Natural 2D layered mineral cannizzarite with anisotropic optical responses

Cannizzarite is a naturally occurring mineral formed by van der Waals (vdW) stacking of alternating layers of PbS-like and Bi2S3-like two-dimensional (2D) materials. Although the PbS-type and Bi2S3-type 2D material layers are structurally isotropic individually, the forced commensuration between these two types of layers while forming the heterostructure of cannizzarite induces strong structural anisotropy. Here we demonstrate the mechanical exfoliation of natural cannizzarite mineral to obtain thin vdW heterostructures of PbS-type and Bi2S3-type atomic layers. The structural anisotropy induced anisotropic optical properties of thin cannizzarite flakes are explored through angle-resolved polarized Raman scattering, linear dichroism, and polarization-dependent anisotropic third-harmonic generation. Our study establishes cannizzarite as a new natural vdW heterostructure-based 2D material with highly anisotropic optical properties for realizing polarization-sensitive linear and nonlinear photonic devices for future on-chip optical computing and optical information processing.


Results
Crystal morphology and chemical composition of cannizzarite. Figure 1a is a picture of a typical naturally formed cannizzarite mineral rock from La Fossa Crater, Vulcano Island, Lipari, Eolie Islands, Messina Province, Sicily, Italy, where silvery-gray, thin-bladed tabular and leafy cannizzarite crystals are attached to gray perlitic andesite matrix. Figure 1b is a schematic showing the simplified crystal structure of cannizzarite, where the crystal is formed by the vdW stacking of the alternated layers of incommensurate lattices of two-atom thick pseudo-tetragonal (Q) layer of (Pb,Bi)(S,Se) and double-octahedral five-atom thick pseudo-hexagonal (H) layer of (Bi,Pb) 2 (S,Se) 3 . Previous studies indicate that cannizzarite exhibits a monoclinic crystal structure belonging to P2 1 /m space group 44,45 . The lattice constants of Q-layer are a Q = 15.48 Å, b Q = 4.09 Å, c Q = 4.13 Å, and β Q = 98.56 • , whereas the lattice constants of the H-layer are a H = 15.46 Å, b H = 4.09 Å, c H = 7.03 Å, β H = 98.00 • . The crystal elongation direction is along the c-axis and the layer stacking period is 15.5 Å along the a-axis. The formula of cannizzarite Pb 46 Bi 54 (S,Se) 127 consists of alternating Q-layer of [(Pb,Bi) 46 (S,Se) 46 ] and H-layer of [(Pb,Bi) 54 (S,Se) 81 ]. It is evident that the two constituent lattices are incommensurable along the c-axis, therefore according to the vernier principle, one super-cell of the cannizzarite crystal is defined by the longrange match between the m number of H-subcells and n number of Q subcells with mc H = nc Q . Previous studies on the material have indicated mH:nQ homologous members of the cannizzarite variable-fit series with 3H:5Q, 7H:12Q, 10H:17Q, 27H:46Q [46][47][48] . Other possible mH:nQ matches have also been predicted for the cannizzarite series, where the change in the number of mQ:nH means a variation in the Pb:Bi ratio and cation:anion ratio in the crystal [46][47][48] . Since the constituent H-layer and Q-layer are periodically stacked and held by weak vdW interaction, cannizzarite can be mechanically exfoliated. The bulk cannizzarite minerals are exfoliated using Nitto tape (SPV 224) to obtain thin flakes with different thicknesses on quartz substrate. Figure 1c is the reflection optical microscope image of one exfoliated cannizzarite flake. The surface topography of the crystal is analyzed using atomic force microscopy (AFM). The height profile of the flake in the AFM image in Fig. 1d shows that the thickness of the flake is 92 nm approximately.
