Supporting Information

Waterproof perovskite-hexagonal boron nitride hybrid

nanolasers with low lasing thresholds and high operating

temperature

Haoran Yu, Xing Cheng, Yang Liu, Kexiu Rong, Yilun Wang, Ziling Li, Yi Wan,

Wenting Gong, Kenji Watanabe, Takashi Taniguchi, Shufeng Wang, Jianjun Chen, Yu

Ye* and Lun Dai*

*Corresponding authors, E-mails: ye_yu@pku.edu.cn, lundai@pku.edu.cn

Section 1. Growth of CsPbI3 nanoplates (NPs) via chemical vapor deposition

(CVD) method

Schematic diagram illustrates the experimental setup of our home-built CVD system

(Figure S1a), and optical image of the typically synthesized CsPbI3 NPs show regular

rectangle shapes as well as smooth surfaces (Figure S1b). Due to the interference

effect between the top and bottom facets, the CsPbI3 NPs with different thicknesses

show different colors.

Figure S1. CVD growth of CsPbI3 NPs. (a) Schematic diagram of the CVD

experimental setup. (b) Optical image of the representative CsPbI3 NPs grown on

muscovite mica substrate. Scale bar: 10 μm.

Section 2. Transfer of hBN capping layer

In order to see the influence of hBN capping on the lasing performance of CsPbI3 NP,

characterization of the pump intensity dependent emission spectra is firstly conducted

on the bare NP (Figure S2a). Afterwards, the hBN capping layer is transferred using

the dry transfer method under the help of a micromanipulator (Figure S2b).1 Firstly,

the hBN flakes are mechanically exfoliated on the viscoelastic PDMS stamp, adhering

to a glass slide. Then, an hBN with proper size and thickness (indicated by its optical

contrast) is selected under the optical microscope. Later, the hBN flake and the

selected NP are aligned under the microscope. The PDMS stamp needs to be pressed

against the target substrate slowly to achieve a bubble-free contact. Finally, the PDMS

stamp is slowly peeled off from the substrate, leaving the hBN capping layer on top of

the CsPbI3 NP (Figure S2c).2 After that, the second pump intensity dependent

emission spectra measurement is conducted on the hBN capped CsPbI3 NP.

Figure S2. Transfer of hBN capping layer onto CsPbI3 NP. (a) Optical image of the

bare CsPbI3 NP on muscovite mica, which is used for lasing characterization before

hBN capping. (b) Diagrams of the deterministic transfer steps of the hBN capping

layer. (c) Optical image of the hBN capped CsPbI3 NP, which is used for lasing

characterization after hBN capping. Length of the scale bars in both a and c is 10 μm.

Section 3. AFM characterizations of the hybrid structure

Using atomic force microscopy (AFM), the topography of the CsPbI3 NP-hBN hybrid

structure is measured (Figure S3a). The height profile along the white dashed line

gives the thicknesses of CsPbI3 NP and hBN flake to be about 177 nm and 57 nm,

respectively (Figure S3b).

Figure S3. AFM characterizations of the hybrid structure. (a) The topography of the

CsPbI3 NP-hBN hybrid structure measured by AFM. (b) Cross section profile of

height variation along the white dashed line in a.

Section 4. Three-dimensional simulation of WGM electric field distribution

We simulate the three-dimensional electric field distribution of the CsPbI3 NP-hBN

hybrid structure. The dimensional parameters of the hybrid structure are duplicated

from the AFM measurement. Refractive indices of hBN flake, CsPbI3 NP and

muscovite mica substrate are 2.173, 3.284 and 1.58, respectively. The planforms along

z-axis and x-axis (Figure S4) indicate that it is transverse electric (TE with the electric

field component parallel to the surface of CsPbI3 NP) whispering gallery mode

(WGM) supported by the hBN capped CsPbI3 NP.

Figure S4. Simulated three-dimensional electric field distribution of the CsPbI3 NP

after hBN capping.

Section 5. Optical images and polarization-dependent emission of the NP above

lasing threshold

The sharp increase of output intensity (‘kink’ behavior in L-L curves) and the sudden

reduction in full-width at half-maximum (FWHM) have confirmed the formation of

high quality WGM nanolaser. To further characterize lasing operation, we take optical

images of the far-field radiation pattern and measure the polarization of the lasing

emission.

