Reversed oxygen sensing using colloidal quantum wells towards highly emissive photoresponsive varnishes

A closer look at the effects of oxygen on the photoluminescence properties of CdSe/CdS quantum dots

We present a detailed study on the effects of oxygen on the photoluminescence properties of CdSe/CdS quantum dots (QDs). We investigated the role of oxygen by performing confocal measurements on thin films as well as on single particles while rapidly exchanging the gaseous environment between oxygen and an inert gas atmosphere. We found that the deionization of negatively charged particles by oxygen depends on both the excitation power and the shell thickness of the QDs. For QDs with thin shells, which exhibit strong photoluminescence blinking, we observed that the presence of oxygen affects both band-edge carrier blinking and hot-carrier blinking.

Highly Transparent, Yet Photoluminescent: 2D CdSe/CdS Nanoplatelet-Zeolitic Imidazolate Framework Composites Sensitive to Gas Adsorption

In this work, thin composite films of zeolitic imidazolate frameworks (ZIFs) and colloidal two-dimensional (2D) core-crown CdSe/CdS nanoplatelet (NPL) emitters with minimal scattering are formed by a cycled growth method and yield highly transparent coatings with strong and narrow photoluminescence of the NPLs at 546 nm (FWHM: 25 nm) in a solid-state composite structure. The porous ZIF matrix acts as functional encapsulation for the emitters and enables the adsorption of the guest molecules water and ethanol. The adsorption and desorption of the guest molecules is then characterized by a reversable photoluminescence change of the embedded NPLs. The transmittance of the composite films exceeds the values of uncoated glass at visible wavelengths where the NPL emitters show no absorption (>540 nm) and renders them anti-reflective coatings. At NPL absorption wavelengths (440-540 nm), the transmittance of the thin composite film-coated glass lies close to the transmittance of uncoated glass. The fast formation of innovative, smooth NPL/ZIF composite films without pre-polymerizing the colloidal 2D nanostructures first provides a powerful tool toward application-oriented photoluminescence-based gas sensing.

Revealing Two Distinct Formation Pathways of 2D Wurtzite-CdSe Nanocrystals Using In Situ X-Ray Scattering

Understanding the mechanism underlying the formation of quantum-sized semiconductor nanocrystals is crucial for controlling their synthesis for a wide array of applications. However, most studies of 2D CdSe nanocrystals have relied predominantly on ex situ analyses, obscuring key intermediate stages and raising fundamental questions regarding their lateral shapes. Herein, the formation pathways of two distinct quantum-sized 2D wurtzite-CdSe nanocrystals - nanoribbons and nanosheets - by employing a comprehensive approach, combining in situ small-angle X-ray scattering techniques with various ex situ characterization methods is studied. Although both nanostructures share the same thickness of ≈1.4 nm, they display contrasting lateral dimensions. The findings reveal the pivotal role of Se precursor reactivity in determining two distinct synthesis pathways. Specifically, highly reactive precursors promote the formation of the nanocluster-lamellar assemblies, leading to the synthesis of 2D nanoribbons with elongated shapes. In contrast, mild precursors produce nanosheets from a tiny seed of 2D nuclei, and the lateral growth is regulated by chloride ions, rather than relying on nanocluster-lamellar assemblies or Cd(halide)2 -alkylamine templates, resulting in 2D nanocrystals with relatively shorter lengths. These findings significantly advance the understanding of the growth mechanism governing quantum-sized 2D semiconductor nanocrystals and offer valuable guidelines for their rational synthesis.

Electrochemical Control over the Optical Properties of II-VI Colloidal Nanoplatelets by Tailoring the Station of Extra Charge Carriers

An electrochemical (EC) method has been successfully applied to regulate the optical properties of nanocrystals, such as reducing their gain threshold by EC doping and enhancing their photoluminescence intensity by EC filling of trap states. However, the processes of EC doping and filling are rarely reported simultaneously in a single study, hindering the understanding of their underlying interactions. Here, we report the spectroelectrochemical (SEC) studies of quasi-two-dimensional nanoplatelets (NPLs), intending to clarify the above issues. EC doping is successfully achieved in CdSe/CdZnS core/shell NPLs, with red-shifted photoluminescence and a reversal of the emission intensity trend. The injection of extra electrons (holes) into the conduction (valence) band edges needs high bias voltages, while the passivation/activation process of trap states with the shift of Fermi level starts at lower EC potentials. Then, we explore the role of excitation light conditions in these processes, different from existing SEC research studies. Interestingly, increasing the laser power density can hinder EC electron injection, whereas decreasing the excitation energy evades the passivation process of trap states. Moreover, we demonstrate that EC control strategies can be used to realize color display and anti-counterfeiting applications via simultaneously tailoring the photoluminescence intensity of red- and green-emitting NPLs.

