Size Of QDs Research Articles

The Luminescence and optical behavior of Cd1-xWxSe QDs synthesized using microwave assisted hydrothermal approach for bioimaging applications

Semiconducting quantum dots are light-emitting nanocrystals that have been flourished as novel kind of fluorescent label for biomedical diagnostics and biomolecular imaging. In comparison to conventional fluorescence probe, quantum dots have exceptional optoelectronic characteristics, narrow and symmetric emission spectra, size-tunable light emission, and wide absorption spectrum that permit the concurrent excitations of numerous fluorescent colors. Furthermore, quantum dots are substantially bright and have high resistance to photo bleaching compared with fluorescent proteins and organic dye. Here, a new family of quantum dots based on Cd1-xWxSe nanocrystals using microwave assisted hydrothermal route has been developed. The X-ray demonstrated the cubic phase of these nanocrystals and the expansion of their lattice dimensions due to the incorporation of W dopant. The TEM inspected the increase of the sizes of the Cd1-xWxSe QDs from 3.5 nm to 10.8 nm due to the increase of W dopant. The W dopant caused redshift of the absorption spectra and reduction of the bandgap from 2.54 eV to 2.32 eV. The inclusion of W dopant in the CdSe improved the intensity of luminescence emission by 8 times with narrow bandwidth and quantum yield of 89 %. These auspicious properties pave the way to use the Cd1-xWxSe QDs as fluorescent label for biomedical diagnostics and biomolecular imaging.

Synthesis, optical properties and tuning size of CdSe quantum dots by variation capping agent

In this study cadmium selenium quantum dots (CdSe QDs) were synthesized with different capping agents namely, l-cysteine, mercaptosuccinic acid and thioglycolic in aqueous solution. The effect of capping agent on the nucleation and particle growth process was investigated. Electronic absorption and emission fluorescence spectroscopy techniques were applied to map out the size evolution of the QDs. The methods of SEM, EDX, XRD and FTIR were also used for characterization of the prepared samples. The average diameters of the synthesized quantum dots were estimated using different methods. Results revealed that the order of particles size was obtained as follow: CdSe-MSA > CdSe-LCY > CdSe-TGA. Results also showed that the TGA has the strongest binding ability with the QDs surface; in contrast MSA has the weakest binding ability. Results indicated that there is a mutual relationship between changing the bandwidth and the rate of growing particle size of the quantum dot, so that the particles with slow growing rate (CdSe-TGA, 12 h) its bandwidth varies smoothly. Also the magnitude of bandwidth of the spectrum is fairly dependent to the size of the CdSe QDs during the growth process. The quantum dots with larger size (CdSe-MSA) have the fluorescence spectra with broader bandwidth. The trend of the particles growth was discussed in details. Results revealed that the kind of capping agents are considerably influence on the nucleation as well as particle growth processes.

Zinc sulfide quantum dots coated with PVP: applications on commercial solar cells

In this study, we report the synthesis and characterization of zinc sulfide quantum dots (ZnS QDs) coated with different concentrations of polyvinylpyrrolidone (PVP), as well as their deployment as luminescent films on the window side of previously characterized commercial silicon solar cells to quantify their influence on the power conversion efficiency (PCE). The synthesis of the semiconductor nanoparticles was carried out by the reaction of zinc nitrate with sodium sulfide in an aqueous solution at room temperature. XRD measurements indicated a cubic sphalerite phase of the QDs crystal structure, which was not modified by the addition of PVP in the synthesis process. However, the PVP concentration was a key parameter to modulate the size distribution and the luminescent intensity of the QDs, suggesting that an increase in the PVP concentration produced a slight decrease in the QDs size and improved their luminescent properties desired for the photovoltaic applications. The obtained nanoparticles presented great absorption of photons with energies above 3.72 eV and a broad intense blue photoluminescent emission centered at around 450 nm, under excitation of ultraviolet light of 325 nm. The implementation of the synthesized ZnS QDs as spectral response enhancer produced improvements on the performance of solar cells, leading to increases of 0.7%, 1.9%, and 6.1% on the efficiencies of commercial polycrystalline solar cells after the deposition of ZnS QDs synthesized without PVP, with 0.05 mM PVP and with 0.10 mM PVP, respectively.