Figure S5. Optical images of the CsPbI3 NP’s emission. (a) and (b), Optical images

of the far-field radiation pattern from CsPbI3 NP under pump intensity below and

above lasing threshold, respectively. Length of the scale bars in both a and b is 5 μm.

Optical image of the emission below lasing threshold shows uniform spontaneous

emission over the whole CsPbI3 NP (Figure S5a). Above threshold, strong emissions

are observed at the four corners, due to the sharp geometry change. In addition, there

exist clear interference fringes (Figure S5b), indicating the coherent light emission.

For the measurement of the polarization-dependent lasing intensity of the CsPbI3 NP,

we use a linear polarizer, and a pinhole (~500 μm) placed at the image plane to

achieve the spatial resolution of ~2.5 μm. We selectively collect the scattering light of

the lasing emission at different positions from a CsPbI3 NP (5.06 μm×5.07 μm, see

Figure S6a), including a corner of the NP (point A) and a midpoint of an edge (point

B). The intensity of the lasing peak (Figure S6b for point A; Figure S6c for point B)

shows strong polarization dependence. However, the degree of linear polarization is

limited by the size of the pinhole, since the scattering light from different positions

show distinct polarizations.

Figure S6. Polarization-dependent emission of the NP above lasing threshold. (a)

Lasing emission image of the measured NP. The measured points A and B are marked

by dashed circles. (b) and (c), The measured polarization-dependent lasing intensities

at points A and B, respectively.

Section 6. FWHM of the resonant peak above lasing threshold before and after

hBN capping of the NP nanolasers

Lasing emission spectra of CsPbI3 NP taken under identical pump intensity above

lasing threshold (375.44 μJ/cm2) before (Figure S7a) and after (Figure S7b) hBN

capping are fitted by two Gaussian functions. The FWHM (Δλ) of the resonant peak

above the lasing threshold increases from 0.63 nm to 1.17 nm after hBN capping.

Figure S7. Fitting of the spectrum before and after hBN capping. (a) Emission

spectrum of CsPbI3 NP before hBN capping. The FWHM of the resonant peak is 0.63

nm. (b) Emission spectrum of CsPbI3 NP after hBN capping The FWHM of the

resonant peak is 1.17 nm.

Section 7. Rate equation analysis

In order to further investigate the influence of hBN capping, we obtain the

spontaneous emission factor, β, of the nanolaser using rate equation analysis.5-7 The

dynamics of the excited electronic state population n and photon number s that couple

into a specific lasing mode can be represented as the following equations:

d𝑛𝑛d𝑡𝑡

= 𝑝𝑝 − 𝐴𝐴𝐴𝐴 − 𝛽𝛽Γ𝐴𝐴𝐴𝐴(𝐴𝐴 − 𝐴𝐴0) (1)

d𝑠𝑠d𝑡𝑡

= 𝛽𝛽𝐴𝐴𝐴𝐴 + 𝛽𝛽Γ𝐴𝐴𝐴𝐴(𝐴𝐴 − 𝐴𝐴0) − 𝛾𝛾𝐴𝐴 (2)

where p is a pumping rate, A is the total spontaneous emission rate into all modes, β is

the spontaneous emission factor, Γ is the optical confinement factor, n0 is the excited

state population at transparency, and γ is the total cavity mode loss rate. For simplicity,

we assume Γ to be one, and n0 to be zero. When it reaches steady state, both d𝐴𝐴 d𝑡𝑡⁄

and d𝐴𝐴 d𝑡𝑡⁄ equal zero, which leads to the following relationship.

𝑝𝑝 = 1+𝛽𝛽𝑠𝑠𝛽𝛽(1+𝑠𝑠)

∙ 𝛾𝛾𝐴𝐴 (3)

We take the pump fluence intensity of the excitation as p, and the output intensity of

the oscillation peak as s. The experimental data can be best fitted with a spontaneous

emission factor, β, of 0.0203 (0.0163) before (after) hBN capping (Figure S8).

Figure S8. Curve fittings to the experimental data using rate equation analysis.

