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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Photoluminescence Manipulation by the Ion Beam Irradiation in CsPbBr Halide Perovskite Single Crystals 3

Vsevolod I. Yudin, Maksim Lozhkin, Anna V. Shurukhina, Alexei V Emeline, and Yury V. Kapitonov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04267 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 6, 2019

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The Journal of Physical Chemistry

Photoluminescence Manipulation by the Ion Beam Irradiation in CsPbBr3 Halide Perovskite Single Crystals Vsevolod I. Yudin, Maksim S. Lozhkin, Anna V. Shurukhina, Alexey V. Emeline, Yury V. Kapitonov*.

Saint-Petersburg State University, ul. Ulyanovskaya 1, Saint-Petersburg 198504, Russia

ABSTRACT

Halide perovskites are promising materials for optoelectronics with an attractive radiation resistance property. In this article, we study the effect of 30 keV Ga+ ions irradiation on the photoluminescence (PL) of CsPbBr3 halide perovskite single crystal. The high crystal

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quality and liquid helium temperature studies make it possible to distinguish the radiationdefect-related PL band. A model to explain the band shifts with radiation dose and pulsed pump intensity has been developed. Bright defect PL allows to visualize the irradiated areas using PL mapping. Manipulation of optical properties using focused ion beams opens wide opportunities for halide perovskites nanofabrication for optoelectronics.

Introduction

Halide perovskites, having burst onto the scene in photovoltaics several years ago1, turned out to be an extremely suitable materials for optoelectronics due to the their high photoluminescence quantum yield2,3, optical amplification ability4,5, and rich possibilities for tuning their optical properties by manipulating their composition6 and dimensionality7. A further necessary step towards the creation of optoelectronic devices based on halide perovskites is the miniaturization and integration of halide-perovskite-based functional elements into photonic circuits, which requires a development of suitable


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nanofabrication methods. One such highly developed method is lithography using Focused Ion Beams (FIB). The most widely used ion sources provide 30 keV Ga+ ion beams focused onto 10 nm spots8 and 35 keV He+ ions focused onto sub-nanometersize spots9,10. Despite the convenience of such ion beams for nanolithography, their application in optoelectronics is rather limited, since in traditional semiconductors such as silicon and gallium arsenide, ion-solid interaction results in the creation of defects serving as non-radiative recombination centers. Their generation leads to the deterioration of both electronic properties, e.g, the carrier mobility decay11,12, and optical properties, e.g. PL quenching13 and exciton resonances broadening14. The examples of successful applications of ion irradiation for optoelectronics include quantum well intermixing15 and the fabrication of resonant diffractive optical elements16.

Recent studies have shown that unlike traditional semiconductors, halide perovskites have high resistance to radiation17,18. In these studies, it was demonstrated that the PL signal is maintained up to 30 keV Ga+ ion irradiation doses leading to the sample milling


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(~1016 cm−2). Presumably, this behavior is caused by the defect tolerance of halide perovskites19 and their self-healing behavior20.

In this work, we will establish the role of radiation-induced defects in the optical properties of the inorganic halide perovskite CsPbBr3. For this, we use the maximally perfect single crystals grown without organic solvents, and conduct optical experiments at liquid helium temperatures. Such a combination allows us to determine the influence of defect-related states on the photoluminescence with great accuracy, and to develop the corresponding theoretical model describing the observed optical effects.

Experimental

Crystal growth

CsPbBr3 halide perovskite single crystals were grown from a solution using concentrated acid: 0.6 g (2.8 mmol) of CsBr and 1.03 g (2.8 mmol) of PbBr2 were


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dissolved in 10 ml of concentrated hydrobromic acid HBr. Solution was left at room temperature for 16 hours until single crystals were obtained. The resulting crystals were filtered, washed with ethanol and stored in a dry atmosphere. Formation of the single CsPbBr3 perovskite phase was confirmed by the X-ray diffraction (see Supporting Information).

Ion beam irradiation

CsPbBr3 single crystal was irradiated by 30 keV Ga+ ions at room temperature in the Zeiss Crossbeam 1540XB workstation combining a Scanning Electron Microscope (SEM) and a Focused Ion Beam (FIB) column. For controlled exposure, an external scan generator Raith ELPHY plus and an electrostatic beam blanker were used. The irradiated pattern was an array of 200×200 μm squares with 200 μm gaps between squares and uniform irradiation doses from 1010 to 1014 cm−2. At the lowest doses, the 200 pA ion beam dwell time was set equal to 0.17 ms, and the dose change was


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achieved by changing the raster spacing. To homogenize the irradiation, the ion beam was defocused to a spot with diameter of the order of hundreds of nanometers. Largest doses were obtained by increasing the dwell time. For the exposure of the grating ion beam was focused to 20 nm spot. Maximum applied doses are two orders of magnitude lower than doses leading to the sample milling by 30 keV Ga+ ions18.

