Article pubs.acs.org/JPCC
Short-Lived, Intense and Narrow Bluish-Green Emitting Gold Zinc Sulfide Semiconducting Nanocrystals Riya Bose,†,‡ Umamahesh Thupakula,‡ J. K. Bal,‡ and Narayan Pradhan*,†,‡ †
Department of Materials Science and ‡Centre for Advanced Materials, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India S Supporting Information *
ABSTRACT: In nanoscale, gold is one of the widely studied metals. It is well-known for its size dependent surface plasmonic absorbance. It has also been reported that clusters of a few atoms of gold can show fluorescence. However, these optical properties of gold are mostly associated with Au(0), and little has been explored for the compounds of gold in nanoscale. Herein, we report a new semiconducting nanocrystalline material involving Au(I), which shows intense, narrow, and stable emission at its bandedge absorption. These are composed of Au, Zn, and S and synthesized by introducing Au to zinc sulfide or Zn to gold(I) sulfide nanocrystals in their aqueous dispersion and under ambient condition. The obtained emission is short-lived and tunable in a short spectral window. These new semiconducting fluorescent gold based nanomaterials are characterized with UV−visible, photoluminescence spectroscopy, TCSPC, HRTEM, STM, and STS experiments. Further, the electrical and optical sensing properties of these nanocrystals have also been measured.
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INTRODUCTION Gold is one of the most important and widely studied materials in nanoscale. It is widely known for its size-dependent plasmonic absorption properties.1−4 The chemical synthesis, dimension-dependent optical properties and possible implementations comprise the multidisciplinary fields that have been enormously studied since Faraday’s report of Au colloid.4−11 Typically, when the size of a gold particle reduces to ∼20 nm, the conduction electrons form size-dependent plasmon bands.5−8 But, on further reduction of the size of these materials (3 nm) have been widely reported as fluorescence quencher and can quench the semiconductor emissions.42 Searching for new nanomaterials for multipurpose applications and with the urge to know more about gold, we report here a new multinary gold zinc sulfide semiconducting nanocrystalline material, which can absorb visible light and emit tunable bandedge emission in visible window. These nanocrystals are synthesized simply by introducing Zn(II) solution to an aqueous mixture of Au(III) and thiol (mercaptopropionic acid, MPA). Alternatively, these are also designed by introducing Au(III) to ZnS nanocrystals in the aqueous dispersion. The obtained emission is intense, stable, narrow, and short-range tunable (blue-green window), and the Received: May 22, 2012 Revised: July 7, 2012 Published: July 10, 2012 16680
dx.doi.org/10.1021/jp3049718 | J. Phys. Chem. C 2012, 116, 16680−16686
The Journal of Physical Chemistry C
Article
particle size remains within 2−2.5 nm (diameter). Such ultrasmall nanocrystals, which are synthesized at room temperature and in presence of air, have an emission intensity (Quantum Yield, 22%) comparable to several leading semiconductor nanocrystals reported so far. Details of the synthesis, sequential optical absorption and emission spectra, analysis of the products, definition of the emission origin, and so on are provided in this communication. Further, the electrical and photoresponse properties of these nanocrystals have been measured and reported here, which support their semiconducting behavior.
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RESULTS AND DISCUSSION Alkyl thiols that have high affinity to gold produce light emitting nanoclusters/nanocrystals when treated with Au(III) solution.16,43−46 This has been reported since decades with several speculation of the origin of this emission. Proceeding one step further, we introduced aqueous solution of Zn(II) to this Au(III)−thiol mixture that quenched the existing emission and generated a new but spectacularly intense emission with completely different characteristics. Details of the synthesis have been provided in the Experimental section. In a typical experiment, aqueous solution of mercaptopropionic acid (MPA) with desired concentration was taken in a test tube along with required amount of PVA solution that acted as a stabilizing agent. Stock solution of NaAuCl4 was added dropwise to the same test tube under vigorous stirring condition at room temperature. Within seconds the solution showed a faint yellow emission centered at 570 nm (excitation at ∼320 nm) whose intensity enhanced to some extent (quantum yield
