Brief introduction of the results
Energy funneling is a phenomenon used to improve the performance of optoelectronic devices based on low-dimensional materials. Based on this,Academician Yang Peidong (corresponding author) of the University of California, Berkeley, etcA new class of two-dimensional (2D) semiconductors characterized by multiple regions of different thicknesses in a single confined nanostructure with uniform composition is reported.
The authors used the structural transformation of bilayer octahedral (n=2) 2D cesium-lead bromide perovskite nanosheets to prepare this "non-integer 2D semiconductor". In the non-integer system, there are no ligands between regions with different perovskite thicknesses, forming a downslope band arrangement without dielectric barriers, which can avoid the obstacles faced by quasi-2D perovskites in the energy funnel. Through time-resolved photoluminescence (PL) and femtosecond transient absorption (FSTA) spectroscopy, the authors report the occurrence of an energy funnel within the material from a thinner region to a thicker region.
Based on atomic-resolution transmission electron microscopy (TEM) imaging and structural modeling, the interface where the transfer occurs is virtually strain-free, unlike the heterostructure composed of different elements. In addition, this study also reveals the constraint as a knob that regulates the local semiconductor interface and properties of individual nanoparticles, and highlights the effect of thickness changes on the optoelectronic properties of semiconductors, indicating direct device applications.
Background:
Metal halide perovskites have the advantages of high absorption coefficients, easily adjustable band gaps, relative defect tolerances, and long carrier diffusion lengths in optoelectronic applications. At present, the photoluminescence quantum yield (PLQY) of lead halide perovskite quantum dots has reached a uniform level, and the efficiency of perovskite-based solar cells has exceeded 33%. Among them, two-dimensional (2D) metal halide perovskites are superior to massive perovskites in terms of optical properties and environmental stability, which is one of the most common problems that limit the diffusion of perovskite materials to mainstream applications.
However, the main challenge of 2D perovskites is that the binding hinders the formation of long-lived free carriers by competing with strongly bound radiation excitons, and the bandgap of 2D perovskites increases with the increase of material thickness. In the energy funnel, the photoexcitation energy is funnelized downward along a bandgap gradient and is formed by the distribution of stacked 2D perovskites with increased thickness and is referred to as quasi-2D perovskites. From the thickest 2D perovskites, the enhanced emission rate can be used for light-emitting diode or laser applications, or the charge in solar cells can be more easily extracted. Based on the dielectric constant of the ligand spacer surrounding the perovskite lattice, a small portion of the photoexcitation energy is lost during each successive transfer event in the energy funnel, driving further research into energy transfer, ligand shell chemistry, and the development of alternative methods to improve the energy funnel.
**Reading guide
In this paper, the authors induce an ideal structural transition by introducing a polar ligand into colloidal cesium lead bromide nanosheets to form non-integer 2D perovskites, taking advantage of the dynamic ligand environment and the instability of perovskites. The non-uniform thickness products of the structural transformation reaction are called "non-integers" because they cannot be fully described by just one integer phase. By modifying the 2D morphology of CSPBB3 nanosheets with cubic lead dibromide octahedral thickness (N=2), strong excitons at room temperature were observed, with an absorption peak at 428 nm and a PL emission peak at 436 nm. TEM imaging showed that the contrast at the edges of the nanosheets increased as the reaction progressed. At the edges of the non-integer structure, the line-scan intensity of the integrated TEM image increases discretely.
Figure 1n=2:3 Characterization of cspbbr3 growth
The authors hypothesize that the attached n=2 and n=3 phases of non-integer perovskites form in-plane i-type arrangement of dual quantum wells, in which the ligand barrier blocks the i-type arrangement. Photoexcitation above the band gap yields a ple spectrum of n=2:3 2d perovskites, which consists of exciton emission from n=2 and n=3 phases.
Direct photoexcitation of n=2 excitons results in greater emission intensity, or sensitization, of n=3 excitons compared to photoexcitation above the band gap. In the PL attenuation plot for 40% n=3 cspbbr3, two distinct regions are seen, corresponding to the n=2 phase in the presence of n=3 phase. This quenching behavior can be observed by measuring the PL attenuation in the n=2:3 cspbbr3 complex as a function of phase composition. The 16 ns PL lifetime of the n=2 exciton in pure N=2 nanosheets was quenched in 40% N=3 cspbbr32 times.
Figure 2Photophysics of cspbbr3 based on PL spectra n=2:3
The phase composition of non-integer 2D perovskites is 10% and 40% as shown by femtosecond transient absorption spectroscopy (FSTA) as shown by N=3. With the increase of the presence of n=3 in the nanostructure, radiation 1 and non-radiation 2 of the n=2 excitons are significantly quenched, and the photoexcitation energy is flowing from the n=2 phase funnel to the n=3 phase. With the increase in the presence of n=3, the excited state absorption at 410 nm is quenched. In addition, in 40% n=3 cspbbr3, n=3 gsb rises more slowly than pure n=3 nanosheets. When a direct transfer channel is included between the conduction bands of the n=2 and n=3 phases, kinetic modeling of non-integer 2d perovskites reproduces this behavior.
Figure 3Photophysics of CSPBB3 based on FSTA spectroscopy n=2:3
Using atomic-resolution transmission electron microscopy (TEM), the authors probed the possible out-of-plane or in-plane structural interfaces in the n=2:3 CSPBBR3 lattice. There is no unique out-of-plane interface between the n=2 and n=3 phases, which confirms that the n=2 and n=3 phases are uniformly attached and there is almost no lattice strain between them. Compared to pure n=2 and n=3 nanosheets, the diffraction pattern of n=2:3 cspbbr3 does not have a Bragg peak. The diffraction pattern of the selected n=2:3 cspbbr3 is a single cubic plot, which confirms that the growth of the third octahedron on the n=2 phase is completely epitaxy. Thus, an energy funnel occurs at the implied n=2:3 interface, demonstrating for the first time efficient transfer in a heterostructure that is not affected by strain.
Figure 4n=2:3 Atomic-resolution characterization of the CSPBBR3 interface
Bibliographic information
energy funneling in a noninteger two-dimensional perovskite. nano lett.,, doi: 10.1021/acs.nanolett.3c03058.