Facile synthesis of highly luminescent Mn-doped ZnS nanocrystals

Manganese-doped zinc sulfide quantum dots (Mn:ZnS d-dots) with high optical quality, pure dopant emission of 55-65% photoluminescence quantum yield, were synthesized in octadecene media with generic starting materials, namely, zinc (manganese) carboxylic acid salts, S powder, and dodecanethiol (DDT) based on a "nucleation doping" strategy. The optical properties and structure of the obtained Mn:ZnS d-dots have been characterized by UV-vis, photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The resulting nearly monodisperse d-dots were found to be of spherical shape with a zinc-blende crystal structure. The influences of various experimental variables, including the reaction temperature for the MnS core nanocluster and ZnS host material, the amount of octadecene (ODE)-S, DDT, as well as Zn/Mn ratio have been systematically investigated. The use of DDT as capping ligand ensured the reproducible access to a stable small-sized MnS core. This paves the way for reproducibly obtaining highly luminescent d-dots. Programmed overcoating temperature for growth of ZnS shell was employed to realize balanced diffusion of the Mn ions in the d-dots.     

Synthesis and characterization of high-quality ZnS, ZnS:Mn2+, and ZnS:Mn2+/ZnS (core/shell) luminescent nanocrystals

High-quality ZnS, ZnS:Mn2+, and ZnS:Mn2+/ZnS (core/shell) nanocrystals (NCs) were synthesized via a high-boiling solvent process and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectra. The monodisperse ZnS NCs (size = 8 nm), which self-assembled into several micrometer-sized domains, were achieved by adopting poly(ethylene glycol) (PEG) in the reaction process (without using a size-selection process). The obtained ZnS:Mn2+ and ZnS:Mn2+/ZnS core/shell NCs are highly crystalline and quasimonodisperse with an average particle size of 6.1 and 8.4 nm, respectively. All of the as-formed NCs can be well dispersed in hexane to form stable and clear colloidal solutions, which show strong visible emission (blue for ZnS and red-orange for ZnS:Mn2+ and ZnS:Mn2+/ZnS) under UV excitation. The growth of a ZnS shell on ZnS:Mn2+ NCs, that is, the formation of ZnS:Mn2+/ZnS core/shell NCs, resulted in a 30% enhancement in the PL intensity with respect to that of bare ZnS:Mn2+ NCs due to the elimination of the surface defects.     

On doping CdS/ZnS core/shell nanocrystals with Mn

This paper presents a mechanistic study on the doping of CdS/ZnS core/shell semiconductor nanocrystals with Mn based on a three-step synthesis, which includes host-particle synthesis, Mn-dopant growth, and ZnS-shell growth. We used a combination of electron paramagnetic resonance spectroscopy (EPR) and inductively coupled plasma atomic emission spectroscopy (ICP) to monitor Mn-doping level and growth yield during doping synthesis at both the dopant-growth and ZnS-shell-growth steps. First, our kinetic study shows that Mn adsorption onto the nanocrystal surface includes the formation of weakly and strongly bound Mn. The formation of weakly bound Mn is associated with a chemical equilibrium between adsorbed Mn species on the nanocrystal surface and free Mn species in growth solution, while the formation of strongly bound Mn exhibits first-order kinetics with an activation-energy barrier of 211 +/- 13 kJ/mol. Second, our results demonstrate that both weakly and strongly bound Mn can be removed from the surface of nanocrystals during ZnS-shell growth. The replacement of strongly bound Mn requires a higher temperature than that of weakly bound Mn. The yield of the replacement of strongly bound Mn is strongly dependent on the temperature of ZnS-shell growth. Third, our results show that the Mn-growth yield is not dependent on the size and crystal structure of nanocrystals. All together, these results suggest a mechanism in which nanocrystal doping is determined by the chemical kinetics of three activation-controlled processes: dopant adsorption, replacement, and ZnS-shell growth.     

Mn:ZnSe/ZnS核-壳结构纳米晶的无膦法制备和光学性质

以溶于十八烯的Se作为Se前驱体,在无膦条件下制备得到了具有较高量子产率的Mn:ZnSe纳米晶.为了进一步提高纳米晶的稳定性和发光强度,运用外延生长的方法进行ZnS壳层包覆并得到了具有核-壳结构的Mn:ZnSe/ZnS纳米晶.X射线衍射、透射电子显微镜及吸收和荧光光谱测试结果表明,该方法合成的Mn:ZnSe纳米晶以及核-壳结构Mn:ZnSe/ZnS纳米晶均为闪锌矿结构,具有良好的单分散性,包覆ZnS外壳层后量子产率可达到60%以上.此外,对ZnS壳层厚度和Mn2+的掺杂量对Mn:ZnSe/ZnS纳米晶发光强度的影响及发光机制也进行了初步讨论.     

