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

Excitation wavelength-dependent quantum yield in water-soluble CdTe quantum dots

The quantum yield (QY) of semiconductor quantum dots (QDs) is severely hampered by the inherent fluorescence intermittency. The QY of QDs typically increases with an increase in the excitation wavelength. Here, we present a distinctive behavior, where the QY is found to decrease with an increase in the excitation wavelength in water-soluble CdTe QDs (CQDs). Single-particle level measurements highlight the increase in permanent single dark particles at longer wavelengths that comprehend the overall QY of the CQDs in bulk solution. Fluorescence correlation spectroscopy further revealed an increase in the number of dark particles at longer wavelengths. As confirmed by D2O/H2O exchange, the presence of H+ ions in water plays an important role in creating variable permanently dark states in the CQDs. This observation was further supported by the cell internalization study of the CQDs, where a much brighter image at a shorter wavelength than at a longer wavelength was observed. A study of the excitation wavelength-dependent QY in QDs may reveal new insights into the applicability of QDs in different device fabrication cases.

Fine-Tuning Crystal Structures of Lead Bromide Perovskite Nanocrystals through Trace Cadmium(II) Doping for Efficient Color-Saturated Green LEDs

Decreasing perovskite nanocrystal size increases radiative recombination due to the quantum confinement effect, but also increases the Auger recombination rate which leads to carrier imbalance in the emitting layers of electroluminescent devices. Here, we overcome this trade-off by increasing the exciton effective mass without affecting the size, which is realized through the trace Cd2+ doping of formamidinium lead bromide perovskite nanocrystals. We observe an ~2.7 times increase in the exciton binding energy benefiting from a slight distortion of the [BX6]4- octahedra caused by doping in the case of that the Auger recombination rate is almost unchanged. As a result, bright color-saturated green emitting perovskite nanocrystals with a photoluminescence quantum yield of 96 % are obtained. Cd2+ doping also shifts up the energy levels of the nanocrystals, relative to the Fermi level so that heavily n-doped emitters convert into only slightly n-doped ones; this boosts the charge injection efficiency of the corresponding light-emitting diodes. The light-emitting devices based on those nanocrystals reached a high external quantum efficiency of 29.4 % corresponding to a current efficiency of 123 cd A-1, and showed dramatically improved device lifetime, with a narrow bandwidth of 22 nm and Commission Internationale de I'Eclairage coordinates of (0.20, 0.76) for color-saturated green emission for the electroluminescence peak centered at 534 nm, thus being fully compliant with the latest standard for wide color gamut displays.

Luminescence enhancement by deuterium

Created literally at the dawn of time, deuterium has been extremely valuable in so many chemistry roles. The subject of this review focuses on one deuterium application in particular: its enhancement of luminescence in many substances. After providing general overviews of both deuterium and luminescence, the early exploration of deuterium's effect on luminescence is described, followed by a number of specific topics. These sections include a discussion of deuterium-influenced luminescence for dyes, proteins, singlet oxygen, and the lanthanide elements, as well as anomalous inverse deuterium luminescence effects. Future directions for this important research topic are also proposed, as well as a summary conclusion.

Raman study of colloidal Cu2ZnSnS4 nanocrystals obtained by "green" synthesis modified by seed nanocrystals or extra cations in the solution

The method of affordable colloidal synthesis of nanocrystalline Cu₂ZnSnS₄ (CZTS) is developed, which is suitable for obtaining bare CZTS nanocrystals (NCs), cation substituted CZTS NCs, and CZTS-based hetero-NCs. For the hetero-NCs, the synthesized in advance NCs of another material are introduced into the reaction solution so that the formation of CZTS takes place preferably on these "seed" NCs. Raman spectroscopy is used as the primary method of structural characterization of the NCs in this work because it is very sensitive to the CZTS structure and allows to probe NCs both in solutions and films. Raman data are corroborated by optical absorption measurements and transmission electron microscopy on selected samples. The CdTe and Ag NCs are found to be good seed NCs, resulting in a comparable or even better quality of the CZTS compound compared to bare CZTS NCs. For Au NCs, on the contrary, no hetero-NCs could be obtained under the given condition. Partial substitution of Zn for Ba during the synthesis of bare CZTS NCs results in a superior structural quality of NCs, while the introduction of Ag for partial substitution of Cu deteriorates the structural quality of the NCs.

