liang Yong

Bandgap engineering of MoS2/MX2 (MX2 = WS2, MoSe2 and WSe2) heterobilayers subjected to biaxial strain and normal compressive strain

Using first-principles calculations, we studied the electronic properties of quasi-2D MoS2/MX2 (MX2 = WS2, MoSe2 and WSe2) heterobilayers, focusing on engineering the band gap via application of in-plane biaxial strain and out-of-plane normal compressive strain (NCS). All heterobilayers show semiconducting characteristics with an indirect band gap except for the MoS2/WSe2 system which exhibits direct band gap character. The band gaps can all be widely tuned through strain and semiconductor–metal transitions can occur. In particular the direct band gap can be tuned and an appropriate compressive strain can tune the direct band gap of MoS2/WSe2 and MoS2/MoSe2, but MoS2/WS2 does not exhibit a direct band gap under any circumstances.

Adsorption sensitivity of graphane decorated with B, N, S, and Al towards HCN: a first-principles study

The geometric structure, adsorption energy, electronic structure, and magnetic properties of hydrogenated graphene (graphane) with the adsorption of a HCN molecule were investigated by first-principles calculations. Compared with graphane, the adsorption of HCN on H-vacancy defected graphane (VHG) exhibited higher stability, which implied that the H-vacancy improved the sensitivity of graphane. However, the small adsorption energies and large bond distance indicated that the weak adsorption of a HCN molecule on the graphane and VHG substrates was due to physisorption. By introducing dopants (B, N, S, and Al), the activity of graphane was significantly improved. The adsorption of HCN changed to chemisorption on the graphane with dopants. Meanwhile, the opening of band gaps by HCN adsorption can be used as an electronic signal to detect HCN gas. Interestingly, the spin polarized density of states (PDOS) results suggested that the adsorption of HCN on VHG and S-doped VHG exhibited magnetic character and half-metallicity behavior. These results could provide useful information to design gas sensors for HCN or spintronic devices based on graphane.

Adsorption of H2S on graphane decorated with Fe, Co and Cu: a DFT study

Herein, density functional theory (DFT) calculations were performed to investigate the adsorption of a H2S molecule on the surface of hydrogenated graphene (graphane). In our results, we found that the appearance of an H-vacancy significantly improved the reactivity of graphane due to the unpaired electrons of the vacancy site. However, small adsorption energy and low charge transfer indicated that the interaction between the H2S molecule and the pure H-vacancy-defected graphane occurred via physisorption. By introducing transition-metal dopants (Fe, Co, and Cu), the adsorption process of the H2S molecule changed to chemisorption. Furthermore, the adsorption of H2S induced a decrease in the band gaps, which could be seen as signal for the detection of H2S gas.

Molecular structure, vibrational spectra, NBO analysis and molecular packing prediction of 3-nitroacetanilide by ab initio HF and density functional theory

Quantum chemical calculations of geometries and vibrational wavenumbers of 3-nitroacetanilide (C8H8N2O3) in the ground state were carried out by using ab initio HF and density functional theory (DFT/B3LYP) methods with 6-31+G(*) basis set. The -311++G(**) basis set is also used for B3LYP level. The scaled harmonic vibrational frequencies have been compared with experimental FT-IR spectra. Theoretical vibrational spectra of the title compound were interpreted by means of potential energies distributions (PEDs) using MOLVIB program. The theoretical spectrograms for IR spectra of the title compound have been constructed. The shortening of C-H bond length and the elongation of N-H bond length suggest the existence of weak C-H⋯O and N-H⋯O hydrogen bonds, which is confirmed by the natural bond orbital analysis. In addition, the crystal structure obtained by molecular mechanics belongs to the P2(1) space group, with lattice parameters Z=4, a=14.9989 Å, b=4.0367 Å, c=12.9913 Å, ρ=0.998 g cm(-3).

Adsorption of gas molecules on a graphitic GaN sheet and its implications for molecule sensors

Motivated by the recent realization of two-dimensional (2D) nanomaterials as gas sensors, we have investigated the adsorption of gas molecules (SO2, NO2, HCN, NH3, H2S, CO, NO, O2, H2, CO2, and H2O) on the graphitic GaN sheet (PL-GaN) using density functional theory calculations. It is found that among these gases, only SO2 and NH3 gas molecules are chemisorbed on the PL-GaN sheet with apparent charge transfer and reasonable adsorption energies. The electronic properties (especially the electric conductivity) of the PL-GaN sheet showed dramatic changes after the adsorption of NH3 and SO2 molecules. However, the strong adsorption of SO2 on the PL-GaN sheet makes desorption difficult, which precludes its application to SO2 sensors. Therefore, the PL-GaN sheet should be a highly sensitive and selective NH3 sensor with short recovery time. Furthermore, the adsorption of NO (or NO2) molecules introduces spin polarization in the PL-GaN sheet with a magnetic moment of about 1 μB, indicating that magnetic properties of the PL-GaN sheet are changed obviously. Based on the change of magnetic properties of the PL-GaN sheet before and after molecule adsorption, the PL-GaN sheet could be used as a highly selective magnetic gas sensor for NO and NO2 detection.

