Fine Structure of Nearly Isotropic Bright Excitons in InP/ZnSe Colloidal Quantum Dots

Chemistry of Materials, 2015

High Energy Chemistry, 2017

⎯Luminescence decay curves have been measured for InP@ZnS colloidal quantum dots (CQDs) synthesized at different ratios between 1-octanethiol and 1-dodecanethiol. The luminescence lifetime distributions reflecting the ratios between the on, off, and grey states of CQDs have been calculated. With an increase in the portion of 1-octanethiol, the distribution is shifted toward the on states, which almost completely suppresses fluorescence blinking at all detection wavelengths.

We demonstrate fine-tuning of the atomic composition of InP/ZnSe QDs at the core/shell interface. Specifically, we control the stoichiometry of both anions (P, As, S, and Se) and cations (In, Zn) and correlate these changes with the resultant steady-state and time-resolved optical properties of the nanocrystals. Anion deposition on the InP QD surface results in a redshift in the absorption, quenching of the excitonic photoluminescence, and a relative increase in the intensity of the broad trap-based photoluminescence, consistent with delocalization of the exciton wavefunction and relaxation of exciton confinement. Time-resolved photoluminescence data show an overall small change in the decay dynamics on the ns timescale, suggesting the relatively low photoluminescence quantum yields may be attributed to the creation of new thermally activated charge trap states. Cluster-model density functional theory calculations show that the presence of core/shell interface anions give rise to el...

Chemistry of Materials

Zinc is routinely employed in the synthesis of InP quantum dots (QDs) to improve the photoluminescence efficiency and carrier mobility of the resulting In(Zn)P alloy nanostructures. The exact location of Zn in the final structures and the mechanism by which it enhances the optoelectronic properties of the QDs are debated. We use synchrotron X-ray absorbance spectroscopy to show that the majority of Zn in In(Zn)P QDs is located at their surface as Zn carboxylates. However, a small amount of Zn is present inside the bulk of the QDs with the consequent contraction of their lattice, as confirmed by combining high-resolution high-angle annular darkfield imaging scanning transmission electron microscopy with statistical parameter estimation theory. We further demonstrate that the Zn content and its incorporation into the QDs can be tuned by the ligation of commonly employed Zn carboxylate precursors: the use of highly reactive Zn acetate leads to the formation of undesired Zn 3 P 2 and the final nanostructures being characterized by broad optical features, whereas Zn carboxylates with longer carbon chains lead to InP crystals with much lower zinc content and narrow optical features. These results can explain the differences between structural and optical properties of In(Zn)P samples reported across the literature and provide a rational method to tune the amount of Zn in InP nanocrystals and to drive the incorporation of Zn either as surface Zn carboxylate, as a substitutional dopant inside the InP crystal lattice, or even predominantly as Zn 3 P 2 .

