Degradation of Black Phosphorus upon Environmental Exposure and Encapsulation Strategies To Prevent It

Along this chapter we discuss about one of the main issues in black phosphorus research : its environmental instability . We will discuss the works were the role of the environmental exposure has been studied and the reported works describing strategies to effectively encapsulate black phosphorus to preserve its pristine properties .


Introduction
In 2013, Yuanbo Zhang and co-workers demonstrated that graphene was not the only elemental two-dimensional material that could be isolated by exfoliation of a bulk layered crystal (1).Atomically thin layers, and even one-atom thick layers, of phosphorus can be isolated by mechanical and chemical exfoliation of bulk black phosphorus, a stable layered allotrope of phosphorus, crystals.Bridgman grew black phosphorus single crystals for the first time in 1914.He was investigating phase transformations of white phosphorus under high pressure and discover the black phosphorus allotrope by serendipity.In his original manuscript Bridgman said: "…Black phosphorus was discovered during an attempt to force ordinary white phosphorus to change into red phosphorus by the application of high hydrostatic pressure…" (2).
Regarding the optical and electronic properties of black phosphorus, in bulk it is a semiconductor with a direct bandgap of 0.35 eV and charge carrier mobilities in the order of 10000 cm 2 /V•s (3).Thin flakes of black phosphorus have been recently used in field-effect transistors (FETs) showing mobility values up to 1000 cm 2 /V•s (4-6) at room temperature and up to ~4000 cm 2 /V•s at low temperatures (7)(8)(9).Moreover, due to its small and direct bandgap black phosphorus has a bright perspective for broadband photodetection where transition metal dichalcogenides are limited because of their large bandgap (10)(11)(12)(13)(14)(15)(16).
Unlike other 2D materials isolated to date, however, black phosphorus is rather environmentally instable.Therefore, the attainment of either stable thin layers or effective passivation methods is required to produce reliable BP-based optoelectronic devices (17)(18)(19).In the following we will discuss about this issue and about the strategies developed to overcome this environmental instability.

