ZnSe Core Research Articles

Constructing ZnTe Spherical Quantum Well for Efficient Light Emission.

ZnTe colloidal semiconductor nanocrystals (NCs) have shown promise for light-emitting diodes (LEDs) and displays, because they are free from toxic heavy metals (Cd). However, so far, their low photoluminescence (PL) efficiency (∼30%) has hindered their applications. Herein, we devised a novel structure of ZnTe NCs with the configuration of ZnSe (core)/ZnTe (spherical quantum well, SQW)/ZnSe (shell). The inner layer ZnTe was grown at the surface of ZnSe core with avoiding using highly active and high-risk Zn sources. Due to the formation of coherently strained heterostructure which reduced the lattice mismatch, and the thermodynamic growth of ZnTe, the surface or interface defects were suppressed. A high PL efficiency of >60% was obtained for the green light-emitting ZnSe/ZnTe/ZnSe SQWs after ZnS outer layer passivation, which is the highest value for colloidal ZnTe-based NCs. This work paves the way for the development of novel semiconductor NCs for luminescent and display applications.

Accumulated photogenerated holes in type-II ZnSe/CdS nanotetrapods for efficient photocatalytic hydrogen evolution

The spatial separation of photogenerated charges and accumulated holes in ZnSe core accelerates hole migration to the surface and the subsequent oxidation reaction, resulting in the exceptional photocatalytic performance of ZnSe/CdS nanotetrapods.

A novel route to fibers with volatile crystalline semiconductor cores part 2: Selenides and phosphides

The flux assisted molten core method, previously employed to fabricate glass-clad crystalline GaAs core fiber, is applied here to realize crystalline selenide and phosphide cores. These semiconductor families show great promise in photonics, optoelectronics, and thermoelectrics, although their volatility has previously prohibited successful fabrication utilizing the conventional molten core method. Through the introduction of a flux, the alloy melt temperature and associated vapor pressure can be sufficiently reduced to permit thermal drawing of the melt inside a glass cladding into fiber. Reported here are demonstrations of crystalline selenide and phosphide cores. Also reported is the CO2 laser post processing of a ZnSe core fluxed with Cu8GeSe6, which resulted in lateral segregation of the ZnSe phase from the flux. X-ray diffraction and electron microscopy were deployed to determine the resulting ZnSe, InP and flux alloy phases and elemental distributions in the fiber core. X-ray diffraction of the InP: flux and most of the ZnSe: flux systems showed intermetallic flux phases that were not present on the phase diagrams reported in the literature. Formation enthalpy and phase diagrams of the intermetallic phases at room temperature were used to determine which observed phases were thermodynamically stable or meta-stable. The results indicate that the influences at play are primarily thermodynamic, but in some cases, kinetics plays a role during the fabrication of these fibers, forming metastable phases. Lastly, a review and discussion of the new demands on cladding glasses is presented for laser post-processing such fibers. This work demonstrates an approach to incorporating volatile and incongruently melting phases, that are not compatible with the standard molten core method, to realize novel in-fiber optoelectronic compositions.

The Strain Effects and Interfacial Defects of Large ZnSe/ZnS Core/Shell Nanocrystals.

The shell growth of large ZnSe/ZnS nanocrystals( is of great importance in the pursuit of pure-blue emitters for display applications, however, suffers from the challenges of spectral blue-shifts and reduced photoluminescence quantum yields. In this work, the ZnS shell growth on different-sized ZnSe cores is investigated. By controlling the reactivity of Zn and S precursors, the ZnS shell growth can be tuned from defect-related strain-released to defect-free strained mode, corresponding to the blue- and red-shifts of resultant nanocrystals respectively. The shape of strain-released ZnSe/ZnS nanocrystals can be kept nearly spherical during the shell growth, while the shape of strained nanocrystals evolutes from spherical into island-like after the critical thickness. Furthermore, the strain between ZnSe core and ZnS shell can convert the band alignment from type-I into type-II core/shell structure, resulting in red-shifts and improved quantum yield. By correlating the strain effects with interfacial defects, a strain-released shell growth model is proposed to obtain large ZnSe/ZnS nanocrystals with isotropic shell morphology.

