Significance A family of strong yet removable 1- to 2-nm-thick ultrathin monolayer is developed as a corrosion inhibitor for 2-dimensional materials that significantly prolong lifetime while protecting optoelectronic properties in both ambient and harsh chemical or thermal environments. This method is low in toxicity and can be applied to arbitrary substrate with no size limit.

Abstract Two-dimensional van der Waals materials have rich and unique functional properties, but many are susceptible to corrosion under ambient conditions. Here we show that linear alkylamines n-C m H 2m+1 NH 2 , with m = 4 through 11, are highly effective in protecting the optoelectronic properties of these materials, such as black phosphorus (BP) and transition-metal dichalcogenides (TMDs: WS 2 , 1T′-MoTe 2 , WTe 2 , WSe 2 , TaS 2 , and NbSe 2 ). As a representative example, n-hexylamine (m = 6) can be applied in the form of thin molecular monolayers on BP flakes with less than 2-nm thickness and can prolong BP’s lifetime from a few hours to several weeks and even months in ambient environments. Characterizations combined with our theoretical analysis show that the thin monolayers selectively sift out water molecules, forming a drying layer to achieve the passivation of the protected 2D materials. The monolayer coating is also stable in air, H 2 annealing, and organic solvents, but can be removed by certain organic acids.

Passivation of materials in air and water is foundational to our civilization (1). When we consider the robust ultrathin passivation of 2D materials (2⇓⇓⇓⇓–7), it should be even more essential because 1) the thickness of passivation layer on 3D materials like Si, Al, Cr, etc. stays 2 to 5 nm over a very long time, which is an insignificant fraction of the remaining unreacted bulk material. However, one cannot say this for thin 2D materials with their total thickness likely comparable to the native oxide passivation layers. Thus, the atomistic details of passivation matter even more here. 2) An ultrathin, electronically insulating layer provides opportunity to engineer extremely thin vertical heterostructures, akin to the SiO 2 /Si gate in metal-oxide-semiconductor field-effect transistors. For these reasons, it is becoming increasingly critical to facilely passivate layered materials such as transition-metal dichalcogenides (TMDs), black phosphorus (BP), silicene, stanine (8⇓⇓⇓–12), etc., which are susceptible to corrosion under ambient conditions with air, water, or even small amounts of acidic or basic contaminants (9, 10, 13⇓⇓⇓⇓⇓–19).

Several passivation strategies have been developed for these layered materials including covering by more robust 2D materials such as graphene (20) and hexagonal boron nitride (21). However, many previous strategies suffer from processability issues and other drawbacks: Metal-oxide coatings are prone to cracking (14, 22); polymers [e.g., poly(methyl methacrylate) (PMMA), polystyrene, Parylene, and perylene-3,4,9,10-tetracarboxylic dianhydride] are readily attacked by organic solvents and offer limited durability (19, 23⇓⇓–26); self-assembled monolayers with silane-terminated octadecyltrichlorosilane are highly toxic (27). Here, we discovered a one-pot scalable process for passivating a large variety of 2D van der Waals materials. It involves coating a nanometer-thick monolayer of linear alkylamines onto the surface of 2D materials, which greatly increases the lifetime of these materials in ambient environments with moisture and can sustain even harsh aqueous and thermal conditions. First-principles simulations suggest that the alkylamine coating significantly slows down the permeation of O 2 , which reacts with the 2D layered material to form an ultrathin oxide passivation layer, and completely blocks H 2 O molecules and shuts down the cycles of oxidation–dissolution, leading to the extended lifetime for many different classes of 2D crystals.

Since BP is the most vulnerable to corrosion among the 2D van der Waals (vdW) materials studied in this work and creates the most challenges for processing, it is used here as an illustrative example of the alkylamine coating. As a representative example of linear alkylamines n-C m H 2m + 1 NH 2 , n-hexylamine (m = 6) coating onto BP is systematically investigated both theoretically and experimentally in its corrosion inhibition mechanism and behaviors.

Discussion Amines with low water solubility have long been known as efficient and reliable corrosion inhibitors for steels (41, 44). It is found here that they also serve as an effective coating for 2D layered materials, by blocking water for the native thin-oxide layer growing at the interface between the 2D material and the alkylamine coating. The photooxidation of bare BP starts with the synergetic effect of oxygen, water, and light, where phosphorus transformed to a layer of acidic phosphorus species. The thin layer of acid then coarsens into a droplet, leaving a fresh phosphorus surface in contact with ambient air, and the oxidation process starts once again (45). n-hexylamine monolayer lowers the permeability of oxygen and strongly blocks the water molecules from directly contacting the oxide passivation layer and phosphorus. Although the first BP layer is still oxidized by O 2 , it is isolated from ambient humidity by the hydrophobic alkyl monolayer, which prevents the water from dissolving this top native oxide that would have perpetuated the corrosion. Our experimental finding of the passivation effect on BP is consistent with the theoretical prediction that mere BP + O 2 reaction forming BP-PO x should be fully stable and self-limiting at ∼1 to 2 nm if no moisture exists (31). In summary, we have developed a strategy to effectively slow down the corrosion of BP by coating of alkylamine monolayer onto its surface. General applicability on a variety of other layered materials is also demonstrated. The alkylamine monolayer is robust in a range of chemical and thermal environments, including ambient air. The facile coating method can be implemented with many different substrates and is compatible with all linear alkylamines no shorter than n-butylamine, thus offering a platform for controlling the surface physics and chemistry of a rich tableau of 2D materials. Because of its simplicity, ecofriendliness, and low cost, we envision it to be scalable and adaptable in various industrial configurations.

Acknowledgments C.S. and Z.Y. would like to thank Philip Kim for granting lab access for the glovebox-enclosed AFM. C.S. would like to thank Greg Lin and Frank Zhao for helpful discussions. Z.Y. thanks Pablo Jarillo-Herrero for providing the glovebox. J.L., C.S., M.D. (MIT), and L.S. acknowledge support by the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0001088. J.L., C.S., and Z.Y. acknowledge support by NSF ECCS-1610806, the Australian Research Council Discovery Project (Grant DP190100295), and the Australian National University (ANU) Futures Scheme (Grant Q4601024). Q.-B.Y. and G.S. acknowledge support in part by the Ministry of Science and Technology of China (Grant 2013CB933401), the National Science Foundation of China (Grant 11474279), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB07010100). H.L. and J.H. acknowledge funding support provided by NSF under Award 1453218. J.H.W. acknowledges the support from the Royal Society. J.K. and C.S. acknowledge support from the US Army Research Office through the Massachusetts Institute of Technology Institute for Soldier Nanotechnologies, under Award 023674. T.Y., N.Z., and K.T. acknowledge support by Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University (program no. 15G0031).

Footnotes Author contributions: C.S., Z.Y., and J.L. designed research; C.S., Z.Y., Q.-B.Y., Z.W., H.L., W.X., T.Y., and X.J. performed research; C.S., Z.Y., L.S., N.Z., K.T., J.H.W., M. Dincă, J.H., M. Dong, G.S., J.K., and J.L. analyzed data; N.Z., K.T., J.H.W., M. Dincă, J.H., M. Dong, G.S., J.K., and J.L. supervised the project; and C.S., Z.Y., and J.L. wrote the paper.

Competing interest statement: US Patent under International Application PCT/US2018/025174 has been filed for technique related to this work.

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