Accelerated Degradation of CrCl3 Nanoflakes Induced by Metal

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Accelerated Degradation of CrCl3 Nanoflakes Induced By Metal Electrodes: Implications for Remediation in Nano Device Fabrication Lixuan Liu, Kun Zhai, Anmin Nie, Weiming Lv, Bingchao Yang, Congpu Mu, Jianyong Xiang, Fusheng Wen, Zhisheng Zhao, Zhongming Zeng, Yongji Gong, Yongjun Tian, and Zhongyuan Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00058 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Nano Materials

Accelerated Degradation of CrCl3 Nanoflakes Induced By Metal Electrodes: Implications for Remediation in Nano Device Fabrication Lixuan Liua,c, Kun Zhaia*. Anmin Niea*, Weiming Lvb, Bingchao Yanga, Congpu Mua, Jianyong Xianga, Fusheng Wena, Zhisheng Zhaoa, Zhongming Zengb*, Yongji Gongc, Yongjun Tiana, Zhongyuan Liua aCenter

for High Pressure Science (CHiPS), State Key Laboratory of Metastable Materials

Science and Technology, Yanshan University, Qinghuangdao 066004, China bKey

Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nanobionics, Chinese Academy of Sciences, Ruoshui Road 398, Suzhou 215123, China

cSchool

of Materials Science and Technology, Beihang University, Beijing 100191, China

ABSTRACT: Two-dimensional (2D) layered magnetic materials have attracted great attention in recent years because of the discovery of long-range order of magnetism down to the monolayer and its expected application in novel devices such as spintronic devices and magnetic sensors. Similar to many other 2D layered nonmagnetic materials, such as black phosphorus, environmental instability presents great challenge for the potential application of 2D layered magnetic materials in the future. In the present work, we report the phenomenon of accelerated

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deterioration in environmental stability of exfoliated CrCl3 nanoflakes in direct contact with metal electrodes. As revealed in our investigations, depending on the metal type of the electrode, immensely accelerated degradation of CrCl3 nanoflake can be induced upon exposure in air. We have further determined that the presence of H2O is indispensable in the expedited degradation. Our study can provide useful guidance on the choice of metal electrodes in the fabrication of CrCl3-based devices.

KEYWORDS: CrCl3 nanoflake, 2D layered magnet, environmental stability, accelerated degradation, electrode, device fabrication Introduction In the past few years, great interest has been aroused in two-dimensional (2D) layered magnetic materials, such as chromium trihalides (CrX3, X=Cl, Br and I), Cr2Ge2Te6, VSe2, and MnSe2, owing to the discovery of long-range order of magnetism with reduction of thickness down to a few layers and even monolayer.1-8 The observed long-range order of magnetism down to monolayer limit has significant importance in fundamental research in magnetism. It will help deepen our understanding of 2D magnetism and bring possibilities in the exploration of new physical phenomena at the 2D scale. In contrast to the traditional magnetic thin films, 2D layered magnetic materials exhibit many unique advantages in the fabrication of novel devices, such as controllability of the ground state by strain, ionic gating, and electric field.9-11 The great potential application of 2D layered magnetic materials has been anticipated in the future design of novel devices, such as heterostructures, magnetic sensors, spintronic devices, and magnetoelectric devices.12-18


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CrX3 (X = Cl, Br, I) are one attractive family of 2D layered magnetic materials owing to their great potential in the fabrication of novel devices.19-21 Similar to the other 2D layered nonmagnetic materials such as black phosphorus,22-25 however, the practical applications of CrX3 face the big challenge of environmental instability. For example, the exfoliated nanoflakes of CrI3 exhibit fast degradation under ambient conditions.1-3 The fabrication of CrI3 based devices thus require very rigorous conditions, such as indispensable protection of insert gas and passivated layers. Even if such meticulous operations were taken, the shelf life of fabricated devices would still be limited.10,18 In contrast to CrBr(I)3, CrCl3 exhibit much better environmental stability when it is handled in air.19 In the study on CrCl3-based devices, however, we have noticed that the exfoliated CrCl3 nanoflake in air can be induced to exhibit accelerated degradation upon direct contact with metal electrodes. In the present work, we have performed detailed studies on the environmental degradation of CrCl3 nanoflakes induced by direct contact with a series of metal electrodes, aiming to reveal the underlying mechanism and provide useful guidance on electrode selection in device fabrication. Experimental Section Synthesis of CrCl3 Crystal: Bulk CrCl3 crystal was grown by a chemical vapor transport method with metal-basis 99.9% CrCl3 powder (Alfa Aesar) sealed in a silica tube. The raw material was heated to 700 °C and the plate-like CrCl3 was obtained at the cooler end of the tube as shown in Scheme 1a. Preparation of CrCl3 nanoflakes: The CrCl3 nanoflakes were mechanically exfoliated from the as-grown CrCl3 crystal in the air or ambient conditions (T = 26 ˚C, Humidity = 30 - 40 %, light) by using standard Scotch tape method. The exfoliated CrCl3 nanoflakes were then transferred to SiO2(300 nm)/Si substrate and metal electrodes on SiO2(300 nm)/Si substrates.


