Debasis Bera

Quantum Dots and Their Multimodal Applications: A Review

Semiconducting quantum dots, whose particle sizes are in the nanometer range, have very unusual properties. The quantum dots have band gaps that depend in a complicated fashion upon a number of factors, described in the article. Processing-structure-properties-performance relationships are reviewed for compound semiconducting quantum dots. Various methods for synthesizing these quantum dots are discussed, as well as their resulting properties. Quantum states and confinement of their excitons may shift their optical absorption and emission energies. Such effects are important for tuning their luminescence stimulated by photons (photoluminescence) or electric field (electroluminescence). In this article, decoupling of quantum effects on excitation and emission are described, along with the use of quantum dots as sensitizers in phosphors. In addition, we reviewed the multimodal applications of quantum dots, including in electroluminescence device, solar cell and biological imaging.

Hybrid polymer-CdSe solar cells with a ZnO nanoparticle buffer layer for improved efficiency and lifetime

We report the use of a solution-processed ZnO nanoparticle buffer layer in hybrid solar cells based on blends of poly(3-hexylthiophene) (P3HT) and CdSe quantum dots. Depending on the size of the CdSe nanocrystal, these devices exhibit 20–70% higher photocurrent output than similar devices without the ZnO nanoparticle layer, which is attributed to a combination of electronic and optical effects. With negligible change in open-circuit voltage and a small increase in the fill factor, the power conversion efficiency of these P3HT:CdSe hybrid solar cells was increased by 30–80% with incorporation of the ZnO nanoparticle layer. The presence of the ZnO nanoparticle layer also drastically improves the stability of these hybrid solar cells. Less than 40% loss in efficiency was observed for such devices after 70 days of exposure to the laboratory ambient without any type of encapsulation.

Electrophosphorescence from triplet excimers in poly-(N-vinylcarbazole)

The authors have observed electrophosphorescence from triplet excimer in poly-(N-vinylcarbazole) (PVK) polymer at room temperature. The triplet excimer appears to be influenced by the PVK molecular conformation. This observation is very important to the understanding of the mechanism of triplet exciton formation in PVK. It is, therefore, an important consideration in selecting materials for phosphor-doped organic light-emitting diode applications.

High efficiency photoluminescence from silica-coated CdSe quantum dots

The effects on the luminescent properties of coating CdSe quantum dots with silica are presented. Coating increased the quantum yields from ∼10%–20% to ∼80% at a maximum. The changes in quantum yields and photoluminescence peak at wavelength were discussed in terms of the effects of surface charge. By neutralizing surface charge, the emission from CdSe quantum dots was initially blueshifted followed by redshifting, and the quantum yield increased dramatically.

White light emission from single layer poly (n-vinylcarbazole) polymeric light-emitting devices by mixing singlet and triplet excimer emissions

White light electroluminescence (EL) was obtained by mixing emission from singlet and triplet excimers from a single poly (n-vinylcarbazole) (PVK) spin cast layer after irradiation of the solution with UV light. With increased UV light irradiation, the intensity from the triplet excimer (red-630 nm) of PVK increased compared with that of the singlet excimer (blue-460 nm) due to an increased population of both adjacent benzene rings being aligned with one another (fully overlapping) versus only one of the adjacent benzene rings being aligned (partially overlapping). The emission color changed from blue to white with increased UV irradiation time while the EL brightness and current density decreased and the turn-on voltage increased.

Time-evolution of photoluminescence properties of ZnO/MgO core/shell quantum dots

In situ and ex situ time-evolution of photoluminescence data during growth of ZnO/MgO core/shell quantum dots (Qdots) were used to study the stability of the green–white (CIE: 0.32, 0.41) luminescence from ZnO defects. The ZnO/MgO Qdots, synthesized by a sol–gel process, exhibited a quantum yield of 13% compared with less than 5% for ZnO Qdots. The growth of ZnO Qdots was arrested and the defect emission was stabilized by formation of the MgO shell. UV–Vis absorption data verified the quantum confinement of ZnO/MgO Qdots. Photoluminescence emission from defects in the ZnO/MgO Qdots was stable ~70 days at room temperature.

Optimization of the Yellow Phosphor Concentration and Layer Thickness for Down-Conversion of Blue to White Light

A cerium-doped gadolinium-yttrium garnet yellow emitting phosphor was used to optimize the phosphor layer for down-conversion of blue light from InGaN inorganic light-emitting diode (iLED) to white. Optical, morphological and elemental characterizations of phosphor were carried out. Various amounts of the phosphor were dispersed in polymethyl methacrylate polymer (PMMA)-chlorobenzene solution. The phosphor layer thickness was optimized by varying the volume of the phosphor-PMMA mixture. Using a blue emitting (458 nm) iLED, a two times larger luminance efficacy (lm/W) was obtained from down-converted white light using a 37.5 μm thick 25 mm × 25 mm film prepared from 30 mg of phosphor in 500 μl of the PMMA-chlorobenzene solution.

Photoluminescence from ZnS∕CdS:Mn∕ZnS quantum well quantum dots

Enhanced photoluminescence is reported from ZnS∕CdS:Mn∕ZnS quantum well quantum dots (QWQDs) prepared with a reverse micelle process. Compared to CdS:Mn∕ZnS core/shell quantum dots, the QWQDs exhibited a much stronger Mn2+ T14-A16 yellow emission and the quantum yield (QY) was increased from 9% to 45%. This increased QY for QWQDs was postulated to result from the quantum well structure suppressing nonradiative relaxations from the Auger recombination at short times and reduced energy transfer between the Mn2+ dopants at longer times.

Temporal evolution of white light emission from CdSe quantum dots

A study of white light emission from magic sized CdSe quantum dots is reported. Quantum dots were synthesized using a hot solution method and exhibited strong white light with a quantum yield in the range of 10-30%. The emission of white light was attributed to photoluminescence (PL) from both excitons and surface states. The wavelength and intensity of the PL from both excitons and surface states were found to depend upon the particle size, reaction time, reaction temperature and precursor ratio.

Effects of ultraviolet light irradiation on poly(vinlycarbazole)

In this paper, we present a study of the photophysical and photochemical effects that influence phosphor-doped polymer light-emitting diodes based on poly(vinlycarbazole). With ultraviolet (UV) light irradiation, the relative concentration of partially overlapping carbazole groups [poly(vinlycarbazole), (p-PVK)] to the fully overlapping conformation (f-PVK) is decreased via photoisomerization, which affects both the energies and the probability of forming singlet versus triplet excitons. As a result, phosphor-doped polymer light-emitting diodes fabricated using UV irradiated PVK exhibited higher efficiencies as compared with control devices (doped PVK without UV light irradiation). These data demonstrate a new route for the production of more efficient electrophosphorescent light-emitting diodes.