Wang Dingguan, Lu Xuefeng, Cai Liangliang, Zhang Lei, Feng Shuo, Zhang Wenjing, Yang Ming, Wu Jishan, Wang Zhuo, Wee Andrew T S
SZU-NUS Collaborative Innovation Center for Optoelectronic Science and Technology, International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, China.
Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore.
ACS Nano. 2022 Jun 28;16(6):9843-9851. doi: 10.1021/acsnano.2c03909. Epub 2022 Jun 3.
Polycyclic hydrocarbons (PHs) share the same hexagonal structure of sp carbons as graphene but possess an energy gap due to quantum confinement effect. PHs can be synthesized by a bottom-up strategy starting from small building blocks covalently bonded into large 2D organic sheets. Further investigation of the role of the covalent bonding/coupling ways on their electronic properties is needed. Here, we demonstrate a surface-mediated synthesis of hexa--hexabenzocoronene (HBC) and its extended HBC oligomers (dimers, trimers, and tetramers) via single- and triple-coupling ways and reveal the implication of different covalent bonding on their electronic properties. High-resolution low-temperature scanning tunneling microscopy and noncontact atomic force microscopy are employed to determine the atomic structures of as-synthesized HBC oligomers. Scanning tunneling spectroscopy measurements show that the length extension of HBC oligomers narrows the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Furthermore, the energy gaps of triple-coupling HBC oligomers are smaller and decrease more significantly than that of the single-coupling ones. We hypothesize that the triple coupling promotes a more effective delocalization of π-electrons than the single coupling, according to density functional theory calculations. We also demonstrate that the HBC oligomers can further extend across the substrate steps to achieve conjugated polymers and large-area porous carbon networks.
多环烃（PHs）与石墨烯具有相同的sp碳原子六边形结构，但由于量子限制效应而具有能隙。PHs可以通过自下而上的策略合成，从小的构建块开始，通过共价键合形成大的二维有机薄片。需要进一步研究共价键合/耦合方式对其电子性质的作用。在这里，我们展示了通过单耦合和三耦合方式在表面介导合成六 - 六苯并蔻（HBC）及其扩展的HBC低聚物（二聚体、三聚体和四聚体），并揭示了不同共价键合对其电子性质的影响。采用高分辨率低温扫描隧道显微镜和非接触原子力显微镜来确定合成后的HBC低聚物的原子结构。扫描隧道谱测量表明，HBC低聚物的长度延伸使最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）之间的能隙变窄。此外，三耦合HBC低聚物的能隙比单耦合的更小，且减小更为显著。根据密度泛函理论计算，我们推测三耦合比单耦合更能促进π电子的有效离域。我们还证明了HBC低聚物可以进一步跨越衬底台阶延伸，以实现共轭聚合物和大面积多孔碳网络。
