Directly Evaluating the Optical Anisotropy of Few‐Layered Black Phosphorus during Ambient Oxidization

Abstract

air causing property degradation, which severely hinders the approaching of pristine properties and the development of BP in practical applications. The degradation of 2D BP in the air was recognized in a very early stage, and a large amount of effort has been devoted to understand the fundamental mechanism of its oxidization in both theoretical and experimental perspectives. [7][8][9][10] Though the complete understanding of the oxidization mechanism under complicated ambient condition is still ongoing, some significant progresses have been made in the past few years. [11] Oxygen, water and light have been determined to be the active species that induce BP oxidization in the air according to numerous studies. [11,12] As confirmed by both theoretical calculations and experiments, a general process of BP oxidization is believed to start from the reactions of oxygen with long pair of surface P atoms while overcoming a certain amount of energy barrier that is dependent on the occupied positions in the phosphorene lattices by chemisorbed or interstitial oxygen atoms. [13,14] The BP could support several oxidization states of P x O y , but some of the oxidization species are metastable, and P 2 O 5 is reported to be the most stable form in the air. [13] Thus light illumination is essential to provide external energy to stimulate the chemical reaction between BP and oxygen, which is in agreement with the experimental observation of lower oxidization rate while storing BP in dark environment. [15] With water involved, the oxidization process becomes much more complex, and the precise roles of water, oxygen, and light are still unclear and in active debate with different mechanisms, such as the oxidization behaviors that took place in water solvent and moisture atmosphere result in different degradation products. [16][17][18][19][20] One of the oxidization mechanisms is reported that the water molecular (humidity) exhibits catalyzed function that first lowers the reaction energy between BP and oxygen, [12] and then reacts with phosphorus oxides forming phosphorus acid, which accelerates the oxidization and etching speed. [11,12,21] An alternative mechanism was proposed by Hu et al., who revealed the BP oxidization occurred even in the dark environment with the coexistence of water and oxygen, which was attributed to the polarization effect of water that lowers the energy level of oxygen, resulting in acceleration of electron transfer from BP to oxygen that triggers BP oxidization. [12] Besides, Zhang et al. found that the hydroxide ion (OH − ) generated from the dissociation reaction of H 2 O Few-layered black phosphorus (BP) is known to be oxidized easily in the air which causes property degradation. However, the optical anisotropy of oxidized BP is yet to be well determined. Here, the optical anisotropy of the oxidized BP is revealed by continuous measurement of ambient exposed BP flake over a month using azimuth-dependent reflectance difference microscopy. The isotropic oxidization process is elucidated by both the di...

A Black Phosphorus‐Based Fabry–Pérot Cavity and Its Application for Reversible Color Switching

“…It was presented that the BP crystal exhibited larger carrier mobility along the AC axis than the ZZ axis. [38,[70][71][72] Under illumination of an external laser source, the BP crystal can create anisotropic excitons along the AC axis, producing more mobile hot carriers. [73,74] Therefore, for the BP laminate in contact with the Au reflector, the asymmetric FP cavity resonance-induced transfer charges primarily distribute along the AC axis.…”

Correlating Optical Microspectroscopy with 4×4 Transfer Matrix Modeling for Characterizing Birefringent Van der Waals Materials

“…However, for some materials such as BP, molybdenum ditelluride (MoTe 2 ) and tungsten ditelluride (WTe 2 ), [10] the commonly established thickness determination methods can only be applied with great restrictions because of the need of a passivation layer to prevent degradation at ambient atmosphere. [11,12] These passivation layers, e.g., poly(methyl methacrylate) (PMMA) or hexagonal boron nitride (hBN), inhibit the otherwise immediately starting degradation [11][12][13][14][15] but further hinder or distort atomic force microscopy (AFM) based measurements. [16] Consequently, the topography is only accessible in vacuum or under an inert atmosphere (for instance in a glovebox) or instead has to be accessed after removing the passivation layer and effectively destroying the device.…”

Quantitatively Exploring Giant Optical Anisotropy of Quasi-One-Dimensional Ta2NiS5

“…Two-dimensional (2D) or quasi-one-dimensional (quasi-1D) materials with in-plane anisotropy, such as PdSe 2 [1], ReS 2 and ReSe 2 [2,3], ZrTe 5 [4], CrPS 4 [5], TiS 3 and ZrS 3 [6], have been hot topics in research since the study of black phosphorus (BP) [7][8][9]. These types of materials not only exhibit richer physical properties, but also provide an additional degree of freedom to modulate physical properties [10], leading to the design of novel electronic, optical, thermal, and optoelectronic devices, a feature that is not available in isotropic materials.…”