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Applied Surface Science

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Full Length Article

Enhanced temperature-tunable narrow-band photoluminescence fromresonant perovskite nanograting

E.Y. Tiguntsevaa,1, Z. Sadrievaa,1, B.V. Stroganovb, Yu.V. Kapitonovb, F. Komissarenkoa,R. Haroldsonc, B. Balachandranc, W. Huc, Q. Guc, A.A. Zakhidova,c, A. Bogdanova, S.V. Makarova,⁎

a ITMO University, St. Petersburg 197101, Russia1b Saint-Petersburg State University, ul. Ulyanovskaya 1, Saint-Petersburg 198504, RussiacUniversity of Texas at Dallas, Richardson, TX 75080, USA

A R T I C L E I N F O

Keywords:Halide perovskitesOptical resonancesNanogratingPhotoluminescenceNanoimprint

A B S T R A C T

Tunable light-emitting nanostructures are prospective for reconfigurable compact optoelectronic devices. In thisregard, halide perovskites are one of the most efficient class of materials, because of their outstanding electronicand optical properties as well as low cost of fabrication and nanostructuring. Here, we study the temperature-tunable reconfiguration of photonic modes in periodically nanostructured perovskite (MAPbI3) film probed byenhanced photoluminescence. We achieved the quality factors of resonances around 500 at wavelengths750–760 nm. The experimental results are in well agreement with theoretical simulations. Our study reveals theorigin of the emission enhancement from such kind of perovskite nanostructures and paves the way to highlydirectional, efficient, and tunable narowband emission from optoelectronic devices.

1. Introduction

Active and tunable nanophotonics has attracted considerable at-tention during last years [1]. Reconfigurable subwavelength light-emitting systems with enhanced characteristics have high potential inoptoelectronics. Previously, resonant nanogratings [2,3] were em-ployed for photoluminescence enhancement from dyes [4], quantumdots [5], and 2D materials [6,7]. Moreover, light emission enhancementvia nanogratings were applied for organic–inorganic (hybrid) per-ovskites, which is a novel rapidly developing class of semiconductormaterials, owing to its outstanding photovoltaic [8,9] and light-emit-ting properties [10]. The hybrid perovskites support excitonic states atroom temperature and their refractive indices is about 2.5 that is en-ough for the efficient excitation of the resonances at optical frequencieseven in subwavelength nanoparticles [11,12]. Perovskites have highdefect tolerance [13] and high quantum yield [14] of photo-luminescence (PL) as well as high optical gain at room temperature[15]. Therefore, the various nanostructures [16] and nanogratings[17–21] made of halide perovskites demonstrate excellent emissionproperties. Since the electrically pumped lasing in resonant perovskitenanostructures was not achieved yet, it is highly desirable to getspectral narrowing of PL in a threshold-less regime for displays with

extremely pure colors [22].On the other hand, that periodic nanostructuring is known to result

in photonic bands formation [23]. Therefore, PL spectrum of the na-nostructure is modified according to the dispersion ω k( ) of the parti-cular eigenmode [24,25] and it becomes angular dependent. Also, theluminescence spectrum of perovskites has strong temperature depen-dence because of the phase transition [26–28]. It means that dependingon the temperature one can get enhancement of luminescence to thedifferent directions governed by the modes structure. However, in mostof the experiments with perovskites mentioned above, the angular re-solution was relatively weak making it difficult to distinguish con-tributions from different narrow resonances to the PL enhancement.Therefore, high-resolution angular dependences of the PL enhancementwould shield light on physics of light emission from resonant perovskitenanogratings, especially, when they support high-Q modes [29].

In this paper, we study the angular and temperature dependences ofthe enhanced PL from halide perovskite (CH3NH3PbI3 or MAPbI3) na-nograting, fabricated by means of high throughput nanoimprint tech-nique. We show that the gratings demonstrate the narrow spectralemission (less than 2.5 nm that corresponds to quality factor Q ≈ 500)even without lasing. Such a high quality factor is originated from thecoupling of the emitted light with resonant states of a shallow grating,

https://doi.org/10.1016/j.apsusc.2018.12.084Received 12 September 2018; Received in revised form 6 December 2018; Accepted 10 December 2018

⁎ Corresponding author.E-mail address: s.makarov@metalab.ifmo.ru (S.V. Makarov).

