What is the role of defects in single-walled carbon nanotubes for nonlinear optical property?

Abstract

In carbon nanotubes (CNTs), there exist various kinds of defects which may influence the electrical, thermal and mechanical properties of carbon nanotubes. However, how the defect influences the nonlinear optical (NLO) property is still unknown. This paper exhibits the role of defects on the static first hyperpolarizability (β₀) for topological Stone-Wales (SW), single vacancy (SV) and double vacancy (DV) defects. For eight carbon nanotubes with defects and one relevant perfect nanotube, the obtained order of the β₀ values is 0 (Perfect) < 5.0 × 10² (DV585-z) < 5.3 × 10² (SW5577-z) < 4.6 × 10³ (DV585-xz) < 6.0 × 10³ (DV555777) < 7.3 × 10³ (SV-xz) < 1.1 × 10⁴ (SV-z) < 3.3 × 10⁴ (SW5577-xz) < 2.2 × 10⁵ a.u. (SW57-75). The results show that the defect plays an important role on greatly enhancing the β₀ value due to lowering the transition energy. Four interesting relationships between the defect and the β₀ value are observed. (1) The defects bring a large increase of the β₀ values. (2) For the SW and DV defects, the direction of the defect is an important factor to enhance the β₀ value. For example, β₀ values of SW5577 structures are 3.3 × 10⁴ (near xz direction) ≫ 5.3 × 10² a.u. (the z direction along the tube axis). (3) Interestingly, not vacancy defects but the topological defect can bring the largest β₀ value (2.2 × 10⁵ a.u. for the SW57-75). (4) For the topological SW defects, the separate pentagon-heptagon pair defect (57–75, the β₀ value of 2.2 × 10⁵ a.u.) may be more efficient than the fused one (5577, 3.3 × 10⁴ a.u.) to enhance the β₀ value. This work suggests a new strategy, introducing the right defect on CNTs to enhance the nonlinear optical response.