This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), energy storage (lithium-ion batteries, hydrogen generation), emissive materials (plasmonics, LEDs, biolabelling), sensors (electrochemical, biochemical), biomedical devices (magnetic resonance imaging, X-ray computer tomography), and medical therapies (photochemothermal therapies, immunotherapy, radiotherapy, and drug delivery). The confluence of advances in the synthesis, assembly, and application of these NCs in the past decade has the potential to significantly impact society, both economically and environmentally.
ABSTRACT:The quaternary copper chalcogenide, Cu 2 ZnSnS 4, is an important emerging material for the development of low cost and sustainable solar cells. Here we report a facile solution synthesis of stoichiometric Cu 2 ZnSnS 4 in size controlled nanorod form (11 ×35 nm). The monodisperse nanorods have a band gap of 1.43 eV and can be assembled into perpendicularly aligned arrays by controlled evaporation from solution.Colloidal semiconductor nanocrystals are a remarkable material set, that can be synthesized and processed as a 'chemical' in high yield while exhibiting optical and electronic properties that are size dependent.1 Applications ranging from bio-labeling to photocatalysis and photovoltaics have emerged exploiting either the discrete or collective properties of these size controlled crystals. 2 The synthesis of the archetypal binary (II-VI) nanocrystals has progressed to the point where precise control over their size, shape, composition and crystal phase is routine thereby rapidly accelerating the advances that utilize these as building blocks.1b,3 Extension of colloidal nanocrystal synthesis to ternary and quaternary semiconductors has the capacity to greatly expand this research platform. 4,5 In particular, copper based ternary and quaternary semiconductors such as CuInS(Se) 2 (CIS), CuIn x Ga 1-x S(Se) 2 (CIGS) and Cu 2 ZnSnS(Se) 4 (CZTS) are of interest due to their high absorption coefficients, low toxicity and suitable band gap for solar energy conversion. [4][5][6] Advances in the colloidal synthesis and shape control of nanocrystal CIS and CIGS have been demonstrated although similar reports with CZTS remain elusive. 4c-4e CZTS is flagged as the material most likely to allow unrestricted PV application on a global scale, given the relatively abundant nature of Zn and Sn in comparison to In and Ga and the promising efficiencies of 9.7%. 6,7 Generating CZTS in nanocrystal form allows absorber layer production by simple solution processes (spin-casting, spraying or printing methods) dramatically offsetting the cost of expensive vacuum processes. 6b,7,8b While synthesis of 0D CZTS nanocrystals in the tetragonal crystal structure, 5a-5e has been achieved, their formation in the more attractive rod geometry remains elusive. In nanorods, maximization of total absorption and directional charge transfer is possible by controlling the length while retaining the diameter dependent properties such as band gap. Moreover, control of orientation and positioning such that each nanorod is vertically aligned and close packed allows their collective properties to be harnessed at a device scale. Herein we describe a colloidal synthesis of monodisperse stoichiometric Cu 2 ZnSnS 4 nanorods in high yield. The quaternary semiconductor nanorods occur in the wurtzite crystal structure with elongation occurring along the [002] direction and exhibit a band gap of 1.43 eV. This crystal phase is attractive not just for shape control but is also known to allow wide range tuning of the band gap due to random distribu...
We observe the assembly of CdS nanorod superlattices by the combination of a DC electric field and solvent evaporation. In each electric field (1 V/um) assisted assembly, CdS nanorods (5 X 30 nm) suspended initially in toluene were observed to align perpendicularly to the substrate. Azimuthal alignment along the nanorod crystal faces and the presence of stacking faults indicate that both 2D and 3D assemblies were formed by a process of controlled super crystal growth.KEYWORDS Nanorod, Superlattice, assembly 2 Self assembly or directed assembly of discrete nanostructures into organized patterns provides a new route to the formation of functional materials. Colloidal nanocrystals are suitable building blocks as they can be synthesized with size and shape control. 1 The assembly of symmetrical nanospheres and nanocubes into superlattices is known; and in the case of silver nanocrystals, 3 nm in diameter, an insulator to metal transition is observed to occur as a function of sphere size and interparticle separation. [2][3][4] In effect, the superlattice functions as a novel nanocrystal solid where it is possible to control the electronic coupling by manipulating the size and position of the quantum confined structural units. The coupling can further be modified through exchange of the insulating organic ligands on the nanocrystal surface with low barrier organics e.g. hydrazine. Talapin and Murray used this approach to convert poorly conducting PbSe nanocrystal solids into n-and p-channel field effect transistors. 5 The ability to direct anisotropic structures such as cylindrical nanorods into superlattices is less well developed but also interesting. In organized nanorod superlattices, it may be possible to simultaneously and independently optimize quantities which depend on the diameter (such as band gap) from quantities which depend on length (total absorption, cross section or conductivity).Superlattice formation with spherical nanocrystals is strongly correlated to size monodispersity and their entropy driven packing under slow evaporation conditions. While dimensional control and monodispersity in nanorods has been achieved, their organization into superlattices is restricted as both positional and orientational ordering is required during assembly. Some progress has been made in preferred nanorod alignment in single layers with nematic and semectic ordering achieved from gentle evaporation of low boiling point solvents. [6][7][8][9] The rods align parallel to substrate in small domain sizes.There is further evidence for the preferred orientation of anisotropic nanostructured-rods, tubes and wires along electric field lines. The strength of the interaction is greatest in metallic nanostructures followed by nanostructures with permanent dipole moments e.g. CdSe, CdS nanorods. 10,11 Anisotropic structures with low polarizability such as silicon nanowires and carbon nanotubes can also be induced to align when the electric field induced torque is greater than the thermal excitation energy (kT). 12,13 The ...
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Supercrystallisation of CdS nanorods (10 nm × 25 nm) into perpendicular superlattices was obtained by controlled evaporation of a nanorod solution trapped between a smooth substrate and a block of highly ordered pyrolytic graphite (HOPG). Hexagonal oriented domains, 2 µm 2 in size were routinely obtained on a variety of substrates without external electric fields.
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