Uniform Quantum Dots Research Articles

Paper No S2.3: Uniform Quantum Dot Light Emitting Diodes Fabricated by Transfer Mold Method

AbstractHighly uniform quantum dot light emitting diodes (QD LEDs) have been developed by using Transfer Mold method to realize highly efficient and bright displays. The brightness of Transfer Mold QD LEDs is as high as 633 times, being compared with those of QD LEDs by using a flat substrate. Transfer Mold QD LEDs have uniform Mold arrays, QDs in the Molds, and SiO2 layer patterned on the flat area having no Molds. The SiO2 layer blocks the leakage current between cathode and anode. Low leakage current due to QDs in the uniform Mold arrays with the patterned SiO2 layer, improves carrier‐injection efficiency. The QD LEDs by using a flat substrate, have non‐uniform QD's area due to the QD particle's aggregation, and have low brightness because of the high leakage current. The Transfer Mold QD LEDs can realize the highly efficient QD LEDs.

Wide-Range Correlated Color Temperature Light Generation From Resonant Cavity Hybrid Quantum Dot Light-Emitting Diodes

This study presents extremely uniform colloidal quantum dot white light-emitting diodes (QD-WLEDs) that demonstrate a high color rendering index (CRI) and correlated color temperatures (CCTs) ranging from 2500 to 4500 K. Experimental results indicate that the structure of the distributed Bragg reflector (DBR) containing a stopband in the UV region enhances the intensity output of both monochromatic QD-LEDs and QD-WLEDs by reflecting the unconverted UV light back onto the package to excite the QDs further. Furthermore, the angular CCT uniformity of the QD-WLEDs also improved considerably because of the dependence of the DBR structure on the incident angle. The angular CCT deviation in the range of –70° to 70° decreased to 39 K and the CRI of the WLED is higher than 90. The high-CRI and uniform angular CCT QD-WLED containing the DBR demonstrates potential applicability as the lighting source of next-generation display devices and solid-state lighting.

A sensitive photoelectrochemical biosensor for AFP detection based on ZnO inverse opal electrodes with signal amplification of CdS-QDs.

In this work, ZnO inverse opals structure (IOs) based photoelectrochemical (PEC) electrode was fabricated for alpha-fetoprotein (AFP) detection. Then, the uniform CdS quantum dots (QDs) were hydrothermally synthesized, which allowed the binding of AFP and glucose oxidase (GOD) on CdS QDs, forming the AFP-CdS-GOD composite. The competitive immunosensor of AFP and the AFP-CdS-GOD composite with anti-AFP antibodies (Ab) immobilized on FTO (fluorine-doped tin oxide) /ZnO IOs electrode was successfully applied to the detection of AFP. GOD could catalyze glucose to produce hydrogen peroxide (H2O2) acting as an electron donor to scavenge photogenerated holes in the valence band of CdS QDs, reducing the recombination of electrons and holes of CdS QDs. Also the effective energy level matching between the conduction bands of CdS QDs and ZnO widened the range of light absorption, allowing for electron injection from excited CdS QDs to ZnO upon visible light irradiation, which enhanced the photocurrent. The results show that the immunosensor of AFP possesses a large linear detection range of 0.1-500 ng/ml with a detection limit of 0.01 ng/ml. It also exhibits excellent anti-interference property and acceptable stability. This work provides a promising method for achieving excellent photoelectrochemical biosensor detection of other proteins.

Soft contact transplanted nanocrystal quantum dots for light-emitting diodes: effect of surface energy on device performance.

To realize the full-color displays using colloidal nanocrystal quantum dot (QD)-based light emitting diodes (QLEDs), the emissive QD layer should be patterned to red (R), green (G), and blue (B) subpixels on a micrometer scale by the solution process. Here, we introduced a soft contact QD-transplanting technique onto the vacuum-deposited small molecules without pressure to pattern the QD layer without any damage to the prior organic layers. We examined the patternability of QDs by studying the surface properties of various organic layers systematically. As a result, we found that the vacuum-deposited 4,4',4″-tri(N-carbazolyl)triphenylamine (TCTA) layer is suitable for QD-transplanting. A uniform and homogeneous QD patterns down to 2 μm could be formed for all the RGB QDs (CdSe/CdS/ZnS, CdSe@ZnS, and Cd1-xZnxS@ZnS, respectively) with this method. Finally, we demonstrated the R, G, and B QLEDs by transplanting each QD onto the soft TCTA layer, exhibiting higher brightness (2497, 14 102, and 265 cd m(-2), respectively) and efficiency (1.83, 8.07, and 0.19 cd A(-1), respectively) than those of the previous QLEDs fabricated by other patterning methods. Because this pressure-free technique is essential for patterning and stacking the QDs onto the soft organic layer, we believe that both fundamental study and the engineering approach presented here are meaningful for the realization of the colloidal QD-based full-color displays and other optoelectronic devices.

