For the interlayer dielectric in microelectronics, light element compounds are preferably accepted due to less electronic polarization. Here, the nontrivial dielectric nature of the Sb 4 O 6 cage-molecular crystal, known as α-antimony trioxide (α-Sb 2 O 3 ), is reported. The gas-phase synthesized α-Sb 2 O 3 nanoflakes are of high crystal quality, from which the abnormal local admittance responses were revealed by scanning microwave impedance microscopy (sMIM). The remarkably low dielectric constant (k), 2.0∼2.5, is corroborated by the analysis of the thickness-dependent sMIM-capacitance signal. In light of the theoretical calculations, the ultralow molecular density and the significantly suppressed ionic polarization are both crucial to the highly reduced k. Combining with the excellent optical band gap, thermal stability, and breakdown strength, α-Sb 2 O 3 is a promising low k dielectric. KEYWORDS: low k, α-Sb 2 O 3 , Sb 4 O 6 cage, molecular crystal, interlayer dielectric

Coronoids as polycyclic aromatic macrocycles enclosing a cavity have attracted a lot of attention due to their distinctive molecular and electronic structures. They can be also regarded as nanoporous graphene molecules whose electronic properties are critically dependent on the size and topology of their outer and inner peripheries. However, because of their synthetic challenges, the extended hexagonal coronoids with zigzag outer edges have not been reported yet. Here, we report the on-surface synthesis of C144 hexagonal coronoid with outer zigzag edges on a designed precursor undergoing hierarchical Ullmann coupling and cyclodehydrogenation on the Au(111) surface. The molecular structure is unambiguously characterized by bond-resolved noncontact atomic force microscopy imaging. The electronic properties are further investigated by scanning tunneling spectroscopy measurements, in combination with the density functional theory calculations. Moreover, the values of the harmonic oscillator model of aromaticity are derived from calculations that suggest that the molecular structure is ideally represented by Clar's model. Our results provide approaches toward realizing a hexagonal coronoid with zigzag edges, potentially inspiring fabrication of hexagonal zigzag coronoids with multiple radical characters in the future.

The cubic-phase antimony trioxide (α-Sb2O3) is a room-temperature stable molecular crystal, composed of cage-like tetraantimony hexoxide (Sb4O6) molecules. Despite its versatile functionality, the van der Waals (vdW) bond-dominated nanomechanics is still unclear. Here, the bending plate-like linear behaviors of high-quality α-Sb2O3 nanoflakes were observed using the nanoindentation method. It is found that the cage-molecular crystal owns a very low in-plane Young’s modulus of 14.9 ± 0.8 GPa and a remarkable maximum tensile strain of 6.0–8.8%, corresponding to a rupture strength of 0.89–1.31 GPa. Elucidated by the atomistic simulations, the compliant elastic modulus and the unexpectedly strong rupture strain are associated with the high-symmetry vdW bonding structure. The vdW nanomechanics is of fundamental and technological relevance to nanoelectronics.

On-surface synthesis has emerged as a powerful approach to the atomically precise fabrication of molecular architectures with potential applications in nanotechnology. However, it is challenging to synthesize molecular structures that can protrude from the surface such as polymer chains forming by the molecules with upright conformations, since most of the on-surface reaction products, particularly the conjugated structures, prefer to adsorb parallel on the surface to maximize the molecule–substrate interaction. Here, we show an up-standing phenanthrene polymer chain with an all-cis configuration obtained by on-surface synthesis upon highly selective C–H activation. Using bond-resolved nc-AFM imaging, the reaction route of polymers from an in-plane to an all-cis upright conformation is fully characterized, and the reaction mechanism is further revealed in combination with first principles calculations. Our results on this selective dehydrogenation induced upright-oriented polymer chains that will enrich the toolbox for the on-surface synthesis of novel molecular structures and may provide new insights on designing optimized precursors for preparing three-dimensional molecular frameworks through on-surface synthesis.