Highly Emissive Colloidal Nanocrystals of a "2.5-Dimensional" Monomethylhydrazinium Lead Bromide

Abstract

The ability to control materials at the nanoscale has advanced optoelectronic devices, such as LEDs, displays, and quantum light sources. A new frontier is controlling exciton properties beyond quantum size confinement, achieved through single monolayer heterostructures. In the prototypical example of transition metal dichalcogenide heterostructures and moiré superlattices, excitons with long lifetimes, strong binding energies, and tunable dipole moments have been demonstrated and are ideal for optoelectronics and quantum applications. Expanding this material platform is crucial for further progress. This study introduces colloidal nanocrystals (NCs) of monomethylhydrazinium lead bromide (MMHPbBr₃), a novel lead halide perovskite (LHP) with a unique "2.5-dimensional" electronic structure. While the spatial dimensionality of the NC extends in all three dimensions, these NCs exhibit excitonic properties intermediate between 2D and 3D LHPs. Density functional theory (DFT) calculations show that MMHPbBr₃ features spatially separated electron and hole wave functions, with electrons delocalized in 3D and holes confined in 2D monolayers. Synthesized via a rapid colloidal method, these NCs were characterized by using techniques such as 4D-STEM and nuclear magnetic resonance, confirming their monoclinic structure. Optical analysis revealed size-dependent properties and 3D quantum confinement effects, with three distinct photoluminescence (PL) bands at cryogenic temperatures corresponding to excitons with varying interlayer coupling. PL spectroscopy of single MMHPbBr₃ NCs reveals their photon emission statistics, expanding their potential for unconventional quantum material designs.

Figure 1

(a–d) Various examples of dimensional reduction of a bulk (a) semiconductor to achieve exciton confinement: morphological (b), molecular (c), and combined molecular and morphological (d). (e) The number of defects in a semiconductor decreases with the number of atoms (volume) and nanostructuring offers a pathway to decrease the number of defects per exciton. (f–h) Examples of confinement potentials: 3D confinement in the whole volume of the material (f), an ordered periodic 2D potential in a layered material (g), and a mixed case, where one of the carriers delocalized in a 2D space and the other in a 3D space (2.5D) (h).

Figure 2

(a) Monomethylhydrazinium (MMH⁺) cation. MMHPbBr₃ has a 3D perovskite lattice (b) with two distinct alternating layers: one distorted layer (depicted in gold), where PbBr₆ octahedra are distorted due to coordination of two MMH⁺ cations (c), and one undistorted layer (depicted in blue) comprising undistorted PbBr₆ octahedra (d).

Figure 3

(a) Electronic band structure of bulk MMHPbBr₃ color-coded with Pb-s, Pb-p, and Br-p contributions. (b–d) Projected atomic orbitals at the CB of MMHPbBr₃ showing a three-dimensional electron wave function delocalization. (e,f) Projected atomic orbitals at VB1 (Y point and E – E_F = 0 eV) showing hole-wave function delocalization only in the plane of the undistorted layer. (g,h) Projected atomic orbitals at VB2 showing hole-wave function delocalization in the plane of the distorted layer.

Figure 4

(a) Room-temperature synthesis of MMHPbBr₃ NCs. (b-e) Size evolution of MMHPbBr₃ NCs with increasing reaction time. (f) Phase contrast reconstruction from 4D-STEM of the MMHPbBr₃ NCs, with (g) depicting a magnified region of (f) to show the atomic structure in high resolution and with the model of the structure projected along the [001] crystallographic direction as an overlay (Pb = orange, Br = black, and MMH = blue/white). (h) PDF analysis of the XRD data of MMHPbBr₃ NCs fitted to the RT crystallographic phase. (i) Solid-state ²⁰⁷Pb NMR of the bulk (black solid line) and NC powder (blue dotted line) of MMHPbBr₃ shows two signals from two different coordination environments of Pb in MMHPbBr₃.

Figure 5

(a) PL and UV–vis spectra of an MMHPbBr₃ NC colloid in n-hexane. (b) RT PL QY and PL peak wavelength of MMHPbBr₃ colloids of various average NC sizes (see Figure S4 for TEM images). (c) Ensemble PL spectrum of a spin-coated MMHPbBr₃ NC thin film at 4 K upon excitation at 405 nm, demonstrating three distinct PL bands. (d–f) PL decay traces measured at the maximum wavelength of the bands PL1 (d), PL2 (e), and PL3 (f), respectively. The insets depict the suggested excitonic nature for each PL band, with blue and gold indicating the undistorted and distorted octahedral layer and the dashed and solid lines sketching the presumed confinement potential wells, respectively.

Figure 6

(a) PL spectra of a single MMHPbBr₃ NC at 297 and 4 K. (b) Correlation of peak energy and line width of single MMHPbBr₃ NC PL at 297 K, derived from NC colloids with an ensemble-averaged NC size of 6 nm (yellow markers) and 10 nm (orange markers), respectively. (c,e) Second-order intensity correlation (g⁽²⁾) and (d,f) PL intensity trace of two representative single NCs. Red traces are the background signal. (g,h) g⁽²⁾ extracted from (f) separately for bright and dim intensity levels defined via the blue and gray shaded areas, respectively.