Yongguang Xiao

Effects of intertube coupling and tube chirality on thermal transport of carbon nanotubes

We study the thermal conductivity of single-walled carbon nanotube bundles and multi-walled carbon nanotubes. It is shown that, for an individual single-walled carbon nanotube, its thermal conductivity is both diameter and chirality dependent. In defect-free bundles and multi-walled carbon nanotubes, the phonon essentially does not transcend the bonds between the constituent walls due to the weak intertube interaction. If, however, the intertube coupling is strong, a substantial reduction in the thermal conductivity maybe implied. Such a low thermal conductivity can be found in several thermal transport experiments of carbon nanotube mats.

Specific heat of single-walled boron nitride nanotubes

Based on a force constant model, we have calculated the phonon spectrum and specific heat of single-walled boron nitride nanotubes. Compared with carbon nanotubes, boron nitride nanotubes have a larger specific heat. The fitting formulas for diameter and chirality dependence of specific heat at 300 K are given.

Enhanced magnetoelectric effect in La0.67Sr0.33MnO3/PbZr0.52Ti0.48O3 multiferroic nanocomposite films with a SrRuO3 buffer layer

Epitaxial thin films of La0.67Sr0.33MnO3/PbZr0.52Ti0.48O3 (LSMO/PZT) were deposited on SrTiO3 substrates with a SrRuO3 buffer layer by employing the pulsed laser deposition method. Its crystal structure, surface morphology, and polarization switching were investigated by using X-ray diffraction, scanning electron microscopy, and piezo-force microscopy measurements, respectively. Ferroelectric behavior along with a remnant polarization (2Pr) of 62 μC/cm2, a saturated magnetization of ∼76 emu/cm3, and a magnetoelectric effect voltage coefficient αME around 202 mV/cm Oe and 300 mV/cm Oe at 1 kHz were obtained successfully for the LSMO/PZT bilayers at room temperature. The result indicated that the coupling effects of electric and magnetic fields exist in the fabricated composite thin films.

Phonon spectrum and specific heat of silicon nanowires

Based on lattice dynamics theory and molecular dynamics simulations, we have investigated the geometrical structures, phonon dispersion relations, and specific heat of silicon nanowires with Stillinger-Weber potential. It was shown that the original Stillinger-Weber potential can reproduce the well-established four acoustical branches. With the calculated spectra, we calculated specific heats of silicon nanowires. It is found that the specific heats of thin nanowires are much higher than those of bulk silicon. According to the partial density of states of surface atoms, the enhancement of specific heats of silicon nanowires can be attributed to the surface effect and phonon confinement effect.

Temperature dependence of magnetoelectric effect in Bi3.15Nd0.85Ti3O12–La0.7Ca0.3MnO3 multiferroic composite films buffered by a LaNiO3 layer

Multiferroic composite thin films were fabricated based on ferroelectric (FE) Bi3.15Nd0.85Ti3O12 (BNT) and ferromagnetic (FM) La0.7Ca0.3MnO3 (LCMO) parents with different growth sequences of BNT–LCMO/LNO/STO (BL) and LCMO–BNT/LNO/STO (LB). Ferroelectric behaviour along with remnant polarization (2Pr) of 50 μC cm2 and 40 μC cm2 at room temperature, saturated magnetization values around 206 emu cm−3 and 192 emu cm−3 at 100 K were measured for BL and LB composite films, respectively. The temperature dependence of the magnetoelectric (ME) coupling effect was investigated and the ME voltage coefficients of 63 mV cm−1 Oe−1 and 60 mV cm−1 Oe−1 at 100 K were respectively obtained for the BL and LB films. The results show that the composite films exhibit both good ferroelectric and ferromagnetic properties, as well as a substantial ME effect. Moreover, it is demonstrated that the layer sequences and temperature have significant impacts on the magnetoelectric coupling behaviour of these double-layered thin films, which is very likely caused by the magnetic–mechanical–electric interaction, substrate clamping and interface coupling.

