Copper chalcogenide-based nanocrystals (NCs) are a suitable replacement for toxic Cd/Pb chalcogenide NCs in a wide range of applications including photovoltaics, optoelectronics, and biological imaging. However, despite rigorous research, direct synthesis approaches of this class of compounds suffer from inhomogeneous size, shape, and composition of the NC ensembles, which is reflected in their broad photoluminescence (PL) band widths. A partial cation exchange (CE) strategy, wherein host cations in the initial binary copper chalcogenide are replaced by incoming guest cations to form ternary/quaternary multicomponent NCs, offers a valuable alternative.

A straightforward synthesis of binary copper chalcogenide NCs[1-2] provides a perfect basement for obtaining fairly monodisperse multicomponent particles by replacing a part of host copper ions by guest cations of choice. This partial CE allows for a precise control of the composition of the exchanging NCs by simple tuning the ratio between host particles and incoming cation precursors, while preserving the size, shape, and crystal structure of template NCs.[3] Host copper ions can easily be replaced by a range of different metal cations, making copper chalcogenides a universal platform to prepare different metal chalcogenide structures, otherwise inaccessible from direct synthesis, including 2D nanomaterials.[4-5] Partial Cu⁺-to-In³⁺ CE yields Cu-In-S[6] and Cu-In-Se[7] NCs with a uniform distribution of all elements over individual particles. At the same time, the simultaneous incorporation of In³⁺ and Zn²⁺ into Cu_2-xSe NCs results in gradient alloyed Cu-Zn-In-Se NCs with a Zn-rich surface.[8] Without an additional shell growth these NCs exhibit near-infrared PL with narrow bands, reaching quantum yields of 20%. These results evidence that synthetic approaches that help to eliminate inhomogeneities in the NC ensembles can lead to narrower PL spectra. The large Stokes shift inherent to these materials, their absorption in the solar range, as well as their near-infrared PL within the biological window make them suitable candidates for applications in the area of solar energy harvesting and bioimaging.

Eco-friendly I-III-VI quantum dots (QDs) are emerging as promising materials for next-generation light-emitting and energy-harvesting devices due to their non-toxic nature, tunable band gap, environmental stability, and water solubility. [1]However, intrinsic defects such as cation and anion vacancies, interstitials, and antisites significantly control their optical properties. Here, we synthesize and study AgInS₂ QDs that exhibit two emissions: a relatively narrow but weak near the bandgap, and a more intense but broad and strongly red-shifted one. The former indicates free exciton emission and the latter defect related luminescence. Coating these QDs with gallium sulfide (GaS_x) to produce AgInS₂/GaS_x core/shell structures leads to a significant suppression of the defect-related emission and great enhancement of the free-exciton luminescence. Using steady-state optical- and ultrafast transient absorption spectroscopy, we further investigate the absorption features of these core and core/shell QDs. Essentially, we find that the emissive defects in these QDs are primarily on the surface and diminishing them enables band-edge excitonic transitions. A comprehensive understanding of such surface defects is therefore crucial for yielding pure and efficient free-exciton emission in I-III-VI QDs.

CuInS₂ quantum dot (QD) has been considered as one of the choicest toxic-metal-free QDs.^[1-3] CuInS₂ QD exhibit large Stokes shift (145 nm /4265 cm^-1 /530 meV) in the solution phase and the FWHM of its PL emission is quite significant (105 nm /2415 cm^-1 /300 meV). Photoluminescence quantum yield (PLQY) of core-only CuInS₂ QD is quite low (~15%). Although a few ultrafast exciton-dynamical (femtosecond-early nanosecond) investigations have been reported in this QD system,^[4-6] however, there is no report of detailed exciton dynamical analyses to understand (a) low PLQY, (b) large magnitude of Stokes shift and (c) large magnitude of FWHM of PL emission. Moreover, although the excited state lifetime of this QD is quite long (~4 microsecond), however, there is no detailed investigation of exciton dynamics from nanosecond to microsecond time domain. Temperature (4K to 300K) dependent steady-state PL measurement exhibits an emissive state just below the band edge. All these detailed steady-state and dynamical investigations and analyses clearly reveal the reasons behind (a) low PLQY, (b) large magnitudes Stokes shift, and (c) large FWHM of PL emission.

