25th anniversary article: Colloidal quantum dot materials and devices: a quarter-century of advances

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

Understanding and controlling the electronic structure of nanomaterials is the key to tailoring their use in a wide range of practical applications. Despite this need, many important electronic states are invisible to conventional optical measurements and are typically identified indirectly based on their inferred impact on luminescence properties. This is especially common and important in the study of nanomaterial surfaces and their associated defects. Surface trap states play a crucial role in photophysical processes yet remain remarkably poorly understood. Here we demonstrate for the first time that broadband electronic sum frequency generation (eSFG) microspectroscopy can directly map the optically bright and dark states of nanoparticles, including the elusive below gap states. This new approach is applied to model cadmium selenide (CdSe) quantum dots (QDs), where the energies of surface trap states have eluded direct optical characterization for decades. Our eSFG measurements show clear signatures of electronic transitions both above the band gap, which we assign to previously reported one- and two-photon transitions associated with the CdSe core, as well as broad spectral signatures below the band gap that are attributed to surface states. In addition to the core states, this analysis reveals two distinct distributions of below gap states, providing the first direct optical measurement of both shallow and deep surface states on this system. Finally, chemical modification of the surfaces via oxidation results in the relative increase in the signals originating from the surface states. Overall, our eSFG experiments provide an avenue to directly map the entirety of the QD core and surface electronic structure, which is expected to open up opportunities to study how these materials are grown in situ and how surface states can be controlled to tune functionality.

Exceptionally stable quantum dot/siloxane hybrid encapsulation material for white light-emitting diodes with a wide color gamut

Herein, we report a luminescent light-emitting diode (LED) encapsulating material using a thermally curable quantum dot (QD)/siloxane hybrid (TSE-QD) color converter, which has superior long-term stability even at elevated temperatures, in high humidity, and in various chemicals. The TSE-QD is cured by a thermal-induced hydrosilylation reaction of an in situ sol-gel synthesized QD dispersed siloxane resin (QD/siloxane resin) without additional ligand-exchange processes. QDs are successfully encapsulated by highly condensed and linear structured siloxane networks with additional chemical linkages between the surface ligands of the QDs and organic functional groups of the siloxane matrix. Moreover, QDs are uniformly distributed within the siloxane matrix retaining their optical properties during the fabrication processes of the TSE-QD. The result is that the stability, as evaluated by the photoluminescence (PL) quantum yield (QY), is greatly improved under harsh conditions, for example, 120 °C/5% relative humidity (RH), ethanol and acetone for 30 days. Based on the exceptionally stable TSE-QD, we demonstrate a white LED using a blue LED chip directly encapsulated by a yellow emitting TSE-QD that shows excellent spectral stability, outstanding reliability at 85 °C/85% RH and a wide color gamut (116% of NTSC).

Stability of Quantum Dots, Quantum Dot Films, and Quantum Dot Light-Emitting Diodes for Display Applications

Quantum dots (QDs) are being highlighted in display applications for their excellent optical properties, including tunable bandgaps, narrow emission bandwidth, and high efficiency. However, issues with their stability must be overcome to achieve the next level of development. QDs are utilized in display applications for their photoluminescence (PL) and electroluminescence. The PL characteristics of QDs are applied to display or lighting applications in the form of color-conversion QD films, and the electroluminescence of QDs is utilized in quantum dot light-emitting diodes (QLEDs). Studies on the stability of QDs and QD devices in display applications are reviewed herein. QDs can be degraded by oxygen, water, thermal heating, and UV exposure. Various approaches have been developed to protect QDs from degradation by controlling the composition of their shells and ligands. Phosphorescent QDs have been protected by bulky ligands, physical incorporation in polymer matrices, and covalent bonding with polymer matrices. The stability of electroluminescent QLEDs can be enhanced by using inorganic charge transport layers and by improving charge balance. As understanding of the degradation mechanisms of QDs increases and more stable QDs and display devices are developed, QDs are expected to play critical roles in advanced display applications.

Enhanced-performance of self-powered flexible quantum dot photodetectors by a double hole transport layer structure

The usefulness of self-powered quantum dot (QD) photodetectors is increased if they are fabricated on flexible substrates. However, the performance of such photodetectors is typically significantly worse than similar devices fabricated on glass substrates due to poor charge transport performance. Here, a novel flexible self-powered CdSexTe1-x QD photodetector with a double hole transport layer of PEDOT:PSS/P-TPD has been fabricated, which achieves a performance comparable to that of rigid devices. The energy level of the P-TPD layer matches well with that of the PEDOT:PSS and QD layers, which significantly enhances photodetection capability across a spectral region that spans the ultraviolet, visible and near infrared (UV-NIR). A low dark current density (1.03 × 10-6 mA cm-2) and a large specific detectivity of approximately 2.6 × 1012 Jones at a wavelength of 450 nm are demonstrated, significantly outperforming previously reported flexible QD-based detectors. This improvement in performance is attributed to both increased hole transport efficiency and the inhibition of electron transport from the QDs into the PEDOT:PSS layer. The photodetector also exhibits good sensitivity under weak illumination, producing a photocurrent of 196 × 10-6 mA cm-2 under an irradiance of 5 μW cm-2. Moreover, no significant performance degradation is observed after 150 bending cycles to an angle of 60 degrees.

