Characterization of highly crystalline lead iodide nanosheets prepared by room-temperature solution processing

Shortly after the discovery of graphene by mechanical exfoliation, other two-dimensional (2D) materials were also isolated using the same approach [1, 2]. Among the different isolated 2D materials, the family of 2D semiconductors is especially relevant for applications as their intrinsic bandgap makes them suitable for different electronic and optoelectronic devices [3, 4]. Although theory predicts a large number of 2D semiconductors that can be produced by mechanical exfoliation (>1000) [5–9], so far the number of compounds that have been experimentally studied is rather limited (<20) [10–12]. The chalcogenides group, and in particular the transition metal dichalcogenides (TMDC) such as molybdenite (MoS₂), is probably the most investigated family of 2D materials [13, 14]. The transition metal halides (TMH) family, on the other hand, is a scarcely explored family of materials in the few-layer limit, although it contains many examples of layered materials [15–18]. In their bulk form, lead-based TMHs are gaining a great deal of attention as they can be used as a source to synthesize metal halide perovskite solar cells [19–21]. Moreover, ultrathin TMHs present several characteristics that make them an interesting complement to TMDCs, such as their direct bandgap in multilayer samples and the absolute value of the bandgap that spans a range not covered by the TMDCs.

In this study, we use solution-based synthesis of lead iodide (PbI₂) followed by mechanical exfoliation to produce thin flakes. We study the isolated layers by micro-reflectance and transmission spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, micro x-ray photoemission spectroscopy, temperature-dependent photoluminescence (PL) spectroscopy and photocurrent spectroscopy. To complement the material characterization we perform ab initio calculations of the thickness dependency of the PbI₂ band structure. Even though many methods have been demonstrated to synthesize PbI₂ [22], such as the Bridgman method, solution processing and vapor deposition studies of this material are still rather scarce [23]. Our results demonstrate that crystalline large-area ultra-thin lead iodide flakes can be easily synthesized at room temperature from an aqueous solution.

Lead iodide is a layered material, belonging to the TMHs family, which has been studied in applications such as source for perovskite solar cells [19], photodetectors [24] and nuclear radiation detectors [25]. Its structure, schematically depicted in figure 1(a) for a single-layer, is formed by covalently bonded I and Pb atoms arranged in an in-plane hexagonal lattice with out-of-plane van der Waals interactions between the different layers [26]. In order to gain insight into the expected material electronic properties, we computed the band structure based on the crystal structure described above (see computational methods section in the supplementary information is available online at stacks.iop.org/NANO/28/455703/mmedia). Figure 1(b) shows the band structure calculated for a PbI₂ single-layer (left) and for a bulk crystal (right). When going from bulk to a single-layer the nature of the bandgap changes, passing from direct in bulk (with a value of 2.26 eV) to indirect in the monolayer (2.64 eV). The magnitude of the direct gap increases monotonically when thinning down the material from a bulk crystal to a single-layer, as can be seen in figure 1(c) and the crossover from a direct to an indirect bandgap happens between one and two layers, in agreement with literature [27].

