Abstract
A semiconducting molecule containing a thiol anchor group, namely 2-(5-mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione (abbreviated as D-A-D-SH), was designed, synthesized, and used as a ligand in nonstoichiometric quaternary nanocrystals of composition Ag1.0In3.1Zn1.0S4.0(S6.1) to give an inorganic/organic hybrid. Detailed NMR studies indicate that D-A-D-SH ligands are present in two coordination spheres in the organic part of the hybrid: (i) inner in which the ligand molecules form direct bonds with the nanocrystal surface and (ii) outer in which the ligand molecules do not form direct bonds with the inorganic core. Exchange of the initial ligands (stearic acid and 1-aminooctadecane) for D-A-D-SH induces a distinct change of the photoluminescence. Efficient red luminescence of nanocrystals capped with initial ligands (λmax = 720 nm, quantum yield = 67%) is totally quenched and green luminescence characteristic of the ligand appears (λmax = 508 nm, quantum yield = 10%). This change of the photoluminescence mechanism can be clarified by a combination of electrochemical and spectroscopic investigations. It can be demonstrated by cyclic voltammetry that new states appear in the hybrid as a consequence of D-A-D-SH binding to the nanocrystals surface. These states are located below the nanocrystal LUMO and above its HOMO, respectively. They are concurrent to deeper donor and acceptor states governing the red luminescence. As a result, energy transfer from the nanocrystal HOMO and LUMO levels to the ligand states takes place, leading to effective quenching of the red luminescence and appearance of the green one.
Highlights
Semiconductor nanocrystals can be considered as inorganic/ organic hybrids consisting of an inorganic core and surfacial capping ligands which ensure their colloidal stability
In our previous research we elaborated new procedures of fabricating these quaternary nanoparticles of different compositions which exhibited strong and tunable photoluminescence covering green and red regions of the spectrum (QY in the range 48−67%).[37−39] We developed an efficient method of exchanging primary hydrophobic ligands for hydrophilic ones such as 11-mercaptoundecanoic acid (MUA), for example
Ag−In−Zn−S nanocrystals studied in this research were prepared by using a reaction mixture that consisted of silver nitrate, indium(III) chloride, zinc stearate, and 1-dodecanethiol (DDT) dissolved in 1-octadecene (ODE) in molar ratios AgNO3/InCl3/zinc stearate/DDT = 1.0/3.4/3.6/5.6
Summary
Semiconductor nanocrystals can be considered as inorganic/ organic hybrids consisting of an inorganic core and surfacial capping ligands which ensure their colloidal stability. Recent progress in the synthesis of ternary and quaternary nanocrystals, including strongly nonstoichiometric alloyed ones, has not induced parallel intensive research on the design of “tailor-made” photo- and electroactive ligands suitable for these nanocrystals as well as on the exchange of primary ligands for functionalized ones. This type of research is highly desirable in the case of quaternary Ag−In−Zn−S nanocrystals, especially, because by changing their composition it is possible to tune their efficient luminescence over the whole visible spectral range. We demonstrate that the exchange of primary, spectroscopically inactive ligands for the conjugated ones induces a hypsochromic shift of the photoluminescence peak by over 200 nm, that is, from the red region of the spectrum to the green one
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