InAs quantum dots as charge storing elements for applications in flash memory devices

Abstract InAs quantum dots (QDs) were grown by metal organic chemical vapor deposition technique to use them as charge storage nodes. Uniform QDs were formed with average diameter 5 nm and height 5–10 nm with a density of 2 × 10 11 cm −2 . The QDs were grown on high-k dielectric layer (ZrO 2 ), which was deposited onto ultra-thin GaP passivated p-GaAs (1 0 0) substrate. A charge storage device with the structure Metal/ZrO 2 /InAs QDs/ZrO 2 /(GaP)GaAs/Metal was fabricated. The devices containing InAs QDs exhibit superior memory window, low leakage current density along with reasonably good charge retention. A suitable electronic band diagram corresponding to programming and erasing operations was proposed to explain the operation.

High-density 1.54 μm InAs/InGaAlAs/InP(100) based quantum dots with reduced size inhomogeneity

Self-assembled InAs quantum dots (QDs) were grown by solid source molecular beam epitaxy. The impact of the growth parameters like the growth temperature of the InGaAlAs nucleation layer, V/III ratio and growth rate during growth of QD layers were carefully investigated by using atomic force microscopy and photoluminescence spectroscopy. The excellent size uniformity of InAs QDs grown on InP substrates are verified by narrow photoluminescence line widths of 17meV for single QD layers and 26meV for stacked QD layers, respectivaly. Both values measured at 10K.

Broadband optical limiting response of a graphene-PbS nanohybrid.

Graphene-based materials have shown promising nonlinear optical properties in the visible range. To extend their nonlinear optical response to the near infrared (NIR) region, we prepared a new nanohybrid consisting of uniform PbS quantum dots (QDs) attached on the reduced graphene oxide, named rGO-PbS, via a facile, low-cost, and phosphine-free method. The rGO-PbS nanohybrid exhibited superior optical limiting properties to either graphene oxide or PbS QDs upon both 532 nm and 1064 nm excitation in the nanosecond laser pulse regime, which is attributed to the synergetic effects stemming from charge transfer between the two components. Meanwhile, the thin films containing the rGO-PbS nanohybrid dispersed in polymethylmethacrylate (PMMA) also showed excellent optical limiting properties with high transparency, implying the potential applications of this hybrid material in broadband nonlinear optical devices.

Luminescence Properties of InAlAs/AlGaAs Quantum Dots Grown by Modified Molecular Beam Epitaxy

Self-assembled InAlAs/AlGaAs quantum dots (QDs) on GaAs substrates were grown by using modified molecular epitaxy beam in Stranski-Krastanov method. In order to study the structural and optical properties of InAlAs/AlGaAs QDs, atomic force microscopy (AFM) and photoluminescence (PL) measurements are conducted. The size and uniformity of QDs have been observed from the AFM images. The average widths and heights of QDs are increased as the deposition time increases. The PL spectra of QDs are composed of two peaks. The PL spectra of QDs were analyzed by the excitation laser power- and temperature-dependent PL, in which two PL peaks are attributed to two predominant sizes of QDs.

Colloidal quantum dot solar cells on curved and flexible substrates

Colloidal quantum dots (CQDs) are semiconductor nanocrystals synthesized with, processed in, and deposited from the solution phase, potentially enabling low-cost, facile manufacture of solar cells. Unfortunately, CQD solar cell reports, until now, have only explored batch-processing methods—such as spin-coating—that offer limited capacity for scaling. Spray-coating could offer a means of producing uniform colloidal quantum dot films that yield high-quality devices. Here, we explore the versatility of the spray-coating method by producing CQD solar cells in a variety of previously unexplored substrate arrangements. The potential transferability of the spray-coating method to a roll-to-roll manufacturing process was tested by spray-coating the CQD active layer onto six substrates mounted on a rapidly rotating drum, yielding devices with an average power conversion efficiency of 6.7%. We further tested the manufacturability of the process by endeavoring to spray onto flexible substrates, only to find that spraying while the substrate was flexed was crucial to achieving champion performance of 7.2% without compromise to open-circuit voltage. Having deposited onto a substrate with one axis of curvature, we then built our CQD solar cells onto a spherical lens substrate having two axes of curvature resulting in a 5% efficient device. These results show that CQDs deposited using our spraying method can be integrated to large-area manufacturing processes and can be used to make solar cells on unconventional shapes.

