van der Waals Transition-Metal Oxide for VisâMIR Broadband

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Functional Inorganic Materials and Devices

Van der Waals transition metal oxide for vis-MIR broadband photodetection via intercalation strategy Ruihui He, Zefeng Chen, Haojie Lai, Tiankai Zhang, Jinxiu Wen, Huanjun Chen, Fangyan Xie, Song Yue, Pengyi Liu, Jian Chen, Weiguang Xie, Xiaomu Wang, and Jianbin Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00181 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Applied Materials & Interfaces

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Van der Waals Transition Metal Oxide for Vis-MIR Broadband

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Photodetection via Intercalation Strategy

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Ruihui He,† Zefeng Chen,§ Haojie Lai,† Tiankai Zhang,§ Jinxiu Wen,‖ Huanjun Chen,‖

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Fangyan Xie,⊥ Song Yue,† Pengyi Liu,† Jian Chen,⊥ Weiguang Xie *,†,‖ , Xiaomu

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Wang *,‡, Jianbin Xu *,§,‖

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† Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials,

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Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New

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Energy Materials, Department of Physics, Jinan University, Guangzhou, Guangdong 510632, People's

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Republic of China

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‡ School of Electronic Science and Technology, Nanjing University, Nanjing 210093, China

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§ Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese

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University of Hong Kong, Hong Kong SAR, China ‖ State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of

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Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University,

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Guangzhou 510275, China

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⊥ Instrumental Analysis & Research Center, Sun Yat-sen University, Guangzhou 510275, P. R.China

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Abstract

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Defects engineering can broaden the absorption band of wide band gap Van der Waals (vdW)

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materials to the visible or Near-IR regime at the expense of material stability and

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photoresponse speed. Herein, we introduce an atomic intercalation method that bring the wide

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band gap vdW α-MoO3 for Vis-MIR broadband optoelectronic conversion. We confirm

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experimentally that intercalation significantly enhance photo absorption and electrical

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conductivity, while bring negligible change to the lattices structure as compared with ion

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intercalation. Charge transfer from the Sn atom to the lattices induces the opto-electrical

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change. As a result, the Sn intercalated α-MoO3 show room temperature, air stable, broadband

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photodetection ability from 405 nm to 10 µm, with photo-responsivity better than 9.0 A/W in

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405 nm to 1500 nm, ~ 0.4 A/W at 3700 nm and 0.16 A/W at 10 µm, and response time of ~

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0.1 s and peak D* of 7.3×107 cm·Hz0.5·W-1 at 520 nm. We further reveal that photo-thermal

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effect dominates in our detection range by real-time photo-thermal-electrical measurement,

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and the materials show a high TCR value of -1.658 %/K at 300 K. These results provide

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feasible route for design of broadband absorption materials for photo-electrical, photo-thermal

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or thermal-electrical application.

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Keywords: 2D material, intercalation, metal oxide, broadband absorption, photodetection,


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1. Introduction

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Broadband spectral absorption materials are favorable in energy conversion, modern

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multispectral detection, telecommunication, molecular and thermal imaging. Traditional

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materials such as Si, and PbS only cover a narrow detection wavelength in ultra-violet (UV)

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to the mid-infrared (MIR) region. Most of them also suffer from expensive and

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environmentally hazardous fabrication process. Two-dimensional (2D) van der Waals (vdW)

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materials have arisen as the attractive platform for photoelectrical conversion due to strong

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light matter interaction in the out-of-plane direction, as well as its superior processing

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compatibility without dedicated consideration of lattice mismatching issue.1-3 Although there

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have existed intensive studies on the broadband response of various types of vdW materials,1,

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4-6

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few-layered Black phosphorous (0.3 eV)9-10 and few-layered noble metal dichalcogenide (0.3

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eV)11 have broadband photoresponse ability from visible to the MIR region. Although a

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moderate bandgap vdW semiconductor is naturally unfavorable as broad band photo

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absorption material, it is proposed that defect engineering is able to extend the DOS deep into

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the bandgap.12 The introduction of lattice defect is effective to broaden the absorption band

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and enhance the responsivity. However, it causes a loss of device stability, as well as the

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speed of photoresponse.7 Exploring of new materials or processing strategies are highly

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desired.