Origin of the Instability under Ambient Conditions
Most of the two-dimensional materials isolated prior to 2014 (graphene, h-BN, MoS 2 , etc…) are rather stable at atmospheric conditions which facilitated the fabrication of devices based on these materials.Black phosphorus, however, shows a relatively large reactivity (20,21).This reactivity is originated by its particular puckered honeycomb crystal structure.In black phosphorus, the phosphorus atoms have 5 valence shell electrons available for bonding with a valence shell configuration 3s 2 3p 3 .Therefore, each phosphorus atom bonds to three neighboring phosphorus atoms through sp 3 hybridized orbitals, making the phosphorus atoms to form a puckered honeycomb lattice (orthorhombic, with space group Cmca) and each phosphorus atom also has a lone pair (a pair of valence electrons that are not shared with another neighbouring atom), which makes phosphorus very reactive to air.In fact, exfoliated flakes of black phosphorus are highly hygroscopic and tend to uptake moisture from air (22).Several initial studies report on quick water accumulation at the surface of exfoliated flakes (23)(24)(25)(26)(27)(28)(29). Figure 1 shows some examples from the literature of atomic force microscopy images of exfoliated black phosphorus flakes left in ambient conditions that progressively accumulates water with time (22,26,27).The electronic properties of black phosphorus are also strongly modified by the environmental exposure as illustrated by Island et al. and Wood et al. through direct electrical transport measurements as a function of the exposure in air (22,25) (see Figure 2b).Interestingly, from these transport measurements one can conclude that there are two different regimes: one at shorttimescale where the modifications are mainly due to physisorption of O 2 and N 2 species and another one at longer timescales were the water absorption starts.In the short-timescale (less than 1 hour) there is an instant shift of the threshold voltage to the left (shown in Figure S6 (e) from the supplementary information file in ref. (22)) and an overall decrease in conductance and these changes are reversible by putting the device in vacuum.A vacuum to air zero bias resistance cycling experiment showed that during the first cycles up to the 7 th , the initial value was recover (figure S4, from the supplementary information file in ref. (22)).At long-timescale the conductance of the devices steadily decreases until the device breakdown.Complementary to the macroscopic electrical transport measurements microscopic measurements with microwave impedance microscopy are a non-destructive, good asset to probe the effect of the environmental exposure on black phosphorus (26,29).As an example to illustrate the importance to study locally the mechanism of interaction between the BP surface and its degradation agents, figure 2c shows an example of a black phosphorus flake capped with a thin layer of Al 2 O 3 that cannot effectively encapsulate completely the black phosphorus flake (26).From the microwave impedance microscopy images one can conclude that the degradation process occurs first at the edges of the flake and then it is extended towards the center of the flake.This study allowed the authors to optimize the stability in air of the BP by capping it with a double layer of Al 2 O 3 /hydrophobic fluoropolymer film, instead of a single Al 2 O 3 layer.Moreno-Moreno et al. demonstrated that exposing thin flakes of black phosphorus to air also degrades its mechanical properties: after 24 h or exposure to ambient the Young's modulus of thin black phosphorus flakes decreases by a factor of two (see Figure 2d) (30).
Regarding the physical and chemical mechanisms behind this environmental degradation of black phosphorus, Favron et al. and Zhou et al. have deduced that the degradation most likely occurs as a result of photo-induced oxidation from oxygen absorbed in accumulated water at the surface of exfoliated flakes exposed to ambient conditions.(28,31,32).Recently, Kim et al. have studied by scanning Kelvin probe microscopy (SKPM) the degradation process of black phosphorus in dark.They have observed that the degradation rate is layer-dependent, since the electronic structure of a 2D material changes with its thickness (33).Zhang et al. have combined experimental results and first-principle calculations to study the reaction kinetics of black phosphorus with oxygen contained in the water.They have concluded that the main products are the PO 2 3-, PO 3 3-and PO 4 3-species.They also showed the possibility to store BP in deoxygenated water under ambient light without degradation for at least 15 days (34).The exact role of water in the oxidation mechanism of the black phosphorus is not fully understood yet.As a consequence, further experimental results and theoretical models are being developed.A recent work has proposed that OH -ions are responsible for the degradation of BP, through experimental observation and density functional theory calculations (DFT).They showed that the degradation of BP has therefore a dependence with the pH, with linear increasing degradation rate from pH 4 to 10 (35).Finally, Luo et al. studied the kinetics of black phosphorus oxidation under 5% O 2 /Ar, 2.3% H 2 O/Ar and 5% O 2 /2.3% H 2 O/ Ar mixture.The highest rate of oxidation was found in the mixture of oxygen and water.The authors concluded that water continued to interact chemically with the oxidized BP surface promoting the oxygen dissociation further (36).The stepwise reactivity of H 2 O with a phosphorene surface once it was oxidized was also studied theoretically (37).

