Lead‐Chalcogenide Colloidal‐Quantum‐Dot Solids: Novel Assembly Methods,Electronic Structure Control,and Application Prospects

The quest for novel semiconductors with easy, cheap fabrication and tailorable properties has led to the development of several classes of materials, such as semiconducting polymers, carbon nanotubes, hybrid perovskites, and colloidal quantum dots. All these candidates can be processed from the liquid phase, enabling easy fabrication, and are suitable for different electronic and optoelectronic applications. Here, recent developments in the field of colloidal‐quantum‐dot solids are discussed, with a focus on lead‐chalcogenide systems. These include novel deposition methods; the recent growing understanding of their fundamental properties, driven by major successes in the control of the nanostructured assembly and surface chemistry; and selected reports on lab‐scale devices showing the technological prospects of these fascinating class of materials.     

Emerging Trends of Carbon-Based Quantum Dots: Nanoarchitectonics and Applications

Carbon-based quantum dots (QDs) have emerged as a fascinating class of advanced materials with a unique combination of optoelectronic, biocompatible, and catalytic characteristics, apt for a plethora of applications ranging from electronic to photoelectrochemical devices. Recent research works have established carbon-based QDs for those frontline applications through improvements in materials design, processing, and device stability. This review broadly presents the recent progress in the synthesis of carbon-based QDs, including carbon QDs, graphene QDs, graphitic carbon nitride QDs and their heterostructures, as well as their salient applications. The synthesis methods of carbon-based QDs are first introduced, followed by an extensive discussion of the dependence of the device performance on the intrinsic properties and nanostructures of carbon-based QDs, aiming to present the general strategies for device designing with optimal performance. Furthermore, diverse applications of carbon-based QDs are presented, with an emphasis on the relationship between band alignment, charge transfer, and performance improvement. Among the applications discussed in this review, much focus is given to photo and electrocatalytic, energy storage and conversion, and bioapplications, which pose a grand challenge for rational materials and device designs. Finally, a summary is presented, and existing challenges and future directions are elaborated.     

Understanding chemically processed solar cells based on quantum dots

Abstract

Photovoltaic energy conversion is one of the best alternatives to fossil fuel combustion. Petroleum resources are now close to depletion and their combustion is known to be responsible for the release of a considerable amount of greenhouse gases and carcinogenic airborne particles. Novel third-generation solar cells include a vast range of device designs and materials aiming to overcome the factors limiting the current technologies. Among them, quantum dot-based devices showed promising potential both as sensitizers and as colloidal nanoparticle films. A good example is the p-type PbS colloidal quantum dots (CQDs) forming a heterojunction with a n-type wide-band-gap semiconductor such as TiO₂ or ZnO. The confinement in these nanostructures is also expected to result in marginal mechanisms, such as the collection of hot carriers and generation of multiple excitons, which would increase the theoretical conversion efficiency limit. Ultimately, this technology could also lead to the assembly of a tandem-type cell with CQD films absorbing in different regions of the solar spectrum.     

Synthesis and modulation of the optical properties of carbon quantum dots using microwave radiation

Abstract

Carbon quantum dots have exhibited highly fluorescent characteristics as nanomaterials. Soluble in water and easily synthesized by multiple simple techniques, there are immense fabrication possibilities by permuting their properties via changing precursors, synthesis route, reaction parameters, etc. As economic and environment-friendly seed material, they are being viewed as an alternative to conventional fluorescent materials in myriad of applications including displays, cancer detection, drug delivery carriers in biomedicine, absorbing material in photovoltaics, etc. In this work, the hydrophilic carbon quantum dots were synthesized from the aqueous solution of citric acid and urea through microwave radiation for varying heat durations. The method is facile, faster and friendly to the environment without any need for high temperature and complicated chemical techniques. It was observed that the bandgap of the fabricated carbon quantum dots and its optical properties namely absorbance, photoluminescence enhanced with an increase in exposure of samples to heat up to an optimum limit, owing to the increase in density of states. However, further exposure to heat for longer duration degraded the absorbance and bandgap while photoluminescence gets saturated. Stokes’ shift revealed that all the synthesized carbon quantum dots possess stable emission. This was reconfirmed from consistent emission peak positions under varying excitation in the samples. The absorbance and PL spectrum exhibited by the synthesized dots makes it a suitable material for boosting the performance of organic solar cell.     

