Nguyen Xuan Nghia

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.

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.

Optical properties and green upconversion emission from Er3+-doped sol-gel silica glasses under red light (632.8-nm) excitation

The sol-gel process has been employed to prepare highly doped Er3+ silica glasses, up to 5-wt% for homogeneous doping distribution. The optical characteristics of the silica glasses including the UV/VIS absorption and fluorescence emission spectra under excitation of 488 nm and 520 nm lights were measured. Micro-Raman spectra of these silica glasses heated at different temperatures from 200 degree(s)C to 1100 degree(s)C under 632.8 nm laser beam excitation of the He-Ne laser also recorded. The green up conversion emission from Er3+ ion doped into the silica glasses heated to 1000 degree(s)C under red light (632.8 nm) excitation has been observed. Two green and weak emission bands, centered at 525 and 550 nm, for the 2H11/2 yields 4I15/2 and 4S3/2 yields 4I15/2 transitions, respectively, have been observed. The blue upconversion emission er3+ at 300 K centered at 460 nm was observed rather strength depends on the He-Ne laser pump powers and the Er3+ concentration. The pump wavelength is far from any electronic absorption from the ground state of the Er3+ ion, but corresponds to the excited state absorption. This upconversion process was interpreted by the excited state absorption and an upconversion mechanisms is proposed.

The status of indochinese grey langur Trachypithcus crepusculus at Xuan Lien nature reserve, Thanh Hoa province

Field surveys and interviewing local residents and forest rangers from 2013 to 2015 in Xuan Lien Nature Reserve have recorded 7 groups/troops of Indochinese grey langur (5 groups were sighted and 2 groups recorded by from interview). Each group consistS of 30-35 individuals as maximum and total of 100-200 individuals of Indochinese grey langurs was estimated to exist in the reserve. For all recorded groups, each group consists of at least on adult male, several adult females and sub-adults, and four groups have juveniles. The langurs were found in 4 little-affected forest types (Limestone evergreen forest, Sub-tropical evergreen forest, Lowland evergreen forest and Mixed wood-bamboo forest) located in 8 forest compartments which are situated far away from human settlements. The population of Indochinese grey langur in Xuan Lien NR is among the largest populations of this species in Vietnam. The population is facing with many threats such as wildlife hunting/trapping; Forest habitat degradation; Illegal timber poaching; Habitat disturbance by illegal encroachment of local residents and free-ranging cattle raising. Recommended measures for conservation of the langur population in Xuan Lien NR includestrengthening enforcement of forest and wildlife protection laws; promotion of natural regeneration of forest habitat; awareness education for local residents; monitoring of the langur population and habitat state; study of the species ecological requirement and investigation of Indochinese grey langur population in adjacent Pu Hoat Nature Reserve for collaborative actions on the species conservation.

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.

Results of the survey of the mammal fauna in the Tamdao national park, Vinhphuc province

Rapid assessments of the mammal fauna in the Tamdao national park (NP) were conducted in December of 2004, January and May of 2005 through village interviews and extensive field investigations. Eight (8) large mammal species were reported to be extirpated from the Tamdao NP. Seventy-four (74) mammal species were recorded to be existing in the Tamdao NP. A new checklist of 77 mammal species belonging to 24 families,18 orders was compiled. Out of these 77 recorded species, 21 species were of conservation concern, including 16 nationally threatened species, 17 globally threatened species and 16 species enlisted in the Government Decree 48/2002/NDCP. Current threats to mammal fauna in the Tamdao NP were illegal wildlife hunting, wildlife trade, exploitation of forest products, forest fire and domestic cattle raising.

The first record of living “Fossil” species (Laonestes aenigmanus) in Phong Nha - Ke Bang, Quang Binh province, Vietnam

Laotian Rock Rat Laonastes aenigmamus was first discovered in Hin Nammo NBCA, Khammoune Province, Lao PDR in 2005 by Jenkins et al. (2005). This species was later recognized as living species of the ancient family Diatomyidae which was thought to be extinct since the Miocene, about 11 millions years ago. Conservation of this species is recommended at the highest priority not only due to its limited range but also due to its status as the single living representative of an ancient family. During biodiversity survey on small mammals in Phong Nha - Ke Bang National Park, Quang Binh Province, Central Vietnam, we obtained 4 specimens of un-usual small animals in Thuong Hoa Commune, Minh Hoa District, Quang Binh Province. Based on analysis of appearance, body measurements and skull charasters of these specimens, we confirm that they belong to Laotian Rock Rat Laonastes aenigmamus which was described in 2005 based on specimens collected in Hin Nam No National Biodiversity Conservation Area, Khammoune Province, Lao PDR . Morphological features and some ecological attributes of this species are provided. This species is proposed to be included in the Checklist of wild mammal species of Vietnam under name as Truong son Rat (Annamite Rat). Main threats to Annamite Rat in Vietnam are wildlife hunting/snaring and forest destruction. Snaring small mammals for food and for trade is common habit of local residents in Thuong Hoa and nearby communes. Urgent conservation actions and further studies on population genetics and ecology of this elusive species are recommended.

Synthesis and Optical Properties of Colloidal CdS/CdSe/CdS Quantum Wells

Colloidal CdS/CdSe/CdS quantum wells were synthesized from TOPSe and cadmium oleate in octadecene, a non-coordinating solvent. Absorption, emission, and Raman scattering spectra of colloidal CdS/CdSe/CdS quantum wells with different thickness of CdSe well were investigated. The effect of thickness of CdSe well on the optical and vibrational properties of colloidal CdS/CdSe/CdS quantum wells was discussed. The expri-mental results provide further evidence for the existence of quantum dot-quantum well structures in CdS/CdSe/CdS type materials.

Synthesis and optical properties of colloidal core-shell semiconductor nanocrystals quantum dots for sensory application

We present results on the synthesis of core-shell CdSe/ZnS semiconductor nanocrystal quantum dots (NQDs) and their optical enhancement properties. A very strong and narrow photoluminescence is observed for sizes varying from 2 to 5 nm. Using an effective mass theory, we model the energy shift for the first excited state for core and core/shell NQDs. Core/shell heterostructure can be used to significantly increase the absorption cross section and decrease the efficiency of Auger recombination. Opal structures prepared from silica beads are also reported. This photonic crystal infiltrated with NQDs shows promise in biological detection.