Spin injection and emission helicity switching in a 2D perovskite/WSe2 heterostructure

Abstract

The initialization and control of a long-lived spin population in lead halide perovskites are prerequisites for their use in spintronic applications. Here, we demonstrate circular polarization of the interlayer exciton emission in a (BA)2PbI4/WSe2 monolayer heterostructure. The helicity of this emission is controlled by tuning the energy of the excitation laser through the manifold of exciton resonances of the WSe2 monolayer, together with an emerging interlayer absorption feature of the heterostructure. Theoretical calculations show that this resonance arises from hybridized (BA)2PbI4/WSe2 states in the valence band. This hybrid character enables its observation in both linear absorption and ultrafast pump-probe spectroscopies, and plays a key role in controlling the sign of the helicity of the interlayer exciton emission. The tunable spin polarization demonstrated here, with the WSe2 monolayer effectively acting as a tunable spin filter, represents an important step toward the use of 2D perovskites in opto-spintronic applications.

Figures (4)

• Figure 1: (a) PL and (b) reflectivity spectrum of the WSe$_2$ monolayer and the heterostructure. A$_{1\text{s}}^{\text{WSe}_2}$ indicates the 1s state of neutral exciton of WSe$_2$, A$_{2\text{s}}^{\text{WSe}_2}$ the 2s state, B$_{1\text{s}}^{\text{WSe}_2}$ the B exciton, XX$^{\text{WSe}_2}$ the biexciton, T$_{\text{T}}^{\text{WSe}_2}$ and T$_{\text{S}}^{\text{WSe}_2}$ the triplet and singlet charged excitons, respectively, XX$^{\text{-WSe}_2}$ the charged biexciton, and L the localized excitons. X$^{\text{CT}}$ designates the charge transfer interlayer exciton, and X$_{\text{LT}}^{\text{(BA)}_2\text{PbI}_4}$ and X$_{\text{HT}}^{\text{(BA)}_2\text{PbI}_4}$ the exciton of the low and high temperature phases of (BA)$_2$PbI$_4$, respectively. (c) Ball and stick model of the WSe$_2$ monolayer/(BA)$_2$PbI$_4$ heterostructure with a schematic depiction of IX and X$^{\text{CT}}$. The horizontal lines indicate the relative position of the band edges. Helicity-resolved PL spectrum of the WSe$_2$ monolayer/(BA)$_2$PbI$_4$ heterostructure for excitation in resonance with (d) the 2s exciton of WSe$_2$ (A$_{2\text{s}}^{\text{WSe}_2}$) and (e) the X$^{\text{CT}}$ transition. The excitonic resonances are indicated by arrows. (f) Schematic band alignment of the TMD-perovskite heterostructure. Spin-conserving charge transfer of the hole from WSe$_2$ to PbI$_4$ and formation of an interlayer exciton are schematically shown.
• Figure 2: (a) First derivative of the reflectivity contrast spectrum measured on the WSe$_2$ monolayer and on the heterostructure. (b) Helicity resolved PL intensity of the interlayer exciton and of the 1s exciton of WSe$_2$ monolayer and (c) degree of circular polarization $P_{\text{c}}$ as a function of the excitation energy. The vertical dashed lines indicate excitonic resonances. (d) Schematic of transfer of the exciton population towards states not driven optically but with the same spin configuration as the optically driven state mediated by Dexter-like coupling. (e) Pictorial view of the reversed circular polarization emission of the interlayer exciton.
• Figure 3: (a) Zoom-in to the valence band of the (BA)$_2$PbI$_4$/WSe$_2$ heterostructure from Fig. \ref{['SI_fig:BandStructure_DOS']} together with the spin expectation values, $\sigma_z$. Bands plotted with full dots originate from the WSe$_2$ monolayer, while states plotted with a coloured shading originate from (BA)$_2$PbI$_4$. VB1 and VB2 indicate the spin-orbit split valence bands of WSe$_2$, while Hyb designates the hybridized states involved in the X$^{\text{CT}}$ transition. (b) Eigenstate of one of the hybridized states involved in the X$^{\text{CT}}$ transition. An isosurface value of 1e-6^-3, which is the probability density of this state, was used. This corresponds to about 2.8e-8electron / ^3. For the valence band maximum of WSe$_2$, these values are 1e-3^-3 and 2.8e-5electron / ^3, respectively
• Figure 4: (a) Schematic of broadband pump-probe measurements performed on the (BA)$_2$PbI$_4$/WSe$_2$ heterostructure. (b) Transient reflectivity spectrum of the heterostructure and of the isolated WSe$_2$ monolayer excited at 2.64 (above the quasi-particle band gap of both materials) and extracted at a delay $\tau = 500fs$. The excitonic resonances are indicated. The resonance corresponding to the WSe$_2$ A exciton (A$_{1\text{s}}^{\text{WSe}_2}$) has been rescaled for increased clarity. (c) Transient differential reflectivity of the B exciton (B$_{1\text{s}}^{\text{WSe}_2}$) and of the interlayer charge transfer exciton (X$^{\text{CT}}$) measured as a function of the pump-probe delay. The line is the fit of an exponential rise and a bi-exponential decay model to the experimental data. Transient circular dichroism $(CD = (\Delta R / R)_{\text{co}} - (\Delta R / R)_{\text{cr}})$ maps of the (d) exciton transition of the low temperature phase of (BA)$_2$PbI$_4$ (X$_{\text{LT}}^{\text{(BA)}_2\text{PbI}_4}$) and (e) X$^{\text{CT}}$ excited in resonance with the WSe$_2$ A exciton obtained by subtracting the cross-polarized transient absorption from the co-polarized transient absorption. (f) Dynamics of the transient circular dichroism of X$^{\text{CT}}$ as a function of the pump-probe delay. The line is the fit to an exponential rise and an exponential decay model.