Wei-Hsuan Tseng

Soliton compression of the erbium-doped fiber laser weakly started mode-locking by nanoscale p-type Bi2Te3 topological insulator particles

We demonstrate the nanoscale p-type Bi2Te3 powder-based saturable absorber-induced passive mode-locking of the erbium-doped fiber laser (EDFL) with sub-picosecond pulsewidth. Such a nanoscale topological insulator powder is obtained by polishing the bulk p-type Bi2Te3 in a commercial thermoelectric cooler (TE cooler). This is then directly brushed onto the end-face of a single-mode fiber patchcord, to avoid any mis-connecting loss caused by laser beam divergence, which can result in a mode-locked pulsewidth of 436 fs in the self-amplitude modulation mode of a TE cooler. To further shorten the pulse, the soliton compression is operated by well-controlling the group delay dispersion and self-phase modulation, providing the passively mode-locked EDFL with a pulsewidth as short as 403 fs.

Application of F4TCNQ doped spiro-MeOTAD in high performance solid state dye sensitized solar cells

Amid the investigation of solid-state dye-sensitized solar cells (SDSSCs), it was found that the incorporation of F4TCNQ into the solid hole-transporting materials (HTMs) spiro-MeOTAD forms a spiro-MeOTAD/F4TCNQ (strong electron acceptor) polaron charge-transfer complex. Careful examination indicates that the formation of the polaron charge-transfer complex not only facilitates the conductivity of HTMs but also inhibits the charge recombination across the interface of the heterojunction, i.e. photoanode/HTMs and/or counter electrode/HTMs. As a result, the performance of SDSSCs has been markedly improved by using the organic dye A2-F. At AM1.5 illumination the short circuit current densities J(SC) increase from 8.29 mA cm(-2) (w/o F4TCNQ) to 10.95 mA (w/F4TCNQ), accompanied by a 20% increase of the overall power conversion efficiency, η, from 4.55% to 5.44%.

Antiferromagnetic iron nanocolloids: a new generation in vivo T1 MRI contrast agent.

A novel T1 agent, antiferromagnetic α-iron oxide-hydroxide (α-FeOOH) nanocolloids with a diameter of 2-3 nm, has been successfully prepared. These nanocolloids, together with a post synthetic strategy performed in mesoporous silica, are a great improvement over the low T1-weighted contrast common in traditional magnetic silica nanocomposites. The intrinsic antiferromagnetic goethite (α-FeOOH) shows very low magnetization (M(z)) of 0.05 emu g(-1) at H = 2 T at 300 K (0.0006 emu g(-1) for FeOOH/WMSN-PEG), which is 2 orders of magnitude smaller than any current ultrasmall iron oxide NPs (>5 emu g(-1)) reported to date, hence ensuring the low r2 (∝ Mz) (7.64 mM(-1) s(-1)) and r2/r1 ratio (2.03) at 4.7 T. These biodegradable α-FeOOH nanocolloids also demonstrate excellent in vitro cellular imaging and in vivo MR vascular and urinary trace imaging capability with outstanding biocompatibility, which is exceptionally well secreted by the kidney and not the liver as with most nanoparticles, opening up a new avenue for designing powerful antiferromagnetic iron T1 contrast agents.

Enhancement of current injection in organic light emitting diodes with sputter treated molybdenum oxides as hole injection layers

The enhancement of current density and luminance in organic light emitting diodes is achieved by treating molybdenum oxide (MoO3) hole-injection-layers with slight argon ion sputtering. The sputter treated MoO3 layers provide improvement in current injection efficiency, resulting in better current density which is about ten times higher than that of the reference devices. Photoemission spectroscopy shows that molybdenum in MoO3 is reduced to lower oxidation states after sputter treatment due to the removal of oxygen. As a result, gap states are formed to enhance metallic characteristics of the sputter treated MoO3 surface and facilitate better hole injection efficiency.

Biofunctionalized magnetic nanoparticles for in vitro labeling and in vivo locating specific biomolecules

In this work, we developed processes to biofunctionalize magnetic nanoparticles dispersed in phosphate buffer saline solution. For future clinical utility, magnetic nanoparticles were biofunctionalized with anti-vascular cell adhesion molecule-1 (VCAM-1) to label the VCAM-1 molecule, which served as an indicator for the lesions prone to vulnerable atherosclerotic plaque formation. The biofunctionalized magnetic nanoparticles were used to magnetically label, in vitro, cells expressing VCAM-1, as well as to locate the vulnerable aortic lesions of hypercholesterolemic rabbits with the aid of magnetic resonance imaging. In addition to demonstrating the feasibility of using biofunctionalized magnetic nanoparticles for biomolecule assays, the relevant physical mechanisms are discussed.

Self-amplitude and self-phase modulation of the charcoal mode-locked erbium-doped fiber lasers

With the intra-cavity nano-scale charcoal powder based saturable absorber, the 455-fs passive mode-locking of an L-band erbium-doped fiber laser (EDFL) is demonstrated. The size reduction of charcoal nano-particle is implemented with a simple imprinting-exfoliation-wiping method, which assists to increase the transmittance up to 0.91 with corresponding modulation depth of 26%. By detuning the power gain from 17 to 21 dB and cavity dispersion from -0.004 to -0.156 ps² of the EDFL, the shortening of mode-locked pulsewidth from picosecond to sub-picosecond by the transformation of the pulse forming mechanism from self-amplitude modulation (SAM) to the combining effect of self-phase modulation (SPM) and group delay dispersion (GDD) is observed. A narrower spectrum with 3-dB linewidth of 1.83-nm is in the SAM case, whereas the spectral linewidth broadens to 5.86 nm with significant Kelly sideband pair can be observed if the EDFL enters into the SPM regime. The mode-locking mechanism transferred from SAM to SPM/GDD dominates the pulse shortening procedure in the EDFL, whereas the intrinsic defects in charcoal nano-particle only affect the pulse formation at initial stage. The minor role of the saturable absorber played in the EDFL cavity with strongest SPM is observed.