Improved quantum dot stacking for intermediate band solar cells using strain compensation

We use thin tensile-strained AlAs layers to manage compressive strain in stacked layers of InAs/AlAsSb quantum dots (QDs). The AlAs layers allow us to reduce residual strain in the QD stacks, suppressing strain-related defects. AlAs layers 2.4 monolayers thick are sufficient to balance the strain in the structures studied, in agreement with theory. Strain balancing improves material quality and helps increase QD uniformity by preventing strain accumulation and ensuring that each layer of InAs experiences the same strain. Stacks of 30 layers of strain-balanced QDs exhibit carrier lifetimes as long as 9.7 ns. QD uniformity is further enhanced by vertical ABAB… ordering of the dots in successive layers. Strain compensated InAs/AlAsSb QD stacks show great promise for intermediate band solar cell applications.

Highly uniform, multi-stacked InGaAs/GaAs quantum dots embedded in a GaAs nanowire

We demonstrate a highly uniform, dense stack of In0.22Ga0.78As/GaAs quantum dot (QD) structures in a single GaAs nanowire (NW). The size (and hence emission energy) of individual QD is tuned by careful control of the growth conditions based on a diffusion model of morphological evolution of NWs and optical characterization. By carefully tailoring the emission energies of individual QD, dot-to-dot inhomogeneous broadening of QD stacks in a single NW can be as narrow as 9.3 meV. This method provides huge advantages over traditional QD stack using a strain-induced Stranski-Krastanow growth scheme. We show that it is possible to fabricate up to 200 uniform QDs in single GaAs NWs using this growth technique without degradation of the photoluminescence intensity.

Toward weak confinement regime in epitaxial nanostructures: Interdependence of spatial character of quantum confinement and wave function extension in large and elongated quantum dots

In this paper we present a comprehensive and detailed analysis of carrier/exciton wave function extension in large low-strain ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}$As quantum dots (QDs). They exhibit rather shallow confinement potential with electron/hole localization energy below 30 meV and confinement strength substantially weakened in comparison to typical epitaxial quasi-zero-dimensional semiconductor nanostructures. The aim of this study is to investigate the influence of different factors on the wave function (probability density distribution) for carriers or excitons in this regime, i.e., object shape anisotropy as well as strain, piezoelectricity, and Coulomb interactions, and to identify the physical mechanisms determining the properties of optical emission. To probe the wave function symmetry, polarization-resolved photoluminescence has been performed, and the spatial extensions of the corresponding probability densities have been verified in magneto-optical measurements. The observed diamagnetic coefficients in the range of (15--31) $\ensuremath{\mu}\mathrm{eV}/{\mathrm{T}}^{2}$ reflect large in-plane QD size. These studies also enable us to investigate the importance of light hole states admixture to the valence band ground state in such nanostructures, which can be addressed via the degree of linear polarization of emission as well as the exciton ${g}_{X}$ factor. The linear-polarization-resolved measurements revealed an exceptionally low exciton fine structure splitting of $5 \ensuremath{\mu}\mathrm{eV}$ on average as well as a low emission polarization degree of \ensuremath{-}0.05, with the polarization perpendicular to the QD elongation direction dominating. The increased light hole contribution to the lowest energy hole level is reflected in the decreased exciton ${g}_{X}$ factor (in the range of 0--1) and is consistent with the results of the eight-band k\ifmmode\cdot\else\textperiodcentered\fi{}p modelling. Based on the temperature dependence of the diamagnetic coefficient, the problem of individual QD uniformity has additionally been discussed. To evaluate the impact of the confinment potential and the structure geometry on the optical properties of the QDs, a comparison between the investigated dots and InAs/InGaAlAs/InP quantum dashes exhibiting a much deeper confining potential is presented.

