Stephen V. Kershaw

Color-Switchable Electroluminescence of Carbon Dot Light-Emitting Diodes

Carbon-dot based light-emitting diodes (LEDs) with driving current controlled color change are reported. These devices consist of a carbon-dot emissive layer sandwiched between an organic hole transport layer and an organic or inorganic electron transport layer fabricated by a solution-based process. By tuning the device structure and the injecting current density (by changing the applied voltage), we can obtain multicolor emission of blue, cyan, magenta, and white from the same carbon dots. Such a switchable EL behavior with white emission has not been observed thus far in single emitting layer structured nanomaterial LEDs. This interesting current density-dependent emission is useful for the development of colorful LEDs. The pure blue and white emissions are obtained by tuning the electron transport layer materials and the thickness of electrode.

Control of Emission Color of High Quantum Yield CH3NH3PbBr3 Perovskite Quantum Dots by Precipitation Temperature

Emission color controlled, high quantum yield CH3NH3PbBr3 perovskite quantum dots are obtained by changing the temperature of a bad solvent during synthesis. The products for temperatures between 0 and 60 °C have good spectral purity with narrow emission line widths of 28–36 nm, high absolute emission quantum yields of 74% to 93%, and short radiative lifetimes of 13–27 ns.

Narrow bandgap colloidal metal chalcogenide quantum dots: synthetic methods, heterostructures, assemblies, electronic and infrared optical properties

The chemistry, material processing and fundamental understanding of colloidal semiconductor nanocrystals (quantum dots) are advancing at an astounding rate, bringing the prospects of widespread commercialization of these novel and exciting materials ever closer. Interest in narrow bandgap nanocrystals in particular has intensified in recent years, and the results of research worldwide point to the realistic prospects of applications for these materials in solar cells, infrared optoelectronics (e.g. lasers, optical modulators, photodetectors and photoimaging devices), low cost/large format microelectronics, and in biological imaging and biosensor systems to name only some technologies. Improvements in fundamental understanding and material quality are built on a vast body of experience spread over many different methods of colloidal synthetic growth, each with their own strengths and weaknesses for different materials and sometimes with regard to particular applications. The nanocrystal growth expertise is matched by a rapidly expanding, and highly interdisciplinary, understanding of how best to assemble these materials into films or hybrid composites and thereby into useful devices, and again there are many different strategies that can be adopted. In this review we have attempted to survey and compare the recent work on colloidal synthesis, film and nanocrystal composite material fabrication, concentrating on narrow bandgap chalcogenide materials and some of their topical applications in the solar energy and biological fields. Since these applications are attracting rising interest across a wide range of disciplines, from the biological sciences, device engineering, and materials processing fields as well as the physics and synthetic chemistry communities, we have endeavoured to make the review of these narrow bandgap nanomaterials both comprehensive and accessible to newcomers to the area.

