Encapsulation-Free Stabilization of Few-Layer Black Phosphorus

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Encapsulation-free stabilization of few-layer black phosphorus Christopher Elbadawi, Roger Tormo Queralt, Zai-Quan Xu, James Bishop, Taimur Ahmed, Sruthi Kuriakose, Sumeet Walia, Milos Toth, Igor Aharonovich, and Charlene J Lobo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04180 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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ACS Applied Materials & Interfaces

Encapsulation-free stabilization of few-layer black phosphorus Christopher Elbadawi1, Roger Tormo Queralt1, Zai-Quan Xu1, James Bishop1, Taimur Ahmed2,3, Sruthi Kuriakose2,3, Sumeet Walia2,3, Milos Toth1, Igor Aharonovich1 and Charlene J. Lobo1* 1

School of Mathematical and Physical Sciences, University of Technology Sydney, Ultimo, NSW, 2007, Australia 2 School of Engineering, RMIT University Melbourne, VIC 3001, Australia 3 Functional Materials and Microsystems Research Group and Micro Nano Research Facility, RMIT University Melbourne, VIC 3001, Australia *E-mail: [email protected] Keywords: phosphorene, black phosphorus, environmental scanning electron microscopy, sensing, reactive oxygen species ABSTRACT Under ambient conditions and in H2O and O2 environments, reactive oxygen species (ROS) cause immediate degradation of the mobility of few-layer black phosphorus (FLBP). Here, we show that FLBP degradation can be prevented by maintaining the temperature in the range ~125-300 ºC during ROS exposure. FLBP devices maintained at elevated temperature show no deterioration of electrical conductance, in contrast to the immediate degradation of pristine FLBP held at room temperature. Our results constitute the first demonstration of stable few-layered black phosphorus in the presence of ROS without requiring encapsulation or a protective coating. The stabilization method will enable applications based on surface properties of intrinsic FLBP.

Few-layer black phosphorus is a van der Waals material consisting of multiple layers of two-dimensional (2D) phosphorene. This material comprises puckered anisotropic layers consisting of sp3-hybridised bonds, and can be thinned down to a monolayer using mechanical 1 ACS Paragon Plus Environment

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and liquid exfoliation techniques1. Optoelectronic properties that include high carrier mobility (up to 1000 cm2/Vs for holes2) and a tunable thickness-dependent direct band gap varying from ~0.3 to ~1.8eV3 have made black phosphorus of all thicknesses a promising material for applications in field effect and thin film transistors4-5, batteries6, ultraprecise gas sensors7-8, photonic9, and biomedical devices10-11. Unfortunately, the deployment of FLBP in these applications is hampered by its high instability in ambient conditions, resulting in rapid degradation within a few days12-14. Several studies have examined the origins of this environmental degradation and have concluded that H2O and O2 exposure in combination with high-energy electromagnetic (photon or electron) irradiation are required for the degradation reaction to occur15-16. This reaction proceeds through the rapid (< 2 days) formation of a surface oxide layer (PxOy) which is converted to phosphoric acid (H3PO4) upon interaction with atmospheric water vapour or reactive oxygen species (ROS)15, 17-18, thereby accelerating the degradation process and leading to complete etching of FLBP within 1 week in ambient environment19. Thus, in order to be employed in practical device applications, FLBP must be stabilized, which is typically achieved by encapsulation in materials that provide an effective barrier to the diffusion of water and ROS, such as ionic liquids, surface coordinating ligands, or amide solvents16, 20-21. The strategies that have so far resulted in longterm passivation (as indicated by preservation of electronic conductance up to 3 months in ambient) have been atomic layer deposition (ALD) of Al2O322, encapsulation in hexagonal boron nitride (hBN) using dry transfer in inert environments23, or protection with oxygensequestering ionic liquids16. However, these fabrication and protection strategies are both complicated and time-consuming. Moreover, they are incompatible with many applications such as gas sensing and catalysis because they modify the surface properties of the material. Recent studies have shown that ROS exposure via photoactivated oxidation induces an immediate and permanent reduction in the electron and hole mobility of FLBP24. Electronbeam irradiation in H2O vapour is known to generate ROS (such as *O2-, *OH and OH-) that 2 ACS Paragon Plus Environment