First, the crystal structure of mechanically exfoliated cannizzarite flakes is characterized by TEM analysis, where the exfoliated flakes are transferred onto a TEM copper grid. Figure 1e is the TEM image of a cannizzarite flake where an in-plane striped pattern of periodic darker and lighter areas is clearly observed, indicating an out-of-plane rippling modulated perpendicular to the surface. These rippled structures known as interlayer moiré patterns are common in vdW heterostructures made by stacking incommensurate 2D material layers. In cannizzarite, the rippled structures are formed to force the commensuration between the incommensurate Q-type (Pb,Bi)(S,Se) lattice and H-type (Bi,Pb) 2 (S,Se) 3 lattice. The constituent Q-type and H-type lattices are incommensurate along the c-axis of the crystal, which signifies the direction of rippling. The out-of-plane rippling also induces strong in-plane structural anisotropy. Similar interlayer moiré patterns have been observed in other natural vdW heterostructures of cylindrite 31,32 , franckeite 33,34,37 , and lengenbachite 35 . The zoomed-in high-resolution (HR) TEM image in Fig. 1f shows that the fringe period along the rippling direction of the c-axis  Fig. 1i aquired from the highlighted area shown in the dark-field TEM image in www.nature.com/scientificreports/ the inset of Fig. 1h indicates that each element is uniformly distributed. A homogeneous distribution of all the elements in the crystal is a further indication of cannizzarite being a vdW heterostructure of (Pb,Bi)(S,Se) and (Bi,Pb) 2 (S,Se) 3 individual layers.
The surface composition and stability of the air-aged exfoliated cannizzarite flakes are investigated through XPS measurement. Figure 2a shows the averaged XPS survey spectrum, which confirms the presence of all the major elements Pb, Bi, S, and Se. The fact that there is no oxygen peak in the XPS spectrum suggests that the exfoliated cannizzarite crystals are stable in ambient conditions. The compositional stoichiometry of the cannizzarite flake under study is shown in Table 2, giving the approximate surface chemical formula of cannizzarite as Pb 47 Bi 56 S 107 Se 20 . The deduced formula is very close to that of the previously reported charge-balanced formula of cannizzarite. A slight deviation from the actual formula might be due to the heavy superposition between the Bi 4f. and S 2p peaks. Further, the oxidation states of each element are determined from the XPS results. Figure 2b-d   www.nature.com/scientificreports/ are the high-resolution spectra around Pb 4f., Bi 4f., S 2p, and Se 3d binding energy regions. Figure 2b suggests that Pb is present in the crystal at 2 + and 4 + states at 89% to 11% ratio. While Bi atoms are present at 3 + state according to Fig. 2c. S and Se are solely present at 2 − state as shown in Fig. 2c and d. Figure 3a shows the Raman spectra acquired from three cannizzarite flakes with the thicknesses of 92, 127, and 173 nm by exciting the flakes with a 632. 8  . The 83 cm −1 peak is assigned as the B 1g mode of Bi 2 S 3 (81 cm −1 ). The 129 cm −1 peak is related to the combination of longitudinal optical phonons of PbSe (138 cm −1 ) and the E g mode of Bi 2 Se 3 (133 cm −1 ). The peak at 167 cm −1 is attributed to the B 1g mode of Bi 2 S 3 (168 cm −1 ) and the A 1g mode of Bi 2 Se 3 (175 cm −1 ). The 276 cm −1 peak corresponds to the B 1g mode of Bi 2 S 3 (277 cm −1 ) and the first overtone of the longitudinal optical phonons of PbSe (274 cm −1 ). The peak at 342 cm −1 is associated with the higher-order Raman scattering of PbSe (350 cm −1 ). The 454 cm −1 peak represents the first overtone of the longitudinal optical phonons of PbS (454 cm −1 ). The structural anisotropy in the monoclinic crystal of cannizzarite is further probed by the angle-resolved polarized Raman spectroscopy. Previously, angle-resolved polarized Raman spectroscopy has been employed to determine the structural anisotropy in monoclinic 2D crystals such as MoTe 2 57 and GeAs 18 . Moreover, the technique has also been used for probing the structural anisotropy in other natural   35 , and nagyágite 58 . The contour color map in Fig. 3b shows the Raman intensity variation in the parallel polarization configuration as a function of the incident polarization angle ( θ ) with respect to the x-axis ( θ = 0 • ). The flake used for this study is the same 92 nm flake shown in Fig. 1c. Both A g and B g Raman modes are present, which is expected in case of monoclinic crystals. It is evident that the intensities of the Raman modes vary periodically depending on the excitation polarization angle. By fitting such angle-dependent Raman intensities with the Raman tensor of the monoclinic crystal system of cannizzarite, the c-axis of the crystal can be determined. Considering that cannizzarite exhibits a monoclinic crystal structure belonging to P2 1 /m space group, and the c, b , and a-axes of the crystal are oriented along x -, y -, and z -directions, the Raman tensors of the A g and the B g mode can be written as,

Determination of structural anisotropy in cannizzarite by Raman spectroscopy.
