Temperature−Dependent Raman Scattering Investigation on vdW Epitaxial PbI₂/CrOCl Heterostructure

Abstract

Van der Waals (vdW) epitaxial growth provides an efficient strategy to prepare heterostructures with atomically and electronically sharp interfaces. Herein, PbI₂ was in situ thermally deposited onto exfoliated thin−layered CrOCl nanoflakes in high vacuum to fabricate vdW PbI₂/CrOCl heterostructures. Optical microscopy, atomic force microscopy, X−ray diffraction, and temperature−dependent Raman spectroscopy were used to investigate the structural properties and phonon behaviors of the heterostructures. The morphology of PbI₂ films on the CrOCl substrate obviously depended on the substrate temperature, changing from hemispherical granules to 2D nanoflakes with flat top surfaces. In addition, anomalous blueshift of the $A_g^1$ and $A_u^2$ modes as the temperature increased in PbI₂/CrOCl heterostructure was observed for the first time. Our results provide a novel material platform for the vdW heterostructure and a possible method for optimizing heterostructure growth behaviors.

1. Introduction

van der Waals (vdW) epitaxy provides an efficient strategy to prepare heterostructures with atomically and electronically sharp interfaces [1,2,3]. The two−dimensional (2D) vdW heterostructures can be synthesized via chemical vapor deposition / physical vapor deposition (CVD/PVD), molecular beam epitaxy, or other synthesizing methods [4,5,6,7]. It is reported that 2D vdW heterostructures with different energy band alignments can play a key role in the field of optoelectronic devices. Heterostructures with type−Ⅱ band alignment may be applied in photovoltaics and optoelectronics, while those with type−Ⅰ and type−Ⅲ band alignment can be used for light−emitting and tunneling field−effect transistors, respectively [8,9,10].

Layered lead iodide (PbI₂) consists of three atomic planes, which are bonded in the sequence of I−Pb−I repeating units and stacked along the c−axis, as shown in Figure 1a. It is reported that PbI₂ exhibits strong light adsorption and emission with a band gap continuously tunable with the number of layers, having an indirect band gap over 3.72 eV for monolayer PbI₂ and a direct band gap of 2.38 eV for multilayer PbI₂ [11,12,13]. Heterostructures based on PbI₂ exhibit good potential in optoelectronic applications, such as 2D–1D vdW heterostructure photodetector [14] and flexible or stretchable electronics [5]. In addition, PbI₂ is an important precursor for perovskite luminescent materials [15]. Layered chromium oxide chloride (CrOCl) has D112h point group symmetry [16], and the Cr–O layers sandwiched by Chlorine atom layers are coupled to each other by vdW forces, as shown in Figure 1a. CrOCl is a kind of wide band gap P−type semiconductor with strong in−plane optical anisotropy and out−of−plane antiferromagnetic order [17,18,19]. In low−temperature ranges, because of the similar crystal structure with CrSBr, CrOCl has a temperature−dependent magnetic order structure and can exhibit three thermodynamic transitions [20]. In our previous studies, vdW PbI₂−TMDCs (MoS₂ and MoSe₂) were fabricated through thermal deposition under high vacuum conditions and showed type−Ⅱ band alignment [21,22]. Different from the heterostructures formed between isotropic 2D materials, photoelectric devices based on MoS₂/CrOCl heterostructures have various applications, such as nonlinear optics in mid−IR band [18] and spin−dependent photoelectric device [17], owing to the strong in−plane anisotropy and the out−of−plane antiferromagnetic order of CrOCl. In addition, ultraviolet photoelectric detectors have attracted the attention of researchers because of their potential applications in solar ultraviolet radiation detection, environmental monitoring, and so on. Because of the wide band gap and high thermal conductivity of CrOCl [18,23,24], PbI₂/CrOCl heterostructures have great potential to be used in ultraviolet detectors and thermal transport devices in the future.

In this article, we report our investigation on multilayered PbI₂/CrOCl vdW heterostructures. The heterostructures were fabricated via thermal deposition in high vacuum and investigated ex situ using optical microscopy (OM), atomic force microscopy (AFM), X−ray diffraction (XRD), and temperature−dependent Raman spectroscopy (TDRS). The morphology of PbI₂ film on CrOCl nanoflakes changed from granules to 2D nanoflakes with flat and smooth top surfaces with increasing substrate temperature. Zoomed−in AFM images show that triangular PbI₂ nanoflakes grew on top with the [$\overline{1}$100] direction along the CrOCl [010] direction, confirming epitaxy. Furthermore, we observed that the $A_g^1$ and $A_u^2$ modes of PbI₂ in heterostructure represented anomalous blueshift as the temperature increased.

