Nature of photoexcited states in ZnO-embedded graphene quantum dots

The combination of wide-band gap semiconductors like zinc oxide (ZnO) and graphene quantum dots (GQDs) is a promising strategy to tune optoelectronic properties of GQDs and to develop new functionalities. Here we report on a theoretical design of not-yet-synthesized hybrid materials composed of ZnO clusters surrounded by carbon moieties, hereinafter referred to as ZnO-embedded graphene quantum dots. Their structure and light absorption properties are presented, with an in-depth analysis of the nature of the photoexcited states. The stability of the (ZnO)nC96-2n system with n=1, 3, 4, 7, 12 and 27 is investigated by estimating cohesive energy and performing vibrational mode analysis. A strong dependence of the structural and optoelectronic properties of the hybrid material on the amount of ZnO pairs is revealed and discussed. A strong light absorption and unexpected enhancement of Raman modes related to the vibrations in carbon moiety are observed for highly symmetric (ZnO)27C42 system that makes it an ideal study subject. Complementary excited state analysis, charge density difference (CDD) analysis and interfragment charge transfer analysis enabled reaching deep insights into the nature of the excited states. A dominating contribution of doubly degenerate locally excited states in broadband light absorption by (ZnO)27C42 is identified. The present results are helpful to elucidate the nature of the fundamental internal mechanisms underlying the light absorption in ZnO-embedded graphene quantum dots, thereby providing a scientific background for future experimental study of low-dimensional metal-oxygen-carbon materials family.


Introduction
Graphene quantum dots (GQDs)nanosized graphene fragmentsare currently considered as an exclusive organic material that brings together the beneficial properties of graphene (large surface area, high carrier mobility and conductivity) and the excellent lightemitting ability dictated by its semiconducting character. Possessing tuneable band gap, low toxicity, and good chemical stability, GQDs find applications in biomedicine, optoelectronics, energy, and sensing technologies. A further integration of zero-dimensional (0D) GQDs with inorganic compound semiconductors (ICS) is a winning strategy to create advanced organicinorganic hybrid nanomaterials with novel functionalities. Among different ICS, zinc oxide (ZnO) is of special importance for many applications due its unique physical properties (direct bandgap of 3.3 eV at room temperature; a strong radiation hardness; large exciton binding energy of 60 meV; antimicrobial activity etc.), relative growth/synthesis simplicity, and morphological diversity 1,2 .
Over the recent years a great deal of progress has been made in synthesis, material characterization and conceptualization of ZnO-GQDs nanohybrids. More specifically, a combination of both materials enables to design high-performance sensors for detection of different substances including H2O2 3 , CO2 4 , ammonia 5 , ethanol 6 , hydroquinone 7 , zearalenone in mildewing cereal crops 8 , acetone 9,10 , H2S 11 , acetic acid 12 , chemo-therapeutic agent (6mercaptopurine) 13 , and NO2 14,15 . Leaving behind brackets various sensing principles that have been applied for quantification of gas or liquid molecules, it should be emphasized that in most cases sensitive hybrid materials were constructed through functionalization or decoration of host ZnO matrix with undoped or doped GQDs. Another example of a beneficial synergy between GQDs and ZnO features concerns a high promise of related nanohybrids for design of next-generation ultraviolet (UV) photodetectors 16−29 . On one hand, the presence of GQDs may favour separation and transport of the photogenerated carriers, thus reducing both the electronhole recombination and the photoresponse time. Since GQDs have a good response to UV light, sensitization with GQDs may, on the other hand, enhance the overall UV absorption in ZnO and hence the photoconductivity of ZnO-based photodetector. The pronounced charge separation at the GQDs/ZnO interface also provides good prerequisites of using this hybrid material in perovskite-, dye sensitized-, quantum-dot-sensitized-and inverted polymer solar cells 30−43 . ZnO-graphene quantum dots system is promising for designing optoelectronic devices (namely light-emitting diodes, LEDs) 44−49 , playing a role of either electron transport layer or active emissive component. Particularly, a brightness of 798 cd•m −2 was achieved for white LEDs based on ZnO cores wrapped in a shell of GQDs 49 .
One more important application of ZnO-GQDs hybrid nanomaterials is related to their excellent photocatalytic properties. The enhanced light absorption in such combined systems and, consequently, increased number of photo-induced charge carriers make it possible to reach synthesis of tetrasubstituted propargylamines 50 , improved solar-driven water splitting 51  Since such a material system is still unexplored, we employ density-functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to explore the nature of photoexcited states in (ZnO)nC96−2n system with n=1, 3, 4, 7, 12 and 27.