Paramagnetic Mn:CdS/ZnS quantum dots: synthesis,luminescence, and magnetic properties

Russian Chemical Bulletin -     

Monodisperse lanthanide fluoride nanocrystals: synthesis and luminescent properties

Three types of high-quality, monodisperse lanthanide fluoride colloidal nanocrystals (NCs) including LnF(3) (Ln = La-Pr), NaLnF(4) (Ln = Sm-Er), and Na(5)Ln(9)F(32) (Ln = Tm-Lu) with two crystal phases (hexagonal and cubic) and a rich variety of morphologies have been synthesized in high boiling organic solvents oleic acid and 1-octadecene, via a thermal decomposition pathway. The as-synthesized NCs were well characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), and photoluminescence (PL) spectra, respectively. It is found that the as-synthesized NCs consist of monodisperse nanoparticles with diverse shapes and narrow size distribution, which can easily disperse in nonpolar cyclohexane solvent. Additionally, a possible mechanism of NC nucleation and growth has been proposed. The results reveal that the formation of monodisperse NCs closely correlates with the inherent nature of lanthanide series from La to Lu. Under 980 nm NIR excitation, as-synthesized Yb(3+)/Ln(3+) (Ln = Er, Tm, Ho)-doped NaGdF(4) and Na(5)Lu(9)F(32) colloidal NCs show the respective characteristic up-conversion (UC) emissions of Er(3+), Tm(3+), and Ho(3+), which are promising for applications in biolabels, bioimaging, displays, and other optical technologies.     

Hybrid approach to the synthesis of highly luminescent CdTe/ZnS and CdHgTe/ZnS nanocrystals

We report the synthesis of highly luminescent CdTe/ZnS and CdHgTe/ZnS core/shell semiconductor nanocrystals (NCs). A hybrid of two synthesis routes leads to novel nanocrystal compositions and small core/shell sizes (4-5 nm) that emit in the far-red and near-infrared regions. These particles exhibit higher resistance to oxidation and photobleaching, have high quantum yields, and could be used for biological labeling and imaging.     

表面修饰CdS和(CdS)ZnS纳米晶的性能研究

在水相中合成了CdS纳米微粒,以ZnS对其进行表面修饰,得到具有核壳结构的(CdS)ZnS水溶性纳米晶。采用红外光谱、X射线衍射(XRD)、透射电镜(TEM)表征其粒度和形貌,紫外-可见吸收光谱(UV)、荧光光谱表征其光学特性。制得的CdS近似呈球形,直径为8nm;CdS纳米颗粒表面经ZnS修饰后,其荧光发射峰强度显著增强,表面态发射减弱。     

One-pot synthesis of highly luminescent InP/ZnS nanocrystals without precursor injection

InP/ZnS core/shell nanocrystals are prepared using a single-step heating-up method relying on the difference in reactivity of the applied InP and ZnS precursors. The obtained particles exhibit size-dependent emission in the range of 480-590 nm, a fluorescence quantum yield of 50-70%, and high photostability.     

Solvothermal synthesis and luminescence properties of CdS:Mn nanorods

CdS:Mn nanorods have been produced via a solvothermal approach in the nonaqueous solvent of ethylenediamine. An absolutely dominant single Mn²⁺ emission originating from the d-d (⁴T₁-⁶A₁) transition was obtained in CdS:Mn nanocrystals at room temperature. The effects of varying reaction temperature, molar ratio of S/Cd, and reaction time on the crystallinity and luminescence of CdS:Mn nanocrystals were systematically investigated. 1% Mn²⁺-doped CdS nanorods without any other additives were synthesized at 130°C for 10 h with an S/Cd molar ratio of 2:1. They show a rod-like shape, and their luminescence intensity around 593 nm is almost the strongest of all the nanorod samples investigated. CdS:Mn nanorods promise potential applications in nanoscale electronic and photonic devices.     

Mn2+-doped ZnS–CdS alloy nanocrystals for the photocatalytic hydrogen evolution reaction

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Synthesis and characterization of highly luminescent CdSe-core CdS/Zn0.5Cd0.5S/ZnS multishell nanocrystals

We report on the preparation and structural characterization of CdSe nanocrystals, which are covered by a multishell structure from CdS and ZnS. By using the newly developed successive ion layer adhesion and reaction (SILAR) technique, we could gradually change the shell composition from CdS to ZnS in the radial direction. Because of the stepwise adjustment of the lattice parameters in the radial direction, the resulting nanocrystals show a high crystallinity and are almost perfectly spherical, as was investigated by X-ray diffraction and electron microscopy. Also, due to the radial increase of the respective valence- and conduction-band offsets, the nanocrystals are well electronically passivated. This leads to a high fluorescence quantum yield of 70-85% for the amine terminated multishell particles in organic solvents and a quantum yield of up to 50% for mercapto propionic acid-covered particles in water. Finally, we present experimental results that substantiate the superior photochemical and colloidal stability of the multishell particles.     

