Origins of photoluminescence decay kinetics in CdTe colloidal quantum dots

Chemical Reactivity-Controlled Synthesis of Silver Chalcogenide Colloidal Quantum Dots for Efficient Shortwave Infrared Photodetectors

Eco-friendly Ag2Te colloidal quantum dots (CQDs) have emerged as promising candidates for shortwave infrared (SWIR) optoelectronic applications owing to their size-tunable bandgaps with high optical properties. However, conventional synthesis methods relying on high temperatures and long reaction times yield low-quality Ag2Te CQDs because of their low chemical stability, resulting in decomposition under synthetic conditions and, thus, a non-uniform size distribution. Here, chemical reactivity-controlled synthesis is presented to regulate the crystal size and bandgap of Ag2Te CQDs. This involves adjusting the concentration and type of ligands, as well as the precursor ratio. The rapid termination of the reaction in this method prevents Ag2Te CQD decomposition, yielding monodisperse CQDs with a 1.66 peak-to-valley ratio at the first exciton absorption peak (≈1440 nm) and enabling absorption and emission in the 1100-1600 nm range. Furthermore, polar antisolvents in the purification process cause surface ligand removal from Ag2Te CQDs, resulting in surface defects and CQD aggregation. To mitigate these issues by enhancing their chemical stability, core/shell-type Ag2Te/Ag2S CQDs are synthesized. The photoluminescence (PL) intensity of Ag2Te/Ag2S CQDs significantly increased fivefold compared to Ag2Te core CQDs, and after purification, their size distribution remained uniform with preserved PL intensity. This is attributed to a significant reduction in surface defects. Consequently, the Ag2Te/Ag2S CQD-based SWIR photodetector exhibits a high external quantum efficiency of 8.4% and a specific detectivity of 1.1 × 1011 Jones at 1550 nm, with a fast response time of 38 ns.

Effect of trifluoroacetic acid on InP/ZnSe/ZnS quantum dots: mimicking the surface trap and their effects on the photophysical properties

Understanding the precise effects of defects on the photophysical properties of quantum dots (QDs) is essential to their development with near-unity luminescence. Because of the complicated nature of defects in QDs, the origins and detailed roles of the defects still remain rarely understood. In this regard, we used detailed chemical analysis to investigate the effect of surface defects on the optical properties of InP/ZnSe/ZnS QDs by introducing shell defects through controlled trifluoroacetic acid (TFA) etching. TFA treatment on the InP/ZnSe/ZnS QDs partially removed the ZnS shell as well as ligands and reduced the quantum yield by generating energetically deep surface traps. The surface defects of QDs by TFA cause charged trap sites inducing an Auger recombination process with a rate of ca. 200 ps. Based on these results, we proposed possible trap-assisted non-radiative decay pathways between the band-edge state and surface deep traps in InP/ZnSe/ZnS QDs.

Indirect to Direct Band Gap Transformation by Surface Engineering in Semiconductor Nanostructures

Indirect band gap semiconductor materials are routinely exploited in photonics, optoelectronics, and energy harvesting. However, their optical conversion efficiency is low, due to their poor optical properties, and a wide range of strategies, generally involving doping or alloying, has been explored to increase it, often, however, at the cost of changing their material properties and their band gap energy, which, in essence, amounts to changing them into different materials altogether. A key challenge is therefore to identify effective strategies to substantially enhance optical transitions at the band gap in these materials without sacrificing their intrinsic nature. Here, we show that this is indeed possible and that GaP can be transformed into a direct gap material by simple nanostructuring and surface engineering, while fully preserving its "identity". We then distill the main ingredients of this procedure into a general recipe applicable to any indirect material and test it on AlAs, obtaining an increase of over 4 orders of magnitude in both emission intensity and radiative rates.

Increasing the Energy Gap between Band-Edge and Trap States Slows Down Picosecond Carrier Trapping in Highly Luminescent InP/ZnSe/ZnS Quantum Dots

Non-toxic InP-based nanocrystals have been developed for promising candidates for commercial optoelectronic applications and they still require further improvement on photophysical properties, compared to Cd-based quantum dots (QDs), for better device efficiency and long-term stability. It is, therefore, essential to understand the precise mechanism of carrier trapping even in the state-of-the-art InP-based QD with near-unity luminescence. Here, it is shown that using time-resolved spectroscopic measurements of systematically size-controlled InP/ZnSe/ZnS core/shell/shell QDs with the quantum yield close to one, carrier trapping decreases with increasing the energy difference between band-edge and trap states, indicating that the process follows the energy gap law, well known in molecular photochemistry for nonradiative internal conversion between two electronic states. Similar to the molecular view of the energy gap law, it is found that the energy gap between the band-edge and trap states is closely associated with ZnSe phonons that assist carrier trapping into defects in highly luminescent InP/ZnSe/ZnS QDs. These findings represent a striking departure from the generally accepted view of carrier trapping mechanism in QDs in the Marcus normal region, providing a step forward understanding how excitons in nanocrystals interact with traps, and offering valuable guidance for making highly efficient and stable InP-based QDs.