Colloidal CdSe/CdS Core/Crown Nanoplatelets for Efficient Blue Light Emission and Optical Amplification

The application of CdSe nanoplatelets (NPLs) in the ultraviolet/blue region remains an open challenge due to charge trapping typically leading to limited photoluminescence quantum efficiency (PL QE) and sub-bandgap emission in core-only NPLs. Here, we synthesized 3.5 monolayer core/crown CdSe/CdS NPLs with various crown dimensions, exhibiting saturated blue emission and PL QE up to 55%. Compared to core-only NPLs, the PL intensity decays monoexponentially over two decades due to suppressed deep trapping and delayed emission. In both core-only and core/crown NPLs we observe biexciton-mediated optical gain between 470 and 510 nm, with material gain coefficients up to 7900 cm^-1 and consistently lower gain thresholds in crowned NPLs. Gain lifetimes are limited to 40 ps, due to residual ultrafast trapping and higher exciton densities at threshold. Our results provide guidelines for rational optimization of thin CdSe NPLs toward lighting and light-amplification applications.

Measuring the Ultrafast Spectral Diffusion and Vibronic Coupling Dynamics in CdSe Colloidal Quantum Wells using Two-Dimensional Electronic Spectroscopy

We measure the ultrafast spectral diffusion, vibronic dynamics, and energy relaxation of a CdSe colloidal quantum wells (CQWs) system at room temperature using two-dimensional electronic spectroscopy (2DES). The energy relaxation of light-hole (LH) excitons and hot carriers to heavy-hole (HH) excitons is resolved with a time scale of ∼210 fs. We observe the equilibration dynamics between the spectroscopically accessible HH excitonic state and a dark state with a time scale of ∼160 fs. We use the center line slope analysis to quantify the spectral diffusion dynamics in HH excitons, which contains an apparent sub-200 fs decay together with oscillatory features resolved at 4 and 25 meV. These observations can be explained by the coupling to various lattice phonon modes. We further perform quantum calculations that can replicate and explain the observed dynamics. The 4 meV mode is observed to be in the near-critically damped regime and may be mediating the transition between the bright and dark HH excitons. These findings show that 2DES can provide a comprehensive and detailed characterization of the ultrafast spectral properties in CQWs and similar nanomaterials.

2D II-VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II-VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II-VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II-VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron-hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

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.

Multifunctional Platform Based on a Copper(I) Complex and NaYF4:Tm3+,Yb3+ Upconverting Nanoparticles Immobilized into a Polystyrene Matrix: Downshifting and Upconversion Oxygen Sensing

This work presents an innovative approach to obtain a multifunctional hybrid material operating via combined anti-Stokes (upconversion) and Stokes (downshifting) emissions for oxygen gas sensing and related functionalities. The material is based on a Cu(I) complex exhibiting thermally activated delayed fluorescence emission (TADF) and infrared-to-visible upconverting Tm³⁺/Yb³⁺-doped NaYF₄ nanoparticles supported in a polystyrene (PS) matrix. Excitation of the hybrid material at 980 nm leads to efficient transfer of Tm³⁺ emission in the ultraviolet/blue region to the Cu(I) complex and consequently intense green emission (560 nm) of the latter. Additionally, the green emission of the complex can also be directly generated with excitation at 360 nm. Independently of the excitation wavelength, the emission intensity is efficiently suppressed by the presence of molecular oxygen and the quenching rate is properly characterized by the Stern-Volmer plots. The results indicate that the biocompatible hybrid material can be applied as an efficient O₂ sensor operating via near-infrared or ultraviolet excitation, unlike most optical oxygen sensors currently available which only work in downshifting mode.

Binary temporary photo-response of ZnSe:Mn/ZnS quantum dots for visible time-domain anti-counterfeiting

The development of multi-level anti-counterfeiting techniques is of great significance for economics and security issues, particularly the newly emerged temporal-domain techniques based on lifetime coding. However, the intricate reading methods required to obtain temporal-level information are inevitably cumbersome and expensive, which greatly limits the practical applications of these techniques. Herein, we report a novel, unclonable time-domain anti-counterfeiting strategy for the first time, which is achieved using photo-responsive ZnSe:Mn/ZnS quantum dots (QDs) with dynamic luminescence and can be authenticated by the naked eye. Through introducing electron traps and constructing cascade electron channels in the QDs, the binary temporary photo-response is tailored and manifested as distinctive response rates between the band-edge and Mn ⁴T₁-⁶A₁ transition emissions. Impressively, the generated photo-response is instantaneous, is capable of delayed recovery, and can be visibly detected under UV irradiation. The prospective use of colorless, nontoxic aqueous-phase ZnSe:Mn/ZnS QDs provides a new idea and important guidance for developing the next generation of multi-level anti-counterfeiting techniques without the need for complex time-gated decoding instrumentation.

Nonblinking Colloidal Quantum Dots via Efficient Multiexciton Emission

Nonblinking colloidal quantum dots (QDs) are significant to their applications as single-photon sources or light-emitting materials. Herein, a simple heat-up method was developed to synthesize high-qualityWZ-CdSe/CdS core-shell colloidal QDs, which achieved a near-unity photoluminescence quantum yield (PLQY). It was found that the blinking behavior of such QDs was completely suppressed at high excitation intensities, and ultra-stable PL emission was observed. For this reason, a systematic investigation was conducted, revealing that the complete blinking suppression was attributed mainly to the efficient multiexciton emission at high excitation intensities. Such high-quality QDs with nonblinking behaviors and nearly ideal PL properties at high excitation intensities have massive potential applications in various robust conditions, including QD display screens, single-particle tracks, and single-photon sources.

