Doped ZnSe Nanocrystals Research Articles

Synthesis and size control via variation of temperature of ZnSe:Fe nanocrystals in a microemulsion assisted hydrothermal system

A reverse micelle microemulsion assisted hydrothermal approach has been developed for synthesizing iron (Fe2+) doped ZnSe nanocrystals (NCs). The synthetic process contains a two-step procedure, a chemical reaction between Zn2+ and Se using a reducing agent NaBH4 in a Triton X-100 surfactant contained microemulsion, followed by a hydrothermal treatment. The ZnSe NCs have a well-defined crystallinity and the Fe2+ ions have been effectively incorporated into the ZnSe matrix as shown by the compositional mappings from scanning transmission electron microscope (STEM). As revealed by transmission electron microscope (TEM) and powder X-ray diffraction (XRD), the as-synthesized nanocrystals have a cubic zinc blende ZnSe structure with high purity and homogeneity. The sizes of the as-synthesized ZnSe:Fe NCs can be well controlled by changing temperature in the hydrothermal treatment procedure. If the temperature in the hydrothermal system is set lower, the synthesized NCs can be rendered smaller. The size of the ZnSe NCs can be controlled over a wide range from 3.6 nm to 10.4 nm with the temperature range from 90 °C to 140 °C, respectively. The correlation of NC size versus temperature is nearly linear, which can be used to predict and tune NC sizes. This manuscript reports for the first time that the size of Fe2+ doped ZnSe nanocrystals can be controlled by hydrothermal treatment after synthesis via a microemulsion process. Controlling size of the ZnSe:Fe nanocrystals is critical for developing new sensor materials and mid-infrared laser media in the infrared region based on tunable optical properties of the nanomaterials.

Influence of Mg on Structural and Optical Properties of ZnSe Nanocrystals Synthesized by Microwave Assisted Technique

Abstract ZnSe is an important semiconductor material having applications in electronic and optoelectronic fields like nonlinear optics, light emitting diodes (LEDs), flat panel displays, lasers, logic gates, transistors, etc. Pure ZnSe and Mg doped ZnSe have been synthesized by microwave assisted method. In the synthesis of pure and Mg doped ZnSe nanoparticles the chemicals used were Zinc Chloride (ZnCl2), Selenium Powder, ethylenediamine and Magnesium Chloride. The synthesized nanoparticles have been characterized to investigate the morphological, structural and optical properties. The crystal structure of the nanocrystallites have been calculated by X- ray diffraction pattern and the size of the crystalliltes have been observed using Debye-Scherrer formula. The crystallite size was obtained around 6 nm for pure ZnSe and around 11 nm for Mg doped ZnSe nanocrystals. UV- Visible spectrophotometer was used for the measurement of optical bandgap. The value of bandgap was found to be 3.42 eV for ZnSe and it was 3.48 eV for Mg-doped ZnSe. The XRD spectrum also confirms the crystallinity of the samples.

Facile synthesis of manganese (II)-doped ZnSe nanocrystals with controlled dimensionality.

Doping is one of the key technologies in modern semiconductor science and industry. However, the synthetic control of doped nanocrystals is difficult to achieve. Here, we report the facile synthesis of manganese (II) doped ZnSe nanocrystals with controlled dimensionality. A strong Lewis acid-base reaction using air-stable and environmentally friendly metal chlorides as precursors can readily produce a large amount of quantum-confined ZnSe:Mn2+ nanocrystals. A combination of primary and secondary amines is used to control the synthetic chemistry, which enables the shape of the doped nanocrystals to be controlled. The final doping concentration of the products can be finely tunable, which is critical for carrier relaxation dynamics. It turns out that the threshold doping level for the maximum photoluminescence intensity of doped nanocrystals highly depends on their shape. Furthermore, this simple synthetic method is extendable to obtain various Mn2+-doped II-VI semiconductor nanocrystals such as CdS:Mn2+ and ZnS:Mn2+. Our study will facilitate the fundamental understanding of the doped semiconductor nanocrystals with different shapes, which is potentially useful for a wide range of applications such as lighting, photocatalysis, and bioimaging.