Facile preparation of Gd3+ doped carbon quantum dots: Photoluminescence materials with magnetic resonance response as magnetic resonance/fluorescence bimodal probes

There are a few bimodal molecular imaging probes constructed by gadolinium (3+) ions in combination with carbon quantum dots (CQDs), and the reported ones show such obvious drawbacks as low luminous efficiency and weak MRI contrast. In the paper, a kind of CQDs photoluminescence materials with magnetic resonance response was prepared by hydrothermal method and employing gadopentetate monomeglumine (GdPM) as a precusor. Here, the GdPM plays a role of not only carbon source, but also gadolinium (3+) sources. When the GdPM aqueous solution with a concentration of 4 mg mL⁻¹ was pyrolyzed under 220 °C and 2.0 MPa for 8 h, an optimal CQDs was obtained which are doped with gadolinium (3+) ions in both chelates and Gd₂O₃ (named as Gd³⁺-CQDs). The average diameter of the Gd³⁺-CQDs is about 1.6 nm, which show a high photoluminescence quantum yield of 7.1%, as well as high longitudinal relaxivity (r₁) of 9.87 mM⁻¹ s⁻¹. And owing to the unconspicuous cell toxicity, the Gd³⁺-CQDs show big possibility for clinical application in magnetic resonance/fluorescence bimodal molecular imaging.     

Size‐Controlled Soft‐Template Synthesis of Carbon Nanodots toward Versatile Photoactive Materials

Size‐controlled soft‐template synthesis of carbon nanodots (CNDs) as novel photoactive materials is reported. The size of the CNDs can be controlled by regulating the amount of an emulsifier. As the size increases, the CNDs exhibit blue‐shifted photoluminescence (PL) or so‐called an inverse PL shift. Using time‐correlated single photon counting, ultraviolet photoelectron spectroscopy, and low‐temperature PL measurements, it is revealed that the CNDs are composed of sp² clusters with certain energy gaps and their oleylamine ligands act as auxochromes to reduce the energy gaps. This insight can provide a plausible explanation on the origin of the inverse PL shift which has been debatable over a past decade. To explore the potential of the CNDs as photoactive materials, several prototypes of CND‐based optoelectronic devices, including multicolored light‐emitting diodes and air‐stable organic solar cells, are demonstrated. This study could shed light on future applications of the CNDs and further expedite the development of other related fields.     

Infrared Colloidal Quantum Dots for Photovoltaics: Fundamentals and Recent Progress

Colloidal quantum dots (CQDs) are solution‐processed semiconductors of interest in low‐cost photovoltaics. Tuning of the bandgap of CQD films via the quantum size effect enables customization of solar cells’ absorption profile to match the sun's broad visible‐ and infrared‐containing spectrum reaching the earth. Here we review recent progress in the realization of low‐cost, efficient solar cells based on CQDs. We focus in particular on CQD materials and approaches that provide both infrared and visible‐wavelength solar power conversion CQD photovoltaics now exceed 5% solar power conversion efficiency, achieved by the introduction of a new architecture, the depleted‐heterojunction CQD solar cell, that jointly maximizes current, voltage, and fill factor. CQD solar cells have also seen major progress in materials processing for stability, recently achieving extended operating lifetimes in an air ambient. We summarize progress both in device operation and also in gaining new insights into materials properties and processing – including new electrical contact materials and deposition techniques, as well as CQD synthesis, surface treatments, film‐forming technologies – that underpin these rapid advances.     

Carbon Dots-Based Ultrastretchable and Conductive Hydrogels for High-Performance Tactile Sensors and Self-Powered Electronic Skin

Smart tactile sensing materials have excellent development prospects, including wearable health-monitoring equipment and energy collection. Hydrogels have received extensive attention in tactile sensing owing to their transparency and high elasticity. In this study, highly crosslinked hydrogels are fabricated by chemically crosslinking polyacrylamide with lithium magnesium silicate and decorated with carbon quantum dots. Magnesium lithium silicate provides abundant covalent bonds and improves the mechanical properties of the hydrogels. The luminescent properties endowed by the carbon dots further broaden the application of hydrogels for realizing flexible electronics. The hydrogel-based strain sensor exhibits excellent sensitivity (gauge factor 2.6), a broad strain response range (0–2000%), good cyclicity, and durability (1250). Strain sensors can be used to detect human motions. More importantly, the hydrogel can also be used as a flexible self-supporting triboelectric electrode for effectively detecting pressure in the range of 1–25 N and delivering a short-circuit current (I_SC) of 2.6 µA, open-circuit voltage (V_OC) of 115 V, and short-circuit transfer charge (Q_SC) of 29 nC. The results reveal new possibilities for human–computer interactions and electronic robot skins.     