1 The authors made equal contribution.

Applied Surface Science 473 (2019) 419–424

Available online 15 December 20180169-4332/ © 2018 Published by Elsevier B.V.

T

which is fabricated by means of high throughput nanoimprint tech-nique. The enhanced emitted light is linearly polarized and highly di-rected. We demonstrate that the observed emission is temperature-tunable due to sharp phase transition around 130 K from tetragonal toorthorhombic phases of MAPbI3 [30]. The proposed concept of tem-perature-tunable PL enhancement from a perovskite nanograting isshown in Fig. 1.

2. Sample fabrication

2.1. Films fabrication

The perovskite (MAPbI3) thin film was fabricated by solvent en-gineering method [31]. Thickness of thin film after preparation isaround 500 nm. Solution of perovskite precursor (MAPbI3) is preparedin a dry-box in the following way: methylammonium iodide (MAI) inGBL:DMSO with the concentration of 1.2 M is used to dissolve PbI2(1.2 M). The solution was stirred and heated (70 °C) overnight and usedafter filtration through 0.45μm PTFE syringe filter. Depositing per-ovskite layers is performed by solvent-engineering technique inside thedry-box. Silicon substrates are washed with sonication, in deionizedwater, toluene, acetone, and isopropanol, consecutively. Spin-coating ofMAI+ PbI2 solution runs in two steps at 1000 rpm and 5000 rpm. Inthe middle of second step toluene are dripped on top of a rotatingsubstrate. Each film is annealed at 100 °C for 10min.

2.2. Nanostructuring of perovskite thin films

Generally, there are different methods of thin film perovskite na-nostructuring: focused ion beam lithography [32,33] with removingpart of material from surface, electron beam etching [34,35], nanoim-print lithography with temperature and pressure control [36]. Na-noimprint lithography method allowed to obtain nanostructures atlarge scale (from cm-scale and bigger) easily and rapidly, which de-monstrated photoluminescence enhancement at room temperature[19], and photodetectors efficiency enhancement [36].

In the experiments, the perovskite thin film is structured by thenanoimprint lithography. The commercial Si mold with period 600 nmand ridges height 270 nm is first treated with 1H,1H,2H,2H-per-fluorodecyltrichlorosilane (FDTS) in N-heptone solvent for 5min andthen cleaned with acetone and blow-dried with N2. The mold is thenannealed at 100 °C for 20min. Monolayer FDTS is deposited on the Simold, which served an antiadhesive layer in the nanoimprint process.The Si nanostripe mold is then placed on the perovskite thin-film-coated substrate at different areas in a single process. The imprint re-presents a multistep process of 90 s at a temperature of 35 °C and a

pressure of 2MPa, 180 s at a temperature of 55 °C and a pressure of4MPa, 180 s at a temperature of 75 °C and a pressure of 6MPa, andthen 1200 s at a temperature of 100 °C and a pressure of 7MPa. Thepressure is kept at 7MPa, while the chamber is cooled to a temperatureof 35 °C. The nanoimprint process is then finished, and perovskite na-nograting is formed with negative replication of the Si molds.

The perovskite films and nanoimprinted structure were visualizedand inspected using a field-emission scanning electron microscopy(SEM, JEOL, Cross Beam). The SEM images of obtained nanogratingsare showed in Fig. 2. The geometrical parameters of the nanostructureswere estimated from the SEM images taken for several different anglesof view: the height of pedestal is around 410 nm, the height of ridges isaround 270 nm, and the period is around 600 nm.

