ZnSe Core Research Articles

Antimicrobial and biocompatibility of highly fluorescent ZnSe core and ZnSe@ZnS core-shell quantum dots

Present study reports synthesis of Cd-free core-only and core-shell quantum dots (ZnSe of size 3.60 ± 0.12 nm and ZnSe@ZnS of size 4.80 ± 0.20 nm) having excellent fluorescent properties, stability, and aqueous solubility. The fluorescence behavior of these quantum dots (QDs) was utilized for cancer cell imaging and their comparative toxicity in cancerous (HeLa) and normal (HEK-293) cell lines was evaluated. The LC50 parameter of ZnSe (1.8 and 2.6 mg/ml) was significantly lower than that of ZnSe@ZnS (3.8 and 4.5 mg/ml) for the aforesaid cell lines which indicated higher toxicity of the core-only QD in comparison to the core-shell structure. Further, the cellular uptake and fluorescence intensity of core-shell QD was significantly higher. Furthermore, the antimicrobial property of both type of QDs was evaluated on microbes (Escherichia coli and Staphylococcus aureus) and core-shell structure was found to possess higher antimicrobial property towards gram-positive bacteria S. aureus, but was non-toxic to E. coli. These results suggested that surface modification of ZnSe with ZnS shell helps to enhance the fluorescence property and make these more biocompatible for biomedical applications. The non-toxicity towards E. coli also makes it suitable for labeling the microbial cell surface and for mapping the cellular metabolism.

Magnetic field insensitive photoluminescence decay of ZnSe/CdS core/shell type-II colloidal quantum dots

We have synthesized ZnSe/CdS core/shell type-II colloidal quantum dots, where an electron and a hole are separated in the CdS shell and the ZnSe core, respectively. Our theoretical model has revealed that absorbance spectrum of bare ZnSe quantum dots in 2 nm radius becomes broadened with a large redshift (∼1.15 eV) when the electron in ZnSe core is separated by 3.2 nm CdS shell. Also, we found that our type-II QDs are insensitive to an external magnetic field up to 5 T in terms of central emission energy, degree of polarization, and photoluminescence decay time. This can be attributed to the electron–hole charge separation in a type-II structure, whereby the suppressed exchange interaction gives rise to a magnetic insensitivity with a small energy difference between the bright and dark exciton states.

Phosphine-Free, Highly Emissive, Water-Soluble Mn:ZnSe/ZnS Core–Shell Nanorods: Synthesis, Characterization, and in Vitro Bioimaging of HEK293 and HeLa Cells

Phosphine-free, highly luminescent one-dimensional Mn2+ ion-doped ZnSe(core)/ZnS(shell) nanorods (NRs) were synthesized by heating-up method (core) followed by hot injection route (shell). Effect of Mn2+ doping and shell thickness on structural and optical properties is reported. The NRs were formed with wurtzite-structured Mn:ZnSe core (diameter 2.52 nm) encapsulated epitaxially by a wurtzite-structured ZnS shell (thickness of 3.51 nm) with 3.1% lattice mismatch that alters the band alignment of the overall core–shell structure. A redshift was observed in optical absorption and photoluminescence (PL) emission due to an overall size increase with increasing shell thickness. Because of the reduction of defects/traps by surface passivation, the maximum photoluminescence quantum yield (QY) was obtained to be 49.35%. The exciton radiative lifetime for the core–shell NRs (1.678 ms) was more prolonged than that of the core (0.573 ms). A clear dependence of QY and lifetime was established on the Mn2+ content and...

Structural and optical properties of ZnSe:Eu/ZnS quantum dots depending on interfacial residual europium

A multimodal emitter comprising of ZnSe:Eu/ZnS (core/shell) quantum dots (QDs) by adding a ZnS precursor in situ during synthesis. ZnSe/Eu2+/Eu3+/ZnS actives both core and core/shell. QDs prepared with the ZnS precursor displayed a luminescence intensity three times that of ZnSe QDs due to the passivation effect of the Shell. While the core QDs display the 450–550nm emission of Eu2+ (4F65D1→4F7), the core/shell system showed no Eu2+ emission but only the sharp peaks in the red at 579, 592, 615, 651, and 700nm due to the electronic transitions of 5D0→7Fn (n=0–4) depending on leisurely decreased with increased reaction time. These results are in agreement with Eu 3d spectra of XPS analysis results. Microscopic analyses show that the core and core/shell QDs both have a zinc blende structure, and their respective sizes were about 3.19 and 3.44nm. The lattice constant in the central portion of the core/shell QDs are around d111=3.13Å, which is between the outside and inside ring patterns (d111=3.27 and 3.07Å, respectively). This shows the effective over-capping of shell onto the core QDs. The core/shell structure may contain Eu2O3 bonding the over-coated ZnS surface on the Eu3+-doped ZnSe core.

