Space-confined microwave synthesis of ternary-layered BiOCl crystals with high-performance ultraviolet photodetection

School of Materials Science and Engineering, Nanyang Technological University, Singapore CINTRA CNRS/NTU/THALES, Singapore Centre for OptoElectronics and Biophotonics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore Department of Materials Science and Engineering, National University of Singapore, Singapore Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, China


| INTRODUCTION
Since the discovery of graphene, much academic attention has been drawn to the nanoscale in the materials realm. 1,2 In the near past, this intriguing type of material has been recognized and termed distinguishingly as two-dimensional (2D) materials. The family of 2D materials has grown, displaying a broad range of unique properties. 3,4 Recently, in a rather attractive direction, 2D ternary materials have drawn more and more attention and thoughts. 5,6 Compared with 2D binary compounds, three-element 2D materials have more freedom to tailor their chemical and physical properties through the variation of element species. To date, some exciting properties have been successfully found in 2D ternary-layered compounds, including excitonic insulating phase in Ta 2 NiSe 5 , 7 high electron mobility in Bi 2 O 2 Se, 8 and intrinsic ferromagnetism in Cr 2 Ge 2 Te 6 and Fe 3 GeTe 2 . 9,10 Photodetectors based on the Ga 2 In 4 S 9 flakes also display an excellent performance. 11 In order to expand the family of 2D ternary materials and investigate their special properties, exploring new ternary-layered compounds and developing reliable synthetic methods are extremely urgent and necessary.
As a new layered ternary material, bismuth oxyhalide BiOX (X = Cl, Br, and I) has attracted a great deal of concern and interest in the field of photocatalytic energy conversion and environmental remediation in recent years. 12 Barium oxyhalide is a typical multicomponent metal halide oxide, which belongs to group V-VI-VII, with high chemical stability, nontoxicity, and corrosion resistance. 13,14 Most recently, Feng et al 15 have reported a facile synthesis of large-area atomically thin BiOI by space-confined chemical vapor deposition (CVD), and for the first time attempted a visible light photodetector based on 2D BiOI nanosheets with good photo response and high sensitivity. Compared with BiOI, BiOCl possesses a relatively big band gap close to 3.2 eV with pronounced photosensitivity, which shows promise for ultraviolet (UV) light detection. 16 Up to now, most of the BiOCl synthesis works focus on the welldeveloped liquid phase method. 15 The surface contamination and low quality of the BiOCl obtained by hydrothermal obstruct their practical application in photoelectric conversion and semiconductor device areas. Unfortunately, there are also no reports on the growth of 2D ternary BiOCl flakes with controllable stoichiometry on the surface of a flat substrate by CVD method. A detrimental reason is that the common precursors for the synthesis of BiOCl, such as BiCl 3 and Bi (NO 3 ) 3 are extremely easy to hydrolyze in air and decompose to bismuth oxide when the control of heating temperature is unsuitable. 17 Unlike conventional CVD heating, microwave heating utilizes polar molecules rotating in the electromagnetic field to realize the conversion of electromagnetic energy to thermal energy. 18,19 Thus, the distinctive character of microwave heating is quick and efficient, which opens the new way to synthesize some special material using microwave technology.
Here, we reported an extremely fast and reliable approach to the synthesis of 2D ternary-layered BiOCl crystals by the space-confined microwave-assisted heating process. The detailed characterizations, including Raman, atomic force microscopy (AFM), X-ray absorption spectra (XPS), and scanning transmission electron microscopy-annular dark field (STEM-ADF), were recorded and confirmed that the as-obtained BiOCl samples were a single crystal with high quality. UV photodetectors based on as-grown 2D BiOCl crystals were also fabricated and showed a highly sensitive detection. Figure 1A shows the structure models of BiOCl. Its crystal structure belongs to the PbFCl type tetragonal system. 20 BiOCl can also be regarded as a layered structure in which the [Bi 2 O 2 ] 2+ layer and the double slabs of halogen ions [Cl] −1 layer are interlaced in the c-axis direction. The inner layer atoms are in the form of strong covalent bonds, while the interlayers are weak van der Waals forces. 16,21 As shown in Figure 1B, the BiOCl crystals were synthesized using an ordinary household microwave oven operating at 2.45 GHz with 700 W of power. Solid bismuth chloride (BiCl 3 ) powder was chosen as the precursor, which was readily to react with the moisture in the air to form the intermediate hydrate BiCl 3 ÁH 2 O. 17 Two SiO 2 /Si substrates were stacked together directly to create a narrow gap and a small amount of BiCl 3 powder was located in the confined space. 22,23 Turning on the microwave, the BiCl 3 ÁH 2 O was heated by a molecular interaction with the electromagnetic field and then decomposed to square shape bismuth oxychloride flakes on the SiO 2 /Si substrate. The whole process can be finished within 3 minutes, and the details are described in Section 4. Figure 1C shows a representative optical image of the asobtained BiOCl crystals grown on the SiO 2 /Si substrate. These BiOCl flakes are square-shaped with the edge size up to 10 μm and the usual thickness is about 248 nm according to the AFM image ( Figure 1D). The scanning electron microscope (SEM) image of a BiOCl nanosheet ( Figure 1E) shows a clean and uniform surface, without granular deposit, indicating a good crystalline morphology. Note that besides SiO 2 /Si, other substrates, such as bare silicon and mica, can also be employed (Supporting Information Figure S1). Moreover, the BiOCl crystals grown on the mica show much thinner with the thickness down to 8.7 nm (Figure S1B, C), which could be due to no dangling bonds on the mica surface. The free migration of adatoms on a mica surface is much easier than on SiO 2 /Si substrates during the growth. 13 However, the fabrication of devices on insulating mica is often hindered by incompatibility between the silicon technology. 24 So, we focus on the direct growth of BiOCl crystals on SiO 2 /Si. In addition, the confined space and growth time are critical factors to obtain uniform BiOCl crystals. When the microwave synthesis process takes place in open space ( Figure S2) or prolonged the reaction time to 5 and 8 minutes ( Figure S3), the obtained BiOCl crystals become thicker and unhomogeneous. The terraced structures could also be found on some BiOCl crystals.