Water Effects on Colloidal Semiconductor Nanocrystals: Correlation of Photophysics and Photochemistry

With high-quality CdSe/CdS core/shell nanocrystals as the main model system and under a controlled atmosphere, responses of photoexcited semiconductor nanocrystals to two active species (water and/or oxygen) in an ambient environment are studied systematically. Under photoexcitation, although high-quality semiconductor nanocrystals in either thin solid films or various solutions have a near-unity photoluminescence quantum yield, there is still a small probability (∼10^-5 per photon absorbed) to be photoreduced by the water molecules efficiently accumulated in the highly hydrophilic nanocrystal-ligands interface. The resulting negatively charged nanocrystals are the starting point of most photophysical variations, and the hydroxyl radical─key photo-oxidation product of water─plays the main role for initiating various photochemical processes. Depending on the supplementation of water to the interface, accessibility to oxygen, photoirradiation power, type of matrices, type of measurement schemes, and solubility of nanocrystals in the solution, various photophysical/photochemical phenomena─either reported or not reported in the literature─are reproducibly observed. Results confirm that photophysical properties and photochemical reactions can be well-correlated, offering a unified and unique basis for fundamental studies and the design of processing techniques in industry.

Optical-Gain-based Sensing Using Inorganic-Ligand-Passivated Colloidal Quantum Dots

Thanks to their extremely large surface-to-volume ratio, colloidal quantum dots are potential high-performance sensing materials. However, previous sensing works using their spontaneous emission suffer from low sensitivities. The absence of an amplification process and the presence of the steric hindrance of long-chain organic ligands are two possible causations. Herein we propose that these two issues can be circumvented by using the amplified spontaneous emission of colloidal quantum dots capped by short-chain inorganic ligands. To exemplify this concept, we performed humidity sensing and observed a ∼31 times enhancement in sensitivity. Meanwhile, we found that the amplified spontaneous emission threshold power was reduced by 34% in a high humidity environment. On the basis of our transient absorption measurements, we attribute these observations to the mitigation of ultrafast subpicosecond trapping processes, which are enabled by the absorption of water molecules.

Colloidal Synthesis of Laterally Confined Blue-Emitting 3.5 Monolayer CdSe Nanoplatelets

The typical synthesis protocol for blue-emitting CdSe nanoplatelets (NPLs) yields particles with extended lateral dimensions and large surface areas, resulting in NPLs with poor photoluminescence quantum efficiency. We have developed a synthesis protocol that achieves an improved control over the lateral size, by exploiting a series of long-chained carboxylate precursors that vary from cadmium octanoate (C₈) to cadmium stearate (C₁₈). The length of this metallic precursor is key to tune the width and aspect ratio of the final NPLs, and for the shorter chain lengths, the synthesis yield is improved. NPLs prepared with our procedure possess significantly enhanced photoluminescence quantum efficiencies, up to 30%. This is likely due to their reduced lateral dimensions, which also grant them good colloidal stability. As the NPL width can be tuned below the bulk exciton Bohr radius, the band edge blue-shifts, and we constructed a sizing curve relating the NPL absorption position and width. Further adjusting the synthesis protocol, we were able to obtain even thinner NPLs, emitting in the near-UV region, with a band-edge quantum efficiency of up to 11%. Results pave the way to stable and efficient light sources for applications such as blue and UV light-emitting devices and lasers.

Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

The surface of nominally diamagnetic colloidal CdSe nanoplatelets can demonstrate paramagnetic behaviour owing to the uncompensated spins of dangling bonds, as we reveal here by optical spectroscopy in high magnetic fields up to 15 T using the exciton spin as a probe of the surface magnetism. The strongly nonlinear magnetic field dependence of the circular polarization of the exciton emission is determined by the magnetization of the dangling-bond spins (DBSs), the exciton spin polarization as well as the spin-dependent recombination of dark excitons. The sign of the exciton-DBS exchange interaction depends on the nanoplatelet growth conditions.

Ligand-conjugated quantum dots for fast sub-diffraction protein tracking in acute brain slices

Semiconductor quantum dots (QDs) have demonstrated utility in long-term single particle tracking of membrane proteins in live cells in culture. To extend the superior optical properties of QDs to more physiologically relevant cell platforms, such as acute brain slices, we examine the photophysics of compact ligand-conjugated CdSe/CdS QDs using both ensemble and single particle analysis in brain tissue media. We find that symmetric core passivation is critical for both photostability in oxygenated media and for prolonged single particle imaging in brain slices. We then demonstrate the utility of these QDs by imaging single dopamine transporters in acute brain slices, achieving 20 nm localization precision at 10 Hz frame rates. These findings detail design requirements needed for new QD probes in complex living environments, and open the door to physiologically relevant studies that capture the utility of QD probes in acute brain slices.