3. Optical measurements

3.1. Photoluminescence measurements

The measured at room temperature PL spectra of smooth and im-printed perovskite film are shown in Fig. 3a. The film was excited by λ= 530 nm with spectral width 10 nm generated from filtered super-continuum source (repetition rate 80MHz, pulse duration 7 ps, modelFianium SC 400) with average power 10 mW. Signal collected from thetop of sample by an 50× objective (Mitutoyo M Plan APO NIR,NA=0.42), sent to Horiba LabRam HR spectrometer, and projectedonto a thermoelectrically cooled charge-coupled device (CCD, AndorDU 420A-OE 325) with a 150 g/mm diffraction grating.

As shown in Fig. 3(a), the presence of nanostructuring surface leadsto 3-fold PL enhancement. These results on PL enhancement at roomtemperature are in a good agreement with previous work [19]. Also,one can observe slight reshaping of the red wing of the PL line, which isusually related to amplified spontaneous emission (ASE) from per-ovskite, possessing high optical gain (in range 200–3200 cm−1 [16]).Indeed, the intensity in our experiments is in range 0.001–1 kW/cm2,which is lower than for lasing at cw-irradiation 10 kW/cm2 [30], butmight be enough to compensate material losses.

3.2. Temperature dependence

The temperature dependence measurements were obtained in thefollowing way. The samples were placed in cryo-chamber with closedcycle allowing for temperature-scan from T=300 K down to 8 K. ForPL measurements, we used a CW-laser with λ =641 nm, power up to7400 μW, focused by 100mm lens. The Beam diameter on the samplewas approximately 28μm. The PL signal was collected by the same lensand filtered by an IR-filter (”KS-19”, block wavelengths λ < 700 nm),sample was placed normally to the lens optical axis.

At the temperature range from 12 K to 130 K at two polarizations,we observed two narrow peaks with the width less than 2.5 nm in PLspectra from the nanograting [Fig. 3(b)]. This spectral width is com-parable with some of previously reported lasing designs [37,38].Moreover, these spectrally narrow peaks are cross-polarized, whereasthe regular broader peak of PL is not polarized at all. The achieved PLenhancement at these peaks is up to 7 times relatively to the PL signal

heig

htperoid

λ(T)

α(T)

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Fig. 1. Schematic illustration of the nanoimprinted perovskite grating andprinciple of temperature-dependent emission.

(a) (b)

1 μm1 μm

Fig. 2. SEM images of a perovskite nanograting on a glass substrate at an angleof 90 degrees (a) and 45 degrees to surface (b).

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from the smooth perovskite film.The perovskite MAPbI3 is a hybrid material with a phase transition

from tetragonal to orthorhombic phases [26–28] around T=150 Kduring the decreasing temperature from room temperature to liquidhelium one [Fig. 3(c)]. It is clearly seen in Fig. 3(c) that the tetragonalphase (approximately >150 K) in MAPbI3 gives rise to the emissionpeak around 780 nm, whereas orthorhombic phase (<150 K) exhibitemission peak around 750 nm, which yields the low-temperature ASEthreshold at a fluence of 10μJ/cm2 under femtosecond laser excitation[15], and around an intensity of 10 kW/cm2 at cw-laser irradiation[30]. The red shift of emission wavelength such as in perovskite withincreasing temperature (from 12 to 160 K) is related to phase transition[27]. In our experiments, the sharp appearance of two peaks around130 K [Fig. 3(d)] is related to the phase transition in MAPbI3.

Observation of the same phase transitions in perovskite crystals [39]and smooth perovskite films means that spectral changes are related tothe material properties rather than optical modes reconfiguration in thenanograting. Regarding ultrafast emission dynamics, we observed dif-ferent decay times for tetragonal and orthorhombic phases: from 4down to 0.25 ns, respectively. The temperature dependence and otherdetails for PL decay see in Supporting information, Fig. S4.

It is worth noting, that the two narrow peaks in PL spectra[Fig. 3(b,d)] are unlikely to be lasing, due to the absence of thethreshold intensity in the output power growth and spectral narrowing,as shown in Fig. 3(e,f) (for details, see Supplementary information Fig.S3). At intensities approaching 1 kW/cm2, the temperature of the per-ovskite film increases, and narrow peaks disappear at higher intensities,as shown in Fig. 3(f). In comparison, the nanograting made of the samematerial but with the other period (500 nm) does not demonstrate suchnarrow lines in PL spectrum (see Fig. S2 in Supplementary informa-tion), owing to the absence of high-Q resonances within the PL spectralband.

