Size Segregation and Atomic Structural Coherence in Spontaneous Assemblies of Colloidal Cesium Lead Halide Nanocrystals

Abstract

Colloidal nanocrystals (NCs) of fully inorganic lead halide perovskites (LHPs) are the latest generation of quantum-dot materials with compelling optical characteristics. Favored by the nearly regular cubic shape of NCs, they readily self-organize into highly ordered supercrystals (SCs) that have been stimulating in-depth investigation of uncommon collective properties at the fundamental level. Herein, we explore LHP NC SCs formed in colloidal suspensions and as solid materials, without any prior rigorous NC size-selection process. The spontaneous self-organization manifests itself in synchrotron wide-angle X-ray total scattering (WAXTS) measurements of randomly oriented ensembles of SCs by virtue of the substructure in the 100 C peak (in cubic notation). By developing atomistic models of SCs, we find an evidence that, in spite of the initial NCs size polydispersity (up to 20%), SCs of CsPbBr 3 and CsPbBr 1.5 I 1.5 NCs form through a size segregation process. The SCs exhibit high spatial structural coherence, that is, crystallographic alignment between NCs, along two and three directions, a degree of orientational order (the SCs mosaicity) below 2°, and coherent sizes on the order of tens of nanometers. In situ coupled photoluminescence (PL) and WAXTS experiments evidence pronounced electronic coupling between NCs in SCs, seen as an ca. 50−70 meV red-shifted emission in dried samples compared to the corresponding dilute solution of NCs.

Photoluminescence Properties of CsPbCl3 and CsPbBr3 Nanocrystals synthesized by LARP Method with Various Ligands and Anti-solvents

“…The crystal structure is cubic for both CsPbCl3 and CsPbBr3 NCs with space group Pm-3m (221), as seen Figure 3. The lattice constants have been calculated from their XRD patterns and are listed in hot injection method, i.e., 5.60 Å for CsPbCl3, [10] and the mechanochemical method, i.e., 5.84 Å for CsPbBr3 [11]. LARP method is the more practical for synthesizing CsPbCl3 and CsPbBr3 NCs because the synthesis can be carried out at room temperature rather than the hot injection method which requires high temperature.…”

Collective Diffraction Effects in Perovskite Nanocrystal Superlattices

“…The literature on metal halide nanocrystals is rich with XRD patterns showing Bragg peaks split in fringes − or having asymmetric shapes, − which are typical signatures of multilayer diffraction. However, only a few of these studies concern nanocrystal superlattices (refs and − ), while most did not target or mention the formation of self-assembled structures. On the other hand, multilayer diffraction was not reported for any of the highly ordered superlattices that have been grown for decades from nanocrystals of other materials.…”

Diffraction from Nanocrystal Superlattices

“…XRPD is a simple technique and quite well suited to perform X-ray scattering experiments on SCs as a powder or floating in suspension, with real-time in-situ monitoring of the coalescence and simultaneously measuring other properties of the system. Modeling of SL disorder can be essential to reproduce observed features, as in Bertolotti et al [ 15 ] and to correlate them to photon properties. The modeling techniques here demonstrated can be used to understand the powder diffraction signal.…”

“…The use of wide-angle X-ray scattering data (WAXS) is far less common in the study of SLs, although it has been shown that information about structural coherence along two and three directions, degree of orientational order, and coherent sizes can be retrieved [ 15 ].…”