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Abstract
In this work, we report a femtosecond mode-locking Erbium-doped fiber laser using mechanically exfoliated rhenium disulfide (ReS2) deposited onto the polished surface of a D-shaped optical fiber. By performing the polarization and saturable absorption measurements, the sample exhibited a polarization extinction ratio of 10 dB (90%) and nonlinear transmittance variation of 3.40%. When incorporated into the cavity as a saturable absorber (SA), the passive mode-locking performance of 220 fs was achieved. This is the best mode-locking performance ever reported in literature achieved with all-fiber based ReS2 SA. By using density functional theory (DFT) calculations, we obtained the electronic states and the optical absorption spectrum at 1550 nm attributed by defects in the ReS2 structures, which is consistent with its linear and nonlinear optical absorption in the laser mode-locking mechanism.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrafast mode-locked (ML) Erbium-doped fiber lasers (EDFLs) have been widely studied and developed in the past few decades for research, medicine, industry, photonics, and optical communications applications due to remarkable alignment-free structure and high stability features [1]. In their simple configuration, the passive mode-locking technique has often been used to investigate the nonlinear optical properties of nanomaterials, especially saturable absorption, as the main nonlinear ultrafast pulse generation mechanism in these lasers. At this time, SESAM [2] and carbon nanotubes [3–7] were the first saturable absorbers incorporated in the pulsed fiber laser field due to ultrafast response time and large modulation depth parameters but showing it high-cost material production and bandwidth limitation at infrared optical fiber bands, respectively.
In this concept, graphene [6–16] as the recent-discovered two-dimensional (2D) nanomaterial and its derivatives (graphene oxide and reduced graphene oxide) [17, 18], topological insulators [19, 20], transition metal dichalcogenides (TMDs) [21–26] and black phosphorus [27–29], have emerged as new layered nanomaterials. Novel nanotechnology platforms for the photonics, optoelectronics, and optical communications fields due to their outstanding optical properties such as broadband operation, ultrafast carrier dynamics, and large third-order nonlinear susceptibility are related to saturable absorption properties. As a particular family of 2D nanomaterials and like allotrope carbon-based graphene, the layered semiconductor TMDs consist of a plane of hexagonally arranged transition metal atoms (molybdenum - Mo, tungsten - W, rhenium - Re) sandwiched between two hexagonal planes of chalcogenide atoms (sulfur - S, selenium - Se, telluride - Te). This results in the most popular nanomaterials of group VI such as MoS2, WS2, MoSe2, WSe2 with strong in-plane chemical bounds and weak Van der Waals interaction between layers. In most of these fascinating materials, indirect-to-direct bandgap electronic transitions by tuning the layer number from bulk to monolayer forms have been widely used for optoelectronics and visible-infrared photonics fields. However, these become a major challenge in the manufacture of electronic components and mode-locked fiber lasers based on these nanomaterials as well, as they are optically and electronically sensitive to the number of layers.
Among the traditional TMDs, 2D semiconductor rhenium disulfide (ReS2) has been recently discovered as a novel and promising nanomaterial with unusual physical, chemical, and optical properties for nanotechnology and photonics applications [30–39]. Differing from TMDs family members, the layered ReS2 exhibits a unique distorted 1 T triclinic crystal structure, which provides a unique layer-independent direct electronic transition from bulk (1.35 eV) to monolayer (1.43 eV) forms at visible and infrared optical regions. Also, in contrast to in-plane isotropic TMDs structure, highly anisotropic properties [30–39] of ReS2 have been demonstrated for several promising applications such as sensors and polarization-based electronic devices [36]. More environmentally stable than 2D in-plane-anisotropic black phosphorous, excitonic transitions localized at 1.5-1.6 eV range have shown high layer-polarization dependence [30–39], which is attractive as wavelength tuning for near-infrared optoelectronics.
