Excitons in ultrathin organic-inorganic perovskite crystals

We demonstrate the formation of large sheets of layered organic-inorganic perovskite (OIPC) crystals, as thin as a single unit cell, prepared by mechanical exfoliation. The resulting two-dimensional OIPC nanosheets of 2.4 nm thickness are direct semiconductors with an optical band gap of 2.4 eV. They exhibit unusually strong light-matter interaction with an optical absorption as high as 25% at the main excitonic resonance, as well as bright photoluminescence. We extract an exciton binding energy of 490 meV from measurement of the series of excited exciton states. The properties of the excitons are shown to be strongly influenced by the changes in the dielectric surroundings. The environmental sensitivity of these ultrathin OIPC sheets is further reflected in the strong suppression of a thermally driven phase transition present in the bulk crystals.

Figure 1

Schematic representation of 2D OIPC structure (after Ref. [8]). A is an organic ammonium cation, M is a divalent metal cation, and X is a halogen anion. The dashed line indicates the VdW interface.

Figure 2

(a) Typical optical micrograph of exfoliated (${\mathrm{C}}_4{\mathrm{H}}_9{\mathrm{NH}}_3$)${}_2{\mathrm{PbI}}_4$ films deposited on ${\mathrm{SiO}}_2$(280 nm)/Si substrates. (b) A magnified image of the area indicated by the black rectangle in (a). In this image, only the red channel of the camera is presented and a monochrome presentation is used. The inset of (b) shows the reflectance intensity profile along the white dotted line. Equal steplike changes are observed, as expected for materials of a few monolayers thickness on a ${\mathrm{SiO}}_2$/Si substrate [30]. (c) AFM topography image for the black rectangle indicated in (b), including the boundary between the two areas with the lowest contrast. The height profile (inset) shows a step of 2.4 nm height at the boundary.

Figure 3

(a) Room-temperature optical absorption and photoluminescence spectra of an exfoliated (${\mathrm{C}}_4{\mathrm{H}}_9{\mathrm{NH}}_3$)${}_2{\mathrm{PbI}}_4$ NS. (b) Optical absorption and PL from an OIPC-NS at 5 K. The contributions from the two phases are indicated by (I) and (II).

Figure 4

(a) Derivative of the absorption spectrum of the OIPC-NS sample at the temperature of $T=5$ K (from the data in Fig. 3). The data are rescaled in the spectral range of the ground state resonance for clarity. The optically active exciton states are denoted by $1s$, $2s$, $3s$, and $4s$. (b), (c) Derivatives of the reflectance contrast spectra of two additional OIPC NS samples at $T=5$ K. (d) Low-temperature reflectance spectrum of a thick bulk sample. The exciton ground state and the first excited state are indicated by $1s$ and $2s$, respectively. (e) Energies of the excited states of the exciton relative to the $1s$ transition for an OIPC-NS compared with the results for the bulk crystals (literature data taken from Ref. [21]). The averaged data (open circles) are presented together with the results from individual measurements on different NS samples (closed circles). The quasiparticle band gap in the OIPC-NS, obtained from the fit of $n=3,4$ states, is indicated by the dashed line. (f) Effective dielectric screening for the measured exciton states. The error bars originate from the experimental uncertainties in the extracted $3s$ and $4s$ peak positions. The dashed lines indicate the averaged dielectric constant of fused silica substrate ($\epsilon =2.13$) and vacuum ($\epsilon =1$) at optical frequencies.

Figure 5

(a) Reflectance spectra for the two types of NSs at $T=5$ K. (b) The corresponding evolution of exciton peak energies with temperature.