Photoluminescence study

To measure the low-temperature photoluminescence (PL) and reflection, the single crystal was mounted in a Montana Instruments closed-cycle helium cryostat and cooled down to 4 K. Photoluminescence was pumped using a Ti:Sapphire laser Spectra Physics Mai Tai HP. Laser pulses with duration of 100 fs, 80 MHz repetition rate and 800 nm wavelength were converted to 400 nm using BBO crystal. The laser beam was focused on the sample using 10x Mitutoyo microobjective in a 10-μm-diameter spot. The same objective was used to collect luminescence into a spectrometer with a CCD-


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detector. The reflection spectrum was measured using the same optical pathway by a halogen lamp with a longpass filter. PL mapping was carried out by the illumination of the sample by a 450 nm cw-laser and collecting PL by a color CMOS-camera with different longpass filters.

Theoretical calculations

To simulate ion scattering and defect formation, the Monte Carlo method implemented in the SRIM 2013 program21 was used. Modeling by this method does not take into account the crystal structure of the material and the ion channeling. We model a beam of 30 keV Ga+ ions (68.93 amu) hitting the 200-nm-thick CsPbBr3 target normal to the surface. The density of the material was taken to be 4.856 g/cm3 22. The target consisted of 20% Cs (132.91 amu), 20% Pb (207.19 amu) and 60% Br (79.904 amu). For all elements, the lattice binding energy was set to 3 eV and displacement energy was 25 eV. Detailed calculations with full damage cascades were performed for 106


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incident ions. Sputtering effects were not considered. Based on the simulations, the vacancy generation yields V(d) (Vacancies/(Angstrom·Ion)) for Cs, Pb and Br atoms at a given target depth d were obtained. Multiplying this value by the layer thickness and the irradiation dose provides the vacancy density in the layer at the given depth d. Nuclear energy loss predicted by the method used Ref.21 is 780 eV/nm. Corresponding projected range of Ga+ ions is 24 nm.

Results and discussion

Crystal growth in the absence of organic solvents provides very homogeneous CsPbBr3 single crystals of high optical quality. To minimize the effects of temperature broadening of resonances, all optical studies were carried out at a temperature of 4 K. Figure 1(a) shows the normal reflection and photoluminescence (PL) spectra of the non-irradiated sample area. A free exciton (FE) resonance is observed in the reflection spectrum around 2.335 eV. The large linewidth and the resonant reflection coefficient up to 0.7


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indicate large oscillator strength of the free exciton resonance. PL spectra are governed by a bound exciton (BE) emission red-shifted by 10 meV with respect to the free exciton, with the full width at half maximum of 1.4 meV estimated by the high-energy edge of the emission line. Excitonic nature of this emission line was identified also in Ref. 23 and 24. Such narrow linewidth and the very weak defect-related luminescent tail at lower energies confirm the high optical quality of the crystal. Similar spectral properties were observed over the entire sample several millimeters in size, making it a good starting point for the ion irradiation effect studies.

The sample was irradiated by Ga+ ions with 30 keV energy. The Monte-Carlo method was used to simulate the interaction of ions with the CsPbBr3 target. For such ion energies, secondary cascades play an important role, so they were taken into account in the simulation. For each primary gallium ion, about 600 vacancies are generated in the sample. Thus, the implantation of gallium atoms could have a smaller role on the optical properties of the sample. Figure S4 (see Supporting Information) shows the distribution of implanted gallium ions. Around 90% of the ions are absorbed in the layer


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50 nm thick, which corresponds to a maximum concentration of gallium atoms of about 1019 cm−3 at the maximum irradiation dose 1014 cm−2. In Figure 1(b) a vacancy generation yield V as a function of the layer depth d is shown for different atoms of the target. The results demonstrate that due to the small atomic mass, the number of defects associated with Br atoms is several times higher than for Pb and Cs atoms. In our experiments, the depth of the defects formation is comparable with the light absorption depth, which allows us to apply photoluminescence for monitoring the alternation of the optical properties of the sample after formation of radiation-induced defects.


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Figure 1. (a) Normal reflection KR (red) and photoluminescence PL (blue) spectra from the non-irradiated sample at 4 K. (b) Calculated vacancy generation yield V as a function of the layer depth d for cesium (blue dotted), lead (red dashed) and bromine (green solid curve) atoms.

We have studied the effect of uniform 30 keV Ga+ ion irradiation doses D ranging from 1010 to 1014 cm−2, which are two orders of magnitude lower than the milling doses for the same ion species18. For PL pumping we used femtosecond laser pulses, which made it possible to study PL spectra in a wide range of excitation densities.

Figure 2(a) shows the dependence of PL spectra on the irradiation dose D at a fixed pump intensity 100 μW. Ion irradiation of the sample leads to the appearance of a band lower in energy than the BE peak. As the dose increases, this band broadens and experiences an effective red shift. At the same time, the integrated PL intensity decreases only slightly, mainly due to the quenching of the BE peak. Figure 2(b) shows the PL spectra as a function of the pump intensity I at a fixed ion irradiation dose 1014 cm−2. The PL spectra are normalized with respect to the pump intensity. The


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increase in pump intensity is shown to lead to an effective blue shift of the irradiationinduced PL band.