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As a typical wide bandgap vdW semiconductor, molybdenum trioxide (α-MoO3) has wide

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optoelectrical applications such as gas sensing,13-14 solar cell,15-16 photodetector,17-18 and field

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effect devices.19-20 The few layered MoO3 shows high k value allowing high carrier mobility

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(>103 cm2 V-1 s-1),19-20 which is beneficial for fabrication of high speed electronic devices.

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Recently, α-MoO3 was found to be a superior hyperbolic materials, in which tunable, in-plane

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anisotropic and ultra-low-loss polaritons was demonstrated in the MIR wavelength.21-22 It

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provides the opportunity to fabricate nanophotonic device beyond the diffraction limit.

by far, only several zero/narrow bandgap materials, such as graphene (zero band gap),7-8



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Although there is superior electrical and optical properties, photo-electrical coupling and

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conversion still can not be realized till the MIR region. Intrinsic layered MoO3 has a wide

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bandgap of ∼3.0 eV, resulting in low carrier concentration and the limited photo-detection

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applications in ultraviolet region.23 It has been exemplified that the light detection region can

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be extended to visible range via the introduction of substantial defects.17,

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generated defects also provide active sites for O2 and H2O absorption and reaction, which

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cause instability in air. Even in vacuum condition, the defects slow down the response to tens

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of seconds.17

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In 2D van der Waals layered crystal, there is an increasing interest in guest species

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intercalation into the van der Waals gap.25-29 The intercalated materials may show some

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intriguing properties, such as superconductivity,30 tunable transparency26 and conductivity.31-

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32

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structural and unique properties of monolayer in a bulk structure.29 However, ion intercalation

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always leads to a great volume extension, which generate defects as well. Herein, broadband

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photoresponse from visible to the MIR regime from α-MoO3 treated by intercalation of Sn

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atoms is demonstrated. The merit of this approach is that it maintains the lattices structure of

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host MoO3 so that it avoids the degradation of electrical transport and materials instability. In-

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situ photo-electrical-thermal measurement reveals that bolometric effect is predominated for

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the observed characteristics.

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However, the

The intercalated strategy is also attractive because of its possibility to preserve the

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2. EXPERIMENTAL METHODS

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2.1 Materials Synthesis and Characterization

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The growth of layered α-MoO3 were prepared by physical vapor deposition (PVD) method

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previously reported.24 The details of growth and intercalation are documented in Figure S1.

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The size, thickness, surface topography and element distribution of MoO3-Sn nanosheets were

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verified by optical microscopy, AFM (NT-MDT NTEGRA) and high-resolution HTEM with 4 ACS Paragon Plus Environment

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EDS (FEI Titan G2 60-300). Raman spectroscopy of MoO3 nanosheets were achieved from a

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confocal Raman microscope (Renishaw inVia Reflex system) excited by a laser of 532 nm.

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The crystalline and electronic structure of MoO3 after intercalation were characterized using

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XRD, SAED and XPS (Thermo VG ESCALAB 250Xi). Optical performance of MoO3-Sn

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between 300-2500 nm was characterized by UV-Vis spectrophotometer (Thermo Scientific

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350 UV-Vis) using an integrating sphere with a reference of Teflon. A Fourier transform

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infrared spectrometer (Thermo Scientific Nicolet iN10) equipped with a supersensitive MCT

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detector cooling with liquid nitrogen was applied to measure the transmittance of sample on a

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IR non-absorption diamond substrate.

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2.2 Device Fabrication and Characterization

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Optical images of three types of intercalated α-MoO3 devices, including nanosheet, thin film

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and single crystal are showed in Figure S1. For α-MoO3 nanosheet, a MoO3 micro belt with

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width of several micrometer was used as shadow mask. For thin film and single crystal, they

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are isolated with the substrate by a spacer of about 0.2 mm. Plastic plate with width of about 1

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mm was used as shadow mask. Both metal electrodes (typically 80 nm Au or Ag) were

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deposited by thermal evaporation.

