High-Density and Uniform Lead Halide Perovskite Nanolaser Array on

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High-density and Uniform Lead Halide Perovskite Nanolaser Array on Silicon Kaiyang Wang, Zhiyuan Gu, Shuai Liu, Wenzhao Sun, Nan Zhang, Shumin Xiao, and Qinghai Song J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01072 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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High-density and Uniform Lead Halide Perovskite Nanolaser Array on Silicon Kaiyang Wang†, Zhiyuan Gu†, Shuai Liu†, Wenzhao Sun†, Nan Zhang†, Shumin Xiao*,‡, and Qinghai Song*,† †

National Key Laboratory on Tunable Laser Technology, Department of Electrical and

Information Engineering and ‡ Department of Material Science and Engineering, Harbin Institute of Technology, Shenzhen, 518055, China. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Q. H. Song). *E-mail: [email protected] (S. M. Xiao).

ABSTRACT The realization of high density and highly uniform nanolaser arrays in lead halide perovskite is quite challenging, especially on silicon. Herein we demonstrate a simple way to form lead halide nanolaser array on silicon chip with high density and uniform lasing wavelengths. By positioning a perovskite microwire onto a silicon grating, only the suspended parts can hold high quality (Q) resonances and generate laser emissions. As the perovskite microwire is periodically segmented by the silicon grating, the transverse lasers are divided into a periodic nanolaser array and the lasing wavelengths from different subunits are almost the same. The transverse laser has

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been observed in an air gap as narrow as 420 nm, increasing the density of nanolasers to about 1250 per millimeter (800 nm period in experiment). We believe this research shall shed light on the development of perovskite microlaser & nanolaser arrays on silicon and their applications.

TOC Graphic

Lead halide perovskites have been intensively studied due to their potentials in high efficient solar cells.1 In just four years, the light conversion coefficient has been dramatically improved from a few percent to around 22.1%.2,3 In principle, the exceptional performances of lead halide perovskites in solar cells are attributed to their relatively long carrier lifetimes and carrier diffusion lengths. Interestingly, such unique properties are also important for the applications of perovskites as lasing materials.4,5 In past two years, a series of lead halide perovskite microlasers have been successfully demonstrated in microplates,6-11 microwires,12-14 and nanowires15-17 with either single-photon or two-photon excitation.12 Compared with the top-down etched devices, the synthesized microwires, nanowires, and microplates are usually dislocation free single crystals that have atomically flat facets. Consequently, record high Q factor (~ 3600), near unity quantum efficiency, and low threshold have been experimentally achieved by Zhu et al..15 Despite these excellent performances,18 the single crystalline lead halide perovskite devices still remain a major challenge in the realization of microlaser array with uniform lasing wavelengths, which are


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essential for practical applications such as high throughput bio-sensing, ultrahigh resolution imaging, and medical diagnosis.19-21 While the microwires and nanowires can be post-synthesis tailored with tungsten needle or optical tweezer,13,14 the assembled samples usually cannot function as microlaser array. This is because lasing actions in lead halide perovskite wires are strongly dependent on their morphological parameters, which are usually random numbers within particular ranges.12 Actually, the uniform wavelength of microlaser array is also a severe challenge for conventional semiconductor nanowire arrays, combs, and tetrapods.22-24 Recently, Feng et al. have successfully demonstrated the whispering gallery (WG) mode based microlaser arrays with uniform size and programmed location which are constituted of single-crystalline perovskite square microplates.25 In 2016, Wang et al. have reported the lasing actions in the transverse plane of lead halide perovskite microstructures.13 Compared with the axial Fabry-Perot (FP) modes, the transverse lasers have several intrinsic advantages. The light can be confined by total internal reflection in the transverse plane, then forming transverse WG modes. Thus higher Q factor (> 5000) and much narrower linewidth (~ 0.1 nm) can be simply achieved (unpublished results). Most importantly, the facets of single-crystalline microstructures are atomically flat and their transverse sizes are extremely uniform. In this sense, the wavelengths of transverse lasers at different locations should be very close, making lead halide perovskite microwire and microplates to be nice candidates for uniform microlaser or nanolaser array. Liao et al. have demonstrated similar laser array by cutting a J-aggregation microbelt into several transverse FP lasers by two-photon processing technique.21 However, as the refractive indices of semiconductors are always lower than silicon, this technique is hard to integrate the microlaser arrays onto silicon. And the subunits are usually around 10 m, which are too large for high density laser array. Therefore, it is highly desirable to find a simple



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way to generate uniform and dense lead halide perovskite based laser arrays. Herein we report the formation of uniform and high-density lead halide perovskite microlaser array on silicon chip.

