Luminescence and photoelectrochemical properties of size-selected aqueous copper-doped Ag-In-S quantum dots.

Alexandra Raevskaya,Volodymyr Dzhagan,Dietrich R T Zahn,Alexander Eychmüller,Oleksandr Selyshchev,Anastasiya Novikova,Oleksandr Stroyuk,Nikolai Gaponik,Oksana Rozovik,Irina Goryacheva

doi:10.1039/c8ra00257f

Abstract

Ternary luminescent copper and silver indium sulfide quantum dots (QDs) can be an attractive alternative to cadmium and lead chalcogenide QDs. The optical properties of Cu–In–S and Ag–In–S (AIS) QDs vary over a broad range depending on the QD composition and size. The implementation of ternary QDs as emitters in bio-sensing applications can be boosted by the development of mild and reproducible syntheses directly in aqueous solutions as well as the methods of shifting the photoluminescence (PL) bands of such QDs as far as possible into the near IR spectral range. In the present work, the copper-doping of aqueous non-stoichiometric AIS QDs was found to result in a red shift of the PL band maximum from around 630 nm to ∼780 nm and PL quenching. The deposition of a ZnS shell results in PL intensity recovery with the highest quantum yield of 15%, with almost not change in the PL band position, opposite to the undoped AIS QDs. Size-selective precipitation using 2-propanol as a non-solvent allows discrimination of up to 9 fractions of Cu-doped AIS/ZnS QDs with the average sizes in the fractions varying from around 3 to 2 nm and smaller and with reasonably the same composition irrespective of the QD size. The decrease of the average QD size results in a blue PL shift yielding a series of bright luminophors with the emission color varies from deep-red to bluish-green and the PL efficiency increases from 11% for the first fraction to up to 58% for the smallest Cu-doped AIS/ZnS QDs. The rate constant of the radiative recombination of the size-selected Cu-doped AIS/ZnS QDs revealed a steady growth with the QD size decrease as a result of the size-dependent enhancement of the spatial exciton confinement. The copper doping was found to result in an enhancement of the photoelectrochemical activity of CAIS/ZnS QDs introduced as spectral sensitizers of mesoporous titania photoanodes of liquid-junction solar cells.

Highlights

Metal chalcogenide nanocrystals with a size smaller than the doubled Bohr exciton radius, or quantum dots (QDs), reveal in many cases a unique combination of intense light absorbance, photoluminescence (PL) emission with high quantum yields (QYs), and a strong dependence of the bandgap (Eg) and the energies of the charge carriers on the QD size
In the present paper we summarize our studies on colloidal solution (Cu)-doped AIS (CAIS) and CAIS/ZnS QDs synthesized in aqueous solutions and subjected to post-synthetic size selection
Colloidal CAIS and CAIS/ZnS QDs are characterized by an average hydrodynamic size of $4 nm and $4.2 nm, respectively (ESI, Fig. S1†), with no larger formations present indicating the individual character of each QD in the colloidal ensemble

Summary

Introduction

Metal chalcogenide nanocrystals with a size smaller than the doubled Bohr exciton radius, or quantum dots (QDs), reveal in many cases a unique combination of intense light absorbance, photoluminescence (PL) emission with high quantum yields (QYs), and a strong dependence of the bandgap (Eg) and the energies of the charge carriers on the QD size. CIS and AIS QDs can emit strong PL with the spectral parameters broadly varying with the CIS/AIS QD composition, size, doping, etc.[7,8,11,12,13,14,15] Such ternary compounds revealed a number of quite unique properties differing drastically from those of the binary CdX/PbX compounds, in particular, the immensely broad deviations from the stoichiometry while preserving the crystal lattice symmetry and quality, the capability to form a plethora of solid solutions via a partial substitution of sulfur with Se or Te, In – with Ga, and by alloying CIS (AIS) with ZnS, as well as the lattice preservation at a heavy doping with “alien” metal cations.[7,9,10,11,12,13,14,15,16] These special features open many options for tailoring the band structure of the ternary compounds with related consequences as to the spectral sensitivity range, the charge carriers energies, the conductivity and the carrier mobility, etc., which are unattainable for the binary compounds This unprecedented variability of properties becomes even broader in the nanometer crystal size range, where size dependences of the electronic properties become expressed distinctly.[9,10,11,12,13,14,15,16]

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Conclusion