects in SiC epilayers. They act as nucleation sites for single layer Shockley-type stacking faults which account for the degradation of the bipolar devices operating under forward bias. It is well documented that most of the BPDs in the SiC epilayers propagate from the substrates. However, two characteristic types of BPDs were suggested to be due to either nucleation or multiplication during epitaxy, including interfacial dislocations and short BPD arrays connected to the epilayer surface by threading segments. Combining molten KOH etching, plan-view transmission x-ray topography, and photoluminescence mapping, both types are determined to be two parts of one defect produced by the sideway glide of a BPD under the influence of shear stress. During the glide, the down-step end of the BPD frequently produces a series of short BPD segments at the moving growth front. These BPD segments will grow into an array of dislocation half loops. At the same time, the sideway glide of the BPD in the epilayer leaves an edge-type BPD segment at the epilayer∕substrate interface, which is the interfacial dislocation. The defect morphology provides the evidence of significant level of shear stresses present in SiC homoepitaxy of typical power device structures. The magnitude of such stresses is estimated.

A transmission electron microscopy study of defects formed through the capping layer of self-assembled InAs∕GaAs quantum dot samples

Plan-view and cross-sectional transmission electron microscopy have been used for a detailed study of the defects formed in capped InAs∕GaAs quantum dot (QD) samples. Three main types of defects, V-shaped defects, single stacking faults, and stacking fault pyramids, were found to form under growth conditions that led to either very large, indium enriched, or coalesced islands. All three types of defects originate at the buried quantum dot layer and then travel through the GaAs cap to the surface on the {111} planes. The V-shaped defects were the most common and typically consisted of two pairs of closely spaced 60° Shockley partials with a ⟨211⟩ line direction. The two pairs originate together at the buried QD layer and then travel in “opposite” directions on different {111} planes. The second type of defect is the single stacking fault which consists of a single pair of partial dislocations separated by an ≈50nm wide stacking fault. Finally, both complete and incomplete stacking fault pyramids were observed. In the case of the complete stacking fault pyramid the bounding dislocations along the [110], [11¯0], [101¯], and [101] directions were identified as stair rods. A possible mechanism for the stacking fault pyramid formation, which can also account for the creation of incomplete stacking fault pyramids, is presented.

Electronic driving force for stacking fault expansion in4H‐SiC

Trapping of electrons in stacking fault (SF) interface states may lower the energy of a SF more than it costs to form the SF. This ``electronic stress'' driving force for SF expansion is evaluated for single and double stacking faults in $4H\text{\penalty1000-\hskip0pt}\mathrm{SiC}$ in terms of a two-dimensional free-electron density of states model based on first-principles calculations. In contrast with previous work, which claimed that the number of electrons that can be trapped in the SF is severely limited by the potential barrier arising from the space-charge region adjacent to the SF, we find that the potential barrier is strongly reduced by screening and its effect is negligible.

4,5,6-Triamino-2-(methylsulfanyl)pyrimidine: π-stacked hydrogen-bonded sheets ofR22(8),R22(10) andR66(32) rings

In the title compound, C5H9N5S, the three independent C-NH2 units are all somewhat pyramidal. The molecules are linked by a combination of one N-H...S and two N-H...N hydrogen bonds into sheets containing three types of ring motif, viz. R(2)(2)(8), R(2)(2)(10) and R(6)(6)(32), all of them centrosymmetric. Adjacent sheets are linked by a single pi-pi stacking interaction.

Pressure-dependent Ni–O phase transitions and Ni oxide formation on Pt(111): An in situ STM study at elevated temperatures

Growth, atomic structure and O2 partial pressure dependent phase transitions of Ni-O structures and thin NiO films on Pt(111) have been studied using scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). In situ STM experiments were performed during film growth by reactive metal deposition at elevated temperatures (400-550 K) and variable O2 pressure. Depending on the substrate temperature, one-dimensional network-like Ni-O structures and islands with (7x1) and (4x2) reconstructions are formed during the initial stages of growth. These structures transform reversibly to a (2x2) reconstruction in a narrow O2 pressure range of 1.5-2x10(-6) mbar and can be monitored by in situ STM. Upon reduction of the O2 pressure to <10(-10) mbar pseudomorphic Ni monolayers are obtained. The defect-free ordering of Ni atoms on Pt(111) in a single stacking domain indicates an O-surfactant induced growth mode. The structural properties of the O2 pressure-dependent Ni-O phases are discussed in a simple model assuming NiO(001)-like atomic arrangements in the adsorbate overlayer. At higher coverage stable (111)-oriented NiO islands grow in a three-dimensional mode.