Composition- and band-gap-tunable synthesis of wurtzite-derived Cu₂ZnSn(S(1-x)Se(x))₄ nanocrystals: theoretical and experimental insights

Structure-Property Correlations in CZTSe Domains within Semiconductor Nanocrystals as Photovoltaic Absorbers

Semiconductor nanocrystals (NCs) are promising materials for various applications. Two of four recently identified CuαZnβSnγSeδ (CZTSe) domains demonstrate metallic character, while the other two exhibit semiconductor character. The presence of both metallic and semiconductor domains in one NC can hugely benefit future applications. In contrast to traditional band gap studies in the NC community, this study emphasizes that NC domain interfaces also affect the electronic properties. Specifically, the measured band gap of a tetrapod-shaped CZTSe NC is demonstrated to originate from two specific domains (tetragonal I4 ¯ $\bar 4$ and monoclinic P1c1 Cu2ZnSnSe4). The heterojunction between these two semiconductor domains exhibits a staggered type-II band alignment, facilitating the separation of photogenerated electron-hole pairs. Interestingly, tetrapod NCs have the potential to be efficient absorber materials with higher capacitance in photovoltaic applications due to the presence of both semiconductor/semiconductor interfaces and metal/semiconductor "Schottky"-junctions. For the two photo-absorbing domains, the calculated absorption spectra yield maximum photon-absorption coefficients of about 105 cm-1 in the visible and UV regions and a theoretical solar power conversion efficiency up to 20.8%. These insights into the structure-property relationships in CZTSe NCs will guide the design of more efficient advanced optical CZTSe materials for various applications.

Improving the performance of kesterite solar cells by solution germanium alloying

Cation substitution is an effective strategy to regulate the defects/electronic properties of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) absorbers and improve the device photovoltaic performance. Here, we report Ge alloying kesterite Cu2Zn(Sn,Ge)(S,Se)4 (CZTGSSe) via a solution approach. The results demonstrate that the same chemical reaction of Ge4+ to Sn4+ ensures homogeneous Ge incorporation in the whole range of concentrations (from 0 to unit). Ge alloying promotes grain growth and linearly enlarges the absorber band gap by solely raising the conduction band minimum, which maintains a "spike" conduction band offset at the heterojunction interface until 15% alloying concentration and thus facilitates effective charge carrier collection. A promising efficiency of 11.57% has been achieved at 15% Ge alloying concentration with a significant enhancement in open-circuit voltage and fill factor. By further 10% Ag alloying to improve the absorber film morphology, a champion device with an efficiency of 12.25% has been achieved without an antireflective coating. This result emphasizes the feasibility of achieving homogeneous and controllable Ge alloying of kesterite semiconductors through the solution method, paving the way for further improvement and optimization of device performance.

Polymorphic Control of Solution-Processed Cu2SnS3 Films with Thiol-Amine Ink Formulation

There is increasing demand for tailored molecular inks that produce phase-pure solution-processed semiconductor films. Within the Cu-Sn-S phase space, Cu₂SnS₃ belongs to the I₂-IV-VI₃ class of semiconductors that crystallizes in several different polymorphs. We report the ability of thiol-amine solvent mixtures to dissolve inexpensive bulk Cu₂S and SnO precursors to generate free-flowing molecular inks. Upon mild annealing, polymorphic control over phase-pure tetragonal (I4̅2m) and orthorhombic (Cmc2₁) Cu₂SnS₃ films was realized simply by switching the identity of the thiol (i.e., 1,2-ethanedithiol vs 2-mercaptoethanol, respectively). Polymorph control is dictated by differences in the resulting molecular metal-thiolate complexes and their subsequent decomposition profiles, which likely seed distinct Cu_2-x S phases that template the ternary sulfide sublattice. The p-type tetragonal and orthorhombic Cu₂SnS₃ films possess similar experimental direct optical band gaps of 0.94 and 0.88 eV, respectively, and strong photoelectrochemical current responses. Understanding how ink formulation dictates polymorph choice should inform the development of other thiol-amine inks for solution-processed films.

Solvent engineering to regulate the phase of copper zinc tin sulfide nanocrystals

Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) often shows two phases in synthesis, i.e. kesterite and wurtzite structures. Our research shows that the phase of CZTS can be regulated by the chemical environment of Cu in a two-step heating process.