Inverse-designed semiconductor nanocatalysts for targeted CO2 reduction in water

The most commonly used photocatalyst for CO₂ reduction is TiO₂. However, this semiconductor material is far from being ideally suited for this purpose, owing to its inefficient energy harvesting (it absorbs in the UV), low reduction rates (it exhibits short carrier lifetimes), and lack of selectivity with respect to competing reactions (such as the nearly isoenergetic and kinetically more favourable water reduction). In this work we compile a wish-list of properties for the ideal photocatalyst (including high reaction selectivity, availability of multiple redox equivalents at one time, large contact area for CO₂ adsorption with independently tunable band gap, and availability of electrons and holes at different locations on the surface for the two redox reactions to take place), and, using the principles of inverse design, we engineer a semiconductor nanostructure that not only meets all the necessary fundamental criteria to act as a catalyst for CO₂ reduction, but also exhibits all the wish-list properties, as confirmed by our state-of-the-art atomistic semi-empirical pseudopotential modelling. The result is a potentially game-changing material.

Competition between Photoinduced Electron Transfer and Resonance Energy Transfer in an Example of Substituted Cytochrome c-Quantum Dot Systems

Colloidal quantum dots (QDs) are nanoparticles that are able to photoreduce redox proteins by electron transfer (ET). QDs are also able to transfer energy by resonance energy transfer (RET). Here, we address the question of the competition between these two routes of QDs’ excitation quenching, using cadmium telluride QDs and cytochrome c (CytC) or its metal-substituted derivatives. We used both oxidized and reduced versions of native CytC, as well as fluorescent, nonreducible Zn(II)CytC, Sn(II)CytC, and metal-free porphyrin CytC. We found that all of the CytC versions quench QD fluorescence, although the interaction may be described differently in terms of static and dynamic quenching. QDs may be quenchers of fluorescent CytC derivatives, with significant differences in effectiveness depending on QD size. SnCytC and porphyrin CytC increased the rate of Fe(III)CytC photoreduction, and Fe(II)CytC slightly decreased the rate and ZnCytC presence significantly decreased the rate and final level of reduced FeCytC. These might be partially explained by the tendency to form a stable complex between protein and QDs, which promoted RET and collisional quenching. Our findings show that there is a net preference for photoinduced ET over other ways of energy transfer, at least partially, due to a lack of donors, regenerating a hole at QDs and leading to irreversibility of ET events. There may also be a common part of pathways leading to photoinduced ET and RET. The nature of synergistic action observed in some cases allows the hypothesis that RET may be an additional way to power up the ET.

Charge Dynamics in Quantum-Dot-Acceptor Complexes in the Presence of Confining and Deconfining Ligands

Nanocrystal surface functionalization is becoming widespread for applications exploiting fast charge extraction or ultrasensitive redox reactions. A variety of molecular acceptors are being linked to the dot surface via a new generation of organic ligands, ranging from neutral linkers to charge delocalizers. Understanding how core states interact with these molecular orbitals, localized outside the dot, is paramount for optimizing the design of efficient nanocrystal-acceptor conjugates. Here we look at two examples of this interaction: charge transfer to a molecular acceptor linked through either an exciton-delocalizing ligand or a more conventional localizing molecule. We find that such transfer can be described in terms of an Auger-mediated process whose rates can be tuned within a window of a few orders of magnitude (for the same dot-ligand-acceptor conjugate) by a suitable choice of the dispersion solvent and nanocrystal's dielectric environment. This result provides clear guidelines for charge extraction rate engineering in nanocrystal-based devices.

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation

Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II-VI and III-V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl₃ or CdCl₂ afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II-VI and III-V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation

Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II–VI and III–V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl₃ or CdCl₂ afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II–VI and III–V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.