Advanced Optical Materials, 2022

environmentally friendly constituent elements (lower toxicity), outstanding sizedependent monochromaticity, bandgap tunability, and high photoluminescence quantum yield (PLQY). [1-3] InP QDs have been considered as ideal alternative materials to toxic Cd-containing QDs for a wide range of applications such as lightemitting diodes (LEDs), lasers, solar cells, bio-maker, and sensors. [4-7] Breakthroughs in materials and synthetic-process development for highly luminescent InP QDs (green and red) have enabled the highest optical efficiencies reaching PLQY of 95% [8] and 100%, [9] respectively, comparable to the high-performance Cd-based QDs. Blue InP QDs are regarded as breakthrough materials for an ideal future QDbased display. [10] Since 2008, [11] intensive efforts on the synthesis of blue InP QDs have been made. In particular, the high reactivity between tris(trimethylsilyl)phosphine (TMSP) and indium precursors allowed to form a magicsized cluster as an intermediate, which enabled a uniform InP core formation with tunable emission wavelengths in the blue color region. [12,13] In 2008, the first step forward in the synthesis of blue InP@ZnS QDs (20% PLQY at 480 nm) was demonstrated using zinc carboxylate complexes that facilitate the stabilization of nanocrystal surfaces and the reduction of critical nuclei size during the growth of InP core. [11,14] Similar observations for the same QD materials were also reported in 2012 (36% PLQY at 430 nm) [15] and 2017 (44% PLQY at 488 nm). [16] Besides, size reduction of grown InP cores by acetic acid (a byproduct of core growth process with TMSP at 150 °C) enabled the emission in the blue wavelength region (5% and 10% PLQYs at 475 and 485 nm, respectively). [17] InPZnS alloys grown with a long injection of TMSP [18] were also demonstrated with 24.5% PLQY at 495 nm. However, despite the tremendous efforts on the synthesis of the blue InP QDs using the TMSP, the issue is far from resolved because of their low PLQY (around 40%) along with emission at a greenish-blue light wavelength (over 488 nm). Here, we demonstrate a synthetic method for engineering the core size of InP@ZnS core@shell QDs suitable for blue emission and unveil the associated significance of shell thickness and materials via first-principles simulations based on Blue indium phosphide quantum dot (InP QD) is an emerging colloidal semiconductor nanocrystal, considered as a promising next-generation photoactive material for light-emitting purposes. Despite the tremendous progress in blue InP QDs, the synthetic method for tailoring InP core size to realize the blue-emissive QDs still lags behind. This work suggests a synthetic method for blue-emitting InP QDs by engineering the core size with an incipient ZnS (i-ZnS) shell. The formation of i-ZnS complexes, before the tris(trimethylsilyl) phosphine injection (e.g., before core growth process), restrains the overgrowth of InP nuclei by rapidly forming a ZnS shell on its surface, thereby resulting in further dwarfed InP cores. With additional ZnS shell coating, the blue QDs exhibit a photoluminescence quantum yield of ≈52% at 483 nm. The origin of bandgap diminution with the increase of shell thickness, or with the utilization of ZnSe shell is unraveled via the first-principles density functional theory simulations. Simulational evidence on InP-core densification with the shell coating, along with accompanying changes in chemical and structural properties, is presented. The blue-emitting InP QD device shows a maximum luminance of 1162 cd m −2 and external quantum efficiency of 1.4%.

Journal of Materials Chemistry, 2007

This paper describes the synthesis of highly water-soluble and fluorescent ZnSe(S)-alloyed quantum dots (QDs). We used zinc perchlorate hexahydrate, sodium hydrogen selenide as precursors and mercaptopropionic acid as stabilizer to synthesize ZnSe QDs in aqueous solution at 160 uC for 9 h. The as-prepared ZnSe QDs possess a quantum yield (QY) of 8.1% and high trapped emission. After UV irradiation using a 100 W Hg-Xe lamp for 0.5 h, ZnSe(S) QDs having a QY of 19.0% are formed from ZnSe QDs. However, aggregation of ZnSe(S) QDs under longer UV irradiation (. 0.5 h) takes place, leading to instability and irreproducibility. To overcome this, additional thiol compounds (mercaptopropionic acid, mercaptosuccinic acid, 11-mercaptoundecanoic acid, and thioglycolic acid) were separately added to ZnSe QD solutions during UV irradiation. UV irradiation and oxygen accelerate the release of S 22 from the thiol compounds, leading to the formation of ZnSe(S) QDs. Among the thiol compounds, mercaptosuccinic acid is the most suitable in terms of stability and photoluminescence intensity. We suggest that the size and functional group of the thiol compounds play an important role in determining the optical properties and stability of ZnSe(S) QDs. The as-prepared ZnSe(S) QDs fluoresce strongly (QY up to 44.0%) at 407 nm with a narrow bandwidth (W 1/2 , 25 nm) when excited at 325 nm.

Journal of Materials Science: Materials in Electronics

In this article, InP quantum dots (QDs) are synthesized with a green methodology. The preparation of the InP QDs is demonstrated by varying the ratios of the precursors used such as InCl3 and trioctylphosphine (TOP). These QDs are fabricated through the application of TOP (as phosphorous source and reducing agent for the In salt) for forming indium and to develop the targeted InP QDs, which are characterized using HRTEM, XRD, PL, FTIR, UV–Vis, and time resolved spectroscopy. A rapid reaction time (~ 30 min) procedure in a single pot at a temperature of ~ 310 °C is developed. InP QDs with particle sizes varying from 4 to 6 nm have been measured with different concentrations of InCl3 and TOP. InP QDs yield reached 23%. Zincblende crystal structure is recognized for InP with high orientation plane of (220) as confirmed with XRD and confirmed with SAED. Solar cell devices are built by anchoring the InP QDs onto a TiO2 layer and measure the photovoltaic performance and spectral response ...