Zoom In Zoom Out Reset image size

Thin PbI₂ crystals, with lateral dimensions of up to approximately 100 μm and thicknesses ranging from a few nanometers up to hundreds of nanometers, can be synthesized with a simple precipitation process [23]. We use as a starting material PbI₂ in powder (99% purity, Sigma-Aldrich). The solubility of PbI₂ in water strongly depends on the temperature in the range between 20 °C and 100 °C, with PbI₂ being more soluble in hot water (4.1 g l⁻¹ at 100 °C) than in cold water (0.8 g l⁻¹ at 20 °C) [28, 29]. Panels 1 and 2 of figure 2(a) show a vial containing a supersaturated solution of PbI₂ powder (≈0.1 g) in Milli-Q water (20 ml) respectively at 25 °C and at 100 °C. The change in solubility of PbI₂ is reflected by the change in the color of the liquid. During the cooling down of the solution, dissolved Pb and I species start to aggregate and form crystals of PbI₂, which appear as a fine dust in panels 3–5 of figure 2(a) (see the comparison between the top and bottom rightmost panels in figure 2(a)). To isolate the PbI₂ nanosheets, a 1 ml droplet of the saturated PbI₂ solution at 100 °C is drop-cast onto the surface of a polydimethylsiloxane (PDMS) stamp. When the drop cools down, PbI₂ crystals of various sizes and thicknesses grow, as shown in the optical micrographs recorded at different times shown in figure 2(b). The crystallization process can last from a few seconds to several minutes and by monitoring the process with a microscope (20× magnification, working distance of 20.5 mm) we can draw up the liquid at the desired time from the PDMS surface with a pipette in order to reach the desired density of crystals on the PDMS. The advantage of this method is that the as-produced crystals can be readily transferred to another substrate or onto pre-patterned nanoscale structures using the deterministic transfer method [30]. Moreover, thinner flakes can be achieved by mechanical exfoliation of the as-crystallized flakes with another PDMS stamp. The crystallization process is reproducible and gives rise to PbI₂ crystallites with hexagonal shape (see supplementary information), consistent with previous studies using different PbI₂ syntheses [27, 31, 32].

Zoom In Zoom Out Reset image size

To relate the thickness of the PbI₂ crystals to the optical properties of the material we first characterized the PbI₂ flakes by a combination of optical and AFM. Figures 3(a)–(e) show optical micrographs of different flakes that have been transferred onto a SiO₂/Si substrate with 285 nm oxide. Similar to the case of graphene and other 2D materials, different flakes deposited on SiO₂/Si present different colors according to their thickness because of thin-film interference effects [33]. We have used AFM measurements to accurately determine the thickness of the flakes in figure 3. From comparison of the AFM data to the microscope pictures we extract a calibration chart, shown in figure 3(f), which can be used to estimate the thickness of PbI₂ flakes from their color at first glance. More quantitative information can be obtained from the analysis of the red/green/blue channels of the optical images. Figure 3(g) shows the optical contrast of the PbI₂ flakes on SiO₂ calculated for different flakes from the intensity of the red/green/blue channels according to C = (I_R,flake − I_R,sub)/(I_R,flake + I_R,sub), where C is the optical contrast and I_R,flake (I_R,sub) is the intensity of the light reflected by the PbI₂ flake (substrate). When the contrast is zero the flake is not detectable, while when the contrast is positive (negative) the flake appears brighter (darker) than the substrate. In all the three channels the optical contrast of PbI₂ flakes does not present a monotonic dependence on the thickness, but instead it shows oscillations and it explores both positive and negative values because of thin film interference effects. Notice that the PbI₂ flakes in panels (d), (e) and (g) of figure 3 have been mechanically exfoliated from thicker flakes.

Zoom In Zoom Out Reset image size

Another simple method that is used to estimate the thickness of ultra-thin 2D materials is the analysis of a photograph of the flake under study deposited on a transparent substrate and recorded in transmission illumination mode with a white light source. The inset of figure 3(h) shows a microscope picture of a PbI₂ flake on PDMS illuminated in transmission mode. Different colors and shades correspond to different values of thickness. We extract the transmission, T = I_T,flake/I_T,sub, for red/green/blue channels at five different points on the flake. After recording the transmission illumination mode microscope optical photograph we transfer the flake onto a SiO₂/Si substrate to perform AFM measurements and extract the thickness of the flake. Figure 3(h) shows the local transmission of the flake as a function of the thickness for red/green/blue channels. Due to the large band gap of the PbI₂, the transmission of the flakes remains close to unity in the red and green channels even for thicker flakes. By contrast, the transmission extracted from the blue channel shows a monotonic dependence on the thickness and it can be used to have a rough and quick estimation of the thickness of ultra-thin PbI₂ crystals.

