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 charg...

Aqueous Based Semiconductor Nanocrystals

Among the armoury of photovoltaic materials, thin film heterojunction photovoltaics continue to be a promising candidate for solar energy conversion delivering a vast scope in terms of device design and fabrication. Their production does not require expensive semiconductor substrates and high temperature device processing, which allows reduced cost per unit area while maintaining reasonable efficiency. In this regard, superstrate CdTe/CdS solar cells are extensively investigated because of their suitable bandgap alignments, cost effective methods of production at large scales and stability against proton/electron irradiation. The conversion efficiencies in the range of 6–20% are achieved by structuring the device by varying the absorber/window layer thickness, junction activation/annealing steps, with more suitable front/back contacts, preparation techniques, doping with foreign ions, etc. This review focuses on fundamental and critical aspects like: (a) choice of CdS window layer and CdTe absorber layer; (b) drawbacks associated with the device including environmental problems, optical absorption losses and back contact barriers; (c) structural dynamics at CdS–CdTe interface; (d) influence of junction activation process by CdCl2 or HCF2Cl treatment; (e) interface and grain boundary passivation effects; (f) device degradation due to impurity diffusion and stress; (g) fabrication with suitable front and back contacts; (h) chemical processes occurring at various interfaces; (i) strategies and modifications developed to improve their efficiency. The complexity involved in understanding the multiple aspects of tuning the solar cell efficiency is reviewed in detail by considering the individual contribution from each component of the device. It is expected that this review article will enrich the materials aspects of CdTe/CdS devices for solar energy conversion and stimulate further innovative research interest on this intriguing topic.

Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects

Besides nanostructured materials, individual particles are key elements for nanosciences. The structuring properties of liquid crystals (LCs) are appealing to assemble them, to organize them on substrates or to design functional composites. We present here an overview on particles/LC systems, the size of the dispersed particles being larger than the typical LC length. We first summarize the large number of advances made these last 10 years concerning microparticles assemblies. We then discuss the evolution of the relevant interactions between nanoparticles (NPs) dispersed in LCs when their size decreases from micrometers to nanometers. Various NPs assemblies obtained, either in LC bulk, at interfaces or within LC distorted areas or topological defects are then reported and discussed. Finally, we consider the recent possibilities to use NPs as building elements of complex fluids. We discuss accordingly the LC phases, which can be obtained with pure inorganic NPs in concentrated solution, as well as the sel...

Ordering nano- and microparticles assemblies with liquid crystals

Liquid crystal nanoscience, a field exploring the mutually beneficial combination of the unique properties of nanoscale materials and fluid, yet ordered liquid crystalline phases, is increasingly focusing on semiconductor quantum dots. In one major research thrust, the anisotropic properties of the liquid crystal host are sought to facilitate the assembly of quantum dots into arrays, in another, both size- and shape-dependent optical and electronic properties of quantum dots are used to manipulate optical, electro-optical and alignment properties of liquid crystalline materials. This feature article reviews recent accomplishments and new insights in this fascinating area of soft matter nanocomposites including work from our laboratory on a series of CdSe and CdTe quantum dots as additives in nematic liquid crystal hosts.

Quantum dots as liquid crystal dopants

In the past decades, great strides have been made in the synthesis of hybrid nanocrystals (HNCs) consisting of two or more disparate subunits (such as metals and/or semiconductors) joined through nanointerfaces, which are intriguing due to their exceptional functionalities that cannot be achieved by single-component nanosystems. The promising and versatile applications of these HNCs are closely dependent on the structural and electronic properties of the nanointerface between subunits. This is because the compatibility of the lattice structures between subunits not only determines the synthetic accessibility and growth mechanisms of the HNCs on the thermodynamic basis but also influences their interfacial characteristics (atomic arrangement, lattice mismatch-induced strain or defects), configurations, crystallinity, and the synergistic interplay of different subunits at the nanoscale. As a result, nanointerface chemistry has attracted intense scientific endeavors worldwide and spurred the rapid development of the lattice-mismatch-directed precise synthesis. This review gives an overview of the main strategies developed for delicate design and fabrication of core-shell HNCs under different degrees of lattice mismatch (from 0.2% to larger than 50%), including epitaxial seeded growth, nanoscale cation exchange, cation exchange-facilitated nonepitaxial growth, etc. Moreover, as for the core-shell HNCs with small ( 20%), the challenges in precise synthesis, the promising solutions enabled by cation exchange-facilitated nonepitaxial growth, and the enhanced applications of the resulting HNCs with strain-free nanointerface are elaborated. We conclude with a personal perspective on the significance and urgency of fully harnessing the effects of lattice mismatch to further advance the science of synthesis and application of HNCs.

