Optical properties and optoelectronic applications of black phosphorus

Introduction The recent isolation of black phosphorus (a layered material composed of phosphorus atoms held together by strong in-plane bonds forming layers that interact between them through weak van der Waals forces) has unleashed the interest of the community working on 2D materials because of its interesting and attractive electronic properties including: narrow intrinsic gap, ambipolar field effect, and high carrier mobility [1-11]. The outstanding electrical properties of black phosphorus, discussed in detail in the previous chapter, has already motivated the fabrication of black phosphorus-based devices such as field-effect transistors [1, 3, 4, 12-21], inverter amplifiers [22]. Apart from its electrical properties, black phosphorus also displays remarkable optical properties [23-27], unmatched by any other 2D materials that have been isolated to date, further motivating the recent surge of works on this novel material. This chapter reviews the main optical properties of black phosphorus, focusing on its thickness-dependent band gap and its unusual in-plane anisotropy. Then, recent works demonstrating black phosphorus optoelectronic devices (phototransistors, high-speed photodetectors and solar) are summarized. Finally, the future perspectives of black phosphorus’ application in optoelectronic are discussed. Optical Properties In this section, the main optical properties of black phosphorus will be reviewed, stressing the thickness dependence of the band gap and the characteristic anisotropic optical properties of black phosphorus. Thickness-Dependent Band Gap In spite of its morphological similarity to graphene, black phosphorus differs considerably from graphene in its electronic properties. While graphene is a zero-gap semiconductor, black phosphorus presents a sizeable gap which makes it attractive for optoelectronic applications [28-30]. Moreover, similarly to other 2D semiconducting materials, the band-gap value strongly depends on the number of layers due to the quantum confinement effect in the out-of-plane direction [31-33]. In order to illustrate this effect, Fig. 23.1(a) compares the band structures of monolayer, bilayer, and trilayer black phosphorus obtained through ab initio calculations with the GW approximations [34]. A general feature of all the calculated band structures is that the band gap remains direct at the Γ point of the Brillouin zone. In semiconducting transition, metal dichalcogenides, on the other hand, the gap is at the K point and it is only direct for single-layers (becoming and an indirect gap semiconductor for multilayer samples) [31, 32].