Jun-Sheng Yu

Selective synthesis of CdTe and high luminescence CdTe/CdS quantum dots : The effect of ligands

This paper describes the selective syntheses of high luminescence CdTe and core-shell CdTe/CdS quantum dots (QDs) in aqueous solution by simple heating refluxing at 100 degrees C. CdTe QDs are prepared by using three kinds of ligands (thioglycolic acid-TGA, tiopronin-TP, and glutathione-GSH) as stabilizer, respectively. The results of refluxing for 10 min to several hours indicate that GSH-capped CdTe QDs have higher photoluminescence quantum yields (QY 54%) than TGA (QY 41%)- and TP (QY 24%)-stabilized CdTe QDs. Further, using TP-CdTe as core template and GSH as stabilizer and sulfur source, high luminescence GSH-capped CdTe/CdS core-shell QDs have been successfully synthesized in aqueous solution by simple refluxing at 100 degrees C. The GSH-CdTe/CdS QDs exhibit high fluorescence QYs about 55% over a broad spectral range of 530-588 nm, with the best QY of 83%. TP-stabilized CdTe/CdS QDs are also synthesized with TP as stabilizer and thioacetamide (TAA) as sulfur source, and with the best QY of 80%. GSH-stabilized CdTe and CdTe/CdS QDs are highly biocompatible, monodispersed, and stable under physiological conditions. The method of QDs prepared using GSH is simple and environmentally friendly, and it can be easily extended to the large-scale, aqueous-phase production of QDs.

In situ synthesis of highly luminescent glutathione-capped CdTe/ZnS quantum dots with biocompatibility

This paper focuses on the in situ synthesis of novel CdTe/ZnS core-shell quantum dots (QDs) in aqueous solution. Glutathione (GSH) was used as both capping reagent and sulfur source for in situ growth of ZnS shell on the CdTe core QDs. The maximum emission wavelengths of the prepared CdTe/ZnS QDs can be simply tuned from 569 nm to 630 nm. The PL quantum yield of CdTe/ZnS QDs synthesized is up to 84%, larger than the original CdTe QDs by around 1.7 times. The PL lifetime results reveal a triexponential decay model of exciton and trap radiation behavior. The average exciton lifetime at room temperature is 17.1 ns for CdTe (2.8 nm) and 27.4 ns for CdTe/ZnS (3.7 nm), respectively. When the solution of QDs is dialyzed for 3 h, 1.17 ppm of Cd(2+) is released from CdTe QDs and 0.35 ppm is released from CdTe/ZnS. At the dose of 120 microg/ml QDs, 9.5% of hemolysis was induced by CdTe QDs and 3.9% was induced by CdTe/ZnS QDs. These results indicate that the synthesized glutathione-capped CdTe/ZnS QDs are of less toxicity and better biocompatibility, so that are attractive for use in biological detection and related fields.

How do nitrogen-doped carbon dots generate from molecular precursors? An investigation of the formation mechanism and a solution-based large-scale synthesis

A bottom-up method, using monoethanolamine (MEA) as both a passivation agent and a solvent, has been developed for rapid and massive synthesis of nitrogen-doped carbon dots (N-C-dots) from citric acid under heating conditions. This method requires a relatively mild temperature (170 °C) without special equipment, and affords one-pot large-scale production (39.96 g) of high-quality N-C-dots (quantum yield of 40.3%) in a few minutes (10 minutes). Significantly, an interesting formation process of N-C-dots, for the first time, has been monitored by transmission electron microscopy, ultraviolet-visible absorbance spectroscopy, photoluminescence spectroscopy, Fourier transformed infrared spectroscopy, and thermogravimetric analysis, and a corresponding formation mechanism, including polymerization, aromatization, nucleation, and growth, is proposed. It is important that the MEA-based synthesis of N-C-dots can be extended to various precursors, such as glucose, ascorbic acid, cysteine, and glutathione, which show general universality. Furthermore, the N-C-dots with strong fluorescence, excellent optical stability, and low cytotoxicity are successfully applied as fluorescent probes for bioimaging.

Transfer from trap emission to band-edge one in water-soluble CdS nanocrystals.

