Abstract
Properties of inorganic–organic interfaces, such as their interface dipole, strongly depend on the structural arrangements of the organic molecules. A prime example is tetracyanoethylene (TCNE) on Cu(111), which shows two different phases with significantly different work functions. However, the thermodynamically preferred phase is not always the one that is best suited for a given application. Rather, it may be desirable to selectively grow a kinetically trapped structure. In this work, we employ density functional theory and transition state theory to discuss under which conditions such a kinetic trapping might be possible for the model system of TCNE on Cu. Specifically, we want to trap the molecules in the first layer in a flat-lying orientation. This requires temperatures that are sufficiently low to suppress the reorientation of the molecules, which is thermodynamically more favorable for high dosages, but still high enough to enable ordered growth through diffusion of molecules. On the basis of the temperature-dependent diffusion and reorientation rates, we propose a temperature range at which the reorientation can be successfully suppressed.
Highlights
Metal−organic interfaces act as a basis for a variety of possible nanotechnological applications, such as molecular switches,[1,2] thermoelectrics,[3,4] memories,[5] transistors,[6−8] or spintronic devices.[9]
We focus exclusively on two fundamental aspects in the low coverage growth regime: The diffusion and the reorientation of individual molecules on the surface
To propose experimental conditions that prevent the reorientation of flat-lying molecules in the first adsorbate layer to the thermodynamically favored upright-standing positions, we studied kinetic processes of tetracyanoethylene (TCNE) molecules on a Cu(111) surface
Summary
Metal−organic interfaces act as a basis for a variety of possible nanotechnological applications, such as molecular switches,[1,2] thermoelectrics,[3,4] memories,[5] transistors,[6−8] or spintronic devices.[9] Owing to the advances in computational material design, possibilities for developing functional interfaces with tailored physical properties and functionalities have increased in the last decades.[10,11] the functionality of these interfaces does not depend on the choice of the metal and the organic component alone. The structure the organic component assumes on the surface plays a decisive role. A prime example are molecular acceptors that undergo a (coverage-dependent) reorientation from flat-lying to uprightstanding positions, such as hexaazatriphenylene-hexacarbonitrile (HATCN) and dinitropyrene-tetraone (NO2-Pyt) on Ag(111).[12,13] Because the electron affinity of organic films depends on their orientation,[14] this is accompanied by significant changes of the charge transfer and interface work functions.[13,15] In the two examples above, the structural transition causes a change of the work function of more than 1 eV, illustrating how important control over the structure is
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