Absorption and emission in the visible range by ultra-small PbS quantum dots in the strong quantum confinement regime with S-terminated surfaces capped with diphenylphosphine

The synthesis and characterization of PbS QDs absorbing and emitting in the visible range is a very challenging task, since it requires the QDs to be within the strong quantum confinement regime (QD size<2.5 nm). It implies not only having “small” QDs, but also stable and monodisperse; characteristics that have been elusive to achieve for many researchers. In the current work, ultra-small PbS QDs (size ~2 nm) were synthesized based on a modification of the Hines method, controlling the reaction time, and adding diphenylphosphine (DPP) which serves as a catalyst and a protective agent in the reaction synthesis. Novel ultra-small PbS QDs with S-terminated surfaces were obtained, which formed at the early stages of the synthesis reaction and are stabilized by the DPP; as it was suggested by the TEM, FTIR and Raman results. The ultra-small PbS QDs display a maximum peak of optical absorption at ~532 nm, with a corresponding optical band gap of 1.82 eV; a maximum peak of emission at ~679 nm, which results in a Stokes shift of 119 nm, smaller than the Stokes shift observed in “larger” PbS QDs. These ultra-small QDs displayed an average size of ~2 nm, with a standard deviation of ~0.3 nm, which was the smallest among the synthesized samples, based on TEM measurements. Finally, the LUMO and HOMO levels were measured by means of cyclic voltammetry and optical absorption spectroscopy. The values of the optical band gap and the energies measured for the LUMO and HOMO levels of these ultra-small PbS QDs were affected by their atomistic surface arrangement and the capping ligand interacting with their surface. Producing variations in their values that doesn’t follow the trends established for quantum confinement effects related to size variation only. Thorough physical and chemical characterization of such ultra-small PbS QDs are crucial in understanding the origin of their optoelectronic properties, which will contribute to better delineate possible future applications.

Tunable luminescence of Cu-In-S/ZnS quantum dots-polysaccharide nanohybrids by environmentally friendly synthesis for potential solar energy photoconversion applications

Colloidal semiconductor nanomaterials (QDs) have been systematically investigated as innovative alternatives for green energy production through solar photoconversion devices. Thus, we developed novel fluorescent QD nanostructures based on Cu-In-S (CIS) quantum dot core and ZnS layer (ZCIS) stabilized with carboxymethyl cellulose (CMC) biopolymer ligand for potential applications in solar energy photoconversion. The physicochemical, optical, morphological, and surface properties of the nanostructures were comprehensively characterized by TEM-EDX, AFM, SAED, XPS, XRD, FTIR, DLS, and zeta potential analyses. The results demonstrated that monodispersed CIS QDs (size 3.5 nm) were produced with adjustable emission properties spanning from visible to NIR by altering the concentrations of metallic cations and sulfides. Moreover, the quantum yield presented a drastic enhancement of over 1500% due to the amalgamation of chemical composition and the passivation of surface defects by the formation of quaternary nanostructures (ZCIS). As a proof of concept, a solid-liquid junction solar cell was built based on the photoanode composed of TiO2 sensitized by ZCIS-CMC nanohybrids, which proved photoelectrochemical activity upon visible light irradiation. Thus, these nanohybrids produced strictly via green chemistry colloidal process can be envisioned as promising candidates for the development of solar cells based on quantum dot-sensitized nanostructures.

Nonlinear Optical Properties of CdSe Quantum Dots Having Different Sizes.

We synthesis different sizes of cadmium selenide quantum dots (CdSe QDs) and their linear and nonlinear optical properties. Size-controlled CdSe QDs are characterized by ultraviolet-visible absorption, photoluminescence, transmission electron microscope (TEM), and Z-scan techniques. Redshift was observed in the linear absorption and photoluminescence with an increase in the size of CdSe QDs. TEM images show the sizes of the QDs are 2.2 nm-4.9 nm. Bandgap tuning CdSe QDs of third-order nonlinear optical properties are investigated at 532 nm with an open aperture Z-scan technique using a picosecond laser. These CdSe QDs can be used as optical limiters.