To explore its linear and nonlinear optical properties at 1550 nm (0.8 eV) EDFLs, the introduction of lattice imperfections such as defects in the ReS2 2D structure has been theoretically and experimentally investigated as the main mechanism to induce the creation of new localized electronic states and promote optical absorption at near-infrared spectrum [40], as reported with other TMDs [41–45]. As a result, ML EDFLs based on ReS2 SA have been reported in this range. The first one was demonstrated by Liu et al. using chemical deposition vapor (CVD) ReS2 deposited onto a D-shaped optical fiber surface. In this work, the linear and nonlinear optical responses were experimentally investigated, which resulted in a picosecond EDFL mode-locking performance as SA [46]. Next, Mao et al. demonstrated both passively Q-switched and ML EDFLs using liquid-phase exfoliated (LPE) ReS2 nanosheets as SA, generating picosecond pulses [47]. In the same period, the first harmonic mode-locking (HML) generation in an EDFL using microfiber-based ReS2 SA was reported operating at 168th harmonic frequency [48]. In this configuration, pulses of 2.55 ps could be achieved. Then, Q-switched mode-locking was both reported at 1.53 μm EDFL [49] and 2.8 μm solid-state laser [50] using mechanical and LPE ReS2. In the same line, Q-switched and mode-locked solid-state lasers operating from 0.65 to 2 μm were experimentally demonstrated to confirm the ReS2 broadband absorption due to defects from liquid exfoliated flakes. In this work, the first femtosecond (323 fs) mode-locked laser was achieved at 1 μm using this material as SA [51]. And recently, the same LPE ReS2 was incorporated as SA into an EDFL, generating stable multi-wavelength bright-dark solitons [52] and Q-switching mode-locking at 1.3 μm [53].
Here, we report a femtosecond pulse generation from an ML EDFL using mechanically exfoliated ReS2 deposited onto the side-polished surface of a D-shaped optical fiber. Unlike the previous Refs. [54], we used density functional theory (DFT) calculations to determine the energy bands and absorption spectra at 1550 nm (0.8 eV), the latter obtained for the first time, introducing S and Re monovacancies defects induced via mechanical exfoliation process onto the ReS2 structure. For experimental characterization, optical images, Raman spectroscopy, atomic force microscope (AFM), scanning electronic microscope (SEM), polarization, and nonlinear saturable absorption measurements of the mechanically exfoliated ReS2/D-shaped optical fiber sample were performed. As a 1550-nm polarizer/saturable absorber, the sample exhibited high polarization relative extinction ratio of 10 dB (90%) and nonlinear transmittance variation of 3.40%, resulting from the strong light evanescent field interaction with the ReS2 flakes along the fiber-polished surface and the best hybrid mode-locking performance of 26 nm broadest bandwidth and 220 fs shortest pulse duration ever reported in the literature using all-fiber based ReS2 SA.
2. Density functional theory calculations of ReS2
2.1 Computational tools
Our density functional theory (DFT) calculations were performed through SIESTA code [55] which implements standard norm-conserving pseudopotentials factorized in the Kleinman-Bylander form and flexible linear combinations of atomic orbitals (LCAO) basis set. In the current study, was used double-ζ plus polarization orbitals (DZP) as a basis set. Exchange and correlation are treated within the generalized gradient approximation (GGA), according to Perdew-Burke-Enrzerhof (PBE) functional [56]. The real-space grid of 250 Ry was defined to project the basic functions and the electron density to calculate the exchange-correlation and Hartree potentials. All configurations were optimized with forces smaller than 0.025 eV/Å. To avoid interactions between periodic images, we set a vector of 25 Å spacing basal planes. Electronic properties were calculated with integrations over the Brillouin zone, with a 25 × 25 × 1 Monkhorst-Pack grid [57] and a 55 × 55 × 1 grid for density of states (DOS) calculations.
The optical properties were calculated by using the first-order time-dependent perturbation theory of the frequency-dependent complex dielectric function (ε) = ωR(ε)+iωI(ε). In the simple dipole approximation, the imaginary part is given by [58]
2.2 Defect electronic states
We investigated the structural, electronic, and optical absorption properties of ReS2, considering induced monovacancy (one atom) defects by using the density functional theory (DFT) framework. Different supercells with 3 × 3 repetitions of the ReS2 unit cell in the 1T” phase (Fig. 1(a) and 1(c)) were built.
Fig. 1. (a), (c) ReS2 supercells and (b), (d) band structure/projected density of states (PDOS) obtained with VS(s) (orange atoms) and VRe (blue atoms) monovacancies. The green, blue, and purple arrows represent the electronic transitions at 0.6 and 0.7 eV for VS(s) and 0.5, 0.7 and 0.9 eV for VRe, respectively.