An electrochemical tyrosinamide aptasensor using a glassy carbon electrode modified by N-acetyl-l-cysteine-capped Ag-In-S QDs

This paper reports an aptamer-based green approach for the electrochemical evaluation of tyrosinamide (Tyr-NH₂). In this regard, at the first step, an aqueous synthetic strategy for preparing N-acetyl-l-cysteine (NAC)-capped Ag-In-S (AIS) quantum dots (QDs) with bright yellow/orange emission was developed. The conjugation of AIS QDs to NAC-biomolecules provides opportunities for using them as luminescent contrast agents for living cell tracking and labeling or sensing studies. In the next step, the design stage of the aptasensor, the glassy carbon electrode (GCE) was modified with the AIS QDs and then the Tyr-NH₂ special aptamer, which has an amine group at its end, interacts with silver and indium ions at the surface of the AIS QDs and through the formation of covalent bonding of AgN and InN, attaches to the GCE surface modified with the AIS QDs. In this approach, for the first time, NAC-capped AIS QDs have been used to modify the electrode surface in the aptamer-based electrochemical sensor. The response changes of the [Fe(CN)₆]^4-/3- as redox probe, during the modification of GCE surface, the fabrication and assessment of proposed aptasensing, using the cyclic voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy were recorded. The designed aptasensor for the Tyr-NH₂ evaluation showed good linearity from 0.01 to 2.81 nM and 2.81-10.81 nM, and low detection limit of 3.34 pM. The obtained results of the stability, reproducibility and selectivity investigations implying that the reported aptasensor as the first aptamer-based electrochemical assay for Tyr-NH₂, can be reliable for the determination of Tyr-NH₂ in serum samples.

High-Speed Colloidal Quantum Dot Photodiodes via Accelerating Charge Separation at Metal-Oxide Interface

With ever-growing technological demands in the imaging sensor industry for autonomous driving and augmented reality, developing sensors that can satisfy not only image resolution but also the response speed becomes more challenging. Herein, the focus is on developing a high-speed photosensor capable of obtaining high-resolution, high-speed imaging with colloidal quantum dots (QDs) as the photosensitive material. In detail, high-speed QD photodiodes are demonstrated with rising and falling times of τ_r = 28.8 ± 8.34 ns and τ_f = 40 ± 9.81 ns, respectively, realized by fast separation of electron-hole pairs due to the action of internal electric field at the QD interface, mainly by the interaction between metal oxide and the QD's ligands. Such energy transfer relations are analyzed and interpreted with time-resolved photoluminescence measurements, providing physical understanding of the device and working principles.

Sulfur-free synthesis of size tunable rickardite (Cu3-x Te2) spheroids and planar squares

We report a novel synthesis of monodisperse samples of copper telluride with crystallinity and stoichiometry corresponding to forms of rickardite, Cu_3-x Te₂ (x < 1). This synthesis makes use of a ligand balanced reaction to allow control over shape and size by varying the relative and absolute concentration of oleylamine to stearic acid. The rickardite samples presented here display size dependent plasmon peaks in the near infrared and direct energy band gaps between 1.7 and 2.3 eV. As such they may find utility in photovoltaic, thermoelectric or as novel optical materials for study of surface plasmons.

Comparing Semiconductor Nanocrystal Toxicity in Pregnant Mice and Non-Human Primates

Rationale: Despite growing use of engineered nanomaterials (ENM) in applications from electronics to medicine, the potential risk to human health remains a critical concern within clinical use. ENM exposure during pregnancy can potentially cause reproductive toxicity even at levels that produce no measurable harm to animals in normal conditions. Methods: Phospholipid micelle-encapsulated CdSe/CdS/ZnS semiconductor nanocrystals with an average hydrodynamic diameter of 60 nm were intravenously injected during pregnancy in both rodent and nonhuman primate animal models. Cadmium concentration levels and maternal haematological and biochemical markers were determined, along with histopathological examination of major organs. Results: Nanocrystals were found to have crossed the placenta from mother to fetus in both rodents and nonhuman primates. However, the animal models display different responses with respect to reproductive toxicity. In the rodent model, toxicity symptoms are absent in treated subjects, with no observed gestational or fetal abnormalities and complications. A significantly higher miscarriage rate of 60% is recorded for macaques after prenatal nanoparticle administration. There was a miscarriage rate of 15% in the general population despite only ~0.16% of the initial cadmium dose present in the fetus. Blood and biochemical markers of treated macaques indicate acute hepatocellular injury within a week after nanoparticle administration. Histology of major organs of the miscarried macaque fetuses show no abnormalities. Conclusion: The potential of nanomaterials to cross the placenta and impact fetal survival in primates suggest the necessity of precautionary measures to prevent gestational exposure of ENMs.