Stable near-infrared photoluminescence from silicon quantum dot–bovine serum albumin composites

Abstract

Mutual effects of protein corona formation on CdTe quantum dots

Future biomedical applications of nanoparticles will encounter these particles with patients’ serum which might affect the properties and stability of quantum dots and serum proteins at the desired site of action. Therefore, it is essential to clarify the patient-specific serum components, serve as major interaction partners, the spatial distribution of these, and consequently the time-dependent effects of nanoparticle–protein interaction. Here, a biochemical and structural study was performed on the protein corona formation and the corresponding interaction of different sizes of CdTe QDs with human serum proteins to determine if the mutual effects on optical properties by using electrophoresis, chemiluminescence, and fluorescence spectroscopy. The results revealed that interaction with human serum significantly enhanced the stability and photoluminescence of quantum dots. Structural studies of HSA-coated CdTe QDs also showed that corona formation has no adverse effects on protein structure, and the reduction in fluorescence emissions of HSA is due to the direct quenching of aromatics residues by the quantum dot. Improving nanoparticle properties, as well as the lack of structural changes in HSA, can be very useful in biomedical applications and in vivo studies where stability is important.

Size-Dependent and Enhanced Photovoltaic Performance of Solar Cells Based on Si Quantum Dots

Recently, extensive studies have focused on exploring a variety of silicon (Si) nanostructures among which Si quantum dots (Si QDs) may be applied in all Si tandem solar cells (TSCs) for the time to come. By virtue of its size tunability, the optical bandgap of Si QDs is capable of matching solar spectra in a broad range and thus improving spectral response. In the present work, size-controllable Si QDs are successfully obtained through the formation of Si QDs/SiC multilayers (MLs). According to the optical absorption measurement, the bandgap of Si QDs/SiC MLs shows a red shift to the region of long wavelength when the size of dots increases, well conforming to quantum confinement effect (QCE). Additionally, heterojunction solar cells (HSCs) based on Si QDs/SiC MLs of various sizes are presented and studied, which demonstrates the strong dependence of photovoltaic performance on the size of Si QDs. The measurement of external quantum efficiency (EQE) reveals the contribution of Si QDs to the response and absorption in the ultraviolet–visible (UV-Vis) light range. Furthermore, Si QDs/SiC MLs-based solar cell shows the best power conversion efficiency (PCE) of 10.15% by using nano-patterned Si light trapping substrates.

Colour tunability in a bimodal fluorescent hybrid nanostructure UCNPs@AuNPs@QDs

In the present work, lysine modified NaY0.78Er0.02Yb0.2F4 upconversion nanoparticles (UCNPs, positively charged) and lysine modified ZnSe:Mn2+ quantum dots (QDs, positively charged) are attached onto the surface of citrate reduced gold nanoparticles (AuNPs, negatively charged). The gold nanoparticles not only entangle the QDs and the UCNPs, through electrostatic interaction, but also tune the optical properties of UCNPs through the effect of surface plasmon resonance. The hybrid nanostructure gives green emission both through photoluminescence (under UV excitation) and through photon upconversion (under IR light excitation) process. The colour tuning is observed through variation in the size of QDs and through plasmonic effect of gold nanoparticles. In both the cases, the colour of emission gradually changes from green to red. The colour tunability and bi-modal photon conversion property of this material could be useful for its application in the field of bio-imaging and solar energy harvesting.

