Chang-Ching Tu

Visible electroluminescence from hybrid colloidal silicon quantum dot-organic light-emitting diodes

We demonstrate hybrid colloidal silicon quantum dot (SiQD)-organic light-emitting diodes with electroluminescence (EL) in the visible wavelengths. The device using blue photoluminescence (PL) SiQDs as emitters shows multiple EL peaks which are attributed to carrier recombination in the core quantum confinement states, the hole-transport-layer and the surface trap states, respectively. However, the red PL SiQD device shows a single EL peak consistent with the PL peak. These findings are in agreement with the previous report that large Stokes shift were observed for oxidized blue emission SiQDs due to oxide states while red emission SiQDs show negligible PL shift after oxidation.

High efficiency photodetectors fabricated by electrostatic layer-by-layer self-assembly of CdTe quantum dots

We demonstrate high-performance photodetectors from multilayers of CdTe quantum dots (QDs). The QDs are synthesized and dispersed in aqueous solution with either 2-mercaptoethylamine (positively charged) or thioglycolic acid (negatively charged) as capping stabilizers. By electrostatic attraction, the charged QDs are self-assembled layer by layer on an indium tin oxide substrate modified with (3-aminopropyl)triethoxysilane. This process allows control of active layer thickness by self-assembly, and can in principle be applied to a wide range of substrates. The photodetector exhibits high responsivity (0.18A∕W) under 0.1V bias due to extremely short capping ligands of QDs, which have high internal quantum efficiency, and the densely packed multilayer structure.

Red-emitting silicon quantum dot phosphors in warm white LEDs with excellent color rendering

We demonstrate red-emitting silicon quantum dot (SiQD) phosphors as a low-cost and environment-friendly alternative to rare-earth element phosphors or CdSe quantum dots. After surface passivation, the SiQD-phosphors achieve high photoluminescence quantum yield = 51% with 365-nm excitation. The phosphors also have a peak photoluminescence wavelength at 630 nm and a full-width-at-half-maximum of 145 nm. The relatively broadband red emission is ideal for forming the basis of a warm white spectrum. With 365-nm or 405-nm LED pumping and the addition of green- and/or blue-emitting rare-earth element phosphors, warm white LEDs with color rendering index ~95 have been achieved.

Colloidal quantum dot photodetectors enhanced by self-assembled plasmonic nanoparticles

Self-assembled colloidal plasmonic silver nanoparticles monolayers were integrated into solution-processed colloidal quantum dot (QD) photodetectors. We observed plasmon enhancement of photodetector responsivity over a board spectrum range (400–600 nm), with a 1.2–1.6 fold enhancement for a 440-nm-thick QD film device and a 2.4–3.3 fold enhancement for a 100-nm-thick QD device. The enhancement behavior was wavelength dependent with higher enhancement factor at longer wavelengths. The origins of responsivity enhancement were discussed.

Brightly photoluminescent phosphor materials based on silicon quantum dots with oxide shell passivation

We demonstrate silicon-based phosphor materials which exhibit bright photoluminescence from near-infra-red to green. The colloidal composites which are composed of silicon quantum dots (SiQDs) attached on micro-size silicon particles are synthesized by electrochemical etching of silicon wafers and then dispersed in ethanol. Subsequently, isotropic etching by HF/HNO3 mixture controls the size so as the emission wavelength of SiQDs, and forms an oxide passivating shell. The phosphors can further react with alkoxysilanes to form a stable suspension in non-polar solvents for solution-processing. The resulting red-light-emitting SiQD-based phosphors in chloroform exhibit photoluminescence external quantum efficiency of 15.9%. Their thin films can be efficiently excited by InGaN light-emitting diodes and are stable in room condition.

Solution-processed photodetectors from colloidal silicon nano/micro particle composite

We demonstrate solution-processed photodetectors composed of heavy-metal-free Si nano/micro particle composite. The colloidal Si particles are synthesized by electrochemical etching of Si wafers, followed by ultra-sonication to pulverize the porous surface. With alkyl ligand surface passivation through hydrosilylation reaction, the particles can form a stable colloidal suspension which exhibits bright photoluminescence under ultraviolet excitation and a broadband extinction spectrum due to enhanced scattering from the micro-size particles. The efficiency of the thin film photodetectors has been substantially improved by preventing oxidation of the particles during the etching process.

Surface passivation dependent photoluminescence from silicon quantum dot phosphors.

We demonstrate wavelength-tunable, air-stable and nontoxic phosphor materials based on silicon quantum dots (SiQDs). The phosphors, which are composed of micrometer-size silicon particles with attached SiQDs, are synthesized by an electrochemical etching method under ambient conditions. The photoluminescence (PL) peak wavelength can be controlled by the SiQD size due to quantum confinement effect, as well as the surface passivation chemistry of SiQDs. The red-emitting phosphors have PL quantum yield equal to 17%. The SiQD-phosphors can be embedded in polymers and efficiently excited by 405 nm light-emitting diodes for potential general lighting applications.

Fluorescent porous silicon biological probes with high quantum efficiency and stability

We demonstrate porous silicon biological probes as a stable and non-toxic alternative to organic dyes or cadmium-containing quantum dots for imaging and sensing applications. The fluorescent silicon quantum dots which are embedded on the porous silicon surface are passivated with carboxyl-terminated ligands through stable Si-C covalent bonds. The porous silicon bio-probes have shown photoluminescence quantum yield around 50% under near-UV excitation, with high photochemical and thermal stability. The bio-probes can be efficiently conjugated with antibodies, which is confirmed by a standard enzyme-linked immunosorbent assay (ELISA) method.