Mid-Infrared Emission of Transition Metal Co2+-Doped ZnSe Nanocrystals at Room Temperature via Hydrothermal Preparation

A series of Co2+:ZnSe and Co2+:ZnSe/ZnSe nanocrystals emitting mid-infrared (mid-IR) fluorescence centered at around 3.4 and 4.7 μm were successfully synthesized via a simple hydrothermal method. The core/shell structure and post-heat treatment under reducing atmosphere were adopted to decrease the nanocrystal’s surface quenching centers and defects and improve the mid-IR photoluminescence. By analysis of the X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, Fourier transform IR, absorption, and mid-IR emission measurements of serial concentrations of Co2+-doped ZnSe nanocrystals, the crystal phase, grain size, core/shell structure, distribution of Co2+ ions, impurities on the surface, and mid-IR emission performance were investigated in detail. By use of optimum Co2+ concentration, core/shell structure construction, and proper post-heat treatment, the mid-IR emission intensities of nanocrystals were enha...

Temperature- and Mn2+ Concentration-Dependent Emission Properties of Mn2+-Doped ZnSe Nanocrystals.

Mn2+-doped ZnSe nanocrystals (Mn:ZnSe d-dots) with high optical quality-high dopant emission quantum yield with monoexponential dopant-emission decay dynamics-enable systematic and quantitative studies of temperature- and Mn2+ concentration-dependent optical properties of the dopant emission, especially its relationship with magnetic coupling. While temperature-dependent steady-state and transient dopant emission of d-dots with dilute Mn2+ concentrations originated from isolated Mn2+ ions, and can be quantitatively treated as a result of exciton-phonon coupling of isolated paramagnetic emission centers. Dopant emission of d-dots with high Mn2+ concentrations (up to 50% of Zn2+ ions being replaced by Mn2+ ions in the core nanocrystals) are found solely related to magnetically coupled Mn2+ emission. Magnetic coupling effects on steady-state dopant emission of d-dots with high Mn2+ concentrations are much stronger than those observed for doped bulk semiconductors, which is found to follow a strong and universe shell-thickness dependence for the epitaxial ZnSe and/or ZnS shells of the d-dots. By exciting the magnetically coupled Mn2+ ions directly, dopant-emission of d-dots with high Mn2+ concentrations exhibit monoexponential decay dynamics. In addition to this emission channel, a minor channel with slightly longer decay lifetime appears when the host nanocrystals with high Mn2+ concentrations are excited, which is barely visible at room temperature and increases its fraction by decreasing temperature.

Controlling Anisotropic Growth of Colloidal ZnSe Nanostructures.

Semiconductor nanocrystals serve as outstanding model systems for studying quantum confined size and shape effects. Shape control is an important knob for controlling their properties but so far it has been well developed mainly for heavy-metal containing semiconductor nanocrystals, limiting their further widespread utilization. Herein, we report a synthesis of heavy-metal free ZnSe nanocrystals with shape and size control through utilization of well-defined molecular clusters. In this approach, ZnSe nanowires are synthesized and their length and shape control is achieved by introduction of controlled amounts of molecular clusters. As a result of [Zn4(SPh)10](Me4N)2 clusters (Zn4 clusters) addition, short ZnSe nanorods or ZnSe nanodots can be obtained through tuning the ratio of Zn4 clusters to ZnSe. A study using transmission electron microscopy revealed the formation of a hybrid inorganic-organic nanowire, whereby the ligands form a template for self-assembly of ZnSe magic size clusters. The hybrid nanowire template becomes shorter and eventually disappears upon increasing amount of Zn4 clusters in the reaction. The generality of the method is demonstrated by using isostructural [Cu4(SPh)6](Me4N)2 clusters, which presented a new approach to Cu doped ZnSe nanocrystals and provided also a unique opportunity to employ X-ray absorption fine structure spectroscopy for deciphering the changes in the local atomic-scale environment of the clusters and explaining their role in the process of the nanorods formation. Overall, the introduction of molecular clusters presented here opens a path for growth of colloidal semiconductor nanorods, expanding the palette of materials selection with obvious implications for optoelectronic and biomedical applications.