Observation of Electron Shakeup in CdSe/CdS Core/Shell Nanoplatelets

While ensembles of CdSe nanoplatelets (NPLs) show remarkably narrow photoluminescence line widths at room temperature, adding a CdS shell to increase their fluorescence efficiency and photostability causes line width broadening. Moreover, ensemble emission spectra of CdSe/CdS core/shell NPLs become strongly asymmetric at cryogenic temperatures. If the origin of these effects were understood, this could potentially lead to stable core/shell NPLs with narrower emission, which would be advantageous for applications. To move in this direction, we report time-resolved emission spectra of individual CdSe/CdS core/shell NPLs at 4 K. We observe surprisingly complex emission spectra that contain multiple spectrally narrow emission features that change during the experiment. With machine-learning algorithms, we can extract characteristic peak energy differences in these spectra. We show that they are consistent with electron "shakeup lines" from negatively charged trions. In this process, an electron-hole pair recombines radiatively but gives part of its energy to the remaining electron by exciting it into a higher single-electron level. This "shakeup" mechanism is enabled in our NPLs due to strong exciton binding and weak lateral confinement of the charge carriers. Time-resolved single-photon-counting measurements and numerical calculations suggest that spectral jumps in the emission features originate from fluctuations in the confinement potential caused by microscopic structural changes on the NPL surface (e.g., due to mobile surface charges). Our results provide valuable insights into line width broadening mechanisms in colloidal NPLs.

In Situ Photoluminescence of Colloidal Quantum Dots During Gas Exposure-The Role of Water and Reactive Atomic Layer Deposition Precursors

Colloidal quantum dots (QDs) are a promising material for optoelectronic applications. Typically, device integration requires QDs to be embedded in a host material. Atomic layer deposition (ALD) is often considered as a deposition technique for such purposes. However, it is known that ALD and vacuum processes often influence the optical properties of QDs in a negative way. Here, we describe an in situ photoluminescence (PL) measurement setup and use it to monitor the PL of QDs under vacuum and during ALD. For CdSe-based core/shell QDs, a reduction in the QD PL was observed upon exposure to vacuum. Water was identified as crucial for maintaining a high PL as evidenced by re-exposure to different gases. Furthermore, we addressed the influence of vacuum, different plasmas (O₂, H₂O, H₂, H₂S/Ar, and Ar), precursors (trimethylaluminum, diethylzinc, tetrakis(dimethylamido)titanium, and tetrakis(ethylmethylamido)hafnium), reactants (H₂O, H₂S, and O₃), and ALD processes (Al₂O₃, TiO₂, HfO₂, and ZnS) on QDs. We observed a PL reduction by up to 90% upon plasma treatments. Furthermore, we found that trimethylaluminum and diethylzinc reduced the PL efficiency by more than 70% while exposure to tetrakis(dimethylamido)titanium and tetrakis(ethylmethylamido)hafnium lowered the PL by only 10-20%. Surprisingly, tetrakis(dimethylamido)titanium and H₂O, which by themselves had only a minor influence on the QD PL, still caused an 80% drop of the PL efficiency when combined as an ALD process. On the other hand, ALD growth of HfO₂ by combining tetrakis(ethylmethylamido)hafnium and O₃ preserved 80% of the initial PL quantum yield, making it a promising process for QD embedding. These results put forward in situ PL measurements as a versatile technique to identify suitable precursors, reactants and ALD processes for QD embedding and investigate the interaction between QDs and reactive gaseous species in general.

O2 as a molecular probe for nonradiative surface defects in CsPbBr3 perovskite nanostructures and single crystals

Lead halide perovskites, owing to their flexible, scalable chemistry and promising physical properties are attracting increasing attention for solution-processed optoelectronic and photonic technologies. Despite their well-known 'defect tolerant' electronic structure, studies highlighted the active role of shallow and deep defect states, as well as of oxidative environmental conditions, on the optical and electrical behavior of perovskite nanocubes, films and single bulk crystals. To date, however, no in-depth systematic study of the surface trap-mediated processes in perovskite materials of different dimensionality has been conducted. In this work, we aim to bridge this gap by using O2 as a molecular probe for the effects of surface states on the exciton recombination processes of nanocubes (NCs), nanowires (NWs), nanosheets (NSs) and bulk single crystals (SCs) of CsPbBr3 perovskite. Continuous wave and time-resolved photoluminescence (PL) experiments in a controlled O2 atmosphere reveal the opposite optical response of NCs with respect to higher dimensional perovskites directly deriving from the different nature of the material surfaces. Specifically, O2 passivates surface hole-traps in NWs, NSs and SCs, leading to PL brightening with unaltered recombination dynamics. Conversely, NCs appear to be free from such surface hole-traps and exposure to O2 leads to direct extraction of photogenerated electrons that competes with radiative exciton recombination, leading to dimmed PL efficiency in atmospheric conditions. This opposite oxygen PL response demystifies the critical role of surface passivation in perovskite NCs in stark contrast to higher dimensional nanostructures and single crystals.

Optoelectronic Properties of CuI Photoelectrodes

Detailed mechanistic understanding of the optoelectronic features is a key factor in designing efficient and stable photoelectrodes. In situ spectroelectrochemical methods were employed to scrutinize the effect of trap states on the optical and electronic properties of CuI photoelectrodes and to assess their stability against (photo)electrochemical corrosion. The excitonic band in the absorption spectrum and the Raman spectral features were directly influenced by the applied bias potential. These spectral changes exhibit a good correlation with the alterations observed in the charge-transfer resistance. Interestingly, the population and depopulation of the trap states, which are responsible for the changes in both the optical and electronic properties, occur in a different potential/energy regime. Although cathodic photocorrosion of CuI is thermodynamically favored, this process is kinetically hindered, thus providing good stability in photoelectrochemical operation.