To investigate the energetic stability of these supercells with monovacancy, we used the formation energy (${E_f}$) equation described by
where $E_{total}^{Vac},E_{total}^{pristine}$, n and ${\mu _i}$ are the total energy of the layer with S/Re vacancy, the pristine layer total energy, the numbers of vacancies for each atom and the chemical potential, respectively. This equation represents the cost of creating a defect into a perfect ReS2 layer and computes in terms of chemical potentials $({{\mu_i}} )$ for different sources of Re and S atoms. The formation energy is the energy needed to remove an atom from the layer and take it to a reservoir. We assume our reservoir as the bulk form for S and Re, which are defined as the chemical potentials.Due to the ReS2 lattice distorted structure, we considered two different monovacancies for S atoms, both localized in the site allowing a long length $({V_{S(l )}})\; $and short length $({V_{S(s )}})\; $bonds between S and Re atoms, and one monovacancy for Re (${V_{Re}})$.
Our DFT calculations predicted that ${V_{S(l )}}$ are more likely to occur in the pristine layer than either ${V_{S(s )}}$ or ${V_{Re}}$ vacancies, being in good agreement with the Horzum et al. [60]. In terms of electronics states, these structural defects in the ReS2 layer may have a significant influence on its electronic, optical, and mechanical properties. Our calculation shows a flat level inside the bandgap when we include a monovacancy in the layer, having its defect energy level in the band structure dependent on which atom was removed. For the system with ${V_{S(s )}}$ and ${V_{Re}}$ vacancies, new states inside the bandgap arise with low energy required to electronic transition from 0.6 (green arrow) to 0.7 eV (blue arrow) for ${V_{S(s )}}$ and from 0.5 (purple arrow) to 0.9 eV (green/blue arrows) for ${V_{Re}}$, as depicted in Fig. 1(b) and 1(d) along with its projected density of states (PDOS). Even though these two types of vacancies are not the most stable, for instance, these monovacancies may arise in the mechanical exfoliation process.
2.3 Optical absorption spectrum
Once these monovacancies are present in the ReS2, optical absorption can be accessed from these new states on the electronic structure. In Fig. 2(a) and 2(b) are shown the theoretical optical absorption spectra for configurations with ${V_{S(s )}}$ and ${V_{Re}}$ vacancies. From the pristine supercell without defects on which the absorption peak is around 1.5 eV, two small peaks arise in the low absorption energy range (0.6 and 0.7 eV) due to transitions between the new defect states (one below and two above the Fermi level) present in the DOS (Fig. 1(b) and 1(d)), in the case of ${V_{S(s )}}.$ A significant change in the absorption spectrum is also observed for the system with ${V_{Re}}$ vacancy, in which some extra peaks also appear in the same range as with ${V_{S(s )}}$ vacancy.
Fig. 2. ReS2 optical absorption spectrum obtained with (a) VS(s) and (b) VRe monovacancies. The color arrows correspond to the PDOS electronic transitions shown Fig. 1.
Because PBE calculations usually underestimate Egap, the whole PBE spectrum may be red shifted, compared with the experimental spectrum [61]. Despite this limitation, we believe that it is still fruitful to point changes due to structural modifications in the optical spectrum of large systems, where the computational cost of more elaborated methods can be impeditive.
3. ReS2 sample fabrication and characterization
3.1 SEM and AFM characterization
We used ReS2 flakes provided by 2D Semiconductors and applied the micromechanical exfoliation technique using scotch tape [62]. First, we characterized the morphology and thicknesses of the exfoliated flakes using SEM and AFM, respectively. As shown in Fig. 3(a) and 3(b), the SEM images showed a few to a hundred micrometric-sized exfoliated ReS2 flakes in the whole substrate surface area. By analyzing some of these flakes, we could identify a wide range of few (thinner than 10 nm - 15 layers) and multilayer flakes (thicker than 100 nm - 142 layers), as illustrated in the AFM images of Fig. 3(c) and 3(d).
Fig. 3. (a), (b) SEM images of micrometric sizes exfoliated ReS2 flakes measured at 450X and 1400X magnitudes (10 μm scale bar) and (c), (d) its AFM measurements with few and multilayer structures.