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The metal electrodes were prepared by patterning to resist layer using electron beam lithography, followed by electron-beam evaporation of metals such as Cu, Ti, Ni, Al, Cr, Ag, Au, Pd, and Pt. All the prepared metal electrodes have a thickness of 50 nm. The schematic illustration of transfer method is shown in Scheme 1b. Characterizations: The chemical compositions and atomic structure of CrCl3 nanoflake were examined via EDX elemental maps, SAED patterns and atomic scale HAADF images inside a probe aberration corrected FEI Themis Z 300 TEM, equipped with Super X EDX system. The HAADF images were acquired using a 24-mrad-probe convergence semi-angle, as well as a 60mrad-inner and 200-mrad-out detector angles. The environmental stability of CrCl3 nanoflakes in air or ambient conditions was investigated via optical, AFM and XPS measurements. The optical images were taken in a Leica DM4000 M LED microscope. The AFM measurements were carried out in a Bruker Multimode 8 by using ScanAsyst mode with silicon nitride SCANASYST-AIR-HR tips. The XPS spectra were collected in a ThermoFisher ESCALAB™ 250Xi with a monochromatic Al Kα line (source energy = 1486.69 eV), source size of 650 μm and analyzer pass energy of 30 eV. The contaminant carbon (284.8 eV) was used as a reference for calibration. For the exposure performed under anaerobic condition (O2 level ≤ 0.1 %), the CrCl3 nanoflakes and Fe-based O2 absorbent were placed together into a sealed vessel, which was initially filled with air. For the exposure carried out at the greatly increased level of humidity (> 90 %), the CrCl3 nanoflakes were put into a sealed vessel with water on the bottom. A humidometer was also placed into the sealed vessel to monitor the humidity inside. All the optical and AFM measurements were carried out in air.

Results and Discussion


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The CrCl3 nanoflakes were mechanically exfoliated from bulk CrCl3 crystal grown by chemical vapor transport (CVT). The chemical compositions and atomic structure of exfoliated CrCl3 flakes were first characterized by Transmission Electron Microscopy (TEM). Figure 1a shows a typical high angle annular dark field (HAADF) image of individual CrCl3 nanoflake. Owing to atomic number and thickness dependence of the intensity of HAADF image,26 the layered feature of CrCl3 nanoflake is distinctly illustrated in Figure 1a. The corresponding energy dispersive X-ray (EDX) elemental maps (Figures 1b and c) indicate a uniform elemental distribution of Cl and Cr. Figures 1d-f show the selected area electron diffraction (SAED) patterns of CrCl3 nanoflake taken along different directions. As shown in Figure 1d, the typical SAED pattern taken from the top of CrCl3 flake is determined to be along the [103] zone axis of a monoclinic crystal structure (C2/m space group, JCPDS 73-0309). Noticeably, some forbidden spots are observed in the SAED pattern of Figure 1d, forming superstructure reflections. The SAED pattern of Figure 1e from one side of CrCl3 nanoflake is determined to be along the [010] zone axis of the monoclinic CrCl3 crystal. Interestingly, a streaking feature of the reflections along the [103] direction (perpendicular to the (001) plane) appears in the SAED of Figure 1e, implying the existence of stacking faults of the CrCl3 nanoflake. Streaking lines are also observed in the SAED pattern in Figure 1f taken from another side of CrCl3 nanoflake. Apart from the arranged weak spots in the streaking line, the SAED pattern in Figure 1f could belong to both the [100] and [110] (also the equivalent [1-10]) zone axes of monoclinic CrCl3. Figures 1g-i show the corresponding atomic scale HAADF images of CrCl3 flake acquired along different zone axes. As indicated in Figure 1g, superlattices are observed in the atomic scale HAADF image along the [103] zone axis, differing from the atomic structure of perfect monoclinic CrCl3 crystal projected along the [103] direction (Inset in Figure 1g). Interestingly,