where a, b, c, d, f, and g are the complex components of the Raman tensors. Mostly, these components are treated as real values since the imaginary part of the Raman susceptibility is negligible in transparent materials. However, for cannizzarite flakes with significant absorption, these components are complex elements, and can be represented as R ij = R ij e iφ R ij . Therefore, the Raman intensity for a particular incident polarization is calcu- where the unit polarization of the incident beam is e i = cos θ sin θ 0 , and the unit polarization of the scattered beam is expressed as e s = cos θ sin θ 0 for the parallel polarization. Here, θ = 0 • corresponds to the incident linear polarization being along the x-direction. Therefore, the angle-dependent Raman intensities of the parallel polarization components of the A g and B g modes can be written as 58 , where φ cb is the phase difference between c and b . From Eq. (3), it is evident that the variation of I A g will exhibit a two-fold rotational symmetry, while I A g reaches the maximum as the incident linear polarization is either along the c-axis ( θ = 0 • ) or b-axis ( θ = 90 • ). Also I B g will display a four-fold rotational symmetry with the incident polarization angle, where the intensity will be the minimum when the incident linear polarization is along any of the crystal axes. Figure 3c-h display the polar plots of the Raman intensity variations of the parallel polarization components for the Raman modes at 83, 129, 167, 276, 342, and 454 cm −1 as a function of the incident polarization angle. The Raman mode at 83 cm −1 is a B g mode as it shows a four-fold rotational symmetry, while all the other modes are A g modes giving a two-fold rotational symmetry. The A g modes at 167, 276, 342, and 454 cm −1 attain a maximum intensity when the incident polarization is along the x-direction ( θ = 0 • ), while the A g mode at 129 cm −1 gets a maximum intensity when the incident light is polarized along the y-direction ( θ = 90 • ). Therefore, it can be inferred that the direction of rippling which corresponds to the c-axis of the crystal is oriented along either the x-direction or the y-direction.
Linear dichroism in cannizzarite crystal. Next, the effect of structural anisotropy on the anisotropic optical properties of cannizzarite crystal is explored by performing polarization-dependent absorption measurements in the 92 nm-thick flake. Figure 4a plots the measured reflectance (R), transmittance (T), and absorbance (A) spectra in the 420-800 nm wavelength range when the incident white light is linearly polarized along the x-direction at θ = 0°. The reflectance spectrum (black curve) shows a broad dip around 650 nm, while the transmittance spectrum (blue curve) also shows a shallow dip around 550 nm. The absorbance spectrum of the cannizzarite flake on the quartz substrate is then obtained using the relationship A = 1 − R − T. The effect of absorption in the quartz substrate is measured by obtaining R (cyan curve), T (magenta curve), and A (brown curve) from the bare quartz substrate. Finally, the absorbance spectrum of the cannizzarite flake (red curve) is obtained by subtracting the absorbance of the bare quartz from the absorbance recorded from the cannizzarite flake on the quartz. The absorbance of cannizzarite shows a broad peak centered around 640 nm. The contour color map in Fig. 4b displays a series of absorbance spectra for different incident linear polarization angles varied from 0 • to 360 • . A periodic variation in absorbance as a function of the incident polarization angle is observed, which directly reflects the structural anisotropy in cannizzarite crystal. Further, the imaginary part of the refractive index of the cannizzarite flake is extracted from the reflectance and transmittance spectra by using the equation where d is the thickness of the flake. The estimated k values as a function of the wavelength are plotted in Fig. 4c when the incident white light is linearly polarized along the x-direction ( k x ) and the y-direction ( k y ), together www.nature.com/scientificreports/ with the k x /k y ratio representing the quantification of linear dichroism. It is observed that the k x /k y ratio is greater than 1 in the entire wavelength range, while its value gradually increases from approximately 1 at 420 nm to 2.52 at 750 nm, suggesting that cannizzarite crystal exhibits strong linear dichroism. It should be noted that for the estimation of the k