A novel electrochemical immobilization of Hg in CdSe QDs - modified graphite electrode and its improved performance in voltammetric determination of lead (II) ion

A novel metal ion sensor was fabricated through electrochemically immobilized Hg spheres on surface-unpassivated CdSe quantum dots modified paraffin wax impregnated graphite electrode. The size and surface morphology of the CdSe QDs and CdSe QDs - Hg modified electrode was characterized by transmission electron microscope and field emission scanning electron microscopy. The modified electrode was also characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The modified electrode was used for the determination of trace level lead ion which was preconcentrated onto the modified electrode by differential pulse anodic stripping voltammetry (DPASV). Numerous key variables affecting the current response of lead ion have been optimized. The proposed electrode has a linear concentration range of 5.2 ng/L–4.2 µg/L with a optimized deposition time of 180 s and the detection limit is found to be 1.7 ng/L (S/N = 3). The electroanalytical performance of the modified electrode was successfully tested in a real sample of tap water and sea water spiked with Pb(II) ion. The proposed electrode shows excellent selectivity and sensitivity over other heavy metals, such as copper, zinc, manganese and tin. Hg(II) immobilized CdSe QDs modified electrode has shown great potential application in detecting metal ions at very low concentrations.

Thioglycolic acid modified chitosan: a template for in-situ synthesis of CdSe QDs for cell imaging

We prepared CdSe QDs on thioglycolic acid modified chitosan (CdSe@Chit-TGAc). It showed bright yellow fluorescence under long wave UV radiation. Optical properties and growth of crystal was checked by ultraviolet-visible spectroscopy and photoluminescence spectroscopy. HRTEM and XPS results demonstrated that CdSe QDs were successfully prepared and decorated on thiol modified chitosan templet and size of CdSe QDs was also successfully stabilized within the quantum dot size range. Biocompatibility of CdSe@Chit-TGAc was checked on in-vitro system in two cell lines i.e., normal lung cell line (WI-38) and pulmonary cancerous cell line (A549) which exhibited significant uptake and florescence of CdSe@Chit-TGAc with WI-38. The concentration-dependent cytotoxicity in A549 cells for the QDs was also studied. Based on preliminary bioassays, further mechanistic studies can be done to exploit the properties of CdSe@Chit-TGAc as fluorescent agent for cell imaging, targeted drug delivery, nano-medicine etc.

CdTe quantum dots in a glassy carbon electrochemical platform modified by N-substituted polypyrrole: Increasing the functional active surface for conjugation

We developed a biosensing platform by electropolymerizing 2-(1H-Pyrrol-1-yl)ethanamine on glassy carbon (GC) surfaces, followed by the immobilization of carboxyl-coated CdTe QDs onto the films. The immobilization of QDs was studied by applying two procedures: adsorption (electrostatic interactions) and covalent (using crosslinking agents). At a later stage, the covalent immobilization of IgG antibodies was also tested in platforms prepared with and without QDs. The formation of the polymeric film was confirmed by scanning electron microscopy. Furthermore, all surface modifications were monitored using cyclic voltammetry and electrochemical impedance spectroscopy, and K3[Fe(CN)6]/K4[Fe(CN)6] as the redox probe. The covalent crosslinking showed reproducibility and effective QD immobilization. Our study also showed that the QD coating by carboxylic groups leads to a decrease in the charge transfer process. The nanometric size of QDs and their active surfaces highly increased the functional platform area, enabling a high number of available sites for IgG antibodies. Indeed, in the absence of QDs, the platform was not able to detect the anti-IgG molecules. These results were supported by saturation curves constructed from differential pulse voltammetry. Thus, the use of carboxyl-coated CdTe QDs combined with 2-(1H-Pyrrol-1-yl)ethanamine polymer favored the development of a promising and sensitive biosensing electrochemical platform.

