Specific Quantum Dots Research Articles

One-step fast fabrication of multi-layer quantum dot diffusion plate for stable display and ultra-long life, a novel quantum dot packaging strategy

Quantum dots (QDs), renowned for their distinctive optical and physical attributes, are extensively employed in diverse fields such as lighting, displays, electronics, sensors, and solar cells. Nonetheless, the susceptibility to light, oxygen, and moisture is a major obstacle to the development of QDs. Achieving simple, efficient, and universal modification or encapsulation for enhanced stability remains a challenge. Herein, a one-step fast fabrication of multi-layer quantum dot diffusion plate (QD-DP) with excellent barrier and optical properties was proposed, which is alternately stacked by MS-EVOH and MS-QDs layers. The barrier phase morphology undergoes a fiber-flake transition induced by the bidirectional tensile force field during multiplication, and layer-by-layer encapsulation synergistically enhances the barrier performance, photo-oxygen stability, thermal-oxygen stability, and fluorescence lifetime of QD-DP in backlight applications. Under high-power UV irradiation, the fluorescence intensity ofMS/MS-QD 128L dropped to 90 % (T90) in only about 10 minutes, while the T90 of MS-E/MS-QD 128L was close to 216 hours, improved more than 1200 times. Concurrently, the quantum dot diffusion plate (QD-DP), characterized by an alternating multi-layer structure, demonstrated an elevated backlight color gamut value of ∼ 120 % NTSC, outperforming conventional quantum dot light emitting diodes (QLED). Compared with existing QDs backlight modules, a simpler structure, more efficient preparation process, and longer fluorescence lifetime of QD-DP were achieved.Moreover, unlike prevalent modification or packaging technologies that only target specific QD types, our alternating multilayer coextrusion packaging technology is applicable to all kinds of QDs and is simple, efficient, and universal.It was believed that this work could provide new opportunities for all kinds of QDs encapsulation and application in many fields and will be the dawn of large-scale commercialization of QDs.

Orbital Mixing between Colloidal Quantum Dots and Surface-Bound Molecules

Orbital mixing is paramount to chemistry as it plays a central role in bond formation. It is also important for technologies such as molecular doping of polymers, where the concept of fractional charge transfer is essentially orbital mixing between dopants and hosts. Likewise, it would be both fundamentally interesting and technologically relevant to investigate orbital mixing in emerging hybrid materials containing both inorganic and organic moieties. Here we report experimental observation of orbital mixing between valence band levels of strongly confined PbS quantum dots (QDs) and lowest unoccupied molecular levels of surface-bound high-electron affinity molecules (F4TCNQ), manifested as both an absorption blue-shift of PbS and the emergence of visible and infrared signatures of the fractional charge-transfer species of F4TCNQ. The degree of mixing can be controlled by varying the QD size or by varying the molecule/QD ratio for a specific QD size and can be quantitatively reproduced by a nondegenerate, two-level perturbation model.

(Invited) Chemical Modulation of Precursor Reactivity for Systematic Synthesis of High-Quality Quantum Dots

With their size-dependent electronic properties, solution processability, and synthetic tunability, quantum dots (QDs) have been heavily studied for fundamental research and commercial applications. The performance of QD-based devices and QD probes are greatly affected by their size distribution and structural quality. To date, extensive studies have investigated size control, allowing us to grow QDs with less than a single monolayer difference in their size variation. In contrast, high quality QD growth is achieved mostly by trial-and-error and a reaction condition optimized for a specific QD system cannot be adapted to QDs of different sizes or materials. To address this critical gap in QD synthesis, we first identified the key parameters contributing to the structural quality of QDs and examined how they affect QD growth. Our study shows that the reaction temperature governs the instability of the QD surface and the level of lattice relaxation, while precursor reactivity impacts the reaction kinetics at the surface as well as satellite particle formation. Systematic optimization of growth conditions requires independent tuning of these two parameters. Unfortunately, conventional precursors do not allow temperature-independent modulation of their reactivity, requiring new precursors to be synthesized for different reaction conditions. This approach is inherently low throughput and labor-intensive.To enable temperature-independent modulation of precursor reactivity, we designed a new sulfur precursor that has a reactivity that can be chemically tuned in a predictable manner. The additives used for this reaction are commercially available, making the new chemistry highly accessible. The new precursor chemistry represents a new paradigm for QD growth: chemically-induced precursor conversion. We show that the new precursor allows systematic growth of high quality QDs of various sizes and materials for both core-only and core-shell QDs. Moreover, a close examination of the growth results provided invaluable insight into the nanoparticle growth process, enabling logical tuning of the reaction condition. We also show that this new precursor chemistry employs completely different reaction mechanisms for nucleation and growth. By effectively separating growth and nucleation even at a high concentration of precursors, the new precursor allows heat-up-based growth of high quality shells that are comparable to those created by the injection method. Our study inspires the design of other precursors to systematically grow high quality nanocrystals and achieve the heat-up-based growth of high quality shells, a highly economical and scalable approach for large-scale synthesis.