Phosphine-free synthesis and characterization of type-II ZnSe/CdS core-shell quantum dots

A phosphine-free route for synthesis of type-II ZnSe/CdS core-shell quantum dots, using green, low cost and environmentally friendly reagents and phosphine-free solvents such as 1-octadecene (ODE) and liquid paraffin has been reported. Hot-injection technique has been used for the synthesis of ZnSe core quantum dots. The CdS shell quantum dots prepared by reaction of CdO precursor and S powder in 1-octadecene (ODE). The ZnSe/CdS core-shell quantum dots were synthesized via successive ion layer adsorption and reaction (SILAR) technique. The characterization of produced quantum dots were performed by absorption and fluorescence spectroscopy, X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). The results showed the formation of type-II ZnSe/CdS core-shell quantum dots with FWHM 32 nm and uniform size distribution.

Purple-, Blue-, and Green-Emitting Multishell Alloyed Quantum Dots: Synthesis, Characterization, and Application for Ratiometric Extracellular pH Sensing

We report the synthesis of a series of CdxZn1–xSe/CdyZn1–yS/ZnS and ZnSe/CdyZn1–yS/ZnS multishell alloyed luminescent semiconductor quantum dots (QDs) with fluorescence maxima ranging from 410 to 530 nm which cover the purple, blue, and green portion of the spectrum. Their subsequent surface modification to prepare water-soluble blue-emitting QDs, characterization, and application for ratiometric pH sensing in aqueous buffers and in an extracellular environment are further described. QDs were synthesized starting from ZnSe cores, and the fluorescence peak positions were tuned by (i) cation exchange with cadmium ions and/or (ii) overcoating with CdyZn1–yS layers. The as-prepared QDs had reasonably high fluorescence quantum yields (∼30–55%), narrow fluorescence bands (fwhm ∼25–35 nm), and monodispersed semispherical shapes. Ligand exchange with hydrophilic compact ligands was successfully carried out to prepare a series of water-soluble blue-emitting QDs. QDs coated with the hydrophilic compact ligands pres...

ZnSe core and ZnSe@ZnS core-shell quantum dots as platform for folic acid sensing

This report demonstrates a quantum dot (QD)-based selective and fast sensor platform for detection of folic acid (FA). This electrochemical platform provides a good linear relation between the anodic and cathodic peak currents (i pa and i pc ) in the FA concentration range of 12 to 96 nM, and the minimum detection limit (MDL) achieved was 10 nM. As an extension, absorbance and fluorescence methods were also used for the detection of FA in solutions. Core-shell QDs provided better binding than core-only ZnSe quantum dots, and showed twofold increment in binding constant. A detailed comparative evaluation of the three methods (absorbance, fluorescence, and electrochemical) is presented vis-a-vis real samples. Therefore, in principle absorbance and fluorescence spectroscopy can also be used for detecting folic acid with high selectivity and sensitivity. The MDL can be extended to be 4–7 nM level by using fluorescence and absorbance spectroscopy. FA metabolism occurs in the intestine, where the pH conditions are basic. Hence, sensing of FA under physiological conditions is relevant, which was achieved in our case. Earlier methods have reported sensing under acidic or neutral pH conditions. Considering the importance of folic acid in physiology, the significance of the present study can be hardly stressed.

Composition and strain effects in Type I and Type II heterostructure ZnSe/Cd(Zn)S and ZnSe/Cd1-xZnxS core/shell quantum dots

We investigated the effect of ternary shell alloy composition on the bandgap and diameter of core of ZnSe/Cd1−xZnxS heterostructure core/shell quantum dots, which were synthesized by using a simple colloidal technique. Characterization by using the x-ray diffraction (XRD), transmission electron microscopy (TEM), UV–Vis absorption and fluorescence emission spectroscopic techniques indicate that (i) there is a transition of ZnSe/Cd0.6Zn0.4S Type-I heterostructure (electrons and holes tend to localize in core) to ZnSe/Cd0.75Zn0.25S quasi-Type-II heterostructures (holes tend to localized in the core and electrons are delocalized) and (ii) then after large red shift and Stock-shift in PL emission spectra but not a distinct absorption peak in UV spectra become noticeable in ZnSe/Cd0.75Zn0.25 S quasi-Type II and ZnSe/CdS Type II heterostructures (electrons are localized in core and holes are localized in shell). Furthermore, the increase of Cd:S ratio in shell alloy composition shifts the XRD peaks to lower 2θ degrees which corresponds to tensile strain in the ZnSe core. Finally, the hydrostatic interfacial strain has effect on the squeezing or stretching the capped core: A decrease of compressive force on core from ZnSe/ZnS to tensile force in ZnSe/CdS with increase in Cd:S ratio indicates that transition of compressive strain to tensile strain takes place with the transition from Type-I to II heterostructure.

