Heterointerface Effects on Carrier Dynamics in Colloidal Quantum Dots and Their Application to Light-Emitting Diodes.

Colloidal quantum dots (QDs) are promising candidates for next-generation display technology because of their unique optical properties and have already appeared in the market as a high-end product. On the basis of their extraordinary properties, QD emissions with a given chemical composition can be tailored in a wide spectral window due to quantum size effects, which constitutes a key advantage of QDs in the display field. Specifically, investigations of structure-dependent and composition-dependent characterizations outside the quantum confinement effect have become an important part of practical applications.

Inhibition of Photoconversion Activity in Self-Assembled ZnO-Graphene Quantum Dots Aggregated by 4-Aminophenol Used as a Linker.

The aggregation of zinc oxide nanoparticles leads to an increased absorbance in the ultraviolet-visible region by an induced light scattering effect. Herein, we demonstrate the inhibition of photoconversion activity in ZnO-graphene core-shell quantum dots (QD) (ZGQDs) agglomerated by 4-aminophenol (4-AP) used as a linker. The ZnO-graphene quantum dots (QD) aggregates (ZGAs) were synthesized using a facile solvothermal process.

Simultaneous growth of three-dimensional carbon nanotubes and ultrathin graphite networks on copper.

A new way to simultaneously grow carbon nanotubes (CNTs) and ultrathin graphite on copper (Cu) foils has been investigated. This one-step growth process yields three-dimensional networks of CNTs on graphitic layers (3D CNTs/G) on Cu foils. Their synthesis conditions and growth mechanism are discussed in detail taking their structural properties into account.

Characterization and Photocatalytic Performance of Potassium-Doped Titanium Oxide Nanostructures Prepared via Wet Corrosion of Titanium Microspheres.

Potassium doped titanium oxide (KTiO) nanowires were prepared by the wet corrosion process (WCP) and their photocatalytic effects were systematically characterized. For the synthesis of KTiO, the potassium hydroxide concentration of the WCP was varied in order to obtain nanostructures with different surface area and surface charge. Structural and crystalline properties of KTiO were studied by means of X-ray diffraction, scanning and transmission electron microscopy.

Unveiling the composite structures of emissive consolidated p-i-n junction nanocells for white light emission.

Hybrid organic-Red-Green-Blue (RGB) color quantum dots were incorporated into consolidated p(polymer)-i(RGB quantum dots)-n(small molecules) junction structures to fabricate a single active layer for a light emitting diode device for white electroluminescence. The semiconductor RGB quantum dots, as an intrinsic material, were electrostatically bonded between functional groups of the p-type polymer organic material core surface and the n-type small molecular organic material shell surface. The ZnCdSe/ZnS and CdSe/ZnS quantum dots distributed uniformly and isotropically surrounding the polymer core which in turn was surrounded by small molecular organic materials.

Inverted quantum dot light emitting diodes using polyethylenimine ethoxylated modified ZnO.

Colloidal quantum dots (QDs) are an emerging class of new materials due to their unique physical properties. In particular, colloidal QD based light emitting diodes (QDLEDs) have been extensively studied and developed for the next generation displays and solid-state lighting. Among a number of approaches to improve performance of the QDLEDs, the most practical one is optimization of charge transport and charge balance in the recombination region.

Emissive ZnO-graphene quantum dots for white-light-emitting diodes.

Hybrid nanostructures combining inorganic materials and graphene are being developed for applications such as fuel cells, batteries, photovoltaics and sensors. However, the absence of a bandgap in graphene has restricted the electrical and optical characteristics of these hybrids, particularly their emissive properties. Here, we use a simple solution method to prepare emissive hybrid quantum dots consisting of a ZnO core wrapped in a shell of single-layer graphene.

Charging and discharging mechanisms of organic bistable devices based on ZnO nanoparticles capped with a poly N-vinylcarbazole polymer.

