Figure 2(a-c) show the microscope images of the MAPbI3 nanoplatelets adhere to the thermal release tape obtained by mechanical exfoliating after pressing nanoplatelets with pressures of 0.45, 0.50, 0.55 MPa for 3 minutes, respectively. Under the pressure of 0.45 MPa, only one MAPbI3 nanoplatelet fragment is exfoliated. As shown in in Figure 2(a), a yellow MAPbI3 nanoplatelet fragment with length longer than 20 mm adheres to the tape. When the pressure is increased to 0.50 MPa, more than 3 undamaged MAPbI3 nanoplatelets adhere to the thermal release tape after the mechanical exfoliation as can be seen in Figure 2(b). As the pressure increased to 0.55 MPa, a growing number of MAPbI3 nanoplatelets adhere to the thermal release tape after the mechanical exfoliation as can be seen in Figure 2(c). Unfortunately, the surface of MAPbI3 nanoplatelets are covered with mica sheets. And the number of damaged nanoplatelets increased due to the large pressure. Therefore, pressing nanoplatelets/tape with a pressure of 0.50 MPa can exfoliate the nanoplatelets without damage and residual mica.
Figure 2(d-f) shows the micrograph of MAPbI3 nanoplatelets adhere to the heat-release tape exfoliated by pressing the nanoplate/tape with a pressure of 0.50 MPa and pressing time of 2, 3 and 4 min, respectively. As can be seen from the Fig. 2d, few MAPbI3 nanoplatelets adhere to thermal release tape when mechanical exfoliated after pressing nanoplatelets/tape for 2 min. As the nanoplatelets/tape pressing time increased to 3 min, more than 4 regular shape MAPbI3 nanoplatelets were adhere to the thermal release tape after mechanical exfoliation. However, as the nanoplatelets/tape pressing time increased to 4 min, not only MAPbI3 nanoplatelets but also mica fragments adhere to the thermal release tape after mechanical exfoliation, and more nanoplatelets were broken during the mechanical exfoliation. It can be seen that the adhesion can be precisely controlled by the pressing time of the nanoplatelets/tape. The adhesion increases with the growing pressing time. Therefore, appropriate adhesion that can mechanical exfoliates nanoplatelets without damage and mica fragments can be realized by controlling the pressure and the pressing time. Our studies show that pressing the nanoplatelets/tape with a pressure of 0.5 MPa and a pressing time of 3 min before mechanical exfoliation can provide an appropriate adhesion that can transfer nanoplatelets to the thermal release tape without damage and mica fragments.
In situ TEM observation of the thermal response of perovskite-base solar cell shows that perovskite layer can be stable for short times until 423 K. In order to avoid heat induced degradation of the perovskite during the thermal release process, we investigated the thermal stability of the perovskite nanoplatelets. During the experiment, the thermal stability of the perovskite nanoplatelets was tested with continuous heating at 367 K for 2 min. We monitored the XRD pattern of the perovskite nanoplatelets before heating and after heating. Figure 3 shows a detailed XRD pattern of the pristine perovskite nanoplatelets. Before heating, the specimen exhibits tetragonal perovskite crystal structure with small (202), (210), and (221) peaks and also small (100) and (003) peaks of PbI2 as can be seen in upper panel in Fig. 3. Then, the specimen is heated at 367 ℃ for 2 min in ambient air condition. The resulting XRD pattern (lower panel in Fig. 3) reveals that the relative intensity of the tetragonal perovskite crystal structure peaks and PbI2 peaks almost unchanged. The MAPbI3 does not degrade to PbI2 during the heating process in our experiment. Therefore, the CVD synthesized perovskite nanoplatelet is stable under moderate heating at 367 K for short time (~2 min).
Thermal release tapes with undamaged MAPbI3 nanoplatelets were pressed on the target substrate gently to ensure they are in full contact. After heating nanoplatelets/tape at 366 K for 2min with a hotplate, the thermal release tape was removed at a sufficiently low separation speed of 0.6 mm/s to ensure the nanoplatelet adhere preferentially to the target substate and separate from the thermal release tape. As can be seen, the nanoplatelet on mica substrate (Fig. 4a) are transferred to SiO2 substrate (Fig. 4b) without any damage.