In Vivo NIR Fluorescence Imaging,Biodistribution, and Toxicology of Photoluminescent Carbon Dots Produced from Carbon Nanotubes and Graphite

Oxidization of carbon nanotubes by a mixed acid has been utilized as a standard method to functionalize carbon nanomaterials for years. Here, the products obtained from carbon nanotubes and graphite after a mixed‐acid treatment are carefully studied. Nearly identical carbon dot (Cdot) products with diameters of 3–4 nm are produced using this approach from a variety of carbon starting materials, including single‐walled carbon nanotubes, multiwalled carbon nanotubes, and graphite. These Cdots exhibit strong yellow fluorescence under UV irradiation and shifted emission peaks as the excitation wavelength is changed. In vivo fluorescence imaging with Cdots is then demonstrated in mouse experiments, by using varied excitation wavelengths including some in the near‐infrared (NIR) region. Furthermore, in vivo biodistribution and toxicology of those Cdots in mice over different periods of time are studied; no noticeable signs of toxicity for Cdots to the treated animals are discovered. This work provides a facile method to synthesize Cdots as safe non‐heavy‐metal‐containing fluorescent nanoprobes, promising for applications in biomedical imaging.     

碳纳米管气敏传感器响应机理研究进展

碳纳米管以其优异的表面化学性能和良好的电学性能成为制作化学传感器的理想材料.论述了碳纳米管的结构特点及其气敏响应机理的最新进展,包括气敏吸附和电学传感机理和复合型碳纳米管气敏材料响应机理研究的进展.     

Cover Picture: Separation Techniques for Carbon Nanotubes (Adv. Eng. Mater. 3/2005)

The cover shows a single‐walled carbon nanotube in a schematic drawing. How these fascinating materials can be separated from semiconducting tubes in substantial quantities is described in the review article by R. Krupke and F. Hennrich on page 111.     

Grafting the surface of carbon nanotubes and carbon black with the chemical properties of hyperbranched polyamines

Controlling the chemistry on the surface of new carbon materials is a key factor to widen the range of their applicability. In this paper we show a grafting methodology of polyalkylamines to the surface of carbon nanomaterials, in particular, carbon nanotubes and a carbon black. The aim of this work is to reach large degrees of covalent functionalization with hyperbranched polyethyleneimines (HBPEIs) and to efficiently preserve the strong chelating properties of the HBPEIs when they are fixed to the surface of these carbon materials. This functionalization opens new possibilities of using these carbon nanotubes-based hybrids. The results show that the HBPEIs are covalently attached to the carbon materials, forming hybrids. These hybrids emerge from the reaction of amine functions of the HBPEIs with carbonyls and carboxylic anhydrides of the carbon surface which become imine and imide bonds. Thus, due to the nature of these bonds, the pre-oxidized samples with relevant number of C=O groups showed an increase in the degree of functionalization with the HBPEIs. Furthermore, both the acid-base properties and the coordination capacity for metal ions of the hybrids are equivalent to that of the free HBPEIs in solution. This means that the chemical characteristics of the HBPEIs have been efficiently transferred to the hybrids. To reach this conclusion we have developed a novel procedure to assess the acid-base and the coordination properties of the hybrids (solids) by means of potentiometric titration. The good agreement of the values obtained for the hybrids and for the free HBPEIs in aqueous solution supports the reliability of the procedure. Moreover, the high capacity of the hybrids to capture Ni²⁺ by complexation opens new possibilities of using these hybrids to capture high-value metal ions such as Pd²⁺ and Pt²⁺.     