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The photoresponse measurements was performed using a Keithley 2612A dual-channel digital

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source meter and illuminated by five semiconductor laser (405 nm, 520 nm, 638 nm, 860 nm

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and 1550 nm), MIR laser 3.7 µm (M Squared Firefly-IR-LP-C-BB-1) and 10 µm, (Pranalytica

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Monolux ). All photoresponse measurements were operated in air or vacuum with a pressure

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about 10-3 mbar at room temperature. The noise spectrum was measured using a spectrum

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analyzer (Stanford Research System SR770) with a measuring bandwidth of 100 kHz at 0.1 V

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bias at room temperature. Seebeck coefficient was measured by Seebeck coefficient/resistance

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measurement system (Joule Yacht). Thermal infrared imager (FOTRIC 220s) with a M20-

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macro-lens was utilized to record the real-time temperature. The TCR of the MoO3-Sn single

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devices was measured using HFS Probe systems in the THMS600 heating and freezing stages. 5 ACS Paragon Plus Environment


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3. RESULTS AND DISCUSSION

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3.1 Structural & optoelectrical properties

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Intrinsic α-MoO3 is composed of double layer stacked up through weak van der Waals force

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along the [010] direction, in which linked distorted MoO6 octahedras share corners in the

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directions of [100] and [001] (Figure 1a). Intercalation of Sn atom into the van der Waals gap

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is realized by reported disproportionation reaction,26 and the results are shown in Figure 1b &

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Figure S1-S2. As-prepared MoO3 in Figure 1c shows strong diffraction peaks at 12.8°, 25.76°,

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and 39.04°, which corresponds to (020), (040), and (060) planes of α-MoO3 (JCPDS: 05-

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0508), respectively. The only diffraction of (0k0) observed suggests that the as-grown

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Figure 1. Crystalline structure and optical properties of α-MoO3 nanosheet with Sn

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intercalation a) Lattice structure of MoO3. b) AFM morphology of partially Sn-intercalated

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MoO3 nanosheet. The inset shows the optical image. The Raman spectra of the intrinsic (●)

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and intercalated (○) areas in the inset are shown in Figure S4. c) The XRD spectra of MoO3

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and MoO3-Sn thin film. d) & e) SAED patterns of the intrinsic and Sn-intercalated part of

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MoO3. The elemental mapping is shown in Figure S3. (f) Absorption spectra of MoO3 before 6 ACS Paragon Plus Environment

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and after Sn intercalation.

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α-MoO3 are highly b-axis-oriented. Figure 1d shows that lattices spacing is 3.94 Å along the

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[100] direction and 3.77 Å along the [001] direction, consistent with that in well-crystallized

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α-MoO3. The (001) and (100) spots are not observed due to interference cancellation of

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electron beam. It is found that after reaction, the (0k0) peaks left shift in Figure 1c.

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Calculation showed that the spacing of (010) plane was enlarged from 13.82 Å to 13.99 Å,

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which proves successful intercalation of Sn atoms into the van der Waals gap. In intercalated

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MoO3, diffraction spots of (100) and (001) are observed (Figure 1e). Especially, along the

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[001] direction, bright and dark spots appeared alternately, which implied periodic distortion

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of lattices due to the intercalation. Lattices distortion is also supported by Raman spectra in

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Figure S4. Intercalation of Sn atom into the MoO3 caused a significant change in color from

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optical microscope (inset in Figure 1b and Figure S2). AFM in Figure 1b shows that there is

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no obvious morphology boundary between the intrinsic (light green in the center) and

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intercalated (dark green at both sides) areas. This is totally different from those annealed or

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hydrogenated cases, where line defects are clearly observed by AFM24, and those

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electrochemical intercalation, where a significant increase of thickness can be observed.28-29

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Figure 1f and Figure S5 show the optical spectra of MoO3 before and after Sn intercalation.

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As prepared MoO3 shows a wide optical bandgap at 3.03 eV (409 nm). After Sn intercalation,

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the absorption increases significantly in the wavelength range from 300 nm to 15000 nm. It

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shows that the MoO3 have broadband absorption ability after Sn intercalation.

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The observed structural distortion in MoO6 octahedral is predicted theatrically to induced a

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hybrid p-d band in the bandgap due to overlap of Mo 3d and O 2p (Figure 2a).33-34 After

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intercalation, the Sn 3d5/2 and 3d3/2 core levels lie at 487.44 and 495.95 eV respectively,

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corresponding to the Sn4+ (Figure 2b). As-grown MoO3 nanosheets only show strong Mo 3d5/2

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and 3d3/2 peaks located at 232.11 eV and 235.21 eV, which were assigned to the +6 oxidation 7 ACS Paragon Plus Environment


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states (Figure 2c). After intercalation, both Mo 3d5/2 and 3d3/2 peaks were separated to

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doublet, arising from the appearance of Mo +5 and +6 oxidation states (Figure 2d). This

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change of charge states meant that there was electron transfer from the Sn atoms to the d

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orbital of Mo atoms (Figure 2a):

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Mo6+O3 + e- = Mo(6-)+O3

(1)

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The electron doping will fill the hybrid gap states, which can be observed by the change in the

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valance band structure in Figure 2e. The valence band edge shifts from 3.1 eV to 2.7 eV, and

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the p-d hydride band centered at 0.75 eV appeared. This interband state extends to the Fermi

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level, which explains the increase of absorption in the IR region.

