Paper MN+2D-WeM13
Anisotropic Thermal Conductivity of Suspended Black Phosphorous Probed by Opto-thermomechanical Resonance Spectromicroscopy

Wednesday, November 1, 2017, 12:00 pm, Room 16

Two-dimensional (2D) black phosphorus (P) exfoliated from its layered bulk crystals has attracted great attention due to its unique in-plane anisotropic properties along armchair (AC) and zigzag (ZZ) directions [1-2]. Probing the anisotropic properties in the black P is important for both exploring fundamental science and engineering device performance . Here, we employ 2D nanoelectromechanical systems (NEMS) platform to study anisotropic thermal conductivity (k) of black P.

In this study, for the first time, we use thermomechanical motion with localized laser heating (Fig. 1a) (optothermomechanical spectromicroscopy) in combination with finite element modeling (FEM) to precisely determine anisotropic k_AC and k_ZZ of black P. We fabricate a black P circular drumhead resonator (thickness of t=~80nm and diameter of d=~9µm) using a dry-transfer method [3]. Before resonance measurement, polarized reflectance measurement is performed to determine the crystal orientation of the black P flake (Fig. 1b) [4]. We then employ a 633nm laser (laser power of P=1.6mW, spot size of 1µm) to photothermally heat up the device and interferometrically detect Brownian motion. We obtain the fundamental mode frequency at f_res=~9MHz when the laser is located on the center of the device. We move the laser spot location along AC/ZZ on the resonator, and track f_res along the path. We find that measured f_res values are higher when laser spot is moving in AC direction (f_res,AC) than that in ZZ direction (f_res,ZZ)(Fig. 1c) at same distance from the center. This can be attributed to anisotropic k_AC and k_ZZ, which dictates different temperature distribution on the device as the laser is moving along AC/ZZ, providing uneven biaxial thermal expansion thus frequency shift.

We employ FEM simulation to model the coupling between thermal transport from optothermal heating and resonance characteristics of the black P drumhead resonator. By fitting the modeling to the experimental results, we are able to determine anisotropic thermal conductivities along AC/ZZ orientations (k_AC=15Wm^-1K^-1 and k_ZZ=55Wm^-1K^-1) (Fig. 1c and 1d), which are consistent with k_AC and k_ZZ obtained by other methods [2].

References:

[1] X. Ling, et al.,Proc. of the Nat. Acad. of Sci., 112(15), pp.4523-4530 (2015).

[2] B. Smith, et al.,Adv. Mat. 29, 1603756 (2017).

[3] R. Yang, et al., J. Vac. Sci. & Tech. B 32, 061203 (2014).

[4] A. Islam et al., Manuscript in Preparation (2017).