2. Experimental

Sample preparation: CrOCl nanoflakes were mechanically exfoliated using Scotch tape from bulk crystals (Shanghai Prmat Technology Co., Ltd., Shanghai, China, http://www.prmat.com/ accessed on 15 September 2021) onto 300 nm SiO₂/Si substrates. PbI₂ film was deposited on the CrOCl substrate by thermal evaporation under a pressure of 1.0 × 10⁻⁵ Pa. There was a temperature−controlled k−cell at the bottom of the vacuum system, loaded with PbI₂ (Aldrich, Shanghai, China, 98+ %) powder. The CrOCl substrate was placed on a heating stage in the vacuum chamber and heated using the resistive method. PbI₂ was evaporated at 498 K at a nominal rate of 1.2 nm/min, and the growth time was fixed at 16 min. To improve the quality of the PbI₂ film, the substrate temperature was optimized.

Characterization: The morphologies and topography of PbI₂/CrOCl heterostructures were characterized using OM (CaiKang DMM−200C, Shanghai, China) and AFM (Agilent, Palo Alto, CA, USA) in the tapping mode [25,26]. XRD patterns were collected with a Bruker D8 Advance diffractometer. Raman measurements were performed in the inVia Qontor system (Renishaw, London, UK) using a 532 nm laser with a spot size of less than 1 μm and 1800 lines/mm grating [27,28]. During the temperature−dependent Raman spectroscopy measurements, the samples were mounted in a programmable stage THMS600 (Linkam Scientific Instrument, Salfords, UK) to control the sample temperatures. The measurements were started from 80 K and increased to 300 K in steps of 10 K. At each step, the samples were held for 5 min for temperature stability.

3. Results and Discussion

Owing to the small exfoliation energy of CrOCl of 0.2 J/m² [23], thin−layered nanoflakes can be easily exfoliated from bulk crystals. Figure 1b shows a representative AFM image of the exfoliated thin−layered CrOCl nanoflake, and the corresponding thickness was ~7 nm. The obvious layered fracture of exfoliated nanoflake confirmed the low vdW force between the layers [18]. The roughness of the CrOCl nanoflake surface was 0.41 nm, representing the positive air stability [29] and the cleanliness, which were helpful for the interface coupling in PbI₂/CrOCl heterostructures. The corresponding OM image is shown at the top right corner in Figure 1b. The fracture directions of the CrOCl nanoflake represent two mutually perpendicular ones because the ideal tensile strength was higher [30].

To identify the accurate crystalline directions of CrOCl nanoflakes, Raman scattering measurements were performed. Figure 1c shows two typical Raman spectra of CrOCl nanoflake with the polarization of incident laser along the long straight edge (blue) and the short straight edge (red) at room temperature. There are three peaks located at 207, 414, and 455 cm⁻¹ ascribed to the $A_g^1$, $A_g^2$, and $A_g^3$ modes of CrOCl, respectively, in agreement with previous works [17]. The corresponding atomic vibrations of these three optical phonon modes were inserted. The $A_g^1$ and $A_g^2$ modes belong to the out−of−plane vibration model along the [001] direction, while the $A_g^3$ mode belongs to the in−plane mode along the [100] direction [17]. The relative intensity of these modes is totally different along the two given directions. The difference in maximum intensity direction between the $A_g^1$ and $A_g^2$ modes might be caused by the electronic states involved in Raman scattering affected by phonon energy [31]. The additional peak located at 300 cm⁻¹ arose from the strain inside the SiO₂/Si substrate [32]. It is reported that the crystalline orientations of low symmetry 2D materials, such as ReS₂, b−As, and MoO₃, can be determined by angle−resolved polarization Raman measurements [33,34,35]. The orientation of the $A_g^1$ mode could be decomposed to the components along the [001] and [100] directions, respectively [17,36]. The maximum intensity of the $A_g^1$ mode existed while the incident light was parallel to the [100] direction in polarized Raman spectra. The inserted intensity polar plot of the $A_g^1$ mode in Figure 1c shows a period of 180° and maximum intensity along the short straight edge, indicating that the long straight edge was along the [010] direction while the short one was along the [100] direction. The Raman intensity mapping image of the $A_g^1$ mode in Figure S1 (Supplementary Materials) presents a uniform contrast, indicating that the CrOCl nanoflake possessed a high crystalline degree. These results guarantee that the high quality of the exfoliated CrOCl nanoflakes is suitable for our further investigations.