Tuning the optical properties of colloidal Quantum Dots using thiol group capping agents and its comparison

Mercapto Succinic Acid (MSA), Mercapto Propionic Acid (MPA) and Thio Glycolic Acid (TGA) capped Cadmium Telluride Quantum Dots (CdTe QDs) were synthesized by aqueous phase method. The growth mechanisms of the prepared CdTe QDs were observed to be dependent on the molecular structure of the surfactants used, which in turn had impact on the CdTe QDs growth kinetics, size and shape. The functional groups attached to the surfactant were found to get influenced by the diffusion length, electron confinement and charge transfer. Henceforth, the photo physical and electronic properties of these CdTe QDs were also determined by its molecular structure. In addition, the refluxing time during the synthesis played a significant role in defining the CdTe QDs size and its luminescence effect. Based on the selection of surfactants, the luminescence property of CdTe QDs can be altered, which will make these materials a suitable candidate for extensive variety of applications. Using UV–Visible and Photo Luminescence spectrum the growth kinetics of the CdTe QDs with three different capping agents has been explored first time and this can be utilized for different applications.

Hybrid 2D–0D SnS2 Nanoflakes/CTS QDs-Based Broadband (UV-Visible-NIR) Photodetector

This article reports the SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoflakes/(Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) CTS Qds-based hybrid ultrawideband photodetector (PD). The fabricated device detects the illumination in the 300-1100-nm spectrum (UV-Vis-NIR). The synergetic effect of UV detection characteristics of SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoflakes and NIR detection characteristics of CTS Qds is utilized to obtain ultrawideband detection in the UV to NIR region. Low cost and simple solvothermal technique has been used to synthesize good quality of 2D SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoflakes (size ~ 146 nm) and 0D CTS Qds (size, ~ 3.2 nm). In addition to being an active semiconductor for UV light, the SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoflakes layer in the device has an additional task of being a carrier transport layer for the photogenerated carriers in CTS Qds. This 2D/0D (SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoflakes/CTS Qds) PD structure fabricated on a silicon substrate has shown excellent performance in the entire range, that is, UV-Vis-NIR. The sensitivity of the fabricated CTS Qds/SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoflake-based PD was found to be 296.2, 240.7, and 368.05 for illumination wavelengths of 320, 640, and 970 nm, respectively. The reported CTS Qds/SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoflake-based 2D-0D hybrid detector has a responsivity of 53.88, 5.16, and 12.61 A/W for 320, 640, and 970-nm light wavelengths, respectively, and best detectivity has been observed in the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> Jones for UV light.

Study of Physico-Chemical Changes of CdTe QDs after Their Exposure to Environmental Conditions.

The irradiance of ultraviolet (UV) radiation is a physical parameter that significantly influences biological molecules by affecting their molecular structure. The influence of UV radiation on nanoparticles has not been investigated much. In this work, the ability of cadmium telluride quantum dots (CdTe QDs) to respond to natural UV radiation was examined. The average size of the yellow QDs was 4 nm, and the sizes of green, red and orange QDs were 2 nm. Quantum yield of green CdTe QDs-MSA (mercaptosuccinic acid)-A, yellow CdTe QDs-MSA-B, orange CdTe QDs-MSA-C and red CdTe QDs-MSA-D were 23.0%, 16.0%, 18.0% and 7.0%, respectively. Green, yellow, orange and red CdTe QDs were replaced every day and exposed to daily UV radiation for 12 h for seven consecutive days in summer with UV index signal integration ranging from 1894 to 2970. The rising dose of UV radiation led to the release of cadmium ions and the change in the size of individual QDs. The shifts were evident in absorption signals (shifts of the absorbance maxima of individual CdTe QDs-MSA were in the range of 6–79 nm), sulfhydryl (SH)-group signals (after UV exposure, the largest changes in the differential signal of the SH groups were observed in the orange, green, and yellow QDs, while in red QDs, there were almost no changes), fluorescence, and electrochemical signals. Yellow, orange and green QDs showed a stronger response to UV radiation than red ones.