3.2 Microscope images and Raman spectroscopy
For the transference of ReS2 flakes onto D-shaped optical fiber, a stacked substrate composed by 1 μm thickness of water-soluble polyvinyl alcohol (PVA) as a sacrificial layer and 300 nm thickness of polymethyl methacrylate (PMMA), as the host of the exfoliated ReS2 flakes, was used. With the flakes transferred onto the PMMA layer, we attached a concentric scotch tape in a chosen region of the sample. After this step, the PVA layer was dissolved from the stacked substrate with deionized water, resulting in a ReS2/PMMA film attached to the tape. In the last step, the sample was transferred to a 17 mm polishing length D-shaped optical fiber with 1 μm core to polished surface distance by using a tweezer [11, 15, 16, 23].
The exfoliated ReS2/D-shaped fiber sample characterization was realized by using a conventional microscope and a confocal Raman spectrometer Witec Alpha 300R. In Fig. 4(a) and 4(b), are shown the microscope images of D-shaped optical fiber with exfoliated ReS2 deposited along its polishing surface length in some cladding/core regions obtained with 10X and 20X objective lens. To ensure the long-length evanescent field interaction between the light and the material, the fiber polishing length was fully covered by the flakes.
Fig. 4. Optical microscope images of exfoliated bulk ReS2 onto the polished surface of D-shaped optical fiber using (a) 10X and (b) 20X objective lens and (c) its Raman spectrum (red marker in Fig. 4(b)).
For Raman spectroscopy characterization, we could identify the Raman bands from the exfoliated ReS2 flakes (red marker in Fig. 4(b)) under a 532 nm solid-state laser excitation wavelength, as shown in Fig. 4(c). Because of ReS2 unit cell consisting of 12 atoms (4 Re and 8 S atoms) [30], the material exhibits 18 active Raman vibrational modes, most of them intralayer modes localized in higher frequencies, in contrast to its interlayer modes localized at lower frequency range (< 50 cm-1) [63], which could not be observed due to limited equipment resolution. Among these 18 modes, ${A_g}$, ${E_g}$, and ${C_p}$ are Raman active phonons, where ${A_{1g}}$ is the out-of-plane, ${E_g}$is the in-plane, and ${C_p}$ is the mix of in-plane and out-of-plane coupled vibrational modes. Analyzing the Raman spectrum in Fig. 4(c), we identified the Re atoms $4\; {A_g}$(132 (1), 142 (2), 153 (3) and 161 (4) cm-1) + $2\; {E_g}$(212 (5) and 235 (6) cm-1), Re + S atoms $2\; {C_p}$ (275 (7) and 283 (8) cm-1), and S atoms$\; 6\; {A_g}$(306 (9), 309 (10), 320 (11), 324 (12), 347 (13) and 368 (14) cm-1) + $4\; {E_g}$(376 (15), 407 (16), 417 (17) and 439 (18) cm-1) vibrational modes, which was in good agreement with some experimental data of literature for bulk ReS2 structure [39, 63], as observed in most of flakes deposited along the fiber polished surface.
3.3 Absorption spectrum and polarization measurements
The spectral absorption characterization of mechanically exfoliated ReS2 onto a D-shaped optical fiber sample was performed using a broadband source with a wavelength detection range from 1000 to 1700nm, as shown in Fig. 5(a). As result, we could observe a high optical absorption (∼3.56 dB) of exfoliated ReS2 flakes onto fiber side-polished surfaces at 1550 nm.
Fig. 5. (a) Absorption spectra and (b) transmitted power as a function of beam polarization angle through the D-shaped optical fiber without (black filled circles) and with exfoliated ReS2 (red filled circles) shown in polar coordinates.
The sample polarization performance was measured by using a polarization setup, as described in [13]. A 1550-nm continuous-wave EDFL beam was collimated through a 20X objective lens, linearly polarized by Glan-Thompson cube, and directed by a half-wave plate (HWP). By coupling the polarized beam to the sample with another 20X objective lens, its output power was measured by rotating the linearly polarized beam in steps of 10° from 0° to 360° polarization angles. In polar coordinates, the polarization measurements of D-shaped optical fiber without (black filled circles) and with exfoliated ReS2 (red filled circles) are shown in Fig. 5(b), respectively. From the curves, we observed a periodic variation of the power with respect to polarization angle, indicating its strong evanescent field light interaction behavior. From the difference between the maximum and minimum absorption of orthogonal polarization components, the corresponding polarization relative extinction ratio value was ∼ 10 dB (90%), very close to our previous reports [13–18, 23].
