Finn Purcell-Milton

Quantum dots for Luminescent Solar Concentrators

There is a great need for solar cell based energy resources due to the rapid growth in price of fossil fuels and the great danger of increasing greenhouse effect caused by carbon dioxide emission. Luminescent Solar Concentrators (LSCs) offer a very useful and promising approach to improve solar energy harvesting and increase the efficiency of photovoltaic cells. In this feature article, we review the progress on the utilisation of semiconducting nanocrystals or quantum dots (QDs) in LSCs. We consider basic operating principles and parameters of LSCs, requirements of QD characteristics theoretical modelling, design and main approaches for fabrication of QD-based LSCs. Finally, we provide a brief outlook on the future development of this research area, particularly preparation of new core–shell QD structures and creation of second-generation LSC devices for the future solar cell market.

Intrinsic Chirality of CdSe/ZnS Quantum Dots and Quantum Rods.

A new class of chiral nanoparticles is of great interest not only for nanotechnology, but also for many other fields of scientific endeavor. Normally the chirality in semiconductor nanocrystals is induced by the initial presence of chiral ligands/stabilizer molecules. Here we report intrinsic chirality of ZnS coated CdSe quantum dots (QDs) and quantum rods (QRs) stabilized by achiral ligands. As-prepared ensembles of these nanocrystals have been found to be a racemic mixture of d- and l-nanocrystals which also includes a portion of nonchiral nanocrystals and so in total the solution does not show a circular dichroism (CD) signal. We have developed a new enantioselective phase transfer technique to separate chiral nanocrystals using an appropriate chiral ligand and obtain optically active ensembles of CdSe/ZnS QDs and QRs. After enantioselective phase transfer, the nanocrystals isolated in organic phase, still capped with achiral ligands, now display circular dichroism (CD). We propose that the intrinsic chirality of CdSe/ZnS nanocrystals is caused by the presence of naturally occurring chiral defects.

Molecular Recognition of Biomolecules by Chiral CdSe Quantum Dots.

Molecular recognition is one of the most important phenomena in Chemistry and Biology. Here we present a new way of enantiomeric molecular recognition using intrinsically chiral semiconductor nanocrystals as assays. Real-time confocal microscopy studies supported by circular dichroism spectroscopy data and theoretical modelling indicate an ability of left-handed molecules of cysteine and, to a smaller extent, histidine and arginine to discriminate between surfaces of left- and right-handed nanocrystals.

Enantioselective cellular uptake of chiral semiconductor nanocrystals

The influence of the chirality of semiconductor nanocrystals, CdSe/ZnS quantum dots (QDs) capped with L- and D-cysteine, on the efficiency of their uptake by living Ehrlich Ascite carcinoma cells is studied by spectral- and time-resolved fluorescence microspectroscopy. We report an evident enantioselective process where cellular uptake of the L-Cys QDs is almost twice as effective as that of the D-Cys QDs. This finding paves the way for the creation of novel approaches to control the biological properties and behavior of nanomaterials in living cells.

Application of semiconductor quantum dots in bioimaging and biosensing

In this review we present new concepts and recent progress in the application of semiconductor quantum dots (QD) as labels in two important areas of biology, bioimaging and biosensing. We analyze the biologically relevant properties of QDs focusing on the following topics: QD surface treatment and stability, labeling of cellular structures and receptors with QDs, incorporation of QDs in living cells, cytotoxicity of QDs and influence of the biological environment on the biological and optical properties of QDs. Initially, we consider utilization of QDs as agents in high-resolution bioimaging techniques that can provide information at the molecular levels. The diverse range of modern live-cell QD-based imaging techniques with resolution far beyond the diffraction limit of light is examined. In each technique, we discuss the pros and cons of QD use and deliberate how QDs can be further engineered to facilitate their application in the respective imaging techniques and to produce significant improvements in resolution. Then we review QD-based point-of-care bioassays, bioprobes, and biosensors designed in different formats ranging from analytic biochemistry assays and ELISA, to novel point-of-care smartphone integrated QD-based biotests. Here, a wide range of QD-based fluorescence bioassays with optical transduction, elecrochemiluminescence and photoelectrochemical assays are discussed. Finally, this review provides an analysis of the prospects of application of QDs in selected important areas of biology.

Colloidal quantum dots for optoelectronics

This review is focused on new concepts and recent progress in the development of three major quantum dot (QD) based optoelectronic devices: photovoltaic cells, photodetectors and LEDs. In each application, we discuss recent champion devices with a range of architectures and discuss in detail the chronological steps taken to produce significant improvements in efficiency. We consider this relative to developments in colloidal quantum dots and their effects on these devices, covering alloyed, doped and core/shell QDs, quaternary Cu–Zn–In–S QDs, graphene and silicon QDs, and the wide range of highly promising NIR QDs. The diverse range of novel device designs is examined, including all-quantum dot devices, ternary hybrid compounds, plasmonic enhancements, and nano-heterojunction architectures. In addition, we analyse recent advances in charge transport layers, blocking layers, nanostructured photoanode fabrication and the importance of QD surface treatments. Throughout, we emphasise the use of hybrid composite materials including combinations of QDs with metal oxides, plasmonic nanoparticles, graphene and others. Finally, this review provides an analysis of prospects of these important selected quantum dot-based optoelectronic devices.

