Neumann Christof, Wilhelm Richard A, Küllmer Maria, Turchanin Andrey
Institute of Physical Chemistry, Friedrich Schiller University Jena, 07743 Jena, Germany.
Faraday Discuss. 2021 Apr 1;227:61-79. doi: 10.1039/c9fd00119k. Epub 2020 Dec 9.
Aromatic self-assembled monolayers (SAMs) can be cross-linked into molecular nanosheets - carbon nanomembranes (CNMs) -via low-energy electron irradiation. Due to their favorable mechanical stability and tunable functional properties, they possess a high potential for various applications including nanosensors and separation membranes for osmosis or energy conversion devices. Despite this potential, the mechanistic details of the electron irradiation induced cross-linking process still need to be understood in more detail. Here, we studied the cross-linking of 4'-nitro-1,1'-biphenyl-4-thiol SAM on gold. The SAM samples were irradiated with different electron energies ranging from 2.5 to 100 eV in ultra-high vacuum and subsequently analysed by complementary techniques. We present results obtained via spectroscopy and microscopy characterization by high-resolution X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction with micrometre sized electron beams (μLEED) and low-energy electron microscopy (LEEM). To demonstrate the formation of CNMs, the formed two-dimensional molecular materials were transferred onto grids and oxidized wafer and analyzed by optical, scanning electron microscopy (SEM) and atomic force microscopy (AFM). We found a strong energy dependence for the cross section for the cross-linking process, the rate of which decreases exponentially towards lower electron energies by about four orders of magnitude. We conduct a comparative analysis of the cross sections for the C-H bond scission via electron impact ionization and dissociative electron attachment and find that these different ionization mechanisms are responsible for the variation of the cross-linking cross section with electron energy.
芳香族自组装单分子层（SAMs）可通过低能电子辐照交联成分子纳米片——碳纳米膜（CNMs）。由于其良好的机械稳定性和可调的功能特性，它们在包括纳米传感器、渗透分离膜或能量转换装置等各种应用中具有很高的潜力。尽管有这种潜力，但电子辐照诱导交联过程的机理细节仍需更详细地了解。在此，我们研究了金表面4'-硝基-1,1'-联苯-4-硫醇SAM的交联。在超高真空下，用2.5至100 eV范围内的不同电子能量对SAM样品进行辐照，随后通过互补技术进行分析。我们展示了通过高分辨率X射线光电子能谱（XPS）、微米级电子束低能电子衍射（μLEED）和低能电子显微镜（LEEM）进行光谱和显微镜表征获得的结果。为了证明CNMs的形成，将形成的二维分子材料转移到网格和氧化晶圆上，并通过光学、扫描电子显微镜（SEM）和原子力显微镜（AFM）进行分析。我们发现交联过程的截面具有很强的能量依赖性，其速率朝着较低电子能量呈指数下降约四个数量级。我们对通过电子碰撞电离和离解电子附着导致的C-H键断裂的截面进行了比较分析，发现这些不同的电离机制是交联截面随电子能量变化的原因。