In Situ Measurement and Control of the Fermi Level in Colloidal Nanocrystal Thin Films during Their Fabrication

In engineering a high-performance semiconductor device, understanding of the Fermi level position is critical. Here, we demonstrate that open-circuit potential (OCP) measurements can be used to quantify the Fermi level in nanocrystal thin films in situ during their solution-based fabrication. We use this method to study the influence of (1) a metal contact and (2) nanocrystal surface termination on the Fermi level of the nanocrystal film, and find that oxidization or reduction of the nanocrystals as well as surface terminations with dipoles can be used to tune the Fermi level over large energy ranges. Finally, to emphasize the compatibility of the technique with device fabrication, we show that we can use blends of ligands to design the Fermi level landscape in a nanocrystal film. Our work highlights that OCP measurements can be used to gain insight into existing device operation and direct further optimization of optoelectronic devices.

Metal-enhanced fluorescence in polymer composite films with Au@Ag@SiO2 nanoparticles and InP@ZnS quantum dots

For white light-emitting diode (LED) applications, semiconductor quantum dots (QDs) have been widely utilized as efficient down-converters to change the blue color of the light source into different emission colors. Because QDs offer spectral tunability over the entire visible light range, as well as improved color purity, they have rapidly replaced conventional phosphor-based white LEDs. However, for the sustainable growth of QD-mediated LEDs, the amount of QDs required must be reduced by enhancing the color-conversion efficiency. For this purpose, we prepared poly(lauryl methacrylate) (PLMA) composite films by the photo-crosslinking polymerization of lauryl methacrylate monomers in the presence of Au@Ag@SiO2 nanoparticles (NPs) and InP@ZnS QDs. In the PLMA composites, the Au@Ag NPs not only amplified the blue light source but also modified the relaxation of the excited QDs via localized surface plasmon resonance. This resulted in a maximum 12.9-fold enhancement in the QD fluorescence. Because the blue light source in this study can be easily replaced by blue LEDs, the enhanced efficiency of QD emissions via the plasmonic effect could potentially increase the performance of QDs for display applications.

Tunable colloidal quantum dot distributed feedback lasers integrated on a continuously chirped surface grating

We report a colloidal quantum dot (CQD) distributed feedback (DFB) laser structure containing a chirped grating. The device exhibits single-mode DFB lasing, of which the wavelength is spatially dispersed in a single chip. A period-chirped surface grating is fabricated using a modified Lloyd-type laser interference lithography setup, where a flat Lloyd's mirror is replaced with a concave one. A dense red-emitting CdSe/CdS/ZnS CQD film is prepared on a temporary substrate by spin-coating, which is subsequently released and wet-transferred onto a period-chirped quartz surface grating. Upon optical excitation, the fabricated DFB laser device lases in a single mode at a laser threshold of ∼400 μJ cm-2, with its lasing wavelength shifted linearly (in proportion to the grating pitch) along the chirp direction from 613.4 nm to 623.2 nm over a distance of ∼5.6 mm.

The influence of ionothermal synthesis using BmimBF4 as a solvent on nanophosphor BaFBr:Eu2+ photoluminescence

Ionothermal synthesis is a strongly growing research area due to the outstanding physical and chemical properties of ionic liquids (ILs) in the design of new materials, however, the interaction of these IL molecules with optical materials and their effect on the luminescence properties of the materials are poorly known. In this work, it is shown that the imidazolium based BmimBF4 ionic liquid, used as a solvent during the synthesis, tethers at the surface of the nanoparticles and influences the photoluminescence of the nanophosphor BaFBr:Eu2+ materials. In time resolved spectra, two different emissions are observed for the material synthesized by the ionothermal approach, one corresponding to Eu(ii) 5d-4f transitions in the BaFBr host with a decay time of 843 ns and the other with a much faster decay time compared to the ionic liquid BmimBF4.

Investigating exciton structure and dynamics in colloidal CdSe quantum dots with two-dimensional electronic spectroscopy

Two-Dimensional Electronic Spectroscopy (2DES) is performed on CdSe colloidal quantum dots. These experiments reveal new observations on exciton structure and dynamics in quantum dots, expanding upon prior transient absorption measurements of excitonics in these systems. The 2DES method enables the separation of line broadening mechanisms, thereby better revealing the excitonic lineshapes and biexcitonic interactions. 2DES enables more information rich spectral probing of coherent phonons and their coupling to excitons. The data show spectral modulations and drifts, with differences based upon whether one monitors the excitation energy (E₁) or emission energy (E₃). These measurements reveal both homogeneous and inhomogeneous broadenings, as well as static and dynamic line broadening. The longitudinal optical phonon modulates the dynamic absorption spectrum both in energy and linewidth. These experiments enable measurement of hot exciton cooling with improved resolution in energy and time. These 2DES results are consistent with prior excitonic state-resolved transient absorption measurements, albeit with the addition of contributions due to coherent phonons. Finally these 2DES experiments enable disentangling of coupling versus relaxation contributions to the signals, further offering a test of electronic structure theory.

