Chunhe Yang

Facile One-Step Synthesis and Transformation of Cu(I)-Doped Zinc Sulfide Nanocrystals to Cu1.94S–ZnS Heterostructured Nanocrystals

A facile one-pot heating process without any injection has been developed to synthesize different Cu-Zn-S-based nanocrystals. The composition of the products evolves from Cu(I)-doped ZnS (ZnS:Cu(I)) nanocrystals into heterostructured nanocrystals consisting of monoclinic Cu1.94S and wurtzite ZnS just by controlling the molar ratios of zinc acetylacetonate (Zn(acac)2) to copper acetylacetonate (Cu(acac)2) in the mixture of n-dodecanethiol (DDT) and 1-octadecene (ODE). Accompanying the composition transformation, the crystal phase of ZnS is changed from cubic zinc blende to hexagonal wurtzite. Depending on the synthetic parameters including the reaction time, temperature, and the feeding ratios of Zn/Cu precursors, the morphology of the as-obtained heterostructured nanocrystals can be controlled in the forms of taper-like, matchstick-like, tadpole-like, or rod-like. Interestingly, when the molar ratio of Cu(acac)2 to Zn(acac)2 is increased to 9:1, the crystal phase of the products is transformed from monoclinic Cu1.94S to the mixed phase composed of cubic Cu1.8S and tetragonal Cu1.81S as the reaction time is further prolonged. The crystal-phase transformation results in the morphological change from quasi-spherical to rice shape due to the incorporation of Zn ions into the Cu1.94S matrix. This method provides a simple but highly reproducible approach for synthesis of Cu(I)-doped nanocrystals and heterostructured nanocrystals, which are potentially useful in the fabrication of optoelectronic devices.

Shape-controlled synthesis of PbS nanocrystals via a simple one-step process.

A one-step colloidal process was adopted to prepare face-centered-cubic PbS nanocrystals with different shapes such as octahedral, starlike, cubic, truncated octahedral, and truncated cubic. The features of this approach avoid the presynthesis of any organometallic precursor and the injection of a toxic phosphine agent. A layered intermediate compound (lead thiolate) forms in the initial stage of the reaction, which effectively acts as the precursor to decompose into the PbS nanocrystals. The size and shape of the PbS nanocrystals can be easily controlled by varying the reaction time, the reactant concentrations, the reaction temperatures, and the amount of surfactants. In particular, additional surfactants other than dodecanethiol, such as oleylamine, oleic acid, and octadecene, play an important role in the shape control of the products. The possible formation mechanism for the PbS nanocrystals with various shapes is presented on the basis of the different growth directions of the nanocrystals with the assistance of the different surfactants. This method provides a facile, low-cost, highly reproducible process for the synthesis of PbS nanocrystals that may have potential applications in the fabrication of photovoltaic devices and photodetectors.

Heating-up synthesis of cadimum-free and color-tunable quaternary and five-component Cu–In–Zn–S-based semiconductor nanocrystals

A simple heating-up colloidal approach has been developed to synthesize quaternary Cu–In–Zn–S (CIZS) nanocystals with different emission colors, which can be tuned by varying the Cu : In : Zn precursor ratios. The as-obtained products have a quasi-triangular shape with a small size distribution, and their crystal phase can be varied from cubic zinc-blende to hexagonal wurtzite structure with an increase of Cu stoichiometry. The inductively coupled plasma optical emission spectrometry (ICP-OES) results reveal that the as-obtained CIZS nanocrystals are Cu-deficient, and the Cu-vacancies have a significant effect on their optical properties. Moreover, the photoluminescence (PL) spectra of the CIZS nanocrystals recorded at different growth periods indicate that the partial cation exchange of Cu+ and In3+ with Zn2+ takes charge of the growth process. After coating a ZnS shell over the CIZS nanocrystals, the PL quantum yield (PLQY) is improved greatly and a blue-shift of the PL peak is observed as compared to that of plain CIZS nanocrystals. More interestingly, the highly luminescent CIZS/ZnS core/shell nanocrystals with different colors have been incorporated into poly(dimethylsiloxane) (PDMS) to be fabricated on an engraved “BJTU” glass substrate, and the as-obtained PDMS membranes exhibit a bright emission under UV light, which also show good stability when they are bent arbitrarily or immersed in water. This heating-up method has also been extended to the preparation of five-component Cu–Ag–In–Zn–S and Cu–Mn–In–Zn–S nanocrystals, which possess a cubic zinc-blende crystal structure and have bright luminescence. Our work may shed light on the synthesis of multi-component semiconductor nanocrystals.

