Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today’s strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenom...

Prospects of Nanoscience with Nanocrystals

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), en...

Compound Copper Chalcogenide Nanocrystals

We review the field of multinary metal chalcogenide nanocrystals, which has gained strongly increasing interest in the quest for novel narrow band gap semiconductors. Small (2–4 nm) CuInS2 and CuInSe2 nanocrystals, for example, exhibit size dependent luminescence in the visible and near infrared range. Their quantum yield can exceed 50% after growth of a ZnS shell, which makes them appealing emitters for lighting, displaying and biological imaging applications. Cu2ZnSnS4 (CZTS) nanocrystals, on the other hand, can be used as solution processed absorbing materials in thin film solar cells showing high power conversion efficiencies (currently around 8–10%). These examples illustrate that multinary metal chalcogenide nanocrystals have high potential for replacing classical cadmium and lead chalcogenide quantum dots in many fields. We give an overview of the chemical synthesis methods of the different systems reported to date, classifying them according to the obtained crystal structure. Next, we discuss their photophysical properties and give a brief description of the main fields of application. Finally, we conclude by outlining current challenges and related future directions of this exponentially growing domain.

Ternary and quaternary metal chalcogenide nanocrystals: synthesis, properties and applications

We review the synthesis of semiconductor nanocrystals/colloidal quantum dots in organic solvents with special emphasis on earth-abundant and toxic heavy metal free compounds. Following the Introduction, section 2 defines the terms related to the toxicity of nanocrystals and gives a comprehensive overview on toxicity studies concerning all types of quantum dots. Section 3 aims at providing the reader with the basic concepts of nanocrystal synthesis. It starts with the concepts currently used to describe the nucleation and growth of monodisperse particles and next takes a closer look at the chemistry of the inorganic core and its interactions with surface ligands. Section 4 reviews in more detail the synthesis of different families of semiconductor nanocrystals, namely elemental group IV compounds (carbon nanodots, Si, Ge), III–V compounds (e.g., InP, InAs), and binary and multinary metal chalcogenides. Finally, the authors’ view on the perspectives in this field is given.

Synthesis of Semiconductor Nanocrystals, Focusing on Nontoxic and Earth-Abundant Materials.

Semiconductor nanocrystals possess size-dependent properties, which make them interesting candidates for a variety of applications, e.g., in solar energy conversion, lighting, display technology, or biolabelling. However, many of the best studied nanocrystalline materials contain toxic heavy metals; this seriously limits their potential for widespread application. One of the possible less toxic alternatives to cadmium- or lead-containing semiconductors is copper indium disulfide (CIS), a direct semiconductor with a bandgap in the bulk of 1.45 eV and a Bohr exciton radius of 4.1 nm. This Review gives an overview of the methods developed during the last years to synthesize CIS nanocrystals and summarizes the possibilities to influence their shape, composition and crystallographic structure. Also the potential of the application of CIS nanocrystals in biolabellling, photocatalysis, solar energy conversion, and light-emitting devices is discussed.

Synthesis and Application of Colloidal CuInS2 Semiconductor Nanocrystals

Monodisperse wurtzite CuIn(x)Ga(1-x)S(2) nanocrystals have been synthesized over the entire composition range using a facile solution-based method. Depending on the chemical composition and synthesis conditions, the morphology of the nanocrystals can be controlled in the form of bullet-like, rod-like, and tadpole-like shapes. The band gap of the nanocrystals increases linearly with increasing Ga concentration, with band gap values for the end members being close to those observed in the bulk. Colloidal suspensions of the nanocrystals are attractive for use as inks for low-cost fabrication of thin film solar cells by spin or spray coating.

Synthesis of shape-controlled monodisperse wurtzite CuIn(x)Ga(1-x)S2 semiconductor nanocrystals with tunable band gap.

