Shape- and Phase-Controlled Synthesis of Monodisperse, Single-Crystalline Ternary Chalcogenide Colloids through a Convenient Solution Synthesis Strategy

Direct Synthesis and Characterization of Hydrophilic Cu-Deficient Copper Indium Sulfide Quantum Dots

Copper indium sulfide (CIS) nanocrystals constitute a promising alternative to cadmium- and lead-containing nanoparticles. We report a synthetic method that yields hydrophilic, core-only CIS quantum dots, exhibiting size-dependent, copper-deficient composition and optical properties that are suitable for direct coupling to biomolecules and nonradiative energy transfer applications. To assist such applications, we complemented previous studies covering the femtosecond-picosecond time scale with the investigation of slower radiative and nonradiative processes on the nanosecond time scale, using both time-resolved emission and transient absorption. As expected for core particles, relaxation occurs mainly nonradiatively, resulting in low, size-dependent photoluminescence quantum yield. The nonradiative relaxation from the first excited band is wavelength-dependent with lifetimes between 25 and 150 ns, reflecting the size distribution of the particles. Approximately constant lifetimes of around 65 ns were observed for nonradiative relaxation from the defect states at lower energies. The photoluminescence exhibited a large Stokes shift. The band gap emission decays on the order of 10 ns, while the defect emission is further red-shifted, and the lifetimes are on the order of 100 ns. Both sets of radiative lifetimes are wavelength-dependent, increasing toward longer wavelengths. Despite the low radiative quantum yield, the aqueous solubility and long lifetimes of the defect states are compatible with the proposed role of CIS quantum dots as excitation energy donors to biological molecules.

Crystal-Phase Engineering in Heterogeneous Catalysis

The performance of a chemical reaction is critically dependent on the electronic and/or geometric structures of a material in heterogeneous catalysis. Over the past century, the Sabatier principle has already provided a conceptual framework for optimal catalyst design by adjusting the electronic structure of the catalytic material via a change in composition. Beyond composition, it is essential to recognize that the geometric atomic structures of a catalyst, encompassing terraces, edges, steps, kinks, and corners, have a substantial impact on the activity and selectivity of a chemical reaction. Crystal-phase engineering has the capacity to bring about substantial alterations in the electronic and geometric configurations of a catalyst, enabling control over coordination numbers, morphological features, and the arrangement of surface atoms. Modulating the crystallographic phase is therefore an important strategy for improving the stability, activity, and selectivity of catalytic materials. Nonetheless, a complete understanding of how the performance depends on the crystal phase of a catalyst remains elusive, primarily due to the absence of a molecular-level view of active sites across various crystal phases. In this review, we primarily focus on assessing the dependence of catalytic performance on crystal phases to elucidate the challenges and complexities inherent in heterogeneous catalysis, ultimately aiming for improved catalyst design.

Compound Copper Chalcogenide 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), 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.

Synthetic strategies and biomedical applications of I–III–VI ternary quantum dots

In this review, we discuss recent advances of I–III–VI QDs with a major focus on synthesis and biomedical applications; advantages include low toxicity and fluorescent tuning in the biological window.

Colloidal Synthesis of Quaternary Wurtzite Cu3 AlSnS5 Nanocrystals and Their Photoresponsive Properties

Novel wurtzite Cu₃ AlSnS₅ nanocrystals with uniform size and shape are synthesized for the first time using a facile colloidal synthetic approach. The nanocrystals are characterized in detail and have a band gap of 1.35 eV. The photoresponsive behavior indicates their potential application in solar energy conversion devices.

A simple solution-phase approach to synthesize high quality ternary AgInSe2 and band gap tunable quaternary AgIn(S1-xSex)2 nanocrystals

A facile solution-phase route for the preparation of AgInSe2 nanocrystals was developed by using silver nitrate, indium stearate, and oleylamine-selenium (OAm-Se) as precursors. The evolution process of the AgInSe2 nanocrystals is discussed in detail and different reaction conditions all have a great impact on the growth and morphology of the nanocrystals. Alloyed AgIn(S1-xSex)2 nanocrystals with controlled composition across the entire range (0 ≤ x ≤ 1) was also successfully prepared by modulating the S/Se reactant mole ratio. X-ray diffraction (XRD), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were used to confirm that the alloyed AgIn(S1-xSex)2 nanocrystals are homogeneous. The UV-vis absorption spectra revealed that the band gap energies of the alloyed AgIn(S1-xSex)2 nanocrystals could be continuously tuned by increasing the Se content.

Facile synthesis of CuInGaS2 quantum dot nanoparticles for bilayer-sensitized solar cells

TiO₂@CuIn_0.7Ga_0.3S₂ QDs (2–5 nm) were firstly synthesised by a vacuum one-pot-nanocasting process without long-chain ligands.

Synthesis and application of colloidal CuInS2 semiconductor nanocrystals

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.

Cu(In(1-x)Ga(x))S2 nanocrystals and films: low-temperature synthesis with size and composition control

We demonstrate a single-step X-ray irradiation process that yields high-quality Cu(In1-xGax)S2 nanocrystals in colloidal solutions, with complete control of size and composition. Thin films produced by drop-casting exhibit high-quality photoresponse, confirming that our process is suitable for microelectronics applications.

