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Rational Tuning the Optical Properties of Metal Sulfide Nanocrystals and Their Applications Shuling Shen, and Qiangbin Wang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 19 Nov 2012 Downloaded from http://pubs.acs.org on November 21, 2012
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Chemistry of Materials
Rational Tuning the Optical Properties of Metal Sulfide Nanocrystals and Their Applications Shuling Shen, Qiangbin Wang* Division of Nanobiomedicine and i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123 China. KEYWORDS: metal sulfide, band gap tuning, optical property, applications ABSTRACT: This short review provides an overview of recent progress in rational tuning the optical properties of metal sulfide nanocrystals. Three kinds of tuning strategy are discussed in detail: composition control, doping and manufacturing heterostructured metal sulfide nanocrystals. We lay emphases on the tuning of optical properties of metal sulfide nanocrystals and the novel properties generating during these tuning processes that have provided scientists with new opportunities for tailoring and designing the properties of metal sulfide nanocrystals. Examples of applications of metal sulfide nanomaterials with unique optical properties are provided to demonstrate the end goals of such research.
1. Introduction As a typical and important class of semiconductor, metal sulfide (MS) nanocrystals (NCs) have attracted much attention over the past decade due to their unique optical properties, as well as multiple potential applications in biological labels,1-3 light-emitting devices,4-7 photovoltaic devices,8-10 and catalyst,11-13 etc. Developing high quality MS NCs with precisely tunable optical properties is highly desirable for both fundamental studies and practical applications. The optical properties of MS NCs are fundamentally determined by the intrinsic band gap energy of MS. Having the ability to adjust the band gap can be extremely useful for developing MS NCs with novel optical properties and applications. Quantum confinement occurs when one dimension or more dimensions of a semiconductor NC is or are close to or smaller than the exciton Bohr radius, which is the limit size that a material maintains the continuous band structure.14-17 So the band gap of MS NCs can be finely tuned by simply controlling their sizes without altering the chemical compositions of MS, which is the reality of binary MS NCs. Based on this, during last decades, there are a number of novel techniques that have been developed for band gap tuning of binary MS through simple artificial size control.18-26 However, the tuning of the band gap by changing the size could cause a series of problems, practically the instability of the extremely small NCs in some applications and the vanishing of quantum confinement effect while NCs form a dense film or sinter into a polycrystalline layer. In addition, the improvement and multifunctional optical properties of MS usually cannot be achieved only by size control of binary MS. Recent advances have led to the exploration of tunable optical properties of MS by changing their composition, doping impurities in MS NCs, designing heterostructured MS NCs, or fabricating superstructures
constructed from individual MS nanocrystals, which offer researchers more control and more flexibility on tuning the optical properties of MS NCs.27-29 By combining these strategies, the optical properties of MS NCs can be widened or novel properties can be further brought. Some comprehensive reviews on tuning the semiconductor properties through the band gap engineering by changing composition have been published.30-33 However, most of them focused on the selenide or telluride NCs. In this short review article, we try to give an overview of recent progress in rational tuning the optical properties of MS NCs. Three kinds of tuning strategy are discussed in detail: composition control, doping and manufacturing heterostructured MS NCs. We lay emphases on the tuning of optical properties of MS NCs and the novel properties generating during these tuning processes that have provided scientists with new opportunities for designing and tailoring the properties of MS NCs. We also illustrate the possible applications of the as-obtained MS NCs, which may be suggestive for the exploration of this interesting and promising field. 2. Alloyed metal sulfide nanocrystals Alloying of MS NCs with different band gap energies at the nanometer scale produces new materials that display properties distinct from the parent binary MS NCs. The alloyed MS NCs can be classified as cation alloyed MS, anion alloyed MS and cation-anion alloyed MS. The optical properties of alloyed MS NCs can be tuned by changing the stoichiometries of cation constituents in cation alloyed MS, anion constituents in anion alloyed MS or cation and anion constituents simultaneously in cation-anion alloyed MS, which open up new possibilities for developing new family of materials with distinct optical properties. a. Cation alloyed metal sulfide nanocrystals
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Chemistry of Materials
