Researchers have reported an organic metal chalcogenide (OMC) birefringent film that retains 77% of the optical anisotropy of its bulk crystal, offering a potential route to high-performance birefringent films for integrated photonic devices.

The research, led by Prof. XU Gang from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences, was published in Angewandte Chemie International Edition.

With the development of integrated optics, there is a growing need to transform birefringent materials from bulk crystals into thin films. However, when traditional birefringent materials are processed into films, their performance often deteriorates significantly, with their optical anisotropy usually retaining only about 20% of that of their bulk counterparts. This limits their use in integrated photonic devices.

OMCs synthesized through coordination chemistry represent a new type of two-dimensional van der Waals (2D-vdW) material, characterized by alternating inorganic and organic layers that form a "molecular-scale superlattice". OMCs exhibit strong structural anisotropy due to the difference between in-plane covalent bonding and out-of-plane van der Waals interactions. Their compatibility with solution-based processing also provides a route to preparing high-quality films.

To develop a birefringent material that can better preserve its optical anisotropy in film form, the researchers synthesized noncentrosymmetric PbHBT crystals using a solvothermal method. The crystal structure of PbHBT consists primarily of alternating PbS inorganic layers and -Ph-OH organic layers. The inorganic PbS layers are covalently anchored to the -Ph-OH organic groups through S-C covalent bonds. These -Ph-OH-decorated PbS layers are further stacked along the c-axis via van der Waals interactions between adjacent organic groups, forming the 3D layered architecture of PbHBT.

The researchers found that the periodic molecular-scale superlattice of inorganic and organic layers produced a strongly anisotropic electron distribution, yielding a bulk birefringence of 0.39 at 546 nm.

The researchers then employed in situ spin-coating layer-by-layer liquid-phase epitaxy based on coordination chemistry to fabricate the films. Each growth cycle enabled self-limiting, self-healing assembly at the molecular scale, allowing film thickness to be controlled at the ~1.3 nm scale while maintaining high crystallographic orientation and no obvious defects.

The resulting films exhibited a birefringence of 0.65 in the visible region and 0.43 in the near-infrared, which the researchers reported as exceeding those of existing films and commercial crystals. At 546 nm, the PbHBT film exhibited a birefringence of 0.3, retaining 77% of the bulk crystal value.

According to the researchers, the findings demonstrate the potential of OMCs as a platform for developing high-birefringence films. Unlike conventional materials that lose much of their anisotropy upon film formation, OMCs can retain molecular-scale order through coordination chemistry and liquid-phase assembly.

The researchers suggest that this principle may extend beyond PbHBT to other OMCs, providing a potential route to birefringent films with high intrinsic birefringence and high retention of bulk optical anisotropy.

Organic metal chalcogenide PbHBT film with giant birefringence. (Image by Prof. XU's group)