Wachter Christoph, Werkovits Anna, Hofmann Oliver T
Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria.
J Chem Phys. 2025 May 28;162(20). doi: 10.1063/5.0252855.
Numerous properties of organic/metal interfaces strongly depend on their polymorphism. To computationally predict the thermodynamically most stable polymorph at finite temperature and pressure, the state-of-the-art method is ab initio thermodynamics. However, several approximations made within ab initio thermodynamics have been developed with small adsorbates in mind, which are comparable in size to a substrate unit cell and have no or only few internal degrees of freedom, and it is unclear how well these approximations translate to larger, more complex organic adsorbates. In this work, we study this question exemplarily for the model system of tetracyanoethylene on Cu(111), which shows a Blyholder-like charge transfer that in amount and shape depends sensitively on the structure of the organic adsorbate. In particular, we investigate how the phase boundaries between different stable polymorphs are affected by the (temperature-dependent) vibrational and entropy terms. Based on these investigations, we find that, while the configurational entropy is de facto negligible for organics, the vibrational free energy can shift phase boundaries by several tens of kelvins. Moreover, we discuss how the vibrational free energy can be approximated and why said approximations work well for organic/metal interfaces.
有机/金属界面的众多性质在很大程度上取决于它们的多晶型性。为了通过计算预测在有限温度和压力下热力学上最稳定的多晶型物，目前最先进的方法是从头算热力学。然而，从头算热力学中所做的一些近似是在考虑小分子吸附质的情况下发展而来的，这些吸附质的大小与衬底晶胞相当，并且没有或只有很少的内部自由度，目前尚不清楚这些近似对于更大、更复杂的有机吸附质的适用性如何。在这项工作中，我们以四氰基乙烯在Cu(111)上的模型系统为例研究了这个问题，该系统显示出类似布莱德霍尔德的电荷转移，其数量和形状对有机吸附质的结构敏感地依赖。特别是，我们研究了不同稳定多晶型物之间的相界如何受到（温度相关的）振动项和熵项的影响。基于这些研究，我们发现，虽然对于有机物来说构型熵实际上可以忽略不计，但振动自由能可以将相界移动几十开尔文。此外，我们讨论了振动自由能如何可以被近似以及为什么所述近似对于有机/金属界面效果良好。