Highly stable water-soluble ZnSe:Cu quantum dots coated with doubly ZnS shell

Doped ZnSe quantum dots (QDs) have become a potential substitute for Cd-based QDs. In this paper, center-doped monodisperse Cu-doped ZnSe (ZnSe:Cu) QDs are synthesized as the first step in aqueous solution by co-nucleation doping method, and then the double-layered ZnS shell is grown by successive growth. The prepared ZnSe:Cu/ZnS/ZnS QDs have high photoluminescence(PL) intensity and excellent stability in the blue region. The crystal structure has not been changed and the PL intensity maintains 95% after 365 days stored in the air. The outstanding performance of the prepared QDs benefit from the ZnS shell grown outside the core of ZnSe:Cu, especially the second layer of the ZnS shell can effectively limit excitons and Cu in the ZnSe core and isolate O2. The strategy in our work paves a new path to elevate the stability of QDs.

Cation Exchange during the Synthesis of Colloidal Type-II ZnSe-Dot/CdS-Rod Nanocrystals.

Cation exchange is known to occur during the synthesis of colloidal semiconductor heteronanoparticles, affecting their band gap and thus altering their optoelectronic properties. It is often neglected, especially when anisotropic heterostructures are discussed. We present a study on the role of cation exchange inevitably occurring during the growth of anisotropic dot-in-rod structures consisting of a spherical ZnSe core enclosed by a rod-shaped CdS shell. The material combination exhibits a type-II band alignment. Two reactions are compared: the shell-growth reaction of CdS on ZnSe and an exchange-only reaction of ZnSe cores to CdSe. Transmission electron microscopy and a comprehensive set of optical spectroscopy data, including linear and time-resolved absorption and fluorescence data, prove that cation exchange from ZnSe to CdSe is the dominant process in the initial stages of the shell-growth reaction. The degree of cation exchange before significant shell growth starts was determined to be about 50%, highlighting the importance of cation exchange during the heteronanostructure growth.

Blue Lasing from Heavy‐Metal‐Free Colloidal Quantum Dots

AbstractColloidal quantum dots (CQDs) have emerged as promising candidates for solution‐processed optical gain materials. Despite the substantial progress made with Cd or Pb‐contained CQDs, lasing from the environmentally friendly CQDs remains underexplored. Here, the highly emissive ZnSeTe/ZnSe/ZnS and ZnSe/ZnS CQDs is developed. The optical gain and lasing action from such Te‐alloyed and intrinsic ZnSe/ZnS CQDs across the blue spectral region is achieved. Te‐alloyed ZnSe core manifests a quasitype‐II band alignment with the ZnSe intermediate layer, favorable for suppressing the Auger recombination and also lasing wavelength tuning. The transient absorption spectroscopy reveals evidence of a blue optical gain with a threshold of one exciton per dot and a lifetime of 171 ps in the alloyed CQDs. Under a quasicontinuous‐wave pumping, lasing is observed from the blue CQDs in Fabry–Pérot cavities with a lower threshold fluence of 12.5 µJ cm−2 as compared with the traditional counterparts with heavy metals. The demonstration of heavy‐metal‐free CQD lasing opens up new opportunities in the application of CQDs as efficient and environmental‐friendly gain media.

