Jen-Bin Shi

Synthesis and Characterization of Tin Disulfide (SnS2) Nanowires

The ordered tin disulfide (SnS2) nanowire arrays were first fabricated by sulfurizing the Sn nanowires, which are embedded in the nanochannels of anodic aluminum oxide (AAO) template. SnS2nanowire arrays are highly ordered and highly dense. X-ray diffraction (XRD) and corresponding selected area electron diffraction (SAED) patterns demonstrate the SnS2nanowire is hexagonal polycrystalline. The study of UV/Visible/NIR absorption shows the SnS2nanowire is a wide-band semiconductor with three band gap energies (3.3, 4.4, and 5.8 eV).

Dielectric studies in T* and T′ structures of (La, Gd)2CuO4

Abstract The real (e′) and imaginary (e″) parts of dielectric constant in some T* (La1.15Gd0.85CuO4) and T′ (La0.3Gd1.7CuO4) copper-oxide samples were investigated as a function of temperature (15–300 K) and frequency (102–106 Hz). The e′ reveals a very gigantic value at low frequencies in both cases. These two T* and T′ samples show a thermally activated process with a small activation energy of 0.034 eV for T* sample and 0.044 eV for T′ sample, respectively. We have found α value of 0.11 for the T′ sample at 50 K and 0.48 for the T* sample at 60 K. These temperature and frequency effects in e′, e″ and σ can be nicely interpreted in terms of dipole polarization associated with the 3D hopping motion of localization charge centers.

Dielectric properties of Gd2CuO4

Abstract The complex dielectric response of the Gd 2 CuO 4 samples measured from 20 Hz to 1 MHz and from 20 K to 300 K reveals very high real part of the dielectric constant e ′>10 4 at 300 K and >5×10 3 above 200 K. The large polarizability results from a thermally activated process with a very small activation energy of 0.065 eV and extra additional polarization. We observed one frequency-dependent and one frequency-independent peak in dissipation factor D ( T , ω ) curves and corresponding features in imaginary part of the dielectric constant e ″ curves. The first peak occurs at T ∼230 K and is thought to be due to the occurrence of antiferromagnetic transition in the sample and the second one occurs over the range of 50 to 120 K and is related to the change of conductivity mechanism.

Anodic Aluminum Oxide Membrane-Assisted Fabrication of β-In2S3Nanowires

In this study, β-In2S3nanowires were first synthesized by sulfurizing the pure Indium (In) nanowires in an AAO membrane. As FE-SEM results, β-In2S3nanowires are highly ordered, arranged tightly corresponding to the high porosity of the AAO membrane used. The diameter of the β-In2S3nanowires is about 60 nm with the length of about 6–8 μm. Moreover, the aspect ratio of β-In2S3nanowires is up to 117. An EDS analysis revealed the β-In2S3nanowires with an atomic ratio of nearly S/In = 1.5. X-ray diffraction and corresponding selected area electron diffraction patterns demonstrated that the β-In2S3nanowire is tetragonal polycrystalline. The direct band gap energy (Eg) is 2.40 eV from the optical measurement, and it is reasonable with literature.

Critical current densities in K3C60/Rb3C60 powders determined from AC/DC susceptibility measurements

Abstract We report the magnetic J c ( T ) in K 3 C 60 and Rb 3 C 60 powder samples for temperatures from 5 K to T c . J c near T c was obtained from AC measurements whereas J c at low temperatures was obtained from DC measurements. In the AC measurements, two χ″ peaks were observed on some of the samples. This is attributed to weak links in granular superconductors. The intragranular J c is much larger than the intergranular J c . The J c ( T ) data are fitted by J c ( T ) = J c (0)(1 − T / T c ) γ , with J c (0) and γ as fitting parameters. In the DC measurements on Rb 3 C 60 , J c depends approximately linearly on temperature. Extrapolating the zero-field DC data to 0 K yields J c (0) = 3.1 × 10 6 A/cm 2 . This is a rather large J c for a powdered superconductor, suggesting strong pinning in these materials. Finally, the values of γ for the intergranular and intragranular components are compared with the predictions of theories.

