Morphological Control Over Gel Structures of Mixed Semiconductor‐Metal Nanoparticle Gel Networks with Multivalent Cations

Abstract

The main aspect being, that interparticle connections essentially govern the properties in nanoparticle assemblies, thus, the control over nanocrystal connections is the main tool toward tailored gel structures. Manifold gelation techniques have been explored in the recent decade: via addition of oxidative agents, [3,5] cryogelation, [23,24] ion-induced, [25][26][27][28] or via light irradiation. [6,22,29] All gelation methods are based on the concept of overcoming the interparticle repulsion forces in a stable colloidal solution of nanocrystals facilitating their controlled assembly. The resulting structures are as different as their methods, but overall they still deliver structures with large surface areas, low densities, high and open porosity. These properties unite all these structures, even if they differ greatly in their composition and the interaction between the building blocks. Especially in case of nanoparticle assemblies consisting of two or more different components (in the following named heteroassemblies), the connection itself between the particles has a great impact on the optical properties evolving in the gel networks, e.g., spatial separation of photoexcited charge carriers. [30][31][32][33] In the present work, we give an insight into the importance of choosing the most suitable gelation agent for specific purposes requiring different gel morphologies. As recent studies have mostly dealt with ion-induced gelation of In this work, the influence of two different types of cations onthe gel formation and structure of mixed gel networks comprised of semiconductor (namely CdSe/CdS nanorods NR) and Au nanoparticles (NP) as well as on the respective monocomponent gels is investigated. Heteroassemblies built from colloidal building blocks are usually prepared by ligand removal or crosslinking, thus, both the surface chemistry and the destabilising agent play an essential role in the gelation process. Due to the diversity of the composition, morphology, and optical properties of the nanoparticles, a versatile route to fabricate functional heteroassemblies is of great demand. In the present work, the optics, morphology, and gelation mechanism of pure semiconductor and noble metal as well as their mixed nanoparticle gel networks are revealed. The influence of the gelation agents (bivalent and trivalent cations) on the structure-property correlation is elucidated by photoluminescence, X-ray photoelectron spectroscopy, and electron microscopy measurements. The selection of cations drastically influences the nano-and microstructure of the prepared gel network structures driven by the affinity of the cations to the ligands and the nanoparticle surface. This gelation technique provides a new platform to control the formation of porous assemblies based on semiconductor and metal nanoparticles.

Controlled Morphological Arrangement of Anisotropic Nanoparticles via Oxidation or Ionic Cross‐Linking

“…[5,8,9] While H 2 O 2 is decomposed by oxidizing the surface ligands and possibly the CdS surface itself, YCl 3 residues can be found in the resulting network structures, as we showed in our former publications. [9,10] For side-byside arranged nanoparticle networks, the PLQY is decreasing as well. The strongest decrease in PLQY can be observed for BaCl 2induced hydrogel formation.…”

“…Self-supporting networks such as lyogels and aerogel have gained significant attention in the recent years due to their unique structural, [1][2][3][4][5][6] optical, [5,[7][8][9][10] and electronic properties. [8,9,[11][12][13][14][15][16][17][18][19] Among these assemblies, semiconductor nanoparticle networks have been studied for their potential applications in various fields, including sensing.…”

“…[18,20] The gel formation of semiconductor nanoparticles involves the formation of 3D structures by connecting individual nanoparticles, which can be achieved via various methods such as ligand oxidation, [3,5] cryogelation, [21,22] light irradiation, [6,23,24] or ion-induced assembly. [10,[25][26][27][28] The resulting assemblies exhibit unique properties, such as enhanced mechanical stability compared to individual nanoparticles, spatial charge-carrier separation after photoexcitation, [8,[29][30][31] and high porosity. These properties are influenced by multiple factors, including the size and shape of the building blocks, the nature of ligands and therefore the surface chemistry, and the method of assembly.…”

“…The new technique is able to cross-link nanorods in this geometrical structure without negatively impacting fluorescence properties such as lowered photoluminescence quantum yields (PLQY), as was observed in, e.g., Ba 2þ -ion-induced gelation. [10] In the present work, we give an insight into the importance of choosing the most suitable gelation agent for specific purposes requiring different gel morphologies. As recent studies have mostly dealt with ion-induced gelation, [26,27,32] this work compares the influence of various gelation agents (demonstrating oxidative and non-oxidative gelation treatments) on the structure and property of the nanoparticle gel networks.…”

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

“…55,61,62 It was shown before, that Y 3+ -ions remain in the network aer gelation and consequent washing steps. 33,63 Therefore, the inuence of these ions onto the optoelectronic properties of the assemblies have to be considered. At this point the precise optoelectronic interactions of the ions with the nanocrystals have not been investigated, because the introduction of the ions always leads to network formation.…”