Ultrathin two-dimensional materials and corresponding quantum dots, such as graphene and phosphorene, exhibiting outstanding physical and chemical properties and holding potential applications in electronics and optoelectronics, have attracted world-wide attention. In the process of fabrication or application of materials, we need to tune and control their properties to meet various requirements by changing the size of materials, doping, surface modification, or heterojuncting. Moreover, the preparation of material can also introduce defects which may influence the properties of materials. To make clear how these factors influence the electronic and optical properties, in this thesis, we systematically study graphene or black phosphorene quantum dots, graphitic zinc oxide/blue phosphorene heterostructure and arsenene-based heterostructure with molybdenum disulfide or organic molecules within the framework of density functional theory (DFT) or time-dependent (TD) DFT in combination with many-body Green’s function (GW). The underlying mechanism such as absorption and emission in nitrogen doping graphene quantum dots, anomalous size dependence of optical properties and photo-absorption tolerance of defects in black phosphorus quantum dots are revealed.
1) Revealing the underlying absorption and emission mechanism of nitrogen doping graphene quantum dots. Nitrogen-doped graphene quantum dots (N-GQDs) hold promising application in electronics and optoelectronics because of excellent photo-stability, tunable photoluminescence and high quantum yield. However, the absorption and emission mechanisms have been debated for years. Here, by employing TDDFT, we demonstrate that different N-doping types and positions give rise to different absorption and emission behaviors, which successfully addresses the inconsistence observed in different experiments. Specifically, the center doping creates mid-states, rendering as non-fluorescent, while the edge N-doping modulates the energy levels of excited states and increases the radiation transition probability, thus enhances the fluorescence strength. More importantly, the even hybridization of frontier orbitals between edge N-doping and GQD leads to the blue-shift for both absorption and emission spectra, while the uneven hybridization of frontier orbitals induces the red-shift. Solvent effects on N-GQDs are further explored by the conductor-like screening model and it is found that strong polarity of solvent can cause the red-shift and enhance the intensity of both absorption and emission spectra.
2) Anomalous size dependence of optical properties in black phosphorus quantum dots. Understanding electron transitions in black phosphorus nanostructures plays a crucial role for applications in electronics and optoelectronics. By employing TDDFT calculations, we systematically study the size-dependent electronic, optical absorption and emission properties of black phosphorus quantum dots (BPQDs). Both the electronic gap and the absorption gap follow an inversely proportional law to the diameter of BPQDs in conformity to the quantum confinement effect. In contrast, the emission gap exhibits anomalous size dependence in the range of 0.8-1.8 nm which is blue-shift with the increase of size. The anomaly, in fact, arises from the structure distortion induced by the excited state relaxation and it leads to huge Stokes shift in small BPQDs.
3) Photo-absorption tolerance of intrinsic point defects and oxidation in black phosphorus quantum dots. BPQDs exhibit excellent optical and photothermal properties and promising applications in optoelectronics and biomedicine. However, various intrinsic structural defects and oxidation are nearly unavoidable in preparation of BPQDs and how they affect the electronic and optical properties remains unclear. By employing TDDFT, we reveal that there are two types of photo-absorption in BPQDs for both point defects and oxidation. A close structure-absorption relation is unraveled: BPQDs are defect-tolerant and show excellent photo-absorption as long as the coordination number (CN) of defective P atoms is 3; In contrast, the unsaturated or oversaturated P atoms with CN≠3 create in-gap-states (IGSs) and completely quench the optical absorption. An effective way to eliminate the IGSs and repair the photo-absorption of defective BPQDs via sufficient hydrogen passivation is further proposed.
4) Efficient carrier separation in graphitic zinc oxide and blue phosphorus van der waals heterostructure. Efficient carrier separation is the key to the application of photoelectric device. However, photo-generated electron-hole pairs in simplex semiconductors generally occupy the same regions spatially and are easy to recombine. We design a graphitic zinc oxide (g-ZnO) based intrinsic type-II heterostructure, g-ZnO/blue phosphorus (Blue-P), based on first-principles calculations. The type-II band offsets and large built-in electric field ensure the photogenerated electrons easily migrating from g-ZnO to Blue-P, which significantly enhances the separation of electron-hole pairs. Improved optical absorption is also observed in the heterostructure. Furthermore, the perpendicular external electric field can greatly modulate band edges and achieve a direct band gap at Г point, which provides further promotion in the separation of carriers.
5) Arsenene-based heterostructures: high-efficient bifunctional candidates for photovoltaic and photocatalytic applications based on accurate band alignments. Utilizing many-body perturbation GW method with an extrapolation technique, we obtain accurate band edges and find that arsenene-based heterostructures constructed with molybdenum disulfide or ornanic molecules tetracyano-quinodimethane and tetracyanonaphtho-quinodimethane possess suitable band gap, energy level of band edges which can satisfy the requirements of photocatalytic water splitting. Moreover, the power conversion efficiency of these arsenene-based heterostructures is predicted to be ~ 20% in photovoltaic solar-cell application. The arsenene serves as the donor, and other materials as acceptor. The bifunctional performance of arsenene-based heterostructures possess promising potential in optoelectronics applications.