Nanointerface Chemistry: Lattice-Mismatch-Directed Synthesis and Application of Hybrid Nanocrystals.

We compare the spectroscopy of two different morphologies of CdTe/CdSe type-II nanocrystals. Core/tetrapod and spherical core/shell particles are grown from identicalCdTe cores, and both morphologies exhibit type-II spectroscopic behavior. The two morphologies show very different oscillator strengths for the lowest (luminescent) transition; the core/tetrapod particles exhibit larger oscillator strengths for the same amount of spectral shift. A model is presented that explains this difference and accurately predicts emission wavelengths and relative oscillator strengths for the spherical particles. This model uses an elastic continuum treatment to consider strain induced by lattice mismatch at the core/shell interface. The CdTe–CdSe lattice mismatch results in a calculated core pressure of about 2.9 GPa for the particles with the thickest shells. This pressure causes a change in the CdTe conduction band energy of about 0.24 eV and a change in the CdTe lattice parameter of 2.3%. The change in the lattice pa...

Spectroscopy of CdTe/CdSe Type-II Nanostructures: Morphology, Lattice Mismatch, and Band-Bowing Effects

We demonstrate the formation and spatial modulation of strongly coupled gallium selenide quantum dot (QD) nanoassemblies suspended in a nematic liquid-crystal (NLC) matrix at room temperature. Using static and dynamic optical techniques we show that the coupled QDs aggregate with a well-defined directionality commensurate with the NLC director axis. This results in highly anisotropic spectral properties of the QD assembly. The spatial orientation of the aggregates is selectively controlled in situ by the application of in-plane electric fields. The strong interdot coupling further increases the excitonic recombination rate which is both direction and electric field dependent. This electrical modulation, a noninvasive process, could potentially be an important functionality for the design and creation of building blocks for novel optoelectronic devices.

https://faculty.ucmerced.edu/sites/default/files/sghosh/files/docs/Electrical%20modulation%20of%20static%20and%20dynamic%20spectroscopic%20properties%20of%20coupled%20nanoscale%20GaSe%20quantum%20dot%20assemblies_.pdf

Electrical modulation of static and dynamic spectroscopic properties of coupled nanoscale GaSe quantum dot assemblies

Precise spatial organization and electronic coupling between quantum dots are pivotal for many potential applications. Typical spherical quantum dots in assemblies are separated by organic ligands and hence weakly coupled. GaSe nanoparticles are disk-like particles that are four atoms thick with tunable lateral dimensions. Previous spectroscopic investigations indicate the formation of nanoscale aggregates in which the quantum dots are strongly coupled. In this report, we show that the anisotropic properties of these particles may be exploited to assemble surface-stabilized superstructures with well-defined distances between the quantum dots. By changing the ligands adsorbed on the nanoparticle edges, three distinct aggregate morphologies can be produced. The surface chemistry of GaSe orients the nanoparticles on a gold surface and induces stacking in the surface normal direction. The discrete heights of such stacked aggregates suggest that the layers are held together by van der Waals interactions with a...

Nanoscale Organization of GaSe Quantum Dots on a Gold Surface

GaSe nanoparticles have been synthesized in the absence of tightly binding edge ligands. These particles are ligated primarily with TOP and TOPO ligands that are easily displaced by far less bulky alkyl aldehydes. Ligand replacement results in nanoparticle aggregates in which the lowest energy optical transitions are strongly coupled, resulting in a large red shift of the absorption spectrum (about 2600 cm−1) and a reversal of the singlet and triplet states. The reversal of the spin states results in changes in the polarization spectroscopy and a dramatic decrease in the radiative lifetime. Specifically, the exciton singlet states are linear oscillators, and time-resolved fluorescence polarization spectroscopy gives an initial anisotropy very close to the linear oscillator limit of 0.4. The radiative lifetime of this fluorescence is about 12 ns, compared to about 82 ns for the monomers. Upon aggregation, the fluorescence quantum yield increases from 4.7 to 61%.

Hoda Mirafzal

Papers

Spectroscopy of CdTe/CdSe Type-II Nanostructures: Morphology, Lattice Mismatch, and Band-Bowing Effects

Electrical modulation of static and dynamic spectroscopic properties of coupled nanoscale GaSe quantum dot assemblies

Nanoscale Organization of GaSe Quantum Dots on a Gold Surface

Singlet/Triplet Reversal in Strongly-Coupled GaSe Nanoparticle Aggregates