| RESULTS AND DISCUSSION
Further characterizations were performed to investigate the composition and bandgap of the as-obtained BiOCl crystals. In Figure 2A, typical Raman spectra with a 532 nm excitation laser show two obvious resonance peaks at 142 and 198 cm −1 , corresponding to the A 1g and E g internal Bi Cl stretching modes, respectively. 21 The uniform color contrast of Raman mapping image at A 1g modes (≈142 cm −1 ) (inset in Figure 2A) suggests a high quality of the as-grown crystal structure. The characteristic RM peaks at 142 and 198 cm −1 are also good consistency with the prior reports, 14,21 implying that high-purity BiOCl crystals have been synthesized. Then, the BiOCl platelets grown on SiO 2 /Si substrate were transferred to the fused quartz surface by a PS-assisted method to do the Ultraviolet-Visible (UV-Vis) absorption and X-ray diffraction (XRD) analysis. The detailed transfer process is described in Section 4. As shown in Figure 2B, the band gap value of BiOCl crystals was calculated to be 3.18 eV using a classical Tauc approach  Figure S2). 25 The XRD pattern of the resulting BiOCl platelets is shown in Figure 2C. Three distinct diffraction peaks are corresponding to the (001), (002), and (003) planes of the tetragonal BiOCl crystals, concluded by the standard tetragonal BiOCl pattern (JCPDS card 06-0249). These diffraction peaks are intense and sharp, indicating that our material is well-crystallized. In addition, all three peaks (001, 002, and 003) are assigned to the (001) family planes, which could be attributed to the [001] direction of the as-obtained crystals is perpendicular to the growth substrate. 20 To investigate the surface states and chemical compositions of the as-grown BiOCl flakes, XPS measurements were carried out. The high-resolution XPS spectra of Bi 4f, Cl 2p, and O 1s are given in Figure 2D-F, respectively. As shown in Figure 2D, a typical 4f 7/2 (157.4 eV) and 4f 5/2 (162.7 eV) doublet in Bi 4f spectra are accounted for by the spin-orbit coupling effect, which shows good consistency with the reported values for BiOCl powders. 20 Two characteristic peaks for chloride anions at 196.0 and 197.7 eV in Cl 2p ( Figure 2E) are assigned to Cl 2p 3/2 and Cl 2p 1/2 , respectively. The O1s peaks at 528.1 and 529.6 eV are corresponding to the oxygen anions in the Bi O bond from BiOCl and the oxygen anions in the Si O bond from SiO 2 /Si substrate, respectively. 13 No other doublet found in Bi 4f and Cl 2p spectra and the stoichiometric ratio of Bi/Cl is quite close to 1 indicating that the BiOCl is the high purity phase.
Next, we applied STEM-ADF imaging to investigate the crystallinity of the BiOCl crystals. The contrast of STEM-ADF image is dependent on the Z atomic number as approximately $Z 1.6-1.7 , thereby it was widely employed for analyzing crystal structures and atomic defects in 2D materials. 26,27 From the theoretical atomic model ( Figure 3A), we can see that BiOCl crystal shows a perfect tetragonal lattice along the [001] zone axis. All atomic columns are equivalent containing periodic O Bi Cl atomic planes. A typical STEM image and its enlarged image of the as-grown BiOCl crystal along the [001] direction are shown in Figure 3B,C. It can be seen that the tetragonal phase contrast patterns are highly homogenous suggesting that the crystal has no noticeable structural defects. Corresponding fast Fourier transform (FFT) pattern ( Figure 3D) reveals one set of spots that further corroborate the single crystallinity of BiOCl crystals. In addition, we applied energy dispersive X-ray spectroscopy F I G U R E 2 A, Typical Raman spectrum of BiOCl nanoplates, the inset shows the optical image of an individual BiOCl platelet and the corresponding Raman