Formation Pathway of Wurtzite-like Cu2ZnSnSe4 Nanocrystals

Cu₂ZnSnSe₄ is a direct band gap semiconductor composed of earth-abundant elements, making it an attractive material for thin-film photovoltaic technologies. Cu₂ZnSnSe₄ crystallizes in the kesterite structure type as a bulk material, but it can also crystallize in a metastable wurtzite-like crystal structure when synthesized on the nanoscale. The wurtzite-like polymorph introduces unique and useful properties to Cu₂ZnSnSe₄ materials, including widely tunable band gaps and superior compositional flexibility as compared to kesterite Cu₂ZnSnSe₄. Here, we investigate the formation pathway of colloidally prepared wurtzite-like Cu₂ZnSnSe₄ nanocrystals. We show that this quaternary material forms through a chain of reactions, starting with binary Cu₃Se₂ nanocrystals that, due to both kinetic and thermodynamic reasons, preferentially react with tin to yield hexagonal copper tin selenide intermediates. These ternary intermediates then react with zinc to form the resulting wurtzite-like Cu₂ZnSnSe₄ nanocrystals. Based on this formation pathway, we suggest synthetic methods that may prevent the formation of unwanted impurity phases that are known to hamper the efficiency of Cu₂ZnSnSe₄-based optoelectronic devices.

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.

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.

Aqueous-based Binary Sulfide Nanoparticle Inks for Cu2ZnSnS4 Thin Films Stabilized with Tin(IV) Chalcogenide Complexes

Cu₂ZnSnS₄ (CZTS) is a promising semiconductor material for photovoltaic applications, with excellent optical and electronic properties while boasting a nontoxic, inexpensive, and abundant elemental composition. Previous high-quality CZTS thin films often required either vacuum-based deposition processes or the use of organic ligands/solvents for ink formulation, which are associated with various issues regarding performance or economic feasibility. To address these issues, an alternative method for depositing CZTS thin films using an aqueous-based nanoparticle suspension is demonstrated in this work. Nanoparticles of constituent binary sulfides (Cu_xS and ZnS) are stabilized in an ink using tin(IV)-based, metal chalcogenide complexes such as [Sn₂S₆]⁴⁻. This research paper provides a systematic study of the nanoparticle synthesis and ink formulation via the enabling role of the tin chalcogenide complexing power, the deposition of high-quality CZTS thin films via spin coating and annealing under sulfur vapor atmosphere, their structural characterization in terms of nanocrystal phase, morphology, microstructure, and densification, and their resultant optoelectronic properties.

Solar light harvesting with multinary metal chalcogenide nanocrystals

The paper reviews the state of the art in the synthesis of multinary (ternary, quaternary and more complex) metal chalcogenide nanocrystals (NCs) and their applications as a light absorbing or an auxiliary component of light-harvesting systems. This includes solid-state and liquid-junction solar cells and photocatalytic/photoelectrochemical systems designed for the conversion of solar light into the electric current or the accumulation of solar energy in the form of products of various chemical reactions. The review discusses general aspects of the light absorption and photophysical properties of multinary metal chalcogenide NCs, the modern state of the synthetic strategies applied to produce the multinary metal chalcogenide NCs and related nanoheterostructures, and recent achievements in the metal chalcogenide NC-based solar cells and the photocatalytic/photoelectrochemical systems. The review is concluded by an outlook with a critical discussion of the most promising ways and challenging aspects of further progress in the metal chalcogenide NC-based solar photovoltaics and photochemistry.

Aqueous Synthesis of High-Quality Cu2ZnSnS4 Nanocrystals and Their Thermal Annealing Characteristics

Copper zinc tin sulfide (CZTS) nanocrystal inks are promising candidates for the development of cheap, efficient, scalable, and nontoxic photovoltaic (PV) devices. However, optimization of the synthetic chemistry to achieve these goals remains a key challenge. Herein we describe a single-step, aqueous-based synthesis that yields high-quality CZTS nanocrystal inks while also minimizing residual organic impurities. By exploiting simultaneous redox and crystal formation reactions, square-platelet-like CZTS nanocrystals stabilized by Sn₂S₆^4- and thiourea are produced. The CZTS synthesis is optimized by using a combination of inductively coupled plasma analysis, Raman spectroscopy, Fourier transform infrared spectroscopy, and synchrotron powder X-ray diffraction to assess the versatility of the synthesis and identify suitable composition ranges for achieving phase-pure CZTS. It is found that mild heat treatment between 185 and 220 °C is most suitable for achieving this because this temperature range is sufficiently high to thermalize existing ligands and ink additives while minimizing tin loss, which is problematic at higher temperatures. The low temperatures required to process these nanocrystal inks to give CZTS thin films are readily amenable to production-scale processes.

