Pham Thu Nga

Experimental Determination of the Fluorescence Quantum Yield of Semiconductor Nanocrystals.

Many studies have considered the luminescence of colloidal II–VI nanocrystals, both in solution at a collective scale and at an individual scale by confocal microscopy. The quantum yield is an important figure of merit for the optical quality of a fluorophore. We detail here a simple method to determine the quantum yield of nanocrystals in solution as a function of the absorption. For this purpose, we choose rhodamine 101 as a reference dye to measure the nanocrystal fluorescence quantum yield. The influence of the concentration on quantum yield is therefore studied for both the reference and the solutions of nanocrystals and is found to be critical for the acuity of the method. Different types of nanocrystals are studied to illustrate different quantum yield evolutions with the concentration.

Shell layer dependence of photoblinking in CdSe/ZnSe/ZnS quantum dots

We report on the influence of shell addition, shell thickness, and shape of CdSe/ZnSe/ZnS colloidal quantum dots (QDs) on luminescence, decay lifetime, and photoblinking. The addition of shell layers appreciably increased (decreased) the time interval of on (off) time, while the increase of outer shell thickness only increases the time interval of on-time. The photoblinking was suppressed as the QD shape changes from spherical to ellipsoidal probably due to the reduction of Auger process as the wave function degeneracy of the identical states of the two holes is lifted. We also suggest that interface traps might enhance the Auger recombination.

Detection of the pesticide by functionalised quantum dots as fluorescence–based biosensor

In this paper, we present the new results of biosensor that is made from the surface–modified quantum dots with acetylcholinesterase enzymes (AChE) for optical detection of the pesticides. The quantum dots (QDs) mentioned in this study are CdTe, CdSe/ZnS and CdSe/ZnSe/ZnS - the thick shell QDs are totally new. The results pointed out that all of the quantum dots are fit for the role of transducers in biosensor. In the biosensor, the QD– streptavidine - AChE is used as the substrate for the detection of pesticide. The pesticides used in this work are Parathion Methyl (PM) and Acetamiprid. The acetylthiocholine (ATCh) is used as an indicator of the activity of the AChE enzyme. Alternatively, the organophosphorus (OP) pesticides are the inhibitor for the AChE enzymes. Therefore, the mixture of the pesticide and ATCh is used for the goal of the specification of pesticide. We can detect pesticides by the change in PL intensity of QDs biosensor, with the content ranges from 0.05 ppb to 10 ppb.

Effect of reaction temperature and ligand concentration on the shape of CdSe nanocrystals

Colloidal CdSe nanocrystals were synthesised in octadecene solvent using oleic acid and trioctylphosphine as ligands for cadmium and selenium precursors respectively. The reaction parameters, including temperature and ligand concentration, were investigated. The morphology, crystalline structure and optical properties of CdSe nanocrystals were studied by transmission electron microscopy, X-ray diffraction, optical absorption and photoluminescence spectra. It was observed that the reaction parameters greatly affect the shape of nanocrystals. Dot-shaped CdSe nanocrystals were synthesised at high reaction temperature and low oleic acid concentration. In contrast, CdSe tetrapods were formed at lower reaction temperature and higher oleic acid concentration. Simultaneously, the ligand concentration results in the changes in core diameter and aspect ratio of tetrapod arms. The formation of CdSe nanocrystals of various shapes is discussed in relation to the nucleation and growth of CdSe nanocrystals in noncoordinating solvent. The obtained results demonstrate the possibility for controlling the shape of CdSe nanocrystals in single-step synthesis.

Measurement and modelization of silica opal optical properties

We present the synthesis process and optical characterization of artificial silica opals. The specular reflection spectra are analyzed and compared to band structure calculations and finite difference time domain (FDTD) simulations. The silica optical index is a key parameter to correctly describe an opal and is usually not known and treated as a free parameter. Here we propose a method to infer the silica index, as well as the silica spheres diameter, from the reflection spectra and we validate it by comparison with two independent infrared methods for the index and, scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements for the spheres diameter.

Optical Detection of the Pesticide by Functionalized Quantum Dots as Fluorescence-Based Biosensor

In this work, the results on using biosensor composed from quantum dots as transducer and acetylcholinesterase enzymes (AChE) to detect pesticides optically are presented. The used quantum dots were CdTe, CdSe/ZnS 10 monolayer (ML) and CdSe/ZnSe2ML/ZnS 8 ML – the brand new thick-shell quantum dots (QD). The study results pointed out that the CdSe/ZnS 10 ML and CdSe/ZnSe 2ML/ZnS 8ML quantum dots best fit for the role of transducers in biosensors. In the biosensor, acetylthiocholine (ATCh) is used as indicator for the AChE enzymes to work, since it is a very powerful hydrolyte with the presence of AChE enzymes. Moreover, the organophosphorus (OP) pesticides are the inhibitors for the AChE enzymes, thus, by the biosensors that we designed, we can detect pesticides by the change in photoluminescence (PL) intensity of QDs, with the detection of OP like parathion methyl is 0.05 ppm, and acetamiprid is 2.5 ppm.