Section 8. TRPL measurements of another CsPbI3 NP

For further investigations about the dynamics of lasing in CsPbI3 NPs and the

influences of hBN capping, we conducted time-resolved photoluminescence (TRPL)

measurements using a 517 nm femtosecond laser over a time window of about 1.34 ns

on another CsPbI3 NP (Figure S9). The TRPL is measured four times for a CsPbI3 NP:

(a) excitation intensity below lasing threshold, before hBN capping; (b) excitation

intensity (identical to a) below lasing threshold, after hBN capping; (c) excitation

intensity above lasing threshold, before hBN capping; (d) excitation intensity

(identical to c) above lasing threshold, after hBN capping. Below lasing threshold, we

perform mono-exponential fitting to the PL decay curve. Above lasing threshold, we

perform bi-exponential deconvolution fitting to the experimental data. Under low

pump intensity below lasing threshold, the spontaneous emission lifetime increases

from 729.9 ps to 859.2 ps after hBN capping (Figure S10a, b and c). Besides, there

exists a remarkable PL enhancement after hBN capping. Above the lasing threshold,

there is a new fast decay component appears both before (27.20 ps, 85.01%) and after

(28.92 ps, 79.25%) hBN capping, which decays more rapidly than the other

component (386.96 ps, 14.99% before hBN capping; 347.28 ps, 20.75% after hBN

capping, see Figure S10a, d and e). These results are consistent with the data

presented in the main text.

Figure S9. Optical images of another measured sample before and after hBN capping.

(a) Optical image of another CsPbI3 NP before hBN capping. (b) Optical image of the

CsPbI3 NP after hBN capping. Length of the scale bars in both a and b is 10 μm.

Figure S10. Time-resolved photoluminescence (TRPL) measurements of another

sample. (a) Typical TRPL decay transients following photo-excitation of another

CsPbI3 NP with pump fluence below and above lasing threshold, before and after

hBN capping. The instrument response function is plotted with the gray dashed line.

(b-e) Images of spectrum vs. time (collected over a time window of about 1.34 ns)

taken under different conditions. (b) Below lasing threshold, before hBN capping; (c)

Below lasing threshold (with identical pump intensity as in B), after hBN capping; (d)

Above lasing threshold, before hBN capping; (e) Above lasing threshold (with

identical pump intensity as in d), after hBN capping.

Here we further analyze the reason behind PL enhancement. The phenomenological

lifetime 𝜏𝜏 depends on the radiative recombination lifetime 𝜏𝜏𝑟𝑟 and the total lifetime

resulted from different nonradiative recombination pathways 𝜏𝜏𝑛𝑛𝑟𝑟 as the following

equation:

𝜏𝜏−1 = 𝜏𝜏𝑟𝑟−1 + 𝜏𝜏𝑛𝑛𝑟𝑟−1 (4)

At room-temperature, since 𝜏𝜏𝑛𝑛𝑟𝑟 ≪ 𝜏𝜏𝑟𝑟 (PL quantum yield ~8.3%8), 𝜏𝜏 is

approximately to be equal to 𝜏𝜏𝑛𝑛𝑟𝑟. The PLQY can be estimated by the ratio of radiative

rate to total rate as:

𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 =radiative rate

total rate ~

1𝜏𝜏𝑟𝑟

1𝜏𝜏𝑟𝑟

+ 1𝜏𝜏𝑛𝑛𝑟𝑟

=1

1 + 𝜏𝜏𝑟𝑟𝜏𝜏𝑛𝑛𝑟𝑟

(5)

With the surface passivation brought by hBN capping, surface traps induced

nonradiative recombination rate becomes slower, resulting a longer 𝜏𝜏𝑛𝑛𝑟𝑟, and

accordingly, higher PLQY, which leads to a higher PL emission intensity.

