Characterizing Topological Flat Bands in Tin-Phthalocyanine Nanosheets Using Photoelectric Detections

Topological Photonics, exploring the topologically nontrivial states in photonics, is a fast-progressing research topic. Topological flat bands, fascinating features in topological photonics, have attracted much attention recently. Metal-phthalocyanine (MPc) molecular semiconductors are formed by molecular chains, offering a great platform to study topological states in low dimension. Previous work shows that the MPc molecular chain could be used to realize one-dimensional topological excitons based on the two-level systems. Therein, the topological flat band is a key feature in this type of topological excitons. Here, we have synthesized tin-phthalocyanine (SnPc) nanosheets. The ultraviolet–visible (UV–vis) and photoelectric experiments on these SnPc nanosheets are performed to characterize the one-dimensional topological flat bands. We demonstrate the strong evidence that the two photocurrent peaks at the wavelengths of 455 and 850 nm could originate from the topological flat bands within the B- and Q-spectral regions, respectively, according to our excitonic model. In addition, the photocurrent is improved by up to two orders using the nanosheets compared to the raw materials. Moreover, the previous excitonic model consisting of two-level systems is further generalized to incorporate both the B- and Q-bands by using three-level systems, interpreting the UV–vis spectra and the photocurrent peaks properly. The combination of our experimental and theoretical research shows that the stable and high broadband photoelectric performance of the SnPc nanosheets not only provides a new material platform for the organic photodetectors but also opens up a new research avenue for the observation of the topological flat band using molecular materials.