Kipil Lim

Bright dual-mode green emission from selective set of dopant ions in β-Na(Y,Gd)F 4 :Yb,Er/β-NaGdF 4 :Ce,Tb core/shell nanocrystals

Bright dual-mode green-emitting core/shell nanoparticles (NPs) were synthesized by doping selective set of lanthanide ions. Up-conversion (UC) green-emitting β-NaY0.2Gd0.6F4:Yb0.18,Er0.02 NPs (8.3 nm) were used as core material. Bright down-conversion (DC) green-emitting β-NaGd0.8F4:Ce0.15,Tb0.05 NPs showed ca. 31 times higher photoluminescence (PL) intensity than β-NaGdF4:Tb NPs and they were served as shell material with their excellent PL properties. The UC/DC core/shell NPs showed bright green light under excitations of 980 nm near infrared (NIR) light and 254 nm ultraviolet (UV) light, respectively. The UC/DC core/shell NPs showed ca. 11 times higher UC PL intensity than core UCNPs. Consequently, the core/shell NPs doped with selective set of lanthanide ions showed bright dual-mode green emission under excitations of NIR light and UV light, indicating that they are promising for application to optical imaging.

Highly bright multicolor tunable ultrasmall β-Na(Y,Gd)F4:Ce,Tb,Eu/β-NaYF4 core/shell nanocrystals

Herein, we report highly bright multicolor-emitting β-Na(Y,Gd)F₄:Ce,Tb,Eu/β-NaYF₄ nanoparticles (NPs) with precise color tunability. First, highly bright sub-20 nm β-Na(Y,Gd)F₄:Ce,Tb,Eu NPs were synthesized via a heating-up method. By controlling the ratio of Eu(3+) to Tb(3+), we generated green, yellow-green, greenish yellow, yellow, orange, reddish orange, and red emissions from the NP solutions via energy transfer of Ce(3+)→ Gd(3+)→ Tb(3+) (green) and Ce(3+)→ Gd(3+)→ Tb(3+)→ Eu(3+) (red) ions under ultraviolet light illumination (254 nm). Because of Ce(3+) and Gd(3+) sensitization, Tb(3+) ions exhibited strong green emission. The decay time of Tb(3+) emission decreased from 4.0 to 1.4 ms as the Eu(3+) concentration was increased, suggesting that energy was transferred from Tb(3+) to Eu(3+). As a result, Eu(3+) emission peaks were generated and the emission color was transformed from green to red. Monodisperse sub-6 nm β-Na(Y,Gd)F₄:Ce,Tb,Eu NPs were synthesized through a simple reduction of the reaction temperature. Although fine color tunability was retained, their brightness was considerably decreased owing to an increase in the surface-to-volume ratio. The formation of a β-NaYF₄ shell on top of the sub-6 nm NP core to produce β-Na(Y,Gd)F₄:Ce,Tb,Eu/β-NaYF₄ significantly increased the emission intensity, while maintaining the sub-10 nm sizes (8.7-9.5 nm). Quantum yields of the ultrasmall NPs increased from 1.1-6.9% for the core NPs to 6.7-44.4% for the core/shell NPs. Moreover, highly transparent core/shell NP-polydimethylsiloxane (PDMS) composites featuring a variety of colors, excellent color tunability, and high brightness were also prepared.

Facile synthesis and optical properties of colloidal silica microspheres encapsulating a quantum dot layer

We present colloidal silica microspheres encapsulating a homogeneous quantum dot layer at radial equidistance from the centre by utilizing electrostatic interaction between surface-engineered silica microspheres and QDs. The microspheres show dramatically enhanced optical absorption and emission with an appropriate silica shell thickness.

Electric-Field-Induced Mass Movement of Ge2Sb2Te5 in Bottleneck Geometry Line Structures

We report an electric-field-induced directional mass movement of Ge 2 Sb 2 Te 5 in bottleneck geometry. Under high-electric-stress circumstances (>10 6 A cm -2 ), a mass of Ge 2 Sb 2 Te 5 tends to move toward the cathode (-) by the remaining mass depletion at the anode (+). The high electric stress induces an asymmetric compositional separation such that Sb is distributed toward the cathode (-) whereas Te is distributed toward the anode (+). Ionicity in Ge 2 Sb 2 Te 5 at high temperature and high electric stress can be one of the origins of the asymmetric behavior. The electric-field-induced mass movement may provide insight on the device reliability of phase-change random access memory.

Fabrication of silicon nanopillar teradot arrays by electron-beam patterning for nanoimprint molds.