Upon alloy-shelling of this core CuInS₂ QD, the PLQY of the core/alloy-shell (CAS) CuInS₂ QD increases enormously and a near unity PLQY (0.96) could be achieved.^[7] More interestingly, such high PLQY of CAS CuInS₂ QD remains nearly stable for at least one year.^[7] Ultrasensitive single particle spectroscopic investigation with such highly luminescent and stable QD reveal highly suppressed blinking (>80% ON fraction).^[7] Interestingly, exciton trapping and detrapping rates and their ratios have been observed to be dependent on excitation energy/wavelength.^[8] To the best of our knowledge, so far this is the only QD system in which very significant amplitude variations of both hole trapping and detrapping have been observed.^[8]  Unlike electron trapping, hole trapping in this CAS CuInS₂ QD has been shown to be beneficial.^[8] Simultaneous electron and hole trapping have been shown to be leading to very long ON time (130 s) without blinking, which is so far the highest for a toxic-metal-free QD.^[8]

All these detailed dynamical analyses and the results will be elaborated.

Ternary chalcopyrite quantum dots, such as AgInS₂ (AIS) or CuInS₂ (CIS), as well as quaternary kesterite quantum dots (QDs), reveal unique properties with no analogs in the realm of conventional binary A^IIB^VI QDs, in particular, high tolerance to compositional non-stoichiometry, doping and alloying, as well as remarkable composition- and size-dependent spectral characteristics, photoluminescence (PL) mechanisms, charge carrier dynamics, etc. The chalcopyrite CIS and AIS QDs revealed volcano-shaped dependencies of the efficiency of PL emission and interfacial charge on their composition, with the highest values reached far from stoichiometry, at In:Ag(Cu) ratios of 4-5, highlighting a unique potential of such non-stoichiometric QDs. The realization of this potential crucially depends on the availability and versatility of synthetic approaches, allowing the QD composition, size, surface chemistry, and ligand shell structure to be tailored to specific light harvesting/conversion applications. The present talk shows that conventional heating-up/hot injection syntheses in high-boiling-point coordinating solvents can be successfully rivaled by mild and relatively “green” approaches of direct synthesis and size-tuning of ternary/quaternary QDs in aqueous solutions.

Aqueous approaches proved to be universal and applicable to many ternary chalcopyrite QDs, such as CIS, AIS, and CAIS, as well as to quaternary kesterite QDs, in particular Cu(Ag)₂ZnSnS₄. These approaches allow the QD composition to be varied in a broad range (for example, In(Sn):Ag(Cu) ratio variable between 1 and 20), protecting QDs with various ligands (mercaptocarboxylic acids, multi-functional ligands such as glutathione or polymers, such as polyethylene imine), and covering QDs with various protective shells (ZnS, CdS, In₂S₃, etc.). When combined with fine size-selective precipitation and post-synthesis solvent transfer, the aqueous approaches deliver unprecedented synthetic flexibility and variability of QD composition and size, allowing the aqueous synthesis to be upscaled to a combinatorial high-throughput robot-assisted regime. The latter is capable of yielding hundreds of samples with different spectral characteristics and PL dynamics within the same synthetic protocol, exemplified in the present talk by a high-throughput PL study of core AIS and core/shell AIS/ZnS QDs.

The combinatorial potential of the aqueous synthesis of ternary/quaternary QDs can be further expanded by their unique tendency to spontaneous alloying, which results in more complex multinary chalcogenide QDs and allows more sophisticated combinatorial synthesis to be performed with several types of ternary QDs used as sacrificial precursors. This approach is exemplified by a high-throughput robot-assisted synthesis of non-stoichiometric chalcopyrite CuAg-In-SSe (CAISSe) QDs from individual CIS, AIS, CISe, and AISe QDs as precursors. The CAISSe QDs revealed unexpected volcano-shaped dependences of PL intensity and lifetime on both Cu:Ag and S:Se ratios, advocating a large potential of the high-throughput screening of aqueous QDs for the discovery of new materials with advanced functionalities.

The broad range of instruments for composition and size variation of multinary QDs available in aqueous syntheses, including precise size selection, spontaneous alloying of QDs, as well as various cation, ligand, and solvent exchanges are currently incorporated into a unified concept of high-throughput robot-assisted screening capable of delivering tens of thousands of different QD species to be tested as light emitters and photovoltaic absorber materials. The present talk highlights our developing strategy of combining high-throughput aqueous syntheses with accelerated characterization and machine-learning-assisted analysis of QD properties as functions of their composition and size. The latter is expected to provide meaningful feedback to select new compositions and steer the synthesis toward the desired QD properties, closing the “synthesis-characterization-analysis” loop and enabling automated high-throughput material discovery within the domain of multinary metal-chalcogenide QDs.

