Ab initio quantum dynamics of charge carriers in graphitic carbon nitride nanosheets

Abstract

Graphitic carbon nitride (g-C3N4), a metal-free and visible light responsive photocatalyst, has garnered much attention due to its wide range of applications. In order to elucidate the role of dimensionality on the properties of photo-generated charge carriers, we apply nonadiabatic (NA) molecular dynamics combined with time-domain density functional theory to investigate nonradiative relaxation of hot electrons and holes, and electron–hole recombination in monolayer and bulk g-C3N4. The nonradiative charge recombination occurs on a nanosecond timescale and is faster in bulk than the nanosheet, in agreement with the experiment. The difference arises due to the smaller energy gap and participation of additional vibrations in the bulk system. The long carrier lifetimes are favored by small NA coupling and rapid phonon-induced loss of quantum coherence between the excited and ground electronic states. Decoherence is fast because g-C3N4 is soft and undergoes large scale vibrations. The NA coupling is small since electrons and holes are localized on different atoms, and the electron–hole overlap is relatively small. Phonon-driven relaxation of hot electrons and holes takes 100–200 fs and is slightly slower at higher initial energies due to participation of fewer vibrational modes. This feature of two-dimensional g-C3N4 contrasts traditional three-dimensional semiconductors, which exhibit faster relaxation at higher energies due to larger density of states, and can be used to extract hot carriers to perform useful functions. The ab initio quantum dynamics simulations present a comprehensive picture of the photo-induced charge carrier dynamics in g-C3N4, guiding design of photovoltaic and photocatalytic devices.

Ultrafast Interlayer Charge Separation, Enhanced Visible‐Light Absorption, and Tunable Overpotential in Twisted Graphitic Carbon Nitride Bilayers for Water Splitting

“…Recently, Prezhdo and co-workers discussed the charge carrier dynamics in monolayer g-C 3 N 4 using NAMD. [38] Their results showed that due to the significant orbital overlap, the photoexcited electrons in the higher conduction band orbitals will relax to the bottom of the conduction band in 100 to 200 fs which is close to our results attributed to the interlayer interaction. Ultrafast interlayer charge transfer and good spatial separation in the twisted bilayer system imply a potential enhancement in photocatalytic performance.…”

Harnessing the Potential of Graphitic Carbon Nitride for Optoelectronic Applications

“…Nanosheets of 1-2 layers, [50,165] few layers, [166,167] and 6-12 layers have been previously studied, [49,75] and monolayers have been modeled using first-principle calculations. [168][169][170] However, these sporadic reports do not provide a clear picture of how the thickness of nanosheets affects their properties. In 2D transition metal dichalcogenides with thicknesses of one to several layers, exciton consideration is of great significance to exploit these materials in high-end photonic and plasmonic applications.…”

“…[171] In the case of 2D perovskites, sample thickness can have a profound effect on exciton-exciton annihilation. [172] Current knowledge indicates that the charge carrier dynamics of monolayer g-CN is very different from that of bulk CN, [169] suggesting that corrugation has a significant influence on g-CN properties (bandgap and optical absorption) and thus influences optoelectronic and catalytic performance, [170,173] as in the case of graphene. [174] Some other interesting aspects of g-CN also deserve a closer look.…”

Extending the Time Scales of Nonadiabatic Molecular Dynamics via Machine Learning in the Time Domain

“…Depending on the level of theory used and the size of the systems studied, such calculations may be quite expensive. Conventionally, few-picosecond trajectories are computed for systems of few hundreds of atoms while using Kohn–Sham (KS) orbitals and pure functionals such as the Perdew–Burke–Ernzerhof (PBE). ,,− Recently, we have extended this procedure to go beyond the KS picture via the time-dependent density functional theory (TD-DFT) calculations of excited states and computing the corresponding couplings. , However, one major drawback of such an approach is that the TD-DFT still relies on the short-ranged exchange component of the PBE functional and hence is incapable of fully capturing excitonic effects. To capture such effects, the TD-DFT calculations with hybrid functionals are desirable.…”