3.3. Angular dependence

In order to prove the photonic origin of the observed two narrowpeaks at low temperature, we carried out the multiple measurements of

intensity, angular, and temperature dependencies for the both TE andTM polarizations of light emission from the resonant perovskite nano-grating. To do this, we put analyzer in the collection channel of theoptical scheme (for details, see Supporting information Fig. S1). Thisallows us to experimentally reconstruct dispersions of the optical modesin the nanogratings [Fig. 4(a,b)]. Namely, one can see that the dis-persion branches for TM- and TE-polarizations are slightly separatedfrom each other ( ≈EΔ 1meV approximately), that is discussed in nextsection.

In Fig. 4(a), the intensity dependence exhibits considerable tuningof the emission lines at various angles and polarizations. The reason ofslight tuning of energy (wavelength) value is temperature increasingdue to local heating of rise pump density fluence. Fig. 4(b,c) confirmthis assumption, where PL lines are shifted strongly by varying tem-perature from 4 K up to 140 K. The spectral width of these two peaksalmost does not change with the temperature increase from 4 K up to100 K, being around 2–3meV, and increases around 150 K by 1meV.This spectral broadening correlates with some saturation of the spectralshift at temperatures around 100–140 K, as shown in Fig. 4(c). One canalso see that at the fixed PL wavelength, changing of temperature re-sults in steering of PL angle. Thus for photon energy 1.64 eV, thecooling of perovskite nanograting from 140 to 60 K results in thechange of PL angle from around 1 to 8 degree, as can be estimated fromFig. 4(c).

All aforementioned optical effects originate from the hypothesis thatthe narrow polarized peaks in PL are the parts of spectrally broad PLemission enhanced at certain wavelengths and polarizations owing tothe excitation of planar waveguide modes in the perovskite layer withshallow nanograting. To prove the hypothesis on the waveguide originof the narrowband PL enhancement, we carried out full-wave numericalsimulations of eigenmodes in the perovskite nanogratings.

4. Simulations

According to previous studies of PL enhancement by nanogratings,there are two main contributions [24]. Photonic density of states is akey parameter in the Fermi‘s golden rule, which affects the efficiency of

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Fig. 3. Experimental study of photoluminescence spectra. PL spectra from the areas of a thin MAPbI3 film with nanograting (red) and without (black) at roomtemperature 300 K (a) and at 12 K (b). PL temperature maps for the plane perovskite film (c) and for perovskite film with nanograting (d). (e) Intensity dependence ofPL spectra for the perovskite film with nanograting. (f) PL intensity as a function of incident CW-laser power from 4.7μW to 7400μW.

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PL. On the other hand, the grating period governs the direction (angle)of the emitted light outcoupling, making it stronger at some particulardirections. Therefore, to identify the luminescence peaks observed inthe experiment and to explain the selective enhancement of the PLsignal, we calculated the band structure of the grating, Q-factors andelectromagnetic field distribution of the grating modes.

The perovskite grating design for theoretical study is shown inFig. 5(a). The simulations were carried out in COMSOL Multiphysics

software realizing finite element method. We calculated eigenmodes ofthe grating using Electromagnetic Wave module with Frequency Do-main Interface. The eigensolver allows us to calculate complex fre-quencies = +f f if1 2, field distribution of the eigenmodes and Q factors

as =Q ff21

2. The geometric parameters of the grating used in the simu-

lations are shown in Fig. 5(a). The parameters were extracted from theSEM images (see Fig. 2). As far as material losses in MAPbI3 perovskite

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T= 4K T= 4K

P= 500 μW

4K100K130K140K

P= 500 μW

Fig. 4. Experimental dependence of photoluminescence. Dependence of PL photon energies on angle of signal collection for two polarizations (TE and TM) atdifferent laser pump powers at 4 K (a) and temperatures (b). (c) Dependence of PL photon energies on temperature at different angles of signal collection andpolarizations.