Improvement of resistive switching performances via an amorphous ZrO2 layer formation in TiO2-based forming-free resistive random access memory

We present the effects of an amorphous ZrO2 layer on the TiO2-based bipolar resistive switching memory device where the ZrO2 layer plays an important role as a supplementary reservoir of oxygen vacancies. Compared with Pt/TiO2/Pt monolayer device, a remarkably improved uniformity of switching parameters such as switching voltages and resistances in high/low states is demonstrated in the Pt/ZrO2/TiO2/Pt system. The resistive switching mechanism of memory devices incorporating the ZrO2/TiO2 bilayer structure can be attributed to multiple conducting filaments through the occurrence of redox reactions at the ZrO2/TiO2 surface.

Impact of total ionizing dose irradiation on electrical property of ferroelectric-gate field-effect transistor

P-type channel metal-ferroelectric-insulator-silicon field-effect transistors (FETs) with a 300 nm thick SrBi2Ta2O9 ferroelectric film and a 10 nm thick HfTaO layer on silicon substrate were fabricated and characterized. The prepared FeFETs were then subjected to 60Co gamma irradiation in steps of three dose levels. Irradiation-induced degradation on electrical characteristics of the fabricated FeFETs was observed after 1 week annealing at room temperature. The possible irradiation-induced degradation mechanisms were discussed and simulated. All the irradiation experiment results indicated that the stability and reliability of the fabricated FeFETs for nonvolatile memory applications will become uncontrollable under strong irradiation dose and/or long irradiation time.

Resistive switching properties of Ce and Mn co-doped BiFeO3 thin films for nonvolatile memory application

The Ce and Mn co-doped BiFeO3 (BCFMO) thin films were synthesized on Pt/Ti/SiO2/Si substrates using a sol-gel method. The unipolar resistive switching (URS) and bipolar resistive switching (BRS) behaviors were observed in the Pt/BCFMO/Pt device structure, which was attributed to the formation/rupture of metal filaments. The fabricated device exhibits a large ROFF/RON ratio (>80), long retention time (>105 s) and low programming voltages (<1.5 V). Analysis of linear fitting current-voltage curves suggests that the space charge limited leakage current (SCLC) and Schottky emission were observed as the conduction mechanisms of the devices.

Influence of substrates on resistive switching behaviors of V-doped SrTiO3 thin films

V-doped SrTiO3 (V:STO) thin films on Si and Pt/Ti/SiO2/Si substrates are synthesized by sol–gel method to form metal–insulator–metal (MIM) structures. Bipolar resistive switching (RS) characteristics were investigated in Pt/V:STO/Si and Pt/V:STO/Pt structures respectively. The enhancement of resistive switching behavior in Pt/V:STO/Pt/Ti/SiO2/Si structures were demonstrated in terms of the maximum operation voltage reduced from 20 to 2 V and the improved ROFF/RON ratio increased from 102 to 103. The electrochemical migration of oxygen vacancies resulted from the metal–oxide interfaces was applied to explain the resistive switching behaviors. On the basis of current–voltage characteristics, the switching mechanisms for the low resistance state (LRS) and high resistance state (HRS) currents of V:STO films are considered as Ohmic and trap-controlled space charge-limited current (SCLC) behavior, respectively.

Electric-field tuning of ferromagnetic resonance in CoFeB/MgO magnetic tunnel junction on a piezoelectric PMN-PT substrate

We investigated the strain-induced modulation of in-plane magnetic anisotropy in single crystal Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) with a (011) orientation/seed layer/IrMn/CoFeB/MgO/CoFeB/capping layer structure using spin-torque ferromagnetic resonance (ST-FMR) at room temperature. An in-plane biaxial strain is produced by an electric field applied across the substrate, which results in the tuning of the magnetic anisotropy of the CoFeB layer. As the change in the electric field value progresses, the MR loop becomes slanted, indicating the change in the easy axis direction. A strain-induced FMR frequency shift dependence of 145 MHz⋅cm⋅kV−1 is obtained by analyzing the experimental FMR spectra. An analytical model of the potential dynamic mechanism based on the strain induced reorientation of magnetization was discussed. Our results suggest that PMN-PT based magnetic tunnel junctions can have applications in electric-field driven spintronic devices.