Figure 1. (a) The dependence of Q factor on the refractive index of substrate (ns). The insets show the field patterns of transverse resonances in CH3NH3PbBr3 microwire in air and on AZ film with ns = 1.6. (b) The SEM image of the measured sample, where the scale bar is 20 m. The top inset shows the cross-sectional SEM image, where the scale bar is 2 m. The bottom insets are the fluorescent microscope images of transverse lasers on AZ film (left) and in air (right). (c) The experimental recorded laser thresholds of transverse lasers in CH3NH3PbBr3 microwire on different substrates with ns = 1, 1.66, and 3.46, respectively. (d) The emission spectra of the perovskite microwire on different substrates. Here the pump density is fixed at 7.1 J/cm2.

The key to our finding is the dependence of transverse resonances on the refractive index of substrate (ns). As the transverse shapes of microwires are usually close to squares, WG like modes can be simply formed by the coupling between FP resonances along x and y directions.26,27 When the microwire is suspended in air, the light along WG orbit can be totally reflected at four facets and thus form very high Q factors. Once the microwires are placed onto substrate, the light confinement in transverse plane shall be significantly changed. This can be clearly seen from the simulation results in Figure 1(a). When the microwire is suspended in the air, the transverse electric


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(TM) polarized WG resonance (see the field pattern in the inset of Figure 1(a)) has a Q factor more than 3.6×104. With the increase of ns from 1 to 1.6 (for AZ photoresist, see below), the Q factor is quickly spoiled to less than one thousand. The normalized electric field distributions of microwire in air and on AZ photoresist are shown in the insets of Figure 1(a), where the substrate induced light leakage can be clearly seen from the bottom inset. Further increasing ns to above 2.2 can introduce another dramatic reduction of Q factor, which relates to the broken of total internal reflection at microwire/substrate interface.28 From the dependence of Q factor on the ns as shown in Figure 1(a), it is easy to know that the laser threshold of microwire laser on substrate should be much higher than the one that is suspended in air. This information has been experimentally verified. Here the CH3NH3PbBr3 microwire and microplate were synthesized by solution processed precipitation method12 (see details in Figure S1 in Supporting Information (SI)). The synthesized CH3NH3PbBr3 microwire and microplate were positioned onto different substrates via micromanipulation.13,14 Taking a microwire as an example, the emission properties were characterized by optical pumping with a Ti: Sapphire laser (100fs, 1k Hz repetition rate, see schematic picture in Figure S2 in SI). All the results are shown in Figure 1. The SEM image of transferred perovskite microwire is shown in Figure 1(b), where the left part of the microwire is positioned onto a 1.8 m thick AZ photoresist film and the right part is suspended in air. The fluorescence images of partially pumped microwire above threshold are shown as insets in Figure 1(b), where the pump area is marked by the dashed box. When the pump density is the same, we can see that the suspending part exhibits much brighter emission than the left part on AZ photoresist. Figure 1(c) shows the log-log plot of the integrated intensity as a function of pump density on different substrates, where the “S” curve indicates the lasing thresholds of 5.9 J/cm2 and 6.8 J/cm2 in the air and on AZ photoresist,



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respectively. The 15% increase in lasing threshold and the reduced slope above threshold on AZ substrate results from the substrate induced leakage loss in Figure 1(a). In addition, when the microwire was transferred onto silicon substrate, no “S” curve can be observed due to the substrate induced strong leakages of modes in lead halide perovskite waveguide. Representative spectra at 7.1J/cm2 above both lasing thresholds are shown in Figure 1(d), where the microwire suspending in the air displays much larger emission intensity. Practically, such substrate dependent lasing actions can be simply employed to selectively excite the suspended subunits and to form a microlaser array.

Figure 2. Microlaser array on AZ grating. (a) top-view SEM image of the CH3NH3PbBr3 microwire on AZ grating, where the scale bar is 10 m. (b) Laser spectra of CH3NH3PbBr3 microwire at different pump density. The inset shows the fluorescent microscope image of laser spectrum. (c) The output intensity (dots) and linewidth (squares) as a function of pump density. (d) The polarization of CH3NH3PbBr3 microwire laser.

