Rationalizing and Controlling the Surface Structure and Electronic Passivation of Cesium Lead Halide Nanocrystals

Abstract

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as versatile photonic sources. Their processing and luminescent properties are challenged by the lability of their surfaces, i.e., the interface of the NC core and the ligand shell. On the example of CsPbBr₃ NCs, we model the nanocrystal surface structure and its effect on the emergence of trap states using density functional theory. We rationalize the typical observation of a degraded luminescence upon aging or the luminescence recovery upon postsynthesis surface treatments. The conclusions are corroborated by the elemental analysis. We then propose a strategy for healing the surface trap states and for improving the colloidal stability by the combined treatment with didodecyldimethylammonium bromide and lead bromide and validate this approach experimentally. This simple procedure results in robust colloids, which are highly pure and exhibit high photoluminescence quantum yields of up to 95-98%, retained even after three to four rounds of washing.

Figure 1

(a) Size-dependent anion/lead ratio (X/Pb) of cubic CsPbX₃ NCs, where Cs, Pb, and X (=halide) atoms are depicted by green, orange, and gray spheres, respectively. Unlike in the bulk, in a NC the anion/lead ratio deviates from three, with the upper bound (green circles) and lower bound (orange circles) given by CsX and PbX₂ termination, respectively. The inset shows commonly found experimental sizes, for which the anion/lead ratio should vary only between about 2.8 and 3.2, indicated by the yellow shaded area. (b) As explained in the main text, and to aid the discussion of aging, the NC is further (arbitrarily) divided into core, inner, and outer shell. For a realistic NC requiring colloidal stability, the outermost layer is commonly replaced either by ligand pairs {AX′}, where A = cationic ligand (e.g., oleylammonium) and X′ = anionic ligand (e.g., bromide, oleate), respectively, or more recently also with zwitterionic ligands {AX′}, e.g., sulfobetaines. In either case, the anion/lead ratio (now [X + X′]/Pb) still falls within the yellow shaded area depicted in (a), and green circles correspond to full capping by the {AX′} ligand shell.

Figure 2

Plausible surface transformations illustrated for a 3.6 nm perovskite NC. The initial state “1” can be described as consisting of a [CsPbX₃] core (continuous black line), surrounded by a shell of k (PbX₂) moieties (dashed orange line) and capped with n AX′-type ligands (dotted green line). To enhance clarity, the core is shown slightly smaller than in reality. Partial removal of m {AX′} units (m < n, state “2”) or its complete elimination (m = n, state “3”) leads to a (PbX₂)-terminated NC. Further detachment of l (PbX₂) units (l < k, state “4”) eventually leads to a bare [CsPbX₃] core (l = k, state “1”, analogous to “1”). During the aging process, the anion/lead ratio oscillates from initially anion-rich (state “1”) to Pb-rich (state “3”) and back to anion-rich (state “1”).

Figure 3

Computed geometry and electronic structure of CsPbX₃ NCs during the aging process introduced in Figure 2, at the DFT/PBE level of theory. (a) Loss of the outermost {AX′}-ligand shell of a [CsPbX₃](PbX₂)_k{AX′}_n(1–x) NC, with X = X′ = Br, A = Cs, and 0 < x < 1. The top row depicts the aging-induced changes to the geometry, where Pb, Cs, and Br atoms are depicted by orange, green, and gray spheres, respectively. The middle row shows the respective evolution of the valence band electronic structure from the HOMO (E – E_HOMO = 0) up to 3 eV below the HOMO, where relative orbital contributions by the core and surface (defined as the outermost, one-atom-thick layer) are depicted by blue and red bars, respectively. Surface-localized states only appear after loss of more than 75% of the {AX′} shell, indicated by a dashed ellipse and visualized in the bottom row via the associated localized valence band edge molecular orbitals (HOMOs). (b) Subsequent loss of the now exposed (PbX₂) shell. Localized states already appear at only 25% loss of the (PbX₂) shell. (c) Schematics of the aging/damage and recovery of trap-free [CsPbX₃](PbX₂)_k{AX′}_n NCs.

Figure 4

(a) Comparison of the steady-state absorption and PL spectra for the starting colloid and the same colloid subjected to several treatments (for details see sample numbers 1, 2, 9, and 11 in the Supporting Information): untreated starting colloid of OLA/OA-stabilized NCs (black line, sample 1), starting colloid precipitated with the acetone as a nonsolvent (containing OLA and OA) and redispersed in toluene (dark blue line, sample 2), starting colloid treated with the mixture of DDAB + PbBr₂ (gray line, sample 9), and purified colloid (sample 2) treated with the mixture of DDAB + PbBr₂ (blue line, sample 11). The inset magnifies the PL peak region. All colloids have been colloidally stable and their visual brightness under UV irradiation (see (b)) clearly reflected the variation of the measured QYs. (c) TEM image of sample 11, indicating the retention of structural integrity. (d) TRPL spectra for the same samples. For samples 1 and 9, the effect of aging by several days is indicated by dashed arrows.