Section 9. Simulation of thermal dissipation

To demonstrate the efficient thermal dissipation induced by hBN capping, we

simulate the whole dissipation process within the first pump cycle, with an initial

temperature set to be 25 ℃ (for both of the device and the environment). To simplify

the calculation, we take the NP as a circular disk, since the shape of the sample will

not change the nature of thermal dissipation. In this way, a two-dimensional

axisymmetric model is set up. The dimension of the model’s geometry is close to the

actual size of the NP, i.e. the radius of the perovskite circular disk is 2.5 μm. The

boundaries are set at 15 μm (30 μm) away from the center of the disk in the vertical

(lateral) direction, which is far enough to be taken as infinity. Temperatures along the

boundaries keep constant at 25 ℃ during the simulation. The laser spot is large

enough (~ 21 μm) to cover the whole NP. So, the heating induced by nonradiative

recombination is approximately uniform in the CsPbI3 NP.9 We assume that the pump

power intensity is constant at 200 kW/cm2 during the 4.5 ns pulse duration. Since both

hBN and mica are almost transparent, the absorption can be ignored. We assume that

the energy of the excitation photons will either be converted into the energy of the

emitted photons or into heat.

The relation between heating power intensity 𝑃𝑃ℎ and pump power intensity 𝑃𝑃𝑝𝑝= 200

kW/cm2, can be taken as:

𝑃𝑃ℎ = 𝑃𝑃𝑝𝑝 �1 − 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 ∙ 𝜆𝜆𝑝𝑝𝜆𝜆𝐸𝐸� (6)

where PLQY is about 8.3% for CsPbI3 NP.8 λp = 532 nm and λE ~ 700 nm are the

wavelengths of excitation laser and the PL emission from the CsPbI3 NP, respectively.

Here, we assume the absorption to be approximately 100% (with absorption

coefficient α ≈ 1.5×105 cm−1 at 532 nm10 and CsPbI3 NP’s thickness of 177 nm). The

mesh for calculation and time steps are set to be identical for the CsPbI3 before and

after hBN capping.

Thicknesses of different layers are given in Figure S3b. The parameters of hBN

capping layer, CsPbI3 NP, and muscovite mica substrate are listed in Table S1. An

anisotropic thermal conductivity of hBN is applied, which is much higher than those

of the air and CsPbI3. Thus, the thermal dissipation is largely improved after hBN

capping, and the clear distinction can be seen in the main text (Figure 3a)

The heat flux (at time of 50 ns) inside the structure before (Figure S11a) and after

(Figure S11b) hBN capping, with arrows depicting the magnitude and direction,

clearly shows that the heat dissipates mostly through hBN layer and more rapidly

after hBN capping. Before hBN capping, thermal dissipation is mainly through the

CsPbI3 crystal and mica substrate, since the thermal conductivity of air is very low.

There is a large portion of heat passing through the CsPbI3 NP, increasing the

temperature of the material. However, after hBN capping, heat flux is mostly confined

inside the highly anisotropic thermal conductive hBN layer, with the in-plane

component of the heat flux larger than the out-of-plane component. Since there exists

a new and effective pathway for thermal dissipation after hBN capping, heat conveyed

into the CsPbI3 NP is reduced enormously, leading to a slower temperature increase

and a faster cooling process inside the CsPbI3 NP.

Figure S11. The heat flux (at time of 50 ns) inside the hybrid structure before and

after hBN capping. (a) Illustration of heat flux before hBN capping. (b) Illustration of

heat flux after hBN capping.

Section 10. More experimental results of lasing under high temperature

a. Lasing characterization under high temperature before and after hBN capping

of the same CsPbI3 NP

In order to further verify the effect of hBN capping on CsPbI3 NP lasing under high

temperature, we adopt two different samples. Both of them are measured before and

after hBN capping under identical excitation power range, with one sample measured

at 56.8 ℃ (Figure S12a) and the other measured at 60.7 ℃ (Figure S12b).

Figure S12. Lasing characterization under high temperature. (a) Normalized spectral

maps of a CsPbI3 NP before and after hBN capping measured at 56.8 ℃ under

identical excitation range. Before hBN capping, the sample can hardly lase. However,

clear lasing behavior can be seen from the sample after it is covered by an hBN flake.

(b) Spectral maps of the other CsPbI3 NP before and after hBN capping measured at

60.7 ℃ under identical excitation range. The amplified spontaneous emission appears

in the sample, after it is covered by an hBN flake.