Environmentally friendly, highly efficient, and large stokes shift-emitting ZnSe:Mn2+/ZnS core/shell quantum dots for luminescent solar concentrators

In this study, heavy-metal-free orange light-emitting ZnSe:Mn2+/ZnS doped-core/shell (d-C/S) quantum dots (QDs) were synthesized using a nucleation doping strategy. To synthesize high quality d-C/S QDs with high photoluminescence (PL) quantum yield (QY), the Mn2+ concentration was optimized. The resulting ZnSe:Mn2+(5%)/ZnS d-C/S QDs showed a high PL QY of 83.3%. The optical properties of the synthesized QDs were characterized by absorption and PL spectroscopy. Their structural and compositional properties were studied by X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. After doping Mn2+ into a ZnSe core, the ZnSe:Mn2+/ZnS d-C/S QDs showed a large Stokes shift of 170 nm. The ZnSe:Mn2+/ZnS d-C/S QDs were embedded in a poly(lauryl methacrylate) (PLMA) polymer matrix and the ZnSe:Mn2+/ZnS-based polymer film was fabricated. The fabricated ZnSe:Mn2+/ZnS-PLMA film was highly transparent in the visible spectral region (transmittance > 83.8% for λ ≥ 450 nm) and it exhibited bright orange light under air mass (AM) 1.5G illumination using a solar simulator. The optical path-dependent PL measurement of the ZnSe:Mn2+/ZnS-PLMA film showed no PL band shift and minimal PL decrease under variation of excitation position. These results indicate that the highly efficient and large Stokes shift-emitting ZnSe:Mn2+/ZnS QDs are promising for application to luminescent solar concentrators.

An alloyed mid-shell strategy assisted realization of thick-shell violet-blue ZnSe/ZnSexS1-x/ZnS quantum dots with high color purity

Environmentally friendly zinc selenide (ZnSe) based core/shell quantum dots (QDs) are promising emitting materials for special light sources and displays. However, ZnSe-based core/shell QDs with violet-blue emission that simultaneously possess thick shells and narrow emission linewidths are still challenging thus far. We herein use a gradient ZnSexS1-x/ZnS shell strategy to target this goal. The ZnSexS1-x mid shell helps the lattice strain relaxation during the growth, whereas the wide bandgap ZnS outer shell benefits the optical properties by passivating the emitting cores and improving the quantum confinement effect. These ZnSe core QDs show a unity zinc blende structure and high crystallinity. The optimized ZnSe/ZnSexS1-x/ZnS QDs exhibit tunable emission wavelengths (415–445 nm), narrow photoluminescence linewidths (10.2–14.2 nm), thick shells (>3.9 nm), and high photoluminescence quantum yields (>50%). We postulate that these excellent features are due to the use of alloyed ZnSexS1-x layer at the core/shell interfaces. This study provides a feasible idea for the development of blue ZnSe-based QDs with perfect emitting properties.

Tunable electronic, optical and magnetic characteristics in Mn-doped inverted type-I ZnSe/CdSe core/shell nanocrystals: Atomistic tight-binding model

Employing the atomistic tight-binding theory and sp-d exchange interaction, Mn-doped ZnSe cores encapsulated by a CdSe thin and thick shell are examined as the functional of the applied magnetic fields. The spin-degenerate states are broken in the applied fields because of the sp-d exchange interaction. The electronic, optical and magnetic characteristics of Mn-doped inverted type-I ZnSe/CdSe core/shell nanocrystals are sensitive with the applied magnetic fields caused by the sp-d exchange interaction, while those depend on the shell thickness on account of the quantum confinement phenomena. The optical properties are enhanced in Mn-doped ZnSe/CdSe@Thick core/shell nanocrystals inside the high magnetic fields. For the validation, the calculated Zeeman splitting energies agree well with the experimental data. Overall, these determined materials combine the advantages of nanocrystals and magnetic ions, thus opening alternative opportunities for a wide-ranging applications in the forthcoming.