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are responsible for the degradation of FLBP in ambient environment16, but FLBP stability has not been assessed during prolonged exposure to other gaseous environments. Here, we study the stability of FLBP in H2O, O2, NF3 and NH3 environments using environmental scanning electron microscopy (ESEM) and in situ electrical conductance measurements. In this approach, an electron beam is used for ESEM imaging, and also to generate reactive species such as *O, *OH, *F and *H that can drive spatially-localized chemical reactions at the sample surface25. Few-layered black phosphorus (FLBP) flakes were micro mechanically exfoliated from commercial black phosphorus crystals (Smart Elements) onto a silicon substrate with a 100 nm thermal oxide layer. These pristine FLBP flakes were shielded from light and stored in a low vacuum desiccator to prevent reaction with air or moisture. In order to study the degradation process in controlled gaseous environments, untreated FLBP flakes with average thickness of 50 nm (Fig. S1a) were subjected to electron beam irradiation in oxidizing (8 Pa H2O, O2 or NF3) and reducing (8 Pa NH3) environments at room temperature (RT). Electron beam irradiation results in efficient dissociation of surface-adsorbed molecules, and enables real-time imaging of the resulting surface reactions using a magnetic-field-assisted helix gaseous secondary electron detector (Fig. S2 and Refs. 16, 26). Following irradiation in 8 Pa H2O for 60 minutes, the degraded FLBP flake (Fig. 1a, right) has a much lower Raman intensity (Fig 1e, green trace) than the pristine flake (Fig. 1a, left and Fig 1e, red trace). In addition, the Ag1/Ag2 peak ratio of the H2O-irradiated FBLP was reduced from 0.9 to 0.4, which is indicative of basal plane oxidation15. No degradation was observed in H2O in the absence of electron beam irradiation, consistent with other observations that humidity on its own does not cause degradation12.

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Figure 1. a-d) SEM images of FLBP before (left) and after (right) 60 minute electron beam irradiation in H2O, O2, NF3, and NH3 environments at RT and 8 Pa chamber pressure. Irradiation and imaging were conducted using an accelerating voltage of 15kV and electron beam flux of 2.3x1019 electrons cm-2min-1. The scale bar is 1 µm. e) Normalised Raman spectra taken along the b axis of pristine FLBP (top), and from each of the flakes shown in (a)-(d). Ag1/Ag2 peak ratios of < 0.6 are indicative of basal plane oxidation15. Phosphorus oxides were not detected due to the overlap with Raman peaks originating from the Si substrate (Fig. S3).

Degradation of FLBP in H2O proceeded via the formation of PxOy bubbles13, 15, 17 (Fig S4a and S4b), followed by further reaction of these bubbles with water (Fig. S4c). When H2O molecules diffuse onto the PxOy bubbles, phosphoric acid (H3PO4) is produced in a highly exothermic reaction, resulting in a reaction-diffusion front that rapidly leads to complete degradation of the FLBP after 18 minutes (Fig. S4d). Energy dispersive x-ray (EDX) mapping of a FLBP flake that had been irradiated in H2O (boxed region in Fig. S4e) showed a


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reduction in the phosphorus signal and corresponding increase in the oxygen signal (Fig. S4f, g) in the beam-irradiated region. As is the case for H2O, electron induced dissociation of O2 can create ROS such as superoxide, *O2-. However, bubbles did not appear on the surface of FLBP irradiated in 8 Pa O2 (Fig 1b, right), although some surface roughening was observed at the edges of the rastered beam pattern, most likely due to diffusion of residual H2O adsorbates towards the electron beam irradiated area (residual H2O is a common impurity in electron microscopy chambers and has been implicated in a range electron-beam-driven chemical reactions27). The lack of degradation in an O2 environment is confirmed by analysis of the Raman spectrum, which shows a much smaller reduction in both Raman intensity and Ag1/Ag2 peak ratio (from 0.9 to 0.7) than observed under equivalent pressures of H2O. Rather than being a result of oxidation, the slight reduction in peak ratio in an O2 environment may be due to the observed surface roughening at the edges of the irradiated region. A previous DFT study predicted that unlike oxidized BP, fully fluorinated flakes should be dynamically stable, while hydrogenated BP should be unstable28. In order to test this prediction, pristine FLBP was irradiated in 8 Pa NF3 and NH3 under identical conditions as irradiation in H2O and O2. We find that electron beam irradiation in NF3 does not cause substantial degradation of the FLBP (Fig. 1c). As is the case for electron irradiation in O2, roughening occurred at the edges of the electron-beam-irradiated area, and is attributed to residual H2O adsorbates. The Raman signal has a slightly greater Ag1/Ag2 peak ratio than pristine FLBP (1.1 compared to 0.9), which may be due to electron induced surface fluorination29. Electron beam irradiation in an NH3 environment resulted in etching and volatilization of the FLBP in a highly localized manner. Unlike the H2O process, neither roughening at the flake edge nor a reaction-diffusion front was observed. The electron beam -induced dissociation of NH3 results in the production of hydrogen radicals30, which likely react with 5 ACS Paragon Plus Environment


FLBP to form phosphine (PH3) gas. This NH3 induced etching mechanism is much more localised than the H2O mediated process, potentially enabling electron beam patterning of FLBP with nanoscale resolution, as we have recently demonstrated for hBN26. Having shown that FLBP degradation in the dark only occurs under exposure to ROS created by the electron-induced dissociation of H2O, we explored the utility of thermal treatment to prevent or halt FLBP degradation during ROS exposure. A series of electron irradiations in 8 Pa H2O was performed at temperatures ranging from RT to 150ºC (Fig. 2a-d). Due to the increasing reaction rate with increasing temperature, the onset time for degradation of the FLBP (extracted from analysis of videos taken during the irradiation process) decreased from 12 minutes at RT (Fig. 2a) to just over 2 minutes at 100ºC. Above 125ºC, (Fig. 2c) FLBP degradation completely stopped. This is attributed to reduced residence times of surface-adsorbed water and/or ROS species (as the thermal desorption rates increase exponentially with temperature). When the temperature was reduced below 50ºC after being held at 150ºC for 90 minutes, FLBP degradation resumed (Fig. 2d). However, the degradation onset time (68 minutes) was greatly increased compared to that of a pristine FLBP flake irradiated at 50ºC without thermal treatment (6 mins) (Fig 2b). This is most likely due to desorption of intercalated water when the temperature was raised above 100ºC. (Note that the bright spot in Fig. 2b is due to contamination rather than being part of the FLBP flake).