Graphene/TiO2 nanocomposite electrodes sensitized with tin sulfide quantum dots for energy issues

Abstract In this work, graphene (Gr)/TiO2 nanocomposite electrodes sensitized with tin sulfide quantum dots (SnS QDs) for energy issues have been investigated. Different sizes of SnS QDs as non-toxic and earth-abundant semiconducting materials are synthesized onto Gr/TiO2 nanocomposite electrodes using successive ionic layer adsorption and reaction (SILAR) technique for “n” cycles (n: 1 to 8). The structural properties of the prepared SnS(n) QDs photoanodes are studied using an X-ray diffractometer. The XRD measurements ensure the formation of the crystalline structure of orthorhombic SnS QDs. The optical properties of the synthesized SnS(n) QDs photoanodes are characterized using a UV–visible spectrophotometer. The estimated direct (indirect) energy band gap (Eg) of the prepared SnS(n) photoanodes is tuned from 2.36 (1.57) eV to 1.76 (1.30) eV by controlling the number of SILAR cycles from 1 to 8. The photovoltaic performance of the assembled SnS(n) quantum dots sensitized solar cells (QDSSCs) has been studied under a solar power illumination of AM1.5 conditions. The optimal photovoltaic performance of the assembled SnS(n) QDSSCs is achieved at 6 SILAR deposition cycles. As compared with previous studies, enhancement of 48% in the energy conversion efficiency η has been achieved due to the active Gr incorporation. SnS(n) QDSSCs shows high reproducibility and sensitivity undercutting ON-OFF the solar illumination. Gr plays the role of kids slide for the photo-generated electrons and facilitates their transportation.

Switchable Multi-Color Solution-Processed QD-laser

In this paper, for the first time, the switchable two-color quantum dot laser has been realized considering solution process technology, which has both simultaneous and lonely lasing capability exploiting selective energy contacts. Furthermore, both channels can be modulated independently, which is a significant feature in high-speed data transmission. To this end, utilizing superimposed quantum dots with various radii in the active layer provides the different emission wavelengths. In order to achieve the different sizes of QDs, solution process technology has been used as a cost-effectiveness and fabrication ease method. Moreover, at the introduced structure to accomplish the idea, the quantum wells are used as separate selective energy contacts to control the lasing channels at the desired wavelength. It makes the prominent device have simultaneous lasing at different emission wavelengths or be able to lase just at one wavelength. The performance of the proposed device has been modeled based on developed rate equation by assuming inhomogeneous broadening of energy levels as a consequence of the size distribution of quantum dots and considering tunnel injection of carriers into the quantum dots via selective energy contacts. Based on simulation results, the simultaneous lasing in both or at one of two wavelengths 1.31 μm and 1.55 μm has been realized by the superimposition of two different sizes of InGaAs quantum dots in a single cavity and accomplishment of selective energy contacts. Besides, controlling the quantum dot coverage leads to managing the output power and modulation response at the desired wavelengths. By offering this idea, one more step is actually taken to approach the switchable QD-laser by the simple solution process method.

The effects of strain and composition on the conduction-band offset of direct band gap type-I GeSn/GeSnSi quantum dots for CMOS compatible mid-IR light source

A direct band gap type-I Ge0.75Sn0.25/Ge0.682Sn0.158Si0.16 QD heterostructure is proposed for mid-IR light sources. The effects of strain and composition are theoretically investigated to optimize the band offset of the direct band gap GeSn QDs. It is found that the introduction of tensile strain and Si incorporation in the barrier layers are effective to increase the band offset in the conduction band. Besides, the band offset is expected to increase with the increase of Sn composition in QDs and the size of QDs.

Фотолюминесценция гетероструктур CdTe/ZnTe с номинальными толщинами слоев CdTe от 1 до 8 монослоев, выращенных методом атомного наслаивания

The luminescence of CdTe layers with a nominal thickness of 1, 2, 4, and 8 monolayers grown by atomic layer epitaxy in ZnTe matrix was studied. It is shown that layers with a thickness of 1 and 2 monolayers are homogeneous, while layers with a nominal thickness of 4 and 8 monolayers are the planar QD arrays. The sizes of QDs and their size dispersion increase with an increase in the nominal thickness of CdTe layer. The luminescence excitation spectra of CdTe layer in the samples vary significantly. It has been shown that, depending on the energy distance between the CdTe and ZnTe exciton levels, the ratio of contributions to the energy transfer via the exciton ZnTe and charge carriers varies greatly.