Restriction‐In‐Motion of Surface Ligands Enhances Photoluminescence of Quantum Dots—Experiment and Theory

AbstractThe relationship between emission and ligand restriction of a series of ZnSe/ZnS quantum dots (QDs) encapsulated in nanoparticles is investigated systematically via experiments and quantum theory. The QDs have a ZnSe core and a ZnS shell, capped with hydrophobic ligands (triotylphosphine oxide/hexadecylamine), allowing them to be entrapped in a model biomembrane, bicelle, made of zwitterionic dipalmitoyl and dihexanoyl phosphatidylcholines and charged dipalmitoyl phosphatidylglycerol. Enhanced photoluminescence is observed upon encapsulation, depending on the QD‐to‐lipid ratio. Transmission electron microscopy and small‐angle X‐ray scattering confirm that QDs are preferably situated at the rim of bicellar discs. A simplified quantum dissipation heat‐bath theory is proposed to correlate the enhancement with slower nonradiative processes caused by the restriction‐in‐motion (RIM) of the surface ligands. However, Förster resonance energy transfer due to QD aggregation counteracts the effect.

Phosphine-Free-Synthesized ZnSe/ZnS Core/Shell Quantum Dots for White Light-Emitting Diodes

Blue-light-emitting ZnSe core (C) and ZnSe/ZnS core/shell (C/S) quantum dots (QDs) were synthesized with phosphine-free precursors by a thermal decomposition method in paraffin oil solvent and applied to QD-converted light-emitting diodes (LEDs). The optical properties of the synthesized ZnSe C and ZnSe/ZnS C/S QDs were characterized by absorption spectroscopy and photoluminescence spectroscopy. Additionally, the quantum efficiency of the QDs was investigated. Their structural properties were studied with X-ray crystallography and transmission electron microscopy. The ZnSe/ZnS C/S QDs showed deep-blue light peaking at 425 nm. The blue-light-emitting ZnSe/ZnS C/S QDs were used as color-converting materials for near-ultraviolet LED-pumped blue LEDs and combined with yellow-light-emitting Zn-Cu-In-S/ZnS C/S QDs to fabricate white LEDs. The white LEDs showed warm white light [(CIE x, CIE y) = (0.4088, 0.3987)], Tc = 3488 K, and Ra = 61.2]. The results indicate that the ZnSe/ZnS C/S QDs have good potential for white light application after further improvements to their optical properties.

Correlating ZnSe Quantum Dot Absorption with Particle Size and Concentration.

The focus on heavy metal-free semiconductor nanocrystals has increased interest in ZnSe semiconductor quantum dots (QDs) over the past decade. Reliable and consistent incorporation of ZnSe cores into core/shell heterostructures or devices requires empirical fit equations correlating the lowest-energy electron transition (1S peak) to their size and molar extinction coefficients (ε). While these equations are known and heavily used for CdSe, CdTe, CdS, PbS, etc., they are not well established for ZnSe and are nonexistent for ZnSe QDs with diameters <3.5 nm. In this study, a series of ZnSe QDs with diameters ranging from 2 to 6 nm were characterized by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), UV-vis spectroscopy, and microwave plasma atomic emission spectroscopy (MP-AES). SAXS-based size analysis enabled the practical inclusion of small particles in the evaluation, and elemental analysis with MP-AES elucidates a nonstoichiometric Zn:Se ratio consistent with zinc-terminated spherical ZnSe QDs. Using these combined results, empirical fit equations correlating QD size with its lowest-energy electron transition (i.e., 1S peak position), Zn:Se ratio, and molar extinction coefficients for 1S peak, 1S integral, and high-energy wavelengths are reported. Finally, the equations are used to track the evolution of a ZnSe core reaction. These results will enable the consistent and reliable use of ZnSe core particles in complex heterostructures and devices.

Bulk-like ZnSe Quantum Dots Enabling Efficient Ultranarrow Blue Light-Emitting Diodes.