Deciphering hot- and multi-exciton dynamics in core-shell QDs by 2D electronic spectroscopies

2D electronic spectroscopy maps acquired in different configurations unveil intraband hot carrier cooling and interband multi-exciton recombination dynamics.

Although the harnessing of multiple and hot excitons is a prerequisite for many of the groundbreaking applications of semiconductor quantum dots (QDs), the characterization of their dynamics through conventional spectroscopic techniques is cumbersome. Here, we show how a careful analysis of 2DES maps acquired in different configurations (BOXCARS and pump–probe geometry) allows the tracking and visualization of intraband Auger relaxation mechanisms, driving the hot carrier cooling, and interband bi- and tri-exciton recombination dynamics. The results obtained on archetypal core–shell CdSe/ZnS QDs suggest that, given the global analysis of the resulting datasets, 2D electronic spectroscopy techniques can successfully and efficiently dispel the intertwined dynamics of fast and ultrafast recombination processes in nanomaterials. Hence, we propose this analysis scheme to be used in future research on novel quantum confined systems.

Engineering interactions in QDs-PCBM blends: a surface chemistry approach

Here we present a comprehensive study on the photophysics of QDs-fullerene blends, aiming to elucidate the impact of ligands on the extraction of carriers from QDs. We investigated how three different ligands (oleylamine, octadecanethiol and propanethiol) influence the dynamics of charge generation, separation, and recombination in blends of CdSe/CdS core/shell QDs and PCBM. We accessed each relevant process directly by combining the results from both optical and magnetic spectroscopies. Transient absorption measurements revealed a faster interaction dynamics in thiol-capped ligands. Through phenomenological modeling of the interaction processes, i.e., energy transfer and electron transfer, we estimated the suppression of exciton migration and the enhancement of electron transfer processes when alkyl-thiols are employed as ligands. Contextually, we report the profound impact of the ligands' alkyl chain length, leading to strengthened interactions with PCBM acceptors. Quantitatively, we measured a 10-fold increase in the electron transfer rate when oleylamine ligands were exchanged with propanethiol ligands. EPR spectroscopy gave access to subtle details regarding both the enhanced charge generation and lower binding energy of charge-transfer states in blends compared to PCBM alone. Moreover, through pulsed EPR techniques, we inferred the localization of deep electron traps in localized sites close to QDs in the blends. Therefore, our thorough characterization evidenced the essential role of ligands in determining QD interactions. We believe that these discoveries will contribute to the efficient incorporation of QDs in existing organic PV technologies.

Surface Engineering of Quantum Dots for Remarkably High Detectivity Photodetectors

Ternary alloyed CdSe _xTe_{1- x} colloidal QDs trap-passivated by iodide-based ligands (TBAI) are developed as building blocks for UV-NIR photodetectors. Both the few surface traps and high loading of QDs are obtained by in situ ligand exchange with TBAI. The device is sensitive to a broad wavelength range covering the UV-NIR region (300-850 nm), showing an excellent photoresponsivity of 53 mA/W, a fast response time of ≪0.02s, and remarkably high detectivity values of 8 × 10¹³ Jones at 450 nm and 1 × 10¹³ Jones at 800 nm without an external bias voltage. Such performance is superior to what has been reported earlier for QD-based photodetectors. The photodetector exhibits excellent stability, keeping 98% of photoelectric responsivity after 2 months of illumination in air even without encapsulation. In addition, the semitransparent device is successfully fabricated using a Ag nanowires/polyimide transparent substrate. Such self-powered photodetectors with fast response speed and a stable, broad-band response are expected to function under a broad range of environmental conditions.

Windowless Observation of Evaporation-Induced Coarsening of Au-Pt Nanoparticles in Polymer Nanoreactors

The interactions between nanoparticles and solvents play a critical role in the formation of complex, metastable nanostructures. However, direct observation of such interactions with high spatial and temporal resolution is challenging with conventional liquid-cell transmission electron microscopy (TEM) experiments. Here, a windowless system consisting of polymer nanoreactors deposited via scanning probe block copolymer lithography (SPBCL) on an amorphous carbon film is used to investigate the coarsening of ultrafine (1-3 nm) Au-Pt bimetallic nanoparticles as a function of solvent evaporation. In such reactors, homogeneous Au-Pt nanoparticles are synthesized from metal-ion precursors in situ under electron irradiation. The nonuniform evaporation of the thin polymer film not only concentrates the nanoparticles but also accelerates the coalescence kinetics at the receding polymer edges. Qualitative analysis of the particle forces influencing coalescence suggests that capillary dragging by the polymer edges plays a significant role in accelerating this process. Taken together, this work (1) provides fundamental insight into the role of solvents in the chemistry and coarsening behavior of nanoparticles during the synthesis of polyelemental nanostructures, (2) provides insight into how particles form via the SPBCL process, and (3) shows how SPBCL-generated domes, instead of liquid cells, can be used to study nanoparticle formation. More generally, it shows why conventional models of particle coarsening, which do not take into account solvent evaporation, cannot be used to describe what is occurring in thin film, liquid-based syntheses of nanostructures.

