Light‐Induced Ambient Degradation of Few‐Layer Black Phosphorus: Mechanism and Protection

Abstract

The environmental instability of single-or few-layer black phosphorus (BP) has become am ajor hurdle for BPbased devices.T he degradation mechanism remains unclear and finding ways to protect BP from degradation is still highly challenging.Based on ab initio electronic structure calculations and molecular dynamics simulations,athree-step picture on the ambient degradation of BP is provided:g eneration of superoxide under light, dissociation of the superoxide,a nd eventual breakdown under the action of water.T he wellmatched band gap and band-edge positions for the redox potential accelerates the degradation of thinner BP.F urthermore,i tw as found that the formation of P-O-P bonds can greatly stabilizet he BP framework. Ap ossible protection strategy using afully oxidized BP layer as the native capping is thus proposed. Suchafully oxidization layer can resist corrosion from water and leave the BP underneath intact with simultaneous high hole mobility.

Degradation of black phosphorus is contingent on UV–blue light exposure

“…It is theoretically predicted that the exposure of up to 3-5 layers thick BP to ambient light and molecular oxygen leads to the generation of superoxide radicals (where the reaction proceeds at −4.11 eV with respect to the vacuum level), which is further expedited in the presence of H 2 O. 13,[30][31][32][33][34] The generated superoxide radicals are proposed to interact with the BP surface to produce P x O y , resulting in the eventual loss of pristine BP. 10,13 The process of superoxide radical formation cannot be sustained in BP that is beyond 3-5 layers in thickness due to the changes in the band positions in thicker BP flakes.…”

“…13,[30][31][32][33][34] The generated superoxide radicals are proposed to interact with the BP surface to produce P x O y , resulting in the eventual loss of pristine BP. 10,13 The process of superoxide radical formation cannot be sustained in BP that is beyond 3-5 layers in thickness due to the changes in the band positions in thicker BP flakes. 13 In this scenario, however, when thicker BP flakes are photoirradiated, they can efficiently participate in the generation of OH…”

“…After a lot of debate, there is a growing consensus that the main cause of BP degradation is photo-oxidation which is expedited in the presence of moisture. 10,[12][13][14][15] It is also generally agreed upon that the photodegradation of BP is thickness-dependent. However, there is a mismatch between theoretical and experimental studies in regards to BP stability.…”

“…10 This has also been corroborated using theoretical calculations. 13 However, experimental investigations consistently show that even thicker layers of BP are prone to ambient degradation. It is noteworthy that the theoretical models, being resource-intensive, are restricted to mono-layer to few-layer BP, wherein they suggest the involvement of superoxide radicals (O 2 − ) as the dominant reactive oxygen species (ROS) participating in BP photooxidation.…”

“…It is noteworthy that the theoretical models, being resource-intensive, are restricted to mono-layer to few-layer BP, wherein they suggest the involvement of superoxide radicals (O 2 − ) as the dominant reactive oxygen species (ROS) participating in BP photooxidation. 13 As such, several reports of BP degradation have surfaced recently and passivation procedures are now being explored to protect the material. [16][17][18][19][20][21][22][23] A knowledge gap exists in terms of (i) which part of the light spectrum degrades BP the most, and (ii) involvement of additional ROS in BP degradation, particularly in the case of thicker BP flakes.…”