Carbon Dots‐in‐Matrix Boosting Intriguing Luminescence Properties and Applications

As a new class of luminescent nanomaterials, carbon dots (CDs) have aroused significant interest because of their fascinating photoluminescence properties and potential applications in biological, optoelectronic, and energy‐related fields. Strikingly, embedding CDs in host matrices endow them with intriguing luminescent properties, in particular, room temperature phosphorescence and thermally activated delayed fluorescence, due to the confinement effect of the host matrix and the H‐bonding interactions between CDs and the matrix. Here, the state‐of‐the‐art strategies for introducing CDs in various host matrices are summarized, such as nanoporous materials, polyvinyl alcohol, polyurethane, potash alum, layered double hydroxides, amorphous silica, etc. The resultant luminescent properties of the composites and their emission mechanisms are discussed. Their applications in bioimaging, drug delivery/release, sensing, and anticounterfeiting are also presented. Finally, current problems and challenges of CDs‐based composites are noted for future development of such luminescent materials.     

In vivo Quantum‐Dot Toxicity Assessment

Quantum dots have potential in biomedical applications, but concerns persist about their safety. Most toxicology data is derived from in vitro studies and may not reflect in vivo responses. Here, an initial systematic animal toxicity study of CdSe–ZnS core–shell quantum dots in healthy Sprague–Dawley rats is presented. Biodistribution, animal survival, animal mass, hematology, clinical biochemistry, and organ histology are characterized at different concentrations (2.5–15.0 nmol) over short‐term (<7 days) and long‐term (>80 days) periods. The results show that the quantum dot formulations do not cause appreciable toxicity even after their breakdown in vivo over time. To generalize the toxicity of quantum dots in vivo, further investigations are still required. Some of these investigations include the evaluation of quantum dot composition (e.g., PbS versus CdS), surface chemistry (e.g., functionalization with amines versus carboxylic acids), size (e.g., 2 versus 6 nm), and shape (e.g., spheres versus rods), as well as the effect of contaminants and their byproducts on biodistribution behavior and toxicity. Combining the results from all of these studies will eventually lead to a conclusion regarding the issue of quantum dot toxicity.     

Supramolecular Chemistry of Carbon-Based Dots Offers Widespread Opportunities

Carbon dots are an emerging class of nanomaterials that has recently attracted considerable attention for applications that span from biomedicine to energy. These photoluminescent carbon nanoparticles are defined by characteristic sizes of <10 nm, a carbon-based core and various functional groups at their surface. Although the surface groups are widely used to establish non-covalent bonds (through electrostatic interactions, coordinative bonds, and hydrogen bonds) with various other (bio)molecules and polymers, the carbonaceous core could also establish non-covalent bonds (π π stacking or hydrophobic interactions) with π-extended or apolar compounds. The surface functional groups, in addition, can be modified by various post-synthetic chemical procedures to fine-tune the supramolecular interactions. Our contribution categorizes and analyzes the interactions that are commonly used to engineer carbon dots-based materials and discusses how they have allowed preparation of functional assemblies and architectures used for sensing, (bio)imaging, therapeutic applications, catalysis, and devices. Using non-covalent interactions as a bottom-up approach to prepare carbon dots-based assemblies and composites can exploit the unique features of supramolecular chemistry, which include adaptability, tunability, and stimuli-responsiveness due to the dynamic nature of the non-covalent interactions. It is expected that focusing on the various supramolecular possibilities will influence the future development of this class of nanomaterials.     

Synthesis of fluorescent carbon quantum dots from aqua mesophase pitch and their photocatalytic degradation activity of organic dyes

A synthesis strategy of fluorescent carbon quantum dots (CQDs) with high quantum yield (QY) using aqua mesophase pitch (AMP) as the carbon source has been developed via the hydrothermal method in this study. The hydrothermal temperature and soaking time have important effects on the morphology and QY of CQDs. As-prepared CQDs at 120 °C holding for 24 h (CQDs-120-24) have the uniform size of about 2.8 nm, and the QY can reach 27.6%. The obtained CQDs are successfully modified with ammonia and thionyl chloride, respectively, and they exhibit an excellent photocatalytic performance on degrading rhodamine B (Rh B), methyl blue (MB) and indigo carmine (IC). Importantly, the degradation percentage of N-CQDs on Rh B under natural light for 4 h reaches 97% with the degradation rate constant of 0.02463 min⁻¹ and it can maintain 93% after repetitively used 5 times. The results indicate that these as-prepared CQDs have the potential application in degrading organic dyes.     