3.4 Nonlinear saturable absorption measurement
The saturable absorption of exfoliated ReS2/PMMA onto a D-shaped fiber sample was characterized in a balanced twin-detector experiment using a 1560 nm ultrafast laser with 89 MHz repetition rate and 920 fs pulse duration. Figure 6 shows the nonlinear saturation curves of ReS2/PMMA (black open circles) and only PMMA (red open circles) onto D-shaped fiber. At low peak intensities, the linear transmittance of the ReS2/PMMA sample is 35.6% (insertion loss = 4.49 dB), which corresponds to a relative transmittance of 93.7% considering non-saturable transmittance of 39%. Under high peak intensities, up to 700 MW/cm2, we observe a 3.40% transmittance increase (ΔTmeasured) for the sample. According to the saturable absorber model (blue solid line), the estimated saturation intensity (Isat) is ∼590 MW/cm2 and the maximum transmittance variation (ΔT) is 6.40%, which shows we were not able to fully saturate our ReS2 saturable absorber sample. This transmittance increase is due to the ReS2 saturable absorption at 1560 nm, and there is no contribution or change in transmittance from the PMMA film or the D-shaped fiber alone. These results demonstrate the strong nonlinear absorption saturation of exfoliated ReS2 onto D-shaped optical fiber and show its promising potential for ultrashort pulse generation at 1560 nm.
Fig. 6. Relative nonlinear transmittance curves (relative to non-saturable transmittance) comparing exfoliated ReS2/PMMA (black open circles), fitted by the SA model (blue solid line), and only PMMA (red open circles) onto D-shaped optical fiber.
4. Laser results
4.1 Erbium-doped fiber laser setup
A ring-cavity ML EDFL based on exfoliated ReS2/D-shaped optical fiber SA was built as schematically depicted in Fig. 7.
We used a 2-m Erbium doped fiber (absorption coefficient @1530 nm = 47.6 dB/m, D = -57 ps/nm/km; β2 = +73.6 ps2/km @1550 nm) as laser gain medium and optically pumped by a 980-nm diode laser via 980/1550-nm wavelength division multiplexer (WDM) coupler. The cavity was also consisted of a 50-dB optical isolator, a polarization controller, a 15% output coupler, and the fabricated in-line exfoliated ReS2/D-shaped optical fiber, having all components based on standard single mode fiber (D = +17 ps/nm/km; β2 = -22 ps2/km), and resulting in total length of 12.3 m with low anomalous accumulated (DACCUMULATED = +0.061ps/nm; β2(ACCUMULATED) = -0.078 ps2) and average (DAVERAGE = +4.96 ps/nm/km; β2(AVERAGE) = -6.42 ps2/km) dispersion values. The laser performance was measured by using an optical spectrum analyzer (Yokogawa AQ6370B, 600-1700 nm), a sampling oscilloscope (Keysight, 350 MHz), radio frequency (RF) spectrum analyzer (Keysight N9951A, 44 GHz) coupled to a InGaAs photodetector (Newport 818-BB-35F, 12 GHz), a power meter (Thorlabs) and an interferometric autocorrelator (Femtochrome FR-103XL).
4.2 Laser mode-locking performance
Incorporating the exfoliated ReS2/D-shaped optical SA into the EDFL cavity and tuning the polarization controller, the best mode-locking performance was obtained at a pump power of 70 mW. As result, we could generate a stretched-pulse laser spectrum profile [64–66] with the broadest bandwidth of 26 nm (Fig. 8(a)). In this point, we measured the respective pulse duration of ∼ 435 fs with an output power of 0.640 mW, corresponding to 0.590 kW, 257 pJ, and 737 MW/cm2 of intracavity peak power, pulse energy, and intracavity peak intensity values, respectively. As stretched lasers are typically chirped, we compressed the pulse to 220 fs optimal value using 1.20 m SMF piece at the laser output (operating at the fundamental cavity repetition rate of ∼16.26 MHz), assuming a gaussian profile (Fig. 8(b)), reducing its time-bandwidth product (TBP) from 1.39 to 0.703.
Fig. 8. (a) Laser spectrum bandwidth (inset – log scale spectrum) and (b) pulse autocorrelation trace (inset – pulse train).