Passivation Methods: Encapsulation and Functionalization
These works focused on the air-induced degradation of black phosphorus have motivated works to find effective methods to encapsulate black phosphorus at the early stages of isolation/fabrication to increase its longevity (25,26,38).Several encapsulation techniques have been reported to date including polymer dielectrics (10,39), atomic layer deposited (ALD) oxides (25,26,(40)(41)(42), hybrid metal organic chemical vapor deposition (MOCVD)/ALD coating of BN/Al 2 O 3 (43), electronbeam evaporated metallic thin films oxidized under ambient air (44), van der Waals heterostructures using boron nitride, MoS 2 and graphene flakes (7-9, 38, 45-48), and chemical functionalization (49)(50)(51)(52).In two different works, a thin native oxide layer on top of the black phosphorus surface has been formed by oxygen plasma (53) or thermal annealing etching (54).In both works, the presence of the oxide proved to stop the degradation of BP.Tian et al have produced a thin native P ox layer around 2 nm thick on the bottom side of black phosphorus films of typically 20 nm thick.This bottom oxide layer was formed under exposure in air for 30 min during the mechanical exfoliation and metallic contacts deposition.The authors used the charge transfer induced at the bottom P ox /BP interface to simulate synaptic behavior along the 45°angle spaced electrodes of a field-effect transistor, exploiting the in-plane anisotropy of the material.After fabrication and electrical measurements at vacuum, from room to high temperature, the devices were stored in a N 2 box under dark and the authors checked that the bottom oxide layer was not modified for a few months (55).Another work demonstrated the use of ozone generated by ultraviolet light under atmospheric pressure, to form phosphorus oxide on top of thick black phosphorus films.This oxide is etched away by rinsing with deionized water, leaving a thinner BP surface, which presents a retarded degradation effect in air.They found an etching rate of 12 ± 2 nm/h and could achieve a single-layer phosphorene film under this process (56).Figure 3a-b shows the results of encapsulating black phosphorus using a thick AlO x capping layer (deposited through atomic layer deposition) (25,26).While uncapped field-effect transistors break down in a few days, field-effect transistors with encapsulation show performances (e.g.mobility and conductance) that do not change over several days (25).Microwave impedance microscopy measurement technique confirms that black phosphorus devices encapsulated with a thick AlO x layer shows little to no degradation (26).Sandwiching black phosphorus flakes between boron nitride flakes (bottom and top encapsulation, see Figure 3c) is a more advanced route toward passivation which has proven to be a method to obtain exquisite electronic properties for graphene and MoS 2 devices.This enhancement of the device performance is due to the atomically flat, dangling-bond-free and defect free surface of hexagonal boron nitride (47,57,58).Thanks to this improved performance in these fully encapsulated devices it has been possible to observe interesting physical phenomena not observable in low-mobility electronic devices such as the Shubnikov-de Haas oscillations and the integer quantum Hall effect (7-9, 46, 47). Figure 4a) shows the passivation effects of a P x O y layer formed by an oxygen plasma to thin down the initial exfoliated thick phosphorene layer (orange curve).This oxide layer was found to have a dual passivation role: its presence itself slowed down the degradation rate under ambient exposure.Then, for a dual passivation process depositing further a Al 2 O 3 layer by ALD to increase the lifetime of the device, the oxide served as a protection layer to prevent the reaction of the phosphorene with the ALD precursors (pink curve) The optical images reinforce the longer time stability of double capped flakes compared to one that was also covered by the oxide layer (53).Another strategy pursued to improve the stability of the black phosphorus under ambient conditions and consequently, its performance as device is the functionalization of its surface with different elements and molecules.The adsorption of Ag + ions on the black phosphorus surface via cation-π interactions constitutes an example of such approach.To produce such metal ion modified surface, a mechanically exfoliated BP flake transferred to a 300 nm SiO 2 /Si substrate was immersed in a N-methyl-2-pyrrolidine (NMP) solution with silver nitrate (1x10 -6 M).They observed an improvement on the hole mobility from 796 to 1593 cm 2 /V s and the ON/OFF ratio from 5.9x10 4 to 2.5x10 6 after 1 h immersion, compared to the original device before passivation (59).In another work, the edges of BP nanosheets were functionalized by covalent bonding with C 60 molecules (see Figure 4c).The fullerenes were chosen due to its high hydrophobicity.These C 60 functionalized black phosphorus films showed higher stability against oxidation and good photocurrent characteristics (60).The functionalization of a black phosphorus surface with antioxidant molecules following processes such as fluorination (61) or treatment in imidazolium-based ionic liquids (62) were pursued.A sulfur doped thin layer BP field effect transistor showed around 77% of the mobility and an I ON /I OFF ratio of 10 3 still after 21 days under ambient exposure (63).Aluminum doped black phosphorus flakes were used to produce FETs that presented an n-doping effect and unnoticeable degradation after 10 days under ambient conditions (64).Finally, density functional theory calculations were performed to estimate the ratio of BP doping with scandium atoms at which the flake was protected from the oxygen and water molecules, but also retained the semiconductor behavior (65).
An alternative approach to address the environmental instability issue consists on using liquid phase exfoliation to obtain suspensions of black phosphorus nanosheets.It has been demonstrated that liquid phase exfoliation can produce large quantities of high-quality few-layer black phosphorus nanosheets.These black phosphorus nanosheets are surprisingly stable in suspension (even when deoxygenated water is used as solvent for the liquid phase exfoliation), probably due to the solvation shell protecting the nanosheets from reacting with oxygen (Figure 5) (66)(67)(68)(69)(70)(71).
In summary, in this chapter we have discussed the issue of the environmental instability in black phosphorus and the works devoted to develop strategies to effectively encapsulate black phosphorus to preserve its pristine properties.