A simple and rapid route to water-soluble CdS nanocrystals stabilized by citrate was reported, and the transfer of citrate-stabilized CdS NCs from trap emission to band-edge one was studied systematically for the first time. It was found that heating in air, alkaline activation and illumination, all efficiently manipulated surface states of CdS NCs and controlled the emission states, leading to transferring CdS NCs from a broad trap emission (FWHM ~125 nm) to their strong, narrow band-gap emission (FWHM ~25 nm), comparable to that of CdS NCs synthesized by organic routes. Lifetime decay kinetic studies demonstrated that the average lifetimes for CdS NCs before and after transferred were 131.1 and 32.7 ns, respectively. The freshly-synthesized NCs were predominated by trap emission (~94%), while the transferred CdS NCs with well cubic structure dominated by band-edge emission (up to 91%). The tunable emissions of CdS NCs from violet to green could be achieved by controlling emission states of CdS NCs with different Cd/S molar ratios. The transfer mechanisms of CdS NCs from trap to band-edge emission were proposed to be epitaxial growth of a Cd(OH)(2) shell on CdS NCs core. The transition probability of energy states before and after transferred was further investigated.

Synthesis of stable near-infrared emitting HgTe/CdS core/shell nanocrystals using dihydrolipoic acid as stabilizer

HgTe/CdS core/shell nanocrystals (NCs) with highly-stable near-infrared (NIR) emissions were synthesized by employing dihydrolipoic acid (DHLA) as a stabilizer. This synthetic route was performed using Te powder as the tellurium source to prepare HgTe NCs, and H2S generated by the reaction of Na2S–H2SO4 as the sulfur source for synthesizing HgTe/CdS core/shell NCs at room temperature. The fluorescence emission peaks of DHLA-capped HgTe/CdS NCs could be facilely tuned from 910 nm to 1200 nm by varying the reflux time at 100 °C, with a maximum photoluminescence quantum yield of 52%. The obtained HgTe/CdS NCs exhibited an NIR stable emission when heated at 75 °C. Correspondingly, these HgTe/CdS core/shell NCs displayed excellent colloidal dispersion stability and long exciton lifetimes that reached up to 47.6 ns in an aqueous medium. The resultant HgTe/CdSe NCs have been successfully used to fabricate NIR emitting thin films on the surface of polymers and to perform fluorescence imaging in a live animal.

Aqueous synthesis and characterization of glutathione-stabilized β-HgS nanocrystals with near-infrared photoluminescence

A simple, rapid and green aqueous approach to near-infrared (NIR)-emitting β-HgS nanocrystals (NCs) was demonstrated for the first time by using glutathione (GSH) as the stabilizer at room temperature. The resulting HgS NCs with zinc blend structure exhibited strong quantum size effect, and the emission peak could be tuned in a wide NIR region from ca. 775 to 1041 nm. As compared with early achievements, the emission intensity of GSH-stabilized HgS NCs enhanced, with the maximum quantum yield reaching ~2.8%. It was also found that the stability of the GSH-HgS NCs was improved noticeably, the PL peak red-shifting only 9 nm and 23 nm after stored at 4°C for 4 months and 25°C for 7 days, respectively. The better stability of the HgS NCs was elucidated by FT-IR due to the multiple coordination of GSH molecule to surface Hg of the NCs. The emission range of GSH-stabilized HgS NCs was located between the visible region (500-800 nm) and IR region (1000-1600 nm) of HgS NCs as reported previously, extending the emission region of HgS nanomaterial. Therefore, the continuous emission from visible to IR spectral ranges provided HgS material more potential applications.

Highly Fluorescent, Near-Infrared-Emitting Cd2+-Tuned HgS Nanocrystals with Optical Applications

Bulk HgS itself has proven to be a technologically important material; however, the poor stability and weak emission of HgS nanocrystals have greatly hindered their promising applications. Presently, a critical problem is the uncontrollable growth of HgS NCs and their intrinsic surface states which are susceptible to the local environment. Here, we address the issue by an ion-tuning approach to fabricating stable, highly fluorescent Cd:HgS/CdS NCs for the first time, which efficiently tuned the band-gap level of HgS NCs, pushing their intrinsic states far away from the surface, reducing the strong interaction of the environment with surface states and hence drastically boosting the exciton transition. As compared to bare HgS NCs, the obtained Cd:HgS/CdS NCs exhibited tunable luminescence peaks from 724 to 825 nm with an unprecedentedly high quantum yield up to 40% at room temperature and excellent thermal and photostability. Characterized by TEM, XRD, XPS, and AAS, the resultant Cd:HgS/CdS NCs possessed a zinc-blende structure and was composed of a homogeneous alloyed HgCdS structure coated with a thin-layer CdS shell. The formation mechanism of Cd:HgS/CdS NCs was proposed. These bright, stable HgS-based NCs presented promising applications as fluorescent inks for anticounterfeiting and as excellent light converters when coated onto a blue-light-emitting diode.