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undulating atomic layers are observed in the atomic HAADF image along the [010] zone axis in Figure 1e, indicating disordered stacking and weak Van der Waals force between CrCl3 layers. The atomic scale HAADF image in Figure 1i shows that CrCl3 flake possesses different domains projected along the [100], [110], and [1-10] zone axes, which are labeled with white, red, and green lines, respectively. The multi-oriented domains in a single nanoflake indicate that the layered CrCl3 crystal usually contains stacking faults and also explain the observed superstructure along the [103] direction. In the present work, the mechanically exfoliated CrCl3 nanoflakes were transferred onto SiO2 (300 nm)/Si substrate, and they were exposed in air, i.e., ambient conditions (T = 26 ˚C, Humidity = 30 - 40 %, light) to test the environmental stability. The optical images (Figure S1 in Supporting Information (SI)), which were taken immediately after the transfer and after 12-day exposure in air, exhibit no obvious change in morphology on the surface. As shown in the corresponding images of atomic force microscopy (AFM) (Figure S1 in SI), small bumps are observed on the nanoflake’s surface, and the 12-day exposure led to increase in number and growth in size. The extracted surface roughness (Rq) exhibits a slight increase from 1.1 to 1.7 nm. In a parallel experiment where the humidity was increased to above 90 %, no obvious change on the nanoflake’s surface can be observed in the optical images taken with the increase of exposure time (Figure S2 in SI). In the corresponding AFM images (Figure S2 in SI), the small bumps observed on the nanoflake’s surface show a faster increase in number and growth in size with the increased exposure time. Note that, we use surface roughness as a parameter to semi-quantitatively describe the degradation degree. After the 7-day exposure, the surface roughness (Rq) is increased from 1.0 to 1.7 nm (Figure S2f in SI). Therefore, the H2O level, i.e., humidity in the enviroment is a significant factor effecting the environmental stability of CrCl3


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nanoflake. Yet overall, the exposure-induced change is not fast on the surface of CrCl3 nanoflake even at the greatly increased humidity, implying the good environmental stability of CrCl3 particularly in comparison to CrBr(I)3.19 Layered CrCl3 has been demonstrated to exhibit the long-range order of magnetism when the thickness is reduced to a few layers and even monolayer. This discovery stimulates great anticipation about the application of CrCl3 nanoflakes in novel devices.27-31 Generally, device fabrication involves the use of metal electrodes. Unfortunately, we have noticed expedited degradation of exfoliated CrCl3 nanoflake in contact with metal electrodes under exposure in air. The exfoliated CrCl3 nanoflakes were transferred onto Cu electrodes on SiO2/Si substrate, and they were exposed to ambient conditions (T = 26 ˚C, Humidity = 30 - 40 %, light). The optical and AFM images were collected with an increase of exposure time. Even in the optical images (see Figure S3 and Movie S1 in SI), obvious change in contrast or color is observed on the nanoflake’s surface in just a few minutes of exposure. The color change initially occurs on the electrode’s edge and rapidly expands toward the surrounding regions over the Cu electrode with the increase of exposure time. After exposure of 1 hour, the change in color occurs all over the part of the nanoflake in direct contact with the Cu electrode. With the further increase of exposure time, the nanoflake in direct contact with Cu electrode becomes deeper and deeper in contrast, and it ultimately looks dark in color. For the part of the nanoflake in contact with SiO2/Si substrate, the initial color or contrast is well retained with the increase of exposure time. When the exposure time is long enough (> 34 h), however, localized change in color is noted to appear. These observations in the optical images imply that the CrCl3 nanoflake can be induced to show fast degradation if it is in contact with Cu electrode. The AFM images as shown in Figure 2a provide further confirmation of the exposure-produced degradation of CrCl3 nanoflake


