Stable and Size Tunable CsPbBr₃ Nanocrystals Synthesized with Oleylphosphonic Acid

Abstract

We employed oleylphosphonic acid (OLPA) for the synthesis of CsPbBr 3 nanocrystals (NCs). Compared to phosphonic acids with linear alkyl chains, OLPA features a higher solubility in apolar solvents, allowing us to work at lower synthesis temperatures (100 °C), which in turn offer a good control over the NCs size. This can be reduced down to 5.0 nm, giving access to the strong quantum confinement regime. OLPA-based NCs form stable colloidal solutions at very low concentrations (∼1 nM), even when exposed to air. Such stability stems from the high solubility of OLPA in apolar solvents, which enables these molecules to reversibly bind/unbind to/from the NCs, preventing the NCs aggregation/precipitation. Small NCs feature efficient, blue-shifted emission and an ultraslow emission kinetics at cryogenic temperature, in striking difference to the fast decay of larger particles, suggesting that size-related exciton structure and/or trapping-detrapping dynamics determine the thermal equilibrium between coexisting radiative processes.

Guidelines for the characterization of metal halide nanocrystals

“…The characterization of MH NCs is performed via several techniques aimed at probing the crystal structure (X-ray diffraction, 3D electron diffraction, high-resolution transmission electron microscopy, high-angle annular dark-field imaging performed in a scanning transmission electron microscope), surface termination (X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy), ligand shell (NMR, nuclear Overhauser-effect spectroscopy, diffusion-ordered spectroscopy), and optical features (optical absorption, photoluminescence, time-resolved photoluminescence, photoluminescence quantum yield, transient absorption), whose output, elaborated also via computational models, is used to retrieve a complete picture of the NCs' core and organic shell and, possibly, the surface or bulk trapping states arising from defects. Adapted, with permission, from [7,9,19,39,46,60,69,102]. temperature.…”

“…Nuclear magnetic resonance (NMR) spectroscopy is currently the most suitable technique to investigate the ligand-surface interactions in colloidal NCs, including both classical and MH systems (Figure 3) [3,16,[43][44][45]. 1 H NMR (or 13 C and/or 31 P NMR) can be readily exploited to identify bound ligands: their resonances are slightly up-field shifted and broadened with respect to those of the free molecules in solution [3,11,16,17,[43][44][45][46]. Such line broadening has been attributed to the limited degrees of freedom (mainly restricted rotational mobility) experienced by a ligand bound to a NC surface.…”

“…In such cases, a possible strategy comprises choosing ad hoc ligand pairs characterized by non-overlapping NMR signals; for example, oleylamine and phenyl acetic acid, the former characterized by alkenyl protons and the latter by aromatic ones, as done in a recent work of Manna and colleagues [17]. To assess whether a species is actually bound, 1 H− 1 H 2D nuclear Overhauser-effect spectroscopy (NOESY) analysis can be performed [3,8,11,16,17,[43][44][45][46][48][49][50]. The 1 H− 1 H NOE Trends Trends in in Chemistry Chemistry signals' sign and intensity depend on the size of the molecule: signals are small and positive for small molecules, while large and negative signals are observed for large molecules, such as a ligand interacting with the surface of a NC [11,16,17,46,51].…”

Unusually Strong Biexciton Repulsion Detected in Quantum Confined CsPbBr3 Nanocrystals with Two and Three Pulse Femtosecond Spectroscopy

“…Recent synthetic advances have enabled production of QC MLHP crystals 17,18,19 with tight size distributions which exhibit unique TA signatures. 20 As in other QC NC samples, above band gap excitation leads to rapid bleaching at the BE accompanied by an induced absorption at longer wavelengths (A1), both often assigned to bi-exciton interaction of the pump and probe related excitons.…”

The Role of Subsurface Valence Band Localization in the Passivation of Perovskite Nanocrystals

“…Other successful ligand variants include short‐chain amines, [ 20–23 ] branched amines, [ 24 ] bidentate ligands, [ 8 ] phospholipid ligands, [ 25 ] and phosphonate ligands. [ 26–29 ] While it is certainly well established that these ligands are highly effective in providing the nanocrystals with near‐perfect quantum yields, their mechanistic role in the passivation and the long‐term degradation behavior of perovskites is not extensively studied. In particular, we still do not fully understand why perovskite nanocrystals can be so resistant to degradation despite possessing only a modest organic ligand shell, and why certain ligands are empirically better than others in offering long‐term protection and stability.…”