To extract more quantitative information about the optical properties of PbI₂ we study the thickness dependence of the optical contrast of PbI₂ on SiO₂/Si resolved in energy. To this end, we employ a micro-reflectance spectroscopy set-up that was previously described elsewhere [34, 35]. Briefly, the flakes are illuminated in normal incidence with the white light produced by a tungsten halogen lamp, and the reflected light is collected with an optical fiber (core diameter of 105 μm), which acts as a confocal pinhole, placed at the image plane of the trinocular of an optical microscope. The other end of the fiber optic cable is connected to a compact charge-coupled device (CCD) spectrometer (Thorlabs), allowing to measure the spectrum of the reflected light from 400–900 nm with ≈1 nm resolution. By measuring the reflectance spectrum of the bare substrate (SiO₂/Si) and that of the PbI₂ flake, we can determine the optical contrast C in a broad range of the visible and near-infrared part of the electromagnetic spectrum. Figure 4(a) shows the optical contrast spectra recorded for four flakes with different thickness. A comparison of the different spectra shows that the largest positive contrast can be achieved at a wavelength of ≈550 nm, independent of the PbI₂ thickness. At longer wavelengths the optical contrast spectra reverse their sign, going from positive to negative, and present a dip with the largest negative contrast at a wavelength in the range between 600 and 800 nm depending on the thickness of PbI₂. Figure 4(b) shows the optical contrast as a function of the PbI₂ thickness, extracted at different wavelengths between 450 and 700 nm. The contrast data have an oscillatory behavior with a larger period for longer wavelengths, which can be well fit to the Fresnel equation represented by dashed lines in figure 4(b) [33]. From each spectrum taken at a certain wavelength λ, we extract the complex refractive index at that wavelength n(λ).

Zoom In Zoom Out Reset image size

From the Fresnel equation fit of figure 4(b), by fitting hundreds of curves with different wavelengths between 400 and 800 nm, we calculate the wavelength-resolved PbI₂ complex refractive index n = n − iκ. Figure 5(a) shows the refractive index n and the extinction coefficient κ of PbI₂ as a function of the wavelength. The refractive index is a slowly varying function of the wavelength while the extinction coefficient, which is related to the absorption, shows sharper features. For wavelengths longer than 510 nm, κ takes values close to zero, indicating that the material does not absorb at these wavelengths. On the other hand, at wavelengths shorter than 510 nm the extinction coefficient steadily increases, which is due to direct band-to-band transitions. Finally, we use the estimated complex refractive index to calculate the optical properties of a thin film of PbI₂ under normal incidence using the Fresnel equation. Figure 5(b) displays a colormap that represents the calculated optical contrast C of a monolayer PbI₂ as a function of the illumination wavelength and the thickness of the SiO₂ dielectric layer. For a given excitation wavelength, the contrast oscillates around zero with a period that increases with the wavelength of the incident light. The maximum contrast is achieved for SiO₂ thicknesses of approximately 70, 220 and 350 nm. Notice that the abrupt change in the colormap around 510 nm is due to a change in the magnitude of the optical contrast caused by the strong absorption of PbI₂ at the bandgap (see the graph of κ versus wavelength).

Zoom In Zoom Out Reset image size

Raman spectroscopy, which has been demonstrated to be a very powerful tool to characterize the quality and the thickness of 2D materials [36, 37], was employed to characterize the flakes previously studied in figures 3 and 4. Figure 6(a) shows the Raman spectra collected for flakes 23–180 nm thick whose intensities are all normalized by the intensity of the peak at 520 cm⁻¹, arising from transverse optical modes of the Si substrate. The excitation wavelength is 532 nm (2.33 eV), which is close to resonance with the fundamental absorption band (≈2.3 eV). The spectra show an intense and broad peak centered at approximately 214 cm⁻¹ that is characteristic of the ${{{\rm{A}}}_{1}}^{2}$ vibrational mode of hexagonal PbI₂ and is indicative of the crystallinity of the sample [38–40]. The intensity of this peak increases with the PbI₂ thickness and can be used to estimate the thickness of the material without the need of topographic measurements. Figure 6(b) shows the ratio between the ${{{\rm{A}}}_{1}}^{2}$ peak and the Si 520 cm⁻¹ peak as a function of the PbI₂ thickness. Due to the strong background in the Raman spectra we cannot access the energy range below 150 cm⁻¹ which contains additional Raman peaks of PbI₂ that are sensitive to the crystalline phase of the material [41, 42].