Intra- and inter-nanocrystal charge transport in nanocrystal films

The exploitation of semiconductor nanocrystal (NC) films in novel electronic and optoelectronic applications requires a better understanding of charge transport in these systems. Here, we develop a model of charge transport in NC films, based on a generalization of the concept of transport energy level ET to nanocrystal assemblies, which considers both intra- and inter-NC charge transfer processes. We conclude that the role played by each of these processes can be probed from temperature-dependent measurements of charge carrier density n and mobility μ in the same films. The model also enables the determination of the position of the Fermi energy level EF with respect to ET, an important parameter of charge transport in semiconductor materials, from the temperature dependence of n. Moreover, we provide support to an essentially temperature-independent intra-NC charge carrier mobility, considered in the transport level concept, and consequently the frequently observed temperature dependence of the overall mobility μ in NC films results from a temperature variation of the inter-NC charge transport processes. Importantly, we also conclude that the temperature dependence of conductivity in NC films should result in general from a combination of temperature variations of both n and μ. By applying the model to solution-processed Si NC films, we conclude that transport within each NC is similar to that in amorphous Si (a-Si), with charges hopping along band tail states located below the conduction band edge. For Si NCs, we obtain values of ET - EF of ∼0.25 eV. The overall mobility μ in Si NC films is significantly further reduced with respect to that typically found in a-Si due to the additional transport constraints imposed by inter-NC transfer processes inherent to a nanoparticulate film. Our model accounting for inter- and intra-NC charge transport processes provides a simple and more general description of charge transport that can be broadly applied to films of semiconductor NCs.

Balancing Charge Carrier Transport in a Quantum Dot P-N Junction toward Hysteresis-Free High-Performance Solar Cells

In a quantum dot solar cell (QDSC) that has an inverted structure, the QD layers form two different junctions between the electron transport layer (ETL) and the other semiconducting QD layer. Recent work on an inverted-structure QDSC has revealed that the junction between the QD layers is the dominant junction, rather than the junction between the ETL and the QD layers, which is in contrast to the conventional wisdom. However, to date, there have been a lack of systematic studies on the role and importance of the QD heterojunction structure on the behavior of the solar cell and the resulting device performance. In this study, we have systematically controlled the structure of the QD junction to balance charge transport, which demonstrates that the position of the junction has a significant effect on the hysteresis effect, fill factor, and solar cell performance and is attributed to balanced charge transport.

Quantum-dot antibody conjugation visualized at the single-molecule scale with high-speed atomic force microscopy

Conjugates of semiconductor quantum dots (QDs) and antibodies have emerged as a promising bioprobes due to their great combination of QD's efficient fluorescence and the high specificity of antigen-antibody reactions. For further developments in this field, it is essential to understand the molecular conformation of the QD-antibody conjugates at the single-molecule scale. Here, we report on the direct imaging of QD-antibody conjugates at the single-molecule scale by using high-speed atomic force microscopy (HS-AFM). Owing to the high spatiotemporal resolution of HS-AFM, we observed the dynamic splitting of individual antibodies during the conjugation process. QD-antibody conjugates were also clearly visualized at the single-molecule scale details. Several important features were even discovered through dynamic observation of the QD-antibody conjugates. We observed an intermediate state of conjugation, where the antibodies attached and detached to QDs repeatedly. We also revealed that the attached antibodies were not steady but drastically fluctuated in their recognition areas due to the Brownian motion. We also demonstrated that HS-AFM observation is useful for the quantitative analysis of fabricated conjugates.

Colloidal Quantum Nanostructures: Emerging Materials for Display Applications

Colloidal semiconductor nanocrystals (SCNCs) or, more broadly, colloidal quantum nanostructures constitute outstanding model systems for investigating size and dimensionality effects. Their nanoscale dimensions lead to quantum confinement effects that enable tuning of their optical and electronic properties. Thus, emission color control with narrow photoluminescence spectra, wide absorbance spectra, and outstanding photostability, combined with their chemical processability through control of their surface chemistry leads to the emergence of SCNCs as outstanding materials for present and next-generation displays. In this Review, we present the fundamental chemical and physical properties of SCNCs, followed by a description of the advantages of different colloidal quantum nanostructures for display applications. The open challenges with respect to their optical activity are addressed. Both photoluminescent and electroluminescent display scenarios utilizing SCNCs are described.

Self-doped colloidal semiconductor nanocrystals with intraband transitions in steady state

The tunable bandgap energy has been recognized as a prominent feature of the colloidal semiconductor nanocrystal, also called the colloidal quantum dot (CQD).

