Optical properties of NIR photoluminescent PbS nanocrystal-based three-dimensional networks

Abstract

The assembly of nanocrystals (NCs) into three-dimensional network structures is a recently established strategy to produce macroscopic materials with nanoscopic properties. These networks can be formed by the controlled destabilization of NC colloids and subsequent supercritical drying to obtain NC-based aerogels. Even though this strategy has been used for many different semiconductor NCs, the emission of NC-based aerogels is limited to the ultraviolet and visible and no near-infrared (NIR) emitting NC-based aerogels have been investigated in literature until now. In the present work we have optimized a gelation route of NIR emitting PbS and PbS/CdS quantum dots (QDs) by means of a recently established gel formation method using trivalent ions to induce the network formation. Thereby, depending on the surface ligands and QDs used the resulting network structure is different. We propose, that the ligand affinity to the nanocrystal surface plays an essential role during network formation, which is supported by theoretical calculations. The optical properties were investigated with a focus on their steady-state and time resolved photoluminescence (PL). Unlike in PbS/CdS aerogels, the absorption of PbS aerogels and their PL shift strongly. For all aerogels the PL lifetimes are reduced in comparison to those of the building blocks with this reduction being especially pronounced in the PbS aerogels.

Fig. 1. Schematic synthesis route from PbS QDs stabilized with oleic acid (OA) ligands to PbS nanocrystal based aerogels. The QDs are phase transferred to the aqueous phase via ligand exchange with 11-mercaptoundecanoic acid (MUA) or 3-mercaptopropionic acid (MPA), leading to different gel structures. TEM image of a PbS aerogel, assembled from (a) MUA and (b) MPA stabilized PbS QDs.

Fig. 2. The network structure of the PbS aerogels depends on the surface ligands (MUA or MPA) used during the phase transfer step. TEM images of a PbS aerogel synthesized from QDs and with YCl₃, stabilized with (a and c) MUA, and (b and d) MPA.

Fig. 3. The PbS/CdS aerogels show, similarly to the PbS aerogels, a dependency of the surface ligands onto the obtained network structure. In addition, the particle type is also relevant. TEM images of a PbS/CdS aerogel synthesized from QDs, stabilized with (a and c) MUA and (b and d) MPA.

Fig. 4. Illustration of four surface structures used to calculate absorption energies. (a) and (b) show the PbS rock salt surfaces with (100) and (111) surfaces respectively, whereas (c) and (d) show the CdS zinc blende (100) and (111) surfaces respectively. Neighboring simulation cells were show with transparent atoms. The thiol ligand is composed of a sulfur atom, a n-hexylchain and a trimethylamine group (S–(CH₂)₆–N–(CH₃)₃). For (a) an adsorption site of the ligand sulfur atom on top of the lead atom is found. For all other three structures bridged adsorption sites were found where the sulfur atom is bound to two Pb/Cd atoms.

Fig. 5. The variety of QDs used during the gelation (PbS or core/shell PbS/CdS) has an impact on the optical properties of the resulting aerogels manifested in bathochromic shifts of the emission and decreased PL lifetimes. Absorption (dotted line) and steady state emission (solid line) spectra and PL decays (dots) (a and c) of PbS OA QDs, PbS MUA aerogel and PbS MPA aerogel and (b and d) core/shell PbS/CdS OA QDs, core/shell PbS/CdS MUA aerogel and core/shell PbS/CdS MPA aerogel. The PLQY and average PL lifetimes are shown. The mono-exponential (for PbS and core/shell PbS/CdS OA QDs) and bi-exponential (for aerogels) fits of the PL decays are displayed as dashed lines.