Wet Chemical Synthesis of Highly Luminescent HgTe/CdS Core/Shell Nanocrystals**

We have recently reported the wet chemical synthesis of colloidal HgTe nanocrystals in aqueous solution at room temperature. This novel material exhibits very strong and broad photoluminescence (PL) in the near infrared, with the exact wavelength depending on the synthetic conditions. The preparation was an extension to the previously published work on cadmium chalcogenide nanocrystals, i.e., CdS, CdSe, and CdTe, and used 1-thioglycerol as the size-regulating capping agent. Particles with a large distribution of sizes in the 3±6 nm diameter range were produced. Luminescence quantum efficiencies (QEs) of around 50 %, which are among the highest ever reported for a colloidal system, and stability towards oxidation indicate that the surface of the HgTe nanocrystals is extremely well-passivated. Typically, the PL from the freshly prepared material is around 1050 nm, but this moves to longer wavelengths with time, accompanied by an apparent drop in QE, until after two weeks the luminescence has shifted out to around 1200 nm. This aagingo process continues indefinitely, albeit at a slower rate, and presents real cause for concern for the long-term stability of such materials in device applications. In addition, if the colloidal nanocrystal solutions are heated, then a similar but more rapid process occurs with the PL shifting to beyond 1500 nm and the QE dropping to < 1 % with just a few minutes refluxing at 100 C. Again, this lack of stability to heating could be problematic in certain applications if high optical pump powers are used in laser or amplifier devices. A similar, although less-pronounced, effect is also observed in 1-thioglycerol-stabilized CdTe nanoparticles, which undergo a red shift of the PL under prolonged reflux. Whether this shift is caused by a gradual aripeningo of the nanocrystals with heat and/or time, or actually represents a compositional change will be discussed at a later date. Needless to say, the effect is undesirable and for HgTe to be a useful material, operating at the strategic near-infrared telecommunications wavelengths, the problem has to be eliminated. A possible way of achieving this is to cap the surface of the nanocrystals with a higher bandgap inorganic layer. These acore/shello composite materials can increase the luminescence quantum yield due to improved passivation of the surface, and also tend to be more physically robust than the abareo organically passivated clusters. This should therefore produce a much more stable material where not only the aagingo process is suppressed, but which is more tolerant to the processing conditions necessary for incorporation into useful devices. Examples of such core/shell structures include CdSe/ZnS, CdS/ZnS, CdSe/CdS, CdSe/ZnSe, and CdS/HgS with reports of roomtemperature QEs of up to 50 % for the CdSe/ZnS material. It was therefore decided to attempt to overcoat our HgTe nanocrystals with a layer of CdS in order to produce a physically stable core/shell heterostructure. The abareo 1-thioglycerol-stabilized HgTe colloidal nanocrystals were prepared in aqueous solution at room temperature using the method described previously. In order to coat these abareo organically capped nanocrystals with a layer of CdS, H2S gas buffered in N2 was passed through a vigorously stirred, dilute aqueous solution of HgTe nanocrystals and cadmium(II) perchlorate at a pH of around 10 in the presence of extra 1-thioglycerol stabilizer. The mixture was initially slightly turbid, but this cleared upon injection of the H2S, resulting in a clear, goldenbrown solution. This solution was then refluxed for about 30 min as a check of the robustness of the HgTe/CdS system. In order to fully characterize the material, optical absorption and photoluminescence spectra were recorded at all stages of the synthesis, as well as X-ray diffraction (XRD) patterns and high-resolution transmission electron microscopy (HRTEM) images to confirm the presence of HgTe/CdS core/shell particles in the final solution. The optical absorption data is shown in Figure 1, comparing the freshly prepared abareo HgTe prior to capping, and the subsequent HgTe/CdS material. A clear aexcitonico peak is visible for the bare HgTe sample at 850 nm, which clearly shifts to the red when the CdS capping is introduced. The increased optical density at shorter wavelengths could be indicative of some discrete CdS or HgCdTeS alloyed nanoparticles being formed in addition to the core/shell material. The effect of aging and/or heating the samples, both capped and uncapped, is to red-shift and broaden the absorption edges into the region where water absorption caused by a slight cell-mismatch prevents the recording of good-quality spectra. These are, therefore,

Hydrothermal synthesis of hierarchical SnO2 microspheres for gas sensing and lithium-ion batteries applications: Fluoride-mediated formation of solid and hollow structures

Hierarchical solid and hollow microspheres composed of oriented aligned cone-like SnO2 nanoparticles are prepared by a hydrothermal route using either NH4F or NaF, as morphology controlling agents. Their structures and morphology evolution are comprehensively characterized by TEM, SEM, XRD, XPS and the Brunauer–Emmett–Teller (BET) method, and a formation mechanism is proposed. Both solid and hollow SnO2 microspheres are formed via an Ostwald ripening process undergoing different reorganization paths in the presence of either NH4F or NaF. The solid spheres preferentially recrystallize starting from the cores and grow by consuming adjacent smaller particles, while the hollow spheres preferentially recrystallize starting from outer shells and grow by consuming the entrapped core materials via the mechanism of solid evacuation. As gas sensing materials, both solid and hollow SnO2 microspheres demonstrate sensitive and selective response to several hazardous gases, such as formaldehyde, ammonia, benzene, acetone, and methanol. As lithium storage materials, the hierarchical SnO2 hollow spheres show a higher charge/discharge capacity and better cyclic performance than the hierarchical SnO2 solid spheres. The discharge capacity of the hierarchical SnO2 hollow spheres is 187 mAh g−1 higher than the solid spheres for up to 50 discharge/charge cycles.