Based on above analysis and experimental results, we transferred a CH3NH3PbBr3 microwire onto an AZ grating (see the scanning electron microscope (SEM) image in Figure 2(a)) and studied its optical properties via optical excitation. The grating was fabricated with standard photolithography.29 It has a periodicity around 5 microns and the width of each AZ strip is about


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1.5 micron. The thickness of grating is similar to above AZ film (1.8 m). According to the SEM image, the length and width of transferred CH3NH3PbBr3 microwire are 36.01 m and 1.43 m, respectively. Due to the periodicity of AZ grating, the CH3NH3PbBr3 microwire has been separated by AZ strips into 7 subunits. Each subunit includes two parts of microwires that are attached to AZ strip and suspended in air, respectively. Figure 2(b) shows the recorded emission spectra at different pump density. Here only subunit-1 (marked by red dashed box in the inset of Figure 2(b)) is excited. When the pump power is low, the emission is a broad photoluminescence peak centered at ~ 542 nm (dashed line). And the output intensity increases slowly with a slope ~ 1.7 (see Figure 2(c)). When the pump density is above 6.5 J/cm2, a narrow peak with full width at half maximum (FWHM) below 1 nm appears at around 554.4 nm. The corresponding slope is dramatically increased to around 15.2. All these changes show the onset of lasing actions in the transverse plane. Figure 2(d) shows the polarizations of laser emissions. We can see that the emission is polarized perpendicular to the longitudinal axis of CH3NH3PbBr3 microwire, indicating the transverse electric (TE, E is in plane) polarization well. We note that both TE polarized and TM polarized laser emissions have been experimentally observed from suspended CH3NH3PbBr3 microwire. The inset in Figure 2(b) shows the fluorescence microscope image of CH3NH3PbBr3 microwire. Similar to previous reports,23 bright spots can be observed at the lateral facets of microwire in the pump area, whereas no emission can be observed at the two end-facets. This is consistent with the occurrence of transverse WG modes in CH3NH3PbBr3 microwire. Interestingly, the bright spots are well confined within the suspended region in subunit-1. While all of the suspended segments show bright spots on their sides, those regions attached to AZ strips are quite dark. This is consistent with the distinguished threshold in two regions and confirms that the grating can separate the CH3NH3PbBr3 microwire into discrete parallel light sources.



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Figure 3. The uniform of lasing wavelengths. (a) and (c) are the laser spectra from different single subunit. (b) and (d) are the laser spectra of two and four subunits. Here the pump density is fixed at 8.1 J/cm2.

For a uniform laser array, the most important characteristic is the uniform lasing wavelengths in different modules. We have also studied the laser emissions from different subunits by changing the pump area. All the results are summarized in Figure 3, where the insets of these figures are the corresponding fluorescence microscope images. Here the pump density is fixed at 8.1 J/cm2, which is above the lasing threshold. It is clear to see that all the laser spectra have the lasing modes at 554.2 nm. The lasing wavelength shift is smaller than 0.4 nm, which is close to the resolution limit of spectrometer (0.46 nm) and is negligible in practical applications. Importantly, the intensities of laser peaks from different individual subunit are also very close (see Figure 3(a) and (c)). This is another very essential characteristic for microlaser array. Figure 3(b) and (d) show the laser spectra of two and four segments simultaneously. We can see that the main changes occur at the output intensity. The lasing wavelengths and the FWHMs are still very close to the ones from individual subunit.


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The results in Figures 2 and 3 show the transverse lasing actions in lead halide perovskite microwire can be applied as laser array with uniform laser wavelength. This technique can be applied in a number of materials such as silica and polymer. However, the performances of such laser array are still limited due to the following reasons. i) The pump density must be carefully selected to be higher than the threshold of suspending part and lower than that attached to AZ stripe. Then the total outputs are restricted. ii) As the CH3NH3PbBr3 microwire can form waveguide mode on AZ, the FP modes along the axial direction can also be excited and compete with the transverse lasers (see Figure S3 in SI) when the entire sample is pumped. To solve above limitations, we consider transferring the CH3NH3PbBr3 microwire onto silicon, which is the most important platform for photonic devices. As the refractive index of silicon is higher than lead halide perovskite, no high Q resonances and waveguide mode can be formed in the transverse plane and along the axial direction of the microwire. This fact can be seen in Figure. 1(c) and (d). The microwire on silicon only shows the photoluminescence till it is damaged by the high pump power. Consequently, if a CH3NH3PbBr3 microwire or a CH3NH3PbBr3 microplate is transferred onto a silicon grating, the parts on the silicon strips cannot support lasing actions. Then the microlaser array that is based on the transverse lasers can also be formed in the air-suspending subunits. And it won’t support the axial FP lasers when the entirely microwire is excited.