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are neglectable at the wavelength around red wing of the exciton, wedid not take the imaginary part of the refractive index into account,while the real part is taken as =n 2.5 [40,41]. In order to estimate theeffect of a substrate on the spectra of the grating we consider thegrating with and without substrate [see Fig. 5(a)].

The band structure of the grating without substrate is shown inFig. 5(c). As the grating supposed to be infinite along the trenches thespectrum is divided into TE- and TM-polarized modes. In the center ofthe first Brillouin zone (Γ-point), the incident angle is zero and both TMand TE modes are divided into odd and even with respect to the verticalplane of the symmetry of the unit cell. The distribution of the electricfield amplitude for even and odd TE and TM modes in the Γ-point isshown in Fig. 5(e). In virtue of the symmetry, the odd modes have zeroradiation losses and, therefore, infinite radiation quality factor. Suchmodes are known as optical bound states in the continuum (BICs) [42].In real systems due to material losses and roughness, their Q factorbecomes finite [43]. In our case, the role of the substrate is essentialsince it opens diffraction channels and destroys BICs [43]. Nevertheless,the Q-factors remain quite big. The spectrum of the grating on a glasssubstrate is shown in Fig. 5(d). One can see that the substrate results ina slight blue-shift of the resonances. It can be shown easily from thediffraction condition that in order to avoid leakage into the glass sub-strate (n=1.5) at wavelength about 760 nm, the period of the gratingshould not exceed ≈ 500 nm, as the shorter periods will close the alldiffraction channels into the substrate. Tuning other geometricalparameters one can obtain the resonant state at the excitonic wave-length.

To improve theoretical results and define the geometric parametersof the sample we used the planar waveguide approach. It is assumedthat the planar slab waveguide is made from perovskite material, thethickness should not exceed approximately 600 nm. The results areshown in Fig. 5(b). For the effective waveguide thickness equal to500 nm and dielectric function correction by 5 percent, we redefine thegrating depth as about 171 nm. It is hardly surprising that the obtainedgrating depth is less than the stamp‘s depth, because the fabricationprocess does not allow to fully control the depth. One can see that thefrequency gap for TE and TM modes in the Γ-point is quite small. It

means that periodic potential is weak and the grating could be consideras a shallow one. Indeed, Fig. 5(b) shows the spectrum of the gratingunder the assumption that the height of the grating’s ridge is vanish-ingly small – the model of the empty lattice. One can see that such arough approximation gives adequate position of the resonances.

5. Conclusion

In conclusion, we have demonstrated polarized and narrowband PLfrom perovskite nanograting enhanced by two waveguiding modes (TEand TM) with high Q-factors (up to 500) in the nanoimprinted struc-ture. The measured temperature and angular dependencies show sharpPL enhancement (up to 7 times) at the resonant modes in the nano-grating owing to the phase transition in MAPbI3 around 130 K.Thresholdless appearance of the narrow peaks makes this design ap-plicable for the low-intensity applications. Our study helps to designoptimized surface structures for enhanced outcoupling from perovskite-based light-emitting devices.

Acknowledgments

We are thankful to Y.Kivshar and M. Franckevicius for useful dis-cussions. The theoretical part of this work was partially supported byRussian Foundation for Basic Researches (17-03-00621). The experi-mental parts and numerical simulation were supported by the RussianScience Foundation (17-12-01581). A.Z. also acknowledges a partialsupport from the Welch Foundation (grant AT 16-17). E.T. also ac-knowledges Russian Federation President Scholarship for YoungScientists. This work was carried out using equipment of SPbU resourcecenter “Nanophotonics”.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in theonline version, at https://doi.org/10.1016/j.apsusc.2018.12.084.

Fig. 5. Numerical calculations. (a) Schematic drawing of a perovskite slab with one-dimentional periodicity. (b) Band structure of the grating calculated by planarwaveguide approach. Calculated band structure of grating (c) and (d) grating on glass substrate. (e) Near-filed distributions for the modes in the nanograting at theΓ-point without a glass substrate.

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