Such good results could be assigned to the hybrid mode-locking mechanism [16, 20] between nonlinear polarization rotation (NPR) due to the high polarization extinction ratio of the sample, as previously reported by our group using other nanomaterials [13–18, 23], and ReS2 saturable absorption, as theoretically shown in our DFT calculations and experimentally demonstrated in some Refs. [46, 47, 52], but other effects such as edge states [67] also can be contributing to both nonlinear optical absorption at 0.8 eV and the laser hybrid mode-locking mechanism.
To verify the ReS2 saturable absorption influence in the laser, we measured the mode-locking performance using a 1550 nm in-line polarizer (>30 dB polarization extinction ratio) at the same previous cavity conditions (dispersion, pump power, repetition rate) but inducing only nonlinear polarization rotation (NPR) mechanism. The results are depicted in Fig. 9(a) and 9(b).
Fig. 9. (a) Laser spectrum bandwidth and (b) pulse autocorrelation trace of NPR (red dashed line) and NPR + ReS2 hybrid (blue continuous line) mode-locking mechanisms in the laser.
By comparing the spectrum bandwidths and pulse durations of both regimes, we observed better mode-locking performance of the NPR + ReS2 hybrid mode-locking with broader bandwidth and shorter pulse duration as compared to NPR alone, showing the ReS2 saturable absorption influence. Such results are comparable to ref. 20, which showed the direct comparison between NPR and hybrid mode-locking using topological nanomaterial as saturable absorber.
In Table 1, we summarize all reported EDFL performances using ReS2-based SA in comparison to our work.
Table 1. Mode-locked EDFL performances using ReS2 SA
4.3 RF spectrum and long-term laser stability
For verifying the stability of the laser mode-locking regime with the exfoliated ReS2/D-shaped optical fiber sample, the RF spectrum was measured at 2 kHz-narrow frequency spans with 1 Hz resolution bandwidth (RBW). As expected from the fundamental cavity repetition rate observed in the oscilloscope, the corresponding RF frequency of the laser was observed at 16.2596 MHz, with a high signal-to-noise ratio (SNR) around 70 dB, as illustrated in Fig. 10(a).
Fig. 10. (a) Output RF spectrum measured around the fundamental cavity repetition rate at 16.2596 MHz and (b) spectral bandwidth of the long-term laser mode-locking stability over time at 32.5 MHz obtained with ReS2 SA.
The long-term stability of the laser mode-locking was measured via optical spectrum and repetition rate monitoring over time. Because of the short time stability of the fundamental repetition rate condition (∼16.26 MHz) due to environmental fluctuations, we could achieve the optimal long-term laser stability at double pulse operation (∼32.5 MHz) by increasing the pump power from 80 to 90 mW, which provided stable at the same mode-locking performance (∼26 nm) for 300 minutes long time (Fig. 10(b)).
5. Conclusions
We presented a femtosecond ML EDFL using mechanically exfoliated ReS2 deposited onto the polished surface of a D-shaped optical fiber. As a 1550-nm polarizer/saturable absorber device, high polarization extinction ratio of 10 dB (90%) and nonlinear transmittance variation of 3.40% were obtained, respectively, which resulted in a passive mode-locking performance of 220 fs when incorporated into an EDFL cavity as SA. Allied with nonlinear polarization rotation mechanism, this is the best hybrid mode-locking performance ever reported in literature achieved with all-fiber based ReS2 SA. The linear and nonlinear optical properties at 1550 nm are consistent with DFT calculations which show that (S/Re) monovacancy introduces defect states in the energy gap leading to new absorption peaks in low energies.
Funding
Universidade Presbiteriana Mackenzie (MackPesquisa (201037)); Universidad de Antioquia (2018-22676, G8-2020 3906); Conselho Nacional de Desenvolvimento Científico e Tecnológico (306422/2017-4); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (1716664, Print process 88887.310281/2018-00); Fundação de Amparo à Pesquisa do Estado de São Paulo (2012/50259-8, 2015/11779-4, 2016/25836-2, 2017/14705-7, 2018/08988-9).
Acknowledgments
All authors thank Prof. Hugo Luis Fragnito for fruitful discussions, and Prof. Leandro Seixas and SDumont/LNCC for computational support and high-performance computational facilities.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 supporting content on sample's EDS characterization and PDOS results.
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