The structural properties of vertical PbI₂/CrOCl heterostructures were characterized using AFM. Firstly, PbI₂ was deposited onto the substrate maintained at room temperature. Different from our previous reports on MoS₂ and MoSe₂ [21,22], PbI₂ aggregated into semispherical granules but not 2D flat films on CrOCl nanoflakes, as shown in the typical AFM image in Figure 2a. The bright protrusions over the whole substrate surface indicate that PbI₂ had similar growth behaviors on CrOCl as those on SiO₂ at room temperature. It is clearer in the zoomed−in AFM image from the white−dashed box area in Figure 2b. The statistical grain sizes of PbI₂ clusters on CrOCl and SiO₂ substrates were almost the same as shown in green and red bars, respectively, in Figure 2g. The same is true for the nucleation densities. The corresponding surface roughness was 1.25 and 1.55 nm, respectively. This might be related to the large diffusion barrier that induced limited diffusion lengths of PbI₂ on both surfaces. Post−annealing would possibly improve the quality of the PbI₂ films. The as−deposited samples were sequentially annealed up to ~473 K in steps of ~50 K for 1 h at each step. It was found that annealing at ~423 K caused the PbI₂ on CrOCl to recrystallize into a 2D form with an atomically flat top surface, as shown in Figure 2c,d, while those on SiO₂ were still in granule form but larger, as shown by the yellow bars in Figure 2g. Post−annealing at ~473 K led to PbI₂ desorption, while post−annealing at ~373 K had a less obvious effect on the films, as shown in Figure S2. The corresponding surface roughness for annealing at ~423 K was reduced to 0.53 and 0.96 nm on CrOCl and SiO₂, respectively. The still rough surface of PbI₂ on CrOCl was ascribed to the single−layer deep (0.70 nm) pits, as shown in the profile along the black line in Figure 2d. The statistical graph in Figure S3 shows that the number of pits in the post−annealed sample was significantly less than that in the as−deposited sample. The above results confirm that the growth of PbI₂ on CrOCl is diffusion−limited at room temperature.

To realize the vdW epitaxy of PbI₂ on CrOCl, the substrate temperature was kept at 423 K during the deposition. As shown in the AFM image in Figure 2e,f, 2D films with a surface roughness of 0.74 nm with some triangular features on top were prepared on CrOCl, while the larger and isolated PbI₂ islands were prepared on SiO₂, as shown in the blue bars in Figure 2g. The corresponding line profile shows that the triangular features had thicknesses of 0.70 nm, the same as that of the single−layer PbI₂. Some of them have thicknesses of multiples of 0.70 nm, meaning multilayers. The size of PbI₂ islands is much larger than those in the previous two cases, as shown in Figure 2g. Interestingly, the triangular features show an obvious orientation preference. Figure S4 shows the statistical diagram of the orientation of PbI₂ on CrOCl in triangular islands. We define the included angle between one side of the triangular island and the [010] direction of CrOCl as the twist angle. The statistical diagram shows 180° intervals, which is similar to the case of the highly oriented PbI₂ on mica observed by Debjit Ghoshal et al. [37], indicating the existence of vdW epitaxy here. Such uniform orientation of PbI₂ nanoflakes ensures they merge into a single−crystalline flake on CrOCl during the growth process. According to the crystal structure of triangular and hexagonal single−crystal PbI₂ nanoflakes fabricated using the PVD method [38,39], the sides of the PbI₂ triangular nanoflakes on CrOCl could be determined to be [$\overline{1}$100]. Thus, the epitaxial relationship between PbI₂ and CrOCl could be determined to be that the [$\overline{1}$100] direction of PbI₂ was parallel to the [010] direction of the CrOCl nanoflake. The lattice spacing values of PbI₂ in [$\overline{1}$100] and [$\overline{1}\overline{1}$20] were 4.56 and 3.95 nm, respectively, and those for CrOCl in [010] and [100] were 3.25 and 3.94 nm, respectively. The atomic model built according to the above results in Figure 1a shows that the lattice mismatch was 0.21% and 0.25% along the CrOCl [010] and [100] directions, respectively, which were small enough, justifying our proposed atomic model.