Impact of Shell Thickness on Photoluminescence and Optical Activity in Chiral CdSe/CdS Core/Shell Quantum Dots

Core/shell quantum dots (QDs) are of high scientific and technological importance as these nanomaterials have found a number of valuable applications. In this paper, we have investigated the dependence of optical activity and photoluminescence upon CdS shell thickness in a range of core-shell structured CdSe/CdS QDs capped with chiral ligands. For our study, five samples of CdSe/CdS were synthesized utilizing successive ion layer adsorption and reaction to vary the thickness of the CdS shell from 0.5 to 2 nm, upon a 2.8 nm diameter CdSe core. Following this, a ligand exchange of the original aliphatic ligands with l- and d-cysteine was carried out, inducing a chiroptical response in these nanostructures. The samples were then characterized using circular dichroism, photoluminescent spectroscopy, and fluorescence lifetime spectroscopy. It has been found that the induced chiroptical response was inversely proportional to the CdS shell thickness and showed a distinct evolution in signal, whereas the photoluminescence of our samples showed a direct relationship to shell thickness. In addition, a detailed study of the influence of annealing time on the optical activity and photoluminescence quantum yield was performed. From our work, we have been able to clearly illustrate the approach and strategies that must be used when designing optimal photoluminescent optically active CdSe/CdS core-shell QDs.

Enantioselective cytotoxicity of ZnS:Mn quantum dots in A549 cells

Chirality strongly influences many biological properties of materials, such as cell accumulation, enzymatic activity, and toxicity. In the past decade, it has been shown that quantum dots (QDs), fluorescent semiconductor nanoparticles with unique optical properties, can demonstrate optical activity due to chiral ligands bound on their surface. Optically active QDs could find potential applications in biomedical research, therapy, and diagnostics. Consequently, it is very important to investigate the interaction of QDs capped with chiral ligands with living cells. The aim of our study was to investigate the influence of the induced chirality of Mn-doped ZnS QDs on the viability of A549 cells. These QDs were stabilized with D- and L-cysteine using a ligand exchange technique. The optical properties of QDs were studied using UV-Vis, photoluminescence (PL), and circular dichroism (CD) spectroscopy. The cytotoxicity of QDs was investigated by high content screening analysis. It was found that QDs stabilized by opposite ligand enantiomers, had identical PL and UV-Vis spectra and mirror-imaged CD spectra, but displayed different cytotoxicity: QDs capped with D-cysteine had greater cytotoxicity than L-cysteine capped QDs.

Induction of Chirality in Two-Dimensional Nanomaterials: Chiral 2D MoS2 Nanostructures

Two-dimensional (2D) nanomaterials have been intensively investigated due to their interesting properties and range of potential applications. Although most research has focused on graphene, atomic layered transition metal dichalcogenides (TMDs) and particularly MoS2 have gathered much deserved attention recently. Here, we report the induction of chirality into 2D chiral nanomaterials by carrying out liquid exfoliation of MoS2 in the presence of chiral ligands (cysteine and penicillamine) in water. This processing resulted in exfoliated chiral 2D MoS2 nanosheets showing strong circular dichroism signals, which were far past the onset of the original chiral ligand signals. Using theoretical modeling, we demonstrated that the chiral nature of MoS2 nanosheets is related to the presence of chiral ligands causing preferential folding of the MoS2 sheets. There was an excellent match between the theoretically calculated and experimental spectra. We believe that, due to their high aspect ratio planar morphology, chiral 2D nanomaterials could offer great opportunities for the development of chiroptical sensors, materials, and devices for valleytronics and other potential applications. In addition, chirality plays a key role in many chemical and biological systems, with chiral molecules and materials critical for the further development of biopharmaceuticals and fine chemicals, and this research therefore should have a strong impact on relevant areas of science and technology such as nanobiotechnology, nanomedicine, and nanotoxicology.

Large area quantum dot luminescent solar concentrators for use with dye-sensitised solar cells

Luminescent solar concentrators (LSCs) have the potential to significantly contribute to solar energy harvesting strategies in the built environment. For the practical realisation of LSC technology, the ability to create large area devices, which contain considerable volumes of high quality luminescent species, is paramount. Here, we report the development of large area (90 cm2 top face), planar LSCs doped with green-emitting CdSe@ZnS/ZnS core–shell quantum dots (QD) with a composition gradient shell. The champion LSC demonstrates an optical efficiency of 1.2%, for a geometric factor of 7.9, under full spectrum illumination (AM1.5G). It was observed that inhomogeneity in the edge emission is a feature of large area devices and that an appropriate measurement geometry should be used to account for this when determining the optical efficiency. The LSCs exhibit excellent optical stability under accelerated testing conditions and display reasonably low optical reabsorption losses. Proof-of-principle integration of the QD-LSC with a planar, thin strip DSSC is demonstrated to generate an enhanced photocurrent. These results not only highlight the promise of composition gradient shell QDs for the practical realisation of large area LSCs, but indicate that we should look beyond conventional silicon cells and towards emerging photovoltaic (PV) technologies for the design of hybrid LSC-PV systems for the urban environment.