Enhancement of the Fluorescence Quantum Yield of Thiol-Stabilized CdTe Quantum Dots Through Surface Passivation with Sodium Chloride and Bicarbonate

Colloidal quantum dots (QDs) have potential for several applications, e.g. as novel light sources; as photoluminescent probes; and for solar energy conversion devices, but their sensitivity towards their environmental surroundings, and the presence of surface defects may still degrade their emission properties. Herein, we present a post-synthetic treatment of CdTe QDs stabilized by mixed thiol ligands using chloride and bicarbonate ions to achieve near-complete surface passivation, resulting in a two-fold increase of the photoluminescence quantum yield (PL QY) and significant suppression of non-radiative recombination. Time-resolved PL measurements reveal fluorescence lifetime and PL QY trends did not both track identically; in the most favorable cases a suppression of non-radiative recombination and a slight increase in the radiative recombination rates after the salt treatment took place. The optimized conditions demonstrated here are proven to work for different sizes of CdTe QDs, and also show a dependence on the composition of the mixed ligand systems used.

Pure white-light emitting ultrasmall organic-inorganic hybrid perovskite nanoclusters

Organic-inorganic hybrid perovskites, direct band-gap semiconductors, have shown tremendous promise for optoelectronic device fabrication. We report the first colloidal synthetic approach to prepare ultrasmall (∼1.5 nm diameter), white-light emitting, organic-inorganic hybrid perovskite nanoclusters. The nearly pure white-light emitting ultrasmall nanoclusters were obtained by selectively manipulating the surface chemistry (passivating ligands and surface trap-states) and controlled substitution of halide ions. The nanoclusters displayed a combination of band-edge and broadband photoluminescence properties, covering a major part of the visible region of the solar spectrum with unprecedentedly large quantum yields of ∼12% and photoluminescence lifetime of ∼20 ns. The intrinsic white-light emission of perovskite nanoclusters makes them ideal and low cost hybrid nanomaterials for solid-state lighting applications.

Impact of D2O/H2O Solvent Exchange on the Emission of HgTe and CdTe Quantum Dots: Polaron and Energy Transfer Effects

We have studied light emission kinetics and analyzed carrier recombination channels in HgTe quantum dots that were initially grown in H2O. When the solvent is replaced by D2O, the nonradiative recombination rate changes highlight the role of the vibrational degrees of freedom in the medium surrounding the dots, including both solvent and ligands. The contributing energy loss mechanisms have been evaluated by developing quantitative models for the nonradiative recombination via (i) polaron states formed by strong coupling of ligand vibration modes to a surface trap state (nonresonant channel) and (ii) resonant energy transfer to vibration modes in the solvent. We conclude that channel (i) is more important than (ii) for HgTe dots in either solution. When some of these modes are removed from the relevant spectral range by the H2O to D2O replacement, the polaron effect becomes weaker and the nonradiative lifetime increases. Comparisons with CdTe quantum dots (QDs) served as a reference where the resonant energy loss (ii) a priori was not a factor, also confirmed by our experiments. The solvent exchange (H2O to D2O), however, is found to slightly increase the overall quantum yield of CdTe samples, probably by increasing the fraction of bright dots in the ensemble. The fundamental study reported here can serve as the foundation for the design and optimization principles of narrow bandgap quantum dots aimed at applications in long wavelength colloidal materials for infrared light emitting diodes and photodetectors.

Integration of CdSe/CdSexTe1-x Type-II Heterojunction Nanorods into Hierarchically Porous TiO2 Electrode for Efficient Solar Energy Conversion

Semiconductor sensitized solar cells, a promising candidate for next-generation photovoltaics, have seen notable progress using 0-D quantum dots as light harvesting materials. Integration of higher-dimensional nanostructures and their multi-composition variants into sensitized solar cells is, however, still not fully investigated despite their unique features potentially beneficial for improving performance. Herein, CdSe/CdSe_xTe_1−x type-II heterojunction nanorods are utilized as novel light harvesters for sensitized solar cells for the first time. The CdSe/CdSe_xTe_1−x heterojunction-nanorod sensitized solar cell exhibits ~33% improvement in the power conversion efficiency compared to its single-component counterpart, resulting from superior optoelectronic properties of the type-II heterostructure and 1-octanethiol ligands aiding facile electron extraction at the heterojunction nanorod-TiO₂ interface. Additional ~32% enhancement in power conversion efficiency is achieved by introducing percolation channels of large pores in the mesoporous TiO₂ electrode, which allow 1-D sensitizers to infiltrate the entire depth of electrode. These strategies combined together lead to 3.02% power conversion efficiency, which is one of the highest values among sensitized solar cells utilizing 1-D nanostructures as sensitizer materials.

Exciton–phonon scattering and nonradiative relaxation of excited carriers in hydrothermally synthesized CdTe quantum dots

The strength of the exciton–LO-phonon coupling, as reflected in the Huang–Rhys parameter ‘S’, is found to increase from 1.13 to 1.51 with a reduction in CdTe QD size from 4.8 to 3.0 nm.