Tailoring Cu+ for Ga3+ Cation Exchange in Cu2-xS and CuInS2 Nanocrystals by Controlling the Ga Precursor Chemistry

Effect of the incorporation of gallium ions into silver indium sulfide nanocrystals

Gallium ion incorporation into silver indium gallium sulfide nanocrystals is investigated by various methods, including photoluminescence (PL) and X-ray photoelectron spectroscopy. The ZnS shell-growth enhances a PL quantum yield of up to 16%, with which the quantum dot light-emitting diode was successfully fabricated.

Cation exchange synthesis of AgBiS2 quantum dots for highly efficient solar cells

Silver bismuth sulfide (AgBiS2) nanocrystals have emerged as a promising eco-friendly, low-cost solar cell absorber material. Their direct synthesis often relies on the hot-injection method, requiring the application of high temperatures and vacuum for prolonged times. Here, we demonstrate an alternative synthetic approach via a cation exchange reaction. In the first-step, bis(stearoyl)sulfide is used as an air-stable sulfur precursor for the synthesis of small, monodisperse Ag2S nanocrystals at room-temperature. In a second step, bismuth cations are incorporated into the nanocrystal lattice to form ternary AgBiS2 nanocrystals, without altering their size and shape. When implemented into photovoltaic devices, AgBiS2 nanocrystals obtained by cation exchange reach power conversion efficiencies of up to 7.35%, demonstrating the efficacy of the new synthetic approach for the formation of high-quality, ternary semiconducting nanocrystals.

Synthesis and hybridization of CuInS2 nanocrystals for emerging applications

Copper indium sulfide (CuInS2) is a ternary A(I)B(III)X(VI)2-type semiconductor featuring a direct bandgap with a high absorption coefficient. In attempts to explore their practical applications, nanoscale CuInS2 has been synthesized with crystal sizes down to the quantum confinement regime. The merits of CuInS2 nanocrystals (NCs) include wide emission tunability, a large Stokes shift, long decay time, and eco-friendliness, making them promising candidates in photoelectronics and photovoltaics. Over the past two decades, advances in wet-chemistry synthesis have achieved rational control over cation-anion reactivity during the preparation of colloidal CuInS2 NCs and post-synthesis cation exchange. The precise nano-synthesis coupled with a series of hybridization strategies has given birth to a library of CuInS2 NCs with highly customizable photophysical properties. This review article focuses on the recent development of CuInS2 NCs enabled by advanced synthetic and hybridization techniques. We show that the state-of-the-art CuInS2 NCs play significant roles in optoelectronic and biomedical applications.

I-III-VI Quantum Dots and Derivatives: Design, Synthesis, and Properties for Light-Emitting Diodes

Quantum dots (QDs) are important frontier luminescent materials for future technology in flexible ultrahigh-definition display, optical information internet, and bioimaging due to their outstanding luminescence efficiency and high color purity. I-III-VI QDs and derivatives demonstrate characteristics of composition-dependent band gap, full visible light coverage, high efficiency, excellent stability, and nontoxicity, and hence are expected to be ideal candidates for environmentally friendly materials replacing traditional Cd and Pb-based QDs. In particular, their compositional flexibility is highly conducive to precise control energy band structure and microstructure. Furthermore, the quantum dot light-emitting diodes (QLEDs) exhibits superior prospects in monochrome display and white illumination. This review summarizes the recent progress of I-III-VI QDs and their application in LEDs. First, the luminescence mechanism is illustrated based on their electronic-band structural characteristics. Second, focusing on the latest progress of I-III-VI QDs, the preparation mechanism, and the regulation of photophysical properties, the corresponding application progress particularly in light-emitting diodes is summarized as well. Finally, we provide perspectives on the overall current status and challenges propose performance improvement strategies in promoting the evolution of QDs and QLEDs, indicating the future directions in this field.