Blue-emitting heavy-metal free QDs simultaneously exhibiting photoluminescence quantum yield close to unity and narrow emission line widths are essential for next-generation electroluminescence displays, yet their synthesis is highly challenging. Herein, we develop the synthesis of blue-emitting QDs by growing a thin shell of ZnS on ZnSe cores with their size larger than bulk Bohr diameter. The bulk-like size of ZnSe cores enables the emission to locate in the blue region with a narrow emission width close to its intrinsic peak width. The obtained bulk-like ZnSe/ZnS core/shell QDs display high quantum yield of 95% and extremely narrow emission width of ∼9.6 nm. Moreover, the bulk-like size of ZnSe cores reduces the energy level difference between QDs and adjacent layers in LEDs and improves charge transport. The LEDs fabricated with these high-quality QDs show bright pure blue emission with an external quantum efficiency of 12.2% and a relatively long operating lifetime.

Fabrication of highly efficient pure blue-emitting electroluminescent devices using ZnSe/ZnSexS1-x/ZnS QDs

Since electroluminescent (EL) quantum dots (QDs) are considered a key component of the next-generation display, and large-scale production of environment-friendly QDs is required for their wide use in commercial displays. Therefore, several studies on non-cadmium QDs, such as indium phosphide (InP) QDs in the III-V category, graphene QDs, and copper indium sulfide (CuInS2) or silver indium sulfide (AgInS2) QDs in the I-III-VI2 category, have been conducted owing to their non-toxicity and good optical properties. Subsequently, significant results have been reported for green and red colors. However, for synthesis of blue QDs, pure blue emission in the range of 440-460 nm has been achieved with few materials. Among them, zinc selenide (ZnSe) is a promising candidate for synthesizing blue QDs. However, owing to the wide band gap (2.7 eV) of ZnSe, highly effective QDs were attained in the violet region (420-440 nm). Here, for the first time, we have synthesized ZnSe/ZnSexS1-x/ZnS QDs emitting at a wavelength of 444 nm with high photoluminescence quantum yield (PLQY) of 77.2%. Also, full width at half maximum (FWHM) of 23.3 nm ensured its excellent color purity. Use of a gradient intermediate shell of ZnSeS in the original ZnSe/ZnS QDs was the key factor behind this achievement. The intermediate gradient shell of ZnSeS around the core delocalizes the electrons, weakening the quantum confinement effect (QCE), hence rendering the emission color of the QDs tunable from violet to blue by manipulating the ratio of selenium (Se) and sulfur (S) in the composites. A blue emission peak centered at 452 nm was observed for the quantum dot light-emitting diodes (QD-LEDs) fabricated using the above-mentioned QDs, and an external quantum efficiency (EQE) of 5.32%, current efficiency of 1.51 cd/A, and power efficiency of 0.74 lm/W were reported. Furthermore, our fabricated device exhibited a maximum brightness of 3,754 cd/m2 and a half operational time (LT50) with 100 cd/m2 initial luminance of 1.27 h, which are the highest values of these parameters to be reported till date for a blue QD-LED fabricated using ZnSe core based QDs in pure blue region.

Aluminosilicate glasses for zinc selenide tunable fiber laser cladding

AbstractA set of novel potassium‐calcium‐aluminosilicate glasses are developed to serve as the cladding material for a doped ZnSe core infrared fiber laser. The compositions exhibit high glass transition temperatures between 753°C and 918°C and high thermal expansion coefficients between 5.75 ppm/°C and 8.21 ppm/°C. We demonstrate successful application of these glass compositions as a cladding for a ZnSe tunable fiber laser.

Paired electrosynthesis of ZnSe/ZnS quantum dots and Cu2+ detection by fluorescence quenching