Zoom In Zoom Out Reset image size

To characterize the crystal structure of the fabricated PbI₂ flakes we study them with high-resolution TEM. The samples are fabricated by simply drop casting the saturated hot solution of PbI₂ and water onto a nickel TEM grid with 6 μm holes, which leads to crystallization of thin hexagonal flakes covering a few holes. Figure 7(a) shows a high-resolution TEM image (20 nm × 20 nm) of a PbI₂ flake where individual Pb atoms are visible and in the inset a low-magnification image (3 μm × 3 μm) of the same flake is shown. To better analyze the atomic arrangement we show a zoom of the TEM image in figure 7(b) where we superimposed a drawing of the PbI₂ lattice in the 2H polytype. We find a perfect match between the atomic arrangements in the TEM image and the PbI₂ 2H polytype generated lattice, which is in agreement with the minimum energy structure found in the ab initio calculations and with previous studies with similar synthesis [26, 43]. Notice that because of differences in the notation, the 2H polytype in PbI₂ is identical in stacking to the 1T polytype in TMDC such as MoS₂ or WSe₂ [44].

Zoom In Zoom Out Reset image size

Additionally, using a Matlab algorithm we extract the distance between all the pairs of prime- and second-nearest neighbor atoms in figure 7(a) (see the supplementary information). We find an average distance of (2.65 ± 0.03) Å for the nearest neighbors, which is consistent with the lead–lead distance and corresponds to the lattice spacing of the (100) planes. The second-nearest neighbor pairs of atoms have an average distance of (4.56 ± 0.02) Å which matches perfectly the lattice constant of PbI₂ [15]. The 2D Fourier transform of the TEM image, shown in figure 7(c), reveals the long-range order of the imaged lattice. Finally, the clear diffraction spots of the electron diffraction image of figure 7(d) are consistent with the single-crystalline nature and long-range order of the fabricated PbI₂ flakes.

We also perform micro-XPS and low-energy electron microscopy measurements of PbI₂ crystallites transferred onto a platinum substrate. The results, discussed in the supplementary information, indicate a steady desorption of iodine with time when the sample is placed in ultrahigh vacuum (10⁻¹⁰ mbar) and under x-ray irradiation. We observe a full desorption of the iodine atoms when the samples were annealed at temperatures larger than 150 °C in ultrahigh vacuum conditions.

After establishing the crystallinity of the PbI₂ ultra-thin crystals, we investigate their optical properties with temperature-dependent PL measurements. The PbI₂ flakes grown from solution on PDMS are deterministically transferred onto a SiO₂/Si substrate and subsequently studied in a self-built micro-PL system, details are published elsewhere [45]. Briefly, a continuous-wave laser (central wavelength 375 nm) is coupled into a 100× microscope objective and focused to a sub-micron spot on the sample, which is mounted in a He-flow cryostat. The PL emission from the sample is collected by the same microscope objective and coupled into a spectrometer with a Peltier-cooled CCD sensor.

