Many-Body Green’s Function Theory Studies On The Excited States Of Biologic And Material Systems

The excited-state properties, including ionization energy, electron affinity, energy band, optical gap, absorption spectrum, and emission spectrum, of some biologic and material systems have been studiedby using many-body Green’s function theory (MBGFT) in this researches. The first-principle MBGFT have been widely and successfully used for describingopticalexcitationsinbulkcrystals, clusters, polymers, inorganic molecules, biologicmolecules, etc. As is known to all, MBGFT is based on a series of Green’s function equations.The electron’ sself-energyΣ, which includes the electron and hole exchange-correlationinteractions, is obtained by Hedin’s GW approach, and the electron-hole interaction kernel, which is described by the Bethe-Salpeter equation (BSE). The accuracy of MBGFT for orbital energies and excitation energies is usually within 0.1～0.2 eV. MBGFT have advantages in studying nonlocal excitations, since the long-range Coulomb potential between electron and hole is well expressed through dielectric function which can be constructed from first principles.GW approach can predict molecular orbital energy levels with much higher accuracy than DFT.At present GW approach and BSE method are included in many codes, and most of them are based on plane-wave basis sets, which limit the computational efficiency. In this paper, in order to reducecomputational effort, a set ofGaussian-orbital based codesare used to performed all the MBGFT calculations. We can study the excited-state propertiesof the large molecular systems, as many as 200 atoms. Here, we mainly study some biologic and material systems, including DNA bases, diamond nanoparticles (DNPs), single-walled carbon nanotubes (SWCNTs), and ZnO surfaces. Calculation examplesare also given and some conclusions are drawn. Our work demonstrates that MBGFT is a promising method in the study of excited states of many systems from both the accuracy and efficiency.The main contents and conclusions are summarized as follows:1. As we know, DNA, as an informational molecule in the cells of living things, plays an important role in the biological hereditary process. In organisms, prolonged exposure to solar UV radiation may induce a series of photochemistry and photophysics processes in the excited states of DNA, resulting in DNA damages and carcinogenic mutations. Intra-and interstrandCT states are dark states, so their energies cannot be determined directly from experiment yet. This hinders our understanding of the properties of CT statesand also the excited-state dynamics related to the UV absorption and excimerstates. Theoretical studies may provide us some useful information on the CT states. However, A huge discrepancy still existsbetween experiment and theory. By usingmany-bodyGreen’s function theory, together with classical molecular dynamics simulations, we confirm the existenceof CT states at the lower energy side of the optical absorption maximum in aqueous DNA as observed inexperiments. We find that the hydration shell can exert strong effects (-1 eV) on both the electronicstructure and CT states of DNA molecules through dipole electric fields. Our calculations clearly show that solvent effects are muchmore important than base-stacking and base-pairing interactions for CT states. To study the CT states of DNA in aqueoussolution theoretically, one should either consider enough watermolecules in the calculation or choose a suitable solvationmodel which can reproduce the modifications of molecularorbitals and energy levels as observed in this work.2. Diamond nanoparticles (DNPs), as an important photoelectric material, are widely used inoptoelectronic devices and sensor. In contrast to other semiconductor nanocrystals, such as Si and Ge, which exhibit clearquantum confinement (QC) effects, the optical properties ofDNPs, e.g., optical gaps and oscillator strength, are determined by not only the size but also the shape and symmetry ofthe nanoparticles.Here, we calculate the photoelectricpropertiesof the nine kinds of DNPs by MBGFT method.These DNPs have been well studied in recent experiments. The calculatedionization energies, optical gaps and absorption spectrum are in agreement withexperiments, with the average error about 0.2 eV. The electron affinity is negative and the lowest unoccupied molecularorbital is rather delocalized. Precise determination of the electron affinity requires one to take theoff-diagonal matrix elements of the self-energy operator into account in the GW calculation.3. GW-BSE method has been widely used to study the excited-state properties of SWCNTs. SWCNTshave promising applications in optoelectronics, biological imaging and sensing. Optical excitations of the complexes, which are generated by SWCNT coupling with small molecular systems, are studied using ab-initio many-body perturbation theory (within the GW approximation and Bethe-Salpeter equation). Inclusion of small molecules in the surfaceof the SWCNT can modify the optical selection rules surprisingly. The completely optically forbidden exciton (low-lying HOMO �' LUMO transition) of SWCNT may be brightened by the couplinginteraction between the dye molecules and SWCNT. Our findings may be useful for expanding thelight spectrum and enhancing the PL efficiency for the SWCNT. The predictions reported here would guide us anew direction of the modification of the SWCNTS optical properties through the noncovalent binding of dyemolecules.4. Owing to the unique properties, Zinc Oxide (ZnO) is an importantphotoelectric material in solar celldevices.ZnO is a direct wide-gap semiconductor with n-type conductivity and the experimental band gap is 3.4 eV. Recently, the band gap calculated by DFT-LDA is only 0.7 eV, which has a large errorwith experiment. In order to obtain precise band gap, a lot of hybrid functionals and DFT+U method have been used in the quantum chemical calculations. GW approach can increase the band gap of ZnO by about 2.15 eV, e.g.,2.85 eV GW gap. However, if the renormalization constant Z=1, the GW band gap is 3.25 eV, which is in agreement with experimental result. Some researches have shown that self consistent GW calculationis needed to get accurate band gap of ZnO. Here, the number of κ-points has a great influence on the excitation energies and exciton binding energies of ZnO in the BSE calculations. So we need to consider as many BSE-κ-points as we can to describe the small exciton binding energies of ZnO. In this paper, we have also studied the change of energy bandstructureof ZnO surface through the inclusion of surface defects and organicthiophenemolecule by DFT. Next, we will calculate the excited-state properties of ZnO surface by MBGFT method.