Ab initio calculations were carried out for the naphthalene dimer and naphthalene–anthracene complex to determine their stable geometries and binding energies. Two medium-size basis sets of 6-31G*(0.25) and 6-31+G* were employed at the MP2 level. Five local minima were found for the naphthalene dimer, three of which were parallel-displaced type and the other two T-shaped type. The global minimum geometry was a parallel-displaced structure of a two-layer graphitic type (Ci point group), not the crossed form (D2d). Its energy calculated by the 6-31G*(0.25) and 6-31+G* basis sets was −7.62 and −6.36 kcal/mol, respectively. The naphthalene–anthracene complex showed four local minima, two of which were parallel-displaced type and the other two T-shaped type. The global minimum was a twisted parallel-displaced form (C2), in which the centers of both molecules lie on the same z-axis with their two long axes skewed at an angle of ≈40°. Its energy was −11.30 and −9.52 kcal/mol with the 6-31G*(0.25) and 6-31+G* basis sets, respectively. From these results a set of general rules for the stable geometry of the polycyclic aromatic hydrocarbon clusters were derived, which turned out to be the same as those previously deduced from other systems less directly relevant to polycyclic aromatic hydrocarbons: (1) a face-to-face configuration is unstable, (2) the T-shaped structure is stable, (3) the parallel-displaced structure is also stable. We also found some additional rules: (4) the energies of the T-shaped and parallel-displaced structures are quite comparable when the molecules are small, but (5) the parallel-displaced structure becomes more stable than the T-shaped one as the molecules become larger due to the nature of the π–π interaction. The interplanar distance of stable parallel-displaced structures was about 3.3–3.4 Å, while the plane-to-center distances of T-shaped structures was about 5.0–5.1 Å. We also discovered what we would call the integer rule for the binding energy of the polycyclic aromatic hydrocarbon clusters in that the binding energy varied linearly as the number of overlapping hexagons in the parallel-displaced structures. The ratio of binding energies for the benzene dimer, benzene–naphthalene complex, naphthalene dimer, and naphthalene–anthracene complex were nearly 1:2:3:4.

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Recent success in experimental and theoretical works on metal thiophosphates (MTPs) paved the way to add multiple functionalities of complex oxides, such as ferroelectricity, in 2D materials. To realize multiferroicity and magnetoelectric coupling on layered van der Waals materials, incorporating magnetic ions in the ferroelectric framework is desirable. Unfortunately, replacing the metal ion with a magnetic one in MTPs results in antiferroelectricity in which spontaneous macroscopic polarization is absent. Herein, the emergence of a tunable local ferroelectric state in antiferroelectric CuCrP2S6 possessing magnetic Cr3+ ion is reported. The spontaneous macroscopic polarization is observed, which is switchable by an external poling field through controlling a defect‐dipole polarization in the quasi‐antipolar state. The observations suggest that the formation of defect dipoles, which is facilitated by an order‐disorder‐type structural transition, is likely related to a metastable Cu site within the van der Waals gap and therefore is a smoking gun of the existence of a uniaxial quadruple potential well. The interaction between the defect‐dipole polarization and dipoles in the antipolar matrix may lead to the emerging local ferroelectricity in antiferroelectric CuCrP2S6. The findings suggest a possibility of utilizing the local ferroelectricity of multiferroic MTPs for novel 2D applications.

We report p-type tin monoselenide (SnSe) single crystals, grown in double-sealed quartz ampoules using a modified Bridgman technique at 920 °C. X-ray powder diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) measurements clearly confirm that the grown SnSe consists of single-crystal SnSe. Electrical transport of multi-layer SnSe nanoflakes, which were prepared by exfoliation from bulk single crystals, was conducted using back-gated field-effect transistor (FET) structures with Au and Ti contacts on SiO2/Si substrates, revealing that multi-layer SnSe nanoflakes exhibit p-type semiconductor characteristics owing to the Sn vacancies on the surfaces of SnSe nanoflakes. In addition, a strong carrier screening effect was observed in 70−90-nm-thick SnSe nanoflake FETs. Furthermore, the effect of the metal contacts to multi-layer SnSe nanoflake-based FETs is also discussed with two different metals, such as Ti/Au and Au contacts.

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