Fabrication of Fe2+:ZnSe nanocrystals and application for a passively Q-switched fiber laser

Fe2+-doped ZnSe nanocrystals (NCs) were fabricated by femtosecond laser ablation in solution (FLAS). The synthesized Fe2+:ZnSe NCs was characterized by XRD and SEM, which demonstrated the nanocrystalline characteristic of the sample. By spin-coating colloidal Fe2+:ZnSe solution on a multiple-layer dielectric film, a Fe2+:ZnSe saturable absorber (SA) was prepared. Employing the as-prepared Fe2+:ZnSe NCs SA in an Er3+:ZBLAN fiber laser, a passively Q-switched laser was successfully demonstrated at ~2.78 μm, with a recorded minimum pulse width of ~0.52 μs, maximum repetition rate of 127.46 kHz, peak power of 7.30 W and pulse energy of 3.81 μJ. This work shows Fe2+:ZnSe NCs can be successfully fabricated by FLAS, and after ablation Fe2+:ZnSe NCs can be efficient miniaturized SA device candidates for mid-IR fiber lasers and have great potential to be prepared on a fluoride fiber end for a compact all-fiber passively Q-switching laser at room temperature.

One-pot room temperature synthesizing Cu- and Mn-doped ZnSe nanocrystals by a rapid photochemical method

In this work, a one-pot, rapid, green and room temperature photochemical synthesis of transition metal (TM; Cu, Mn)-doped ZnSe nanocrystals (NCs) was reported. NCs were successfully characterized using Fourier transform-infrared (FT-IR), photoluminescence (PL) and UV-Visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), X-ray diffractometry (XRD) and energy dispersive X-ray spectra (EDX). FT-IR spectra confirmed the capping of ZnSe by thioglycolic acid (TGA) molecules. XRD and TEM analysis demonstrated zinc blend phase NCs with an average size of around 3 nm. Band gap of ZnSe NCs was about 3.6 eV which it was decreased by increasing the illumination time. PL spectra of ZnSe NCs showed a broad emission with two peaks located at 380 nm and 490 nm related to excitonic and trap states emission, respectively. For ZnSe:Cu NCs, excitonic emission disappeared completely and PL intensity of trap states emission increased with the increase in the Cu[Formula: see text] ion concentration so that for precursor ratio of Cu:Zn 1%, optimal value of PL intensity was obtained. For ZnSe:Mn NCs, the excitonic emission decreased gradually with the increase in the impurity concentration whereas trap state emission increased. Moreover, a peak about 590 nm was appeared from 4T1-6A1 transition of the Mn[Formula: see text] impurity, demonstrating the Mn incorporation inside the ZnSe NCs structure.

Doped Semiconductor-Nanocrystal Emitters with Optimal Photoluminescence Decay Dynamics in Microsecond to Millisecond Range: Synthesis and Applications.

Transition metal doped semiconductor nanocrystals (d-dots) possess fundamentally different emission properties upon photo- or electroexcitation, which render them as unique emitters for special applications. However, in comparison with intrinsic semiconductor nanocrystals, the potential of d-dots has been barely realized, because many of their unique emission properties mostly rely on precise control of their photoluminescence (PL) decay dynamics. Results in this work revealed that it would be possible to obtain bright d-dots with nearly single-exponential PL decay dynamics. By tuning the number of Mn2+ ions per dot from ∼500 to 20 in Mn2+ doped ZnSe nanocrystals (Mn:ZnSe d-dots), the single-exponential PL decay lifetime was continuously tuned from ∼50 to 1000 μs. A synthetic scheme was further developed for uniform and epitaxial growth of thick ZnS shell, ∼7 monolayers. The resulting Mn:ZnSe/ZnS core/shell d-dots were found to be essential for necessary environmental durability of the PL properties, both steady-state and transient ones, for the d-dot emitters. These characteristics combined with intense absorption and high PL quantum yields (70 ± 5%) enabled greatly simplified schemes for various applications of PL lifetime multiplexing using Mn:ZnSe/ZnS core/shell d-dots.

Fast doping of Cu into ZnSe NCs by hydrazine promoted cation exchange in aqueous solution at room temperature.