Spontaneous Hetero-attachment of Single-Component Colloidal Precursors for the Synthesis of Asymmetric Au-Ag2X (X = S, Se) Heterodimers

Finding simple, easily controlled, and flexible synthetic routes for the preparation of ternary and hybrid nanostructured semiconductors is always highly desirable, especially to fulfill the requirements for mass production to enable application to many fields such as optoelectronics, thermoelectricity, and catalysis. Moreover, understanding the underlying reaction mechanisms is equally important, offering a starting point for its extrapolation from one system to another. In this work, we developed a new and more straightforward colloidal synthetic way to form hybrid Au-Ag₂X (X = S, Se) nanoparticles under mild conditions through the reaction of Au and Ag₂X nanostructured precursors in solution. At the solid-solid interface between metallic domains and the binary chalcogenide domains, a small fraction of a ternary AuAg₃X₂ phase was observed to have grown as a consequence of a solid-state electrochemical reaction, as confirmed by computational studies. Thus, the formation of stable ternary phases drives the selective hetero-attachment of Au and Ag₂X nanoparticles in solution, consolidates the interface between their domains, and stabilizes the whole hybrid Au-Ag₂X systems.

Triple crosslinking conductive hydrogels with digitally printable and outstanding mechanical stability for high-resolution conformable bioelectronics

Soft, conductive, and stretchable hydrogels offer a broad variety of applications, including skin-interfacing electrodes, biomonitoring patches, and electrostimulation. Despite rapid developments over the last decades, a combination of good electrical and mechanical properties, low-cost fabrication, and biocompatibility is yet to be demonstrated. Also, the current methods for deposition and patterning of these hydrogels are manual, and there is a need toward autonomous and digital fabrication techniques. In this work, we demonstrate a novel Gallium (Ga) embedded sodium-alginate-polyacrylamide-LAPONITE® (Ga-SA-PAAM-La) hydrogel, that is ultra-stretchable (Maximum strain tolerance of∼985%), tough (toughness ∼30 kJ m^-3), bio-adhesive (adhesion energy ∼216 J m^-2), conductive, and digitally printable. Ga nanoparticles are used as radical initiators. By adjusting the sonication parameters, we control the solution viscosity and curing time, thus allowing us to prepare pre-polymers with the desired properties for casting, or digital printing. These hydrogels benefit from a triple-network structure due to the role of Ga droplets as crosslinkers besides BIS (N,N'-methylene-bis-acrylamide) and LAPONITE®, thus resulting in tough composite hydrogels. The inclusion of LAPONITE® into the hydrogel network improved its electrical conductivity, adhesion, digital printability, and its mechanical properties, (>6× compared to the same hydrogel without LAPONITE®). As electrodes in the electrocardiogram, the signal-to-noise ratio was surprisingly higher than the medical-grade Ag/AgCl electrodes, which are applied for monitoring muscles, heart, respiration, and body joint angle through EMG, ECG, and bioimpedance measurements. The results obtained prove that such digitally printed conductive and tough hydrogels can be used as potential electrodes and sensors in practical applications in the next generation of printed wearable computing devices.

Subsuming the Metal Seed to Transform Binary Metal Chalcogenide Nanocrystals into Multinary Compositions

Direct colloidal synthesis of multinary metal chalcogenide nanocrystals typically develops dynamically from the binary metal chalcogenide nanocrystals with the subsequent incorporation of additional metal cations from solution during the growth process. Metal seeding of binary and multinary chalcogenides is also established, although the seed is solely a catalyst for nanocrystal nucleation and the metal from the seed has never been exploited as active alloying nuclei. Here we form colloidal Cu-Bi-Zn-S nanorods (NRs) from Bi-seeded Cu_2-xS heterostructures. The evolution of these homogeneously alloyed NRs is driven by the dissolution of the Bi-rich seed and recrystallization of the Cu-rich stem into a transitional segment, followed by the incorporation of Zn²⁺ to form the quaternary Cu-Bi-Zn-S composition. The present study also reveals that the variation of Zn concentration in the NRs modulates the aspect ratio and affects the nature of the majority charge carriers. The NRs exhibit promising thermoelectric properties with very low thermal conductivity values of 0.45 and 0.65 W/mK at 775 and 605 K, respectively, for Zn-poor and Zn-rich NRs. This study highlights the potential of metal seed alloying as a direct growth route to achieving homogeneously alloyed NRs compositions that are not possible by conventional direct methods or by postsynthetic transformations.

