Real-space investigation of surface reactions of organic adsorbates on silicon surfaces using a combination of fast laser heating and scanning tunneling microscopy

在大多数情况下，有机分子在半导体表面的吸附是通过控制反应和最终产品的中间状态进行的。对于含有氮或氧等杂原子的分子，中间体通常是通过杂原子和表面未占据的悬挂键状态之间的化学键形成的。该反应进一步通过所谓的转换势垒进入表面上的共价束缚终态。常规实验通常只解决中间态和终态之间转换障碍最低的途径；较高活性的反应通道仅在与STM或光谱学实验的典型时间尺度相比总体速率太高的温度下才对反应速率有显著贡献。在拟议的研究中，将重点研究醇、胺和醚等模型类型有机分子在Si(001)面上的势能面，重点是较高活性的反应通道。为了做到这一点，我们将通过纳秒时间尺度上的激光感应加热脉冲来加热表面；在这些快速加热循环之间，将通过扫描隧道显微镜(STM)来跟踪表面重排。因此，快加热周期与慢测量周期是解耦的，可以以原子精度观测到高达10^91/S的速率。因此，实验可以获得更宽的温度范围；然后从作为温度函数的终态分布推导出各个反应通道的活化能。因此，不仅可以访问活性最低的反应通道，而且可以访问势能面的较大部分。我们以这样一种方式选择分子，不同的变量被分开处理。例如，在乙醚分子吸附的情况下，局部表面电子结构对最终构型的影响将是最重要的，因为反应将主要通过O-C裂解进行。另一方面，对于醇的吸附，O-H和O-C裂解之间的竞争将是我们研究的重点。被研究的分子可以被看作是广泛的吸附体系在Si(001)上的模型系统，因此结果应该有助于更好地理解半导体表面的一般表面反应，特别是关于潜在的驱动力。此外，这些结果对于具有多官能团有机分子的半导体表面的功能化很重要：在这种情况下，对反应的控制，例如通过吸附温度，需要关于更高活性的反应通道的知识。

The adsorption of organic molecules on semiconductor surfaces proceeds in most cases via an intermediate state which controls the reaction and the final product. For molecules containing a heteroatom such as nitrogen or oxygen, the intermediate is typically formed by a dative bond between the heteroatom and the unoccupied dangling bond state of the surface. The reaction further proceeds via the so-called conversion barrier into the covalently bound final state on the surface. Conventional experiments typical address the pathway with the lowest barrier for conversion between intermediate and final state only; higher-activated reaction channels contribute significantly to the reaction rate only at temperatures at which the overall rate is too high compared to the typical time scale of STM or spectroscopy experiments. In the proposed study, the potential energy surface of model-type organic molecules, such as alcohols, amines, and ethers, on Si(001) will be investigated with focus on the higher-activated reaction channels. In order to do so, we will heat the surface by means of laser-induced heating pulses on the nanosecond timescale; between these fast heating cycles, rearrangement on the surface will be followed by means of STM. The fast heating cycles are thus decoupled from the slow measurement cycle and rates as high as 10^9 1/s can be observed with atomic precision. In consequence, a wider range of temperature is accessible by the experiments; from the distribution of final states as a function of temperature the activation energies for the respective reaction channels are then deduced. Thus not only the lowest-activated reaction channel but a larger part of the potential energy surface can be accessed. We choose the molecules in such a way that the different variables a separately addressed. E.g., in the case of the adsorption of ether molecules, the influence of the local surface electronic structure on the final configurations will be most important since the reaction will predominantly take place via O-C cleavage. On the other hand, in the case of the adsorption of alcohols, the competition between O-H and O-C cleavage will be in the focus of our studies.The molecules to be investigated can be seen as model systems for a wide range of adsorbate systems on Si(001) and the results should thus lead to a better understanding of surface reactions on semiconductor surfaces in general, especially with respect to the underlying driving forces. Furthermore, the results are important with respect to the functionalization of semiconductor surfaces with multi-functional organic molecules: In that case, control of the reaction, e.g., by the adsorption temperature, requires the knowledge on the higher-activated reaction channels.