Controllable doping is an effective way of tuning the properties of semiconductor nanocrystals (NCs). In this work, a simple strategy of fast doping Cu ions into ZnSe NCs under ambient conditions was proposed. The principle of doping is based on hydrazine (N2H4) promoted cation exchange reaction. By direct addition of Cu ion stock solution into the preformed ZnSe NCs, Cu doped ZnSe NCs can be obtained. Furthermore, the emission of doped NCs can be tuned by changing the amount of impurity ion addition. The cation exchange reaction is facilitated by three factors: 1) N2H4 addition, 2) fast impurity ions, and 3) partial stabilizer removal. The proposed cation exchange reaction in aqueous solution could be an alternate route for NC doping as well as synthesis of ionic NCs.

Structural and optical properties of Ni doped ZnSe nanoparticles

In the present work synthesis of ZnSe:Ni nanoparticles using a simple solvothermal method has been discussed. The structural characterizations of as synthesized materials were done by powder X-ray diffraction (XRD), Transmission electron microscope (TEM) and High resolution transmission microscope (HRTEM) imaging techniques, which revealed formation of core–shell nanoparticles with crystallite size 2–4nm. The structural parameters such as lattice constants, internal strain, dislocation density etc. of ZnSe and Ni doped ZnSe nanocrystals were estimated. Nickel doping in ZnSe host is verified by the Raman spectroscopy. Optical properties were diagnosed by UV–vis absorption and photoluminescence (PL) techniques. The observed blue-shift in UV–vis absorption edge of the prepared sample of ZnSe as compared to its value for the bulk counterpart indicates formation of nanosized particles. PL spectra of Ni2+ doped samples indicate red-shift and improved emission intensity.

Zero-Reabsorption Doped-Nanocrystal Luminescent Solar Concentrators

Optical concentration can lower the cost of solar energy conversion by reducing photovoltaic cell area and increasing photovoltaic efficiency. Luminescent solar concentrators offer an attractive approach to combined spectral and spatial concentration of both specular and diffuse light without tracking, but they have been plagued by luminophore self-absorption losses when employed on practical size scales. Here, we introduce doped semiconductor nanocrystals as a new class of phosphors for use in luminescent solar concentrators. In proof-of-concept experiments, visibly transparent, ultraviolet-selective luminescent solar concentrators have been prepared using colloidal Mn(2+)-doped ZnSe nanocrystals that show no luminescence reabsorption. Optical quantum efficiencies of 37% are measured, yielding a maximum projected energy concentration of ∼6× and flux gain for a-Si photovoltaics of 15.6 in the large-area limit, for the first time bounded not by luminophore self-absorption but by the transparency of the waveguide itself. Future directions in the use of colloidal doped nanocrystals as robust, processable spectrum-shifting phosphors for luminescent solar concentration on the large scales required for practical application of this technology are discussed.

Theoretical and experimental investigation of stability and spectra of doped Ag:ZnSe nanocrystals

In experiment, doped Ag:ZnSe nanocrystals (NCs) had better stability than that of ZnSe nanocrystals under ambient atmospheres in the presence of air and light illumination. However, it is difficult to explain the mechanism of better stability of Ag:ZnSe nanocrystals from the experiment perspective for doped nanocrystals are more unstable than corresponding pure nanocrystals in general. Using B3LYP/LANL2DZ method, we have investigated the geometrical structures, bonding characters, and molecular orbitals (MOs) of hexagonal and tetrahedral Ag doped ZnSe structures in theory. The results showed that the good stability of Ag:ZnSe nanocrystals can be attributed to the stronger binding between Ag and Se. Moreover, we have proved that Ag doped ZnSe nanocrystals synthesized in experiment should be substituting doped but not vacuity doped. Substituting Ag doped ZnSe molecules have the same configuration as that of the ZnSe structure, but vacuity doped Ag:ZnSe have completely different configuration than ZnSe structure due to the big size of Ag atom. In addition, through contrast of MO of ZnSe and Ag doped ZnSe, we have testified that Ag easily formed bonds with Se. The high binding energy and high probability of forming bonds with Se atom make Ag doped ZnSe nanocrystals have better stability than that of ZnSe nanocrystals.