Controllable synthesis of silver and silver sulfide nanocrystals via selective cleavage of chemical bonds

A one-step colloidal process has been adopted to prepare silver (Ag) and silver sulfide (Ag₂S) nanocrystals, thus avoiding presynthesis of an organometallic precursor and the injection of a toxic phosphine agent. During the reaction, a layered intermediate compound is first formed, which then acts as a precursor, decomposing into the nanocrystals. The composition of the as-obtained products can be controlled by selective cleavage of S-C bonds or Ag-S bonds. Pure Ag₂S nanocrystals can be obtained by directly heating silver acetate (Ag(OAc)) and n-dodecanethiol (DDT) at 200 ° C without any surfactant, and pure Ag nanocrystals can be synthesized successfully if the reaction temperature is reduced to 190 ° C and the amount of DDT is decreased to 1 ml in the presence of a non-coordinating organic solvent (1-octadecene, ODE). Otherwise, the mixture of Ag and Ag₂S is obtained by directly heating Ag(OAc) in DDT by increasing the reaction temperature or in a mixture of DDT and ODE at 200 ° C. The formation mechanism has been discussed in detail in terms of selective S-C and Ag-S bond dissociation due to the nucleophilic attack of DDT and the lower bonding energy of Ag-S. Interestingly, some products can easily self-assemble into two- or three-dimensional (2D or 3D) highly ordered superlattice structures on a copper grid without any additional steps. The excess DDT plays a key role in the superlattice structure due to the bundling and interdigitation of the thiolate molecules adsorbed on the as-obtained nanocrystals.

Synthesis of Cu2−xS nanocrystals induced by foreign metal ions: phase and morphology transformation and localized surface plasmon resonance

High-quality rhombohedral digenite (Cu1.8S) nanocrystals (NCs) were synthesized using a facile one-pot approach, in which foreign metal ions (Zn2+ and Cd2+) were added to drive a transformation in the phase and morphology of the as-obtained Cu2−xS NCs. During this process, the as-obtained products could transform from monoclinic djurleite (Cu1.94S) NCs to rhombohedral Cu1.8S NCs in the presence of Zn2+ and Cd2+ ions by extending the reaction time and increasing the reaction temperature, which could provide sufficient energy to overcome activation energy barriers. Moreover, we further studied the evolution of the plasmonic properties of the as-obtained Cu2−xS NCs, and the near-infrared (NIR) localized surface plasmon resonance (LSPR) absorption wavelength could be tuned by varying the amount of Zn2+ and Cd2+ ions added into the reaction system, which was in close association with the change in copper deficiencies and free hole densities induced by foreign metal ions.

Tunable near-infrared localized surface plasmon resonances of djurleite nanocrystals: effects of size, shape, surface-ligands and oxygen exposure time

Colloidal djurleite nanocrystals exhibit a well-defined and strong localized surface plasmon resonance absorption in the near-infrared region, which arises from the excess free holes in the valence band. The near-infrared localized surface plasmon resonance absorption wavelength of the as-obtained djurleite nanocrystals can be modulated by varying their size and shape, which are controlled through the variation of the reaction conditions during the synthesis. For a given size, the plasmonic behavior of the spherical nanocrystals exhibits an obvious surface-dependent shift due to the different electron-donating abilities of the surface ligands, which leads to the change of hole density. Moreover, the plasmonic band of the djurleite nanocrystals shifts to a shorter wavelength upon exposure to air for longer time, during which no crystal structure is altered, and this blue-shift may be attributed to the increasing density of copper vacancies. The experimental results of the near-infrared plasmonic behavior are in good agreement with the calculated results based on the Mie–Drude model.

