STM study of exfoliated few layer black phosphorus annealed in ultrahigh vacuum

Abstract

Black Phosphorus (bP) has emerged as an interesting addition to the category of two-dimensional materials. Surface-science studies on this material are of great interest, but they are hampered by bP's high reactivity to oxygen and water, a major challenge to scanning tunneling microscopy (STM) experiments. As a consequence, the large majority of these studies were performed by cleaving a bulk crystal in situ. Here we present a study of surface modifications on exfoliated bP flakes upon consecutive annealing steps, up to 550 • C, well above the sublimation temperature of bP. In particular, our attention is focused on the temperature range 375 • C -400 • C, when sublimation starts, and a controlled desorption from the surface occurs alongside with the formation of characteristic well-aligned craters. There is an open debate in the literature about the crystallographic orientation of these craters, whether they align along the zigzag or the armchair direction. Thanks to the atomic resolution provided by STM, we are able to identify the orientation of the craters with respect to the bP crystal: the long axis of the craters is aligned along the zigzag direction of bP. This allows us to solve the controversy, and, moreover, to provide insight in the underlying desorption mechanism leading to crater formation.

Anisotropic thermal expansion of black phosphorus from nanoscale dynamics of phosphorene layers

“…From an electronic point of view, the loss of the pseudotetrahedral local symmetry at the Phosphorus sites, associated to the flattening of the layers, correspond to a weakening in the valence shell orbital hybridization and to a consequent decrease of the stabilizing sp 3 orbital mixing, consistently with the significant increase of α a observed above 706 K. The originating instability is here proposed to be at the origin of the decomposition of the Phosphorene layers. This interpretation is in agreement with the work of Kumar et al 31 in which the thermal decomposition of few layer Phosphorene, studied by STM microscopy, is reported to proceed along the "zig-zag" (a) direction with the formation of crater-like holes. This is consistent with the observed higher LTEC along this direction, particularly above 706 K and with the loss of the layers stability due to their flattening.…”

Computational scanning tunneling microscope image database

“…STM imaging is particularly well-suited to studying two-dimensional (2D) materials, such as graphene 16 , MoS 2 17 , NbSe 2 18 , WSe 2 19 , WTe 2 20 , FeSe 21 , black-phosphrous 22 , 23 and SnSe 24 . 2D materials 25 , 26 has opened diverse areas of application, such as sub-micron level electronics 27 , flexible and tunable electronics 28 , superconductivity 29 , photo-voltaics 30 , water-purification 31 , sensors 32 , thermal-management 33 , energy-storage 34 , medicine 35 , quantum dots 36 , 37 and composites 38 – 40 .…”

“…We simulate computational STM images of 716 exfoliable materials (E f < 200 meV/atom) using the Tersoff-Hamann approach. We compare computational STM images with those from experiments for graphene 16 , 2H-MoS 2 17 , 2H-NbSe 2 18 , 2H-WSe 2 19 , 1T’-WTe 2 20 , FeSe 21 , black-P 22 , 23 , SnSe 24 , Bismuth 64 , 65 . We chose these systems because we could find well-characterized experimental images in the literature.…”

“…], ( g ) DFT-STM image of black phosphorous, ( h ) experimental STM image of black-P [Adapted with permission from ref. 22 , Copyright 2019 American Physical Society], ( i ) DFT-STM image of FeSe, ( j ) experimental STM image of FeSe [Reprint from ref. 21 with the permission of AIP Publishing.…”

“…6b that the model performs excellently for hexagonal, centered rectangle and square lattices, and less well for the rectangle and oblique lattice types. Moving beyond simulated STM images, as an initial validation, we apply the model to nine experimental images discussed above for an initial more realistic test step for graphene 16 , 2H-MoS 2 17 , 2H-NbSe 2 18 , 2H-WSe 2 19 , 1T’-WTe 2 20 , FeSe 21 , black-P 22 , 23 , SnSe 24 , Bismuth 64 , 65 . We find that the model predicts the correct class for seven of them.…”