Figure 1 .
Figure 1.a)-c) Selected AFM scans of three BP flakes in air taken at different times after exfoliation.a) Adapted with permission from (22).Copyright 2015 IOP Publishing Ltd.b) Adapted with permission from (26) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).c) Gamage, S.; Li, Z.; Yakovlev, V. S.; Lewis, C.; Wang, H.; Cronin, S. B.; Abate, Y. Nanoscopy of Black Phosphorus Degradation.Adv.Mater.Interfaces 2016, 3 (12), 1600121.Copyright Wiley-VCH Verlag GmbH & Co. KGaA.Reproduced with permission.Apart from the topographical changes induced by the moisture uptake of the black phosphorus flakes exposed to air, the environmental exposure also induces degradation of the optical, electrical and mechanical properties of black phosphorus.In fact, Favron et al. measured the evolution of the Raman intensity upon exposure to different atmospheres showing a clear degradation of black phosphorus in air (Figure 2a) (28).

Figure 2 .
Figure 2. a) Integrated intensity of the A g 2 Raman mode as a function of time under different exposure conditions: air, vacuum, O 2 /H 2 O mixture and 300 nm thick parylene layer-capped flake in air.The faster decay rate observed in air (5.5 min -1 ) compared to under the O 2 /H 2 O mixture exposure (36 min -1 ) is due to the photon flux (1.7 × 10 4 W cm -2 and 1.8 × 10 3 W cm -2 , respectively).Reproduced with permission from (28).Copyright 2015 Springer Nature.b) Transfer characteristics (IV g ) of a BP FET device at selected times over the first hour of exposure (curves are offset by 100 nA for clarity).Adapted with permission from (22).Copyright 2015 IOP Publishing Ltd.c) Local sheet resistance maps of a 24 nm thick flake capped by ~3 nm Al 2 O 3 layer.Adapted with permission from (26) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).d) F(δ) curves for a 5.3 nm thick BP drumhead in high vacuum and after 3, 7, 11 and 26 h of exposure to ambient conditions.Adapted with permission from (30) under a Creative Commons License (http://creativecommons.org/licenses/by/3.0/).

Figure 3 .
Figure 3. a) Hole mobility for encapsulated and unencapsulated BP FETs versus ambient exposure time.Reproduced with permission from (25).Copyright 2014 American Chemical Society.b) Sheet resistance maps of a 16 nm-thick flake capped by 25 nm Al 2 O 3 .Adapted with permission from (26) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).c) (Left) Schematic of the BN-BP-BN heterostructure device fabrication process.(Right) Optical image of the BN-BP-BN heterostructure after O 2 -plasma etching (the etched area is enclosed within the white line) and of the BN-BP-BN Hall-bar device.Adapted with permission from (47) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).

Figure 4 .
Figure 4. a) Time dependence of the normalized PL intensity of the monolayer phosphorene samples treated by different methods: exfoliated 1L phosphorene (red), 1L phosphorene with P x O y (~11 nm) capping layer produced by O 2 plasma etching (green) and 1L phosphorene with dual passivation layers of P x O y and 5 nm of ALD Al 2 O 3 (black).All PL was measured in ambient condition under the same laser excitation.Inset: zoom in plot for the exfoliated 1L phosphorene sample.Adapted with permission from (53) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).b) Optical images of flakes covered by a thin P x O y layer and a Al 2 O 3 + P x O y double layer after fabrication and respectively, after 3 days and 30 days.The images show the higher passivation effect of the double capping layer.Adapted with permission from (53) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).c) HRTEM image of the hybrid structure, where the C 60 molecules can be seen passivating the edges of the BP layer.Inset: low magnification TEM image (top).STEM image (middle) and EDX elemental mapping (bottom) of the C 60 -BP hybrid structure to show the successful presence of the fullerenes along the borders.Adapted with permission from (60) under a Creative Commons License (http://creativecommons.org/ licenses/by/4.0/).

Figure 5 .
Figure 5. a) Preparation process to obtain few-layer phosphorene enriched aqueous dispersions.The workflow is represented by the test tubes, going the order of the different steps from left to right.Deoxygenated water with 2% (wt/vol) SDS was prepared by ultrahigh-purity Ar purging.The exfoliation of the BP crystal was performed in a sealed container using tip ultrasonication.Then, this solution was centrifuged to remove unexfoliated BP crystals.The remaining FL-BP dispersion was ultracentrifuged to precipitate large flakes.The supernatant was finally redispersed in deoxygenated water.Reproduced with permission from (67).Copyright 2016 National Academy of Sciences.b) Relative absorbance as a function of time, measured at 465 nm for: the standard few-layer BP dispersion exfoliated in CHP, in NMP, in IPA and the BP exfoliated in CHP in a glovebox (CHP GB).Adapted with permission from (66) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).