CdSe-based Quantum Dots Research Articles

(Invited) Luminescent InP Quantum Dots: They're Not Just Like CdSe

There are significant similarities and differences in the spectroscopy and photophysics between CdSe-based and InP-based quantum dots. One of the more important differences is the presence of reversibly populated hole traps in InP/ZnSe quantum dots (QDs). These traps affect many aspects of the overall excited state dynamics. In this paper we elucidate the hole relaxation dynamics in high quality InP/ZnSe/ZnS QDs at room temperature and focus on the role of these traps. Specifically, we show that the presence of transiently populated traps results in extremely slow (tens of picoseconds to nanoseconds) hole cooling as evidenced by a significant fraction of the photoluminescence exhibiting a slow risetime following photoexcitation. Through a combination of chemical derivatization studies and density functional theory calculations, they are assigned to substitutional indium adjacent to a zinc vacancy, In3+/VZn 2-, in the ZnSe shell.

Enhanced Spin Polarization from Biaxially Strained Colloidal Quantum Dots.

Electron and hole spin polarization is crucial for quantum dots to be used in spin lasers and quantum information processing. However, the degree of spin polarization in II-VI and III-V semiconductor quantum dots is low because of the degenerated valence band. Here, we increase the light and heavy hole degeneracy by introducing biaxial strain into CdSe-based quantum dots, enabling the degree of spin polarization to be increased from 20% to 50% under photoexcitation. The optical gain threshold measurement further reveals that the increase in polarization helps to reduce the gain threshold.

Numerical comparison of quantum-confined Stark effect on emission spectra between InP- and CdSe-based colloidal quantum dots

Numerical comparison of quantum-confined Stark effect on emission spectra between InP- and CdSe-based colloidal quantum dots

Biexciton Blinking in CdSe-Based Quantum Dots.

Experiments on single colloidal quantum dots (QDs) have revealed temporal fluctuations in the emission efficiency of the single-exciton state. These fluctuations, often termed "blinking", are caused by opening/closing of charge-carrier traps and/or charging/discharging of the QD. In the regime of strong optical excitation, multiexciton states are formed. The emission efficiencies of multiexcitons are lower because of Auger processes, but a quantitative characterization is challenging. Here, we quantify fluctuations of the biexciton efficiency for single CdSe/CdS/ZnS core-shell QDs. We find that the biexciton efficiency "blinks" significantly. The additional electron due to charging of a QD accelerates Auger recombination by a factor of 2 compared to the neutral biexciton, while opening/closing of a charge-carrier trap leads to an increase of the nonradiative recombination rate by a factor of 4. To understand the fast rate of trap-assisted recombination, we propose a revised model for trap-assisted recombination based on reversible trapping. Finally, we discuss the implications of biexciton blinking for lasing applications.

Spectroelectrochemistry of CdSe/CdxZn1-xS Nanoplatelets.

We report an unexpected enhancement of photoluminescence (PL) in CdSe-based core/shell nanoplatelets (NPLs) upon electrochemical hole injection. Moderate hole doping densities induce an enhancement of more than 50% in PL intensity. This is accompanied by a narrowing and blue-shift of the PL spectrum. Simultaneous, time-resolved PL experiments reveal a slower luminescence decay. Such hole-induced PL brightening in NPLs is in stark contrast to the usual observation of PL quenching of CdSe-based quantum dots following hole injection. We propose that hole injection removes surface traps responsible for the formation of negative trions, thereby blocking nonradiative Auger processes. Continuous photoexcitation causes the enhanced PL intensity to decrease back to its initial level, indicating that photocharging is a key step leading to loss of PL luminescence during normal aging. Modulating the potential can be used to reversibly enhance or quench the PL, which enables electro-optical switching.

Contributions of exciton fine structure and hole trapping on the hole state filling effect in the transient absorption spectra of CdSe quantum dots.