The interest in CuInS₂ (CIS) quantum dots (QDs) has increased significantly in the past few years. CIS QDs have been studied for many applications like photodynamic therapy or solar cells[1]. They show exciting optoelectronic properties, such as broad photoluminescence (PL) with a large Stokes shift and long charge carrier lifetimes. Several mechanisms for the radiative recombination in CIS QDs have been proposed.

The most popular explanation is that radiative recombination results from an electron in the conduction band and a hole in a so-called confined hole state (CHS) related to Cu [2-3]. The range of such possible states would explain the broad PL and large Stokes shift. In addition, different synthetic characteristics, such as Cu:In stoichiometry, Zn doping or the passivation of the QDs can greatly affect the photophysical properties including the formation process of the CHS [4], as well as the structure.

X-ray techniques can be very useful tools to explore these kinds of systems from a new angle. X-ray absorption and emission spectroscopies provide information with element and oxidation state specificity, which can be useful to observe processes such as charge localization and transfer. These can be complemented with X-ray diffraction to have a more general view of the structure.

With the development of X-ray free electron lasers (XFELs), these techniques have become available in the ultrafast time-domain, allowing us to incorporate them in the study of the photophysics in many systems.

We will review our recent results obtained through a combination of laser, synchrotron and XFEL techniques. We focused on following the oxidation state of Cu through time-resolved Cu K-edge XANES and comparing the structure of the different samples through steady-state XANES and EXAFS at the Cu, Zn and S K-edges. This allowed us to gain insights into the structure, the surface passivation and the Zn incorporation through different synthetic routes. This was then complemented by optical studies [5].

Near-infrared to visible photon upconversion holds great promise for a diverse range of applications. Current photosensitizers for triplet-fusion upconversion across this spectral window often contain either precious or toxic elements, and have relatively low efficiencies. Although colloidal semicondcutor nanocrystals have emerged as versatile photosensitizers, the only family of nanocrystals discovered for near-infrared upconversion is the highly-toxic lead chalcogenides. Here we report zinc-doped CuInSe2 nanocrystals as a low-cost and lead-free alternate, allowing for near-infrared to yellow upconversion with an external quantum efficiency reaching 16.7%. When directly merged with photoredox catalysis, this system enables efficient near-infrared-driven organic synthesis and polymerization, which in turn solves the issue of photon reabsorption loss for nanocrystal-sensitized upconversion. Moreover, the broadband light capturing of these nanocrystals allows for very rapid reactions under indoor sunlight. Extending the reach of "solar synthesis" into the near-infrared may realize the century-long dream of conducting high added-value chemical transformations using sunlight.

While most research on I-III-VI nanomaterials has focused on stoichiometric compositions with equimolar amounts of Group I (Cu, Ag) and Group III (In, Ga) metals, we have taken a different approach by developing Indium-rich I-III-VI colloids (>50 at.% Indium in the cationic sublattice). [1],[2] These materials, though non-stoichiometric, remain perfectly charge-balanced semiconductors due to the presence of cationic vacancies and long-range atomic ordering. Notably, Indium-rich I-III-VI compositions are go-to materials for thin-film photovoltaics, as they exhibit fewer defects in CIGS grains and at interfaces.

In this talk, we will explore the potential of Indium-rich I-III-VI quantum dots. In addition to their size-dependent properties, these non-stoichiometric colloids offer broad solubility ranges (i.e., tunable Group I to Group III ratios), enabling precise control over the optical bandgap and photoluminescence wavelength. [3] We will examine how atomic ordering influences these properties in a non-linear fashion, leading to particularly efficient photoluminescence at select compositions. [2],[4] We will also discuss a generalizable synthetic method for Indium-rich I-III-VI nanocrystals, [3] capable of batch-upscaling to >10 grams of size- and composition-uniform quantum dots in the laboratory settings. [5]