Figure 2. (a) Photoluminescence spectra at 4 K for the fixed pump intensity 100 μW as a function of the ion irradiation dose D. (b) Photoluminescence normalized on the pump intensity for the fixed ion irradiation dose 1·1014 cm−2 as a function of the pump intensity

I.

Ion irradiation leads to the generation of crystal structure defects. The preservation of integral PL up to dose 1014 cm−2 and higher suggests that the emerging defects do not


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lead to the appearance of new non-radiative recombination channels. Such resistance to the radiation is a manifestation of defect tolerance of halide perovskites19 and possibly their self-healing ability20, which distinguishes them from traditional semiconductor materials such as silicon or gallium arsenide. The luminescent band arising due to the ion irradiation could be attributed to the recombination of excitons localized at shallow irradiation-induced defects or donor-acceptor pair recombination23. The observed localization depth is in good agreement with the theoretical modeling of defect transition levels for various point defects in CsPbBr3 19. Similar blue shift of the defect-related luminescence with the pump intensity growth in CsPbBr3 single crystals was also reported in Ref. 23.

To explain the observed dependences of the defect-related PL band on the pump intensity and on the ion irradiation dose, we develop a model of transitions in the system described by the energy level diagram shown in Figure 3. The pump laser pulse excites the system in the fundamental absorption region. As a result of charge carriers thermalization the bound excitons (BE) are formed (possibly through the free exciton


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(FE) states). The main contribution to the PL spectrum for an non-irradiated sample is the emission from these states with a rate we denote as r0. In our model, we consider BE as a reservoir with exciton concentration n0, which is instantly filled by a pump pulse at time t = 0 to the initial concentration I0 proportional to the pump intensity. Ion irradiation with a dose D leads to the appearance of further localized defect-related states with a localization energy denoted as E. Taking into account the relatively small ion irradiation dose, we assume that the density of such states linearly depends on the dose and is determined by the expression Dnm(E), where nm(E) is the localized states distribution function. We will consider the following possible transitions: a nonradiative transition from a reservoir to a defect-related state E at a rate k0(E), a nonradiative transition between defect-related states from E' to E at a rate k(E',E), and radiative transition from the state E at a rate r(E). The experimentally measured quantity is the number of photons emitted from the state E designated as I(E). In this model, we neglect the dependence of all above parameters on the ion irradiation dose (except the defect-related states distribution scaling) and the pump intensity, as well as on the nonradiative recombination pathways.


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Figure 3. Energy level diagram for the proposed model. Green arrow denotes pump light absorption, red arrows – radiative recombination, blue arrows – nonradiative transitions. FE – free exciton state, BE – bound exciton state. r0 and r(E) – radiative recombination rates for the BE reservoir and defect-related states respectively. k0(E) and k(E',E) – nonraditive transitions rates for the transition from the reservoir to a defect-related state and between defect-related states respectively. Grey vertical gradient schematically shows the defect-related states distribution Dnm(E).

The described model can be expressed by the following system of integro-differential equations:


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(1)

with initial conditions at t = 0: n0 = I0, n(E) = 0, I(E) = 0. The experimentally measured time-integrated PL intensity corresponds to I0 and I(E) in the limit t → +∞. The equations above do not have an analytical solution. However the qualitative behavior of its solutions can be obtained by numerical methods. In the case of k(E',E) = 0, r(E) = const and k0(E) = const, all defect-related states are completely equivalent, and the spectral behavior of I(E) does not depend on I and D.

The experimentally observed red shift (I(E2) > I(E1) at t → +∞ for E2 > E1) with an increase in D and a decrease in I0 can be obtained with the following possible deviations from the “indifferent” case described above:


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

Relaxation from the reservoir occurs faster to deeper localized states (k0(E2) >

k0(E1) for E2 > E1). This can be explained by a larger trapping cross section of deeper defects.



Transitions between defect-related states are directed towards increasing of the localization degree (k(E1,E2) > 0 for E2 > E1). At zero temperature, such a mechanism can be provided by the tunneling from a shallower localized state to an excited state of a deeper localized state followed by a thermalization to its ground state. Non-zero temperature can accelerate this process by exciting excitons out of shallow-localized states. Both of these processes can be accelerated with an increase in the irradiation dose due to a decrease in the average distance between defects.



Radiative transitions are suppressed for more deeply localized states (r(E2)
E1). The localization of excitons at deeper levels leads to the decrease in the spatial spread their wave function, and thus to the decrease in their local oscillator strength and the radiative decay rate.


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Thus, the most general and probable assumptions about the physical mechanisms provides a qualitative explanation of the dependences observed in the experiments.

One can analyze the limiting cases of the proposed model. In the case of high pump intensities, the concentration of localized states quickly reaches a state saturation concentration nm(E) and remains at this level until the reservoir is exhausted. In this case, I(E) ~ Dre(E)nm(E). This situation is demonstrated in Figure 4(a), where spectra for the irradiation dose D = 1014 cm−2 are shown for various pump intensities. As shown with increasing pump intensity the shape of the spectrum approaches the exponential form (shown by a dash-dotted line) I(E) ~ exp(−E/Em) where Em = 23 meV.

Another limiting case is the low pump intensity. In this case, n(E)

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