Manipulating Cation Exchange Reactions in Cu2-xS Nanoparticles via Crystal Structure Transformation

Copper-deficient Cu_2-xS nanoparticles (NPs) are extensively exploited as a superior cation exchange (CE) template to yield sophisticated nanostructures. Recently, it has been discovered that their CE reactions can be facilely manipulated by copper vacancy density, morphology, and NP size. However, the structural similarity of usually utilized Cu_2-xS somewhat limits the manipulation of the CE reactions through the factor of crystal structure because it can strongly influence the process of the reaction. Herein, we report a methodology of crystal structure transformation to manipulate the CE reactions. Particularly, roxbyite Cu_1.8S nanodisks (NDs) were converted into solid wurtzite CdS NDs and Janus-type Cu_1.94S/CdS NDs by a "full"/partial CE reaction with Cd²⁺. Afterward, the roxbyite Cu_1.8S were pseudomorphically transformed into covellite CuS NDs. Unlike Cu_1.8S, the CuS was scarcely exchanged because of the unique disulfide (S-S) bonds and converted into hollow wurtzite CdS under a more reactive condition. The S-S bonds were gradually split and CuS@CdS core@shell-type NDs were generated. Therefore, our findings in the present study provide not only a versatile technique to manipulate CE reactions in Cu_2-xS NPs but also a better comprehension of their reaction dynamics and pathways.

Crystal structure dependent cation exchange reactions in Cu2-xS nanoparticles

Because of high mobility of Cu⁺ in crystal lattice, Cu_2-xS nanoparticles (NPs) utilized as cation exchange (CE) templates to produce complicated nanomaterials has been extensively investigated. Nevertheless, the structural similarity of commonly used Cu_2-xS somewhat limits the exploration of crystal structure dependent CE reactions, since it may dramatically affect the reaction dynamics and pathways. Herein, we select djurleite Cu_1.94S and covellite CuS nanodisks (NDs) as starting templates and show that the crystal structure has a strong effect on their CE reactions. In the case of djurleite Cu_1.94S NDs, the Cu⁺ was immediately substituted by Cd²⁺ and solid wurtzite CdS NDs were produced. At a lower reaction temperature, these NDs were partially substituted, giving rise to the formation of Janus-type Cu_1.94S/CdS NDs, and this process is kinetically and thermodynamically favorable. For covellite CuS NDs, they were transformed into hollow CdS NDs under a more aggressive reaction condition due to the unique disulfide covalent bonds. These disulfide bonds distributed along [0 0 1] direction were gradually ruptured/reduced and CuS@CdS core-shell NDs could be obtained. Our findings suggest that not only the CE reaction kinetics and thermodynamics, but also the intermediates and final products are intimately correlated to the crystal structure of the host material.

Binary Cu2-xS Templates Direct the Formation of Quaternary Cu2ZnSnS4 (Kesterite, Wurtzite) Nanocrystals

Kesterite Cu₂ZnSnS₄ (k-CZTS) nanocrystals have received attention for their tunable optoelectronic properties, as well as the earth abundance of their constituent atoms. However, the phase-pure synthesis of these quaternary NCs is challenging due to their polymorphism, as well as the undesired formation of related binary and ternary impurities. A general synthetic route to tackle this complexity is to pass through intermediate template nanocrystals that direct subsequent cation exchange toward the desired quaternary crystalline phase, particularly those that are thermodynamically disfavored or otherwise synthetically challenging. Here, working within this model multinary system, we achieve control over the formation of three binary copper sulfide polymorphs, cubic digenite (Cu_1.8S), hexagonal covellite (CuS), and monoclinic djurleite (Cu_1.94S). Controlled experiments with Cu⁰ seeds show that selected binary phases can be favored by the identity and stoichiometry of the sulfur precursor alone under otherwise comparable reaction conditions. We then demonstrate that the nature of the Cu_2-xS template dictates the final polymorph of the CZTS nanocrystal products. Through digenite, the cation exchange reaction readily yields the k-CZTS phase due to its highly similar anion sublattice. Covellite nanocrystals template the k-CZTS phase but via major structural rearrangement to digenite that requires elevated temperatures in the absence of a strong reducing agent. In contrast, we show that independently synthesized djurleite nanorods template the formation of the wurtzite polymorph (w-CZTS) but with prominent stacking faults in the final product. Applying this refined understanding to the standard one-pot syntheses of k- and w-CZTS nanocrystals, we identify that these reactions are each effectively templated by binary intermediates formed in situ, harnessing their properties to guide the overall synthesis of phase-pure quaternary materials. Our results provide tools for the careful development of tailored nanocrystal syntheses in complex polymorphic systems.