When the temperature is set at 56.8 ℃, we can see a clear comparison between the

two spectral maps measured before and after hBN capping (Figure S12a). Lasing

behavior is observable after hBN capping. It is noted that the bare CsPbI3 NPs without

hBN capping layer can be damaged by laser irradiation to some extent at high

temperature. These damages, appearing as dark spot on the CsPbI3 NP’s top surface,

can only be discerned in optical images after hBN capping. When the temperature

reaches 60.7 ℃, most of the samples are so badly damaged during the first

measurement before hBN capping, that the lasing behavior can be hardly observed

even after hBN capping (Figure S12b).

b. Temperature-dependent lasing characterization with hBN capping

In the main text, the experiments about lasing under high temperature are done on a

selected sample from the previously measured sixteen ones. In the optical image after

hBN capping, no clear damage is found. In this case, a clear lasing behavior at 75.6 ℃

can be observed.

c. Failure of observation of lasing in three bare CsPbI3 NP at 60 °C

Although all-inorganic lead halide perovskites are reported to have better thermal

stability, the bare CsPbI3 NPs without hBN capping can hardly lase when temperature

reaches 60 ℃, even under a pump power intensity which is large enough to damage

the material (Figure S13a, b). In our three measured samples, the resonant peaks are

strongly suppressed and do not show any sign of the super-linear increase with the

input power. Notable, we observe two sharp peaks appearing on the short wavelength

side of the spontaneous emission (Figure S13c) with peak positions of 693.1 nm and

691.9 nm (Figure 13d) from all of these three samples. The peak positions do not shift

as pumping intensity and temperature increase. The origins of these two sharp peaks

are still unknown, which warrant future investigation.

Figure S13. Failure in lasing of three bare CsPbI3 NP at 60 °C. (a) Optical image of a

bare CsPbI3 NP before laser illumination. (b) Optical image of the damaged CsPbI3

NP after being pumped at 60 ℃ with pump intensity increasing but no lasing behavior

occurs. Length of the scale bar in both a and b is 10 μm. (c) Normalized emission

spectral maps of three different bare CsPbI3 NPs at 60 ℃. All these three samples

cannot lase. (d) Spectrum zoom-in with two odd peaks clearly observed.

Section 11. Encapsulation of CsPbI3 NPs by hBN flakes and its effective

protection from polar solvents and traditional device fabrication processes

We mechanically exfoliate a bottom hBN flake on a 285 nm SiO2/Si substrate, which

is pre-prepared by oxygen plasma cleaning for five minutes.16 The bottom hBN needs

to be large enough, guaranteeing large enough contact area with the top hBN flake to

form an encapsulation. A PDMS viscoelastic stamp is firstly pressed onto the

muscovite mica substrate with the grown CsPbI3 NPs on its top surface, and then

peeled off rapidly.2 In this way, some CsPbI3 NPs can be occasionally separated from

the mica substrate. Once a CsPbI3 NP is picked up by the stamp, we use a

micromanipulator to transfer it onto the bottom hBN flake prepared earlier (Figure

5a).1,2 The last step is to transfer the top hBN flake to cover the NP1 (see SI Section 2

and Figure 5b). The top hBN flake is the larger the better so as to fully cover the

entire structure.

As for lasing characterization in water, the SiO2/Si substrate, with the hBN

encapsulated CsPbI3 NP on its top, is adhered on a glass slide by a double-sided tape.

A water layer is maintained between the substrate and an upper cover slip. Air bubbles

are carefully removed (Figure S14). The encapsulated nanolaser is then continuously

pumped above lasing threshold for an hour, with emission spectra recorded

simultaneously. After that, we immerse the encapsulated CsPbI3 NP in water for 24

hours. The emission spectra from the sample is collected after being stored in water

for 3, 5, and 24 hours.

Figure S14. Experimental setup for the measurement of lasing in water.

Apart from water, we also do the lasing characterizations for the encapsulated hybrid

nanolaser (Figure S15a) in other polar solvents, including isopropanol, glycol, and 38%

dextrose in water (D38W). Clear lasing behavior is observed (Figure S15b) in all

cases, which further confirms the effective protection of hBN encapsulation against

polar solvents.

Figure S15. Lasing characterization of the encapsulated hybrid nanolaser in different

polar solvents. (a) Optical image of the encapsulated CsPbI3 NP. Scale bar: 10 μm. (b)

Lasing spectra of the sample in different polar solvents.