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Figure 2. Electronic structure of Sn-intercalated MoO3: a) Schematic energy band

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structure of MoO3 caused by Sn intercalation. b) Sn 3d XPS core level spectra in the MoO3-

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Sn. c) & d) Mo 3d XPS core level spectra of the intrinsic and MoO3-Sn thin film respectively.

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e) XPS valence band structure of MoO3 and MoO3-Sn.

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3.2 Photoresponse of Sn intercalated MoO3

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To demonstrate the application of broadband absorption and enhanced electrical properties,

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two terminal device based on MoO3-Sn nanosheet was fabricated, which show prominent

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photoresponse to the under-bandgap illumination of 520 nm (Figure S6). It reflects the 9 ACS Paragon Plus Environment


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contribution from the hybrid band as illustrated in Figure 2. Intercalated single crystal α-

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MoO3 device shows similar photoresponse. The power-dependent responsivity (R)

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characteristics is shown in Figure 3a & Figure S7. Under illumination power between 1 µW –

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100 µW, the photocurrent increases linearly with increasing light power, leading to a slowly

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changing R around 13.9 A/W in Figure 3a. Figure 3b shows the current noise (in) power

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spectra that is dominated by the 1/f noise, but not thermal noise. 1/f noise originates from the

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fluctuations of local electronic sates induced by disorders or defects.11 Wavelength

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dependence of the responsivity is measured from 405 nm to 10µm. In the visible and near-IR

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regions, the R value is better than

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Figure 3 Broadband Photoresponse of Sn intercalated MoO3 single crystal. a) Light

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intensity dependent responsivity (left) and photocurrent (right) under 520 nm; b) The current

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noise power spectrum. c) Wavelength dependence of the responsivity and specific detectivity

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D*. The R is taken from the linear range from the power dependent photocurrent curves. d)

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Response speed of a typical device. The rise/fall time was defined as the photocurrent

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increased/decreased from 10/90 percent to 90/10 percent of the stable photocurrent. The inset

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shows the free standing structure for heat isolation from the substrate. e) Photoresponse under

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vacuum and air condition. 10 ACS Paragon Plus Environment

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9.0 A/W. In the mid-IR region, the R decrease to ~ 0.4 A/W at 3700 nm, and 162 mA/W at 10

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µm. The specific detectivity D* calculated by6

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D* = (A BW)1/2 / NEP

(2)

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where A is the area of device, BW is the bandwidth, and NEP is the noise equivalent power

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derived from in/R at 1Hz.9, 35 The peak D* is 7.3×107 cm·Hz0.5·W-1 at 520 nm. The peak D*

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value is close to the state-of-the-art bolometer (108 cm·Hz0.5·W-1),9 and is better than

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graphene bolometer D* of 6×104 cm•Hz0.5•W-1,36 showing the high performance of MoO3-

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Sn as room temperature bolometer. The decrease of R and D* in the mid-IR region results

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from the decrease of optical absorption above ~3 µm in Figure S5. As a bolometric material

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(Discussed in next section), this can be further improved by additional absorption layer.

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The intercalation technique has shown the capability of converting a wide bandgap MoO3 into

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a narrow bandgap semiconductor with broadband photoresponse ability as that of defect

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engineering. Moreover, it avoids the destruction of lattices and improve the stability. The

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MoO3-Sn nanosheets were found to maintain good photoresponse in air, and its electrical

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performance can be maintain for more than 100 days in air (Figure S8). Traditional

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approaches, including annealing in vacuum,17 hydrogenation,13 and irradiation in aqueous

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environment,37 cause oxygen vacancies defects. Their photoresponse is generally poor and

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unstable in air.17, 24 Intercalation improves the rise time up to 0.084 s and fall time up to 0.143

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s (Figure 3d, Figure S9). Figure 3e shows that moving the device from air to vacuum

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substantially increases the photocurrent without sacrificing the response time. These superior

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properties distinguish it from the traditional defective sample, whose response time is over 10

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s, and strongly depends on the ambient condition because of the high activity of defect sites.