value of cannizzarite we have not included the effect of multiple reflections of light within the cannizzarite flake at the cannizzarite-quartz and cannizzarite-air interfaces. Therefore, the estimated k value would be slightly different from the actual k value in the crystal. Nevertheless, the current estimated k x /k y ratio clearly shows that cannizzarite crystal has strong linear dichroism effect. The polar plot in Fig. 4d shows the evolution of the average absorbance integrated over the measured wavelength range as a function of incident polarization angle. The two-fold rotational symmetry in the evolution of the absorbance is fitted with a periodic function of the form A = A x cos 2 θ + A y sin 2 θ , where A x and A y are the absorbance values when the incident polarization is along the x-direction and the y-direction, respectively. The absorbance reaches the highest value as the incident linear polarization is along the x-direction. Previously, other natural vdW heterostructure materials such as cylindrite 32 , franckeite 37 and lengenbachite 35 have showed the similar characteristics with the absorption being maximum when the incident linear polarization is along the rippling direction of the c-axis of the crystal. Therefore, it can be concluded that the c-axis along the direction of ripples of the cannizzarite crystal is oriented along the x-direction. Simultaneously, the absorbance exhibits the lowest value when the incident linear polarization is along the y-direction, which follows the b-axis of the crystal. The average A x /A y ratio within the wavelength region is 1. 44. Figure 4e and f show the polar plots of the evolution of the absorbance at two particular wavelengths of 640 nm and 520 nm, with the A x /A y ratio of 1.47 and 1.27, respectively.
Polarization-dependent anisotropic THG from cannizzarite flakes. The in-plane structural anisotropy of the crystal also induces anisotropic nonlinear optical properties, as reported in other natural vdW heterostructures of cylindrite 32 , franckeite 36 , and lengenbachite 35 . With that hindsight, we investigate the polarization-dependent anisotropic THG response from the mechanically exfoliated cannizzarite flakes. Figure 5a shows the transmission optical microscope image of the THG emission from the 92 nm-thick cannizzarite flake shown Fig. 1c. The flake is illuminated by a 1560 nm fundamental laser with the spot size of 1.5 µm. The recorded THG spectrum in Fig. 5b shows that the spectral response peaks at 520 nm, which is exactly one-third of the excitation wavelength. The log-scale plot of the average THG power as a function of the average pump power in Fig. 5c shows a cubic power-law dependence confirming the THG process. For the average pump power of 4 mW corresponding to a peak pump irradiance of 27.2 GW cm −2 , the achieved THG conversion efficiency for the 92 nm-thick flake is around 1.34 × 10 −9 . When the crystal axes c , b , and a are oriented along the x , y , and z -directions, the contracted form of the third-order susceptibility tensor of the monoclinic crystal system of cannizzarite can be written as 59 , = E 0 cos θ x + sin θ y , with x and y being the unit vectors along the x-and y-axes. Since the polarization of the excitation electric field only remains in the x-y plane, the contribution from the χ (3) components including z terms to the THG emission is negligible. Therefore, the x-and y-polarized components of the THG intensity can be written as, Figure 5d plots the evolution of the TH power as a function of the incident linear polarization angle with respect to the x-axis at the pump power of 2.5 mW. The desired incident polarization is obtained by placing a linear polarizer oriented along the x-axis (the c-axis of the crystal) and a rotating half-wave plate. The measured x-and y-polarized components (black and blue data points) of the TH power are theoretical fitted (solid curves) using Eq. (7), showing a good agreement between the theoretical prediction and the measurements. The red data point and the curve correspond to the total TH power. The polarization-dependent THG response from the cannizzarite flake displays an anisotropic two-fold rotational symmetry, with the maximum THG emission occurring at the incident linear polarization angle of θ = 0 • , which is parallel to the rippling direction along the c-axis of the crystal. Similar trends are also observed in other flakes with thicknesses of 127 and 173 nm, as shown in Fig. 5e and