Figure 2. a-d) Temperature dependence of the electron-beam-induced FLBP reaction with H2O. SEM images were taken before and after 60 min. electron irradiation (15 kV, 4.42x1017 electrons cm-2min-1) in 8 Pa H2O at (a) RT (b) 50ºC, (c) 150ºC, and (d) 50ºC 6 ACS Paragon Plus Environment

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following a single heat treatment (HT) in which the temperature was held at 150ºC for 90 minutes. The onset time at which degradation began is indicated as tdeg. FLBP aged for 2 weeks in the dark, heat-treated (HT) at 200 ºC to remove intercalated H2O, and subjected to electron beam irradiation remains unoxidized, as demonstrated by an Ag1/Ag2 peak ratio of 0.9 (Fig. 3). Thus, thermal treatment can be used to stabilize FLBP during ROS exposure. Subsequent exposure to ambient conditions showed that both pristine FLBP and aged, heat treated FLBP began to degrade after approximately 4 weeks, while similar aged FLBP flakes irradiated in O2 or NF3 began to degrade after 14 days (Fig. S5).

Figure 3. Compositional analysis of FLBP aged for 2 weeks, heat-treated at 200 ºC to remove intercalated H2O, and subjected to electron beam irradiation (15 keV and 2.3x1019 electrons cm-2min-1, 1 hr). The FLBP flake shown in (a) was analysed by EDX for (b) P, (c) Si, and (d) O at 5 keV. (e) A Raman spectrum taken after the heat treatment, electron beam irradiation and EDX confirms that HT FLBP is unoxidized.

To further establish preservation of FLBP properties during and after thermal treatment in the presence of moisture, in situ electrical measurements were performed on thin FLBP flakes (thickness 8-10 nm as shown in Fig S1b), under electron beam irradiation in 8 Pa H2O at both 280ºC and RT (Fig. 4). An SEM image of the FLBP taken during in situ conductance measurements is shown in Fig. 4a. At 280ºC, the conductance is constant at 3.5x10-5 S for at 7 ACS Paragon Plus Environment


least 20 minutes, when the experiment was terminated due to electrode delamination (Fig 4b, red line). However, after 15 min. continuous electron irradiation in H2O at RT, the conductance irreversibly declined, reaching a baseline of 10-6 S (Fig 4b, black line). In the same experiment, IV curves were also acquired before (green curve) and after (red curve) electron beam irradiation in H2O at 280ºC, and after cooling down to room temperature (black) (Fig. 4c). The IV characteristics after the 280ºC heat treatment were the same as that of a pristine flake, showing that the heat treatment effectively restores the few-layered black phosphorus to original condition. Analogous electrical characterization results are shown in Fig. S6 for the ~50 nm FLBP flakes shown in Fig S1a. These thicker FLBP flakes were stable for ~ 1hr under continuous ROS exposure at 150ºC, and have similar conductance and IV characteristics to pristine FLBP.

Figure 4. a) SEM screen captures taken during electron beam irradiation (15keV at a flux of 4.42x1017 electrons/cm2/min) of FLBP flakes at high temperature (HT) and room temperature (RT). b) Conductance of a ~8 nm FLBP flake as a function of irradiation time in 8 Pa H2O at 280ºC (red) and RT (black). (c) IV characteristics before (green) and after (red) electron irradiation in 8 Pa H2O at 280ºC, and after cooling down to RT (black).

In conclusion, we have shown that FLBP exposed to ambient environment can be returned to its original state by a short (~1 hr) heat treatment at ~125-300 ºC. Heat treatment also enables FLBP to be kept stable during accelerated ROS exposure via electron beam irradiation in H2O, and for a further 28 days in ambient environment. This finding is 8 ACS Paragon Plus Environment

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confirmed by electrical conductance measurements of FLBP devices, which show no deterioration upon ROS exposure at elevated temperatures, in contrast to irreversible degradation upon ROS exposure at room temperature. This is the first demonstration of stable few-layered black phosphorus in the presence of ROS without requiring a protective coating, and will open up new applications of FLBP in catalysis, optoelectronic, and sensing devices.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: Description of experimental methods, additional data and experimental characterization (PDF). ACKNOWLEDGEMENTS This work was funded by FEI Company and the Australian Research Council (grant number DP140102721).

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Encapsulation-Free Stabilization of Few-Layer Black Phosphorus

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