Highly Efficient Liquid-Quantum Dot/Melamine- Modified Urea-Formaldehyde Microcapsules for White Light-Emitting Diodes

A liquid matrix is beneficial for improving the luminescence performance of quantum dots (QDs), although it is difficult to be packaged with light-emitting diode (LED) chips. In this study, liquid-QDs with chlorobenzene solution and melamine-modified urea-formaldehyde (MUF) were used as the core and shell structures, respectively, successfully yielding liquid-QD/MUF microcapsules for white LED packaging. The concentration of the liquid-QDs in the microcapsules was optimized, demonstrating an increase of 15.9% in the luminous efficiency, as compared to the traditional solid-QDs at a specific QD energy proportion. The efficiency roll-off can be attributed to the increased reabsorption and back-scattering loss at higher liquid-QDs and microcapsules concentrations. It was also confirmed that the liquid-QD/MUF microcapsules have an excellent thermal stability below 200 °C, owing to the protection of the MUF shells. Finally, white LEDs compounding the red liquid-QD/MUF microcapsules and YAG phosphor particles were fabricated, achieving a high luminous efficiency of 149.2 lm/W at 20 mA, correlated color temperature of 3301 K, and color rendering index of 88.8. This study will open new research avenues for liquid-QD packaging in white LEDs.

Immunofluorescent labeling of CD20 tumor marker with quantum dots for rapid and quantitative detection of diffuse large B-cell non-Hodgkin's lymphoma.

Fluorescent semiconductor quantum dots (QDs) are newfound nanocrystal probes which have been used in bioimaging filed in recent years. The purpose of this study is to evaluate the diagnostic value of specific QDs coupled to rituximab monoclonal antibody against CD20 tumor markers for patients with diffuse large B-cell lymphoma (DLBCL). In current study rituximab-conjugated quantum dots (QDs-rituximab) were prepared against CD20 tumor markers for detection of CD20-positive cells (human Raji cell line) using flowcytometry. A total of 27 tumor tissue samples were collected from patients with DLBCL and 27 subjects with negative pathological tests as healthy ones, which stained by QD-rituximab. The detection signals were obtained from QDs using fluorescence microscopy. The flowcytometry results demonstrated a remarkable difference in fluorescent intensity and FL2-H + (CD20-positive cells percentage) between two groups. Both factors were significantly higher in Raji in comparison with K562 cell line (P < 0.05). Lot of green fluorescence signals was observed due to the selectively binding of QD-rituximab to CD20 tumor markers which overexpressed in tumor tissues and a few signals observed on the defined healthy ones. Based on these observations the cut-off point was 46.8 dots and the sensitivity, specificity, positive predictive value, and negative predictive value were 100%, 89.5%, 91.3%, and 100%, respectively (LR+, 9.52; LR-, 0). The QD -rituximab could be beneficial as a bioimaging tool with high sensitivity to provide an accurate molecular imaging technique for identifying CD20 tumor markers for early diagnosis of the patients with DLBCL.

Specific Radius Change of Quantum Dot inside the Lipid Bilayer by Charge Effect of Lipid Head-Group

We studied the quantum dot-liposome complex (QLC), which is the giant unilamellar vesicle with quantum dots (QDs) incorporated in its lipid bilayer. A spin coating method in conjunction with the electroformation technique yielded vesicles with highly homogeneous unilamellar structure. We observed QD size dependence of the QLC formation: QLCs form with blue, green and yellow-emission QD (core radius ~1.05 nm, 1.25 nm and 1.65 nm) but not with red-emission QD (core radius ~2.5 nm). In order to explain this size dependence, we made a simple model explaining the QD size effect on QLC formation in terms of the molecular packing parameter and the lipid conformational change. This model predicts that QDs below a certain critical size (radius ≈ 1.8 nm) can stably reside in a lipid bilayer of 4 - 5 nm in thickness for Egg-PC lipids. This is consistent with our previous experimental results. In the case of red-emission QD, QD-aggregations are only observed on the fluorescent microscopy instead of QLC. We expected that the reduction of packing parameter (P) would lead to the change of specific QD radius. This prediction could be verified by our experimental observation of the shift of the specific QD size by mixing DOPG.