Fluorescent nanothermometers are nanoscale materials that posses temperature dependent specrtoscopic properties. These nanostructures are expected to revolutionize research of cell functions and provide strategies for early diagnostics if nontoxic, stable materials allowing for accurate and precise temperature measurement can be fabricated. In this work, we study temperature-dependent photoluminescence (PL) properties of CuInS2/ZnS core/shell colloidal quantum dots (QDs) encapsulated in micelles for solubility in aqueous environments. We demonstrate four properties that can be used for temperature readout: (i)  intensity quenching, (ii) PL decay acceleration, (iii) peak energy shift, and (iv) change in the excitation efficency ratio. We explain the physical mechanisms responsible for the four modes and demonstrate single mode nanothermometer performance. Crucially, using multiple linear regression (MLR), we combine the four modes into a single multiparameter readout mode. We unambiguously demonstrate that the MLR mode significantly boosts the nanothermometer performance. Namely, the sensitivities are increased by up to a factor of 7, while the precision is improved by a factor of 3. We discuss the implications of these results to other nanothermometer materials. Our results show that CuInS2/ZnS QDs are excellent nanomaterials for intracellular in vivo thermometry and provide guidelines for further optimization of their performance.

Copper indium sulfide (CIS) QDs present a promising alternative to traditional Pb and Cd based quantum dots [1] due to their low toxicity, photostability, and tunable optical properties. CIS-QDs can crystallize in chalcopyrite or zinc blende-like structures, allowing for compositional tuning and doping, which enhances stability and functionality. Recent efforts have focused on aqueous synthesis using water-soluble stabilizers to improve their colloidal stability and applicability in bioimaging.[2]

Our work involves the hydrothermal synthesis of manganese-doped Cu-In-Zn-S (Mn-CIZS​) QDs, which exhibit dual emission from Mn²⁺ (~ms lifetime) and the CIZS host (~μs lifetime). Mn²⁺ emission depends on the host composition and involves an intermediate energy state. Mn-CIZS-QDs also exhibit stable, reversible temperature sensitivity under physiological conditions, making them promising as water-soluble luminescent temperature probes.[3]

The second topic of our work is related to hybrid systems of azobenzene-functionalized CIS QDs. These systems demonstrate quantitative photoisomerization to the (Z)-isomer under UV-visible light (from 365 nm to 533 nm) without back photoisomerization. This unique behavior is attributed to the direct interaction between the (Z)-isomer and CIS QDs, a novel light-induced ligand exchange mechanism, where photoactive azobenzene derivatives repleace ligands on the CIS surface promoting functionalization upon light irradiation.[4]

Semiconductor nanocrystals, also known as quantum dots, given their robust photoluminescent properties, single source excitation and multicolor emission properties have been employed for multiplexing and long-term imaging studies. [1] On the other hand, cation exchange (CE) reactions consisting of the replacement of cations in the nanocrystalline structure with different metal ions while maintaining in place the anion framework of th nanocrystals, have been extensively used for the synthesis of nanocrystals at different compositions. Here, we exploit CE reactions to radiolabel cadmium-free semiconductor NCs of ZnS, ZnSe and chalcopyrite (CuFeS₂) NCs with Cu-64 radioisotope. [2] We have developed a one-step CE protocol that is straightforward and highly efficient and by tuning the type of ligand coating to be chosen as water soluble stabilizer agents we kept the NC colloidal stability. The amount of Copper-64 to be exchanged could be also tuned in a controlled manner from partial to 100 % cation exchange. This unique approach of CE reaction enables to tune the specific activity in a wide range (from 2 to 100 TBq/g ) with an unprecedentedly record value of specific activity up to 100 TBq/g. This protocol also enables to obtaining ⁶⁴Cu:CuFeS₂ with high radiolabeling yield which do not require any further work out for the purification thus speeding up the radiolabeled NCs preparation. In addition, among the NCs explored, CuFeS₂ NCs even after partial-CE reaction with Copper-64 preserve a plasmonic resonance band, which peaks in the near infrared region making them promising NCs for photo-thermal therapy (PPT). The synergic toxicity of photo-hyperthermia and ⁶⁴Cu mediated radiotherapy ionization here is used to prove the damage to glioblastoma and epidermoid carcinoma tumor cells as demonstrated in vitro on cell culture model. A modified version of this protocol has been also established to obtain sub nanometer copper-64 radio-clusters possessing also radio and photoluminescent properties. Finally, we have also extended the same CE reaction concept to radiolabel lanthanide-based nanoparticles (NPs), consisting of NaLnF4 composition (Ln= Gd, Lu) with Yttrium 90 (⁹⁰Y ). Throughout this presentation, for the best performing materials, preclinical results to evaluate therapeutic efficacy and bio-distribution will be also discussed.

A short introduction to the RSC’s portfolio covering the breadth of energy, materials, and catalysis, with particular focus on our nanoscience journals, Nanoscale and Nanoscale Horizons.