MS-2-P-3218 TEM electron diffraction analysis of few-layer Black Phosphorus
Black phosphorus (BP) is an allotrope of Phosphorus characterized by a layered structure. It has been recently shown [1] that, similarly to graphene, it can be mechanically exfoliated to isolate atomically thin layers which have very interesting electrical and photonic properties. Single-layer BP is in fact an intrinsic semiconductor with a direct bandgap (~2 eV) and it has been employed in the fabrication of field-effect transistors with large current on-off ratios and high mobilities (100-3000 cm2/Vs) [1].
Given the rising interest in this layered material, an extensive TEM analysis of few-layer BP was performed [2].
We have investigated the Electron Diffraction (ED) pattern of few-layer black phosphorus transferred on a holey Silicon Nitride membrane with 1 µm holes diameter (see Figure 1(a)). An HRTEM image from a multilayer area of the sample is shown in Figure 1(b). The uniformity in this image indicates that the lattice contains no extended defects (single vacancies cannot be detected). We found that electron diffraction patterns depend on the number of layers and thus ED can be employed to determine the thickness of the BP flakes. We simulated electron diffraction patterns finding that the ratio between the 101 and 200 reflections depends on the number of black phosphorus: in particular this ratio is > 1 for single layer BP and decreases rapidly with the number of layers. The table shown in Figure 2 summarizes the simulated 101/200 intensity ratios for different number of layers, together with the experimental data acquired. Figure 3(a) and 3(b) show an ED taken from a thin region and a thick region of the flake, with 101/200 intensity ratios of 0.4 and 0.01, respectively.
We also noticed the presence of “forbidden” reflections (h+l = 2n+1) in the thin sample, which was not accounted in our simulations. This could be explained by the presence of adatoms on the surface of the black phosphorus layer or a slight distortion of the lattice.
References:
[1] Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y.
Preprint at arXiv:1401.4117 (2014)
[2] “Isolation and characterization of few-layer black phosphorus”, Castellanos-Gomez, Andres; Vicarelli, Leonardo; Prada, Elsa; Island, Joshua O.; Narasimha-Acharya, K. L.; Blanter, Sofya I.; Groenendijk, Dirk J.; Buscema, Michele; Steele, Gary A.; Alvarez, J. V.; Zandbergen, Henny W.; Palacios, J. J.; van der Zant, Herre S. J. Preprint at arXiv 1403.0499 (2014)
The research leading to these results has received funding from the European Research Council, ERC Project n. 267922
Fig. 1: (a) Optical image of a black phosphorus flake transferred onto a holey silicon nitride membrane. (b) High resolution transmission electron microscopy image of the multilayered region of the flake (~ 13-21 layers). |
Fig. 2: Thickness dependence of the electron diffraction patterns. We display the thickness dependence of the intensity ratio between the 101 and 200 reflections. The experimental data acquired on two spots of the thin flake and one spot of the thicker area has been included for comparison. |
Fig. 3: (a) and (b) are the electron diffraction patterns acquired with a 400 nm spot on the thin (~ 2 layers) and on the thick (~ 13-21 layers) region of the flake, respectively. |
Fig. 4:
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