Anisotropic thermal conductivity in direction-specific black phosphorus nanoflakes
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Research Letter
Anisotropic thermal conductivity in direction-specific black phosphorus nanoflakes Heguang Liu , School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China† Jianxi Liu, State Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, Northwestern Polytechnical University, Xi’an 710072, China† Ruixuan Jing, and Caiyin You, School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China Address all correspondence to Heguang Liu at [email protected] (Received 3 July 2019; accepted 5 September 2019)
Abstract Herein, the authors report our pioneering demonstration of the anisotropic thermal properties of black phosphorus (BP) nanoflakes. The nanoflakes were produced using a scotch tape-based mechanical exfoliation technique. Their thickness was characterized using Atomic Force Microscopy The anisotropic direction of the nanoflakes was determined by the Raman Spectroscopy equipped with a polarized laser. Then, a temperature-dependent Raman spectroscopy method was utilized to study the thermal transport properties of the BP nanoflakes. The results indicated that the thermal conductivities of zigzag BP and armchair nanoflakes are 30.6 and 12.6 W/m·K, respectively. This fundamental thermal study gives insight into the future fabrication of nanoscale electronic devices with thermal properties that can be well controlled.
Introduction Two-dimensional (2D)-layered nanomaterials, such as graphene and molybdenum disulfide (MoS2), have become increasingly popular because of their outstanding electrical, optical, and mechanical properties.[1–3] The discovery and success of these materials have inspired black phosphorus (BP) to be viewed recently as a new nanomaterial in the 2D family. Unlike graphene with zero band gap[4] and MoS2 which has a band gap of ∼1.8 eV as a monolayer,[5] BP has been shown to have a thickness-dependent direct band gap varying from ∼0.3 eV in its bulk form to greater than 1.5 eV as a monolayer.[6] The structure of BP is highly asymmetric and provides unusually strong in-plane anisotropy in orthogonal directions along what are called the Zigzag (ZZ) and Armchair (AC) directions.[7–9] Moving along the two different lattice directions renders different properties to be exhibited based on the phonon–phonon interactions among other attributes. Although it shows promise as a semiconductor material, its elementary electrical and thermal conductivities have yet to be fully characterized and documented. Monolayer and few-layer BP nanoflakes can be produced by a traditional Scotch tape-based mechanical exfoliation technique often used in obtaining graphene from graphite.[10] The repeated “peeling” of bulk BP using scotch tape causes the puckered layers of BP to come apart. Compared to graphene,
† These authors contributed equally to this work.
the van der Waals forces are greater between the BP layers and make it difficult to obtain a monolayer, but few-layer and multi-layer BP are readily availa
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