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Figure 4. Microlaser array on silicon grating. (a) top-view SEM image of the CH3NH3PbBr3 microwire on silicon grating, where the scale bar is 10 m. The length, width and thickness of microwire are 33.85 m, 3.31 m and 1.34 m, respectively. (b) The lasing spectra at different pump densities. The inset shows the output intensity as a function of pump density. (c)-(e) are the laser spectra recorded from subunits 1, 1-2 and 1-3. (f) shows the laser spectrum of microwire that is entirely pumped. The insets in (c) –(f) are the corresponding fluorescent microscope images. The pump density in (c)-(f) is fixed at 12.25 J/cm2.

Figure 4(a) shows the top-view SEM image of the examined sample. It consists of a CH3NH3PbBr3 microwire and a silicon grating. The silicon grating is patterned by standard photolithography, which is followed by an anisotropic silicon etching with CHF3 and SF6 (SF6:CHF3=1:2, ICP power 1200W and RF power 30W). The width of the silicon strip and the period of grating are 1.95 m and 5 m, respectively. The etched depth is about 5.6 m. Following above researches, the microwire is divided by silicon grating into 5 subunits. Then the sample is optically excited by a Ti:Sapphire laser and its laser properties have been studied. With the increase


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of pump power, we can see that the broad emission peak transits to a narrow peak (see Figure 4(b)). Meanwhile, the slope of total output also increases from ~ 1.4 to more than 17, indicating the threshold behavior at ~ 10.34 J/cm2 well. Similar to Figure 3, the laser spectra at different subunits have also been analyzed. As shown in Figure 4(c)-Figure 4(f), the lasing wavelengths at subunits 1, 1-2, 1-3 and full subunits are almost the same. All these phenomena are consistent with the results on AZ gratings. The main difference happens in the case that the CH3NH3PbBr3 microwire is entirely excited. Owning to the strong loss of perovskites on silicon strip and their extremely flat facets, the lasing wavelength keeps almost the same as a single subunit and no FP lasers have been formed. In this sense, the CH3NH3PbBr3 microwire on a silicon grating can form an even better microlaser array.

Figure 5. Transverse lasing actions of CH3NH3PbBr3 microplate on silicon nanogaps. (a) The SEM image of CH3NH3PbBr3 microplate on nanogaps, where the scale bar is 10 m. (b) The enlarged SEM images of 500 nm (top) and 400 nm (bottom) gaps and their microscope image. The scale bar in (b) is 200 nm. (c) and (d) are the lasing spectra from microplate on 500 nm and 400 nm gaps, respectively. Here the pump density is fixed at 5.4 J/cm2. The corresponding fluorescent microscope images are shown as insets in (c) and (d).



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Compared with the formation of uniform microlaser array, the density of microlasers is also a crucial parameter for practical applications such as ultrahigh resolution imaging and diagnosis. In Figures 2-4, the lengths of each subunit, which includes the lasing area and separation strip, are both 5 m. This value is almost half of the one in organic microlaser array that is fabricated by two-photon cutting.21 But it is still too large and hinders the high-density microlaser or nanolaser array. To improve the integration density, we have fabricated a series of micro- and nano-slots on silicon wafer. Here the slots are defined with E-beam lithography (Raith E-line 30 kV, see Figure S4 in SI) and also etched with ICP. The width of the slots changes from 100 nm to 3000 nm with a step of 100 nm. With the decrease of gap width from 3000 nm to 400 nm, we can clearly see the bright spots at the side of microplate.30 In our experiment, we have measured transverse lasing actions from the CH3NH3PbBr3 microplate that is suspended on a narrow gap. The results are shown in Figure 5, where microplate on 400 nm and 500 nm gaps are studied (see SEM images in Figure 5(a) and (b)). Note that in practical experiment, broadening emerges, which results in the larger air gap (420nm). As shown in Figure 5(c) and (d), when individual subunits are excited above the threshold, the laser spectra taken from different positions are almost the same except the intensities. The laser characteristics and the bright spots on two sides of CH3NH3PbBr3 microplate confirm that transverse lasing actions happen on a gap as narrow as 420 nm. And the bright spots are well confined within the gap area, which is too small to be resolved in a typical microscope image (see Figure 5(d)). Therefore, considering its narrow gap width and tiny thickness, we know that a nanolaser has been generated by suspending a microplate or microwire onto a silicon based air gap.