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I-III-VI ternary alloyed semiconductors exhibiting suitable band gaps, less toxicity and nonlinear optical properties have been attracted tremendously attentions as potential candidate materials for optoelectronic device, solar cells, light-emitting diodes and lasers.34 Among these semiconductors, CuInS2 and AgInS2 NCs have attracted more and more attention as environmentally friendly nanomaterials. CuInS2 (Eg=1.5 eV) and AgInS2 (Eg=1.8 eV) have direct band gap with energies well matching to the solar spectrum, high radiation stability, high absorption coefficient, and low toxicity, which endow them potentially suitable materials for optoelectronic devices. Up to date, several methods have been developed for the synthesis of CuInS2 and AgInS2 NCs and preliminary studies on their photoluminescence (PL) properties were carried out.35-45 For example, Castro et al.35 used a single molecular source and obtained CuInS2 NCs with quantum yield (QY) of 5%, but they were unable to tune the PL wavelength in a wide range despite their ability to control particle size within 2-4 nm, less than the exciton Bohr radius (~4.1nm for CuInS246). Recently research indicated that the optical absorption of the Cu-In-S NCs could be adjusted from ~560 nm to ~870 nm with the increase of Cu content, which corresponded to the tunable band gaps ranging from 2.30 eV to 1.48 eV.47 Vittal et al. synthesized metastable orthorhombic phase AgInS2 polyhedral NCs by decomposing the precursor [(Ph3P)2 AgIn (SCOPh)4] in a mixture organic solvents, which exhibited significant third order non-linear optical properties.48 Burda et al. reported the synthesis of AgInS2 NCs and their photo physical properties based on intrinsic versus surface states. The as-obtained AgInS2 NCs possessed long-lived excitons due to abundant intrinsic trap states, which may provide potential applications in photocatalysis and photovoltaics.49 Peng et al. prepared AgInS2 nanoparticles with tunable PL peak in the range of 570-720nm and PL QY of ~8%.34 The PL QYs of CuInS2 and AgInS2 NCs are significantly lower than the PL QY of the II-VI semiconductors (>50%). Other ternary alloyed MS NCs, especially I-V-VI group MS NCs such as AgBiS2 with radius of 4.2 nm and a band gap of 1.4 eV;50 CuSbS2 crystalline thin-films with a direct band gap tunable between 0.9-1.9 eV;51 Cu3SbS3 with a band gap of 1.84 eV;52 Cu12Sb4S13 with a band gap of 1.72 eV and Cu3SbS4 with band gaps of 0.8 eV53 have also been reported. However, their fundamental properties and performance have not been fully characterized. Indeed, the underlying bulk and electronic structure of these ternary MS materials is complex and it is urgent to understand their composition- and structure-property relationships. Multinary MS NCs with more flexible constituents have currently received considerable attention. For example, to improve the luminescence properties of CuInS2 and AgInS2 NCs, multicomponent solid-solution such as CuInS2-ZnS and AgInS2-ZnS were intensively investigated.54 The band gap of CuInS2-ZnS solid solution could be tuned in the broad range of 1.5 to 3.7 eV by changing the ratio of CuInS2 to ZnS.55-57 With further overcoating a ZnS shell, the PL emission wavelength of the resulting CuInS2-ZnS/ZnS NCs could cover from 500 to 850 nm with the maximum PL QY up to 50%80%.58-61 Torimoto and co-workers reported the preparation of AgInS2-ZnS quantum dots (QDs) with maximum QY of 24%. The PL wavelength of AgInS2-ZnS QDs could be tuned from 540 to 720 nm by changing the chemical composition.62 Further coating a ZnS shell on AgInS2-ZnS QDs also resulted
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in the highest QY of 80%.63 As a typical quaternary MS, Cu2ZnSnS4 NCs also attracted wide interest. Cu2ZnSnS4 nanowires with a band gap of 1.5 eV were synthesized via a facile solvothermal approach using anodic aluminum oxide (AAO) as a hard template.64 Hierarchical nanostructured Cu2ZnSnS4 particles have been prepared by changing the reaction conditions via the convenient solvothermal method. Compared with sphere-like Cu2ZnSnS4 nanoparticles, the hierarchical nanostructured Cu2ZnSnS4 with a band gap of 1.47 eV had stronger optical absorption in the visible wavelength region, disclosing their suitability for the photovoltaic application.65 Aydil et al. synthesized Cu2ZnSnS4 NCs with tunable diameters ranging from 2 to 7 nm and band gaps in the range of 1.5-1.8 eV by varying the amount of oleylamine and the growth temperature. They firstly observed the quantum confinement effect of Cu2ZnSnS4 NCs with diameters less than 3 nm.66 The optical properties of alloyed MS NCs can be tuned by changing the stoichiometries of cation constituents in cation alloyed MS. For example, high-quality cation alloyed ZnxCd1−xS (0≤x≤1) NCs have been obtained through the reaction of a mixture of CdO- and ZnO-oleic acid complexes with sulfur at elevated temperatures. The obtained ZnxCd1−xS alloy NCs possessed superior optical properties with QY of 25-50%, especially the extremely narrow emission spectral width (FWHM=14 nm).67 Zhang et al. synthesized porous CdxZn1-xS nanosheets with controlled pore size and adjustable composition by a facile cation-exchange strategy. The porous CdxZn1-xS nanosheets with narrow band gap and strong absorption ability of photons, the intrinsic single-crystal-like properties, and the unique configuration showed a higher performance for the photocatalytic H2 evolution from water splitting.68 Lu et al. introduced band gap sensitive elements, gallium and thallium, into the Cu1.0In2.0S3.5 NCs to form homogeneous quaternary Cu1.0GaxIn2-xS3.5 and Cu1.0InxTl2-xS3.5 (0≤x≤2) NCs. It was found that the introduction of gallium enabled precise tuning of the band gaps of alloyed Cu1.0In2.0S3.5 NCs in the range of 1.43 to 2.42 eV by increasing the indium content. However, due to the similar band gaps of Cu1.0In2.0S3.5 (1.45 eV) and Cu1.0Tl2.0S3.5 (1.37 eV) NCs, the Cu1.0InxTl2-xS3.5 alloyed NCs only presented slight band gap variation at different compositions.69 Gupta et al. firstly reported the synthesis of monodisperse wurtzite structured CuInxGa1-xS2 (0≤x≤1) NCs with bullet-like, rod-like, and tadpole-like shapes. The band gap of obtained wurtzite CuInxGa1-xS2 NCs also could be systematically tuned from 1.53 eV for CuInS2 to 2.48 eV for CuGaS2 by varying the In/Ga ratio.70 Recently, Kanatzidis and coworkers demonstrated the synthesis of Pb2−xSnxS2 (0.4
