Probing the Physical Origin of Anisotropic Thermal Transport in Black Phosphorus Nanoribbons

Abstract

van der Waals interactions, making the material suitable for exfoliation. Layerdependent bandgap has also been reported previously [2,3] in BP like in the transition metal dichalcogenides (TMDCs). Because of its special puckered honeycomb structure formed by covalently-bonded phosphorus atoms, BP shows interesting angle-dependent transport properties. [1,2,6] Thus far, while extensive studies on electrical and optoelectronic applications of BP have been carried out, thermal transport measurement of BP is rarely explored.Theoretically, it has been calculated that phonon transport in BP is anisotropic, where the thermal conductivity along the zigzag (ZZ) direction is larger than that along the armchair (AC) direction [7,8] and an anisotropy around 3 was estimated by first-principles calculation. [9] Nevertheless, results from experimental studies on the thermal transport anisotropy of BP diverge significantly. By means of micro-Raman spectroscopy, Luo et al. [10] measured the thermal conductivity of BP thin films to yield an anisotropy ratio around 2 between ZZ and AC directions and this value dropped to ≈1.5 for thinner films. Similarly, a thermal conductivity anisotropy ratio of 1.85 was obtained using the thermal Black phosphorus (BP) has emerged as a promising candidate for next-generation electronics and optoelectronics among the 2D family materials due to its extraordinary electrical/optical/optoelectronic properties. Interestingly, BP shows strong anisotropic transport behavior because of its puckered honeycomb structure. Previous studies have demonstrated the thermal transport ani sotropy of BP and theoretically attribute this to the anisotropy in both the phonon dispersion relation and the phonon relaxation time. However, the exact origin of such strong anisotropy lacks clarity and has yet to be proven experimentally. Here, the thermal transport anisotropy of BP nanoribbons is probed by an electron beam technique. Direct evidence is provided that the origin of this anisotropy is dominated by the anisotropic phonon group velocity, verified by Young's modulus measurements along different directions. It turns out that the ratio of the thermal conductivity between zigzag (ZZ) and armchair (AC) ribbons is almost same as that of the corresponding Young modulus values. The results from first-principles calculation are consistent with this experimental observation, where the anisotropic phonon group velocity between ZZ and AC is shown. These results provide fundamental insight into the anisotropic thermal transport in low-symmetry crystals.Black phosphorus (BP) has attracted considerable attention as a promising 2D material due to its high hole mobility [1,2] and tunable bandgap. [3][4][5] Similar to graphene and other 2D materials, atomic layers in black phosphorus are stacked together through Figure 5. Calculated phonon dispersion of bulk black phosphorus.Γ-X corresponds to the armchair direction of the primitive cell, and Γ-Y denotes the zigzag direction. According to Jain and McGaughey, [9] the sound velocity ...

Thermal Transport in 2D Semiconductors—Considerations for Device Applications

“…c) Thickness dependent Young's modulus, including both theoretical calculations and experimental measurement. Reproduced with permission . Copyright 2018, Wiley‐VCH.…”

“…The role of phonon relaxation time for thermal transport anisotropy is doubtful since dominant direction‐dependent phonon dispersion and similar intrinsic phonon scattering rate along ZZ and AC directions were demonstrated. More recently, Zhao et al decoupled the dominant role of phonon group velocity from phonon relaxation time by linking the anisotropic phonon group velocity to anisotropic sound velocity, as verified by measuring the Young's modulus of black phosphorene along both ZZ and AC directions using method of three‐point bending. It turns out that the anisotropy ratio between κ ZZ and κ AC is comparable to that of the Young's modulus value along these two directions.…”

Piezoelectricity in Multilayer Black Phosphorus for Piezotronics and Nanogenerators

“…Black phosphorus (BP), which has only recently been investigated as a 2D material, has been a research focus due to its thickness‐dependent bandgap and high carrier mobility . The anisotropic characteristics of BP, including its optical, thermal, mechanical, ionic transport and electronic properties, are oriented according to its anisotropic atomic structure. These characteristics encouraged further investigation leading to the discovery of the piezoelectricity of BP.…”

Remarkable Reduction of Interfacial Thermal Resistance in Nanophononic Heterostructures

“…We select BP as an example because it has drawn a great deal of attention due to its unique thermal properties. For example, a strong anisotropy was found in thermal transport behavior of BP in both ballistic [40] and diffusive regimes, [4,[41][42][43][44][45][46] attributed to the puckered honeycomb structure. Moreover, benefited by the orthogonal prominent electrical and thermal conductance, high thermoelectric performance is expected in BP, [47,48] which can be further enhanced by strain.…”