An overview on enhancing the stability of lead halide perovskite quantum dots and their applications in phosphor-converted LEDs

Beyond the unprecedented success achieved in photovoltaics (PVs), lead halide perovskites (LHPs) have shown great potential in other optoelectronic devices. Among them, nanometer-scale perovskite quantum dots (PQDs) with fascinating optical properties including high brightness, tunable emission wavelength, high color purity, and high defect tolerance have been regarded as promising alternative down-conversion materials in phosphor-converted light-emitting diodes (pc-LEDs) for lighting and next-generation of display technology. Despite the promising applications of perovskite materials in various fields, they have received strong criticism for the lack of stability. The poor stability has also attracted much attention. Within a few years, numerous strategies towards enhancing the stability have been developed. This review summarizes the mechanisms of intrinsic- and extrinsic-environment-induced decomposition of PQDs. Simultaneously, the strategies for improving the stability of PQDs are reviewed in detail, which can be classified into four types: (1) compositional engineering; (2) surface engineering; (3) matrix encapsulation; (4) device encapsulation. Finally, the challenges for applying PQDs in pc-LEDs are highlighted, and some possible solutions to improve the stability of PQDs together with suggestions for further improving the performance of pc-LEDs as well as the device lifetime are provided.

Effect of Post Thermal Annealing on the Optical Properties of InP/ZnS Quantum Dot Films

The enhancement of optical properties via thermal annealing on InP/ZnS core/shell quantum dot (QD) film was investigated in this work. The increase of emission intensities of the QD films was observed after thermal annealing at 180 °C for 5 min. Through temperature dependence photoluminescence (TDPL) and power dependence photoluminescence (PL) measurement, the peak located at the low-energy shoulder was confirmed to be localized state emission and the high energy one comes from free-carrier emission. Moreover, from the TDPL spectra of the sample annealed at 180 °C for 5 min, the full width at half maximum (FWHM) of localization state emission was nearly the same before which is 250 K and then decreased with increasing temperature. However, the FWHM was decreased significantly when temperature increased in the untreated sample. We conclude that the escape of localization states with increasing temperature contributes to this anomaly phenomenon. Our studies have significance on the application of QDs in electroluminescence devices and down-conversion light-emitting devices.

Chloride-Induced Thickness Control in CdSe Nanoplatelets

Current colloidal synthesis methods for CdSe nanoplatelets (NPLs) routinely yield samples that emit, in discrete steps, from 460 to 550 nm. A significant challenge lies with obtaining thicker NPLs, to further widen the emission range. This is at present typically achieved via colloidal atomic layer deposition onto CdSe cores, or by synthesizing NPL core/shell structures. Here, we demonstrate a novel reaction scheme, where we start from 4.5 monolayer (ML) NPLs and increase the thickness in a two-step reaction that switches from 2D to 3D growth. The key feature is the enhancement of the growth rate of basal facets by the addition of CdCl₂, resulting in a series of nearly monodisperse CdSe NPLs with thicknesses between 5.5 and 8.5 ML. Optical characterization yielded emission peaks from 554 nm up to 625 nm with a line width (fwhm) of 9-13 nm, making them one of the narrowest colloidal nanocrystal emitters currently available in this spectral range. The NPLs maintained a short emission lifetime of 5-11 ns. Finally, due to the increased red shift of the NPL band edge photoluminescence excitation spectra revealed several high-energy peaks. Calculation of the NPL band structure allowed us to identify these excited-state transitions, and spectral shifts are consistent with a significant mixing of light and split-off hole states. Clearly, chloride ions can add a new degree of freedom to the growth of 2D colloidal nanocrystals, yielding new insights into both the NPL synthesis as well as their optoelectronic properties.

MnII-Doped Cesium Lead Chloride Perovskite Nanocrystals: Demonstration of Oxygen Sensing Capability Based on Luminescent Dopants and Host-Dopant Energy Transfer

The design of photoluminescence-quenching probes for molecular oxygen (O₂) is always a large space to explore. Luminescent semiconductor nanocrystals (NCs) have been proposed as emerging oxygen-responsive probes, but the inherent O₂ sensing of phosphorescent semiconductor NCs has not been reported so far. Here, we demonstrate the O₂ sensing capability of Mn^II-doped CsPbCl₃ nanocrystals (Mn:CsPbCl₃ NCs) and reveal the role of O₂ on the optical de-excitation process of such perovskite nanocrystals (PNCs). By adjusting the amount and distribution of Mn^II dopants, as well as the host-dopant energy transfer process in PNCs, we highlight that O₂ can reversibly quench the Mn^II emission due to the temporary disturbance to the ligand field of near-surface Mn^II dopants in PNCs. In phosphorescence mode, the photoluminescence intensity of the Mn:CsPbCl₃ NCs is quenched by 53% on increasing O₂ concentration from 0 to 100%. The Stern-Volmer plot shows a good linear in the 0-12% O₂ concentration range. High sensing reversibility and rapid signal response are also achieved. In our perception, the mechanism study makes our PNCs candidates for the optical probes of O₂, and it is enlightening to explore more possibilities of the inherent O₂ sensing based on the semiconductor-doped NCs (not restricted to Mn^II-doped PNCs) with phosphorescence emission.