Influence of InxGa1-xAs Underlying Layer on the Structural of the In0.5Ga0.5As Quantum Dots Grown by MOCVD

The single layer In0.5Ga0.5As quantum dots (QDs) were grown on a thin InxGa1-xAs underlying layer by metal-organic chemical vapor deposition(MOCVD) via Stranski-Krastanow growth mode. The effect of different indium composition in the InxGa1-xAs underlying layer was investigated usingatomic force microscopy (AFM). AFM images show that the QDs structures were formed on the surface. The dots formation on the surface changes withdifferent composition of InxGa1-xAs underlying layer. Increasing indium composition in the underlying layer resulted to formation of higher density andsmaller size dots. Several large dots were also formed on the surface. Growing of underlying layer reduces the lattice mismatch between In0.5Ga0.5As andGaAs, and decreases the critical thickness of the dots. This strongly influences the dots nucleation on the surface. Growth of quantum dots usingunderlying layer is one way to modify dot formation in order to achieve uniform QDs of right size and high density, which are essential for QDs deviceapplications.

Quantum dot loading in strong alkaline: improved performance in quantum-dot sensitized solar cells

For the first time, we demonstrate that the conversion efficiency of CdTe quantum-dot sensitized solar cells could be effectively improved by using a strong alkaline environment during deposition of quantum dots (QDs) onto the TiO2 mesoporous electrode. Alkalis play three unique roles during the deposition: (i) decreasing the inter-particle electrostatic force between TiO2 nanoparticles and QDs to improve QD deposition; (ii) spontaneous formation of Cd(OH)2 during the deposition, which contributes to improvement of device efficiency; (iii) enhancing QD stability by hindering ligands' detachment from QD surface. With these advantages, improved QD loading onto a TiO2 photoanode has been achieved, from barely loading to dense, uniform QD loading with an optimized NaOH addition. Using this method, the overall efficiency of CdTe sensitized solar cell exceeds 2.1% when coupled with a Cu2S cathode—an almost 40% increase of efficiency compared with QDs deposited under a relatively low pH environment.

Modeling temperature-dependent shift of photoluminescence peak of In(Ga)As quantum dots with acoustic and optical phonons as two oscillators

We demonstrate that all the available experimental data of temperature (T)-dependent shift of photoluminescence (PL) peak of In(Ga)As quantum dots (QDs) can be fitted successfully by using a two-oscillator model if and only if the whole temperature interval (0–300 K) is divided into a few parts (at most four parts), depending on dispersion degree of the PL peak from a monotonic behavior. Analysis of the numerical results show that excitons mostly interact (inelastically) with acoustic (AC) or optical (OP) phonons separately. Increasing QDs uniformity, by using some improved growth techniques, results in decreasing or removing the sigmoidal behavior, enhancing total AC phonon contribution and the maximum temperature that AC phonons contribute to the T-dependent redshift of the PL peak. Elevation of the zero bandgap (ZBG) energy up to a critical value about 1.4 eV, for In(Ga)As QDs grown using molecular-beam epitaxy, results in enhancement of QD symmetry and total OP phonon contribution and decline of QDs uniformity and total AC phonon contribution, while a rollover happens for further increase of the ZBG. Therefore we find that the highest QD symmetry and the lowest exciton fine structure splitting correspond to this critical value of ZBG, in accordance with previous experimental results.

Self-healing Encapsulation Strategy for Preparing Highly Stable, Functionalized Quantum-Dot Barcodes

Quantum dot (QD) barcodes are becoming an urgent requirement for researchers and clinicians to obtain high-density information in multiplexed suspension (bead-based) assay. However, how to improve the stability of quantum dot barcodes is a longstanding issue. Here, we present a new self-healing encapsulation strategy to generate functionalized uniform quantum dots barcodes with high physical and chemical stability. This efficient and facile strategy could make porous polymer microspheres self-heal to encapsulate QDs via the thermal motion and interaction of the molecular chains. Consequently, the new strategy solved especially the QDs leakage problem and improved the chemical stability under different pH physiological conditions as well as the longtime storage stability. In the meantime, the encoding capacity and the spatial distribution uniformity of quantum dots could be also improved. Furthermore, immunofluorescence assays for alpha fetoprotein (AFP) detections indicated that carboxyl groups on the surface of QD-encoded microspheres could facilitate efficient attachment of biomacromolecules.

