Defect states at organic–inorganic interfaces: Insight from first principles calculations for pentaerythritol tetranitrate on MgO surface

Abstract

Light-responsive organic-inorganic interfaces offer experimental opportunities that are otherwise difficult to achieve. Since laser light can be manipulated very precisely, it becomes possible to engineer selective, predictive, and highly controlled interface properties. Photochemistry of organic-inorganic energetic interfaces is a rapidly emerging research field in which energy absorption and interface stability mechanisms have yet to be established. To explore the interaction of the laser irradiation with molecular materials, we performed first principles calculations of a prototype organic-inorganic interface between a nitroester (pentaerythritol tetranitrate, PETN, C 5 H 8 N 4 O 12 ) and a magnesium oxide (MgO) surface. We found that the light absorption is defined by the band alignment between interface components and interfacial charge transfer coupled with electronic states in the band gap, generated by oxide surface defects. Hence the choice of an oxide substrate and its morphology makes the optical absorption tunable and governs both the energy accumulation and energy release at the interface. The obtained results offer a possible consistent interpretation of experiments on selective laser initiation of energetic materials, which reported that the presence of metal oxide additives triggered the photoinitiation by an excitation energy much lower than the band gap. We suggest that PETN photodecomposition is catalyzed by oxygen vacancies (F 0 centers) at the MgO surface. Our conclusions predict ways for a complete separation of thermo-and photo-stimulated interface chemistry of molecular materials, which is imperative for highly controllable fast decomposition and was not attainable before. The methodology described here can be applied to any type of molecular material/wide band gap dielectric interfaces. It provides a solid basis for novel design and targeted improvements of organic-inorganic interfaces with desired properties that promise to enable vastly new concepts of energy storage and conversion, photocatalysis, and molecular electronics.

Photochemistry of the α-Al2O3-PETN Interface

“…The calculated binding energy for the PETN molecule adsorbed on the α-Al 2 O 3 surface in the configuration-1 (1.59 eV) is two times higher than that obtained from the configuration-2 (0.80 eV). A comparison of the adsorbtion energies obtained for α-Al 2 O 3 -PETN with those reported earlier for MgO-PETN system (0.43 eV) [22] indicates significantly stronger binding of the PETN molecule to Al 2 O 3 than to MgO surface. This actually means that the Al 2 O 3 -PETN interface should be more stable than MgO-PETN.…”

“…Taking into account that the electronic structure of an organic molecular crystal is nearly fully defined by the electronic structure of constituting molecules, we constructed simplified model supercells, which consist of an individual PETN molecule positioned at the (0001) α-Al 2 O 3 surface (Figure 1). Similar model structures have been recently employed to investigate the photo-chemistry of the PETN-MgO interfaces [22] and interactions of various nitro-containing molecules with silicon [24], aluminum [25], and aluminum oxide [26] surfaces.…”

“…Our current research was inspired by an idea of achieving a highly controllable explosive decomposition chemistry initiated by photo-excitation with standard available lasers. In a series of recent experiments [18,19,20,21] and quantum-chemical simulations [22,23], it was fundamentally established that the decomposition of a high explosive material, PETN, can be reliably triggered by laser irradiation once PETN crystals are mixed with a small fraction of micron size metal oxide (e.g., MgO) powders. This demonstrated that there is a strong dependence of the initiation process on the presence of the oxide.…”

“…However, once they form an interface, or a composite, their optical, electronic, and photochemical properties significantly change. Thus, oxygen vacancies at the MgO surface facilitate strong chemical adsorption of PETN on MgO, induce a charge transfer, and consequently may initiate the PETN decomposition by laser light with the photon energy of 1.17 eV, with an unusually low activation barrier [22,23].…”

Molecular Theory of Detonation Initiation: Insight from First Principles Modeling of the Decomposition Mechanisms of Organic Nitro Energetic Materials

“…Because energetic materials are densely packed, we predict that there are many other manifestations of the crystal arrangement governing the decomposition chemistry and hence the sensitivity. To mention just a few vivid illustrations, we recall the shear-strain effect in DADNE [21,124,141], the aspects of autocatalysis analyzed for DADNE and TATB [124], polarization-induced charge transfer in δ-HMX [96], enhanced optical absorption on PETN-MgO interfaces [142], and photocatalytic dissociation of PETN molecules adsorbed on F 0 -centers on the α-Al 2 O 3 (0001) surfaces [143]. A variety of structural and electronic defects also play a role in changing decomposition chemistry in organic molecular crystals [19,122,133,144,145,146].…”