Efficient Solution-Processed Nanoplatelet-Based Light-Emitting Diodes with High Operational Stability in Air

Colloidal nanoplatelets (NPLs), owing to their efficient and narrow-band luminescence, are considered as promising candidates for solution-processable light-emitting diodes (LEDs) with ultrahigh color purity. To date, however, the record efficiencies of NPL-LEDs are significantly lower than those of more-investigated devices based on spherical nanocrystals. This is particularly true for red-emitting NPL-LEDs, the best-reported external quantum efficiency (EQE) of which is limited to 0.63% (EQE = 5% for green NPL-LEDs). Here, we address this issue by introducing a charge-regulating layer of a polar and polyelectrolytic polymer specifically engineered with complementary trimethylammonium and phosphonate functionalities that provide high solubility in orthogonal polar media with respect to the NPL active layer, compatibility with the metal cathode, and the ability to control electron injection through the formation of a polarized interface under bias. Through this synergic approach, we achieve EQE = 5.73% at 658 nm (color saturation 98%) in completely solution processed LEDs. Remarkably, exposure to air increases the EQE to 8.39%, exceeding the best reports of red NPL-LEDs by over 1 order of magnitude and setting a new global record for quantum-dot LEDs of any color embedding solution-deposited organic interlayers. Considering the emission quantum yield of the NPLs (40 ± 5%), this value corresponds to a near-unity internal quantum efficiency. Notably, our devices show exceptional operational stability for over 5 h of continuous drive in air with no encapsulation, thus confirming the potential of NPLs for efficient, high-stability, saturated LEDs.

Excitonic pathway to photoinduced magnetism in colloidal nanocrystals with nonmagnetic dopants

Electronic doping of colloidal semiconductor nanostructures holds promise for future device concepts in optoelectronic and spin-based technologies. Ag⁺ is an emerging electronic dopant in III-V and II-VI nanostructures, introducing intragap electronic states optically coupled to the host conduction band. With its full 4d shell Ag⁺ is nonmagnetic, and the dopant-related luminescence is ascribed to decay of the conduction-band electron following transfer of the photoexcited hole to Ag⁺. This optical activation process and the associated modification of the electronic configuration of Ag⁺ remain unclear. Here, we trace a comprehensive picture of the excitonic process in Ag-doped CdSe nanocrystals and demonstrate that, in contrast to expectations, capture of the photohole leads to conversion of Ag⁺ to paramagnetic Ag²⁺. The process of exciton recombination is thus inextricably tied to photoinduced magnetism. Accordingly, we observe strong optically activated magnetism and diluted magnetic semiconductor behaviour, demonstrating that optically switchable magnetic nanomaterials can be obtained by exploiting excitonic processes involving nonmagnetic impurities.

Electron and Hole g-Factors and Spin Dynamics of Negatively Charged Excitons in CdSe/CdS Colloidal Nanoplatelets with Thick Shells

We address spin properties and spin dynamics of carriers and charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells. Magneto-optical studies are performed by time-resolved and polarization-resolved photoluminescence, spin-flip Raman scattering and picosecond pump-probe Faraday rotation in magnetic fields up to 30 T. We show that at low temperatures the nanoplatelets are negatively charged so that their photoluminescence is dominated by radiative recombination of negatively charged excitons (trions). Electron g-factor of 1.68 is measured, and heavy-hole g-factor varying with increasing magnetic field from -0.4 to -0.7 is evaluated. Hole g-factors for two-dimensional structures are calculated for various hole confining potentials for cubic- and wurtzite lattice in CdSe core. These calculations are extended for various quantum dots and nanoplatelets based on II-VI semiconductors. We developed a magneto-optical technique for the quantitative evaluation of the nanoplatelets orientation in ensemble.

Spectro-electrochemical Probing of Intrinsic and Extrinsic Processes in Exciton Recombination in I-III-VI2 Nanocrystals

Ternary CuInS₂ nanocrystals (CIS NCs) are attracting attention as nontoxic alternatives to heavy metal-based chalcogenides for many technologically relevant applications. The photophysical processes underlying their emission mechanism are, however, still under debate. Here we address this problem by applying, for the first time, spectro-electrochemical methods to core-only CIS and core/shell CIS/ZnS NCs. The application of an electrochemical potential enables us to reversibly tune the NC Fermi energy and thereby control the occupancy of intragap defects involved in exciton decay. The results indicate that, in analogy to copper-doped II-VI NCs, emission occurs via radiative capture of a conduction-band electron by a hole localized on an intragap state likely associated with a Cu-related defect. We observe the increase in the emission efficiency under reductive electrochemical potential, which corresponds to raising the Fermi level, leading to progressive filling of intragap states with electrons. This indicates that the factor limiting the emission efficiency in these NCs is nonradiative electron trapping, while hole trapping is of lesser importance. This observation also suggests that the centers for radiative recombination are Cu²⁺ defects (preexisting and/or accumulated as a result of photoconversion of Cu¹⁺ ions) as these species contain a pre-existing hole without the need for capturing a valence-band hole generated by photoexcitation. Temperature-controlled photoluminescence experiments indicate that the intrinsic limit on the emission efficiency is imposed by multiphonon nonradiative recombination of a band-edge electron and a localized hole. This process affects both shelled and unshelled CIS NCs to a similar degree, and it can be suppressed by cooling samples to below 100 K. Finally, using experimentally measured decay rates, we formulate a model that describes the electrochemical modulation of the PL efficiency in terms of the availability of intragap electron traps as well as direct injection of electrons into the NC conduction band, which activates nonradiative Auger recombination, or electrochemical conversion of the Cu²⁺ states into the Cu¹⁺ species that are less emissive due to the need for their "activation" by the capture of photogenerated holes.