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3.3 Origin of the photoresponse



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Figure 4. Spatial photocurrent distribution by scanning photocurrent microscopy

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(SPCM) on a single crystal devices under focused laser illumination of 1550 nm at power

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of 0.2 mW. a) & b) Photocurrent distribution at 0 and +20 mV respectively. c) Photocurrent

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line profile along the dash line in a). d) The current transport and energy diagram at 0 V. A

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Schottky junction appears at the contact. e) Photocurrent line profile at various bias along the

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dash line in b). Photocurrent at zero bias is deducted in all lines in order to remove the contact

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effect. (The original data are showed in Figure S12)

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Photoresponse of a material may come from photovoltaic (PV) effect, and photo-thermal

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effect including thermoelectric (TE) effect and bolometric (BOL) effect.6 To reveal the

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photoresponse mechanism, we first map the spatial distribution of photocurrent across the

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channel in Figure 4. When the channel is illuminated at zero bias, the photocurrent could

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appear by PV effect, where photo-induced electron-hole pairs diffused to the contact area and

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separated by the Schottky junction (Figure 4d); or by TE effect, where the asymmetric

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thermal distribution inside the channel gives rise to the asymmetric current flow across the

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channel. Figure 4a & 4c shows that at zero bias, the photocurrent appears across the channel

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with two opposite photocurrent peaks at the contacts. This proves the existence of the above

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effect. However, KPFM and thermoelectric measurements in Figure S10 & Figure S11 show 12 ACS Paragon Plus Environment

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that the contact potential barrier of ~ 200 mV is much higher than the thermoelectric voltage

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of ~369.6 µV, which supports the PV effect over TE effect, and the energy diagram is

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depicted in Figure 4d.

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If the above PV effect dominates in biased device, the photocurrent peaks with increasing bias

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will be pinned at the electrode when the contact is Schottky type, or move to the opposite

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electrode when it is Ohmic contact.38 However, these are not observed in our devices. When a

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small bias of +20 mV is applied in Figure 4b, the change of local current in the channel

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(Figure S12) is much higher than the peak current at the contact (Figure 4c). As this voltage

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is much smaller than the contact potential barrier of 200 mV, this significant change in current

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cannot be simply attributed to the PV effect. It implied that BOL effect, where the local

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resistance via electron heating may appear under illumination variation. To extract the BOL

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effect, the current profile at different bias in Figure S12 is deducted at zero bias and shown in

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Figure 4e. It is found that the change in the current is the same across the channel. This

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means that the change in the local resistance is the same, which supports the inference of BOL

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effect.

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To further illustrate the thermal effect, a real-time heating and electrical measurement system

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with infrared thermal imager was assembled to obtain the relationships between photocurrent,

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sample temperature and illumination power (Figure S13). The temperature was obtained from

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the infrared thermal image from the back of the MoO3-Sn thin film device to avoid the

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obstruction from the incident light. Inset in Figure 5a shows that when the light illuminated

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on the front surface, the thermal image became bright, meaning that the temperature increased.



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Figure 5 Bolometric effect in MoO3-Sn thin film under 638 nm laser illumination at 0.1

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V bias voltage. a) Power density dependence of photocurrent and average sample temperature.

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Inset exhibits infrared thermal image of the back of the thin film. b) Typical thermal-

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resistance characteristic of a single crystal MoO3-Sn and the calculated TCR. (c) Schematic of

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the photodetection mechanism.