Effect of the nature the carbon precursor on the physico-chemical characteristics of the resulting activated carbon materials

Carbon materials, including amorphous carbon, graphite, carbon nanospheres (CNSs) and different types of carbon nanofibers (CNFs) [platelet, herringbone and ribbon], were chemically activated using KOH. The pore structure of carbon materials was analyzed using N₂/77 K adsorption isotherms. The presence of oxygen groups was analyzed by temperature programmed desorption in He and acid–base titration. The structural order of the materials was studied by X-ray diffraction and temperature programmed oxidation. The morphology and diameter distribution of CNFs and CNSs were characterized by transmission electron microscopy. The materials were also characterized by temperature-desorption programmed of H₂ and elemental composition. The ways in which the different structures were activated are described, showing the type of pores generated. Relationships between carbon yield, removed carbon, activation degree and graphitic character were also examined. The oxygen content in the form of oxygen-containing surface groups increased after the activation giving qualitative information about them. The average diameter of both CNFs and CNSs was decreased after the activation process as consequence of the changes produced on the material surface.     

Simultaneous adsorption of hydrophilic and lipophilic dyes on iron-doped hydroxyapatite/carbon dots nanocomposites

Nanocomposites of iron-doped hydroxyapatite and carbon dots were synthesized through green synthesis processes, using Yerba Mate (Ilex paraguariensis) as a precursor. A thorough characterization of the particles was carried out by HRTEM, XRD, FTIR, DLS, XPD, and static contact angle. The characterization indicated that the crystallinity, surface charge, and wettability depend on the synthesis parameters.Carbon dots-doped materials, YMnAp@CD and YMFenAp@CD, showed amphiphilic properties with the capacity to adsorb Crystal violet (CV) and Oil Blue A (OBA) from aqueous or toluene-based solutions. The mentioned materials showed the highest number of adsorption sites, with values of 2.95 ± 0.05 and 3.18 ± 0.04 mmol CV/g_ads and 1.65 ± 0.02 0.97 ± 0.06 mmol OBA/g_ads on monocomponent systems, and through a multi-component analysis, the competition for adsorption sites was elucidated.     

Facile and Scalable Preparation of Fluorescent Carbon Dots for Multifunctional Applications

The synthesis of fluorescent nanomaterials has received considerable attention due to the great potential of these materials for a wide range of applications, from chemical sensing through bioimaging to optoelectronics. Herein, we report a facile and scalable approach to prepare fluorescent carbon dots (FCDs) via a one-pot reaction of citric acid with ethylenediamine at 150 °C under ambient air pressure. The resultant FCDs possess an optical bandgap of 3.4 eV and exhibit strong excitation-wavelength-independent blue emission (λ_Em = 450 nm) under either one- or two-photon excitation. Owing to their low cytotoxicity and long fluorescence lifetime, these FCDs were successfully used as internalized fluorescent probes in human cancer cell lines (HeLa cells) for two-photon excited imaging of cells by fluorescence lifetime imaging microscopy with a high-contrast resolution. They were also homogenously mixed with commercial inks and used to draw fluorescent patterns on normal papers and on many other substrates (e.g., certain flexible plastic films, textiles, and clothes). Thus, these nanomaterials are promising for use in solid-state fluorescent sensing, security labeling, and wearable optoelectronics.     

量子点/碳复合材料在碱金属离子电池的应用进展EI北大核心

碱金属离子电池作为可充电电池,是目前重要的储能设备之一。它凭借能量密度大、工作电压高、无“记忆效应”、自放电小、绿色无污染等优点在近些年来受到人们的广泛关注。电极材料是影响碱金属离子电池电化学性能的重要因素之一,因此,寻求比容量高、结构稳定的电极材料是推动碱金属离子电池发展的关键。量子点/碳复合材料(QDs/C)集合量子点与碳材料的优势,是碱金属离子电池优异的候选电极材料。本文首先对量子点进行简要介绍,然后分别综述单质量子点/碳复合材料、化合物量子点/碳复合材料及异质结构量子点/碳复合材料在碱金属离子电池中的应用进展。最后,分析量子点/碳复合材料作为碱金属离子电池电极材料的优势与不足,针对目前存在的问题提出了未来发展的方向:(1)探索新型方法,解决量子点及其复合材料的团聚问题;(2)研究SEI膜的结构性能等,解决首次库仑效率偏低的问题;(3)明确反应机理,获取更优异的电化学性能。