mappings at 142 cm −1 . B, (αE photon ) 1/2 vs E photon curves of the as-prepared BiOCl crystals. α is the absorption coefficient and E photon is the discrete photo energy. C, XRD pattern of BiOCl transferred on a glass slide (red) compared with the standard tetragonal BiOCl pattern (PDF#06-0249) (black). XPS spectr of the BiOCl crystals for Bi 4f D, Cl 2P E, and O 1 second F binding energies. XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction (EDS) mapping ( Figure 3E) and found that the distribution of each element is homogenous suggesting that macroscopically the BiOCl is free of large-scale structural defects.
In order to investigate the potential optoelectronics applications of our BiOCl crystals, photodetectors based the individual BiOCl flakes were fabricated on SiO 2 /Si substrate. The schematic of the BiOCl detector is a photoconductive field-effect transistor with effective illumination area about 100 μm 2 as shown in Figures S5 and S6. The device fabrication process and more details are described in Section 4. Figure 4A illustrates the energy band diagram of the device under 266 nm laser illumination (hv = 4.66 eV) which is adequate to excite the carriers in the valance band to the conduction band. As shown in Figure 4B, the photocurrent increases with the increase in the driven voltage and laser power. The responsivity (R) is calculated to be $8 A/W under continuous wave 266 nm laser illumination with a driven voltage of 5 V. The result indicates the potential application of BiOCl nanosheet as a UV detector. In order to determine the intrinsic response time of the BiOCl nanosheet, a $500 ps, 266 nm pulse laser is employed via a two-pulse photocurrent correlation method as described in Section 4. In the experiment, we apply a small driven voltage between the source and drain electrodes to measure the photocurrent together with a lock-in amplifier. The photocurrent is calculated by eliminating the dark current from the current when the light was focused on the sample. The clear dip in the photocurrent curve as shown in Figure 4C indicates fast response of the BiOCl nanosheet between two temporally nearby pulse excitations. It could further evaluate the extraction speed of the photocarriers from the first pulse excitation before the second pulse's arrival. 28,29 When the pump and probe pulses overlap with nearly zero-time delay, the photocurrent is minimum due to the sublinear powerdependent photocurrent. However, when time delay increases, the photocurrent dynamically increases accordingly and saturate when the photocarriers are completely extracted from the pump pulse. 30 The response time is calculated to be $18 ps with the exponential fitting of the rising curve. We assume that the fast response is originated from the short electron-hole lifetimes in BiOCl. 31 The linear dependence of the photocurrent on the incident laser pulse fluence ( Figure 4D) indicates that the number of photogenerated carriers increases with the increase of the incident laser power and also demonstrates that the photocarrier dynamics involved both traps/defects and mid-gap states. 32 Further investigation is required to interpret the complex-carrier recombination and scattering processes of the photogenerated electron/hole pairs under UV laser illumination. On the other hand, the device is quite stable after being exposed in ambient air within 3 weeks and the response is almost unchanged during the measurement as shown in Figure 4E, indicating the potential applications of the 2D BiOCl photodetectors. For example, the detector arrays can be transferred to a polydimethylsiloxane substrate by a facial method ( Figure 4F), which is promising for wearable UV detector for daily UV illumination monitor.