The optoelectronic properties of quantum confined semiconductor nanocrystals depend critically on the band edge electron and hole levels and their exciton fine structures. Transient absorption (TA) spectroscopy has been widely used to probe the dynamics of photogenerated electrons, holes, and excitons in these materials through their state filling induced bleach of the band edge exciton transition. Such effects, in principle, reflect the band edge fine structures and are well understood for the conduction band electrons. However, the valence band hole state filling signals remain poorly understood due to the complexity of the valence band level structure and the presence of fast hole trapping in many materials. Herein, we report a study of the valence band hole state filling effect by comparing the TA spectra of CdSe quantum dots (QDs) with different degrees of hole trapping and by selective removal of the conduction band electrons to adsorbed methyl viologen molecules. We observe that in CdSe/CdS core/shell QDs with a high photoluminescence quantum yield of 81%, the valence band hole contributes to 22% ± 1% of the exciton bleach, while a negligible hole state filling signal is observed in CdSe core only QDs with a photoluminescence quantum yield of 17%. This hole state filling effect can be explained by a simplified valence band edge hole model that contains two sets of twofold degenerate hole levels that are responsible for the higher energy bright exciton and lower energy dark exciton states, respectively. Our result clarifies the TA spectral features of the valence band holes and provides insights into the nature of single hole states in CdSe-based QDs.

Efficient, Stable, and Photoluminescence Intermittency-Free CdSe-Based Quantum Dots in the Full-Color Range

Efficient, Stable, and Photoluminescence Intermittency-Free CdSe-Based Quantum Dots in the Full-Color Range

Unusual Spectral Diffusion of Single CuInS2 Quantum Dots Sheds Light on the Mechanism of Radiative Decay.

The luminescence of CuInS2 quantum dots (QDs) is slower and spectrally broader than that of many other types of QDs. The origin of this anomalous behavior is still under debate. Single-QD experiments could help settle this debate, but studies by different groups have yielded conflicting results. Here, we study the photophysics of single core-only CuInS2 and core/shell CuInS2/CdS QDs. Both types of single QDs exhibit broad PL spectra with fluctuating peak position and single-exponential photoluminescence decay with a slow but fluctuating lifetime. Spectral diffusion of CuInS2-based QDs is qualitatively and quantitatively different from CdSe-based QDs. The differences reflect the dipole moment of the CuInS2 excited state and hole localization on a preferred site in the QD. Our results unravel the highly dynamic photophysics of CuInS2 QDs and highlight the power of the analysis of single-QD property fluctuations.

CdSe-Based Quantum Dots as In Situ Pressure and Temperature Non-intrusive Sensors in Elastohydrodynamic Contacts

We present a new technique designed for in situ measurement of pressure and temperature in lubricating films. An innovative methodology has been developed, based on the photoluminescence properties of non-intrusive CdSe-based nanosize sensors (quantum dots). The sensitivity to pressure and temperature of these sensors dispersed in a carrier fluid was established through calibrations performed in diamond anvil cells. Elastohydrodynamic (EHD) contacts of different combinations of contacting solids (glass-steel, glass-Si3N4, sapphire-steel and sapphire-Si3N4) and submitted to various operating conditions were studied through in situ experiments and numerical simulations. Isothermal experiments were performed first: both experimental central pressures and pressure profiles were obtained, with a very good agreement with the values predicted by the numerical model. A series of non-isothermal experiments were then carried out to perform temperature measurements. Temperature rises in the central zone of EHD contacts involving various material pairs were measured and compared to predictions, leading to a very satisfying agreement. Overall, the deviation between measurements and predictions remained smaller than the uncertainty of the measurement method. Therefore, these findings proved the potential of the methodology to probe in situ pressure and temperature in EHD contacts. Comparative performance with competing techniques was examined in terms of intrusiveness, level of reliability, spatial resolution, accuracy and complexity. As this work is a pioneering development, the technique may be improved in the near future, opening an avenue for even more accurate or faster measurements for example, and eventually offering a better understanding of the mechanisms at work in this type of lubricated interface.

High-resolution patterning of colloidal quantum dots via non-destructive, light-driven ligand crosslinking

Establishing multi-colour patterning technology for colloidal quantum dots is critical for realising high-resolution displays based on the material. Here, we report a solution-based processing method to form patterns of quantum dots using a light-driven ligand crosslinker, ethane-1,2-diyl bis(4-azido-2,3,5,6-tetrafluorobenzoate). The crosslinker with two azide end groups can interlock the ligands of neighbouring quantum dots upon exposure to UV, yielding chemically robust quantum dot films. Exploiting the light-driven crosslinking process, different colour CdSe-based core-shell quantum dots can be photo-patterned; quantum dot patterns of red, green and blue primary colours with a sub-pixel size of 4 μm × 16 μm, corresponding to a resolution of >1400 pixels per inch, are demonstrated. The process is non-destructive, such that photoluminescence and electroluminescence characteristics of quantum dot films are preserved after crosslinking. We demonstrate that red crosslinked quantum dot light-emitting diodes exhibiting an external quantum efficiency as high as 14.6% can be obtained.