Tuning the plasmonic resonance of Cu2−xS nanocrystals: effects of the crystal phase, morphology and surface ligands

A simple low-cost and phosphine-free colloidal method was developed to prepare disk-shaped Cu2−xS nanocrystals (NCs) with different crystal structures and basal planes, which could be manipulated effectively by varying the Cu : S feed molar ratios and the amount of surfactants. The near-infrared (NIR) localized surface plasmon resonance (LSPR) wavelength could be tuned from 1133 to 1512 nm with the crystal phase changing from CuS (covellite) to monoclinic Cu7S4 (roxbyite) and Cu31S16 (djurleite). Phase transformation had more important effects on tuning the plasmonic resonance than the surface interaction of the deprotonated carboxyl functional group of oleic acid. Moreover, the crystal phase could transform from covellite to djurleite and the morphology underwent a transition from nanodisks to nanospheres when the post-treatment temperature of CuS nanodisks by dodecanethiol (DDT) was increased to 120 °C, but only a size decrease took place at room temperature. As a result, a slight red-shift of the LSPR wavelength was observed at room temperature, but an obvious red-shift from 1140 to 1910 nm with the change in crystal structure and morphology. The post-treatment temperature played an important role in tuning the plasmonic resonance of the products, which was closely associated with the variation of the crystal structure, morphology and surface properties.

Tunable near-infrared localized surface plasmon resonances of heterostructured Cu 1.94 S-ZnS nanocrystals

Strong near-infrared (NIR) localized surface plasmon resonances (LSPRs) have been observed in spherical Cu1.94S nanocrystals and matchstick-like Cu1.94S-ZnS heterostructured nanocrystals, which have been synthesized using a simple one-pot approach without any injection and pre-synthesis of metal precursors. The LSPRs peak of the Cu1.94S nanocrystals could be tuned from 1680 nm to 1375 nm by heterogrowth of ZnS onto the Cu1.94S nanocrystals due to the increase of free carriers (holes). The LSPRs absorbance can be optimized to 1322 nm by prolonging the growth time of the heterostructured nanocrystals, which may be used as a light absorbing agent for photothermal therapy.

Understanding the roles of metal sources and dodecanethiols in the formation of metal sulfide nanocrystals via a two-phase approach

We present a two-phase approach for controllable synthesis of metal sulfide nanocrystals, such as Cu2−xS, Ag2S and Ni9S8, in which the reaction takes place at the water/oil interface. This method is environmental-friendly and economical due to the use of low-toxicity and low-cost reagents. The crystal phase and composition can be manipulated by varying the amount of n-dodecanethiol (DDT) in the organic phase and the type of metal salt in the aqueous phase. For example, CuO nanocrystals can be obtained in the presence of low DDT concentration and hydroxyl ions, and Cu2−xS nanocrystals are successfully synthesized in the presence of relatively high DDT concentration. The XPS spectra and near-infrared (NIR) absorption spectra have been collected to confirm the formation of CuO and Cu2−xS nanocrystals. Moreover, the Cu2−xS nanocrystals often exhibit an obvious localized surface plasmon resonance band due to their excess free holes in the valence band. A plausible formation mechanism has been proposed to study the synthesis of Cu2−xS and CuO nanocrystals. This two-phase approach has also been extended to prepare Ag2S and Ni9S8 nanocrystals, and the Ag and Ag2S nanocrystals often coexist in the products which are closely associated with cleavage of S–C bonds or the Ag–S bond, which relies on the DDT dosage and the reactivity of the metal sources. This two-phase approach may open up a simple and environmental-friendly strategy for synthesis of metal sulfide nanocrystals with controllable crystal phase and morphology.

Effects of a heteroatomic benzothienothiophenedione acceptor on the properties of a series of wide-bandgap photovoltaic polymers

A series of benzodithiophene (BDT) and benzothienothiophenedione (BTTDO) alternating wide-bandgap (WBG) copolymers, PBDT-O1, PBDT-S1 and PBDT-Se1, were designed and synthesized, in which heteroatoms (O, S and Se) were incorporated into the electron-deficient BTTDO motif. The effect of heteroatoms on the thermal stability, absorption spectra, energy level, charge carrier mobility, and photovoltaic properties of these WBG polymers was systematically studied. The results indicated that upon increasing the size of the heteroatoms, the maximum absorption peaks were red-shifted and the optical bandgap decreased. Solar cells with a conventional structure of ITO/PEDOT:PSS/polymers : PC70BM (1 : 1, w/w)/Ca/Al were fabricated. Among the three polymers, PBDT-S1 achieved the best photovoltaic performance, with a high power conversion efficiency (PCE) of 9.0%, with an open-circuit voltage (Voc) of 0.91 V, a short-circuit current (Jsc) of 12.99 mA cm−2, and an unprecedented fill factor (FF) of 74.9%.