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 (PbI
2) 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 PbI
2 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 PbI
2 and a direct band gap of 2.38 eV for multilayer PbI
2 [
11,
12,
13]. Heterostructures based on PbI
2 exhibit good potential in optoelectronic applications, such as 2D–1D vdW heterostructure photodetector [
14] and flexible or stretchable electronics [
5]. In addition, PbI
2 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 PbI
2−TMDCs (MoS
2 and MoSe
2) 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 MoS
2/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], PbI
2/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 [100] direction along the CrOCl [010] direction, confirming epitaxy. Furthermore, we observed that the and modes of PbI2 in heterostructure represented anomalous blueshift as the temperature increased.
3. Results and Discussion
Owing to the small exfoliation energy of CrOCl of 0.2 J/m
2 [
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 PbI
2/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
,
, and
modes of CrOCl, respectively, in agreement with previous works [
17]. The corresponding atomic vibrations of these three optical phonon modes were inserted. The
and
modes belong to the out−of−plane vibration model along the [001] direction, while the
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
and
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 SiO
2/Si substrate [
32]. It is reported that the crystalline orientations of low symmetry 2D materials, such as ReS
2, b−As, and MoO
3, can be determined by angle−resolved polarization Raman measurements [
33,
34,
35]. The orientation of the
mode could be decomposed to the components along the [001] and [100] directions, respectively [
17,
36]. The maximum intensity of the
mode existed while the incident light was parallel to the [100] direction in polarized Raman spectra. The inserted intensity polar plot of the
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 map** image of the
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 PbI
2/CrOCl heterostructures were characterized using AFM. Firstly, PbI
2 was deposited onto the substrate maintained at room temperature. Different from our previous reports on MoS
2 and MoSe
2 [
21,
22], PbI
2 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 PbI
2 had similar growth behaviors on CrOCl as those on SiO
2 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 PbI
2 clusters on CrOCl and SiO
2 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 PbI
2 on both surfaces. Post−annealing would possibly improve the quality of the PbI
2 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 PbI
2 on CrOCl to recrystallize into a 2D form with an atomically flat top surface, as shown in
Figure 2c,d, while those on SiO
2 were still in granule form but larger, as shown by the yellow bars in
Figure 2g. Post−annealing at ~473 K led to PbI
2 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 SiO
2, respectively. The still rough surface of PbI
2 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 PbI
2 on CrOCl is diffusion−limited at room temperature.
To realize the vdW epitaxy of PbI
2 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 PbI
2 islands were prepared on SiO
2, 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 PbI
2. Some of them have thicknesses of multiples of 0.70 nm, meaning multilayers. The size of PbI
2 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 PbI
2 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 PbI
2 on mica observed by Debjit Ghoshal et al. [
37], indicating the existence of vdW epitaxy here. Such uniform orientation of PbI
2 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 PbI
2 nanoflakes fabricated using the PVD method [
38,
39], the sides of the PbI
2 triangular nanoflakes on CrOCl could be determined to be [
100]. Thus, the epitaxial relationship between PbI
2 and CrOCl could be determined to be that the [
100] direction of PbI
2 was parallel to the [010] direction of the CrOCl nanoflake. The lattice spacing values of PbI
2 in [
100] and [
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 PbI
2 on CrOCl and SiO
2 were 41.16 and 50.58 nm, respectively. Upon annealing at 423 K, the average grain size of PbI
2 on SiO
2 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 PbI
2 films on both sides were compact. For the last case, the line profile shows ~22 nm bumps because PbI
2 nucleated into larger clusters with an exposed SiO
2 surface. According to the good epitaxial relationship between PbI
2 and CrOCl, we speculate that there were no interfaces or stacking layers which might trap or aggregate defect trap** 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 PbI
2 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 PbI
2, 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 PbI
2. According to the well−known Debye−Scherrer formula [
41],
where
is the grain size,
is the wavelength of the scattered radiation,
is the width−width at half maxima of the diffraction peak, and
is the scattering angle, the average PbI
2 thickness in the three cases are calculated to be 10 ± 2 nm.
Figure 3b shows the typical normalized Raman spectra of PbI
2/CrOCl heterostructure in the above three cases, as well as that of pure PbI
2 on SiO
2/Si substrate (solid green curve). The peaks were observed at 70, 95, and 110 cm
−1, corresponding to the
(in−plane),
(out−of−plane), and
(longitudinal acoustic mode) modes of PbI
2, respectively [
42,
43]. Different from the
and
modes belonging to optical phonon modes, the
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 PbI
2 films have the interlayer restoring force, which results in the nonzero frequencies, the
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 (
)/I (
)) could be used to quickly determine the thicknesses of the PbI
2 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. It can be seen that the mode positions in all the PbI
2 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
mode increased in the last case.
TDRS measurements were performed on the PbI
2/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 PbI
2 are basically unchanged through the whole temperature range, especially for the
mode. However, the
mode displays an obvious enhancement in intensity as the temperature increases. The measured peak positions of the
,
, and
modes as functions of temperature are plotted in
Figure 4b–d, respectively. The
mode exhibits linear redshift, while the
mode shows linear blueshift as the temperature increases. The abnormal blueshift of the
mode at temperatures above 260 K could have come from the slip between PbI
2 and CrOCl substrate induced by thermal expansion mismatch, similar to previous reports [
43,
47]. The
mode moved from 74.3 to 71.4 cm
−1, while the
mode moved from 96.2 to 95.6 cm
−1 over the whole temperature range, indicating the temperature dependence of the
mode was stronger than that of the
mode. The temperature dependence of the Raman mode position could be fitted by the
model [
48]:
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
= −(1.44 ± 0.05) × 10
−2 cm
−1 K
−1,
= −(6.48 ± 0.05) × 10
−3 cm
−1 K
−1 (only extracting the measured data at a temperature range below 240 K) and
= (1.48 ± 0.06) × 10
−2 cm
−1 K
−1, respectively. For comparison to a previous study of PbI
2/SiO
2/Si [
43] (
= −1.80 × 10
−2 cm
−1 K
−1,
= −0.60 × 10
−2 cm
−1 K
−1 = −0.10 × 10
−2 cm
−1 K
−1), the first−order coefficients of the
and
modes in PbI
2/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
mode and the abnormal blueshift of the
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
,
, and
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
mode of the CrOCl substrate. In agreement with previous studies [
17,
23], the
mode exhibits redshift as the temperature increases, and the fitted first−order temperature coefficient is
= −(1.89 ± 0.04) × 10
−2 cm
−1 K
−1.