Discovery of a Wurtzite-like Cu2FeSnSe4 Semiconductor Nanocrystal Polymorph and Implications for Related CuFeSe2 Materials

I₂-II-IV-VI₄ and I-III-VI₂ semiconductor nanocrystals have found applications in photovoltaics and other optoelectronic technologies because of their low toxicity and efficient light absorption into the near-infrared. Herein, we report the discovery of a metastable wurtzite-like polymorph of Cu₂FeSnSe₄, a member of the I₂-II-IV-VI₄ family of semiconductors containing only earth-abundant metals. Density functional theory calculations on this metastable polymorph of Cu₂FeSnSe₄ indicate that it may be a superior semiconductor for solar energy and optoelectronics applications compared to the thermodynamically preferred stannite polymorph, since the former displays a sharper dispersion of energy levels near the conduction band minimum that can enhance electron mobility and suppress hot electron cooling. The experimental optical band gap was measured by the inverse logarithmic derivative method to be direct, in agreement with theory, and in the range of 1.48-1.59 eV. Mechanistic studies reveal that this metastable phase derives from intermediate Cu₃Se₂ nanocrystals that serve as a structural template for the final hexagonal wurtzite-like product. We compare the chemistry of wurtzite-like Cu₂FeSnSe₄ to the related CuFeSe₂ material system. Our experimental and computational comparisons between Cu₂FeSnSe₄ and CuFeSe₂ help explain both the crystal chemistry of CuFeSe₂ that prevents it from forming wurtzite-like polymorphs and the essential role of Sn in stabilizing the metastable structure of Cu₂FeSnSe₄. This work provides insight into the importance of elemental composition when designing syntheses for metastable materials.

Pb-Doped Ag2 Se Quantum Dots with Enhanced Photoluminescence in the NIR-II Window

Ag₂ Se quantum dots (QDs) as an effective biological probe in the second near-infrared window (NIR-II, 1000-1700 nm) have been widely applied in bioimaging with high tissue penetration depth and high spatiotemporal resolution. However, the ions deficiency and crystal defects caused by the high Ag⁺ mobility in Ag₂ Se crystals are mainly responsible for the inefficient photoluminescence (PL) of Ag₂ Se QDs. Herein, a tailored route is reported to achieve controllable doping of Ag₂ Se QDs in which Ag is exchanged by Pb via cation exchange (CE), which is unattainable by direct synthetic methods. The Pb-doped Ag₂ Se QDs (denoted as Pb:Ag₂ Se QDs) present fire-new optical features with significantly enhanced PL intensity of 4.2 folds. Photoelectron spectroscopy confirms that Pb acts as an n-type dopant for Ag₂ Se QDs and therefore the electronic impurities provide additional carriers to fill the traps. Moreover, the general validity of this method is demonstrated to convert different sized Ag₂ Se into Pb:Ag₂ Se QDs, so that a wide range of NIR-II PL with high intensity is obtained. The bright NIR-II emission of Pb:Ag₂ Se QDs is further successfully performed in lymphatic system mapping.

