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The acoustic phonon transport through an asymmetric y-branch semiconductor quantum structure is investigated by use of the scattering-matrix method. It is found that the quantum structure exhibits mode-splitting behavior, resonant transmission, and the noninteger quantized thermal conductance at very low temperatures. The thermal transport behavior sensitively depends on the geometric configuration of the structure. Our work suggests that such a y-branch structure with specific structural parameters could be used as a splitter of phonon modes.

We propose and demonstrate a spin-filter diode based on semimagnetic semiconductor ZnSe/Zn1−xMnxSe/Zn1−yMnySe/ZnSe heterostructures. It is found that the degree of spin polarization greatly depends on the electric field direction and its magnitude. There is significant difference of the spin polarization between under forward bias and under reverse bias. It is also found that the spin polarization will reverse under relatively small magnetic field, which is originated from resonant enhancement effect for spin-up electrons tunneling through effective steplike potential of the corresponding structure.

By use of first-principles calculations, we investigate the effect of introducing substitutional boron or nitrogen atom in the tip on field-emission properties of capped single-wall carbon nanotubes. We find that the substitution in the tip can significantly enhance the localized density of states at the Fermi level, which is important for field emission. The substitution of nitrogen introduces unsaturated dangling bond states at the tip, yielding a reduction of the work function of the tip. Our results suggest that the capped tube with substitutional nitrogen or boron atom in the tip could have much larger field-emission current than the tube without substitution.

We find that the conventional method to calculate the soft-mode frequency of the pseudospin model is incorrect, and present a valid modified method. It is demonstrated that the conventional method does not show the soft-mode frequency going to zero at a critical temperature in the presence of random internal fields, while the frequency calculated by our modified method goes to zero in random fields at a critical temperature. For the field-induced phase transition relaxor ferroelectrics, the soft-mode frequency decreases to zero at the first-order phase transition temperature though the symmetry has been destroyed at high temperatures under an external field.

In this letter, by use of the first-principles calculations, field emission properties of the body are investigated for a series of dimension-variational single-walled carbon nanotubes (SWNTs). We find that in the course of increasing tube diameter, at low emission currents, field emission properties worsen, chemical stability enhances and binding energy increases, which indicates that optimized field emitter could be achieved; while at high applied voltages, the highest emission current density may appear for the SWNT with some peculiar dimension. We predict that the body of the SWNT could behave as a peculiar field emitter with high threshold voltage and high current density.

The low-energy theory for finite long carbon nanotube is derived and numerically examined. It shows that the electronic structure is dominated by the quantum confining, which determines the profile of wave functions as well as the eigen energies; while the details of the wave functions are resolved by the structure of the nanotubes. This behavior is attributed to the peculiar electronic structure of the nanotubes. Because of the slow variation of the profile of electron wave functions, the measured conductance is NOT independent of the position to measure it, which is evident in the multiprobe experiment.

A method to prepare silicon nitride nanoscale rods using carbon nanotube as a template has been presented in this letter. The products of the reaction of carbon nanotubes with a mixture of Si and SiO2 powder in nitrogen atmosphere are β-Si3N4, α-Si3N4, and Si2N2O nanorods. The sizes of the nanorods are 4–40 nm in diameter and up to several microns in length. The formation mechanism of the nanorods has also been discussed.

The dielectric constant e*(ω,T)=e′(ω,T)+ie″(ω,T) of relaxor ferroelectrics is simulated by the Monte Carlo method. It is proved that the polar microregions switch among different polarization directions as a whole under the electrical field, giving rise to the high dielectric constant. The relaxor characteristics are caused by the randomly distributed interactions between the polar microregions. Factors which influence the dielectric response process are discussed.

From first-principles calculations, we find that in a (110) Si film with surface etching along the [001] direction, the holes can be confined underneath the patterned layer. This effect arises from the interplay between the anisotropic carriers and the patterning-induced quantum confinement. An anisotropy coefficient K=mz/my, which is the ratio between the out-of-plane effective mass and the in-plane effective mass of the charge carriers in the film, is introduced to explain the orientation dependence. We propose that a modulation-dopinglike effect can be achieved in the (110) nanopatterned Si film by selective doping in the top patterned layer.