"Quantized" Doping of Individual Colloidal Nanocrystals Using Size-Focused Metal Quantum Clusters

The insertion of intentional impurities, commonly referred to as doping, into colloidal semiconductor quantum dots (QDs) is a powerful paradigm for tailoring their electronic, optical, and magnetic behaviors beyond what is obtained with size-control and heterostructuring motifs. Advancements in colloidal chemistry have led to nearly atomic precision of the doping level in both lightly and heavily doped QDs. The doping strategies currently available, however, operate at the ensemble level, resulting in a Poisson distribution of impurities across the QD population. To date, the synthesis of monodisperse ensembles of QDs individually doped with an identical number of impurity atoms is still an open challenge, and its achievement would enable the realization of advanced QD devices, such as optically/electrically controlled magnetic memories and intragap state transistors and solar cells, that rely on the precise tuning of the impurity states (i.e., number of unpaired spins, energy and width of impurity levels) within the QD host. The only approach reported to date relies on QD seeding with organometallic precursors that are intrinsically unstable and strongly affected by chemical or environmental degradation, which prevents the concept from reaching its full potential and makes the method unsuitable for aqueous synthesis routes. Here, we overcome these issues by demonstrating a doping strategy that bridges two traditionally orthogonal nanostructured material systems, namely, QDs and metal quantum clusters composed of a "magic number" of atoms held together by stable metal-to-metal bonds. Specifically, we use clusters composed of four copper atoms (Cu₄) capped with d-penicillamine to seed the growth of CdS QDs in water at room temperature. The elemental analysis, performed by electrospray ionization mass spectrometry, X-ray fluorescence, and inductively coupled plasma mass spectrometry, side by side with optical spectroscopy and transmission electron microscopy measurements, indicates that each Cu:CdS QD in the ensemble incorporates four Cu atoms originating from one Cu₄ cluster, which acts as a "quantized" source of dopant impurities.

Role of Nonradiative Defects and Environmental Oxygen on Exciton Recombination Processes in CsPbBr3 Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) are emerging as optically active materials for solution-processed optoelectronic devices. Despite the technological relevance of tracing rational guidelines for optimizing their performances and stability beyond their intrinsic resilience to structural imperfections, no in-depth study of the role of selective carrier trapping and environmental conditions on their exciton dynamics has been reported to date. Here we conduct spectro-electrochemical (SEC) experiments, side-by-side to oxygen sensing measurements on CsPbBr3 NCs for the first time. We show that the application of EC potentials controls the emission intensity by altering the occupancy of defect states without degrading the NCs. Reductive potentials lead to strong (60%) emission quenching by trapping of photogenerated holes, whereas the concomitant suppression of electron trapping is nearly inconsequential to the emission efficiency. Consistently, oxidizing conditions result in minor (5%) brightening due to suppressed hole trapping, confirming that electron traps play a minor role in nonradiative decay. This behavior is rationalized through a model that links the occupancy of trap sites with the position of the NC Fermi level controlled by the EC potential. Photoluminescence measurements in controlled atmosphere reveal strong quenching by collisional interactions with O2, which is in contrast to the photobrightening effect observed in films and single crystals. This indicates that O2 acts as a scavenger of photoexcited electrons without mediation by structural defects and, together with the asymmetrical SEC response, suggests that electron-rich defects are likely less abundant in nanostructured perovskites than in the bulk, leading to an emission response dominated by direct interaction with the environment.

Single-Particle Ratiometric Pressure Sensing Based on "Double-Sensor" Colloidal Nanocrystals

Ratiometric pressure sensitive paints (r-PSPs) are all-optical probes for monitoring oxygen flows in the vicinity of complex or miniaturized surfaces. They typically consist of a porous binder embedding mixtures of a reference and a sensor chromophore exhibiting oxygen-insensitive and oxygen-responsive luminescence, respectively. Here, we demonstrate the first example of an r-PSP based on a single two-color emitter that removes limitations of r-PSPs based on chromophore mixtures such as different temperature dependencies of the two chromophores, cross-readout between the reference and sensor signals and phase segregation. In our approach, we utilize a novel "double-sensor" r-PSP that features two spectrally separated emission bands with opposite responses to the O₂ pressure, which boosts the sensitivity with respect to traditional reference-sensor pairs. Specifically, we use two-color-emitting dot-in-bulk CdSe/CdS core/shell nanocrystals, exhibiting red and green emission bands from their core and shell states, whose intensities are respectively enhanced and quenched in response to the increased oxygen partial pressure that effectively tunes the position of the nanocrystal's Fermi energy. This leads to a strong and reversible ratiometric response at the single particle level and an over 100% enhancement in the pressure sensitivity. Our proof-of-concept r-PSPs further exhibit suppressed cross-readout thanks to zero spectral overlap between the core and shell luminescence bands and a temperature-independent ratiometric response between 0 and 70 °C.