Mercury chalcogenide colloidal quantum dots for infrared photodetection: from synthesis to device applications

Commercial infrared (IR) photodetectors based on epitaxial growth inorganic semiconductors, e.g. InGaAs and HgCdTe, suffer from high fabrication cost, poor compatibility with silicon integrated circuits, rigid substrates and bulky cooling systems, which leaves a large development window for the emerging solution-processable semiconductor-based photo-sensing devices. Among the solution-processable semiconductors, mercury (Hg) chalcogenide colloidal quantum dots (QDs) exhibit unique ultra-broad and tuneable photo-responses in the short-wave infrared to far-wave infrared range, and have demonstrated photo-sensing abilities comparable to the commercial products, especially with advances in high operation temperature. Here, we provide a focused review on photodetectors employing Hg chalcogenide colloidal QDs, with a comprehensive summary of the essential progress in the areas of synthesis methods of QDs, property control, device engineering, focus plane array integration, etc. Besides imaging demonstrations, a series of Hg chalcogenide QD photodetector based flexible, integrated, multi-functional applications are also summarized. This review shows prospects for the next-generation low-cost highly-sensitive and compact IR photodetectors based on solution-processable Hg chalcogenide colloidal QDs.

In situ-Synthesized cadmium sulfide quantum dots in pore-forming protein and polysaccharide matrices for optical biosensing applications

The main limitation for practical implementation of quantum dots-based sensors and biosensors is the possible contamination of sensing media with quantum dots (QDs) moved out from the sensor structure, being critical for living systems measurements. Numerous efforts have addressed the challenge of pre-synthesized QDs incorporation into porous matrix provide, on the one hand, proper fixation of quantum dots in its volume and preserving a free analyte transfer from the sensing media to them - on the other hand. Here, we propose an alternative insight into this problem. Instead of using preliminary synthesized particles for doping a matrix, we have in situ synthesized cadmium sulfide QDs in porous biopolymeric matrices, both in an aqueous solution and on a mica substrate. The proposed technique allows obtaining QDs in a matrix acting simultaneously as a ligand passivating surface defects and preventing QDs aggregation. The conjugates were used as a photoluminescence sensor for the metal ions and glutathione detection in an aqueous media. Different kinds of sensor responses have been found depending on the analyte nature. Zinc ions' presence initiates the intraband QDs emission increases due to the reduction of non-radiative processes. The presence of copper ions, in contrast, leads to a gradual photoluminescence decrease due to the formation of the non-luminescent copper-based alloy in the QDs structure. Finally, the presence of glutathione initiates a ligand exchange process followed by some QDs surface treatment enhancing defect-related photoluminescence. As a result, three different kinds of sensor responses for three analytes allow claiming development of a new selective QD-based sensor suitable for biomedical applications.

Establishing design principles for emissive organic SWIR chromophores from energy gap laws

Rational design of bright near and shortwave infrared (NIR: 700-1000 SWIR: 1000- 2000 nm) emitters remains an open question with applications spanning imaging and photonics. Combining experiment and theory, we derive an energy gap quantum yield master equation (EQME), describing the fundamental limits in SWIR quantum yields (ϕ F ) for organic chromophores. Evaluating the photophysics of 21 polymethine NIR/SWIR chromophores to parameterize the EQME, we explain the precipitous decline of ϕ F past 900 nm through decreasing radiative rates and increasing nonradiative losses via high frequency vibrations relating to the energy gap. Using the EQME we develop an energy gap independent ϕ F NIR/SWIR chromophore comparison metric. We show electron donating character on polymethine heterocycles results in relative increases in radiative efficiency obscured by a simultaneous redshift. Finally, the EQME yields rational chromophore design insights shown by how deuteration (backed by our experimental results) or molecular aggregation increases SWIR ϕ F .

