Theoretical Investigations on the Optical Properties of Graphene Quantum Dots and Novel Two Dimensional Materials

Author:Li Yun Hai

Supervisor:yang yong hong wang jin lan


Degree Year:2018





The success of graphene has triggered intensive investigations on graphene-derived nano-materials and other two-dimensional materials.Graphene quantum dots(GQDs)have promis-ing application potentials in solar cells,photo-detectors,bio-imaging,and catalysts.Layered Indium Selenide(InSe)has high on/off ratio,carrier mobility and stability favoring its applica-tions in opto-electronics.Organic-inorganic perovskites have increased the power conversion efficiency of solar cells based on them to 22.1%in just a few years.They are also promis-ing in light-emitting diodes and laser devices.The application of these novel nanomaterials in opto-electronic devices requires thorough knowledge into their electronic structure and optical properties.In this dissertation,the electronic structure and optical properties of GQDs,mono-layer InSe and two-dimensional MAPbBr3perovskite have systematically investigated utilizing density functional theory and many-body perturbation theory,in combination with tight-binding models and quantum well models.The main conclusions of this dissertation are summarized as following:(1)Influence of many-body effects on the electronic and optical properties of GQDs.Many-body interactions have profound effects on the electronic and optical properties of low-dimensional materials.However,their role in GQDs were not clear.We inspected the role of many-body effects considering hexagonal GQDs with armchair and zigzag edges respectively.Calculations show that many-body effects are significant in GQDs,yielding quasi-particle cor-rections and exciton binding energies as high as several electron volts,much larger than that in other carbon allotropes,as the consequence of quantum confinement and reduced screening.The absorption spectrum shifts towards lower energies when electron-hole interaction is taken into consideration,with a prominent excitonic peak emerging below the absorption onset of the spectrum without electron-hole interaction.We also find that the HOMO-LUMO energy gap,exciton binding energy and spin singlet-triplet splitting are closely related to the size and shape of GQDs.The spin singlet-triplet splitting in GQDs are larger than other carbon allotropes,indicating a better photoluminescence performance.(2)Influence of edge-attached organic functional groups on the electronic and optical properties of GQDs.The synthesis process or GQDS relies heavily on strong oxidants,which unavoidably introduce oxygen-containing functional groups.Moreover,functional groups such as ammonia and halide atoms have been deliberately introduced in to GQDs for tuning the in-trinsic properties.However,the influence of such organic functional groups was not clear.We propose a mechanism based on the competition and collaboration between frontier orbitals mix-ing(FOM)and charge transfer(CT)between the functional group and GQD moiety to interpret the influence of edge-attached functional groups.For the electronic structure,FOM lowers the energy gap via asymmetric shift of the energy levels,while CT enlarges the energy gap by reduc-ing electronic screening.However,FOM and CT collaborate to lower the excitation energy of the first bright exciton when it comes to optical properties.We also find that functional groups containing C=O double bond are more effective in tailoring both the electronic and optical prop-erties of GQDS.Ammonia group can only affect the optical properties due to too large charge transfer,while hydroxyl group and halide atoms have negligible effects on both electronic and optical properties of GQDs.(3)Influence of lateral strain on the band structure and optical properties of mono-layer InSe.The photoluminescence of free-standing InSe is almost quenched due to its in-direct band gap,which can then be converted to direct band gap under lateral strain and the photoluminescence performance may be improved.The underlying mechanism underlying this indirect-direct band gap was,however,not clarified,and the influence of strain on the excitation energies,transition matrix elements and selection rules had not been investigated.We system-atically studied the involution of band structure of monolayer InSe under lateral strain along armchair,zigzag and biaxial directions.We find that compressive strain can enhance the cou-pling between Se 4px/4pybands compared to Se 4pzband and reverse the band ordering,leading to the indirect-direct band gap conversion.The selection rules in single-particle regime are still valid when electron-hole interaction is taken into consideration,with the lowest excited state being bright under incident light polarized vertically to the surface.Under compressive strain the lowest exciton becomes active under laterally polarized light,with significant increment of transition matrix element,indicating an enhancement in the photoluminescence performance.We also observe a linear strain-band gap relation,which can provide guides for tailoring the band gap and photoluminescence wavelength.(4)Excitons in MAPbBr3 perovskite platelets.Excitons are well known to be dominant in the optical properties of low-dimensional materials.However,the application of many-body perturbation theory to low-dimensional perovskites is intractable due to the huge computational cost.We cooperated with experimental researchers to prepare crystalline MAPbBr3perovskite platelets,and studied the excitons with a generalized quantum well model.The results show that the exciton radii are in the range of 2.2 nm-4.4 nm for perovskites with the thicknesses of 5.9nm–26.2 nm,twice the size of that in polycrystalline MAPbBr3perovskite films,indicating that the quality of crystallization is a key factor in determining the excitonic effects in perovskites.Further calculations reveal that increasing the dielectric constants of the barrier and perovskite,and the sample thickness,will decrease the exciton binding energy.For exciton radius the trend is the opposite.Our results can not only provide guides for tailoring the excitonic effects,but also prove that the quantum well model is an accurate yet cheap method to study the excitonic effects in low-dimensional perovskites.