Water-soluble Ag:ZnSe nanocrystals with excellent stability via internal doping of donor-type cation impurity

Aqueous internally doped ZnSe nanocrystals (NCs) are a recent promising Cd-free NC system. One major problem for this NC system is the intrinsic poor stability of NCs in aqueous environments due to the promoted oxidation of NC surface ligands by acceptor-type impurity. In this work, we successfully solve this problem by doping a donor-type Ag impurity instead of an acceptor-type impurity inside aqueous ZnSe NCs. Proper doping ratio and solution pH are keys for preparing high quality Ag:ZnSe NCs. Under similar synthesis conditions, as-prepared Ag:ZnSe NCs show quite different optical properties from acceptor-type impurity-doped ZnSe NCs, suggesting the donor nature of Ag impurity. In comparison to the weak stability of acceptor-type impurity-doped ZnSe NCs moreover, as-prepared Ag:ZnSe NCs show strong photochemical and luminescent stability, making this new type of NCs available for LED, optical coding, multicolor bio-imaging and so on.

Highly reactive, flexible yet green Se precursor for metal selenide nanocrystals: Se-octadecene suspension (Se-SUS)

A suspension of fine selenium powder (100 or 200 mesh) in octadecene (Se-SUS) has proven to be a high-performance, versatile, convenient, reproducible, yet green selenium precursor. The advantages of Se-SUS arise from its highly reactive chemical nature and flexibility. These two features made it possible to carry out the synthesis of high quality metal selenide nanocrystals with diverse compositions and structures, including binary, core/shell, transition metal doped, and complex composition nanocrystals. These successes further demonstrated that Se-SUS is a powerful Se precursor for solving a few long-standing challenges in the synthesis of high quality selenide nanocrystals. For instance, Se-SUS was successfully employed as a Se precursor for shell growth in high quality core/shell nanocrystals to replace expensive and highly toxic precursors, such as Se-phosphine and bis-trimethylsilyl selenide, with greatly lowered epitaxial temperatures (as low as 150 °C) to avoid alloying. As another example, Se-SUS enabled “co-nucleation doping” as a means of preparing high quality Mn doped ZnSe nanocrystals with pure, stable, and highly efficient dopant fluorescence. Open image in new window

Material Diffusion and Doping of Mn in Wurtzite ZnSe Nanorods

Light-emitting transition metal ion doped 1D nanorods can be a suitable candidate for fabrication of the advanced opto-electronic-based nanodevices. Among various doped nanocrystals, Mn doped ZnSe nanocrystals are widely studied for their intense Mn d–d emission. However, this is mostly performed in the zinc blende phase of spherical ZnSe quantum dots. But, herein we study the strategy to dope Mn ions in wurtzite phase of 1D ZnSe nanorods. To achieve this, it is essential to control the 1D crystal growth of ZnSe to facilitate the adsorption of dopants. The anisotropic 1D nanostructures are designed following thermally controlled material diffusion process rather than the most widely expected kinetically driven crystal growth protocol, and the dopants are introduced at the appropriate stage of the growth for their adsorption. Using preformed magic size wurtzite ZnSe nanowires as the source material and fragmenting them at higher reaction temperature, ZnSe nanorods with variable aspect ratios are designed. ...

Synthesis of a White-Light-Emitting ZnSe:Mn Nanocrystal via Thermal Decomposition Reaction of Organometallic Precursors