Controllable synthesis of non-layered two-dimensional plate-like CuGaSe2 materials for optoelectronic devices

CuGaSe2 semiconductor materials, as an important member of the I-III-VI2 family, have sparked tremendous attention due to their fascinating structure-related properties and promising applications in solar energy storage and conversion. Nevertheless, the controllable preparation of two-dimensional (2D) CuGaSe2 structures is still a daunting challenge owing to the intrinsic non-layered crystal structure and inaccessible reactivity-matching of multiple reaction precursors, which will seriously impede the much deeper research progress on their properties and applications. Herein, non-layered 2D CuGaSe2 plates possessing high crystallinity, and uniform size and morphology have been first synthesized by a feasible cation exchange strategy. Because the fabrication of 2D CuGaSe2 crystals is rarely reported, a particular highlight is laid on the compositional analysis, structural characterization, and formation mechanism. Furthermore, the optical absorption and optoelectronic measurements reveal that the as-synthesized CuGaSe2 plates exhibit high light harvesting capacity and excellent photoelectric performance. This study opens up a new avenue for the feasible fabrication of non-layered CuGaSe2 plates possessing a high-quality crystalline structure and provides a promising candidate for the development of novel solar energy conversion and storage devices.

Seeded Growth Combined with Cation Exchange for the Synthesis of Anisotropic Cu2-x S/ZnS, Cu2-x S, and CuInS2 Nanorods

Colloidal copper(I) sulfide (Cu_2-x S) nanocrystals (NCs) have attracted much attention for a wide range of applications because of their unique optoelectronic properties, driving scientists to explore the potential of using Cu_2-x S NCs as seeds in the synthesis of heteronanocrystals to achieve new multifunctional materials. Herein, we developed a multistep synthesis strategy toward Cu_2-x S/ZnS heteronanorods. The Janus-type Cu_2-x S/ZnS heteronanorods are obtained by the injection of hexagonal high-chalcocite Cu_2-x S seed NCs in a hot zinc oleate solution in the presence of suitable surfactants, 20 s after the injection of sulfur precursors. The Cu_2-x S seed NCs undergo rapid aggregation and coalescence in the first few seconds after the injection, forming larger NCs that act as the effective seeds for heteronucleation and growth of ZnS. The ZnS heteronucleation occurs on a single (100) facet of the Cu_2-x S seed NCs and is followed by fast anisotropic growth along a direction that is perpendicular to the c-axis, thus leading to Cu_2-x S/ZnS Janus-type heteronanorods with a sharp heterointerface. Interestingly, the high-chalcocite crystal structure of the injected Cu_2-x S seed NCs is preserved in the Cu_2-x S segments of the heteronanorods because of the high-thermodynamic stability of this Cu_2-x S phase. The Cu_2-x S/ZnS heteronanorods are subsequently converted into single-component Cu_2-x S and CuInS₂ nanorods by postsynthetic topotactic cation exchange. This work expands the possibilities for the rational synthesis of colloidal multicomponent heteronanorods by allowing the design principles of postsynthetic heteroepitaxial seeded growth and nanoscale cation exchange to be combined, yielding access to a plethora of multicomponent heteronanorods with diameters in the quantum confinement regime.

Multinary copper-based chalcogenide nanocrystal systems from the perspective of device applications

Multinary chalcogenide semiconductor nanocrystals are a unique class of materials as they offer flexibility in composition, structure, and morphology for controlled band gap and optical properties. They offer a vast selection of materials for energy conversion, storage, and harvesting applications. Among the multinary chalcogenides, Cu-based compounds are the most attractive in terms of sustainability as many of them consist of earth-abundant elements. There has been immense progress in the field of Cu-based chalcogenides for device applications in the recent years. This paper reviews the state of the art synthetic strategies and application of multinary Cu-chalcogenide nanocrystals in photovoltaics, photocatalysis, light emitting diodes, supercapacitors, and luminescent solar concentrators. This includes the synthesis of ternary, quaternary, and quinary Cu-chalcogenide nanocrystals. The review also highlights some emerging experimental and computational characterization approaches for multinary Cu-chalcogenide semiconductor nanocrystals. It discusses the use of different multinary Cu-chalcogenide compounds, achievements in device performance, and the recent progress made with multinary Cu-chalcogenide nanocrystals in various energy conversion and energy storage devices. The review concludes with an outlook on some emerging and future device applications for multinary Cu-chalcogenides, such as scalable luminescent solar concentrators and wearable biomedical electronics.