optical, [6][7][8] and thermal properties. [9][10][11][12][13][14][15][16][17][18] BP has an orthorhombic structure with puckered layers of sp 3 -hybridized phosphorus atoms along the armchair direction and van der Waals interactions between layers. [ 2 ] The structure of BP makes it the most thermodynamically stable phosphorus allotrope at ambient temperature and pressure. Both monolayer (also called phosphorene) and few-layer BP fl akes obtained from mechanical exfoliation [ 19,20 ] have proven to be anisotropic 2D semiconductors with high carrier mobility, [ 5,[19][20][21] possessing a direct band gap tunable from 0.3 to 1.0 eV. [ 22 ] This combination of properties makes BP a great candidate for use in the next generation nanoelectronic and nanophotonic devices, compared even favorably to other well-studied 2D materials such as graphene, hexagonal boron nitride, and molybdenum disulfi de. [23][24][25] One unique feature of BP is the inverse dependence of its in-plane electronic and thermal transport on the crystalline orientation: a higher electrical conductivity is associated with a lower thermal conductivity in the armchair direction and vice versa for the zigzag direction, which has been predicted in previous studies. [ 4,9 ] To date, experimental investigations on the anisotropic thermal properties of BP mainly focused on thin fi lms; thermal