Showing papers by "Paddy K. L. Chan published in 2010"

Nonvolatile organic transistor-memory devices using various thicknesses of silver nanoparticle layers

Abstract: We demonstrate the modification of the memory effect in organic memory devices by adjusting the thickness of silver nanoparticles (NPs) layer embedded into the organic semiconductor. The memory window widens with increasing Ag NPs layer thickness, a maximum window of 90 V is achieved for 5 nm Ag NPs and the on/off current ratio decreases from 105 to 10 when the Ag NPs layer thickness increases from 1 to 10 nm. We also compare the charge retention properties of the devices with different Ag NPs thicknesses. Our investigation presents a direct approach to optimize the performance of organic memory with the current structure.

Physical mechanisms for hot-electron degradation in GaN light-emitting diodes

Abstract: We report investigations on the degradation of GaN-based light-emitting diodes due to high dc current stress by examining two types of devices with the same fabrication procedures except for the growth conditions for the InGaN quantum wells (QWs). Higher trimethylindium and triethylgallium fluxes are used for type A devices resulting in a threefold increase in the InGaN QWs growth rate compared to type B devices. Detailed structural and optoelectronic properties of the devices are investigated by transmission electron microscopy, atomic force microscopy, thermal imaging, I-V measurements, and the low-frequency noise properties of the devices as a function of the stress time, tS. The experimental data show that the QWs in type B devices are dominated by spiral growth and they have substantially higher strain nonuniformity than type A devices. The highly strained GaN/InGaN interfaces in device B are also responsible for the faster increase in the defect density due to hot-electron injection. The defects enhance the trap-assisted tunneling in the multiple quantum wells (MQWs) resulting in the development of hot spots among type B devices after high current stressing of the MQWs. This in turn leads to an increase in the defect generation rate resulting in a thermal run-away condition that ultimately resulted in the failure of the device. The data show that an increase in the growth rate in the InGaN layer led to the domination by the step flow growth mode over the spiral growth mode in the MQWs. This is the main reason for the reduction in the dislocation density in type A devices and hence their increase in device reliability.We report investigations on the degradation of GaN-based light-emitting diodes due to high dc current stress by examining two types of devices with the same fabrication procedures except for the growth conditions for the InGaN quantum wells (QWs). Higher trimethylindium and triethylgallium fluxes are used for type A devices resulting in a threefold increase in the InGaN QWs growth rate compared to type B devices. Detailed structural and optoelectronic properties of the devices are investigated by transmission electron microscopy, atomic force microscopy, thermal imaging, I-V measurements, and the low-frequency noise properties of the devices as a function of the stress time, tS. The experimental data show that the QWs in type B devices are dominated by spiral growth and they have substantially higher strain nonuniformity than type A devices. The highly strained GaN/InGaN interfaces in device B are also responsible for the faster increase in the defect density due to hot-electron injection. The defects enh...

A Continuum Model of the Van der Waals Interface for Determining the Critical Diameter of Nanopumps and its Application to Analysis of the Vibration and Stability of Nanopump Systems

Abstract: Carbon nanotubes make ideal nanopumps for the transport of fluid. To analyze the vibration and stability of nanopump systems with inner fluid effectively, it is necessary to incorporate nanoscale effects into continuum-based simulations. This paper first proposes a continuum model for the van der Waals (vdW) interface between a single-wall carbon nanotube (SWCNT) and incompressible inner fluid to determine the critical tube diameter above which continuum fluid mechanics may be reasonably applied to that inner fluid. Then, with overall consideration of the scale effects, including the nonlocal effects of the carbon nanotube, the surface tension of the inner fluid and the vdW interface, an improved Euler beam/plug fluid model is developed to investigate the vibration and stability of the nanopump system. The two models are both validated by comparing with molecular dynamic simulations. The results show that the critical diameter for water flow is about 1.8 nm. Nanopump stability is noticeably enhanced by the surface tension of the inner fluid for a high slenderness ratio. Both coaxial vibration frequency and stability decline as the system temperature is increased. Moreover, the proposed models predict that the transverse vibration of the inner fluid inside a nearly rigid SWCNT occurs due to the existence of the vdW interface gap and the negligible bending rigidity of the fluid.

The effect of intertube van der Waals interaction on the stability of pristine and functionalized carbon nanotubes under compression

Abstract: This paper investigates the effect of intertube van der Waals interaction on the stability of pristine and covalently functionalized carbon nanotubes under axial compression, using molecular mechanics simulations. After regulating the number of inner layers of the armchair four-walled (5, 5)@(10, 10)@(15, 15)@(20, 20) and zigzag four-walled (6, 0)@(15, 0)@(24, 0)@(33, 0) carbon nanotubes, the critical buckling strains of the corresponding tubes are calculated. The results show that each of the three inner layers in the functionalized armchair nanotube noticeably contributes to the stability of the outermost tube, and together increase the critical strain amplitude by 155%. However, the three inner layers in the corresponding pristine nanotube, taken together, increase the critical strain of the outermost tube by only 23%. In addition, for both the pristine and functionalized zigzag nanotubes, only the (24, 0) layer, among the three inner layers, contributes to the critical strain of the corresponding outermost tube, by 11% and 29%, respectively. The underlying mechanism of the enhanced stability related to nanotube chirality and functionalization is analyzed in detail.

Intermolecular electronic coupling in organic molecular thin films measured by temperature modulation spectroscopy

Abstract: Temperature modulation spectroscopy is used to obtain the temperature dependences of oscillator strength, exciton transition energy, and linewidth for a copper phthalocyanine thin film. With increasing temperature, the oscillator strength exhibits a pronounced decrease for charge transfer (CT) excitons, making this technique suitable for differentiating exciton types. From the measured magnitude and temperature dependence of the CT oscillator strength, we obtain estimates for the intermolecular electronic coupling and its exponential decay coefficient.

Theoretical and experimental studies on the electric impedance of active piezoelectric sensors bonded on cracked beams

Abstract: The electric impedance of symmetrically surface-bonded piezoelectric sensors on a cracked beam is studied. To investigate the effect of the crack on the electric impedance in a convenient fashion, an analytical expression is derived that is correlated to the physical parameters of the crack and the host beam. The beam segment covered with piezoelectric patches and the cracked region are regarded as a bimorph segment and an equivalent spring, respectively, and the entire beam system is then represented by three elastic beam segments and a bimorph segment together with the spring. Electric impedance experiments are also conducted for uncracked beams and for cracked beams with single-edge or double-edge cracks. The experimental results agree with those generated by the analytical expression. The crack depth has little effect on the corresponding mode frequency for cracks located at the mode node of a beam. For cracks located away from the mode node, the corresponding mode frequency decreases as the crack depth increases. Moreover, the closer the crack to the anti-node of the mode, the greater the decrease in the corresponding mode frequency. The mechanism of these changes is discussed. The findings should prove helpful for structural health monitoring using active piezoelectric sensors.

Interfacial nature of resistive switching effect in perovskite-oxide thin film devices

Abstract: Resistive switching effect has been demonstrated in LaNiO 3 \Pa 0.7 Ca 0.3 MnO 3 \Ti top-down device structures. Hysteretic Current-voltage (I–V) characteristic was observed by applying potential differences in the order of 5 V across the electrodes. I–V characteristics with different combinations of top and bottom electrodes were measured, in order to identify the location contributing to the switching. Our results suggested that the interface between PCMO and the electrodes are responsible for the resistive switching effect.