Biomimetic Inspired Core-Canopy Quantum Dots: Ions Trapped in Voids Induce Kinetic Fluorescence Switching

Closely packed hollow spheres connected through pillars to a CdSe quantum dot (QD) core produce channels through which ions navigate. This particular structure is well represented by [CdSe@CarbOPH(O)]@Cl/[N(Caprylyl)₃ Me₁ ] indicating that in the channels between the canopy made by the carboranyl spheres (carboranylphosphinate, CarbOPH(O)) and the CdSe core exist chloride anions. Due to the close packing, the spheres produce openings. These are converted into gates because [N(Caprylyl)₃ Me₁ ] acts as a plug. The [CdSe@CarbOPH(O)]@Cl/assembly is negatively charged because the Cd positive charges are outnumbered by the negative charges due to the Se, the phosphinic acid and, very importantly, the trapped chloride anions, and this negative load is compensated by the cationic surfactant. Here, it is shown that this synergism produces an unprecedented phenomenon, namely, kinetic fluorescence switching. It is observed that the material shines brightly then loses its brightness and, upon the application of kinetic energy, shines back to the maximum power. This process continues for an extended period of time, up to half a year, at least. This new type of architecture in QDs is named as core-canopy QDs. In this case, this study demonstrates one property, the kinetic fluorescence switching, as a consequence of the trapping of Cl^- in the QDs channels, but other properties can be envisaged with the judicious choice of the anions or even the pillar connecting the hollow sphere with the ground.

Butterfly-inspired micro-concavity array film for color conversion efficiency improvement of quantum-dot-based light-emitting diodes

Inspired by the Papilio blumei butterfly, quantum-dot (QD) film coupled with micro-concavity array (MCA) films is proposed in this Letter to enhance color conversion efficiency (CCE) of QD-based light-emitting diodes (LEDs). The diameter, aspect ratio, and pitch of the MCA are optimized in the optical simulations. Both the simulation and experimental results show that the scattering and double reflection effects are the key to the CCE improvement of QD films. The results show that the CCEs are increased from 19.98% to 21.59% and 21.78% (350 mA) for single-sided microstructured QD film and double-sided microstructured QD film configurations, respectively. Overall, the MCA film is a promising solution to enhance the CCE of QD-based LEDs.

Synthesis of highly stable quantum-dot silicone nanocomposites via in situ zinc-terminated polysiloxane passivation

Quantum dot (QD) silicone nanocomposites are promising luminescent materials for developing high performance light-emitting diodes (LEDs). However, their practical application still faces a critical issue of strong fluorescence quenching in commercial silicone, which is normally induced by the agglomeration of QDs and the impurities such as a Pt-catalyst and oxygen in the silicone matrices. This article reports the development of zinc-terminated polydimethylsiloxane (Zn-PDMS) to passivate CdSe/CdS/ZnS QDs via an in situ approach. The Zn-PDMS passivation protects the QDs from reacting with impurities and provides the mono-dispersion of QDs in silicone resin, leading to over 80% quantum efficiency as well as effective anti-quenching properties for the QD-silicone nanocomposite under an ambient atmosphere. A high performance warm-white LED prototype with direct on-chip packaging using the as-prepared QDs is developed.

Fully-Inorganic Trihalide Perovskite Nanocrystals: A New Research Frontier of Optoelectronic Materials

All-inorganic trihalide perovskite nanocrystals (NCs) are emerging as a new class of superstar semiconductors with excellent optoelectronic properties and great potential for a broad range of applications in lighting, lasing, photon detection, and photovoltaics. This article provides an up-to-date review on the developments of fully-inorganic trihalide perovskite NCs by emphasizing their controllable solution fabrication strategies, structural phase transformation, tunable optoelectronic properties, stability, as well as their photovoltaic and optoelectronic applications. Among the properties to be surveyed, particular focus is on the size-, shape-, and composition-dependent photoluminescence properties. Finally, by identifying new challenges, suggestions are provided for further research and potential development of this area.

Patterning and fluorescence tuning of quantum dots with haptic-interfaced bubble printing

Semiconductor quantum dots (QDs) are attractive for a wide range of applications such as displays, light-emitting devices, and sensors due to their properties such as tunable fluorescence wavelength, high brightness, and narrow bandwidth. Most of the applications require precise patterning of QDs with targeted properties on solid-state substrates. Herein, we have developed a haptic-interfaced bubble printing (HIBP) technique to enable high-resolution (510 nm) high-throughput (>10⁴ μm s^-1) patterning of QDs with strong emission tunability and to significantly enhance the accessibility of the technique via a smartphone device. The scalability and versatility of the HIBP are demonstrated in our arbitrary patterning of QDs on plasmonic substrates. A detailed study of plasmonic and photothermal interactions is performed via programmed stage movements to realize tunability of the emission wavelength and lifetime. Finally, the influence of the hand movement on the properties of the printed QDs in terms of emission wavelength tuning from yellow to blue is established. This work provides a single-step macroscale platform to manipulate nanoscale properties at high resolution and high throughput.