We also characterized the surface morphology of nanoplatelet on mica substrate and on transferred nanoplatelet on SiO2 substrate by atomic force microscope. Figure 5a shows a detailed surface morphology of the pristine perovskite nanoplatelets. As can be seen, the pristine perovskite nanoplatelet has a smooth surface with an RMS roughness less than 0.96 nm. After transfer printing, the surface morphology of transferred nanoplatelet on SiO2 substrate is well preserved with an RMS roughness less than 1.18 nm as shown in Fig. 5b. The slightly higher roughness is caused by increased scanning range of the transferred nanoplatelet on SiO2 substrate.
Pristine perovskite nanoplatelet on mica substrate before transfer printing and perovskite nanoplatelets on SiO2 substrate after transfer printing were optically pumped at room temperature by a femtosecond-pulsed laser through a home-built fluorescence microscope. For comparison, we select a pristine perovskite nanoplatelet on mica substrate. As can be seen in Fig. 6a, at low pump density P (<28.19 µJ cm−2), each emission spectrum shows a broad peak centered at ~ 770 nm with a full-width at half-maximum (FWHM) of Δλ = 48 nm, which corresponds to spontaneous emission (SPE). At higher pump density P(>28.19 µJ cm−2), the emission spectrum changes from a broad spectrum to a narrow spectrum which is one of the key properties of lasing. At PTh = 28.78 µJ cm−2, a sharp peak at 783.67 nm appears and grows rapidly with increasing P, and the intensity of the broad SPE peak (non-lasing) remains almost constant. The FWHM at P = 28.78 µJ cm−2, at which pump density the lasing peak dominates, is 0.5 nm. It indicates that the laser has a FWHM linewidth more than an order of magnitude narrower than the typical linewidth (10 nm) of ASE in an organic semiconductor. The light-in-light-out curve in Fig. 6b shows a slow increase in emission intensity with increasing pump density below the pump density of ~28.78.98 µJ/cm2, and then a faster increase in emission intensity thereafter. The nonlinear dependence of the emission intensity on the pump intensity is another the key properties of lasing. Therefore, the transfer printed perovskite nanoplatelet laser have a narrow linewidth of 0.5 nm and a clear threshold at 28.19 µJ cm−2.
Lasing performances of a perovskite nanoplatelet on SiO2 substrate after transfer printing was also tested. As can be seen in Fig. 6c, at low pump density P (<27.59 µJ cm−2), each emission spectrum shows a broad peak centered at ~ 770 nm with a full-width at half-maximum (FWHM) of Δλ = 48 nm, which corresponds to spontaneous emission (SPE). At higher pump density P(>27.59 µJ cm−2), the emission spectrum changes from a broad spectrum to a narrow spectrum which is one of the key properties of lasing. At PTh = 27.89 µJ cm−2, a sharp peak at 774.06 nm appears and grows rapidly with increasing P, and the intensity of the broad SPE peak (non-lasing) remains almost constant. The FWHM at P = 27.89 µJ cm−2, at which pump density the lasing peak dominates, is 0.3 nm. It indicates that the laser has a FWHM linewidth more than an order of magnitude narrower than the typical linewidth (10 nm) of ASE in an organic semiconductor. The light-in-light-out curve in Fig. 6d shows a slow increase in emission intensity with increasing pump density below the pump density of ~27.89 µJ/cm2, and then a faster increase in emission intensity thereafter. The nonlinear dependence of the emission intensity on the pump intensity is another the key properties of lasing. Therefore, the transfer printed perovskite nanoplatelet laser have a narrow linewidth of 0.3 nm and a clear threshold at 27.89 µJ cm−2. It can be seen that the transfer printed perovskite nanoplatelet on SiO2 substrate shows almost similar lasing performances as the pristine perovskite nanoplatelets on mica substrate. It indicates that the transfer printing of perovskite nanoplatelets by thermal release tape can provide an efficient method to combine the perovskite nanoplatelets with various substate for laser applications.