Graphene quantum dots as a highly efficient solution-processed charge trapping medium for organic nano-floating gate memory.

A highly efficient solution-processible charge trapping medium is a prerequisite to developing high-performance organic nano-floating gate memory (NFGM) devices. Although several candidates for the charge trapping layer have been proposed for organic memory, a method for significantly increasing the density of stored charges in nanoscale layers remains a considerable challenge. Here, solution-processible graphene quantum dots (GQDs) were prepared by a modified thermal plasma jet method; the GQDs were mostly composed of carbon without any serious oxidation, which was confirmed by x-ray photoelectron spectroscopy. These GQDs have multiple energy levels because of their size distribution, and they can be effectively utilized as charge trapping media for organic NFGM applications. The NFGM device exhibited excellent reversible switching characteristics, with an on/off current ratio greater than 10(6), a stable retention time of 10(4) s and reliable cycling endurance over 100 cycles. In particular, we estimated that the GQDs layer trapped ∼7.2 × 10(12) cm(-2) charges per unit area, which is a much higher density than those of other solution-processible nanomaterials, suggesting that the GQDs layer holds promise as a highly efficient nanoscale charge trapping material.

Fabrication of graphene flakes composed of multi-layer graphene sheets using a thermal plasma jet system.

We have developed a method to fabricate graphene flakes composed of high quality multi-layer graphene sheets using a thermal plasma jet system. A carbon atomic beam was generated by injecting ethanol into Ar plasma continuously; the beam then flowed through a carbon tube attached to the anode. Graphene was made by epitaxial growth where a carbon atomic beam, having the proper energy, collided with a graphite plate. The graphene fabricated was very pure and showed a relatively good crystalline structure. We have demonstrated that the number of layers of graphene sheets could be controlled by controlling the rate of ethanol injection. Our process is a continuous process with a relatively high yield (approximately 8%).

Minimum enhancement of surface-enhanced Raman scattering for single-molecule detections.

We have calculated the minimum enhancement factor for single-molecule detections from the surface-enhanced Raman scattering (SERS) data measured from well-defined silver nanorod arrays. Silver nanorods were fabricated by electrodepositing them evenly near the mouth of the pores of anodic aluminum oxide templates with a very shallow depth. The SERS intensity increased almost linearly with an increase in the concentration of the mother solution. From the data of the enhancement and the number of molecules irradiated by the laser beam at the detection limit, the minimum SERS enhancement factor for nonresonant single-molecule detections was calculated to be approximately 10(11).

Multi-Shaped Ag Nanoparticles in the Plasmonic Layer of Dye-Sensitized Solar Cells for Increased Power Conversion Efficiency.

The use of dye-sensitized solar cells (DSSCs) is widespread owing to their high power conversion efficiency (PCE) and low cost of manufacturing. We prepared multi-shaped Ag nanoparticles (NPs) and introduced them into DSSCs to further enhance their PCE. The maximum absorption wavelength of the multi-shaped Ag NPs is 420 nm, including the shoulder with a full width at half maximum (FWHM) of 121 nm. This is a broad absorption wavelength compared to spherical Ag NPs, which have a maximum absorption wavelength of 400 nm without the shoulder of 61 nm FWHM. Therefore, when multi-shaped Ag NPs with a broader plasmon-enhanced absorption were coated on a mesoporous TiO2 layer on a layer-by-layer structure in DSSCs, the PCE increased from 8.44% to 10.22%, equivalent to an improvement of 21.09% compared to DSSCs without a plasmonic layer. To confirm the plasmon-enhanced effect on the composite film structure in DSSCs, the PCE of DSSCs based on the composite film structure with multi-shaped Ag NPs increased from 8.58% to 10.34%, equivalent to an improvement of 20.51% compared to DSSCs without a plasmonic layer. This concept can be applied to perovskite solar cells, hybrid solar cells, and other solar cells devices.

Ag Nanoparticle-Functionalized Open-Ended Freestanding TiO2 Nanotube Arrays with a Scattering Layer for Improved Energy Conversion Efficiency in Dye-Sensitized Solar Cells.