Scanning electron microscopy and energy-dispersive spectrometer images of hybrid nanocomposites of ZnO nanoparticles capped with a poly N-vinylcarbazole (PVK) that was fabricated using the spin-coating technique showed that the ZnO nanoparticles were capped with a PVK polymer layer. The measurement of the current-voltage (I-V) of the Al/ZnO nanoparticles capped with a PVK layer/indium-tin-oxide/glass devices at 300 K showed electrical bistability and negative differential resistance, which indicate the nonvolatile nature of the memory effect of the electron captured in the ZnO nanoparticles. The charging and discharging mechanisms of the organic bistable devices that were fabricated using hybrid nanocomposites of ZnO nanoparticles capped with a PVK layer are described based on the I-V results.

Electrical bistabilities and memory mechanisms of organic bistable devices fabricated utilizing SnO2 nanoparticles embedded in a poly(methyl methacrylate) layer.

Organic bistable devices fabircated utilizing SnO2 nanoparticles embedded in a poly(methyl methacrylate) (PMMA) polymer layer were formed by using a spin coating method. Transmission electon microscopy images and photoluminescence spectra showed that synethized SnO2 nanoparticles were randomly distributed in the dibutyl ehter solution. Current-voltage (I-V) measurements on the Al/SnO2 nanoparticles embedded in PMMA layer/ITO devices at 300 K showed current bistability due to the existence of SnO2 nanoparticles.

Operating mechanisms of organic bistable devices containing ZnO nanoparticles embedded in a poly-4-vinyl-phenol layer.

Scanning electron microscopy images showed that self-assembled ZnO nanoparticles were created inside a poly-4-vinyl-phenol (PVP) layer. Current-voltage (I-V) measurements on the Al/ZnO nanoparticles embedded in a PVP layer/indium tin oxide (ITO)/glass device fabricated by using a simple spin coating method at 300 K showed an electrical hysteresis behavior, indicative of an essential feature for a bistable device. The data fitting results of the I-V curves showed that the carrier transport mechanisms at low and high voltages were attributed to the space charge limited current and the Fowler-Nordheim tunneling processes, respectively.

Carrier transport mechanisms of the writing and the erasing processes for Al/ZnO nanoparticles embedded in a polymethyl methacrylate layer/C60/p-Si diodes.

Transmission electron microscopy images showed that ZnO nanoparticles were randomly distributed inside the polymethyl methacrylate (PMMA) layer. Capacitance-voltage (C-V) measurements on the Al/ZnO nanoparticles embedded in a PMMA layer/C60/p-Si diode at 300 K showed a clockwise hysteresis with a flatband voltage shift due to existence of the ZnO nanoparticles and a C60 buffer layer. The insertion of the C60 layer enlarged the memory window of the device containing the ZnO nanoparticle, as estimated by the flatband voltage shift in the C-V hysteresis.

Flexible organic bistable devices based on graphene embedded in an insulating poly(methyl methacrylate) polymer layer.

The electrical properties of flexible nonvolatile organic bistable devices (OBDs) fabricated with graphene sandwiched between two insulating poly(methyl methacrylate) (PMMA) polymer layers were investigated. Current-voltage (I-V) measurements on the Al/PMMA/graphene/PMMA/indium-tin-oxide/poly(ethylene terephthalate) devices at 300 K showed a current bistability due to the existence of the graphene, indicative of charge storage in the graphene. The maximum ON/OFF ratio of the current bistability for the fabricated OBDs was as large as 1 x 10(7), and the endurance number of ON/OFF switchings was 1.

Enhancement of the memory effects for nonvolatile memory devices fabricated utilizing ZnO nanoparticles embedded in a Si3N4 layer.