Interaction of plasma proteins with ZnSe and ZnSe@ZnS core-shell quantum dots

Herein, we report, interaction of water-soluble ZnSe core only (size=3.60±0.12nm, zeta potential=−45mV), and ZnSe@ZnS core-shell (size=4.80±0.20nm, zeta potential=–38mV) quantum dots (QDs) with three globular plasma proteins (Human Serum Albumin (HSA), Bovine Serum Albumin (BSA) and β-lactoglobulin (β-Lg)). It was found that QDs effectively quenched the fluorescence of proteins that was size-dependent. The nature of quenching was static which resulted from the generation of QDs-Protein complexes. It was found that the binding affinity followed: BSA>HSA>β-Lg for both ZnSe and ZnSe@ZnS samples. Interestingly, the secondary and tertiary structure of proteins remained unaffected after interactions.

Cadmium-free aqueous synthesis of ZnSe and ZnSe@ZnS core–shell quantum dots and their differential bioanalyte sensing potential

Herein we report a facile and cadmium-free approach to prepare water-soluble fluorescent ZnSe@ZnS core–shell quantum dots (QDs), using thioglycolic acid (TGA) ligand as a stabilizer and thiourea as a sulfur source. The optical properties and morphology of the obtained core–shell QDs were characterized by UV–vis and fluorescence spectroscopy, transmission electron microscopy (TEM), energy-dispersive x-ray analysis (EDX), x-ray diffraction (XRD), electrophoresis and dynamic light scattering (DLS) techniques. TEM analysis, and electrophoresis data showed that ZnSe core had an average size of 3.60 ± 0.12 nm and zeta potential of −38 mV; and for ZnSe@ZnS QDs, the mean size was 4.80 ± 0.20 nm and zeta potential was −45 mV. Compared to the core ZnSe QDs, the quantum yield of these core–shell structures was higher (13% versus 32%). These were interacted with five common bioanalytes such as, ascorbic acid, citric acid, oxalic acid, glucose and cholesterol which revealed fluorescence quenching due to concentration dependent binding of analytes to the core only, and core–shell QDs. The binding pattern followed the sequence: cholesterol < glucose < ascorbic acid < oxalic acid < citric acid for ZnSe, and cholesterol < glucose < oxalic acid < ascorbic acid < citric acid for core–shell QDs. Thus, enhanced binding was noticed for the analyte citric acid which may facilitate development of a fluorescence-based sensor based on the ZnSe core-only quantum dot platform. Further, the hydrophilic core–shell structure may find use in cell imaging applications.

Synthesis and characterization of CuInSe2 core–shell quantum dots

A synthesis of 1-dodecanetiol stabilized colloidal quantum dots of CuInSe2 exhibiting photoluminescence in the range of 700–900 nm has been described. The effect of the shell on the energy levels of electrons in CuInSe2–ZnS and CuInSe2–ZnSe core–shell quantum dots has been investigated by quantum mechanical calculations.

Aqueous-phase synthesis and color-tuning of core/shell/shell inorganic nanocrystals consisting of ZnSe, (Cu, Mn)-doped ZnS, and ZnS

We report synthesis of colloidal nanocrystals based on ZnSe core, (Cu,Mn)-doped ZnS inner-shell, and ZnS outer-shell by using an eco-friendly method and their optical properties. Synthesis of core/shell/shell nanocrystals was performed by using a one-pot/three-step colloidal method with 3-mercaptopropionic acid as a stabilizer in aqueous phase at low temperature. A double-shell structure was employed with inner-shell as a host for doping and outer-shell as a passivation layer for covering surface defects. Copper and manganese were introduced as single- or co-dopants during inner-shell formation, providing an effective means to control the emission color of the nanocrystals. The synthesized nanocrystals showed fluorescent emission ranging from blue to green, to white, and to orange, adjusted by doping components, amounts, and ratios. The photoluminescence quantum yields of the core/doped-shell/shell nanocrystals approached 36%.