Characteristics and interactions of threading dislocations in GaN films grown on (0001) sapphire substrates with or without short-period superlattice insertion

In this study, transmission electron microscopy was employed to investigate the characteristics of threading dislocations (TDs) in GaN films grown on the (0001) sapphire substrates with or without the insertion of Al0.3Ga0.7N (2 nm)/GaN (2 nm) short period superlattices (SPSLs). By using gb = 0 invisibility criterion, it was found that most of the TDs were type a TDs in a GaN film either containing SPSL or having no SPSL insertion. Type a + c TDs were found to nucleate through the interactions between type a and type c TDs in GaN near the GaN/sapphire interface. Some of the type a TDs were observed to bend along GaN basal plane because of the influence of biaxial strain near the GaN/SPSL interface.

Scaling law for vortex dynamics in Rb3C60 superconductor

Abstract AC susceptibility of the Rb3C60 superconductor shows a weak dependence on ac magnetic-field frequency f. The temperature, Tp where χ″ is maximum, rises with increasing frequency and the effect is enhanced as the dc magnetic-field amplitude Hdc is increased. The temperature is found to obey a scaling law (1 − T p /T c ) 3 2 = 1.9 × 10 −4 H dc [ ln (f 0 /f) + 23] + 8 × 10 −5 [ ln (f 0 /f) + 23] , where f0 (≈ 1010 s−1) is a characteristic frequency and Hdc is in tesla. The scaling relationship is in good agreement with the predicted equation based on the model of thermally activated flux motion.

Resistance switching behavior of ZnO resistive random access memory with a reduced graphene oxide capping layer

In this work, we investigate the characteristics of ZnO resistive random access memory (RRAM) with a reduced graphene oxide (rGO) capping layer and the polarity effect of the SET/RESET bias on the RRAM. The rGO film insertion enhances the stability of the current–voltage (I–V) switching curve and the superior resistance ratio (~105) of high-resistance state (HRS) to low-resistance state (LRS). Using the appropriate polarity of the SET/RESET bias applied to the rGO-capped ZnO RRAM enables the oxygen ions to move mainly at the interface of the rGO and ZnO films, resulting in the best performance. Presumably, the rGO film acts as an oxygen reservoir and enhances the easy in and out motion of the oxygen ions from the rGO film. The rGO film also prevents the interaction of oxygen ions and the Al electrode, resulting in excellent performance. In a pulse endurance test, the rGO-capped ZnO RRAM reveals superior endurance of up to 108 cycles over that of the ZnO RRAM without rGO insertion (106 cycles).

Observation of high dielectric permittivity in single-crystal Bi2Sr2CoO6+δ

Abstract We have measured the complex dielectric constants of a c -axis-oriented single-crystal insulating sample of Bi 2 Sr 2 CoO 6+δ as a function of frequency (10 2 –10 6 Hz) and temperature (80–330 K). A large real dielectric constant (> 10 4 ) was found at high temperature and low frequency. This ferroelectric-like polarization results from a thermally activated process with an activation energy of 0.25 eV plus an exponentially increasing temperature-dependent background. Our results support the idea that the high dielectric constant may be a common feature of the high-temperature oxide family.

Characteristics of a VHF H2 plasma at high pressures for different discharge gaps

A capacitively coupled VHF H2 plasma (60 MHz) was produced at high pressures, and plasma parameters were examined as a function of pressure and power at different discharge gaps (10, 15, and 20 mm). Here, a balanced power feeding method was adopted to avoid anomalous discharges. It was found that the electron density peaked at a certain pressure. As the discharge gap decreased, the peak pressure increased. The electron temperatures at the discharge gaps of 10 and 20 mm were approximately 8 and 3 eV at high pressures, respectively. In addition, we found that the sheath potential corresponding to the ion bombardment energy with the discharge gap of 10 mm was lower than the theoretical value calculated on the basis of the sheath theory, suggesting the existence of negative ions. Estimation of negative ion concentration was attempted on the basis of the sheath theory taking into consideration negative ions.