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Final Report Summary - DECIMOL (Decoupled Insulator-Supported Molecules)

Logic circuitry for information processing at the single molecular level is a vision since decades. A fundamental yet open question is the molecule-conductor coupling and their mutual interaction – aspects that ultimately will en- or disable device functionalization. After the initial presentation of rather undisturbed molecular orbitals of single pentacene molecules deposited on a bilayer NaCl film [Repp et al., Phys. Rev. Lett. 94, 026803 (2005)], thin insulating films deposited on metallic surfaces gained increasing attention for the study of decoupled molecules. Motivated from fundamental scientific questions as well as fuelled by the essential requirement of insulating layers in semiconducting device technology, DECiMOL aims for establishing an analogue to this famous insulator-on-metal system on a semiconducting substrate, namely CaF_2 thin films grown on Si(111) surfaces.

In order to gain important insights into the correlations which exist between electronic structure, interaction forces, molecular geometry, and electronic transport in molecule-insulator-semiconductor systems, DECiMOL outlined a combination of three experimental techniques – namely scanning tunnelling microscopy (STM), dynamic force microscopy (DFM, also known as non-contact atomic force microscopy), and photoemission spectroscopy (PES) techniques, and supplements these techniques by theoretical calculations at the density functional theory (DFT) level. The scientific objectives and achievements of DECiMOL fall into three key research topics: (1) the fabrication and characterisation of CaF_1 and CaF_2 thin films on Si(111), (2) the investigation of molecule-substrate electronic coupling across the thin insulating CaF_2 film using comparably inert molecules and (3) the study of functionalised molecules forming specific bonds to the insulating layer.

Within the first research topic, DECiMOL could contribute important details to the understanding of microscopic properties of this sample system. Two different CaF_1 island types with different morphologies were revealed during the initial growth phase of the interface layer (see also Figure 1(a-e) in attached document), one of them likely being the result of Si(111) surface etching by excess fluorine. From high-resolution DFM imaging, DECiMOL was furthermore able to investigate the point and line defect structures observed within the CaF_1 interface layer and especially identified the point defects being of atomic size. As the growth of stoichiometric CaF_2-on-Si films is not only important within DECiMOL, but also in the application-oriented context of resonant tunnelling diodes, the identification of inhomogeneity that could cause non-stoichiometric growth is an important result from the DECiMOL project.

For the growth of thicker insulating layers, DECiMOL tested different approaches with the aim to grow multi-layered CaF_2 islands on the CaF_1 interface layer, suitable for molecule deposition in the other work packages. A two-step deposition method (see Figure 1(f) in attached document), where in a first step a high-quality interface layer was formed on Si(111) and, second, further CaF_2 layers were added at reduced substrate temperatures, carved out to be most suitable for studying molecular adsorption in the second and third key area of DECiMOL.

The second key area of the DECiMOL project focusses on the investigation of molecular decoupling within the CaF_2-on-Si system of a comparably inert molecule, namely Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). First, DECiMOL identified different molecular assemblies on the two CaFx phases: while mostly single molecules were found on the CaF1 interface layer – likely nucleated and trapped at defect sites – the formation of ultrasmall molecular clusters in CaF_2 multilayer areas was observed, see also Figure 2(a-c) in attached document. Second, and at the heart of the project, DECiMOL revealed a distinct difference between the molecular orbital structure of molecules adsorbed on the Si cornerholes – where strong coupling between the molecules and the Si substrate leads to significant electronic modification – and on the CaF_1 interface layer, see Figure 2(d,e) in attached document. As the orbital structure mapped at negative sample bias on CaF_1 has remarkable similarities with the LUMO level of the PTCDA molecule, the results suggest that the molecular orbitals are significantly shifted, i.e., the decoupling efficacy on the interface layer is still limited. In contrast, on multilayer CaF_2 islands PTCDA molecules can be found that are imaged at negative sample bias with an orbital structure more likely related to the HOMO.

This analysis of PTCDA orbital structures delivers important insights into electronic decoupling within the molecule-CaF_2-Si system: While the CaF_1 interface layer reduces the molecule-surface interaction when compared to the strong PTCDA-Si bond on the Si cornerholes, our data suggest that electronic decoupling of PTCDA requires at least a CaF_2/CaF_1 layer structure. We envision that this finding will form an important basis, and could even start a new field, on further studies that involve electronic decoupling of molecules from semiconducting substrates.

The third work package within DECiMOL addresses an inherent issue with molecule-on-insulator systems, namely the necessity to firmly anchor molecules by specific molecule-surface bonds. We selected a functional molecule with two carboxylic acid groups as a ‘probe particle’ and found stable adsorption of small molecular clusters at room temperature. From a statistical analysis of the molecule orientations on CaF_2 areas we furthermore revealed a preferred binding orientation of the molecules with respect to the substrate. DFT calculations scheduled for this system are expected to reveal further insights into atomistic details of this molecule-insulator bond formation. However, finding a suitable ‘match’ between functional organic molecules and insulator surfaces that allows for ordered and stable assembly of molecular superstructures is one of the current challenges in the field of molecular self-assembly and in the context of, i.e., hybrid organic-inorganic devices. Consequently, DECiMOL delivers a promising strategy for firmly anchoring molecules to CaF_2 surfaces, not only for fundamental research, but due to the importance of CaF_2 in optics possibly also in an industry-relevant context such as coating or functionalization of optical systems by organic materials.

Further details about DECiMOL are available on the project website available under

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Paul Cartledge, (Head of Research Contracts)
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