Photogenerated carriers transport behaviors in L-cysteine capped ZnSe core-shell quantum dots

The photoexcited carrier transport behavior of zinc selenide (ZnSe) quantum dots (QDs) with core–shell structure is studied because of their unique photoelectronic characteristics. The surface photovoltaic (SPV) properties of self-assembled ZnSe/ZnS/L-Cys core–shell QDs were probed via electric field induced surface photovoltage and transient photovoltage (TPV) measurements supplemented by Fourier transform infrared, laser Raman, absorption, and photoluminescence spectroscopies. The ZnSe QDs displayed p-type SPV characteristics with a broader stronger SPV response over the whole ultraviolet-to-near-infrared range compared with those of other core–shell QDs in the same group. The relationship between the SPV phase value of the QDs and external bias was revealed in their SPV phase spectrum. The wide transient photovoltage response region from 3.3 × 10−8 to 2 × 10−3 s was closely related to the long diffusion distance of photoexcited free charge carriers in the interfacial space–charge region of the QDs. The strong SPV response corresponding to the ZnSe core mainly originated from an obvious quantum tunneling effect in the QDs.

ZnO-core/ZnSe-shell nanowire UV photodetector

ZnO–ZnSe core–shell nanowires were synthesized by the thermal evaporation of a mixture of ZnO and graphite powders and ZnSe powders sequentially. Subsequently, multiple networked ZnO–ZnSe core–shell nanowire photodetectors were fabricated. The diameters and thicknesses of the ZnO-core/ZnSe-shell nanowires ranged from 50 to 150 nm and from 15 to 25 nm, respectively, and the lengths of the nanowires ranged up to a few hundreds of micrometers. Our results showed that the photoconductivity and response time of the ZnO nanowires were enhanced significantly and reduced, respectively, by encapsulating them with a ZnSe thin film. Under a 2 V applied voltage, the dark current and photocurrent of the ZnO NW photodetector were 2.04 × 10−7 A and 9.94 × 10−6 A, respectively. In contrast, the dark current and photocurrent of the ZnO-core/ZnSe-shell nanowire photodetector were 7.50 × 10−8 A and 1.75 × 10−5 A, respectively. The rising and decaying times of the core–shell nanowires were 6.2 and 5.8 s, respectively, whereas both the rising and decaying times of the pristine ZnO nanowires were 7.3 s. The photoconduction mechanism in the core–shell nanowires is discussed.

Tunable luminescent emission characterization of type-I and type-II systems in CdS–ZnSe core–shell nanoparticles: Raman and photoluminescence study

We used wet chemical methods to synthesize core–shell nanocrystalline samples CdS(d)/ZnSeN, where d = 3–6 nm and N = 1–5 are the size of CdS cores and the number of monolayers grown on the cores, respectively. By annealing typical CdS(d)/ZnSeN samples (with d = 3 and 6 nm and N = 2) at 300 °C for various times tan = 10–600 min, we created an intermediate layer composed of Zn1−xCdxSe and Cd1−xZnxS alloys with various thicknesses. The formation of core–shell structures and intermediate layers was monitored by Raman scattering and UV–vis absorption spectrometers. Careful photoluminescence studies revealed that the as-prepared CdS(d)/ZnSeN samples with d = 5 nm and N = 2–4, and the annealed samples CdS(3 nm)/ZnSe2 with tan ≤ 60 min and CdS(6 nm)/ZnSe2 with tan ≤ 180 min, show the emission characteristics of type-II systems. Meanwhile, the other samples show the emission characteristics of type-I systems. These results prove that the partial separation of photoexcited carriers between the core and shell is dependent strongly on the engineered core–shell nanostructures, meaning the sizes of the core, shell, and intermediate layers. With the tunable luminescence properties, CdS–ZnSe-based core–shell materials are considered as promising candidates for multiple-exciton generation and single-photon sources.