In figure 8(a) we plot the PL spectra of three different flakes with thickness from 23 to 395 nm in a semi-logarithmic representation recorded at 4 K and with the same excitation power of 10 μW (see the supplementary information for optical pictures of the flakes). The spectra present common features, starting from high energy we find a peak centered at 2.49 eV. The 395 nm thick flake shows a shoulder on the low-energy side of the first peak. Additionally, in all three spectra a weaker peak is present at 2.42 eV and a broad peak or double peak is centered at lower energies between 2.1 and 1.8 eV. The peak at 2.49 eV is the fundamental excitonic peak due to direct band-to-band transition, and its energy is in good agreement with the onset of absorption observed in the refractive index data discussed above [46, 47]. We note that all flakes investigated in the PL measurements are so thick that neither changes of the band structure nor exciton confinement effects which would shift the transition energies are to be expected. Remarkably, we observe that the thinnest flake (23 nm) shows the highest PL intensity. The peak at 2.42 eV can be interpreted as due to bound exciton by a comparison with literature [40]. On the other hand, the lowest peak at 2.05 eV, which is called G band, does not have an excitonic origin, but is due to defects in the surface of the PbI₂ samples and especially to the presence of Pb⁺ ions [40, 42]. Figure 8(b) shows the PL spectra of the 23 nm thick flake recorded at different excitation powers. We extract the intensity of the free and bound excitonic peaks and of the G band for different laser powers and figure 8(c) shows the results for the 23 nm thick flake. From the log–log plot one can see that for all the peaks the emission intensity I displays a power-law dependence on the excitation power, which can be described by the formula I ≈ P^k. The dependency of the PL intensity on the laser power is an indicator of the nature of the different recombination processes. The intensity of the peaks at 2.49 and 2.42 eV can be fit with exponents k = 1.6 and 1.5 respectively, indicating a super-linear scaling of I versus P. By contrast, the G band intensity scales sub-linearly with the excitation power with exponent k = 0.8. According to literature, a value of k in the range between 1 and 2 is related to exciton-like transitions while a value of k smaller than 1 is related to recombination of trapped with free carriers [48, 49], confirming that the G band is due to defects in the crystal structure and that the higher energy peaks are due to excitonic transitions.

Zoom In Zoom Out Reset image size

We also investigate the temperature dependence of the PL emission. Figure 8(d) shows the PL spectra of a 23 nm thick PbI₂ flake recorded between 4 and 295 K. Between 4 and 50 K, we observe a slight blueshift for the 23 and 32 nm thick flakes (see figure 8(e)), indicative of a de-trapping of excitons bound to shallow defects such as charged surface adsorbates. As the temperature is increased further, the main excitonic peak shows an energetic redshift and its intensity decreases. This temperature dependence of the excitonic peak reflects the thermally induced change in the bandgap of PbI₂ and the graph of the excitonic peak center versus temperature of figure 8(e) can be fit to the Bose–Einstein model, see supplementary information.

Finally, we study the optoelectronic properties of thin PbI₂ flakes by fabricating photodetector devices. In total we fabricated five PbI₂-based photodetectors, four of which were fabricated on a SiO₂/Si substrate while one on a transparent and flexible polycarbonate substrate (see section 6 of the supplementary information). We transfer a 15 nm thick PbI₂ flake from a PDMS substrate, see figure 9(A), bridging drain and source Au electrodes pre-patterned on a SiO₂/Si substrate (see supplementary information for the AFM profile of the flake). The Si is heavily doped in order to use it as a back gate electrode and in the measurements discussed below it is kept at zero voltage. An optical picture of the final device is shown in figure 9(b). We record current versus drain–source voltage (IV_SD) characteristics of the device in dark and under illumination. We use various high-power LEDs, with central wavelengths ranging from 375 to 1050 nm, that are focused on the surface of PbI₂ on spot diameter of 200 μm providing a homogenous illumination over the entire device area. Figure 9(c) shows IV_SD characteristics acquired in dark and under illumination using a 405 nm LED with increasing optical power. The optical power indicated in the plots, P_D, is the power of the radiation falling on the device, calculated by dividing the total beam power by the spot area and multiplying the result for the area of the PbI₂ transistor channel (684 μm²) estimated from the microscope picture. The photocurrent (difference between the current upon illumination and in dark) generated in the device displays a sub-linear dependence on the incident power (exponent equal to 0.7, see supporting information) which suggests the influence of defects in the dynamics of the charge carriers [50–52]. The semi-logarithmic representation of the IV_SD of figure 9(d) reveals an on–off ratio larger than 2000 for the largest optical power.