Perovskites’ instability in polar solvents is the major obstacle for their practical

applications. Here, an encapsulated CsPbI3 NP is put through a series of traditional

fabrication processes, including electron beam lithography, development,

metallization, and lift-off (Figure S16a). Clear lasing behavior is still observed from

the encapsulated CsPbI3 NP after all these processes (Figure S16b), indicating that the

hybrid structure can survive from complex device fabrication procedures. Our results

promise a bright future for the practical application of perovskite materials.

Figure S16. Lasing characterization of the hBN encapsulated CsPbI3 NP after

traditional device fabrication processes. (a) Optical image of the hBN encapsulated

CsPbI3 NP after device fabrication. The electrodes are fabricated through traditional

methods. Scale bar: 10 μm. (b) Optically pumped lasing spectrum of the sample after

the device fabrication.

References:

(1) Castellanos-Gomez, A.; Buscema, M.; Molenaar, R.; Singh, V.; Janssen, L.; van

der Zant, H. S. J.; Steele, G. A. 2D Mater. 2014, 1, 011002.

(2) Meitl, M. A.; Zhu, Z.; Kumar, V.; Lee, K. J.; Feng, X.; Huang, Y. Y.; Adesida, I.;

Nuzzo, R. G.; Rogers, J. A. Nat. Mater. 2005, 5, 33-38.

(3) Madelung, O. Semiconductors-Basic Data (Springer, Berlin Heidelberg, 1996).

(4) Berdiyorov, G. R.; Kachmar, A.; El-Mellouhi, F.; Carignano, M. A.; El-Amine

Madjet, M. J. Phys. Chem. C 2016, 120, 16259-16270.

(5) Ye, Y.; Wong, Z. J.; Lu, X.; Ni, X.; Zhu, H.; Chen, X.; Wang, Y.; Zhang, X. Nat.

Photonics 2015, 9, 733-737.

(6) Saxena, D.; Mokkapati, S.; Parkinson, P.; Jiang, N.; Gao, Q.; Tan, H. H.; Jagadish,

C. Nat. Photonics 2013, 7, 963-968.

(7) Ma, R.; Oulton, R. F.; Sorger, V. J.; Zhang, X. Laser Photonics Rev. 2012, 7, 1-21.

(8) Guo, P.; Hossain, M. K.; Shen, X.; Sun, H.; Yang, W.; Liu, C.; Ho, C. Y.; Kwok, C.

K.; Tsang, S.; Luo, Y. et al. Adv. Opt. Mater. 2017, 6, 1700993.

(9) Cadelano, M.; Sarritzu, V.; Sestu, N.; Marongiu, D.; Chen, F.; Piras, R.; Corpino,

R.; Carbonaro, C. M.; Quochi, F.; Saba, M. et al. Adv. Opt. Mater. 2015, 3,

1557-1564.

(10) Xiao, Z.; Meng, W.; Wang, J.; Mitzi, D. B.; Yan, Y. Mater. Horiz. 2017, 4,

206-216.

(11) Wang, Y.; Xu, L.; Yang, Z.; Xie, H.; Jiang, P.; Dai, J.; Luo, W.; Yao, Y.; Hitz, E.;

Yang, R. et al. Nanoscale 2018, 10, 167-173.

(12) Lee, W.; Li, H.; Wong, A. B.; Zhang, D.; Lai, M.; Yu, Y.; Kong, Q.; Lin, E.;

Urban, J. J.; Grossman, J. C. et al. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 8693-8697.

(13) Kovalsky, A.; Wang, L.; Marek, G. T.; Burda, C.; Dyck, J. S. J. Phys. Chem. C

2017, 121, 3228-3233.

(14) Hsieh, W. P.; Chen, B.; Li, J.; Keblinski, P.; Cahill, D. G. Phys. Rev. B 2009, 80,

180302.

(15) Krupka, K. M.; Robie, R. A.; Hemingway, B. S. Am. Mineral. 1979, 64, 86-101.

(16) Huang, Y.; Sutter, E.; Shi, N. N.; Zheng, J.; Yang, T.; Englund, D.; Gao, H.;

Sutter, P. ACS Nano 2015, 9, 10612-10620.