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Figure 6. High density uniform perovskite microlaser array. (a) Top-view SEM image of the microplate on high density silicon grating, where the scale bar is 10 m. Here the period of grating is 802 nm and the width of air gap is 460 nm. (b) The microscope and fluorescent microscope images of the microplate on silicon grating that is pumped above threshold. Bright light spots periodically appear at each subunit. (c) and (d) show the corresponding lasing threshold curve and laser spectrum. The pump density in (c) is 5.4J/cm2.

Based on above the lasing action on 420 nm air gap, we thus can expect the high density microlaser (or nanolaser) array. To confirm this expectation, we have also fabricated a high density silicon grating with E-beam lithography and ICP etching. Then a CH3NH3PbBr3 microplate was transferred onto the grating and aligned well along direction perpendicular to the grating. Figure 6(a) shows the top-view SEM image of CH3NH3PbBr3 microplate on the grating. Here the width of air gap is fixed at 460 nm (400 nm in design) and the period is 800 nm. The length and width of the CH3NH3PbBr3 microplate are 28.56 m and 9.71m, respectively. When the sample is pumped above threshold (Figure 6(c)), two laser peaks can be observed in the emission spectra even if the microplate is entirely pumped (see Figure 6(d)). And this laser spectrum has the same lasing wavelength with the one taken from a single subunit. This confirms that the onset of transverse laser in CH3NH3PbBr3 microplate and also verifies their uniformity in lasing wavelengths. It should be noted that WG mode lasing from total internal reflection in the transverse



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plane is very sensitive to the cavity size and configuration. Thus the lasing wavelength and Q factor in Figure 2, 4, 5 and 6 are calculated based on finite element method in Figure S5 (see supporting information). The information of high density is shown in the corresponding fluorescent microscope image in Figure 6(b). While the whole sample is excited, we can still see the periodic bright spots along the sides of CH3NH3PbBr3 microplate. The period of the bright spots are the same as the period of silicon grating. Thus we know that the density of nanolaser array can be as high as 1250 per millimeter. In conclusion, we have studied the formation of high density and uniform microlaser and nanolaser array on silicon. Based on the distinguished laser thresholds of microlasers and nanolasers on different substrates, the transverse lasers of a single-crystalline CH3NH3PbBr3 microwire and microplate have been divided by a silicon grating into tiny subunit. And all the subunits produce the same lasing spectra owning to the high quality of the synthesized CH3NH3PbBr3 microwire and microplate. The smallest subunit period is 800 nm in our experiment, giving an integration density of nanolasers as high as 1250 per millimeter. We believe our finding shall be important for the development of lead halide perovskite nanolaser arrays on silicon and their applications. We note 800 nm is still not the ultimate limit of subunit, the integration density can be further increased by replacing the micromanipulation with more precise nano-manipulation.

ASSOCIATED CONTENT Notes


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The authors declare no competing financial interests. ACKNOWLEDGMENT This research is supported by National Nature Science Foundation of China under the Grant No. NSFC11374078. It is also supported by the Shenzhen fundamental research plan under the grant Nos. JCYJ20140417172417110, JCYJ20140417172417096. The authors also would like to thank the supports from Shenzhen Aerospace Micro- and Nano- Device Fabrication and Test Platform. Supporting Information. The solution processed synthesis; optical characterization setup; axial Fabry-Perot (FP) modes in microwire on AZ grating; the process of E-beam lithography; finite element method based simulation results. (word) REFERENCES (1) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506-514. (2) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim,Y. C.; Ryu, S.-C.; Seo, J.-W.; Seok, S. I. Highperformance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234-1237. (3) Research Cell Efficiency Records http://www.nrel.gov/ncpv (National Renewable Energy Laboratory, accessed 1 April 2016). (4) Xing, G. C.; Mathews , N.; Lim S. S.; Yantara, N.; Liu, X. F.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-temperature Solution-processed Wavelength-tunable Perovskites for Lasing. Nature Mater. 2014, 13, 476-480.



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High-Density and Uniform Lead Halide Perovskite Nanolaser Array on

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