Green synthesis of ZnSe and core–shell ZnSe@ZnS nanocrystals (NCs) using a new, rapid and room temperature photochemical approach

In this work, ZnSe and core–shell ZnSe@ZnS nanocrystals (NCs) were synthesized using a one-pot, rapid and room temperature photochemical method. UV illumination provided the required energy for the chemical reactions. Synthesized NCs were characterized using X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM), UV–vis and photoluminescence (PL) spectroscopy. XRD pattern indicated cubic zinc blende structure for ZnSe NCs and the TEM image indicated round-shaped particles, most of which had a diameter of about 3nm. Band gap of ZnSe NCs was obtained as about 3.6eV, which was decreased by increasing the illumination time. Synthesized NCs indicated intensive and narrow emission in the UV-blue area (370nm) related to the excitonic recombination and a broad band emission with a peak located at about 490nm originated from the DAP (donor–acceptor pairs) recombination. ZnS shell was grown on ZnSe cores using a reaction based on the photo-sensitivity of Na2S2O3. For ZnSe@ZnS core–shell NCs, XRD diffraction peaks shifted to higher angles. TEM image indicated a shell around cores and most of the ZnSe@ZnS NCs have a diameter of about 5nm. After the ZnS growth, ZnSe excitonic emission shifted to the longer wavelength and PL intensity was increased considerably. PL QY was obtained about 11% and 17% for ZnSe and ZnSe@ZnS core–shell QDs respectively.

Enhanced Electron Lifetime of CdSe/CdS Quantum Dot (QD) Sensitized Solar Cells Using ZnSe Core–Shell Structure with Efficient Regeneration of Quantum Dots

Research on development of efficient passivation materials for high performance and stable quantum dot sensitized solar cells (QDSCs) is highly important. While ZnS is one of the most widely used passivation material in QDSCs, an alternative material based on ZnSe which was deposited on CdS/CdSe/TiO2 photoanode to form a semi-core/shell structure has been found to be more efficient in terms of reducing electron recombination in QDSCs in this work. It has been found that the solar cell efficiency was improved from 1.86% for ZnSe0 (without coating) to 3.99% using 2 layers of ZnSe coating (ZnSe2) deposited by successive ionic layer adsorption and reaction (SILAR) method. The short circuit current density (Jsc) increased nearly 1-fold (from 7.25 mA/cm2 to13.4 mA/cm2), and the open circuit voltage (Voc) was enhanced by 100 mV using ZnSe2 passivation layer compared to ZnSe0. Studies on the light harvesting efficiency (ηLHE) and the absorbed photon-to-current conversion efficiency (APCE) have revealed that the Z...

Single-phase dual emissive Cu:CdS–ZnSe core–shell nanocrystals with “zero self-absorption” and their application in white light emitting diodes

Single phase dual emissive Cu:CdS–ZnSe NCs exhibited flexible two emission bands in the whole visible region. This novel phosphor shows potential for use as color convertors in white light emitting diodes (WLEDs).

Tunable Band Gap and Conductivity Type of ZnSe/Si Core-Shell Nanowire Heterostructures.

The electronic properties of zincblende ZnSe/Si core-shell nanowires (NWs) with a diameter of 1.1–2.8 nm are calculated by means of the first principle calculation. Band gaps of both ZnSe-core/Si-shell and Si-core/ZnSe-shell NWs are much smaller than those of pure ZnSe or Si NWs. Band alignment analysis reveals that the small band gaps of ZnSe/Si core-shell NWs are caused by the interface state. Fixing the ZnSe core size and enlarging the Si shell would turn the NWs from intrinsic to p-type, then to metallic. However, Fixing the Si core and enlarging the ZnSe shell would not change the band gap significantly. The partial charge distribution diagram shows that the conduction band maximum (CBM) is confined in Si, while the valence band maximum (VBM) is mainly distributed around the interface. Our findings also show that the band gap and conductivity type of ZnSe/Si core-shell NWs can be tuned by the concentration and diameter of the core-shell material, respectively.

Facile synthesis and characterization of highly luminescent UV-blue-emitting ZnSe/ZnS quantum dots via a one-step hydrothermal method

The synthesis of water-soluble blue-emitting quantum dots (QDs) in aqueous solutions has attracted much attention recently, and here we report the synthesis of high-quality core/shell ZnSe/ZnS QDs through a one-step hydrothermal route with thiol ligand N-acetyl-L-cysteine (NAC) as the stabilizer. The impacts of various synthesis factors on the fluorescence properties of the prepared QDs have been systematically investigated. Under optimal conditions, the as-prepared QDs exhibit high photoluminescence quantum yield (∼39%), narrow full-width at half-maximum, and high photostability, due to the formation of a protective ZnS shell on the ZnSe core through the decomposition of NAC under high temperature. The core/shell structure of the ZnSe/ZnS QDs was verified by UV irradiation experiment, X-ray powder diffraction, and electron diffraction spectroscopy.