As the substrate temperature increases, the molecules gain thermal energy to merge into larger particles [40]. The statistical results in Figure 2g show that the average grain sizes of the as−deposited PbI₂ on CrOCl and SiO₂ were 41.16 and 50.58 nm, respectively. Upon annealing at 423 K, the average grain size of PbI₂ on SiO₂ increased to 70.65 nm, while that deposited at 423 K enlarged to 130.54 nm. This can be further demonstrated in the line profiles across the CrOCl edges for the three cases, as shown in Figure 2h. For the first two cases, the line profiles look smooth because the PbI₂ films on both sides were compact. For the last case, the line profile shows ~22 nm bumps because PbI₂ nucleated into larger clusters with an exposed SiO₂ surface. According to the good epitaxial relationship between PbI₂ and CrOCl, we speculate that there were no interfaces or stacking layers which might trap or aggregate defect trapping during the fabrication process.

Figure 3a shows the XRD patterns taken from the three kinds of samples. The positions of diffraction peaks are basically the same. The peak located at 11.5° belongs to the (001) plane of the CrOCl nanoflake [17]. The high intensity of this peak in the samples of PbI₂ films deposited at room temperature was caused by the high coverage of exfoliated CrOCl nanoflake. The other characteristic peaks located at 12.6°, 25.4°, and 38.7° belong to the (001), (002), and (003) crystal planes of 2H−phase PbI₂, respectively, in agreement with previous studies [11]. Different from our previous reports [22], all three peaks appear much broader, maybe ascribed to the smaller thickness of the deposited PbI₂. According to the well−known Debye−Scherrer formula [41],

$$D=\frac{0.94 \lambda }{\beta cos \theta } ,$$

(1)

where $D$ is the grain size, $\lambda$ is the wavelength of the scattered radiation, $\beta$ is the width−width at half maxima of the diffraction peak, and $\theta$ is the scattering angle, the average PbI₂ thickness in the three cases are calculated to be 10 ± 2 nm.

Figure 3b shows the typical normalized Raman spectra of PbI₂/CrOCl heterostructure in the above three cases, as well as that of pure PbI₂ on SiO₂/Si substrate (solid green curve). The peaks were observed at 70, 95, and 110 cm⁻¹, corresponding to the $E_g$ (in−plane), $A_g^1$ (out−of−plane), and $A_u^2$ (longitudinal acoustic mode) modes of PbI₂, respectively [42,43]. Different from the $E_g$ and $A_g^1$ modes belonging to optical phonon modes, the $A_u^2$ mode is a kind of non−Raman−active acoustic mode with vibration along the [001] direction, which is hard to observe most times [44]. However, because the deposited PbI₂ films have the interlayer restoring force, which results in the nonzero frequencies, the $A_u^2$ mode could be observed at the backscattering Raman configuration [44,45]. Because the out−of−plane phonon intensity decreased faster than the in−plane phonon intensity as the thickness increased, the ratio of Raman intensity (I ($A_g^1$)/I ($E_g$)) could be used to quickly determine the thicknesses of the PbI₂ films [46]. The ratios for the three cases were 1.83, 2.10, and 1.54, respectively, consistent with the above discussions. The fitted phonon mode positions and widths of the three samples are summarized in

Table 1. Phonon mode information of PbI₂/CrOCl heterostructure and PbI₂.

Phonon Modes	Peak Position (cm−1)				Peak Width (cm−1)
	PbI2/CrOCl (Deposited at RT)	PbI2/CrOCl (Annealed at 423 K)	PbI2/CrOCl (Deposited at 423 K)	PbI2/SiO2	PbI2/CrOCl (Deposited at RT)	PbI2/CrOCl (Annealed at 423 K)	PbI2/CrOCl (Deposited at 423 K)	PbI2/SiO2
$E_g$	70.0	70.5	71.8	70.1	10.7	9.6	8.0	8.3
$A_g^1$	95.4	95.3	95.1	95.1	9.2	8.9	8.0	8.8
$A_u^2$	110.2	109.9	109.9	109.8	18.0	18.7	15.6	17.0

. It can be seen that the mode positions in all the PbI₂ samples were basically the same, but the mode peak widths for the last case were obviously smaller than the others, confirming the improved crystal quality. Therefore, the peak intensity of $A_u^2$ mode increased in the last case.

