Electronic structure modification of copper phthalocyanine (CuPc) induced by intensive Na doping

Investigation into electronic interaction and work function tuning of phthalocyanine molecules and graphene interfaces

Understanding the interfacial electronic structures of organic semiconductor phthalocyanines (MePc) and graphene is essential for their practical application in various fields. In this study, we investigated the electronic structure and the tuning of the work function of free metal phthalocyanine (Pc4), cobalt phthalocyanine (CoPc), and copper phthalocyanine (CuPc) molecules deposited on a graphene monolayer using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). We found that the functionalization of MePc molecules induces p-doping to the graphene substrate due to the charge transfer mechanism with the MePc molecule. XPS analysis shows that the electronic interaction at the interface between Pc molecules and graphene substrate mainly occurs through the pyrrole species. Comparatively, it was observed that the electronic coupling between graphene and CuPc is relatively stronger than that between CoPc and graphene. The electronic interaction of the CuPc/Gr sample principally occurs through the Cu2+ 3d states of CuPc, the molecule, and the graphene 2p π states. This electronic interaction increases the work function of the graphene substrate to 5.75 eV, 6.05 eV, and 6.12 eV after depositing the Pc4, CoPc, and CuPc molecules, respectively. These results show that the MePc/Gr samples investigated in this study can be considered promising candidates for applications requiring a high work function to increase hole injection efficiency.

A curious interplay in the films of N-heterocyclic carbene Pt(II) complexes upon deposition of alkali metals

The recently synthesized series of Pt^II complexes containing cyclometallating (phenylpyridine or benzoquinoline) and N-heterocyclic carbene ligands possess intriguing structures, topologies, and light emitting properties. Here, we report curious physicochemical interactions between in situ PVD-grown films of a typical representative of the aforementioned Pt^II complex compounds and Li, Na, K and Cs atoms. Based on a combination of detailed core-level photoelectron spectroscopy and quantum-chemical calculations at the density functional theory level, we found that the deposition of alkali atoms onto the molecular film leads to unusual redistribution of electron density: essential modification of nitrogen sites, reduction of the coordination Pt^II centre to Pt⁰ and decrease of electron density on the bromine atoms. A possible explanation for this is formation of a supramolecular system “Pt complex-alkali metal ion”; the latter is supported by restoration of the system to the initial state upon subsequent oxygen treatment. The discovered properties highlight a considerable potential of the Pt^II complexes for a variety of biomedical, sensing, chemical, and electronic applications.

Multi-Component Organic Layers on Metal Substrates

Increasingly high hopes are being placed on organic semiconductors for a variety of applications. Progress along these lines, however, requires the design and growth of increasingly complex systems with well-defined structural and electronic properties. These issues have been studied and reviewed extensively in single-component layers, but the focus is gradually shifting towards more complex and functional multi-component assemblies such as donor-acceptor networks. These blends show different properties from those of the corresponding single-component layers, and the understanding on how these properties depend on the different supramolecular environment of multi-component assemblies is crucial for the advancement of organic devices. Here, our understanding of two-dimensional multi-component layers on solid substrates is reviewed. Regarding the structure, the driving forces behind the self-assembly of these systems are described. Regarding the electronic properties, recent insights into how these are affected as the molecule's supramolecular environment changes are explained. Key information for the design and controlled growth of complex, functional multicomponent structures by self-assembly is summarized.

Decacyclene Trianhydride at Functional Interfaces: An Ideal Electron Acceptor Material for Organic Electronics

We report the interface energetics of decacyclene trianhydride (DTA) monolayers on top of two distinct model surfaces, namely, Au(111) and Ag(111). On the latter, combined valence band photoemission and X-ray absorption measurements that access the occupied and unoccupied molecular orbitals, respectively, reveal that electron transfer from substrate to surface sets in. Density functional theory calculations confirm our experimental findings and provide an understanding not only of the photoemission and X-ray absorption spectral features of this promising organic semiconductor but also of the fingerprints associated with the interface charge transfer.

Spin tuning of electron-doped metal-phthalocyanine layers

The spin state of organic-based magnets at interfaces is to a great extent determined by the organic environment and the nature of the spin-carrying metal center, which is further subject to modifications by the adsorbate-substrate coupling. Direct chemical doping offers an additional route for tailoring the electronic and magnetic characteristics of molecular magnets. Here we present a systematic investigation of the effects of alkali metal doping on the charge state and crystal field of 3d metal ions in Cu, Ni, Fe, and Mn phthalocyanine (Pc) monolayers adsorbed on Ag. Combined X-ray absorption spectroscopy and ligand field multiplet calculations show that Cu(II), Ni(II), and Fe(II) ions reduce to Cu(I), Ni(I), and Fe(I) upon alkali metal adsorption, whereas Mn maintains its formal oxidation state. The strength of the crystal field at the Ni, Fe, and Mn sites is strongly reduced upon doping. The combined effect of these changes is that the magnetic moment of high- and low-spin ions such as Cu and Ni can be entirely turned off or on, respectively, whereas the magnetic configuration of MnPc can be changed from intermediate (3/2) to high (5/2) spin. In the case of FePc a 10-fold increase of the orbital magnetic moment accompanies charge transfer and a transition to a high-spin state.

Site- and orbital-dependent charge donation and spin manipulation in electron-doped metal phthalocyanines

Chemical doping offers promise as a means of tailoring the electrical characteristics of organic molecular compounds. However, unlike for inorganic semiconductors used in electronics applications, controlling the influence of dopants in molecular complexes is complicated by the presence of multiple doping sites, electron acceptor levels, and intramolecular correlation effects. Here we use scanning tunnelling microscopy to analyse the position of individual Li dopants within Cu- and Ni-phthalocyanine molecules in contact with a metal substrate, and probe the charge transfer process with unprecedented spatial resolution. We show that individual phthalocyanine molecules can host at least three distinct stable doping sites and up to six dopant atoms, and that the ligand and metal orbitals can be selectively charged by modifying the configuration of the Li complexes. Li manipulation reveals that charge transfer is determined solely by dopants embedded in the molecules, whereas the magnitude of the conductance gap is sensitive to the molecule-dopant separation. As a result of the strong spin-charge correlation in confined molecular orbitals, alkali atoms provide an effective way for tuning the molecular spin without resorting to magnetic dopants.

Tailoring the spin functionality of a hybrid metal-organic interface by means of alkali-metal doping

We employ a recently developed purpose-made technique based on spin-resolved two-photon photoemission spectroscopy to study the influence of alkali-metal doping (Cs and Na) on the spin functionality of the interface between a cobalt thin film and the organic semiconductor copper phthalocyanine. We find two alkali-metal-induced effects. First, alkali-metal atoms act as impurities and increase the spin-flip probability for the electrons crossing the interface (detrimental effect). Second, they allow one to modify the interface energy level alignment and, consequently, to enhance the efficiency of spin injection at an arbitrary energy above the Fermi level of the cobalt (intrinsic effect). We show that the intrinsic effect dominates over the detrimental one, opening the possibility to actively tailor the spin functionality of the considered hybrid metal-organic interface by changing the doping concentration.