25th Anniversary Article: Ion Exchange in Colloidal Nanocrystals

We review the progress in ion exchange in a variety of nanocrystal structures from the earliest accounts dating back over two decades ago to the present day. In recent years the number of groups using this method to form otherwise difficult or inaccessible nanoparticle shapes and morphologies has increased considerably and the field has experienced a resurgence of interest. Whilst most of the early work on cation exchange centered on II-VI materials, the methodology has been expanded to cover a far broader range of semiconductor nanocrystals including low toxicity I-III-VI materials and the much less facile III-V materials. The extent of exchange can be controlled leading to lightly doped nanoparticles, alloys, core-shells, segmented rods and dots-in-rods. Progress has been driven by a better understanding of the underlying principles of the exchange process - from thermodynamic factors (differences in cation solubilities); the interactions between ions and transfer agents (solvents, ligands, anions, co-dopants); ionic in-diffusion mechanisms and kinetics. More recent availability of very detailed electron microscopy coupled with image reconstruction techniques has been a valuable tool to investigate the resulting heterostructures and internal interfaces. We start by surveying the range of synthetic approaches most often used to carry out ion exchange, mainly focusing on cation replacement strategies, and then describe the rich variety of nanostructures these techniques can bring forth. We also describe some of the principles that are used to establish the relative ease of exchange and to systematically improve the process where the basic energetics are less favorable. To help further the understanding of the underlying fundamentals we have gathered together useful data from the literature on solubilities, cation and anion hardness, ligand and solvent Lewis acid or base strengths for a wide range of chemical species generally used. We offer a perspective on the outlook for the field in terms of the emerging applications and the ion exchange derived materials that will enable them.

Aqueous Based Semiconductor Nanocrystals

This review summarizes traditional and recent nonconventional, bioinspired, methods for the aqueous synthesis of colloidal semiconductor quantum dots (QDs). The basic chemistry concepts are critically emphasized at the very beginning as these are strongly correlated with the selection of ligands and the optimal formation of aqueous QDs and their more sophisticated structures. The synergies of biomimetic and biosynthetic methods that can combine biospecific reactivity with the robust and strong optical responses of QDs have also resulted in new approaches to the synthesis of the nanoparticles themselves. A related new avenue is the recent extension of QD synthesis to form nanoparticles endowed with chiral optical properties. The optical characteristics of QD materials and their advanced forms such as core/shell heterostructures, alloys, and doped QDs are discussed: from the design considerations of optical band gap tuning, the control and reduction of the impact of surface traps, the consideration of charge carrier processes that affect emission and energy and charge transfer, to the impact and influence of lattice strain. We also describe the considerable progress in some selected QD applications such as in bioimaging and theranostics. The review concludes with future strategies and identification of key challenges that still need to be resolved in reaching very attractive, scalable, yet versatile aqueous syntheses that may widen the scope of commercial applications for semiconductor nanocrystals.

Colloidally Prepared CdHgTe and HgTe Quantum Dots with Strong Near-Infrared Luminescence

Several series of CdHgTe composite nanocrystals were prepared using thiol-capped CdTe nanocrystal precursors to which subsequent layers of HgTe and CdTe were added. The position of the ‘excitonic’ photoluminescence peak measured at room temperature was red-shifted to the near infrared to give emission wavelengths ranging from 800 to 1100 nm depending on the quantum dot composition, with quantum efficiency (QE) significantly increased over the pure CdTe material (QE up to around 40%). Thiol-capped HgTe nanocrystals synthesized in aqueous solution show a broad photoluminescence with a QE of ∼ 50%. It has been shown using D2O as a solvent that by varying the synthesis conditions it is possible to tune the luminescence of HgTe quantum dots to the desired wavelength in the range of 900–2000 nm. HgTe nanocrystals passivated at the surface with a thick CdS layer have been shown to be much more robust towards heating and “aging” of the optical properties.

Magnetically Engineered Semiconductor Quantum Dots as Multimodal Imaging Probes

Light-emitting semiconductor quantum dots (QDs) combined with magnetic resonance imaging contrast agents within a single nanoparticle platform are considered to perform as multimodal imaging probes in biomedical research and related clinical applications. The principles of their rational design are outlined and contemporary synthetic strategies are reviewed (heterocrystalline growth; co-encapsulation or assembly of preformed QDs and magnetic nanoparticles; conjugation of magnetic chelates onto QDs; and doping of QDs with transition metal ions), identifying the strengths and weaknesses of different approaches. Some of the opportunities and benefits that arise through in vivo imaging using these dual-mode probes are highlighted where tumor location and delineation is demonstrated in both MRI and fluorescence modality. Work on the toxicological assessments of QD/magnetic nanoparticles is also reviewed, along with progress in reducing their toxicological side effects for eventual clinical use. The review concludes with an outlook for future biomedical imaging and the identification of key challenges in reaching clinical applications.

Colloidal CdTe/HgTe quantum dots with high photoluminescence quantum efficiency at room temperature

We have used an aqueous colloidal growth technique to form hybrid CdTe/HgTe quantum dots with a broad, strong fluorescence in the infrared (800–1200 nm). The quantum efficiency is high, around 44%, when pumped in the visible (488 nm), and the excited state lifetime is around 130 ns, making the material interesting as an optical amplifier medium. Using a pump-probe experiment, we have demonstrated weak optical amplification in a dilute aqueous suspension of CdTe/HgTe dots in the short wavelength wing of the emission spectrum at 808 nm.