Selective epitaxial growth of zinc blende-derivative on wurtzite-derivative: the case of polytypic Cu2CdSn(S(1-x)Se(x))4 nanocrystals

Wurtzite CuIn(SxSe1-x)2 Nanocrystals: Colloidal Synthesis and Band-Gap Engineering

CuIn(SxSe1-x)2 nanocrystals as an emerging class of functional materials present huge potential for industrial applications; however, the synthesis of CuIn(SxSe1-x)2 nanocrystals remains a formidable challenge in achieving both tunable band gap and phase. Here, we reported a facile hot-injection method for synthesizing a family of wurtzite CuIn(SxSe1-x)2 nanocrystals, enabling manipulation of the S and Se contents across the entire compositional range (0 ≤ x ≤ 1). The obtained nanocrystals exhibit band gaps ranging from 1.21 to 1.58 eV, which vary depending on the S/Se ratios in the products. This approach can be readily extended to other scenarios involving chalcogenide nanomaterials, thereby facilitating the advancement of next-generation functional materials and applications.

Polytypic metal chalcogenide nanocrystals

By engineering chemically identical but structurally distinct materials into intricate and sophisticated polytypic nanostructures, which often surpass their pure phase objects and even produce novel physical and chemical properties, exciting applications in the fields of photovoltaics, electronics and photocatalysis can be achieved. In recent decades, various methods have been developed for synthesizing a library of polytypic nanocrystals encompassing IV, III-V and II-VI polytypic semiconductors. The exceptional performances of polytypic metal chalcogenide nanocrystals have been observed, making them highly promising candidates for applications in photonics and electronics. However, achieving high-precision control over the morphology, composition, crystal structure, size, homojunctions, and periodicity of polytypic metal chalcogenide nanostructures remains a significant synthetic challenge. This review article offers a comprehensive overview of recent progress in the synthesis and control of polytypic metal chalcogenide nanocrystals using colloidal synthetic strategies. Starting from a concise introduction on the crystal structures of metal chalcogenides, the subsequent discussion delves into the colloidal synthesis of polytypic metal chalcogenide nanocrystals, followed by an in-depth exploration of the key factors governing polytypic structure construction. Subsequently, we provide comprehensive insights into the physical properties of polytypic metal chalcogenide nanocrystals, which exhibit strong correlations with their applications. Thereafter, we emphasize the significance of polytypic nanostructures in various applications, such as photovoltaics, photocatalysis, transistors, thermoelectrics, stress sensors, and the electrocatalytic hydrogen evolution. Finally, we present a summary of the recent advancements in this research field and provide insightful perspectives on the forthcoming challenges, opportunities, and future research directions.

A library of polytypic copper-based quaternary sulfide nanocrystals enables efficient solar-to-hydrogen conversion

Designing polytypic homojunction is an efficient way to regulate photogenerated electrons and holes, thereafter bringing desired physical and chemical properties and being attractive photocatalysts for solar-to-hydrogen conversion. However, the high-yield and controllable synthesis of well-defined polytypes especially for multinary chalcogenide - the fundamental factor favoring highly efficient solar-to-hydrogen conversion - has yet to be achieved. Here, we report a general colloidal method to construct a library of polytypic copper-based quaternary sulfide nanocrystals, including Cu₂ZnSnS₄, Cu₂CdSnS₄, Cu₂CoSnS₄, Cu₂MnSnS₄, Cu₂FeSnS₄, Cu₃InSnS₅ and Cu₃GaSnS₅, which can be synthesized by selective epitaxial growth of kesterite phase on wurtzite structure. Besides, this colloidal method allows the precise controlling of the homojunction number corresponding to the photocatalytic performance. The single-homojunction and double-homojunction polytypic Cu₂ZnSnS₄ nanocrystal photocatalysts show 2.8-fold and 3.9-fold improvement in photocatalytic hydrogen evolution rates relative to the kesterite nanocrystals, respectively. This homojunction existed in the polytypic structure opens another way to engineer photocatalysts.

While polytypic semiconductors are promising for solar-to-fuel applications, preparing homojunction nanomaterials has proven challenging. Here, authors obtain a library of polytypic copper-based quaternary sulfide nanocrystals by selective epitaxial growth of kesterite phase on wurtzite structure.