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The average temperature increased linearly with the increase of light intensity as that of

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photocurrent, exhibiting high consistency between photocurrent and temperature. Another

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important proof is the time response of the photocurrent and sample temperature under

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illumination in Figure S14, which are highly synchronous at the beginning of illumination

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change. It showed that the photocurrent was determined by the thermal dissipation of the

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sample. 14 ACS Paragon Plus Environment

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Figure 5c summarizes the mechanisms of photoresponse of MoO3-Sn. After intercalation of

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Sn atoms, a mid-band appeared in the gap. During illumination, electrons in the gap states can

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be excited to a higher energy level above the conduction band. ( ① in Figure 5c) These

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electrons have high energy and they are thermally relaxed to the minimum of conduction band

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in a short time. The relaxation process is a thermalization process, when the electrons will

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interact with phonons, losing their energies to the lattice atoms and raise the local temperature

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(② in Figure 5c). The increase of local temperature causes the excitation of electron-hole pair

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from the gap states to the conduction band (②’ in Figure 5c), which further raise the density

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of local carriers and thus the decrease of the resistance. As the visible light is more effectively

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absorbed than the IR light (Figure 1f), and the excited electron has a higher energy for

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thermally relaxation, the device shows higher responsivity in the visible range.

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As a bolometer materials, the most important parameter is the temperature coefficient of

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resistance (TCR): TCR 

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1 dR R dT

(1)

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where R is the resistance of single crystal MoO3-Sn and T is the temperature. Figure 5b

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shows the typical temperature dependence from single crystal MoO3-Sn. At temperature

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below 373 K, the resistance decreases sharply with increasing temperature. At higher

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temperature, the change of resistance decreased, which may result from the motion of Sn

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atoms. At 300 K, the TCR is -1.658 %/K. This value is close to that of VO2 (-2%/K - -3%/K),

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which is widely used in commercialized bolometric type IR detector.39 Recently, studies

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reported that graphene have ultra-high TCR (Table R), however, the D* of MoO3-Sn is better,

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which should be attributed to low noise level due to the high structural quality of the MoO3-

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Sn.

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4. CONCLUSION 15 ACS Paragon Plus Environment


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In summary, we report the alteration of optoelectronic properties of the van der Waals α-

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MoO3 via atomic intercalation and its application in broadband photoresponse. We

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experimentally demonstrated that intercalation of Sn atoms into the van der Waals gap

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induced gap states with enhancing light absorption without causing the destruction of lattices.

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The striking enhancement in conductivity and absorption of MoO3 nanosheets causes a

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broadband photoresponse from 405 nm to 10 µm at room temperature. The two terminal

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device presents a response time of ~ 0.1 s, high responsivity around 9 A/W and the peak

8

detectivity up to 107 cm·Hz0.5·W-1 in the near-IR region. We further demonstrate that effect of

9

photo-thermal-electrical coupling in the materials, and bolometric effect dominate the

10

photoresponse. This work demonstrates the possibility to tune optoelectronic properties of

11

layered transition metal oxide and make them a potential candidate of broadband absorption

12

materials for different kinds of energy conversion through an effective and non-destructive

13

intercalation process.

14

ASSOCIATED CONTENT

15

Supporting Information

16

The Supporting Information is available free of charge on the ACS Publications website at

17

DOI: 10.1021/acsami.XXX.

18

Fabrication of MoO3-Sn devices; In-situ observation of Sn atoms intercalation; The TEM and

19

EDS of intercalated MoO3 nanosheets; Raman spectra of MoO3 nanosheets; Transmission

20

spectrum in IR range; Photoresponse of Sn-intercalated MoO3 nanosheet; The responsivity

21

characteristics; The stability exploration of intercalated MoO3 nanosheets device; Comparison

22

of photoresponse of free-standing and contacted samples; KPFM measurement; Seeback

23

coefficient; Photocurrent cross-sectional profile; The real-time heating and photoelectrical

24

response measurement system; Real time measurement of the response of photocurrent and

25

average sample temperature;

26

AUTHOR INFORMATION

27

Corresponding Authors

28

*E-mail: [email protected]

29

*E-mail: [email protected] 16 ACS Paragon Plus Environment

Page 16 of 22

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1

*E-mail: [email protected]

2

Notes

3

The authors declare no competing financial interest

4

ACKNOWLEDGEMENTS

5

This work was financially supported by the National Natural Science Foundation of China

6

(Grants Nos. 11574119, 61775092, 11474364, 61229401, 21576301, and 51290271), Science

7

and Technology Project of Guangdong Province (Grants No. 2017B030314031), Science

8

and Technology Program of Guangzhou (Grants No. 201804010143), the Research Grants

9

Council of Hong Kong (Grant Nos. AoE/P-03/08, T23-407/13-N, AoE/P-02/12, 14207515,

10

14204616) and the CUHK Group Research Scheme, as well as Innovation and Technology

11

Commission (Grant No. ITS/088/17).

12 13

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