Dye-sensitized solar cells (DSSCs) were fabricated using open-ended freestanding TiO2 nanotube arrays functionalized with Ag nanoparticles (NPs) in the channel to create a plasmonic effect, and then coated with large TiO2 NPs to create a scattering effect in order to improve energy conversion efficiency. Compared to closed-ended freestanding TiO2 nanotube array-based DSSCs without Ag or large TiO2 NPs, the energy conversion efficiency of closed-ended DSSCs improved by 9.21% (actual efficiency, from 5.86% to 6.40%) with Ag NPs, 6.48% (actual efficiency, from 5.86% to 6.24%) with TiO2 NPs, and 14.50% (actual efficiency, from 5.86% to 6.71%) with both Ag NPs and TiO2 NPs. By introducing Ag NPs and/or large TiO2 NPs to open-ended freestanding TiO2 nanotube array-based DSSCs, the energy conversion efficiency was improved by 9.15% (actual efficiency, from 6.12% to 6.68%) with Ag NPs and 8.17% (actual efficiency, from 6.12% to 6.62%) with TiO2 NPs, and by 15.20% (actual efficiency, from 6.12% to 7.05%) with both Ag NPs and TiO2 NPs. Moreover, compared to closed-ended freestanding TiO2 nanotube arrays, the energy conversion efficiency of open-ended freestanding TiO2 nanotube arrays increased from 6.71% to 7.05%. We demonstrate that each component-Ag NPs, TiO2 NPs, and open-ended freestanding TiO2 nanotube arrays-enhanced the energy conversion efficiency, and the use of a combination of all components in DSSCs resulted in the highest energy conversion efficiency.

Size-controllable and low-cost fabrication of graphene quantum dots using thermal plasma jet.

We report a size-controllable and low-cost fabrication method of graphene quantum dots (GQDs) using a thermal plasma jet. A carbon atomic beam was generated by injecting a large amount (2.5 L/min) of ethylene gas continuously into Ar plasma. The beam was then flowed through a carbon tube (5-20 cm in length) attached to the anode and then dispersed into a chamber. Carbon materials including GQDs were made by a gas phase collision reaction. The production rate of carbon soot was 40 g/h for a 2.5 L/min injection rate. Almost all of the carbon soot dispersed in ethanol by sonication, while isolated GQDs were dispersed in ethanol by stirring with a stirring rod. The weight percent of GQDs in carbon soot, based on the amount extracted in ethanol, was about 10%. This means that the production rate of GQDs was about 4 g/h. The average size of GQDs, with a relatively narrow size distribution, was controlled by varying the length of the carbon tube attached. It was about 10, 14, and 19 nm when the length was 5, 10, and 20 cm, respectively. The electric structure based on the photoluminescence data of our GQDs had a singlet ground state and was in good agreement with that of carbyne. Our GQDs will disperse in organic solvents such as toluene, but not in water. The dispersion properties also support that our GQDs have carbyne-like edges. We proposed that the PL peaks observed can be attributed to electronic transitions between energy levels of the GQDs having carbyne-like edges.

Enhancement at the junction of silver nanorods.

The enhancement of surface enhanced Raman scattering (SERS) at the junction of linearly joined silver nanorods (31 nm in diameter) deposited in the pores of anodic aluminum oxide templates was studied systematically by excitation with a 632.8 nm laser line. The single and joined silver nanorod arrays showed a similar extinction spectrum when their length was the same. Maximum enhancement was observed from the junction system of two nanorods of the same size with a total length of 62 nm. This length also corresponded to the optimum length of single nanorods for SERS by excitation with a 632.8 nm laser line. The enhancement at the junction was approximately 40 times higher than that of the 31 nm single nanorod, while it was 4 times higher than that of the 62 nm single nanorod. The enhancement factor at the junction after oxide removal was approximately 3.9 x 10 (9).

Clean carbon nanotube field emitters aligned horizontally.

Horizontally aligned carbon nanotube (CNT) field emitters, which strongly adhere to the substrate and show good field emission properties, were fabricated by electrophoresis deposition and fissure formation techniques. A thin film of CNTs was deposited on a substrate, by electrophoresis, from an aqueous mixture of CNT and detergent, and then the detergent was deposited also by electropholysis. CNTs with a clean surface were exposed in the fissures produced by firing. The field emission was increased significantly due to the additional deposition of the detergent. When the CNTs were cut by increasing the firing time, the field emission increased significantly, while their stability decreased considerably. Our method does not require any further treatment for field emission.

Assembly of metal nanoparticle-carbon nanotube composite materials at the liquid/liquid interface.

Carbon nanotubes (CNTs)-mediated self-assembly of metal (Au and Ag) nanoparticles at the liquid/liquid interface in the form of a stable nanocomposite film is reported. The metallic luster results from the electronic coupling of nanoparticles, suggesting the formation of closely packed nanoparticle thin films. The interfacial film could be transferred to mica substrates and carbon-coated transmission electron microscopy (TEM) grids. The transferred films were very stable for a prolonged time. The samples were characterized by UV-vis spectroscopy, scanning electron microscopy (SEM), TEM, and X-ray photoelectron spectroscopy (XPS). SEM and TEM results show that the films formed at the liquid/liquid interface are indeed composite materials consisting of CNTs and nanoparticles. XPS measurements further indicate the presence of the interaction between nanoparticles and CNTs.