The formation of high-density, nanometer-scale dot (nanodot) arrays is a challenging task. Such arrays are considered important not only for scientific study of the fundamental quantum-mechanical behavior of materials, but also for achieving a practical goal of ultrahigh-density data storage and manipulation devices. [1‐3] In particular, methods for producing isolated high-density magnetic nanodot arrays with a pitch of 25nm or less have been extensively studied with the aim of fabricating the next generation of patterned magnetic media with a recording density of up to 1 terabit inch � 2 . [4‐6] With this application in mind, various fabrication attempts have been made, such as those based on a block copolymer, anodized aluminum oxide, colloid lithography, or laser interference lithography with extreme UV light. [7‐10] How

Synthesis of blue emitting InP/ZnS quantum dots through control of competition between etching and growth

Blue (<480 nm) emitting Cd-free quantum dots (QDs) are in great demand for various applications. However, their synthesis has been challenging. Here we present blue emitting InP/ZnS core/shell QDs with a band edge emission of 475 nm and a full width at half maximum of 39 nm (215 meV) from their quantum confined states. The drastic temperature drop immediately after mixing of the precursors and holding them at a temperature below 150 °C was the critical factor for the synthesis of blue emitting QDs, because the blue QDs are formed by the etching of ultra-small InP cores by residual acetic acid below 150 °C. Etching was dominant at temperatures below 150 °C, whereas growth was dominant at temperatures above 150 °C. ZnS shells were formed successfully at 150 °C, yielding blue emitting InP/ZnS QDs. The colour of the InP/ZnS QDs depicted on the CIE 1931 chromaticity diagram is located close to the edge, indicating a pure blue colour compared to other InP-based QDs.

Facile synthesis of intense green light emitting LiGdF4:Yb,Er-based upconversion bipyramidal nanocrystals and their polymer composites

A pathway for achieving intense green light emitting LiGdF4:Yb,Er upconversion nanophosphors (UCNPs) via Y(3+) doping is demonstrated. It was revealed that Y(3+) doping initiated the formation of a tetragonal phase and affected the particle size. Single tetragonal-phase LiGd0.4Y0.4F4:Yb(18%),Er(2%) (LGY0.4F:Yb,Er) UCNPs exhibited strong upconversion (UC) green luminescence and tetragonal bipyramidal morphologies. They showed 1325 and 325-fold higher photoluminescence intensity than the 0 and 80 mol% Y(3+)-doped LiGdF4:Yb,Er UCNPs, respectively. Additionally the particle size (edge length) of LiGdF4:Yb,Er-based upconversion tetragonal bipyramids (UCTBs) was controlled from 60.5 nm to an ultrasmall size of 9.3 nm with varying Y(3+) doping concentration. In an LGY0.4F:Yb,Er UCTB, uniform distribution of all constituent elements was directly confirmed by using high-angle annular dark-field scanning transmission electron microscopy and energy-filtered transmission electron microscopy (EFTEM) image analyses. In particular, existence of activator Er(3+) ions with extremely small quantity was clearly seen over a particle on the EFTEM image. Moreover, the LGY0.4F:Yb,Er UCTBs were successfully incorporated into the polydimethylsiloxane (PDMS) polymer and the highly transparent UCTB-PDMS composites showed bright green light under the excitation of 980 nm infrared light.

The fabrication scheme of a high resolution and high aspect ratio UV-nanoimprint mold

We propose a new scheme of fabricating molds for UV-nanoimprint lithography (UV-NIL) that is both high resolution and has a high aspect ratio. The scheme involves the utilization of a hydrogen silsesquioxane (HSQ) electron beam resist for high resolution patterning and the sputter-deposited alpha-Si layer that defines the high-aspect-ratio mold pattern obtained from the high etch selectivity between the HSQ and the alpha-Si. We obtained high resolution line patterns and dot patterns with feature sizes of 40 nm and 25 nm, respectively. The aspect ratio of the patterns was about 3.5 for line patterns and about 5 for dot patterns. These molds also demonstrate successful UV-nanoimprint patterning.

Selective Incorporation of Colloidal Nanocrystals in Nanopatterned SiO2 Layer for Nanocrystal Memory Device

CdSe colloidal nanocrystals with a size of ∼5 nm were selectively incorporated in SiO 2 nanopatterns formed by a self-assembled diblock copolymer patterning through a simple dip-coating process. The selective incorporation was achieved by capillary force, which drives the nanocrystals into the patterns during solvent evaporation in dip-coating. The capacitor structures of an Al-gate/ atomic layer deposition-Al 2 O 3 (27 nm)/CdSe (5 nm)/patterned SiO 2 (25 nm)/p-Si substrate were fabricated to characterize the charging/discharging behavior for a memory device. The flatband voltage shift was observed by a charge transport between the gate and the nanocrystals. It demonstrates the colloidal nanocrystal application to a memory device through selective incorporation in regularly ordered nanopatterns by a simple dip-coating process.

Fabrication of ultra-high-density nanodot array patterns (∼3 Tbits/in.2) using electron-beam lithography

The authors fabricated 15 nm pitch scale high-density dot patterns on a Si substrate using a hydrogen silsesquioxane electron-beam (e-beam) resist, vacuum treatment as a prebake, and vertical sidewall etching. The e-beam lithography was performed at 100 keV. The dot density fabricated was close to 3 Tbits/in.,2 which is one of the highest density patterns reported thus far. The process window was quite wide and the result can be easily and routinely duplicated.