Exciton-Photon Interactions in Semiconductor Nanocrystals: Radiative Transitions, Non-Radiative Processes and Environment Effects

In this review, we discuss several fundamental processes taking place in semiconductor nanocrystals (quantum dots (QDs)) when their electron subsystem interacts with electromagnetic (EM) radiation. The physical phenomena of light emission and EM energy transfer from a QD exciton to other electronic systems such as neighbouring nanocrystals and polarisable 3D (semi-infinite dielectric or metal) and 2D (graphene) materials are considered. In particular, emission decay and FRET rates near a plane interface between two dielectrics or a dielectric and a metal are discussed and their dependence upon relevant parameters is demonstrated. The cases of direct (II–VI) and indirect (silicon) band gap semiconductors are compared. We cover the relevant non-radiative mechanisms such as the Auger process, electron capture on dangling bonds and interaction with phonons. Some further effects, such as multiple exciton generation, are also discussed. The emphasis is on explaining the underlying physics and illustrating it with calculated and experimental results in a comprehensive, tutorial manner.

Bright CsPbI3 Perovskite Quantum Dot Light-Emitting Diodes with Top-Emitting Structure and a Low Efficiency Roll-Off Realized by Applying Zirconium Acetylacetonate Surface Modification

Zirconium acetylacetonate used as a co-precursor in the synthesis of CsPbI₃ quantum dots (QDs) increased their photoluminescence quantum efficiency to values over 90%. The top-emitting device structure on a Si substrate with high thermal conductivity (to better dissipate Joule heat generated at high current density) was designed to improve the light extraction efficiency making use of a strong microcavity resonance between the bottom and top electrodes. As a result of these improvements, light-emitting diodes (LEDs) utilizing Zr-modified CsPbI₃ QDs with an electroluminescence at 686 nm showed external quantum efficiency (EQE) of 13.7% at a current density of 108 mA cm^-2, which was combined with low efficiency roll-off (maintaining an EQE of 12.5% at a high current density of 500 mA cm^-2) and a high luminance of 14 725 cd m^-2, and the stability of the devices being repeatedly lit (cycled on and off at high drive current density) has been greatly enhanced.

Tailoring spontaneous infrared emission of HgTe quantum dots with laser-printed plasmonic arrays

Chemically synthesized near-infrared to mid-infrared (IR) colloidal quantum dots (QDs) offer a promising platform for the realization of devices including emitters, detectors, security, and sensor systems. However, at longer wavelengths, the quantum yield of such QDs decreases as the radiative emission rate drops following Fermi's golden rule, while non-radiative recombination channels compete with light emission. Control over the radiative and non-radiative channels of the IR-emitting QDs is crucially important to improve the performance of IR-range devices. Here, we demonstrate strong enhancement of the spontaneous emission rate of near- to mid-IR HgTe QDs coupled to periodically arranged plasmonic nanoantennas, in the form of nanobumps, produced on the surface of glass-supported Au films via ablation-free direct femtosecond laser printing. The enhancement is achieved by simultaneous radiative coupling of the emission that spectrally matches the first-order lattice resonance of the arrays, as well as more efficient photoluminescence excitation provided by coupling of the pump radiation to the local surface plasmon resonances of the isolated nanoantennas. Moreover, coupling of the HgTe QDs to the lattice plasmons reduces the influence of non-radiative decay losses mediated by the formation of polarons formed between QD surface-trapped carriers and the IR absorption bands of dodecanethiol used as a ligand on the QDs, allowing us to improve the shape of the emission spectrum through a reduction in the spectral dip related to this ligand coupling. Considering the ease of the chemical synthesis and processing of the HgTe QDs combined with the scalability of the direct laser fabrication of nanoantennas with tailored plasmonic responses, our results provide an important step towards the design of IR-range devices for various applications.