TDRS measurements were performed on the PbI₂/CrOCl vdW heterostructure with the highest quality to demonstrate the possible substrate effect. Four selected spectra are shown in Figure 4a. The peak positions of PbI₂ are basically unchanged through the whole temperature range, especially for the $A_g^1$ mode. However, the $A_u^2$ mode displays an obvious enhancement in intensity as the temperature increases. The measured peak positions of the $E_g$, $A_g^1$, and $A_u^2$ modes as functions of temperature are plotted in Figure 4b–d, respectively. The $E_g$ mode exhibits linear redshift, while the $A_u^2$ mode shows linear blueshift as the temperature increases. The abnormal blueshift of the $A_g^1$ mode at temperatures above 260 K could have come from the slip between PbI₂ and CrOCl substrate induced by thermal expansion mismatch, similar to previous reports [43,47]. The $E_g$ mode moved from 74.3 to 71.4 cm⁻¹, while the $A_g^1$ mode moved from 96.2 to 95.6 cm⁻¹ over the whole temperature range, indicating the temperature dependence of the $E_g$ mode was stronger than that of the $A_g^1$ mode. The temperature dependence of the Raman mode position could be fitted by the $\mathrm{Gr}\ddot{\mathrm{u}}\mathrm{neisen}$ model [48]:

ω = ω₀ + χT

(2)

where ω₀ is the peak position extended to 0 K and χ is the first−order temperature coefficient. The first−order temperature coefficients are calculated to be ${\chi }_{E_g^{}}^{\mathit{PbI}2/\mathit{CrOCl}}$ = −(1.44 ± 0.05) × 10⁻² cm⁻¹ K⁻¹, ${\chi }_{A_g^1}^{\mathit{PbI}2/\mathit{CrOCl}}$ = −(6.48 ± 0.05) × 10⁻³ cm⁻¹ K⁻¹ (only extracting the measured data at a temperature range below 240 K) and ${\chi }_{A_u^2}^{\mathit{PbI}2/\mathit{CrOCl}}$ = (1.48 ± 0.06) × 10⁻² cm⁻¹ K⁻¹, respectively. For comparison to a previous study of PbI₂/SiO₂/Si [43] (${\chi }_{E_g^{}}^{\mathit{PbI}2}$ = −1.80 × 10⁻² cm⁻¹ K⁻¹, ${\chi }_{A_g^1}^{\mathit{PbI}2}$ = −0.60 × 10⁻² cm⁻¹ K⁻¹ ${\chi }_{A_u^2}^{\mathit{PbI}2}$ = −0.10 × 10⁻² cm⁻¹ K⁻¹), the first−order coefficients of the $A_g^1$ and $A_u^2$ modes in PbI₂/CrOCl heterostructure obviously increased. The interfering factor of the temperature dependence of the Raman peak position could be attributed to anharmonic phonon–phonon coupling, thermal expansion, and the substrate effect [49,50,51]. The increase of the first−order coefficient in the $A_g^1$ mode and the abnormal blueshift of the $A_u^2$ mode position could have originated from the increase in the number of up−conversion channels in the anharmonic phonon–phonon coupling, which could be induced by the CrOCl substrate, but more research is needed to prove this [52,53]. The peak widths of the $E_g$, $A_g^1$, and $A_u^2$ modes broadened as the temperature increased, originating from the anharmonic phonon–phonon coupling [54,55], as shown in Figure S5. In addition, Figure 4e shows the temperature dependence of the $A_g^3$ mode of the CrOCl substrate. In agreement with previous studies [17,23], the $A_g^3$ mode exhibits redshift as the temperature increases, and the fitted first−order temperature coefficient is ${\chi }_{A_g^3}^{\mathit{CrOCl}}$ = −(1.89 ± 0.04) × 10⁻² cm⁻¹ K⁻¹.

4. Conclusions

In summary, the investigation of multilayered PbI₂/CrOCl vdW heterostructures was reported in this paper. The morphology of PbI₂ films on CrOCl nanoflakes changed from granules to oriented 2D nanoflakes with flat and smooth top surfaces while being fabricated at an elevated substrate temperature of 423 K. In addition, both the $A_g^1$ and $A_u^2$ modes in PbI₂ showed anomalous blueshift as the temperature increased in the PbI₂/CrOCl heterostructure in the temperature−dependent Raman spectra.