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in contact with Cu electrode. The initial AFM image was collected in the air immediately after the transfer of the exfoliated CrCl3 nanoflake to the Cu electrode, and the scan took 4.5 minutes. Even during the several-minute scan, a cluster of nanopillars appeared on the surface of the nanoflake’s region in contact with Cu electrode. For simplicity in description, 0 min is used to denote the initial AFM image. As indicated in the AFM images, with the increase of exposure time, more clusters of nanopillars are grown along the edge of Cu electrode, while the initial cluster of nanopillars expanded quickly toward the surrounding regions and simultaneously grew in height and diameter. At exposure of up to 60 minutes, the nanopillars are observed to appear nearly all over the part of the nanoflake in contact with Cu electrode. The surface roughness (Rq) was extracted from the part of the nanoflake in contact with Cu electrode, as shown in Figure 2b. Rq exhibits quick increase with the increase of exposure time. Just 60 min exposure leads to the increase of Rq by over 500 %. For the part of the nanoflake in contact with SiO2/Si substrate instead of Cu electrode, a gradual increase in surface roughness can be also noted when the exposure time is over 25 min, consistent with the observed color change in the optical images. Then we studied the stability of CrCl3 with some frequently used metals to give guidance on choosing electrode materials based on experiments. The exfoliated CrCl3 nanoflakes were also transferred to eight other metal electrodes including Al, Cr, Ti, Ni, Ag, Au, Pd and Pt, and the effect of metal type on CrCl3 environmental stability in air has been investigated. The optical images were collected immediately after the transfer of CrCl3 nanoflakes and after exposure of up to 7 days, which are presented as Figures S4 and S5 in SI. For the CrCl3 nanoflakes in contact with Pt and Pd electrodes, no obvious sign of degradation is identifiable in the optical images even after 7-day exposure. For the CrCl3 nanoflakes in contact with Ag, Au, Al, Cr, Ti and Ni electrodes, the degradation can be clearly observed in the optical images, though it is developed


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much more slowly with the increased exposure time in comparison to those in contact with Cu electrode. The degradation speed is strongly dependent on the metal species. Among the investigated metal electrodes, the Cu electrode induces the fastest degradation, and the degradation speed can be qualitatively recognized from the optical images with the sequence of Cu ≫Ag, Au, Al> Cr, Ti, Ni ≫ Pt, Pd. In order to elucidate if O2 and/or H2O are involved or play vital roles in the degradation of CrCl3 nanoflake in contact with metal electrodes, we have performed investigations on the exposure of exfoliated CrCl3 nanoflakes on Au electrodes under five different conditions. The choice to use Au electrode is because it induces obvious but not too fast degradation of CrCl3 nanoflake under exposure in air. The five different exposure environments are selected as: I) Ambient condition with light (T = 26 ˚C, Humidity = 30 - 40 %, light); II) Dark condition, i.e. ambient condition without light; III) Anaerobic condition, i.e. ambient condition with reduced O2 level of ≤ 0.1 %; IV) Dry condition, i.e. ambient condition with reduced humidity of ~ 0 %; and V) High humidity condition, i.e. ambient condition with increased humidity of > 90 %. For comparison, the AFM images were collected immediately after the sample preparation and exposure under the five selected environments, as shown in Figures 3a-e. Figure 3f gives the extracted surface roughnesses after the exposure under the five selected environments, which are normalized with the initial ones. After 3-day exposure under conditions I, II and III, the AFM images in figures 3a, b and c demonstrate obvious morphological changes on the surface of CrCl3 nanoflakes. As revealed in figure 3f, the surface roughnesses are comparable after 3-day exposure under conditions I and II, being increased by ~ 9 times, while the surface roughness after 3-day exposure under condition III is increased by ~ 12 times. The slightly higher surface roughness is actually ascribed to the rise of humidity due to the removal of O2 by using Fe-based


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O2 absorbent. These observations indicate that the roles of O2 and light can be basically ruled out, or are insignificant, in the environmental degradation of CrCl3 on metal electrodes. Under dry condition (IV), however, no morphological change can be obviously observed on the surface of CrCl3 nanoflake even after the exposure of up to 17 days (Figure 3d), and the surface roughness exhibit no increase (normalized Rq = 1, Figure 3f). Under condition V, with the greatly increased humidity of > 90 %, fast growth in surface roughness is observed in the AFM image just after 1 h exposure (Figure 3e, also see Figure S6 in SI). The surface roughness is increased by 40 times (Figure 3f). Therefore, H2O is indispensable in the accelerated environmental degradation of CrCl3 nanoflake in contact with the metal electrode. These observations strongly suggest that H2O and contact with metal electrodes play the synergistic roles in the accelerated degradation of CrCl3 nanoflake under ambient conditions. To gain deeper insight into the possibly formed species during CrCl3 degradation with the synergistic involvement of metal electrode and H2O, we have performed the studies of X-ray photoelectron spectroscopy (XPS) on CrCl3 nanoflakes in contact with Cu, Ag, Au, and Pd electrodes. The XPS curves were taken after the exposure under ambient conditions with the normal and increased humidity. Figure 4a gives the C1s, O1s, Cu2p and Cl2p XPS spectra, which were taken from the CrCl3 nanoflakes on Cu electrodes after exposure of 7 days at the increased humidity of > 90 %. In the C1s spectrum, three peaks are observed at 284.80, 286.53 and 288.49 eV, and the one at 284.80 eV is associated with the contamination carbon.32 The O1s spectrum is deconvoluted into four peaks at 530.99, 531.86, 532.73 and 533.67 eV. The peaks at 532.73 and 533.67 eV can be resolved to arise from SiO2/Si substrate and adsorbed H2O, respectively.33,34 By taking into consideration of both the C1s and O1s spectra together, the C1s peak at 288.49 eV and the O1s peak at 531.86 eV can be assigned to chemically absorbed,