Exciton relaxation in carbon nanotubes via electronic-to-vibrational energy transfer

Covalent functionalization of semiconducting single-wall carbon nanotubes (CNTs) introduces new photoluminescent emitting states. These states are spatially localized around functionalization sites and strongly red-shifted relative to the emission commonly observed from the CNT band-edge exciton state. A particularly important feature of these localized exciton states is that because the exciton is no longer free to diffusively sample photoluminescent quenching sites along the CNT length, its lifetime is significantly extended. We have recently demonstrated that an important relaxation channel of such localized excitons is the electronic-to-vibrational energy transfer (EVET). This process is analogous to the Förster resonance energy transfer except the final state of this process is not electronically, but vibrationally excited molecules of the surrounding medium (e.g., solvent). In this work, we develop a theory of EVET for a nanostructure of arbitrary shape and apply it to the specific case of EVET-mediated relaxation of defect-localized excitons in a covalently functionalized CNT. The resulting EVET relaxation times are in good agreement with experimental data.

Oxalic Acid Enabled Emission Enhancement and Continuous Extraction of Chloride from Cesium Lead Chloride/Bromide Perovskite Nanocrystals

All-inorganic cesium lead halide perovskite nanocrystals (NCs) have demonstrated excellent optical properties and an encouraging potential for optoelectronic applications; however, mixed-halide perovskites, especially CsPb(Cl/Br)₃ NCs, still show lower photoluminescence quantum yields (PL QY) than the corresponding single-halide materials. Herein, anhydrous oxalic acid is used to post-treat CsPb(Cl/Br)₃ NCs in order to initially remove surface defects and halide vacancies, and thus, to improve their PL QY from 11% to 89% for the emission of 451 nm. Furthermore, due to the continuous chelating reaction with the oxalate ion, chloride anions from the mixed-halide CsPb(Cl/Br)₃ perovskite NCs could be extracted, and green emitting CsPbBr₃ NCs with PL QY of 85% at 511 nm emission are obtained. Besides being useful to improve the emission of CsPb(Cl/Br)₃ NCs, the oxalic acid treatment strategy introduced here provides a further tool to adjust the distribution of halide anions in mixed-halide perovskites without using any halide additives.

Using Polar Alcohols for the Direct Synthesis of Cesium Lead Halide Perovskite Nanorods with Anisotropic Emission

Semiconductor nanorods (NRs) offer the useful property of linearly polarized light emission. While this would be an attractive functionality for strongly emitting perovskite nanoparticles, to date, there has been limited success in demonstrating a direct chemical synthesis of cesium lead halide perovskite NRs. In this work, we realized the direct synthesis of CsPbBr₃ NRs with an average width of around 5 nm and average lengths of 10.8 and 23.2 nm, respectively, in two samples, which show a high photoluminescence quantum yield of 60-76% and reasonably high emission anisotropy of about 0.2 for longer rods. Both CsPbCl₃ and CsPbI₃ NRs with similar dimensions have then been derived from the CsPbBr₃ NRs by anion-exchange reactions. Remarkably, the synthesis of the NRs has been achieved in polar alcohols, a class of solvents not usually found to be beneficial in classical perovskite nanoparticle synthesis. This work not only offers the possibility to control the shape of chemically synthesized perovskite nanocrystals but also constitutes the hitherto less common strategy of synthesizing perovskite nanoparticles in polar rather than nonpolar or only weakly polar solvents.

Zn-Alloyed CsPbI3 Nanocrystals for Highly Efficient Perovskite Light-Emitting Devices

We alloyed Zn²⁺ into CsPbI₃ perovskite nanocrystals by partial substitution of Pb²⁺ with Zn²⁺, which does not change their crystalline phase. The resulting alloyed CsPb_0.64Zn_0.36I₃ nanocrystals exhibited an improved, close-to-unity photoluminescence quantum yield of 98.5% due to the increased radiative decay rate and the decreased non-radiative decay rate. They also showed an enhanced stability, which correlated with improved effective Goldschmidt tolerance factors, by the incorporation of Zn²⁺ ions with a smaller radius than the Pb²⁺ ions. Simultaneously, the nanocrystals switched from n-type (for CsPbI₃) to nearly ambipolar for the alloyed nanoparticles. The hole injection barrier of electroluminescent LEDs was effectively eliminated by using alloyed CsPb_0.64Zn_0.36I₃ nanocrystals, and a high peak external quantum efficiency of 15.1% has been achieved.