Methodology
All quantum chemistry calculations were conducted by means of Gaussian 16 Rev. C.01 program package 70 under gas-phase conditions. Circumcircumcoronene molecule (C96H24) demonstrated in Figure S1 (ESI) was chosen as a representative of GQDs and a host organic molecule for ZnO clusters. ZnO-embedded graphene quantum dots, hereinafter referred to as (ZnO)nC96−2n systems, were constructed through replacing carbon (C) atoms with zinc (Zn) and oxygen (O) atoms (see Figure S2, ESI) in a way that ensures balanced substitutional doping of the system of hexagonally arranged carbon atoms in C96H24. This is analogous to a symmetric doping of two sublattices of infinite graphene model. As a result, there were equal numbers of zinc and oxygen atoms, indicating that the stoichiometry of ZnO clusters embedded in graphene quantum dots is 1:1. Geometry optimization of the (ZnO)nC96−2n systems was performed using Perdew-Burke-Ernzerhof exchange-correlation hybrid functional (PBE0, also called PBE1PBE) 71 , which combines 25% exact Hartree-Fock (HF) exchange with 75% of PBE exchange. PBE0 is a reasonable choice to correctly reproduce the experimentally observed electronic and optical properties of ZnO 72 and GQDs 73,74,75 . 6-31G* basis sets 76 were used for carbon, hydrogen, and oxygen atoms, while the SDD basis set developed by the Stuttgart-Dresden-Bonn group 77 was used for zinc atom.
(ZnO)nC96−2n structures corresponding to the lowest energy minima were obtained by enabling a full relaxation of all coordinates using the default convergence criteria. Vibrational mode calculations were additionally carried out to make sure there are no imaginary frequencies and to confirm the equilibrium of the ground-state geometry. Finally, absorption spectra of the (ZnO)nC96−2n hybrid materials were predicted by using the time-dependent density functional theory (TD-DFT) approach at the same level of theory (PBE1PBE/6-31G*, SDD). The lowest 100 excited states were considered. The analysis of excited states was performed by using Multiwfn program 78 . The VESTA program 79 was employed to visualize the structures of (ZnO)nC96−2n. The stability of (ZnO)nC96−2n structures was tested by estimating the cohesive energy per atom 80 : where ( ) 96−2 is the total energy of the relaxed (ZnO)nC96−2n structure, , , , is the energy of isolated carbon, hydrogen, zinc, and oxygen atoms, respectively. The summation indices i, j, k and l correspond to the total amount of carbon, hydrogen, zinc, and oxygen atoms ( , , , ) in (ZnO)nC96−2n system with n=1, 3, 4, 7, 12 and 27.

Results and discussion
The optimized geometric structures of (ZnO)nC96−2n are illustrated in Figure 1, while the cohesive energies and selected structural parameters are reported in Table 1   compared to that of (ZnO)12C72 can be explained by the abundance of conduction electrons participating in collective oscillations of free charges and hence contributing to the plasmon resonance.
From Table 1, it is also seen that the HOMO-LUMO energy gap decreases upon increasing the amount of ZnO molecules embedded into C96 matrix. The minimum energy gap of ~1.24 eV is achieved for (ZnO)27C42 system (Figure S4, ESI). The observed energy gap narrowing is mainly affected by the upward shift of the HOMO level, while the downward shift of LUMO acquires a less pronounced character.     (Figure S5, ESI). Particularly, it was revealed that hole and electrons are delocalized over the entire area of pristine C96 system, and no charge separation occurs. Table 2 also summarises the electronic transitions that are involved in these excited states.
The absorption spectrum of the ZnO-free pristine molecule undergoes substantial changes after ZnO incorporation (Figure 2b, Table 2 A more detailed consideration of each of the presented spectra shows that in the case of ZnOC94 system the most intense band extending from 400 nm to 900 nm is related to S3-S5 excited states, respectively. The complementary analysis of the CDD (Figure 3a), Sr, D and t parameters ( Table 2) shows that all three excitations are LE-CT hybrid states. Interestingly, the electrons and holes are more separated in S3 and S5 states than in S4 state (that is confirmed by larger D and less negative t), indicating a dominant CT character for these excited states.
For (ZnO)3C90 system, the distance between centroids of holes and electrons for three most probable transitions is very low because they occupy the same space (Figure 3b). Therefore, S10, S28 and S29 states are mostly local excitations. (ZnO)4C88 exhibits a relatively complex series of spectral features, with dominant contribution from S5, S10 and S16 excited states. First two states can be identified as LE-CT hybrid states with prevailing CT character (Figure 3c), while the S16 state is characterized by low D and rather negative t values, which are peculiar to LE state with minimal charge separation. The absorption spectrum (ZnO)7C82 is dominated by a wide band peaked at 365 nm (with major contribution from S85←S0 transition) followed by the weak shoulder at ~612 nm that can be attributed to the S8←S0, and S11←S0 transitions.
Unexpectedly, all three transitions have completely different nature (see Table 2 and Figure   3d). S8 and S85 can be regarded as local excitations, while S11←S0 has obvious charge transfer characteristics. Furthermore, it is important to note that among all considered systems only     (Figure 4a). Even though the charge-transfer components (C42 → (ZnO)27 and (ZnO)27 → C42, respectively) contribute to the total absorption spectra, it is seen that the band at 614 nm is mostly originating from the electron transitions within the C42 moiety, additionally pointing to the domineering role of local excitations in this spectral region. Therefore, it can be stated that zinc and oxygen atoms are minimally involved in the corresponding excitations since the electrons and holes are mostly localized at the peripheral carbon moieties. In contrast to this, the analysis of the weak absorption band at 366 nm that is dominated by doubly degenerate S62,63 states (Figure 2b), demonstrates a significant role of the charge transfer from the C42 moiety to the (ZnO)27 part in the S62,63 excitations and a small contribution of electronic transitions within the (ZnO)27 fragment. CDD analysis provides additional evidence of the origin of S62,63 states (Figure 4b). Particularly, we notice that the electron accumulation region is mainly located at the (ZnO)27 fragment, while the obvious charge depletion region is observed at C42 moiety.