Water-soluble silica-overcoated CdS:Mn/ZnS semiconductor quantum dots

Highly luminescent and photostable CdS:Mn/ZnS core/shell quantum dots are not water soluble because of their hydrophobicity. To create water-soluble quantum dots by an appropriate surface functionalization, CdS:Mn/ZnS quantum dots synthesized in a water-in-oil (W/O) microemulsion system (reverse micelles) were consecutively overcoated with a very thin silica layer ( approximately 2.5 nm thick) within the same reverse micellar system. The water droplet serves as a nanosized reactor for the controlled hydrolysis and condensation of a silica precursor, tetraethyl orthosilicate (TEOS), using an ammonium hydroxide (NH4OH) catalyst. Structural characterizations with transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) indicate that the silica-quantum dot nanocomposites consist of a layered structure. Owing to the amorphous, porous nature of a silica layer, the optical and photophysical properties of silica-overcoated CdS:Mn/ZnS quantum dots are found to remain close to those of uncoated counterparts.     

Radial-position-controlled doping in CdS/ZnS core/shell nanocrystals

In this paper, we report a new doping approach using a three-step synthesis to make high-quality Mn-doped CdS/ZnS core/shell nanocrystals. This approach allows precise control of the Mn radial position and doping level in the core/shell nanocrystals. On the basis of this synthetic advance, we have demonstrated the first example in which optical properties of Mn-doped nanocrystals strongly depend on Mn radial positions inside the nanocrystals. In addition, we have synthesized nanocrystals with a room-temperature Mn-emission quantum yield of 56%, which is nearly twice as high as that of the best Mn-doped nanocrystals reported previously. Nanocrystals with such a high-emission quantum yield are very important to applications such as nanocrystal-based biomedical diagnosis.     

Formation of monodispersed PVP-capped ZnS and CdS nanocrystals under microwave irradiation

Poly (N-vinyl-2-pyrrolidone) (PVP)-capped ZnS, CdS nanoparticles were prepared by a microwave method from Zn(Ac)₂ or Cd(Ac)₂ and thiourea in N,N-dimethylformamide (DMF) solution. The reaction process was monitored by the temporal evolution of the absorption spectrum. With PVP as stabilizer, monodispersed semiconductor nanoparticles, which showed high quantum size effect, have been obtained. Further study showed that the microwave irradiation could influence selectively the nucleation and growing rates of different semiconductor nanoparticles.     

Encapsulation of ZnS:Mn2+ nanocrystals with diblock copolymers

The encapsulation of the nanocrystalline manganese‐doped zinc sulfide (ZnS:Mn) in poly(styrene‐b‐2vinylpyridine) (PS‐PVP) diblock copolymers is reported. Below the critical micelle concentration in the absence of nanocrystals (NCs), inverse micelles of PS‐PVP were induced by adding ZnS:Mn NCs, the presence of which was confirmed by scanning force microscope and dynamic light scattering. In toluene, a PS‐selective solvent, the less‐soluble PVP blocks preferentially surround the ligand‐coated ZnS:Mn NCs. For PS‐PVP encapsulated ZnS:Mn NCs, the ratio of blue emission to orange emission of ZnS:Mn NCs is dependent on both the concentration of PS‐PVP and the solvent quality. The pyridine of PVP blocks form complexes with the Zn atoms via the nitrogen lone pair and thus the sulfur vacancies are passivated. As a result, the defect‐related blue emission is selectively quenched even when the micelles are not formed. As the concentration of PS‐PVP encapsulating the ZnS:Mn NCs increases, the intensity of blue emission decreases. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3227–3233, 2006     

Emission properties of manganese-doped ZnS nanocrystals

We have performed steady-state and time-resolved fluorescence studies on undoped and Mn-doped ZnS nanocrystals with approximately 16 A diameter. While there is no band-edge emission, the intensity of the steady-state blue fluorescence from ZnS surface states decreases upon Mn incorporation, which gives rise to an orange emission. These results show that Mn incorporation competes very effectively with the donor-acceptor surface states for the energy transfer from the electron-hole pair excited across the band gap. In both undoped and doped samples, the time-resolved fluorescence studies establish the presence of a distribution of decay lifetimes possibly due to a number of emission centers in the nanocrystals. A faster short-time decay of the blue emission in the Mn-doped samples compared to that in the undoped sample suggests an additional decay channel for the surface states via an energy transfer from these states to the dopant levels.     

Solid-state synthesis of luminescent silicon nitride nanocrystals

Silicon nitride nanocrystals (NCs) have been prepared via in situ nitridation of magnesium followed by a metathesis reaction with sol-gel derived silica particles. Highly luminescent, freestanding β-Si(3)N(4) NCs with complex surface chemistry dominated by Si-H and N-H moieties were isolated upon etching with hydrofluoric acid.     

One-pot synthesis of high-quality zinc-blende CdS nanocrystals

This paper reports a one-pot synthetic method for producing CdS nanocrystals. We have demonstrated that the nanocrystal nucleation and growth stages can be automatically separated in a homogeneous system with the presence of nucleation initiators. Accelerators used for more than 70 years in rubber vulcanization (i.e., tetraethylthiuram disulfides, and 2,2'-dithiobisbenzothiazole) were found to be effective nucleation initiators for CdS nanocrystal synthesis. The as-prepared CdS nanocrystals are highly monodisperse and possess a zinc blende crystal structure. The quantum yield of the band-gap photoluminescence is up to 12% when the surface-trap emission was totally eliminated after a gentle oxidation under laboratory fluorescent light.