Using spin-polarized density functional theory calculations, we demonstrate that hybrid BN–C nanotubes (BN-CNTs) have diverse electronic and magnetic properties depending on their percentage of carbon and BN components. Typically, a BN-CNT is converted from a nonmagnetic semiconductor to a spin-polarized metal and then to a nonmagnetic semiconductor by increasing the ratio of BN component. The intrinsic half-metallicity could be achieved when the percentage of carbon component in the tube is within a certain ratio, and is insensitive to the tube curvature. Our findings suggest that BN-CNTs may offer unique opportunities for developing nanoscale spintronic materials.

Our first-principles calculations indicate the possibility of preparing spin-polarized scanning tunneling microscopy (SP-STM) probes from Fe-doped capped carbon nanotubes (CNTs). The structural stability, magnetic moment, and electronic property of hybrid systems are found to depend on the Fe adsorption site, which is attributed to the hybridization between Fe 3d and C 2p orbitals. The CNTs with Fe atoms adsorbed at the tip-top are demonstrated to be promising candidates for the SP-STM probe, with a high spin polarization leading to a completely spin-polarized current at lower voltages. In contrast, the CNTs encapsulating Fe atom are basically nonmagnetic, and thus useless for the SP-STM probe application in nature.

Using first-principle electronic structure calculations, we show a metal- semiconductor transition of a metallic graphene nanoribbon with zigzag edges induced by substitutional doping of Nitrogen or Boron atoms at the edges. A field effect transistor consisting of a metal-semiconductor-metal junction can then be constructed by selective doping of the ribbon edges. The current-voltage characteristics of such a prototype device is determined by the first-principle quantum transport calculations., 4 pages, 3 figures. Accepted in APL

First-principles calculations of crystalline silicon nanotubes SiNTs show that nonuniformity in wall thickness can cause sizable variation in the band gap as well as notable shift in the optical absorption spectrum. A unique quantum confinement behavior is observed: the electronic wave functions of the valence band maximum and conduction band minimum are due mainly to atoms located in the thicker side of the tube wall. This is advantageous to spatially separate the doping impurities from the conducting channel in doped SiNTs. Practically, the performance of the SiNT-based transistors may be substantially improved by selective p/n doping in the thinner side of the tube wall in the spirit of modulation doping. © 2007 American Institute of Physics. DOI: 10.1063/1.2778756

Density functional calculations are performed to investigate the functionalization of carbon nanotubes with the vacancy and carboxyl group that always appear in pairs after chemical oxidation (such as oxidative acid treatment). It is found that the geometrical structures and electronic properties of the nanotubes are changed significantly, with the formation of a strong covalent bond between the carboxyl and the defected tube. The electrical conductivity of the system could be considerably enhanced, which sensitively depends upon the axial density of vacancy-carboxyl pairs. A promising approach, controllable oxidative acid treatment, is designed to make carbon nanotube-based chemical and nanoelectronic devices.

First-principles calculations are performed to study the mechanical properties, electronic structure, and uniaxial-stress effects of β-SiC nanowires (NWs). It is found that the band gap of SiC NWs becomes larger as their diameter decreases because of the quantum confinement effect, but increases (decreases) slightly with increasing tensile (compressive) stress up to about 12GPa. The calculated Young’s modulus and tensile strength of SiC NWs are about 620 and 52GPa, respectively, in accordance with the experimental data. The characteristics of their mechanical and electronic properties suggest that β-SiC NWs may be used in electronic composites as reinforcement nanomaterials or in nanoscale electronic/photoelectric devices under harsh environments.

Ab initio calculations reveal that ultrashort-period epitaxial KNbO3∕(KTaO3)m superlattice exhibits typical ferroelectricity when m=1 or 2; while it becomes paraelectric when m⩾3. We observe considerable and tantamount dipole moment in each cell, which can be attributed to the continuity of dipole moment at the interface and the short period of the heterostructure. The polarization behavior of KNbO3∕KTaO3 superlattices sensitively depends upon the ratio of two constituents. Our results directly support the experimental finding of the antiferroelectric behavior in the 1×1 KNbO3∕KTaO3 superlattice.

Using a first-principles-based approach, we investigate the strain–temperature phase diagram of Pb(Zr0.5Ti0.5)O3 ultrathin film without surface charge screening. We find that the compressive strain dramatically affects the sequence of phase transitions, and observe a rich variety of ferroelectric phases, including a monoclinic phase and a stripe domain forbidden in the bulk crystal. The vortex stripe structure leads to an unusual characteristic of the phase diagram.