Anion exchange induced formation of kesterite copper zinc tin sulphide-copper zinc tin selenide nanoheterostructures

We report the colloidal synthesis of quaternary kesterite CZTS-CZTSe heterostructures via anion exchange reactions on a kesterite CZTS template. The crystal phase selectivity during the synthesis (kesterite vs. wurtzite) is due to the initial nucleation of cubic Cu9S5 seeds, followed by incorporation of Zn and Sn. Upon injection of Se-precursor, which triggered simultaneous anion exchange and overgrowth of the pristine CZTS template, sandwich CZTS-CZTSe (core-tip) nanoheterostructures were obtained. X-ray photoelectron spectroscopy (XPS) and optical band gap measurement results suggest a change of intrinsic electronic structure of CZTS by Se-treatment. Our study not only provides insight into mechanisms of formation of kesterite CZTS nanocrystals (NCs) and subsequent anion exchange reactions, but also opens doors to access novel CZTSSe nanostructures for potential applications.

Single crystalline quaternary sulfide nanobelts for efficient solar-to-hydrogen conversion

Although solar-driven water splitting on semiconductor photocatalysts is an attractive route for hydrogen generation, there is a lack of excellent photocatalysts with high visible light activity. Due to their tunable bandgaps suitable for superior visible-light absorption, copper-based quaternary sulfides have been the important candidates. Here, we first assessed the preferred facet of wurtzite Cu-Zn-In-S for photocatalytic hydrogen evolution reaction using the relevant Gibbs free energies determined by first principle calculation. We then developed a colloidal method to synthesize single crystalline wurtzite Cu-Zn-In-S nanobelts (NBs) exposing (0001) facet with the lowest reaction Gibbs energy, as well as Cu-Zn-Ga-S NBs exposing (0001) facet. The obtained single crystalline Cu-Zn-In-S and Cu-Zn-Ga-S NBs exhibit superior hydrogen production activities under visible-light irradiation, which is composition-dependent. Our protocol represents an alternative surface engineering approach to realize efficient solar-to-chemical conversion of single crystalline copper-based multinary chalcogenides.

Quaternary sulfides are important candidates for solar-to-H₂ conversion due to tunable bandgaps for controllable light absorption. Here, authors prepare single crystalline wurtzite Cu-Zn-In-S and Cu-Zn-Ga-S nanobelts with (0001) facets that show strong photocatalytic H₂ production performances.

Optical Properties, Synthesis, and Potential Applications of Cu-Based Ternary or Quaternary Anisotropic Quantum Dots, Polytypic Nanocrystals, and Core/Shell Heterostructures

This review summaries the optical properties, recent progress in synthesis, and a range of applications of luminescent Cu-based ternary or quaternary quantum dots (QDs). We first present the unique optical properties of the Cu-based multicomponent QDs, regarding their emission mechanism, high photoluminescent quantum yields (PLQYs), size-dependent bandgap, composition-dependent bandgap, broad emission range, large Stokes’ shift, and long photoluminescent (PL) lifetimes. Huge progress has taken place in this area over the past years, via detailed experimenting and modelling, giving a much more complete understanding of these nanomaterials and enabling the means to control and therefore take full advantage of their important properties. We then fully explore the techniques to prepare the various types of Cu-based ternary or quaternary QDs (including anisotropic nanocrystals (NCs), polytypic NCs, and spherical, nanorod and tetrapod core/shell heterostructures) are introduced in subsequent sections. To date, various strategies have been employed to understand and control the QDs distinct and new morphologies, with the recent development of Cu-based nanorod and tetrapod structure synthesis highlighted. Next, we summarize a series of applications of these luminescent Cu-based anisotropic and core/shell heterostructures, covering luminescent solar concentrators (LSCs), bioimaging and light emitting diodes (LEDs). Finally, we provide perspectives on the overall current status, challenges, and future directions in this field. The confluence of advances in the synthesis, properties, and applications of these Cu-based QDs presents an important opportunity to a wide-range of fields and this piece gives the reader the knowledge to grasp these exciting developments.