Effect of Core/Shell Interface on Carrier Dynamics and Optical Gain Properties of Dual-Color Emitting CdSe/CdS Nanocrystals

Two-color emitting colloidal semiconductor nanocrystals (NCs) are of interest for applications in multimodal imaging, sensing, lighting, and integrated photonics. Dual color emission from core- and shell-related optical transitions has been recently obtained using so-called dot-in-bulk (DiB) CdSe/CdS NCs comprising a quantum-confined CdSe core embedded into an ultrathick (∼7-9 nm) CdS shell. The physical mechanism underlying this behavior is still under debate. While a large shell volume appears to be a necessary condition for dual emission, comparison between various types of thick-shell CdSe/CdS NCs indicates a critical role of the interface "sharpness" and the presence of potential barriers. To elucidate the effect of the interface morphology on the dual emission, we perform side-by-side studies of CdSe/CdS DiB-NCs with nominally identical core and shell dimensions but different structural properties of the core/shell interface arising from the crystal structure of the starting CdSe cores (zincblende vs wurtzite). While both structures exhibit dual emission under comparable pump intensities, NCs with a zincblende core show a faster growth of shell luminescence with excitation fluence and a more readily realized regime of amplified spontaneous emission (ASE) even under "slow" nanosecond excitation. These distinctions can be linked to the structure of the core/shell interface: NCs grown from the zincblende cores contain a ∼3.5 nm thick zincblende CdS interlayer, which separates the core from the wurtzite CdS shell and creates a potential barrier for photoexcited shell holes inhibiting their relaxation into the core. This helps maintain a higher population of shell states and simplifies the realization of dual emission and ASE involving shell-based optical transitions.

CdSe Nanoplatelets: Living Polymers

Colloidal CdSe nanoplatelets are considered to be excellent candidates for many applications in nanotechnology. One of the current challenges is to self-assemble these colloidal quantum wells into large ordered structures to control their collective optical properties. We describe a simple and robust procedure to achieve controlled face-to-face self-assembly of CdSe nanoplatelets into micron-long polymer-like threads made of up to ∼1000 particles. These structures are formed by addition of oleic acid to a stable colloidal dispersion of platelets, followed by slow drying and re-dispersion. We could control the average length of the CdSe nanoplatelet threads by varying the amount of added oleic acid. These 1-dimensional structures are flexible and feature a "living polymer" character because threads of a given length can be further grown through the addition of supplementary nanoplatelets at their reactive ends.

Band-Edge Exciton Fine Structure and Recombination Dynamics in InP/ZnS Colloidal Nanocrystals

We report on a temperature-, time-, and spectrally resolved study of the photoluminescence of type-I InP/ZnS colloidal nanocrystals with varying core size. By studying the exciton recombination dynamics we assess the exciton fine structure in these systems. In addition to the typical bright-dark doublet, the photoluminescence stems from an upper bright state in spite of its large energy splitting (∼100 meV). This striking observation results from dramatically lengthened thermalization processes among the fine structure levels and points to optical-phonon bottleneck effects in InP/ZnS nanocrystals. Furthermore, our data show that the radiative recombination of the dark exciton scales linearly with the bright-dark energy splitting for CdSe and InP nanocrystals. This finding strongly suggests a universal dangling bonds-assisted recombination of the dark exciton in colloidal nanostructures.

Temperature-Dependent Emission Kinetics of Colloidal Semiconductor Nanoplatelets Strongly Modified by Stacking

We systematically studied temperature-dependent emission kinetics in solid films of solution-processed CdSe nanoplatelets (NPLs) that are either intentionally stacked or nonstacked. We observed that the steady-state photoluminescence (PL) intensity of nonstacked NPLs considerably increases with decreasing temperature, whereas there is only a slight increase in stacked NPLs. Furthermore, PL decay time of the stacked NPL ensemble is comparatively much shorter than that of the nonstacked NPLs, and this result is consistent at all temperatures. To account for these observations, we developed a probabilistic model that describes excitonic processes in a stack using Markov chains, and we found excellent agreement between the model and experimental results. These findings develop the insight that the competition between the radiative channels and energy transfer-assisted hole trapping leads to weakly temperature-dependent PL intensity in the case of the stacked NPL ensembles as compared to the nonstacked NPLs lacking strong energy transfer. This study shows that it is essential to account for the effect of NPL stacking to understand their resulting PL emission properties.

Modular Polymer Biosensors by Solvent Immersion Imprint Lithography

We recently demonstrated Solvent Immersion Imprint Lithography (SIIL), a rapid benchtop microsystem prototyping technique, including polymer functionalization, imprinting and bonding. Here, we focus on the realization of planar polymer sensors using SIIL through simple solvent immersion without imprinting. We describe SIIL's impregnation characteristics, including an inherent mechanism that not only achieves practical doping concentrations, but their unexpected 2-fold enhancement compared to the immersion solution. Subsequently, we developed and characterized optical sensors for detecting molecular O₂. To this end, a substantially high dynamic range is reported, including its control through the immersion duration, a manifestation of SIIL's modularity. Overall, SIIL exhibits the potential of improving the operating characteristics of polymer sensors, while significantly accelerating their prototyping, as it requires a few seconds of processing and no need for substrates or dedicated instrumentation. These are critical for O₂ sensing as probed by way of example here, as well as any polymer permeable reactant.