Improved photostability of silica bead impregnated with CdSe-based quantum dots prepared through proper surface silanization

The incorporation of emitting quantum dots (QDs) into silica and their sustained photostability have been the subject of research over the last two decades. This report focuses on the synthesis of luminescent silica beads by directly coating silica on the hydrophobic CdSe-based QDs by exchanging the original ligand (oleic acid) through partially hydrolyzed tetra-alkoxysilane, using a surface silanization approach. It was found that the ratio of QD/alkoxide upon silanization must be optimized to get uniform coating of silica. The thickness of silica layer was controlled judiciously by a subsequent reverse micelle method. The photostability of silica beads in water thus prepared was evaluated systematically by a newly developed, and facile method using high irradiation intensity (ca. 45 W/cm2). When the thickness of the silica layer attained a value of 5 nm, the photostability of QDs improved drastically, probably due to a change in the glass structure. We have found that the addition of small amount of water increased the degree of surface silanization, resulting in ca. 46 % enhancement of the photoluminescence (PL) degradation time of QDs. We also discuss the advantages of this direct coating method with tetra-alkoxysilanes in comparison with other organo-alkoxysilanes as primers.

Sensing of Ozone Gas by Using CdSe-Based Photoluminescent Quantum Dots and Effects of Combining Noble Metals and Quantum Dots

Sensing of Ozone Gas by Using CdSe-Based Photoluminescent Quantum Dots and Effects of Combining Noble Metals and Quantum Dots

Stoichiometry-Controlled InP-Based Quantum Dots: Synthesis, Photoluminescence, and Electroluminescence.

We introduce stoichiometry control within both core and shell regions of InP/ZnSe/ZnS core/shell/shell quantum dots (QDs) to advance their properties drastically, approaching those of state-of-the-art CdSe-based QDs. The resulting QDs possess near-unity photoluminescence quantum yield, monoexponential decay dynamics, narrow line width, and nonblinking at a single-dot level. Quantum-dot light-emitting diodes (QLEDs) with the InP/ZnSe/ZnS core/shell/shell QDs as emitters exhibit a peak external quantum efficiency of 12.2% and a maximum brightness of >10 000 cd m-2, greatly exceeding those of the Cd/Pb-free QLEDs reported in literature. These results pave the way toward Cd/Pb-free QDs as outstanding optical and optoelectronic materials.

Structural dependent, dielectric and conduction analysis of CdSe based quantum dots

Nowadays, the effect of temperature on nanocrystal’s dielectric has become a routine study while their structural effects on dielectric properties, lacks the attention of researchers. The correlation of different shell arrangement on dielectric enhancement in CdSe based quantum dots, is addressed in this letter. Detrimental impact of surface defects on dielectric properties of quantum dots is investigated and shown that surface defects of bare core increases dielectric loss, dissipation factor and decreases conductivity of the quantum dots. Dielectric properties of quantum dots is evaluated after deposition of different shells and multishells. It is observed that loss factor is declined in core/shell(multishell) quantum dots due to passivation of surface defects and consequently ease in charge carrier tunneling. Higher conductivity of the core/shell and multishell structures has also been demonstrated here. Structural and optical investigation of the quantum dots after shell deposition is also considered.

Tuning of nonlinear absorption in highly luminescent CdSe based quantum dots with core-shell and core/multi-shell architectures.

We present our effort on an efficient way of tuning the nonlinear absorption mechanisms in ultra-small CdSe based quantum dots by implementing core-shell and core/multi-shell architectures. Depending on the size, architecture and composition of the QDs, these materials exhibited resonant and near-resonant nonlinear optical absorption properties such as saturable (SA) and reverse saturable (RSA) absorption for 5 ns pulses of 532 nm. These QDs exhibited a non-monotonic dependence of the effective two-photon absorption coefficient (β) under nanosecond excitation with a maximum value for a thinner shell. We obtained a nonlinear absorption enhancement of an order of magnitude by adopting the core-shell architecture compared to their individual counterparts. Interestingly, CdSe QDs exhibit SA and/or RSA depending on their size and show a switching over from SA to RSA as the input intensity increases. We explained the enhanced nonlinear absorption in core-shell QDs compared to their individual counterparts in view of enhanced local fields associated with the core-shell structure. Thus, the present nanostructured materials are excellent candidates as saturable absorbers and optical limiters.

