Kumar Dhaneesh, Hellerstedt Jack, Lowe Benjamin, Schiffrin Agustin
School of Physics and Astronomy, Monash University, Clayton, Victoria A-3800, Australia.
ARC Centre of Excellence in Future Low-Energy Electronics Technologies, Monash University, Clayton, Victoria A-3800, Australia.
Nanotechnology. 2023 Feb 28;34(20). doi: 10.1088/1361-6528/acba20.
Two-dimensional (2D) nanostructures and nanomaterials offer potential for a wide range of technological applications in electronics, optoelectronics, data storage, sensing and catalysis. On-surface molecular self-assembly-where organic molecules act as building blocks and where surfaces play the role of supporting templates-allows for the bottom-up synthesis of such 2D systems with tuneable atomically precise morphologies and tailored electronic properties. These self-assembly protocols are well established on metal surfaces, but remain limited on electronically gapped substrates (insulators, semiconductors). The latter are useful for preventing electronic coupling (that is, hybridization between molecular assembly and underlying surface) and for avoiding quenching of optical processes, necessary for prospective electronic and optoelectronic applications. In particular, molecular self-assembly on surfaces other than weakly interacting metals can be challenging due to substrate reactivity, defects and inhomogeneities, resulting in intricate energy landscapes that limit the growth kinetically and hampers the synthesis of large-area defect-free 2D systems. Here, we demonstrate the self-assembly of a 2D, atomically thin organic molecular film on a model wide bandgap 2D insulator, single-layer hexagonal boron nitride (hBN) on Cu(111). The molecular film consists of flat, aromatic 9,10-di-cyano-anthracene (DCA) molecules. Our low-temperature scanning tunnelling microscopy and spectroscopy measurements revealed mesoscopic (>100 × 100 nm), topographically homogeneous crystalline molecular domains resulting from flat molecular adsorption and noncovalent in-plane cyano-ring bonding, with electronically decoupled molecular orbitals (MOs) lying within the hBN electronic gap. These MOs exhibit an energy level spatial modulation (∼300 meV) that follows the moiré work function variation of hBN on Cu(111). These findings pave the way for large-area, atomically precise, highly crystalline 2D organic (and metal-organic) nanomaterials on electronically functional wide bandgap insulators.
二维（2D）纳米结构和纳米材料在电子、光电子、数据存储、传感和催化等广泛的技术应用中具有潜力。表面分子自组装——有机分子作为构建单元，表面充当支撑模板——允许以自下而上的方式合成具有可调节的原子精确形态和定制电子特性的此类二维系统。这些自组装协议在金属表面已得到充分确立，但在带电子能隙的衬底（绝缘体、半导体）上仍然有限。后者对于防止电子耦合（即分子组装与下层表面之间的杂化）以及避免光学过程的猝灭很有用，这对于预期的电子和光电子应用是必要的。特别是，由于衬底的反应性、缺陷和不均匀性，在除弱相互作用金属之外的表面上进行分子自组装可能具有挑战性，这会导致复杂的能量景观，从而在动力学上限制生长并阻碍大面积无缺陷二维系统的合成。在这里，我们展示了在模型宽带隙二维绝缘体、Cu(111) 上的单层六方氮化硼（hBN）上自组装二维原子级薄有机分子膜。该分子膜由扁平的芳香族 9,10 - 二氰基蒽（DCA）分子组成。我们的低温扫描隧道显微镜和光谱测量揭示了介观（>100×100 nm）、地形均匀的晶体分子域，这是由扁平分子吸附和平面内非共价氰基环键合产生的，其电子解耦分子轨道（MOs）位于 hBN 电子能隙内。这些 MOs 表现出能级空间调制（约 300 meV），其遵循 hBN 在 Cu(111) 上的莫尔功函数变化。这些发现为在电子功能宽带隙绝缘体上制备大面积、原子精确、高度结晶的二维有机（和金属有机）纳米材料铺平了道路。
