Nanoscopy of Black Phosphorus Degradation

DOI: 10.1002/admi.201600121 degradation caused by ambient medium. By performing simultaneous topographic and optical time series imaging over several months, we show that a nanolayer BP exposed to ambient environment degrades at a steadily increasing rate until saturation begins, so that the degraded area and volume of degraded regions as functions of time follow the well-known S-shaped growth curve (sigmoid growth curve). [ 12 ] Phenomenological modeling of experimental results suggests a strong infl uence of degraded areas on adjacent BP. Our model is advantageous since it is based on elementary probabilities that can be related to the O 2 and H 2 O content in the ambient medium, as well as to the chemical reaction processes that result in oxidized phosphorus species. Therefore, our phenomenological model can, in principle, be used to predict the degradation time under different environmental conditions. Our work incorporates experimentally measured geometric patterns of BP degradation with model prediction allowing us to determine the growth saturation value, steepness of the growth, and the mid-point of the growth, and the relative importance of neighboring interactions. Mechanical exfoliation was used for BP sample preparation on an Si substrate. After the exfoliation, the substrates were loaded in to ALD chamber (Cambridge NanoTech, Savannah 200) immediately for the Al 2 O 3 deposition with TMA (trimethyl aluminum) and water as precursors. For the fi rst three rounds, only TMA is fl owed through the chamber to prevent oxidization of the BP fl akes. The ALD was performed at 200 °C. Various thicknesses (1, 5, 10, 20, 50, and 100 nm) of Al 2 O 3 were coated on the BP fl akes using ALD. Topographic and near-fi eld optical images were acquired using a commercial s-SNOM system (neaspec.com). An AFM tip coated with PtIr, oscillating at resonance frequency of f ≈ 280 kHz, is irradiated by a focused quantum cascade laser (QCL) laser at 45° with respect to the sample surface ( Figure 1 a). The scattered signal from the tip– sample interface region is demodulated at high harmonics of the tip resonance frequency ( nf, n > 1 ) and detected by phasemodulation (pseudo-heterodyne) interferometry, producing simultaneous topography, optical amplitude, and phase images. We performed infrared (IR) optical amplitude and phase resolved near-fi eld spectroscopy imaging of unencapsulated BP fl ake left to degrade in order to chemically identify the degraded region in BP in the laser wavelength range, λ = 5.5 – 10.8 μm. Figure 1 b shows a topography image and Figure 1 c simultaneously recorded IR third harmonic amplitude and phase optical images of a 10 nm thick BP fl ake left to degrade for some time in ambient environment. Although the various AFM height distributions shown in the topography and line profi le images in Figure 1 b are not suffi cient to identify the chemical composition of nano-islands, taller topographic protrusions on BP samples exposed to ambient environment have been used to Among numerous van der Waals materials that have been identifi ed lately, black phosphorus (BP) has generated intense interest for various optoelectronic applications due to its outstanding physical properties. Its tunable, direct, and narrow band gap makes it an attractive semiconductor for nanoelectronics and nanophotonics applications. [ 1–3 ] Similar to other 2D layered materials, BP can be prepared by simple mechanical exfoliation with additional exotic features such as its extraordinary in-plane anisotropic electrical, optical, and vibrational properties. [ 4–7 ] The major impediment to research and prospective application of single/few-layer BP is its chemical degradation under ambient conditions. [ 8 ] An extensive understanding of the degradation process and reliable ways to achieve stable and scalable degradation protection is necessary in order to realize the full potential of this material in its singleand few-layer forms. Recent studies have shed light on the underlying mechanism and speed of degradation by atomic force microscopy (AFM) and spectroscopic techniques. [ 8,9 ] Few passivation methods have also been investigated with various degree of success to slow down and prevent degradation. [ 3 ]