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negatively charged CO2.32 For the C1s peak at 286.53 eV and O1s peak 530.99 eV, the source cannot be exactly determined just from the XPS data. Possibly, they could be produced by a new complex compound, which may be formed with the involvement of H2O and CO2 in the complex reactions on the surface of CrCl3 nanoflake. In the Cu2p spectrum, two split 2p3/2 peaks are observed at 933.22 and 935.63 eV. The peak at 933.22 eV is assigned to the Cu electrode.35,36 In the Cl2p spectrum, the 2p3/2 peak can be deconvoluted to two peaks at 199.78 and 198.79 eV. The peak at 199.78 eV comes from the CrCl3 nanoflakes.37 For the Cu2p3/2 peak at 935.63 eV and Cl2p3/2 peak at 198.79 eV, they are recognized to arise from copper (II) chloride (CuCl2),38 which is most possibly formed at the interface between Cu electrode and the CrCl3 nanoflake. For the CrCl3 nanoflakes on Cu electrodes after the 7-day exposure at the normal humidity of 30 - 40 %, the XPS measurements give the similar C1s, O1s, Cu2p, and Cl2p spectra (see Figure S7 in SI). In the Cu2p and Cl2p spectra, the formation of CuCl2 is still observed but in a much lower amount owing to the lower humidity of 30 - 40 %. For CrCl3 nanoflake on Pd electrode after exposure under ambient conditions, no sign of degradation can be observed in the optical images (see Figure S5 in SI). In the XPS measurements, however, the exposure-induced degradation can be identified (see Figure S8 in SI). Traces of PdCl2 can be recognized in the Pd3d and Cl2p XPS spectra, and the increased humidity leads to the formation of PdCl2 in a higher amount.39 Similarly, traces of Ag(Au)Cl are also observed in the Cl2p XPS spectra from samples exposed under ambient conditions with normal (30 - 40 %) and highly increased (> 90 %) humidity.40,41 For comparison, the Cl2p XPS spectra of CrCl3 nanoflakes on Cu, Ag, Au and Pd electrodes are presented together in Figures 4a and b, which were collected after the 7-day exposure in the air with the greatly increased humidity of > 90 %. Obviously, the quantity of formed metal chloride depends on the metal species. Figure 4c shows the determined Cl2p3/2 peak intensity ratio of


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metal chloride over CrCl3. The quantity of metal chloride decreases from Cu, Ag, Au, and Pd in sequence, consistent with the observed surface change in the optical and AFM images. In the formation of metal chloride after exposure of CrCl3 nanoflakes on metal electrodes in the air, CrCl3 is the only source of Cl. One possibility is the presence of complicated oxidationreduction reaction at the interface of the metal electrode and CrCl3 nanoflakes, which leads to the formation of the corresponding metal chloride and CrCl2. In this case, CrCl2 is possibly formed inside CrCl3 nanoflakes as well. CrCl2 is very hygroscopic under ambient environment and will form the hydrate CrCl2(H2O)4.42 It also has strong redox ability due to the bivalent Cr2+. Owing to the presence of CrCl2, the CrCl3 nanoflake can be induced to become quite unstable under ambient environment. Based on this and our analysis of XPS data, we believe complex reactions are able to occur on the surface of CrCl3 nanoflake in contact with metal electrode, where the metal itself and H2O play synergistic roles. Such reactions will lead to the formation of trace amounts of CrCl2 and CrCl2(H2O)4, which will trigger further degradation of the CrCl3 nanoflake. From the sign of CO2δ- based on XPS data , we also propose that apart from H2O, CO2 may also be involved in the further degradation of CrCl3, to result in a complicated mixture of Cr compounds with the possible formula of Cr(H2O)x(CO2)yClz . CONCLUSION In summary, we have determined the stacking sequence in the CrCl3 crystal grown by CVT method and carried out investigations on the environmental stability of exfoliated CrCl3 nanoflake on SiO2/Si substrate and a series of metal electrodes via the optical, AFM and XPS measurements. The CrCl3 nanoflake on SiO2/Si substrate shows pretty good stability under exposure to the ambient environment. However, the CrCl3 nanoflakes in direct contact with metal electrodes become worse in environmental stability. With exposure to air, the contact with


