Congyan Lu

Charge-order breaking and ferromagnetism in La0.4Ca0.6MnO3 nanoparticles

La(0.4)Ca(0.6)MnO(3) nanoparticles of grain size as small as similar to 20 nm are prepared and their magnetic behaviors are investigated in order to understand the size effect of the charge ordering in manganites. The highly stable charge-ordered state can be significantly suppressed upon reduction of the grain size down to nanometer scale, while the ferromagnetism is enhanced. The magnetic phase separation due to the competition between ferromagnetic state and charge-ordered state as well as the surface spin disordering is responsible for the spin-glass-like state at low temperature

Specific heat anomalies and possible Griffiths-like phase in La0.4Ca0.6MnO3 nanoparticles

The specific heat of La(0.4)Ca(0.6)MnO(3) in bulk and nanoparticle (similar to 60 nm in grain size) forms was investigated. It is found that the charge-ordered state highly stabilized in the bulk samples can be significantly suppressed in the nanoparticle. The low temperature specific heat data reveal a Schottky-like anomaly at similar to 5 K for the bulk sample, while a large electronic linear term (gamma=17.8 mJ/mole K(2)) was identified for the nanoparticle samples. The magnetic measurements unveil the small magnetic entropy as low as similar to 0.255 J/kg K and the possible existence of a Griffiths-like phase in the nanoparticle samples. We argue that the physics underlying the size effect is associated with the dimension-dependent interactions based on which the ferromagnetic/charge-ordering transition occurs. (c) 2008 American Institute of Physics.

Ru doping induced quantum paraelectricity in ferroelectric Sr0.9Ba0.1TiO3

We investigate the effect of Ru doping on the dielectric and ferroelectric behaviors of polycrystalline Sr0.9Ba0.1TiO3. It is observed that the Ru doping significantly suppresses the ferroelectric mode, as evidenced by the Raman spectroscopy at various temperatures. The enhanced quantum fluctuation associated with the incorporation of Ru4+ at B site is demonstrated, which transforms Sr0.9Ba0.1TiO3 from the ferroelectric state into a typical quantum paraelectric state.

Contact-Length-Dependent Contact Resistance of Top-Gate Staggered Organic Thin-Film Transistors

In this letter, a contact-length-dependent contact resistance model for top-gate staggered organic thin-film transistors (OTFTs) is proposed. The presented model can well describe the contact resistance increase as the distance by which the gate electrode overlaps the source and drain contact (contact length) decrease. Based on the contact resistance model, the contact-length-dependent cutoff frequency of OTFTs is described, and an optimized contact length can be obtained for high-frequency OTFT applications.

Analysis of the contact resistance in amorphous InGaZnO thin film transistors

Contact resistance has great impact on the performance of oxide thin film transistors (TFTs) and their applications. In this letter, temperature, gate voltage, and electrode dependences of the contact resistance were investigated in amorphous InGaZnO (a-IGZO) TFTs. We found that gate voltage dependent contact resistance made a large contribution to or even dominated the “field effect” of oxide TFTs. After separating the influence of contact resistance, the intrinsic temperature dependent field effect mobility of the a-IGZO TFTs was obtained. Furthermore, the experimental data of the contact resistance can be well described by an optimized transmission line model, and the height of the Schottky barrier in the interface between the metal electrode and a-IGZO semiconductor was found to be related to the gate voltage and account for the contact resistances dependence on the gate voltage.

Organic Programmable Resistance Memory Device Based on

In this letter, an organic memory cell based on tris(8-hydroxyquinolinato)aluminum doped with gold nanoparticles is reported. The device can achieve good resistive switching properties, such as a high on/off current ratio of about 104, and good retention (4 h at 0.1 V). The device remains in either state even after the power has been turned off and shows a good stability under stress test. The current-voltage characteristics are comprehensively investigated, and a possible mechanism is proposed and well fitted with the experiment data. The results show that the organic memory devices have promising potentials for future flexible electronic systems.

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In this letter, high-performance organic phototransistors and photoswitches based on an ultraclosely π-stacked organic semiconductor, i.e., titanyl phthalocyanine, are fabricated. The p-type transistor exhibited high performance with a hole mobility of ~ 4. 5 cm²·V^-1·s^-1 and an on/off ratio higher than 3.1 × 10⁷. Phototransistors exhibited high-photoresponse properties with a responsivity at 10.5 A/W and a photosensitivity up to 2.7 × 10⁵. The high performance of the devices showed their potential application in optoelectronic organic integrated circuits.

Structure

There are two critical issues involved in voltages of organic thin-film transistors (OTFTs), i.e., high operating voltages Vop and random distributed threshold voltages VTH, which lead to high power consumption and unreliable logic function in organic integrated circuits, respectively. This paper demonstrates that both issues can be solved in a single OTFT device by using very thin insulator films. The operating voltages could be reduced to be less than 3 V, without lowering the mobility and the on/off ratio. Using light-enhanced bias stress effect, the threshold voltages can be tuned into the range of -4 to 4 V in OTFTs with SiO2 or Al2O3 as dielectric films, demonstrating that light-enhanced bias stress effect in OTFTs is independent of the dielectric material. The ΔVTH/Vop ratio is about 3, which is one of the largest threshold-voltage-shifting ratios. It is found that the threshold voltage shifting has a linear relationship with the bias voltage and the light illumination time. This tuning technology is a post fabrication and noninvasive method with negligible influence on the mobility and on/off current ratios.

Phototransistors and Photoswitches From an Ultraclosely

We investigate the magnetization and electrical transport properties of Sm-based ternary magnetic bulk metallic glasses at low temperatures. A cluster spin-glass phase is evidenced below T(f) similar to 25 K, which is probably ascribed to competition among the multifold magnetic interactions and intrinsic structural inhomogeneity, and further demonstrated by the magnetic hysteresis under field-cooling conditions. It is shown that the short-range magnetic clusters play as scattering centers for conductive electrons and result in a Kondo-like transport behavior in highly spin disordered Sm(50)Al(25)Co(25) at low temperature

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Contact resistance plays an important role in amorphous InGaZnO (a-IGZO) thin film transistors (TFTs). In this paper, the surface potential distributions along the channel have been measured by using Kelvin probe force microscopy (KPFM) on operating a-IGZO TFTs, and sharp potential drops at the edges of source and drain were observed. The source and drain contact resistances can be extracted by dividing sharp potential drops with the corresponding drain to source current. It is found that the contact resistances could not be neglected compared with the whole channel resistances in the a-IGZO TFT, and the contact resistances decrease remarkably with increasing gate biased voltage. Our results suggest that the contact resistances can be controlled by tuning the gate biased voltage. Moreover, a transition from gradual channel approximation to space charge region was observed through the surface potential map directly when TFT operating from linear regime to saturation regime.