Zoom In Zoom Out Reset image size

We also study the time response of the device by recording the current at a fixed source–drain voltage as a function of time (It) while modulating the light intensity with a ≈0.5 Hz square wave. Figure 9(e) report the current at V_SD = 1 V in dark and under modulated illumination. We cannot directly resolve the switching times of the device when the illumination is turned on or off with our experimental time resolution. Instead, we can define an upper boundary of 20 ms for the device switching time. By recording It traces with different excitation wavelength all set at the same optical power we can study the energy dependence of the PbI₂ responsivity. Figure 10(a) shows three It traces recorded at wavelengths λ = 405, 455 and 505 nm. The device displays the same fast response to all the wavelengths and has the largest response for the shortest wavelength. We extract the photocurrent generated at each wavelength I_Ph(λ) in the on state and we calculate the responsivity according to R(λ) = I_Ph(λ)/P_D. In the top panel of figure 10(b) we plot the responsivity extracted from the It traces as a function of excitation wavelength. The responsivity is zero for wavelengths larger than 505 nm and it steadily increases at shorter wavelengths with the largest value recorded equal to 1.3 × 10⁻³ A W⁻¹, which is a low value compared to other multilayer 2D materials [52]. The behavior of the responsivity is consistent with the bandgap of PbI₂ of 2.38 eV, corresponding to a wavelength of 521 nm, extracted from the Tauc plot of the absorbance (with Tauc exponent for a direct gap) shown in the bottom panel of figure 10(b) [32]. The different photodetectors built from PbI₂ and shown in the supplementary information have comparable responsivities to the main text device.

Zoom In Zoom Out Reset image size

In conclusion, we have discussed an easy route to synthesize highly crystalline nanosheets of PbI₂ from an aqueous solution and we presented a thorough characterization of the resulting material. We studied the reflectance of PbI₂ as a function of the crystal thickness to extract the refractive index components n and κ of the material and found that the onset of absorption spectrally matches the main emission peak we observed in PL. Additionally, temperature- and power-dependent PL measurements revealed defect-related emission and a redshift of the main emission peak with increasing temperature due to a decreasing band gap. Raman spectroscopy, TEM and electron diffraction measurements demonstrated the crystallinity of the as-grown and exfoliated PbI₂ samples. Finally we fabricated photodetectors based on PbI₂ thin flakes and studied their optoelectronic properties. The experiments have been compared to ab initio calculations of the electronic band structure of the material.

We acknowledge financial support from the European Commission under the Graphene Flagship (CNECTICT-604391), and European Research Council (ERC-StG-MINT 307609), the MINECO, the Comunidad de Madrid, the Netherlands Organisation for Scientific Research (NWO), and the German Science Foundation (DFG). We are also grateful to B Galler and R Zeisel (OSRAM OS, Regensburg) for technical assistance in the photoluminescence experiments. JLL and JFR acknowledge financial support by Marie-Curie-ITN 607904-SPINOGRAPH. JFR acknowledges financial support from MEC-Spain (MAT2016-78625-C2).

The authors declare no competing financial interests.

ACG European Commission under the Graphene Flagship: contract CNECTICT-604391. ACG MINECO: Ramón y Cajal 2014 program RYC-2014-01406 ACG MINECO: program MAT2014-58399-JIN. ACG Comunidad de Madrid: MAD2D-CM program (S2013/MIT-3007). RF Netherlands Organisation for Scientific Research (NWO): Rubicon 680-50-1515. DPdL MINECO: program FIS2015-67367-C2-1-p. EMP European Research Council (ERC-StG-MINT 307609). EMP MINECO: CTQ2014-60541-P. EG AMAROUT II fellowship program: grant for transnational mobility (Marie Curie Action, FP7-PEOPLE-2011-COFUND (291803)). PN, TK: DFG: GRK 1570 and KO3612/1-1. JML: DFG SFB 689 JLL, JFR Marie-Curie-ITN 607904-SPINOGRAPH. JFR MINECO-Spain MAT2016-78625-C2. LA and MF: MINECO: MAT2015-64110-C2-2-P.