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.

Tuning Bandgap of p-Type Cu2Zn(Sn, Ge)(S, Se)4 Semiconductor Thin Films via Aqueous Polymer-Assisted Deposition

Bandgap engineering of kesterite Cu₂Zn(Sn, Ge)(S, Se)₄ with well-controlled stoichiometric composition plays a critical role in sustainable inorganic photovoltaics. Herein, a cost-effective and reproducible aqueous solution-based polymer-assisted deposition approach is developed to grow p-type Cu₂Zn(Sn, Ge)(S, Se)₄ thin films with tunable bandgap. The bandgap of Cu₂Zn(Sn, Ge)(S, Se)₄ thin films can be tuned within the range 1.05-1.95 eV using the aqueous polymer-assisted deposition by accurately controlling the elemental compositions. One of the as-grown Cu₂Zn(Sn, Ge)(S, Se)₄ thin films exhibits a hall coefficient of +137 cm³/C. The resistivity, concentration and carrier mobility of the Cu₂ZnSn(S, Se)₄ thin film are 3.17 ohm·cm, 4.5 × 10¹⁶ cm^-3, and 43 cm²/(V·S) at room temperature, respectively. Moreover, the Cu₂ZnSn(S, Se)₄ thin film when used as an active layer in a solar cell leads to a power conversion efficiency of 3.55%. The facile growth of Cu₂Zn(Sn, Ge)(S, Se)₄ thin films in an aqueous system, instead of organic solvents, provides great promise as an environmental-friendly platform to fabricate a variety of single/multi metal chalcogenides for the thin film industry and solution-processed photovoltaic devices.

Phase-controlled synthesis of orthorhombic and tetragonal AgGaSe2 nanocrystals with high quality

Orthorhombic AgGaSe₂ nanocrystals with high quality have been successfully synthesized for the first time, and their crystalline phase could be tuned by adjusting the reaction conditions.

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.

Mechanism of versatile catalytic activities of quaternary CuZnFeS nanocrystals designed by a rapid synthesis route

Quaternary alloyed nanocrystals (NCs) composed of earth abundant, environment friendly elements are of interest for energy-harvesting applications. These complex NCs are useful as catalysts for the degradation of multiple refractory organic pollutants as well as nitro-organic reduction at a rapid rate. Here, a remarkably fast (∼30 s) and facile synthesis of crystalline quaternary chalcopyrite copper-zinc-iron-sulfide (CZIS) NCs is reported. These NCs show excellent catalytic properties by degrading a number of refractory organic dyes and converting nitro-compounds at a rapid rate. The valence and conduction band information of the newly designed NCs are extracted using scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy, which reveal energy levels suitable for performing redox chemistry by generating reactive radicals establishing NCs as efficient catalyst with multiple uses. Rapid synthesis of high quality phase-controlled CZIS NCs with robust catalytic activities could be useful for organic waste treatment.

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.

Colloidal nanocrystals of orthorhombic Cu2ZnGeS4: phase-controlled synthesis, formation mechanism and photocatalytic behavior

The orthorhombic polymorph of Cu2ZnGeS4 (CZGS) is a metastable wurtzite-derived phase that can only be prepared in the bulk form by extensive heating at high temperatures (≥790 °C) when using the conventional solid-state reaction route. By employing a facile solution-based synthetic strategy, we were able to obtain phase-pure orthorhombic CZGS in nanocrystalline form at a much lower reaction temperature. Prior to this work, the colloidal synthesis of single-phase orthorhombic CZGS on the nanoscale has never been reported. We find that the use of an appropriate combination of coordinating solvents and precursors is crucial to the sole formation of this metastable phase in solution. A possible formation mechanism is proposed on the basis of our experimental results. Because CZGS consists of environmentally benign metal components, it is regarded as a promising alternative material to the technologically useful yet toxic cadmium-containing semiconductors. The orthorhombic CZGS nanocrystals display strong photon absorption in the visible spectrum and are photocatalytically active in dye degradation under visible-light illumination.

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.