In this work, ZnSe and ZnSe/ZnS quantum-dots (QDs) stabilized by 3-mercaptopropionic acid (MPA) were synthesized by an electrochemical method, in aqueous medium (pH 7). A paired electrolysis (i = 30 mA) of elemental selenium and zinc rod sacrificial anode was carried out in an electrochemical cavity cell, simultaneously producing Se2− (cathode) and Zn2+ (anode), which reacts in the central compartment of the cell, in a controlled way. Nanoparticles were characterized by UV–vis and fluorescence spectroscopy, XRD, and HRTEM; hydrodynamic diameter and zeta potential were also determined. ZnSe-MPA nanoparticles of 3.2 nm average diameter were obtained with a zinc blend crystal structure and luminescence QY of 4.8%. The same electrochemical procedure was carried out to synthesize ZnS-MPA, which was used in the preparation of core/shell QDs. The ZnSe/ZnS-MPA nanoparticle were prepared with diameter of 4.8 nm, and the luminescence quantum yield was increased to 9.8% due to the decrease on the number of surface defects of the ZnSe core. ZnSe/ZnS-MPA presented efficiency for Cu2+ sensing by fluorescence quenching method, showing good sensitivity with LoD = 0.17 μmol L−1. A static quenching mechanism was proposed, based on the quenching rate constant (Kq).

ZnSe/ZnS Core/Shell Quantum Dots with Superior Optical Properties through Thermodynamic Shell Growth.

Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the key strategy for achieving high fluorescence quantum efficiency and essential stability for optoelectronic applications and biotagging with emissive QDs. Herein we investigate the effect of shell growth rate on the structure and optical properties in blue-emitting ZnSe/ZnS QDs with narrow emission line width. Tuning the precursor reactivity modifies the growth mode of ZnS shells on ZnSe cores transforming from kinetic (fast) to thermodynamic (slow) growth regimes. In the thermodynamic growth regime, enhanced fluorescence quantum yields and reduced on–off blinking are achieved. This high performance is ascribed to the effective avoidance of traps at the interface between the core and the shell, which are detrimental to the emission properties. Our study points to a general strategy to obtain high-quality core/shell QDs with enhanced optical properties through controlled reactivity yielding shell growth in the thermodynamic limit.

ZnSe:Te/ZnSeS/ZnS nanocrystals: an access to cadmium-free pure-blue quantum-dot light-emitting diodes.

Cadmium-free quantum dots (QDs) are attracting considerable research attention because of their low toxicity. However, the bandgap of most cadmium-free QDs avoids the pure-blue region, which leads to difficulty in realizing pure-blue quantum-dot light-emitting diodes (QLEDs). In this work, we successfully tuned the emission wavelength of ZnSe/ZnS quantum dots from the violet region (∼420 nm) to the pure-blue region (450-460 nm) by doping Te into the ZnSe core. The ZnSe:0.03Te/ZnSeS/ZnS QD sample with emission at 450 nm and a quantum yield of 30% was the most balanced formula. To overcome the energy gap between the hole-transfer layer and QD layers, a specific hole-transfer layer was developed for normal-structure QLEDs. A QLED with such a structure with ZnSe:0.03Te/ZnSeS/ZnS QDs achieved the pure-blue light emission at 455 nm, a low turn-on voltage of 4.4 V, and an external quantum efficiency of 0.33%. Overall, our cadmium-free QLED achieved pure-blue emission, revealing the potential of ZnSe-based pure-blue QLEDs for future displays.

A Tunneling Dielectric Layer Free Floating Gate Nonvolatile Memory Employing Type-I Core-Shell Quantum Dots as Discrete Charge-Trapping/Tunneling Centers.

A nonvolatile memory with a floating gate structure is fabricated using ZnSe@ZnS core-shell quantum dots as discrete charge-trapping/tunneling centers. Systematical investigation reveals that the spontaneous recovery of the trapped charges in the ZnSe core can be effectively avoided by the type-I energy band structure of the quantum dots. The surface oleic acid ligand surrounding the quantum dots can also play a role of energy barrier to prevent unintentional charge recovery. The device based on the quantum dots demonstrates a large memory window, stable retention, and good endurance. What is more, integrating charge-trapping and tunneling components into one quantum dot, which is solution synthesizable and processible, can largely simplify the processing of the floating gate nonvolatile memory. This research reveals the promising application potential of type-I core-shell nanoparticles as the discrete charge-trapping/tunneling centers in nonvolatile memory in terms of performance, cost, and flexibility.