Junku Liu

Why lanthanum-substituted bismuth titanate becomes fatigue free in a ferroelectric capacitor with platinum electrodes

Bi4Ti3O12, (BTO) a Bi-layered perovskite oxide, shows fatigue after repeated ferroelectric polarization reversals. On the other hand, Bi3.25La0.75Ti3O12 (BLT) is fatigue free. From an extensive transmission electron microscopy study, it was found that there is a high density of antiphase boundaries (APBs) in BLT like in the fatigue-free SrBi2Ta2O9 but not in BTO. It is proposed that the existence of APBs possibly plays a key role in the fatigue-free behavior of Bi-layered perovskite oxides.

The Dependence of Graphene Raman D-band on Carrier Density

Raman spectroscopy has been an integral part of graphene research and can provide information about graphene structure, electronic characteristics, and electron-phonon interactions. In this study, the characteristics of the graphene Raman D-band, which vary with carrier density, are studied in detail, including the frequency, full width half-maximum, and intensity. We find the Raman D-band frequency increases for hole doping and decreases for electron doping. The Raman D-band intensity increases when the Fermi level approaches half of the excitation energy and is higher in the case of electron doping than that of hole doping. These variations can be explained by electron-phonon interaction theory and quantum interference between different Raman pathways in graphene. The intensity ratio of Raman D- and G-band, which is important for defects characterization in graphene, shows a strong dependence on carrier density.

Fast Adaptive Thermal Camouflage Based on Flexible VO2/Graphene/CNT Thin Films

Adaptive camouflage in thermal imaging, a form of cloaking technology capable of blending naturally into the surrounding environment, has been a great challenge in the past decades. Emissivity engineering for thermal camouflage is regarded as a more promising way compared to merely temperature controlling that has to dissipate a large amount of excessive heat. However, practical devices with an active modulation of emissivity have yet to be well explored. In this letter we demonstrate an active cloaking device capable of efficient thermal radiance control, which consists of a vanadium dioxide (VO2) layer, with a negative differential thermal emissivity, coated on a graphene/carbon nanotube (CNT) thin film. A slight joule heating drastically changes the emissivity of the device, achieving rapid switchable thermal camouflage with a low power consumption and excellent reliability. It is believed that this device will find wide applications not only in artificial systems for infrared camouflage or cloaking but also in energy-saving smart windows and thermo-optical modulators.

Transmission electron microscopy study on ferroelectric domain structure in SrBi2Ta2O9 ceramics

A transmission electron microscopy investigation has been conducted on the domain structure in SrBi2Ta2O9 ceramics. From the 90° rotation relationship of the electron diffraction pattern of the [001] zone axis, a 90° domain wall can be confirmed. It is due to the failure of Friedel’s law that the contrast of a 180° domain wall can be identified. The antiphase boundary can be seen clearly in the dark-field image, which is taken by the (300) superlattice reflection. The 90° domain wall, as well as antiphase the boundary (APB), has an irregular configuration. The APB combined with the 90° domain wall is also identified.

Ferroelectric switching mechanism in SrBi2Ta2O9

The ferroelectric switching mechanism in strontium bismuth tantalate [SrBi2Ta2O9 (SBT)] has been studied using in situ transmission electron microscopy observations of the nucleation and growth of polarization domains, such as 180° and 90° domains. Thank to this high density of the antiphase boundary (APB), a switching mechanism in SBT based on the nucleation of new polarization domains at APBs as well as the electrode interfaces put forward and the fatigue-free behavior of SBT with a platinum electrode is explained.

Fabrication of All-Carbon Nanotube Electronic Devices on Flexible Substrates Through CVD and Transfer Methods

SWNT thin films with different nanotube densities are fabricated by CVD while controlling the concentration of catalyst and growth time. Three layers of SWNT films are transferred to flexible substrates serving as electrodes and channel materials, respectively. All-carbon nanotube TFTs with an on/off ratio as high as 10(5) are obtained. Inverters are fabricated on top of the flexible substrates with symmetric input/output behavior.

Retention characteristics of SrBi2Ta2O9 thin films prepared by metalorganic decomposition

Polycrystalline SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized on Pt/Ti/SiO2/Si substrates by metalorganic decomposition. Electric measurements demonstrate that the polarization decay increases with increasing the write/read voltage within the first second. This could be attributed to the depolarization fields, which increases with increasing the retained polarization. However, we found that the polarization loss is insignificant with different write/read voltages over a range of 1–30 000 S. Furthermore, experiment indicates that there is weak pinning of domain walls existing in SBT, which plays an important role for SBT thin film over a range of 1–30 000 S with a low write/read voltage.

Trap-state-dominated suppression of electron conduction in carbon nanotube thin-film transistors.

The often observed p-type conduction of single carbon nanotube field-effect transistors is usually attributed to the Schottky barriers at the metal contacts induced by the work function differences or by the doping effect of the oxygen adsorption when carbon nanotubes are exposed to air, which cause the asymmetry between electron and hole injections. However, for carbon nanotube thin-film transistors, our contrast experiments between oxygen doping and electrostatic doping demonstrate that the doping-generated transport barriers do not introduce any observable suppression of electron conduction, which is further evidenced by the perfect linear behavior of transfer characteristics with the channel length scaling. On the basis of the above observation, we conclude that the environmental adsorbates work by more than simply shifting the Fermi level of the CNTs; more importantly, these adsorbates cause a poor gate modulation efficiency of electron conduction due to the relatively large trap state density near the conduction band edge of the carbon nanotubes, for which we further propose quantitatively that the adsorbed oxygen-water redox couple is responsible.

Stacking faults and their effects on ferroelectric properties in strontium bismuth tantalate

The stacking faults and their effects on ferroelectric properties in strontium–bismuth–tantalate SrBi2Ta2O9 have been studied by transmission electron microscopy (TEM) and ferroelectric hysteresis loop measurement. The structure of SrBi2Ta2O9 consists of Bi2O2 layers and double perovskite type TaO6 octahedral units. There are four possible types of nonstiochiometric stacking faults: two intrinsic faults by removing either one or two perovskite layers and two extrinsic faults by inserting either one or two perovskite layers. TEM investigation reveals that the stacking faults are the extrinsic type. The extrinsic stacking faults in high density might destroy the ferroelectricity, and the reasons are discussed. With excess Bi, the density can be held low efficiently, and with deficient Sr the stacking faults change its form.

Linearly polarized light emission from quantum dots with plasmonic nanoantenna arrays.

Polarizers provide convenience in generating polarized light, meanwhile their adoption raises problems of extra weight, cost, and energy loss. Aiming to realize polarizer-free polarized light sources, herein, we present a plasmonic approach to achieve direct generation of linearly polarized optical waves at the nanometer scale. Periodic slot nanoantenna arrays are fabricated, which are driven by the transition dipole moments of luminescent semiconductor quantum dots. By harnessing interactions between quantum dots and scattered fields from the nanoantennas, spontaneous emission with a high degree of linear polarization is achieved from such hybrid antenna system with polarization perpendicular to antenna slot. We also demonstrate that the polarization is engineerable in aspects of both spectrum and magnitude by tailoring plasmonic resonance of the antenna arrays. Our findings will establish a basis for the development of innovative polarized light-emitting devices, which are useful in optical displays, spectroscopic techniques, optical telecommunications, and so forth.