Yuki Tani

Ultraviolet electroluminescence from colloidal ZnO quantum dots in an all-inorganic multilayer light-emitting device

We report ultraviolet (UV) electroluminescence (EL) at 3.30 eV of colloidal ZnO quantum dots (QDs) in an inorganic multilayer thin-film EL device. The EL spectrum was identical to the photoluminescence spectrum of the source solution of ZnO QDs, and the emission is attributable to quantum confined electron hole pair recombination. The UV emission was successful when the ZnO QD layer was sandwiched by thin MgO layers, while only a defect-related visible emission appeared without MgO layers. The type-I quantum well structure of MgO/ZnO/MgO and surface passivation of ZnO QDs by MgO must be important for the UV EL emission.

Quantum Dot Activated All-inorganic Electroluminescent Device Fabricated Using Solution-Synthesized CdSe/ZnS Nanocrystals

We report a low temperature fabrication and light-emitting characteristics of inorganic electroluminescent (EL) devices in which a CdSe/ZnS quantum dot (QD) layer was formed by a new ion beam deposition process. The deposition process was developed for fabrication of surfactant- and solvent-free QD structured films from colloidally dispersed QD solution. The electroluminescence spectra of the alternating field driven devices were found to be identical to the photoluminescence spectra of the source solution. The finding paves the way for the development of full color, transparent, flexible, and low-cost large area flat panel displays.

Ion Beam Deposition Technique for Fabricating Luminescent Thin Films from a Solution of Nanocrystalline Semiconductor Dots

We report a new technique for fabricating organic-free luminescent thin films from solution-synthesized surfactant-capped nanocrystalline semiconductor quantum dots with a core/shell structure. Colloidal solutions of CdSe/ZnS nanocrystals with diameters of 2.1 to 5.1 nm were made into ion beams and deposited on polycrystalline substrates under high vacuum; we applied the ion beam direct deposition technique with an electrospray ionizer, supersonic jet emitter, and energy analyzer. The nanocrystalline structure was preserved in the thin films; the photoluminescence spectra were identical to those of the source solution, although the thin films were essentially surfactant- and solvent-free.

Characterization of electrospray ion-beam-deposited CdSe/ZnS quantum dot thin films from a colloidal solution

Colloidal semiconductor quantum dot (QD) nanocrystals can be deposited in the form of inorganic thin films using the ion beam direct deposition method. To simultaneously preserve the nanocrystal configuration and remove the organics derived from the ligand and solvent, the authors used an electrospray technique and an ion beam technique. These techniques provided a soft-ionization process to obtain nanocrystalline ions and a collision process to attain a nonequilibrium state of the deposits, respectively. Because of the nature of the soft-ionization process, the electrospray phenomenon resulted in various forms of QD ions that depended on the preparation of the colloidal solution source and spraying conditions. The authors concentrated on finding operational conditions of the system that deposited thin films with reduced organics concentrations by examining the correlation between fast Fourier transform infrared absorption spectroscopy and photoluminescence intensity. The morphology of the deposited films...

Ion Beam Deposition of Quantum Dots from Colloidal Solution

A newly developed ion beam deposition technique for fabrications of organic free quantum dot (QD) structured thin films from colloidal solutions of surfactant-capped nanocrystalline provides a uniform speed to electrosprayed QD ions independently of its form. A CdSe/ZnS QDs ion beam was generated from a colloidal solution source to deposit onto a substrate in a vacuum chamber. The energy spectra of the quantum dots ion beam were measured using an energy analyzer. We discuss appropriate conditions for a valid deposition onto a substrate in terms of ion energy. The ion energy provided from a free jet accelerator was set at 6 keV, or 0.25 eV/atom (Cd, Se). A cross-sectional transmission electron microscopy image of the deposited film shows that the deposited QDs are densely packed in a polycrystalline thin film configuration. Photoluminescence (PL) collected from the deposited film provides evidence that the deposited QDs are preserved. These suggest the soft landing of the QDs with appropriate surface passivations.

Electric field effects on radiative transition in quantum dot inorganic electroluminescent devices

Our challenge is to fabricate practical all-inorganic electroluminescent devices using colloidal quantum dots with presenting new aspects of electroluminescence mechanisms. The electric field effect on the excitation of a quantum dot with a hot electron that results in saturated chromatic electroluminescence was studied by time-resolved spectral transient properties at room temperature. Fast and very slow radiative recombination transition pathways are newly distinguished in the electroluminescence related to carrier confinement in the quantum dot under an electric field.

Ion Beam Deposition from Colloidal CdSe/ZnS Nanocrystals Solutions for Fabricating Electroluminescent Device

We have already reported AC-drive CdSe/ZnS quantum dot (QD) inorganic electroluminescent (EL) devices from colloidal solution. The EL spectra of the devices reproduced photoluminescent spectra of the source QD solutions. Furthermore, the observation of time resolved luminescent properties in ambient atmosphere at room temperature, are analyzed for improvements and projection of brightness and efficiency. The results suggest that the lifetimes of excited states in quantum dots are affected by the electric field in the electroluminescent devices.