Electrospun polymer/quantum dot composite fibers as down conversion phosphor layers for white light-emitting diodes

Color control without severe photoluminescence (PL) quenching is one of main issues in white light emission technology. White light emission was successfully achieved using phosphor layers made of electrospun quantum dot (QD) embedded polymer fibers as color down-conversion layers of blue light-emitting diodes (LEDs). Down-conversion from blue to longer wavelength was characterized by fluorescence microscopy and photoluminescence (PL) spectroscopy. Using orange QD-embedded fiber-based phosphor layers, a broad spectrum of white-light was demonstrated with the CIE coordination of (0.367, 0.367). The QDs in the polymer fiber matrix did not show the aggregation of QDs unlike the QDs in a thin film matrix. Furthermore, from Time-Correlated Single Photon Counting (TCSPC) analysis, the QDs in fiber mats have longer PL lifetime (∼3.95 ns) than that in a thin film matrix (∼3.20 ns) due to the lower aggregation-induced luminescence concentration quenching. Our results suggest that the simple electrospinning method may be a very good method to obtain uniform and bright QD phosphors for white LEDs which can be used for solid-state illumination sources and lighting devices.

One-step vapor diffusion synthesis of uniform CdS quantum dots/reduced graphene oxide composites as efficient visible-light photocatalysts

CdS quantum dots (QDs)/reduced graphene oxide (RGO) composites were synthesized through a one-step vapor diffusion process in the presence of ethylene glycol. The in situ growth of CdS QDs and the reduction of graphene oxide (GO) were completed simultaneously. Fourier transform infrared spectra, X-ray diffraction patterns, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the reduction of GO. Electron microscopy indicated uniform CdS QDs with size around 4–7 nm were well distributed on the RGO sheets. The transient photocurrent response, electrochemical impedance spectroscopy and diffuse reflectance UV-visible spectra of CdS QDs/RGO composites and CdS were tested to explain the role of RGO for the photocatalytic reaction. As-obtained composites exhibited better photocatalytic properties than pure CdS under visible light irradiation. The influence of different contents of GO on photocatalytic performance was also investigated. A possible photocatalytic mechanism of CdS QDs/RGO composites was proposed.

Enhanced solar cell efficiency and stability using ZnS passivation layer for CdS quantum-dot sensitized actinomorphic hexagonal columnar ZnO

We report the photoanodes consisting of CdS quantum-dots (QDs) sensitized actinomorphic hexagonal columnar ZnO (CdS QDs/AHC-ZnO) with ZnS passivation layer are applied for solar cells. Simple chemical solution synthesized AHC-ZnO films on transparent conducting glass substrates, and then, AHC-ZnO is functionalized with uniform CdS and ZnS QDs via successive ionic layer adsorption and reaction (SILAR) method. The as-prepared materials were characterized by XRD, SEM, TEM, UV–vis diffused reflectance absorption spectra and photovoltaic performances analysis. Photovoltaic performances results indicate the quantity of CdS QDs as well as the visible light absorption threshold can be effectively controlled by varying the coating cycles during the SILAR process, and the photocurrent density (Jsc) is greatly improved by increasing the amount of ZnS. By optimizing the AHC-ZnO with the amount of CdS and ZnS, the best efficiency of 1.81% was achieved for solar cell under AM 1.5G illumination with Jsc=7.44mA/cm2, Voc=0.57V and FF=43%.

Ultrasensitive electrochemiluminescent immunoassay for morphine using a gold electrode modified with CdS quantum dots, polyamidoamine, and gold nanoparticles

We report on a novel electrochemiluminescent (ECL) immunoassay for the ultrasensitive determination of morphine by making use of a gold electrode which was modified with a nanocomposite film containing self-assembled polyamidoamine (PAMAM) CdS quantum dots and electrodeposited gold nanoparticles (Au-NPs). The highly uniform and well-dispersed quantum dots were capped with PAMAM dendrimers. Due to the synergistic effect of the modified quantum dots and the electrodeposited Au-NPs, the ECL response is dramatically enhanced. Under optimal experimental conditions, the immunoreaction between morphine and anti-morphine antibody resulted in a decrease of the ECL signal because of steric hindrance. The calibration plot is linear in the morphine concentration range from 0.2 to 180 ng•mL−1, with a detection limit as low as 67 pg•mL−1. The sensor was successfully applied to the determination of morphine in blood plasma. This kind of assay is expected to pave new avenues in label-free drug assays.