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metal electrodes leads to obvious degradation of CrCl3 nanoflake, and the degradation speed depends on the metal species. For some metals such as Cu, the degradation can be induced to occur immediately under exposure in ambient conditions. The presence of H2O is identified to be indispensable in the degradation of CrCl3 nanoflakes in direct contact with metal electrodes. During the degradation of CrCl3 nanoflakes in contact with metal electrodes, corresponding metal chlorides are recognized to be formed at the interface, implying the possible reduction of CrCl3 to CrCl2. The formation of CrCl2 within the CrCl3 nanoflakes is believed to result in rapid degradation of CrCl3 and the environmental instability in air. The different degradation degree of metals may attribute to the complex reaction involved with CO2 and H2O. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figures for optical microscope images, AFM images, and XPS data. AUTHOR INFORMATION Corresponding Authors: *E-mail:[email protected],

*

E-mail:[email protected],

*

E-mail:[email protected]

ORCID Kun Zhai: 0000-0002-1465-6778 Congpu Mu: 0000-0003-1656-716X


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Jianyong Xiang: 0000-0001-9374-7127 Fusheng Wen: 0000-0002-6827-5348 Zhongming Zeng: 0000-0001-7240-2058 Zhongyuan Liu: 0000-0002-5600-8998 Author Contributions: B.C.Y did the growth of the CrCl3 crystal. W.M.L. prepared the metal electrodes on SiO2/Si substrates. L.X.L. performed the optical, AFM and XPS measurements, analysis of the data. A.M.N. did the TEM measurements and the analysis. K. Z., A.M.N., Z.M.Z. and Z.Y. L. designed the experiment together. L.X.L., K. Z., and Z.Y.L. wrote the manuscript together. All the authors joined in the discussion about experimental measurements and analysis.

ACKNOWLEDGMENTS We would like to appreciate the financial support from the National Natural Science Foundations of China (Nos. 51732010, 51761145025). REFERENCES (1) Huang, B.; Clark, G.; Navarro-Moratalla, E., Klein D. R.; Cheng, R.; Seyler, K. L.; Zhong, D.; Schmidgall, E.; McGuire, M. A.; Cobden D. H.; Yao, W.; Xiao, D.; Jarillo-Herrero, P.; Xu. X. Layer-Dependent Ferromagnetism in a van der Waals Crystal Down to the Monolayer Limit. Nature 2017, 546, 270-273. (2) Lin, G. T.; Luo, X.; Chen, F.C.; Yan, J.; Gao, J.J.; Sun, Y.; Tong, W.; Tong, P.; Lu, W.J.; Sheng, Z. G.; Song, W.H.; Zhu, X.B.; Sun, Y.P. Critical Behavior of Two-dimensional Intrinsically Ferromagnetic Semiconductor CrI3. Appl. Phys. Lett. 2018, 112, 072405.