Bi-dipole emission via plasmon modes of Au/Ag nanoshell

The far-field and near-field responses of Au/Ag nanoshell (NS) irradiated by a plane wave are studied theoretically using the Mie theory. The locally enhanced electric fields at the plasmon modes of NS are shown useful to excite a pair of nearby quantum dots (QDs) simultaneously. The dipole mode can be utilized for inducing an anti-symmetric radial bi-dipole and the quadrupole mode for the symmetric radial one. Moreover, the radiative and nonradiative powers of the anti-symmetric and symmetric emitting bi-dipoles in the presence of Au/Ag NS are analyzed using the dyadic Green's functions. The results show that the Purcell factors of NS on the emission of these bi-dipoles are increased dramatically at these plasmon modes. These results illustrate that NS possesses dual functions at these modes: a nanolens for focusing irradiating light locally to excite any nearby QD and an efficient nanoantenna for enhancing the emission of the excited emitter in the proximity of NS. According to the apparent quantum yield, the dipole mode is a bright mode, while the quadrupole mode is a dark one. In addition, these plasmon modes of Au/Ag NS, whose wavelengths are in between those of Au and Ag NSs, are tunable by adjusting the percentage of Au. This advantage allows us to tailor NS for matching these modes with the emission spectrum of specific QD.

Effect of Structure on the Fluorescent Properties of the Hydrophilic Modified Quantum Dots

Quantum dots (QDs), as a new class of biological nanoprobe, its fluorescence properties are re- lated to the detection sensitivity. QDs with different core-shell structures vary in the resistance to the fluo- rescence quenching during surface modification. In this study, a series of core and core-shell QDs with dif- ferent structure and composition were designed and synthesized. And they were modified with amphiphilic polymer. Their fluorescence properties during the surface modification were monitored by fluorescence spectroscopy. Experimental results show core QDs are the most easily quenched during the surface modifi- cation; the ability of resistance to quenching is enhanced after shell coating on core QDs, and this ability in- creases with the shell layers increase. Results also indicate the structure and composition of shells affect the resistance to fluorescence quenching. The resistance to fluorescence quenching is greatly enhanced when the core-shell QDs has a reasonable shell structure and composition. Furthermore, specific QD nanoprobes are constructed based on selecting appropriate structure of QDs and coupling with folic acid. The prepared QD nanoprobes are used for detection of breast cancer cells on flow cytometry. The results show that by opti-

A methodology for the fabrication by FIB of needle-shape specimens around sub-surface features at the nanometre scale

A novel methodology for the preparation by focused ion beam (FIB) of needle-shape specimens in specific sites underneath the sample surface for their study by electron tomography (ET) is proposed. In particular, we demonstrate this methodology for the fabrication of needles containing InAs/InP quantum dots (QDs). The main challenge of this methodology is the location of specific QDs in the FIB equipment, as they are not visible with the secondary electrons detector. In order to overcome this difficulty, a series of marks visible both in conventional transmission electron microscopy and in the FIB are introduced before the preparation of the needles. The conditions for the fabrication by FIB of needles with optimized characteristics for their study by ET are also detailed.

Studies of Intracorneal Distribution and Cytotoxicity of Quantum Dots: Risk Assessment of Eye Exposure

The cornea is a potential route of exposure and drug administration for nanoparticles. In this work, we use noninvasive two-photon microscopic imaging to study the distribution and permeability pathway of CdSe/ZnS core/shell quantum dots (QDs) capped with three different functional groups through the cornea. With no additional staining, the two-photon image clearly discloses that fluorescent QDs penetrate and reside within the interlamellar space of second harmonic generating collagenous stroma when the corneal epithelium barrier is injured. An in vitro cytotoxicity test using bovine corneal stromal cells incubated individually with all three kinds of QDs indicates that the cell viability decreases significantly as the QD concentration and incubation period increased. The results also show that the specific QDs influence corneal stromal cell viability up to a significant magnitude of 50% under a relatively low concentration (5-20 nM) and short exposure period (24-48 h). Furthermore, two-photon imaging shows that QDs can be retained within the cornea up to 26 days in an in vivo mouse model. On the basis of our in vivo and in vitro data, we conclude that QDs can penetrate and be retained within cornea long enough to cause consequential cytotoxicity, under the circumstance in which the corneal epithelium barrier is injured. Since corneal abrasion is quite a common situation in daily life, our work raises public attention to the potential risk of eye exposure to nanoparticles.