Influence of Cd:Se precursor ratio on optical properties of colloidal CdSe tetrapods prepared in octadecene

Colloidal CdSe tetrapods were synthesized by chemical method in octadecene with the initial Cd:Se precursor ratios from 6:1 to 1:10 and the different monomer concentrations. Their stoichiometry and optical properties were investigated using the energy dispersion X-ray, absorption and photoluminescence spectroscopy. It was found that the CdSe tetrapods are Cd rich for all of the obtained sizes. Their stoichiometry was almost not changed during the growth and depends on the initial precursor ratio. The double-peak structure corresponding to the optical transitions in the core and arms of CdSe tetrapods was observed clearly in both the optical absorption and photoluminescence spectra for the Cd:Se precursor ratios from 2:1 to 1:2. The influence of the initial Cd:Se precursor ratio and monomer concentration on the growth of CdSe tetrapods and their optical properties has been analysed and discussed.

Manipulating emission of CdSe/ZnS nanocrystals embedded in synthetic opals

Photonic crystals (PCs) are the object of great interest due to the possibility, for appropriate PCs, to modify and control light propagation and even to influence the emission properties of an emitter, such as its emission diagram and its life time. One of the most common approaches to prepare 3D PCs takes advantage of the spontaneous self-organisation of spherical colloidal particles. Various self-assembly techniques such as sedimentation, convective or Langmuir-Blodgett ones have been studied as they provide a low cost and relatively easy protocol to obtain artificial opals. SiO2 opals exhibit a pseudo-band gap. Nevertheless the coupling of II–VI nanocrystal emitters in such PCs allows one to recognize and study some basic problems. Large opals have been prepared by the sedimentation method and the size of the balls has been adjusted so that the pseudo-band gap of those PCs lies in the same region as the emission band of CdSe/ZnS nanocrystals. Diagrams of radiation and the modification of the spontaneous life time of the embedded nanocrystals will be presented and discussed. Introducing well-defined defects in PCs which are necessary to guide the photons through the crystal remains a hard technological challenge. Several top-down methods have been investigated. We will present different bottom-up routes proposed by different groups to engineer planar defects into colloidal PCs.

SYNTHESIS AND COMPARATIVE PHOTOLUMINESCENCE OF CdZnSe / ZnS AND CdZnSe / ZnSeS ALLOY QUANTUM DOTS

In order to search for new structures and compositions of quantum dots, suppress the blinking photoluminescence (random changes between high emission state ( on) and low emission status ( off ) under continuous photo- excitation ) and serve the application purposes in bio- medical and in optoelectronic devices , we have studied the fabrication of new alloy quantum dots ( QDs ). In this paper, we present new results on alloy core / shell quantum dots, with changed alloy shell composition, that was CdZnSe/ZnSe x S 1-x a ML with x ( x = 0 , 0.2, 0.4, 0.5 , 0.6, 0.8) and the shell thickness in monolayer (ML) ( a = 2 , 4 , 6 ) . The emission spectra and the intensity change according to the composition of the alloy shell. The full width a half maximum ( FWHM ) of the emission spectra of quantum dots CdZnSe is 25.5 nm. Covered with a shell layer, the emission intensity of the CdZnSe core increases along with the shell thickness. For comparison purpose, two different shell materials have been used, which are ZnS and ZnSeS alloy. With the same shell thickness, the emission wavelengths and intensity of the QDs change when the shell’s composition changes.The photoluminescence (PL) decay and the PL blinking of the alloy QDs was studied. It was shown that the alloy QDs spent by the nanocrystal in the ON state ranged typically between 20 and 40 %, and was dependent on the core composition. Detailed discussions on the experiment results are presented.

Sensitive Organophosphorus and Carbamate Pesticides Biosensor Based on Acetylcholinesterase and CdZnSe/ZnS Ternary Alloy Quantum Dots

This report presents the results of ternary alloy quantum dots (TQDs) manufacturing and its superior optical properties to fabricate the biosensor for pesticide detection. The explanation for the changes in PL intensity based on the change of electronic charges that alters the pH value of the surrounding environment, and the types of interaction on the surface of CdZnSe/ZnS TQDs were discussed. We show the decomposition process of the indicator (ATCh) with the catalysis from acetylcholinesterase enzyme (AChE) that inhibited by organophosphorus and carbamate pesticides including Trichlorfon (TF), Cypermethrin (CH), Abamectin (AT), and Carbosulfan (CS) with concentrations from 0.05 ppm to 10 ppm.