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(20) McGuire, M.A.; Dixit, H.; Cooper, Sales, B.C. Coupling of Crystal Structure and Magnetism in the Layered, Ferromagnetic Insulator CrI3. Chem. Mater. 2015, 27, 612-620. (21) Abramchuk, M.; Jaewski, S.; Metz, K. R.; Osterhoudt, G. B.; Wang, Y.; Burch, K. S.; Tafti, F. Controlling Magnetic and Optical Properties of the van der Waals Crystal CrCl3−xBrx via Mixed Halide Chemistry Adv. Mater. 2018, 30, 1801325. (22) Wang, Y.; Yang, B.; Wan, B.; Xi, X.; Zeng, Z.; Liu, E.; Wu, G.; Liu, Z.; Wang, W. Degradation of Black Phosphorus: A Real-Time 31P NMR Study. 2D Mater. 2016, 3, 035025. (23) Island, J.O.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Environmental Instability of Few-layer Black Phosphorus. 2D Mater. 2015, 2, 011002. (24) Yang, B.; Wan, B.; Zhou, Q.; Wang, Y.; Hu, W.; Lv W.; Chen, Q.; Zeng, Z.; Wen, F.; Xiang, J.; Yuan, S.; Wang, J.; Zhang, B.; Wang, W.; Zhang, J.; Xu, B.; Zhao, Z.; Tian, Y.; Liu, Z. Te-Doped Black Phosphorus Field-Effect Transistors. Adv. Mater. 2016, 28, 9408. (25) Li, P.; Zhang, D.; Liu, J.; Chang, H.; Sun, Y.; Yin, N. Air-Stable Black Phosphorus Devices for Ion Sensing. ACS Appl. Mater. Interface 2015, 7, 24396-24402. (26) Pennycook, S.J.; Jesson, D.E. High-resolution Z-constant Imaging of Crystals. Ultramicroscopy 1991, 37, 14-38. (27) Zhang, W.-B.; Qu. Q.; Zhu, P.; Lam, C.H. Robust Intrinsic Ferromagnetism and Half Semiconductivity in Stable Two-dimensional Single-Layer Chromium Trihalides. J. Mater. Chem. C 2015, 3, 12457. (28) Liu, J.; Sun, Q.; Kawazoe, Y.; Jena, P. Exfoliating Biocompatible Ferromagnetic Crtrihalide Monolayers. Phys. Chem. Chem. Phys. 2016, 18, 8777. (29) Lado, J. L.; Fernández-Rossier, J. On the Origin of Magnetic Anisotropy in Twodimensional CrI3. 2D Mater. 2017, 4, 035002.


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(41) Kitagawa, H.; Kojima, N.; Nakajima, T. Studies of Mixed-valance States in Threedimensional Halogen-bridged Gold Compounds, Cs2AuIAuIIX6, (X = Cl, Br or I). Part 2. X-Ray Photoelectron Spectroscopic Study. J. Chem. Soc. Dalton Trans. 1991, 11, 3121-3125. (42) Dellien, I.; Hall, F. M.; Hepler, L. G. Chromium, Molybdenum, and Tungsten: Thermodynamic Properties, Chemical Equilibria, and Standard Potentials. Chem. Rev. 1976, 76, 283-310.


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Page 20 of 30

Scheme 1. Schematic illustration of (a) CrCl3 growth method and (b) CrCl3 transfer process.


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Figure 1. Chemical compositions and atomic structure of CrCl3 nanoflake. (a-c) Typical HAADF image of CrCl3 nanoflake and corresponding EDX elemental maps. (d-f) SAED patterns of CrCl3 nanoflake taken along the [103], [010] and [100] zone axes, respectively. (g-i) Atomic-scale HAADF images of CrCl3 nanoflake viewed along the [103], [010] and [100] zone axes, respectively. Insets show the unit cell of monoclinic CrCl3 projected along the corresponding directions (Red ~ Cl, Green ~ Cr).


21


(b)

(a) 100 8

nm

70 0 min

10 min

15 min

20 min

25 min

30 min 456 181

50 min

60 min

Rq (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

35

nm

720 min

0

10 100 Time (min)

800

Figure 2. AFM characterization. AFM images (a) collected immediately after the transfer of CrCl3 nanoflake on Cu electrode (0 min) and after exposure in ambient conditions (T = 26 ˚C, Humidity = 30 - 40 %, light) for different lengths of time. The surface roughness (Rq) and its variation with the increase of exposure time (b). The surface roughness was extracted from the nanoflake’s region in direct contact with Cu electrode.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) I

nm

0d

(b) II

3d

135 48

nm

3d

(d) IV

nm

191 75

260 101

3d (f) nm

167 67

100 45

0d

0d

nm

419 166

(e) V nm

nm

182 72

0d

0d nm

17 d

(c) III nm

107 41

nm

167 67

700 280

Rq (t=t) / Rq (t=0) (×100 %)

Page 23 of 30

1h

40 30 20 10

3d 3d 3d 17d

0

1h

I II III IV V Condition

Figure 3. AFM characterization of CrCl3 nanoflakes transferred to Au electrodes. AFM images (a-e) were taken immediately after transfer and after exposure under five different conditions IV. The five different conditions I-V are defined as: I. Ambient condition with light (T = 26 ˚C, Humidity = 30 - 40 %, light); II. The dark condition, i.e. ambient condition without light; III. The anaerobic condition, i.e. ambient condition with reduced O2 level of ≤ 0.1 %; IV. The dry condition, i.e. ambient condition with reduced humidity of ~ 0 %; and V. High humidity condition, i.e. ambient condition with increased humidity of > 90 %. The surface roughness (Rq) after exposure under the five different conditions I-V (f), which was extracted from the nanoflake’s region in direct contact with Au electrode and normalized with the initial roughness obtained immediately after the initial transfer.