f. As the incident linear polarization is along the c-axis of the crystal, the measured THG conversion efficiency from the three flakes with the thicknesses of 92, 127, and 173 nm are 6.01 × 10 −10 , 4.80 × 10 −10 , and 3.52 × 10 −10 respectively, at the pump power of 2.5 mW with the incident spot size of 1.5 µ m, which corresponds to a pump power irradiance of 17 GW cm −2 . The THG anisotropy ratio I (3ω) www.nature.com/scientificreports/ maintains a constant value of 3.24 ± 0.14 for all the three flakes with different thicknesses. The anisotropy in the THG response is further revealed by extracting the average relative magnitudes of χ (3) tensor elements for the cannizzarite flakes with different thicknesses as χ 11 : χ 18 : χ 22 : χ 29 = 1 : 0.34 : 0.57 : 0.22. Finally, to compare the THG response of cannizzarite with other existing nonlinear 2D materials, we estimate the effective scalar value of the third-order nonlinear susceptibility χ (3) eff by using the following expression where ǫ 0 is the vacuum permittivity, d is the flake thickness, P (3ω) represents the average TH power, n 1 and n 3 are the real part of the refractive index of cannizarite at the fundamental wavelength 1 = 1560 nm and the TH wavelength 3 = 520 nm, while k 3 is the imaginary part of the refractive index at 3 . The parameters of the fundamental beam are given by spot size W = 1.5 μm, repetition rate f rep = 80 MHz, pulse width τ = 90 fs, while P (ω) is the pump power. The value of k 3 is estimated to be 0.48 from the absorbance measurement in Fig. 4. It is noted that the refractive index of cannizzarite crystal is unknown, but the refractive indices of its component binary sulfides are available, with n 1 = 4.24 and n 3 = 4.34 for PbS, n 1 = 1.9 and n 3 = 2.6 for Bi 2 S 3 60 . By assuming an averaged refractive index of n 1 = n 3 = 3.5 for cannizzarite crystal, the estimated χ

Discussion
In summary, we have demonstrated that ultrathin layers of vdW heterostructures composed of alternating PbS-type and Bi 2 S 3 -type 2D material layers can be obtained by the mechanical exfoliation of natural layered mineral cannizzarite. Compared to other natural vdW heterostructure materials such as cylindrite, franckeite and lengenbachite, it is relatively difficult to exfoliate cannizzarite crystal into ultrathin flakes. The smallest thickness achieved in this study by mechanical exfoliation is 92 nm, corresponding to 59 pairs of layers of the vdW heterostructure. A better controlled mechanical exfoliation or other exfoliation approaches such as liquid phase exfoliation could be utilized to produce 2D vdW heterostructures of molecular thicknesses. Through the TEM analysis, we have established that the forced commensuration between the constituent lattices in cannizzarite crystal leads to in-plane structural anisotropy. The bulk and surface chemical composition of cannizzarite crystal is further determined by the EDXS and XPS analysis as Pb 59 Bi 67 S 97 Se 30 and Pb 47 Bi 56 S 107 Se 20 , respectively. It is demonstrated that the structural anisotropy in thin cannizzarite flakes can be optically probed by angleresolved polarized Raman spectroscopy, which is used to identify the direction of the ripples along the c-axis of the crystal. By performing the polarization-dependent absorption measurements, it is found that cannizzarite crystal exhibits strong linear dichroism, which might be useful for designing polarization-sensitive photodetectors and modulators. Furthermore, the effect of structural anisotropy on the nonlinear optical properties of cannizzarite is studied by demonstrating anisotropic THG response from thin cannizzarite flakes. The TH power for the incident polarization along the rippling direction of the crystal is found to be almost 3.24 times stronger compared to that along the direction perpendicular to it. These results provide deeper understanding of the origin of optical anisotropy in the natural vdW heterostructure of cannizzarite, even though its constituent 2D material layers are structurally isotropic in nature. The third-order nonlinear susceptibility of cannizzarite is measured as 1.61 ×10 −20 m 2 V −2 , indicating that cannizzarite exhibits strong THG response as other existing nonlinear anisotropic vdW materials. The study presented here may be harnessed for building polarizationsensitive anisotropic linear and nonlinear photonic devices used in future on-chip optical computing, optical information processing and communication.