Diorganyl dichalcogenides as useful synthons for colloidal semiconductor nanocrystals

The ability to synthesize colloidal semiconductor nanocrystals in a well-controlled manner (i.e., with fine control over size, shape, size dispersion, and composition) has been mastered over the past 15 years. Much of this success stems from careful studies of precursor conversion and nanocrystal growth with respect to phosphine chalcogenide precursors for the synthesis of metal chalcogenide nanocrystals. Despite the high level of success that has been achieved with phosphine chalcogenides, there has been a longstanding interest in exploring alternate chalcogenide precursors because of issues associated with phosphine chalcogenide cost, purity, toxicity, etc. This has resulted in a large body of literature on the use of sulfur and selenium dissolved in octadecene or amines, thio- and selenoureas, and silyl chalcogenides as alternate chalcogenide precursors for metal chalcogenide nanocrystal synthesis. In this Account, emerging work on the use of diorganyl dichalcogenides (R-E-E-R, where E = S, Se, or Te and R = alkyl, allyl, benzyl, or aryl) as alternate chalcogenide precursors for the synthesis of metal chalcogenide nanocrystals is summarized. Among the benefits of these dichalcogenide synthons are the following: (i) they represent the first and only common precursor type that can function as chalcogen transfer reagents for each of the group VI elements (i.e., to make metal oxide, metal sulfide, metal selenide, and metal telluride nanocrystals); (ii) they possess relatively weak E-E bonds that can be readily cleaved under mild thermolytic or photolytic conditions; and (iii) the organic substituents can be tuned to affect the reactivity. These combined attributes have allowed dichalcogenide precursors to be employed for a wide range of metal chalcogenide nanocrystal syntheses, including those for In2S3, SnxGe1-xSe, SnTe, Cu2-xSySe1-y, ZnSe, CdS, CdSe, MoSe2, WSe2, BiSe, and CuFeS2. Interestingly, a number of metastable phases of compositionally complex semiconductors can be kinetically accessed through syntheses utilizing dichalcogenide precursors, likely as a result of their ability to convert at relatively low temperatures. These include the hexagonal wurtzite phases of CuInS2, CuInSe2, Cu2ZnSn(S1-xSex)4, and Cu2SnSe3 nanocrystals. The discovery of crystal phases on the nanoscale that do not exist in their bulk analogues is a developing area of nanocrystal chemistry, and dichalcogenides are proving to be a useful synthetic tool in this regard. The most recent application of dichalcogenide synthons for semiconductor nanocrystals is their use as precursors for surface ligands. While there is a rich history of using thiol ligands for semiconductor nanocrystals, the analogous selenol and tellurol ligands have not been studied, likely because of their oxidative instability. Dichalcogenides have proven useful in this regard, as they can be reduced in situ with diphenylphosphine to give the corresponding selenol or tellurol ligand that binds to the nanocrystal surface. This chemistry has been applied to the in situ synthesis and ligand binding of selenols to PbSe nanocrystals and both selenols and tellurols to CdSe nanocrystals. These initial studies have allowed the photophysics of these nanocrystal-ligand constructs to be investigated; in both cases, it appears that the selenol and tellurol ligands act as hole traps that quench the photoluminescence of the semiconductor nanocrystals.

Heterovalent substitution to Enrich electrical conductivity in Cu2CdSn(1-x)GaxSe4 series for high thermoelectric performances

Serials of Ga doping on Sn sites as heterovalent substitution in Cu2CdSnSe4 are prepared by the melting method and the spark plasma sintering (SPS) technique to form Cu2CdSn(1-x)GaxSe4 (x = 0, 0.025, 0.05, 0.075, 0.01, and 0.125). Massive atomic vacancies are found at x = 0.10 by the heterovalent substitution, which contributes significantly to the increase of electrical conductivity and the decrease of lattice thermal conductivity. The electrical conductivity is increased by about ten times at 300 K after Ga doping. Moreover, the seebeck coefficient only decreases slightly from 310 to 226 μV/K at 723 K, and a significant increase of the power factor is obtained. As a result, a maximum value of 0.27 for the figure of merit (ZT) is obtained at x = 0.10 and at 723 K. Through an ab initio study of the Ga doping effect, we find that the Fermi level of Cu2CdSnSe4 is shifted downward to the valence band, thus improving the hole concentration and enhancing the electrical conductivity at low doping levels. Our experimental and theoretical studies show that a moderate Ga doping on Sn sites is an effective method to improve the thermoelectric performance of Cu2CdSnSe4.

Complete study of the composition and shape evolution in the synthesis of Cu2ZnSnS4 (CZTS) semiconductor nanocrystals

This article describes a complete study of the evolution of composition (from binary to quaternary) and shape (0D–1D) during the synthesis of CZTS nanocrystals.