Optical excitations of defects in realistic nanoscale silica clusters: Comparing the performance of density functional theory using hybrid functionals with correlated wavefunction methods

Abstract

Optical excitations of low energy silica (SiO(2))(4) clusters obtained by global optimization, as opposed to constructed by hand, are studied using a range of theoretical methods. By focusing on the lowest energy silica clusters we hope to capture at least some of the characteristic ways by which the dry surfaces of silica nanosystems preferentially terminate. Employing the six lowest energy (SiO(2))(4) cluster isomers, we show that they exhibit a surprisingly wide range of geometries, defects, and associated optical excitations. Some of the clusters show excitations localized on isolated defects, which are known from previous studies using hydrogen-terminated versions of the defect in question. Other clusters, however, exhibit novel charge-transfer excitations in which an electron transfers between two spatially separated defects. In these cases, because of the inherent proximity of the constituent defects due to the small cluster dimensions, the excitation spectrum is found to be very different from that of the same defects in isolation. Excitation spectra of all clusters were calculated using time-dependent density functional theory (TD-DFT) and delta-SCF DFT (DeltaDFT) methods employing two different hybrid density functionals (B3LYP and BB1K) differing essentially in the amount of incorporated Hartree-Fock-like exchange (HFLE). In all cases the results were compared with CASPT2 calculated values which are taken as a benchmark standard. In line with previous work, the spatially localized excitations are found to be well described by TD-DFT/B3LYP but which gives excitation energies that are significantly underestimated in the case of the charge-transfer excitations. The TD-DFT/BB1K combination in contrast is found to give generally good excitation energies for the lowest excited states of both localized and charge-transfer excitations. Finally, our calculations suggest that the increased quality of the predicted excitation spectra by adding larger amounts of HFLE is mainly due to an increased localization of the excited state associated with the elimination of spurious self-interaction inherent to (semi-)local DFT functionals.

TD-DFT benchmarks: A review

“…There exists a significant number of works comparing TD‐DFT and wavefunction transition energies, the latter being often obtained with the CC2 or CCSD methods. Most of these investigations are focused on a rather small panel of compounds and we briefly summarize selected examples in this Section.…”

“…These errors were found to decrease when corrections for the asymptotic behavior of the XCF were included . Zwijnenburg et al compared CAS‐PT2, B3LYP, and BB1K descriptions of the ES of (SiO 2 ) 4 clusters and found that the latter functional better reproduced CAS‐PT2 values . For DNA bases and base pairs, Shukla and Leszczynski, compared CC2/def2‐TZVP excitation energies to TD‐DFT values in both gas and solvent phases .…”

“…Recently, most CT investigations focussed on pinpointing an adequate hybrid (typically a RSH) . Nguyen and coworkers investigated 19 ES in donor‐acceptor complexes between TCE and aromatic compounds with a large panel of RSH and the cc‐pVDZ approach .…”

Concentration gradient induced morphology evolution of silica nanostructure growth on photoresist-derived carbon micropatterns

“…Figure 2g shows the typical HRTEM image of one nanowire without any crystalline domains, indicating that the nanowire is amorphous, which is confirmed by the selected area electron diffraction (SAED) pattern with only diffusive rings instead of diffraction spots, shown as an inset in Figure 2g. Optical devices will have more functions on each chip if silica nanostructures with different morphologies originating from silica nanowires that are integrated [11,13,14]. …”

“…Rope-like silica nanowire assemblies showing reversible blue light emission behavior in photoluminescence and infrared analysis could be used to fabricate effective optoelectronic devices and optical signal humid sensors [12]. Theoretical calculations have also been conducted to explore some interesting optical properties of silica nanoclusters [13,14]. Up to the present, many approaches have been introduced to synthesize silica nanostructures [15-19], and various morphologies were reported, such as nanowire assemblies [17,20], nanoflowers [21], nanotubes [22], and nanosprings, [23] etc.…”