Rapid fabrication of submillimeter ultrathin CdI2 flakes via a facile hot plate-assisted vapor deposition method

As an important member of cadmium-based halides, two-dimensional (2D) CdI2 has drawn widespread attention due to its excellent optoelectronic properties. However, the large-size growth of ultrathin CdI2 flakes remains a huge challenge. Here, a facile hot plate-assisted vapor deposition (HPVVD) method is developed to synthesize submillimeter ultrathin CdI2 flakes. The photodetectors based on CdI2 flakes exhibit extremely low dark current (0.53 pA), ultrafast response speed (<2 ms) and good stability. The proposed HPVVD method may open a new avenue to prepare 2D layered metal halides.


Introduction
2D layered metal halides have attracted increasing attention recently in the fields of optics, electronics and magnetism, owning to high absorption coefficient, direct bandgap and spin freedom properties [1]. Cadmium-based halides (CdX 2 : X = I, Br, Cl), as typical metal halides, have been studied widely in light emitting devices and ultraviolet optoelectronic devices because of layered van der Waals (vdW) structure, wide band gap and excellent luminescent properties [2,3]. Specially, CdI 2 , has gained dramatical research interest in luminescence, mainly thanks to its remarkable properties including intrinsic n-type semiconductor, wide band gaps, and strong optical absorption [4].
Currently, three main methods have been used for preparing 2D CdI 2 , namely chemical bath deposition, thermal evaporation, and horizontal physical vapor deposition (PVD) [3,5,6]. However, the chemical bath deposition method is generally complex and time-consuming, resulting in irregular morphology, poor crystallinity and small size of the products; the thermal evaporation method suffers from thick, uneven and amorphous samples; although horizontal PVD has been recently adopted to grow high-quality 2D CdI 2 nanosheets, the small size, thick thickness, and harsh preparation conditions are inevitably caused. Therefore, it is urgent to develop a simple and efficient preparation approach to synthesize large-area, large-size, ultrathin 2D CdI 2 high-quality single crystals, in order to meet the practical application in semiconductor industry in future.
Herein, a facile HPVVD approach has been developed to synthesize large-area, submillimeter-size, ultrathin CdI 2 flakes with high quality on mica substrates. The resulting CdI 2 flakes with preferred (001) crystal plane are high-quality single crystals, which have been proved by a series of characterizations. Moreover, the photodetectors based on individual CdI 2 flake show excellent ultraviolet photoelectric properties, such as extremely low dark current (0.53 pA), high on/off current ratio (56.70), high responsibility (5.55 mA/W), ultrafast response speed (<2 ms) and good stability.

Experimental section
2.1 Synthesis of CdI 2 flakes: First, trace amounts of CdI 2 powder was placed on clean glass sheets and put together on a hot plate. Meanwhile, another two glass sheets with thickness of 1 mm were placed on either side of the above glass sheet as a bracket for mica substrate. The temperature of the hot plate is then raised to the set temperature (360-400℃), and CdI 2 flakes began to grow on the mica immediately. The whole growth process taken only 1-2 min in air at room temperature. Except for mica, CdI 2 flakes can also grow on SiO 2 /Si and quartz substrates.

Characterization of CdI 2 flakes:
The morphology, size and thickness of CdI 2 flakes were characterized by optical microscope (OM) and atomic force microscope (AFM). The crystal structure, phase and chemical composition, fluorescence and optical absorption of the samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) system, Raman and UV-vis absorption spectrum.

Fabrication and performance measurement of CdI 2 devices:
The CdI 2 flakes-based photodetectors were fabricated by transfer electrodes method. In short, Au electrodes, which were firstly deposited on a SiO 2 /Si substrate by the thermal evaporation machine (Nexdep, Angstrom Engineering), were peeled off assisted by the liquid mental Ga and transferred to the desired CdI 2 flake. The photodetection test was performed on the probe station (Lakeshore, TTPX) connected to a semiconductor device analyzer (Keysight, B1500A) and 365 nm laser. Figure 1a shows the HPVVD experimental setup, which is quite simple devoid of harsh experimental conditions as required by traditional PVD. As shown in Figure 1(b, c), CdI 2 is a typical layered material with a hexagonal unit cell, and belongs to space group P63mc (186) Figure 2a displays the XRD patterns of CdI 2 flakes. Four sharp XRD peaks can all be well indexed to {001} family planes ((002), (004), (006), (008)) of hexagonal phase CdI 2 (PDF#33-0239), suggesting the (001) plane preferred orientation and high crystalline quality of CdI 2 flakes. As exhibited in Figure 2b, the HRTEM image and the corresponding SAED pattern show clear lattice fringes and only single set of sharp diffraction spots in hexagonal symmetry, further indicating the excellent single crystalline nature of CdI 2 flakes. The lattice spacing with 0.37 nm is corresponded to (110) family planes, suggesting (001) plane is the preferred growth orientation, in accordance with the XRD results. The corresponding EDS spectrum (Figure 2c) demonstrates strong signals of Cd and I with atomic ratio close to stoichiometric ratio of CdI 2 (1:2). The Raman spectrum of CdI 2 flakes (Figure 2d) presents two prominent peaks around 45 and 110 cm -1 , corresponding to the in-plane (E g ) and the out-of-plane (A 1g ) phonon vibration mode, respectively, in conformity to the recently reported 2D CdI 2 crystals [4,6]. The inset is the typical Raman intensity maps of E g and A 1g modes, confirming highly uniform crystalline quality of CdI 2 flake. In addition, the PL spectrum in Figure 2e  As shown in Figure 3a, the CdI 2 flake-based photodetector was constructed. Figure 3 (b, c) demonstrates I-V and I-T curves under the dark and 365 nm laser illumination with different power intensity, indicating remarkable photoresponse. Specially, the device reveals an ultralow dark current (0.53 pA) even at V bias = 5 V. The corresponding on/off current ratio is 56.70 under 16.66 mW/cm 2 . On basis of the equation: R = I ph /PS, where I ph , P, S represent photocurrent, light indensity, and effective illuminated areas, respectively, the responsivity (R) can be reach up to 5.55 mA/W. Moreover, the photocurrent as a function of the power density is fitted as I ph = αP 0.97 (Figure 3d), implying the photocurrent is dominated by photoconductive mechanism. In addition, the repeatable photocurrent is displayed in Figure 3e, suggesting the good stability of the device. Furthermore, the obtained rising and decay time from enlarged I-T curve is < 2 ms, is shown in Figure 3f. Thus, the outstanding optoelectronic performance is ascribed to the high-quality single-crystalline feature of CdI 2 flakes.

Conclusion
In summary, large-size ultrathin CdI 2 flakes with high quality were successfully synthesized by a facile HPVVD method for the first time. The CdI 2 flakes-based photodetectors achieve outstanding optoelectronic performance, including extremely low dark current (0.53 pA), high on/off current ratio (56.70), high responsibility (5.55 mA/W), ultrafast response speed (<2 ms) and good stability, implying potential application in optoelectronics. This work provides a simple approach to grow 2D metal halides.