Low-dimensional nanosized semiconductor materials, such as quantum dots, have attracted considerable interest in the past decade. Due to their size-dependent physical and optical properties, these nanosized semiconductor materials offer various technological applications in photoelectronic devices. Among the II-VI semiconductor materials, zinc selenide (ZnSe) is of special interest due to its intense UV blue light emission at 460 nm wavelength with a band gap of 2.7 eV, which has not been observed in other well-known semiconductor materials, such as CdS and CdSe. In addition, it has been reported that when the surface of ZnSe is passivated by another semiconductor nanocrystal layer such as ZnS, a ZnSe/ZnS core shell quantum dot can be formed, and the quantum yield of the ZnSe/ZnS quantum dot is much higher (about 20 times) than that of bare ZnSe nanocrystallite. The preparation method of ZnSe or ZnSe/ZnS quantum dot often includes a thermal decomposition reaction of organometallic precursors in a hot coordinating organic solvent such as trioctylphosphineoxide (TOPO), leading to a hydrophobic surface on the resulting nanocrystal. Previously, in this lab, water-dispersible ZnSe and ZnSe/ZnS nanocrystals were also prepared by exchanging the hydrophobic nanocrystal surface with polar organic ligands such as mercaptoacetic acid (MAA) and ethylenediaminetetraacetic acid (EDTA) molecules. Manganese-ion doped ZnSe nanocrystals have been prepared in various media. In most cases, emission wavelengths of the ZnSe:Mn nanocrystals were far shifted from that of the undoped ZnSe nanocrystal. They usually emit orangecolored lights (580-600 nm) due to the dopant Mn ions. In this paper we report on the synthesis of Mn ion-doped ZnSe nanocrystal via thermal decomposition reaction from organometallic precursors, and unexpected white light emission from the prepared ZnSe:Mn nanocrystal. Figure 1 presents an HR-TEM image of ZnSe:Mn nanocrystal. In the picture, the measured and calculated average particle size was 3.5 nm. In addition, the appearance of distinct lattice planes in the fringe image with an approximate spacing of 3.4 A suggests that all the solid samples were made of single crystals rather than poly-crystalline aggregate mixtures. An energy dispersive X-ray spectroscopy diagram (EDXS, in Fig. 2) was also obtained to confirm the elemental compositions of the ZnSe:Mn nanocrystal in the solid state. The obtained doping concentration of manganese(II) ions in the ZnSe:Mn nanocrystal was 2.3 atomic %. To determine the doping concentration of Mn ions more precisely, Inductively Coupled Plasma-Atomic Emission Spectrometry (ICPAES) analysis was performed. Three trials of the sample measurements revealed that the average elemental proportion of the Mn ions relative to ZnSe parent crystal was Figure 1. HR-TEM image of ZnSe:Mn nanocrystal, the scale bar represents 5 nm.

Redox Brightening of Colloidal Semiconductor Nanocrystals Using Molecular Reductants

Chemical reductants of sub-conduction-band potentials are demonstrated to induce large photoluminescence enhancement in colloidal ZnSe-based nanocrystals. The photoluminescence quantum yield of colloidal Mn(2+)-doped ZnSe nanocrystals has been improved from 14% to 80% simply by addition of an outer-sphere reductant. Up to 48-fold redox brightening is observed for nanocrystals with lower starting quantum yields. These increases are quickly reversed upon exposure to air and are temporary even under anaerobic conditions. This redox brightening process offers a new and systematic approach to understanding redox-active surface "trap states" and their contributions to the physical properties of colloidal semiconductor nanocrystals.

Aqueous synthesis and bio-imaging application of highly luminescent and low cytotoxicity Mn 2+-doped ZnSe nanocrystals

In this paper, Mn 2+-doped ZnSe quantum dots (Mn:ZnSe d-dots) are synthesized successfully by a nucleation–doping method in aqueous solution with 3-Mercaptopropionic acid as the stabilizer and sodium selenite as the Se source for the first time in contrast to the use of oxygen-sensitive NaHSe or H 2Se as Se source. The obtained quantum dots performed strong band-edge luminescence, narrow size distribution and weak trap emission without post-treatments. The results of transmission electron microscopy and X-ray diffraction demonstrated the small particle size (3–4 nm), good monodispersity and ZnSe(S) alloyed structure of as-prepared quantum dots. Finally, the biological application of luminescent Mn 2+-doped ZnSe nanocrystals to PK 15 cell imaging was also illustrated, which showed excellent biocompatibility and low cytotoxicity, implying their potential as a new generation of fluorescent labels for biological assays, tissues, and even in vivo investigations.

Utilizing boron nitride sheets as thin supports for high resolution imaging ofnanocrystals

We demonstrate the use of thin BN sheets as supports for imaging nanocrystals using lowvoltage (80 kV) aberration-corrected high resolution transmission electron microscopy. Thisprovides an alternative to the previously utilized 2D crystal supports of graphene andgraphene oxide. A simple chemical exfoliation method is applied to get few layer boronnitride (BN) sheets with micrometer-sized dimensions. This generic approach of using BNsheets as supports is shown by depositing Mn doped ZnSe nanocrystals directly onto theBN sheets and resolving the atomic structure from both the ZnSe nanocrystals and the BNsupport. Phase contrast images reveal moiré patterns of interference between the beamsdiffracted by the nanocrystals and the BN substrate that are used to determinethe relative orientation of the nanocrystals with respect to the BN sheets andinterference lattice planes. Double diffraction is observed and has been analyzed.