This paper investigates strain effects on the electronic properties of single-layer and bilayer graphene using a first-principles method. The deformation significantly alters energy dispersion, band overlap, band gap, and the band edges of graphenes. Fermi velocity behaves both linearly and nonlinearly with the strains, depending on the types of deformation and the direction of the Fermi velocity. In bilayer graphene, the uniaxial strain enhances the band overlap by 2 orders of magnitude. A semimetal–insulator transition occurs when bilayer graphene is under a compressive uniaxial strain along the zigzag chain direction. These strain-dependent results are useful for acquiring the intralayer and interlayer atomic relations or Slonczewski–Weiss–McClure parameters. The intralayer coupling γ0 under the H-strain and interlayer couplings γ1, γ3, and γ4 under the P-strain decrease dramatically as the strain increases. Nevertheless, interlayer couplings vary more slowly with the H-strain than the P-strain.

This study evaluates the effect of crystallinity and point defects on time-dependent photoresponsivity and the cathodoluminescence (CL) properties of β-Ga₂O₃ epilayers. A synchrotron high-resolution X-ray technique was used to understand the crystalline structure of samples. Rutherford backscattering spectroscopy was used to determine the net chemical composition of the samples to examine the type and ratio of their possible point defects. The results show that in functional time-dependent photoresponsivity of photodetectors based on β-Ga₂O₃ epilayers, point defects contribution overcomes the contribution of crystallinity. However, the crystalline structure affects the intensities and emission regions of CL spectra more than point defects.

[structure: see text] A new donor-acceptor, 1,4,5,8,9,12-hexaazatriphenylene HATCNOR(n), is described. The synthesis of HATCNOR1 and HATCNOR6 is achieved by the regioselective displacement of 1,4,5,8,9,12-hexaazatriphenylene hexacarbonitrile (HATCN) with an alkoxy group. The X-ray analysis revealed self-assembly of HATCNOR1 in the solid state. HATCNOR6 is the new difunctionalized hexaazatriphenylene discotic liquid crystal.

Using a first-principles method, I have studied the static and vibrational properties of the lonsdaleite-silicon. The frequencies of the Raman active modes of the lonsdaleite-Si at ambient pressure agree very well with the experimental results. Among these Raman active modes, the E 2g mode can be an indication mode of the lonsdaleite-Si. The high-pressure behavior of the zone-center modes of the lonsdaleite-Si is first studied and a pressure-induced mode softening is found. The E 2u mode of the lonsdaleite-Si exhibits red shift under compression and its frequency decreases to zero as the applied pressure increases to around 14 GPa. The pressureinduced mode softening of the E 2u mode indicates that the lonsdaleite-Si has a phase transition before the applied pressure reaches 14 GPa.Silicon has been found to have no less than 12 polymorphs, 1-6 and high pressures can produce most of the polymorphs. That is just the reason why high-pressure behavior of silicon has attracted attention over three decades. 7-9 Silicon exhibits the following phase-transition sequence when the pressure is applied up to 240 GPa: cubic diamond ͑Si I͒ to ␤-Sn ͑Si II͒ around 11 GPa, 2,5 ␤-Sn to Imma ͑Ref. 10͒ and then to simple hexagonal ͑sh͒ at 13-16 GPa, 2,5,10 sh to double-hexagonal close packed ͑dhcp͒ at 34 GPa, 4-6 dhcp to hexagonal close pack ͑hcp͒ at 42 GPa, 4,6 and finally to face-centered cubic ͑fcc͒ at 79 GPa. 4,6,10 When the pressure is released to ambient, no evidence shows these high-pressure phases transforming back to the diamond phase, which is the most stable phase of silicon, instead several metastable phases have been observed. Two metastable phases ͑Si VIII and Si IX͒ have been reported on very fast ͑less than 0.1 sec͒ pressure release from 12 and 15 GPa to ambient pressure. 11 Under slow decompression from ␤-Sn phase, an intermediate R8 phase is firstly formed 8,12 and then the metastable phase BC8 is obtained at 2 GPa. 13 On heating the BC8 silicon to temperatures in the range 200-600°C, it transforms to the other metastable phase: the lonsdaleite ͑hexagonal diamond͒ silicon ͑Si IV͒. 1,14,15 The lonsdaleite-Si also can be induced during hardness indentation at elevated temperature 9 and has been found in the CVD silicon. 16 In addition to being a polymorph of silicon, the lonsdaleite-Si is also a polytype of the diamond-Si ͑Si I͒. The first observation of the lonsdaleite-Si was made by Wentorf and Kasper 1 from annealing the BC8 Si and they resolved the structure by powder-diffraction techniques. The layers of the lonsdaleite-Si are stacked in the wurtzite manner with four atoms per unit cell. Being the metastable phases of silicon, the lonsdaleite-Si has attracted less attention than the BC8 phase. The phase transition between the BC8 and the lonsdaleite structures were studied by their electrical properties. 14,17 The experimental results on the Raman spectrum of the lonsdaleite-Si have been reported, 17,18 where theoretical studies based on empirical models were also given, while no reliable theoretical study has bee...