Ligand-Free, Colloidal, and Luminescent Metal Sulfide Nanocrystals

We report here a different kind of binary and ternary colloidal inorganic nanocrystals, where no capping ligand is used. The surfaces of these ligand-free nanocrystals were designed to exhibit negative charges and, therefore, electrostatically repel each other, forming a colloidal dispersion in a polar solvent. Undoped and Mn-doped ZnxCd1-xS nanocrystals were studied both in solution and close-packed films. While undoped samples exhibit poor luminescence because of surface defects, Mn-doped nanocrystals show strong luminescence both in solution (20% quantum efficiency) and the film. Strong Mn emission arises from the inner-core d electrons, which are less sensitive to nonradiative decay channels on the nanocrystal surface. Ligand-free AgInS2 nanocrystals exhibit donor-acceptor type luminescence. Our preliminary results suggest the possibility of electronic coupling between ligand-free nanocrystals in their film, and therefore, they are expected to be more suitable for electronic and optoelectronic applications compared to organic-capped nanocrystals.

Synthesis and high-pressure transformation of metastable wurtzite-structured CuGaS2 nanocrystals

The metastable wurtzite nanocrystals of CuGaS(2) have been synthesized through a facile and effective one-pot solvothermal approach. Through the Rietveld refinement on experimental X-ray diffraction patterns, we have unambiguously determined the structural parameters and the disordered nature of this wurtzite phase. The metastability of wurtzite structure with respect to the stable chalcopyrite structure was testified by a precise theoretical total energy calculation. Subsequent high-pressure experiments were performed to establish the isothermal phase stability of this wurtzite phase in the pressure range of 0-15.9 GPa, above which another disordered rock salt phase crystallized and remained stable up to 30.3 GPa, the highest pressure studied. Upon release of pressure, the sample was irreversible and intriguingly converted into the energetically more favorable and ordered chalcopyrite structure as revealed by the synchrotron X-ray diffraction and the high-resolution transmission electron microscopic measurements. The observed phase transitions were rationalized by first-principles calculations. The current research surely establishes a novel phase transition sequence of disorder → disorder → order, where pressure has played a significant role in effectively tuning stabilities of these different phases.

A facile and general approach to polynary semiconductor nanocrystals via a modified two-phase method

Cu(2)ZnSnS(4) nanocrystals were synthesized through a modified two-phase method and characterized with transmission electron microscopy (TEM), powder x-ray diffraction (XRD) and UV-vis spectroscopy. Inorganic metal salts were dissolved in the polar solvent triethylene glycol (TEG) and then transferred into the non-polar solvent 1-octadecene (ODE) by forming metal complexes between metal ions and octadecylamine (ODA). Since nucleation and growth occur in the single phase of the ODE solution, nanocrystals could be produced with qualities similar to those obtained through the hot-injection route. Balancing the reactivity of the metal precursors is a key factor in producing nanocrystals of a single crystalline phase. We found that increasing the reaction temperature increases the reactivity of each of the metal precursors by differing amounts, thus providing the necessary flexibility for obtaining a balanced reactivity that produces the desired product. The versatility of this synthesis strategy was demonstrated by extending it to the production of other polynary nanocrystals such as binary (CuS), ternary (CuInS(2)) and pentanary (Cu(2 - x)Ag(x)ZnSnS(4)) nanocrystals. This method is considered as a green synthesis route due to the use of inorganic metal salts as precursors, smaller amounts of coordinating solvent, shorter reaction time and simpler post-reaction treatment.

Noninjection gram-scale synthesis of monodisperse pyramidal CuInS2 nanocrystals and their size-dependent properties

CuInS2 nanocrystals are viewed as very good candidates for solar harvesting and light emitting applications. Here we report an optimized noninjection method for the synthesis of monodisperse pyramidal CuInS2 nanocrystals with sizes ranging from 3 to 8 nm. This synthetic route is able to yield large amounts of high quality nanoparticles, usually in the gram scale for one batch experiment. The structure and surface studies showed that the resulting nanocrystals are pyramids of CuInS2 tetragonal phase with well-defined facets, while their surface is functionalized with dodecanethiol capping ligands. Spectroscopic and electrochemical measurements revealed size-dependent optical and electrical properties of CuInS2 nanocrystals, demonstrating quantum confinement effects in these systems. The size-dependent optical bandgaps of CuInS2 nanocrystals were found to be consistent with the finite-depth well effective mass approximation (EMA) calculations, which provide a convenient method to estimate the diameter of CuInS2 pyramids. Additionally we have also determined some important physical parameters, including bandgaps and energy levels, for this system, which are crucial for the integration of CuInS2 nanocrystals in potential device applications.

Sulfide nanocrystal inks for dense Cu(In1-xGa(x))(S1-ySe(y))2 absorber films and their photovoltaic performance

Recent developments in the colloidal synthesis of high quality nanocrystals have opened up new routes for the fabrication of low-cost efficient photovoltaic devices. Previously, we demonstrated the utility of CuInSe(2) nanocrystals in the fabrication of CuInSe(2) thin film solar cells. In those devices, sintering the nanocrystal film yields a relatively dense CuInSe(2) film with some void space inclusions. Here, we present a general approach toward eliminating void space in sintered nanocrystal films by utilizing reactions that yield a controlled volume expansion of the film. This is demonstrated by first synthesizing a nanocrystal ink composed of Cu(In(1-x)Ga(x))S(2) (CIGS). After nanocrystal film formation, the nanocrystals are exposed to selenium vapor during which most of the sulfur is replaced by selenium. Full replacement produces a approximately 14.6% volume expansion and reproducibly leads to good dense device-quality CIGSSe absorber films with reduced inclusion of void space. Solar cells made using the CIGSSe absorber films fabricated by this method showed a power conversion efficiency of 4.76% (5.55% based on the active nonshadowed area) under standard AM1.5 illumination.