Monodisperse nanocrystals of a new wurtzite phase of Cu2CoSnS4 (CCTS) have been synthesized using a simple solution-based method. The wurtzite CCTS nanocrystals grow in the shape of nanorods with an average length of 32 ± 2.0 and width of 16 ± 1.5 nm. The more stable stannite phase of CCTS has also been synthesized by increasing the reaction temperature or by a post-high-temperature annealing process. The band gap of wurtzite CCTS nanocrystals is determined to be 1.58 eV. Thin films prepared from the nanocrystal suspension display photoresponse behaviour with white light from a solar simulator, suggesting the potential use of CCTS as an active layer in low-cost thin-film solar cells.

https://iopscience.iop.org/article/10.1088/0957-4484/24/10/105706/pdf

Colloidal synthesis of wurtzite Cu2CoSnS4 nanocrystals and the photoresponse of spray-deposited thin films

Cu2CdxZn1−xSnS4 is a likely interfacial layer in Cu2ZnSnS4/CdS based thin film solar cells. It is thus important to investigate and understand the photoresponse behaviour of Cu2CdxZn1−xSnS4 in order to further improve the cell efficiency and to develop alternative buffer layer materials containing no heavy metals. We have synthesized monodispersed wurtzite phase Cu2CdxZn1−xSnS4 nanorods over the entire composition range by a facile solution based method and observed a systematic increase of the nanorod aspect ratio with increasing cadmium concentration. On the other hand, the optical band gap of Cu2CdxZn1−xSnS4 nanorods decreases linearly from 1.56 eV (x = 0) to 1.39 eV (x = 1). Smooth, uniform and dense nanorod thin films have been coated on Mo-coated glass substrates using nanorod dispersions as nano-inks by a spray deposition method. Current (I)–Voltage (V) measurements of the annealed films show the highest photoresponse for Cu2Cd0.75Zn0.25SnS4 composition. We further demonstrate that the dipole–dipole interaction between the solvent and the nanorods can be manipulated for long-range vertical self-assembly of the nanorods for the different compositions. This simple method is promising for large area coating of the absorber layer in thin film solar cells and can be extended for use with other technologically important materials.

Synthesis, photoconductivity and self-assembly of wurtzite phase Cu2CdxZn1−xSnS4 nanorods

The effects of excess SrO on the thermoelectric properties of n-type SrTiO3 have been investigated through a comparative study of different polycrystalline ceramic samples. These include Gd-doped SrTiO3 with varying SrO, nominally in the form of Ruddlesden-Popper phase of SrO(SrTiO3)n with n = 5, 10, and 20, and previously reported analogues with n = 1, 2, and ∞ (i.e., stoichiometric SrTiO3). As compared with stoichiometric SrTiO3, with increasing SrO excess (i.e., decreasing n value), the electrical conductivity is found to decrease more substantially than the thermal conductivity, while the Seebeck coefficient remains almost unaffected with n in the range of 5–20.

Influence of excess SrO on the thermoelectric properties of heavily doped SrTiO3 ceramics

Wurtzite-structured CuInxGa1-xS2 (0 ≤ x ≤ 1) is synthesized by rapidly injecting a mixture of 1-dodecanethiol and tert.-dodecanethiol into a solution of Cu(acac)2, In(acac)3, Ga(acac)3, trioctylphosphine oxide, and oleylamine at 150 °C with nitrogen purging, followed by heating at 280—290 °C for 30 min.

Xiaoyan Zhang

Papers

Synthesis of shape-controlled monodisperse wurtzite CuIn(x)Ga(1-x)S2 semiconductor nanocrystals with tunable band gap.

Colloidal synthesis of wurtzite Cu2CoSnS4 nanocrystals and the photoresponse of spray-deposited thin films

Synthesis, photoconductivity and self-assembly of wurtzite phase Cu2CdxZn1−xSnS4 nanorods

Influence of excess SrO on the thermoelectric properties of heavily doped SrTiO3 ceramics

Synthesis of Shape‐Controlled Monodisperse Wurtzite CuInxGa1‐xS2 Semiconductor Nanocrystals with Tunable Band Gap.