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282

531.86 530.99

533.67

Intensity (arb.u.)

534

Cu2p

Cl2p

532

960

950

940

930

199.78 198.79 CuCl2

201.22

202

530 933.22 935.63

200

198

Binding Energy (eV)

Intensity (arb.u.)

285 532.73

Intensity (arb.u.) Intensity (arb.u.)

288

Cl2p

Cl2p

(c) Intensity Ratio (%)

291

286.53

O1s

(b)

284.80

288.49

Intensity (arb.u.)

C1s

Intensity (arb.u.)

(a)

Intensity (arb.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

199.98

Ag

201.55 198.61 AgCl

Au

200.46

201.45 198.92 AuCl

Cl2p

200.26

Pd

201.85 198.76 PdCl2

202

200

198

Binding Energy (eV)

Cu 60 40

Au

20

Ag

Pd 0

Metallic Electrode

Figure 4. XPS characterization of CrCl3 nanoflakes on metal electrodes after the exposure of 7 days under ambient conditions with the increased humidity of > 90 %. C1s, O1s, Cu2p and Cl2p XPS spectra of CrCl3 nanoflakes on Cu electrode (a). Cl2p XPS spectra of CrCl3 nanoflakes on Ag, Au, and Pd electrodes (b). The intensity ratio of IMClx over ICrCl3 (c), where ICrCl3 and IMClx represent the integrated intensities of Cl2p3/2 for CrCl3 and the formed metal chloride, respectively.


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Page 26 of 30

(a) Source End:

powder 1 CrCl T = 700 °C 2 (b) 3Glass 4 5 6 7 8 3

Growth End: CrCl3 flakes T = 550 °C CrCl3 nanoflake

Mechanical exfoliation Humidity = 30 - 40 %,

T = 26 ˚C, light PDMS metal electrode (Au, Cu, Ag, etc.) SiO2/Si substrate


CrCl3 on metal electrodes

Page 27 of 30ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16


(a)

(b)


nm

1 2 3 4 5 6 7 8 9 10

Page 28 of 30

70

0 min

10 min

15 min

Rq (nm)

100 8

20 min

25 min

35

30 min 456 181

nm

ACS Paragon Plus Environment 50 min

60 min

720 min

0

10

100 Time (min)

800

10 d 2 3 4 53 d 6 (d) IV 7 8 9 10 0d 11 12 13 14 17 d 15

(b) II

(c) III

nmACS Applied Nano Materials nm 107 182 41 72

135 48

nm

3d

nm 419 166

(f) nm

167 67

0d nm

260 101

100 45

1h

40 30 20

nm 167 10 700 67 Paragon Plus Environment 280 ACS

1h

191 75

3d (e) V

nm

nm

0d

0d nm

Rq (t=t) / Rq (t=0) (×100 %)

(a) I Page 29 of 30

0

3d 3d 3d 17d I II III IV V Condition

ACS Applied Nano(b) Materials

291

Intensity (arb.u.) Intensity (arb.u.) Intensity (arb.u.)

286.53

O1s

288

285 532.73

282

531.86 530.99

533.67

Intensity (arb.u.)

534

Cu2p

Intensity (arb.u.)

Cl2p

532

530 933.22 935.63

960

950

930

198.79 CuCl2

201.22

202

940

199.78

200

Page 30 of 30 199.98

Cl2p

Ag

201.55 198.61 AgCl

Au

200.46

Cl2p 201.45

198.92 AuCl 200.26

Cl2p

Pd

201.85 198.76 PdCl2

202

(c) Intensity Ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

284.80

288.49

Intensity (arb.u.)

C1s

Intensity (arb.u.)

(a)

200

198

Binding Energy (eV)

Cu 60 40

Au

20

Ag

Pd

ACS Paragon Plus Environment 0 198

Binding Energy (eV)

Metallic Electrode

Accelerated Degradation of CrCl3 Nanoflakes Induced by Metal

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