Internal electronic structure and fine structure of multiexcitons in semiconductor quantum dots

We perform a detailed theoretical study of the characteristic internal electronic structure of various multiexcitons $({N}_{h},{N}_{e})$, where ${N}_{h}$ is number of holes and ${N}_{e}$ is the number of electrons in the self-assembled semiconductor quantum dots (QDs). For each of the leading $({N}_{h},{N}_{e})$ excitonic complexes we start from the single-particle configuration (e.g., a specific occupation pattern of $S$ and $P$ electron and hole levels by a few carriers) and then show the many-particle multiplet levels for the initial state of emission $({N}_{h},{N}_{e})$ and the final state of emission $({N}_{h}\ensuremath{-}1,\text{ }{N}_{e}\ensuremath{-}1)$. We denote which states are dark and which are bright; the order and multiplicity, the leading single-particle character of each multiplet state, and the fine-structure splittings. These are of general utility. We also show explicit numerical values for distances between various transitions for four specific QDs. Here the presented information is important and potentially useful for a few reasons: (i) the information serves as a guide for spectroscopic interpretation; (ii) the information reveals non-Aufbau cases, where the dot does not have Aufbau occupation of carriers' levels; (iii) the information shows which transitions are sensitive to random-alloy fluctuations (if the dot is alloyed) and importance of this effect. We show that because of such alloy information, distances between peaks cannot be used to gauge structural information.

Optical Biopsy of Cancer: Nanotechnological Aspects

Naturally occurring differences in the optical properties of normal and cancerous tissue have been exploited frequently in optical detection systems. However, optical biopsy of cancer can be improved by using targeted, optically active and bright contrast agents to enhance the optical signal from disease-specific molecular markers. Nanotechnology has advanced greatly in recent years and can be applied to variety of biomedical research areas, as well as optical biopsy in clinical settings. Quantum dots (QDs) are stable, bright fluorophores that, under ideal conditions, can have high quantum yields, narrow fluorescence emission bands, high absorbency, very large effective Stokes shifts, high resistance to photobleaching, and can provide excitation of several different emission colours using a single wavelength for excitation. Optically efficient, cancer specific QDs provide a new tool to enable non-invasive visualization of disease-specific molecular and tissue changes with subcellular spatial resolution. Nanotechnology is in a unique position to transform cancer diagnostics and to produce a new generation of fluorescent markers and medical imaging techniques with higher sensitivity and precision of recognition.

Structural and optical investigations of 1-, 2-, and 3-dimensional InAs quantum dot arrays

We have overgrown GaAs (0 0 1) substrates, patterned with dense square arrays of round-shaped nanometer-sized holes and trenches with stacked InAs quantum dots (QDs). Different periods of square hole arrays were overgrown at the same time and we observe a strong dependence of the evolving surface morphology on the period of the pre-pattern. QD pairs are observed only for large periods, whereas predominately single QDs form on patterns with smaller periods. We are able to assign the photoluminescence (PL) emission to specific QD layers and correlate the observed PL spectra of these QD arrays with the surface morphology. Furthermore, a comparison of the PL emission between conventionally grown and laterally ordered QDs reveals a comparable optical intensity of the ordered QDs.

Facets, indium distribution, and lattice distortion of InGaAs/GaAs quantum dots observed by three-dimensional scanning transmission electron microscope

Multiazimuth 360° observation of InGaAs/GaAs quantum dots (QDs) was performed with a 300 kV scanning transmission electron microscope, where both cross-sectional and plan-view images of the same specific QDs can be taken for a single specimen. The facet structure of truncated pyramids was reconstructed from facet-enhanced bright-field images newly observed with the incident axes slightly off from 〈−552〉 or 〈−332〉, resulting in high contrast for the lattice distortion on (110) facets of InGaAs QDs. Dark-field images for a large QD clearly indicate indium distribution inside the large QD, originating from the coalescence of two small QDs during growth. Localized relaxation of the lattices was observed, for the same large QD with indium content fluctuation, as disturbed/disappeared moire-fringes in the images taken with the incidence around 〈−552〉.