Methods
Sample preparation. The quartz substrate is sonicated in deionized water, acetone, and isopropanol one after the other. The cannizzarite thin flakes are mechanically exfoliated from the bulk natural cannizzarite mineral (from La Fossa Crater, Vulcano Island, Lipari, Eolie Islands, Messina Province, Sicily, Italy) using Nitto tape (SPV 224) and transferred directly onto the cleaned quartz substrate. For preparing the TEM sample, the exfoliated thin flakes are first transferred onto a thermal release tape (Revalpha 319Y-4LS). Then the flakes from the thermal release tape are transferred on to a copper TEM grid by placing the tape on it and heating at 100 • C for 30 s.
TEM and EDXS measurement. The TEM imaging and EDXS measurements of the transferred cannizzarite flakes on a copper TEM grid are performed using a JEOL 2100 scanning/transmission electron microscope (S/TEM) operating at 200 keV, with a Bruker Quantax X-Flash detector for EDXS compositional analysis. XPS measurement. The XPS spectra from the exfoliated cannizzarite flakes are obtained using the Thermo Scientific Nexsa XPS spectroscopy system. The instrument is equipped with a monochromatic Al Kα X-ray (8)  www.nature.com/scientificreports/ source. The highest energy resolution of the recorded XPS spectra are < 0.48 eV. Small spot spectroscopy is achieved by inserting an aperture into the electrostatic lens column forming a virtual probe at the surface.
Optical setup. The Raman spectrum is acquired by using a 632.8 nm He-Ne laser beam, which is focused on the cannizzarite flake using a 40 × objective lens (NA = 0.6). The linear polarization of the laser beam is determined using a linear polarizer and a rotating half-wave plate. The back-reflected light is further collected by the same objective lens and guided towards a spectrometer (Horiba, iHR 550) using a beam splitter. The Rayleighscattered laser light is filtered out by placing a long pass filter (Semrock, LP02-633RE-25) in front of the spectrometer. The parallel and perpendicular polarization components of the Raman spectrum are analyzed using another linear polarizer before the spectrometer. For the polarization-dependent absorption measurements, white light beam from a broadband white light source (Thorlabs, SLS201L, 360-2600 nm) is passed through a linear polarizer and focused on the cannizzarite flake using a 100 × objective lens (NA = 0.7). The reflection spectrum is analyzed from the back-reflected light from the sample which is collected by the same objective lens and directed towards the spectrometer with a beam splitter. The transmission spectrum is collected using another 100 × objective lens (NA = 0.7). An iris is used to spatially filter out a small area of the flake in both the cases. The reflectance (R) and transmittance (T) are measured by normalizing the reflection and transmission spectra with the light source spectra. Finally, the absorbance (A) spectrum is obtained by using the relationship A = 1 − R − T.
The THG measurements are performed by designing a nonlinear transmission microscopy system. The pump laser beam at the fundamental wavelength of 1560 nm (Calmar fiber laser, pulse width 90 fs, repetition rate 80 MHz) is focused on the cannizzarite flake using a 40 × objective lens (NA = 0.65). The excitation linear polarization of the pump laser is determined by using a linear polarizer and a half-wave plate in front of the objective lens. The transmitted THG emission from the flake is collected by a 100 × objective lens (NA = 0.7) and analyzed using a spectrometer (Horiba, iHR 550). A short pass filter (Thorlabs, FESH 900) is used before the spectrometer to remove the transmitted fundamental pump laser beam. The polarization state of the collected THG spectrum is resolved by using another linear polarizer before the spectrometer.

Data availability
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.