Luminescence kinetics of CdSe-based quantum dots in toluene

Luminescence kinetics of quantum dots (CdSe-CdS core-shell nanoparticles) in toluene were studied. Quantum dots were obtained through colloidal synthesis method in aqueous-organic media with the average size estimated as 2 nm and 2.9 nm from the position of the exciton absorption peak. A biexponential luminescence decays with time constants of a few and tens of picoseconds were observed via up-conversion technique

Development of Technologies for Sensing Ozone in Ambient Air

Ozone (O3) gas is widely used as a strong oxidizing agent for many purposes, such as the decomposition/removal of organic contaminants and photoresist, and the deodorization/disinfection of air and water. However, ozone is highly toxic to the human body when the air concentration exceeds about 1 ppm. Therefore, there is increasing demand for simple, sensitive, reliable, and cost-effective techniques for sensing ozone gas. This article describes the features, advantages, and disadvantages of the available, practical techniques for sensing ozone gas in ambient air. The advantages of optical gas sensors as next-generation sensors is specifically introduced. The features of photoluminescent, semiconductor nanoparticles (quantum dots, QDs) as bright phosphors with the potential for various applications is further explored. Lastly, recent research results demonstrating the ozone sensitivity of photoluminescent CdSe-based core-shell quantum dots are presented. These results strongly suggest that optical ozone sensing using photoluminescent quantum dots is a promising technique.

Photonic Crystal Phosphors Integrated on a Blue LED Chip for Efficient White Light Generation.

Following the proof-of-concept experiment in the unit structure level, photonic crystal (PhC) phosphors-structurally engineered phosphor materials based on the nanophotonics principles-are integrated with a blue light-emitting diode (LED) chip to demonstrate a compact and efficient white light source. Red- or green-emitting CdSe-based colloidal quantum dots (CQDs) are coated on a Si3 N4 thin-film grating to fabricate PhC phosphors. The underlying PhC structure is designed such that the photonic band-edge modes at the zone center (k∣∣ = 0) are tuned to the energy of the blue excitation photons. By progressively stacking the PhC phosphor plates on a blue LED chip, the blue, green, and red emission intensities can be tightly controlled to obtain white light with the desired properties. The chromaticity coordinates, (0.332, 0.341), and correlated color temperature, 5500 K, are obtained from a stack of 3 red and 11 green PhC phosphor plates; in contrast, a stack of 5 red and 16 green reference phosphor plates are required to generate a similar white light. Overall, the PhC phosphors produce 8% higher total emission intensity out of 33% less amount of CQDs than the reference phosphors.

Hybrid downconverters with green perovskite-polymer composite films for wide color gamut displays

We propose to use a hybrid downconverter system comprising low-cost green perovskite-polymer composite films for liquid crystal display (LCD) backlight unit (BLU) to realize wide color gamut. Recently, ultrastable, highly luminescent CH3NH3PbBr3 (MAPbBr3) organic-inorganic perovskite-polymer composite films have been developed. These films exhibit outstanding color quality with a full-width-at-half-maximum (FWHM) of only 18 nm and a peak wavelength of 530 nm, which makes them promising candidates as green downconverters. Two configurations to hybridize these green films with state-of-the-art red emitting downconverters, including CdSe-based quantum dots (QDs) and narrow peak phosphors, are proposed. Color and efficiency analyses indicate that the hybridization of green perovskite-polymer films with red K2SiF6:Mn4+ (KSF) phosphor could lead to wide color gamut coverage of nearly 90% Rec. 2020 and high total light efficiency (TLE) of around 20 lm/W while maintaining low cost.

Sensing of ozone based on its quenching effect on the photoluminescence of CdSe-based core-shell quantum dots

Thin films of CdSe-based core-shell type quantum dots (CdSe/CdZnS, CdSe/ZnS and CdSeTe/ZnS) deposited on glass substrate were found to undergo a reversible change in photoluminescence (PL) on exposure to ozone in concentrations as low as 0.1 ppm in air. PL decreased rapidly on adding ozone and fully recovered after changing back to pure air. The PL of the CdSe/ZnS quantum dots (QDs) was not quenched by pure oxygen, nitrogen, argon, carbon dioxide, and air containing hydrogen. A comparison of various CdSe-based core-shell type QDs with different emission colors showed that green-emitting QDs with smaller size were more sensitive to ozone, while red-emitting QDs (of larger size) were more resistant to high concentrations of ozone. The response time of the sensor (for a 90 % signal change) was in the order of 10–20 min. The reversible, reproducible and selective response to ozone at room temperature under atmospheric pressure suggested that these QDs have a great potential in terms of fluorescent sensing of ozone.