ACS Applied Materials & Interfaces, 2014

Environmentally friendly nanocrystals (NCs) such as InP are in demand for various applications, such as biomedical labeling, solar cells, sensors, and light-emitting diodes (LEDs). To fulfill their potential applications, the synthesis of such high-quality "green" InP NCs required further improvement so as to achieve better stability, higher brightness NCs, and also to have a more robust synthesis route. The present study addresses our efforts on the synthesis of high-quality In(Zn)P/ZnS core−shell NCs using an air-and moisture-stable ZnS single molecular precursor (SMP) and In(Zn)P cores. The SMP method has recently emerged as a promising route for the surface overcoating of NCs due to its simplicity, high reproducibility, low reaction temperature, and flexibility in controlling the reaction. The synthesis involved heating the In(Zn)P core solution and Zn(S 2 CNR 2) (where R = methyl, ethyl, butyl, or benzyl and referred to as ZDMT, ZDET, ZDBT, or ZDBzT, respectively) in oleylamine (OLA) to 90− 250°C for 0.5−2.5 h. In this work, we systematically studied the influence of different SMP end groups, the complex formation and stability between the SMP and oleylamine (OLA), the reaction temperature, and the amount of SMP on the synthesis of high-quality In(Zn)P/ZnS NCs. We found that thiocarbamate end groups are an important factor contributing to the lowtemperature growth of high-quality In(Zn)P/ZnS NCs, as the end groups affect the polarity of the molecules and result in a different steric arrangement. We found that use of SMP with bulky end groups (ZDBzT) results in nanocrystals with higher photoluminescence quantum yield (PL QY) and better dispersibility than those synthesized with SMPs with the shorter alkyl chain groups (ZDMT, ZDET, or ZDBT). At the optimal conditions, the PL QY of red emission In(Zn)P/ZnS NCs is 55 ± 4%, which is one of the highest values reported. On the basis of structural (XAS, XPS, XRD, TEM) and optical characterization, we propose a mechanism for the growth of a ZnS shell on an In(Zn)P core.

Materials Science and Engineering: B, 1998

Quantum dots of ZnSe have been precipitated in the presence of polyvinyl pyrrolidine using a modified chemical bath method. Addition of the polymeric capping agent serves to limit particle growth and stabilize nanocrystals. Analysis of X-ray diffraction (XRD) measurements indicates that capped particles attain an average particle diameter of only 15 versus 34 Å for the uncapped material. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) results show nanocrystals to be single crystals encapsulated in polymeric material, while SAED patterns confirm that the cubic phase of ZnSe was obtained with this synthesis route. Observation of a blue shift in the optical absorption and luminescence data is consistent with the reported reduction in nanocrystal size below the ZnSe bulk exciton Bohr diameter of 90 Å .

Nano Letters, 2012

We propose and demonstrate the fabrication of flexible, freestanding films of InP/ZnS quantum dots (QDs) using fatty acid ligands across very large areas (greater than 50 cm × 50 cm), which have been developed for remote phosphor applications in solid-state lighting. Embedded in a poly(methyl methacrylate) matrix, although the formation of stand−alone films using other QDs commonly capped with trioctylphosphine oxide (TOPO) and oleic acid is not efficient, employing myristic acid as ligand in the synthesis of these QDs, which imparts a strongly hydrophobic character to the thin film, enables film formation and ease of removal even on surprisingly large areas, thereby avoiding the need for ligand exchange. When pumped by a blue LED, these Cd-free QD films allow for high color rendering, warm white light generation with a color rendering index of 89.30 and a correlated color temperature of 2298 K. In the composite film, the temperature-dependent emission kinetics and energy transfer dynamics among different-sized InP/ZnS QDs are investigated and a model is proposed. High levels of energy transfer efficiency (up to 80%) and strong donor lifetime modification (from 18 to 4 ns) are achieved. The suppression of the nonradiative channels is observed when the hybrid film is cooled to cryogenic temperatures. The lifetime changes of the donor and acceptor InP/ZnS QDs in the film as a result of the energy transfer are explained well by our theoretical model based on the exciton−exciton interactions among the dots and are in excellent agreement with the experimental results. The understanding of these excitonic interactions is essential to facilitate improvements in the fabrication of photometrically high quality nanophosphors. The ability to make such large-area, flexible, freestanding Cd-free QD films pave the way for environmentally friendly phosphor applications including flexible, surfaceemitting light engines.