Close-Packed Ultrasmooth Self-assembled Monolayer of CsPbBr₃ Perovskite Nanocubes

Abstract

The use of colloidal self-assembly to form the complex multiscale patterns in many optoelectronic devices has been a long-standing dream of the nanoscience community. While great progress has been made using charged colloids in polar solvents, controlled assembly from nonpolar solvents is much more challenging. The major challenge is colloidal clustering caused by strong van der Waals (vdW) attraction between long-chain surface capping ligands passivating the surface of nanocrystals. Such clustering degrades ordering in packing during the self-assembly process. While ligand exchange to provide colloidal stability in polar phases is often an option, this is not the case for the exciting new class of halide perovskites due to the material’s solubility in essentially all polar solvents. Here, we report surface-functionalized self-assembly of luminescent CsPbBr 3 perovskite nanocubes by partially replacing long-chain oleyl groups (18 carbon chain) with short-chain thiocyanate (SCN – ). This enables the fabrication of ultrasmooth monolayer thin films of nanocubes with a root-mean-square (RMS) roughness of around 4 Å. This ultrasmooth large area self-assembled layer could act as high-efficiency optoelectronic devices like solar cells, light-emitting diodes (LEDs), transistors, etc. We correlate our experimental results with simulations, providing detailed predictions for lattice constants with chain conformations showing reduced free energy for cubes grafted with short-chain thiocyanate compared to long-chain oleyl groups, thus facilitating better self-assembly.

State of the Art and Prospects for Halide Perovskite Nanocrystals

“…The forces that drive NC self-assembly range from hard- to soft-particle interactions. Taking advantage of previous knowledge gained on the self-assembly of conventional monodisperse colloidal NCs, a large variety of self-assembly techniques have been reported over the years such as evaporation-driven or destabilization-driven approaches, as well as spontaneous and template-assisted self-assembly. ,, Recently, these techniques have been extended to self-assembly of halide perovskite NCs into highly ordered SLs for exploration of their collective properties that can be very different from their individual constituents. ,,,,,,,− Near monodispersity of NCs and high shape uniformity are important factors to obtain long-range ordered NC SLs. ,, Fortunately, these conditions are easily met for all-inorganic CsPbX 3 perovskite NCs as they are often prepared with near monodispersity regardless of the synthesis method, as discussed in the synthesis section. ,,,, As a result, these perovskite nanocubes tend to self-assemble into 1D or 2D SLs on a TEM grid upon solvent evaporation from a droplet of high concentrated colloidal solution. Initial examples of CsPbBr 3 nanocube SLs date back to 2017, when 2D and 3D assemblies were obtained by the solvent evaporation method. ,, In reference small superlattice domains on TEM grids exhibit a simple cubic packing of the nanocubes with a lattice constant of ≈12.5 nm and an interparticle separation of ≈2.3 nm.…”

“…It is very important to consider that the NCs of SLs can coalesce into larger structures, and this can significantly affect their PL properties by energy-transfer process . Despite significant progress toward the fabrication of high-quality CsPbBr 3 nanocube 3D SLs, only a few studies have been published on the preparation of 2D and 1D SLs. ,, Very recently, Patra et al demonstrated the preparation of ultrasmooth self-assembled monolayers using CsPbBr 3 nanocubes functionalized with short-chain thiocyanate ligands (SCN – ) …”

“…83 Despite significant progress toward the fabrication of high-quality CsPbBr 3 nanocube 3D SLs, only a few studies have been published on the preparation of 2D and 1D SLs. 681,683,694 Very recently, Patra et al demonstrated the preparation of ultrasmooth self-assembled monolayers using CsPbBr 3 nanocubes functionalized with short-chain thiocyanate ligands (SCN − ). 683 Device applications of SLs will most likely require control over their dimensionality and positioning on a given solid substrate.…”

“…681,683,694 Very recently, Patra et al demonstrated the preparation of ultrasmooth self-assembled monolayers using CsPbBr 3 nanocubes functionalized with short-chain thiocyanate ligands (SCN − ). 683 Device applications of SLs will most likely require control over their dimensionality and positioning on a given solid substrate. However, it is extremely difficult to fulfill these conditions using the self-assembly techniques discussed above.…”

Self-Assembly and Regrowth of Metal Halide Perovskite Nanocrystals for Optoelectronic Applications

“… 77 Furthermore, assisted by surface-functionalized self-assembly, an ultrasmooth monolayer nanocube thin film with a root-mean-square roughness of ∼4 Å has been fabricated. 78 The short-chain ligands introduced into the system enable more efficient charge transport and substantially reduce the NC–NC interactions, thereby greatly promoting self-assembly. Similarly, a bipolar-shell-resurfacing strategy has been proposed to stabilize CsPbBr 3 NCs, where ligand exchange leads to close-packed films with a long diffusion length, high carrier mobility, and reduced trap density.…”