ZnO nanoparticles embedded in a Si3N4 layer by using spin-coating and thermal treatment were fabricated to investigate the feasible applications in charge trapping regions of the metal/oxide/nitride/oxide/p-Si memory devices. The magnitude of the flatband voltage shift of the capacitance-voltage (C-V) curve for the Al/SiO2/ZnO nanoparticles embedded in Si3N4 layer/SiO2/p-Si memory device was larger than that of Al/ZnO nanoparticles embedded in SiO2 layer/p-Si and Al/SiO2/Si3N4/SiO2/p-Si devices. The increase in the flatband voltage shift of the C-V curve for the Al/SiO2/ZnO nanoparticles embedded in Si3N4 layer/SiO2/p-Si memory device in comparison with other devices was attributed to the existence of the ZnO nanoparticles or the interface trap states between the ZnO nanoparticles and the Si3N4 layer resulting from existence of ZnO nanoparticles embedded in the Si3N4 layer.

White light-emitting diodes fabricated utilizing hybrid polymer-colloidal ZnO quantum dots.

White light-emitting diodes (WLEDs) with a hybrid poly N-vinylcarbazole (PVK) and poly(methyl methacrylate) (PMMA) polymer and ZnO quantum dots (QDs) were fabricated by a spin-coating technique. Transmission electron microscopy images showed that the ZnO QDs were predominantly distributed at the circumference of the surface of the PVK polymer. Electroluminescence spectra for hybrid polymer-QD WLEDs showed a broad peak around 600 nm.

Electroluminescence of a single active layer polymer-nanocrystal hybrid light-emitting diode with inversion symmetry.

A hybrid polymer-nanocrystal (NC) light-emitting diode (LED) device with a single active layer structure is simply fabricated by a spin coating. From a high-resolution transmission electron microscopy (HRTEM) study, each PVK polymer particle is observed to be capped with TPBi molecules and CdSe/ZnS NCs are mainly distributed along the circumference of PVK and TPBi surfaces, resulting in a core-shell polymer-NC hybrid of [CdSe/ZnS]/TPBi/[CdSe/ZnS]/PVK. An Al/[CdSe/ZnS]/TPBi/[CdSe/ZnS]/PVK/indium-tin oxide(ITO)/glass LED shows electroluminescence (EL) centered at around 585 nm at the forward bias of +10 V, which clearly reveals that CdSe/ZnS NCs existing at the interface between PVK and TPBi act as recombination centers for excitons.

Carrier transport in flexible organic bistable devices of ZnO nanoparticles embedded in an insulating poly(methyl methacrylate) polymer layer.

The bistable effects of ZnO nanoparticles embedded in an insulating poly(methyl methacrylate) (PMMA) polymer single layer by using flexible polyethylene terephthalate (PET) substrates were investigated. Transmission electron microscopy (TEM) images revealed that ZnO nanoparticles were formed inside the PMMA polymer layer. Current-voltage (I-V) measurement on the Al/ZnO nanoparticles embedded in an insulating PMMA polymer layer/ITO/PET structures at 300 K showed a nonvolatile electrical bistability behavior with a flat-band voltage shift due to the existence of the ZnO nanoparticles, indicative of trapping, storing, and emission of charges in the electronic states of the ZnO nanoparticles.

Electrical bistabilities and operating mechanisms of memory devices fabricated utilizing ZnO quantum dot-multi-walled carbon nanotube nanocomposites.

Transmission electron microscopy images showed that the ZnO quantum dots (QDs) were conjugated with multi-walled carbon nanotubes (MWCNTs). Bistable memories utilizing an ensemble of the ZnO QD-MWCNT heterostructures were developed and the storage capability of the devices was significantly enhanced due to the conjugation of the ZnO QDs and the MWCNTs. Operating mechanisms of memory devices fabricated utilizing the ZnO QD-MWCNT heterostructures are described on the basis of the current-voltage results.

Flexible photovoltaic cells fabricated utilizing ZnO quantum dot/carbon nanotube heterojunctions.