Cesium Lead Chloride/Bromide Perovskite Quantum Dots with Strong Blue Emission Realized via a Nitrate-Induced Selective Surface Defect Elimination Process

Cesium lead halide perovskites emitting blue light in the 460-470 nm range of wavelengths have so far been plagued with rather poor luminescent performance, placing inevitable limitations on the development of perovskite nanocrystal-based blue light-emitting devices. Herein, a selective surface defect elimination process with the help of hydrated nitrates was introduced into the perovskite/toluene solution to strip the undesired surface defects and vacancies and to boost the photoluminescence quantum yield of true-blue-light-emitting (at 466 nm) CsPb(Cl/Br)₃ perovskite nanocrystals to the impressive value of 85%. Unlike the conventional passivation strategy, the anionic nitrate ions are able to desorb the undesired surface metallic lead and combine with excess surface metal ions, leaving perovskite quantum dots with better crystallinity and fewer surface defects.

Solvent- and Wavelength-Dependent Photoluminescence Relaxation Dynamics of Carbon Nanotube sp3 Defect States

Photoluminescent sp³ defect states introduced to single wall carbon nanotubes (SWCNTs) through low-level covalent functionalization create new photophysical behaviors and functionality as a result of defect sites acting as exciton traps. Evaluation of relaxation dynamics in varying dielectric environments can aid in advancing a more complete description of defect-state relaxation pathways and electronic structure. Here, we exploit helical wrapping polymers as a route to suspending (6,5) SWCNTs covalently functionalized with 4-methoxybenzene in solvent systems including H₂O, D₂O, methanol, dimethylformamide, tetrahydrofuran, and toluene, spanning a range of dielectric constants from 80 to 3. Defect-state photoluminescence decays were measured as a function of emission wavelength and solvent environment. Emission decays are biexponential, with short lifetime components on the order of 65 ps and long components ranging from around 100 to 350 ps. Both short and long decay components increase as emission wavelength increases, while only the long lifetime component shows a solvent dependence. We demonstrate that the wavelength dependence is a consequence of thermal detrapping of defect-state excitons to produce mobile E₁₁ excitons, providing an important mechanism for loss of defect-state population. Deeper trap states (i.e., those emitting at longer wavelengths) result in a decreased rate for thermal loss. The solvent-independent behavior of the short lifetime component is consistent with its assignment as the characteristic time for redistribution of exciton population between bright and dark defect states. The solvent dependence of the long lifetime component is shown to be consistent with relaxation via an electronic to vibrational energy transfer mechanism, in which energy is resonantly lost to solvent vibrations in a complementary mechanism to multiphonon decay processes.

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.

Carrier Multiplication Mechanisms and Competing Processes in Colloidal Semiconductor Nanostructures

Quantum confined semiconductor nanoparticles, such as colloidal quantum dots, nanorods and nanoplatelets have broad extended absorption spectra at energies above their bandgaps. This means that they can absorb light at high photon energies leading to the formation of hot excitons with finite excited state lifetimes. During their existence, the hot electron and hole that comprise the exciton may start to cool as they relax to the band edge by phonon mediated or Auger cooling processes or a combination of these. Alongside these cooling processes, there is the possibility that the hot exciton may split into two or more lower energy excitons in what is termed carrier multiplication (CM). The fission of the hot exciton to form lower energy multiexcitons is in direct competition with the cooling processes, with the timescales for multiplication and cooling often overlapping strongly in many materials. Once CM has been achieved, the next challenge is to preserve the multiexcitons long enough to make use of the bonus carriers in the face of another competing process, non-radiative Auger recombination. However, it has been found that Auger recombination and the several possible cooling processes can be manipulated and usefully suppressed or retarded by engineering the nanoparticle shape, size or composition and by the use of heterostructures, along with different choices of surface treatments. This review surveys some of the work that has led to an understanding of the rich carrier dynamics in semiconductor nanoparticles, and that has started to guide materials researchers to nanostructures that can tilt the balance in favour of efficient CM with sustained multiexciton lifetimes.