| CONCLUSIONS
In conclusion, we reported a space-confined microwave synthesis of single crystalline tetragonal BiOCl platelets. Contrary to the traditional CVD process, the microwaveassisted heating process shows an extremely fast growth of few-layer BiOCl within 3 minutes in air. Through XRD, XPS, and HRTEM studies, we confirmed the single phase of BiOCl. The BiOCl-based UV photodetector exhibits an excellent performance with high responsivity and fast response characteristic. We assume that the fast response is originated from the short electron-hole lifetime in high crystalline quality BiOCl flakes. The mild growth conditions and excellent UV detection performance of BiOCl suggest that 2D BiOCl is a promising material for optoelectronic applications and other fundamental properties investigations. Furthermore, our microwave-assisted heating process offers more choices for the structure-control growth of other atomically 2D materials due to the simplicity and speediness of the process.

| Microwave synthesis of BiOCl crystals
The microwave synthesis process is schematically depicted in Figure 1B. In detail, the primary equipment is a conventional household microwave oven (Midea M1-L213B) with a frequency of 2.45 GHz and a power of 700 W. Two SiO 2 /Si substrates were stacked together to form a confined space, and the precursors BiCl 3 powders were located between the silicon substrates. Then they were placed at the center of microwave and on high (100%) for 1-3 minutes. After the heating, we wear heat protective gloves to take out the samples and place them under running water to remove unreacted BiCl 3 powder. Finally, the final product was dried by nitrogen.

| Characterizations of 2D BiOCl crystals
The as-obtained BiOCl platelets were further characterized by optical microscopy (Olympus BX53M), AFM (Asylum Research Cypher Scanning Probe Microscope system with a tapping mode), SEM (JEOL JSM-7600F), XRD (Rigaku DMAX 2500), RM (WITEC alpha 300R Confocal Raman system using a 532 nm laser as the excitation source), UV-vis spectra (PerkinElmer Lambda 950 UV-vis spectrometer), and XPS (Kratos AXIS Supra spectrometer with a monochromatic Al K-alpha source). ADF-STEM imaging was conducted on an aberration-corrected JEOL ARM-200F equipped with a cold field emission gun, operating at 80 kV, and an advanced STEM corrector probe corrector.

| PS-assisted transfer of BiOCl onto a fused quartz substrate
PS was first spin-coated on the BiOCl on the SiO 2 /Si substrate at 2000 rpm for 60 seconds and then baked at 90 C for 10 minutes. Then, the BiOCl supported by the PS film was peeled off from the SiO 2 /Si substrate by tweezers under a drop of DI water. Next, it was transferred onto the fused quartz substrate and dried at 80 C for 1 hour. Finally, the PS film was removed by immersing into toluene for 24 hours, and then successfully transferred the BiOCl samples were dried by nitrogen gas.

| Device fabrications and measurements
First, the BiOCl flakes grown on SiO 2 /Si substrate were spin-coated with 950 K PMMA (MicroChem) at 3000 rpm for 1 minute and baked on a 120 C hotplate for 5 minutes. Then, the BiOCl FET devices were fabricated by standard optical lithography and deposited Ti/Au (20/80 nm) as contact electrodes using electron-beam evaporation. The final devices were annealed for 2 hours with the protection of nitrogen in order to remove resist residues and enhance the metallic contacts.
The electrical characteristics were examined by a semiconductor analyzer (Agilent, B1500A). The photoresponsivity measurement was performed in a digital deep level transient spectroscopy (BIORAD) system with an externally triggerable high power picosecond laser (versatile picosecond laser module from PICOQUANT) by two-pulse photocurrent correlation experiment. The two 266 nm optical pulses from an 80-MHz repetition rate laser were mechanically chopped at 1.85 KHz and then split into two pulses by a 50/50 beam splitter and cross-polarized to minimize interference and focused onto the devices using a ×100 objective lens. The time delay between the two pulses was controlled by a linear translation stage. The photocurrent was collected by a lock-in amplifier with a 5-MΩ input resistance (SR830). Noise spectra are acquired by a spectrum analyzer (Keysight M9018A) with biased supplied by Agilent 1500A at ambient conditions.

| Calculation of response time
The responsivity of the detector is determined by eliminating the dark current from the current when the light was focused on the sample and calculated as I ph (I ph = I lluminated -I dark ). Then, the responsivity (R) is calculated by R = I ph /P a , where P a is the laser power illuminated on the active area of the detector and is calculated by P a = P t × S 0 /S t (P t is the total laser power measured by the power meter, S o is the area of the device, and S t is the area of the laser spot).