Investigation of anti-solvent induced optical properties change of cesium lead bromide iodide mixed perovskite (CsPbBr3-xIx) quantum dots

Cesium lead halide (CsPbX₃, X=Cl, Br, I) perovskites are a new material system that has attracted a lot of research focus. Its tunable band gap and better thermal stability than organic lead halide perovskite give it the potential for applications in optoelectronic devices such as light-emitting diodes and solar cells. Here we have synthesized CsPbBr_3-xI_x perovskite quantum dots (QDs) via a solution process, and then have selected three different anti-solvents to purify the product. A significant effect on optical properties of CsPbBr_3-xI_x was found after the centrifugation process. Up to a ∼40nm shift was observed in mixed halide CsPbBr_3-xI_x QDs in both absorbance and PL spectra after purification while there was no obvious change in pure CsPbBr₃ when it was subjected to the same purification steps. XPS analysis shows that the Br:I ratio of the CsPbBr_3-xI_x QDs had changed as a result of exposure to the anti-solvent, causing the change of the band gap and shift of the spectra. It is also shown that iodine can be removed more easily than bromine during the anti-solvent purification. Ab-initio simulations of small CsPbBr_3-xI_x atomic clusters suggest that exposed Cs ions on Cs-terminated facets are the first species to be attacked by hydrophilic molecules, likely dragging halide ions into solution with them to maintain overall charge neutrality in the material. Charge carrier recombination rates were found to be unchanged and all samples maintained a good PL quantum yield which was more than 44%.

Hybrid tandem quantum dot/organic photovoltaic cells with complementary near infrared absorption

Monolithically integrated hybrid tandem solar cells that effectively combine solution-processed colloidal quantum dot (CQD) and organic bulk heterojunction subcells to achieve tandem performance that surpasses the individual subcell efficiencies have not been demonstrated to date. In this work, we demonstrate hybrid tandem cells with a low bandgap PbS CQD subcell harvesting the visible and near-infrared photons and a polymer:fullerene-poly (diketopyrrolopyrrole-terthiophene) (PDPP3T):[6,6]-phenyl-C₆₀-butyric acid methyl ester (PC₆₁BM)-top cell absorbing effectively the red and near-infrared photons of the solar spectrum in a complementary fashion. The two subcells are connected in series via an interconnecting layer (ICL) composed of a metal oxide layer, a conjugated polyelectrolyte, and an ultrathin layer of Au. The ultrathin layer of Au forms nano-islands in the ICL, reducing the series resistance, increasing the shunt resistance, and enhancing the device fill-factor. The hybrid tandems reach a power conversion efficiency (PCE) of 7.9%, significantly higher than the PCE of the corresponding individual single cells, representing one of the highest efficiencies reported to date for hybrid tandem solar cells based on CQD and polymer subcells.

Spectroscopic Insights into Carbon Dot Systems

The controversial nature of the fluorescent properties of carbon dots (CDs), ascribed either to surface states or to small molecules adsorbed onto the carbon nanostructures, is an unresolved issue. To date, an accurate picture of CDs and an exhaustive structure-property correlation are still lacking. Using two unconventional spectroscopic techniques, fluorescence correlation spectroscopy (FCS) and time-resolved electron paramagnetic resonance (TREPR), we contribute to fill this gap. Although electron micrographs indicate the presence of carbon cores, FCS reveals that the emission properties of CDs are based neither on those cores nor on molecular species linked to them, but rather on free molecules. TREPR provides deeper insights into the structure of carbon cores, where C sp² domains are embedded within C sp³ scaffolds. FCS and TREPR prove to be powerful techniques, characterizing CDs as inherently heterogeneous systems, providing insights into the nature of such systems and paving the way to standardization of these nanomaterials.

Solution Synthesis of Atomically Precise Graphene Nanoribbons

Bottom-up fabrication of narrow strips of graphene, also known as graphene nanoribbons or GNRs, is an attractive way to open a bandgap in semimetallic graphene. In this chapter, we review recent progress in solution-based synthesis of GNRs with atomically precise structures. We discuss a variety of atomically precise GNRs and highlight theoretical and practical aspects of their structural design and solution synthesis. These GNRs are typically synthesized through a polymerization of rationally designed molecular precursors followed by a planarization through a cyclodehydrogenation reaction. We discuss various synthetic techniques for polymerization and planarization steps, possible approaches for chemical modification of GNRs, and compare the properties of GNRs that could be achieved by different synthetic methods. We also discuss the importance of the rational design of molecular precursors to avoid isomerization during the synthesis and achieve GNRs that have only one possible structure. Significant attention in this chapter is paid to the methods of material characterization of solution-synthesized GNRs. The chapter is concluded with the discussion of the most significant challenges in the field and the future outlook.