Wurtzite CZTS nanocrystals and phase evolution to kesterite thin film for solar energy harvesting

A quaternary indium- and gallium-free kesterite (KS)-based compound, copper zinc tin sulfide (Cu₂ZnSnS₄, CZTS), has received significant attention for its potential applications in low cost and sustainable solar cells.

Band gap tunable Zn2SnO4 nanocubes through thermal effect and their outstanding ultraviolet light photoresponse

This work presents a method for synthesis of high-yield, uniform and band gap tunable Zn₂SnO₄ nanocubes. These nanocubes can be further self-assembled into a series of novel nanofilms with tunable optical band gaps from 3.54 to 3.18 eV by simply increasing the heat treatment temperature. The Zn₂SnO₄ nanocube-nanofilm based device has been successfully fabricated and presents obviously higher photocurrent, larger photocurrent to dark current ratio than the previously reported individual nanostructure-based UV-light photodetectors, and could be used in high performance photodetectors, solar cells, and electrode materials for Li-ion battery.

Synthesis of wurtzite-zincblende Cu2ZnSnS4 and Cu2ZnSnSe4 nanocrystals: insight into the structural selection of quaternary and ternary compounds influenced by binary nuclei

Nearly monodispersed wurtzite-dominant Cu2ZnSnS4 and zincblende-dominant Cu2ZnSnSe4 nanocrystals were successfully synthesized by mixing metal salts with heated thiourea or selenourea in oleylamine. A perspective of the structural relationship between quaternary and ternary semiconductors was investigated through the application of different anion sources to prepare Cu2SnS3 and Cu2SnSe3 nanocrystals. Investigations on copper-based binary compounds found that CuSe (or CuS) and Cu2Se (or Cu1.96S, Cu9S5) nuclei were primarily responsible for the formation of zincblende or wurtzite structures, respectively. Further management over these binary intermediates corresponded to slight structural transformations of the quaternary nanocrystals which could be observed not only in XRD patterns, but from optical and electrical properties as well. According to these results, Cu2ZnGeS4 nanocrystals with wurtzite-dominant structures were first reported using SC(NH2)2, which also verified that the binary semiconductors are the determinative factors.

Polarity-driven polytypic branching in cu-based quaternary chalcogenide nanostructures

An appropriate way of realizing property nanoengineering in complex quaternary chalcogenide nanocrystals is presented for Cu2CdxSnSey(CCTSe) polypods. The pivotal role of the polarity in determining morphology, growth, and the polytypic branching mechanism is demonstrated. Polarity is considered to be responsible for the formation of an initial seed that takes the form of a tetrahedron with four cation-polar facets. Size and shape confinement of the intermediate pentatetrahedral seed is also attributed to polarity, as their external facets are anion-polar. The final polypod extensions also branch out as a result of a cation-polarity-driven mechanism. Aberration-corrected scanning transmission electron microscopy is used to identify stannite cation ordering, while ab initio studies are used to show the influence of cation ordering/distortion, stoichiometry, and polytypic structural change on the electronic band structure.

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

Polytypic nanocrystals with zinc blende (ZB) cores and wurtzite (WZ) arms, such as tetrapod and octopod nanocrystals, have been widely reported. However, polytypic nanocrystals with WZ cores and ZB arms or ends have been rarely reported. Here, we report a facile, solution-based approach to the synthesis of polytypic Cu2CdSn(S1-xSex)4 (CCTSSe) nanocrystals with ZB-derivative selectively engineered on (000±2)WZ facets of WZ-derived cores. Accordingly, two typical morphologies, i.e., bullet-like nanocrystals with a WZ-derivative core and one ZB-derivative end, and rugby ball-like nanocrystals with a WZ-derivative core and two ZB-derivative ends, can be selectively prepared. The epitaxial growth mechanism is confirmed by the time-dependent experiments. The ratio of rugby ball-like and bullet-like polytypic CCTSSe nanocrystals can be tuned through changing the amount of Cd precursor to adjust the reactivity difference between (0002)WZ and (000-2)WZ facets. These unique polytypic CCTSSe nanocrystals may find applications in energetic semiconducting materials for energy conversion in the future.

In situ fabrication of Cu2ZnSnS4 nanoflake thin films on both rigid and flexible substrates

Pure CZTS thin film is formed directly at a temperature of 250 °C, the lowest temperature of any current fabrication system, on both flexible stainless steel and rigid FTO substrates.