High-efficiency small-molecule-based organic light emitting devices with solution processes and oxadiazole-based electron transport materials.

We demonstrate high-efficiency small-molecule-based white phosphorescent organic light emitting diodes (PHOLEDs) by single-active-layer solution-based processes with the current efficiency of 17.3 cdA(-1) and maximum luminous efficiency of 8.86 lmW(-1) at a current density of 1 mA cm(-2). The small-molecule based emitting layers are codoped with blue and orange phosphorescent dyes. We show that the presence of CsF/Al at cathodes not only improves electron transport in oxadiazole-containing electron transport layers (ETLs), but also facilitates electron injection through the reacted oxadiazole moiety to reduce interface resistance, which results in the enhancement of current efficiency. By selecting oxadiazole-based materials as ETLs with proper electron injection layer (EIL)/cathode structures, the brightness and efficiency of white PHOLEDs are significantly improved.

Enhancing the incorporation compatibility of molybdenum oxides in organic light emitting diodes with gap state formations

The enhancement of injection current and luminance in organic light emitting diodes is achieved by annealing molybdenum oxide (MoO3) hole injecting layers prior to the deposition of hole transport layers. While there is no benefit by the incorporation of non-annealed MoO3 in devices using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as the hole transport layers, the annealed MoO3 layers exhibit a significant improvement in hole injection from indium tin oxide anodes to TAPC. X-ray photoemission spectroscopy reveals the change of oxidation states of Mo atoms in MoO3 films due to the annealing process. The gap state formation is verified by ultra-violet photoemission spectroscopy. A more energetically favorable band alignment is obtained at the interface between the annealed MoO3 and TAPC, resulting in improved hole injection efficiency. The overall performance of OLEDs can be enhanced by adopting annealed MoO3 in most of the hole transport layers.

One-Step, Room-Temperature Synthesis of Glutathione-Capped Iron-Oxide Nanoparticles and their Application in In Vivo T1-Weighted Magnetic Resonance Imaging

The room-temperature, aqueous-phase synthesis of iron-oxide nanoparticles (IO NPs) with glutathione (GSH) is reported. The simple, one-step reduction involves GSH as a capping agent and tetrakis(hydroxymethyl)phosphonium chloride (THPC) as the reducing agent; GSH is an anti-oxidant that is abundant in the human body while THPC is commonly used in the synthesis of noble-metal clusters. Due to their low magnetization and good water-dispersibility, the resulting GSH-IO NPs, which are 3.72 ± 0.12 nm in diameter, exhibit a low r2 relaxivity (8.28 mm(-1) s(-1)) and r2/r1 ratio (2.28)--both of which are critical for T1 contrast agents. This, together with the excellent biocompatibility, makes these NPs an ideal candidate to be a T1 contrast agent. Its capability in cellular imaging is illustrated by the high signal intensity in the T1-weighted magnetic resonance imaging (MRI) of treated HeLa cells. Surprisingly, the GSH-IO NPs escape ingestion by the hepatic reticuloendothelial system, enabling strong vascular enhancement at the internal carotid artery and superior sagittal sinus, where detection of the thrombus is critical for diagnosing a stroke. Moreover, serial T1- and T2-weighted time-dependent MR images are resolved for a rats kidneys, unveiling detailed cortical-medullary anatomy and renal physiological functions. The newly developed GSH-IO NPs thus open a new dimension in efforts towards high-performance, long-circulating MRI contrast agents that have biotargeting potential.

Comprehensive study of medium-bandgap conjugated polymer merging a fluorinated quinoxaline with branched side chains for highly efficient and air-stable polymer solar cells

A new medium-bandgap conjugated copolymer comprising a rigidly fused benzo[1,2-b:4,5-b′]-dithiophene (BDT) unit and a fluorinated quinoxaline moiety through a thiophene π-spacer has been rationally designed and synthesized by Stille coupling polymerization and thoroughly evaluated for use as a donor material in bulk-heterojunction polymer solar cells (BHJ PSCs). A comprehensive study of the structure-function relationship in the PSCs was also explored. The PDBTQEH copolymer exhibits good solubility in a wide range of organic solvents and has a high hole mobility. Introduction of an highly electronegative fluorine atoms to quinoxaline moiety further lowers both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of polymer, which is beneficial for attaining higher open-circuit voltage (Voc) and long-term stability. Conventional architecture BHJ PSCs using PBDTQEH:PC71BM (1 : 1, w/w) displays a high power conversion efficiency (PCE) of 5.90%. Compared with the same composition, the device in the inverted configuration reveals a rather high PCE of 6.36% with a Voc of 0.78 V, a short-circuit current density (Jsc) of 12.72 mA cm−2, and a high fill factor (FF) of 64.3%. The inverted device also demonstrates outstanding air stability; without any encapsulation, the solar efficiency of the device remains above 74% of the original value after storage in air for 1000 h.