Based on density functional theory, we investigated the magnetic properties of 5d transition metal (TM) atoms at the porous sites of graphene-like carbon nitride (g-C3N4).

No abstract

Considering the three-phonon process, we calculate the thermal conductivity of zigzag tubes. It is found that thermal conductivity of an isolated ͑6, 0͒ single-walled carbon nanotube increases with the increase of temperature at low temperature, and would show a peak behavior at about 85 K before falling off at high temperature. Moreover, thermal conductivity is high for single-walled carbon nanotubes with small diameters as compared to the tubes with large diameters. The thermal conductivity at 300 K is approximately inversely proportional to the tube's diameter.Potential applications of carbon-nanotube-based devices, such as diodes, 1 field-effect transistors, 2 single-electron transistors, 3 as well as elementary logic circuits 4 rely on an effective way of removing high density of excess heat from the device active regime. 5-7 Removal of heat, actually, depends on the thermal conductivity of carbon nanotubes and the related compounds. 8 Thermal conductivity of singlewalled carbon nanotubes ͑SWCN's͒ is predicted to be unusually high by molecular-dynamics simulations at room temperature. 9-11 The measurement on ropes of SWCN's indicates a linear temperature dependence up to 30 K and an upward bend near 30 K on the curve of thermal conductivity (T). 12 In contrast to that of SWCN's, for multiwall carbon nanotube ͑MWNT͒ ropes increases with temperature in a parabolic fashion at low temperature up to ϳ120 K. 13 In order to reveal temperature dependence behavior of , Kim 14 performed scanning electron microscopy technique on individual MWNT. It is found that at low temperature (8 KϽT Ͻ50 K), increases following a power law with an exponent 2.50, and then increases quadratically when T Ͻ150 K. Although there is a vast literature concerning thermal transport in SWCN's, it is necessary to describe phonon characteristics for understanding the physical essence of the various experimental observations. In this paper, considering the three-phonon umklapp process, we analyze the physical mechanics of the thermal transport in a perfect isolated SWCN.As heat in SWCN's is mostly carried by acoustic phonons, it is reasonable to neglect the electronic component of thermal conductivity. The expression for lattice thermal conductivity at a given temperature T can be written as 15,16where , C , and V are phonon relaxation time, specific heat, and phonon group velocity of phonon mode , respectively. In a perfect isolated SWCN, phonon relaxation time is mainly controlled by boundary scattering and three-phonon umklapp scattering process. So, the total phonon relaxation time is usually given by the Matthiessen rule as 17with the relaxation-time parameters for boundary scattering B and for three-phonon umklapp scattering process U . Here, we choose B ϭ50 ps to be independent of temperature and phonon energy. 12,14,[19][20][21] From the first-order perturbation theory, the relaxation time U in the three-phonon umklapp process for thermal modes q is given by 18where ␥ is the Gruneisen parameter, ប is the Planck constant, M is atomic mas...

Through systematic first-principles calculations, we found extraordinarily high piezoresistance coefficients in a pristine Si͑111͒ nanowire. This stems from interplay between the presence of two surface states with different localizations and unusual surface relaxation. Lattice compression along the axis causes switch between light and heavy surface states and, subsequently, alters the effective masses of carriers. Fascinatingly, model calculations produced main features of experimental data ͓R. R. He and P. D. Yang, Nat. Nanotechnol. 1, 42 ͑2006͔͒ despite much difference in wire sizes and surface conditions.

The modification of the material surface by the second-phase particles enables the electron interaction on the Fermi level or the energy band between different materials, which can achieve the improvement of gas-sensing properties. Herein, a novel composite of PbS quantum-dots-modified MoS 2 (MoS 2 / PbS) is synthesized by combination of hydrothermal method with chemical precipitation and fabricated into the gas sensor to investigate its enhanced gas-sensing properties caused by the modification of PbS quantum dots at room temperature. It is found that the responsivity of MoS 2 /PbS is obviously higher than that of pure MoS 2 gas sensor throughout the whole test range, and MoS 2 /PbS gas sensor has better selectivity compared with pure MoS 2 gas sensor at room temperature. The response of MoS 2 / PbS gas sensor is about 50 times higher than that of MoS 2 gas sensor at 100 ppm NO 2 concentration. The recovery behavior is greatly improved, and the resistance of MoS 2 /PbS gas sensor can return completely with almost no drift (the recovery ratio is more than 99%). The enhanced gas-sensing properties of MoS 2 /PbS, which are superior to those of pure MoS 2 , are ascribed to the large surface area of MoS 2 combined with the high responsivity of PbS quantum dots for NO 2 . The formation of heterojunctions leads to the competitive adsorption of the target gases, which can prevent MoS 2 from being oxidized, further improving the stability of gas sensor. Furthermore, to profoundly discuss the enhanced performances and the sensing mechanism, the molecular models of adsorption systems are constructed to calculate the adsorption energies and the diffusion characters of NO 2 via density functional theory. We expect that our work can offer a useful guideline for enhancing the gassensing properties at room temperature.

Using the full potential linearized augmented plane wave method, we explored the magnetic anisotropy energies of metallophthalocyanine molecules with Mn, Fe, Co, and Ni. Their spin magnetic moments can be expressed as 8−nμB (n=5,6,7,8 for Mn, Fe, Co, and Ni). Interestingly, Mn-phthalocyanine has a large orbital magnetic moment of 0.19μB and a sizable magnetic anisotropy energy of 2.72 meV useable for applications.

No abstract

No abstract