In situ growth of ZnO quantum dots (QDs) on the surface of multi-walled carbon nanotubes (MWCNTs) was presented, and their application in photovoltaic cells by using flexible polyethylene terephthalate substrates was demonstrated. High-resolution transmission electron microscopy images revealed the conjugation of ZnO QDs with MWCNTs. Photoluminescence spectra indicated that the charge transfer efficiency at ZnO QD-MWCNT heterojunctions was above 90%, as confirmed by time-resolved photoluminescence measurements.

Carrier transport mechanisms of bistable memory devices fabricated utilizing core-shell CdSe/ZnSe quantum-dot/multi-walled carbon nanotube hybrid nanocomposites.

Transmission electron microscopy images showed that conjugation between single core-shell CdSe/ZnSe quantum dots (QDs) and oxidized multi-walled carbon nanotubes (MWCNTs) was achieved through the complexation reaction. Current-voltage (I-V) measurements on Al/CdSe:MWCNT conjugated nanocomposite/indium-tin-oxide devices at 300 K showed that the on/off ratio of the current bistability was as large as about 10(4), which was significantly increased due to an enhancement of the carrier transfer efficiency between the CdSe/ZnSe QDs and the MWCNTs. Carrier transport mechanisms of the bistable memory devices fabricated utilizing CdSe/ZnSe QD/MWCNT hybrid nanocomposite are described on the basis of the I-V results.

Structural and electronic properties of ZnO nanoparticles grown on p-Si and Al2O3 substrates by using spin coating and thermal treatment.

ZnO nanoparticles were formed on p-Si and Al2O3 substrates by using spin coating and thermal treatment method. Scanning electron microscopy images and X-ray energy dispersive spectrometry profiles showed that ZnO nanoparticles were formed on p-Si and Al2O3 substrates. X-ray diffraction patterns showed that ZnO nanoparticles formed on the p-Si substrates had polycrystalline hexagonal wurtzite structures and that those formed on the Al2O3 substrates had a c-axis preferential orientation.

Single active-layer structured dual-function devices using hybrid polymer-quantum dots.

We demonstrate hybrid polymer-quantum dot dual-function devices with a single active-layer structure consisting of CdSe/ZnS semiconductor quantum dots dispersed with poly N-vinylcarbazole (PVK) and 1,3,5-tirs-(N-phenylbenzimidazol-2-yl) benzene (TPBi) fabricated on an indium-tin-oxide (ITO)/glass substrate by using a simple spin-coating technique. The dual-function devices are composed of light-emitting diodes (LED) on the top side and nonvolatile organic bistable memory devices (OBD) on the bottom side and can show electroluminescence (EL) along with electrical bistability concurrently. Both the functionality of LEDs and OBDs can be successfully achieved by adding an electron transport layer (ETL) TPBi to the OBD to attain an LED in which the lowest unoccupied molecular orbital (LUMO) level of TPBi is positioned at the energy level between the conduction band of CdSe/ZnS and the LiF/Al electrode.

Nonvolatile flexible organic bistable devices fabricated utilizing CdSe/ZnS nanoparticles embedded in a conducting poly N-vinylcarbazole polymer layer.

The bistable effects of CdSe/ZnS nanoparticles embedded in a conducting poly N-vinylcarbazole (PVK) polymer layer by using flexible poly-vinylidene difluoride (PVDF) and polyethylene terephthalate (PET) substrates were investigated. Transmission electron microscopy (TEM) images revealed that CdSe/ZnS nanoparticles were formed inside the PVK polymer layer. Current-voltage (I-V) measurement on the Al/[CdSe/ZnS nanoparticles+ PVK]/ITO/PVDF and Al/[CdSe/ZnS nanoparticles+ PVK ]/ITO/PET structures at 300 K showed a nonvolatile electrical bistability behavior with a flat-band voltage shift due to the existence of the CdSe/ZnS nanoparticles, indicative of trapping, storing and emission of charges in the electronic states of the CdSe nanoparticles.