Anisotropic Optical, Mechanical, and Thermoelectric Properties of Two-Dimensional Fullerene Networks

Abstract

Nanoclusters like fullerenes as the unit to build intriguing two-dimensional (2D) topological structures is of great challenge. Here we propose three bridged fullerene monolayers and comprehensively investigate the novel fullerene monolayer (α-C60-2D) as synthesized experimentally [Nature2022606507510] by state-of-the-art first-principles calculations. Our results show that α-C60-2D has a direct band gap of 1.55 eV close to the experimental value, an optical linear dichroism with strong absorption in the long-wave ultraviolet region, a small anisotropic Young’s modulus, a large hole mobility, and an ultrahigh Seebeck coefficient at middle–low temperatures. It is unveiled that the anisotropic optical, mechanical, electrical, and thermoelectric properties of α-C60-2D originate from the asymmetric bridging arrangements between C60 clusters. Our study promises potential applications of monolayer fullerene networks in lots of fields.

Stability and Strength of Monolayer Polymeric C60

“…The two-step structure relaxation mimics the experimental procedure to remove the charged ions introduced during synthesis. 1,4 The computed lattice constants for all three phases are given in Table 1, which are in good agreement with previous results, 10,11,15 therefore confirming the reliability of the present calculations.…”

“…To confirm the mechanical stability, the elastic constants are calculated by finite differences through finite distortions of the lattice. , There are different ways to define the 2D elastic constants from the computed 3D coefficients. ,, Here the 2D coefficients C ij 2D are renormalized by the c lattice constant (the spacing between 2D layers): , i.e., C ij 2D = c × C ij 3D . The obtained 2D elastic constants (including ionic relaxations) are given in Table using the Voigt notation, 1 – xx , 2 – yy , 6 – xy , and the present results agree well with previous calculations. , According to Born-Huang’s lattice dynamic theory, , in monoclinic crystals (qTP1 and qHP with space groups P 2/ m and Pc , respectively), the mechanical stability criteria are given by C 11 > 0 , .25em C 22 > 0 , .25em C 66 > 0 , .25em C 11 + C 22 + 2 C 12 > 0 …”

“…On the other hand, a two-step structural relaxation, starting from monolayer qTP Mg 2 C 60 and then removing the Mg ions before the second relaxation, obtains a tightly bound qTP monolayer (qTP2), as shown in Figure b. The two-step structure relaxation mimics the experimental procedure to remove the charged ions introduced during synthesis. , The computed lattice constants for all three phases are given in Table , which are in good agreement with previous results, ,, therefore confirming the reliability of the present calculations.…”

“…Ever since the discovery of C 60 , the formation mechanism and stability of the fullerene molecules are far from being completely understood. − When forming structural units of C 60 clusters in a 2D plane, it is unclear whether ordered structures of monolayer polymeric C 60 are stable under ambient conditions such as strain and temperature. Recent first-principles calculations have investigated various structural phases of monolayer C 60 with different bonding characters. − The mechanical stability of several phases has been confirmed. ,, More recently, the thermal stability of monolayer C 60 has been addressed using molecular dynamics simulations, showing that both qTP and qHP C 60 monolayers can remain stable at temperatures near 800 K, which is partially consistent with the experimental result that monolayer qHP C 60 does not decompose at 600 K . However, previous analyses based on mechanical and thermal stability cannot explain why the qTP monolayers have not yet been exfoliated experimentally.…”

“…10−15 The mechanical stability of several phases has been confirmed. 10,11,14 More recently, the thermal stability of monolayer C 60 has been addressed using molecular dynamics simulations, showing that both qTP and qHP C 60 monolayers can remain stable at temperatures near 800 K, 16 which is partially consistent with the experimental result that monolayer qHP C 60 does not decompose at 600 K. 1 However, previous analyses based on mechanical and thermal stability cannot explain why the qTP monolayers have not yet been exfoliated experimentally. Furthermore, the dynamic stability of monolayer fullerene networks with respect to lattice vibrations, which indicates whether the crystal structure is in a local minimum of the potential energy surface, 17−20 is still unexplored.…”