Reversible Electrochemistry of Mercury Chalcogenide Colloidal Quantum Dot Films

The absolute positions of the energy levels of colloidal quantum dots of Hg(S, Se, Te), which are of interest as mid-infrared materials, are determined by electrochemistry. The bulk valence bands are at -5.85, -5.50, and -4.77 eV (±0.05 eV) for zinc-blend HgS, HgSe, HgTe, respectively, in the same order as the anions p-orbital energies. The conduction bands are conversely at -5.20, -5.50, and -4.77 eV. The stable ambient n-doping of Hg(S, Se) quantum dots compared to HgTe arises because the conduction band is sufficiently lower than the measured environment Fermi level of ∼ -4.7 eV to allow for n-doping for HgS and HgSe quantum dots even with significant electron confinement. The position of the Fermi level and the quantum dots states are reported for a specific surface treatment with ethanedithiol and electrolyte environment. The positions are however sensitive to different surface treatments, providing an avenue to control doping. Electrochemical gating is further used to determine the carrier mobility in the films of the three different systems as a function of CQD size. HgSe and HgS show increasing mobility with increasing particle sizes while HgTe shows a nonmonotonous behavior, which is attributed to some degree of aggregation of HgTe QDs.

Direct Patterning of Colloidal Quantum-Dot Thin Films for Enhanced and Spectrally Selective Out-Coupling of Emission

We report on a template-stripping method for the direct surface patterning of colloidal quantum-dot thin films to produce highly luminescent structures with feature sizes less than 100 nm. Through the careful design of high quality bull's-eye gratings we can produce strong directional beaming (10° divergence) with up to 6-fold out-coupling enhancement of spontaneous emission in the surface-normal direction. A transition to narrow single-mode lasing is observed in these same structures at thresholds as low as 120 μJ/cm². In addition, we demonstrate that these structures can be fabricated on flexible substrates. Finally, making use of the size-tunable character of colloidal quantum dots, we demonstrate spectrally selective out-coupling of light from mixed quantum-dot films. Our results provide a straightforward route toward significantly improved optical properties of colloidal quantum-dot assemblies.

Perovskite nanocrystals: synthesis, properties and applications

Halide perovskites have emerged as superstar materials for optoelectronic devices. Besides the fever of research in solar cells, these materials show great promise on light emitting diodes (LEDs), photodetectors and lasers as well. Rapid advances in bulk perovskite materials aroused universal interest for the development of perovskite nanocrystals, inspired by the great progress of classic colloidal semiconductor quantum dots. Perovskite nanocrystals have been synthesized based on solution process and exhibited high luminescence quantum yield, sharp emission peak, and emission color tunability. Significant progresses have been made about the application of perovskite nanocrystals for LED and lasers in recent years. In this paper, we will comprehensively introduce the synthesis strategies, physical and chemical characteristics, as well as their applications in optoelectronic devices.

Freestanding carbon nanodots/poly (vinyl alcohol) films with high photoluminescent quantum yield realized by inverted-pyramid structure

Carbon nanodots (C-dots) have attracted great attention for their biocompatibility and strong tunable photoluminescence (PL). However, aggregation-induced PL quenching blocks their practical application in solid-state optoelectronics. Here, we report a luminescent C-dots freestanding film with a substantially enhanced high quantum yield (QY) of 72.3%. A facile template method, rather than complicate lithography and etching technique is proposed to fabricate the C-dots composite films with large-area (8 inch × 8 inch) ordered micro-scale inverted-pyramid patterns on the surface. The control experiment and theoretical analysis demonstrate the key success to QY enhancement lies in the separation of C-dots and the pattern of surface inverted-pyramid structure. This work realizes the QY enhancement simply by geometrical optics, not the chemical treatment of luminescent particles. It provides a general approach to fabricate large-area freestanding luminescent composite film with high QY.

Quantum state readout of individual quantum dots by electrostatic force detection

Electric charge detection by atomic force microscopy (AFM) with single-electron resolution (e-EFM) is a promising way to investigate the electronic level structure of individual quantum dots (QDs). The oscillating AFM tip modulates the energy of the QDs, causing single electrons to tunnel between QDs and an electrode. The resulting oscillating electrostatic force changes the resonant frequency and damping of the AFM cantilever, enabling electrometry with a single-electron sensitivity. Quantitative electronic level spectroscopy is possible by sweeping the bias voltage. Charge stability diagram can be obtained by scanning the AFM tip around the QD. e-EFM technique enables to investigate individual colloidal nanoparticles and self-assembled QDs without nanoscale electrodes. e-EFM is a quantum electromechanical system where the back-action of a tunneling electron is detected by AFM; it can also be considered as a mechanical analog of admittance spectroscopy with a radio frequency resonator, which is emerging as a promising tool for quantum state readout for quantum computing. In combination with the topography imaging capability of the AFM, e-EFM is a powerful tool for investigating new nanoscale material systems which can be used as quantum bits.

Method to determine radiative and non-radiative defects applied to AgInS2–ZnS luminescent nanocrystals

The systematic measurement of the photoluminescence quantum yield and the recombination lifetime of a given phosphor allows for the quantification of both radiative and non-radiative recombination rates.

Microwave-assisted one-pot synthesis of hybrid nanosystems based on CdS quantum dots functionalized with organic chromophores: effect of the chromophore nature on the nanosystem composition

Quantum dot/organic dye nanosystems were synthesised via a one-pot technique. Different dependences of the nanosystem composition on the synthetic conditions for different dyes were observed.