Quantum-chemical studies of the structure and performance properties of 5-(1,2,4-triazol-C-yl)tetrazol-1-ols

Density functional theory was used to investigate IR spectra, heat of formation, and thermal stability of three energetic 5-(1,2,4-triazol-C-yl)tetrazol-1-ol compounds substituted at position 5 of the triazole ring. The detonation velocity and pressure were evaluated by using the Kamlet–Jacobs equations based on the packed density and solid-state heat of formation. The bond dissociation energies for the weakest bonds were analyzed to investigate the thermal stability of the title compounds. IR analysis shows that there are four main characteristic regions for the three compounds. Detonation velocity and pressure of the nitro derivative are higher than those of known explosive HMX, while the same characteristics of the nitroamino and azido derivatives are comparable to those of HMX. Bond dissociation calculations show that the N(5)–N(7) bond is the trigger bond during pyrolysis for all three compounds and the order of their thermal stability is azido > nitroamino > nitro. In addition, the energy gaps between the HOMO and LUMO of the studied compounds were also investigated and the obtained conclusion consistent with that of bond dissociation energy analysis.

The Zn 12 O 12 cluster-assembled nanowires as a highly sensitive and selective gas sensor for NO and NO 2

Motivated by the recent realization of cluster-assembled nanomaterials as gas sensors, first-principles calculations are carried out to explore the stability and electronic properties of Zn12O12 cluster-assembled nanowires and the adsorption behaviors of environmental gases on the Zn12O12-based nanowires, including CO, NO, NO2, SO2, NH3, CH4, CO2, O2 and H2. Our results indicate that the ultrathin Zn12O12 cluster-assembled nanowires are particularly thermodynamic stable at room temperature. The CO, NO, NO2, SO2, and NH3 molecules are all chemisorbed on the Zn12O12-based nanowires with reasonable adsorption energies, but CH4, CO2, O2 and H2 molecules are only physically adsorbed on the nanowire. The electronic properties of the Zn12O12-based nanowire present dramatic changes after the adsorption of the NO and NO2 molecules, especially their electric conductivity and magnetic properties, however, the other molecules adsorption hardly change the electric conductivity of the nanowire. Meanwhile, the recovery time of the nanowire sensor at Tu2009=u2009300u2009K is estimated at 1.5 μs and 16.7 μs for NO and NO2 molecules, respectively. Furthermore, the sensitivities of NO and NO2 are much larger than that of the other molecules. Our results thus conclude that the Zn12O12-based nanowire is a potential candidate for gas sensors with highly sensitivity for NO and NO2.

Theoretical investigation on vibrational spectra, first order hyperpolarizability and NBO analysis of 4-Phenylpyridinium hydrogen squarate

The vibrational frequencies of 4-Phenylpyridinium hydrogen squarate (4PHS) in the ground state have been investigated by using B3LYP/6-311++G(d,p) level. The analysis of molecular structure, natural bond orbitals and frontier molecular orbitals was also performed. The IR spectra were obtained and interpreted by means of potential energies distributions (PEDs) using MOLVIB program. NBO analysis proved the presence of C-H⋯O and N-H⋯O hydrogen bonding interactions, which is consistent with the analysis of molecular structure. The dipole moments and first-order hyperpolarizability (βtot) are calculated and are 5.856 D and 4.72×10(-30) esu, respectively. The high βtot value and the low HOMO-LUMO energy gap (4.062eV) are responsible for the optical and electron-transfer properties of 4PHS molecule. The photoresponse-related results indicate that 4PHS molecule is an excellent organic candidate of photon-responsive materials.

Influence of vacancy defects and 3d transition metal adatoms on the electronic and magnetic properties of graphene

Based on the density functional theory (DFT) method, we investigated the geometry stability, electronic and magnetic properties of vacancy-defected graphene with and without the adsorption of transition metal (TM) adatoms (V, Cr, and Mn). The results indicated that the appearance of vacancy, which broke the π-band, induced a magnetic property due to the unpaired electrons. After adsorbing the TM atoms, the electronic and magnetic properties were interestingly modified by the impurity states and the spin-polarized electrons of TM atoms. Moreover, the projected density of states (PDOS) results suggested that the magnetism of systems was mainly dominated by the 2p orbitals of C atoms around the vacancy and the 3d orbital of the TM adatoms.

DFT studies on a high-energy density cage compound 1, 3, 5, 7, 9, 11-hexo(N(CH3)NO2)-2, 4, 6, 8, 10, 12-hexaazatetracyclo[5, 5, 0, 0, 0] dodecane

The infrared and Raman spectra, heat of formation (HOF) and thermodynamic properties were investigated by B3LYP/6-31G** method for a new designed polynitro cage compound 1,3,5,7,9,11-hexo(N(CH3)NO2)-2,4,6,8,10,12-hexaazatetracyclo[5,5,0,0,0]dodecane. The detonation velocity (D) and pressure (P) were predicted by the Kamlet–Jacobs equations based on the theoretical density and condensed HOF. The bond dissociation energies and bond orders for the weakest bonds were analysed to investigate the thermal stability of the title compound. The computational result shows that the detonation velocity and pressure of the title compound are superior to those of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), but inferior to those of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) and hexanitrohexaazaisowurtzitane (HNIW). And the analysis of thermal stability shows that the first step of pyrolysis is the rupture of the N7–NO2 bond. The crystal structure obtained by molecular mechanics belongs to the P21 space group, with the lattice parameters Z = 2, a = 11.8246 Å, b = 10.4632 Å, c = 15.9713 Å, ρ = 1.98 g cm−3.